ASTM F2129 Cyclic-Potentiodynamic Polarization Testing of Nitinol Stents Indicated for Use in the GI Tract
Abstract:
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 tract. Establishing absolute acceptance criteria for corrosion resistance of metallic implants presents a unique challenge in this constantly changing environment.
Cyclic potentio-dynamic polarization corrosion tests per ASTM F2129 were conducted in solutions covering the range of GI tract environments. Breakdown voltage of braided nitinol stents were shown to change by a factor of 10 with changes in the test solution chemistry and pH for a given use environment. Stents for use in the pancreatobiliary system were tested in simulated intestinal fluid adjusted to pH 4.2 and experienced pitting breakdown in less than 15% of devices at Eb greater than 0.900 V. Identical stents tested in 0.9 w/v % NaCl adjusted to pH 4.2 experienced pitting breakdown in 100% of devices at Eb of 0.155 V. Eb results for esophageal, gastric, duodenal and colonic environments show similar trends to lower breakdown voltage as pH decreases and saline content increases.