Not only lepidopterists but also chemists have long been enthralled by the bright blue hues of morpho butterfly wings. The complicated nanoscale protein structures layered inside the wings pique the latter’s curiosity: their shapes behave as prisms, representing a beautiful cerulean colour. Materials scientists plan to use the properties of these structures to create things like paint and cosmetics that are free of toxic dyes and pigments to human health and the environment.
It’s been challenging to figure out how to adapt this “structural “colour definition to a wide variety of goods at a reasonable cost. However, chemists at Cypris Materials, a Berkeley, Calif.–based start-up, say they are close to finding a way to do so. They claim to have produced colourants that can create hues in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum without using dyes or pigments.
That can be used in anything from car paints to printer inks and nail polish. “They’ve made strides in an area in dire need of new ideas, and it appears that they’ve made the transition from the lab to the real world. That’s incredible,” says John Warner, a distinguished research fellow at Zymergen and a green chemistry expert who was not interested in Cypris Materials’ work.
Traditional pigments and dyes have been used to add color to fabrics, paints, and other daily items for decades. They give off different hues depending on how they absorb or reflect light.
The molecular components called chromophores in red dye, for example, capture the wavelengths of light of all colours except red, so red is what we see. However, some of the compounds used, such as azo dyes, contain components that have been related to cancer and other negative consequences. They have been outlawed in several nations.
On the other hand, structural color is created by microscopic structures of differing shapes that absorb or refract light in various ways. The tiny scales on the wings of a morpho butterfly, for example, are covered with small ridges with cross-ribs, similar to the profile of a fir tree.
These patterns emit a bright blue by refracting unique light wavelengths. Other structures produce similar effects: helically structured cellulose microfibers, for example, make marble berries gleam with a gleaming, metallic blue.
Scientists are still studying the finer points of how structural colour manifests in nature. However, they already know that the sizes and volumes of the features in a structure and the material are the primary determinants of how light bounces off that structure and the colour that results.
Cypris Materials use self-assembling block copolymers to make structure-based colourants. They are long chains of molecules that join two distinct forms of widely used plastics, such as polyacrylates and polyesters, in this case.
These copolymers assemble themselves into neatly arranged layered structures that refract light when formed into paint or ink and applied to a surface. Consider gummy worms that are half yellow and half orange, with each colour reflecting a different kind of plastic.
A multilayered arrangement may be formed if a large bag of these worms is organized such that each gummy’s yellow and green bits just reached the same colored sections of other gummies.
When placed in a solution, Cypris’ copolymers do this on their own, with the length of the polymer chain deciding the wavelength of light reflected.
Shorter wavelengths of light, such as ultraviolet, blues, and greens, are refracted by small copolymer strings. In the garden, violet, and near-infrared portions of the spectrum, longer chains refract longer wavelengths.
Self-assembling copolymers aren’t recent, but the firm claims it has made two significant advances: the materials self-assemble under normal environmental conditions, such as when the paint is added and form long chains that reflect long wavelengths of light.
“No one has been able to bring it out into the redder colours,” says Robert Grubbs, a Nobel Prize-winning chemistry professor at the California Institute of Technology and a co-founder of the organization who serves in an advisory capacity. “They’ve come a long way from where I thought they’d be able to go.”
According to Cypris Materials, the new colourants are available in a powdered form integrated into industrial systems, such as adding to paint in an automobile spray gun or mixing into nail polish.
Colourants also serve as binders, which are additives added to keep pigments and dyes in solution as the paint dries on aboard. Ryan Pearson, the company’s co-founder and acting CEO says, “We’re simplifying paint.”
In the past, “people had to apply pigment.” They’ll even need to apply additional additives enough to keep the colour stable. Not only can we eliminate the need for pigments and dyes, but we also eliminate the need for all of the material that surrounds it to keep it stable.”
Surfactants are another popular chemical applied to paints to help preserve the pigments. It’s unclear if the new colourants would eliminate the need for such additives, but “whether they’re able to substitute surfactants or formulate without alkylphenols,” [chemicals commonly used in paints to avoid separation], it will be a good step in our opinion,” says Teresa McGrath, chief scientific officer at the Healthy Building Network.
This nonprofit group encourages the use of safer and less harmful materials that are unaffiliated with Cypris Materials.
Other uses with structural paint, such as those used to make films that can be added to windows to boost insulation and save energy, contrast with the new method.
This technique entails making and mixing several layers of film to achieve a structure that reflects infrared light. However, because those films can only be used on specific surfaces, the new structural colour technology could be more versatile.
Colourants developed by the start-up company do face challenges and potential limitations. They will not compete on price for other pigments, such as titanium dioxide, for example.
To ensure that its colourants are cleaner and more environmentally friendly than conventional dyes and pigments, the corporation must conduct a thorough review of its copolymers and production processes. Warner admits, “It’s not going to be fine.” The fine, on the other hand, is the enemy of the excellent. These sorts of moves ahead are how science works.”
