Multiscale Architecture Governs Stability in Suction-Actuated Variable Stiffness Catheters

Endovascular procedures require devices with widely varying mechanical properties: flexibility for navigating tortuous vessels, and rigidity for stable therapeutic delivery. Suction-actuated variable stiffness sheaths attempt to address this challenge by incorporating axial wire string arrays that couple mechanically under suction to increase flexural rigidity. However, prototype devices achieve stiffening ratios well below theoretical predictions, suggesting that string array positioning and interlayer mechanics require investigation. We therefore investigated whether interweaving expanded polytetrafluoroethylene (PTFE) tape within the string array can enhance flexural rigidity modulation and improve bending uniformity along the catheter length. Prototypes with varying PTFE wrap configurations were fabricated and evaluated using three complementary approaches. First, flexural testing revealed that flexural rigidity in the actuated and unactuated states was largely unaffected by wrapping for small deformations. Second, curvature stability testing revealed that deformation through acute simulated vascular bends was higher than expected due to two failure mechanisms: slip, in which string arrays migrate after overcoming wrap-imposed friction, and buckling, in which arrays become locally pinned and deflect against the outer lumen. Finally, a mathematical model characterized stability limits as functions of the wrapping architecture and device mechanical properties, revealing criteria for which device performance improved. Results suggest design principles for intracatheter wrapping that can narrow the gap between theoretical and achieved flexural rigidity ratios, potentially contributing to the development of endovascular devices capable of single-sheath navigation and intervention.