Hidden Images Revealed When Bent: Transparent Security Film Developed [Reading Science]

Vivid Structural Colors Achieved with Just a 1-Degree Angle Change
Enhanced Anti-Counterfeiting Performance
From Currency and ID Cards to Pharmaceutical Packaging and Flexible Displays

A next-generation optical security film that is transparent under normal conditions but reveals hidden images when bent has been developed by a domestic research team. This technology allows the film's color to change distinctly with just a 1-degree shift in viewing angle, enabling it to contain far more security information than conventional films. As a result, it is expected to be utilized in anti-counterfeiting technologies for currency, ID cards, and high-end products.

Structural color change and angular responsiveness according to observation angle. (a) (i) Schematic diagram of optical measurement to observe structural color changes by angle for (i) regular film, (ii) symmetrical circular pattern pair, and (iii) asymmetrical circular pattern pair. (b) Reflectance spectrum measurement results by observation angle for each structure. (c) Structural color changes according to observation angle for each structure. (d) Comparison of structural color spectral responsiveness according to viewing angle. Courtesy of the research team

On July 8, the Ulsan National Institute of Science and Technology (UNIST) announced that a research team led by Professor Taesung Kim from the Department of Mechanical Engineering has developed a transparent optical security film. This film uses a geometrically designed micro-wrinkle structure, creating images that are invisible under normal conditions but appear in structural color only when the film is bent.

The research results were published in the international journal Advanced Functional Materials, which specializes in functional materials.

Hidden Images Achieved by Changing Wrinkle Direction

Structural color is not produced by dyes, but rather by the microstructure of a surface reflecting or diffracting light. The research team used micro-wrinkles that form only when the film is bent to create images that are otherwise invisible under normal conditions.

The key lies in maintaining a constant wrinkle interval while diversifying the direction of the wrinkles. The team designed part of the wrinkle layer to have circular voids, allowing both straight and curved wrinkles to form together. As a result, light is diffracted in multiple directions, so even a slight rotation of the film produces a vivid color change.

Experimental results showed that only about a 7-degree change in angle was needed to express the full visible spectrum from violet to red. In contrast, conventional linear-wrinkle films required about a 30-degree rotation to achieve the same color change.

Potential Applications in Anti-Counterfeiting, Sensors, and Displays

Using this technology, the research team fabricated a security film that reveals a parrot image only when bent. Unlike existing films, where images can be seen only from specific angles, the new film continuously displays both structural color and the image as it is rotated from 0 to 90 degrees.

Furthermore, even for films with the same image, the positions where micro-wrinkles split or terminate differ for each product, making it possible to use these features as a unique "optical fingerprint," similar to a human fingerprint.

Research team photo. (From left) Professor Taesung Kim, Dr. Kalianan Thiyagarajan, Researcher Seongjun Ji. Courtesy of UNIST

The film also demonstrated excellent durability. After being bent 500 times, it consistently maintained the same structural color and wrinkle pattern.

The research team formed a chitosan thin film on a soft silicone material called polydimethylsiloxane (PDMS) and used ultraviolet light and a photomask to create the microstructures. Due to the difference in rigidity between the two materials, bending the film induces micro-wrinkles in the desired direction on the surface.

Professor Kim stated, "By designing the wrinkles to form in multiple directions, we achieved rapid color changes with even small angle shifts. This technology could be used not only for anti-counterfeiting marks on currency, ID cards, high-value products, and pharmaceutical packaging, but also for optical sensors that detect subtle movements through color changes and for flexible displays."

This content was produced with the assistance of AI translation services.