University of Michigan researchers have created a composite plastic that's as strong as steel but lighter and transparent. The composite plastic is made of layers of clay nanosheets and a water-soluble, glue-like polymer. UM engineering professor Nicholas Kotov and others have 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.

The UM researchers created the composite plastic with a robotic machine that builds materials one nanoscale layer after another. In this experiment, the machine's arm held a piece of glass about the size of a stick of gum on which it built the new material. The arm dipped the glass into the glue-like polymer solution and then into a liquid that was a dispersion of clay nanosheets. It took 300 layers of each of the polymer and the nanosheets to create a piece of this material as thick as a piece of plastic wrap.

The polymer used in the experiment, 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, causing "the Velcro effect." If such bonds are broken, they can easily reform in a new place. The composite plastic could be used in microelectromechanical devices, microfluids, biomedical sensors and valves, and unmanned aircraft.

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