Learning from pangolins and peacocks: Researchers explore next-gen structural materials

In a new review paper, a team of international researchers have laid out how engineers are taking inspiration from the biological world -- and designing new kinds of materials that are potentially tougher, more versatile and more sustainable than what humans can make on their own.

"Even today, nature makes things way simpler and way smarter than what we can do synthetically in the lab," said Dhriti Nepal, first author and a research materials engineer at the Air Force Research Laboratory in Ohio.

Nepal along with Vladimir Tsukruk from Georgia Institute of Technology and Hendrik Heinz of the University of Colorado Boulder served as co-corresponding authors for the new analysis. The team published its findings Nov. 28 in the journal Nature Materials.

The researchers, who come from three countries, delve into the promise and challenges behind "bioinspired nanocomposites." These materials mix together different kinds of proteins and other molecules at incredibly small scales to achieve properties that may not be possible with traditional metals or plastics. Researchers often design them using advanced computer simulations or models. Examples include thin films that resist wear and tear by incorporating proteins from silkworm cocoons; new kinds of laminates made from polymers and clay materials; carbon fibers produced using bioinspired principles; and panes of glass that don't easily crack because they include nacre -- the iridescent lining inside many mollusk shells.

Such nature-inspired materials could, one day, lead to new and better solar panels, soft robots and even coatings for hypersonic jets, said Heinz, professor of chemical and biological engineering at CU Boulder. But first, researchers will need to learn how to build them from the bottom up, ensuring that every molecule is in the right place.

"One of the main challenges in this field is how do we structure these materials down to the atomic level," Heinz said. "We need to know how nature does it so we can try it in the lab and use guidance from computational models."

The amazing keratin

In the new study, Nepal, Tsukruk, Heinz and their colleagues take a close look at keratin, one of nature's most adaptable building blocks.

These simple proteins, which often form into twisting helical shapes like DNA, can join together in different ways to make a huge variety of structures -- from human fingernails and hair to porcupine quills, rhinoceros horns and the overlapping scales of pangolins.

"Keratin is everywhere, and we've hardly even begun to appreciate its utility," Nepal said.

That's one of nature's secrets, she added: Biological materials can exhibit a wide array of complex architectures at many levels -- what engineers call "hierarchical" engineering. Some of those structures are large enough to see with the naked eye, while others are so small researchers need powerful microscopes to study them.

The keratin in pangolin scales, for example, takes on a wavy pattern that makes the scales hard to crack. Peafowl feathers, meanwhile, are made up of melanin rods embedded in a matrix of keratin, which allows these adornments to be both colorful and stiff at the same time -- perfect for peacocks that want to spread their tail feathers.

"One of the biggest things we can learn from nature is how these materials exhibit multiple functions that work together in perfect synergy," Nepal said.

From atoms up

Making advanced synthetic materials with multiple functions in the lab, however, can get tricky.

"Most of current human-made materials are simple, single-component materials with simplistic uniform morphology and composition," Tsukruk said. "And what we learnt from nature is that much more complex and sustainable organization is required to make new bio-inspired materials for advanced applications in the near future."

One of the biggest challenges, Heinz said, comes down to models. His research group uses these tools to simulate new kinds of materials at the scale of a few hundred to millions of atoms. But taking those kinds of tiny designs and scaling them up to the size of something you can actually see becomes an increasingly difficult task.

"From the scale of atoms to the millimeter or even centimeter scale, there are so many levels of organization in natural materials," Heinz said.

Heinz noted that NASA has recently invested in exploring hierarchically-engineered materials for aerospace applications -- such as stronger and more lightweight panels of nanostructured carbon for use in spacecraft to carry life supplies to Mars. Heinz, for example, is part of a $15 million effort funded by NASA to study these kinds of "ultrastrong composites."

Engineers, he added, are also discovering new ways to make nanocomposites in large quantities in a manufacturing setting. Today, researchers often use tools like 3D printers to make these materials, laying them down drop by drop.

Heinz, Tsukruk, Nepal, and their colleagues are optimistic. Nature, they report, has had millions of years to learn how to construct materials like pangolin scales or oyster nacre as efficiently as possible. Engineers may be able to take clues from pangolins and oysters to build materials without creating a lot of harmful waste in the process.

"If we learn from nature, we can find alternatives to many current energy-intensive manufacturing processes or hazardous chemicals," Heinz said.

Krishan Kanhaiya, a recent PhD graduate in chemical and biological engineering at CU Boulder, also served as a co-author on the new study. Other co-authors include researchers from Georgia Institute of Technology; Carnegie Mellon University; Duke University; MIT; University College London; Johns Hopkins University; Deakin University; Tufts University; University of Michigan; University of Cambridge; University of Oxford; University of California San Diego; and Rice University.