Color-Changing Paper: Cellulose Nanofiber Films Incorporating Mechanochromic Fluorenylidene-Acridane

Color-changing paper using a cellulose nanofiber (CNF) film with mechanical pressure-sensing capability was produced using mechanochromic fluorenylidene-acridane (FA), which exhibits color change upon mechanical pressure. This flexible film exhibits a visible color change from yellow to green in response to a mechanical force. Surface investigations, such as scanning electron microscopy and contact angle analysis, revealed that the repeatability of the color change was dependent on the CNF diameter. Furthermore, the FA-CNF film exhibited a linear response to mechanical pressure within the range of 25–300 MPa, as quantitatively measured by mechanochromism using a combination of nanoimprint testing and UV–vis spectroscopy. This measurement suggested that the flexibility of the film improved mechanochromic sensitivity. When compared with the conventional method using UV–vis spectroscopy, a simpler image processing method employing CYM color analysis was able to visualize the distribution of the mechanical pressure.