Non-Invasive Microwave Glucose Sensor by Using a Hybrid Sensor Composed of a Frequency Selective Surface and Microstrip Patch Antenna

In this paper, a hybrid sensor composed of a frequency selective surface (FSS) and microstrip patch antenna is investigated numerically for non-invasive glucose sensing in the microwave region. The sensing method relies on detecting changes in the dielectric constant of the sample under test (SUT) in response to variations in the concentration of glucose-deionized water solutions. The SUT consists of the area between the microstrip patch antenna and FSS. Four glucose-deionized water solutions (i.e., 72 mg/dL, 216 mg/dL, 330 mg/dL, and 600 mg/dL) are tested in the SUT. The dielectric properties of the solutions are determined using the Debye model. For the sensitivity analyses, the return losses (|S11| (dB)) of the proposed sensor and the dependence of the resonance frequencies on the volume percentage of glucose in glucose-deionized water solutions have been noted. When the glucose-deionized water solutions changed from 72 mg/dL to 600 mg/dL, the resonance frequency of the sensor blue shifted from 11.281 GHz to 11.296 GHz. The sensitivity of the glucose sensor is calculated by absolute resonance frequency shift in response to glucose-deionized water solution concentration change. When the FSS structure is removed from the hybrid sensor (i.e., the sensor structure has consisted of just an antenna), the sensitivity value drops from 28.409 kHz/mg dL-1 to 18.939 kHz/mg dL-1 (i.e., 33.3%). Furthermore, the sensitivity of the sensor obtained by removing the antenna part from the hybrid sensor (i.e., the sensor consisting only of FSS) is calculated as 15.152 kHz/mg dL-1 in simulations. The results show that the proposed hybrid sensor structure exhibits heightened sensitivity compared to sensors solely reliant on antennas or FSS structures. This proposed novel hybrid sensor structure that is lightweight, low-cost, easy-to-fabricate, portable, and easy to integrate with microwave-integrated circuits will contribute to the non-invasive glucose sensor literature.