The Use of Silicone/Polyurethane Graft Polymers as a Means of Eliminating Surface Cracking of

The long-term biodegradation of various polyurethanes with and without surface modifications was evaluated by implanting small porous filamentous patches of these materials subcutaneously in the backs of dogs for one month. Data were compared to those obtained with spun polyurethane vascular grafts of similar materials implanted in the aorto-iliac position in dogs. The ex-tremely high surface area of approximately 7 m2/cm3 of these porous filamen-tous patches provided numerous sites for surface cracking and the very fine filaments (10 microns in diameter) provided an easily identifiable structure to study the cracking phenomenon. Results from numerous one month implants clearly demonstrated that the subcutaneous implant model effectively repro-duced the biodegradation behavior observed in vascular graft implants. The degradation was most pronounced in the softer Shore 80A polyurethanes and less pronounced in the harder 55D and 75D polyurethanes. The degradation could not simply be stopped by stress annealing the polyurethane and the degradation did not require the presence of metallic ions. Antioxidants, surface adsorbed albumin, poly(2-hydroxyethyl-methacrylate) grafting, silicone copoly-merization, tetrafluoroethylene plasma discharge and the addition of urea linkages to the polymer were also shown to be ineffective in stopping the bio-degradation process. In contrast, covalent bonding or grafting of silicone polymer to the surface of the urethane successfully inhibited the biodegrada-tion process