Comedian Dmitri Martin once dubbed glitter the herpes of the craft world thanks to its virus-like ability to stick around forever. It’s also the litter of the rest of the world. Like other microplastics ground down from bags and bottles, those tiny, shiny pieces get swept down drains and blown around by the wind. Microplastics wind up in the air and in raindrops. They are scattered across the Arctic wilderness and buried deep in sediment at the bottom of the ocean. Studies show babies ingest them at alarmingly high rates, and the rest of us are consuming plenty, too.
Now, researchers think they may have a solution, at least to the glitter part of the problem: a version that’s biodegradable, could be produced using less energy, and even grows on trees. It’s cellulose: teeny bits of the same substance that makes up the cell walls of plants. When cellulose is assembled into crystals, it reflects light, so those same bits of cellulose not only provide structure to plants but also give butterflies their bright, iridescent wings and make peacocks’ colorful tails so luminous. The plant version can be easily extracted from materials that would otherwise be trash, like wood pulp, mango skins, and coffee grounds.
Researchers at the University of Cambridge are figuring out how to produce these nanocrystals on a larger scale than ever, although the process remains painfully slow. “We can make them in different sizes and, depending on the size, we think the particles that we make can replace different products,” says Benjamin Drouget, a PhD student in chemistry and first author on the paper describing his team’s process, published in November in Nature Materials. Large pieces could be used in place of ordinary craft glitter, while smaller particles could be mixed into cosmetics.
Even though these glittery pieces of plastic are tiny, the European cosmetics industry uses up to 5,500 tons of microplastics every year. And other plastic glitter replacements have proved to be problematic. One popular mineral, titanium, is a carcinogen which will be banned in Europe next year. Mica, another option, is often mined using child labor and can be toxic to acquatic environments.
Some kinds of color are created by using pigments. Grind up a rock like lapis lazuli and mix it with water or egg yolk and you’ve got blue dye or tempera paint. To change the color, you have to change the material, says Silvia Vignolini, a chemistry professor at Cambridge and the head of Droguet’s research group. But there’s another way to create color: structural coloration. This means that the color is an artifact of the material’s microscopic shape, rather than a characteristic of the material itself. Vignolini gives the example of a soap bubble. “You start with something that is water, it’s transparent,” she says. “But as soon as you have the structure, then you get the coloration.”
For cellulose nanocrystals to create color, they need to stack on top of each other, making 360-degree spirals, like steps in winding staircases. Depending on the difference in height between the steps, and on the angle of the staircase, the crystals will refract different wavelengths of light, creating different colors. A peacock’s feather, for example, is studded with tiny, hairlike structures filled with photonic crystals whose different structures reflect green, blue, yellow, and brown.