Spatially and Temporally Programmable Transparency Evolutions in Hydrogels Enabled by Metal Coordination toward Transient Anticounterfeiting

Abstract Hydrogels have emerged as promising candidates for anticounterfeiting materials, owing to their unique stimulus‐responsive capabilities. To improve the security of encrypted information, efforts are devoted to constructing transient anticounterfeiting hydrogels with a dynamic information display. However, current studies to design such hydrogel materials inevitably include sophisticated chemistry, complex preparation processes, and particular experimental setups. Herein, a facile strategy is proposed to realize the transient anticounterfeiting by constructing bivalent metal (M²⁺)‐coordination complexes in poly(acrylic acid) gels, where the cloud temperature (T_c) of the gels can be feasibly tuned by M²⁺ concentration. Therefore, the multi‐T_c parts in the gel can be locally programmed by leveraging the spatially selective diffusion of M²⁺ with different concentrations. With the increase of temperature or the addition of a complexing agent, the transparency of the multi‐T_c parts in the gel spontaneously evolves in natural light, enabling the transient information anticounterfeiting process. This work has provided a new strategy and mechanism to fabricate advanced anticounterfeiting hydrogel materials.