In a new review paper, a team of international researchers has laid out how engineers are taking inspiration from the biological world for the design of new 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, a research materials engineer at the Air Force Research Laboratory in Ohio.
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The researchers, who come from three countries, delve into the promise and challenges behind so-called ‘bioinspired nanocomposites’, materials that mix together different kinds of proteins and other molecules at incredibly small scales to achieve properties that aren’t exhibited by traditional metals or plastics. Such materials are often designed 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 fibres produced using bioinspired principles; and panes of glass that don’t easily crack because they include nacre, the iridescent lining present inside many mollusc shells.
Such nature-inspired materials could, one day, lead to new and better solar panels, soft robots and even coatings for hypersonic jets, said Hendrik Heinz, a professor of chemical and biological engineering of the University of Colorado 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.
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‘One of the main challenges in this field is how do we structure these materials down to the atomic level,’ he said. ‘We need to know how nature does it so we can try it in the lab and use guidance from computational models.’
In the new study, the researchers take a close look at keratin, one of nature’s most adaptable building blocks. These simple proteins, which often form into twisting helical shapes similar to that of 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 (pictured above). ‘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, something engineers call ‘hierarchical’ engineering. Some of those structures are large enough to see with the naked eye, while others are so small that researchers need powerful microscopes to study them.
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The keratin in pangolin scales, for example, takes on a wavy pattern that makes the scales hard to crack. Peacock feathers, meanwhile, are made up of melanin rods embedded in a matrix of keratin, which allows them to be both colourful and stiff at the same time. ‘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.
Making advanced synthetic materials with multiple functions in the lab, however, can get tricky.
‘Most current human-made materials are simple, single-component materials with simplistic uniform morphology and composition,’ said Vladimir Tsukruk from the Georgia Institute of Technology. ‘And what we learnt from nature is that much more complex and sustainable organisation is required to make new bio-inspired materials for advanced applications in the near future.’
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One of the biggest challenges, Heinz said, comes down to models, which his research group uses 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 millimetre or even centimetre scale, there are so many levels of organisation in natural materials,’ he 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 supplies to Mars. Heinz is taking part in a US$15million NASA-funded effort to study these ultra-strong composites.
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Engineers, he added, are also discovering new ways to make nanocomposites in large quantities in a manufacturing setting. Today, researchers often use tools such as 3D printers to make these materials, laying them down drop by drop.
The review has been published in Nature Materials.