A Stretchable, Conductive Thread-Based Sensor Towards Wearable Monitoring of Muscle Atrophy

We present the first wearable sensor designed for frequent monitoring of muscle atrophy and validate performance upon canonical phantoms. Methods: Our approach relies on Faraday's law of induction and exploits the dependence of magnetic flux density on cross-sectional area. We employ wrap-around transmit and receive coils that stretch to fit changing limb sizes using conductive threads (e-threads) in a novel zig zag pattern. Changes in the loop size result in changes in the magnitude and phase of the transmission coefficient between loops. Results: Simulation and in vitro measurement results are in excellent agreement. As a proof-of-concept, a cylindrical calf model for an average-sized subject is considered. The frequency of 60 MHz is selected via simulation for optimal limb size resolution in magnitude and phase while remaining in the inductive mode of operation. We can monitor muscle volume loss of up to 51%, with an approximate resolution of 0.17 dB and 1.58 degrees per 1% volume loss. In terms of muscle circumference, we achieve resolution of 0.75 dB and 6.7 degrees per centimeter. Thus, we can monitor small-scale changes in overall limb size. Conclusion: This is the first known approach for monitoring muscle atrophy with a sensor designed to be worn. Additionally, this work brings forward innovations in creating stretchable electronics from e-threads (as opposed to inks, liquid metal, or polymer). Significance: The proposed sensor will provide improved monitoring for patients suffering from muscle atrophy. The stretching mechanism can be seamlessly integrated into garments which creates unprecedented opportunities for future wearable devices.