Thermochromic Photonic Crystal Paper with Integrated Multilayer Structure and Fast Thermal Response: A Waterproof and Mechanically Stable Material for Structural‐Colored Thermal Printing

Abstract Thermochromic photonic crystals are promising materials for thermal printing due to their unfaded colors under chemical/illuminated environments and the absence of toxic chemicals. However, the slow thermochromic response, the multistep printing procedures, the use of inks or developing liquids, and the requirement of expensive parts in printers limit their applications. Here, a thermochromic polyurethane/hydrophobic‐SiO 2 photonic crystal/paraffin (PU/HPO‐SiO 2 ‐PC/Para) film with an integrated multilayer structure is fabricated for all‐solid‐state and single‐step thermal printing that is fully compatible with commercial printers. The fast thermochromic response in milliseconds enables high‐resolution and grayscale printing as the paraffin infiltration and the color change can be finely controlled in a microscale range. The integrated and hydrophobic multilayer structure renders the thermochromic film good stability in daily liquids, which addresses the long‐existing concern of print fading. Meanwhile, the integrated multilayer structure also enhances the mechanical stability when it is deposited on fibrous paper so that people can fold, cut, or staple the thermal papers, and make notes confidently in practical usage.