A Phase-Change-Material-Based Variable Stiffness Sheath Inspired by a Multi-Layer Wave Spring Structure for Flexible Upper Gastrointestinal Endoscopic Robots
Dezhi Song, xiangyu luo, xiangyang Yu, Bo Zhang, Zhengbao Yang, Chengzhi Hu, Chaoyang Shi
AI summary
Problem
Existing phase-change material variable stiffness methods for endoscopic robots are typically slow and rely on simple cylindrical structures that neglect how structural geometry impacts stiffness modulation and bending performance.
Approach
The researchers designed a thin-walled, multi-layer wave spring-inspired sheath from a temperature-sensitive thermoplastic and integrated it with an active hot/cold water circulation system, optimizing its geometry via orthogonal design to balance rapid thermal switching with mechanical performance.
Key results
- Achieved a 16.5-fold stiffness change ratio within 30 seconds
- Maintained an average positioning error of 1.48 mm across a ±90° bending range
- Resisted a 400g external payload with under 6 mm deflection in the rigid state
- Successfully validated through ex-vivo porcine stomach experiments
Why it matters
Provides a fast, structurally optimized variable stiffness solution that enhances the safety, precision, and operational stability of minimally invasive gastrointestinal endoscopic robots.
Abstract
Continuum robots in flexible gastrointestinal en- doscopy require transitioning between flexible and rigid states. Phase-change-material-based variable stiffness (VS) methods ex- hibit 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 cir- culation 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 1.48 mm 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 Received 14 January 2025; accepted 5 May 2025. Date of publication 9 May 2025; date of current version 20 May 2025. This letter was recommended for publication by Associate Editor J. Kim and Editor J. Burgner-Kahrs upon evaluation of the reviewers’ comments. This work was supported in part by the National Natural Science Foundation of China under Grant 92148201 and Grant 52475029, and in part by the International Institute for Innovative Design and Intelligent Manufacturing of Tianjin University, Zhejiang, Shaoxing 312000, China. (Corresponding author: Chaoyang Shi.) Dezhi Song, Xiangyu Luo, and Chaoyang Shi are with the Key Laboratory of Mechanism Theory and Equipment Design of Ministry of Education, School of Mechanical Engineering, Tianjin University, Tianjin 300072, China (e-mail: chaoyang.shi@tju.edu.cn). Xiangyang Yu is with the Department of Gastrointestinal Surgery, Tianjin Hospital of ITCWM/Tianjin Nankai Hospital, Tianjin 300100, China. Bo Zhang is with Future Robotics Organization, Waseda University, Tokyo 1620044, Japan. Zhengbao Yang is with the Department of Mechanical and Aerospace Engi- neering, The Hong Kong University of Science and Technology, Hong Kong SAR 999077, China. Chengzhi Hu is with the Department of Mechanical and Energy Engineering, Southern University of Science and Technology, Shenzhen 518055, China. This article has supplementary downloadable material available at https://doi.org/10.1109/LRA.2025.3568564, provided by the authors. Digital Object Identifier 10.1109/LRA.2025.3568564 than 6 mm. The efficacy of this design has been validated through ex-vivo experiments on a porcine stomach.