Keratin–Melanin Complexation Enables Robust, Delamination‐Resistant Feather‐Inspired Structural Color Coatings for Flexible and Curved Substrates

ABSTRACT In nature, multiple biopolymers form complex structures, and their cooperative interactions result in functions that are otherwise unattainable by individual components alone. A prime example is the keratin–melanin complex found in bird feathers, in which keratin provides mechanical strength and flexibility, while melanin granules control optical properties. Inspired by this natural design, we developed mechanically robust structural color coatings applicable to flexible and curved substrates based on the keratin–melanin complex. A sulfonated keratin (s‐keratin) aqueous solution forms a thin film upon solvent evaporation. Melanin particles synthesized using polydopamine—a melanin‐mimetic polymer—were dispersed in the s‐keratin‐containing solution, which was then coated onto a substrate. After solvent evaporation, melanin particles formed an amorphous structure within the keratin matrix when the s‐keratin concentration exceeded a certain threshold, generating an angle‐independent structural color. Diverse functional groups of keratin contributed to interparticle and substrate adhesion, thereby yielding mechanically robust coatings, without requiring additional binders, crosslinkers, or complex fabrication processes. Inspired by hierarchical structures found in nature, this approach combines mechanical and optical functions within a single material. This concept, which mimics cooperative interactions found in nature, provides a new pathway for the development of multifunctional photonic materials.