Characterization of Hysteretic Mechanical Behavior of Fiber Bragg Gratings-Based Curvature Sensors for Biomechanical Applications

Fiber Bragg grating (FBG) sensors have recently emerged as a versatile tool for curvature sensing because of their high sensitivity, small size, and immunity to electromagnetic interference. However, the hysteretic mechanical behavior of FBG-based sensors, influenced by the viscoelastic properties of fiber coatings and polymeric casings in which they could be encapsulated, poses challenges to accuracy and reliability in real-world applications. This study investigates the hysteretic behavior of FBG-based curvature sensors intended for wearable devices with three different fiber coatings - acrylate, Ormocer11Registered trademark., and polyimide - embedded in 3D printed thermoplastic polyurethane (TPU) samples. Cyclic bending tests were conducted to evaluate the responses of the realized sensors, focusing on metrics such as wavelength ratio, hysteresis error, and damage index. The results of this study show significant differences in mechanical compliance, energy dissipation, and residual strain accumulation between coatings. Acrylate exhibited the lowest hysteresis error (9.55 %) and residual strain (-0.037 nm), indicating superior stability, while polyimide demonstrated initial hardening and aa very low damage indexin increasing testing cycless (0.1 % after the sixth cycle). These results underscore the remarkable role of fiber coating selection in optimizing sensor performance for biomedical and biomechanical applications, where precision and durability are of paramount importance.