Rapid Battery-Free Glucose Sensing with Phenylboronic Acid Hydrogel and Flexible Interdigitated Capacitor

A battery-free glucose sensor based on phenylboronic acid hydrogel and the flexible interdigitated capacitor was fabricated for rapid detection of postprandial hyperglycemia (PPHG). Excessive elevation of the postprandial glucose level is a general risk factor for type 2 diabetes, obesity, hypertension, and severe organ diseases. The postprandial glucose level reaches its peak at 30 to 60 minutes after food intake, and PPHG should be detected within 30 minutes. Battery-free flexible glucose sensors are a promising tool for the detection of PPHG and continuous monitoring with a relatively low impact on biological tissues and living bodies because they are lighter and more flexible than conventional battery-driven glucose sensors. Existing battery-free glucose sensors, however, are inadequate for the practical diagnosis of PPHG due to their long reaction time (&gt; 1 h) and response variability. To bridge this gap, we developed a battery-free glucose sensor with a response time sufficient for detecting PPHG. A previous study demonstrated the detection of an elevated glucose level using a sensor that forms a layer of glucose-responsive phenylboronic acid hydrogel on a field-effect transistor (FET). However, this sensor requires a battery attached to the sensor and is thus not wearable. We overcame this limitation by using PBA hydrogel embedded on a flexible, interdigitated capacitor (IDC) instead of a battery-driven FET without compromising the speed of signal transduction (<i>i.e.,</i> &lt; 5 minutes). Here, we present a battery-free flexible glucose sensor that combines phenylboronic acid hydrogel with an inkjet-printed IDC. We achieved a 5-minute response to an increased glucose level (from 0 mg/dL to 216 mg/dL) by using the IDC to detect the dielectric transition inside a 200-µm-thick layer of phenylboronic acid hydrogel. To demonstrate battery-free sensing, the sensing data are transferred to a vector network analyzer using inductive coupling. When the glucose level increases, the peak of the transferred signal, namely the resonant frequency, shifts lower by about 3% under an in-vitro setting and about 60% lower under an ex-vivo setting within 5 minutes. The proposed design experimentally shows an increase in the response speed for battery-free glucose sensing and has great potential to extend the range of target biomarkers by replacing glucose-responsive hydrogel with other types of the stimuli-responsive hydrogel. It can extend the lifetime of wearable glucose sensors over 2 weeks, resulting in a significant benefit to health monitoring for the longer term and early prediction of risk factors associated with diabetes.