Chemical hydrogel sensors based on the bimorph effect with short response time

Hydrogel sensors are well suited to measuring the concentration of substances in liquids, and, because the hydrogel is biocompatible, they are ideal for medical use. Hydrogels change their volume in response to stimuli. The larger the hydrogel, the more pronounced the measurement signal. However, a larger volume also leads to slower swelling due to the longer diffusion paths. One method of determining the degree of swelling is to measure the swelling pressure using a piezoresistive pressure sensor. With current approaches, measurement times of several minutes can be achieved. By exploiting the bimorph effect, we were able to reduce the thickness of the hydrogel and, thus, reduce the response time of the entire sensor to less than 1 min. The aim of this paper is now to show how a sensor with short response times can be designed and manufactured and, in particular, how to find a suitable hydrogel composition, how to appropriately structure the hydrogel layer and how a robust adhesion of the hydrogel to the sensor chip can be achieved. As a result, we were able to show that such hydrogel sensors with response times of just a few seconds are possible.