A Closed-Loop Hydrogel-Based Chemical Sensor

State-of-the-art hydrogel-based chemical sensors directly convert the volume change of a hydrogel, induced by an external environmental chemical trigger such as pH, into an electrical or optical output signal. Because hydrogels show relaxation behavior and the network expansion is governed by slow cooperative diffusion, their long-term properties are poor and the sensor parameters, such as response time, are limited. To overcome these drawbacks, we present a closed-loop hydrogel-based pH sensor, which uses the compensation method. The hydrogel sensor itself is part of a feedback circuit comprising a proportional-integral controller and an actuator in the feedback loop. In our approach, the hydrogel volume is kept constant at the steady state 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 or shrink. This counter pressure is a direct measure of the applied pH-value even though the hydrogel remains under isochoric conditions. Here, the novel closed-loop sensor approach and sensor technology are described, as well as measurement results that confirm the applicability of the compensation method to hydrogel sensors are shown.