Noninvasive Detection of Glucose and NaCl Solutions With Environment Correction Using a Dual IDC-Based Microwave Sensor

In this article, a microwave-based sensor for noninvasive glucose detection is proposed. Considering the concentrations of substances other than glucose in the blood, sodium chloride (NaCl) is selected, and the dielectric properties of glucose and NaCl solutions are measured and analyzed. Due to the sensitivity of water to temperature, the dielectric properties are examined concerning temperature variations. As the temperature-induced dielectric properties are found to dominate over those due to changes in the concentrations of glucose and NaCl solutions, a switching circuit is employed to eliminate environmental effects, including temperature. To sensitively detect concentration changes in the solution, a resonator based on an interdigital capacitor (IDC) is designed, generating strong fringing fields between fingers. Dual sensing is achieved by creating a reference resonator for detecting only environmental effects and a sensing resonator for detecting both environmental effects and concentration changes. The difference in reflection coefficients between the two resonators is continuously stored in real time, enabling the continuous detection of concentration changes in glucose and NaCl solutions with environment correction. The reflection coefficient changes for glucose (0-400 mg/dL) and NaCl (500-900 mg/dL) are 0.41 and 1.43 dB, respectively. Through the analysis of dielectric properties and measurement results for glucose and NaCl concentrations, it is confirmed that to detect small changes in glucose concentration in mixtures like blood, where various substances are present, the influence of other substances with dominant dielectric property variations over glucose must be eliminated.