Fracture of Polymer-Coated Nitinol During Gamma Sterilization

Abstract

After gamma sterilization of a packaged medical device, fractures were discovered in the superelastic nitinol wire used as part of the assembly. The nitinol wire was encased in fluorinated ethylene propylene (FEP) shrink tube. The only fractures occurred where the encased wire was held under strain during gamma sterilization. A study was conducted to determine the susceptibility of nitinol to this type of failure. The variables studied included wire diameter, wire surface finish, wire oxide layer, quantity of wires encased, type of tubing, and strain level during gamma sterilization. The greatest susceptibility to fracture occurred to single wire samples with a light oxide layer held under high strain in FEP shrink tube. Gamma sterilization experiments were conducted to isolate and confirm this failure mechanism. Scanning electron microscopy was used to analyze the fractured samples. Chemical analysis was performed in an attempt to detect trace elements to determine the root cause of the failures. Stress corrosion cracking caused by the liberation of fluorine due to the degradation of the polymer during gamma sterilization is suspected.