Instruments used in endoscopic surgery (colonoscopy surgery or natural orifice transluminal endoscopic surgery (NOTES)) are flexible to be advanced in human body. However, when the end of the instrument reaches the target, the instrument should be rigid enough to hold its shape against external forces for better surgical accuracy. In order to obtain these two properties, a variable stiffness over tube based on low-melting-point-alloy (LMPA) is proposed in this paper. The structure exploits the phase transformation property of the LMPA which enables the stiffness change of the over tube by heating and cooling. A prototype was fabricated using a special molding method, and experiments were carried out to evaluate its variable stiffness property and response characteristics. According to experimental results, it costs 17 s to make the over tube transform from rigid state to flexible state and 18 s to make the over tube transform from flexible state to rigid state. The experimental results also indicated that the over tube is very rigid in rigid state and flexible in compliant state. A heat insulation layer was assembled to prevent human tissue from thermal damage. The temperature of the outer wall of the over tube was 42.5 °C when hot water of 80 °C was pumped into the over tube continually with the help of the heat insulation layer.

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