A groundbreaking security film has been developed that remains transparent under normal conditions but reveals hidden images when bent.
On Wednesday, researchers from the Ulsan National Institute of Science and Technology (UNIST) announced a breakthrough in optical security technology. Led by Professor Kim Tae-sung from the Department of Mechanical Engineering, the team has created a film that displays an array of structural colors depending on the viewing angle, utilizing an innovative wrinkled structure.
Unlike traditional dyes, structural colors emerge from the surface’s microstructure, which reflects or diffracts light. In this novel film, surface wrinkles act as the key microstructure.
The security feature activates only when the film is flexed, causing wrinkles to form and reveal a previously invisible image in vibrant color.
This cutting-edge film maintains consistent wrinkle spacing while allowing for multidirectional wrinkle formation. Consequently, even slight changes in viewing angle produce distinct color shifts.
To control wrinkle direction, the team ingeniously incorporated circular voids into specific sections of the wrinkled surface layer.
This design creates straight wrinkles away from the circular spaces, while nearby areas exhibit wrinkles that radiate outward in multiple directions, resembling wheel spokes.
This innovative approach ensures the security image remains stable regardless of film rotation or changes in viewing position.
Demonstrating the technology’s potential, the team created a security film that reveals a parrot image only when bent. This advanced film consistently displayed the image and structural colors throughout a full 90-degree rotation.
While films may display identical security images, subtle variations in wrinkle patterns serve as unique optical fingerprints for each product.
The film also boasts remarkable durability, maintaining stable wrinkle formation and structural color responses even after 500 bending cycles.
Professor Kim envisions wide-ranging applications for this technology, from anti-counterfeiting measures for currency, identifications (IDs), luxury goods, and pharmaceutical packaging to optical sensors detecting minute movements and flexible display technologies.
The groundbreaking research, co-authored by Dr. Kaliannan Thiyagarajan and researcher Ji Sung-jun from UNIST, was published in the prestigious journal Advanced Functional Materials on June 25.
This innovative study was made possible through support from the Ministry of Science and ICT, the National Research Foundation of Korea, and UNIST.