Tech Note: Study on the Effects of Oxide Layer Thickness and Composition on Corrosion in Nitinol Medical Devices

Confluent Medical Technologies has published the first part of an animal study in collaboration with the U.S. Food and Drug Administration (FDA) to understand the effects of oxide layer composition on corrosion resistance and biocompatibility of Nitinol medical devices.

Due to its superelastic and shape-memory properties, Nitinol is a commonly used material in medical devices, including peripheral stents, stent grafts, and heart valves. However, correlations between surface processing of Nitinol, in-vitro corrosion behavior, and in-vivo corrosion had yet to be established.

This was the first controlled study to correlate in-vitro corrosion testing with in-vivo corrosion performance of cardiovascular stents. While previous studies have shown the significance of surface processing on oxide layer composition and its effect on in-vitro localized corrosion behavior in Nitinol, the impact on nickel ion release and biocompatibility had yet to be studied.

The study found that Nitinol stent groups with breakdown potentials lower than 200 mV (millivolts) exhibited in-vivo corrosion after six months of implantation. Conversely, Nitinol stents with pits initiating at potentials higher than approximately 600 mV showed no signs of in-vivo corrosion after six months of implantation.

“The findings demonstrate that functional corrosion testing combined with a detailed understanding of the surface characteristics of a Nitinol medical device can provide insight into in-vivo corrosion resistance,” said Christine Trepanier, Vice President of Product Development & Engineering. “Elucidating the link between in-vitro corrosion and pre-clinical characterization can aid in improved prediction of clinical safety and performance of Nitinol devices.”

As part of the study, Confluent Medical fabricated four groups of Nitinol stents using different processing methods to create unique surface properties. The stents were implanted into iliac arteries of minipigs for six months and explanted for corrosion analysis. Scanning electron microscopy and energy dispersive X-ray spectrometry analyses indicated that stents with a thick complex thermal oxide and high corrosion resistance in-vitro were free from detectable corrosion in-vivo and exhibited no changes in Ni/Ti ratio when compared to non-implanted controls. This result was also found in mechanically polished stents with a thin native oxide.

In contrast, stents with a moderately thick thermal oxide and low corrosion resistance in-vitro possessed corrosion with associated surface microcracks in-vivo. In addition, Ni/Ti ratios in corroded regions were significantly lower compared to non-corroded adjacent areas on explanted stents. When stents were minimally processed (i.e., retained native tube oxide from the drawing process), a thick thermal oxide was present with low in-vitro corrosion resistance resulting in extensive in-vivo pitting.

The findings highlight the importance of surface characterization in Nitinol devices in addition to performance testing and provide in-vitro pitting corrosion levels that can induce in-vivo corrosion in Nitinol stents.

The first phase of the study, which was concluded in 2015, investigated the relationship between post-manufacturing surface composition and the impact of post-manufacturing handling to simulate the impact of loading implantable devices onto a delivery system on in-vitro corrosion behavior. It established for the first time the complex relationship between oxide layer composition and its impact on nickel ion release.

The details of this study have been published in Acta Biomaterialia (Volume 62, 15 October 2017, Pages 385-396). For more information about Confluent Medical Technologies, please contact sales@confluentmedical.com.