Shape Memory Alloy Actuators for Upper-Limb Prosthetic Applications: A Review

Abstract

Nickel-titanium (NiTi) shape memory alloy (SMA) actuators have attracted considerable interest in upper-limb prosthetic design because they can produce useful motion within compact and quiet mechanisms. However, results obtained from isolated SMA wires or springs do not always represent their performance in a complete prosthetic system, where cooling, packaging, control hardware, and user safety must also be considered. This review examines thermally activated SMA actuators used in upper-limb prostheses and related wearable devices. Applications of superelastic NiTi are considered separately when the material functions as a passive structural component rather than a heat-activated actuator. The reviewed systems are organized according to actuator configuration and application, including straight wires, coiled springs, antagonistic arrangements, and hybrid designs. The available evidence shows that slow cooling, thermal hysteresis, fatigue, sensing drift, and surface-temperature limits continue to restrict repeated or sustained operation. Thermally activated SMA is therefore better suited to intermittent and space-constrained functions, such as individual finger motion, occasional wrist positioning, and compact locking or release mechanisms. Motor-driven systems generally remain more appropriate for rapid and repetitive grasping. Hybrid systems may provide a practical alternative by assigning frequent motion to motors and reserving SMA for functions in which compact size, quiet operation, or local compliance provides a clear system-level advantage.