Hand Rehabilitation Dynamic Splint with Variable Force via Pneumatic Artificial Muscle Actuators

Purpose: Dynamic hand splints are rehabilitation orthoses equipped with elastic bands/springs that provide passive resistance to finger movements. To enable real-time controllability of rehabilitative exercises and potentially improve therapeutic outcomes, it would be advantageous to replace the elastic components with soft actuators, enabling finger movements to be exercised against controllable loads. The ideal soft actuators for dynamic hand splints should generate large displacements at moderate forces, have a compact size and low specific weight, and be electrically safe. Methods: Here, we describe a dynamic splint embedded with soft pneumatic actuators functioning as “inverse artificial muscles”, which elongate when pressurised, reducing the force that resists finger flexions. A prototype system was assembled using off-the-shelf materials. An actuator was fabricated and mounted on a forearm brace, connecting one extremity to a finger via a tendon and the other one to an onboard load cell, to monitor the force exerted during finger motions. The system was tested via two psychophysical tests. Results: A dynamic psychophysical test on perceptual differences in force demonstrated that subjects were able to reliably perceive force variations, with a sensitivity (slope of the psychometric curve) of ~ 110%/bar at 50% probability. In a static psychophysical test with a fully flexed finger, the maximum average force was ~ 5.2 N at 0 bar, whilst the minimum average force when subjects reported no perception of force, by progressively increasing the pressure, was as low as ~ 0.7 N at 3 bar, indicating good accuracy of the system. Conclusion: The developed active splint has the potential to enable dynamically controllable hand rehabilitation tasks.