Bio‐Inspired, Water‐Soluble to Insoluble Self‐Conversion for Flexible, Biocompatible, Transparent, Catecholamine Polysaccharide Thin Films

In nature, a variety of functional water‐insoluble organic materials are biologically synthesized in aqueous conditions without chemical additives and organic solvents. Insect cuticle, crustacean shells, and many others are representative examples. The insoluble materials are prepared by enzyme reactions and programmed self‐assembly in water from water‐soluble precursors. If the water‐basis could be adapted, environment‐friendly strategy developed in nature, many problems caused by the vast consumption of petroleum‐based olefin materials could be solved or significantly attenuated. Here, the spontaneous formation of water‐insoluble, biocompatible films from a water‐soluble polymer is demonstrated without using any chemical additives and organic solvents. It is found that a water‐soluble chitosan–catechol polymeric precursor is spontaneously self‐converted to flexible water‐insoluble thin film by simple dehydration. The preparation of mechanically robust, water‐insoluble, flexible, transparent chitosan–catechol film is a completely unexpected result because most water‐soluble polymers exist as powders when dehydrated. The film can be used as a bag similar to polyvinyl one and is multifunctional and biocompatible for drug delivery depots and tissue engineering applications.