Soft Everting Prosthetic Hand and Comparison with Existing Body-Powered Terminal Devices

TL;DR

This work tackles the need for low-cost, user-friendly body-powered prosthetic hands by introducing a soft inverting-everting toroidal hydrostat hand integrated with a body-powered elbow-driven transmission. The study compares the Everting hand to the Kwawu 3D-printed hand and the Hosmer hook across cable tension, grasp security, and a pilot dexterity-focused user study (Box and Blocks, Jebsen-Taylor) with six able-bodied participants. Key findings show the Everting hand requiring only about 1.6 N of actuation tension and achieving higher grasp resistance (~15.8 N) than the comparators, while excelling in object-shape variety tasks but facing limitations in precision tasks like writing. Overall, the soft hydrostat design demonstrates promising adaptability and practical usability as a prosthetic solution, with clear directions for improvements in visibility, fine motor control, and durability before testing with individuals with limb differences.

Abstract

In this paper, we explore the use of a soft gripper, specifically a soft inverting-everting toroidal hydrostat, as a prosthetic hand. We present a design of the gripper integrated into a body-powered elbow-driven system and evaluate its performance compared to similar body-powered terminal devices: the Kwawu 3D-printed hand and the Hosmer hook. Our experiments highlight advantages of the Everting hand, such as low required cable tension for operation (1.6 N for Everting, 30.0 N for Kwawu, 28.1 N for Hosmer), limited restriction on the elbow angle range, and secure grasping capability (peak pulling force required to remove an object: 15.8 N for Everting, 6.9 N for Kwawu, 4.0 N for Hosmer). In our pilot user study, six able-bodied participants performed standardized hand dexterity tests. With the Everting hand compared to the Kwawu hand, users transferred more blocks in one minute and completed three tasks (moving small common objects, simulated feeding with a spoon, and moving large empty cans) faster (p~$\leq$~0.05). With the Everting hand compared to the Hosmer hook, users moved large empty cans faster (p~$\leq$~0.05) and achieved similar performance on all other tasks. Overall, user preference leaned toward the Everting hand for its adaptable grip and ease of use, although its abilities could be improved in tasks requiring high precision such as writing with a pen, and in handling heavier objects such as large heavy cans.

Figures (7)

• Figure 1: Body-powered prosthetic hand designs with different terminal devices: our Everting hand design (left), the 3D printed Kwawu hand kwawu_hand from e-NABLE enable (middle), and the Hosmer hook (right).
• Figure 2: The three prosthetic terminal devices with body-powered actuation. Our Everting hand is attached to the right hand of an able-bodied user through a custom-designed prosthetic simulator, and it is connected to the body powered transmission system (top). The Kwawu hand (bottom left) and the Hosmer hook (bottom right) connect to the same prosthetic simulator and transmission system.
• Figure 3: Measurement setup and experimental results of the cable tension measurement. The plot shows the measured cable tensions as a function of elbow angle for the three terminal devices and, in the case of the Everting hand, also for two different orientations. The photos illustrate the measurement setup with the mannequin arm in the horizontal configuration and with the Everting hand as the terminal device. A force sensor is connected to the actuator cable at the arm attachment to record the cable tension at different elbow angles. The Everting hand requires lower cable tension than the other two devices, and it can operate over a larger range of elbow angles.
• Figure 4: Experimental setup and results of grasp force measurement. (a-c) The measurement setup for the Everting hand, Kwawu hand, and Hosmer hook, respectively. The setup was used to grasp an object (blue cube) with each terminal device and measure the pulling force (indicated with red arrows) required to pull the object out of the grasp. (d) The mean and the standard deviation of six measurements for each of the devices, showing that the Everting hand resisted higher peak force than the other two.
• Figure 5: Setup for the Box and Blocks Test with the Everting hand (left), and results of the user study participants using the three terminal devices (right). Error bars indicate the standard deviation among users, and statistical significance at the level of p $\leq$ 0.01 in a post-hoc test is denoted with two asterisks. The Everting hand had the highest block transfer rate, highlighting the advantage of its adaptable grip in repetitive movements, while the Kwawu hand had the lowest rate due to alignment and grip control challenges.