Biomedical Materials, Second Edition
Edited by Roger Narayan
1 Metallic Biomaterials 1
Robert M. Pilliar
2 Polymeric Biomaterials 49
Sreenu Madhumanchi, Teerapol Srichana, and Abraham J. Domb
3 Ceramics and Glasses 101
Irene G. Turner
4 Biobased Materials for Medical Applications 139
Otto C. Wilson, Jr.
5 Shape Memory Biomaterials and Their Clinical Applications 195
Yufeng Zheng, Jianing Liu, Xili Lu, and Yibo Li
6 Natural and Synthetic Polymeric Scaffolds 257
Diana M. Yoon and John P. Fisher
7 Magnetic Nanomaterials 285
8 Mechanical Properties 303
Damien Lacroix and Josep A. Planell
9 Metal Corrosion 337
10 Wear 365
Chunming Jin and Wei Wei
11 Inflammation, Carcinogenicity, and Hypersensitivity 383
12 Protein Interactions at Material Surfaces 399
Janice L. McKenzie and Thomas J. Webster
13 Biocompatibility Testing 423
Kirsten Peters, Ronald E. Unger, and C. James Kirkpatrick
14 Biomaterials for Dental Applications 455
Sarit B. Bhaduri and Prabaha Sikder
15 Ophthalmic Biomaterials 495
Rachel L. Williams and David Wong
16 Hip Prostheses 517
17 Burn Dressing Biomaterials and Tissue Engineering 537
Lauren E. Flynn and Kimberly A. Woodhouse
18 BioMEMS 581
19 Additive Manufacturing and 3D Printing 621
K. Chua, K. F. Leong, and J. An
20 Sterility and Infection 653
Showan N. Nazhat, Anne M. Young, and Jonathan Pratten
21 Manufacturing Issues 675
Robert M. Pilliar
1.1 Introduction – Why Metals?
Metallic biomaterials continue to be used extensively for the fabrication of surgical implants primarily for the same reason that led to their initial selection many decades ago. The high strength and resistance to fracture that this class of material can provide, assuming proper processing, gives reliable long-term implant performance in major load-bearing situations such as those experienced in certain orthopedic and dental implant applications. In addition, the good electrical conductivity of metals makes them a good material for making neuromuscular stimulation devices (e.g. cardiac pacers). These characteristics, coupled with relative ease of fabrication using well-established and widely available techniques (e.g. casting, forging, machining), as well as more recently developed manufacturing methods (i.e. additive manufacturing using selected laser melting or sintering) has promoted and continues to favor metal use in the fields of orthopedics, dentistry, and cardiovascular surgery, in particular. The favorable properties (good fracture resistance, electrical conductivity, formability) are related to the interatomic bonding and atomic arrangements that characterize metals. While the purpose of this chapter is to focus on the important issues pertaining to the processing and performance of metallic biomaterials and to review the metals that are currently used for implant fabrication, a brief review of fundamental issues related to the structure–property relations of metals, in general, follows.
Metal processing procedures determine metal microstructures that in turn determine material properties (elastic constants being an exception since these are structure-insensitive parameters dependent only on interatomic bond type and equilibrium atom packing as noted below) [1, 2]. An understanding of material properties and processes used to achieve these properties during fabrication of metallic components is critical for achieving desired performance of implants. While mechanical failure is unacceptable for most engineered structures, it is particularly so for surgical implants where failure can result in patient pain, the need for complicated and life-threatening revision surgery and, in certain cases, death (e.g. heart valve component fracture).
1.2 Metallic Interatomic Bonding
Interatomic bonding in solids occurs by strong primary (ionic, covalent, and/or metallic) and weaker secondary interatomic bonding (van der Waals and hydrogen bonding). Metals are characterized by metallic interatomic bonding with valence shell electrons forming a “cloud” of electrons around individual atoms/ions. This is a consequence of the high coordination number, N, (i.e. number of nearest neighboring atoms) that characterize metals (N = 12 or 8 for many metals). As a result of this close positioning of neighboring atoms and the shared valence electrons, the interatomic bonds are nondirectional and electron movement within metal crystal lattices is easier than in ionic or covalently bonded materials. This fundamental distinguishing characteristic of metals results in the relative ease of plastic deformation (i.e. permanent deformation on loading above a yield stress) as well as the high electrical and thermal conductivities of metals. Most metals used for implant fabrication have either close-packed atomic structures with N = 12 with face-centered cubic (fcc) or hexagonal close-packed (hcp) unit cells, or nearly close-packed structures with N = 8 forming body-centered cubic (bcc) structures. Less commonly, tetragonal and orthorhombic as well as other unit cells do occur with some metallic biomaterials. The equilibrium distance between atoms defining the unit cells of these crystals and the strength of their interatomic bonding are determined by intrinsic factors such as atom size and valency as well as extrinsic factors (temperature, pressure). In addition to ease of deformation to desired shapes, the ability to deform plastically at high loads results in another very important feature namely the ability of most metals to blunt sharp discontinuities (through plastic deformation) thereby reducing local stress concentrations thereby resulting in relatively high fracture toughness that most metals display. As noted below, these desirable characteristics are dependent on proper selection of processing conditions during material and part preparation.
1.3 Crystal Structures – Atom Packing in Metals
The most common metallic biomaterials (i.e. stainless steel, Co-based alloys, and Ti and its alloys) form either face-centered cubic, hexagonal close-packed, or body-centered cubic unit cells at body temperature and during different stages of their thermal treatment with ideal crystal lattice structures such as those shown in (Fig. 1.1). Real metal crystals, in contrast to these ideal atomic arrangements, contain lattice defects throughout (vacancies, dislocations, grain boundaries – Fig. 1.2).
The presence of these defects (point, line, and planar defects) has a strong effect on mechanical, physical, and chemical properties. Using a simple solid sphere model to represent atom packing, arrangement of spheres in the closest packed arrangement shown in Fig. 1.3. results in either a face-centered cubic structure (2-D planar layer stacking sequence as ABCABC… – Fig. 1.3a) or a hexagonal close-packed structure (ABABAB… stacking sequence – Fig. 1.3b). The selection of the preferred arrangement for a close-packed metal depends on the lowest free energy form under given extrinsic conditions (temperature and pressure). Regions of substitution of one stacking sequence for the other can occur locally and these represent another type of lattice defect (a stacking fault with its borders defined by partial dislocations ). While many metals used for implant applications form close-packed structures over a certain temperature range (e.g. Ti and its alloys are hcp below about 900 °C, Co-based alloys form fcc crystalline structures above approximately 850 °C, 316L stainless steel is fcc from its forging temperature ~ 1050 °C down to room temperature), others form less closely packed structures (Ti and Ti-based alloys form bcc structures at elevated temperatures). Lowest free energy determines which crystallographic arrangement will exist under given conditions of temperature and pressure. Understanding the nature of the transformations that may occur during metal processing is important for achieving desired properties, but may also result in secondary phases with undesirable properties leading to unacceptable properties. A good understanding of constitutional (equilibrium) phase diagrams is important for the design of processing methods for forming metal implants. It should be appreciated, however, that these often, oversimplified equilibrium phase diagrams (i.e. limited to two- or three-element alloys rather than the multielemental compositions of most practical alloys) indicate, even for these simple compositions, equilibrium structures that may not be achieved during processing because of kinetic considerations as discussed below.