Design and modelling of a variable stiffness manipulator for surgical robots

In Natural Orifice Transluminal Endoscopic Surgery (NOTES), a surgical robot that can access the human colon or stomach via natural orifices should have sufficient flexibility to pass through tortuous paths and to be operated in a confined space. In addition, the robot should possess an acceptable stiffness level to hold payloads during the surgery. This paper presents a new design concept for variable stiffness manipulators using thermoplastic material Polyethylene Terephthalate (PET) and a flexible stainless steel sheath as a heating media. The stiffness phases of PET can be actively adjusted through temperature. Experiments at different conditions showed that the proposed design was at least as flexible as a typical commercial endoscope in compliant mode and at least 9 times stiffer than the endoscope in stiff mode. In addition, flexural modulus of the proposed manipulator with respect to temperature, current, and time was modeled and validated through both simulation and experiments. A tendon-driven flexible robotic arm integrated with a variable stiffness spine was also developed, and ex vivo tests on fresh porcine tissue were conducted. The manipulator in compliant mode can be easily controlled through the tendons, and it is able to hold its shape against considerably large loads in stiff mode. The results demonstrate not only the high potential of the design concept for the future medical application but also the first steps toward building a complete surgical robotic system with fully controlled variable stiffness.