BioFlexShirt: A SMA-Driven Shoulder Exoskeleton and Its Assisted Motion Mechanism Study

Loss of shoulder mobility severely impairs upper limb function and reduces quality of life. Here we present BioFlexShirt, a wearable soft exoskeleton driven by shape memory alloy (SMA) actuators, designed to assist shoulder movements across three degrees of freedom. Inspired by the anatomy of major shoulder muscles, BioFlexShirt integrates bionic actuator alignment, a displacement amplification mechanism, and fabric-based modular design to enhance anatomical conformity and assistive performance. A coupled kinematic model was developed based on an ellipsoidal thorax approximation, while dynamic simulations using OpenSim confirmed consistency with natural shoulder-humeral rhythms. Vicon motion capture experiments revealed significant improvements in joint displacement and coordination at the GH, AC, ST, and SA landmarks. All root mean square errors remained below 5 mm, validating the system's spatial alignment and assistive fidelity. This study demonstrates a viable pathway for SMA-based wearable exoskeletons in rehabilitation, enabling anatomically aligned, lightweight, and efficient upper limb assistance.