A novel method to aid stent and catheter design based on practical investigations observing in-vitro endoluminal delivery and 3-D movement of Nitinol wire during the Shape Memory Effect

The original purpose of this work was to investigate a stent design to accommodate the acute curvature of the aorta negating the need for linking multiple stents. Shape Memory Alloy (SMA) stents are used to dilate or strengthen hollow organs including sections of the aortic arch. Typically a SMA stent utilising the Shape Memory Effect (SME) is endoluminally delivered through a catheter to the stent site, whilst maintaining the stent temperature below the stent transition temperature range. A limiting factor when multiple stents must be linked together is the acute radius of curvature of the aorta compared with the radius of curvature which can be achieved by the linked multiple stents. A preliminary case study to determine the feasibility of manufacture and delivery of a custom made SMES was undertaken using NiTi material. Computer Tomography 3D data was used to determine basic stent dimensions of an aortic arch from which a bespoke SMES was manufactured. Preliminary design investigations concluded that control of SMES temperature during delivery and 3D path taken by SMES material as it reconfigures through transition temperature range As-Af, are critical single points of failure. Using classical heat transfer equations a mathematical model to determine temperature of SMES during transfemoral delivery from femoral artery to aortic arch region was compared with in vitro data and validated. 2D and 3D movement paths of NiTi wire samples during reconfiguration was observed and recorded. Results indicate a direct relationship between stress applied to SMA wire in its martensitic phase and 3D path taken by said SMA wire during reconfiguration through TTR from Austenite start to Austenite finish phase. The basis of this study suggests predictability of 3D behaviour and therefore safety of SMES catheter delivery using through flushing coolant and design of stent reconfiguration geometries may be considered in design. The temperature model may be used to aid NiTi stent delivery catheter design. The relationship between production of stress induced martensite (SIM) before production of temperature induced martensite (TIM) and actual movement path during TIM may be used as an aid in future SMES endoluminal delivery design.