3D printable shape-customized hydrogel thermocells

Hydrogel thermocells based on the thermogalvanic effect hold significant promise for energy harvesting and flexible electronics due to their excellent heat-to-electric conversion and stretchability. However, mismatched contact surfaces between bulk hydrogel thermocells and complex heat-source geometries often lead to poor heat utilization and reduced conversion efficiency. While 3D printing can create structures that match these complex interfaces, direct printing of hydrogel thermocells remains challenging due to issues such as water evaporation and the incompatibility of redox couples with photopolymer inks. To overcome these challenges, we combine 3D digital light processing with a thermoelectric (TE) solvent-exchange strategy, enabling the precise customization of stretchable hydrogel thermocells (DHFGs) that conform to specific heat-source geometries. Nonvolatile deep eutectic solvents in the photopolymer ink precursors form robust eutectogel networks through ultraviolet-initiated polymerization, ensuring high structural fidelity (down to 130 μm). The resulting 3D-printed thermocells exhibit excellent TE performance (3.5 mV K^-1) and can be tailored to complex geometries. This conformal design extends the effective working ambient temperature range by 6.0 K and boosts output power to ∼350% of that of an unmatched DHFG. Additionally, the microstructured DHFG demonstrates superior pressure sensitivity (0.35 kPa^-1) within a low-pressure range (<1.0 kPa), at ∼4.4 times higher than its unstructured counterparts. This approach optimizes thermal-energy utilization through geometric matching, simplifies assembly and paves the way for flexible thermocells with high TE performance, complex architectures and multifunctionality.