In Vivo and In Vitro Nitinol Corrosion Properties

Regulatory authorities often require in vitro testing on medical devices prior to approval. Current standardized corrosion testing methods (ASTM F2129) require testing in a non-physiologic, de-oxygenated solution for a pre-exposure time of approximately one hour; however, no correlations between the prescribed simulated environment and whole blood conditions have been elucidated. This study compared open circuit potential (OCP), breakdown potentials (E_b), Eb - OCP, and cyclic polarization curves tested in vivo (OCP only) and in vitro in whole blood to those tested in phosphate-buffered saline (PBS). Two oxide thicknesses of Nitinol, two solution oxygen contents (deaerated and aerated solutions), and two pre-exposure durations (acute and chronic) were investigated. The in vitro OCP in whole blood was not significantly different than the in vivo OCP, suggesting that whole blood in vitro can be used to determine baseline corrosion behavior of medical implants. E_b - OCP tested per ASTM F2129 was comparable to acute whole blood and was conservative compared to chronic whole blood for both oxide thicknesses. However, OCP, E_b, and cyclic polarization curves were not always comparable to whole blood. Testing in aerated PBS achieved E_b, E_b - OCP, and cyclic polarization curves that were comparable to or more conservative than whole blood testing, regardless of pre-exposure duration and oxide thickness.