Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 80

Biomaterials are typically three‐dimensional solids composed of an enormous number of atoms. A titanium (Ti) implant of volume 1 cm³, for example, contains ~5 × 10²² atoms. Consequently, we are concerned with the structural characteristics at different levels or length scales. Bonding between atoms to form molecules and the way in which the atoms pack together to form a controllable structure determine the intrinsic properties of a solid (Chapter 2). If the atoms pack together in a regularly repeating three‐dimensional pattern, the structure is described as crystalline. On the other hand, if the atoms pack together in a random non‐ordered pattern, the structure is said to be amorphous. Metals and ceramics in their pure state have a crystalline structure. On the other hand, when cooled at a rapid enough rate from the molten state, some metals and ceramics develop an amorphous structure and the solid is said to be a glass or to have a glassy structure. Whereas glasses are, by definition, amorphous, some materials derived from glasses, called glass‐ceramics, consist of a mixture of crystalline and amorphous phases. Many polymers have an amorphous structure but some have a structure composed of a combination of crystalline and amorphous regions, sometimes described as a semicrystalline structure. Whether a material is crystalline or not is important because a crystalline phase has properties that are different and often better than an amorphous phase of the same composition.

A perfect crystal is an idealization. Imperfections, often called defects, occur in the crystal due to packing irregularities of the atoms or the addition of other types of atoms. There are two main types of defects in crystals, classified in terms of whether they occur at one or a few atomic positions, called point defects, or over a more extended one‐dimensional line of atomic positions, called line defects or, more commonly, dislocations. These defects control the rate at which atoms can migrate through the crystal in response to a stimulus, such as mechanical stress or temperature. The attractive mechanical property of ductility in metals arises from the presence of dislocations in the crystals that make up the metal.

Unless they are formed by special and, often, expensive methods, crystalline solids are not composed of just one crystal. Instead, they are composed of a large number of small crystals, called grains, of size smaller than 1 μm to several tens of micrometers. These solids are said to be polycrystalline. The grains and, if present, amorphous phase and porosity make up the microstructure that determines the engineering properties of a material relevant to its application (Chapter 2).

In this chapter, we will discuss the following structural features that, in addition to atomic bonding, are important for the design, properties, and applications of biomaterials:

 Ways in which atoms pack to form a crystalline or an amorphous solid

 Defects that are present in crystals

 Microstructure of materials