Effect of Adhesive Bonding Techniques on Fiber Bragg Gratings-Based Flexible Insoles for Plantar Pressure Measurements

The measurement of plantar pressure plays a crucial role in various typologies of application, particularly in high-performance and demanding activities such as classical dance. Ballet dancers normally experience intense biomechanical loads, making precise pressure assessment essential for injury prevention, technique and performance improvement. Fiber Bragg grating (FBG) sensors represent an effective pressure sensing technology due to their inherent advantages. However, their per-formance is highly dependent on the adhesive bonding methodology used to attach the optical fiber to flexible insoles. This study investigates the effect of two different adhesive bonding methods - dot glue and continuous glue - on the performance of FBGs-based flexible insoles for plantar pressure monitoring. The insoles were fabricated using a 3D-printed thermoplastic polyurethane (TPU) structure embedded with eight FBG sensors. Mechanical tests were performed using a compression static test machine to assess pressure response, sensitivity, hysteresis, and crosstalk between sensors under different loading conditions. Results indicate that the continuous glue configuration generally provides higher average sensitivity (0.4-1.2 pm/kPa; R2 equal to 0.96), instead dot glue exhibits lower hysteresis error (4.38 %) for five sensors out of eight. These findings underscore the importance of selecting the appropriate bonding technique based on the specific application requirements, balancing sensitivity and minimizing interference.