Subcutaneous implants of polypropylene filaments.
Extruded filaments of unmodified polypropylene (PP) with and without antioxidant were implanted subcutaneously in hamsters in order to determine their rate of degradation. Specimens were removed periodically during a 5 month test period and analyzed by infrared spectroscopy and dynamic mechanical testing. The analyses show that degradation beigns to occur after only a few days. Although the reaction sequence is not known, several factors suggest that the in vivo degradation process is similar to autoxidation which occurs in air or oxygen. The infrared data indicate that the hydroxyl content of the implants increases at a rate of 0.061 mg/g polypropylene per day during the initiation phase of the reaction. An induction time of 108 days was extablished. Carbonyl bonds appear after an implantation time of 50--90 days and increase therafter. Mechanical tests indicate a decrease in the dynamic loss tangent, tan delta, during the first month of implantation for unmodified polypropylene. No change in the infrared spectra or tan delta was observed, however, for implants containing an antioxidant. Thus, it is apparent that polypropylene filaments implanted subcutaneously in hamsters degrade by an oxidation process which is retarded effectively by using an antioxidant. While the findings reported are specific to subcutaneous polypropylene implants, they suggest that degradation of other systems may involve similar processes. This notion suggests directions for further research on increasing the in vivo stability of synthetic polymers. Long-term effects of polymer implantation upon tissue were not studied in this work.