Swelling-Induced Information Camouflage and Optical Decryption on a Transparent Recoverable Hydrogel Surface.

Information camouflage and decryption on hydrogels rely on chemical stimuli such as pH, ultraviolet light, and chemical reactions, in which the cyclability is limited. This work develops a simpler yet effective physical method that can achieve the information camouflage on hydrogels by water swelling and decrypt it under white light. The information camouflage and decryption can proceed with unlimited cycles. To successfully reach the information camouflage, the hydrogel is synthesized with the water swelling ratio in weight as high as 250, which is enabled by the strong electrostatic repulsion of cationic moieties inside the network. At such a high water-swollen state, the hydrogel is still robust and elastic, which provides a mechanical basis to maintain the stability of the camouflaged information. We write information on the hydrogel surface by laser cutting. Upon immersing the hydrogel in water, the high swelling results in huge expansion of the hydrogel, thus inducing the information camouflage. With exposure to white light, the information can be decrypted and becomes visible again. Our protocol utilizes a simple physical process to enable the camouflage and decryption of complex information, which might open an alternative pathway for the development of hydrogel materials in the application of informatics.