A novel miniaturizable closed-loop hydrogel-based pH sensor

State-of-the-art hydrogel-based sensors are devices that use the hydrogel swelling due to a specific environmental trigger (e.g. pH-value). They directly convert the volume change of the hydrogel to an electrical output signal. Because hydrogels show relaxation behavior their long-term properties are poor and the sensor parameters are limited. To overcome these drawbacks we present a closed-loop hydrogel-based pH sensor that uses the compensation method. The hydrogel sensor itself is part of a feedback circuit comprising a PI controller and an actuator in the feedback loop. In our approach, the hydrogel volume is kept constant during the whole measuring process. This is accomplished by applying an appropriate counter pressure that compensates the swelling pressure of the hydrogel, and thus, its attempt to swell/shrink. This counter pressure is a direct measure of the applied pH-value. Here, preliminary measurement results, that confirm the applicability of the compensation method to hydrogel sensors, are shown.