Comparison of the Fatigue Performance of Commercially Produced Nitinol Samples Versus Sputter Deposited Nitinol

Self expanding vascular implants are typically manufactured from Nitinol tubing, using laser cutting, shape setting and electropolishing processing. The mechanical and fatigue behaviour of those devices is affected by the raw material and its processing, for example the melting process and subsequent warm and cold forming processes. Current trends focus on the use of raw material with less inclusions to improve the fatigue performance. Further device miniaturization and higher fatigue life requirements will drive the need towards smaller inclusions.

Micro tensile test samples and micro-diamond-shape samples were produced by laser cutting from commercial sheet materials and also using a sputter deposition and lithographic process. All micro tensile test samples received a thermo-mechanical or heat treatment processing that was geared towards achieving a comparable Af-temperature (austenite finished temperature). Tensile testing was performed at 37°C to determine mechanical properties, such as elastic modulus, tensile strength, and plateau strengths. Differential scanning calorimetry and metalographical investigations were performed to assess the impact of cold work and the final annealing steps as well as the thermal shape-setting. Finally, the fatigue life of these materials was compared using micro-diamond-shape test samples with an accelerated fatigue test based on a fatigue-to-fracture test method.

All results were put into a matrix in order to correlate the fatigue results with the overall material process and thermo-mechanical impact such as nucleated precipitations, inclusion sizes & densities and fracture morphology. In general, however, the data suggest that the sputter deposited Nitinol thin film material exhibits an improved fatigue-life in comparison to standard Nitinol.