The Effect of Long Term Exposure of Medical Implant Materials to Simulated Body Fluids on Breakdown Potentials

Current methods to evaluate the corrosion resistance of small medical implants are typically based on short time in-vitro tests such as ASTM F2129 and F746.  While the duration of these tests are kept to a minimum to make it feasible to evaluate a large number of samples in a reasonable time, these methods do not account for the long term changes that can occur in the oxides of metals exposed to biological fluids.  The question of whether these changes lead to a decrease in the separation between the open circuit potential and the breakdown potential are core to the debate on a minimum breakdown potential criterion for ASTM F2129.  Proposals for minimum breakdown potentials, such as the absolute 600mV or 600mV delta to open circuit potential criteria, are based upon the assumption that breakdown potentials are a fundamental material parameter and do not vary with exposure time. This debate highlights the need for greater understanding of the effects of long term exposure to both simulated fluids, such as phosphate buffered saline, and body fluids.  Data on nitinol and 316 stainless steel wire samples and nitinol stents show that not only do the breakdown potentials change with time, but the difference between the breakdown potential and open circuit potential did not decrease for exposures up to 3 months.