Physical Simulation of the Random Failure of Implanted Braided NiTi Stents

Gastrointenstinal, esophaegal and/or tracheal NiTi stents braided from NiTi wires have been reported in the medical literature to suffer occasionally from unexpected embrittlement and subsequent failure which does not seem to be directly related to mechanical fatigue.  The NiTi wire (bare or covered with polymer) in these stents is exposed to aggressive biological environment and undergoes significant deformation when implanted in human body. This randomly encountered failure is assumed to be due to synergic effects of corrosion in biological fluids and long term mechanical cycling in combined bending and torsion. The exact mechanism, however, has not been yet known.

   In this work, as a kind of physical simulation of the mechanochemical fatigue of the implanted NiTi stent, cyclic deformation of superelastic helical NiTi springs submerged in various biological fluids at 37°C was systematically investigated using a selfmade testing set up.  NiTi springs were shape set using the same technology as the braided stents. The deformation state of NiTi wire in the cycled spring is adjusted by setting geometrical characteristics of the spring and cyclic test parameters. Fatigue to failure approach was employed in the fatigue testing, in spite of the large number of cycles (tensile cycling of spring between two preset limits) reaching the range of several tens of millions of cycles for lower stroke amplitudes. The role of three different surface oxide layers, one of them corresponding to the actual heat treatment employed in the stent production, was investigated. The quality and integrity of the surface oxides on the cycled NiTi springs were characterized by electrochemical potenciodynamic tests, local electron microscopy and local chemical analysis. Mechanism of the embrittlement and fracture of cycled NiTi helical springs is discussed based on the results and generalized towards rationalization of the random clinical failure of the braided NiTi stents.