Hydrogel-coated fiber Bragg grating sensors for finger motion capture: exploring the potential of optical fibers in biomechanics

Abstract Accurate motion capture of small joints such as the thumb remains challenging in biomechanics and rehabilitation due to limitations of conventional optical and inertial systems in wearable settings. This study presents a feasibility investigation of a hydrogel-coated fiber Bragg grating (FBG) sensor for thumb joint motion capture. A compliant poly(vinyl alcohol)–borate (PVA–borate) hydrogel layer was employed as a skin-compatible strain-transfer interface to enhance mechanical coupling between the skin surface and the optical fiber and to reduce micro-slippage during movement. The hydrogel-embedded FBG sensor was applied to the dorsal surface of the thumb to capture motions at the carpometacarpal, metacarpophalangeal, and interphalangeal joints. Strain-derived joint angle estimates were synchronized and compared against reference angles obtained using an open-source video-based motion analysis pipeline. Experimental results demonstrated stable and repeatable wavelength shifts corresponding to functional thumb movements. Linear regression-based angle estimation yielded moderate agreement with vision-based measurements, with mean RMSE values of approximately 6°–7° across joints and movements and Pearson correlation coefficients in the range of r = 0.5–0.6. The results indicate that hydrogel-coated FBG sensors can feasibly capture thumb kinematics with sufficient temporal fidelity for wearable motion monitoring. While further multi-subject validation is required, the proposed approach shows promise as a lightweight, drift-free, and electromagnetically immune alternative for biomechanical sensing in applications such as rehabilitation monitoring, wearable robotics, and human–machine interfaces.