Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 112

The atoms at the free surface of a solid have a different atomic environment and, often, a different composition than those within the solid itself. On this basis, we can divide the properties of materials into two broad categories:

 Bulk properties, that is, the properties of the three‐dimensional solid itself

 Surface properties, the properties of the surface of the solid that interfaces with its environment, such as the biological environment in vivo.

Bulk properties include mechanical properties such as strength and elastic modulus, electrical properties, magnetic properties, and optical properties, commonly grouped together as physical properties. In comparison, surface properties include topography (surface roughness or smoothness), contact angle and surface charge. Together, the bulk properties and surface properties determine the performance of a biomaterial in vivo.

In this chapter, we will consider the physical properties of three‐dimensional solids that are relevant to their use as biomaterials. Surface properties are discussed in the next chapter. To round off our treatment of the combination of properties relevant to the applications of biomaterials, chemical properties such as degradation and corrosion are discussed in Chapters 14 and 15 while biocompatibility phenomena are discussed in Chapters 16–22.

Mechanical properties are often among the most important physical properties because many biomaterials are subjected to loads (forces) at some point or during almost their entire application and catastrophic failure of an implant in vivo is a serious complication. Biomaterials should have the requisite mechanical properties to support physiological loads when used, for example, as a prosthetic hip and knee joint, or to withstand the pressure pulsations of blood flow through the arterial vessels when used as a stent. In addition, biomaterials are typically designed to have some desirable combination of properties that normally depend on the intended application and not just adequate mechanical properties (Chapter 1). The electrodes in a cardiac pacemaker, for example, have to deliver an electric charge to the heart muscle. Consequently, in addition to having adequate mechanical reliability, the electrode material should have a strong ability to conduct electrical signals, that is, a high electrical conductivity. Dental restorations such as crowns and bridges, designed to restore the function and appearance of tooth structure, should have not just the requisite mechanical properties to avoid fracture but a low thermal conductivity comparable to the tooth itself and the right color for esthetic appearance as well.

The physical properties of materials cover a very broad field and, consequently, we will discuss only the properties relevant to the applications of biomaterials in this chapter. Whereas mechanical properties form the major part of this discussion because of its aforementioned importance, other properties such as electrical, magnetic, thermal, and optical properties are discussed as well. In general, the following main issues will be considered:

 Response of a biomaterial to a physical stimulus, such as a mechanical force, an electric or magnetic field, or a change in temperature

 The origins of this response and its dependence on atomic bonding and microstructure of the biomaterial

 Relation among the physical properties, application, and performance of biomaterials.