Hydrogel-Actuated Capacitive Transducer for Wireless Biosensors

This article introduces a new type of transducer that combines capacitive pressure sensing techniques with biosensitive hydrogels, using an adaptable MEMS fabrication platform. Hydrogel swelling in response to analyte concentration exerts contact pressure on a deformable conducting diaphragm, producing a capacitance change. Initial results are reported for testing device feasibility. Uncrosslinked PHEMA hydrogel was tested for swelling pressure in response to calcium nitrate tetrahydrate. Diaphragm deflection due to applied air pressure was measured on NiTi-based diaphragms and compared with hydrogel-actuated deflections of the same diaphragms to determine the pressure generating characteristics of the hydrogel. The PHEMA sample exhibited greatest sensitivity in the concentration range 0–0.5 M, generating an average of 110 mN/M/μl. The device was incorporated into a passive LC resonant circuit. Resonance frequency was measured as a function of applied air pressure, in the range of pressures generated by hydrogel swelling. Resonance frequency shifted from 66 MHz to 33 MHz over the pressure range 0–32 kPa, corresponding to an estimated average sensitivity of 66 Hz per μmol of calcium nitrate tetrahydrate over the range 0–0.5 M.