Fracture of Polymer Coated Nitinol During Gamma Sterilization

After gamma sterilization of a packaged medical device, fractures were discovered in the superelastic Nitinol wire used to as part of the assembly.  The Nitinol wire was encased in FEP (fluorinated ethylene propylene) 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 is isolate and confirm this failure mechanism.  Analysis of the test samples will be provided to explain this phenomenon.