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

Nowadays, robust sensors for glucose sensing application is almost a necessity for every diabetic individual. In this work, a (20 mm x 10 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 (UHF) 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 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.