PEDOT–PSS Surface-Coated Warp-Knitted Spacer Structure for Thermoelectric–Piezoelectric Dual-Mode Flexible Sensor

Abstract

Intelligent wearable fabrics featuring multimodal sensors capable of detecting human physiological signals have attracted considerable attention for next-generation health monitoring applications. Piezoelectric and thermoelectric functional polymers can be simultaneously engineered into fiber-based architectures; however, their sensitivity and synergistic response under multiple physical fields remain insufficiently understood and require systematic investigation. In this work, a three-dimensional flexible PPSF sensing unit with dual thermoelectric and piezoelectric response characteristics was developed by uniformly coating PEDOT/PSS onto the surface and PVDF spacer filaments of a 3D spacer fabric via in situ polymerization combined with phase separation, followed by post-treatment with dimethyl sulfoxide (DMSO) and ethylene glycol (EG). The fabrication process, surface morphology, and thermoelectric and mechanical properties, as well as air permeability of the PPSF, were comprehensively characterized. The Seebeck coefficient, electrical conductivity, and power factor were systematically evaluated. The optimized PPSF (5 vol % DMSO, 2 h EG treatment, 3 mm thickness) achieved a Seebeck coefficient of 25.6 μV K–1, an electrical conductivity of 0.33 S·cm–1, a power factor of 84.7 μW·m–1·K–2 at room temperature, and a rapid response time of approximately 1 s under ΔT = 1 K. Thirty p-/n-type PPSF sensing units were integrated in thermal parallel and electrical series to fabricate a pressure–temperature dual-mode flexible sensor, delivering output voltages of 15.3 mV (ΔT = 10 K) and 60.1 mV (ΔT = 40 K). Output voltage stability tests showed a slight initial decline within 5 min, followed by a stable plateau. In finger-pressing experiments, the flexible sensor independently and simultaneously responded to temperature and pressure stimuli, generating distinct voltage and current signals.