Printing of Responsive Photonic Cellulose Nanocrystal Microfilm Arrays

Interactive materials capable of changing appearance upon exposure to external stimuli, such as photonic inks, are generally difficult to achieve on a large scale as they often require self‐assembly processes that are difficult to control macroscopically. Here this problem is overcome by preparing arrays of cellulose nanocrystal (CNC) microfilms from discrete nanoliter sessile droplets. The obtained microfilms show extremely uniform and intense color, enabling exceptional consistency in optical appearance across the entire array. The color can be controlled through the initial ink formulation, enabling the printing of polychromatic dot‐matrix images. Moreover, the high surface‐to‐volume ratio of the microfilms and the intrinsic hydrophilicity of the natural building block allow for a dramatic real‐time colorimetric response to changes in relative humidity. The printed CNC microfilm arrays overcome the existing issues of scalability, optical uniformity, and material efficiency, which have held back the adoption of CNC‐based photonic materials in cosmetics, interactive‐pigments, or anticounterfeit applications.