Biostability of materials and implants.

The purpose of this paper is to discuss some important parameters that are involved in the biostability of materials and implants aimed at the eventual development of improved in vitro testing and evaluation procedures. In view of the fact that certain terms have been the subject of misunderstandings, definitions are offered for the terms "biomaterials", "bioerosion", "biostability", and "bioresorption". Following brief descriptions of various classes of materials used in biomedical applications, the in vitro and in vivo degradations of selected materials are discussed. The main conclusions are as follows: (1) most synthetic polymers degrade in vivo by nonenzymatic hydrolysis. Hence, it is recommended that initial, in vitro testing schemes of most synthetic polymers and implants omit the use of enzyme solutions. The use of enzyme solutions is appropriate in the case of natural biopolymers as well as synthetic biodegradable polymers that contain peptidic-, glycosidic-, or phosphatidic bonds. (2) The biostability of materials and implants may be greatly affected by the simultaneous presence of stresses and active components in the physiologic environment that may lead to environmental stress cracking. (3) The biostability of polymeric materials is influenced not only by adsorption but also by the absorption of components in the physiologic environment.