Clinical studies show that Nitinol (Nita) stent fracture occurs in a surprisingly high percentage (65%). The non-benign cases are asymptomatic, and the association of thrombosis and restenosis with stent fracture was observed. To improve current stenting technology, new shape memory alloys (Smash) with superior fatigue properties must be developed. Recently, a new understanding has been reached on the origins of reversibility of phase transformations, and this understanding translates well into an alloy development program; several new very low hysteresis alloys have recently been discovered by this method. There is a sound scientific basis for the idea that low hysteresis correlates with long fatigue life. We propose to exploit these very recent developments in the context of combinatorial synthesis, to discover new Smash with resistance to crack initiation comparable to 316L stainless steel. The specific hypothesis is that a high degree of reversibility of SMA alloys is dictated by certain special lattice parameters of the two phases that permit an exceptionally high degree of matching of the phases. Precise criteria for low hysteresis emerge from this idea.
The specific aims are to: 1) Use advanced methods of combinatorial synthesis to identify low hysteresis alloys. This involves the creation of a library of SMA samples with various compositions. The methods of combinatorial thin film synthesis involving high throughput sample preparation integrated with synchrotron x-ray analysis will be used. These powerful new methods allow us to quickly establish the mapping from composition to lattice parameters of the phases. The theoretical criteria can then be used to identify exceptionally low hysteresis alloys. We propose to apply these methods to NiTiX, where X = Cu, Pd, Pt, Au. 2) Characterize the fracture of the selected samples under conditions of cyclic transformation, and establish the correlation between fracture resistance and low hysteresis. To establish a quantitative scientific basis for selection of alloys, we wilI (I) select a series of compositions with the theoretical criteria more and more closely satisfied and (ii) systematically measure the crack initiation stress intensity threshold of the selected samples. Timely discovery of a more fatigue resistant SMA would result in significantly improved devices for patients. Specifically, for peripheral stents, improving the alloy to meet fatigue requirements would substantially lessen the existing extensive trade-offs in vessel support and deliverability. ? ? ?
