An experimental evaluation of the mechanics of bare and polymer-covered self-expanding wire braided stents

Self-expanding wire braided stents have been used in a wide-range of medical implant applications due to the distinct flexibility offered by the wide-range of tunable design parameters, which includes braid angle, wire diameter and braid pattern. Recently, there has been increasing attention on developing covered stent systems in endovascular repair, whereby the stent frame is wrapped with a graft or textile material, typically made from expanded polytetrafluoroethylene (ePTFE) or polyester (PET, Dacron). However, the addition of a polymeric cover to a wire braided stent fundamentally changes its mechanism(s) of deformation and there is distinct lack of understanding how the functional performance of these systems compares to their bare-metal counterparts. This paper presents the first systematic evaluation of the effect of a polymeric cover on braided stent mechanics using radial compression, axial compression and tension, kink deformation and stent elongation testing. Nitinol wire braided stents were manufactured with braid angles of α = 30°, α = 45°, and α = 60°, and subsequently covered with a polyurethane-silicone composite polymer with cover thicknesses of t = 25 μm and t = 100 μm. Results demonstrate that the response of both bare-metal and covered wire braided stents is heavily influenced by braid angle across all loading regimes. In particular, it was shown that the bare-metal stents exhibited higher stiffness under radial and axial loading when the direction of loading was closer aligned to the orientation of the wires. It was shown that covering stents with a polymeric cover led to a stiffer response across all braid angles and, in some cases, this could be up to two orders of magnitude greater when thicker covering systems were considered (t = 100 μm). Covered wire braided stents with braid angles of α = 30° and α = 45° show excellent potential for use in femoropopliteal applications, where the addition of 25 μm cover increased the radial resistive force but did not have any negative effects in terms of flexibility. The current analysis shows that use of a cover in braided stent mechanics is another variable parameter which can be used to produce optimum stent properties tailored to an application.