Hydrogels for medical and related applications

Hydrogels for Medical and Related Applications In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symp...

Author: Joseph D. Andrade

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Hydrogels for Medical and Related Applications

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


Hydrogels for Medical and Related Applications Joseph D . Andrade, EDITOR University of Utah

A symposiu by the Division of Polymer Chemistry, Inc. at the 170th Meeting of the American Chemical Society, Chicago Ill., August 27-28, 1975

ACS

SYMPOSIUM

SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C.

1976


31

Library of Congress

Data

Hydrogels for medical and related applications. (ACS symposium series; 31 ISSN 0097-6156) Includes bibliographical references and index. 1. Colloids in medicine—Congresses. 2. Colloids— Congresses. I. Andrade, Joseph D., 1941II. American Chemical Society. Division of Polymer Chemistry. III. Title. IV. Series: American Chemical Society. ACS symposium series; 31. R857.C66H9 ISBN 0-8412-0338-5

610'.28

76-28170 ACSMC8 31 1-359

Copyright © 1976 American Chemical Society All Rights Reserved. No part of this book may be reproduced or transmitted in any form or by any means—graphic, electronic, including photocopying, recording, taping, or information storage and retrieval systems—without written permission from the American Chemical Society. PRINTED IN THE UNITED STATES OF AMERICA


ACS Symposium Series Robert

F.

Gould,

Editor

Advisory Kenneth B. Bischoff Jeremiah P. Freeman E. Desmond Goddard Jesse C. H. Hwa Philip C. Kearney Nina I. McClelland John B. Pfeiffer Joseph V. Rodricks Aaron Wold


FOREWORD The ACS S Y M P O S I U SERIES was founded in 1974 to provide a medium for publishin format of the SERIES parallels that of the continuing A D V A N C E S I N C H E M I S T R Y SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published in the ACS S Y M P O S I U M SERIES are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.


PREFACE here is considerable interest and activity in the application of synthetic and biological polymers in medicine. Synthetic polymers are widely used as surgical and dental implants as well as for blood bags, syringes, tubing, etc. Most of the materials used in medicine have properties greatly different from the tissue with which they are interfaced or replacing. Excepting bones, nails, and the outer layers of skin, mammalian tissues are highly aqueous materials, with water contents ranging up to 90% (blood plasma) Hydrogels are three dimensional networks of hydrophilic polymers, generally covalently or ionically cross-linked, which interact with aqueous solutions by swelling to some equilibrium value. Aqueous gel networks can be relatively strong (such as in celluose dialysis membranes) or relatively weak, generally becoming weaker as the water content increases, although such variables as the nature of the cross-linker, polymer network, tacticity, and crystallinity can significantly influence the mechanical behavior. Interest has focused on the utilization of the bulk or the surface properties of hydrogels for biomedical applications. The bulk property of swelling is of particular interest for "swelling implants," i.e., implants which can be implanted in a small dehydrated state via a small incision and which then swell to fill a body cavity and/or to exert a controlled pressure. The swelling of synthetic and natural gels may also help elucidate swelling and osmotic mechanisms in biological tissues. A related bulk property is the permeability of hydrogels for low molecular weight solutes. Solute diffusivity in gels can be used in sustained drug release applications and in the transport of solutes to gelentrapped macromolecules, particularly enzymes immobilized in the gel network. Ion interactions or partitioning within the gels are important in bone interfacing applications. Aqueous gels are relatively subtle systems which equilibrate with and "follow" many environmental changes. The properties of such gels can be highly dependent on cross-linker levels, impurity levels of comonomers, catalyst residues, and stereoregularity. These variables are discussed for hydrophilic methacrylate ester monomers and their gels. These systems receive the most attention in this volume. Monomer XI


analysis and purification, free radical initiator effects, and tacticity are discussed for the poly(hydroxyethyl methacrylate) system. The bulk properties of synthetic hydrogels can be widely varied and perhaps tailored to specific end uses. Though the poly (hydroxy ethyl methacrylate ) system is the most widely studied for medical applications, a large repertoire of gel types are available and have been applied to varying degrees in medicine. Questions of long term biostability or intentional biodegradeability are also very important, but are not discussed here. However, a number of fairly basic studies of gel networks and a number of practical applications based on network properties are presented. The surface and interfacial properties of hydrogels are somewhat similar to those of natural biological gels and tissues. A number of analogies or comparisons have been made between the hydrogel/water interface and the living cell/physiologic solution interface. Such interfaces are difficult to stud classical surface chemistry cannot be applied. The effect of the gel/water interface on interfacial fluid dynamics and the use of neutral gel coatings to reduce local fluid movement ( electroosmosis ) in particle electrophoresis experiments are discussed. The interface electrokinetic potential (as measured by the streaming potential) can be varied by use of appropriate comonomer compositions. The wettability of hydrogel surfaces is rather complicated, perhaps because of the mobility of polymer segments in the interfacial zone. Protein adsorption at certain gel interfaces is also discussed and related to interfacial free energy arguments. Aqueous gel surfaces can be used in blood-contact applications, as tubing, catheters, and vascular devices. Such applications depend on the nature of the gel/blood interface. As hydrogels generally lack suitable mechanical properties for vascular device applications, there has been considerable activity in coating or grafting the gels to mechanically suitable supports. In discussing blood compatibility of gels, the application of hydrogels as a substrate for in vitro cell or tissue culture studies should be considered. Data on the blood compatibility of hydrogels is available, particularly for polyacrylamide, and there is considerable interest and activity among medical research scientists and a number of commercial firms in hydrogel-coated catheters, drains, and other conduits. Aqueous gel networks have an important role to play in biomedicine, not only as inert blood conduits and devices, but as biochemically and pharmaceutically active elements. The role of ion and solute partitioning or interactions with gels will prove important for such applications as enzyme entrapment, sustained drug or ion release, and bone induction or xii


calcification, as well as for the more popular applications of blood and tissue interfacing. Suitably equilibrated and solute "stocked" gels should prove very important for cell and tissue culture applications. Control of hydration/dehydration phenomena, particularly hysteresis effects, is needed in order to ship and store hydrogel products in a dry state, thus reducing bulk and the possibility of microorganism contamination. This volume is dedicated to the memory of Dr. Willem Prins, a pioneer and leader in the study of gel networks, who died in a boating accident in 1974. I am particularly grateful to Buddy D . Ratner and Allan S. Hoffman for the review paper which begins this volume and to Donald Gregonis for aid in assembling the papers for publication. The participants in the symposium were also the reviewers of the papers. I thank them all for speedy but critical reviews. The secretarial assistance of Terry Smith is gratefully acknowledged University of Utah Salt Lake City, Utah March 5, 1976

JOSEPH

D.

ANDRADE

xiii


1 Synthetic Hydrogels for Biomedical Applications BUDDY D. RATNER and ALLANS.HOFFMAN Departments of Chemical Engineering and Bioengineering, University of Washington, Seattle, Wash. 98195

It is the intention of this paper to review the literature concerning the preparation tions of synthetic hydroge subject is now voluminous and rapidly expanding (e.g. this symposium) it is difficult to produce a completely comprehensive review of the subject. However, a large number of articles will be examined which should give a useful overview of research interests and directions in this growing, multidisciplinary field. This review will be organized as follows: The introduction will discuss general aspects of synthetic hydrogels and point out similarities between the various distinctly different chemical structures which fit into this category. A short section is included within the introduction on problems related to the measurement and description of the biocompatibility of materials. The following six classes of hydrogel materials will then each be treated individually: poly(hydroxyalky1 methacrylates); poly(acrylamide), poly(methacrylamide) and derivatives; poly (N-vinyl-2-pyrrolidone); anionic and cationic hydrogels; polyelectrolyte complexes; and poly(vinyl alcohol). Sections on surface coated hydrogels, the characterization of imbibed water within hydrogels and immobilization or entrapment of biologically active molecules on and within hydrogels for biomedical applications are also included. 1.

Introduction

A. General Aspects of Synthetic Hydrogels. A hydrogel can be defined as a polymeric material which exhibits the ability to swell in water and retain a significant fraction (e.g., > 20%) of water within i t s structure, but which w i l l not dissolve in water. Included in this definition are a wide variety of natural materials of both plant and animal origin, materials prepared by modifying naturally occurring structures, and synthetic polymeric materials. This review article w i l l consider only synthetic hydrogel systems which are being used as, or have 1


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HYDROGELS FOR

MEDICAL AND

RELATED APPLICATIONS

p o t e n t i a l f o r use, as b i o m a t e r i a l s . This c o n s t r a i n t i s not i n tended to imply that b i o m a t e r i a l s prepared from n a t u r a l b i o polymers are unimportant or u n i n t e r e s t i n g . Examples of modified n a t u r a l biopolymers which are p r e s e n t l y r e c e i v i n g a t t e n t i o n f o r biomedical a p p l i c a t i o n s i n c l u d e Cuprophan, c r o s s - l i n k e d Dextrans, and c r o s s - l i n k e d , e n z y m a t i c a l l y t r e a t e d c o l l a g e n s . For a review of n a t u r a l t i s s u e s used as b i o m a t e r i a l s see the recent a r t i c l e ly K i r a l y and Nosé ( 1 ) . Hydrogel m a t e r i a l s resemble i n t h e i r p h y s i c a l p r o p e r t i e s l i v i n g t i s s u e more so than any other c l a s s of s y n t h e t i c b i o m a t e r i a l . In p a r t i c u l a r , t h e i r r e l a t i v e l y h i g h water contents and t h e i r s o f t , rubbery consistency give them a s t r o n g , superf i c i a l resemblance to l i v i n g s o f t t i s s u e . Based upon these p r o p e r t i e s a number of advantages, some o b v i o u s l y r e a l and others somewhat s p e c u l a t i v b c i t e d f o hydrogel m a t e r i a l s With respect to the r e a F i r s t , the expanded natur hydroge p e r m e a b i l i t y to s m a l l molecules a l l o w s p o l y m e r i z a t i o n i n i t i a t o r molecules, i n i t i a t o r decomposition products, p o l y m e r i z a t i o n s o l vent molecules and other extraneous m a t e r i a l s to be e f f i c i e n t l y e x t r a c t e d from the g e l network before the hydrogel i s placed i n contact w i t h a l i v i n g system. The i n v i v o l e a c h i n g of a d d i t i v e s used d u r i n g the f a b r i c a t i o n of polymeric m a t e r i a l s has been c i t ed as a cause of inflammation and eventual r e j e c t i o n of implanted b i o m a t e r i a l s £2). Second, the r a t h e r s o f t and rubbery c o n s i s t ency of most hydrogels can c o n t r i b u t e to t h e i r b i o c o m p a t i b i l i t y by minimizing mechanical ( f r i c t i o n a l ) i r r i t a t i o n to surrounding c e l l s and t i s s u e . The most i n t r i g u i n g of the p o t e n t i a l advantages f o r hydrog e l s i s the low i n t e r f a c i a l t e n s i o n which may be e x h i b i t e d between a hydrogel s u r f a c e and an aqueous s o l u t i o n . This low i n t e r f a c i a l t e n s i o n should reduce the tendency of the p r o t e i n s i n body f l u i d s to adsorb and to u n f o l d upon a d s o r p t i o n (3). Minimal p r o t e i n i n t e r a c t i o n may be important f o r the b i o l o g i c a l acceptance of f o r e i g n m a t e r i a l s as the d e n a t u r a t i o n of p r o t e i n s by surfaces may serve as a t r i g g e r mechanism f o r the i n i t i a t i o n of thrombosis or f o r other b i o l o g i c a l r e j e c t i o n mechanisms. The a b i l i t y of s m a l l molecules to d i f f u s e through hydrogels may a l s o be advantageous f o r hydrogel i n v i v o performance. The d i f f u s i o n of important low molecular weight metabolites and ions through the implant and to the surrounding t i s s u e would occur w i t h hydrogels, but not w i t h r e l a t i v e l y hard, impermeable plastics. A number of biomedical a p p l i c a t i o n s f o r hydrogels which have been mentioned i n the l i t e r a t u r e are l i s t e d i n Table I . The wide range of biomedical a p p l i c a t i o n s f o r hydrogels can be a t t r i buted both to t h e i r s a t i s f a c t o r y performance upon i n v i v o i m p l a n t a t i o n i n e i t h e r blood c o n t a c t i n g or t i s s u e c o n t a c t i n g s i t u a t i o n s and to t h e i r a b i l i t y to be f a b r i c a t e d i n t o a wide range of morphologies. The ease w i t h which the p h y s i c a l form


1.

RATNER AND H O F F M A N

Synthetic Hydrogels

3

of a hydrogel can be a l t e r e d a l l o w s the p h y s i c a l p r o p e r t i e s of the hydrogel to be adjusted s p e c i f i c a l l y f o r a g i v e n a p p l i c a t i o n . Hydrogels can o f t e n be prepared i n the form of porous sponges, non-porous g e l s , o p t i c a l l y transparent f i l m s , l i q u i d s which can be subsequently c r o s s l i n k e d t o form g e l s and c o a t i n g s bound by e i t h e r covalent bonds or non-covalent f o r c e s to a s u b s t r a t e p o l y mer m a t e r i a l . I t should be emphasized, however, t h a t a p a r t i c u l a r hydrogel composition s u i t a b l e f o r one b i o m e d i c a l a p p l i c a t i o n may have to be s i g n i f i c a n t l y m o d i f i e d i n composition and form f o r a d i f f e r e n t a p p l i c a t i o n . That i s , the hydrogel system must be matched t o each b i o m e d i c a l use. I n d i v i d u a l c l a s s e s of hydrogel b i o m a t e r i a l s are d i s c u s s e d i n S e c t i o n I I . Table I Potentia Biomedical A p p l i c a t i o n Coatings

"Homogeneous" M a t e r i a l s

Sutures Catheters IUD s Blood Detoxicants Sensors ( e l e c t r o d e s ) Vascular g r a f t s Electrophoresis c e l l s C e l l C u l t u r e Substrates

Enzyme TheraElectrophoresis gels p e u t i c Systems Contact Lenses Artificial A r t i f i c i a l Corneas Organs V i t r e o u s Humor ReplaceDrug D e l i v e r y ments Systems Estrous-Inducers Breast or other S o f t T i s s u e Substitutes Burn Dressings Bone Ingrowth Sponges Dentures Ear Drum Plugs Synthetic Cartilages Hemodialysis Membranes P a r t i c u l a t e C a r r i e r s of Tumor A n t i b o d i e s

f

Devices

Although the presence of imbibed water w i t h i n a polymeric system i s not a guarantee of b i o c o m p a t i b i l i t y , i t i s b e l i e v e d that the r e l a t i v e l y l a r g e f r a c t i o n of water w i t h i n c e r t a i n hydrogel m a t e r i a l s i s i n t r i n s i c a l l y r e l a t e d to t h e i r h i g h b i o c o m p a t i b i l i t y (4), (see S e c t i o n I V ) . However, these h i g h l y hydrated and w a t e r - p l a s t i c i z e d polymer networks are u s u a l l y mechanically weak. Furthermore, the higher the water content of the g e l , the poorer the mechanical p r o p e r t i e s of the g e l become. F o r t u n a t e l y , there are a number of approaches which can be taken to minimize problems due to the poor mechanical p r o p e r t i e s of these g e l s . Probably the s i m p l e s t of these approaches c o n s i s t s of forming the hydrogel over or surrounding a s t r o n g polymeric mesh or woven f a b r i c . Other methods i n v o l v e c o a t i n g a device or m a t e r i a l w i t h a l a y e r of hydrogel. In order t h a t the c o a t i n g


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HYDROGELS FOR

MEDICAL AND


remain i n t a c t i t must e i t h e r be used i n a non-solvent environment, be c r o s s l i n k e d , or be i n t i m a t e l y and/or c o v a l e n t l y bound to the support m a t e r i a l . Because of the p o t e n t i a l importance and comp l e x i t y of techniques designed to anchor hydrogel c o a t i n g s , the whole area of hydrogels coated onto other surfaces w i l l be d i s cussed s e p a r a t e l y i n S e c t i o n I I I . F i n a l l y , hydrogels u s u a l l y have a l a r g e number of p o l a r r e a c t a b l e s i t e s on which other molecules may be immobilized by a v a r i e t y of chemical techniques. In a d d i t i o n , b i o l o g i c a l l y a c t i v e molecules can be entrapped w i t h i n the network s t r u c t u r e of c r o s s l i n k e d hydrogels. I m m o b i l i z a t i o n of b i o l o g i c a l l y a c t i v e molecules to and w i t h i n s y n t h e t i c hydrogels w i l l be discussed i n more det a i l i n S e c t i o n V. B. B i o c o m p a t i b i l i t y - O p e r a t i o n a the b i o c o m p a t i b i l i t y o countered i n p r o p e r l y d e f i n i n g the terminology used to d e s c r i b e the response of a l i v i n g system to an implanted f o r e i g n m a t e r i a l . Thus, terms such as "non-thrombogenic", "blood compatible", and "biocompatible" are o f t e n i n d i s c r i m i n a t e l y used to d e s c r i b e a wide range of b i o l o g i c a l responses. Bruck has itemized a number of f a c t o r s which might be d e l e t e r i o u s to the performance of m a t e r i a l s used f o r long-term i n t e r n a l biomedical a p p l i c a t i o n s (5). Based upon h i s d e s c r i p t i o n , the i d e a l b i o m a t e r i a l ( i n terms of b i o l o g i c a l response) could be defined as one which does not cause thrombosis, d e s t r u c t i o n of c e l l u l a r elements, a l t e r a t i o n of plasma p r o t e i n s , d e s t r u c t i o n of enzymes, d e p l e t i o n of e l e c t r o l y t e s , adverse immune responses, damage to adjacent t i s s u e , cancer and/ or t o x i c or a l l e r g i c r e a c t i o n s . No s y n t h e t i c m a t e r i a l developed f u l l y s a t i s f i e s these c r i t e r i a . A l s o , no s i n g l e t e s t method f o r e v a l u a t i n g b i o m a t e r i a l s i s capable of measuring t h i s wide range of f a c t o r s r e l e v a n t to b i o m a t e r i a l response. Based upon the l i m i t a t i o n s imposed by a v a i l a b l e t e s t i n g procedures, d i s c u s s i o n i n t h i s review paper concerning the b i o l o g i c a l performance of b i o m a t e r i a l s w i l l be o r i e n t e d towards the t e s t methods which have been used to evaluate the m a t e r i a l s . With respect to the most commonly used t e s t methods, the f o l l o w i n g comments should a l l o w a more c r i t i c a l reading of t h i s review. Lee White Test (and other r e l a t e d s t a t i c , i n v i t r o coagulat i o n time a s s a y s ) : The Lee White t e s t compares the c o a g u l a t i o n time of r e c a l c i f i e d whole blood i n a t e s t tube made of or coated w i t h the m a t e r i a l to be evaluated w i t h the c o a g u l a t i o n time of blood i n a standard c o n t r o l tube ( u s u a l l y g l a s s ) . V a r i a b l e s which can a f f e c t the r e s u l t s from t h i s t e s t i n c l u d e changing the donor, changes i n the d i e t or medication of the donor, storage time of the blood, venipuncture technique, and v a r i a t i o n s i n the experimental technique used to measure the c l o t t i n g times. The t e s t has a l s o been c r i t i c i z e d because of the l a r g e b l o o d - a i r i n t e r f a c e which i s exposed. The v a l i d i t y of r e s u l t s , p a r t i c u l a r l y as they apply to s i t u a t i o n s i n v o l v i n g contact w i t h f l o w i n g


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5

blood i n an i n v i v o or ex v i v o s i t u a t i o n , have f r e q u e n t l y been questioned. Methods f o r the i n v i t r o e s t i m a t i o n of the blood c o m p a t i b i l i t y of m a t e r i a l s have been c r i t i c a l l y reviewed (6). Vena Cava Ring Test Q) : This t e s t method i n v o l v e s the i m p l a n t a t i o n of s t r e a m l i n e d r i n g s made of or coated w i t h the m a t e r i a l to be evaluated i n t o the vena cava of dogs, u s u a l l y f o r 2 hour and 2 week t e s t p e r i o d s . The f o l l o w i n g terminology has been used to d e s c r i b e the experiment r e s u l t s : "Thrombogenid i s used to designate those m a t e r i a l s (formed i n t o t e s t r i n g s ) which are completely occluded w i t h thrombus a f t e r only 2 hours i m p l a n t a t i o n . "Moderately thromboresistant" i s used to d e s c r i b e m a t e r i a l s which remain patent a f t e r 2 hour i m p l a n t a t i o n but show s i g n i f i c a n t adherent thrombus a f t e r the two week t e s t p e r i o d . The d e s i g n a t i o n " h i g h l y thromboresistant" i s reserved f o r mater i a l s which show l i t t l two week i m p l a n t a t i o n p e r i o d d i f f e r e n t procedure f o r d e s c r i b i n g the r e s u l t s has r e c e n t l y been published ( 8 ) . The vena cava r i n g t e s t does not d i s t i n g u i s h between those m a t e r i a l s which are t r u l y non-thrombogenic and those which cause thrombus formation but are non-thromboadherent. Renal Embolus Ring Test ( 9 ) : This t e s t u t i l i z e s r i n g s f a b r i c a t e d from the m a t e r i a l s to be evaluated implanted i n the canine descending a o r t a j u s t above the r e n a l a r t e r i e s . A cons t r i c t i o n i s made i n the a o r t a below the r e n a l a r t e r i e s to f o r c e a l a r g e f r a c t i o n of the blood f l o w i n g through the t e s t r i n g i n t o the kidneys. A f t e r a p e r i o d of i m p l a n t a t i o n ( u s u a l l y 3-6 days) the r i n g s are examined f o r adherent thrombi and the kidneys are d i s s e c t e d and examined f o r i n f a r c t s presumably caused by thrombi shed from the r i n g s u r f a c e . This t e s t should be a b l e to d i s t i n guish between those m a t e r i a l s which are t r u l y non-thrombogenic and those which are o n l y non-thromboadherent. Almost a l l mater i a l s examined to date have been shown to cause some i n f a r c t damage to the t e s t animal's kidneys. Soft Tissue C o m p a t i b i l i t y Tests: For examining the response of the body to m a t e r i a l s implanted i n s o f t t i s s u e areas (not i n d i r e c t contact w i t h the blood stream) there has been l i t t l e e f f o r t extended towards adopting standardized t e s t procedures. Autian has surmised i n a l i t e r a t u r e review t h a t intramuscular implantat i o n may be the most s e n s i t i v e s i t e f o r e v a l u a t i o n of t h i s t i s s u e response (10). Coleman, King and Andrade have r e c e n t l y d e s c r i b e d a comprehensive p r o t o c o l f o r the e v a l u a t i o n of t i s s u e response to standardized i n t r a m u s c u l a r i m p l a n t a t i o n s (11). However, the b u l k of the papers i n the l i t e r a t u r e do not f o l l o w any p a r t i c u l a r standardized t e s t procedure. The term " t i s s u e compatible", f o r the purposes of t h i s review paper, w i l l be used to d e s c r i b e those m a t e r i a l s which, upon i m p l a n t a t i o n , show a normal acute inflammat i o n r e a c t i o n and then r a p i d l y "heal i n " to a p a s s i v e s t a t e wherei n the implant i s surrounded by a t h i n , uniform f i b r o u s capsule i n which m u l t i n u c l e a t e d g i a n t c e l l s and other inflammatory c e l l s are g e n e r a l l y absent. In v i t r o t e s t s u s i n g c u l t u r e d c e l l s have 1


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HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS

a l s o been u t i l i z e d w i t h v a r y i n g degrees o f success t o evaluate the t o x i c i t y and, t o a s m a l l e r e x t e n t , the b i o c o m p a t i b i l i t y o f b i o m e d i c a l polymers. This l i t e r a t u r e has been covered i n a review by A u t i a n (10). II.

B i o m e d i c a l l y Important Hydrogels. Hydrogels may be prepared by v a r i o u s p o l y m e r i z a t i o n t e c h niques o r by conversion of e x i s t i n g polymers. Tablœ I I and I I I l i s t examples o f monomers and polymers used i n p r e p a r i n g these m a t e r i a l s . Although g e n e r a l i z a t i o n s can be made about hydrogels, i t should be apparent from these t a b l e s t h a t t h i s category o f m a t e r i a l s covers a wide range of chemical compositions. There are major d i f f e r e n c e s f o r each type of m a t e r i a l w i t h respect t o s y n t h e s i s , p r o p e r t i e s and b i o c o m p a t i b i l i t y . Therefore, each o f the more important type individually. A. P o l y ( h y d r o x y a l k y 1 m e t h a c r y l a t e s ) . Included i n t h i s class of compounds are p o l y ( 2 - h y d r o x y e t h y l methacrylate) (P-HEMA), p o l y ( g l y c e r y l methacrylate) (P-GMA), and poly(hydroxypropyl methac r y l a t e ) (P-HPMA)* A review a r t i c l e w i t h p a r t i c u l a r emâsis upon h y d r o x y a l k y l methacrylate hydrogels has been p u b l i s h e d (12). P o l y ( 2 - h y d r o x y e t h y l methacrylate) (P-HEMA) hydrogels were f i r s t d e s c r i b e d and s y n t h e s i z e d by Lim and W i c h t e r l e i n the e a r l y I960's (13). Although t h i s polymer was prepared by DuPont s c i e n t i s t s as e a r l y as 1936 (14), they d i d not polymerize the monomer i n the presence of c r o s s l i n k i n g agent i n aqueous s o l v e n t media as did L i m and W i c h t e r l e . To date, P-HEMA hydrogels have probably been among the most w i d e l y s t u d i e d and used of a l l the s y n t h e t i c hydrogel m a t e r i a l s . A d e s c r i p t i o n of the s y n t h e s i s and p r o p e r t i e s o f P-HEMA hydrogels was p u b l i s h e d i n 1965 by Refojo and Yasuda (15). I f HEMA monomer c o n t a i n i n g c r o s s l i n k i n g agent i s polymerized i n the presence o f a good s o l v e n t f o r both monomer and polymer (e.g., ethylene g l y c o l , ethylene glycol-HÔ) an o p t i c a l l y transparent (homogeneous) hydrogel i s formed. I f the monomer p l u s c r o s s l i n k i n g agent i s polymerized i n a poor s o l v e n t system f o r the r e s u l t i n g polymer, an opaque, spongy, white (heterogeneous) hydrog e l i s formed. As the HEMA monomer i s an e x c e l l e n t s o l v e n t f o r P-HEMA, i f the c o n c e n t r a t i o n of water (which i s by i t s e l f a nons o l v e n t f o r P-HEMA) i n the water-HEMA mixture i s 43% o r l e s s by weight, a homogeneous g e l w i l l be formed (16)(17). A s o l u b l e P-HEMA polymer which i s s u i t a b l e f o r d i p c o a t i n g a p p l i c a t i o n s can be prepared by p o l y m e r i z i n g t o a low degree o f conversion i n d i l ute e t h a n o l s o l u t i o n monomer from which most o f the contaminating c r o s s - l i n k i n g agent has been removed (18,19). One o f the problems encountered i n the p r e p a r a t i o n o f P-HEMA hydrogels f o r b i o m e d i c a l a p p l i c a t i o n s i s the p u r i t y o f monomer used i n these systems. T y p i c a l i m p u r i t i e s found i n commercial grades o f HEMA monomer are m e t h a c r y l i c a c i d , ethylene g l y c o l


Synthetic Hydrogels


1.

7

T A B L E 3L MONOMERS

CH

HYDROXYALKYL

= C

2

METHACRYLATES

„CH V

USED

IN

HYDROGELS ACIDIC

OR

ANIONIC

3

C02-R

ACRYLIC ACID, DERIVATIVES (R=-H,-CH )

CH

=C-C0 H

CROTONIC

CH - -C=CH-C0 H

2

2

3

R =-CH CH OH, 2

2

-CHo-CH-OH.-CHo-CH-CHo-OH 1 I CH OH

SODIUM

3

ACID

3

STYRENE

CH

SULFONATE

BASIC 2,4

CH

PENTADIENE-I-OL

(R = - H ,

2

X

t

3

VINYL 2

C0 "C H 2

2

4

Ν-R

(R,R' R" = - H , - C H , - C H )

-CH,)

(R,R"=H,-CH -C H , — CH CHOHCH 5

CATIONIC

METHACRYLATE, C H = C

DERIVATIVES

C - C O - N - R R"

2

S O 3 No

2

CH2 =

3

= CH - < w ) -

2

= C H - C H =CH — C H O H

2

AMINOETHYL ACRYLAMIDE DERIVATIVES

OR

(withVAc)

2

4

9

CH

CH

3

—Ν — VINYL

P Y R R O L I DON Ε

C H = CH- Ν 2

HYDROPHOBIC ACRYLICS

ETHYLENEGLYCOL DIMETHACRYLATE DERIVATIVES

CHj) = C ^

CH = C 2

,

3

4

9

C=CH

Ο

3

(R =-CH ,-C H ,

Rv

1

CO

(USED A S COMONOMERS) (R=-H,-CH ) -OCH ,-CN,-OCH CH OCH 3

2

2

(CH CH 6) 2

2

X

3

CH =CH

METHYLENE-BISACRYLAMIDE

CH=CH

2

CO

CO

I

NH ^CHp

I

n2

Table I I I EXAMPLES OF CONVERTED POLYMERS USED AS HYDROGELS -f-CH -CH-)-

-fCH -CH*

2

2

Ο

OH

C=0 CH

3

-f C H - C H = CH-CH -)" 2

4 CH -CH-CH-CH 4OH OH

2

2

2

POLYELECTROLYTE COMPLEX

-iCh^-ÇHiCN

·•

-fCH ~CH-) CH 2

fCH —CH4—(CH -CH-)C= 0 C0 H NH 2

2

2

2

C H

2

=C

C°

H3

+

C0 C H OH 2

2

-30>31). The s w e l l i n g o f P-HEMA g e l s has a l s o been s t u d i e d w i t h respect t o the fundamental equations d e s c r i b i n g the s w e l l i n g o f c r o s s l i n k e d networks (32). The mechanical p r o p e r t i e s o f P-HEMA hydrogels have r e c e i v e d some a t t e n t i o n (33). These g e l s are p a r t i c u l a r l y s u i t e d f o r s t u d i e s on the fundamental aspects o f rubber e l a s t i c i t y , as the t o p o l o g i c a l aspects o f the c r o s s l i n k e d networks can be e a s i l y v a r i e d by changing the c r o s s l i n k i n g agent, c r o s s l i n k i n g agent c o n c e n t r a t i o n , t o t a l monomer c o n c e n t r a t i o n o r p o l y m e r i z a t i o n s o l v e n t . A l s o , co-monomers are e a s i l y i n c o r p o r a t e d i n t o the system. This l i t e r a t u r e has r e c e n t l y been reviewed by Janacek (33). Biomedical s t u d i e s u t i l i z i n g P-HEMA hydrogels are numerous (34-64). Many of these are l i s t e d i n Table IV. Fundamental s t u d i e s on the h e a l i n g o f d i s k s o f P-HEMA subcutaneously implanted i n r a t s and p i g s were performed p r i m a r i l y by researchers at the I n s t i t u t e o f Macromolecular Chemistry, Prague (31-36). Homogeneous P-HEMA and heterogeneous P-HEMA m a t e r i a l s w i t h varying 5


1.


9

Synthetic Hydrogek

degrees o f p o r o s i t y were examined. I n g e n e r a l , most o f implants were w e l l t o l e r a t e d and d i d not provoke unfavorable r e a c t i o n . However, P-HEMA g e l s which were extremely macroporous (those prepared w i t h greater than 70% water i n the p o l y m e r i z a t i o n mixture) demonstrated poor mechanical p r o p e r t i e s , and c e r t a i n u n d e s i r a b l e h e a l i n g c h a r a c t e r i s t i c s i n c l u d i n g c a l c i f i c a t i o n a t the margins o r center o f the implant. I t was t h e r e f o r e concluded t h a t homogeneous o r microporous heterogeneous P-HEMA g e l s might be more suitable f o r implant purposes. Table IV Biomedical Studies U t i l i z i n g P-HEMA Hydrogels Description E v a l u a t i o n o f Tissue Respons Antibiotic Delivery: From Coated Suture From Coated U r e t h r a l Catheter In Otolaryngology Anti-tumor Drug D e l i v e r y Device Coated I n t r a u t e r i n e Device L i v e r Resection Blood C o m p a t i b i l i t y Coated Sutures Corneal Surgery Soft Contact Lens Ureter P r o s t h e s i s Breast Augmentation Latex Spheres f o r C e l l Surface Studies Hemodialysis and Hemoperfusion Ligament P r o s t h e s i s Bone Formation i n P-HEMA Sponges

Reference

(41,42) (41.43,44) (45) (46) (47) (48) (41,49-52) (41,42,50) (53) (54,55) (56) (57) (58) (59-62) (63) (40,64)

Further problems were noted w i t h the h e a l i n g o f porous P-HEMA sponge m a t e r i a l s by Winter and Simpson (40). They found that pieces o f P-HEMA sponge implanted f o r 62 days i n the s k i n o f young p i g s showed evidence of woven bone formation. The e f f e c t was found t o be r e a d i l y r e p r o d u c i b l e . The h e a l i n g process f o r these porous m a t e r i a l s i s unusual i n that the sponges are appare n t l y w e l l t o l e r a t e d f o r the f i r s t month o f i m p l a n t a t i o n . However, from 31 days onwards m u l t i n u c l e a t e g i a n t c e l l s are found i n the implant (64). Rubin and M a r s h a l l attempted t o u t i l i z e the observed bone formation i n implanted spongy P-HEMA t o anchor i n t o the femur and t i b i a a k n i t t e d Dacron-porous P-HEMA a n t e r i o r c r u c i a t e ligament p r o s t h e s i s (63). F i x a t i o n i n t o the bone was not achieved although bone formation was noted i n p a r t s o f the polymer. Due t o the poor mechanical p r o p e r t i e s o f P-HEMA g e l s , there have always been experimental d i f f i c u l t i e s i n e v a l u a t i n g the blood c o m p a t i b i l i t y o f these m a t e r i a l s by accepted i n v i v o t e s t i n g


10


techniques (e.g., the vena cava r i n g t e s t o r the r e n a l embolus r i n g t e s t ) . However, the blood c o m p a t i b i l i t y o f P-HEMA was observed i n a few s t u d i e s e i t h e r i n v i t r o , o r , u s i n g somewhat l e s s c o n v e n t i o n a l techniques, i n v i v o (41, 49-52). I t was found, i n a l l cases, t o be a r e l a t i v e l y "non-thrombogenic" (or a t l e a s t non-thromboadherent) m a t e r i a l , although the i n v i v o t e s t s t h a t were used do not a l l o w a systematic grading o f thrombogenicity, and do not compare the performance o f P-HEMA t o a number o f other commonly used m a t e r i a l s . More systematic s t u d i e s o f the blood c o m p a t i b i l i t y of P-HEMA are discussed i n the s e c t i o n on s u r f a c e coated hydrogels. The primary c l i n i c a l use f o r P-HEMA hydrogels has been f o r f l e x i b l e , h y d r o p h i l i c contact l e n s e s . This i s a l s o the o n l y a p p l i c a t i o n which has r e c e i v e d U.S. Food and Drug A d m i n i s t r a t i o n approval ( s p e c i f i c a l l y f o S o f l e n s f o r the c o r r e c t i o Lambert poly(HEMA-co-ethylene d i m e t h a c r y l a t e - c o - m e t h a c r y l i c a c i d co-g-Povidone) Soft-con as a t h e r a p e u t i c 'bandage"). The propert i e s and a p p l i c a t i o n s f o r such contact lenses have been reviewed (54). Studies d e a l i n g s p e c i f i c a l l y w i t h oxygen p e r m e a b i l i t y through hydrogel contact l e n s e s (65), and h y d r a t i o n and l i n e a r expansion o f h y d r o p h i l i c contact Tenses (66) have been p u b l i s h e d . In another study methyl methacrylate contact l e n s e s and P-HEMA contact lenses were observed w i t h respect t o t h e i r e f f e c t upon the growth o f c o r n e a l e p i t h e l i a l t i s s u e i n c u l t u r e . The t o l e r ance o f the t i s s u e t o continuous exposure t o the contact lenses was found t o be s i g n i f i c a n t l y higher f o r the h y d r o p h i l i c lenses A number o f problems have been noted w i t h hydrophilic contact l e n s e s . These i n c l u d e low v i s u a l a c u i t y as compared t o hard l e n s e s , accumulation o f m a t e r i a l on and w i t h i n the l e n s e s , suscept i b i l i t y t o mechanical damage, low p e r m e a b i l i t y t o oxygen, ( r e v e r s i b l e ) changes i n the shape and r e f r a c t i v e index o f the l e n s e s , and the need f o r frequent s t e r i l i z a t i o n (54*67)· The p r i n c i p l e advantages o f these hydrogel lenses are t h a t they are easy t o f i t , w e l l t o l e r a t e d and can be used f o r t h e r a p e u t i c purposes other than simple r e f r a c t i v e c o r r e c t i o n . Concerning the degree t o which these lenses are t o l e r a t e d , i t has been r e c e n t l y reported that the Bausch and Lomb Soflens can be worn c o n t i n u o u s l y 10 days w i t h few or no problems (6£) . However, another study r e p o r t s that i t i s unadvisable t o wear s o f t contact lenses c o n t i n u o u s l y f o r long periods o f time due t o the development o f c o r n e a l edema (68). Other hydroxyalky1 methacrylates have not r e c e i v e d n e a r l y as much a t t e n t i o n as P-HEMA. Poly(hydroxypropyl methacrylate) hydrog e l s c o n t a i n i n g a mixture o f the two isomers can be r e a d i l y p r e pared (25). Although the water content of these hydrogels i s f a i r l y low (^25%) they demonstrate e x c e l l e n t mechanical properties. P o l y ( g l y c e r y l methacrylate) hydrogels e x h i b i t a c o n s i d e r a b l y higher e q u i l i b r i u m water content than P-HEMA g e l s (28,60-71). Therefore i t has o f t e n been thought t h a t t h i s m a t e r i a l might show


1.


Synthetic Hydrogels

11

a higher degree of b i o c o m p a t i b i l i t y than P-HEMA. This has not been described i n the l i t e r a t u r e to date. I t has been suggested that dehydrated P-GMA might be swollen i n s i t u i n cases where i t i s necessary to r e p l a c e the v i t r o u s humor of the eye (70). Β· P o l y ( A c r y l a m i d e ) , Poly(Methacrylamide) and D e r i v a t i v e s . Gels of poly(acrylamide) (PAAm) and of some N - s u b s t i t u t e d d e r i v a t i v e s of PAAm can be r e a d i l y prepared by the f r e e r a d i c a l p o l y m e r i z a t i o n of an aqueous s o l u t i o n of acrylamide c o n t a i n i n g a s m a l l f r a c t i o n of c r o s s l i n k i n g agent ( o f t e n N, N-methylenebisacrylamide). The g e l s formed are o p t i c a l l y t r a n s p a r e n t , mechanically weak, and can have extremely h i g h water contents (> 95%). The water content i s dependent upon the per cent c r o s s l i n k e r i n the system, u n l i k e the homogeneous P-HEMA g e l system. Another technique was r e c e n t l y p r i m a r i l y of polyacrylamid ( a c r y l o n i t r i l e ) and p o l y ( a c r y l i c a c i d ) (72). This technique i n v o l v e s p o l y m e r i z i n g a c r y l o n i t r i l e i n concentrated aqueous z i n c c h l o r i d e and then p a r t i a l l y h y d r o l y z i n g the r e s u l t i n g g e l . The high water contents which are a t t a i n a b l e w i t h acrylamide g e l s prepared by s o l u t i o n p o l y m e r i z a t i o n or by h y d r o l y s i s make them a t t r a c t i v e f o r b i o m e d i c a l a p p l i c a t i o n s (see S e c t i o n I V ) . A number of s t u d i e s have been c a r r i e d out on the h y d r o l y s i s of acrylamide and methacrylamide polymers (73-75). Hydrolysis occurs at s i g n i f i c a n t r a t e s at elevated temperatures i f the polymers are i n a c i d i c or b a s i c s o l u t i o n s . Thus, polyamide g e l s should be r e l a t i v e l y s t a b l e under c o n d i t i o n s of p h y s i o l o g i c a l pH and temperature. However, problems might occur a t steam auto c l a v e temperatures and t h e r e f o r e t h i s procedure might be c o n t r a indicated. The t i s s u e c o m p a t i b i l i t y of p o l y ( N - s u b s t i t u t e d acrylamides) was i n v e s t i g a t e d by Kopecek, et a l . (76). N - s u b s t i t u t e d a c r y l a mides were used f o r t h i s study because of t h e i r s u p e r i o r hydrol y t i c s t a b i l i t y compared to p o l y ( a c r y l a m i d e ) . I t was found that d i s c s of p o l y ( N , N - d i e t h y l a c r y l a m i d e ) , p o l y ( N - a c r y l y l morpholine), p o l y ( N - e t h y l acrylamide) and a l s o the methacrylamide d e r i v a t i v e , poly[N-(2-hydroxypropyl) methacrylamide], implanted subcutaneously i n r a t s were w e l l t o l e r a t e d by the animals and d i d not provoke unfavorable r e a c t i o n . The long term b i o l o g i c a l i n t e r a c t i o n of these hydrogels w i t h the t e s t animals was described as being s i m i l a r to t h a t observed w i t h implanted P-HEMA m a t e r i a l s , which were a l s o i n v e s t i g a t e d by t h i s group. The thrombogenicity of a number of PAAm g e l s was i n v e s t i g a t e d i n v i t r o u s i n g the Lee-White c o a g u l a t i o n time t e s t (52). Where the c o a g u l a t i o n time of f r e s h blood samples i n g l a s s tubes was approximately 12 minutes, blood i n PAAm tubes prepared u s i n g s i n g l y r e c r y s t a l l i z e d acrylamide monomer showed a c l o t t i n g time of ^45 minutes. When g e l s formed from t r i p l y r e c r y s t a l l i z e d acrylamide monomer were t e s t e d they showed c l o t t i n g times i n excess of 24 hours. The d e l e t e r i o u s e f f e c t s of incomplete removal -


12


of i n i t i a t o r by-products on the thrombogenicity o f acrylamide m a t e r i a l s was a l s o noted i n t h i s study. The e f f e c t of v a r y i n g the c r o s s l i n k i n g agent c o n c e n t r a t i o n i n the PAAm g e l was found not t o s i g n i f i c a n t l y a l t e r the Lee-White c l o t t i n g times f o r the system. I n t e r e s t i n g e f f e c t s of other co-monomers i n c o r p o r a t e d i n t o the PAAm g e l s on the c l o t t i n g times were noted. Some o f these e f f e c t s a r e d i s c u s s e d i n S e c t i o n I I F. I n g e n e r a l , the PAAm system prepared w i t h t r i p l y r e c r y s t a l l i z e d monomer gave the best r e s u l t s , although systems c o n t a i n i n g dimethylaminoethy1 methacrylate were shown t o demonstrate e x c e l l e n t Lee-White c l o t t i n g times. The thrombogenicity o f these p a r t i c u l a r g e l s was found t o be extremely s e n s i t i v e t o c r o s s l i n k e r c o n c e n t r a t i o n and total gel solids. Further i n f o r m a t i o n about PAAm g e l s i s contained i n a r e c e n t l y p u b l i s h e d review c o v e r i n (4). I n p a r t i c u l a r th discussed i n r e l a t i o n t o v a r i o u s measures o f i t s b i o c o m p a t i b i l i t y . C. P o l y ( N - V i n y l - 2 - P y r r o l i d o n e ) . P o l y (N-vinyl-2-pyrrolidone) (P-NVP) i s a somewhat unique polymer i n t h a t , i n i t s uncrossl i n k e d form, i t i s extremely s o l u b l e i n water and i s a l s o s o l u b l e i n many other p o l a r and non-polar s o l v e n t s . Because o f i t s s t r o n g i n t e r a c t i o n w i t h water i t can be used f o r preparing g e l s which w i l l e x h i b i t h i g h water contents. P-NVP g e l s a r e a l s o o f i n t e r e s t f o r biomedical a p p l i c a t i o n s as the s o l u b l e polymer has had a long h i s t o r y o f use i n the medic a l and pharmaceutical f i e l d s . One of the most important uses f o r P-NVP s o l u t i o n s has been as a plasma expander (77). When i n f u s e d i n t r a v e n o u s l y P-NVP i s non-toxic and non-thrombogenic and can be used t o maintain c i r c u l a t o r y f l u i d volume i n cases o f s e vere i n j u r y o r trauma. Some r e t e n t i o n o f P-NVP i n the l i v e r , s p l e e n , lungs and kidneys has been noted (78). I n a review o f the world l i t e r a t u r e on P-NVP i n 1962 i t was concluded t h a t P-NVP could be used o r a l l y o r i n t r a v e n o u s l y w i t h complete s a f e t y (79). However, a t the present time P-NVP i s no longer used as a plasma expander i n humans because i t i s not metabolized and i s not r e t a i n e d i n c i r c u l a t i o n as w e l l as other plasma expanders (e.g., Dextrans). The r e s i s t a n c e o f P-NVP t o d i g e s t i o n by l y s o somal enzymes has been e x p l o i t e d i n a recent study i n which the k i n e t i c s o f uptake by p i n o c y t o s i s o f I l a b e l l e d P-NVP i n t o r a t y o l k sac c u l t u r e d i n v i t r o was measured(80). P-NVP has a l s o been used i n blood volume determinations (81) and i n the p r e s e r v a t i o n of blood and blood components (82). I n the pharmaceutical f i e l d i t has been used as a t a b l e t b i n d e r , a t a b l e t c o a t i n g and f o r the s o l u b i l i z a t i o n and s t a b i l i z a t i o n o f drugs. An extensive b i b l i o graphy l i s t i n g P-NVP medical and pharmaceutical a p p l i c a t i o n s i s a v a i l a b l e (83). Hydrogels c o n s i s t i n g only o f P-NVP have not o f t e n been described i n the b i o m e d i c a l l i t e r a t u r e p o s s i b l e because h i g h concentrations o f c r o s s l i n k e r (5-20%) are needed t o produce a 1

2

5


1.


Synthetic Hydrogels

13

m a t e r i a l w i t h u s e f u l mechanical p r o p e r t i e s . P-NVP g e l s c r o s s l i n k e d w i t h methylene b i s ( 4 - p h e n y l isocyanate) were evaluated f o r use as hemodialysis membranes (84). F a s t e r metabolic waste t r a n s f e r was obtained w i t h these membranes than w i t h conventional c e l l u l o s e f i l m s . P-NVP g e l s c r o s s l i n k e d w i t h 20%(w/w) methylenebisacrylamide have been considered f o r use as an implantable drug d e l i v e r y system (85). In v i t r o e v a l u a t i o n of the thrombogenicity of P-NVP g e l s and P-NVP-PAAm copolymer g e l s by the Lee-White t e s t showed some extension of c l o t t i n g times although problems w i t h r e s i d u a l NVP monomer i n the g e l s was noted (52). F i b r o b l a s t adhesiveness to P-NVP g e l s has a l s o been measured i n v i t r o (86). I t was determined t h a t i n c r e a s i n g the c o n c e n t r a t i o n of the g e l from 40% to 96% renders i t more adhesive to the c e l l s . The d i f f i c u l t i e s i n v o l v e d i n preparing homogeneous P-NVP m a t e r i a l s makes NVP an g r a f t i n g systems. A numbe cussed i n S e c t i o n I I I . D. P o l y e l e c t r o l y t e Complexes. P o l y e l e c t r o l y t e complexes are p o l y s a l t s formed by the c o r e a c t i o n of a c a t i o n i c polymer such as p o l y ( v i n y l benzyltrimethyl-ammonium c h l o r i d e ) and an a n i o n i c p o l y mer such as sodium p o l y ( s t y r e n e s u l f o n a t e ) . The complex formed from these two p a r t i c u l a r p o l y e l e c t r o l y t e s was developed by the Amicon Corporation and i s r e f e r r e d to as l o p l e x 101. I t has r e c e i v e d b i o m e d i c a l e v a l u a t i o n i n a number of s i t u a t i o n s ( 8 7 , 8 8 ) . Due to mechanical s t r e n g t h l i m i t a t i o n s p o l y e l e c t r o l y t e complexes have been g e n e r a l l y used as coatings on f a b r i c s and other supports. In order to be used f o r coatings the g e l must be s o l u b i l i z e d i n a complex, multicomponent s o l v e n t system u s u a l l y cont a i n i n g water, a p o l a r , water s o l u b l e o r g a n i c s o l v e n t , and a strong e l e c t r o l y t e . I o p l e x - s o l v e n t s o l u t i o n s have g e n e r a l l y been s t r o n g l y a c i d i c and have been shown to degrade c e r t a i n p l a s t i c s such as nylon-6,6 thus making the requirements f o r a s u i t a b l e subs t r a t e f o r the p o l y e l e c t r o l y t e complex more s t r i n g e n t (89). D i f f i c u l t i e s have a l s o been found i n the s t e r i l i z a t i o n of Ioplex m a t e r i a l s . Autoclave s t e r i l i z a t i o n of these complexes can r e s u l t i n d i s i n t e g r a t i o n of the g e l s t r u c t u r e (89). Gas s t e r i l i z a t i o n can leave entrapped ethylene oxide i n the m a t r i x . The problems encountered i n s t e r i l i z i n g these hydrogels have been reviewed(4,90). An advantage of these m a t e r i a l s i s the ease w i t h which a net charge ( a n i o n i c or c a t i o n i c ) can be i n c o r p o r a t e d i n the system. This i s done by adding s t o i c h i o m e t r i c a l l y g r e a t e r or l e s s e r amounts of one of the two polymeric components d u r i n g f o r m u l a t i o n . I t was determined u s i n g the i n v i v o vena cava r i n g t e s t that Ioplex 101 c o n t a i n i n g 0.5 meq. excess a n i o n i c component showed the g r e a t e s t thromboresistance (89). However, the performance of the n e u t r a l Ioplex g e l i n t h i s t e s t i n d i c a t e d t h a t i t i s perhaps only s l i g h t l y l e s s thromboresistant than the a n i o n i c g e l (4). C a t i o n i c Ioplexes and g e l s c o n t a i n i n g an increased number of a n i o n i c s i t e s


14


were s i g n i f i c a n t l y more thrombogenic. E. P o l y ( v i n y l a l c o h o l ) . P o l y ( v i n y l a l c o h o l ) (PVA) i s a water s o l u b l e polymer formed by the h y d r o l y s i s of p o l y ( v i n y l acet a t e ) . C r o s s l i n k e d g e l s of PVA have found a number of uses i n the b i o m e d i c a l f i e l d . A c r o s s l i n k e d , h i g h l y porous sponge of PVA can be formed by r e a c t i n g formaldehyde w i t h s o l u b l e PVA and blowing a i r through the s o l u t i o n before the p o l y m e r i z a t i o n - c r o s s l i n k i n g process i s completed. This m a t e r i a l was commercially a v a i l a b l e under the name I v a l o n and had been e x t e n s i v e l y used i n h e r n i a treatment, duct replacement, c a r d i a c - v a s c u l a r surgery, p l a s t i c surgery, and r e c o n s t r u c t i v e surgery. H e a l i n g problems w i t h I v a l o n sponges were encountered and i t was concluded t h a t I v a l o n d i d not meet up to the e a r l y enthusias t h e t i c s would be more s a t i s f a c t o r A hydrogel c o n s i s t i n g of PVA and the a n t i c o a g u l a n t h e p a r i n c r o s s l i n k e d together w i t h glutaraldehyde/formaldehyde mixtures has been s y n t h e s i z e d and demonstrates low thrombogenicity i n i n v i t r o t e s t s (92). Experiments w i t h S l a b e l l e d heparin indicate that heparin does not l e a c h out o f the c r o s s l i n k e d g e l . A potent i a l problem w i t h t h i s m a t e r i a l i s t h a t , p o s s i b l y due t o the presence of h e p a r i n , i t shows a tendency t o adsorb blood p l a t e l e t s . PVA-heparin hydrogels have a l s o been evaluated f o r use as hemodialysis membranes (93). They show promise f o r t h i s a p p l i c a t i o n s i n c e the p e r m e a b i l i t y to "middle-molecular weight" molecules such as i n u l i n i s much h i g h e r f o r PVA-heparin hydrogels than f o r Cuprophan cellophane hemodialysis membranes. R a d i a t i o n c r o s s l i n k e d g e l s of PVA have been proposed f o r use as s y n t h e t i c c a r t i l a g e i n s y n o v i a l j o i n t s (94). The m a t e r i a l prepared f o r t h i s a p p l i c a t i o n i s annealed t o i n c r e a s e the c r y s t a l U n i t y and t h e r e f o r e the p h y s i c a l s t r e n g t h o f the hydrogel. This a p p l i c a t i o n f o r PVA g e l s i s discussed f u r t h e r i n S e c t i o n I I F. A PVA s u r f a c e w i t h a number o f immobilized biomolecules on i t was designed i n an e f f o r t t o simulate the n a t u r a l blood v e s s e l i n t i m a (95). This m a t e r i a l i s described i n S e c t i o n V. 3 5

F. A n i o n i c and C a t i o n i c Hydrogels. A n i o n i c and c a t i o n i c hydrogels are u s u a l l y formed by copolymerizing s m a l l amounts o f a n i o n i c o r c a t i o n i c monomers w i t h n e u t r a l hydrogel monomers (see Table I I ) . However, they can a l s o be prepared by modifying preformed hydrogels such as by the p a r t i a l h y d r o l y s i s of p o l y ( h y d r o x y a l k y l methacrylates) o r , i n the case o f p o l y e l e c t r o l y t e complexes, by adding an excess of the p o l y a n i o n o r p o l y c a t i o n component. The i n t e r e s t i n such hydrogel systems stems from observations on the s u r f a c e charges on blood c e l l s , blood v e s s e l w a l l s , and other t i s s u e types. Under normal c o n d i t i o n s the blood v e s s e l w a l l s and blood c e l l s have a negative charge. Sawyer has presented evidence which i n d i c a t e s that when t h i s charge i s a l t e r e d


1.


Synthetic Hydrogeh

15

by pH changes, o r by drugs the tendency o f the system t o throm bose i s a l s o a l t e r e d (96). I t i s g e n e r a l l y thought t h a t negative l y charged s u r f a c e s should be l e s s thrombogenic than p o s i t i v e l y charged ones, (97) and experiments have been performed which support t h i s c o n t e n t i o n . However, r e s u l t s i n d i c a t i n g decreased thrombogenicity f o r p o s i t i v e l y charged s u r f a c e s have a l s o been presented (52*98)· The r o l e o f charge d e n s i t y (as opposed t o t o t a l charge) has been suggested as an important f a c t o r w i t h respect t o the thrombogenicity o f s u r f a c e s (97). A t the present time the importance o f s u r f a c e charge i n blood-hydrogel i n t e r a c t i o n s o r i n i n v i v o h e a l i n g i s not a t a l l c l e a r . Cerny, e t a l . , i n v e s t i g a t e d the h e a l i n g o f P-HEMA specimens, some o f which contained charged co-monomers, subcutaneously im planted i n s e v e r a l s p e c i e s o f l a b o r a t o r y animals (35). He found that i n four groups o f methacrylic acid, c a l c i f i c a t i o P-HEMA specimens) was i n h i b i t e d . B a r v i c , e t a l . , found t h a t P-HEMA g e l s c o n t a i n i n g 5 weight % o r g r e a t e r o f the co-monomer d i e t h y l a m i n o e t h y l methacrylate gave r i s e t o inflammatory r e a c t i o n s a f t e r three weeks o f i m p l a n t a t i o n i n r a t s (37). S p r i n c l , et a l . , i n a recent study, i n v e s t i g a t e d the h e a l i n g o f P-HEMA specimens c o n t a i n i n g s m a l l amounts o f copolymerized a n i o n i c (methacrylic a c i d ) and c a t i o n i c (Ν, N, dimethylaminoethyl meth a c r y l a t e ) monomers implanted subcutaneously f o r l o n g p e r i o d s (up t o 360 days) i n r a t s (39). They found t h a t the chemical composition o f the g e l s , wTFhin the c o n c e n t r a t i o n ranges s t u d i e d , showed no apparent e f f e c t on long term h e a l i n g . Thus, the o b s e r v a t i o n by B a r v i c , e t a l , , might o n l y be t r u e f o r short term i m p l a n t a t i o n . S p r i n c l , e t a l . , a l s o showed t h a t f o r P-HEMA g e l s , c a l c i f i c a t i o n apparently o n l y depended on the p h y s i c a l form o f the g e l ( i . e . , macroporous g e l s might c a l c i f y where microporous g e l s wouldn't) r a t h e r than on the chemical composition o f the g e l . Therefore, the i n h i b i t i o n o f c a l c i f i c a t i o n noted by Cerny, e t a l . , might be due t o d i f f e r e n c e s i n the p h y s i c a l s t r u c t u r e o f the gels he used r a t h e r than t o the presence o f 4% m e t h a c r y l i c a c i d . The i n v i t r o thrombogenicity of PAAm g e l s copolymerized w i t h dimethylaminoethyl methacrylate, t - b u t y l a m i n o e t h y l methacry l a t e , 2 - s u l f o e t h y l m e t h a c r y l a t e sodium s a l t , 2-hydroxy-3-methacryloloxypropyltrimethylammonium c h l o r i d e , a c r y l i c a c i d , meth a c r y l i c a c i d , 2 - v i n y l p y r i d i n e , 4 - v i n y l p y r i d i n e and 2-methyl5 - v i n y l p y r i d i n e was s t u d i e d by Halpern, et_ a l . , u s i n g the LeeWhite technique. Only the dimethylaminoethyl methacrylateacrylamide hydrogel showed a s i g n i f i c a n t e x t e n s i o n i n c l o t t i n g times. As was mentioned i n S e c t i o n I I D, p o l y e l e c t r o l y t e com plexes c o n t a i n i n g 0.5 meq. excess a n i o n i c component were found t o have the lowest thrombogenicity i n i n v i v o s t u d i e s (99). These two r e s u l t s are somewhat c o n t r a d i c t o r y as the dimethylaminoethyl methacrylate-acrylamide hydrogel i s p o s i t i v e l y charged a t p h y s i o l o g i c a l pH w h i l e the a n i o n i c p o l y e l e c t r o l y t e complex has a negative charge. Other s t u d i e s on the thrombogenicity o f


16

HYDROGELS FOR

MEDICAL AND


charged hydrogels w i l l be d i s c u s s e d i n S e c t i o n I I I . A unique a p p l i c a t i o n f o r a c a t i o n i c hydrogel has been proposed by Bray and M e r r i l l (94,99). They constructed a s y n t h e t i c a r t i c u l a r c a r t i l a g e m a t e r i a l f o r use i n a s y n o v i a l j o i n t by simu l t a n e o u s l y r a d i a t i o n c r o s s l i n k i n g p o l y ( v i n y l a l c o h o l ) and r a d i a t i o n g r a f t i n g to the PVA chains a c a t i o n i c monomer. Such a mater i a l should s t r o n g l y adsorb n e g a t i v e l y charged h y a l u r o n i c a c i d and produce an "osmotically-enhanced, v i s c o u s g e l l a y e r of boundary l u b r i c a n t " (94). C a t i o n i c monomers which have been used f o r t h i s purpose are allyltrimethyl-ammonium bromide and 2hydroxy-3-methacryloxypropyltrimethylammonium c h l o r i d e . III.

Surface Coated Hydrogels There are a number of techniques which can be used to coat substrates with h y d r o p h i l i Aside from the c o n v e n t i o n a of the polymer, a l l other methods i n v o l v e covalent bonding ( g r a f t i n g ) of the h y d r o p h i l i c polymer to the s u b s t r a t e polymer chains. Table V Techniques f o r D e p o s i t i n g Hydrogel 1. 2. 3.

4.

Coatings

Dip-coat i n pre-polymer + s o l v e n t . Dip i n monomer(s) (+ s o l v e n t , polymer) then polymerize u s i n g c a t a l y s t + heat. P r e - a c t i v a t e s u r f a c e ("active vapor," i o n i z i n g r a d i a t i o n i n a i r ) then contact w i t h monomer(s) + heat to polymerize. Irradiate with ionizing radiation while i n contact w i t h vapor or l i q u i d s o l u t i o n of monomer ( s ) .

By c o v a l e n t l y bonding a hydrogel to the s u r f a c e of another polymer a new composite m a t e r i a l i s formed whose mechanical prope r t i e s more c l o s e l y resembles those of the base polymer than the t h i n g r a f t e d hydrogel l a y e r . The most e f f i c a c i o u s technique f o r p r e p a r i n g such m a t e r i a l s i n v o l v e s generating f r e e r a d i c a l s on a p l a s t i c s u r f a c e and then p o l y m e r i z i n g a monomer d i r e c t l y on that s u r f a c e . A number of techniques have been used f o r generating such r a d i c a l s on s u r f a c e s . Some of the more commonly used ones are l i s t e d i n Table VI. There are at l e a s t f i v e advantages to u s i n g g r a f t i n g t e c h niques f o r p r e p a r i n g b i o m e d i c a l hydrogels. The f i r s t and most obvious advantage i s the i n c r e a s e i n mechanical s t r e n g t h over the ungrafted hydrogel which can be obtained. A second advantage i s the permanence and d u r a b i l i t y which should be e x h i b i t e d by the c o v a l e n t l y bound hydrogel c o a t i n g as compared to coatings on devices prepared by d i p p i n g techniques. T h i r d , g r a f t p o l y m e r i z a -


1.


Synthetic Hydrogels

17

T A B L E VI Techniques

ond Reoctions for Generoting Rodicols on Surfaces.

IONIZING RADIATION CH C H 3

CH

3

CH \ΛΛΛΛ

—

Co^

V/////

Electron Irrodiotion

CER1C+1Y ION OH OH OH 1 I I CH CH CH ////////// 2

2

Ο I CH

Ce

2

PEROXIDE FORMATION I

CH CH

Ç3 Ç 3 Ç 3 ///λ/λ/ / / / / / / / / ) / 0 /ionizing rodiotion H

H

3

CH3

2

CH

/)//////////

3

^

3

H

A,Fe

+ I

+

OH OH I I CH CH

2

2

0 C1

CH

3

"

_

2

CH-a

H

3

- +1 ) / / / / / / / / + OH +Fe

2

ACTIVE VAPOR ACTIVATION CH

3

^^3

^^3

)///)//)/

CH CH CH J , , ,I , >I ; +H 2

microwave generated Η·

3

3

2

U.V. GRAFTING C H

3

C

H

3

C

H

CH

3 photosensitizer

2

C H 3 CH3 γ γ + Η: photosensitizer

ΝΟΤΕ·· The precise noture of the radical intermediates formed has not been elucidated in most cases. Representations in this table show schematically radical species which might be formed.


18

HYDROGELS FOR

MEDICAL AND


t i o n techniques make i t p o s s i b l e to prepare complex surfaces formed by s u c c e s s i v e g r a f t i n g s u s i n g d i f f e r e n t monomers. Fourth, the p r e p a r a t i o n of hydrogels g r a f t e d only on the s u r f a c e , p a r t i a l l y i n t o the s u b s t r a t e , or u n i f o r m l y throughout a hydrophobic m a t r i x can be e f f e c t e d by v a r y i n g the p o l y m e r i z a t i o n s o l v e n t and other g r a f t i n g parameters. The l a t t e r type of g r a f t e d hydrogel comprises an i n t e r e s t i n g c l a s s of m a t e r i a l s which should have mechanical and s u r f a c e p r o p e r t i e s r e f l e c t i n g the c h a r a c t e r i s t i c s of both the s u b s t r a t e and the hydrogel. F i n a l l y , u s i n g r a d i a t i o n to prepare a g r a f t e d h y d r o g e l , the a d d i t i o n of an i n i t i a t o r i s not necessary, thereby e l i m i n a t i n g one p o t e n t i a l source of contamination i n the f i n a l product. There are c e r t a i n u n d e s i r a b l e s i d e r e a c t i o n s which can occur w i t h g r a f t p o l y m e r i z a t i o n s , p a r t i c u l a r l y those i n i t i a t e d by i o n i z i n g r a d i a t i o n . Thes i n c l u d polyme degradation crosslinkin and the formation of unwante a c i d s ) . However, degradatio and c r o s s l i n k i n g ca be minimized by u s i n g low doses and the formation of most unwanted f u n c t i o n a l groups can be e l i m i n a t e d by e x c l u d i n g oxygen and r e a c t i v e solvents from the g r a f t i n g system. As has been noted above, g r a f t i n g c o p o l y m e r i z a t i o n techniques provide a convenient means f o r c o n t r o l l i n g composition, penetrat i o n and morphology of the g r a f t e d polymer. Such c o n t r o l should be u s e f u l f o r " t a i l o r i n g " a g r a f t e d polymer to a given biomedical a p p l i c a t i o n . D e t a i l e d d e s c r i p t i o n s of methods which have been used to vary the s u r f a c e c h a r a c t e r of g r a f t e d hydrogels f o r b i o medical uses have been d e s c r i b e d i n a few papers (100-102). Examples of some of the g r a f t i n g parameters which can be used to i n f l u e n c e the p r o p e r t i e s of r a d i a t i o n g r a f t e d hydrogels i n c l u d e r a d i a t i o n dose, dose r a t e , monomer c o n c e n t r a t i o n , g r a f t i n g s o l vent, temperature, and the presence of v a r i o u s metal ions i n the g r a f t i n g system. One of the e a r l i e s t a p p l i c a t i o n s of g r a f t p o l y m e r i z a t i o n techniques to the p r e p a r a t i o n of m a t e r i a l s f o r biomedical a p p l i c a t i o n s was reported i n 1964 by Yasuda and Refojo (103). They g r a f t e d N - v i n y l - 2 - p y r r o l i d o n e to s i l i c o n e rubber i n an e f f o r t to i n c r e a s e the h y d r o p h i l i c i t y of the rubber s u r f a c e . In 1966, L e i n i n g e r and co-workers discussed the p o s s i b i l i t y of g r a f t i n g v i n y l p y r i d i n e to a base p l a s t i c f o r use i n i m m o b i l i z i n g heparin to the s u r f a c e (104). L a i z i e r and Wajs i n 1969 described a transparent polymer prepared by g r a f t i n g NVP to a s i l i c o n e r e s i n which might be s u i t a b l e f o r contact l e n s a p p l i c a t i o n s (105). W i t h i n the l a s t f i v e yeais a number of groups have p u b l i s h e d papers or r e p o r t s d e s c r i b i n g g r a f t e d hydrogels designed f o r b i o medical a p p l i c a t i o n s . Much of the work i n t h i s f i e l d i s summari z e d i n Table V I I . There has been l i t t l e published on the i n v i v o e v a l u a t i o n of t i s s u e response to g r a f t e d hydrogels, i n a p r e l i m i n a r y , and as yet unpublished study, r a d i a t i o n g r a f t e d P-HEMA and PAAm hydrog e l s on s i l i c o n e rubber were found t o be w e l l t o l e r a t e d when



Group

grafting

Electron irradiation, treatments

Acrylic Acid, Methacrylic Acid, Acrylamide, Ethylene S u l f o n i c A c i d , V a r i o u s E s t e r s and Imides

E l e c t r o n i r r a d i a t i o n o f preformed poly-electrolyte coating, C o mutual i r r a d i a t i o n C e r i c IV i o n g r a f t i n g , r a d i a t i o n g r a f t i n g (Mutual i r r a d i a t i o n tech.)

V i n y l A c e t a t e - c o - 2 % Crof.onic A c i d , NVP-co-2% A c r y l i c A c i d

A c r y l a m i d e , HEMA

HEMA

Polysciences, Inc.

U.S. Army M e d i c a l B i o e n g i n e e r i n g Research & Development L a b o r a t o r y

Univ. o f W i s c o n s i n

Hydromed S c i e n c e s

Union C a r b i d e

Corp.

(Mutual i r r a d i a t i o n

(Mutual i r r a d i a t i o n (Mutual i r r a d i a t i o n

6 0

6 0

6 0

Co

Co Co

HEMA

HEMA

6 0

technique)

technique)

technique)

grafting

technique)

A t o m i z e d gas p l a s m a technique

technique)

HEMA, A c r y l a m i d e

Institute

irradiation

Franklin

(Mutual i r r a d i a t i o n (Mutual

Co

6 0

Co

HEMA

Acrylamide

Kearney, e t a l .

6 0

technique)

Andrade, e t a l .

(Mutual i r r a d i a t i o n

6 0

Co

HEMA, NVP, A c r y l a m i d e , Meth acrylamide, Methacrylic a c i d , Ethyl methacrylate

chemical

technique)

Used

Hoffman, e t a l .

eta l .

Miller,

(pre-irradiation

Co

6 0

Radiation

Vinyl

Pyridine

irradiation

Electron

NVP

NVP

Columbus

G r a f t i n g Technique(s)

Monomer(s) Used

L a i z i e r and Wais

Battelle,

Yasuda and R e f o j o

Research

Grafted Hydrogels Prepared For Biomedical A p p l i c a t i o n s

Table V I I

applications

(113-115)

Artificial

(119)

(117) catheters

membrane

Dialysis IUD,

(118)

Burn d r e s s i n g

Blood oxygenator (hollow f i b e r ) , i n t r a (9, 115, aortic assist balloons, assist bladders, 116) a o r t i c implant tubes

h e a r t and o r g a n s

(9, 47, 112,113)

(111)

(102)

(100,101, 107-110)

(106)

(105)

(104)

(103)

Blood & t i s s u e compatible m a t e r i a l s , h e a r t a s s i s t d e v i c e s , I.U.D.

interface

h e a r t components

Blood compatible

Artificial

A r t i f i c i a l h e a r t components, c a t h e t e r s , k n i t t e d a r t e r y prostheses, blood & t i s s u e compatible i n t e r f a c e s

Non-thrombogenic s u r f a c e s

Contact lenses

Non-thrombogenic p l a s t i c s u r f a c e

Medical

Applications

sr

Ci

Ο

133

it

C/D

>

ο

ι

20

HYDROGELS FOR

MEDICAL AND


implanted both i n t r a p e r i t o n e a l l y and subcutaneously i n r a t s . The f i l m s were surrounded by a t h i n non-adhering f i b r o u s capsule a f t e r i m p l a n t a t i o n periods ranging from 5-10 days (120). I n an i n v i t r o study on adhesiveness of c h i c k embryo myoblasts to r a d i a t i o n g r a f t e d P-HEMA and N-VP hydrogels on s i l i c o n e rubber i t was found t h a t the c e l l s adhere very p o o r l y to g r a f t e d hydrogels and that the hydrogel g r a f t e d polymers always adhered fewer c e l l s than the ungrafted polymers (110). Whether such r e s u l t s are meaningful i n terms of i n v i v o c e l l - g r a f t e d hydrogel i n t e r a c t i o n s i s not yet clear. The blood c o m p a t i b i l i t y of g r a f t e d hydrogels has been examined i n a number of cases by the vena cava r i n g t e s t and r e n a l embolus r i n g t e s t . C e r t a i n g e n e r a l i z a t i o n s can be made from these results. 1. P-HEMA or P-NVP hydrogels g r a f t e d to s i l i c o n e rubber w i l l g r e a t l y reduce the thrombogenicit judged by the vena cav g 2. When evaluated by the vena cava r i n g t e s t s e v e r a l d i f f e r ent types of g r a f t e d hydrogels have been found to perform w e l l , but some thrombus i s u s u a l l y noted adhering to the r i n g , p a r t i c u l a r l y a f t e r the two week t e s t p e r i o d . An e x c e p t i o n to t h i s i s g r a f t e d polyacrylamide m a t e r i a l s which have, i n some cases, shown no thrombus a f t e r 14 days (4,121). Recent r e s u l t s , however, u s i n g a m o d i f i c a t i o n of the vena cava r i n g t e s t do show thrombus adhering to g r a f t e d acrylamide r i n g s i n four out of s i x cases ( 8 ) . A 60% sodium ionomer of p o l y ( v i n y l acetate-co-2% c r o t o n i c a c i d ) has, i n c e r t a i n t e s t s i t u a t i o n s , a l s o showed n e g l i g i b l e thrombus accumu l a t i o n a f t e r 14 days (114). 3. In the r e n a l embolus t e s t , although l i t t l e thrombus i s found w i t h i n most hydrogel t r e a t e d t e s t r i n g s , the kidneys almost always show moderate to e x t e n s i v e embolus damage ( 9 ) . Thus, the g e n e r a l l y low thrombogenicity r a t i n g of s y n t h e t i c hydrogels based on the vena cava r i n g t e s t , may, i n f a c t , be due to a low thromboadherance, i . e . a f l a k i n g o f f of thrombus from the hydrogel s u r f a c e . This aspect of hydrogel-blood i n t e r a c t i o n i s much i n need of f u r t h e r i n v e s t i g a t i o n . Recently p l a t e l e t adhesiveness to r a d i a t i o n g r a f t e d P-HEMA hydrogels on c e l l u l o s e a c e t a t e was measured i n an i n v i t r o t e s t c e l l . The per cent p l a t e l e t s adhering was found to decrease w i t h i n c r e a s i n g g r a f t l e v e l ( f i g u r e 1) (117). This trend was very s i m i l a r to t h a t noted f o r f i b r i n o g e n a d s o r p t i o n to P-HEMA g r a f t e d s i l i c o n e rubber s u r f a c e s ( f i g u r e 2) (109). Decreased adhesiveness of c e l l s to hydrogel g r a f t e d s u r f a c e s was a l s o noted i n the c e l l adhesion study d i s c u s s e d above (110). Again, these observat i o n s tend to support the i d e a t h a t hydrogels have a low thromboadherence.



Synthetic Hydrogeh

Ο.ΟΙ

2 GRAFT

3

6

4

(mg/cm2) ACS Advances in Chemistry Series

Figure 2. Fibrinogen

adsorption to radiation grafted HEMA silicone rubber ( 109 )

hydrogels on


22


IV.

C h a r a c t e r i z a t i o n o f the Imbibed Water C l e a r l y , the presence o f l a r g e amounts o f water w i t h i n a polymeric network i s not the s o l e f a c t o r c o n t r o l l i n g the biocomp a t i b i l i t y and thrombogenicity o f such m a t e r i a l s . Thus, g e l a t i n or some p o l y s a c c h a r i d e s which o f t e n have water contents o f 90% or h i g h e r are considered t o be r e l a t i v e l y thrombogenic m a t e r i a l s . A l s o , extremely "open" P-HEMA g e l s which e x h i b i t h i g h water cont e n t s are l e s s t i s s u e compatible than t i g h t e r P-HEMA g e l s w i t h lower water contents (39). S t i l l , the presence o f a s i g n i f i c a n t f r a c t i o n o f water i s considered important f o r the b i o c o m p a t i b i l i t y and low thrombogenicity o f those hydrogels which are biocompatible and "reasonably" non-thrombogenic (or non-thromboadherent) ( 4 ) . The nature o r o r g a n i z a t i o n o f the water w i t h i n the hydrated polymeric network may be one o f the f a c t o r s which w i l l i n f l u e n c e the i n t e r a c t i o n s that occu hydrogel. Problems concerning the o r g a n i z a t i o n of water a t the molecul a r l e v e l (water s t r u c t u r e ) are o f t e n extremely complex and t h e l i t e r a t u r e on the subject i s e x t e n s i v e . There are e x c e l l e n t r e views a v a i l a b l e on water s t r u c t u r i n g (122,123). A l s o , a number of review a r t i c l e s have been w r i t t e n on the b i o l o g i c a l i m p l i c a t i o n s o f s t r u c t u r e d water (124-126) and the p o t e n t i a l importance of the molecular o r g a n i z a t i o n of water t o the performance o f b i o m a t e r i a l s (127,128). The gross t o t a l water contents o f swollen hydrogels are most e a s i l y measured and most o f t e n reported. I n f o r m a t i o n about the molecular nature o f water w i t h i n the network i s not as easy t o o b t a i n ; such water may be (a) p o l a r i z e d around charged i o n i c groups, (b) o r i e n t e d around hydrogen bonding groups o r other d i p o l e s , (c) s t r u c t u r e d i n " i c e - l i k e " c o n f i g u r a t i o n s around hydrophobic groups, and/or (d) imbibed i n l a r g e pores as "normal" b u l k water. Attempts have been made t o separate the t o t a l g e l water content i n t o some o f these c a t e g o r i e s u s i n g NMR techniques (129). Based upon the NMR data g e l water contents would be d i v i d e d i n t o "bulk water" (category d ) , "bound water" ( c a t e g o r i e s a, b) and the remaining water, c a l l e d " i n t e r f a c e water" (category c ) . R e s u l t s f o r P-HEMA g e l s shown i n Table V I I I suggest that the f r a c t i o n s o f b u l k , bound and i n t e r f a c i a l water v a r y w i t h the t o t a l water content o f the g e l . I n c r e a s i n g f r a c t i o n s o f b u l k water and d e c r e a s i n g f r a c t i o n s of bound water are found as the water content o f the g e l i n c r e a s e s . The f r a c t i o n o f i n t e r f a c i a l water undergoes o n l y s m a l l changes w i t h changing g e l water cont e n t . S i m i l a r c o n c l u s i o n s were drawn concerning the s t a t e o f water i n P-HEMA g e l s which were i n v e s t i g a t e d u s i n g the techniques of d i l a t o m e t r y , s p e c i f i c c o n d u c t i v i t y and d i f f e r e n t i a l scanning c a l o r i m e t r y (130). Water s o r p t i o n s t u d i e s which p r o v i d e some i n s i g h t i n t o the k i n e t i c s and thermodynamics o f water i n t e r a c t i o n w i t h hydrogels have a l s o been performed (131 ,132).



1.

23

Synthetic Hydrogeh

Table V I I I The F r a c t i o n of Water i n P-HEMA Gels of D i f f e r e n t T o t a l Water Content (Wt %) [Data from Lee, Andrade and Jhon, (114)] Wt % of T o t a l Water i n the Gel f w

f,

20

25 30

35

40

45

50

55

F r a c t i o n of 0 0 0.09 0.21 0.30 0.37 0.42 Bulk Water 0 F r a c t i o n of 0 0.2 0.33 0.34 0.29 0.26 0.33 0.22 I n t e r f a c i a l Water F r a c t i o n of 1.0 0.8 0.67 0.57 0.50 0.44 0.40 0.36 Bound Water

60 0.47 0.20 0.33

Studies to date o g e l s are only p r e l i m i n a r of water o r g a n i z a t i o n on b i o l o g i c a l i n t e r a c t i o n s remain to be e l u c i d a t e d . I t should be noted t h a t the o r g a n i z a t i o n and content of g e l water w i l l vary s i g n i f i c a n t l y w i t h hydrogel composition, probably most o f t e n i n expected d i r e c t i o n s ( i . e . , g e l s w i t h higher water contents w i l l have lower f r a c t i o n s of bound and i n t e r f a c i a l water). V.

I m m o b i l i z a t i o n and Entrapment of B i o l o g i c a l l y A c t i v e Molecules on and W i t h i n Hydrogels f o r B i o m a t e r i a l A p p l i c a t i o n s . Hydrogels a r e , i n many r e s p e c t s , eminently s u i t e d f o r use as a base m a t e r i a l f o r " b i o l o g i c a l l y a c t i v e " b i o m a t e r i a l s . Examples of c l a s s e s of b i o l o g i c a l l y a c t i v e molecules which can be used i n c o n j u n c t i o n w i t h hydrogels are l i s t e d i n Table IX. Examples of b i o m e d i c a l a p p l i c a t i o n s f o r immobilized enzymes are presented i n Table X. There are a number of d i s t i n c t advantages f o r hydrogels i n these types of systems. Small molecules (drugs, enzyme subs t r a t e s ) can d i f f u s e through hydrogels and the r a t e of permeation can be c o n t r o l l e d by c o - p o l y m e r i z i n g the hydrogel i n v a r y i n g r a t i o s w i t h other monomers. Hydrogels may i n t e r a c t l e s s s t r o n g l y than more hydrophobic m a t e r i a l s w i t h the molecules which are immobilized to or w i t h i n them thus l e a v i n g a l a r g e r f r a c t i o n of the molecules a c t i v e (3)· Hydrogels can be l e f t i n contact w i t h blood or t i s s u e f o r extended p e r i o d s of time without causing r e a c t i o n making them u s e f u l f o r devices to be used i n long-term treatment of v a r i o u s c o n d i t i o n s . F i n a l l y , hydrogels u s u a l l y have a l a r g e number of p o l a r r e a c t i v e s i t e s on which molecules can be immobilized by r e l a t i v e l y simple c h e m i s t r i e s .


24

HYDROGELS FOR MEDICAL AND RELATED

APPLICATIONS

Table IX B i o l o g i c a l l y A c t i v e Molecules which may be Entrapped or Immobilized i n Hydrogels Antibiotics Anticoagulants Anti-Cancer Drugs Antibodies Drug A n t a g o n i s t s Enzymes Contraceptives Estrous-Inducers A n t i - b a c t e r i a Agents

Biomedical A p p l i c a t i o n Immobilized Enzyme(s)

Application

Brinolase Urokinase Streptokinase Asparaginase,Glutaminase Carbonic Anhydrase, Catalase Urease Glucose Oxidase

Non-Thrombogenie Surface Non-Thrombogenic Surface Non-Thrombogenie Surface Leukemia Treatment

(133) (134) (135) (136)

Membrane Oxygenator A r t i f i c i a l Kidney Glucose S e n s o r - A r t i f i c i a l Pancreas A r t i f i c i a l Liver Blood A l c o h o l E l e c t r o d e Removal of A i r b o r n Infections

(137) (138)

Microsomal Enzymes A l c o h o l Oxidase DNase, RNase

Reference

(139) (140) (141) (142)

B i o l o g i c a l l y a c t i v e molecules can be immobilized w i t h i n hydrogels permanently, or t e m p o r a r i l y . I f the hydrogel system i s designed to r e l e a s e the entrapped b i o l o g i c a l l y a c t i v e molec u l e s at a p r e s e t r a t e , these m a t e r i a l s are w e l l s u i t e d f o r use as c o n t r o l l e d drug d e l i v e r y devices. I n the s i m p l e s t example of such a system, hydrogels can be s a t u r a t e d w i t h s o l u t i o n s of v a r ious a n t i b i o t i c s and other drugs which w i l l l e a c h out to the surrounding t i s s u e upon i m p l a n t a t i o n . A number of papers d e s c r i b i n g such hydrogel drug d e l i v e r y systems have already been mentioned (41-46,95). The r a t e of drug d e l i v e r y g e n e r a l l y decreases r a p i d l y w i t h simple homogeneous hydrogels saturated w i t h a drug s o l u t i o n . By u s i n g a b i o c o m p a t i b l e hydrogel membrane device f i l l e d w i t h a drug i n the form of a pure l i q u i d or s o l i d , constant drug d e l i v e r y r a t e s and extended treatment times can be obtained (143,144). A s t i l l more s o p h i s t i c a t e d approach i n v o l v e s the d e s i g n of a h y d r o p h i l i c polymer backbone c h a i n onto which a l t e r n a t e " c a t a l y s t groups and l a b i l e "drug" groups are 11


1.


Synthetic

Hydrogeh

25

HYDROPHILIC BACKBONE POLYMER

C=0 CATALYTIC—• Η Ν'· GROUP R Figure S.

C=0

ΗΝ·· I

R

I

+

0"

DRUG

Drug release from a polymeric chain controlled by intramolecular catalysis

bound (e.g. F i g u r e 3). The k i n e t i c s o f v a r i o u s i n t r a m o l e c u l a r l y c a t a l y z e d polymeric r e a c t i o n s have been described (145). Hydrogels which are prepared by the s o l u t i o n p o l y m e r i z a t i o n of a monomer i n the presence of a c r o s s l i n k i n g agent are w e l l s u i t e d f o r entrapping an a c t i v e biomolecule w i t h i n the network s t r u c t u r e . For the i m m o b i l i z a t i o n o f an enzyme by the entrapment technique, leakage o f the enzyme would be u n d e s i r a b l e . There f o r e , the "pore s i z e o r average i n t e r c h a i n d i s t a n c e o f the g e l should be s m a l l e r than the s i z e o f the a c t i v e enzyme. A "pore" s i z e o f 35 A o r s m a l l e r should be s u i t a b l e f o r r e t a i n i n g most entrapped enzymes (146). For comparison, homogeneous P-HEMA g e l s have estimated "pore" s i z e s o f approximately 4-5 A (5g,147)> w h i l e acrylamide g e l s might have "pore" s i z e s from 7 A - 17 A depending upon the method o f p r e p a r a t i o n (148). However, these pore s i z e s are probably low w i t h an e r r o r estimated a t greater than 25% (4). Recent r e p o r t s on enzyme entrapment i n c l u d e sys tems i n v o l v i n g glucose oxidase entrapped i n P-HEMA and P-NVP(149) glucoamylase, i n v e r t a s e , and 3-galactosidase entrapped i n p o l y (2-hydroxyethyl a c r y l a t e ) and poly(dimethylacrylamide) g e l s (150) and asparaginase and microsomal enzymes entrapped i n P-NVP g e l s (151). The entrapment o f heparin i n a PVA g e l was described i n S e c t i o n I I Ε (92). Techniques f o r the covalent i m m o b i l i z a t i o n o f a c t i v e mole c u l e s t o s u r f a c e s have been the subject o f a number o f recent i n depth reviews (140,152,153). A l a r g e number o f chemical t e c h niques which are p a r t i c u l a r l y a p p l i c a b l e f o r i m m o b i l i z a t i o n t o hydrogels have been developed. Figure 4 shows c h e m i s t r i e s u s e f u l f o r c o u p l i n g biomolecules t o g e l s c o n t a i n i n g c a r b o x y l groups. F i g u r e 5 i l l u s t r a t e s the probable r e a c t i o n s o c c u r r i n g during the i m m o b i l i z a t i o n o f a p r o t e i n t o a polymer which con t a i n s h y d r o x y l groups (154). The Ugi r e a c t i o n i s a four compo nent condensation r e a c t i o n which occurs between an amine, an aldehyde, a c a r b o x y l i c a c i d and an i s o c y a n i d e (155). I t i s o f i n t e r e s t f o r i m m o b i l i z a t i o n t o hydrogels because o f the many 11

Q



6).

5)·

*

+

Figure 4.

-CC^H

-CO

•CO

i-CO?H +

4).

C H

i-COgH-h S 0 C (

3).

2) .

C

C

Η

/ " x

0

-

^

2

2

.COî^CH-CO-NHR

2

-C0 H

V

2

*2

e.g., PROTEIN - C 0 H

PROTEIN

PROTEIN-NH2

PROTEIN-NH

2

2

2

J o ON Η PROTEIN

^-C0 -CH CHOH-R-CHOH-CH -0 C-PROTEIN -CONH - PROTEIN

?

CO -OU-CHOH-R-CH-CH

ί

i - C O N H -

k02-N^J

CONH - PROTEIN

SURFACES 1

Chemistries useful for coupling biomolecules to gels containing carboxyl groups

Η

2

Νφ B F

PROTEIN-NH

V

2

OH

PROTEIN-NH'

ON POLYCARBOXYLIC

e.g., P R O T E I N - N H 2

m

PROTEINS

4-coce

A

— C H - R - C H - C H

V

2

2

1) . ij-copH 3-CO?H + R - C = N=C-R

IMMOBILIZING

2

ο

> ha r* Ο

w

> > ϋ

ο

a

M

S3

3

ο w

§ Ο

to

1.


Synthetic Hydrogels

27

p o t e n t i a l r e a c t i o n s which might be used depending upon which f u n c t i o n a l groups are on the hydrogel and which are on the b i o molecule to be immobilized. The Ugi r e a c t i o n used t o couple a p r o t e i n - NH group t o a c a r b o x y l i c hydrogel i s shown i n F i g u r e 6. Glutaraldehyde i s o f t e n used f o r i m m o b i l i z a t i o n o f biomolecules to polyacrylamide hydrogels although the p r e c i s e mechanism o f c o u p l i n g i s not yet known (156). There have been s u r p r i s i n g l y few r e p o r t s on the development of m a t e r i a l s intended t o be both biocompatible and b i o l o g i c a l l y a c t i v e . Many a c t i v e biomolecules have been bound t o supports such as Sephadex and Sepharose (modified p o l y s a c c h a r i d e s ) which allow l a r g e amounts o f a c t i v e biomolecule to be immobilized but which would not be expected to show s i g n i f i c a n t b i o c o m p a t i b i l i t y . Devices s p e c i f i c a l l y constructed f o r the i m m o b i l i z a t i o n o f enzymes t o be used i n contac of such m a t e r i a l s as poly(methy c h l o r i d e o r polycarbonate (134), a l l o f which are considered to be r a t h e r thrombogenic s u r f a c e s (although c o l l o i d a l g r a p h i t e was used on some o f the s u r f a c e s , presumably to reduce thrombog e n i c i t y ) . One o f the e a r l i e s t papers d e s c r i b i n g an " a c t i v e " b i o m a t e r i a l prepared by combining r a d i a t i o n g r a f t polymerization plus biochemical and medical concepts i s by Hoffman, ert a l . , (135). In t h i s study s t r e p t o k i n a s e , albumin and heparin were immobilized on r a d i a t i o n g r a f t e d hydrogels based upon P-HEMA and P-NVP. Streptokinase immobilized v i a an "arm" demonstrated s i g n i f i c a n t fibrinolytic activity. Immobilized heparin, on the other hand, d i d not seem to r e t a i n b i o l o g i c a l a c t i v i t y when immobilized to these surfaces u s i n g e i t h e r BrCN o r carbodiimide c h e m i s t r i e s . Nguyen and Wilkes have d e s c r i b e d a f i b r i n o l y t i c s u r f a c e made by immobilizing b r i n o l a s e to E n z a c r y l , a p a r t i c u l a t e , crosslinked, modified polyacrylamide (133). S i g n i f i c a n t b r i n o l a s e a c t i v i t y was maintained. The authors suggest a g r a f t e d polymer u t i l i z i n g a c r y l i c a c i d and N - a c r y l o y l para-phenylene diamine as a more p r a c t i c a l m a t e r i a l f o r both immobilizing b r i n o l a s e and c o n s t r u c t ing u s e f u l d e v i c e s . Another approach to preparing a blood compatible s u r f a c e based upon the i m m o b i l i z a t i o n o f biomolecules to hydrogels has been taken by Lee, e t a l . (95). They prepared a three l a y e r support m a t e r i a l w i t h PVA a t the s u r f a c e . Onto the PVA they e s t e r i f i e d f i r s t , h a l f c h o l e s t e r o l e s t e r s of d i c a r b o x y l i c a c i d s and next, the h a l f s i a l i c a c i d e s t e r o f a longer chain dicarboxy l i c a c i d . The s u r f a c e was f i n a l l y t r e a t e d w i t h t i s s u e c u l t u r e medium t o c o n d i t i o n i t w i t h s a l t s and p r o t e i n s found i n the blood. The r a t i o n a l e behind the m a t e r i a l was to simulate the n a t u r a l blood v e s s e l i n t i m a . Vena cava r i n g t e s t s i n d i c a t e d g e n e r a l l y poor thromboresistance f o r these complex s u r f a c e s (113, 115). Renal embolus r i n g s were r e l a t i v e l y f r e e o f thrombus, but the kidneys o f t e s t animals were o f t e n massively i n f a r c t e d (9). I t has been proposed that an " a r t i f i c i a l pancreas" might be constructed by combining a blood glucose sensor w i t h feedback to 2


28


0C0NH

2

Carbamate (inert) ^-0-C=N

OH BrCNL

Activation step

ÔH

/ /

OH Imidocarbonate (reactive)

C = NH Intermediate cyanate structure

4- O - C - N H - Protein II NH -OH

Coupling step

H N-Protein

C = NH

Isourea derivative

N-substituted imidocarbonate

C = N - Protein

2

Imidocarbonate

;-0H Transactions—American Society for Artificial Internal Organs

Figure 5. Immobilization

of a protein to a polymer containing hydroxyl groups (135)

\

H® C=0

+

H N-PROTEIN 2

C = Ν-PROTEIN + H 0 2

1! ® C - N H - PROTEIN

^NH-PROTEIN

R' \ NH N

R

c

\

o

v

o

0'

PROTEIN

C

, -N R V

-°

Enzyme Engineering

Figure 6. The Ugi reaction as might be used for the of a protein to a surface ( 136 )

immobilization


1.


Synthetic Hydrogels

29

an i n s u l i n d e l i v e r y system. P o t e n t i a l l y , other h e a l t h problems could a l s o be t r e a t e d u s i n g a combination of a blood sensor and a drug or hormone d e l i v e r y system. Enzymatic e l e c t r o d e s which can be designed to show h i g h s p e c i f i c i t y f o r a given type of molecule might be p a r t i c u l a r l y w e l l s u i t e d f o r use as blood sensors (157). Techniques have been r e c e n t l y described whereby an enzyme i s simultaneously entrapped w i t h i n a c r o s s l i n k e d hydrogel and coated onto a g l a s s e l e c t r o d e . Polyacrylamide hydrog e l s have been u t i l i z e d f o r t h i s a p p l i c a t i o n (158). Such e l e c t r o d e s might be expected to show both h i g h s p e c i f i c i t y and non-thrombogenicity making them p a r t i c u l a r l y w e l l s u i t e d f o r blood sensor a p p l i c a t i o n s . VI.

Conclusions Hydrogels as a c l a s t i l i t y and e x c e l l e n t performanc c a t i o n s . However, i n the 15 years s i n c e s y n t h e t i c hydrated polymeric networks were f i r s t proposed f o r b i o m a t e r i a l a p p l i c a t i o n s the " s u r f a c e has b a r e l y been s c r a t c h e d " w i t h respect to new types of hydrogels which might by s y n t h e s i z e d , fundamental knowledge concerning how and why hydrogels "work", and new b i o medical a p p l i c a t i o n s f o r these u s e f u l polymers. S p e c i f i c areas which are c l e a r l y i n need of f u r t h e r study before the f u l l p o t e n t i a l of b i o m e d i c a l hydrogels can be r e a l i z e d a r e : (1) Thrombogenicity of hydrogel s u r f a c e s , p a r t i c u l a r l y w i t h respect to emboli formation. (2) C e l l adhesion and i n t e r a c t i o n w i t h hydrated polymer networks, e s p e c i a l l y w i t h c e l l types such as p l a t e l e t s , leukocytes and f i b r o b l a s t s . (3) The s t r u c t u r e of water w i t h i n hydrogels and i t s p o t e n t i a l r e l a t i o n s h i p to b i o l o g i c a l i n t e r a c t i o n s . (4) The behavior of b i o l o g i c a l molecules immobilized to hydrogels. (5) The i n t e r a c t i o n of a n i o n i c and c a t i o n i c hydrogels w i t h blood and other b i o l o g i c a l systems.


30


1. 2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

21. 22. 23. 24. 25. 26.

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2 Vapor Pressure and Swelling Pressure of Hydrogels MIGUEL F. REFOJO Eye Research Institute of Retina Foundation, Boston, Mass. 02114

H y d r o g e l s c o n s i s t o f two c o m p o n e n t s : the polymer network, which i s constant i n q u a n t i t y is variable. At e q u i l i b r i u t h e w a t e r i n t h e g e l and t h e w a t e r s u r r o u n d i n g t h e g e l a r e e q u a l . The a d d i t i o n t o t h e s o l u t i o n s u r r o u n d i n g t h e g e l o f m a c r o m o l e c u l e s t h a t are too l a r g e to penetrate the gel lowers the chemical p o t e n t i a l of the water i n the s o l u t i o n . Water t h u s moves o u t o f t h e g e l and t h e n e t w o r k c o n t r a c t s , d e c r e a s i n g t h e c h e m i c a l p o t e n t i a l o f the water i n the network to the v a l u e o f the water i n the solution. Therefore, at e q u i l i b r i u m the osmotic pressure of the macromolecular s o l u t i o n equals the s w e l l i n g pressure of the hydrogel. The d e g r e e o f h y d r a t i o n t h a t can be a c h i e v e d by e q u i l i b r a t i o n w i t h a m a c r o m o l e c u l a r s o l u t i o n c a n a l s o be o b t a i n e d by compressing the g e l under a mechanical p r e s s u r e e q u i v a l e n t i n magnitude t o the osmotic p r e s s u r e o f the macromolecular s o l u t i o n . The m e c h a n i c a l p r e s s u r e r a i s e s t h e c h e m i c a l p o t e n t i a l o f t h e w a t e r i n t h e g e l , so w a t e r exudes f r o m t h e g e l u n t i l e q u i l i b r i u m i s reached. Thus t h e e q u i l i b r i u m w a t e r c o n t e n t depends on t h e mechanical pressure a p p l i e d to the g e l . The s w e l l i n g phenomena o f g e l s have been t h e o b j e c t o f t h e r m o d y n a m i c a n a l y s i s (1_,2). The o s m o t i c p r e s s u r e a t t r i b u t e d t o the polymer network ( π ) i s the d r i v i n g f o r c e o f s w e l l i n g . The s w e l l i n g p r o c e s s d i s t e n d s t h e n e t w o r k , and i s c o u n t e r a c t e d by t h e e l a s t i c c o n t r a c t i l i t y o f the s t r e t c h e d polymer network ( p ) . Hence t h e s w e l l i n g p r e s s u r e o f n o n i o n i c h y d r o g e l s ( Ρ ) i s t h e r e s u l t o f t h e i m b i b i t i o n o f s o l v e n t d r i v e n by an o s m o t i c p r e s s u r e , c o u n t e r a c t e d by t h e c o n t r a c t i l i t y o f t h e n e t w o r k w h i c h tends to expel the s o l v e n t : Ρ = π - p. A t e q u i l i b r i u m , π = ρ, and t h e s w e l l i n g p r e s s u r e i s z e r o (P = 0 ) . S w e l l i n g p r e s s u r e can be d e f i n e d as t h e p r e s s u r e e x e r t e d by a g e l when s w e l l i n g i s c o n s t r a i n e d b u t s w e l l i n g s o l v e n t i s a v a i l able. In g e n e r a l , t h e f o l l o w i n g e m p i r i c a l r e l a t i o n s h i p (I), d e v e l o p e d by P o s n j a k i n 1912 (3)> a p p l i e s t o a s w e l l i n g s u b stance:

37


38


Ρ = k X C

(I)

where Ρ i s t h e s w e l l i n g p r e s s u r e , k and η a r e c o n s t a n t s whose v a l u e s a r e u s u a l l y between 2 and 3 , and c i s t h e p o l y m e r n e t w o r k concentration. Since n>l, expression (I) i n d i c a t e s the w e l l known f a c t t h a t t h e s w e l l i n g p r e s s u r e , P, i n c r e a s e s r a p i d l y w i t h the c o n c e n t r a t i o n of the polymer i n the g e l . In t h i s p a p e r , t h e s w e l l i n g p r e s s u r e o f two c l a s s e s o f hydrogels of i n t e r e s t to ophthalmology i s i n v e s t i g a t e d . The f i r s t c l a s s i s a g r o u p o f h i g h l y h y d r a t e d h y d r o g e l s (above a b o u t 80% w a t e r a t s w e l l i n g e q u i l i b r i u m i n w a t e r ) w h i c h i s i n t e n d e d f o r s u r g i c a l use. The s e c o n d c l a s consist f hydrogel d manufacture contact l e n s e s 40 t o 75% w a t e r a t s w e l l i n g e q u i l i b r i u E f f e c t o f t h e E x t e r n a l S o l u t i o n upon t h e H y d r a t i o n o f Hydrogels. The m a g n i t u d e o f e q u i l i b r i u m s w e l l i n g o f a h y d r o g e l i n an aqueous medium i s d e t e r m i n e d by t h e c h e m i c a l p o t e n t i a l o f water i n the outside s o l u t i o n . The c h e m i c a l p o t e n t i a l o f w a t e r i n t h e o u t s i d e s o l u t i o n i s d e t e r m i n e d by t h e n a t u r e and c o n c e n t r a t i o n of the solutes i n the s o l u t i o n . The s o l u t e s , d e p e n d i n g on t h e i r m o l e c u l a r s i z e , may o r may n o t p e n e t r a t e t h e p o l y m e r network. Some s o l u t e s w h i c h c a n p e n e t r a t e t h e n e t w o r k may i n t e r a c t w i t h t h e p o l y m e r s e g m e n t s , m o d i f y i n g t h e i r s t r e t c h i n g and contracting properties. Nevertheless, a l l solutes i n the gel w a t e r a f f e c t t h e g e l by l o w e r i n g t h e c h e m i c a l p o t e n t i a l o f i t s water. Solution t o n i c i t y i s a b i o l o g i c a l concept. It is related to osmotic p r e s s u r e , but i t l a c k s e x a c t p h y s i c o c h e m i c a l meaning. T h u s , v a r i o u s i s o t o n i c s o l u t i o n s may s w e l l o r d e s w e l l h y d r o g e l s d e p e n d i n g on t h e p e n e t r a t i o n and i n t e r a c t i o n o f t h e s o l u t e s w i t h t h e n e t w o r k segments ( £ ) ( F i g . 1 ) . S w e l l i n g Pressure o f High Water-Content G l y c e r y l Methac r y l a t e H y d r o g e l s and t h e i r O p h t h a l m i c A p p l i c a t i o n s . The s w e l 1 ing pressure-volume r e l a t i o n s h i p of p o l y ( g l y c e r y l methacrylate) h y d r o g e l s (PGMA) i s o f p r a c t i c a l i n t e r e s t f o r t h e d e v e l o p m e n t o f s w e l l i n g or expanding s u r g i c a l i m p l a n t s . A swelling implant i s a d e v i c e t h a t can be p l a c e d i n s i d e an o r g a n o r t i s s u e t h r o u g h a r e l a t i v e l y small i n c i s i o n . By i m b i b i n g a v a i l a b l e body f l u i d i t w i l l s w e l l t o f i l l a c a v i t y or to a l t e r the form of the s u r rounding t i s s u e s (5). To be u s e f u l , a s w e l l i n g i m p l a n t must s w e l l t o s e v e r a l tTmes i t s d r y volume u n d e r t h e c o n d i t i o n s o f implantation. In some a p p l i c a t i o n s , t h e i m p l a n t must e x e r t s u f f i c i e n t s w e l l i n g pressure to counteract the c o n s t r a i n i n g pressure of the surrounding t i s s u e s . Of i n t e r e s t i s a c o h e r e n t v i t r e o u s s u b s t i t u t e w h i c h c o u l d be used t o f i l l t h e v i t r e o u s c a v i t y o f t h e eye ( £ ) , and i n some c a s e s , t h e e n t i r e e y e b a l l (_7).



10

2CH

30Η

.9%

NaCI 2

CaCI 13% 2

UREA 1.63°/o

GLUCOSE 5.51%

SERUM

SERUM VITREOUS ULTRAFILTRATE

Figure 1. PGMA hydrogel, 98% H 0 at equilibrium swelling in distilled water. The bars repre sent the equilibrium swelling of the same hydrogel in diverse isotonic solutions and physiological fluids.

2

m

1 50Η

Τ- 6θΗ

Ζ ζ

80


40

A n o t h e r t y p e o f s w e l l i n g i m p l a n t i s used i n ophthalmology i n t h e s c l e r a l b u c k l i n g procedure i n r e t i n a l detachment s u r g e r y (8). G l y c e r y l m e t h a c r y l a t e monomer (GMA) p r e p a r e d by h y d r o T y s i s o f g l y c i d y l m e t h a c r y l a t e (9·) y i e l d s GMA w i t h s m a l l amounts o f some s t i l l u n i d e n t i f i e d c r o s s l i n k i n g a g e n t . However, h y d r o g e l s w i t h r e p r o d u c i b l e p h y s i c a l and c h e m i c a l p r o p e r t i e s c a n be o b t a i n e d f r o m GMA p r e p a r e d by t h i s p r o c e d u r e . Polymerization is c a r r i e d o u t i n g l a s s molds f i l l e d w i t h aqueous s o l u t i o n s o f GMA w i t h a redox i n i t i a t o r . The monomer d i l u t i o n i n t h e p r e p o l y m e r m i x t u r e determines the e q u i l i b r i u m degree of s w e l l i n g of the r e s u l t i n g hydrogel. The s w e l l i n g p r e s s u r e o f PGMA h y d r o g e l s was o b t a i n e d by e q u i l i b r a t i o n i n s o l u t i o n s o f d e x t r a n ( P h a r m a c i a , mol wt 2 3 6 , 0 0 0 ) , The PGMA s p e c i m e n s were p l a c e d i n d e x t r a n s o l u t i o n s and a l l o w e d to e q u i l i b r a t e in t i g h t l d jar t temperature E q u i l i b r i u m s w e l l i n g wa i n g on t h e c o n c e n t r a t i o , specimen. When e q u i l i b r i u m s w e l l i n g was r e a c h e d , t h e d e x t r a n c o n c e n t r a t i o n was d e t e r m i n e d f r o m a l i q u o t s o f t h e s o l u t i o n . The o s m o t i c p r e s s u r e o f t h e d e x t r a n a t d i f f e r e n t c o n c e n t r a t i o n s was determined osmometrically (4). The d e x t r a n m o l e c u l e s Tn w a t e r s o l u t i o n have an e l l i p s o i d a l shape. The d i a m e t e r o f t h e d e x t r a n m o l e c u l e s u s e d i n t h e s e e x p e r i m e n t s i s a b o u t 270 Â ( 1 0 ) . The d e x t r a n m o l e c u l e s a r e n o t l i k e l y t o p e n e t r a t e i n t o PGMTThydrogels b o t h b e c a u s e a ) t h e average pore s i z e o f the hydrogels i s s m a l l e r than the s i z e of t h e d e x t r a n m o l e c u l e s ( f o r i n s t a n c e , PGMA h v d r o g e l s o f 94% H 0 have an a v e r a g e p o r e d i a m e t e r o f a b o u t 124 A)(1_1_), and b) when à h i g h l y hydrated gel i s placed i n a dextran s o l u t i o n , the osmotic d e h y d r a t i o n o f the gel i s f a s t e r than the d i f f u s i o n o f the dextran molecules i n t o the g e l . As a g e l d e h y d r a t e s , p o r e s i z e i s r e d u c e d , f u r t h e r l i m i t i n g p e n e t r a t i o n by l a r g e d e x t r a n m o l e c u l e s . F i g u r e 2 g i v e s t h e s w e l l i n g p r e s s u r e o f s e v e r a l PGMA h y d r o gels versus the " s w e l l i n g r a t i o " (q). The s w e l l i n g r a t i o i s d e f i n e d as t h e volume o f t h e s w o l l e n g e l o v e r t h e volume o f t h e same g e l i n t h e d r y s t a t e . The " s w e l l i n g r a t i o " was c a l c u l a t e d f r o m t h e " d e g r e e o f s w e l l i n g " ( γ ) , t h e d e n s i t y o f t h e s w o l l e n g e l ( d ) , and t h e d e n s i t y of the dry gel ( d ) , according to (II): 2

0

γ i s the r a t i o of the weights of the swollen gel to the dry gel (£). The d e n s i t i e s were o b t a i n e d f r o m F i g u r e 3 , w h i c h g i v e s t h e d e n s i t y o f a PGMA h y d r o g e l v e r s u s t h e w e i g h t f r a c t i o n o f w a t e r i n t h e s w o l l e n g e l (Cw = % H 0 / 1 0 0 ) . D e n s i t i e s w e r e d e t e r m i n e d by t h e h y d r o s t a t i c w e i g h i n g method. G e l s were weighed both i n a i r , 2


2.

Vapor Pressure and Swelling

REFOJO

5

10 15 20

30

40

41

Pressure

50

60

70

125.2

q = VOL. SWOLLEN GEL + VOL. DRY G E L

Figure 2. Swelling pressure vs. "swelling ratio" of several PGMA hydrogels. The equilibrium swelling of the PGMA hydrogels in distilled water is given for each swelling pressure curve in the graph.


42


and immersed i n η - h e p t a n e a t room t e m p e r a t u r e , Cw b y :

γ

=

γ i s r e l a t e d to

TO

The d e n s i t y o f t h e d i f f e r e n t PGMA h y d r o g e l s was o b t a i n e d from F i g u r e 3 assuming t h a t the d e n s i t i e s o f the d r y g e l s ( x e r o g e l s ) and h y d r o g e l s were n o t a f f e c t e d a p p r e c i a b l y by t h e amount of crosslinkage. The r e s u l t s g i v e n i n F i g u r e 2 show t h a t a s u b s t a n t i a l amount o f w a t e r i s removed f r o m h i g h l y h y d r a t e d h y d r o g e l s ( j e l l i e s ) when t h e y a r e s u b j e c t e d t o a s l i g h t c o m p r e s s i o n . Thus, the volume o f PGMA h y d r o g e 1 2 5 . 6 ) was more t h a n h a l v e Hg o s m o t i c p r e s s u r e . T h e s e j e l l i e s exude l i q u i d w a t e r when t h e y a r e a l l o w e d t o s t a n d u n d e r t h e i r own w e i g h t i n t h e a i r . As t h e water content i n the hydrogel decreases, the pressure r e q u i r e d to compress w a t e r o u t o f t h e g e l i n c r e a s e s . H e n c e , PGMA w i t h 95% H 0 by w e i g h t (q = 30) a t e q u i l i b r i u m i n w a t e r l o s t a b o u t one t h i r d o f i t s volume o f w a t e r (q = 20) u n d e r o n l y a b o u t 4 mm Hg, b u t PGMA h y d r o g e l w i t h 82% H 0 by w e i g h t (q = 6.9) l o s t p r a c t i c a l l y no w a t e r u n d e r t e n t i m e s as much o s m o t i c p r e s s u r e . The r e l a t i o n s h i p o f h y d r a t i o n by w e i g h t , Cw, and s w e l l i n g r a t i o by volume ( q ) i s g i v e n b y : 2

2

q

"

(

T^CW~

(IV)

H e n c e , when t h e v a l u e o f Cw i s n e a r o n e , s u c h as i n j e l l i e s , s m a l l d i f f e r e n c e s i n h y d r a t i o n r e p r e s e n t s u b s t a n t i a l volume changes. The s w e l l i n g p r e s s u r e p r o p e r t i e s o f j e l l i e s and h y d r o g e l s a r e i m p o r t a n t f r o m t h e p o i n t o f v i e w o f t h e two k i n d s o f s w e l l i n g i m p l a n t s m e n t i o n e d a b o v e , a v i t r e o u s s u b s t i t u t e and a s c l e r a l b u c k l i n g d e v i c e . A v i t r e o u s s u b s t i t u t e must m i m i c t h e n a t u r a l v i t r e o u s body, which i s a h i g h l y h y d r a t e d j e l l y . The g e l i s implanted i n i t s dry s t a t e i n t o the e y e b a l l through the s m a l l e s t p o s s i b l e i n c i s i o n . Then i t a b s o r b s a v a i l a b l e i n t r a o c u l a r f l u i d , s w e l l i n g f r e e l y as l o n g as i t does n o t a d j o i n t h e w a l l s o f t h e eye, u n t i l i t f i l l s the v i t r e o u s c a v i t y . F u l l y swollen, the i m p l a n t must o c c u p y t h e v i t r e o u s c a v i t y w h i l e e x e r t i n g a minimum of pressure against the extremely s e n s i t i v e r e t i n a . The s e c o n d t y p e o f s w e l l i n g i m p l a n t i s p l a c e d on t h e o u t s i d e o f t h e e y e b a l l , i n t h e s c l e r a . T h i s i m p l a n t , upon s w e l l i n g , e x e r t s p r e s s u r e t o b u c k l e t h e w a l l o f t h e eye i n w a r d , t h e r e b y approximating the choroid that c a r r i e s the blood supply to a


2.

REFOJO

43

Vapor Pressure and Swelling Pressure

detached r e t i n a . Such an i m p l a n t must be s o f t t o a v o i d p r e s s u r e n e c r o s i s i n t h e s c l e r a , b u t n o t so f r a g i l e t h a t i t w i l l c r u m b l e under p r e s s u r e . About a f i v e - f o l d s w e l l i n g a g a i n s t t h e c o n s t r a i n ing t i s s u e i s s u f f i c i e n t . For t h i s a p p l i c a t i o n , a hydrogel w i t h 80 t o 85% w a t e r c o n t e n t a t e q u i l i b r i u m s w e l l i n g i n w a t e r a p p e a r s t o be most u s e f u l . V a p o r P r e s s u r e and S w e l l i n g P r e s s u r e o f H y d r o g e l C o n t a c t Lens M a t e r i a l s . S i n c e W i c h t e r l e and L i m ( 1 2 ) f i r s t p r o p o s e d t h e use o f h y d r o g e l s f o r c o n t a c t l e n s e s and otTiêr m e d i c a l d e v i c e s , many new h y d r o g e l s have been d e v e l o p e d ( 1 3 ) . The f i r s t commerc i a l h y d r o g e l c o n t a c t l e n s e s w e r e made o f s l i g h t l y c r o s s l i n k e d p o l y ( 2 - h y d r o x y e t h y l m e t h a c r y l a t e ) (PHEMA), w h i c h i s s t i l l t h e m a t e r i a l most o f t e n used i n t h e s o f t l e n s i n d u s t r y . A second compound used t o make h y d r o g e (VP), i n t h e form o f a L e n s e s w h i c h c o n t a i n V P , b u t no HEMA a r e a l s o made, s u c h as a c o p o l y m e r o f m e t h y l m e t h a c r y l a t e and V P , P(MMA/VP) ( 1 5 ) . As d i f f e r e n t h y d r o g e l c o n t a c t l e n s e s become a v a T T a b l e , i t i s o f i n t e r e s t t o i n v e s t i g a t e and t o compare t h e i r r e l a t i v e w a t e r retention. Differences i n hydration at swelling equilibrium are i m p o r t a n t i n t h e e v a l u a t i o n o f t h e o p t i c a l and p h y s i o l o g i c a l performance o f t h e l e n s e s . This study determined the e q u i l i b r i u m s w e l l i n g o f s e v e r a l hydrogel c o n t a c t l e n s m a t e r i a l s under o s m o t i c and m e c h a n i c a l p r e s s u r e , as w e l l as t h e w a t e r a c t i v i t y o f t h e h y d r o g e l s under v a r i o u s s w e l l i n g c o n d i t i o n s . 1. D e t e r m i n a t i o n o f S w e l l i n g P r e s s u r e o f a PHEMA H y d r o g e l by E q u i l i b r i u m S w e l l i n g i n D e x t r a n S o l u t i o n . PHEMA I was o b t a i n e d by s o l u t i o n p o l y m e r i z a t i o n ( 1 6 ) . S t e q u i l i b r i u m s w e l l i n g i n d i s t i l l e d w a t e r , i t c o n t a i n s 40% H 0 by w e i g h t on a w e t b a s i s . PHEMA I s p e c i m e n s were p l a c e d i n d e x t r a n - 4 0 s o l u t i o n s and a l l o w e d t o e q u i l i b r a t e i n t i g h t l y capped j a r s a t room t e m p e r a t u r e [ D i f f e r e n t v a l u e s have been r e p o r t e d f o r a v e r a g e p o r e d i a m e t e r o f PHEMA I h y d r o g e l s ; t h e maximum i s 35 S ( 1 3 ) . D e x t r a n - 4 0 i n w a t e r has a m o l e c u l a r d i a m e t e r o f a b o u t 105 Â ( 1 0 ) ] . Equilibrium s w e l l i n g was r e a c h e d a f t e r two months. After e q u i l i b r a t i o n , the d e x t r a n c o n c e n t r a t i o n i n t h e j a r was d e t e r m i n e d g r a v i m e t r i c a l l y . The o s m o t i c p r e s s u r e ( π , i n a t m . ) o f d e x t r a n (mol wt 2 6 , 0 0 0 ) was calculated according to equation (V): 2

π = A-jC + A c

2

2

+ A c

(V)

3

3

where c i s t h e c o n c e n t r a t i o n ( i n g » c m " ) and A = 0 . 8 5 2 a t m » c m . g " , A = 1 3 . 5 2 a t m - c m g ~ , and A = 6 6 . 8 a t m - c m - g " a r e t h e v i r i a l c o e f f i c i e n t s a c c o r d i n g t o V i n k ( V 7 ) . The r e s u l t s o f t h e s w e l l i n g p r e s s u r e o f PHEMA I o b t a i n e d by t h i s p r o c e d u r e a r e g i v e n in Figs. 4,5. 3

3

x

1

6

2

2

9

3

3


HYDROGELS FOR MEDICAL AND RELATED APPLICATIONS 1.4 -η

~t ι » ι » » » 11111 » 1111111M 111 f 1111111111 f 11111 111 > π 11111 i i ι » 111111 π 11 n 111 » 11 M » η ι n n I n ι n m 1 I M 11| 10

20

30

40

50

I

60

70

80

90

100

H 0 2

Figure 3. Density of PGMA hydrogel, 96% H 0 at equilibrium swelling, vs. hydration of the gel. The curve was printed by a computer using the least squares method applied to the data points. 2

Φ

I I I I I I

ι

I!

5°'

a of

SI

1/

/

Q

·/# 'ο

^ / /

4 I

Figure 4. Dehydration of two PHEMA hydrogeh under osmotic and mechanical pressure

- ι — ι — ι — r -1 -2 - 3 -4 WEIGHT

*. H

2

0 LOST


2.

REFOJO


45

2. D e t e r m i n a t i o n o f S w e l l i n g P r e s s u r e o f a PHEMA H y d r o g e l by E q u i l i b r i u m S w e l l i n g Under M e c h a n i c a l C o m p r e s s i o n . PHEMA I I was o b t a i n e d by b u l k p o l y m e r i z a t i o n ( 1 6 ) . I t c o n t a i n s 3 8 . 5 % H 0 a t e q u i l i b r i u m swelling i n d i s t i l l e d water. C i r c u l a r pieces o f PHEMA I I h y d r o g e l 20 mm i n d i a m e t e r (1 t o 2 mm t h i c k ) were p l a c e d between two s i n t e r e d g l a s s d i s k s i n a w a t e r b a t h u n d e r i r o n w e i g h t s ( f i v e t o t e n k i l o g r a m s ) , w h i c h were s e p a r a t e d f r o m t h e u p p e r s i n t e r e d g l a s s d i s c by a p l a s t i c c y l i n d e r p r o t r u d i n g above t h e s u r f a c e o f t h e w a t e r b a t h . The w a t e r b a t h c o n t a i n i n g t h e h y d r o g e l d i s c and t h e w e i g h t s on t o p o f i t were k e p t i n a c l o s e d chamber t o p r e v e n t e v a p o r a t i o n . Equilibrium swelling of t h e h y d r o g e l was o b t a i n e d a f t e r s e v e r a l weeks o f c o m p r e s s i o n . The r e l a t i o n s h i p between h y d r a t i o n and s w e l l i n g p r e s s u r e o f PHEMA I I o b t a i n e d by t h i s p r o c e d u r e i s shown i n F i g s . 4 , 5 . While a s l i g h t pressur water-content hydrogel w a t e r by o s m o t i c o r m e c h a n i c a l means f r o m h y d r o g e l s h a v i n g l o w w a t e r - c o n t e n t , s u c h as t h e commonly u s e d PHEMA c o n t a c t l e n s h y d r o g e l s , w h i c h c o n t a i n a b o u t 40% w a t e r a t e q u i l i b r i u m s w e l l i n g ( F i g s . 4 , 5 ) . S i m i l a r r e s u l t s a r e expected from hydrogels c o n t a i n i n g l e s s t h a n 80% w a t e r a t e q u i l i b r i u m ( F i g . 2 ) . Most h y d r o g e l c o n t a c t l e n s m a t e r i a l s c o n t a i n a b o u t 35 t o 75% w a t e r a t equilibrium in physiological saline solution. I t requires subs t a n t i a l p r e s s u r e t o remove w a t e r f r o m h y d r o g e l l e n s e s , w h i c h i s a d v a n t a g e o u s b e c a u s e i f l i d p r e s s u r e were t o s q u e e z e w a t e r f r o m the l e n s e s i n t h e eye t h e i r performance would s u f f e r . Of c o u r s e , t h e o p t i c a l p r o p e r t i e s , t h e s h a p e , and t h e s i z e o f a h y d r o g e l l e n s a r e a l l d e p e n d e n t on i t s w a t e r - c o n t e n t . 2

3. D e t e r m i n a t i o n o f Water A c t i v i t y i n H y d r o g e l s . One way to f a c i l i t a t e water l o s s from hydrogels i s t o decrease t h e r e l a t i v e humidity; t h i s decreases the chemical p o t e n t i a l o f the water vapor i n t h e s u r r o u n d i n g atmosphere t o a low v a l u e . It i s well known t h a t h y d r o g e l s c a n l o s e w a t e r r a p i d l y by e v a p o r a t i o n . When t h i s happens, t h e network, which i s under e l a s t i c t e n s i o n , w i l l contract. As t h e h y d r o g e l d e h y d r a t e s , t h e c h e m i c a l p o t e n t i a l o f t h e w a t e r r e m a i n i n g i n t h e g e l d e c r e a s e s and i s m a n i f e s t e d a s an i m b i b i t i o n p r e s s u r e , which i s equal i n magnitude t o t h e osmotic o r m e c h a n i c a l p r e s s u r e needed t o compress t h e g e l t o t h e same p a r t i a l l y dehydrated s t a t e . The w a t e r r e t e n t i o n , o r w a t e r e s c a p i n g t e n d e n c y ( f u g a c i t y ) o f h y d r o g e l s was d e t e r m i n e d ( F i g s . 6 , 7 ) by m e a s u r i n g t h e e q u i l i b r i u m r e l a t i v e h u m i d i t y (% ERH) o f t h e h y d r o g e l s a t 3 2 ° C , w h i c h i s approximately the surface temperature of the eye. F o u r d i f f e r e n t h y d r o g e l s were u s e d i n t h e s e e x p e r i m e n t s : ( a ) a PHEMA h y d r o g e l w i t h 4 2 . 5 % H 0 a t e q u i l i b r i u m s w e l l i n g ; ( b ) a c o p o l y m e r o f m e t h y l m e t h a c r y l a t e and v i n y l p y r r o l i d o n e , P(MMA/ V P ) , used i n t h e manufacture o f hydrogel c o n t a c t l e n s e s under t h e t r a d e name S a u f I o n ( c o n t a i n i n g a b o u t 70% H 0 a t e q u i l i b r i u m s w e l l i n g i n d i s t i l l e d w a t e r ) ; ( c ) a c o p o l y m e r o f HEMA and VP 2

2



46

-/•A-r 35

36

37

3 8 39

τ — ι 38 39

37

1 — - r 4 0 41

W E I G H T 7o H 2 O

Figure 5. Swelling pressure of two PHEMA hydrogels equilibrated under mechanical pres sure (PHEMA II, 38.5% H 0) and osmotic pressure (PHEMA I, 40% H 0), respectively 2

2

WATER ACTIVITY ia ) w

Figure 6.

Water sorption isotherms of hydrogel contact lens materials


2.

REFOJO

47


[P(HEMA/VP)], known as PHP and a l s o used i n c o n t a c t l e n s e s . It c o n t a i n s a b o u t 45% w a t e r a t e q u i l i b r i u m s w e l l i n g i n w a t e r ; and (d) a c o p o l y m e r o f MMA and GMA [P(GMA/MMA)] w h i c h c o n t a i n s 4 1 % water a t e q u i l i b r i u m s w e l l i n g . The e q u i l i b r i u m r e l a t i v e h u m i d i t y ( w a t e r a c t i v i t y ) was determined w i t h t h e hydrogel i n a c l o s e d c o n t a i n e r having a c a l i b r a t e d h u m i d i t y and t e m p e r a t u r e s e n s o r c o n n e c t e d by c a b l e t o a r e c e i v e r (Hygrodynamics U n i v e r s a l Hygrometer I n d i c a t o r , A m e r i can I n s t r u m e n t C o . , S i l v e r S p r i n g s , M a r y l a n d ) . The chamber c o n t a i n i n g t h e h y d r o g e l and t h e s e n s o r was m a i n t a i n e d i n a c o n s t a n t t e m p e r a t u r e d r y i n c u b a t o r a t 32°C. When r e l a t i v e h u m i d i t y r e a c h e d e q u i l i b r i u m , t h e w e i g h t o f t h e g e l was r e c o r d e d . The o p e r a t i o n was r e p e a t e d f o r d i f f e r e n t s t a t e s o f h y d r a t i o n t o o b t a i n t h e isotherms i n which t h e weight o f water sorbed per u n i t o f d r y p o l y m e r w e i g h t was p l o t t e d w i t h r e f e r e n c e t o w a t e r a c t i v i t y ( F i g . 6 ) . The w a t e c o n d i t i o n s , e x c e p t f o r P(HEMA/VP), resorp tion conditions. The i s o t h e r m s o b t a i n e d have t h e s t a n d a r d s i g m o i d shape o f w a t e r s o r p t i o n i n p o l y m e r s . F i g u r e 7 shows t h e same r e s u l t s o f w a t e r a c t i v i t y v e r s u s w a t e r c o n t e n t i n t h e h y d r o g e l s , e x p r e s s e d i n p e r c e n t h y d r a t i o n on a w e t b a s i s , w h i c h i s c o n v e n t i o n a l l y used i n hydrogel l i t e r a t u r e . ERH ( r e l a t i v e h u m i d i t y o f t h e s p a c e a r o u n d t h e h y d r o g e l , when m o i s t u r e w i l l n e i t h e r l e a v e n o r e n t e r t h e h y d r o g e l ) i s a r a t i o o f e x i s t i n g p a r t i a l vapor pressure o f water i n t h e hydrogel (P ) t o t h e s a t u r a t i o n w a t e r v a p o r p r e s s u r e i n t h e a i r (P ). It i s given by:

% ERH =

15S. χ

100

(VI)

P /Ps i s , o f course, the "water a c t i v i t y " ( a ) i n t h e hydrogel a t the given temperature. Thus, t h e water a c t i v i t y i n hydrogels a t d i f f e r e n t l e v e l s o f h y d r a t i o n c a n be d e t e r m i n e d d i r e c t l y and c o u l d be u s e d t o c a l c u l a t e t h e s w e l l i n g p r e s s u r e ( P ) o f t h e hydrogels a t d i f f e r e n t hydrations according t o : e

w

P = P l n i w w

(VII)

where R i s t h e u n i v e r s a l gas c o n s t a n t , Τ t h e a b s o l u t e t e m p e r a t u r e , and V ^ t h e p a r t i a l m o l a r volume o f w a t e r . Under t h e e x p e r i mental c o n d i t i o n s ( F i g . 7 ) , small d i f f e r e n c e s i n water a c t i v i t y i n t h e h y d r o g e l s w e r e n o t d e t e c t a b l e a t h y d r a t i o n s above a b o u t 30% w a t e r . Not o n l y t h e hydrogels a t e q u i l i b r i u m s w e l l i n g i n w a t e r , b u t l i q u i d w a t e r a s w e l l , gave an ERH j u s t b e l o w t h e t r u e v a l u e o f 100%. T h i s i s a l i m i t a t i o n o f t h e h y g r o s e n s o r u s e d .

Amcnccn Chemical Socisîy Library 1155 16th St. N. 11

In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Washington. 0. C. Society: 20036Washington, DC, 1976.

48


B e c a u s e o f t h e l a r g e v a l u e o f RT/V ( a b o u t 1390 a t m . a t 32°C) a v e r y s m a l l v a p o r - p r e s s u r e decrease can r e s u l t i n a s u b s t a n t i a l increase in swelling pressure. Thus, the hygrometer t e c h n i q u e c a n n o t be used t o d e t e r m i n e t h e s w e l l i n g p r e s s u r e o f h y d r o g e l s near e q u i l i b r i u m h y d r a t i o n . The s o r p t i o n i s o t h e r m s o f f o u r h y d r o g e l s a r e g i v e n i n F i g u r e 6. The s i g m o i d shape o f t h e c u r v e s may be an i n d i c a t i o n o f t h e t h r e e c l a s s e s o f w a t e r w h i c h , a c c o r d i n g t o L e e , J h o n , and A n d r a d e ( 1 8 ) , may be p r e s e n t i n t h e s e h y d r o gels. Because o f t h e d i f f i c u l t y o f o b t a i n i n g t h e e q u i l i b r i u m r e l a t i v e humidity of a x e r o g e l , the s e c t i o n of the curves at l o w e r h y d r a t i o n a r e n o t as s h a r p l y d e f i n e d as t h e o t h e r two sections. The ERH i s a measure o f " f r e e w a t e r - v a p o r p r e s s u r e " w h i c h i s , i n e s s e n c e , a measurement o f t h e " f r e e d o m o f w a t e r " o r i t s "escaping tendency" (19) ments do g i v e a good i n d i c a t i o o f h y d r a t i o n i n h y d r o g e l s , t h a t i s , w a t e r i n t h e aqueous phase o f t h e h y d r o g e l w i t h t h e same v a p o r p r e s s u r e as l i q u i d w a t e r a t t h e same t e m p e r a t u r e . Water a c t i v i t y v e r s u s h y d r o g e l h y d r a t i o n ( F i g . 7) shows t h a t t h e w a t e r o f h y d r a t i o n i n h y d r o g e l s above a b o u t 25 t o 30% has a p p r o x i m a t e l y t h e same v a p o r p r e s s u r e as l i q u i d w a t e r . F i g u r e 8 , t h u s , r e p r e s e n t s t h e amounts o f " f r e e " w a t e r ( a - l ) and somewhat " b o u n d " w a t e r ( a < l ) i n h y d r o g e l s as r e p l o t t e d f r o m F i g u r e 6. The amount o f " b o u n d " w a t e r , a b o u t 30% o f w a t e r i n t h e h y d r o g e l s , i s s i m i l a r t o t h e amounts t h a t L e e , J h o n , and A n d r a d e (18) a s s i g n e d as " b o u n d " p l u s " i n t e r f a c i a l " w a t e r . The r e s t o f the water of hydration i n the hydrogels i s " f r e e " or " b u l k " water. T h u s , t h e c o n t a c t l e n s m a t e r i a l s examined a l l seem s u b j e c t t o l o s i n g s u b s t a n t i a l amounts o f w a t e r by e v a p o r a t i o n . The f a c t t h a t a b o u t 30% o f t h e w a t e r i s r e t a i n e d more t e n a c i o u s l y i n t h e l e n s m a t e r i a l s does n o t seem t o have any p r a c t i c a l i m p o r t a n c e f r o m t h e p o i n t o f v i e w o f w a t e r r e t e n t i o n o f h y d r o g e l l e n s e s and o p t i c a l performance. Of c o u r s e , f r e q u e n t b l i n k i n g and good t e a r s u p p l y a r e e s s e n t i a l f o r good r e s u l t s w i t h a l l h y d r o g e l c o n t a c t lenses. W

w

w

O s m o t i c E f f e c t s Due t o t h e Aqueous Phase o f a H y d r o g e l Contact Lens. In a d d i t i o n t o t h e s w e l l i n g p r e s s u r e ( o r i m b i b i t i o n pressure) of h y d r o g e l s , which i s a p r o p e r t y of the polymer phase o f g e l s , t h e r e a r e o t h e r o s m o t i c e f f e c t s a t t r i b u t a b l e t o t h e aqueous p h a s e t h a t a r e o f p a r t i c u l a r i n t e r e s t i n t h e c o n t a c t lens f i e l d . Most o f t h e aqueous p h a s e o f a h y d r o g e l c a n be f r e e l y e x c h a n g e d w i t h t h e s u r r o u n d i n g aqueous media by d i f f u s i o n . The movement o f w a t e r , i o n s and o t h e r d i s s o l v e d s u b s t a n c e s i s r e s t r i c t e d t o some d e g r e e by f r i c t i o n w i t h t h e p o l y m e r n e t w o r k . H o w e v e r , due t o t h e l a r g e s u r f a c e a r e a o f c o n t a c t l e n s e s r e l a t i v e t o t h e i r t h i c k n e s s , most o f t h e aqueous phase w i l l i n t e r c h a n g e w i t h t h e t e a r s i n a few m i n u t e s .


2.

REFOJO

Vapor

Pressure and Swelling

49

Pressure

70

60

H

_i LU

Ο

s

5 0

RHEMA/VP) 1—1 PHEMA RGMA/MMA)

CL ^

40

WATER ACTIVITY ·

I » = 30

1

Η WATER ACTIVITY < 1

Figure 8. Amounts of "free" water (a ~ I) and "bound" water (a < 1) in hydrogel contact lens materials w

w


50


The aqueous phase i n a c o n t a c t l e n s can be i s o t o n i c , h y p o tonic or hypertonic with respect to tears. When a h y d r o g e l l e n s i s e q u i l i b r a t e d i n i s o t o n i c s a l i n e s o l u t i o n (0.9% N a C l ) , the aqueous phase o f t h e h y d r o g e l i s i s o t o n i c t o t h e normal t e a r s . When s u c h a l e n s i s p l a c e d i n t h e e y e , t h e r e w i l l be an i n t e r change o f i t s aqueous phase and t h e t e a r f i l m , b u t t h e t e a r t o n i c i t y w i l l n o t change i n t h e p r o c e s s o f e q u i l i b r a t i o n o f t h e lens. The p o s s i b l e change i n s i z e and o p t i c s o f a l e n s f r o m e q u i l i b r i u m s w e l l i n g i n i s o t o n i c s o l u t i o n and i n t e a r s i s n e g l i gible. I f the hydrogel lens i s wetted w i t h tap or d i s t i l l e d water p r i o r t o p l a c i n g i t i n t h e e y e , t h e aqueous phase o f t h e l e n s w i l l be h y p o t o n i c t o t e a r s . Water w i l l move f r o m t h e l e n s t o t h e tears. The l e n s w i l l t h e n c o n t r a c t , o f t e n a d h e r i n g t e n a c i o u s l y t o t h e c o r n e a and c a u s i n t h e aqueous phase o f t h i s o t o n i c i t y i s r e a c h e d and t h e n t h e l e n s w i l l r e l e a s e f r o m i t s adhesion to the cornea. I f t h e l e n s i s p l a c e d i n a s o d i u m c h l o r i d e s o l u t i o n o f more t h a n 0 . 9 % , t h e aqueous phase o f t h e h y d r o g e l w i l l be h y p e r t o n i c to t e a r f i l m . As t h e l e n s i s p l a c e d i n t h e e y e , w a t e r w i l l be drawn o s m o t i c a l l y f r o m t h e t e a r f i l m i n t o t h e l e n s , b u t s a l t w i l l a l s o d i f f u s e from the lens i n t o the t e a r . T h i s w i l l change t h e t o n i c i t y of the tears to a hypertonic s t a t e . The l e n s e f f e c t can be q u i t e l a r g e as t h e t o t a l volume o f t h e t e a r f i l m and t h e t e a r meniscus i s roughly comparable to the water c o n t e n t of a hydrogel lens. The h y p e r t o n i c t e a r s w i l l d e h y d r a t e t h e c o r n e a l e p i t h e l i u m r e s u l t i n g i n o c u l a r d i s c o m f o r t ( i t c h i n g ) to the p a t i e n t . As t h e i s o t o n i c i t y of the tears i s r e - e s t a b l i s h e d through d i l u t i o n , the sensation of comfort i s again r e s t o r e d . The e x i s t e n c e o f a t h i n aqueous f i l m between a h y d r o g e l l e n s and t h e c o r n e a l e p i t h e l i u m i s a m a t t e r o f c o n t r o v e r s y . If the lens i s i n d i r e c t contact w i t h the cornea, the osmotic e f f e c t due t o t h e aqueous phase o f t h e h y d r o g e l l e n s w i l l be a c t i n g d i r e c t l y upon t h e c o r n e a l e p i t h e l i u m w i t h t h e same r e s u l t s d i s cussed above.

Abstract The swelling pressure of two types of hydrogels, which were classified according to application and hydration, were investigated. 1. Poly(glyceryl methacrylate) (PGMA) hydrogels which are intended for surgical uses in the eye, are divided into two subgroups, a) hydrogels of about 80 to 85%H Oat equilibrium swelling, for scleral buckling procedures in retinal detachment surgery, and b) hydrogels of above 98%H Oat equilibrium swelling, for vitreous implantation. The swelling pressure-volume relationship of these hydrogels was determined with dextran solutions. 2. Hydrogel contact lens materials (40-75%H Oat equilibrium). The swelling pressure of PHEMA hydrogels was determined by equilibration under osmotic and mechanical pressure. 2

2

2


2.

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Vapor

Pressure and Swelling

Pressure

51

Although slight pressure will remove water from highly hydrated hydrogels, it is very difficult to expel water from medium hydra tion (40-80% HO) hydrogels. The equilibrium relative humidity at different states of hydration of PHEMA, P(HEMA/VP), P(MMA/VP), and P(GMA/MMA) was investigated. The results show that up to 30% HO, the water activity in the hydrogels is below the activity for pure water. However, all water above 30% hydration up to equilibrium swelling has the same vapor pressure as pure water. 2

2

Acknowledgements The author is indebted to Dr. F. Holly for his helpful discussion. Ms. F.L. Leong provided technical assistance. A. Seidman provided editorial assistance. This study was supported b PHS Grant EY-00327 f th National Eye Institute Literature Cited 1. 2. 3.

Katchalsky, Α.: Prog. Bioph. (1954) 4:1 Rijke, A.M., and Prins, W.: J. Polym. Sci. (1962) 59:171 Posnjak, E.: Kolloidchem. Beih. (1912) 3:417, in M.R. Kruyt, Ed. Colloid Science II. Elsevier Publishing Co., New York, 1949,p557 4. Refojo, M.F.: Anales Qui. (Madrid) (1972) 68:697 5. Refojo, M.F.: J. Biomed. Mater. Res. Symposium (1971) 1:179 6. Daniele, S., Refojo, M.F., Schepens, C.L., and Freeman, H.M.: Arch. Ophthal. (1968) 80:120 7. Dohlman, C.H., Refojo, M.F., Webster, R.G., Pfister, R.R., and Doane, M.G.: Arch. Ophthal. (Paris) (1969) 29:849 8. Calabria, G.A., Pruett, R.C., and Refojo, M.F.: Arch. Ophthal. (1971) 86:77 9. Refojo, M.F.: J. Appl. Polym. Sci. (1965) 9:3161 10. Senti, F.R., Hellman, N.N., Ludwig, N.M., Babcock, G.E., Tobin, R., Glass, C.A., and Lamberts, B.L.: J. Polym. Sci. (1955) 17:527 11. Refojo, M.F.: J. Appl. Polym. Sci. (1965) 9:3417 12. Wichterle, O. and Lim, D.: Nature (1960) 185:117 13. Refojo, M.F.: Encycl. Polymer. Sci. Technol. Supplement (in press) 14. Seiderman, M.: U.S. Patent 3,721,657 (1973) 15. Frankland, J.D., and Highgate, D.J.: Ger. Offen. (1973) No. 2,312,470 [in Chem. Abstr. 80:27872z] 16. Refojo, M.F., and Yasuda, H.: J. Appl. Polymer Sci. (1965) 9:2425 17. Vink, H.: European Polym. J. (1971) 7:1411 18. Lee, M.B., Jhon, M.S., and Andrade, J.D.: J. Colloid and Interface Sci. (1975) 51:225 19. Quinn, F.C.: Reprint No. 472, Am. Instrument Co., Div. Travenol Labs, Silver Springs, Md. In Hydrogels for Medical and Related Applications; Andrade, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

3 The Role of Proteoglycans and Collagen in the Swelling of Connective Tissue F. A. MEYER, R. A. GELMAN, and A. SILBERBERG Weizmann Institute of Science, Rehovot, Israel

Connective tissue provide th chemical environment fo most body cells and th tion. It varies in composition according to this function and characteristically involves a f i b r i l l a r skeletal network composed mainly of collagen f i b r i l s and the so-called ground substance , a highly disperse system of proteoglycans, macromolecules involving both protein and carbohydrate chains. Bone, cartilage, vessel walls, skin, tendon, umbilical cord and synovial fluid are typical examples of connective tissues. A l l of them are characterized by a low cell content. The major biological roles fulfilled by connective tissues in the body are physical in nature. They involve mechanical function such as motion and transport and (chemical) communication between cells. In its mechanical role connective tissue transmits energy from the muscles permitting the performance of work and the absorbance of stresses from the environment thereby protecting delicate structures. Clearly, the physico-chemical properties and the structure of connective tissue are the important factors fixing these features. The s t r u c t u r e and chemistry o f the i n d i v i d u a l connective t i s s u e components v i z . c o l l a g e n , e l a s t i n and proteoglycans can be summarized as f o l l o w s : Collagen (1). Collagen i s a f i b r o u s p r o t e i n which i s formed by the assembly o f monomer u n i t s o f t r o p o c o l l a g e n . Tropocollagen i s a rod shaped molecule V300 nm long and 1.4 nm i n diameter w i t h a molecular weight o f 300,000. I t c o n s i s t s o f three chains of equal l e n g t h . F i v e t r o p o c o l l a g e n molecules can pack t o give a m i c r o f i b r i l , the u n i t s are l o n g i t u d i n a l l y staggared around t h e m i c r o f i b r i l a x i s . M i c r o f i b r i l s pack i n a t e t r a g o n a l l a t t i c e t o give f i b r i l s . There i s a gap between consecutive t r o p o c o l l a g e n s i n the f i b r i l which together w i t h the s t a g g e r i n g gives c o l l a g e n a c h a r a c t e r i s t i c 65 nm repeat d i s t a n c e . S t a b i l i t y o f the s t r u c t u r e i s due i n p a r t t o the q u a r t e r s t a g g e r i n g , which optimizes secondary bond i n t e r a c t i o n s and covalent c r o s s l i n k s as w e l l 52


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occur between and w i t h i n t r o p o c o l l a g e n u n i t s . The h i g h l y ordered compact arrangement w i t h i n the f i b e r gives i t a r i g i d character. Large v a r i a t i o n s are seen i n the degree o f c r o s s l i n k i n g , f i b r i l diameter, f i b r i l o r g a n i z a t i o n and f i b r i l c o n c e n t r a t i o n i n v a r i o u s t i s s u e s . Moreover, four g e n e t i c a l l y d i s t i n c t types of c o l l a g e n e x h i b i t i n g some t i s s u e s p e c i f i c i t y have been char a c t e r i z e d which i n p a r t may e x p l a i n the v a r i a t i o n s o f c o l l a g e n morphology seen. Ε las t i n (2) . E l a s t i n i s g e n e r a l l y i n v o l v e d w i t h t h e f i b r i l l a r network. I t contains an unusually h i g h p r o p o r t i o n o f non-polar amino a c i d s and the amino a c i d s , desmosine and i o d o desmosine, which may be unique t o t h i s p r o t e i n . A corpuscular s t r u c t u r e has been propose the peptide chain i s f o l d e l i n k i n g occurs a t the s u r f a c e between such u n i t s . The b a s i s o f •the e l a s t i c nature o f t h i s p r o t e i n a r i s e s from the work r e q u i r e d t o u n f o l d the hydrophobic r e g i o n s . Ground Substance Components (3_) . The major ground substance components are proteoglycans which c o n s i s t o f g l y c o s aminoglycans attached t o p r o t e i n . The most abundant g l y c o s aminoglycans are h y a l u r o n i c a c i d , c h o n d r o i t i n s u l f a t e , keratan s u l f a t e , and dermatan s u l f a t e . The glycosaminoglycans are l i n e a r chains c o n s i s t i n g o f r e p e a t i n g n e g a t i v e l y charged d i saccharide u n i t s . The p r o t e o g l y c a n , h y a l u r o n i c a c i d , i s o f h i g h molecular weight and i n s o l u t i o n adopts a very voluminous random c o i l c o n f i g u r a t i o n . I t i s a s s o c i a t e d w i t h a s m a l l amount of p r o t e i n ( 99%) HEMA was o b t a i n e d as a g i f t from Hydro Med S c i e n c e s (New Brunswick, N.J., U.S.A.). E t h y l e n e d i m e t h a c r y l a t e was o b t a i n e d from BDH ( P o o l e , England) and Koch and L i g h t (Colnbrook, Eng land). Methacrylic acid ethylene g l y c o l 2-hydroxy propyl methacrylate and £-methoxyphenol D i e t h y l e n e g l y c o l m e t h a c r y l a t e was s y n t h e s i s e d (8) by m i x i n g a p p r o p r i a t e amounts o f d i e t h y l e n e g l y c o l (855 g) and methyl m e t h a c r y l a t e (500 g) a t a temperature o f 60°C, a d d i n g a 4 Ν s o l u t i o n o f sodium methanolate i n methanol (10 g) and h e a t i n g t h e r e a c t i o n m i x t u r e f o r a p e r i o d o f time o f 30 min. S u b s e q u e n t l y , t h e m i x t u r e was poured i n t o water (400 g ) , washed w i t h n-hexane and e x t r a c t e d t w i c e w i t h d i e t h y l e t h e r . A f t e r r e p e a t e d washings w i t h w a t e r , t h e e t h e r e x t r a c t was d r i e d and d i e t h y l e n e g l y c o l m e t h a c r y l a t e was d i s t i l l e d a t r e d u ced p r e s s u r e . A l l f u r t h e r c h e m i c a l s used i n t h i s study were r e a g e n t - g r a d e p r o d u c t s , and were used w i t h o u t f u r t h e r purification. T h i n - l a y e r chromatography was c a r r i e d o u t i n Hellendahl s t a i n i n g j a r s , using precoated s i l i c a g e l p l a t e s ( K i e s e l g e l 60 F 9 5 4 ' o ^^ * a p p r o p r i a t e s i z e (4 χ 8 cm ) . Spots were a p p l i e d w i t h a p o i n t e d paper wick and chromatography i s c a r r i e d o u t i n a n o n - s a t u r a t e d atmosphere. A f t e r development o v e r a l e n g t h o f r u n o f a p p r o x i m a t e l y 7 cm, d e t e c t i o n i s done u s i n g t h e me thods r e p o r t e d below. R e p r o d u c i b i l i t y o f t h e Rp v a l u e s i n t h e s o l v e n t systems used i n t h e p r e s e n t study was satisfactory. P r e p a r a t i v e - s c a l e t h i n - l a y e r chromatography was c a r r i e d o u t on 20 χ 20 cm2 2-mm t h i c k p r e c o a t e d s i l i c a g e l p l a t e s ( K i e s e l g e l 60 F 2 5 4 , Merck). 100-150 mg HEMA, as a 20% (v/v) s o l u t i o n i n e t h a n o l , were a p p l i e d w i t h a Camag chromatocharger equipped w i t h a d i s p o s a b l e p l a s t i c s y r i n g e . Development o v e r a d i s t a n c e o f 15-17 cm, w i t h o u t p r i o r a c c l i m a t i s a t i o n , i s done i n a normal r e c t a n g u l a r tank. M e r c

c

u

t

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t

o

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e

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R e f r a c t i v e i n d i c e s were measured on a t h e r m o s t a t t e d (20 + 0.1°C) Abbe r e f r a c t o m e t e r . I R - s p e c t r a were run on a Shimadzu IR 400 s p e c t r o m e t e r , u s i n g c e l l s w i t h NaCl windows. R e s u l t s and D i s c u s s i o n Reference Substances. As has been s t a t e d i n t h i s i n t r o d u c t i o n , HEMA g e n e r a l l y c o n t a i n s s e v e r a l i m p u r i t i e s * * 9 · EDMA, m e t h a c r y l i c a c i d and e t h y l e n e g l y c o l . S i n c e m e t h a c r y l i c a c i d and i t s d e r i v a t i v e s tend t o p o l y m e r i z e even a t room temperature, an i n h i b i t o r such as h y d r o q u i n o n e , £-methoxyphenol, p h e n o t h i a z i n e o r o c t y l p y r o c a t e c h o l i u s u a l l added t thes d u c t s (9). A c c o r d i n samples o b t a i n e d fro 200 pp p-methoxyphenol, and 100 ppm hydroquinone p l u s 100 ppm £-methoxyphenol, r e s p e c t i v e l y ; HEMA from Hydro Med S c i e n c e s , I n c . , c o n t a i n s 36 ppm £-methoxyphenol. Hydroquinone i s a l s o p r e s e n t i n EDMA and m e t h a c r y l i c a c i d samples. M e t h y l m e t h a c r y l a t e may a l s o be p r e s e n t as an i m p u r i t y i n c o m m e r c i a l l y a v a i l a b l e HEMA, s i n c e i t i s one o f t h e s t a r t i n g m a t e r i a l s i n i t s s y n t h e s i s . However, d u r i n g t i c , i t e v a p o r a t e s o f f from t h e c h r o matoplate (b.p., 100°C) and escapes d e t e c t i o n i n q u a l i t a t i v e a n a l y s i s . In p r e p a r a t i v e - s c a l e work, such e v a p o r a t i o n d u r i n g development ensures i t s removal. By d e v e l o p i n g a sample o f pure methyl m e t h a c r y l a t e and s p r a y i n g w i t h water (JL0) immediately a f t e r t h e r u n , we have shown i t s Rp v a l u e t o be c o n s i d e r a b l y h i g h e r than t h a t o f HEMA i n b o t h s o l v e n t systems recommended below. As r e g a r d s t h e presence o f water, one i s r e f e r r e d t o F i g . 3 and t h e accompanying t e x t . As a consequence o f t h e above, i n a d d i t i o n t o HEMA, 5 compounds were i n c l u d e d i n o u r s t u d y , v i z . EDMA, m e t h a c r y l i c a c i d , e t h y l e n e g l y c o l , hydroquinone and £-methoxyphenol ( c f . T a b l e I ) . The p u r i t y o f t h e samples o b t a i n e d as r e f e r e n c e s u b s t a n c e s was t e s t e d , u s i n g two o r more o f t h e s o l v e n t systems d i s c u s s e d below. The samples were found t o be c h r o m a t o g r a p h i c a l l y pure^ a p a r t from t h e presence o f an i n h i b i t o r , w i t h t h e e x c e p t i o n o f EDMA. Here, t i c w i t h n-hexane d i e t h y l e t h e r (1:1) as mobile phase r e v e a l e d t h e p r e s e n c e o f some e i g h t a d d i t i o n a l s p o t s ( F i g s , l a and l b ) . These contaminants were removed by d i s t i l l a t i o n a t reduced p r e s s u r e (b.p. o f EDMA a t 4 mm Hg, 90°C) o r , p r e f e r a b l y , by p r e p a r a t i v e - s c a l e t i c ( F i g s , l c and d ) . e



±

4

2

0.2

0.6

0.6

0.6

KMnO

0.60.6

0.60.6

15

60

15

100

I

with

0.2

0.2

Echtblausalz B

(yg/μΐ)

0.2

0.2

D i a z o t i s e d sulphan i l i c acid

20 F o r HEMA, d has a v a l u e o f 1.070-1.074 U 4 ) . T h e r e f o r e , l y g contaminant/μΐ o f HEMA r o u g h l y c o r r e s p o n d s w i t h 1000 ppm o r 0.1 wt.%.

15

-

Ethylene g l y c o l

£-Methoxyphenol

30

Methacrylic acid

15

30

Ethylene dimethacrylate (EDMA)

Hydroquinone

30

UV

Detection l i m i t

2-Hydroxyethyl methacrylate (HEMA)

Name

T a b l e I . D e t e c t i o n l i m i t s o f HEMA and some o f i t s contaminants,

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Methacrylate

109

With t h e former t e c h n i q u e , a d d i t i o n o f hydroquinone o r o f a m i x t u r e o f KC1, B 1 C I 3 and B 1 I 3 (1_1) i s n e c e s sary i n order t o prevent polymerization. Detection. From among a l a r g e number o f non s p e c i f i c methods o f i d e n t i f i c a t i o n , t h r e e d e t e c t i o n methods were s e l e c t e d f o r f u r t h e r r e s e a r c h . Under U.V.l i g h t (254 nm) a l l compounds b u t e t h y l e n e g l y c o l a r e v i s i b l e as dark s p o t s on a f l u o r e s c e n t background. Treatment w i t h i o d i n e vapor (red-brown s p o t s on a y e l l o w - w h i t e background) i s a s u i t a b l e means t o d e t e c t the g l y c o l t o o . U n f o r t u n a t e l y , t h e s e n s i t i v i t y o f t h e combined U . V . / I p r o c e d u r e i s r a t h e r low (cf:. T a b l e I ) . T h e r e f o r e as an a l t e r n a t i v e , d e t e c t i o n has been done by s p r a y i n g w i t h a White s p o t s a r e forme a l l compounds e x c e p t £-methoxyphenol, which shows up as a p a l e orange s p o t . S i n c e KKn04 r e a c t s w i t h i m p u r i t i e s i n t h e acetone t o form Μηθ2, which mars t h e detec t i o n , t h e r e a g e n t s o l u t i o n s h o u l d be f r e s h l y p r e p a r e d . Keeping i n mind t h a t c h e c k i n g t h e p u r i t y o f HEMA i s done by t i c o f u n d i l u t e d samples, one may c o n c l u d e from t h e d a t a i n T a b l e I t h a t s p r a y i n g w i t h KMn04 a l l o w s t h e d e t e c t i o n o f i m p u r i t i e s down t o a t l e a s t 0.1%. Since the i n h i b i t o r s are present i n the v a r i o u s samples o f HEMA, EDMA and m e t h a c r y l i c a c i d i n concen t r a t i o n s o f up t o 0.0 2% o n l y , they w i l l escape d e t e c t i o n w i t h t h e above p r o c e d u r e s . T h e r e f o r e , two a l t e r n a t i v e methods o f i d e n t i f i c a t i o n (12,13) have been t e s t e d : 1) s p r a y i n g w i t h a f r e s h l y p r e p a r e d 0.5% s o l u t i o n o f t h e d i a z o r e a g e n t E c h t b l a u s a l z Β i n water, and e i t h e r s p r a y i n g w i t h 0.1 Ν NaOH o r , p r e f e r a b l y , exposure t o vapor o f NH3 ; 2) s p r a y i n g w i t h a m i x t u r e o f 0.09 g s u l p h a n i l i c a c i d d i s s o l v e d i n 10 ml 1.1 Ν HC1 and 10 ml o f an aqueous 4.5% NaN02 s o l u t i o n , k e p t a t 0°C and d i l u t e d w i t h an e q u a l volume o f a 10% Na2C03 s o l u t i o n immediately b e f o r e use. With both r e a g e n t s , brownish s p o t s show up on a n e a r l y white back ground. As can be seen from t h e d a t a i n T a b l e I , t h e use o f even these r e a c t i o n s b a r e l y s u f f i c e s f o r t h e i d e n t i f i c a t i o n o f the i n h i b i t o r s i n methacrylic a c i d and i t s d e r i v a t i v e s . F o r t u n a t e l y , improved r e s u l t s a r e o b t a i n e d i f s u l p h a n i l i c a c i d i s used t o d e t e c t t h e p h e n o l i c compounds by a drop t e s t p r o c e d u r e , i n which 1 drop o f t h e d i a z o t i s e d r e a g e n t i s mixed w i t h 1 drop o f sample ( d i s s o l v e d i n a minimum amount o f e t h a n o l , i f n e c e s s a r y ) and 1 drop o f a 10% Na2C03 s o l u t i o n . The d e t e c t i o n l i m i t now i s a p p r o x i m a t e l y 0.01% f o r h y d r o quinone ( g r e e n - t o - y e l l o w s p o t s ) , and 0.001% f o r η methoxyphenol ( r e d - t o - g r e e n s p o t s ) . 2


110


S o l v e n t Systems. A f t e r s e v e r a l t r i a l runs o f the f o u r main components w i t h s i n g l e - s o l v e n t systems r a n g i n g from n o n - p o l a r (n-hexane, cyclohexane) t o p o l a r s o l v e n t s ( d i e t h y l e t h e r , m e t h y l i s o b u t y l ketone (MIBK), a c e t o n e ) , m i x t u r e s o f n-hexane w i t h d i e t h y l e t h e r o r ( a n ) o t h e r c o m p o n e n t ( s T were s e l e c t e d f o r f u r t h e r r e s e a r c h . S a t i s f a c t o r y r e s u l t s were o b t a i n e d w i t h n-hexane - d i e t h y l e t h e r (1:1, v/v) and n-hexaneMIBK - n - o c t a n o l (9:2:1, v/v) ( F i g . 2a and d ) . As a drawback, however, i t was o b s e r v e d t h a t the s p o t due t o m e t h a c r y l i c a c i d tends t o t a i l , e s p e c i a l l y i n the l a t t e r s o l v e n t system. Such t a i l i n g , which i s p a r t i c u l a r l y harmful i n p r e p a r a t i v e - s c a l e t h i n - l a y e r or column chromatography, can e f f e c t i v e l y be reduced by a d d i n g an a c i d i c componen n a t i v e s ' have been i n v e s t i g a t e d i s s a t u r a t e d w i t h an e q u a l volume o f 25% (v/v) H N O 3 . When a h i g h e r p e r c e n t a g e o f a c i d i s used, decomposit i o n o f one o r more o f the compounds under i n v e s t i g a t i o n o c c u r s d u r i n g development, as e v i d e n c e d by the o c c u r r e n c e o f a brown s t r e a k i n g zone on the chromatogram. 2) Development i s done on a s i l i c a g e l p l a t e soaked f o r 15 min w i t h 1% H 2 S O 4 , and s u b s e q u e n t l y d r i e d o v e r n i g h t a t 6 0 ° C . As i s m a n i f e s t from the d a t a i n F i g . 2, w i t h each o f these t e c h n i q u e s , the r e s u l t s i n d e e d improved. F o r q u a l i t a t i v e a n a l y s i s , development w i t h a HNOj s a t u r a t e d mobile phase s h o u l d be p r e f e r r e d t o t i c on an a c i d - i m p r e g n a t e d s u p p o r t , which i s a more time consuming p r o c e d u r e . From F i g . 2 i t i s e v i d e n t t h a t n-hexane - MIBK - n - o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) i s a v e r y p o w e r f u l s o l v e n t system f o r the r e s o l u t i o n o f the f o u r main components. A d d i t i o n o f n i t r i c a c i d t o n-hexane - d i e t h y l e t h e r (1:1) a l s o improves the s e p a r a t i o n o f HEMA from m e t h a c r y l i c a c i d ; however, the s p o t s due t o m e t h a c r y l i c a c i d and EDMA now show an a p p r e c i a b l e o v e r l a p . T h e r e f o r e , as a t e n t a t i v e conc l u s i o n , we recommend the use o f both n-hexane d i e t h y l e t h e r (1:1) and n-hexane - MIBK - n - o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) f o r q u a l i t a t i v e a n a l y s i s . F o r p r e p a r a t i v e - s c a l e t i c , chromatography on H 2 S O 4 -impregnated s i l i c a g e l has been p r e f e r r e d t o development w i t h a HNC^-saturated mobile phase, because s u l p h u r i c a c i d , but not n i t r i c a c i d , has a n e g l i g i b l y s m a l l s o l u b i l i t y i n the s o l v e n t used t o e l u t e HEMA from the s u p p o r t m a t e r i a l ( c f . b e l o w ) . n-hexane d i e t h y l e t h e r (1:1) has been s e l e c t e d as m o b i l e phase r a t h e r than the n-hexane - MIBK - n - o c t a n o l m i x t u r e , because o f the r e l a t i v e l y l a r g e time o f development o f the l a t t e r s o l v e n t system ( a p p r o x i m a t e l y 4 v s . 2.5 h


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111


EDMA

Figure 1. Thin-layer chromatograms of EDMA samples, (a) EDMA from BDH; (b) EDMA from Koch and Light; (c) redis silica gel/n-hexane-diethyl

ether (1:1).

3

α

b

c

d

β

Figure 2. Tic of HEMA, EDMA, methacrylic acid (MA), and ethylene glycol (EG) in the systems: (a) silica gel/n-hexane-diethyl ether (1:1); (b) silica gel/n-hexane-diethyl ether (1:1, satd. with 25% HN0 ); (c) H S0^-impregnated silica gel/n-hex ane-diethyl ether (1:1); (d) silica gel/n-hexaneMIBK-n-octanol (9:2:1); (e) silica gel/n-hexaneMIBK-n-octanol (9:2:1, satd. with 25%> HN0 ). , acid (1), n-octanol (2) and MIBK (3) front. 3

2

3


112


f o r a 16-cm r u n ) . Due t o t h e low v o l a t i l i t y o f n - o c t a n o l , i t s complete removal from t h e t h i n l a y e r i s n o t e a s i l y e f f e c t e d , and p a r t o f i t w i l l be e l u t e d t o g e t h e r w i t h HEMA. Next, a t t e n t i o n was p a i d t o t h e s e l e c t i o n o f a s o l v e n t s u i t e d t o e l u t e t h e s e p a r a t e d component(s) from t h e s i l i c a g e l . A t f i r s t s i g h t , t h i s c h o i c e does not appear t o be v e r y c r i t i c a l , because HEMA d i s s o l v e s f r e e l y i n s o l v e n t s such as water, acetone, d i e t h y l e t h e r , t e t r a c h l o r o m e t h a n e and, l e s s s o , n-hexane. However, when u s i n g any o f t h e f i r s t t h r e e h i g h l y p o l a r s o l v e n t s , t h e a c i d used t o impregnate t h e s i l i c a g e l i s e x t r a c t e d a l o n g w i t h HEMA. Moreover, when d e v e l o p ment i s c a r r i e d o u t on s i l i c a g e l plates, zinc o r i g i n a t i n g from t h p r e s e n t as f l u o r e s c i n s a l t , i s a l s o e x t r a c t e d . The presence o f both t h e a c i d and i t s z i n c s a l t i n p u r i f i e d HEMA have been demons t r a t e d by t i c on p l a i n s i l i c a g e l , w i t h n-hexane d i e t h y l e t h e r (1:1) as d e v e l o p i n g s o l v e n t . Both compounds remain a t t h e o r i g i n and a r e i d e n t i f i e d by s p r a y i n g w i t h a s u i t a b l e a c i d - b a s e i n d i c a t o r , e.g. thymol b l u e (pH range, 1.2-2.8), and 4 - ( 2 - p y r i d y l a z o ) r e s o r c i n o l , r e s p e c t i v e l y . As a consequence, t h e r e l a t i v e l y n o n - p o l a r t e t r a c h l o r o m e t h a n e i s p r e f e r r e d as eluent. When e x a m i n i n g t h e t i c d a t a i n o r d e r t o e v a l u a t e systems s u i t a b l e f o r column chromatography, one s h o u l d c o n s i d e r t h a t development w i t h a HNO^-saturated mobile phase causes t h e f o r m a t i o n o f an a c i d f r o n t a t Rp v a l u e s o n l y s l i g h t l y h i g h e r than those o f HEMA ( F i g . 2b and e ) . C o n s e q u e n t l y , i n chromatography on columns e q u i l i b r a t e d w i t h HNC^-saturated s o l v e n t systems, t h e s e p a r a t i o n o f HEMA from contaminants h a v i n g h i g h e r Rp v a l u e s , presumably w i l l be r a t h e r m a r g i n a l . T h e r e f o r e , p r e f e r e n c e s h o u l d be g i v e n t o chromatography on H 2 S O 4 impregnated s i l i c a g e l ; f o r t h e same reasons as mentioned above, n-hexane - d i e t h y l e t h e r (1:1) i s a g a i n recommended as mobile phase. Application. T h i n - l a y e r chromatograms o f a l l r e f e r e n c e substances and HEMA samples a r e p i c t u r e d i n F i g . 3. F u r t h e r i n f o r m a t i o n i s r e c o r d e d i n T a b l e I I . The chromatograms demonstrate t h a t - a p a r t from the i n h i b i t o r s - e t h y l e n e g l y c o l and/or o t h e r p o l a r m a t e r i a l , EDMA and m e t h a c r y l i c a c i d (BDH sample o n l y ) are p r e s e n t i n t h e HEMA samples from Merck and BDH. D e t e c t a b l e amounts o f m e t h a c r y l i c a c i d and EDMA a r e absent from HEMA o b t a i n e d from Hydro Med S c i e n c e s .



60 40

pM

MA

£-Methoxyphenol

Methacrylic

DGMA

EG/P

HEMA

Diethyleneglycol methacrylate

Ethylene g l y c o l / polar material

Cannot be d e t e c t e d : c f . t e x t .

Drop t e s t

Hy

HEMA

Hydroquinone

acid

70

EDMA

EDMA

00

10

25

25

Pure compounds

Abréviations (cf. F i g . 3)

Compound

hRp o f / i n :

gel/n-hexane - d i e t h y l e t h e r ( 1 : 1 )

00

+

+

+

10

00

10

25

-

25

60

—

HEMA (Hydro)

60

70

HEMA (Merck)

00

10

25

*

40

60

70

HEMA (BDH)

T a b l e I I . Rp d a t a f o r HEMA and i t s p r i n c i p a l c o n t a m i n a n t s i n t h e system

+

-

—

-

25

40

-

Methacrylic acid

silica

114


S t i l l , even t h i s h i g h l y pure p r o d u c t c o n t a i n s a t l e a s t two i m p u r i t i e s . The s p o t w i t h Rp 0.0 a g a i n may be a t t r i b u t e d t o e t h y l e n e g l y c o l and/or o t h e r p o l a r m a t e r i a l . As f o r t h e second s p o t , a compound d i s p l a y i n g Rp a p p r o x i m a t e l y 0.10 i n both chromatographic s o l v e n t systems, has a l s o been o b s e r v e d i n t h e HEMA samples o b t a i n e d from Merck and BDH. M i l l i g r a m amounts have been c o l l e c t e d by p r e p a r a t i v e - s c a l e t i c and c o lumn chromatography. On t h e b a s i s o f t h e mass spec trum (m/e v a l u e o f 144; p r e s e n c e o f 3 oxygen atoms) the unknown compound was i n i t i a l l y thought t o be h y d r o x y p r o p y l m e t h a c r y l a t e . However, comparison o f t h e t i c b e h a v i o r o f t h e unknown compound and a sample o f pure 2 - h y d r o x y p r o p y l m e t h a c r y l a t e demonstrated o u r h y p o t h e s i s t o be i n c o r r e c t o b t a i n e d from Hydr dent t h a t HEMA samples may c o n t a i n s e v e r a l t e n t h s o f a p e r c e n t o f d i e t h y l e n e g l y c o l m e t h a c r y l a t e . The l a t t e r p r o d u c t has been s y n t h e s i s e d i n o u r l a b o r a t o r y ( c f . above) and has been shown t o be i d e n t i c a l w i t h the unknown compound by both t i c and mass s p e c t r o metry . I n a d d i t i o n , we note t h a t a sharp s e p a r a t i o n o f HEMA and d i e t h y l e n e g l y c o l m e t h a c r y l a t e i s o b t a i n e d by c a r r y i n g o u t development w i t h pure d i e t h y l e t h e r i n s t e a d o f n-hexane - d i e t h y l e t h e r (1:1). The Rp v a l u e s now a r e 0.80 and 0.55 r e s p e c t i v e l y . As r e g a r d s t h e i n h i b i t o r s , w i t h n-hexane d i e t h y l e t h e r (1:1), hydroquinone d i s p l a y s an Rp v a l u e a p p r o x i m a t e l y e q u a l t o t h a t o f HEMA and cannot be detected with e i t h e r Echtblausalz Β o r d i a z o t i s e d s u l p h a n i l i c a c i d . When u s i n g n-hexane - MIBK - η o c t a n o l (9:2:1, s a t d . w i t h 25% H N O 3 ) as mobile phase, hydroquinone has a s l i g h t l y f a s t e r m i g r a t i o n r a t e than has HEMA, i t s s p o t o v e r l a p p i n g t h a t o f m e t h a c r y l i c a c i d . However, d e t e c t i o n o f s m a l l amounts o f h y d r o quinone even now i s n o t s u c c e s s f u l . I n s e p a r a t e expe r i m e n t s , we have demonstrated t h a t t h i s i s due t o s e v e r e s t r e a k i n g o f t h e hydroquinone s p o t , e f f e c t e d by t h e p r e s e n c e o f t h e l a r g e e x c e s s o f HEMA. As a consequence, hydroquinone i s d e t e c t e d i n i s o l a t e d i n s t a n c e s ( m e t h a c r y l i c a c i d ! ) o n l y . No such d i f f i c u l t i e s a r e e n c o u n t e r e d w i t h p-methoxyphenol; w i t h t h i s i n h i b i t o r , i d e n t i f i c a t i o n by t i c i s s u c c e s s ful. *As r e g a r d s t h e d i s c r e p a n c y between t h e mass o f d i e t h y l e n e g l y c o l m e t h a c r y l a t e (174) and t h e v a l u e quo t e d above (144), i t i s a well-known f a c t t h a t i n mass spectrometry p o l y g l y c o l s e a s i l y lose a p a r t o f t h e i r m o l e c u l e h a v i n g a mass o f 30; i . e . , t h e m/e v a l u e o f 144 s h o u l d be a t t r i b u t e d t o Im - C H o ] . 2

+


8.

BRiNKMAN E T A L .


115

S t i l l , w i t h both compounds, i n view o f t h e r a t h e r u n f a v o r a b l e d e t e c t i o n l i m i t s quoted i n T a b l e I , i t i s recommended t o a p p l y s e v e r a l m i c r o l i t e r s o f sample s o l u t i o n t o t h e c h r o m a t o p i a t e , u s i n g development o v e r a d i s t a n c e o f 10-15 cm. A l t e r n a t i v e l y , p r e c o n c e n t r a t i o n o f t h e i n h i b i t o r s by means o f t r e a t m e n t o f t h e sample s o l u t i o n w i t h Amberlyst A-27 r e s i n ( c f . below) may be used. I n c o n c l u s i o n , as i s e v i d e n t from t h e d a t a i n T a b l e I I , t h e drop t e s t p r o c e d u r e w i t h s u l p h a n i l i c acid i s e x c e l l e n t l y s u i t e d t o detect the presenc e , though n o t t h e t y p e , o f an i n h i b i t o r . Comparison o f t h e o v e r a l l p i c t u r e o f t h e s e r i e s o f chromatograms o b t a i n e d w i t h n-hexane - d i e t h y l e t h e r (1:1), and t h e s e r i e s o b t a i n e d w i t h n-hexane MIBK - n - o c t a n o l (9:2:1 strates that a smalle when d e v e l o p i n g w i t h t h e l a t t e r s o l v e n t system. T h i s i s m a i n l y due t o t h e f a c t t h a t n - o c t a n o l has low v o l a t i l i t y and cannot e a s i l y be removed from t h e c h r o matogram p r i o r t o i d e n t i f i c a t i o n . As a consequence, upon s p r a y i n g w i t h a KMn04 s o l u t i o n , t h e c o l o r o f t h e background t u r n s brown r a t h e r r a p i d l y , thus o b s c u r i n g s e v e r a l o f t h e s m a l l e r s p o t s . I n summary, n-hexane d i e t h y l e t h e r (1:1) s h o u l d be p r e f e r r e d f o r q u a l i t a t i v e a n a l y s i s , both on account o f i t s f a s t e r m i g r a t i o n r a t e , and t h e b e t t e r d e t e c t i o n a c h i e v e d f o r a c t u a l l y a l l components p r e s e n t i n t h e HEMA and EDMA samples. P r e p a r a t i v e - s c a l e t i c on ^ S C ^ - i m p r e g n a t e d s i l i c a g e l has s u c c e s s f u l l y been c a r r i e d o u t w i t h HEMA samp l e s from Hydro Med S c i e n c e s and BDH. ( F o r a l l compounds s t u d i e d , t h e Rp v a l u e s i n t h i s system a r e e q u a l t o , o r s l i g h t l y h i g h e r than t h o s e i n t h e system s i l i c a gel/n-hexane - d i e t h y l e t h e r , s a t d . w i t h 25% HNCK; c f . F i g . 2 ) . A f t e r development w i t h n-hexane - d i e t h y l e t h e r (1:1) and e l u t i o n w i t h t e t r a c h l o r o m e t h a n e , t h e samples t u r n o u t t o be c h r o m a t o g r a p h i c a l l y pure. I f hydroquinone has been added t o t h e HEMA sample (Merck\ t h i s i n h i b i t o r w i l l s t i l l be p r e s e n t a f t e r chromatog r a p h i c p u r i f i c a t i o n (£f. above). G e n e r a l l y , t h e p r e s e n c e o f an i n h i b i t o r i s n o t h a r m f u l s i n c e i t can be removed q u a n t i t a t i v e l y from t h e PHEMA g e l by p r o l o n g e d washing w i t h water. However, s h o u l d q u a n t i t a t i v e removal o f hydroquinone a t an e a r l y stage be i m p e r a t i v e , then two c o n s e c u t i v e t r e a t m e n t s o f t h e sample t o be p u r i f i e d w i t h Amberlyst A-27 r e s i n (Rohm and Haas, P h i l a d e l p h i a , Pa. 19105, U.S.A.) s u f f i c e t o remove up t o 0.2% o f hydroquinone, and a l s o o t h e r p o l a r compounds; t h e s e can s u b s e q u e n t l y be e l u t e d w i t h methanol. The c o n c e n t r a t e d e x t r a c t so o b t a i n e d i s e x c e l l e n t l y s u i t e d t o d e t e c t t h e presence o f i n h i b i -


116


Table I I I . η

v a l u e s f o r HEMA and some o f i t s

contaminants Compound

η

HEMA

1.4525

I'

Diethyleneglycol methacrylate

1.4568

8

n

ri.4549 U.4549-1.4553

EDMA Ethylene

I,

2, 4 1

glycol

Methacrylic Methyl

Ref.

acid

methacrylate

1.4314

1> 1

1.4142

5

t o r ( s ) , as was r e f e r r e d t o above. A f i n a l word about two f u r t h e r c r i t e r i a which a r e used t o a s s e s s t h e p u r i t y o f HEMA and EDMA, v i z . t h e r e f r a c t i v e i n d e x and t h e i n f r a r e d spectrum. A c c o r d i n g t o o u r e x p e r i e n c e s , n $ v a l u e s , which a r e r a t h e r gene r a l l y quoted i n t h e l i t e r a t u r e , a r e n o t v e r y r e l i a b l e c r i t e r i o n , since the i m p u r i t i e s normally present i n HEMA e x h i b i t n2Q v a l u e s which a r e both h i g h e r (EDMA and d i e t h y l e n e g l y c o l m e t h a c r y l a t e ) and lower (metha c r y l i c a c i d and e t h y l e n e g l y c o l ) than a r e those o f HEMA i t s e l f . As an i l l u s t r a t i o n , a s e r i e s o f d a t a i s recorded i n Table I I I . Besides, the r e f r a c t i v e index o f HEMA s t r o n g l y d e c r e a s e s w i t h i n c r e a s i n g water con t e n t , as i s demonstrated i n F i g . 4. I n f r a r e d s p e c t r o scopy has been recommended t o check t h e p u r i t y o f EDMA. Our r e s u l t s i n d i c a t e t h a t compounds such as HEMA and e t h y l e n e g l y c o l can be d e t e c t e d down t o app r o x i m a t e l y 1 wt.% due t o t h e s t r o n g a b s o r p t i o n o f ^ t h e i r h y d r o x y l groups (broad band a t 2800-3600 cm ) . However, one s h o u l d keep i n mind t h a t t h e i n f r a r e d spectrum does n o t e a s i l y a l l o w a c o n c l u s i o n r e g a r d i n g the n a t u r e o f t h e c o n t a m i n a n t ( s ) - e.g. HEMA, e t h y l e n e g l y c o l o r water. B e s i d e s , a compound such as metha c r y l i c a c i d goes u n d e t e c t e d a t t h e 1% l e v e l . 2


8.

BRiNKMAN E T AL.

117


HEMA

DGMA EG/P

Figure 3. Tic of three HEMA samples on silica gel, using n-hexane-diethyl ether (1:1) (a-c), and n-hexane-M IBK-n-octanol (9:2:1, satd. with 25% HNO ) (d-f) as mobile phases. HEMA samples obtained from BDH (a,d), Merck (b,e), and Hydro Med Sciences (c,f). For abbreviations, see Table II. , acid (1), n-octanol (2), and MIBK (3) front. s

1.45

1.40

1.35

100

80

60

40

20

0 % HEMA

0

20

40

60

80

1 0 0 % water

Figure 4. Dependence of n of HEMA

D

20

on water content


118


Abstract Poly(hydroxyethyl methacrylate) is generally accepted as a biocompatible, safe and stable hydrogel for medical use. The present paper describes the use of tlc for the analysis and small-scale preparation of the initial monomer, 2-hydroxyethyl methacrylate. Tlc on silica gel, with n-hexane - diethyl ether (1:1, v/v) and/or n-hexane - MIBK - n-octanol (9:2:1, v/v, satd. with 25% HNO) as mobile phase is recommen ded for qualitative analysis. Preparative-scale work is preferably carried out on HSO-impregnated silica gel, development being done with n-hexane - diethyl ether (1:1). Inhibitors are detected by means of tlc and of a drop test procedur with diazotised sulpha nilic acid. The natur several commercially available HEMA samples is dis cussed, n D values are reported for the system HEMA- water. 3

2

4

20

Literature Cited 1. Wichterle, O., and Lim, D., Nature (1960) 185, 117. 2. Wichterle, O., and Lim, D., U.S. Patent, (1961) 2, 976, 576. 3. Kopecek, J., Jokl, J., and Lim, D., J. Polymer Sci., [C] (1968) 16, 3877. 4. Refojo, M.F., J. Appl. Polymer Sci., (1965) 9, 3161. 5. Goedberg, A.I., Fertig, J., and Stanley, Η., U.S. Patent, (1962) 3, 059, 024. 6. Refojo, M.F., and Yasuda, H., J. Appl. Polymer Sci., (1965) 9, 2425. 7. Hasa, J., and Janácek, J., J. Polymer Sci., [C] (1967) 16, 317. 8. Kopecek, J., Thesis, Inst. Macromol. Chem., Prague, (1965) p. 35. 9. Mark, M.F., Gaylord, N.G., Bikales, N.M., (eds), "Encyclopedia of Polymer Science and Technology", Vol. 15, 1971, 273. 10. Zweig, G., and Sherma, J. (eds), "Handbook of Chromatography", CRC Press, Interscience, New York, 1971. 11. Crawford, J.W.C., U.S. Patent, (1939) 2, 143, 941. 12. Jatzkewitz, Η., and Lenz, U., Hoppe-Seylers Z. Physiol. Chem., (1956) 305, 53. 13. Jatzkewitz, Η., Hoppe-Seylers Z. Physiol. Chem., (1953) 292, 99. 14. Merck, Ε., Chemicalien, Reagenzien (1974), Darmstadt.


9 Rotational Viscometry Studies of the Polymerization of Hydrophilic Methacrylate Monomers SHAO M. MA, DONALD E. GREGONIS, CHWEN M. CHEN, and JOSEPH D. ANDRADE Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112 T h e r e i s c o n s i d e r a b l e l i t e r a t u r e on t h e m e c h a n i c a l b e h a v i o r of poly (hydroxyethyl methacrylate k n o w l e d g e , no s t u d y ha polymer during the course of p o l y m e r i z a t i o n . I n t h i s s t u d y we a t t e m p t e d t o use t h i s e a s i l y o b t a i n e d p r o p e r t y t o o b t a i n knowledge c o n c e r n i n g (1) t h e r e l a t i v e r a t e o f p o l y m e r i z a t i o n w i t h v a r i o u s i n i t i a t o r s ; ( 2 ) t h e e f f e c t o f p o l y m e r i z a t i o n t i m e on t h e f l o w p r o p e r t i e s o f PHEMA; ( 3 ) t h e r e l a t i o n s between t h e f l o w c u r v e , m o l e c u l a r w e i g h t and t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n . At a given temperature the v i s c o s i t y (η) v s . shear r a t e ( § ) c u r v e f o r c o n c e n t r a t e d p o l y m e r s o l u t i o n s and p u r e p o l y m e r s depends on t h e m o l e c u l a r w e i g h t and m o l e c u l a r w e i g h t d i s t r i b u t i o n o f t h e system. However, an i n c r e a s e i n v i s c o s i t y o f t h e p o l y m e r i z a t i o n mixture indicates only that conversion i s i n c r e a s i n g . How v i s c o s i t y changes d u r i n g t h e c o u r s e o f p o l y m e r i z a t i o n f r o m N e w t o n i a n t o n o n - N e w t o n i a n as a f u n c t i o n o f m o l e c u l a r w e i g h t o r m o l e c u l a r weight d i s t r i b u t i o n i s not t o t a l l y understood. Q u a l i t a t i v e l y , t h e p o l y m e r i z a t i o n m i x t u r e behaves as a N e w t o n i a n f l u i d d u r i n g t h e e a r l y s t a g e o f p o l y m e r i z a t i o n . As t h e degree o f p o l y m e r i z a t i o n o r t h e c o n c e n t r a t i o n o f t h e p o l y m e r i c f r a c t i o n i n c r e a s e s , t h e v i s c o s i t y a l s o i n c r e a s e s and t h e s y s t e m becomes n o n - N e w t o n i a n . Polymers w i t h broad m o l e c u l a r weight d i s t r i b u t i o n s show a h i g h e r v i s c o s i t y dependency on s h e a r r a t e ( 2 , 3 ) ; non-Newtonian b e h a v i o r s t a r t s t o o c c u r a t l o w e r shear r a t e s than s i m i l a r polymers w i t h narrow m o l e c u l a r weight d i s tributions. T h e r e f o r e , the e f f e c t of p o l y m e r i z a t i o n time a t t h i s s t a g e w o u l d depend on t h e p o l y d i s p e r s i t y o f t h e p o l y m e r p r o d u c e d . F u r t h e r p o l y m e r i z a t i o n e n c o u n t e r s b r a n c h i n g and g e l f o r m a t i o n . G r a e s s l e y (4) r e p o r t e d t h a t l o n g - c h a i n b r a n c h i n g a f f e c t s t h e v i s c o s i t y - s h e a r r a t e c u r v e i n a s i m i l a r way as b r o a d e n i n g o f t h e molecular weight d i s t r i b u t i o n . However, t h e e f f e c t s o f b r a n c h i n g c a n n o t be s e p a r a t e d f r o m t h e e f f e c t o f p o l y d i s p e r s i t y i n v i s c o m e t r y measurements. The v i s c o s i t y o f a b r a n c h e d p o l y m e r may be h i g h e r o r lower than a l i n e a r polymer. T h i s depends on w h e t h e r the molecular weight i s higher or lower than the m o l e c u l a r weight

119


120

HYDROGELS

FOR MEDICAL

AND RELATED APPLICATIONS

of t a n g l i n g segments. Because o f t h e c o m p l i c a t e d n a t u r e o f t h e p r o b l e m , no s i n g l e t h e o r y can a d e q u a t e l y d e s c r i b e t h e m e c h a n i c a l behavior of the p o l y m e r i c system during the course of p o l y m e r i z a tion. However, , s e v e r a l t h e o r i e s ( 5 - 7 ) have w o r k e d w e l l f o r l i n e a r p o l y m e r s , w h i c h m i g h t be t h e c a s e f o r PHEMA p r o d u c e d w i t h very l i t t l e c r o s s l i n k i n g agent i n a r e l a t i v e l y poor s o l v e n t s u c h as w a t e r . The p r e s e n t i n v e s t i g a t i o n c o n s i s t s o f : (1) f l o w c u r v e s o b t a i n e d f o r h y d r o x y e t h y l m e t h a c r y l a t e (HEMA)-water m i x t u r e s , p o l y m e r i z e d a t 60°C as f u n c t i o n s o f p o l y m e r i z a t i o n t i m e and i n i t i a t o r ; (2) t h r e e t h e o r i e s o f n o n - N e w t o n i a n v i s c o s i t y , t h e B u e c h e - H a r d i n g method ( 5 j , R e e - E y r i n g a c t i v a t e d - s t a t e t h e o r y (6) and B a r t e n e v ' s e m p i r i c a l method ( 7 ) , a r e b r i e f l y d e s c r i b e d and t h e f l o w p a r a m e t e r s o f o u r s y s t e m s a r e a n a l y z e d w i t h t h e s e t h e o r i e s ; (3) t h e r e l a t i o n r a t e and between f l o w c u r v e d i s t r i b u t i o n and p o l y m e r i z a t i o n t i m e a r e d i s c u s s e d . Material

and

Methods

The m o n o m e r - s o l v e n t m i x t u r e used i n t h i s s t u d y c o n s i s t e d o f s i x p a r t s o f HEMA and t h r e e p a r t s o f w a t e r , by v o l u m e . The i n i t i a t o r s used i n c l u d e ammonium p e r s u l f a t e , a z o b i s i s o b u t y r o n i t r i l e (AIBN), azobis(methyl i s o b u t y r a t e ) , azobis(methoxyethyl i s o b u t y r a t e ) , and a z o b i s ( m e t h o x y d i e t h o x y e t h y l i s o b u t y r a t e ) . A c o n c e n t r a t i o n o f 5.71 m m o l / l i t e r was used f o r ammonium p e r s u l f a t e and 5.21 m m o l / l i t e r f o r t h e o t h e r s . The s y n t h e s i s , p u r i f i c a t i o n and c h e m i c a l c h a r a c t e r i z a t i o n o f t h e PHEMA a r e g i v e n e l s e w h e r e (8). I t s h o u l d be n o t e d t h a t a l t h o u g h HEMA monomer i s c o m p l e t e l y m i s c i b l e w i t h w a t e r , PHEMA i s n o t w a t e r s o l u b l e . A Haake R o t o v i s c o r o t a t i o n a l v i s c o m e t e r was u s e d f o r v i s c o s i t y measurement. P o l y m e r i z a t i o n was c a r r i e d o u t i n a c o a x i a l c y l i n d e r s e n s o r s y s t e m , w i t h a MV cup and a MVI b o b . Temperature was k e p t a t 60°C w i t h a Lauda K-2/R c i r c u l a t i n g b a t h . F l o w c u r v e s were d e t e r m i n e d as f u n c t i o n s o f t i m e a t s h e a r r a t e s f r o m 0 t o 685 s e c " . To a v o i d permanent m e c h a n i c a l b r e a k d o w n , l o w e r r a t e s were used as t h e p o l y m e r i z a t i o n i n c r e a s e d . Non-Newtonian

shear

Viscosity

In n o n - N e w t o n i a n f l o w t h e c h a n c e i n a p p a r e n t v i s c o s i t y as a f u n c t i o n o f s h e a r r a t e ( § ) g e n e r a l l y t a k e s t h e f o r m η η

0

= f(xS),

(η)

Cl]

where η i s t h e v i s c o s i t y a t z e r o s h e a r r a t e , § > and τ i s t h e c h a r a c t e r i s t i c r e l a x a t i o n t i m e , which i s molecular weight-depend ent. 0

0


9.

Polymerization of Hydrophilic Methacrylate Monomers

M A E T AL.

121

Many t h e o r i e s have been p r o p o s e d t o e x p l a i n n o n - N e w t o n i a n b e h a v i o r i n condensed systems. M o l e c u l a r t h e o r i e s b a s e d on t h e e q u i v a l e n c e h y p o t h e s i s ( 9 j , on t h e e n t a n g l e m e n t c o n c e p t ( 1 0 ) , and on t h e a c t i v a t e d - s t a t e model (6) have g a i n e d c o n s i d e r a b l e acceptance. Bueche-Harding Standard Curve. Bueche i n t r o d u c e d a s h e a r r a t e dependence t o Rouse t h e o r y ( 1 1 ) . A c c o r d i n g t o h i s h y p o t h e s i s , macromolecules i n s o l u t i o n under dynamic d e f o r m a t i o n a r e assumed t o behave s i m i l a r l y t o t h o s e u n d e r s t e a d y s h e a r i n g . The change i n v i s c o s i t y w i t h s h e a r r a t e s i s c o n s i d e r e d as due t o t h e r e s u l t s o f d e f o r m i n g and r o t a t i n g o f t h e c o i l i n g p o l y m e r m o l e c u l e s under a s h e a r i n g f o r c e . Below a c e r t a i n c h a r a c t e r i s t i c t i m e , t ^ , (which equals a p p r o x i m a t e l y t h e r e c i p r o c a l o f z e r o shear r a t e , s ) the v i s c o s i t s h e a r r a t e , w h i l e abov y approache value. T h i s r e l a x a t i o n t i m e c a n be c a l c u l a t e d f r o m t h e p r o p e r t i e s of the system at low shear r a t e s . Bueche has i g n o r e d t h e e f f e c t o f c h a i n e n t a n g l e m e n t s on n o n - N e w t o n i a n b e h a v i o r and assumed t h a t t h e l o c a l p r o p e r t i e s o f t h e s y s t e m , s u c h as t h e r e l a x a t i o n t i m e d i s t r i b u t i o n , a r e i n d e pendent o f i t s s t a t e o f m o t i o n . H i s t h e o r y , t h e r e f o r e , does n o t c o r r e l a t e well with experimental data (4, 12-14). G r a e s s l e y ( 1 0 ) p r o p o s e d a t h e o r y by a s s u m i n g t h a t t h e v i s c o s i t y o f a p o l y m e r i c s y s t e m i s c o n t r o l l e d by i n t e r m o l e c u l a r c h a i n e n t a n g l e m e n t s and t h a t an i n c r e a s e i n s h e a r i n d u c e s changes i n t h e n e t w o r k o f e n t a n g l e m e n t s and hence c a u s e s t h e v i s c o s i t y to decrease. T h i s e n t a n g l e m e n t a p p r o a c h has a sound theoretical basis. However, i n f o r m a t i o n c o n c e r n i n g t h e p o l y d i s p e r s i t y and t h e e n t a n g l e m e n t d e n s i t y a r e needed t o c a r r y o u t actual computation. For a l i n e a r c o i l i n g polymer, B u e c h e - H a r d i n g , l a t e r , s u g g e s t e d a method f o r d e t e r m i n i n g i t s a b s o l u t e m o l e c u l a r w e i g h t f r o m i t s f l o w c u r v e ( 5 j . In t h i s method t h e y u s e d a s t a n d a r d curve which f o l l o w s the e m p i r i c a l equation 0

n / n = 1.00 + 0 . 6 0 ( T S ) o

3 / 4

,

[2]

t o match t h e i r e x p e r i m e n t a l d a t a . The v a l u e o f η and s a r e d e t e r m i n e d by s u p e r i m p o s i n g t h e s t a n d a r d c u r v e i n t h e f o r m o f l o g ( η / η ο ) v s . l o g ( S T ) w i t h an e x p e r i m e n t a l c u r v e i n t h e f o r m o f l o g ( η ) v s . l o g ( s ) w h i l e b o t h c u r v e s were p l o t t e d on t h e same scale. The m o l e c u l a r w e i g h t o f t h e p o l y m e r i s t h e n c a l c u l a t e d from t h e e x p r e s s i o n 0

M = 7T NckT/12n

0

s , [3] ο o where N, c , k, and Τ a r e A v o g a d r o ' s number, t h e c o n c e n t r a t i o n o f p o l y m e r i n g / c c , B o l t z m a n n ' s c o n s t a n t , and t h e a b s o l u t e t e m p e r a ture, respectively. 2

o


122


B u e c h e s r e l a x a t i o n t i m e {tw = l / s ) , as p o i n t e d o u t by G r a e s s l e y ( 1 5 ) , g o v e r n s t h e m a g n i t u d e o f t h e s h e a r r a t e when t h e v i s c o s i t y begins to decrease. I t s h o u l d be an i m p o r t a n t p a r a meter f o r those p r o p e r t i e s i n which the l o n g e r r e l a x a t i o n times a r e d e t e r m i n i n g f a c t o r s . The B u e c h e - H a r d i n g s t a n d a r d c u r v e , w h i c h a g r e e d m o d e r a t e l y w e l l w i t h d a t a on u n f r a c t i o n a t e d p o l y s t y r e n e i n benzene and p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA) i n c h l o r o f o r m , s h o u l d p r e d i c t t h e n o n - N e w t o n i a n b e h a v i o r o f any l i n e a r p o l y m e r i c system w i t h not too broad a m o l e c u l a r weight d i s t r i b u tion (5). PHEMA and PMMA have t h e same backbone s t r u c t u r e . S i n c e t h e s h e a r r a t e used i n t h i s s t u d y i s r e l a t i v e l y l o w ; and t h e s y s t e m i s e x p e c t e d t o have low e n t a n g l e m e n t d e n s i t y ( w a t e r i s a p o o r s o l v e n t ) , low b r a n c h i n g and c r o s s l i n k i n g ( t h e d i e s t e r c o n c e n t r a t i o n i s l o w ) , t h e B u e c h e - H a r d i n g method s h o u l d work w e l l f o r our system. 1

0

R e e - E y r i n g s ' s A c t i v a t e d - S t a t e M o d e l . T h i s model (6) assumes t h a t t h e r e e x i s t s i g r o u p s o f f l o w u n i t s w h i c h d i f f e r i n r e l a x a t i o n t i m e and i n g e o m e t r i c a l d i m e n s i o n s . Some o f t h e s e flow u n i t s are Newtonian, others are non-Newtonian. A Newtonian f l o w u n i t i s a m o l e c u l e o r a group o f m o l e c u l e s i s o l a t e d f r o m o t h e r u n i t s , w h i l e a non-Newtonian u n i t i s a Newtonian u n i t bonded ( o r e n t a n g l e d ) w i t h a n o t h e r N e w t o n i a n u n i t o r u n i t s . T h u s , f o r f l o w o f n o n - N e w t o n i a n u n i t s , t h i s bond ( o r e n t a n g l e ment) must be b r o k e n ( o r d i s e n t a n g l e d ) . Based on t h i s c o n c e p t , t h e g e n e r a l i z e d v i s c o s i t y e q u a t i o n i s χ . β. Σ , -^-1 i

η =

1

sinh"

β.s

1

,

Γ4]

β^

a

where χ · i s t h e f r a c t i o n a l a r e a o c c u p i e d by t h e i t h f l o w u n i t on t h e s h e a r s u r f a c e , and η

cu = (λ X X 3 ) . / 2 k T

;

[5]

β. = [(A/X )2k ]T

;

[6]

2

l

Ί

1

1

a., and β. a r e t h e c h a r a c t e r i s t i c s h e a r volume d i v i d e d by kT w h i c h i s r e l a t e d t o t h e i n v e r s e o f t h e s h e a r modulus and t h e r e l a x a t i o n t i m e , r e s p e c t i v e l y , β./α· i s t h e N e w t o n i a n v i s c o s i t y . λ is

the jumping d i s t a n c e ,

λ ι , λ , and λ 2

3

are the m o l e c u l a r

d i m e n s i o n s o f a f l o w u n i t , k' i s t h e j u m p i n g f r e q u e n c y o f t h e f l o w u n i t when t h e r e i s no s t r e s s . According to the theory of rate processes (16): k

'

=

ΊΪ

IT

e

x

p

m , are mostly non-Newtonian i n b e h a v i o r . F o r two s y s t e m s w i t h t h e same m o l e c u l a r w e i g h t i t f o l l o w s t h a t 2

(62)1

(f ) 2

(m )î

2

(Γ )?

2

=

2

=

(62)2

=

.

Ml

(f )i 2

[13]

(Â )i 2

The s u b s c r i p t 2 i n s i d e t h e p a r e n t h e s e s r e f e r s t o t h e n o n N e w t o n i a n f l o w u n i t s and t h subcript 1 d 2 outsid th parentheses r e f e r to system [13] i n d i c a t e s t h a t the average l e n g t h or the m o l e c u l a r w e i g h t o f t h e t a n g l i n g segments i s p r o p o r t i o n a l t o t h e s q u a r e r o o t o f t h e r e l a x a t i o n t i m e o f t h e f l o w u n i t s w h i c h c o n t a i n s u c h segments. T h i s w o u l d a p p r o x i m a t e l y be t h e c a s e when t h e same d e a r e e o f p o l y m e r i z a t i o n was r e a c h e d f r o m d i f f e r e n t i n i t i a l HEMA-water mixtures. On t h e o t h e r hand we can w r i t e (19) Bp = 6

S

(M/m ) (M/rn^ 2

1

/

3

and

^s a

p

(MM,)

[14] (M/m ) 2

where t h e s u b s c r i p t s ρ and s r e f e r t o p r o p e r t i e s f o r t h e p o l y m e r and u n a t t a c h e d k i n e t i c s e g m e n t s , r e s p e c t i v e l y . I f the degree of p o l y m e r i z a t i o n i s s u f f i c i e n t l y l a r g e , i . e . , i f M > m , the v a l u e o f nu , m s h o u l d n o t be a f f e c t e d by f u r t h e r p o l y m e r i z a tion. It follows that 2

2

(0p)ti

where t i and t

2

(M

V t ,

" '

V t .

.

A I B N ; (3) Both R e e - E y r i n g ' s t h e o r y o f n o n - N e w t o n i a n f l o w and B u e c h e H a r d i n g ' s method can d e s c r i b e t h e b e h a v i o r o f HEMA-water mixtures d u r i n g the course of p o l y m e r i z a t i o n . Bartenev's e m p i r i c a l e x p r e s s i o n works l e s s w e l l presumably because t h e r a t i o n/no f o r o u r s y s t e m i s s t i l l some f u n c t i o n o f m o l e c u l a r w e i g h t and c a n n o t be assumed t o depend on s h e a r s t r e s s alone; (4) The m o l e c u l a r w e i g h t o f l i n e a r PHEMA can be o b t a i n e d by u s i n g t h e B u e c h e - H a r d i n g method w i t h i n d e p e n d e n t i n f o r mation concerning c o n c e n t r a t i o n of the polymer. However, we have n o t c a r r i e d o u t c o n c e n t r a t i o n measurements i n t h i s

(1)


136

HYDROGELS

FOR

MEDICAL

AND RELATED

APPLICATIONS


9. M A E T AL.

Polymerization

of Hydrophilic

Methacrylate

Monomers

137

s t u d y and o n l y t h e r a t i o VM i s e s t i m a t e d . The r e l a t i v e m o l e c u l a r w e i g h t a t d i f f e r e n t p o l y m e r i z a t i o n t i m e s c a n be obtained from E y r i n g ' s r e l a x a t i o n t i m e ; (5) Nakagima's theory (20) c o r r e l a t e s t h e f l o w curve w i t h t h e cumulative molecular weight d i s t r i b u t i o n curve o f l i n e a r polymers. One c a n e x p l o r e t h i s t h e o r y f u r t h e r and s e e whether i t i s a p p l i c a b l e t o o u r systems. The work p r e s e n t e d h e r e i s o n l y a p r e l i m i n a r y s t u d y . The d a t a o b t a i n e d w i t h a Haake R o t o v i s c o v i s c o m e t e r a r e n o t a s u f ficient test. To f u r t h e r t e s t t h e a p p l i c a b i l i t y o f t h e t h e o r i e s requires that: ( 1 ) e x p e r i m e n t a l d a t a be o b t a i n e d o v e r a w i d e range o f r a t e s o f s h e a r , a t s e v e r a l d i f f e r e n t t e m p e r a t u r e s and u s i n g v a r i o u s s o l v e n t s ; ( 2 ) c o n c e n t r a t i o n and p o l y d i s p e r s i t y be d e t e r m i n e d s i d e by s i d e w i t h v i s c o m e t r i c measurement s o t h a t r e s u l t s f r o m t h e s e measurement

Abstract Flow curves for hydroxyethyl methacrylate-water mixtures were determined as functions of polymerization time and initia tor. The non-Newtonian behavior of these systems was analyzed by a Bueche-Harding standard curve, by the Ree-Eyring generalized viscosity equation and by Bartenev's empirical equation. The relative rate of polymerization initiated with AIBN and various AIBN esters and the relationships between flow curves, molecular weight, molecular weight distribution and polymerization time are discussed. Literature Cited 1. Janacek, J., J. Macromol. Sci.-Revs. Macromol. Chem. (1973) C9 (1) 1-47. 2. Rudd, J. F., J. Poly. Sci. (1960) 44, 459. 3. Sabia, R., J. Appl. Poly. Sci. (1964) 8, 1053. 4. Graessley, W. W., and Prentice, J. S., J. Poly. Sci., A-2 (1968) 6, 1887. 5. Bueche, F., and Harding, S. W., J. Poly. Sci. (1958) 32, 177. 6. Ree, T., Eyring, H., J. Appl. Phys. (1955) 26, 793, 800. 7. Bartenev, G. Μ., Vysokomolekyluarnye Soedineniya (1964) 6, 2155. 8. Gregonis, D., Chen, C. M., and Andrade, J. D., this symposium. 9. Bueche, F., F. J. Chem. Phys. (1954) 22, 1570. 10. Graessley, W. W., J. Chem. Phys. (1965) 43, 2696. 11. Rouse, P. E., J. Chem. Phys. (1953) 21, 1272. 12. Ballman, R. L., and Simon, R. M., J. Poly. Sci. A-2 (1964) 2, 3557. 13. Graessley, W. W., Hazleton, R. L., and Lindeman, L. R., Trans. Soc. Rheology, (1967) 11, 267.


138


14. Shih, C. K., Tran. Soc. Rheology (1970) 14, 83. 15. Graessley, W. W., J. Chem. Phys. (1967) 47, 1942. 16. Glasstone, S., Laidler, K., and Eyring, Η., "The Theory of Rate Processes," p. 483, McGraw Hill Book Company, New York, 1941. 17. Jhon, M. S., and Eyring, H., private communication. 18. Ma, S. M., Eyring, H., and Jhon, M. S., Proc. Natl. Acad. Sci. (USA) (1974) 71, 3096. 19. Eyring, H., Ree, T., and Hirai, N., Proc. Natl. Acad. Sci. (USA) (1958) 44, 1213. 20. Nakajima, Ν., Proc. 5th Intl. Cong. on Rheology, Ed. by Shigeharu Onogi, (1968) 4, 295. 21. Gabrysh, A. F., Eyring, H., Shimizu, M., and Asay, J., J. Appl. Phys. (1963) 34 261 22. DeWitt, T. W., Markovitz J., J. Colloid Sci


10 The Effect of Tacticity on the Thermal Behavior of Various Poly(methacrylate esters) G. A. RUSSELL, D. E. GREGONIS, A. A. DEVISSER, and J. D. ANDRADE Department of Materials Science and Engineering, University of Utah, Salt Lake City, Utah 84112 D. K. DALLING Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 T a c t i c i t y , t h e s t e r e o c h e m i c a l placement o f pendant s i d e chains along t h e polyme important f a c t o r i n determinin Such p r o p e r t i e s as c r y s t a l l i z a b i l i t y , s o l u b i l i t y , m e l t i n g p o i n t , g l a s s t r a n s i t i o n t e m p e r a t u r e , e t c . have been c o r r e l a t e d w i t h t h e t a c t i c i t y o f a g i v e n polymer ( 1 , 2 , 3 , 4 ) . Introduction of ZieglerN a t t a c a t a l y s t s and o t h e r c a t a l y s t s y s t e m s c a p a b l e o f c o n t r o l l i n g t a c t i c i t y d u r i n g s y n t h e s i s has opened up new a r e a s f o r c o m m e n ç a i d e v e l o p m e n t based on s y s t e m a t i c v a r i a t i o n o f m o l e c u l a r c o n f i g u r a tion. L i t t l e work has been r e p o r t e d , h o w e v e r , c o r r e l a t i n g c h a i n t a c t i c i t y t o t h e s u r f a c e and i n t e r f a c i a l p r o p e r t i e s o f p o l y m e r s . R e c e n t work ( 5 j s u g g e s t s t h a t r e o r i e n t a t i o n o f t h e c h a i n s i n t h e s u r f a c e zone o f t h e p o l y m e r c a n a f f e c t w e t t a b i l i t y o f t h e s u r f a c e If t h a t i s indeed t h e case, t h e t a c t i c i t y o f t h e polymer chains may w e l l have an i n f l u e n c e on w e t t a b i l i t y , s i n c e t h e b a r r i e r s t o chain r o t a t i o n are a function of t a c t i c i t y . Also, the a v a i l a b i l i t y o f h y d r o p h i l i c and h y d r o p h o b i c s i t e s f o r i n t e r a c t i o n a t t h e i n t e r f a c e i s i n f l u e n c e d by t h e c h a i n c o n f i g u r a t i o n and c o n formation. C o n s e q u e n t l y , i t was f e l t t h a t a s t u d y o f t h e i n f l u e n c e o f t a c t i c i t y on b u l k p h y s i c a l and i n t e r f a c i a l p r o o e r t i e s i n t h e p o l y ( h y d r o x y e t h y l m e t h a c r y l a t e ) (pHEMA)/water s y s t e m m i g h t i n d i c a t e some means o f a l t e r i n g t h e i n t e r f a c i a l p r o p e r t i e s o f t h e p o l y m e r by c o n t r o l o f i t s t a c t i c i t y d u r i n g s y n t h e s i s . In t h i s p a p e r we r e p o r t p r e l i m i n a r y r e s u l t s o f some e x p e r i m e n t s i n w h i c h v a r i o u s methods o f a l t e r i n g t a c t i c i t y d u r i n g s y n t h e s i s have been examined and t h e i r e f f e c t c o r r e l a t e d w i t h changes i n t h e thermal p r o p e r t i e s of t h e r e s u l t i n g polymers. S y n t h e s i s o f m e t h y l and o t h e r a l k y ! m e t h a c r y l a t e s o f h i g h s t e r e o r e g u l a r i t y , e i t h e r o f high i s o t a c t i c content o r high s y n d i o t a c t i c c o n t e n t , i s w e l l - k n o w n (6_). U n f o r t u n a t e l y , t h e organomet a l l i c i n i t i a t o r s used f o r p r o d u c t i o n o f s t e r e o r e g u l a r m e t h a c r y l ates are highly r e a c t i v e t o the hydroxyl f u n c t i o n a l i t y i n the s i d e c h a i n o f HEMA. C o n s e q u e n t l y , some means o f b l o c k i n q t h e h y d r o x y l g r o u p d u r i n g p o l y m e r i z a t i o n was r e q u i r e d . Methoxyethyl m e t h a c r y l a t e (MEMA) was c h o s e n as a model f o r a H E M A - l i k e monomer

139



140

w i t h i t s h y d r o x y l g r o u p p r o t e c t e d , even t h o u g h t h e m e t h y l e t h e r l i n k a g e i s t o o s t a b l e t o p e r m i t h y d r o l y s i s back t o pHEMA. A s e c o n d p r o t e c t e d HEMA d e r i v a t i v e used was t r i m e t h y l s i l y l e t h y l m e t h a c r y l a t e (TMSEMA), whose t r i m e t h y l s i l y l p r o t e c t i o n g r o u p can be c l e a v e d r e a d i l y , f o r m i n g HEMA. T h i s a l l o w s p o l y m e r i z a t i o n o f TMSEMA u s i n g an a n i o n i c c a t a l y s t t o p r o d u c e a s t e r e o r e g u l a r pTMSEMA, f o l l o w e d by h y d r o l y s i s t o s t e r e o r e g u l a r pHEMA. P o l y m e r s o f HEMA, MEMA, and TMSEMA were s y n t h e s i z e d by a v a r i e t y o f t e c h n i q u e s , t h e i r t a c t i c i t y was d e t e r m i n e d by C - N M R s p e c t r o m e t r y , and t h e v a r i a t i o n i n t a c t i c i t y was c o r r e l a t e d w i t h changes i n t h e DSC thermograms o f e a c h p o l y m e r . 13

Experimental Monomer P r e p a r a t i o n A l l d i thi stud exceDt TMSEMA were o b t a i n e d c o m m e r c i a l l y methoxyethyl methacrylat (MEMA) supplie y c a l C o . ; 2 - h y d r o x y e t h y l m e t h a c r y l a t e (HEMA) was s u p p l i e d by Hydro-Med S c i e n c e s . TMSEMA was s y n t h e s i z e d i n o u r l a b o r a t o r y by a p r o c e d u r e t o be d e s c r i b e d e l s e w h e r e (7). The h y d r o a u i n o n e i n h i b i t o r was removed f r o m t h e c o m m e r c i a l monomers by e x t r a c t i o n w i t h aqueous NaOH. The monomer was t h e n d r i e d o v e r MgSOi* and d i s t i l l e d from L i A l a n d CuCl. The d i s t i l l e d p r o d u c t was t h e n s t o r e d a t 4°C o v e r 5A m o l e c u l a r s i e v e u n t i l u s e d . The s t r u c t u r e s o f t h e monomers and a b b r e v i a t i o n s used a r e shown i n F i g u r e 1. The p o l y m e r i z a t i o n c o n d i t i o n s used a r e summarized i n T a b l e I. Anionic Polymerization. MMA, MEMA and TMSEMA were each p o l y m e r i z e d a n i o n i c a l l y i n dry t o l u e n e a t -78 C u s i n g η - b u t y l l i t h i u m as i n i t i a t o r . These c o n d i t i o n s had been shown p r e v i o u s l y t o p r o d u c e pMMA and o t h e r a l k y ! m e t h a c r y l a t e s c o n t a i n ing a high percentage of i s o t a c t i c t r i a d s (8). In a l l c a s e s t h e r e a c t i o n was t e r m i n a t e d by a d d i t i o n o f m e t h a n o l , and t h e p o l y m e r p r e c i p i t a t e d by a n o n - s o l v e n t , p e t r o l e u m e t h e r f o r pMMA and pMEMA, w a t e r f o r pTMSEMA. The c r u d e p r o d u c t s were t h e n r e d i s s o l v e d i n benzene (pW1A and pMEMA) o r some s u i t a b l e s o l v e n t , and c e n t r i f u g e d t o remove c r o s s - l i n k e d p o l y m e r and p r e c i p i t a t e d L i OH. The r e s u l t i n g p r o d u c t was t h e n d r i e d o v e r n i g h t i n vacuo a t a b o u t 80°C. Free R a d i c a l P o l y m e r i z a t i o n . Free r a d i c a l p o l y m e r i z a t i o n of HEMA and MEMA were a c c o m p l i s h e d by a d d i n g 1.^ ηα/ml o f a z o b i s m e t h y l i s o b u t y r a t e (AMIB) t o t h e d e g a s s e d monomer. The monomer/ i n i t i a t o r s o l u t i o n was i n j e c t e d i n t o a s p l i t p o l y p r o p y l e n e mold and p l a c e d i n an oven a t 80°C f o r 20 h o u r s . The r e s u l t i n g p o l y mer s h e e t ( 3 - 4 mm t h i c k ) was t h e n removed f o r f u r t h e r c h a r a c t erization. S i n c e t h e pHEMA p r o d u c e d by b u l k p o l y m e r i z a t i o n was t o o h i g h l y c r o s s - l i n k e d t o be r e d i s s o l v e d f o r d e t e r m i n a t i o n o f i t s t a c t i c i t y , t h e monomer, s o l v e n t ( g e n e r a l l y p y r i d i n e ) and i n i t i a t o r were added d i r e c t l y t o a 10 mm NMR t u b e a l o n g w i t h 0 . 1 - 0 . 2 ml o f p - d i o x a n e as an i n t e r n a l s t a n d a r d and t h e r e s u l t i n g m i x t u r e was p o l y m e r i z e d i n s i t u by p l a c i n g t h e t u b e i n t o t h e oven a t 80°C


10.

RUSSELL ET AL.

Thermal Behavior of Poly(methacrylate esters)

141

f o r 20 h o u r s . TMSEMA was p o l y m e r i z e d i n d r y t o l u e n e u s i n g AMIB (TMSEMA/AMIΒ 1:100) as i n i t i a t o r . The p o l y m e r f o r m e d was p r e c i p a t e d i n p e t r o l e u m e t h e r and d r i e d o v e r n i g h t a t 80°C i n v a c u o . TABLE

I.

MONOMERS USED AND POLYMERIZATION CONDITIONS* Monomer Methylmethacrylate

(MMA)

Approx. Yield

Initiator

Solvent

Τ (°C)

n-BuLi n-BuLi

Toluene THF

-78° -78°

80% 20%

60°

>95%

None

2-Hydroxyethylmethacrylate (HEMA)

AMIB

trimethylsilylethylmetha c r y l a t e (TMSEMA)

AMIB n-BuLi

Toluene Toluene

60°C >90% -78° 20%

methoxyethylmethacrylate

n-BuLi AMIB

Toluene None

-78° 60°

*n-BuLi AMIB UV

: : :

30% >95%

η-butyl l i t h i u m azobismethylisobutyrate ultraviolet radiation

One low t e m p e r a t u r e p o l y m e r i z a t i o n o f HEMA was c a r r i e d o u t i n an e f f o r t t o p r o d u c e a h i g h l y s y n d i o t a c t i c pHEMA. I t was p e r formed by d i s s o l v i n g HEMA and AMIB i n methanol and e x p o s i n g t h e s o l u t i o n t o a 254 nm UV s o u r c e f o r s e v e n h o u r s a t -60°C. The r e s u l t i n g p o l y m e r was t h e n p r e c i p i t a t e d i n t o l u e n e and d r i e d o v e r n i g h t a t 60°C i n vacuo p r i o r t o s u b s e q u e n t c h a r a c t e r i z a t i o n . T a c t i c i t y Determination. Samples f o r t a c t i c i t y d e t e r m i n a t i o n were p r e p a r e d by p l a c i n g 0 . 8 - 1 . 0 g o f t h e d r i e d p o l y m e r i n t o a 10 mm NMR t u b e and a d d i n g 1.5-2 ml o f an a p p r o p r i a t e s o l v e n t a l o n g w i t h 0 . 1 - 0 . 2 ml o f p - d i o x a n e as an i n t e r n a l r e f e r e n c e . The t u b e was c a p p e d , and t h e s o l v e n t was a l l o w e d t o s w e l l o r d i s s o l v e the polymer i n the tube. CDC1 was used f o r pMMA and pMEMA, and p y r i d i n e f o r pHEMA and pTMSEMA. The p r o t o n d e c o u p l e d 25,2 MHz C s p e c t r u m was t h e n o b t a i n e d u s i n g a V a r i a n X L F T - Ι Ο Π NMR s p e c t r o m e t e r i n t h e F o u r i e r T r a n s f o r m mode. Spectra obtained at a m b i e n t t e m p e r a t u r e c o n t a i n e d peaks w h i c h were t o o b r o a d t o resolve. C o n s e q u e n t l y , t h e samples were h e a t e d as h i g h as p r a c t i c a l w i t h o u t causing r e f l u x i n g of s o l v e n t . Probe t e m p e r a t u r e s ranged f r o m 50°C f o r MMA and pMEMA i n CDC1 t o 70°C f o r ρHEMA and pTMSEMA i n p y r i d i n e . A t e l e v a t e d t e m p e r a t u r e s s h a r p s n e c t r a were o b t a i n e d i n w h i c h e a c h c a r b o n a b s o r p t i o n was r e s o l v e d . 3

1 3

3



142

A s s i g n m e n t o f t h e peaks i n t h e pMMA s p e c t r a were b a s e d on p u b l i s h e d spectra (10). A s s i g n m e n t o f peaks e x h i b i t i n g s h i f t s due t o t a c t i c i t y f o r pMEMA, pHEMA, e t c . were made by a n a l o g y t o pMMA. Thermal A n a l y s i s . D i f f e r e n t i a l S c a n n i n g C a l o r i m e t r y (DSC) c u r v e s f o r e a c h p o l y m e r were o b t a i n e d o v e r t h e r a n g e - 5 0 to +250 C u s i n g a DuPont Model 990 Thermal A n a l y s i s S y s t e m . Samples were w e i g h e d and p l a c e d i n c o v e r e d aluminum sample p a n s , t h e n p l a c e d i n t o t h e DSC. Each sample was a n n e a l e d above i t s g l a s s t r a n s i t i o n temperature f o r approximately 5 minutes, then c o o l e d t o t h e s t a r t i n g t e m p e r a t u r e f o r the thermogram. The s a m p l e was t h e n h e a t e d a t 10 C/minute u n d e r a n i t r o g e n a t m o s p h e r e . An empty aluminum sample pan was used as a r e f e r e n c e mass f o r e a c h r u n . Results

and

Discussion

pMMA. F i g u r e 2 c o n t r a s t s t h e C - N M R s p e c t r a o f two d i f f e r e n t s a m p l e s o f pMMA, one p r o d u c e d by f r e e r a d i c a l i n i t i a t i o n and t h e o t h e r by low t e m p e r a t u r e a n i o n i c p o l y m e r i z a t i o n . Each peak i n the spectrum i s assigned to the v a r i o u s carbons i n the polymer, and t h e c h e m i c a l s h i f t o f each r e l a t i v e t o t h e p - d i o x a n e s i n g l e t a t 0 . 0 ppm i s shown. Of p a r t i c u l a r i n t e r e s t a r e t h e t h r e e peaks w h i c h c o r r e s p o n d t o t h e a - C H c a r b o n atoms w h i c h a r e i n t h e c e n t e r o f i s o t a c t i c ( i ) , h e t e r o t a c t i c (h) and s y n d i o t a c t i c ( s ) triads, respectively. S i m i l a r s p l i t t i n g i s observed f o r the c a r b o n y l and q u a t e r n a r y c a r b o n a t o m s , a l t h o u g h t h e peaks a r e l e s s w e l l - r e s o l v e d than the a - C H peaks. The a r e a u n d e r e a c h o f t h e peaks i s p r o p o r t i o n a l t o t h e number o f c a r b o n atoms i n e a c h t y p e of t r i a d i n the sample. H e n c e , a r a t i o o f t h e peak a r e a s y i e l d s t h e r e l a t i v e amounts o f each t y p e o f t r i a d . Since the a-CH peaks a r e s p l i t r e l a t i v e l y f a r a p a r t , t h e y have been used t o c a l c u l a t e t a c t i c i t i e s in t h i s study. The f r e e r a d i c a l pMMA s p e c i m e n c o n t a i n s 52%s / 41% h/ 7% i t r i a d s , b a s e d on t h e r e l a t i v e peak a r e a s . The a n i o n i c pMMA c o n t a i n s 10% s/ 19% h/ 71% i triads. The peak a r e a s and c h e m i c a l s h i f t s o f b o t h pMMA s a m p l e s , as w e l l as t h o s e f o r t h e o t h e r p o l y m e r s s t u d i e d , a r e l i s t e d i n T a b l e I I . F i g u r e 3 c o n t r a s t s t h e DSC thermograms o f t h e same two polymers. C l e a r l y , the predominantly i s o t a c t i c a n i o n i c polymer e x h i b i t s d i f f e r e n t thermal behavior from t h a t o f the predominantly s y n d i o t a c t i c f r e e r a d i c a l polymer. The i s o t a c t i c p o l y m e r shows a g l a s s t r a n s i t i o n a t 76 C, whereas t h e s y n d i o t a c t i c p o l y m e r has a Tg o f 110 C. The t h e r m a l b e h a v i o r o f t h e s p e c i m e n s can t h u s be c o r r e l a t e d w i t h t h e t a c t i c i t y o b s e r v e d by NMR. 3

3

3

pMEMA, The b e h a v i o r o f pMMA d e s c r i b e d above c o r r e s p o n d s w e l l w i t h t h a t r e p o r t e d p r e v i o u s l y (1_). C o n s e q u e n t l y , i t was e x p e c t e d t h a t p o l y m e r s o f MEMA p r o d u c e d by f r e e r a d i c a l and a n i o n i c methods w o u l d show a c o r r e s p o n d i n g d i f f e r e n c e i n t a c t i c i t y . The a - C H p o r t i o n s o f t h e C - N M R s p e c t r a o f two d i f f e r e n t pMEMA s a m p l e s a r e shown i n F i g u r e 4. The f i r s t s p e c i m e n was p r o d u c e d u s i n g AMIB as a f r e e r a d i c a l i n i t i a t o r , w h i l e t h e s e c o n d was 13

3


Thermal Behavior of Poly(methacrylate esters)

RUSSELL E T AL. H

/CH c=c

x

/CH

3

0-CH -CH -OH

O-CH3

2

METHYLMETHACRYLATE (MMA)

H

CH C=C

N

7

H

2

2

2

2-HYDROXYETHYLMETHACRYLATE (HEMA)

3

0-CH -CH

3

\

/ 3 C=C C H

0-CH -CH -0-Si-CH I CH

-OCH3

2

2

3

3

TRIMETHYLSILYLETHYLMETHACRYLATE (TMSEMA)

2-METHOXYETHYLMETHACRYLATE (MEMA)

Figure 1.

Monomer structure and nomenclature

-0-CH3

Free Radical pMMA

110.8 ppm

0.0 ppm

50.5 ppm

Anionic pMMA

Ai 109.4 ppm

Figure 2.

0.0 ppm

-45.0 ppm

C NMR spectra of poly(methyl methacrylate)

13



-47.7

—

from

-48.5

— -50.3

-50.2

-49.3

-49.4

-49.4

-49.5

-50.5

-45.7

45.4 71%

W

ΔΡ = pressure d i f f e r e n t i a l between the concave and con vex s i d e s o f the i n t e r f a c e ; γ^γ

= l i q u i d / v a p o r i n t e r f a c i a l t e n s i o n ; and

R,,

R~ = p r i n c i p l e

r a d i i of curvature.


16.

ANDRADE

E T AL.

Hydro gel/Water

Interface

215

D i f f e r e n c e s i n ΔΡ from r e g i o n t o r e g i o n over the i n t e r f a c e correspond t o g r a d i e n t s i n h y d r o s t a t i c ' p r e s s u r e w i t h i n the under l y i n g bulk l i q u i d , and t h e r e f o r e tend t o cause motion i n d i r e c t i o n s which w i l l d i m i n i s h these g r a d i e n t s . Such motion o r y i e l d i n g o f the i n t e r f a c e t o meet the L a p l a c i a n ΔΡ requirement i s n e a r l y always accompanied by a displacement o f the TPL which causes an instantaneous change i n the shape and/or area o f the l i q u i d / v a p o r i n t e r f a c e . The dynamic contact angle i s an i n s t a n taneous measure o f the angle between the l i q u i d / v a p o r i n t e r f a c e and the s o l i d / l i q u i d i n t e r f a c e w h i l e the TPL i s moving. Dynamic contact angles may be measured by e i t h e r spontaneous spreading o r f o r c e d spreading o f a l i q u i d on a s u r f a c e . In the spontaneous spreading method, the contact angle i s observed as a f u n c t i o n o f time and o f d i s t a n c e t r a v e r s e d by the TPL as a drop o f l i q u i d spontaneousl brium contact angle. I extent t o which the γ_,., γ__, and y cos θ. ^ are imbalanced. SV SL 'LV mst. In f o r c e d spreading, the l i q u i d / v a p o r i n t e r f a c e i s moved r e l a t i v e t o the s o l i d surface at a s e r i e s o f constant v e l o c i t i e s by ap p l i c a t i o n o f an e x t e r n a l f o r c e . At each v e l o c i t y the contact angle reaches a steady s t a t e value and a r e l a t i o n s h i p between the TPL v e l o c i t y and contact angle i s thus e s t a b l i s h e d . In t h i s case the d r i v i n g f o r c e i n which the TPL undergoes displacement i s the r e s u l t o f a pressure-curvature imbalance (25). The importance o f dynamic contact angle measurements l i e s i n the phenomenon o f contact angle h y s t e r e s i s , i . e . , the a b i l i t y to change the observed contact angle o f a l i q u i d on a surface without subsequent displacement o f the TPL. This may be exhib i t e d on many s u r f a c e s by measuring the instantaneous contact angle w h i l e , f o r example, i n c r e a s i n g and decreasing the s i z e o f the s e s s i l e drop. Comparison o f the r e s u l t i n g curves o f advanc ing and receeding contact angles i n the case o f most surfaces w i l l show t h a t a h y s t e r e s i s loop i s formed. The most common types o f n o n - i d e a l i t y are e i t h e r a homogeneous but g e o m e t r i c a l l y rough s u r f a c e o r a g e o m e t r i c a l l y smooth but heterogeneous sur face (19). One may a l s o have both h e t e r o g e n e i t i e s and s u r f a c e roughness. P r a c t i c a l l y speaking then, both s u r f a c e roughness and/or s u r f a c e h e t e r o g e n e i t y , both o f which give r i s e t o contact angle v a r i a t i o n s , can be the major undetected cause o f contact angle h y s t e r e s i s . S w e l l i n g , a b s o r p t i o n , and molecular r e o r i e n t a t i o n a t the i n t e r f a c e can a l s o lead t o apparent h y s t e r e s i s (26). The g e l surface may be capable o f molecular r e o r i e n t a t i o n during dynamic contact angle measurements, as has been suggested (28). The g e l / l i q u i d i n t e r f a c e poses s t i l l f u r t h e r d e v i a t i o n s from i d e a l i t y which have yet t o be considered. In a d d i t i o n t o p o s s i b i l i t i e s o f surface roughness and surface h e t e r o g e n e i t y , the t y p i c a l g e l surface i s extremely deformable. Thus i n the case o f contact angle c h a r a c t e r i z a t i o n o f the g e l s u r f a c e , the v e r t i c a l components o f s u r f a c e t e n s i o n should cause a p p r e c i a b l e rXT


216

HYDROGELS FOR

MEDICAL AND


d i s t o r t i o n i n the g e l surface at the TPL and could cause gross m i s r e p r e s e n t a t i o n s of the angular measurements. This e f f e c t would thus s i g n i f i c a n t l y i n f l u e n c e dynamic measurements and thus contact angle h y s t e r e s i s . The contact angles o f l i q u i d s at deformable s o l i d surfaces have been t h e o r e t i c a l l y t r e a t e d by s e v e r a l i n v e s t i g a t o r s (23,27), but have yet to be e x p e r i m e n t a l l y a p p l i e d to the surfaces o f g e l systems. Interface

Potentials

I d e a l l y we would l i k e to be able t o probe the e l e c t r i c a l double l a y e r at g e l / s o l u t i o n i n t e r f a c e s . Perhaps the most s t r a i g h t f o r w a r d way i s to use e l e c t r o k i n e t i c methods (29,30) g e n e r a l l y e l e c t r o p h o r e s i s (31), streaming p o t e n t i a l (32), or e l e c t r o o s m o s i s (29). Such measurement allo t calculat the p o t e n t i a l at the shea number of assumptions Perhaps the b i g g e s t problem i s the assumption i n v o l v i n g the nature o f the f l u i d dynamic boundary l a y e r i n such s t u d i e s and the p o s i t i o n of the shear plane. Even i f the gel surface i s p e r f e c t l y smooth, we s t i l l have the problem of d e f i n i n g an i n t e r f a c e p o s i t i o n f o r a g e l c o n s i s t i n g of h i g h l y mobile segments and chains at the i n t e r f a c e (Z, 3). The shear plane could be o u t s i d e the i n t e r f a c i a l zone, w i t h i n the zone and f r e e d r a i n i n g , or w i t h i n the i n t e r f a c i a l zone and non-free d r a i n i n g , as d i s c u s sed by Brooks (33), and others (34). These same problems are present i n v i s c o m e t r i c or rhéologie c h a r a c t e r i z a t i o n of g e l / liquid interfaces. One must be p a r t i c u l a r l y c a r e f u l with streaming p o t e n t i a l and e l e c t r o o s m o s i s measurements w i t h respect to other f l u i d dynamic assumptions, p a r t i c u l a r l y entrance e f f e c t s and the establishment of p a r a b o l i c flow p r o f i l e s (35). One can a l s o o b t a i n surface p o t e n t i a l i n f o r m a t i o n u s i n g g e l / a i r measurements, such as w i t h a v i b r a t i n g reed e l e c t r o s t a t i c m i l l i v o l t m e t e r (36). These methods are commonly used to characte r i z e monomolecular f i l m s at the l i q u i d / a i r i n t e r f a c e (37), i n c l u d i n g s y n t h e t i c polymers (38). Such methods cannot be e a s i l y a p p l i e d t o the g e l / s o l u t i o n i n t e r f a c e , however. The i n t e r p r e t a t i o n of g e l / s o l u t i o n e l e c t r o k i n e t i c data i s f a r from s t r a i g h t f o r w a r d . One must of course consider a c l a s s i c a l treatment i n terms of f i x e d charges on the polymer " s u r f a c e " and double l a y e r counterions. In a d d i t i o n , the g e l w i l l absorb and p a r t i t i o n ions from s o l u t i o n , even i f the gel i s l a r g e l y "uncharged" i n terms of f i x e d polymer charges, ( t h i s w i l l be discussed l a t e r ) . I f ions are p a r t i t i o n e d between the gel and the s o l u t i o n , i n t e r f a c e p o t e n t i a l s w i l l r e s u l t which w i l l , of course, i n f l u e n c e e l e c t r o k i n e t i c measurements (39).


16.

ANDRADE

ET

AL.

Hydwgel/Wciter

Interface

217

Adsorption o f Polymers One can study the a d s o r p t i o n o f polymers at gel/water i n t e r faces. Much o f the work i n t h i s area has i n v o l v e d plasma p r o t e i n adsorption (40-42). We have discussed the p r o t e i n adsorption behavior of g e l s p r e v i o u s l y , i n terms of i n t e r f a c i a l f r e e energies and water s t r u c t u r e c o n s i d e r a t i o n s (1). One can o b t a i n i n f o r m a t i o n on the nature o f gel/water i n t e r faces by adsorbing g e l molecules on other s u b s t r a t e s and then c h a r a c t e r i z i n g the adsorbed polymer/water i n t e r f a c e . A l a r g e l i t e r a t u r e on polymer adsorption i s a v a i l a b l e , i n c l u d i n g the study of water-soluble polymers (43). The t h e o r i e s of c o l l o i d a l p a r t i c l e s t a b i l i z a t i o n by adsorbed n o n - i o n i c polymers v i a e n t r o p i e and e n t h a l p i c r e p u l s i o n (44,45) may be u s e f u l i n understandin ions . E f f e c t s of s t e r e o r e g u l a r i t y on i n t e r f a c i a l behavior have been observed i n poly(methyl m e t h a c r y l a t e ) , p o l y ( i s o p r o p y l a c r y l a t e ) , p o l y ( 2 - v i n y l p y r i d i n e 1-oxide) and other polymers (46,67). The adsorbed l a y e r * can then be c h a r a c t e r i z e d by the i n s t r u mental techniques discussed i n t h i s paper. Partitioning Gels are very s u b t l e probes of t h e i r environment. They w i l l p a r t i t i o n ions and other s o l u t e s and s w e l l or deswell i n response to t h e i r s o l u t i o n environments. Of p a r t i c u l a r importance t o t h e i r surface p r o p e r t i e s i s i o n p a r t i t i o n i n g i n the g e l s , which may s i g n i f i c a n t l y i n f l u e n c e i n t e r f a c i a l p o t e n t i a l and i n t e r f a c i a l tension studies. Ion p a r t i t i o n i n g i n gels has been observed f o r c e l l u l o s e (47), c r o s s - l i n k e d dextrans (48,49), p o l y ( h y d r o x y e t h y l methacryl a t e ) g e l s (50), and others. Of p a r t i c u l a r i n t e r e s t t o us i s the i o n c o n c e n t r a t i o n p r o f i l e o f the g e l / s o l u t i o n i n t e r f a c e . I d e a l l y we would l i k e to know the concentrations i n the e l e c t r i c a l double l a y e r , i n the gel/water i n t e r f a c i a l r e g i o n , and i n the s u b - i n t e r f a c i a l zone, perhaps t o 1 or 2 microns below the surface. Such measurements are d i f f i c u l t t o make by conventional techniques. One approach i s t o r a p i d l y freeze the gel/water i n t e r f a c e , f r a c t u r e i t , and perform an e l e c t r o n microprobe a n a l y s i s or energy d i s p e r s i v e a n a l y s i s of x-rays (EDAX) i n the SEM u s i n g a c o l d stage t o maintain the sample below -130°C t o avoid i c e c r y s t a l l i z a t i o n and consequent i o n segregation (51). U n f o r t u n a t e l y the s p a t i a l r e s o l u t i o n i s l i m i t e d to 0.1 t o 1.0 micron or l a r g e r , making a high r e s o l u t i o n i n t e r f a c i a l r e g i o n p r o f i l e very d i f f i c u l t . Microautoradiography of f r o z e n samples i s a l s o p o s s i b l e , but i s plagued by t e c h n i c a l d i f f i c u l t i e s . More s p e c u l a t i v e methods of measuring c o n c e n t r a t i o n p r o f i l e s w i l l be discussed later.


218

HYDROGELS FOR

MEDICAL AND


Probing the Outermost Zone There are very few methods w i t h which t o probe the outermost p a r t o f the g e l / s o l u t i o n i n t e r f a c e . We have a l r e a d y d i s c u s s e d the problems w i t h contact angle measurements. Most o p t i c a l spectroscopy methods, p a r t i c u l a r l y IR, probe a zone o f the order of microns i n depth (next s e c t i o n ) . Although e l l i p s o m e t r y can i d e a l l y measure a f i l m t h i c k n e s s of a few Angstroms (52), i t may be d i f f i c u l t t o apply t o g e l s because o f the lack o f s i g n i f i c a n t r e f r a c t i v e index d i f f e r e n c e s between the g e l and the surrounding solution. A v a r i e t y of techniques are a v a i l a b l e f o r d i r e c t l y examining the nature o f the s u r f a c e i t s e l f . Only a l i m i t e d number o f these can be d i s c u s s e d here - see Reference 53 f o r the o t h e r s . The two techniques most promisin are e l e c t r o n spectroscop copy f o r chemical a n a l y s i s ) and secondary i o n mass spectroscopy (SIMS). Both techniques i n v o l v e high vacuum environments. The sample must be f r o z e n i n l i q u i d n i t r o g e n , g e n e r a l l y at Τ < -130°C. Experience w i t h such methods on aqueous and b i o l o g i c a l samples i s very l i m i t e d at present. ESCA was developed l a r g e l y by the e f f o r t s of Siegbahn and h i s c o l l a b o r a t o r s i n Sweden (54). ESCA i s based on the p r e c i s e measurement of the k i n e t i c energy of e l e c t r o n s e j e c t e d from the sample by the a c t i o n o f i n c i d e n t r a d i a t i o n , u s u a l l y x-ray or UV. The b i n d i n g energy of the e l e c t r o n p r i o r t o e j e c t i o n (Eg) i s obtained from the measured k i n e t i c energy (E, ):

where E ^

i s the energy o f the monoenergetic e x c i t a t i o n r a d i a t i o n

(commonly Mg Κα, 1254 eV), and C i s an instrument constant, which i s r e a d i l y determined e x p e r i m e n t a l l y . The h i g h p r e c i s i o n o f the Ε]ς measurement allows one t o not only i d e n t i f y the elements present i n the s u r f a c e but a l s o t o i d e n t i f y t h e i r o x i d a t i o n s t a t e . The p h o t o e l e c t r o n s p e c t r a can be s h i f t e d by up t o 10 eV, depending on the o x i d a t i o n s t a t e of the element (54,55). Modern ESCA instruments sample an area o f the order o f sev e r a l mm . The volume sampled depends on the p h o t o e l e c t r o n escape depth f o r the sample. The escape depth i s o f the order o f 5 t o 15 A over the energy range o f i n t e r e s t f o r most metals (58). Data f o r polymers and low d e n s i t y s o l i d s are not r e a d i l y a v a i l a b l e , though escape depths o f the order of 50 t o 100 A are gen e r a l l y accepted (58). The sampling depth can be decreased by decreasing the angle the escaping e l e c t r o n s make w i t h the sample s u r f a c e . This procedure i s commonly c a l l e d the " g l a n c i n g angle" technique (59). A n o n d e s t r u c t i v e depth p r o f i l e over the 0-50 A range i n polymers can be obtained by i n t e n s i t y r a t i o s o f emitted e l e c t r o n s o f d i f f e r e n t k i n e t i c energies (58). 2


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The SIMS technique u t i l i z e s a focused i o n beam which i s r a s t e r e d or scanned across the s u r f a c e , s p u t t e r i n g o f f the outer 0 to 15 or so Angstrons of the surface and a n a l y z i n g the s p u t t e r ed ions by a very s e n s i t i v e mass spectrometer. A l l elements and t h e i r i n d i v i d u a l isotopes can be detected. An elemental o r i s o t o p i c image of the surface can be obtained w i t h b e t t e r than one micron r e s o l u t i o n . The i n t e r f a c e can be p r o g r e s s i v e l y i o n etched away and reanalyzed, p r o v i d i n g a compositional a n a l y s i s i n t o the sample w i t h about 100 A depth r e s o l u t i o n . SIMS i s i n r e a l i t y a d e s t r u c t i v e technique, as the i o n beam c o n t i n u a l l y s p u t t e r s away the outer surface. The SIMS method has been e x t e n s i v e l y a p p l i e d to i n o r g a n i c samples (56). Very l i m i t e d a p p l i c a t i o n to b i o l o g i c a l samples i s a l s o underway (57). The q u a l i t a t i v e i n t e r p r e t a t i o s t r a i g h t f o r w a r d . Dept however, i s s p e c u l a t i v e at t h i s time and r e q u i r e s a great deal of e m p i r i c a l c a l i b r a t i o n work before i t can be very u s e f u l . Probing the Subsurface Zone The most common method i s the use of m u l t i p l e i n t e r n a l r e f l e c t i o n i n f r a r e d spectroscopy (60). Some depth dependence i s a v a i l a b l e using d i f f e r e n t angles of i n c i d e n c e , though the d i s tance probed i s i n the micron range. C a r e f u l s t u d i e s have given monolayer s e n s i t i v i t i e s of known monolayers deposited d i r e c t l y on the i n t e r n a l r e f l e c t i o n elements ( I R E s ) . The recent development o f F o u r i e r transform i n f r a r e d spectroscopy (FT-IR) (61) overcomes many of the i n t e n s i t y and s e n s i t i v i t y l i m i t a t i o n s of conventional d i s p e r s i v e i n f r a r e d spectroscopy, p a r t i c u l a r l y f o r aqueous s o l u t i o n s (62,65). FT-IR can a l s o be used i n the m u l t i p l e i n t e r n a l r e f l e c t i o n mode (64). To our knowledge FT-IR has not yet been a p p l i e d t o g e l / s o l u t i o n i n t e r face s t u d i e s , however. Raman spectroscopy should be very u s e f u l i n c h a r a c t e r i z i n g g e l / s o l u t i o n i n t e r f a c e s d i r e c t l y , as water i s an i d e a l solvent f o r Raman s t u d i e s (65). Elemental a n a l y s i s of the subsurface zone can be accomplished by energy d i s p e r s i v e a n a l y s i s of x-rays i n the SEM, as prev i o u s l y mentioned, or by the more accurate wavelength d i s p e r s i v e a n a l y s i s of most e l e c t r o n microprobes (51). Subsurface penetrat i o n i s i n the micron r e g i o n and can be adjusted somewhat by v a r y i n g the i n c i d e n t e l e c t r o n beam energy. Again the sample must be r a p i d l y frozen to l i q u i d n i t r o g e n temperatures and maintained below -130°C f o r a n a l y s i s (51). f

Further

Discussion

Other techniques are a v a i l a b l e f o r study of the gel/water or g e l / s o l u t i o n i n t e r f a c e . Most s o l i d surface c h a r a c t e r i z a t i o n


220


techniques (53) can, i n p r i n c i p l e , be a p p l i e d t o the frozen g e l surface. P r a c t i c a l l y a l l o f the microscopic and h i s t o l o g i c techniques used f o r the study o f c e l l surfaces can a l s o be app l i e d , i n c l u d i n g s e l e c t i v e f i x a t i v e s , s t a i n s , e t c . ( 7 ) . Some o f the methods used i n the study o f l i q u i d / l i q u i d i n t e r f a c e s (17,66) can be a p p l i e d t o g e l i n t e r f a c e s . We have a l s o ignored many techniques which are g e n e r a l l y a p p l i c a b l e only f o r high surface area systems. We have d i s c u s s e d p r i m a r i l y those techniques with which we are f a m i l i a r o r which we are c o n s i d e r i n g t o apply t o the study o f gel/solution interfaces. Conclusions A v a r i e t y o f method solution interfaces. D i r e c t i n s i t u methods i n c l u d e : 1. rhéologie o r v i s c o m e t r i c a n a l y s i s 2. ellipsometry 3. contact angles 4. e l e c t r o k i n e t i c s 5. i n f r a r e d spectroscopy 6. Raman spectroscopy 7. o p t i c a l microscopy Dry g e l / a i r i n t e r f a c e methods i n c l u d e : 1. i n f r a r e d spectroscopy 2. scanning, t r a n s m i s s i o n , and o p t i c a l microscopy 3. surface p o t e n t i a l 4. contact angles 5. ESCA 6. SIMS Frozen g e l surfaces can be studied by: 1. scanning, t r a n s m i s s i o n , or o p t i c a l microscopy 2. e l e c t r o n microprobe o r EDAX 3. ESCA 4. SIMS and many o f the other methods. These methods permit one t o probe the g e l / s o l u t i o n i n t e r f a c e w i t h respect t o : 1. i n t e r f a c e energetics 2. i n t e r f a c e p o t e n t i a l s 3. i n t e r f a c e chemical groups and o r i e n t a t i o n s 4. i n t e r f a c e s t r u c t u r e o r morphology 5. i n t e r f a c e elemental composition The subsurface zone can be analyzed f o r : 1. i n t e r f a c e chemical groups and o r i e n t a t i o n s 2. i n t e r f a c e elemental composition In a d d i t i o n , s o l u b l e g e l molecules can be studied as adsorbed f i l m s at s o l i d / l i q u i d i n t e r f a c e s or l i q u i d / a i r i n t e r f a c e s


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Acknowledgements S t i m u l a t i n g d i s c u s s i o n s w i t h S. W. Kim, M. S. Jhon, Y. K. Sung and D. L. Coleman have been most h e l p f u l . The personnel of ETEC, I n c . , Hayward, C a l i f o r n i a were most generous w i t h B i o SEM time w i t h which our f i r s t f r e e z e - e t c h s t u d i e s were done. The t e c h n i c a l a s s i s t a n c e o f R. Middaugh, G. Iwamoto, and D. Kramer i s appreciated. This work has been supported by NIH Grant HL 16921-01.

Abstract The experimental characterization of gel/water interfaces is briefly discussed. Interfacial characteristics discussed in clude topography/morphology interfacial tensio fre interface potential, adsorption nature of the gel surface, and the nature of the subsurface region. Techniques briefly discussed include microscopy, con tact angle methods, electrokinetic methods, surface potentials, infrared spectroscopy - including Fourier transform and Raman, x-ray photoelectron spectroscopy (ESCA), and secondary ion mass analysis (SIMS). Most of these techniques are discussed in suggestive and speculative terms as so few have been applied to the gel/water interface. A variety of techniques are available for studying the gel/water interface either in situ or in the frozen state. Literature Cited 1. Andrade, J. D., Lee, H. B., Jhon, M. S., Kim, S. W., and Hibbs, Jr., J. Β., Trans. Amer. Soc. Artif. Internal Organ., (1973) 19, 1. 2. Silberberg, Α., Polymer Preprints, (1970) 11, 1289. 3. Silberberg, Α., This Symposium. 4. Whitehouse, D. J., in P. F. Kane and G. B. Larrabee, eds., "Characterization of Solid Surfaces," Plenum, 1974. 5. McCrone, S. C., in P. F. Kane and G. B. Larrabee, eds., "Characterization of Solid Surfaces," Plenum, 1974. 6. Lee, Η. Β., Shim, H. S. and Andrade, J. D., Polymer Pre prints, (1972) 13, 729. 7. Koehler, J. D., ed., "Advanced Techniques in Biological Electron Microscopy," Springer-Verlag, 1973. 8. "Freeze Etch Replication," Ε. M. Ventions, Rockville, Mary land. 9. Cluthe, C. Ε., "Microscopical Study of the Structure of Radiation Cross-linked Aqueous Poly-(ethylene oxide) Gels and Frozen Cryoprotective Agents," Ph.D. Thesis, Cornell University, May 1972. 10. Blank, Z. and Reimschuessel, A. C., J. Materials Science, (1974) 9, 1815.


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11. Jhon, M. S. and Andrade, J. D., J. Biomed. Materials Res., (1973) 7, 509. 12. Belavtseva, Y. M., Titova, Y. F., Braudo, Y. Y. and Tolstoguzov, V. B., Biophysics, (1974) 19, #1, 15. (English translation of Biofizika: 19: No. 1 (1974) 19 13. Bohoneck, J., Coll. and Polymer Sci., (1974) 252, 417. 14. Geymayer, W. F., J. Polymer Sci. Symposium, (1974) 44, 25. 15. Matas, B. R., Spencer, W. H., and Hayes, T. L., Arch. Ophthal., (1972) 88, 287. 16. Johari, O. and Samudra, Α. V., in P. F. Kane and G. B. Larrabee, eds., "Characterization of Solid Surfaces," Plenum, 1974. 17. Adamson, A. W., "Physical Chemistry of Surfaces," 2nd ed., Wiley, 1967. 18. Bikerman, J. J., "Physical Surfaces," Academi Press Ne York, 1970. 19. Neumann, A. W., Adv. Coll. Interface Sci., (1974) 4, 105. 20. Wu, S., J. Macromol. Sci.-Revs. Macromol. Chem., (1974) C10, 1, 1. 21. Bikerman, J. J., Proc. 2nd Intern. Congress on Surface Activity III, (1957) 125. 22. Johnson, R. Ε., J. Phys. Chem., (1959) 63, 1655. 23. Lester, G. R., J. Coll. Sci., (1961) 16, 315. 24. Hansen, R. J. and Toong, Τ. Υ., J. Coll. Interf. Sci., (1971) 37, 196. 25. Schwartz, Α. Μ., Adv. in Coll. and Interf. Sci., (1975) 4, 349. 26. Timmons, C. O. and Zisman, W. Α., J. Coll. Interf. Sci., (1966) 22, 165. 27. Braudo, Υ. Υ., Michailow, Ε. N. and Tolstoguzov, V. Β., Phys. Chemie, Leipzig, (1973) 253, 369. 28. Holly, F. J. and Refojo, M. F., J. Biomed. Materials Res., (1975) 9, 315-326. 29. Davies, J. T. and Rideal, Ε. Κ., "Interfacial Phenomena," 2nd ed. Academic Press, 1963. 30. Shaw, D. J., "Electrophoresis" Academic Press, 1969. 31. Nordt, F. Knox, R. J., and Seaman, G. V. F., This Symposium. 32. Ma, S. M., Gregonis, D. Ε., Van Wagenen, R. and Andrade, J. D., This Symposium. 33. Brooks, D. E., J. Coll. Interf. Sci., (1973) 43 687. 34. Kavanagh, Α. Μ., Posner, Α. Μ., and Quirk, J. P., Faraday Disc., No. 59, (1975). 35. Van Wagenen, R., Andrade, J. D. and Hibbs, Jr., J. Β., submitted to the J. Electrochem. Soc. 36. Instruction Manual for Model 162 Electrostatic Millivolt Meter, Monroe Electronics, Inc., Middleport, New York. 37. Gaines, G. L., "Insoluble Monolayers at Liquid-Gas Inter faces," Interscience, 1966. 38. Beredick, Ν., In. B. Ke, ed., "Newer Methods of Polymer Characterization," Wiley, 1964.


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39. Johansson, G., Biochem. Biophys. Acta, (1970) 221, 387. 40. Holly, F. J. and Refojo, M. F., This Symposium. 41. Horbett, T. A. and Hoffman, A. S., Adv. in Chem. Series #145, "Applied Chem. at Protein Interfaces," R. E. Baier, ed., ACS 1975, 230. 42. Kim, S. W., unpublished work. 43. Lipatov, Yu, S. and Sergeeva, L. M., "Adsorption of Polymers," J. Wiley and Sons, 1974. (Translated from Russian by R. Kondor). 44. Bagchi, P., J. Coll. Interfac. Sci., (1974) 47, 86. 45. Hesselink, F. Th., Vrij, A. and Overbeek, J. Th. G., J. Phys. Chem., (1971) 75, 2094. 46. Botham, R. and Thies, C., J. Coll. Interf. Sci., (1969) 31, 1. 47. McGregor, R. and Ezuddin Κ Η. J Appl Polyme Sci. (1974) 18, 629. 48. Kalasz, J . , Nagy, J., and Knoll, J . , J. Anal. Chem., (1974) 272, 22. 49. Marsden, Ν. V. B., Naturwissenschaften, (1973) 60, 257. 50. Adamcova, Ζ., Coll. Czech. Chem. Comm., (1968) 33, 336. 51. Hall, T., Echlin, P., and Kaufmann, R., eds., "Microprobe Analysis as Applied to Cells and Tissues," Academic Press, 1974. 52. Muller, Η., Adv. Electrochem. and Electrochem. Engr., (1973) 9, 167. 53. Kane, P. F. and Larrabee, G. Β., eds., "Characterization of Solid Surfaces," Plenum, 1974. 54. Siegbahn, Κ., et.al., "ESCA--Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy," Almquist and Wiksells, Publ., Uppsala, Sweden, 1967. 55. Dekeyser, W., Fiermans, L., Vanderkelen, G., and Vennik, J . , "Electron Emmission Spectroscopy," D. Reidel Publ. Co., 1973. 56. Werner, H. W., Surface Science, (1975) 47, 201. 57. Galle, P., in Reference 51. 58. Clark, D. T., Feast, W. J., Musgrove, W. K. R., Ritchie, I., J. Polymer Sci., Polymer Chem., (1975) 13, 857. 59. Fadley, C. S., Baird, R. J., Siekhaus, W., Novakov, T., and Bergstrom, S. A. L., J. Electron Spect. Related Phen., (1974) 4, 93. 60. Harrick, N. J., "Internal Reflection Spectroscopy," Interscience, 1967. 61. Koenig, J. L. and Tabb, D. L., Canad. Res. and Dev., (SeptOct, 1974) 25. 62. Low, M. J. D. and Yang, R. T., Spectrochem. Acta, (1973) 29A, 1761. 63. Low, M. J. D. and Yang, R. T., Spectroscopy Letters, (1973) 6, 299. 64. Yang, R. T., Low, M. J. D., Haller, G. L. and Fenn, J . , J. Coll. Interf. Sci., (1973) 44, 249.


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65. Buechler, E. and Turkevich, J., J. Phys. Chem. (1972) 76, 2325. 66. Albertsson, P. Α., "Partition of Cell Particles and Macromolecules," 2nd ed., Wiley, 1971. 67. Dobreva, Μ., Dancheva, N. and Holt, P. F., Brit. J. Ind. Med., (1975) 32, 224.


17 Elimination of Electroosmotic Flow in Analytical Particle Electrophoresis F. J. NORDT, R. J. KNOX, and G. V. F. SEAMAN Department of Neurology, University of Oregon Health Sciences Center, Portland, Oreg. 97201

I n t e r e s t i n surface coatings which w i l l markedly reduce or e l i m i n a t e the z e t a p o t e n t i a p r a c t i c a l i s s u e of e l i m i n a t i n p h o r e s i s . I n a closed c y l i n d r i c a l g l a s s e l e c t r o p h o r e s i s chamber c o n t a i n i n g an e l e c t r o l y t e the negative charge a t the g l a s s w a l l r e s u l t s i n an i n c r e a s e i n c o n c e n t r a t i o n of c a t i o n s c l o s e t o t h i s s u r f a c e . A p p l i c a t i o n of an e x t e r n a l e l e c t r i c a l f i e l d r e s u l t s i n movement of f l u i d near the w a l l (electroosmosis) toward the cathode and a concurrent f o r c e d r e t u r n flow through the center of the tube. I t can be shown from hydrodynamics that there i s a c y l i n d r i c a l envelope ( s t a t i o n a r y l e v e l ) i n the chamber where no net f l o w of f l u i d occurs during e l e c t r o p h o r e s i s . F i g u r e 1 i l l u s t r a t e s the general f e a t u r e s of laminar e l e c t r o o s m o t i c f l u i d f l o w f o r a c l o s e d c y l i n d r i c a l tube i n c l u d i n g the p a r a b o l i c f l u i d f l o w p r o f i l e , regions of e l e c t r o o s m o t i c f l o w , r e t u r n f l u i d flow and l o c a t i o n of the s t a t i o n a r y l e v e l . In a n a l y t i c a l p a r t i c l e e l e c t r o p h o r e s i s true e l e c t r o p h o r e t i c v e l o c i t i e s of p a r t i c l e s may be measured a t the s t a t i o n a r y l e v e l w h i l e v e l o c i t i e s determined elsewhere i n the chamber w i l l be comprised of c o n t r i b u t i o n s from both e l e c t r o p h o r e s i s and e l e c t r o osmosis. I n p r e p a r a t i v e a p p l i c a t i o n s of e l e c t r o p h o r e s i s the boundary of a concentrated suspension (sample) becomes p a r a b o l o i d a l i n contour as a r e s u l t of electroosmosis of the suspending medium i n the chamber. The non-planar sample d i s t r i b u t i o n introduces d i f f i c u l t i e s i n separating or r e s o l v i n g p a r t i c l e populations which d i f f e r i n e l e c t r o p h o r e t i c m o b i l i t y . I n analyt i c a l p a r t i c l e e l e c t r o p h o r e s i s the presence of e l e c t r o o s m o t i c f l o w r e q u i r e s that measurements be c a r r i e d out a t the s t a t i o n a r y l e v e l . Since t h i s l e v e l i s i n f i n i t e l y t h i n e l e c t r o o s m o t i c f l o w w i l l always c o n t r i b u t e t o experimental e r r o r . The w a l l charge i n e l e c t r o p h o r e s i s chambers a r i s e s from e i t h e r the i o n i z a t i o n of surface charge groups or as a consequence of r e d i s t r i b u t i o n of ions from the suspending medium (adsorption or d e s o r p t i o n ) . The w a l l charge may be reduced o r e l i m i n a t e d by: a) use of adherent or adhesive f i l m s ( 1 ) . 225



Figure 1. Electroosmotic fluid flow in a closed cylindrical tube, (a) A longitudinal crossectional view of the fluid velocity profile and fluid streamlines for a tube with radius, R (fluid velocity is plotted in terms of V , the fluid flow at the tube wall), (b) A transverse crossection where maximum electroosmotic fluid flow is shown by dense shading, and the unshaded area shows the region of the stationary level where fluid flow tends to zero, (c) Fluid flow parabola which is a plot of fluid velocity vs. distance from the tube wall. s


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b) covalent bonding m a t e r i a l s ( 2 ) . c) p h y s i c a l adsorption of substances ( 3 ) . E l e c t r o p h o r e t i c t e s t i n g of the s t a b i l i t y and completeness of d i f f e r e n t surface treatments or coatings may be c a r r i e d out i n v a r i o u s ways: i ) A f t e r coating the i n s i d e of the e l e c t r o p h o r e s i s chamber the electroosmotic f l o w i s c a l c u l a t e d from experimental measurements of the e l e c t r o p h o r e t i c v e l o c i t i e s of stand ard p a r t i c l e s a t v a r i o u s d i s t a n c e s from the tube w a l l , i i ) The e l e c t r o p h o r e t i c m o b i l i t i e s of coated or modified p a r t i c l e s made from the same m a t e r i a l s as the e l e c t r o p h o r e s i s chamber are measured by standard a n a l y t i c a l electrophoresis. i i i ) The zeta p o t e n t i a l of coated tubes may be a l s o determined from e l e c t r o o s m o t i ments. The f i r s t approach i s the more d e s i r a b l e t e s t of any coating procedure but f o r screening purposes the second t e s t approach w i l l s i g n i f i c a n t l y reduce the time needed f o r the i n i t i a l survey or t e s t i n g of coatings or m o d i f i c a t i o n procedures. Theoretical The e l e c t r o p h o r e t i c m o b i l i t y , u, i s defined as the e l e c t r o p h o r e t i c v e l o c i t y , v, of a p a r t i c l e per u n i t f i e l d s t r e n g t h , χ: ν

/.\

The r e l a t i o n s h i p between e l e c t r o p h o r e t i c m o b i l i t y , u, and zeta p o t e n t i a l , ζ, f o r nonconducting p a r t i c l e s whose r a d i u s of curvature, a, i s l a r g e i n comparison to the e f f e c t i v e thickness of the e l e c t r i c a l double l a y e r , l/κ, i s u s u a l l y described a c c u r a t e l y f o r Ka > 300 by the Helmholtz-Smoluchowski equation (4):

where ε and η are the d i e l e c t r i c constant and v i s c o s i t y , respec t i v e l y , w i t h i n the e l e c t r i c a l double l a y e r which are assumed t o be the same as the bulk v a l u e s of the suspending medium. E x p e r i mental measurements of ν r e s u l t i n an observed e l e c t r o p h o r e t i c v e l o c i t y , v , which i s subject t o e r r o r as i s the experimentally derived e l e c t r o p h o r e t i c m o b i l i t y , u . A p p l i c a t i o n of an e l e c t r i c f i e l d t o a suspension of charged p a r t i c l e s contained i n a closed c y l i n d r i c a l chamber w i t h a charged w a l l r e s u l t s i n e l e c t r o p h o r e s i s of the p a r t i c l e s and electroosmotic flow of the suspending medium. The observed v e l o c i t y , v , of a p a r t i c l e i n the tube i s thus the sum of i t s e l e c t r o p h o r e t i c v e l o c i t y , v , and the v e l o c i t y of the suspending e

e

Q

e


228

HYDROGELS FOR

MEDICAL AND RELATED APPLICATIONS

medium, v : w

v

=

o

v

e

+

v

w

I t has been shown by Bangham et a l . (5) that the observed v e l o c i t y of the p a r t i c l e i s r e l a t e d to i t s d i s t a n c e , r , from the a x i s of the tube by the expression: v

o = v + v [|j£ " H

(iv)

s

e

where v i s the f l u i d v e l o c i t y adjacent t o the tube w a l l and R i s the r a d i u s of the tube. S o l u t i o n of the flow equation ( i v ) shows that at a d i s t a n c e r = 0.707R from the a x i s there i s no net f l o w of f l u i d , i . e . , a s t a t i o n a r In theory e l e c t r o p h o r e t i made at the s t a t i o n a r y l e v e l are not subject to e r r o r as a r e s u l t of f l u i d flow. However, i n p r a c t i c e e r r o r s a r i s e as a r e s u l t o f : a) the f i n i t e s i z e of the p a r t i c l e s which cannot be c o n t a i n ed i n an i n f i n i t e l y t h i n s t a t i o n a r y l e v e l ; b) f o c u s i n g e r r o r s a t the s t a t i o n a r y l e v e l (requirement f o r appropriate o p t i c a l c o r r e c t i o n s to r e c t i f y the e f f e c t s of r e f r a c t i o n and a b e r r a t i o n , depth of focus, and shape and s i z e of f o c a l f i e l d r e l a t i v e t o the r a d i u s of curvature of the s t a t i o n a r y l e v e l ) ; c) heterogeneous d i s t r i b u t i o n of charge on the tube w a l l r e s u l t i n g i n a s h i f t i n l o c a t i o n of the s t a t i o n a r y l e v e l ; d) Brownian motion and sedimentation of the p a r t i c l e ; and e) thermal convection a r i s i n g from J o u l e h e a t i n g . The magnitude of e r r o r s i n the e l e c t r o p h o r e t i c v e l o c i t y as a r e s u l t of f l u i d flow may be estimated from a d i f f e r e n t i a t e d form of equation ( i v ) : s

dv

0

= *g£

dr

(v)

D i v i s i o n of equation (v) by the f i e l d g r a d i e n t , χ, gives an expression f o r the change i n the experimentally observed e l e c t r o p h o r e t i c m o b i l i t y at s m a l l r a d i a l increments from the s t a t i o n a r y l e v e l i n terms of the e l e c t r o o s m o t i c m o b i l i t y , u : g

Au

4u r . = — f - Ar 2 q

ο n

I f the f r a c t i o n a l e r r o r i n u as 6, then: δ =

, .v (vi)

R

e

due to f l u i d flow i s defined

Au 4u r = Ar u^ R u~

—0

s

9

z


.. (vu)

17.

NORDT E T A L .

Analytical

Particle

Electrophoresis

229

From e x p r e s s i o n ( v i i ) one may c a l c u l a t e the maximum v a l u e of u which w i l l produce f r a c t i o n a l e r r o r s of l e s s than δ i n u at d i s t a n c e s up t o Ar from the s t a t i o n a r y l e v e l . s

e

M a t e r i a l s and Methods Corning #7740 b o r o s i l i c a t e p a r t i c l e s were used as a model system f o r screening the e f f e c t i v e n e s s of p o l y s a c c h a r i d e d e r i v a t i v e s as low z e t a p o t e n t i a l s u r f a c e coatings f o r g l a s s . A l l chemicals were reagent grade and the water was d i s t i l l e d t w i c e i n pyrex ware. The p a r t i c l e s were prepared by g r i n d i n g Corning #7740 g l a s s tubing w i t h water i n an aluminum oxide b a l l m i l l f o r 16 hours. P a r t i c l e s of s u i t a b l e s i z e f o r e l e c t r o p h o r e t i c measurements were obtained by repeated sedimentatio c l e s having a sedimentatio P a r t i c l e s thus obtained were

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