On the Electropolishing and Surface Characteristics of Braided NiTi Shape Memory Alloy Stents

Medical stents are usually being produced by laser machining of tubes. NiTi shape memory alloys are gaining importance for peripheral, cardio- and neurovascular stent applications, because of their unique mechanical properties and easy deployment due to self-expansion. However, NiTi tube production is complicated and therefore expensive, because the material is hard-to-machine. Braiding is an interesting alternative method for the manufacturing of stents. It allows for the continuous production, using wires instead of tubes, which are much cheaper semi-products. In addition, laser machining has its limitations regarding the inner stent diameter, so that a further miniaturization of these implants is possible using the braiding process [1]. Almost all NiTi implants are being electropolished today in order to remove defects, to reduce the surface roughness, and to produce a thin, well-defined titanium dioxide layer on the surface. In this study, we examine the electropolishing of braided stents, which is complicated because of overlapping wires and small wire diameters (down to 20 µm). The goal of this process is to remove the thermal oxide layer after the shape setting and to improve fatigue and corrosion properties. In addition, previous work has shown that local plastic deformation occurs during braiding and we examine whether it is possible to remove these zones [2]. Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) are being used to characterize surfaces before and after electropolishing.