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Metals for biomedical devices
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Related titles: Joint replacement technology: new developments (ISBN 978-1-84569-245-2) Increasing numbers of joint revision and replacement operations drive the demand for improved prostheses. This book comprehensively reviews developments in joint replacement technology, covering the most pertinent materials science and engineering issues, specific joints, clinical trial results and sterilisation techniques. Part I discusses biomechanics, tribology, the chemical environment of the body and an overview of materials and engineering of joint replacement. The second part reviews specific materials, bearing surfaces and bone cements, in addition to the failure mechanisms and lifetime prediction of joints. Part III discusses the biological environment and interaction of replacement joints. Specific joints are analysed in detail in Part IV. Shape memory alloys for biomedical applications (ISBN 978-1-84569-344-2) Shape memory metals are suitable for a wide range of biomedical devices including applications in dentistry, bone repair, urology and cardiology. This book provides a comprehensive review of shape memory metals and devices for medical applications. The first part of the book discusses the materials, primarily Ti–Ni based alloys. Chapters cover the mechanical properties, thermodynamics, composition, fabrication of parts, chemical reactivity, surface modification and biocompatibility. Medical and dental devices using shape memory metals are reviewed in the following section: chapters cover stents, orthodontic devices and endodontic instruments. Finally, future developments in this area are discussed, including alternatives to Ti–Ni based shape memory alloys. Dental biomaterials: imaging, testing and modelling (ISBN 978-1-84569-296-4) Dental biomaterials: imaging, testing and modelling focuses on the techniques required to undertake research in dental biomaterials. The text forms an instructive and practical review of the scientific methods applied to dental biomaterials, with appropriate case studies. The book includes chapters discussing the practicalities of working on dental biomaterials, such as reviewing the mechanisms of cutting tooth tissue and methods for characterising dental hand piece performance. Chapters review optical and electron imaging techniques for biomaterial interfaces. Specific materials, applications and experimental techniques are discussed in addition to the development and application of computer models to this complex area. Details of these and other Woodhead Publishing materials books can be obtained by: ∑ visiting our web site at www.woodheadpublishing.com ∑ contacting Customer Services (e-mail:
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Metals for biomedical devices Edited by Mitsuo Niinomi
CRC Press Boca Raton Boston New York Washington, DC
Woodhead
publishing limited
Oxford Cambridge New Delhi
© Woodhead Publishing Limited, 2010
iv Published by Woodhead Publishing Limited, Abington Hall, Granta Park, Great Abington, Cambridge CB21 6AH, UK www.woodheadpublishing.com Woodhead Publishing India Private Limited, G-2, Vardaan House, 7/28 Ansari Road, Daryaganj, New Delhi – 110002, India www.woodheadpublishingindia.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2010, Woodhead Publishing Limited and CRC Press LLC © Woodhead Publishing Limited, 2010 The authors have asserted their moral rights. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN 978-1-84569-434-0 (book) Woodhead Publishing ISBN 978-1-84569-924-6 (e-book) CRC Press ISBN 978-1-4398-3107-6 CRC Press order number: N10188 The publishers’ policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acidfree and elemental chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Replika Press Pvt Ltd, India Printed by TJ International Limited, Padstow, Cornwall, UK
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Contents
Contributor contact details
xi
Introduction
xv
Part I General introduction 1
Overview of metals and applications
T. Hanawa, Tokyo Medical and Dental University, Japan
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Introduction General properties required for metals in medical devices Stainless steels Cobalt-chromium-based alloys Titanium-based alloys Shape memory and superelastic alloys Noble metals and alloys Other metals References
3 4 8 12 13 18 20 21 22
2
Material selection
25
Y. Okazaki, National Institute of Advanced Industrial Science and Technology, Japan
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Introduction Standardised implantable metals Biocompatibility of various metals Highly biocompatible a+b-type Ti alloy Stability of passive film formed on metals Metal ion release Evaluation of biological properties Fatigue assessment Orthopaedic implant device failure: Analysis of adverse clinical cases
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25 25 28 29 37 40 46 51 53
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Contents
2.10 2.11 2.12
Performance evaluation for orthopaedic devices Future trends References
55 60 62
Part II Mechanical behaviour, degradation and testing of metals for biomedical devices 3
Mechanical properties of metallic biomaterials
T. Nakano, Osaka University, Japan
3.1 3.2 3.3 3.4 3.5
3.8 3.9
Introduction Requirements for mechanical functions in vivo Methods for strengthening metallic biomaterials Phase rule and phase diagram Deformation and recovery, recrystallisation, and grain ripening Microstructure and related mechanical properties in typical metallic biomaterials Development of metallic biomaterials based on biological bone tissues Summary References
93 96 96
4
Corrosion of metallic biomaterials
99
S. Hiromoto, National Institute for Materials Science, Japan
4.1 4.2 4.3 4.4 4.5 4.6
Importance of corrosion Principles of corrosion Corrosion morphology Evaluation methods of corrosion behaviour Biological environments References
99 100 105 108 115 119
5
Fatigue failure of metallic biomaterials
122
M. Niinomi, Tohoku University, Japan
5.1 5.2 5.3 5.4 5.5 5.6 5.7
Introduction Fatigue strength Fatigue crack propagation Fretting fatigue strength in air and in vitro Fatigue strength of wire Summary References
3.6 3.7
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71 71 72 85 87 91 91
122 123 141 147 150 153 153
Contents
6
Mechanical testing of metallic biomaterials
N. Maruyama, National Institute for Materials Science, Japan
6.1 6.2 6.3 6.4 6.5 6.6
Fracture of metal implants and test methods Living body environment Tensile strength of metallic materials Fatigue and fretting fatigue of metallic materials Effect of corrosion on fatigue and fretting fatigue Corrosion fatigue and fretting corrosion fatigue tests in a simulated body environment Results of fatigue and fretting fatigue tests on metallic biomaterials New fatigue tests for metallic biomaterials Acknowledgements References
6.7 6.8 6.9 6.10
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157 157 159 160 161 166 166 170 175 176 177
7
Tribology and tribo-corrosion testing and analysis of metallic biomaterials
Y. Yan, University of Leeds, UK
7.1 7.2 7.3 7.4 7.5 7.6
Introduction to tribology-related testing General testing methods for tribological properties Tribo-corrosion testing Surface analysis for tribology and tribo-corrosion properties Future trends References
178 179 184 192 198 199
8
Biocompatibility and fabrication of in situ bioceramic coating/titanium alloy biocomposites
202
C. Cui, Hebei University of Technology, China
8.1 8.2 8.3 8.4
Introduction 202 Titanium and its alloys 203 Biomedical applications and development of Ti and its alloys 204 Biocompatibility and fabrication of in situ synthesized bioceramic coatings on Ti alloys 211 Acknowledgements 230 References 230
8.5 8.6
178
Part III Processing metals for biomedical applications 9
Forging of metals and alloys for biomedical applications
M. Chandrasekaran, Bio-scaffold International Pte Ltd, Singapore
9.1
Introduction
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Contents
9.2
9.8 9.9
Fundamentals of forging and typical forging process applied to metals and alloys for biomedical applications Properties for forgeability Microstructural development and its consequences on properties Forging of metals and alloys for biomedical applications Die materials and die design for forging Powder metallurgy forging of metals and alloys for biomedical applications Summary Sources of further information and advice
247 248 248
10
Surface treatment
251
R. Thull, University of Wuerzburg, Germany
10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9
Introduction Surface structuring Physical modifications Strength of modifications Interface modulation and biocompatibility Future developments and optimizations Summary Sources of further information and advice References
251 252 254 256 257 257 258 258 259 260
9.3 9.4 9.5 9.6 9.7
236 239 240 241 246
11
Coatings for metallic biomaterials
T. Kasuga, Nagoya Institute of Technology, Japan
11.1 11.2 11.3 11.4 11.5 11.6
Introduction Calcium phosphate ceramic coatings Calcium phosphate glass-ceramic coatings Bioactive surface prepared by chemical treatments Summary References
260 261 267 272 278 279 283
12
Biocompatible polymer assembly on metal surfaces
K. Ishihara and J. Choi, The University of Tokyo, Japan
12.1 12.2
Introduction Phospholipid polymers providing biocompatible surfaces on metals Surface grafting of 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer on titanium alloy MPC polymer assembly on Ti alloy Future trends
12.3 12.4 12.5
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283 284 288 291 298
Contents
12.6 12.7 12.8
Summary Acknowledgement References
299 300 300 303
13
Sterilisation and cleaning of metallic biomaterials
S. Lerouge, Ecole de Technologie Supérieure, Canada
13.1 13.2 13.3
Introduction Concepts and definitions Principal sterilisation methods for biomaterials; advantages and limitations Alternative sterilisation methods New challenges for sterilisation Cleaning Standards and other sources of further information Summary References
13.4 13.5 13.6 13.7 13.8 13.9
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303 304 307 316 319 321 322 322 323
Part IV Specific applications of metals for biomedical devices 14
Orthopaedic applications of metallic biomaterials
T. Matsushita, Chubu University, Japan
14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8
Introduction Total hip replacement Total knee replacement Miscellaneous joint replacement Implants for bone fractures Failure of orthopaedic implants Summary References
329 331 341 345 347 349 351 351 355
15
New-generation metallic biomaterials
T. Narushima, Tohoku University, Japan
15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8
Introduction Brief overview of traditional metallic biomaterials Newer alloys as metallic biomaterials Novel processing technologies for metallic biomaterials Other metallic biomaterials Future trends Sources of further information and advice References
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355 356 361 366 371 372 374 374
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Contents
16
Degradable metallic biomaterials for cardiovascular applications
H. Hermawan, D. Dubé and D. Mantovani, Laval University, Canada
16.1 16.2 16.3
379 381
16.5
Introduction Clinical needs for using degradable metallic biomaterials Studies on degradable metallic biomaterials for cardiovascular applications Lessons from the first ten years of investigation in degradable metallic biomaterials References
Index
405
16.4
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382 395 400
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Contributor contact details
(* = main contact)
Editor and Chapter 5
Chapter 2
Professor Mitsuo Niinomi Department of Biomaterials Science Institute for Materials Research Tohoku University 2-1-1, Katahira Aoba-ku, Sendai 980-8577 Japan
Dr Yoshimitsu Okazaki Advanced Biomaterial Group Institute for Human Science and Biomedical Engineering National Institute of Advanced Industrial Science and Technology 1-2-1 Namiki Tsukuba Ibaraki 305-8564 Japan
Email:
[email protected] Chapter 1
Email:
[email protected] Professor Takao Hanawa Department of Metals Institute of Biomaterials and Bioengineering Tokyo Medical and Dental University 2-3-10 Kanda-surugadai Chiyoda-ku Tokyo 101-0062 Japan Email:
[email protected] Chapter 3 Professor Takayoshi Nakano Course of Materials Science & Engineering Division of Materials & Manufacturing Science Graduate School of Engineering Osaka University 2-1, Yamada-Oka Suita, Osaka 565-0871 Japan Email:
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Contributor contact details
Chapter 4
Chapter 8
Dr Sachiko Hiromoto Biomaterials Center National Institute for Materials Science 1-1 Namiki Tsukuba Ibaraki 305-0044 Japan
Professor Chunxiang Cui School of Materials Science and Technology Hebei University of Technology No. 8, Road No. 1 Dingzigu Hongqiao District Tianjin 300130 China
Email:
[email protected] Email:
[email protected] Chapter 6 Dr Norio Maruyama Biomaterials Center National Institute for Materials Science 1-1 Namiki Tsukuba Ibaraki 305-0044 Japan Email:
[email protected] Chapter 9 Dr Margam Chandrasekaran Chief Scientist Bio-scaffold International Pte Ltd 61, Science Park Road THE GALEN #02-01 Science Park III Singapore 117525 Email:
[email protected] Chapter 7 Dr Yu Yan Institute of Thermofluids, Surfaces and Interfaces (iETSI) School of Mechanical Engineering University of Leeds Leeds LS2 9JT UK Email:
[email protected] Chapter 10 Prof. Dr-Ing. Roger Thull Em. Head of the Chair of Functional Materials for Medicine and Dentistry University of Würzburg Pleicherwall 2 D-97070 Würzburg Germany Email:
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Contributor contact details
Chapter 11
Chapter 14
Professor Toshihiro Kasuga Department of Frontier Materials Graduate School of Engineering Nagoya Institute of Technology Gokiso-cho Showa-ku Nagoya 466-8555 Japan
Professor Tomiharu Matsushita College of Life and Health Sciences Chubu University 1200 Matsumoto-cho Kasugai, Aichi 487-8501 Japan
Email:
[email protected] xiii
Email:
[email protected] Chapter 15
Chapter 12 Dr Kazuhiko Ishihara* and Jiyeon Choi Department of Materials Engineering School of Engineering and Center for NanoBio Integration (CNBI) The University of Tokyo 7-3-1, Hongo, Bunkyo-ku Tokyo 113-8656 Japan Email:
[email protected] Chapter 13 Dr Sophie Lerouge Department of Mechanical Engineering Ecole de Technologie Supérieure (ÉTS) 1100 Notre-Dame West Montreal Quebec, H3C 1K3 Canada Email:
[email protected] Adjunct Professor Department of Radiology University of Montreal Canada
Professor Takayuki Narushima Department of Materials Processing Tohoku University 6-6-02 Aza Aoba Aramaki, Aoba-ku Sendai 980-8579 Japan E-mail:
[email protected] Chapter 16 Professor Diego Mantovani Laboratory for Biomaterials and Bioengineering Department of Mining, Metallurgy and Materials Engineering & Hospital Research Center Pavillon Adrien-Pouliot, room 1745-E Laval University 1065 Ave de la Médecine Québec City QC, G1V 0A6 Canada Email:
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© Woodhead Publishing Limited, 2010
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Introduction
Metallic materials are used as structural materials for medical devices in the fields of orthopedic surgery, blood circulatory system and dentistry. Approximately 70% of the structural materials employed in implants are found to be metallic materials. The elderly population (over 65 years old) is rapidly increasing all over the world, as shown in Fig. 0.1 (Yanno Tsunetarou Memorial Foundation, 2009). The elderly population (over 60 years old) over the world is increasing by approximately 2–3% per year, although the total population over the world is decreasing, as shown in Fig. 0.2 (Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, 2009). In some countries such as Japan, Italy, and Germany, the ratio of the elderly population to the total population 35.0 Japan
Percentage of elderly population
30.0
France Italy
25.0 Sweden 20.0 15.0 10.0 5.0 0.0
England
Canada
China
1980
1990
Korea
2000 2010 Year
USA
2020
2030
0.1 Change and estimation of elderly (over 65 years old) population (%).
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Introduction
Increasing percentage of population
4.00 World population over 65 years old
3.50 3.00 2.50 2.00 1.50
Total world population
1.00 0.50
45 20
35 20
25 20
15 20
05 20
95 19
85 19
75 19
65 19
19
55
0.00
Year
0.2 Change in increasing percentage of world population over 65 years old and total world population with year.
is already over 20%, especially in Japan where it is approximately 22%; Japan is already the super aged society. Therefore, the number of elderly people with failed tissue needing replacement with artificial instrumentations made of metallic biomaterials is growing. The metallic biomaterials are roughly grouped into stainless steels: mainly SUS 316L stainless steel, cobalt (Co)–chromium (Cr) alloys, titanium (Ti; pure titanium) and its alloys; mainly pure titanium and Ti-6mass% aluminum (Al)-4mass% vanadium (V), and precious alloys (gold (Au) based, silver (Ag) based or platinum (Pt) based alloys). The implant materials used for fabricating artificial hip joints, other artificial joints, bone plates and screws, and artificial tooth roots are mainly stainless steel, Co–Cr alloys, titanium and titanium alloys. Precious alloys are mainly used as dental materials, for fabricating dental products such as dentures, crowns, inlays and bridges. In the early stage of the usage of the metallic materials as biomaterials, general structural metallic materials were used. Now we are seeing a large number of metallic materials composed of nontoxic and allergy-free elements and exhibiting mechanical biocompatibility being proposed or under development. The metallic materials used in biomaterials, namely metallic biomaterials, do not possess bio-functionalities such as bone conductivity, bioactiveness and blood compatibility. Hence, surface modification of metallic biomaterials using bioactive ceramics, such as hydroxyapatite and biopolymers, is required. The integration and harmonization of metals, ceramics, and polymers are very important for developing metal-based bio-functional biomaterials;
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Introduction
xvii
these integrated materials should be called biometals. The harmonization of metals, ceramics and polymers also leads to the harmonization of metallic biomaterials and living tissues. If these sorts of metallic biomaterials are established, then metallic biomaterials will become the scaffold used for artificial organs This book begins with an overview of metals used for biomedical applications, including metal selection for biomedical devices. Part II contains a discussion of mechanical behaviors, degradation and testing of metals used for biomedical devices, including physical, mechanical, and corrosion properties, fatigue and failure properties, mechanical testing and wear testing, and biocompatibility of metallic biomaterials. Part III has chapters on metal processing techniques used for biomedical applications, including forging metals and alloys used for biomedical applications, surface treatment of metallic biomaterials, coatings used for metallic biomaterials, biocompatible polymer assembly on metal surfaces, and sterilization/cleaning of metallic biomaterials. Part IV focuses on specific applications of metals used for biomedical devices, including orthopedic applications of metallic biomaterials, new-generation metallic biomaterials, and degradable metallic biomaterials, made of Mg and Fe, used in cardiovascular stents and orthopedic fixation devices. Every chapter in this book is contributed by distinguished experts in the biomaterials field. The editor (M.N. Mitsuo Niinomi) is grateful to all the authors for their outstanding efforts. The editor is also grateful to the staff of Woodhead Publishing Limited for their help and encouragement in publishing this book.
References Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, World Population Prospects: The 2008 Revision, http://esa. un.org/unpp, (2009). Yano Tsunetarou Memorial Foundation, Sekai-kokusei-zue, Kokusei Co. Ltd, (2009), pp. 75–76.
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© Woodhead Publishing Limited, 2010
1
Overview of metals and applications
T. H a n a w a , Tokyo Medical and Dental University, Japan Abstract: This chapter discusses characteristics such as composition, mechanical properties, corrosion resistance and surface properties, of metals and alloys widely used for medical and dental devices. It first reviews the advantages of metals and the properties required for medical devices, as well as the application of metals to the devices, followed by an explanation of the characteristics of each material as well as advanced future materials from the biomedical viewpoint. The chapter will impart a basic knowledge of metallic biomaterials. Key words: metal, alloy, biomaterial, implant, medical device.
1.1
Introduction
The use of metals as raw materials has a long history and it can be said that materials science and engineering have been based on research into metals. However, metals are sometimes thought of as ‘unfavourite materials’ for biomaterials because of memories of the environmental and human damage caused by heavy metals. Since an improvement in the safety of metals for medical use is vital, strenuous efforts have been made to improve corrosion resistance and mechanical durability. In addition, metals are typically artificial materials and have no biofunctions, which makes them fairly unattractive as biomaterials. This viewpoint is short-sighted and is caused by misunderstandings. On the other hand, the fast technological evolution of ceramics and polymers has made it possible to apply these materials to medical devices over the last three decades. In particular, because of their excellent biocompatibility and biofunctions, ceramics and polymers show useful properties as biomaterials; in fact, many devices made from metals have been replaced by others made from ceramics and polymers. In spite of this fact, over 70% of implant devices are still made from metals and this percentage remains unchanged because of their high strength, toughness, and durability. Therefore current metallic biomaterials cannot be replaced with ceramics or polymers at present. In addition, research into their use in regenerative medicine will not be completed for at least another few decades. In other words, artificial materials such as metals will continue to be used as biomaterials in the future. This chapter looks at the utilization and types of metals employed in medicine and dentistry. The properties and problems or the advantages and disadvantages of metals used for medical devices are then explained. In 3 © Woodhead Publishing Limited, 2010
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Metals for biomedical devices
addition, new alloys are introduced that are currently being researched and developed.
1.2
General properties required for metals in medical devices
1.2.1 Biological environment The biological environment that influences the durability of metals is summarised in Fig. 1.1. The concentrations of chloride ions in serum and interstitial fluid are 113 and 117 mEq L–1, respectively, which is 1/3 of the concentration in brine and a seriously corrosive environment for metallic materials (Fig. 1.1). Body fluids contain various amino acids and proteins that influence metallic corrosion (Merritt and Brown, 1998; Williams et al., 1988) because they are electrolytes. In addition, the concentration of dissolved oxygen in venous blood is 1/4 that of air and in intercellular spaces 1/80–1/4 that of air (Black, 1984), which accelerates the corrosion of metallic materials. Changes in the pH of body fluids are small because the fluids are buffered solutions and the pH usually remains between 7.0 and 7.35 (Black, 1984). The pH of the hard tissue into which a material is implanted decreases to approximately 5.2 and then recovers to 7.4 within two weeks (Hench and Ethridge, 1975). However, the local pH may change according
Low dissolved
Chemical environment
Oxygen concentration Inorganic ion Arterial blood 100 mmHg Venous blood 40 mmHg Intercellular space 2 ~40 Water molecule mmHg Body temperature Crevice Active oxygen Cell-material Generation with Material-material macrophage Corrosion
pH Blood 7.15–7.35 Intercellular space 7.0 Cell 6.8
Cell Biomolecule Fibroblast Bacteria Osteoblast Blood cells Macrophage
Metals
Load Cancellous bone 0–4 MPa Load repetition Cortical bone 0~40 MPa Myocardial contraction 5 ¥ 106 –4 ¥ 107/year Arterial wall 0.2–1 MPa Finger joint exercise 105–106/year Myocardium 0–0.02 MPa Ambulation 2 ¥ 106/year Muscle (maximum) 40 MPa Tendon (maximum) 400 MPa Mechanical environment
Fracture
Wear Fretting
1.1 Biological environment of metallic materials implanted into human body.
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Overview of metals and applications
5
to the dissociation of protein in the body fluid and the isoelectric point of protein (usually 5–7). The pH in an oral cavity may decrease to about 2 if carbonated drinks and foods are ingested. The cell is also a kind of charging body that may influence the corrosion of metallic materials. Materials implanted in the human body are intermittently stressed with loads due to weight and activity (Fig. 1.1). In particular, materials in the lower extremities are intermittently loaded with stressors several times heavier than body weight. In addition, loading is repeated a great number of times. Such loads are applied in the chemical environment described above. Under these conditions, material properties such as strength, toughness, rigidity, elasticity, wettability, bioinertness, bioactivity, biodegradability and X-ray imaging are required according to their purpose. Since biomaterials are used in contact with living tissues, they need to be absolutely safe for the human body. Moreover, it must be possible to sterilize biomaterials. Needless to say, durability in the human body is also important. In particular, corrosion resistance is required in metals.
1.2.2 Properties required for metals in medicine and dentistry Materials are categorised as metals, ceramics, and polymers (Fig. 1.2). Metallic materials generally have a multi-crystal body consisting of metal bonds. However, metal oxides, metal salts, metal complexes, etc. contain metal elements, but since these are compounds consisting of ionic bonds or covalent bonds, their properties are completely different from those of metals consisting of metal bonds. In the field of materials engineering, ceramics and metals are clearly distinguished, despite the fact that both of them are categorised as inorganic compounds. Each material has its own advantages and disadvantages, and applications are determined according to their properties. Metals have been utilised for dental restoration and bone fixation for the past 2500 years and have a long history as biomaterials. The properties of metals derive from their metal bonds and their advantages as biomaterials are as follows: Inorganic materials Organic materials
Metals
Metal; Alloy
Ceramics
Ceramics; Glass
Ionic bond; Covalent bond
Polymers
Natural polymer Synthesised polymer
Covalent bond; lonic bond
Metallic bond
Composites
1.2 Category of materials and their chemical bonding.
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Metals for biomedical devices
∑ high strength ∑ high ductility; easy working ∑ high fracture toughness ∑ suitable elasticity and stiffness ∑ high electroconductivity. Metals and alloys are widely used as biomedical materials and are indispensable in the medical field. The advantages of metals compared with ceramics and polymers are their great strength and resistance to fracture. In particular, toughness, elasticity, rigidity, and electrical conductivity are essential properties for metals used in medical devices. The metals used for such devices are listed in Table 1.1. Conventionally, metals have been essential for orthopaedic implants, bone fixators, artificial joints, external fixators, etc., because they can substitute for the function of hard tissues in orthopaedics. Stents and stent grafts are placed at stenotic blood vessels for dilatation. Therefore, these devices require elasticity or plasticity for expansion and rigidity for maintaining dilatation. In dentistry, metals are used for restorations, orthodontic wire, and dental implants. For mechanical reliability, metals must be used and cannot be replaced with ceramics or polymers. The most important property of biomaterials is safety. Therefore, corrosion-resistant materials such as stainless steel, cobalt (Co)–chromium (Cr)–molybdenum (Mo) alloys, metallic titanium (Ti), and Ti alloys are employed. Noble-metal-based alloys, such as gold (Au) alloys and silver (Ag) alloys, are also used in dentistry. Safety to the human body is essential in biomaterials; therefore, no toxic material is used for biomaterials. Metals implanted in tissues do not show any toxicity unless there is metal ion dissolution from corrosion and/ or the generation of debris from wear. Therefore, corrosion resistance is absolutely essential for metals in biomedical use, necessitating the use of noble or corrosion-resistant metals and alloys for medicine and dentistry. Of the noble metals and alloys, Au markers are used for the imaging of stents, Pt is used for embolic coils, and Au alloys and Ag alloys for dental restoratives. Of the base metals and alloys, austenitic stainless steels, Co–Cr alloys, Ti and Ti alloys, whose corrosion resistance is maintained by a passive film of surface oxide, are utilised for implant materials. In addition, wear resistance is required to reduce the generation of wear debris. Co–Cr–Mo alloys have good wear resistance and are used for sliding parts of artificial joints. A comparison of the various properties of metals used for implants is summarised in Table 1.2. The positions of the component elements of metals in biomedical use on the periodic element table are shown in Fig. 1.3.
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Overview of metals and applications
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Table 1.1 Metals used for medical devices Clinical division Medical device
Material
Orthopaedic Spinal fixation surgery Bone fixation (bone plate, screw, wire, bone nail, mini-plate, etc.) Artificial joint; bone head Spinal spacer
316L stainless steel; Ti; Ti–6Al–4V; Ti–6Al–7Nb 316L stainless steel; Ti; Ti–6Al–4V; Ti–6Al–7Nb
Cardiovascular Implant-type artificial medicine and heart (housing) surgery Pace maker (case) (electric wire) (electrode) (terminal) Artificial valve (frame) Stent Guide-wire Embolization wire Clip
Ti
Otorhinology
Pt 316L stainless steel
Artificial inner ear (electrode) Artificial eardrum
Dentistry Filling Inlay, crown; bridge; clasp; denture base Thermosetting resin facing crown; porcelain-fused to-metal Solder Dental implant Orthodontic wire Magnetic attachment
Co–Cr–Mo; Ti–6Al–4V; Ti–6Al–7Nb 316L stainless steel; Ti–6Al–4; Ti–6Al–7Nb
Ti; Ti–6Al–4V Ni–Co Ti; Pt–Ir Ti; 316L stainless steel; Pt Ti–6Al–4V 316L stainless steel; Ti–N; Ta; Co–Cr–Mo 316L stainless steel; Ti–Ni; Co–Cr Pt Ti–6Al–4V; 630 stainless steel; Co–Cr
Au foil; Ag–Sn(–Cu) amalgam Au–Cu–Ag; Au–Cu–Ag–Pt–Pd; Ti; Ti–6Al–7Nb; Co–Cr; 304 stainless steel; 316L stainless steel Au–Pt–Pd; Ni–Cr
Au–Cu–Ag; Au–Pt–Pd Ti; Ti–6Al–4V; Ti–6Al–7Nb; Au 316L stainless steel; Co–Cr; Ti-Ni; Ti-Mo Sm–C; Nd–Fe–B; Pt–Fe–Nb; 444 stainless steel; 447J1 stainless steel; 316L stainless steel
General surgery Treatment device (bar, scaler, 304 stainless steel periodontal probe, dental tweezers, raspatory, etc.) Needle of syringe 304 stainless steel Scalpel 420J1 stainless steel Catheter Ni–Ti; 304 stainless steel; 316L stainless steel; Co–Cr; Au; Pt–In Staple 630 stainless steel
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Metals for biomedical devices
Table 1.2 Comparision of properties of metals for implants Materials
Mechanical Workability Corrosion property —————————————————— resistance Tensile Wear Plasticity Machinability Pitting strength resistance
Stainless steel
Type316L Good
Excellent
Excellent Excellent
Excellent
Co–Cr alloy
Cast Good Annealed Excellent
Good Good
Poor Good
Poor Good
Good Fair
Excellent Excellent
Fair Fair
Fair Excellent
Excellent Excellent
Ti; Ti alloy CP Ti Excellent Ti–6Al–4V Excellent Ti alloy
1
1 2
H Stainless steel Li Be B C N O F Co–Cr Alloy
3
Na Mg
4
2
3
4
K Ca Sc Ti
5
Rb Sr Y
6
Cs Ba La* Hf
7
Fr Ra Ac*
5
V
6
7
8
9 10 11 12 13 14 15 16
Al Si
P
S
He Ne
Cl Ar
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te Ta W Re Os
Ir
I
Xe
Pt Au Hg Tl Pb Bi Po At Rn Au alloy
La* Lanthanide Ac* Actinide
1.3 Position of elements constituting metallic biomaterials in periodic table.
1.3
Stainless steels
1.3.1 Definition and category ‘Stainless steel’ is defined as an iron-based alloy containing over 12– 13mass%Cr and other elements, provided that the content of iron (Fe) is not over 50mass%. Stainless steel is not corroded under an oxygencontaining atmosphere, but is locally corroded and sometimes forms pits in chloride solutions such as body fluids. Nickel (Ni), Mo, copper (Cu), Ti, niobium (Nb), nitrogen (N), etc. are added to stainless steels to improve their corrosion resistance, heat resistance, strength, and workability. The metallurgical structure, strength, and corrosion resistance of stainless steels depend on the concentrations of Ni and Cr they contain and stainless steels are categorised as ferritic (Fe–Cr system), martensitic (Fe–Cr system), and austenitic (Fe–Cr–Ni system), according to their crystal phase. Austenitic-type stainless steels have outstanding corrosion resistance, but do not have great
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Overview of metals and applications
9
strength. Therefore, austenitic stainless steels are strengthened by working and heat treatment, and hardened with the addition of nitrogen (N). The addition of Mo improves their corrosion resistance, because their passive film becomes more stable. The categories of popular stainless steel series are summarised in Fig. 1.4, according to their development history. In AISI numbering, the 200s represent Fe–Cr–Ni–manganese (Mn) alloy systems, the 300s represent Fe–Cr–Ni alloy systems, and the 400s represents Fe–Cr systems. Recently, the stainless steels used for the stems of artificial hip joints and bone fixators have been replaced by Ti alloys. However, stainless steel is still used for internal bone fixators that are retrieved after healing, and for sternal wire and bone fixation wire, because of its excellent torsion property and elongation. Major stents are made from austenitic stainless steel. Stainless steels are also used for treatment and operating equipment and instruments. Type 304 stainless steel is used for medical equipment such as scalpels, forceps and dental tweezers.
Ferritic and martensitic type stainless steel Improving corrosion resistance Decreasing C 410 Fe–13Cr (
