Lightweight and Flexible Prosthetic Wrist With Shape Memory Alloy (SMA)-Based Artificial Muscle and Elliptic Rolling Joint

This letter proposes a novel prosthetic wrist that emulates the anatomical structure of the human wrist, specifically the wrist bones and muscles responsible for wrist movements. To achieve a range of motion (ROM) and load-bearing capacity comparable to the human wrist joint, we designed an elliptic rolling joint as an artificial wrist joint, mimicking the two-row structures of carpal bones. The joint offers two degrees of freedom (DOFs) and can support high loads while also providing adequate ROM. In addition, we designed the artificial muscles using the properties of human muscles, such as moment arm and displacement, and implemented them as shape memory alloy (SMA) spring-based actuators. The resulting prosthetic wrist, incorporating the artificial joint and artificial muscles, is lightweight at only 50g and can perform functional ranges of motion, including 53° for flexion, 50° for extension, 40° for radial deviation, and 42° for ulnar deviation. The use of SMA spring actuators confers restoring force and flexibility to the prosthetic wrist, allowing it to withstand external disturbances. Furthermore, the proposed wrist can be utilized as a robotic wrist, affording two additional DOFs, the ability to lift loads more than 20 times its weight, and variable joint stiffness.