Synergistic Thermo-Hygroscopic Response in Bioinspired CB-TPU Sensors Fabricated via FDM for Respiratory Monitoring

Temperature (T) monitoring is essential across various domains, including biomedical applications, where respiratory rate (RR) can be estimated from the temperature difference between inhaled and exhaled air. Additive manufacturing, particularly fused deposition modeling (FDM), remains underexplored for this purpose despite the potential offered by conductive thermoplastic composites. Integrating conductive fillers into printable polymers provides an effective strategy to develop novel sensors. This study presents the fabrication and characterization of resistive sensors produced by FDM using thermoplastic polyurethane (TPU) filled with carbon black (CB). Three bio-inspired geometries—spider (SP), honeycomb (HC), and flap (FL)—are fabricated with three thicknesses (1, 2, and 3 layers). The electrical responses to temperature and relative humidity (RH) are analyzed to evaluate the influence of design on metrological performance. All sensors exhibit a synergistic behavior, with resistance increasing as T and RH rise. A parabolic response to T variations is observed, with resistance changes up to 125%. T sensitivity increases with thickness, with the HC geometry showing the highest values. Response times range from 15 to 23 s, with 8% hysteresis. In the 30%–70% RH range, sensors provide linear responses. Integration into a commercial face mask enables reliable RR detection during bradypnea, eupnea, and tachypnea.