Scientists and inventors have often been inspired by the changing designs of nature. “For over a billion years,” Akhlesh Lakhtakia says, “many structures have evolved to display interesting and useful properties. This is an idea we humans should exploit.”
The complexity of biological processes makes mechanical reproduction extremely difficult, he acknowledges. But the surface geometry of structures can be readily copied using modern coating technologies. And if the copy is true enough, it may retain properties of the original that are particularly useful in advanced materials. The cost of producing such replicas has been prohibitive. Now, however, Lakhtakia and colleagues Carlo Pantano of Penn State and Raul Martin-Palma of the Universidad Autónoma de Madrid have developed a fast and inexpensive technique for making copies that are accurate down to the nanoscale. As a demonstration, they have used this process, called conformal evaporated film by rotation (CEFR), to produce exact replicas of two of nature’s most delicate designs: a fly’s eye and a butterfly’s wing.
As Lakhtakia explains it, the copying process works this way: Inside a low-pressure chamber, a solid material—in this case glass—is heated until it becomes a vapor. The vapor then settles on a rapidly rotating substrate—eye or wing—and the result is a thin (500 nanometer) layer of glass that perfectly conforms to the object beneath it. “In the case of the eye,” Lakhtakia says, “we coated the entire head of the fly, then blasted out the remains of the fly, leaving only the coating.”
Because a fly’s compound eye is a very efficient collector of light, Lakhtakia explains, a replica such as the one they created might be adapted to fabricate solar-cell covers and other energy-harvesting structures, as well as high-resolution lenses. The researchers are hoping that the development of compound-eye-based miniature cameras and sensors would stimulate applications in many other areas.
In the case of the butterfly, Lakhtakia points to certain species within the genus Morpho, known for their iridescent wings. “These exhibit what is known as structural color,” he says. “Their coloring is a result not of pigment, but of the way light interacts with the wing’s surface structure.” When white light hits this surface, he explains, all visible wavelengths are absorbed, except for a narrow range of blue that is reflected back. The result of this “narrow band gap” is color of great purity and intensity.
“We could say that this is a natural photonic material,” Lakhtakia says, referring to optical properties that have many potential applications. “It’s not perfect, but it’s far better than what we can produce.” Such a material could be useful for building photonic circuits, crucial for light-powered communication devices and sensors. In addition to optics, the wing’s surface may have thermal properties that could be useful for solar cells or heat exchangers.
At Pantano’s suggestion, the first replicas were made using chalcogenide glass, which “has some wonderful optical properties,” Lakhtakia says, and is robust enough to hold up as a thin film. The group is currently experimenting with other glasses as well as with polymers. They have reported their results in the journal Applied Physical Letters and Nanotechnology, and have filed a provisional patent application on their copying process.
As they move forward, Lakhtakia and Martin-Palma are organizing an interdisciplinary conference on biomimetics for next year. “We are past the point of replication for its own sake,” Lakhtakia explains. “The point is to gain a better understanding of natural structures and their properties. As engineers, we need to be able to ally ourselves with people who know something about biology.”
Akhlesh Lakhtakia, Ph.D., is Charles Godfrey Binder (Endowed) Professor of Engineering Science and Mechanics in the College of Engineering. He can be reached at email@example.com. Carlo Pantano, Ph.D., is Distinguished Professor of Materials Science and Engineering, and Director of the Materials Research Institute. Raul Martin-Palma, Ph.D., is professor of applied physics at the Universidad Autónoma de Madrid.