The digital printing of chromatic pattern with a single cellulose nanocrystal ink

Cellulose nanocrystals (CNCs), a type of natural photonic crystal, have been used to develop various optical materials owing to their chiral nematic organization, renewability, sustainability, and abundance. However, scaling up the production of CNC-based photonic materials remains challenging because of their long self-assembly time, inevitable assembly defects, static optical properties, and brittle nature. To address these drawbacks, the current study introduces flexible photonic hydrogels with chromatic patterns that are 3D printed using CNC-based inks. These viscoelastic inks were composed of photopolymerizable monomers and CNCs that harbored high aspect ratios. The luminance and color difference of the patterns in the photonic hydrogels were digitally controlled by varying the printing angles and the number of layers. Furthermore, our technique was validated by printing recognizable QR codes, Tai-chi, and hydrogel color cards. The current study provides a facile, fast, reproducible, accurate, and scalable strategy for fabricating photonic hydrogels from sustainable materials with chromatic patterns visible only in polarized light, thereby demonstrating their potential use in optical devices, anti-counterfeiting applications, and information concealment and dissemination.