Wrap-Around Wearable Coils for Seamless Monitoring of Joint Flexion

Objective: We introduce and validate a new class of wearable coils that seamlessly monitor joint flexion in the individual's natural environment while overcoming shortcomings in state-of-the-art. Methods: Our approach relies on Faraday's law of induction and employs wrap-around transmit and receive coils that get angularly misaligned as the joint flexes. Results: Simulation and in vitro measurement results for both copper and e-thread coils are in excellent agreement. As a proof-of-concept, a cylindrical arm model is considered and feasibility of monitoring the 0 degrees-130 degrees range of motion is confirmed. The operation frequency of 34 MHz is identified as optimal, bringing forward reduced power requirements, enhanced angular resolution, and extreme robustness to tissue dielectric property variations. Performance benchmarking versus state-of-the-art inertial measurement units shows equivalent or superior performance, particularly for flexion angles greater than 20 degrees. Design guidelines and safety considerations are also explored. Conclusion: Contrary to gold-standard camera-based motion capture, the reported approach is not restricted to contrived environments. Concurrently, it does not suffer from integration drift (unlike inertial measurement units), it does not require line-of-sight (unlike time-of-flight sensors), and it does not restrict natural joint movement (unlike bending sensors). Significance: The reported approach is envisioned to be seamlessly integrated into garments and, eventually, redefine the way joint flexion is monitored at present. This promises unprecedented opportunities for rehabilitation, sports, gestural interaction, and more.