A soft hydrogel-based bilayer grating for moisture-driven actuation and optical encoding

Dynamic modulation of optical diffraction is essential in next-generation photonic devices for sensing, display, and anti-counterfeiting applications. The development of hydrogel actuators offers a compelling route to dynamic opto-mechanical coupling, enabling soft matter systems to encode environmental stimuli into tunable optical signals. However, most hydrogel-based photonic structures remain architecturally complex or lack real-time, quantitative optical output. Here, we report a soft bilayer diffraction grating actuator (BDGA) that integrates moisture-responsive mechanical deformation with optical signal modulation. The device comprises a PEGDA200 grating top layer bonded to a swelling-dominant PEGDA600 base. Upon exposure to humidity, asymmetric swelling induces a curvature, reorienting the grating and shifting the diffraction angle in real time. A geometric model quantitatively relates the curvature (κ) to the diffraction angle (θ), with experimental data showing strong agreement over κ = 0.1-0.45 cm^-1 and the corresponding θ shifts from 16.5° to 21.6°. The BDGA exhibits fast, reversible actuation with up to 7.5° diffraction angle modulation under 60% RH, and a visible color change from green to blue under white light. A patterned BDGA label further demonstrates a naked-eye readable, humidity-responsive color transition for secure optical encoding. These results establish a scalable and multifunctional platform that merges mechanical actuation and optical encoding in a single hydrogel architecture, offering new opportunities for its application in interactive displays, smart packaging, and passive environmental sensors.