Wearable Electrically Small Loop Antennas for Monitoring Joint Flexion and Rotation

Wearable electrically small loop antennas (ESLAs) are introduced to monitor joint flexion and rotation while overcoming limitations in the state-of-the-art. The reported approach is not restricted to laboratory environments, does not suffer from integration drift and line-of-sight, and does not impede natural movement. Our previous work introduced wrap-around coils that addressed the challenges above but were limited to monitoring joint flexion. In contrast, a new class of ESLAs is herewith proposed, placed longitudinally across the joint, and operated at 34 MHz to monitor both flexion and rotation. An added advantage versus our previous design is a remarkable improvement in flexion angle resolution: the transmission coefficient dynamic range for 0° - 100° flexion improves by 18.8 dB in this article. Two ESLAs are shown to accurately detect flexion/rotation for angular resolutions up to 10°. However, if higher resolution is desired, ambiguities arise. To tackle this, a three-ESLA system with integrated post-processing is proposed that achieves resolution as high as 2°. Simulations and in vitro experiments are in excellent agreement. Guidelines for system design suited to diverse applications are discussed, and conformance with safety standards is ensured. In future, ESLAs can be seamlessly integrated in garments, enabling transformative benefits to healthcare, sports, and beyond.