Design and Fabrication of a Hybrid Body-Powered Prosthetic Hand With Voluntary Opening and Voluntary Closing Capabilities

Abstract

There are currently over 500,000 people with upper extremity amputations living in the United States. Among this population—despite the introduction of advanced myoelectric technology—body-powered hand prosthetics remain the hand prosthetic of choice because they are inexpensive, durable, and are easier and cheaper to maintain than myoelectric prosthetics. Yet, body-powered prosthetics tend to have less functionality than myoelectric prosthetics because their output is often limited to a single function—voluntary opening (VO) or voluntary closing (VC). Although these functions serve the same purpose—to grasp an object—they are executed in opposite ways, catering to different body movements, hand actions, and rest positions. In reality, a human hand is more adequately modeled by a VO/VC hybrid mechanism than a standalone VO or VC mechanism. There is therefore a critical need to develop prosthetics with combined VO and VC functions in order to augment their capabilities and to more closely mimic the human hand. This paper presents the design, fabrication, and analysis of a combined, hybrid VO and VC prosthetic hand that is simplistic and electronics-free. To realize a hybrid VO and VC prosthetic hand, we designed and fabricated an easy push-pull switching mechanism for changing between VO and VC and investigated the efficacy of this switching mechanism in response to the wide range of force transmission necessitated by the VC and VO functions. The mechanical switching mechanism is activated with a force of 1 to 1.5N. The mechanism itself is constructed using a system of gears that allow for the direction of force to be changed, effectively providing the foundation for which VO and VC functions can exist in a single prosthetic. This switching mechanism could potentially be used in combination with a number of different prehensor types, catering to a wide range of users. We used a 3.5″ TRS-style prehensor for our prototype and designed the switch to apply a 21 N force while in VO mode and up to 100 N while in VC mode. In our design, VO and VC modes offer characteristics that are identical to existing VO and VC designs, with a few exceptions. These exceptions include an increase in weight, as low as 12% energy loss in VC mode and 9% loss in VO mode due to gears, more complex mechanics, larger required space, and a higher spring force in VC mode. Future design improvements will be discussed in this paper.