Impact of Polymer Coverings on the Corrosion Resistance of Nitinol

Nitinol frames are often covered with polymer(s) in cardiovascular medical devices to direct blood flow or maintain structural integrity. Due to their insulating properties, coverings pose a challenge to accurately assess the corrosion resistance of the device. Potentiodynamic polarization testing was performed on electropolished and thermal oxide Nitinol stent rings with either an electrospun TPU, dip-coated silicone, or ePTFE/FEP covering to evaluate its impact on Nitinol corrosion resistance. Stent rings were tested fully covered, partially covered (no silicone group), and with the covering removed. For the silicone and ePTFE/FEP groups, uncovered stent rings with a simulated thermal history representative of their covering processes were also tested. The results indicated that breakdown potentials (E _b ) were similar between the different TPU groups within each surface finish. In contrast, the silicone group displayed lower E _b values for the covering removed and simulated thermal history compared to the as-manufactured electropolished rings. Substantial variability in electrochemical behavior was observed in the ePTFE-covered group, particularly in the covering removed group that possessed significantly lower E _b values compared to the as-manufactured electropolished stent rings. These findings highlight the importance of test article selection for pitting corrosion assessments of Nitinol medical products covered with a non-conductive polymer.