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

Continuum robots in flexible gastrointestinal endoscopy require transitioning between flexible and rigid states. Phase-change-material-based variable stiffness (VS) methods exhibit a significant stiffness change ratio but are typically time-consuming. These materials are commonly fabricated as simple cylindrical or tubular structures and integrated with continuum joints, overlooking how the VS module's structural characteristics affect stiffness modulation and bending performance. To balance motion flexibility and operational stability, this work presents a stiffness-tunable sheath inspired by a multi-layer wave spring structure, fabricated utilizing thermoplastic material. A water-based active heating/cooling method is employed, wherein the circulation of hot/cold water through silicone tubes helically wrapped around 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 rigid-state stiffness and reduce maximum stress during 90° flexion in a flexible state. Experimental results showed the proposed VS sheath achieves 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 a 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 on a porcine stomach.