Biomimicry: Nacre Inspires Transparent Strong As Steel Plastic
Nacre, or Mother of pearl is the iridescent lining you see on the inside of an oyster, mussel, or abalone shell. It is also the same material that creates pearls. The play of light we find so appealing is a result of the little beasties engineering the shell down to the molecular level.
To create a shell that is tough and lightweight (sometimes 3000 times tougher than its component parts), the abalone layers an organic material and a non-organic material into a nano-structure design resembling brick and mortar.
We have seen before how the abalone inspired super tough materials. Now engineering professor Nicholas Kotov and his team from the University of Michigan have created a process similar to our bivalve friends that allows the creation of materials one nano-layer at a time, with impressive results.Made of layers of clay nanosheets and a water-soluble polymer that shares chemistry with white glue, the material is transparent, as strong as steel, yet lighter in weight. Kotov almost dubbed it "plastic steel," but the new material isn't quite as stretchy as steel. Building materials from the bottom-up is a major engineering challenge, with possible big sustainability wins. Controlling the design and structure of the material on the molecular level allows incredible properties, without excessive use of material or energy inputs.
"When you tried to build something you can hold in your arms, scientists had difficulties transferring the strength of individual nanosheets or nanotubes to the entire material," Kotov said. "We've demonstrated that one can achieve almost ideal transfer of stress between nanosheets and a polymer matrix."
"The glue-like polymer used in this experiment, which is polyvinyl alcohol, was as important as the layer-by-layer assembly process. The structure of the "nanoglue" and the clay nanosheets allowed the layers to form cooperative hydrogen bonds, which gives rise to what Kotov called "the Velcro effect." Such bonds, if broken, can reform easily in a new place."
The process is currently dependent on a robotic arm that applies the materials layer by layer, waiting for the glue to dry between layer applications. We still have a long way to go before we can create materials as sustainable and tough as a sea shell, but this proof of principle is a big step, with possible high end applications.