Cellulose nanocrystals (CNCs) derived from biomass spontaneously organize into a helical arrangement, termed a chiral nematic structure. This structure mimics the organization of chitin found in the exoskeletons of arthropods, where it contributes to their remarkable mechanical strength. Here, we demonstrate a photonic sensory mechanism based on the reversible unwinding of chiral nematic CNCs embedded in an elastomer, leading the materials to display stimuli-responsive stretchable optics. Vivid interference colors appear as the film is stretched and disappear when the elastomer returns to its original shape. This reversible optical effect is caused by a mechanically-induced transition of the CNCs between a chiral nematic and pseudo-nematic arrangement.

The human eye can see over one million colors and color plays a critical role in everyday life. We know red lights mean stop and green lights mean go, while in battle a white flag means surrender. These are very simple ways to use colors to codify actions. However, as technology has advanced, the ways we use and produce color have evolved. For example, liquid crystal displays in smart phones, computers and TV screens are central to the communication and entertainment industries. Nature also heavily relies on color for a range of functions, including camouflage, biological ornaments, and warning Over millions of years, animals and plants have evolved complex molecules and structures that endow them with vibrant colors. Among the sources of natural coloration, structural color is prominent in insects, bird feathers, snake skin, plants, and other organisms, where the color arises from the interaction of light with nanoscale features rather than absorption from a pigment. Cellulose nanocrystals (CNCs) are a biorenewable resource that spontaneously organize into chiral nematic liquid crystals having a hierarchical structure that resembles the Bouligand structure of arthropod shells. The periodic, chiral nematic organization of CNC films leads them to diffract light, making them appear iridescent. Over the past two decades, there have been many advances to develop the photonic properties of CNCs for applications ranging from cosmetics to sensors. Here, the origin of color in CNCs, the control of photonic properties of CNC films, the development of new composite materials of CNCs that can yield flexible photonic structures, and the future challenges in this field are discussed. In particular, recent efforts to make flexible photonic materials using CNCs are highlighted.

The self-assembly process in cellulose nanocrystal (CNC) film formation was studied as a function of evaporation time. It is known that the total evaporation time of CNC dispersions affects the structure of the film obtained, but the extension of different phases of the evaporation has not been explored. By extending the evaporation time of CNC suspensions after the onset of liquid crystallinity, the homogeneity of the resulting films could be improved as observed by polarized optical microscopy and scanning electron microscopy. Here, we show that an intermediate stage of self-assembly, between phase separation and gel vitrification, called tactoid annealing, helps explain the discrepancies in order for chiral nematic CNC films dried at varying evaporation times. This intermediate stage of self-assembly may be useful for designing highly ordered and homogenous CNC-based materials.

Responsive photonic crystals have potential applications in mechanical sensors and soft displays; however, new materials are constantly desired to provide new innovations and improve on existing technologies. To address this, we report stretchable chiral nematic cellulose nanocrystal (CNC) elastomer composites that exhibit reversible visible color upon the application of mechanical stress. When stretched (or compressed) the colorless materials maintain their chiral nematic structure but the helical pitch is reduced into the visible region, resulting in coloration of the CNC‐elastomer composite. By increasing the percentage elongation of the material (ca. 50–300 %), the structural color can be tuned from red to blue. The color of the materials was characterized by reflectance optical microscopy and reflectance circular dichroism to confirm the wavelength and polarization of the reflected light. We also probed the mechanism of the structural color using 2D‐X‐ray diffraction. Finally, by either water‐patterning the starting CNC film, or by forming a CNC film with gradient color, through masked evaporation, we were able to prepare encoded stretchable chiral nematic CNC‐elastomers.

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