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

Nitinol guidewires can give superior performance but can exhibit lag and whip depending on the material properties. Finite element modelling is used to simulate torsion of a Nitinol guidewire in an idealised tortuous path which consists of a curved section and a straight section of varying length, representative of geometries encountered in tortuous path navigation. The stress state in the wire during rotation in the curved path, which is dictated by the stress-strain hysteresis, leads to the generation of a net moment within the wire which requires net work to be done during rotation. This phenomenon is not present for a perfectly elastic material with no hysteresis and is the fundamental mechanism behind lag and whip. Results further show that the performance of the guidewire material can be related to a single metric that is given in terms of the energy dissipated during transformation, i.e., the area of the hysteresis loop. This relationship between performance and hysteresis loop size is consistent with experimental observations for Nitinol guidewires. 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 results show that both are accentuated by increasing the length of the straight section. Overall, these results provide a fundamental mechanistic understanding of the lag and whip phenomena and how they relate to the specific mechanical properties, and they are important in guiding Nitinol material formulation and processing to minimise hysteresis as a means to improve guidewire performance.