Ever wondered about the brilliant blue hues of peacock feathers or the shimmering metallic chitin on beetles? These natural wonders are examples of structural colors—a phenomenon in which microscopic structures create vibrant, lasting hues. Inspired by these marvels, a research team from Japan has been exploring structural colors. Their earlier work realized that the preparation of structural color materials from melanin particles mimics the coloration mechanism of peacock feathers. Building on this foundation, the team set out to develop a coating material that captures the brilliance of structural colors using melanin particles, producing non-iridescent color even when viewed from different angles.
The research team, including Professor Michinari Kohri and Ms. Yui Maejima from the Graduate School of Science and Engineering, Chiba University, Japan collaborated with Dr. Shin-ichi Takeda of Takeda Colloid Techno-Consulting Co., Ltd., and Dr. Hiroshi Fudouzi from the National Institute for Materials Science. Their findings were published in Macromolecular Reaction Engineering on December 18, 2024. Dr. Kohri describes his motivation for the study, "We have been studying melanin-based structural color materials inspired by natural organisms for many years. Our motivation was to make these materials more practical by developing methods to create structural color quickly and add functional properties like water repellency."
To achieve this, the team prepared polystyrene particles of three different diameters. They then added a coat of polydopamine (modified melanin particles) followed by the addition of alkyl groups with 18 carbon atoms (octadecyl groups) with a hydrophobic nature through a Michael addition reaction. In this reaction, a negatively charged chemical group adds to an α, β-unsaturated carbonyl compound for introducing hydrophobic groups that enhance water repellency. This was achieved without relying on fluorine compounds which are hydrophobic but can cause major environmental concerns.
The hydrophobicity of the particles was confirmed using the time-domain nuclear magnetic resonance (TD-NMR) method. Once the particles were treated, they were dispersed in hexane, allowing for quick and efficient application onto substrates like glass and melamine laminate. Upon drying, the coatings displayed a contact angle of over 160 degrees, monochromatic hues, and a self-cleaning surface, demonstrating the lotus effect, where water droplets bead and roll off the material without leaving residues.
It was found that the hydrophobic melanin particles obtained by coating with octadecyl groups exhibited almost the same level of hydrophobicity as particles coated with fluorine compounds, which are known to exhibit high hydrophobicity. Highlighting the study's unique findings, Ms. Maejima, the first author, states, "We discovered that super-hydrophobic structural color coatings can be achieved by combining the hydrophobicity of particle surfaces with the hierarchical assembly structure of the particles—all within just a few minutes."
The team focused on creating a straightforward and scalable approach, ensuring that the coatings could be applied in a matter of minutes without the need for elaborate equipment or processes. Ms. Maejima remarks on the practicality of their findings: "This technology has the potential to become a next-generation coating material, ideal for applications like wallpaper or outdoor surfaces, without relying on pigments that fade over time. Its simplicity and efficiency make it highly adaptable for industrial use."
Structural color coatings hold significant promise for various applications. Unlike traditional pigments, these colors are derived from physical light interactions, ensuring longevity and resistance to fading. Their ability to combine durability with self-cleaning properties makes them particularly suited for outdoor use. Walls coated with this material, for instance, could remain vibrant and clean for years, significantly reducing repainting and maintenance costs.
The structural color coatings exhibit color as long as the structure of the periodic arrangement is maintained. The proposed approach is effective and does not require harsh chemical conditions. Structural color coatings also find their potential applications in the fields of food, pharmaceuticals, cosmetics, and industrial products. These materials provide long-lasting, sustainable material designs.
Looking ahead, the research group is focused on enhancing the adhesion and durability of the coatings to expand their usability across diverse surfaces. As the technology matures, it holds the potential to redefine coating materials, seamlessly combining aesthetics, functionality, and environmental responsibility. This innovation underscores the power of research to create solutions that are not only cutting-edge but also aligned with the needs of a sustainable future.
About Professor Michinari Kohri
Dr. Michinari Kohri is a Professor at Chiba University, serving in the Graduate School of Engineering. His research focuses on the development of polymeric materials inspired by biological systems, aiming to create innovative materials that emulate and surpass natural functionalities. He has authored over 100 publications in research areas, including polymeric materials, polymer particles, biomimetics, structural color, molecular assembly, and nanoparticle synthesis.
