Researchers at the University of Michigan in Ann Arbor have discovered how nacre, the rainbow-sheened material that lines the insides of mussel and other mollusk shells, works. The material is the toughest in nature.
More commonly known as mother-of-pearl, nacre’s hardness and resilience has been a mystery for more than 80 years. If humans could mimic it, it could lead to a new generation of ultra-strong synthetic materials for structures, surgical implants, and more.
“We humans can make tougher materials using unnatural environments, for example extreme heat and pressure,” says Robert Hovden, assistant professor of materials science and engineering at U-M. “But we can’t replicate the kind of nano-engineering that mollusks have achieved. Combining the two approaches could lead to a spectacular new generation of materials, and this paper is a step in that direction.”
Researchers have known for decades that nacre is made of “bricks” of a mineral called aragonite held together by organic material. The microscopic arrangement of the material lends strength, but nacre is stronger than its materials suggest.
At U-M’s Michigan Center for Materials Characterization, the researchers used tiny piezo-electric micro-indenters to exert force on the shells of Pinna nobilis, commonly known as the noble pen shell, while they were under an electron microscope and watched in real time.
They found the bricks are multisided tablets only a few hundred nanometers in size. Ordinarily, these tablets remain separate, arranged in layers and cushioned by thin layers of organic material. However, 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 strength or resilience.
Plastics can spring back from an impact, but they lose some of their strength each time. Nacre lost none of its resilience in repeated impacts at up to 80 percent of its yield strength.
If a crack forms, nacre confines it to a single layer rather than allowing it to spread, keeping the shell’s structure intact.
“It’s incredible that a mollusk, which is not the most intelligent creature, is fabricating so many structures across so many scales,” Hovden says. “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 people could use these methods to create nano-engineered composite surfaces that could be dramatically lighter and stronger than those available today.
“Nature is handing us these highly optimized structures with millions of years of evolution behind them,” he says. “We could never run enough computer simulations to come up with these — they’re just there for us to discover.”
The study is published in Nature Communications and is available here. U-M also worked with geochemists from Australia’s Macquarie University and elsewhere. Mediterranean Pinna nobilis mollusk studied is a protected species due to recent ecological threats.
