On-body seismology for continuous monitoring of tissue mechanics

Recent progress in wearable devices provides new opportunities for precise, non-invasive, long-term recording of body mechanics. The soft device incorporating a single MEMS accelerometer captures subtle vibration of the skin with a resolution of 1×10^-4 g/√Hz in the frequency range from 0 to 800 Hz. In this study, we introduce an on-body mechano-acoustic sensing technology based on a skin-mounted accelerometer array to assess the mechanical profiles of subdermal tissues in-vivo, similar to seismology. A system-level wearable device construction, optimized for a comfortable skin interface and high precision, incorporates a broadband dual-accelerometer sensor, an audio actuator, and a Bluetooth System-on-Chip and enables a wireless, automated operation. An automated algorithm, leveraging the spectral analysis of surface waves (SASW) methods, computes the depth-sensitive, elastic modulus information of the propagation media from the mechanical dispersion relationship. Comprehensive theoretical and experimental investigation verifies that the device, together with the automated data-processing platform, provides a robust, calibration-free, and non-invasive evaluation of the elasticity with a resolution of ~5 kPa and depth information in the range of ~2-140 mm of various phantom materials (Young’s modulus E= ~10-1500 kPa). The device also identifies the softening of pork tissues with increasing injected water content and the changes of modulus of muscle under different levels of tension. The results are in agreement with the in-parallel Ultrasound Elastography measurements. Quantitative access to the stiffness profile of the rectus femoris muscle during leg-press exercise demonstrates the continuous monitoring capability in an ambulant environment that can support long-term clinical assessment of different tissues and disease states in a large population.