Effects of fabrication materials and methods on flexible resonant sensor signal quality

Inductor–capacitor (LC) resonant sensors can provide low-cost, contactless measurement in closed environments. In this work, three fabrication methods to produce flexible LC sensors are compared: screen-printed pastes, etched copper-coated polyimide, and wound metal wires. The LC sensors were interrogated by near-field coupled antennas driven by a vector network analyzer. The sensor response was analyzed by extracting the resonant frequency (MHz) and the peak-to-peak amplitude (dB) of the S21 scattering parameter. For screen-printed resonators, varying parameters such as print feature size (250 to 500μm), conductive paste silver content (DuPont PE825 and 5028 for low and high silver content, respectively), curing condition (convection oven temperature and curing time), and paste viscosity (7.20–20.05​ Pa⋅s) were explored. An empirical model was fit to this data to allow for prediction of required sensor line thickness needed to meet an application’s required step-off distance. The screen-printed resonant sensors were then compared to copper-etched and wound metal wires to evaluate the cost and performance tradeoff of the three fabrication approaches. Finally, a use case of the flexible, screen-printed resonant sensors was demonstrated by contact-free measurement of fluid level of in a closed, curved container.