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Metals for Use in Medicine
Implantable materials make critical contributions to modern medicine. Many of the treatments we now take for granted (joint replacement, pacemakers, heart valves, stents) would not have been possible without the advanced metals, polymers, and ceramics that are available to the medical device community.
The human body presents a very challenging environment for materials engineers because of the need for implants that are highly corrosion resistant and, in many applications, able to withstand millions of high stress load cycles.
Any implantable metal must be well tolerated by the host tissue. Therefore, metals of interest must be nontoxic and biocompatible
In this regard, composition and corrosion resistance are the key factors for metallic materials. With the exception of noble metals such as platinum and gold, implantable metals form very thin, passive oxide films that reduce their corrosion rates to extremely low levels.
Implantable materials must exhibit mechanical properties that are appropriate for specific applications. High fatigue strength is needed for devices such as femoral hip stems and pacemaker leads; metallic suture wires must be relatively soft and ductile.
Only a limited number of metals can be safely implanted for long periods of time; the metals made in the greatest commercial quantities (steels, aluminum alloys, copper alloys) are not suitable for use as implants.
In many cases a material is chosen for an implant application because of a unique property or combination of properties. For example, platinum or palladium alloys are used in a variety of applications where radiopacity is important (i.e., marker bands). Wear behavior is a critical aspect of devices that articulate, and the choice of material for these applications is often made on the basis of wear characteristics; cobalt base alloys, oxidized zirconium, and aluminum oxide ceramics are often used in these applications because of their wear behavior. Table 1 provides a broad overview of implantable metals along with their important attributes and typical applications.
Key Metallurgy Concepts
The atoms in metals are arranged in simple, repetitive, long-range, three-dimensional crystal structures. For the metals of interest in this article, the relevant crystal structures are body centered cubic (BCC), face centered cubic (FCC), and hexagonal close packed (HCP). The nature of atomic bonding in metals is quite different from that found in other materials. Metallic bonding involves the sharing of valence electrons between closely packed atomic cores. The mobility of shared electrons is crucial to the electrical and thermal conductivity of metals. As the bonds are not localized, individual metal atoms or planes of atoms can move, or slip, relatively easily with respect to one another.
Permanent, or plastic, deformation of metal crystals occurs most frequently by dislocation motion. Dislocations are defects in the crystal; edge dislocations (an extra half plane of atoms) are a common type (Figure 1). If the applied loads are high enough, atomic bonds break in a sequential fashion and the dislocation moves through the crystal. Dislocations move most favorably on close packed planes within the crystal. Metals with the FCC structure have more close packed planes than BCC and HCP structures and therefore are generally more ductile. While dislocation theory is a complex subject, it is important to note that the approaches used to alter the strength and ductility of metals rely on mechanisms that alter the ability of dislocations to move through the crystal structure. Deformation by twinning is another deformation mode in some metals. The deformation behavior of Nitinol (the major shape memory alloy) is quite different from that of conventional metals; Nitinol deformation involves phase transformations and reversible twinning processes (Nitinol is discussed in Chapter 1.104, Shape Memory Alloys for Use in Medicine from Comprehensive Biomaterials ).
Most metal objects contain large numbers of individual grains. The orientation of the crystal within each grain is generally different from the orientation in adjacent grains. This mismatch between grains is accommodated by grain boundaries, which are thin disordered regions. Grain size is an important factor in the mechanical strength of metals. In general, finer grains lead to higher strength. A well-known equation describing this behavior is the Hall–Petch relationship, σy = σo + kD−1/2, where σY is the yield stress of the metal, σo is a frictional stress required to move dislocations, k is the Hall–Petch slope, and D is the grain size. 1
Pure metals are typically quite soft and ductile; extremely pure metals are also very expensive to produce as it is difficult to avoid contamination by small amounts of impurities. Therefore, the vast majority of metals are alloys with more than one ingredient. The major constituent may be thought of as the solvent and the minor constituents as solute atoms. Metallic alloying elements generally take the place of the solvent atoms at random locations in the crystal structure; they are referred to as substitutional alloying elements. (Nitinol is an exception to random alloying element locations. NiTi has an ordered structure with the Ni and Ti atoms alternating in the lattice.) As the substitutional alloying elements are not the same atomic size as solvent atoms, the crystal lattice is distorted by the addition of these elements. This makes dislocation motion more difficult and increases the strength of the metal. Small atoms such as nitrogen, oxygen, and carbon tend to be located in the regions (or interstices) between the solvent atoms; these are known as interstitial alloying elements. Interstitial alloying elements ‘pin’ dislocations very effectively, so small additions of these elements can have a large impact on mechanical properties. Chemical composition is also an important determinant of the crystal structure of the material. The crystal structure of a metal determines how many and which slip systems are available; this in turn affects deformation behavior and response to mechanical loading.
Another technique to improve the strength of metals is to increase the number, or density, of dislocations within the crystal. There are a number of mechanisms which produce additional dislocations within a metal crystal as the crystal is deformed. As dislocation density increases, the dislocations begin to interact, and further dislocation motion becomes more difficult. This behavior forms the basis for strengthening metals by work hardening (also known as cold working or strain hardening).
To learn more, take a look at the article Metals for Use in Medicine from the Major Reference Work Comprehensive Biomaterials which examines the status of nearly all biomaterials in the field by analyzing their strengths and weaknesses, performance as well as future prospects. This article will also be part of our innovative Reference Module in Materials Science and Materials Engineering. Live on ScienceDirect in December 2015, this visionary resource combines thousands of encyclopedic and comprehensive articles into one interdisciplinary product. It will save you time and energy by displaying the multidisciplinary links across topics in the broad and complicated field of Materials Science and Materials Engineering in one authoritative platform. Hear what our Acquisitions Editor has to say or click here to learn more!
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