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In this paper, we report on the fabrication and characterization of a new hydrogel-based wireless chemical sensor. The basic device structure is a passive LC resonator coupled to a stimuli-sensitive hydrogel which is confined between a stiff porous membrane and a thin glass diaphragm. As small molecules pass through the porous membrane, hydrogel swells and deflects the flexible glass diaphragm which is the movable plate of the variable capacitor in the totally integrated passive LC resonator. The corresponding change in resonant frequency can be remotely detected. pH-sensitive hydrogels were loaded and tested with the measured sensitivity of 394 kHz/pH at the pH of 7.4. The swelling pressure of the confined hydrogel was also determined using the calibration curve with applied air pressure.