Towards a continuum variable-stiffness robotic arm for surgical applications

Abstract

Natural Orifice Transluminal Endoscopic Surgery (NOTES) is a potential paradigm shift of Minimally Invasive Surgery that makes use of natural orifices to access the peritoneum for surgery without leaving visible scars. With NOTES, there are various benefits for patients such as fewer complications, improved precision, and faster recovery. In NOTES, the endoscope needs to be sufficiently flexible to go through the tortuous paths inside the human body to reach the target therapeutic site; once at the target site, the endoscope needs to be stiff enough to withstand external payloads as a stable platform during tissue manipulation without unwanted bending of the endoscope tip. Thus, an endoscope whose stiffness can be adjusted on command is desired. This thesis presents a novel variable-stiffness manipulator/endoscope and its application in NOTES. Two main objectives are identified: (𝑖) design, testing, and modeling of a novel approach for variable-stiffness manipulators, for proof of concepts, using a thermoplastic material whose stiffness is tunable through temperature. The temperature is adjusted through the joule heat generated by applying electric current into flexible metal coils inside the manipulator. A model that represents the relationship between the stiffness and the variables like dimensions of the design, heating time, and applied current will be studied and verified with experiments; and (𝑖𝑖) design, testing, and modeling of a variable-stiffness manipulator by considering critical requirements on sizes, and efficiency of real surgeries in flexible endoscopy and NOTES. Driven by 8 tendons, the robot has two bending sections and can be bent into “S” shapes. The active cooling mechanism is designed to shorten the activation time. The thermal insulation is also included to keep the temperature of the outer surface in the safe range. The comparisons in terms of the stiffness changing ratio and stiffness itself between the proposed design and the commercial endoscopes are shown along with extensive experiments. In particular, the manipulator has a high stiffness-changing ratio (22) between rigid and flexible states while that of its commercial Olympus counterpart is only 1.59. Heating and cooling mathematical models are also derived and evaluated using the experimental data. The cooling time with the active cooling mechanism is 11.9s while that of passive ambient cooling is 100.3s. The thermal insulation layer keeps the temperature of the outer surface within the safe range (below 41˚C) during operation. The results confirm the feasibility of developing a compact variable-stiffness endoscopic manipulator with significantly high stiffness-changing ratio (low stiffness in the flexile mode and high stiffness in the rigid mode), shortened time for switching the modes, and safe thermal insulation. The proposed manipulator may be used as a stable endoscopic platform in general flexible endoscopy and key-hole surgery.