Abstract:As the interface between human and machine becomes blurred, hydrogel incorporated electronics and devices have emerged to be a new class of flexible/stretchable electronic and ionic devices due to their extraordinary properties, such as softness, mechanically robustness, and biocompatibility. However, heat dissipation in these devices could be a critical issue and remains unexplored. Here, we report the experimental measurements and equilibrium molecular dynamics simulations of thermal conduction in polyacrylamide (PAAm) hydrogels. The thermal conductivity of PAAm hydrogels can be modulated by both the effective crosslinking density and water content in hydrogels. The effective crosslinking density dependent thermal conductivity in hydrogels varies from 0.33 to 0.51 Wm −1 K −1 , giving a 54% enhancement. We attribute the crosslinking effect to the competition between the increased conduction pathways and the enhanced phonon scattering effect. Moreover, water content can act as filler in polymers which leads to nearly 40% enhancement in thermal conductivity in PAAm hydrogels with water content vary from 23 to 88 wt %. Furthermore, we find the thermal conductivity of PAAm hydrogel is insensitive to temperature in the range of 25-40 • C. Our study offers fundamental understanding of thermal transport in soft materials and provides design guidance for hydrogel-based devices.

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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 °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.

The Seebeck and Peltier effects have been widely studied and used in various thermoelectric technologies, including thermal energy harvesting and solid-state heat pumps. However, basic and applied studies on the Thomson effect, another fundamental thermoelectric effect in conductors, are limited despite the fact that the Thomson effect allows electronic cooling through the application of a temperature gradient bias rather than the construction of junction structures. In this article, we report the observation of a giant Thomson effect that appears owing to magnetic phase transitions. The Thomson coefficient of FeRh-based alloys reaches large values approaching –1000 μV K−1 around room temperature because of the steep temperature dependence of the Seebeck coefficient associated with the antiferromagnetic–ferromagnetic phase transition. The Thomson coefficient is several orders of magnitude larger than the Seebeck coefficient of the alloys. Using the active thermography technique, we demonstrate that the Thomson cooling can be much larger than Joule heating in the same material even in a nearly steady state. The operation temperature of the giant Thomson effect in the FeRh-based alloys can be tuned over a wide range by applying an external magnetic field or by slightly changing the composition. Our findings provide a new direction in the materials science of thermoelectrics and pave the way for thermal management applications using the Thomson effect.