Non-Reciprocal Whispering-Gallery-Mode Resonator for Sensitive Blood Glucose Monitoring

In this article, a miniaturized low-cost microwave sensor of high sensitivity is proposed for noninvasive blood glucose monitoring. The traveling waves whispering gallery modes (WGM) are excited/launched on a ferrite-made ring resonator when placed nearby a microstrip line (MTL). The sensing structure is designed appropriately to support four distinct WGM modes with the desired coupling in the frequency range of 22-32 GHz. A disk of permanent magnet is incorporated beneath the MTL substrate to generate a bias magnetic field necessary to create a nonreciprocal effect in the resonator. This nonreciprocal behavior is demonstrated in the transmission responses S-12 and S-21, where the WGM resonances of similar modes are triggered at different frequencies over a narrowband. A magneto-static analysis is performed to approximate the internal biasing fields inside the FRR at multiple longitudinal layers. The complex components of the permeability tensor are computed accordingly for each layer, and then integrated into HFSS to simulate the sensor scattering responses. The EM simulations are validated when compared with practical measurements of a magnetized FRR. The latter fabricated prototype is tested in different setups for monitoring the glucose in synthetic blood of concentrations related to diabetes conditions. The proposed sensor shows high detection sensitivity at WGM(600) and WGM(700) to small changes in the dielectric properties of glucose samples as demonstrated by the frequency and phase shifts (0.07 MHz and 0.17 degrees per 1 mg/dL, respectively) of the measured S-21 and S-12 with repeatability error of about +/- 0.2 MHz and +/- 0.5 degrees.