Numerical Design and Experimental Realization of a PNIPAM-Based Micro Thermosensor

Stimuli-responsive polymers are capable of responding to external stimuli and therefore have been widely used for sensing. However, such applications are often based on naïve designs and cannot achieve the desired performance. In this study, we created a micro thermosensor with temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM) hydrogel and temperature-insensitive poly(ethylene glycol) diacrylate (PEGDA) hydrogel using stop-flow lithography. The thermosensor is a bihydrogel microparticle consisting of a NIPAM-rich section and a NIPAM-poor section. Since the sensor is similar to a bimetallic strip in structure, its deformation can be easily identified to indicate temperature. To gain better control over the sensor performance, a numerical model capable of predicting the thermal behavior of the sensor was also developed. The model simulated the mass transfer and polymerization reaction during the fabrication process to determine the distributions of PNIPAM and PEGDA in the sensor. The information was then applied to predict the sensor deformation at various temperatures. We have used the model to access the effects of sensor geometry and fabrication temperature on the performance of the sensor. The sensor made under the guidelines from the numerical model has a working range between 16 and 55 °C. Except at very large deformation, the thermal response of the microsensor measured in experiments follows closely the numerical prediction. We believe such a numerical model can also be used for developing other applications involving stimuli-responsive polymers such as shape-evolving microparticles and origami-based microstructures. With the small size, ease of use, low manufacturing cost, good biocompatibility, and broad sensing range near physiological condition, the PNIPAM-based micro thermosensor should have strong potential to be used for bio-related applications and in a confined environment.