Computational modelling of the mechanical performance of nitinol guidewires in an idealised tortuous path for medical device applications

Abstract

Guidewires are critically important components in medical implant and device delivery systems and their desired clinical performance or “steerability” requires a good torque response, i.e., the rotation of the (distal) guidewire tip should follow exactly the (proximal) applied input rotation. However, guidewires can suffer from phenomena known as lag (tip rotation is significantly less than the input rotation) and whip (the lag is suddenly recovered). Nitinol is a common guidewire material that can give superior performance but has been known to exhibit lag and whip depending on the specific material formulation and processing route. In this study, the torsional response of Nitinol guidewires is investigated using computational modelling (the finite element method) and analytical approaches, with a view to gaining a fundamental understanding of the mechanisms behind the lag and whip phenomena and how these relate to the specific material properties of a range of Nitinol variants and the geometrical configuration of the wire during torsion. An idealised vascular geometry is considered; this consists of a curved section and a straight section of varying length where the curved section is representative of geometries that are encountered in tortuous path navigation in the vasculature.The results capture the experimentally observed phenomena and reveal the relationship between material properties and guidewire performance. The stress state in the wire, which is dictated by the stress plateaus in the Nitinol material response, leads to the generation of a net moment within the wire which requires net work to be done during rotation of the wire. Analysis of idealised hypothetical materials show that the transformation strain is also an important parameter. The analysis reduced the performance of the guidewire material to a single metric that is given in terms of the energy dissipated during transformation, i.e., the area of the hysteresis loop. The results show that the combination of torsion and the bending of the wire in the curved path are critically important in the generation of lag and whip, and that both are accentuated by increasing the length of the straight section once the phenomena are active.