A Japanese research team from Chiba University created a technique that combines structural color coating with extremely water-repellent qualities as a first step in creating advanced materials for functional coatings. The study was published in Macromolecular Reaction Engineering.
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The structural color coating is self-cleaning and does not fade like traditional paints. Hydrophobic melanin particles offer structural color and water-repellence. The study represents a significant advancement in paint and coating advanced materials.
Scientists use the idea of structural colors to develop next-generation coating materials with long-lasting vibrancy and water-repellent qualities.
The phenomenon known as structural colors, in which microscopic structures produce vivid, enduring hues, is exemplified by the natural wonders – the brilliant blue hues of peacock feathers and the shimmering metallic chitin on beetles.
A Japanese research team has been investigating structural colors after being inspired by these wonders. Their previous research revealed that the process of creating structural color materials from melanin particles is similar to how peacock feathers are colored. Building on this base, the team's goal was to create a coating material that uses melanin particles to capture the brilliance of structural colors, resulting in non-iridescent color even from various perspectives.
Dr. Shin-ichi Takeda of Takeda Colloid Techno-Consulting Co., Ltd. and Dr. Hiroshi Fudouzi from the National Institute for Materials Science worked with the research team, which included Professor Michinari Kohri and Ms. Yui Maejima from the Graduate School of Science and Engineering, Chiba University, Japan.
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.
Michinari Kohri, Professor, Graduate School of Science and Engineering, Chiba University
The team made polystyrene particles with three distinct diameters. After applying a layer of polydopamine (modified melanin particles), the team used a Michael addition reaction to add hydrophobic alkyl groups with 18 carbons (octadecyl groups).
This reaction introduces hydrophobic groups that improve water repellency by joining an α, β-unsaturated carbonyl compound with a negatively charged chemical group. This was accomplished without the use of hydrophobic fluorine compounds, which can pose serious environmental risks.
The time-domain nuclear magnetic resonance (TD-NMR) technique was used to verify the particles' hydrophobicity. Following treatment, the particles were dispersed in hexane to enable rapid and effective application onto substrates such as melamine laminate and glass. The lotus effect, in which water droplets roll and bead off the material without leaving residues, was demonstrated by the coatings' self-cleaning surface, monochromatic colors, and contact angle of more than 160 degrees after drying.
Almost the same degree of hydrophobicity was observed in the hydrophobic melanin particles coated with octadecyl groups as in the particles coated with fluorine compounds, which are known to have high hydrophobicity.
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.
Yui Maejima, Study First Author, Chiba University
The team concentrated on developing a simple and scalable method that would allow the coatings to be applied in a few minutes without requiring complex tools or procedures.
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.
Yui Maejima, Study First Author, Chiba University
Structural color coatings have a lot of potential. Since these hues are produced by physical light interactions rather than conventional pigments, the hues are long-lasting and fade-resistant. The hues are especially well-suited to outdoor use because of their capacity to blend durability with self-cleaning qualities. For example, this material could keep walls bright and clean for years, greatly lowering the need for repainting and upkeep.
As long as the periodic arrangement’s structure is preserved, the structural color coatings will remain colored. The suggested method works well and does not call for harsh chemical conditions. Structural color coatings also have potential uses in the food, pharmaceutical, cosmetic, and industrial products. These materials offer durable, environmentally friendly material designs.
The research team is improving the coatings' durability and adhesion to increase their applicability on wider surfaces. As technology advances, it could completely rethink coating materials by fusing environmental responsibility, practicality, and beauty. This invention demonstrates how research can be used to develop innovative solutions that meet the demands of a sustainable future.
The National Institute for Materials Science (NIMS) Joint Hub Program and the Japan Society for the Promotion of Science (JSPS) KAKENHI supported the study.
Journal Reference:
Maejima, Y., et al. (2025) Michael Addition Reaction-Assisted Surface Modification of Melanin Particles for Water-Repellent Structural Color Coating. Macromolecular Reaction Engineering. doi.org/10.1002/mren.202400040