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Arto Hakola, Monty Rakusen, Mark Bridger, Steve Allen, Martin Pickard,iStock/Getty Images

A mantis shrimp’s clubs, a stag’s antlers and an alligator’s armourcould hold the key to better metals

FEATURE 26 September

2018

Hackingnature’s coolestinventions tocreate theperfect metalFrom bamboo stalks to mantis shrimpclubs to teeth, nature marries strengthand toughness with spectacular effect.Copying its secrets could usher in a newage of metals

By Liz Kalaugher

04/10/2018, 21)42Hacking nature's coolest inventions to create the perfect metal | New Scientist

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WHO’S afraid of the peacock mantis shrimp?Brightly coloured, googly-eyed and with a profusionof weedy legs, it doesn’t seem much of a threat.Beneath its ordinary carapace, however, lurks aremarkable weapon. When the mantis shrimp isroused, its clublike appendages punch forward withacceleration greater than a Formula 1 car engine,moving fast enough to make bubbles in thesurrounding water and create a force that shattersaquarium glass.

It is an extraordinary feat, especially when youconsider the shrimp’s deadly appendage is made ofnothing more remarkable than the material in ourbones or teeth. But it is no isolated example. Allacross the natural world, plants and animals havedeveloped ways of using nature’s simple buildingblocks to astonishing ends. Using only the meagreitems on the menu of raw materials – minerals,proteins, sugars – and without any of the industrialmachinery we have at our disposal, they haveproduced structures that can rival anything we haveengineered.

It is a feat we would dearly like to imitate. The tricklies in the way these constituent parts are arrangedon the nanoscale, formations that have untilrecently been impossible to replicate. But as ourmanufacturing techniques and palette of rawmaterials improve, some materials scientists havestarted looking to outdo the natural world. They’reusing nature’s tricks to re-engineer the materialsthat form the backbone of our world – metals.

A metal’s defining qualities – strength andtoughness – may seem like different ways ofexpressing the same thing, but to a materialsscientist they are like chalk and cheese. It is an aptcomparison. Chalk is stronger than cheese, able toresist loads without bending, but it is not tough – itis brittle, snapping easily. Cheese may be weaker,but its ability to deform before breaking makes ittough.

Metals are generally stronger and tougher than

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either chalk or cheese. But they are cursed with aweakness: any attempt to increase a metal’s naturalstrength destroys its toughness. We’d love tochange that, but it hasn’t stopped us using them forthousands of years. “The world is still made ofsteel,” says Robert Ritchie at the University ofCalifornia, Berkeley, but stronger is always better.“If you have high strength, you don’t have to use somuch material,” says Ritchie. That means yourplane, drone or car can weigh less, saving money aswell as fuel and cutting the carbon dioxideemissions that are warming the planet. Newimproved metals could also help make lighter hipreplacements, bionic hands, robots, pipelines,reinforcing bars for concrete and coatings to protectspacecraft from impacts.

Over the past 20 years or so, as our control over thenanoscale has improved, we have sought to boostthe strength of metals by making their constituentgranular crystals smaller. These nanograined metalsare super-strong, but so brittle we can’t use them instructures. “If you made the Golden Gate Bridge outof a high-strength material, you wouldn’t need touse such thick girders, but one day it could just snapand break,” says Ritchie. Nature has managed tosidestep this problem.

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es-cuisine/PhotoAlto/Alamy; Erik Isakson/Getty

Taking inspiration from a mussel’s byssus threads could lead tometals stronger and tougher than copper

Biological materials are generally made up of amixture of a hard biomineral like calcium carbonateor silica and a soft biopolymer, such as a proteinchain or a sugar. The hard part provides thestrength and the soft part the toughness; togetherthey are often more than the sum of their parts.

Take a mantis shrimp club. Beneath a thin, hardcoating of calcium phosphate like that in our bones,it consists of layers of softer, fibre-like sugarmolecules embedded in yet more calciumphosphate. This composite structure already makesfor a better combination of strength and toughnessthan either material could achieve alone. But whatreally gives the clubs the ability to withstandrepeated pummelling is the way those layers changeas you go deeper beneath the surface. The furtherinside the club you go, the less crystalline thecalcium phosphate becomes. The layers of alignedsugar molecules also become thinner, and each isrotated relative to the layers above and below,forcing any would-be crack to follow a complicatedpath.

This gradient is nature’s secret sauce, and it is inalmost every dish. “It’s probably more difficult tothink of structures in the natural world that do nothave gradients,” says Ulrike Wegst of DartmouthCollege in New Hampshire.

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“A mantisshrimp’s punchacceleratesfaster than aFormula 1 car”

Gradients come in a variety of flavours. Theremarkable flexibility of bamboo stalks, forexample, arises from a distribution gradient, withmore cellulose found on the outside of the stems toensure they don’t break. The bony plates thatprotect alligators have a gradient in their structuralarrangement, with four different types of bone,varying in the way their collagen fibres align,creating a hard upper surface, a porous core and asofter base. Fish scales, which are also made ofcollagen, have a gradient in composition. They aremore mineralised at the surface, where hardness isneeded to defend against the teeth of bigger fish,and less so beneath, providing toughness. Even ourown teeth have a gradient between two differentmaterials, dentin and enamel, creating a smoothinterface rather than a sudden change thatconcentrates stress and may initiate breakage.

“Nature has to play these games because it’s notdealing with particularly good materials – collagenand chitin and minerals,” says Ritchie. “We wouldnever use them as structural materials, but naturehas the ability to craft them into these ingeniousstructures.”

Testing our metalIt has taken a while, but we’re just starting tomaster the same ability. Back in 2011, Ke Lu at theShenyang National Laboratory for Materials Sciencein China and his colleagues made a gradientstructure in copper, one with nanoscale-sized grainsat the surface that gradually increased in size intothe bulk of the material. They did this by repeatedlygrinding the surface, keeping the inner structureuntouched but causing the outer layer to subdivideinto smaller grains. This outer skin, which Lu

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compares to the layer of enamel coating our teeth,provided a gradient that made the copper twice asstrong without reducing its toughness. This was abreakthrough, says Ramathasan Thevamaran at theUniversity of Wisconsin-Madison. “It showed apathway to improve both strength and toughness inmetals that are often found to be mutuallyexclusive.”

Ritchie has taken the concept of gradientnanograins a step further. He was inspired bybyssus, the material in the threads produced bymussels to help them cling to rocky surfaces. Byssusis strong and tough. It also has a huge gradient –while the rock-facing end is strong and stiff, themussel end is stretchier and 10 times less stiff. Tocreate this dramatic difference, the form of collagenmaking up the threads changes along their length.

But there is a snag. To replicate the remarkableproperties of byssus, Ritchie needed somethingmore complicated than copper. That is because theouter coating of the fibres consists of a hard layerthat contains even harder granules – a structureimpossible to reproduce in a pure metal.

Instead, Ritchie focused on steel, an alloy of iron,carbon and other elements. As Lu had done before,he battered the surface of the metal, this time withhundreds of small steel balls. As well as creating agradient in grain size, this also made some pocketsof steel shift their structure into a hard phaseknown as martensite. The result was alternatingstripes of hard martensite and a softer phase,austenite, inside each grain: a gradient in grain sizepaired with a two-phase structure.

This combination gives the steel a higher strengththan other steels while stopping it from becomingbrittle. “The hard and soft phases help you with thestrength and ductility and the gradient gives you aharder, stronger outer surface and a tougherunderbelly,” says Ritchie. That being said, he adds,“it’s still very primitive compared to nature”.

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It’s a start, but it only scratches the surface – quiteliterally. For Thevamaran, the key to making theperfect metal lies in reaching deep into thematerial. In 2016, he fired microcubes of silver at arigid silicon surface faster than the speed of sound.The microcubes hit the silicon so hard that theydeformed, causing the changes in grain size he waslooking to create. “With a shock process, you’llpropagate through the material,” says Thevamaran.“You’re likely to get a continuous gradient. Thatcould be one advantage.”

“Nature can craftpoor materialsinto ingeniousstructures”

For all of these laboratory successes, scaling up suchprocesses is a challenge. “One of the problems withemulating nature is nature does it from the bottomup; it takes the atoms or the molecules and it buildsup from there,” says Ritchie. “We go the other way –we take a big chunk of metal and try to generate itdown.”

The solution might be the emerging technology of3D printing, which allows you to build up a materiallayer by layer. This makes small-scale constructionpossible, says Ritchie, although making largerproducts is more challenging. “The problem with 3Dprinting, despite all the hype you hear, is at presentit’s good for making the right shape but it’s verydifficult to make the right properties,” he says.Ritchie thinks it needs another decade or so toimprove.

That said, gradient nanograined steels could start tobe seen in structures in just five years, says Ritchie.And other super-metals could perhaps be availableeven sooner for Formula 1 cars or biomedical uses.

Even then, nature will still be able to do things wecan’t. The peacock mantis shrimp moults and growsa new shell every three or four months, removing allthe damage done to its club from tens of thousands

04/10/2018, 21)42Hacking nature's coolest inventions to create the perfect metal | New Scientist

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of impacts. That ability packs quite a punch.

This article appeared in print under the headline“Natural born metals”

Liz Kalaugher is a science

writer based in Bristol,

UK, and co-author of FurryLogic: The physics of animallife

Magazine issue 3197,

published 29 September

2018

© Copyright New Scientist Ltd.

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