Biocompatible Polymers Used as Hydrophobic Matrices

One particular hydrophobic polymer, EVAc, has been investigated extensively as a matrix system for protein delivery. This polymer is biocompatible, a major consideration because of the interest in developing systems for human health. Other classes of hydrophobic polymers, like silicone elastomers and polyurethanes, may also be useful for controlled protein delivery, although there are fewer examples available in the literature. Nondegradable, hydrophilic polymers, such as poly(2-hydroxyethyl methacrylate) [p(HEMA)], are also biocompatible but usually release proteins over a relatively short period. However, a few examples oflong-term release of peptides and proteins from hydrophilic polymers are available. Longterm release of peptides from devices that employ cross-linked p(HEMA) as rate-limiting barriers has been reported (Davidson et al, 1988). The use of hydrophilic polymers for protein release is discussed in more detail elsewhere in this volume. 

POLY(ETHYLENE-co-VINYLACETATE) EVAc is a random copolymer with the structure 

The most commonly used EVAc (ELVAX-40, Du Pont) consists of approximately 40% vinyl acetate, with a low degree of crystallinity (5-20%). EVAc is hydrophobic it swells less than 0.8% in water (Hsu and Langer, 1985). 

Matrices composed of EVAc and protein can be fabricated by solvent evaporation or compression molding (Siegal and Langer, 1984). In solvent 

The biocompatibility of EVAc matrices has been studied quite extensively. When implanted in the cornea of rabbits, which is sensitive to the edema, white-cell infiltration, and neovascularization associated with inflammation, washed EVAc caused no inflammation unwashed EVAc caused mild inflammation (Langer and Folkman, 1977). After seven months of subcutaneous implantation, only a thin capsule of connective tissue surrounded EVAc implants no inflammation was present and the adjacent loose connective tissue was normal (Brown et al, 1983). When implanted in the brains of rats, EVAc matrices produce only mild gliosis (During et al, 1989). EVAc has shown good biocompatibility in humans over the years and has been approved by the U.S. Food and Drug Administration (FDA) for use in a variety of implanted and topically applied devices. 

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