From Invisible to Visible: Pressure-Responsive Photonic Crystals for Advanced Information Encryption

Counterfeiting has emerged as an increasingly severe global concern, posing significant threats to the security of individuals and society. In this study, a series of multilayered information-encrypted composite materials is developed by embedding a chiral nematic cellulose nanocrystal (cn-CNC) film inside a shape-memory polymer (SMP) matrix to produce pressure-responsive photonic crystals. The design also incorporates a cellulose acetate film, as a thermoset information carrier, and graphene oxide (GO), as a secrecy filter. In its initial state, the composite shows no information, while a pattern becomes visible to reveal the information upon hot-pressing, due to the region-dependent differences in its photonic responsiveness caused by the rigid pattern. A discernible mark remains even after recovery, serving as tangible evidence of the decrypted information. Moreover, the high color fidelity of these materials is confirmed using reflectance spectroscopy and color analysis over multiple cycles, demonstrating their potential in cyclic information encryption. By harnessing thermal and mechanical stimuli for anti-counterfeiting purposes, these composites diversify the strategies available for information encryption.