A next-generation anti-counterfeiting technology that appears transparent under normal conditions but reveals hidden images in vivid colors when bent has been developed by a Korean research team. Because a distinct color change can be recognized with only a minute change in angle, the technology is expected to be widely adopted for authenticating currency, identification cards and high-end luxury goods.
A team led by Professor Kim Tae-sung of the Department of Mechanical Engineering at the Ulsan National Institute of Science and Technology (UNIST) announced Wednesday that it has developed an "optical security film" that displays various structural colors depending on the viewing angle, using geometrically designed wrinkle structures.
Structural color is color produced when micro- and nano-structures on a surface reflect, diffract and interfere with light, without chemical dyes. It offers the advantages of being fade-resistant, highly durable and environmentally friendly. The research team used the surface wrinkles that form when a flexible film is bent as these microstructures.
The core technology lies in freely controlling the direction and curvature of the wrinkles. Existing wrinkle-based structural color films had the limitation that the wrinkles were aligned in one direction or randomly, so color appeared only at specific viewing angles. The team fabricated a double-layer film of ultraviolet-curable chitosan (UVCC) and flexible silicone (PDMS), then devised geometric empty spaces (patterns) such as circles and triangles in part of the chitosan layer.
When the film is bent, wrinkles form due to the difference in stiffness between the two materials. At this point, as the stress around the empty patterns changes, the wrinkles spread out like a fan or curve. Curved wrinkles with different directions diffract light in multiple directions, dramatically improving the angle dependence of the color change.
In actual performance evaluations, while existing straight-wrinkle films required an angle change of about 30 degrees to reproduce all colors in the visible light range, the newly developed film displayed color changes across the entire spectrum, from violet to red, with a tilt of just 7 degrees. It recorded an excellent angular dispersion response of up to 23.8 nm/°, meaning a distinct color change can be detected by changing the viewing angle by just 1 degree.
In addition, even in films with the same image applied, the micro-locations where internal wrinkles branch or terminate form differently in each device. The research team explained that this can be used as a product's unique physical anti-cloning "optical fingerprint." In tests bending the film more than 500 times, the wrinkles and structural color response remained stable, demonstrating outstanding durability.
"We designed the wrinkles to form in multiple directions so that the color changes quickly even with a small angle change," Professor Kim Tae-sung said. "It can be widely used not only for anti-counterfeiting marks on currency, identification cards and pharmaceutical packaging, but also in advanced industries such as optical sensors that detect minute movements as color and angle-responsive displays."
This research was participated in by UNIST Dr. Kalyanan Thiyagarajan and researcher Ji Sung-jun as first authors, and was published on June 25 in Advanced Functional Materials, a world-renowned journal in the materials field. The research was carried out with the support of the National Research Foundation of Korea under the Ministry of Science and ICT and UNIST.
