Medical care in the gastrointestinal (GI) tract is a major global issue. Soft actuators are expected to solve associated issues such as poor accessibility and difficult operability within the GI tract. The actuators will be inserted into the body through the mouth or anus with a small diameter, perform various tasks in the GI tract with a large diameter, and finally be removed again. Therefore, deployability and retractivity are common requirements. Variable stiffness is also required to adjust or maintain forces on weak tissues. We proposed the new deployable and stiffness-variable miniature actuator consisting of a shape memory polymer bar and flexible channel part with water circulation, which is useful for medical applications in the GI tract. We established the design method of the actuator based on derived physical models and the fabrication method of prototypes. We evaluated the performances of thermal response, retractive deformation, and variable stiffness and confirmed the validity of the concept through the demonstration of continuous actuation, including deploying, retracting, and stiffness-varying. Furthermore, as a case study, we verified the feasibility of endoscopic submucosal dissection traction using prototypes and artificial materials. In the future, the actuator mechanism and design method may also contribute to the development of other medical tools interacting with delicate tissues in the GI tract.
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