Tissue response to commercial silicone and polyurethane elastomers after different sterilization procedures

Two different commercial polymeric materials, a silicone and a polyurethane (PUR), were studied with regard to correlations between the chemical and physical compositions of the polymer surfaces and the biological response on implantation. Test specimens of the materials were manufactured according to standard procedures. The specimens were implanted in rats for 10 and 90 days. Before implantation the polymers were sterilized in three different ways, namely, beta irradiation, ethylene oxide sterilization and steam sterilization. The polymers were characterized before and after the implantation with respect to the chemical composition and the morphology of the surfaces. After implantation the biological response was evaluated by counting numbers of macrophages, giant cells, fibroblasts and other cells present at the surfaces. The thickness of the fibrous capsule surrounding the test specimens was measured at the thickest and thinnest parts. PUR surfaces showed signs of degradation already after sterilization and after 10 to 90 days of implantation, pits and cracks appeared, especially in the ethylene oxide sterilized samples. However, differences in the biological responses were small and independent of the sterilization method. After 10 days of implantation the capsule thickness and the amounts of cell material adhering at the surfaces were different, and it appears that the silicone rubber induces more tissue response than PUR. The differences in the early tissue response evened out after 90 days implantation time and a steady state situation evolved, which was similar for the silicone and the polyurethane