Development of a Tendon-Driven Wearable Assist System for Thumb Motion of Hand Paralysis

Thumb movement is very important for object gripping. However, when a cerebrovascular disorder or spinal cord injury causes hand paralysis, which impairs the motor function of the fingers, it becomes impossible to flex and extend the fingers and to move the thumb to the opposition position; this makes it difficult to grip objects in daily life. Several assistive devices have been developed for people with hand paralysis. However, they only assist flexion/extension of the thumb, narrow the type of grip by fixing the thumb position, or have large sizes and weights due to using links and frames to assist thumb adduction and opposition. They are not effective for daily use. Thumb motion assistance that does not hinder the degree of freedom of the thumb is important for gripping objects in everyday life. In this study, we developed a wearable ring and a fingertip-cap that assist two-degree-of-freedom thumb motion by means of a tendon-drive based on the hand's anatomy; the feasibility of the device is confirmed through basic experiments, in which the device was evaluated by a hand in a weakened state. It was confirmed that opposing movement and flexion/extension motion of the thumb were possible. In addition, we confirmed that it was possible to grip thin objects by using only thumb motion assistance. In conclusion, we developed a tendon-driven thumb motion assistive device using a ring-shaped component and confirmed the feasibility of this system to support the opposition/reposition and flexion/extension motions of the thumb.