Abstract

Wrinkle-based diffraction gratings offer a scalable route to pigment-free structural coloration, yet most platforms are limited by globally aligned, linear wrinkles that provide limited spatial programmability and narrow angular selectivity. Here, we report geometrically programmed wrinkle architectonics in a UV-curable chitosan (UVCC)/polydimethylsiloxane (PDMS) bilayer, where lithography-defined, “geometric voids” serve as boundary constraints and stress concentrators to deterministically steer wrinkle trajectories during mechanical deformation. Simple geometric primitives effectively reshape the local stress field through geometrically incompatible confinement, transforming 1D gratings into shape-specific, spatially bifurcated domains characterized by curvilinear orientation and straight-wrinkle fields. We quantify these trajectory fields using the curvature metric and map critical design levers, such as inter-pattern spacing, UVCC thickness, and pattern asymmetry, that modulate curvature magnitude and steering range without altering diffraction periodicity. Circular perimeters sustain extended semi-circular trajectories, whereas polygonal vertices concentrate stress to induce aggressive local curvature. This programmed orientation dramatically amplifies angular dispersion: symmetric circular arrays achieve peak viewing-angle responsivity nearly three times higher than that of unpatterned thin films. A macaw security motif confirms wide-angle azimuthal readability and the formation of device-unique optical fingerprints for anti-counterfeiting. This work transforms a ubiquitous mechanical instability into a geometry-addressable photonic platform for high-level authentication and advanced displays.

Hidden optical images that appear only when a transparent film is bent could provide a new generation of anti-counterfeiting technologies. Researchers at UNIST have demonstrated a strategy for programming the microscopic wrinkle patterns responsible for structural color, enabling optical films with enhanced viewing-angle sensitivity and unique optical fingerprints. 

Led by Professor Taesung Kim of the Department of Mechanical Engineering, the researchers discovered that simple geometric patterns can precisely control how microscopic wrinkles form as the film bends. By reshaping local mechanical stresses, the patterned boundaries guide wrinkle growth into both straight and curved domains while preserving the regular spacing responsible for structural color. This independent control over wrinkle orientation expands the film's optical capabilities. 

The enhanced wrinkle architecture translated directly into a broader optical response. The engineered films required only seven degrees of viewing-angle change to display the full visible color spectrum, compared with approximately thirty degrees for conventional wrinkle-based films. Distinct color changes also appeared with angular shifts as small as one degree, allowing substantially more optical information to be encoded within the same viewing range.

To demonstrate these capabilities, the team fabricated a transparent security film featuring a hidden macaw image that appeared only when the film was bent. Unlike conventional wrinkle-based security films, which are visible only over a narrow range of viewing angles, the new design maintains clear image visibility and vivid structural colors across viewing angles from 0° to 90°.

Each film also developed subtle variations in wrinkle branching, creating device-specific optical fingerprints that provide an additional layer of authentication beyond the visible security image. The films also showed excellent mechanical durability, maintaining stable wrinkle patterns and optical performance after 500 repeated bending cycles.

“Instead of treating wrinkles as an unavoidable mechanical instability, we showed that they can be programmed through geometric design,” said Professor Kim. “By controlling wrinkle orientation while preserving wrinkle periodicity, we significantly broadened the optical response of wrinkle-based structural color films.” 

He added, "The approach could be applied to anti-counterfeiting technologies for banknotes, identification cards, luxury goods, and pharmaceutical packaging, while also providing a versatile platform for optical sensors and flexible display technologies."

The findings were published in  Advanced Functional Materials on June 25, 2026. Dr. Kaliannan Thiyagarajan and Sungjoon Ji from UNIST served as co-first authors of the study. The research was supported by the National Research Foundation of Korea (NRF) and UNIST.

Journal Reference

Kaliannan Thiyagarajan, Sungjoon Ji, Jiseok Han,   et al ., “Geometrically Programmed Wrinkle Architectonics for High Angle-Dependent Structural Coloration,”   Adv. Funct. Mater. , (2026).