New mechanism opens door to more vivid colors

Researchers from the NCCR Bio-Inspired Materials and Yale-NUS College in Singapore have discovered a novel color-generation mechanism in nature, which if harnessed, has the potential to create cosmetics and paints with purer and more vivid hues, screen displays that project the same true image when viewed from any angle, and even reduce the signal loss in optical fibers.

Using high-energy X-rays, scanning electron microscopy, and optical modelling, Yale-NUS Assistant Professor Vinodkumar Saranathan and Dr. Bodo Wilts of the Adolphe Merkle Institute’s Soft Matter Physics group examined the rainbow-colored patterns in the elytra (wing casings) of a snout weevil from the Philippines, Pachyrrhynchus congestus pavonius. They discovered that to produce the rainbow palette of colors, the weevil benefits from a color-generation mechanism that is so far found only in squid, cuttlefish, and octopuses. Their study was published recently in the peer-reviewed journal Small.

The so-called “Rainbow” Weevil is distinctive for its rainbow-colored spots on its thorax and elytra. These spots are made up of nearly-circular scales arranged in concentric rings of different hues, ranging from blue in the center to red at the outside, just like a rainbow. While many insects have the ability to produce one or two colors, it is rare that a single insect can produce such a vast spectrum of colors. Researchers have been investigating the mechanism behind the natural formation of these color-generating structures, as current technology is unable to synthesize structures of this size.

“The ultimate aim of research in this field is to figure out how the weevil self-assembles these structures, because with our current technology we are unable to do so,” explains Saranathan. “The ability to produce these structures, which are able to provide a high color fidelity regardless of the angle you view it from, will have applications in any industry which deals with color production. We can use these structures in cosmetics and other pigmentations to ensure high-fidelity hues, or in digital displays in your phone or tablet which will allow you to view it from any angle and see the same true image without any color distortion. We can even use them to make reflective cladding for optical fibers to minimize signal loss during transmission.”

Saranathan and Wilts examined these scales to determine that the scales were composed of a three-dimensional crystalline structure made from chitin (the main ingredient in insect exoskeletons). They discovered that the vibrant rainbow colors on this weevil’s scales are determined by two factors: the size of the crystal structure which makes up each scale, as well as the volume of chitin used to make up the crystal structure. Larger scales have a larger crystalline structure and use a larger volume of chitin to reflect red light; smaller scales have a smaller crystalline structure and use a smaller volume of chitin to reflect blue light.

According to Saranathan, who previously examined over 100 species of insects and spiders and catalogued their color-generation mechanisms, this ability to simultaneously control both size and volume factors to fine-tune the color produced has never before been shown in insects, and given its complexity, is quite remarkable. “It is different from the usual strategy employed by nature to produce various different hues on the same animal, where the chitin structures are of fixed size and volume, and different colors are generated by orienting the structure at different angles, which reflects different wavelengths of light,” he said.

The research was partly supported though the National Centre of Competence in Research “Bio-Inspired Materials” and the Ambizione program of the Swiss National Science Foundation (SNSF) to Dr. Wilts, and partly through a UK Royal Society Newton Fellowship, a Linacre College EPA Cephalosporin Junior Research Fellowship, and Yale-NUS College funds to Dr. Saranathan.

Original Yale-NUS press release here.

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