A Novel Method of Using Bifilar Spiral Resonator for Designing Thin Robust Flexible Glucose Sensors

Nowadays, robust sensors for glucose-sensing application are almost a necessity for every diabetic individual. In this work, a (20 mm $\times10$ mm) contact-based sensor has been successfully realized on a thin and flexible polyamide substrate, with the sensing area being a bifilar spiral resonator (BSR). The proposed sensor was based on detecting resonance frequencies shifts at ultrahigh frequencies (UHFs) as a result of changes in the glucose concentration. The BSR acts as a cascaded metasurface which, at around the resonant frequency, forms a plasmon dispersion at the interface between a glucose solution and the BSR. An empirical formula has been developed to express the sensing sensitivity as a function of the loss factor, the oscillator strengths and the steady state permittivity. The model has been experimentally validated with an in vitro measurement on an aqueous glucose solution and an ex-vivo measurement on a serum blood solution. The measured results suggest that there exists a positive and linear correlation between glucose concentration and the resonant frequency shift within the clinical diabetic range. The sensor sensitivity was 250 megahertz (MHz)/(mg/mL) for the aqueous glucose solution and 130 MHz/(mg/mL) for the serum blood along with a $Q$ -factor of 20. Overall, the measured results were highly consistent with our theoretical expectation. Moreover, the proposed sensor was highly resistant to bending losses.