By mimicking a brick-and-mortar molecular structure found in
seashells, University of Michigan researchers created a composite
plastic that's as strong as steel but lighter and transparent.
It's made of layers of clay nanosheets and a water-soluble
polymer that shares chemistry with white glue.
Engineering professor Nicholas Kotov almost dubbed it
"plastic steel," but the new material isn't quite stretchy enough to
earn that name. Nevertheless, he says its further development could
lead to lighter, stronger armor for soldiers or police and their
vehicles. It could also be used in microelectromechanical devices,
microfluidics, biomedical sensors and valves and unmanned aircraft.
Kotov and other U-M faculty members are authors of a paper on
this composite material, "Ultrastrong and Stiff Layered Polymer
Nanocomposites," published in the Oct. 5 edition of Science.
The scientists solved a problem that has confounded engineers
and scientists for decades: Individual nano-size building blocks such
as nanotubes, nanosheets and nanorods are ultrastrong. But larger
materials made out of bonded nano-size building blocks were
comparatively weak. Until now.
"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 researchers created this new composite plastic with a
machine they developed that builds materials one nanoscale layer after
The robotic machine consists of an arm that hovers over a
wheel of vials of different liquids. In this case, the arm held a piece
of glass about the size of a stick of gum on which it built the new
The arm dipped the glass into the glue-like polymer solution
and then into a liquid that was a dispersion of clay nanosheets. After
those layers dried, the process repeated. It took 300 layers of each
the glue-like polymer and the clay nanosheets to create a piece of this
material as thick as a piece of plastic wrap.
Mother of pearl, the iridescent lining of mussel and oyster
shells, is built layer-by-layer like this. It's one of the toughest
natural mineral-based materials.
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 Velcro effect is one reason the material is so strong.
Another is the arrangement of the nanosheets. They're stacked like
bricks, in an alternating pattern.
"When you have a brick-and-mortar structure, any cracks are
blunted by each interface," Kotov explained. "It's hard to replicate
with nanoscale building blocks on a large scale, but that's what we've
Collaborators include: mechanical engineering professor Ellen Arruda; aerospace
engineering professor Anthony Waas; chemical, materials
science and biomedical engineering professor Joerg Lahann; and
chemistry professor Ayyalusamy Ramamoorthy. Kotov is a professor of
chemical engineering, materials science and engineering, and biomedical
The nanomechanical behavior of these materials is being
modeled by professor Arruda's group; Waas and his group are working on
applications in aviation.