All that glitters isn’t litter

2021-12-06

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.

Yet while none of this information is new, it’s been hard to use in a lab. Figuring out how to get these microscopic crystals to reliably assemble into vibrant colors is tricky. So is producing them in large quantities. A petri dish of glitter is far from the 10-pound minimum order required by major manufacturers.

This is the problem Droguet’s team set out to solve using cellulose derived from commercially-available wood pulp. First, they had to figure out how to get the crystals to set up in the right way. They will automatically form a structure, but which structure depends on the ionic composition of the water they’re in. To change that composition, “you just add salt, really,” says Vignolini. The salt changes how the molecules are attracted to each other, and dictates the shape they form and subsequently the color of the glitter they make. Just adding five milligrams will change the color of an entire kilogram of cellulose, making the crystals refract shorter wavelengths, like greens and blues. With less salt, they refract longer wavelengths, like red.

The team also figured out how to control the production process carefully so that they can now create meter-long sheets of glitter using a roll-to-roll machine, a common piece of industrial equipment. The machine rolls skeins of a polymer base, or “web,” while a dispenser squirts out even amounts of the nanocrystal solution. The mixture has to be thin enough that it’s easy to deposit on the roll, but viscous enough to leave a deep, even color.

At this point, the mixture is clear, so the team can’t tell if they’ve successfully produced a good batch until they run the web through a hot air dryer. After the water evaporates, only a film of the nanocrystals remains. The color suddenly emerges and deepens. “At the last moment, it’s really fast,” says Droguet, who has made green, blue, red, and gold glitters. The film can then be peeled off the web and ground into craft glitter or mixed into paint. The process requires less energy than manufacturing plastic glitter, and the final product keeps its sparkle even when it’s mixed in soapy water, ethanol, and oil which means it could be used in makeup and even in food. “I think now we have demonstrated that the principles work at a large scale,” Droguet says.

But they haven’t yet tried making industrial quantities. Using the equipment at Cambridge, it currently takes Droguet about two months to make a kilogram of glitter. To increase production, he’ll need funding and access to commercial venues that have bigger roll-to-roll machines. So far it’s been challenging to get companies onboard; Vignolini says manufacturers have been excited but hesitant because this material is so different from the ones they currently use. “It’s radically new,” she says, and companies want to make sure it works.

Vignolini and Droguet also want to run tests to understand how this material breaks down over its lifecycle and how that decomposition could affect the environment. They’ve partnered with Dannielle Green, an ecologist at Anglia Ruskin University in the United Kingdom, who has studied other cellulose-based glitters to see how they affect the growth of algae.

One of the overall problems with glitter, Green says, is that it’s a material that’s meant to be scattered in large quantities at events like festivals and parades. “Where you’re throwing handfuls of the stuff around, then that is going to have a big impact on the environment locally,” she says. Those effects can include things like stunting plant growth, getting into animals’ bodies, and working its way into the food chain. If cellulose nanocrystals break down more quickly than plastic, and without needing certain ideal conditions to decompose, they could keep one source of plastic out of that chain.

But even adding organic matter like cellulose can influence an ecosystem, Green says. As the crystals degrade, they can add biomass to the environment, which can lead to an increase in chemicals like inorganic nitrogen. If present in large quantities, these chemicals can decrease the oxygen available to plants and algae. “I imagine we would need a heavy load for this to happen, so it is unlikely to occur with a small amount of cellulose-based glitter,” she says.

So far, the team hasn’t discovered any problems with their prototype glitter, but they’ll need to keep testing longer before they’ll understand how it ages, and if it produces long-term effects. “We hope that our material is a solution, but at the same time, I think it’s important people understand that we are also thinking about what are the other problems that our material can cause and take them into account,” says Vignolini.

Given the vast scale of microplastics contamination, Green worries that solutions focused on tiny sources of pollution, like glitter, can be a distraction from much bigger contributors, like car tires and synthetic fabrics. But she also says there is a utility to making change where you can. “If you can easily stop a form of litter going into the environment,” she asks, “then why not do it?”

wired.com, 6 December 2021
; https://www.wired.com