ASTM F2129 Cyclic-Potentiodynamic Polarization Testing of Nitinol Stents Indicated for Use in the GI Tract

Nitinol self-expanding stents are used to treat disease in all areas of the human gastrointestinal (GI) tract. Chemistry along the GI tract changes both spatially and temporally with pH ranging from 1.2 in the stomach to 8.5 in the common bile duct. A variety of secretions add ions, digestive enzymes, and proteins to the environment along the GI tract. Establishing absolute acceptance criteria for corrosion resistance of metallic implants presents a unique challenge in this constantly changing environment. In this study, cyclic potentiodynamic polarization corrosion tests were conducted over several years at multiple labs in accordance with the requirements of ASTM F2129. Braided Nitinol wire stents with thermal oxide surface representing a range of GI tract stents with good clinical history were tested in solutions selected to simulate the target anatomy in the human gastrointestinal tract. The results suggest that acceptance criteria for devices tested in simulated vascular environments do not reflect requirements for successful devices used in the varied chemical environments of the GI tract. Breakdown voltage acceptance criterion need to consider the clinical application and in-body use environment. Inherent test variability must also be considered when demonstrating statistical equivalence to clinically successful comparator devices or fixed value specifications.