On Optimal Tendon Routing-Based Design of Biologically Inspired Underactuated Hand Exoskeleton for Gross Grasping

Abstract

This article presents a compact, portable fingertip-to-elbow hand exoskeleton (F-EL-EX) designed to assist in gross grasping activities involving hand opening and closing movements. The design mimics a biological tendon pulley system (TPS) for finger flexion, optimized for the maximum range of flexion while keeping bowstringing and maximum pulley stress under check. The exoskeleton finger integrates a jointless system of phalanges, designed with care to house the TPS while allowing unrestricted motion of the respective finger joints, each with variable centers of rotation. The exoskeleton is hybrid – fabricated with plastic, natural rubber, and metal, with individual or combination of materials used for different palm and forearm regions. Rigid components used for tendon routing help in modeling a relation between tendon excursion and flexion and provide high grasping force capabilities. The soft material on the palm region ensures retaining flexibility during grasping of objects with varied shapes and supports thumb carpometacarpal (CMC) adjustment. Compactness and portability are ensured through a sliding pulley based slack-tolerant differential mechanism (SPDM), driving all fingers with a single actuator and employing a separate actuator for the thumb. The experimental and functional results of the exoskeleton on a healthy subject demonstrate its adaptive, gross grasping abilities with everyday objects through power grasp, lateral pinch, and parallel extension. These findings encourage further exploration in clinical trials, especially for individuals with hand muscle weaknesses.