The arc-lined core, which delineates the insides of mussels and other molluscs, is known as nature's most difficult material. Now a team of researchers led by the University of Michigan has revealed in real time exactly how it works.
Better known as mother-of-pearl, the combination of pearl hardness and resistance has been mystifying scientists for more than 80 years. If people could imitate it, it could lead to a new generation of super-strong synthetic materials for structures, surgical implants and countless other applications.
"We humans can make more difficult materials using an unnatural environment, such as extreme heat and pressure. But we cannot reproduce the kind of nanotechnology that molluscs have achieved. Combining the two approaches can lead to a grand new generation of materials, and this document is a step in that direction, "says Robert Howden, UM Assistant in Materials Science and Engineering.
Researchers have known the basics of the secret of curtains for decades – it was made by microscopic "bricks" of a mineral called aragonite, lined with "mortar" made of organic material, this arrangement of bricks and mortar apparently gives it strength, but the curtain is far stronger than its materials suggest.  The Hovden team, which includes assistant research assistant for JM materials Jiseok Gim, as well as geochemists at the Australian University of Macquarie and elsewhere, worked together
At the Michigan Center for Material Characterization in Michigan, researchers used small piezoelectric micro-indents to exert force on the shells of Pinna nobilis, commonly known as the noble shell of the pen, while under electron microscope. They watched what happened in real time.
They discovered that "bricks" are actually multilateral tablets of only a few hundred nanometers in size. Typically, these tablets remain separate, stacked and thin with an organic mortar layer. But when stress is applied to the shells, the "mortar" slides away and the tablets lock together, forming what is essentially a solid surface. When the force is removed, the structure springs back without losing force or resistance.
This durability separates curtains from even the most modern materials created by man. Plastics, for example, can come back from impact, but they lose some of their power every time. Dacre does not lose its resistance to repeated blows up to 80% of the yield force
Moreover, if a crack is formed, the pearl limits the crack to one layer, but does not allow it to spread, keeping the structure of the shell intact.
"It's amazing that a mollusk, which is not the most intelligent creature, creates so many structures in so many rocks," Hovden said. "It's the production of individual calcium carbonate molecules, arranging them in nanosheets that are glued together with organic material, right down to the shell structure, which combines a pearl with several other materials."
Howden believes that humans could use mussel methods to create nano-created composite surfaces that could be dramatically lighter and healthier than those available today.
"Nature provides us with these highly optimized structures with millions of years of evolution behind them," he said. "We could never run enough computer simulations to invent these – they are just there to find them."
The study was published in by Nature Communications .
The materials teach how the pearl is made
Jiseok Gim et al., Nanoscale deformation mechanics reveals resistance in the curtain of Pinna nobilis shell Nature Communications (2019). DOI: 10.1038 / s41467-019-12743-z
University of Michigan
Breaking down the mystery of nature's most difficult material (2019, October 23)
retrieved 23 October 2019
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