Vat Photopolymerized HEMA/HEA Hydrogels for Solvent‐Responsive Transparency and Optical Encryption

Abstract Vat photopolymerization (VPP) offers a powerful route for fabricating hydrogels with tailored microstructures and multifunctionalities. Here, a photopolymerizable HEMA/HEA resin that produces 3D‐printed hydrogels exhibiting reversible, solvent‐driven optical transitions is presented. Printed structures are transparent in the dry state, become opaque upon immersion in deionized water due to hydration‐induced microphase separation and refractive index heterogeneity, and recover transparency when exchanged into polar organic solvents such as ethanol and dimethyl sulfoxide (DMSO). This optical switching mechanism enables real‐time hydration sensing, solvent‐programmable information encryption, and optical memory functions. Complementary FTIR analysis demonstrates solvent exchange through the appearance of characteristic DMSO peaks and the suppression of water‐related absorptions. Mechanical and rheological characterization reveals high ductility, elastic dominance, and self‐recovery, ensuring robustness under repeated deformation. These results position vat‐photopolymerized HEMA/HEA hydrogels as a versatile platform for multifunctional photonic devices, secure optical data storage, and adaptive materials.