Inductive sensing technique for low power implantable hydrogel-based biochemcial sensors

This paper presents the feasibility study and investigation of an inductive sensing technique with respect to the swelling of hydrogel samples for implantable biochemical sensing applications. A prototype sensing system is implemented based on an ultra-low power tunnel diode oscillator employing a 3 mm-diameter, 10-turn sensing coil positioned at 100 μm away from a 5 mm × 5 mm metallic sensing plate adhered to a hydrogel slab surface. The oscillator achieves a nominal frequency of approximately 28 MHz with a sensitivity of 15 KHz per um of hydrogel displacement while dissipating a DC power of 40 μW. Further characterization with hydrogel samples immersed in DI water demonstrates that the sensing system can detect 29 μm swelling of the hydrogel along vertical direction.