Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 29

Polymers are generally composed of long‐chain molecules formed by repeated bonding of a large number of small molecules. The simplest example is polyethylene (abbreviated PE) whose molecular chains consist a large number (several hundred to several thousand) of ethylene (H₂C=CH₂) molecules bonded together. Polymers show low strength and low elastic modulus. In contrast to metals and ceramics, they show a time‐dependent mechanical response. This means that the measured mechanical properties of polymers, such as strength and elastic modulus, are dependent on the duration or rate of the mechanical testing procedure, or on the temperature at which the test is performed (Chapter 4). A given polymer, such as PMMA, for example, can show a range of mechanical behavior, from ductile to brittle, depending on the rate or the temperature of the mechanical test. Unless they are brittle, polymers typically show good fatigue resistance. Due to their low hardness, the resistance of polymers to abrasive wear is low.

Polymers generally have low density (~1 g/cm³), low electrical conductivity, and low thermal conductivity. A clear advantage of polymers over metals and ceramics is their ease of fabrication. Polymers can be easily formed into the requisite shape and microstructure using conventional processing or additive manufacturing (3D printing) methods. Another advantage of polymers is their compositional flexibility. Polymers can not only be synthesized with the requisite composition but their composition can also be easily modified to achieve more desirable properties, such as degradation rate.

Overall, the human body, except for bone and teeth, is composed of soft tissues (and organs). When compared to metals and ceramics, polymers can be more easily designed to approximate the structure and properties of these soft tissues. Consequently, polymers find considerable use as biomaterials. Because of the ease in synthesizing compositions with a controllable degradation rate, polymers also find considerable use as biomaterials for drug delivery.