Rotary Bend Fatigue of Nitinol to One Billion Cycles.

Nitinol implants, especially those used in cardiovascular applications, are typically expected to remain durable beyond 108 cycles, yet literature on ultra-high cycle fatigue of nitinol remains relatively scarce and its mechanisms not well understood. To investigate nitinol fatigue behavior in this domain, we conducted a multifaceted evaluation of nitinol wire subjected to rotary bend fatigue that included detailed material characterization and finite element analysis as well as post hoc analyses of the resulting fatigue life data. Below approximately 105 cycles, cyclic phase transformation, as predicted by computational simulations, was associated with fatigue failure. Between 105 and 108 cycles, fractures were relatively infrequent. Beyond 108 cycles, fatigue fractures were relatively common depending on the load level and other factors including the size of non-metallic inclusions present and the number of loading cycles. Given observations of both low cycle and ultra-high cycle fatigue fractures, a two-failure model may be more appropriate than the standard Coffin-Manson equation for characterizing nitinol fatigue life beyond 108 cycles. This work provides the first documented fatigue study of medical grade nitinol to 109 cycles, and the observations and insights described will be of value as design engineers seek to improve durability for future nitinol implants.

In Vitro Clot Trapping Efficiency of the FDA Generic Inferior Vena Cava Filter in an Anatomical Model: An Experimental Fluid-Structure Interaction Benchmark.

PURPOSE: Robust experimental data for performing validation of fluid-structure interaction (FSI) simulations of the transport of deformable solid bodies in internal flow are currently lacking. This in vitro experimental study characterizes the clot trapping efficiency of a new generic conical-type inferior vena cava (IVC) filter in a rigid anatomical model of the IVC with carefully characterized test conditions, fluid rheological properties, and clot mechanical properties. METHODS: Various sizes of spherical and cylindrical clots made of synthetic materials (nylon and polyacrylamide gel) and bovine blood are serially injected into the anatomical IVC model under worst-case exercise flow conditions. Clot trapping efficiencies and their uncertainties are then quantified for each combination of clot shape, size, and material. RESULTS: Experiments reveal the clot trapping efficiency increases with increasing clot diameter and length, with trapping efficiencies ranging from as low as approximately 42% for small 3.2 mm diameter spherical clots up to 100% for larger clot sizes. Because of the asymmetry of the anatomical IVC model, the data also reveal the iliac vein of clot origin influences the clot trapping efficiency, with the trapping efficiency for clots injected into the left iliac vein up to a factor of 7.5 times greater than that for clots injected into the right iliac (trapping efficiencies of approximately 10% versus 75%, respectively). CONCLUSION: Overall, this data set provides a benchmark for validating simulations predicting IVC filter clot trapping efficiency and, more generally, low-Reynolds number FSI modeling.