KNEESENSE: a low-cost wearable system with hydraulic filament sensing for real-time knee rehabilitation monitoring

Knee injuries, if not properly rehabilitated, can lead to long-term gait impairment. However, access to rehabilitation is often limited by cost, travel, and adherence challenges, especially in developing countries. Despite advances in soft sensor technologies, current home-based rehabilitation systems often lack the affordability, portability, and usability needed for broad adoption. This study presents a novel, low-cost, and fully wearable wireless system designed to monitor knee joint angles during rehabilitation with high precision and user convenience. At the core of the system is a custom-designed wearable hydraulic filament sensor (WHFS), costing just $5, integrated into a knee pad and paired with a reusable $20 wireless data acquisition (DAQ) unit. The DAQ transmits real-time joint angle data to a companion mobile app, featuring automatic calibration for consistent multi-user performance. The WHFS distinguishes itself through its optimized hydraulic sensing mechanism, offering a minimum detectable displacement of 0.2 mm, a sensitivity of 2.65, and maximum hysteresis of 4.6%. Testing shows robust performance with an RMSE of 4.1° in angle estimation and just 0.57% signal degradation after 100 full-range flexion cycles. By combining affordability, portability, and precision, this system addresses key barriers in home-based rehabilitation and offers a scalable, noninvasive solution for improving recovery outcomes.