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Understand the core materials that create biomedical innovation
Some of the greatest medical advances in recent decades have come in the form of biomedical implants. Whether in the form of traditional orthopedic implants, medical devices for the cardiovascular system, or polymer-based ocular implants, biomedical implants can be lifesaving or life-transforming interventions. The biomaterials which comprise these implants are a vital area of ongoing research, but no prior volume has ever taken comprehensive stock of this subject and its growing applications.
An Overview of Biomedical Implants fills this gap with a thorough overview of all major biomaterials and their role in biomedical implants. Composed for an interdisciplinary audience, the book addresses all scales and areas of application. The result is an essential resource in this critical ongoing area of biomedical research.
An Overview of Biomedical Implants readers will also find:
An Overview of Biomedical Implants is ideal for physicians, scientists, and engineers-those working in the area of biomaterials, medical, biological and chemical and applied physics, pharmaceutical science and as a reference for professors and students in these areas.
Tolou Shokuhfar, PhD, is an Associate Professor of Bioengineering at University of Illinois at Chicago. She is a recipient of an NSF CAREER Award that pioneers the visualization of biomineralization, biological materials, nanomaterial/protein interactions and microorganisms in nanoenclosures. She has also received the SFB Ortho SIG Faculty Abstract award, TMS Young Leaders Award, DSL Young Scientist Award, and "INSIGHT Diversity Award for Inspiring Women in STEM". Dr. Shokuhfar has been the Chair for Orthopedic Biomaterials SIG and Vice Chair of Nanomaterials SIG at the Society for Biomaterials since 2019.
Preface xiii
Acknowledgments xiv
1 Biomaterials for Dental Implants 1
1.1 Introduction: Dental Implants and Current Materials 1
1.1.1 The Need for Better Dental Implants 1
1.1.2 Various Approaches and Biomaterials to Improve Dental Implants 3
1.1.3 Working at the Nanoscale to Improve Dental Implants 6
1.2 Ceramic Dental Implants 8
1.2.1 Zirconia 8
1.2.1.1 Surface Roughness Optimization of Zirconia-Based Implants 9
1.2.1.2 Coating Alternatives for Zirconia-Based Implants 9
1.2.1.3 Using Nanotechnology to Modify Zirconia-Based Implants 10
1.2.1.4 Disadvantages and Advantages of Zirconia-Based Implants 11
1.2.2 Hydroxyapatite 12
1.2.2.1 Antimicrobial Properties of Hydroxyapatite-Containing Materials 12
1.2.2.2 Combination Dental Materials Using Hydroxyapatite 13
1.2.2.3 Nano-Hydroxyapatite 14
1.2.2.4 Disadvantages and Advantages of HA-based Biomaterials 15
1.3 Polyetheretherketone (PEEK) 15
1.3.1 PEEK Composites to Improve Mechanical Properties 16
1.3.2 PEEK Bioactivity 17
1.3.3 Working at the Nanoscale and With Composites to Enhance PEEK Bioactivity 17
1.3.4 Disadvantages and Advantages of PEEK 20
1.4 Peptide Coatings for Dental Implants 20
1.5 Functionally Graded Dental Implants 22
1.5.1 Biological Responses to Functionally Graded Dental Implants 24
1.5.2 Radially Designed FGMs for Dental Implants 25
1.5.3 Disadvantages and Advantages of FGMs 26
1.6 Looking to the Future: State-of-the-Art Biomaterials for Dental Implants 26
1.7 Conclusion 28
References 29
2.1 Biomaterials for Total Hip Implants 35
2.1.1 Introduction 35
2.1.2 History of THA Development 36
2.1.3 Metallic Materials 36
2.1.3.1 Stainless Steel 36
2.1.3.2 Cobalt-Chromium Alloys 37
2.1.3.3 Titanium Alloys 37
2.1.3.4 Alloy Surface Modifications 37
2.1.4 Exploited Materials for Bearing Surface 38
2.1.4.1 Polymers 38
2.1.4.1.1 Ultrahigh Molecular Weight Polyethylene (UHMWPE) 38
2.1.4.1.2 High Cross-Linked UHMWPE (XLPE) 38
2.1.4.1.3 Antioxidant-Doped PE 38
2.1.4.1.4 Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) 39
2.1.4.2 Ceramics 39
2.1.4.2.1 Alumina 39
2.1.4.2.2 Zirconia 39
2.1.4.2.3 Alumina-Zirconia Composites 40
2.1.4.2.4 Silicon Nitride 40
2.1.4.2.5 Hybrid Design of Oxide Ceramic Layer on Metal (Oxinium(TM)) 40
2.1.4.2.6 Ultra-Hard Coatings on Metals 41
2.1.4.2.7 Clinical Aspects of Bearing Surface 41
2.1.4.3 MoP Articulation 42
2.1.4.3.1 Advantages 42
2.1.4.3.2 Disadvantages 42
2.1.4.3.3 Wear Mechanism 42
2.1.4.4 MoM Articulation 42
2.1.4.4.1 Advantages 42
2.1.4.4.2 Disadvantages 43
2.1.4.4.3 Wear Mechanism 43
2.1.4.5 CoC Articulation 43
2.1.4.5.1 Advantages 43
2.1.4.5.2 Disadvantages 44
2.1.4.5.3 Wear Mechanism 45
2.1.4.6 CoP Articulation 45
2.1.4.6.1 Advantages 45
2.1.4.6.2 Disadvantages 45
2.1.4.6.3 Wear Mechanism 45
2.1.5 Orthopedic Wear Debris 46
2.1.6 Conclusion 47
References 47
2.2 Biomaterials for Total Knee Replacement (TKR) and Total Hip Replacement (THR) and Next-Generation Advancements 57
2.2.1 Introduction 57
2.2.2 Ultra-High Molecular Weight Polyethylene (UHMWPE) and Polyethylene (PE) 61
2.2.3 Polyetheretherketone (PEEK) 62
2.2.4 Polymethylmethacrylate (PMMA) 65
2.2.5 Metal Implants 68
2.2.5.1 Metal Implants: Titanium (Ti) Alloys and Cobalt-Chromium (Co-Cr) and Surface Treatments 68
2.2.5.2 Metal Implants: Stainless Steel and Next Generation Surface Treatments 71
2.2.5.3 Porous Coatings 73
2.2.6 Ceramics and New Generation Surface Treatments 76
2.2.7 Advancements in Biomedicine and Nanotechnology: Titanium, Silver Nanoparticles, and More 78
2.2.8 Summary and Conclusion 81
References 83
2.3 Biomaterials for Shoulder Implants 91
2.3.1 Introduction 91
2.3.2 Titanium Alloys 95
2.3.3 Cobalt-Chrome Alloys 95
2.3.4 Ceramics 96
2.3.5 Pyrolytic Carbon 96
2.3.6 Comparison of Shoulder Implants with Different Class Materials 97
2.3.7 Conclusion 97
References 98
3 Biomaterials for Spinal Implants 103
3.1 Introduction 103
3.2 Overview of Implants and Corresponding Material Design Requirements 104
3.2.1 Spinal Cages 104
3.2.2 Spinal Rods 104
3.2.3 Pedicle Screws 105
3.2.4 Plates 105
3.2.5 Disc Replacements 105
3.3 Metals 106
3.3.1 Titanium and its Alloys 106
3.3.2 Stainless Steel 108
3.3.3 Cobalt-Chromium and its Alloys 108
3.3.4 Nitinol 108
3.3.5 Tantalum 109
3.4 Ceramics 109
3.4.1 Silicon Nitride 109
3.5 Polymers 112
3.5.1 Peek 112
3.6 3D-Printed Spinal Implants: Applications and Relevant Materials 118
3.7 Degradable Implants 118
3.8 Discussion 119
3.9 Conclusion 120
References 120
4 Biomaterials in Cochlear Implants 123
4.1 Introduction 123
4.2 Biological Requirements 125
4.3 Electrical Requirements 126
4.4 Mechanical Requirements 126
4.5 New Electrode Biomaterials 127
4.5.1 Dexamethasone (DEX) 127
4.6 Nanoscale Coatings 129
4.7 New Potential Implant Materials 134
4.8 Drug Delivery 135
4.9 Summary and Conclusion 136
References 138
5 Biomaterials for Cardiovascular Implants 141
5.1 Introduction 141
5.2 Different Applications 143
5.2.1 Stents 143
5.2.1.1 Balloon Expandable Stents 144
5.2.1.2 Self-Expandable Stents 146
5.2.1.3 Drug-Eluting Stents 148
5.2.1.4 Biodegradable Stents 150
5.2.1.5 Latest Stent Inventions 152
5.2.2 Prosthetic Heart Valves 153
5.2.2.1 Mechanical Valves 155
5.2.2.2 Bioprosthetic Valves 160
5.2.2.3 Latest Valves Advancements 161
5.3 Summary and Conclusions 162
References 164
6 Biomaterials for Liver and Kidney Implants 173
6.1 Introduction 173
6.2 Liver Biomaterials 175
6.2.1 General View 175
6.2.2 Biomaterials and Structures 177
6.2.2.1 Natural Polymers 178
6.2.2.2 Synthetic Material 179
6.2.2.3 Decellularized Matrix 181
6.2.3 Conventional Therapy and Future 184
6.3 Kidney Biomaterials 186
6.3.1 A General View 186
6.3.2 Biomaterials and Structure 188
6.3.3 Conventional Therapy and Future 190
6.4 Conclusions 193
References 194
7 Biomaterials for Brain Implants 199
7.1 Introduction 199
7.2 Brain Implants Classification 201
7.2.1 Brain Implants for Recording 201
7.2.1.1 Electrocorticography (ECoG) 201
7.2.1.2 Multielectrode Arrays (MEAs) 202
7.2.2 Brain Stimulator Implants 202
7.2.3 Brain Regenerative Medicine 206
7.3 Causes of Failure 207
7.4 Materials for Neural Electrodes 210
7.4.1 Neural Electrodes 210
7.4.2 Materials for Microelectrode Fabrication 211
7.4.2.1 Conductive Polymers 212
7.4.2.2 Carbon Nanotubes 213
7.4.2.3 Graphene 214
7.4.2.4 Hybrid Nanomaterials 216
7.5 Conclusions 217
References 218
8 Biomaterials for Bionic Implants 225
8.1 Introduction 225
8.2 Biomaterials in Bionic Eye and Neural Systems 227
8.2.1 Implant Package 227
8.2.2 Electrodes 229
8.2.2.1 Diamond Electrodes 229
8.2.2.2 Organic Polymer Electrode Coating 233
8.3 Biomaterials in Bionic Limbs 234
8.3.1 Upper Limb 235
8.3.1.1 Electrodes 235
8.3.1.2 Electrode Packaging and Encapsulation 237
8.3.1.3 Arms 238
8.3.2 Lower Limb 239
8.4 Summary Conclusion 241
References 241
9 Final Remarks 249
Index 251
1.1 Introduction: Dental Implants and Current Materials, 1
1.2 Ceramic Dental Implants, 8
1.3 Polyetheretherketone (PEEK), 15
1.4 Peptide Coatings for Dental Implants, 20
1.5 Functionally Graded Dental Implants, 22
1.6 Looking to the Future: State-of-the-Art Biomaterials for Dental Implants, 26
1.7 Conclusion, 28
References, 29
An interdisciplinary approach including surface chemistry, physics, and engineering as well as biomechanics is required to develop successful dental implants [1]. Dental implants have been prevalent throughout the past century; however, evidence of dental implants within ancient Mayan and Egyptian civilizations has been found [2]. This brings us to the first prototype of the modern dental implant, which was created by Greenfield in 1913 and was first described as an implant/prosthetic combination made of an iridium-platinum alloy [2]. In the 1970s, Brånemark's experimentation led to the general acceptance of oral implants and highlighted the importance of osseointegration [3]. We now understand that the success of a dental implant depends on the chemical, physical, mechanical, and topographic characteristics of its surface [4]. As a result of continuous modifications to implant design and surface topography, dental implant placement is a fairly common treatment procedure with high implant survival rates and limited peri-implant bone loss [5]. In fact, the survival rate of dental implants has been reported to be above 90% [6]. Nowadays, implant surface modifications focus on stronger and faster bone healing to further limit dental implant failure [5]. Even with great advancements in the field of dental implantology, there is still a relatively significant number of dental implant failures, many of which are caused by compromised bone conditions that promote implant failure. For example, diabetes, osteoporosis, obesity, and the use of drugs can decrease bone healing around dental implants [6]. Furthermore, complications involving osseointegrated dental implants can arise from inflammatory conditions associated with bacteria, more specifically, peri-implantitis [7].
Peri-implantitis is a pathological condition that occurs in tissues surrounding dental implants [7]. It is characterized by inflammation of the peri-implant connective tissues as well as loss of supporting bone [7]. In other words, plaque and its byproducts lead to hard and soft tissue breakdown and eventually implant failure, which is a prevalent issue [8]. Factors such as smoking or a history of periodontal disease increase the prevalence of peri-implantitis [8]. However, even with the lack of the aforementioned factors, features such as implant placement, material biocompatibility, and material degradation also play important roles in the development of peri-implantitis or osseointegration breakdown [8]. Osseointegration is the formation of bone tissue around the implant without fibrous tissue growth at the bone-implant interface, resulting in direct anchorage of the implant [1]. The osseointegration process can be visualized in Figure 1.1. In fact, the structural and functional union of the implant and living bone is significantly influenced by the surface characteristics of the dental implant [4]. Thus, proper osseointegration is crucial for the success of the implant and is a research topic of great importance. Presently, researchers are finding ways to optimize implant surfaces by studying specific features such as roughness of the implant surface as well as various materials for dental implants in order to promote proper osseointegration and combat peri-implantitis [10].
Figure 1.1 Osseointegration of dental implants over time.
(From Ref. [9], 2021, Springer Nature, CC BY 4.0).
Currently, titanium or titanium alloys are the gold standard in dental applications [10]. Most dental implants marketed in the United States are made from either commercially pure titanium (cpTi) or titanium alloys [e.g. Ti6Al4V (TAV)] [4]. Seconds after titanium (Ti) is machined, adsorbed oxygen molecules form a thin oxide layer, which is what body tissues interact with [11]. This oxide layer allows for biocompatibility, while the rest of the implant material plays a role in the implant's mechanical properties [11]. Chemical processes that occur at the tissue-implant interface include corrosion, adsorption of some biomolecules, denaturing of proteins, and catalytic activity [11]. For instance, TAV implants degrade and result in peri-implant bone loss [12]. The origins of this degradation were revealed by Chen et al. [12] whose results suggest that the observed bone loss is caused by crevice corrosion and the release of consequential by-products. These types of issues are driving scientists to find materials and methods to improve dental implants, specifically, the surface of dental implants.
The material composition and surface topography of implants greatly influence the wound healing processes that follow implantation and thus also influence subsequent osseointegration [13]. It has been found that implants with a rough surface allow for better osseointegration; however, excessive roughness can increase the risk of peri-implantitis and ionic leakage [14]. Thus several methods have been proposed to produce a moderate roughness of 1-2?µm including titanium plasma spraying, particle blasting and acid etching, anodization of the implant surface, and coatings [14]. Examples of these methods are highlighted in Figure 1.2. One method, anodization or anodic oxidation on Ti-based implants, creates an adherent oxide coating that can have a wide range of stoichiometries as well as microporosities and nanoporosities depending on electrolyte selection and condition manipulation [15].
Figure 1.2 Various aspects of dental implant surfaces viewed by scanning electron microscopy. (a) Original machined implant from Nobel Biocare with a smooth surface. (b) Rough surface of a dental implant system developed by the French company ETK implant that was sandblasted, and acid etched. (c) Surface of a Ti UniteTM implant from Nobel Biocare with a thick layer of titanium creating smooth asperities. (d) High magnification of an implant surface after sandblasting and HF acid etching. (e) Surface of a TA6V implant whose surface was sandblasted with corundum particles. (f) Surface of titanium implant, which was sprayed with titanium beads with a plasma torch.
(From Ref. [14], 2016, Elsevier).
Biomaterials of interest that could be used as a coating or as a Ti implant replacement include hydroxyapatite (HA), ceramic materials [e.g. alumina, calcium phosphate (CP), and zirconia], nanoparticulate zinc oxide (nZnO), and polyetheretherketone (PEEK). Each of these materials has their own promising aspects. Some studies have reported benefits of using HA-coated dental implants as well as risks including dissolution of the coating (although they have not shown that dissolution leads to implant loss) [16]. Furthermore, HA coatings may be more susceptible to bacteria as compared to titanium implants [16]. Nevertheless, coating dental implants with HA has helped metallic materials to osseointegrate with the local tissue environment and distribute load stress [17]. Zirconia is a possible alternative to the traditional Ti-based implant systems as it has superior biological, aesthetic, mechanical, and optical properties [18]. However, more long-term and comparative clinical trials are necessary in order to validate zirconia as a viable alternative to titanium implants [18].
There are many other dental implant biomaterials that clinicians may not be familiar with. For example, bioactive dental glass-ceramics (BDGCs) have shown bone-tooth bonding capabilities as well as positive biological reactions at the material-tissue interface [19]. This makes them an attractive implant coating biomaterial. Nanoparticulate zinc oxide is of great interest because of its integration with antimicrobial nanoparticles (NPs) resulting in a coating material that is antibacterial and promotes osteoblast growth, which would help prevent implant failure from aseptic loosening and infection [20]. PEEK possesses excellent mechanical characteristics and may be used in dentistry with surface modification to enhance its osseointegrative characteristics [10]. Another interesting approach to modifying dental implants is using functionally graded materials (FGMs). FGMs are heterogeneous composite materials that have a compositional gradient with continuously varying properties in the thickness direction [21]. Ultimately, these more "novel" biomaterials must be researched in more depth if they are to be used more frequently in the clinic.
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