N. Corlett, Exponent International, Harrogate, United Kingdom; A. Nissan, L. E. Eiselstein, D. L. Steffey, Exponent, Menlo Park, CA
Implantable medical devices need to possess sufficient resistance to localized corrosion to withstand degradation during in vivo service. Manufacturers are typically required to demonstrate a device’s corrosion resistance through in vitro testing. A common measure of nitinol’s susceptibility to localized corrosion is given by the breakdown potential (Eb) relative to the rest potential (Er), as determined by in vitro cyclic potentiodynamic polarization testing. Long-term in vivo exposure, however, may affect the gap between Eb and Er. This study, which builds on previous research that was presented at SMST 2007, investigates the effect of aeration on Er, the long-term potential drift (ΔEr), Eb, and the gap Eb-Er.In this study, mechanically-polished nitinol (MP NiTi) wires were immersed in phosphate-buffered saline (PBS) at 37 ºC for periods of up to thirty days prior to performing cyclic potentiodynamic polarization tests. The polarization tests were performed in accordance with ASTM International Standard F 2129: ‘Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices’. Aeration was achieved using laboratory air and a blood-gas mix. Results are presented showing the effect of immersion time and aeration on Er, ΔEr, Eb, and Eb-Er.
Summary: One of the challenges faced by manufacturers is using short-term in vitro data to evaluate long-term in vivo corrosion performance of implantable medical devices. In this study, which builds on previous research that was presented at SMST 2007, mechanically-polished and electropolished nitinol wires were immersed in phosphate-buffered saline at 37 ºC for periods of up to thirty days prior to performing cyclic polarization tests in accordance with ASTM F 2129. The effect of aeration using laboratory air and a blood-gas mix during immersion was investigated. Results are presented showing the effect of immersion time and aeration on the rest potential, the long-term rest-potential drift, the breakdown potential, and the gap between the breakdown potential and the rest potential.
