Converting Light Into Programmable Temperatures via Janus Hydrogels for Passive Infrared Thermography

Passive thermal management saves energy using natural processes but struggles with precise temperature control in variable environments. This study designs and synthesizes temperature-programmable hydrogels (TPH) based on a bilayer polyvinyl alcohol network with tunable passive heating capacity. The TPH features an upper layer with adjustable transmittance (6.9% to 37.3%) and a bottom layer with variable solar absorption (92.5% to 97%), achieving an energy-free programmable temperature range of 21-39 °C induced by solar heating. Additionally, the TPH has low density (0.5 g cm^-3), low thermal conductivity (0.25 W m^-1 K^-1), and low water loss rate (0.33 kg m^-2 h^-1). As a concept-of-proof, the TPH's application as smart solar heating blocks in passive information encryption and camouflage by passive infrared thermography is demonstrated, which displays letters and camouflage patterns after light illumination. The TPH provides an energy-free strategy for smart thermal management and opens a direction for passive information encryption and camouflage emulation.