Omnidirectional and Size-Adaptive Soft Bending Sensor for Accurate Human Joint Motion Monitoring

Joint-related diseases can cause chronic symptoms and severe pain, which can be prevented and evaluated by accurate joint status monitoring. Soft strain sensors with high skin compatibility can detect strains from joint bending, making them a promising solution for joint health monitoring. Practically, over half of free-moving joints on the human body can perform omnidirectional bending, but current soft strain sensor cannot accurately quantify such angles. Moreover, strain-angle relations vary significantly with joint sizes, making sensor calibration difficult or unfeasible. Herein, this work designed and implemented an omnidirectional and size-adaptive soft bending sensor with a 3-D structure to capture pose angles for arbitrary joint bending. A dice manufacture-inspired procedure is proposed to precisely position four soft strain sensing channels. Moreover, by mathematically decoupling strain for deformation distributions, the sensor can calculate omnidirectional bending angles regardless of different joint sizes for the first time. The ability of the sensor was verified by mounting on three vulnerable joints for motion detection, thereby demonstrating its potential for reliable and accurate joint health monitoring.