Corrosion failure of nitinol stents in the upper gastrointestinal tract: The role of surface finishes and the importance of an appropriate test environment

Esophageal nitinol stents are an established method for treating swallowing difficulties caused by obstructing cancer. As a consequence of improved patient survival, stent failure and an increasing need for re-intervention is a rapidly emerging problem. This research investigates the influence of different quality of raw metal alloys in combination with production technology on corrosion resistance. Four different international stent manufacturers produced samples of their standard stents from nitinol sourced from three different alloy manufacturers. The corrosion resistance was evaluated by cyclic potentiodynamic scans according to the ASTM standards in phosphate buffered saline solution. Because of targeted application of the stent in gastrointestinal tract, the simulated gastric fluid was also used for corrosion measurement. In addition, the surface of the samples was studied by X-ray Photoelectron Spectroscopy and Mott-Schottky analysis. Results demonstrated that both, raw material and certain steps in the manufacturing process affects corrosion resistance. Conversely, other certain measures in the manufacturing process are able to reduce the impact of the base material on corrosion susceptibility. Furthermore, we could demonstrate that a key factor influencing corrosion resistance is the concentration of nickel on the surface. Current accepted standard test methods with phosphate buffered saline solution are inadequate for assessing susceptibility to corrosion by gastric acid and should take the low pH of the implanted environment into account. Understanding the corrosive process and the impact of surface finishes is key to developing stents suitable for long-term palliation.