Sensitivity-Enhanced Fluidic Glucose Sensor Based on a Microwave Resonator Coupled with an Interferometric System for Noninvasive and Continuous Detection

In this paper, a microwave fluidic glucose sensor based on a microwave resonator coupled with an interferometric system is proposed for sensitivity enhancement. The proposed glucose sensor consists of two parts: a sensing part and a sensitivity enhancement part. The former is composed of a rectangular complementary split ring resonator (CSRR), and the latter is composed of a variable attenuator, a variable phase shifter, two hybrid couplers, and an RF power detector. Because the variation in the electrical properties, which is utilized in the microwave detection scheme, with glucose concentration over the possible concentration range in the human body is very small, improvement of the sensitivity is critical for practical use. Thus, the effective sensing area of the rectangular CSRR is determined by considering the electric field distribution. In addition, magnitude and phase conditions for the effective sensitivity enhancement are derived from a mathematical analysis of the proposed interferometric system. In the present experiment, aimed at demonstrating the detection performance as a function of the glucose concentration in the range of 0 mg/dL to 400 mg/dL, the sensitivity is significantly improved by 48 times by applying the derived conditions for effective sensitivity enhancement. Furthermore, the accuracy of the proposed glucose sensor for glucose concentrations at a step of 100 mg/dL is verified by the Clarke error grid. Based on the measurement results, the proposed glucose sensor is demonstrated to be applicable to noninvasive and continuous monitoring in practical environments.