Structural Color Gives Sustainable Sparkle to Plant-Based Materials

Not all that glitters is gold and not all that’s glitter is good for the environment. Used to add sparkle to everything from cosmetics to Christmas ornaments, glitter is getting an environmentally friendly makeover from scientists at the University of Cambridge. The researchers are developing biodegradable glitter made from plant-based, structurally colored cellulose materials. It has all the sparkle found in conventional glitter but none of the toxic materials, and it is for consumer and industrial use.

The photograph shows three vials containing an ensemble of photonic CNC particles dispersed in three different solvents: water; water: ethanol; and ethanol. The particles are the same in the three vials. The color difference between the three vials results from the ability of water to swell the structure of the particles. Higher water content means greater swelling of the cholesteric structures and a redshift of the color of the particles. Courtesy of Benjamin Droguet.

The glitter is sustainably made from cellulose nanocrystals. The nanostructures in the crystals reflect and bend visible light to create vivid, long-lasting, but biodegradable colors. The researchers transformed these commercially available materials into photonic films through a solvent-evaporation-driven self-assembly process.

First the researchers converted the cellulose into liquid suspension, optimizing the way that the cellulose nanocrystal suspension was formed and the deposition and drying conditions. By maximizing the effectiveness of the cellulose solution and the coating parameters, they were able to fully control the self-assembly process.

To scale the material for industrial use, the researchers used roll-to-roll deposition to produce large-area photonic films from the cellulose solution. They performed continuous deposition and drying of the cellulose-containing suspension on a commercial roll-to-roll machine. The large-scale cellulose films were ground to the size used for making glitters and effect pigments. The resulting particles are biodegradable, plastic-free, and nontoxic. The researchers further showed how meter-long, structurally colored films could be processed into effect pigments and glitters that are dispersible, even in water-based formulations.

A close-up photograph of a glass slide that has been covered with gold flakes with high lighting contrast and observed at a larger angle. Courtesy of Benjamin Droguet.

The team has been working on ways to transform cellulose from wood pulp into colorful materials for several years. “The challenge has been how to control conditions so that we can manage all the physical-chemical interactions simultaneously, from the nanoscale up to several meters, so that we can produce these materials at scale,” researcher Benjamin Droguet said.

The roll-to-roll processes used by the team are compatible with existing industrial-scale machines. To the best of the researchers’ knowledge, they are the first group to fabricate cellulose nanocrystals at industrial scale. The demonstration of the fabrication process on commercial equipment is a big step toward making the new glitter and pigments available outside the lab. Once the materials become commercially available, they could be used in place of materials that include toxic or unsustainable compounds.

For example, the plant-based glitter could be used to replace the plastic glitter particles and tiny mineral effect pigments that are widely used in cosmetics. In Europe alone, about 5500 tons of microplastics are used each year by the cosmetics industry. “We believe this product could revolutionize the cosmetics industry by providing a fully sustainable, biodegradable, and vegan pigment and glitter,” professor Silvia Vignolini said.

The photograph shows a film of cellulose nanocrystal that has been successfully peeled from its substrate, over a black background. Courtesy of Benjamin Droguet.

The self-assembly techniques that allow the cellulose to produce intensely colored films are less energy-intensive than the methods used to produce conventional glitters and pigments. “Traditionally, effect pigment minerals have to be heated at temperatures as high as 800 °C to form pigment particles,” Droguet said.

The structurally colored cellulose nanocrystal films and effect pigments developed by the Cambridge team could offer an alternative to current products that are either micropolluting (for example, nonbiodegradable microplastic glitters) or based on carcinogenic, unsustainable, or unethically sourced compounds (for example, titania or mica). “When you consider the quantity of mineral effect pigments that is produced worldwide, you realize that their use is harmful to the planet,” Droguet said.

“Conventional pigments, like your everyday glitter, are not produced sustainably,” Vignolini said. “They get into the soil, the ocean, and contribute to an overall level of pollution. Consumers are starting to realize that while glitters are fun, they also have real environmental harms.”

Although further optimization of the process is needed, the researchers hope to form a spinout company to make their pigments and glitters commercially available in the coming years.

The research was published in Nature Materials (www.doi.org/10.1038/s41563-021-01135-8).