Fabrication and Metrological Characterization of Bare and Integrated 3-D-Printed Single-Layer CB-TPU Strain Sensors

In recent years, additive manufacturing techniques, particularly 3-D printing methods like fused deposition modeling (FDM), have been increasingly explored for the development of systems for physiological monitoring, such as respiratory activity and joint kinematics, while retaining advantages such as rapid prototyping, low costs, and high customizability. This study presents the design, fabrication, and metrological characterization of single-layer strain bare sensor (BS) produced via FDM, with a thickness of only 0.15 mm, composed of a thermoplastic polyurethane (TPU) matrix filled with carbon black (CB) particles. In addition, the work investigates the impact of integrating the BS into flexible substrates—specifically kinesiology tape-integrated sensor (TS) and silicone-integrated sensor (SS)—to enhance mechanical robustness, a factor often neglected in existing literature. Electromechanical characterization was performed through quasi-static and cyclic tensile tests up to 5% strain. The resistance response exhibited nonlinear behavior, with maximum relative resistance changes of 40%, 38%, and 30% for the BS, TS, and SS configurations, respectively. The highest gauge factor (GF) of -14.7 was observed for the TS at 1% strain. During cyclic loading/unloading tests, all configurations demonstrated low hysteresis errors (~4%), even at high frequencies (90 cycles/min), despite the intrinsic piezoresistive nature of the sensors. In hygrothermal characterization, while substrate integration did not significantly mitigate the effect of temperature, silicone encapsulation proved effective in reducing humidity sensitivity, with the SS configuration showing only a 4% variation compared to ~13% for BS and TS. Finally, pilot tests conducted on a healthy volunteer demonstrated the feasibility of using the developed sensors for respiratory monitoring and joint kinematics assessment.