Processing and Characterization of Braided NiTi Microstents

Nickel Titanium shape memory alloys (NiTi SMA) have become an important material class in medicine today. One of its most important applications is for vessel implants, the so called stents. The processing route for these implants is relatively complicated and usually involves the laser cutting of the delicate structure into a thin NiTi tube. However, the production of stent tubes and the subsequent laser cutting are expensive processes, which require significant post-processing for the removal of artifacts and defect (e.g. burrs and heat affected zones). Therefore, the braiding of stents seems to be an interesting alternative method for the production of stents. Braiding uses the cheapest NiTi semi-finished product in the form of wires and offers the potential for continuous manufacturing. As a consequence, braiding of stents is economically very interesting. In literature, little has been published about the processing and the performance of textile NiTi stents. In the present work, the braiding process and the properties of these implants were studied. Mechanical uniaxial tensile tests and pull-pull fatigue cycling were performed at a simulated body temperature of 37°C. The surface of the stents was examined prior and after fatigue, because of the well-known dependence of the surface quality on fatigue life. Finally, we compare the mechanical properties of the braided stents with a similar-sized, laser-cut microstent.

Keywords: NiTi shape memory alloys, pseudoelasticity, stents, braiding, mechanical properties, fatigue