Abstract
Assessing soft tissue hardness is critical for disease diagnosis and motion monitoring. However, existing technologies are confined to skin-level and quasi-static measurements, leaving the dynamic behavior of deeper tissues, such as muscle, inaccessible. This study introduces a wearable system that enables dynamic monitoring of Young's modulus in multilayer tissues containing deep muscle (DMYD). Inspired by the spatial encoding strategy of human mechanoreceptors, DMYD integrates spatially distributed, high-resolution, low-detection-limit iontronic sensing arrays with a load sensor to continuously capture the contact radius and contact force between a hemispherical indenter and the tissue, allowing real-time and accurate modulus estimation based on Hertz contact theory. A simulation-informed indentation strategy optimizes the accuracy of measurements in deep, multilayer tissues while minimizing wearing discomfort. In vitro experiments demonstrate that DMYD achieves high accuracy (>93%), supports dynamic operation, and remains robust to signal drift, sweat, and mechanical fatigue. In postoperative patients, its measurements correlate strongly with clinical edema indicators, while in healthy users, it tracks task-dependent muscle hardness dynamics during rest, loaded elbow flexion, rope skipping, and stretching. Collectively, these results highlight DMYD as a promising platform for personalized and home-based disease management, performance evaluation, injury-risk warning, and training strategy optimization.