Investigation of a passive capstan based grasp enhancement feature in a voluntary-closing prosthetic terminal device

Body-powered prosthetic terminal devices fall into two main categories: voluntary-closing devices, which require the user to exert a force to maintain a grasp, and voluntary opening devices, which generally utilize springs to close and maintain a force. As a result, voluntary-closing devices often have a locking feature that allows the user to relax and transport objects while maintaining a firm grip. In this paper, we examine a new type of capstan-based passive brake mechanism in a voluntary-closing prosthetic terminal device. Three different mechanisms were compared on the benchtop and with human subjects: the passive capstan grasp enhancement, a "pull-to-lock, pull-to-release" mechanism, and a manual cable locking mechanism. Standard tests of prosthetic device dexterity, including the Box and Blocks test and Southampton Hand Assessment Protocol, were performed with an instrumented prosthesis socket simulator with each device. While results are similar across the three mechanisms, the passive capstan mechanism does not require a physical user input to engage or disengage the lock, adding a benefit over the existing mechanisms.