From nanostructure to function: hierarchical functional structures in chitin and keratin

Abstract Chitin and keratin are naturally abundant biopolymers. They hold significant potential for sustainable applications due to their chemical structure, (nano)structural properties, biodegradability and nontoxicity. Chitin, a polysaccharide contained in exoskeletons of arthropods and the cell walls of fungi, forms strong hydrogen bonds that confer mechanical stability, which is ideal for use in protective structures and lightweight composites. Keratin, a fibrous protein found in vertebrate epithelial tissues such as wool, feathers and hair, is characterized by its high sulfur content and the formation of disulfide bonds, which provide both mechanical strength and flexibility. Utilizing chitin and keratin waste materials from the food industry, such as shrimp shells, chicken feathers and sheep wool, offers an eco-friendly alternative to synthetic materials and leverages their inherent biocompatibility. Additionally to the common macroscale reuse of chitin and keratin waste as fertilizer or livestock feed, using chitin and keratin as functional materials adds further uses for these versatile materials. The waste is increasingly being utilized specifically for its superior structural properties resulting from nanoscale functionalities. Chitin and keratin exhibit excellent thermal insulation properties, making them suitable for energy-efficient building materials. Their structural colours (e.g., in butterflies and birds), arising from micro- and nanoscale arrangements, offer non-fading colouration for textiles and coatings without the need for potentially harmful dyes. Additionally, these biopolymers provide lightweight yet strong materials ideal for packaging, consumer products, and – when smartly structured – even passive radiative cooling applications. Biomimetic designs based on chitin and keratin promise advancements across multiple fields by harnessing their natural properties and converting waste into high-value products, thereby addressing recycling issues and promoting sustainability.