Thermally-Responsive Hydrogels Poly(N-Isopropylacrylamide) as the Thermal Switch

The thermal switch is a device that can modulate the heat flux and create a huge gap between the "On" and "Off" state, which has been widely used in many applications. However, owing to weak biocompatibility and complicated structures, most of existing thermal switch devices mostly are difficult to use in some emerging mobile health areas, such as soft electronics and biomedical applications. Herein, it is reported that a poly(N-iopropylacrylamide) (PNIPAm) hydrogels-based thermal switch featuring good biological compatibility and a simple preparation process. The thermal conductivity of the PNIPAm hydrogels at temperatures from 30 to 40 degrees C has been measured using the transient hot wire method. Interestingly, the thermal conductivity drops from 0.51 to 0.35 Wm(-1)K(-1) when the hydrogel is heated above the lower critical solution temperature. Its thermal resistance ratio R-off /R-on, an important criterion to evaluate the performance of the thermal switch, reaches up to 3.6. Furthermore, the effective medium approach is used to evaluate the thermal conductivity of hydrogels with different water content, and molecular simulation analysis reveals that the hydrogen-bonding network among water molecules mainly contributes to heat conduction of the hydrogels. The proposed thermal responsive hydrogel-based thermal switches contribute to the development of non-mechanical-assist devices and show a promising potential in biomedical science due to their biocompatibility.