Biologists would like to understand how and why birds are brightly colored. Some materials scientists, meanwhile, look to nature to find better ways to recreate such colors. A study conducted using the U.S. Department of Energy’s Advanced Photon Source (APS) and published in the journal Proceedings of the National Academy of Sciences of the United States of America points to possible answers to both those questions. This discovery represents the first directly self-assembled single gyroid crystals known to science. The researchers say the results are not only unexpected, but also happen to be highly relevant to current challenges in the engineering of complex nanostructures for advanced applications, potentially opening new photonic technologies.
The bird in this case is the blue-winged leafbird (Chloropsis cochinchinensis), a brightly colored species found only in Asia. Like many animals, the color comes not from pigments, but from an ordered crystal structure in the leafbird’s feathers, which is quite unusual for birds. These photonic crystals, consisting of the structural protein beta-keratin and air, are arranged in a shape known as a single gyroid and of a size scale—roughly 300 nm—that they interact with wavelengths of visible and ultraviolet light. The interfaces between air and protein form a repeating series of changes in refractive index, leading to the feathers’ colors.
Single gyroid crystals are rare. The same researchers discovered the first one in biology in 2010 within the iridescent green scales of certain butterflies, also using the APS, and now in the feathers of only one of 10,000 bird species. The researchers say these crystals are unusual given the way birds normally grow their feather nanostructures. This process seems to resemble the way foam forms in beer, when dissolved carbon dioxide forms bubbles that push their way to the surface. In the case of the feathers, water and beta-keratin undergo phase separation to form the single gyroid, and once the water evaporates the structure of air and protein is left behind.
This is very different from the way either butterflies or engineers create single gyroid crystals. Engineers can make larger crystals, then apply heat to shrink them to a size where they work with wavelengths of visible light, but it’s difficult to do that at useful volumes, and the process can lead to defects. Taking a cue from the birds, humans might instead develop a process that relies on self-assembly to create the crystals. Those could then be used to improve the efficiency of photovoltaic cells, optical fibers, or catalysts.
To study the evolution of the nanostructures, the researchers compared them to nanostructures in related species of birds. They found that the ordered single gyroid structure evolved from a quasi-ordered state. The more ordered versions produce more saturated or purer hues that the birds can easily perceive. That suggests that the evolution of these structures was driven by sexual selection, where birds with more brilliant coloration were seen as more desirable mates.
To reveal the crystalline structure, the scientists performed synchrotron small-angle x-ray scattering (SAXS) at the X-ray Science Division Dynamics & Structure Group’s beamline 8-ID-I at the DOE Office of Science’s APS at Argonne National Laboratory. They performed pinhole SAXS experiments using beams measuring either 10 x 10 or 15 x 15 µm, which allowed them to focus on just a small number of barb cells on the inside of the feathers. They probed cells from the feathers of 10 to 15 species of the leafbird, and from two related species, the fairy bluebird. They also imaged the feathers with a scanning electron microscope and combined the data from the studies so they could see both the short-range order and intermediate structures in the feathers. - Neil Savage
See: Vinodkumar Saranathan1, 2*, Suresh Narayanan3, Alec Sandy3, Eric R. Dufresne4, and Richard O. Prum2, “Evolution of single gyroid photonic crystals in bird feathers,” Proc. Natl. Acad. Sci. U.S.A. 118(23), e2101357118 (2021). DOI: 10.1073/pnas.2101357118
Author affiliations: 1National University of Singapore, 2Yale University, 3Argonne National Laboratory, 4ETH Zürich
Correspondence: * vinodkumar.saranathan@aya.yale.edu
The authors acknowledge support from a Singapore National Research Foundation Award (CRP20-2017-0004), Yale-NUS startup Grant (R-607-265-241-121), Royal Society Newton Fellowship ATRTLO0 (to V.S.), and Yale University W. R. Coe Funds (to R.O.P.). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
See also: "Blue Animals Are Different From All the Rest," APS/User News 6.22.21, The Atlantic, and Quanta
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