Engineers at Columbia University in New York have discovered a new way to create super-strong materials, taking their inspiration from nature.
The shimmering, iridescent coating on the inside of some sea shells is called ‘nacre’ or ‘mother of pearl’. A naturally occurring composite material, nacre is made up of calcium carbonate and protein. Its rigid structure makes shells resistant to cracking, protecting the soft molluscs inside.
Hexagonal plates of aragonite, a type of calcium carbonate, are arranged into continuous sheets that are stacked on top of one another. Sandwiched in between each brittle sheet is a thin layer of chitin, an elastic ‘biopolymer’ made from protein. Together, this ‘brick and mortar’ construction gives oysters extraordinary mechanical properties, such as great strength and resilience.
In a bid to copy the molluscs’ ingenious shell design, researchers mixed their own elastic polymer with aragonite-like nanoparticles. By slowing the polymer’s crystallisation, the team could control how the nanoparticles assembled, forming them into stacked layers. The resulting ordered pattern can make materials ten times stiffer, while the addition of elastic polymers keeps them lightweight and flexible.
The leader of the study, Sanat Kumar, Bykhovsky Professor of Chemical Engineering, said: “Essentially, we have created a one-step method to build a composite material that is significantly stronger than its host material.”
Around three quarters of commercially used polymers are ‘semicrystalline’, like those found in plastic carrier bags and soft drinks bottles. These make weak or brittle plastics that break apart easily and are of little use for reusable or hard-wearing products. As well as improving the strength of everyday plastics, the technique could boost tougher materials such as those found in car bumpers, tyres and fan belts. The lightweight properties of the material are critical to applications in planes and automobiles. In the future, Kumar suggests it could even allow us to create new, super-strong materials to use in buildings.
The study, published in June, marks the first time anyone has successfully tuned the assembly of nanoparticles inside a crystalline polymer. Dan Zhao, Kumar’s PhD student and first author on the paper, described their achievement as the “holy grail” of nanoscience.
The next step for the team is to explore how and why the nanoparticles align the way they do, in an attempt to speed up the process. Currently, the ordering of particles takes several days. Once they have understood this, they can set about testing other applications for the nanocomposites.
“The potential of replacing structural materials with these new composites could have a profound effect on sustainable materials, as well as our nation’s infrastructure,” said Kumar.