Researchers reveal secrets of nature's toughest material

Nacre – or mother of pearl – is the material that lines the insides of mollusc shells and is recognised as nature's toughest material. Now, a team of researchers led by the University of Michigan (U-M) has revealed precisely how it works, in real time.

"We humans can make tougher materials using unnatural environments, for example extreme heat and pressure. But we can't replicate the kind of nano-engineering that molluscs have achieved. Combining the two approaches could lead to a spectacular new generation of materials, and this paper is a step in that direction," said Robert Hovden, U-M assistant professor of materials science and engineering.

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Researchers know that nacre is made of microscopic ‘bricks’ of aragonite, laced together with a ‘mortar’ made of organic material. This bricks-and-mortar arrangement lends strength, but nacre is far stronger than these building blocks suggest.

At U-M's Michigan Center for Materials Characterization, the researchers used piezo-electric micro-indenters to exert force on shells of Pinna nobilis - the noble pen shell - while they were under an electron microscope and observed what happened in real time.

They found that the ‘bricks’ are multisided tablets only a few hundred nanometres in size. According to U-M, these tablets remain separate, arranged in layers and cushioned by a thin layer of organic ‘mortar’. When stress is applied to the shells, the ‘mortar’ squishes aside and the tablets lock together, forming a solid surface. When the force is removed, the structure springs back, without losing any strength or resilience. Nacre lost none of its resilience in repeated impacts at up to 80 per cent of its yield strength.

If a crack does form, nacre confines the crack to a single layer rather than allowing it to spread, keeping the shell's structure intact.

"It's incredible that a mollusc, which is not the most intelligent creature, is fabricating so many structures across so many scales," Hovden said in a statement. "It's fabricating individual molecules of calcium carbonate, arranging them into nano-layered sheets that are glued together with organic material, all the way up to the structure of the shell, which combines nacre with several other materials."

Hovden believes humans could use the mussel's methods to create nano-engineered composite surfaces that could be significantly lighter and stronger than those available today.