Sustainable resistive and capacitive-mode electromagnetic temperature sensing using recyclable composite-loaded lossy transmission lines toward full circularity

Functional materials are typically employed as either capacitive- or resistive-mode sensors. Here, we present a radio frequency electromagnetic temperature-sensing approach based on functional material-loaded lossy transmission lines. The functional material consists of a polydimethylsiloxane–carbon fiber (PDMS–CF) thermistor composite embedded within a coplanar waveguide, filling a gap in the signal line. The composite bridges the signal line, forming a lossy section whose transmission magnitude depends on both the permittivity and conductivity of the sensing material. The proposed configuration, therefore, establishes a direct relationship between the composite’s permittivity and conductivity and the transmission coefficient of the sensor. The PDMS–CF-loaded sensor exhibits a resistance-dominated response in the lower temperature range (below 125 °C) and a capacitance-dominated response in the higher temperature range (above 125 °C). Each temperature region is modeled using a piecewise linear fit, enabling temperature to be determined from the measured S-parameters across the full frequency span of a vector network analyzer (VNA). A simplified sensing platform is demonstrated using a portable, open-source data-acquisition system (nanoVNA), with performance validated against a traceable benchtop VNA under periodic thermal cycling. Finally, we demonstrate a fully circular sensor architecture based on liquid-metal (Galinstan) conductors embedded in 3D-printed channels, providing a viable route for recycling both the conductive elements and the sensing material.