Biostability of Biomedical Polymers
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/SEPTEMBER1991
perspective. The purpose of this review article is to provide an overview and perspectives on the issues of biomédical polymer biostability. Material or Device Variables and Biostability Three major factors affect the biostability/ biodégradation of biomédical polymers: (1) the in vivo service life of the biomédical polymer or device; (2) the loss of function of the device or prosthesis during the in vivo service life; and (3) the présence or absence of adverse effects on physiological Systems, e.g., body fluids, tissues, and organs. Conceptually, a biomédical polymer may biodegrade but depending on the service life, extent of functional loss, and degree of physiological interaction, the biomédical polymer may be considered either biodégradable or biostable. It is important to define the service life and functional requirements of a biomédical polymer before considering its potential for biostability or biodégradation. With the biostability/biodegradation of a biomédical polymer in mind, the final product suitable for application should be the focus. That is, in addition to the biomédical polymer, ail processes involved in creating the final product from this biomédical polymer should be weighed with regard to material or device variables which can influence biostability/biodegradation. Table I provides a partial list of polymer characteristics and properties, processing or fabrication methods, and cleaning and sterilization techniques which can control the properties of the final product. Thèse variables can potentially control the service life, the function of a prosthesis or a device, and the degree of physiological interaction. Physiological Interactions and Biostability Physiological interactions with the bio-
médical polymer surface or bulk may control biostability/biodegradation phenomena. Table II provides a partial list of physiological agents and interactions proposed to lead to biomédical polymer structure or property changes. Thèse structure or property changes hâve been linked to biodégradation phenomena through the loss of function of a material, device, or prosthesis during the in vivo service life or by the présence of adverse effects on physiological Systems. The three most commonly observed physiological interactions with biomédical polymers are adsorption and swelling of biological components, leading to altérations in the topography and geometrical dimensions of the material, acid and enzyme catalyzed reactions, which lead to surface chemistry changes and réductions in molecular weight and molecular weight distribution, and oxidative processes, which also lead to surface chemistry changes and a réduction in molecular weight. The failure of some polyurethane car-
Table I: Materials or Device Variables That Can Influence Biomédical Polymer Biostabillty/Biodegradation. Polymer Characteristics and Properties Chemical structure and chain regularity Molecular weight and molecular weight distribution Cross-linking Residual monomer and/or catalyst residue Additives Antioxidants Diffusion
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