image: (upper left) Photo of DLP 3D-printed hydrogel lattice before (DH) and after soaking in thermoelectric solution (DHFG), along with microstructured such as a microneedle and a flower-shaped hydrogel thermocell. (upper right) Wearable thermocells such as joint sleeve, bracelet, insole, kneepad and forehead protector on a moving body. (bottom left) Schematic showing microstructured surfaces improving contact with irregular heat source. (bottom right) The output voltage of thermocell under the two conditions of perfect match (1) and mismatch (2) at different temperatures of the irregular heat.
Credit: Nankai University
Most of the heat around us goes to waste, from skin to phone chargers and machinery. These “low-grade” sources are usually too cool, scattered and irregular for rigid thermoelectric chips, so the energy is simply lost to the air.
A research team at Nankai University has now shown that this everyday warmth can be turned into useful electricity with 3D-printed hydrogel thermocell patches. Their soft, stretchable “power patches” can be printed in many shapes and worn on fingers, knees, insoles and headbands, or wrapped around small electronic devices. As long as there is a modest temperature difference between the patch and its surroundings, the device generates a steady voltage. These findings are published in National Science Review.
Each patch contains a hydrogel thermocell, a water-rich network of ions that undergo reversible redox reactions. When one side of the gel is at a higher temperature, ions move on both sides and react at the electrodes, thereby generating voltage through the external circuit by electrons. Unlike brittle solid thermoelectric materials, the hydrogel is soft, bendable and skin-friendly, so it keeps good contact with curved and moving surfaces. These devices are hard to make with conventional hydrogel 3D printing, which uses a lot of water that can evaporate or crack delicate structures. Many ion solutions that work well for heat-to-electricity conversion also do not mix with the light-cured printer inks, forcing a trade-off between fine structure and good performance.
The Nankai team solved this with a two-step process. First, they used digital light processing (DLP) 3D printing to make a transparent hydrogel skeleton with precise three-dimensional features. After this framework was stabilized, they soaked it in a tailored thermoelectric solution so active ions diffused into the network and turned the scaffold into a working hydrogel thermocell. Separating “shaping” from “activation” lets them tune structure and chemistry independently. They also carved the gel surface into tiny ridges and columns to help the gel grip rough, bumpy heat sources. Tests show that the electrical energy generated by the microstructure patch is 3.5 times that of the flat patch, and it can expand the effective environmental temperature for collecting human body heat energy to 6 K.
To demonstrate design freedom, the researchers printed wearable devices such as joint sleeves, bracelets and insoles. In laboratory and on-body tests, these custom-fit patches generated tens of millivolts from the modest temperature difference of roughly ten degrees between skin and room air. That is enough to drive some low-power sensors or to be combined with other harvesters in self-powered systems.
The micro-structures do more than improve heat harvesting. They also make the hydrogels highly sensitive to touch and motion, because small deformations change the internal ion distribution and therefore the electrical signal. Arrays of these units can act as simple electronic skin that maps pressure on a hand or wrist. In one demonstration, a hydrogel wristband converted finger taps into pulses that spelled out the Morse code message “I am fine,” hinting at a possible communication aid for people with speech difficulties. In another, a printed sleeve around a phone charger both lowered the surface temperature by several degrees and captured a small voltage from the waste heat.
According to the authors, the same strategy of high-resolution printing followed by chemical activation could be extended to many other soft energy devices and sensors. In the future, custom 3D printed hydrogel patches might be designed for specific users or products, turning the gentle warmth of bodies and gadgets into a quiet, continuous source of clean electricity.
This work is among the recent advances from Prof. Rujun Ma’s group on high-performance flexible thermoelectric materials and devices. Prof. Ma currently leads the Intelligent Thermal Management Laboratory at the School of Materials Science and Engineering, Nankai University. In recent years, his team has achieved a series of important results across multiple interdisciplinary areas, including active/passive cooling materials, flexible thermoelectric materials and devices, high-performance flexible thermal-conductive composites, and energy-conversion materials and devices. Their findings have been published as corresponding or first author in leading international journals such as Science (2), Natl. Sci. Rev., PNAS, Nat. Commun. (3), Joule (2), Chem. Soc. Rev., Adv. Mater. (5), Energy Environ. Sci., Adv. Energy Mater. (2), Nano Lett. (8), ACS Nano (3). The laboratory is equipped with advanced platforms for materials synthesis and systems for electrical and thermal measurements, and conducts research on new energy and intelligent thermal-management technologies under several projects, including the National Key R&D Program. This work was supported by Nankai University, the National Key R&D Program of China (Grant No. 2020YFA0711500), the National Natural Science Foundation of China (Grant Nos. 52473215, 52273248, and 52303238), the Key Project of the Natural Science Foundation of Tianjin City (Grant Nos. S24JQU021 and QN20230102), and the Scientific Research Innovation Capability Support Project for Young Faculty.