An In-Vitro Study of Wireless Inductive Sensing and Robust Packaging for Future Implantable Hydrogel-Based Glucose Monitoring Applications

A wireless inductive sensing technology with a robust packaging scheme is proposed and demonstrated for future implantable hydrogel-based glucose monitoring applications. A prototype wireless glucose sensor employs a glucose-sensitive hydrogel embedding a parylene-coated metallic sensing plate as a key sensing element. The hydrogel changes its volume responding to glucose change; thus displacing the metallic sensing plate that can be sensed by a nearby sensing coil through the coil inductance change. An ultra-low-power tunnel diode oscillator is designed to convert the inductance change to an oscillator frequency change, which can be wirelessly detected by a nearby receiver. The entire sensing system can be energized by RF powering, thus achieving a wireless and battery-less operation. The proposed sensing scheme allows the sensing electronics to be fully encapsulated in a hermetically sealed package, critical for long-term implantable applications. In vitro characterization demonstrates that the prototype sensor operated around 40 MHz can be wirelessly powered by a 2.77 mW 5 MHz power source with a distance of 2 cm, and achieves a sensing resolution of 0.05 mM and 0.3 mM within a glucose range from 0 mM to 2 mM and 8 mM to 10 mM, respectively, limited by the oscillator frequency drift and fluctuation. Furthermore, a hydrogel pillar-based design is promising to substantially shorten the system response time.