A Brace-Integrated Fiber Bragg Grating Sensor with a Multi-Hardness Silicone Strain Transfer Substrate for Knee Flexion Monitoring: A Proof-of-Concept Study

Continuous knee flexion monitoring is important for rehabilitation and gait analysis, but optical motion capture is generally confined to laboratories, and wearable electrical sensors may be affected by drift, misalignment, or electromagnetic interference. This study presents a brace-integrated system based on a fiber Bragg grating (FBG) embedded in a functionally segmented, multi-hardness silicone strain transfer substrate. The substrate was placed in a dedicated brace recess, avoiding a separate rigid transduction mechanism. A portable interrogation unit enabled wireless transmission and real-time brace-derived angle estimation during sit-to-stand and walking tasks. Calibration against brace hinge angles from 0° to 90° showed a nonlinear Bragg wavelength–angle relationship, modeled using a fourth-order polynomial whose order was selected using a separately acquired validation set that was also used for performance assessment. Validation based on 10 angle-wise mean estimates yielded R2 = 0.9992, RMSE = 0.90°, and MAE = 0.68°, with Bland–Altman 95% limits of agreement of −1.46° to +2.05°. The 0° stability record had a wavelength standard deviation of 1.41 pm and a linear trend of −0.193 pm/min, and Allan deviation analysis gave a wavelength noise floor of 0.089 pm at an averaging time of 24 s. Noise-equivalent angular variation ranged from 0.08° to 0.52°. During a sustained 30° hold, the estimated angle drifted at −0.089°/min, but comparison of the 0° and 30° records in wavelength units showed that the undeformed condition reproduced approximately 59% of that drift, although the non-simultaneous comparison does not establish a common-mode thermal contribution. An uncompensated temperature change of 1 K corresponds to between 0.31° and 1.98° of apparent angle change. Proof-of-concept measurements in two participants, both of whom are authors, produced brace-derived profiles qualitatively consistent with expected task kinematics but revealed a cumulative zero-offset shift of 3.2° across two sit-to-stand cycles. Because no simultaneous anatomical reference was acquired, the dynamic results establish feasibility and waveform plausibility rather than measurement accuracy. The results support the engineering feasibility of this brace-integrated optical architecture, while identifying temperature compensation and cyclic loading characterization as prerequisites for anatomical and clinical validation.