Bend it to reveal it: UNIST team develops transparent security film with hidden images

Color changes in a parrot security image across different wrinkle structures. [Provided by UNIST]

A new type of security film that reveals hidden images when bent — and could fundamentally block counterfeiting — has been developed by South Korean researchers.

A team led by Kim Tae-sung, a professor in the Department of Mechanical Engineering at UNIST (Ulsan National Institute of Science and Technology), announced Wednesday that it has developed an "optical security film" in which images invisible under normal conditions appear in vivid structural colors, varying by viewing angle, when the film is bent. The design relies on geometrically engineered wrinkle patterns.

Tunable structural-color technology — in which colors appear or disappear in response to external stimuli — has drawn growing attention in anti-counterfeiting and security authentication. Nano-wrinkle-based structural color is particularly promising: wrinkles form only when a flexible film is bent, and those wrinkles diffract light to produce color. Existing wrinkle-based films, however, have a key limitation — because wrinkles tend to align in a single direction or at random, color changes are visible only from a narrow range of angles.

The new film keeps the spacing between surface wrinkles uniform while varying the direction in which the wrinkles run, producing clear color shifts even with a slight change in viewing angle. To control wrinkle orientation, the team devised a technique that removes circular sections of the wrinkle layer from the surface. Straight wrinkles form in areas far from these circular voids, while wrinkles near the voids fan outward in multiple directions like the ribs of an open fan. The combination of straight and curved wrinkles running in different directions diffracts light across a wider range of angles, causing the color to change with even a small rotation of the film.

Professor Kim Tae-sung (from left), Dr. Kaliyannan Thiyagarajan and researcher Ji Seong-jun. [Provided by UNIST]

In experiments, the team found that an angular shift of just about 7 degrees was enough to display the full range of visible-light colors, from violet to red. Conventional straight-wrinkle films required an angle change of about 5 degrees to produce a clear color shift and about 30 degrees to cover the entire visible spectrum.

Using this technology, the team produced a security film that reveals a parrot image only when bent. Unlike earlier films in which the parrot appeared only from a specific direction, the image and its structural colors remained continuously visible as the film was rotated from 0 to 90 degrees.

Even among films displaying the same security image, the precise locations where wrinkles split or terminate differ subtly — a variation the team says can serve as a unique optical fingerprint for each individual product.

The film also proved highly durable: after 500 repeated bending cycles, the wrinkle orientation and structural-color response recovered stably.

"By controlling only the shape and arrangement of the patterns, we can tune the direction, curvature and angle-dependent color changes of the wrinkles, allowing complex optical signals to be encoded within a single film," Kim said. "Beyond anti-counterfeiting marks for currency, identity documents, high-value products and pharmaceutical packaging, the technology could also be applied to optical sensors that detect small movements through color changes, and to flexible displays."

The findings were published June 25 in Advanced Functional Materials, an international journal in the field of materials science.

nbgkoo@heraldcorp.com