*Invited* Is Resistance to Localized Corrosion Per ASTM F2129 A Good Indicator of Resistance to Crevice Corrosion and Uniform Corrosion Rate of Nitinol Implantable Devices?

There is a need to understand the potential correlation between localized corrosion, resistance to crevice corrosion and nickel ion release of Nitinol devices.  Currently, the FDA Guidance for Intravascular Stents includes the characterization of material composition and corrosion resistance of devices as part of device safety assessments.  ASTM F2129 is a FDA-recognized method for assessing the corrosion resistance of small medical devices using electrical breakdown potentials.  While acceptance criteria for ASTM F2129 have been proposed in the literature, the criteria have not been directly related to other non-clinical methods for device assessment.   This study aims to establish relationships between the proposed F2129 acceptance criteria and additional bench analyses that assess stent corrosion behavior. 

Generic stents designed using the Open Source Stent tool developed by NDC were processed using common manufacturing techniques of thermal oxidation, mechanical polishing, and electropolishing to target specific corrosion breakdown potentials as described in the Table below.

Final Surface treatment	Breakdown potential    avg ±st dev          (mV vs SCE)
Air furnace	90 ± 73
Mechanical Polishing	578 ±269
Salt pot	Oxygen evolution
Electropolishing	Oxygen evolution

Analysis of the oxide layer with Auger Electron Spectroscopy indicates that the surfaces of air furnace and salt pot specimens are covered by a complex oxide layer whereas mechanically polished and electropolished specimens are covered by a simple titanium oxide layer.  Stents from each group were evaluated for their resistance to crevice corrosion and repassivation of localized corrosion by overlapping specimens and testing them per a variation of ASTM F746.    Samples from each processing group were immersed in PBS solution for various time points and the solution was analyzed with ICP-MS to assess Ni ion release with time.  Trends between processing methods, surface characteristics, benchtop test results and the application to in vivo studies will be discussed.