A Phase-Change-Material-Based Variable Stiffness Sheath Inspired by a Multi-Layer Wave Spring Structure for Flexible Upper Gastrointestinal Endoscopic Robots

Abstract

Continuum robots employed in flexible gastrointestinal endoscopy require the capability of transitioning between the flexible and the rigid states. Phase-change-material-based variable stiffness (VS) methods exhibit a significant stiffness change ratio but are typically time-consuming. Besides, these materials are commonly fabricated into simplistic cylindrical or tubular structures and subsequently integrated with continuum joints, overlooking the impact of the intrinsic structural characteristics of the VS module on stiffness modulation and bending performance. To maintain the combination of motion flexibility and operation stability, this work presents a stiffness-tunable sheath inspired by a multi-layer wave spring structure, which is fabricated utilizing thermoplastic material. A water-based active heating/cooling method is employed, wherein the circulation of hot/cold water through silicone tubes helically wound around the exterior of the VS sheath enables rapid thermal regulation. Structural parameters selection of the VS sheath based on the orthogonal design method has been performed to enhance its stiffness in a rigid state and reduce the maximum stress during 90° flexion in a flexible state. Experimental results indicate that the proposed VS sheath can achieve a stiffness change ratio of up to 16.5 times within 30s. After being integrated with a continuum joint, the sheath demonstrates an average positioning error of 1.48mm within a ±90° bending range in a flexible state, without structural compromise or interference with the continuum joint’s bending. In the rigid state, the proposed design can resist 400g external payload with a deflection of less than 6mm. The efficacy of this design has been validated through ex-vivo experiments conducted on a porcine stom