Parametric design of nitinol wire stents with pyFormex

Wire stents are a class of self-expandable stents braided from a set of fine wires and currently manufactured in a wide range of braiding patterns (single or multilayer) and materials (e.g. phynox, nitinol, (biodegradable) polymers). Braided stents tend to be very flexible, having the ability to be placed in tortuous anatomy while maintaining patency. Despite the promising clinical outcome of minimally invasive interventions with these braided wire stents, some drawbacks of this procedure still need further attention (e.g. restenosis, stent migration, artery straightening, side branch covering, etc.) and require additional investigation. Literature dedicated to the simulation of the mechanical behavior of braided wire stents is very scarce and often the stent(graft)s are simplified as virtual single sheets neglecting the actual wire stent composition. This lack of scientific efforts may partially be justified by considering the complexity of building the geometrical model of such a wire stent using classical CAD methodologies and subsequent (numerical) analysis of this family of stents. We developed an innovative modeling tool with in-house developed open-source software, called pyFormex, which allows to quickly build complex geometrical and finite element models of wire stents, useful in finite element simulations for studying the mechanical behavior of wire stents with arbitrary geometry and arbitrary material under complex loading conditions. The proposed modeling strategy easily generates lots of variations of the original geometry, an essential prerequisite for efficient stent design. This virtual design tool is validated both analytically and experimentally, by examining the mechanics of the Urolume (Wall)stent. In future, the same methodology will be used to optimize the behaviour of braided multilayer and knitted nitinol stents.