Biomimetics: The sincerest form of flattery
We humans like to think of ourselves as extremely smart, clever, and inventive creatures. After all, in just a few thousand years of civilization, we’ve gone from inventing the wheel to landing on the Moon. But the more we learn about nature and the universe, the more that knowledge has a way of putting human hubris in its place. We learned that Earth isn’t the center of the universe, that human beings are just another part of the animal kingdom, and that natural forces such as hurricanes and earthquakes can sweep away our grandest achievements in the blink of an eye.
Fortunately, at least some humans are both smart and humble enough to take clues from nature on how to do some things better. Rather than thinking ourselves the masters of nature, we are learning that it’s often wiser to instead consider ourselves its pupils. That idea has given birth to the science of biomimetics, drawing inspiration from nature for human technology. One facet of that science is bio-inspired photonics, which focuses on how living organisms use light. It’s leading to some fascinating technological innovations useful to humans in both the civilian and military realms, including camouflage and sensing devices.
The basic idea of imitating animals in engineering probably goes back to the first person who watched a bird and dreamed of flying. Leonardo da Vinci, for one, certainly thought about it seriously with his designs for flying machines. But why would anyone think of looking to living creatures for ideas about, for example, how to hide, or communicate over great distances, or to see in the dark? The answer, as Duke University biologist Sönke Johnsen explains, is simple: “Animals get there first.”
When it comes to solving problems, compared to humans, living creatures have “a more or less four-billion-year head start,” he points out. “As far as we can tell, animals are co-opting every aspect of physics except radioactivity.” Thanks to what Johnsen calls a sort of “dumb luck,” it turns out that most of the light-utilizing structures in living organisms have evolved to operate at a size scale from near ultraviolet up through near infrared wavelengths—also just the range in which humans see and use light. “That’s all sort of a lucky accident, but it makes biophotonics a really fertile field,” Johnsen says, because humans can use that four-billion-year head start provided by nature.
The idea is beautiful, but translating it into practical technology can be a considerable challenge. “We’re not terribly good at building well-organized structures around the size of a wavelength of light. It’s difficult to make things that small when you’re an engineer, but it’s very easy for an animal to do it,” Johnsen notes. A 2007 journal article by biologists Andrew R. Parker and Helen E. Townley laments that some of the optical nanostructures found in nature, such as iridescent surfaces or highly reflective or antireflective structures for camouflage, “have such an elaborate architecture at the nanoscale that we simply cannot copy them using current engineering techniques. Or, if they can be copied, the effort involved is so great that commercial-scale manufacture would never be cost-effective.”
A siphonophore in the Gulf of California. Photo credit: © 2015 MBARI.
However, all is not lost. “Engineers are brilliant,” observes Dakota McCoy, a biophysicist at the University of Chicago and its Woods Hole, Massachusetts, Marine Biological Laboratory. “They don’t need extra inspiration, but nature can certainly provide it.” Parker and Townley suggest that one approach for biomimetic photonics could be to “let nature manufacture the devices for us through cell-culture techniques,” actually incorporating biological cells into the structure of materials or devices, such as creating photonic crystals to reflect or absorb particular optical wavelengths. But nanofabrication techniques have come a long way since their 2007 article, and sometimes we can get pretty close to nature through artificial means—at least close enough to be useful.
One of the most striking examples is the realm of ultrablack materials, which feature incredibly low reflectance and high absorbance of visible light. For multiple applications, including photovoltaic cells, coating telescope interiors, or camouflaging airplanes, those are highly desirable properties. Although such materials are generally considered to be a product of nanotechnology research, it turns out that nature got there first.
McCoy traces her interest in biomimetics to her first look at creatures that exhibit ultrablack coloration as an undergraduate, when an ornithologist showed her some specimens. “He showed me some of these incredibly black birds and said, I have no idea how these things are doing this, how black they are, how they’re trapping light so efficiently,” she recalls. “And honestly, I’ll never forget walking through the museum with him, he pulls open the drawer and there were all these normal-looking birds, and then five birds where you couldn’t even tell what you were looking at. They didn’t look like birds. They looked like a black hole in the fabric of reality. From that moment on, I was completely hooked.”
In 2014, Surrey NanoSystems invented Vantablack, an ultradark black matte coating consisting of a forest of vertically aligned carbon nanotubes that trap and absorb more than 99 percent of visible light. (“Vanta” is an acronym for vertically aligned nanotube arrays.) Vantablack coatings are quite impressive on cars, aircraft, bowling balls, and other objects, but they’re also difficult to fabricate and susceptible to abrasion and other damage. Researchers including McCoy and Johnsen have been studying how nature does ultrablack in hopes of finding better and more robust ways to achieve it.
“Nature has come up with its own version of Vantablack,” McCoy points out. She mentions a family of birds called birds-of-paradise, which have evolved deep curved feather barbules, little pieces that curve up at the very edges of feathers. At the microscale, the barbules create highly efficient light traps as light scatters between the vertically parallel-oriented pieces of the feather. Because the barbules are made of keratin, the same material as fingernails, the ultrablack bird-of-paradise feathers are very tough. “I can speak from experience that if you drop a super-black feather or accidentally press it with your thumb, it still looks super black. So, it’s not brittle and fragile,” she says.
Ultrablack features aren’t merely the province of feathered creatures. While many deep-sea animals use bioluminescence to get around and find food, their prey needs to be able to hide to survive. “We find animals down there that have reflectance identical to Vantablack,” Johnsen notes. Using modeling microscopy and other techniques, he and his team study how such ultrablack deep sea structures work on a nanostructural scale.
While ultrablack creatures desire camouflage, others have evolved to be seen, especially in difficult light environments, a quality which can also lead to some interesting technological uses. One of McCoy’s favorite examples is the white cabbage butterfly. “It sits out with its wings at a very precise angle, and they’re ultrawhite, incredibly reflective. It’s basically focusing sunlight onto its abdomen to heat up its flight muscles so that it can get going early in the morning,” a behavior known as “reflectance basking.” When researchers at the University of Exeter focused sunlight using actual butterfly wings at the same V-shaped angle on photovoltaic cells, power output was increased by more than 40 percent. Even a simple monolayer of wing cells mounted on adhesive tape gave comparable results. The work not only confirmed that the butterflies are indeed warming themselves up with their wings, but also raised the intriguing prospect of using lightweight and inexpensive reflective materials that imitate butterfly wing structures to replace the heavy and expensive mirror- or lens-based solar concentrators currently in use. As McCoy says, “the butterfly wings, you know, they weigh nothing. They’re tiny and they’re incredibly efficient.”
An owlfish observed by MBARI’s remotely operated vehicle Tiburon in Monterey Canyon at a depth of approximately 1,400 m. Photo credit: © 2002 MBARI.
The deep ocean environment also features some ultra-white organisms. “There are animals that really need to signal well underwater in a lot of different optical habitats,” Johnsen says. “The best way to signal well in those situations without dealing with the changing spectrum of light with depth is to be super white, with extremely high reflectance across all wavelengths. Photonic structures do that.” Such properties could inspire the development of high-visibility materials for low-light environments, for example.
It may seem that a great deal of biomimetics work in the photonics realm centers on marine organisms, and with good reason. Johnsen explains that “life appeared in the oceans several billion years ago, multicellular life probably 800, 900 million years ago. Colonization of land happened quite a bit later.” The fundamental differences between the marine and land environments are also a big factor. Without the buoyancy offered by an underwater environment, land organisms had to evolve to deal more directly with the effects of gravity, not to mention far greater amount of sunlight, including ultraviolet radiation.
Marine biologist Steven Haddock at California’s Monterey Bay Aquarium Research Institute is approaching bio-inspired photonics from more of a basic research perspective. He’s focusing on bioluminescence and biofluorescence in ocean creatures, a fascinating subject in its own right, yet one that’s also yielding some important findings for engineers looking for ideas from nature.
“What’s really amazing to me is how creative the technologists are,” Haddock says. “We do basic research and find a basic component of bioluminescent light, let’s say in an obscure marine animal. And then people just take it, and they come up with all of these ways to apply it to label tumor cells, and to cause light to do all these kinds of things that we could never have really anticipated.”
Under the sea, Haddock explains, “there are three basic categories of functions for bioluminescence: offense, defense, and communication.” An example of an offensive function would be glowing lures deployed by invertebrates called siphonophores to attract prey. Defensive bioluminescence can provide counter-camouflage. The deep-sea vampire squid, for example, ejects a cloud of glowing mucus to distract predators and provide cover for a quick getaway.
As for communication, Haddock notes that blue light transmits best in the oceans, and that, in the deep sea, there’s not competition from sunlight. Most ocean is deep sea located at or below 200 m, representing some 93 percent of the volume of the world’s oceans. It’s an environment that receives at most only twilight illumination. Because the creatures that can see in such conditions have evolved very sensitive eyes, says Haddock, “bioluminescence becomes a really effective way to communicate,” particularly for tiny animals such as dinoflagellates, which are less than a millimeter long yet can effectively communicate over a distance of about 10 m. He observes that for such creatures, optical communication is the most effective, energy efficient, and powerful way to communicate.
The red luminescent lures of the Erenna sirena siphonophore viewed under a microscope. Photo credit: Steven Haddock © 2004 MBARI.
Haddock observes that the study of bioluminescence has led directly to the development of technologies such as OLEDs (organic light emitting diodes) and the Nobel Prize-winning discovery of green fluorescent protein (GFP), employed in biosensing devices and as a biomarker in fluorescence microscopy and first isolated from a species of jellyfish. “I think all of these questions show the importance of supporting basic research,” he says. “People want applied research right out of the gate, but even with green fluorescent protein, the people who first discovered it had trouble getting funding for years afterwards because people thought it was just an esoteric gee-whiz kind of thing. And now it’s a multimillion-dollar industry that’s really revolutionized biological imaging.”
The inventive process of evolution is also providing scientists and engineers with endless ideas. McCoy is now researching the photonic characteristics and capabilities of coral-reef ecosystems, which use sunlight through a symbiotic relationship with algae. “My interest is thinking about how the light environment impacts differently shaped coral. Some coral look like brains, some look like big branching structures, like trees. And beyond that large-scale morphology, they have very interesting micro and nanoscale morphology of their mineral skeleton. I want to carefully understand how those different structures are better or worse adapted to efficiently harness sunlight.” Using laboratory aquacultures, McCoy can precisely adjust the light environment to study how the coral and their algae symbionts react. She believes that the work will not only lead to solutions for the massive problem of coral bleaching, but also for new photonic technologies.
Sönke Johnsen believes that “there’s a lot of promise for both fields in applying physics to biology and biology to physics.” Particularly in bio-inspired photonics, he notes: “It’s a very tidy field in that way. It doesn’t have a lot of ‘how on earth could they even do this?’ Because now we have the ability to know how they do it.” Basically, an interesting photonic phenomenon is observed in nature; biologists study the responsible structures to understand how it’s done; and engineers figure out how to create a similar or identical structure in a photonic application.
With so much of the work already done for us by nature, the key to biomimetics is all about knowing where to look, figuring out how nature achieves an interesting and useful trick, and then translating that into workable technologies. “Evolution is brutal. Natural selection and sexual selection shape [biological structures] over millennia to become incredibly efficient. Any inefficiency is weeded out by the process of evolution,” says McCoy. “We can learn so much from nature.”
And sometimes, she points out, the lessons aren’t always found in the most obvious places. “There are certain types of creatures that are obvious and interesting to look at, like large mammals and birds. But there’s so many weird creatures that very few engineers are thinking about, like tiny little worms that live on reefs, and weird clams that have windows in their shell. So, basically, I think there’s a massive opportunity for engineers to look at the full diversity of life on Earth for very cool inspiration to design new photonic technologies.”
Mark Wolverton is a freelance science writer and author based in Philadelphia.
