"Redesign and novel bi-directional twisted-string actuation of a 3D-printed metamaterial hand orthosis"

Abstract

This thesis presents the development of a bidirectional, twisted-string actuated 3D-printed metamaterial hand orthosis aimed at supporting patients with impaired hand function in performing activities of daily living (ADL). Building upon a hybrid orthosis concept developed in previous work, the design was improved to address limitations such as mechanical inefficiency, spring buckling, and user discomfort, as well as actuate the device for future ADL support application. A novel actuation strategy using a one to one gear system with pre-wound wires, allowing for bidirectional motion and enhanced responsiveness. The actuation system was designed to deliver at least 120 𝑁 of linear force at 80 𝑚𝑚 of displacement, meeting the 15 𝑁 fingertip force requirement for ADL assistance. A closed-loop control system based on PID feedback was integrated, enabling the orthosis to track position targets.
In parallel, the orthosis design was refined to reduce internal friction, improve the input-output force gradient, and minimize twitching during actuation. The redesigned prototype demonstrated a 23.7% improvement in mechanical efficiency compared to the original version. Testing revealed that the system could perform a full hand closing/opening motion within 4 seconds and accurately replicate natural finger trajectories. Retesting the original orthosis under synchronized input/output conditions produced a reliable baseline
dataset, enabling a clear comparison between the legacy and updated systems. While the redesigned prototype encountered minor issues such as spring buckling under high loads due to fabrication tolerances, the overall performance met the core design goals. This work demonstrates the feasibility of integrating the novel bidirectional twisted-string actuation
system with 3D-printed metamaterial structures for wearable assistive devices. It provides a validated platform for further development toward user-ready robotic rehabilitation gloves with improved comfort, responsiveness, and usability.