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.
Traditional thermochromic photonic crystal (PC) usually has a slow and reversible thermal response, which limits its application in thermal printing. Here, the authors develop a thermochromic “SiO 2 -PC/PEG” double layer structure with a responding time of milliseconds for fast thermal printing. Controlled by the print-head, the polyethylene glycol (PEG) melts, infiltrates, and solidifies within the interparticle voids, which instantly and irreversibly changes the refractive index and produces the PC pattern. Multicolor printing can be realized by tuning the size and type of colloidal particles. Resolution as high as 300 DPI is achieved to print the high-resolution patterns and then the grayscale patterns based on the control of pixel densities. Different from fiber thermal paper, the “SiO 2 -PC/PEG” film has no toxic bisphenol A and possesses superior light stability for keeping the images longer. It is fully compatible with the commercial printer, which provides a mature solution for fast and convenient preparation of PC patterns.
Structural-colored thermal printing using the “photonic crystal/phase change material” (PC/PCM) composites as thermal paper is a promising and feasible technique, which possesses superior color stability against chemicals and green characteristics for humans and the environment. However, there are still great challenges to limit its application, including the incapability of multi-color printing, unsatisfactory resolution, and limited greyscale levels. Here, a laser-assisted thermal printing process is developed based on the photothermal conversion from the laser irradiation and the thermochromic effect of the PC/PCM composites. Thanks to the intense laser heating in a small range, fine patterns with practical/theoretical resolution as high as 275/879 DPI can be obtained under fast laser scanning. Using precise control of the laser intensity or the visual fusion of microdots with tunable size and number, 36 greyscale levels are realized by “photomask” or “halftone” strategies. Multicolor patterns are fabricated by stacking the separately printed RGB sub-images, which vertically mix two or three of the RGB colors to produce abundant colors. Based on the tomographic deconstruction of the source image, a clear structural-colored pattern can be printed on the curved substrate by continuous tuning of the laser focal plane and multi-step laser printing on each slice.
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