Predicting the Response of an Underactuated, 3D Printed Prosthetic Hand

The continuing need for hand prostheses has led to incredible developments in this area. However, due to the cost of the most sophisticated models, it is vital to develop simpler, low-cost, underactuated prostheses that can be produced rapidly. 3D printing is an important tool to achieve this goal. In particular, the 3D printed Flexy-Hand 2 has been one of the most common low-cost designs. A key feature of the Flexy-Hand 2 is its use of tendons and thermoplastic polyurethane (TPU) for joints that are strong and flexible. TPUs, however, lack a thorough understanding of their mechanical behavior, which makes it difficult to discern the forces needed to flex the prosthetic hand or to grip an object. The purpose of this research is to develop, and experimentally verify, a model that can predict the relationship between tendon forces and the flexion of the TPU joints of a prosthetic finger. First, a constitutive model for 3D printed TPU, NinjaFlex® was developed. This model accounts for the effect of filament extrusion temperature and material deposition orientations. Next, flexural tests of cantilevered specimens representing the finger joints were conducted to study the bending behavior of the TPU under realistic conditions to simulate finger flexion. Lastly, the results of these two tasks were incorporated in a quasi-static model relating finger flexion to tendon tension inducing it. This model was verified experimentally.