Fully Passive Sensor Coated With Phenylboronic Acid Hydrogel for Continuous Wireless Glucose Monitoring

We developed a fully passive sensor coated with phenylboronic acid (PBA) hydrogel, which allows wireless sensing of the glucose concentration in human blood. An interdigital capacitor (IDC) integrated with an inductor was used to develop a resonator. The former maximizes the contact between the glucose fluid and the electric field from the sensor and, therefore, the sensitivity. The latter generates mutual coupling between the sensor and the coil on the reader side, allowing not only wireless sensing but also continuous monitoring of the glucose concentration in a fully passive mode without any power sources integrated with the sensor. Furthermore, depending on the glucose concentration, the resonant frequency of the measurement system is shifted, and the magnitude of the reflection changes. With such dual-marker capability, the sensor has the potential to provide substantially enhanced accuracy. The measurement results reveal that both the shift in the resonant frequency and the increase in the magnitude of reflection have a linear dependence on the glucose concentration in the range 0–500 mg/dL, with sensitivities of 14 kHz/(mg/dL) and $1\times 10^{-{3}}$ dB/(mg/dL), respectively. Finally, by coating the resonator with PBA hydrogel, a selectivity of up to 100 is achieved between glucose and fructose. The high selectivity over other sugar types that may exist in human blood demonstrated the potential of the proposed sensor for medical applications. All the measured results were validated using simulations and equivalent models. This study successfully demonstrates the feasibility of a simple LC resonator as a fully passive wireless glucose sensor with dual-marker capability. A lightweight, battery-free, and low-cost sensor with a small volume of 25 mm3 is suitable for invasive operations, which adds to the aforementioned advantages of the sensor for implantable applications.