Performance of Fast-Responsive, Porous Crosslinked Poly(N-Isopropylacrylamide) in a Piezoresistive Microsensor

Hydrogel-based microsensors allow us to measure chemical measurands as well as temperature and other quantities. However, long response times due to cooperative diffusion in the hydrogel has thus far prevented the dissemination of this type of sensor. One way to overcome this drawback is to use porous hydrogels. The strong influence of pores on the response time of the sensor was demonstrated exemplarily in a hydrogel-based thermal microsensor setup. Porous crosslinked poly(N-isopropylacrylamide) (PNIPAAm) was synthesized by a surfactant-based template method. The characterization of the hydrogel with scanning electron microscopy proved the formation of macropores with diameters around 3.8 μm. A piezoresistive microsensor with embedded porous PNIPAAm hydrogel showed the response time t90 to be below 20 s when increasing the temperature from 25 °C to 50 °C, whereas t90 is not reached for the sensor with embedded nonporous PNIPAAm hydrogel after several hours. A discussion on much slower response times for decreasing the temperature to 25 °C is provided. The results can be transferred to other hydrogel-based sensor and actuator systems.