Session: 06-09-04: Computational Modeling in Biomedical Applications - IV

Paper Number: 119424

119424 - Computational Modelling of the Mechanics of Nitinol Guidewires in a Tortuous Path for Medical Device Applications 

Guidewires are critically important components in medical implant and device delivery systems and are typically used to facilitate the placement and exchange of interventional devices during diagnostic or therapeutic procedures. Guidewires are often constructed from a Nitinol core enclosed within a polymer jacket, where the core behaviour dominates the mechanical performance. In clinical applications, guidewire “steerability” is critically important and this requires a good torque response, i.e., the resulting 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 as a guidewire material can give superior performance but has been known to exhibit lag and whip depending on the specific mechanical properties of the material, that are directly dependent on the specific material formulation and processing route.

In this work, we investigate the torsional deformation of Nitinol guidewires to generate a fundamental mechanistic understanding of the lag and whip phenomena and how they relate to the specific mechanical properties of the material, and based on this, to make Nitinol mechanical property selection recommendations in order to minimise these undesirable effects. This work makes a significant contribution, in particular in terms of elucidating the underlying mechanism responsible for lag and whip, and how this knowledge can be used to influence Nitinol material formulation selection and processing for this critically important application domain.

Computational modelling, using the finite element method, is used to simulate the torsional deformation of a Nitinol guidewire in an idealised tortuous path, with a focus on the torque transmission or “torquability” of the guidewire. The path geometry consists of a curved section and a straight section of varying length, selected to be representative of geometries encountered in tortuous path navigation. Nitinol typically exhibits loading-unloading hysteresis in this stress-strain response, due to an austenite-martensite phase transformation, and in this work a range of Nitinol variants are considered, including Nitinol in binary alloy and ternary (NiTiCr) alloy forms, all of which exhibit significant hysteresis, and linear superelastic Nitinol, which exhibits significantly reduced hysteresis.  

The results reveal that 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 torquability 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 torquability 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 is critically important in the generation of lag and whip, and results show that both are accentuated by increasing the length of the straight section once the phenomena are active. Overall, the results of this study are important in guiding Nitinol material formulation and processing to minimise hysteresis as a means to improve guidewire torquability.

Presenting Author: Peter McHugh University of Galway

Presenting Author Biography: Peter McHugh is Professor of Biomedical Engineering at University of Galway. He holds a BE in Mechanical Engineering (University of Galway, 1987), an MSc (1990) and PhD (1992) in Solid Mechanics from Brown University, Providence, USA, and a DSc (National University of Ireland, 2021). He joined University of Galway in 1991, where he is currently the Established Professor of Biomedical Engineering. His research is focused on applying computational and experimental methods to investigate the biomechanics of tissue and of medical implants and devices. He has been particularly active in the cardiovascular device domain, and he is an internationally recognised authority on coronary stent mechanics. He was the founding Director of the Biomechanics Research Centre (BioMEC) at University of Galway. He has a significant publication record, with 171 refereed journal publications, 10 book chapters and over 340 conference publications. He has supervised to completion 31 PhD and 23 research masters students. He has generated over €13 m in research funding from national, EU and industry sources, and has generated active research collaborations with international leaders in the field, spanning Europe and the USA. He has received numerous awards, including membership of the Irish Academy of Engineering (2019), Royal Irish Academy (2011), the Presidential Nominee Fellowship of Engineers Ireland in (2009), and the Alexander von Humboldt Fellowship (1995).

Authors:

William Ronan University of Galway
Donnacha Mcgrath University of Galway
Reyhaneh Shirazi University of Galway
Marie Clancy Integer Holdings Corporation
Roger Dickenson Integer Holdings Corporation
Peter McHugh University of Galway