Design of a Novel Propulsion Mechanism for Flexible Endoscopes Inspired by Plant Root Growth

Abstract

When inserting a flexible endoscope into a human colon during colonoscopy, limitations in the endoscope design can cause medical implications such as excessive colon stretching and buckling of the endoscope shaft. This thesis proposes a novel propulsion mechanism design for flexible endoscopes which changes the method of insertion as a way to prevent these implications. As inspiration for the design an analogy is drawn between plant roots growing through tortuous cracks in soil and flexible endoscopes moving through a tortuous human colon. The analogy was found to be relevant, resulting in eight biological features of which seven were suitable as a potential solution in the development of a flexible endoscope. Of these seven suitable solutions, apical extension and variable stiffness were implemented as functions in the proposed propulsion mechanism. The focus of the design process on these two functions ultimately culminated in a proof of concept flexible, extending endoscope design dubbed the Flextendoscope. The Flextendoscope consists of an inverted tube mechanism which can propel the endoscope through apical extension combined with a fiber jamming mechanism that can vary the bending stiffness. The concept design is found very promising as it theoretically allows insertion of a highly compliant shaft which is also able to provide the high tip rigidity required at the surgical location after insertion. Although evaluation of the Flextendoscope prototype validated the feasibility of the proposed proof of concept design, the performance of the apical extension function was of a more limited success. Due to internal frictional resistances the prototype shaft only extended under the lowest evaluated pressure (0.5 bar) and required a high manual force (up to 119 N) to do so. The behavior of the variable stiffness function on the other hand was found to be very desirable. The bending stiffness of the prototype increased with internal pressure level as well as radial deflection, showing initial elastic deformation followed by hysteresis. It showed considerable stiffening at a convenient threshold pressure of 0.7 bar, slightly above the pressure at which the shaft is extended. Future iterations of the Flextendoscope design should focus on overcoming the internal frictional resistances that limit the apical extension of the current design. They can do so by multiplying the number of inverted tubes inside the shaft. This multiplication would enclose the sliding shaft material of each inverted tube within their own central lumen thus avoiding contact between the sliding shaft material and other stationary components in the design, preventing frictional interactions. If the frictional resistance is overcome the Flextendoscope design could prevent colon stretching during insertion of an flexible endoscope and inherently prevent shaft buckling. The prevention of these implications by the Flextendoscope design could ultimately lead to colonoscopy becoming an easier, safer and less painful procedure with a higher success rate.