A Liquid-Metal-Based, Stretchable Inductive Loop Sensor for Muscle Atrophy

We present a sensor toward wearable monitoring of muscle atrophy with improved stretching capabilities and sensing resolution for practical implementation. The operation relies on our previously reported approach, where wrap-around transmit and receive loops at inductive frequencies monitor changes in muscle size via the magnitude and/or phase of the transmission coefficient. The novel aspects of this work entail: 1) a new fabrication approach with stretchable inductive loops made from Gallium Indium eutectic (EGaIn) injected into silicone tubing to improve stretchability, and 2) operation below the defined resonant frequency to improve magnitude resolution of the sensor. Simulation and in vitro measurement results are in excellent agreement. Compared to our previous sensor implementation, the proposed approach: 1) achieves 4.3 times higher stretchability, 2) measures 1.7 times larger muscle volume loss, and 3) improves magnitude resolution by 6 dB. Specifically, we achieve a magnitude and phase resolution of 1.25 dB and 3.8°, respectively, per centimeter of limb circumference, and 0.3 dB and 0.9° per a 1% volume reduction. We also demonstrate repeatable fabrication with a low standard error of 0.19 dB and 0.7°. Because of the improved stretching capabilities and resolution, we can now accurately measure the circumference at any location on the leg of any given member of the population with a single device. Concurrently, this work brings forward innovations in fabricating radio frequency electronics from liquid metals.