Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 19

Biomaterials can also be classified in terms of their ability to degrade, typically in an aqueous medium such as a phosphate‐buffered saline or the physiological fluid. Degradation refers to the conversion of the material into smaller constituents, such as ions, molecules or fine particles. Degradation of biomaterials in an aqueous environment can occur by chemical attack by water molecules (hydrolysis), enzyme‐mediated hydrolysis, dissolution, oxidative reaction, or some combination of these mechanisms. For example, the synthetic polymer polyglycolic acid has a composition that makes it susceptible to degradation by hydrolysis into smaller molecules composed of glycolic acid.

The term biodegradable is often used in the literature instead of, or interchangeably with, degradable. Sometimes it may be used in a more restrictive sense to describe degradation by enzyme‐mediated reactions in the physiological environment, and so would not include unmediated hydrolysis. Another term is resorbable that, like biodegradable, is often used instead of, or interchangeably with, degradable. This term may be used in a more restrictive sense to emphasize what eventually happens to the biomaterial degradation product composed of ions, molecules, or particles. Commonly, the degradation product (or most of it) is removed from the body via renal clearance, hepatic processing, phagocytosis, or by other processes. Alternatively, the degradation product of some biomaterials can be taken up into the body tissues, and it can then be said to be resorbed back into the body tissues. For example, calcium and phosphate ions produced by the degradation of some calcium phosphate biomaterials can be resorbed into bone as these ions are constituents of bone.

In this book, we will simply use the term degradable regardless of the mechanism of degradation or the fate to the degradation products. The terms biodegradable or resorbable will be used only when necessary to emphasize a distinction from the general term degradable.

Whereas all materials show some degree of degradation, no matter how small, over some appropriate duration, the term nondegradable is often used in a qualitative manner to describe a biomaterial that is normally not susceptible to chemical attack in an aqueous environment. Qualitatively, these nondegradable biomaterials can also be said to be chemically inert, chemically stable, or resistant to degradation. In this sense, then, the synthetic polymer PE can be described as nondegradable or chemically stable, even though it can undergo some degree of degradation by oxidative reactions under certain conditions, such as the presence of highly oxidizing molecules in the aqueous environment (Chapter 15).

The term bioinert has often been used to describe these nondegradable biomaterials. However, it is generally recognized that no biomaterial is totally inert in the biological environment. Although they may not degrade themselves when implanted, all biomaterials elicit a response from ions, molecules, cells, or tissues in the body. Consequently, a biomaterial cannot be said to be truly bioinert.