Design of Insertion Mechanism with Friction Wheels for Cannula Flexible Needle Based on TRIZ

Abstract

Background: Flexible needle insertion is one of the minimally invasive surgeries, which takes advantage of the lateral force acted on the bevel tip to make the needle shaft bend when inserted into the tissue. The bending makes the needle avoid the obstacles (like bones, veins, nerves, etc.) in order to reach the target. However, the traditional flexible needle neither can change its curvature of the path, nor can realize a precise control because of the torsional friction between the needle and the tissue. Hence, a cannula flexible needle was proposed to overcome the drawbacks, which consists of a cannula and a stylet. Also, there is a need of an insertion mechanism for the cannula flexible needle in robot-assisted surgery. Objective: The aim of this paper is to innovatively design an insertion mechanism with friction wheels for the cannula flexible needle, which is used as an end-effector in robot-assisted surgery system. The mechanism is supposed to realize the coordinated driving of the cannula and the stylet in order to achieve variable curvatures of paths. Methods: Making references of the patents and research papers on needle insertion mechanisms, and based on the requirement of degree of freedom for the cannula flexible needle insertion, the insertion mechanism for the cannula flexible needle is designed by using the TRIZ theory. The conflicts matrix analysis, the invention principles and the substance-field analysis are used to innovatively design the driving scheme and the synchronous motion mechanism. Results and Conclusion: In this paper, the concrete structure design of the insertion mechanism for the cannula flexible needle is achieved, which is compact and simple. The friction coefficient between the needle and the wheels, and the pretightening force between both wheels are calculated through the data from experiments. The insertion accuracy of friction wheel mechanism is tested and analyzed through experimentation. The results show that the maximum placement error of the needle executed by the insertion mechanism is less than 0.5 mm, which meets the demands of surgical operations.