Physical Simulation of the Random Failure of Implanted Braided NiTi Stents
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A problem of random clinical failures of the braided esophageal NiTi stents has been addressed by performing physical simulation experiments on helical NiTi springs loaded in cyclic tension in air, water, and simulated biological fluid. Strains and stresses involved in spring deformation were analyzed through simulation by FEM implemented SMA model. It was found that the fatigue life of NiTi springs is significantly lower in fluids than in the air pointing toward the corrosion fatigue mechanism. There is, however, a fatigue limit roughly corresponding to the onset of martensitic transformation in the wire, which is not common for corrosion fatigue. It is proposed that surface TiO2 oxide cracking plays major role in that. Once the oxide layer on the NiTi wire surface fractures, typically during the first mechanical cycle, cracks in the oxide layer periodically open and close during subsequent mechanical cycling. This leads to the localization of mechanical and corrosion attacks under the oxide cracked regions. Microcracks within the surface oxide layer crossing over into the NiTi matrix were indeed revealed by scanning electron microscopy of FIB sections of fatigued wires. A corrosion assisted mechanism for fatigue crack nucleation at the interface between the surface oxide and NiTi matrix is proposed based on the available evidence. The approach opens a space for a better assessment of the corrosion fatigue performance of superelastic NiTi and ultimately for estimation of the lifetime of implanted braided NiTi stents.
Keywordselectron microscopy failure analysis mechanical testing
This research has been supported by the 7RPEU project SMARTNETS (FP7-NMP2010-SMALL-262806) and by the projects P107/12/0800 and P108-12-P111 of the Grant Agency of the Czech Republic. The authors would like to express special thanks to Prof. Jan Van Humbeeck from MTM Department KU Leuven for consultancy a providing access to the FEI SEM facility.
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