Stimuli‐Responsive Optical Switching Patterns in Millimetric Hydrogel Frames: Nanomaterial‐Free Anti‐Counterfeiting Technology

Stimuli‐responsive materials are employed in numerous on‐demand applications, such as anti‐counterfeiting. Although innovative hybrid spectral/graphical approaches have gained wide attention, they are limited by the need for user training and sophisticated technologies to decode the covert pattern which can be achieved through toxic organic dyes, pigments, up‐conversion crystals, nanoparticles, or self‐assembly of colloidal particles within a hydrogel framework. These approaches also involve a lengthy multistep fabrication that consumes several days. Although optical nanomaterials, which are responsive to external stimuli, such as magnetic field, can undergo a quicker self‐assembly, additional equipment is required to generate external fields and the use of nanomaterials alone limits this technology's wide applicability in consumer goods especially on food or pharmaceuticals. Herein, a nanomaterial‐free technology is demonstrated that utilizes various microscale motifs to display an optical shifting behavior without the need for passive or active assembly of colloidal particles, and any toxic dyes, pigments, or nanomaterials. The optical shifting property is purely induced by hydrogel porous microstructures and resulting light scattering. The slit channel lithography technique makes it easy for users to authenticate products using a simple smartphone device or inverted microscope by validating the optical switching behaviors of motifs embedded in hydrogel frames.