Wrinkle Tech Creates Pigment-Free Graphics on Films

Abstract

Thin film-based structural coloration predominantly uses an elastomeric substrate and active materials, which exhibit uniform color changes under external stimuli such as bending, heat, and chemical environments. However, generating distinctive structural colors in a single stimulation is challenging, especially for microscale pixel arrays, which interact in more spectral and structural ways than bare thin films or static color pixels. This study presents a two-step photo-cross-linking fabrication approach for monolithic pixel arrays. The pixelated structure is devised using ultraviolet-curable chitosan (UVCC) on polydimethylsiloxane with controlled thickness at the nanoscales. The pixelated device can generate bending-based reversible wrinkles with different sizes according to the thickness of the UVCC. Subsequently, each pixel exhibits thickness-specific structural color covering the entire visible spectrum when bent. By coupling four microscale pixels with different colors, it is demonstrated that a unit of pixels can be seen as a combined color of each pixel color with the bare eyes. To demonstrate the micropixel approach, a traditional Korean Dancheong pattern is adopted and categorized its colors into three different ones. The proposed method generates distinguishable structural colors between the pixels, which makes this approach a strong contender for pixel-based structural color applications such as anti-counterfeiting, camouflage, and security printing.

A research team, led by Professor Taesung Kim from the Department of Mechanical Engineering at UNIST announced the development of a technology that utilizes nanoscale wrinkles formed on transparent films to display or conceal color patterns, such as the Korean traditional Dancheong designs, by folding and unfolding the film.

The developed technology is based on the principle of structural coloration. Structural colors emerge when light interferes with nanostructures. Just like chameleons or peacocks can produce blue colors without blue pigment cells due to their skin or feather nanostructures, this technology uses nanoscale wrinkles to achieve the same effect.

The research team employed these nanostructures to create wrinkles, which only appear when the film is bent, allowing for the display or concealment of colors. By adjusting the spacing and height of the wrinkles, a variety of colors can be generated.

To ensure that wrinkles only appear when the film is bent, a dual-layer film structure was designed. A rigid film is placed over a flexible one, causing nanoscale wrinkles to form on the surface of the rigid film due to the physical property differences between the two layers when force is applied. This principle is similar to how wrinkles form on the epidermis when pinching the skin with the thumb and index finger due to differences in density between the epidermal and dermal layers.

Using a dual photolithography technique, the research team created wrinkle pixels with varying spacing and heights on a single film. The wrinkles had spacings ranging from 800 to 2,400 nm and heights from 100 to 450 nm, which confirmed their ability to produce colors across the entire visible spectrum.

Additionally, the team successfully patterned Dancheong designs based on the wrinkle pixels. These designs only appeared when the transparent film was bent and reverted back to a transparent state when the force was released.

Professor Kim stated, "This technology allows for the creation of variable structural colors through a simple process. Unlike existing dye-based technologies, the colors do not fade over time, making it highly competitive not only for anti-counterfeiting applications but also for stimuli-responsive smart displays."

Furthermore, the technology is set to be licensed to N.B.S.T., a domestic anti-counterfeiting solution company, for commercialization.

This research involves contributions from UNIST researchers, Kaliannan Thiyagarajan and Sungjoon Ji, as co-first authors and has been made available online for pre-publication in Advanced Functional Materials, with formal publication forthcoming. This study has been supported by the Institute for Information & Communications Technology Promotion (IITP), the National Research Foundation of Korea (NRF), and the Ministry of Science and ICT (MSIT).

Journal Reference

Kaliannan Thiyagarajan, Sungjoon Ji, Jiseok Han, et al., "Monolithic Micro-/Nanoarchitectonic Multi-Level Pixel Arrays Via a Two-Step Photo-Cross-Linking Process for Multi-Modal and Transformative Structural Coloration," Adv. Funct. Mater., (2025).