Evaporative hydrogels for high-performance ambient body heat harvesting via thermoelectric

The increasing demand in power of modern environmental and health sensors have spurred the development of ambient energy harvesting to reduce reliance in battery. Thermoelectric generators (TEGs) are a promising technology, which convert waste or body heat into electricity. However, their power output is severely limited by the thermal impedance mismatch, particularly between the human skin and TEG interface. This work introduces a novel approach to overcome this limitation by integrating a hydrogel into the TEG system. It has been revealed that the enthalpy from the hydrogel's water evaporation effectively achieves thermal impedance matching while simultaneously maximizing the heat flux. Furthermore, by optimizing the TEG's fill-factor, we increased the power density by nearly two orders of magnitude compared to conventional TEG systems. As a proof of concept, our device combined body heat with hydrogel evaporation using an optimized fill-factor of 0.48 (using 52 % less material). This setup achieved a power density of 150 μWcm⁻², which was sufficient to power four wireless sensors. This work demonstrates a counter-intuitive synergistic benefit, that is achieving superior thermal matching while significantly reducing thermoelectric material usage. Our findings redefine optimization approach for TEGs and offer a viable pathway toward realizing off-grid, self-powered sensors. © 2026 The Author(s).