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Touch Of Gold Electrifies Polymers

Gold nanoparticles in polymer organize into conducting pathways when stretched

by Bethany Halford
July 18, 2013 | A version of this story appeared in Volume 91, Issue 29

Credit: Joseph Xu
Polyurethane with embedded gold nanoparticles can conduct electricity even when stretched to more than twice its original length.
Stretchable polymer studded with gold nanoparticles.
Credit: Joseph Xu
Polyurethane with embedded gold nanoparticles can conduct electricity even when stretched to more than twice its original length.

Packing gold nanoparticles into polyurethane transforms the polymer into an electrical conductor, one that can be stretched to more than twice its original size (Nature 2013, DOI: 10.1038/nature12401). The new material could find use in flexible electronics and medical implants, where the combination of stretchiness and electrical conductivity is prized. Researchers report that the material is durable too, as its conductivity doesn’t diminish even after 10,000 stretches.

A stretchy, conductive polymer has been tough to come by, explains Nicholas A. Kotov, the University of Michigan, Ann Arbor, chemical engineer who spearheaded the material’s development. When it comes to polymers, those two qualities tend to be mutually exclusive, he adds. Stretching increases gaps between a polymer’s conducting elements, thereby reducing conductivity. And adding conducting elements into the polymeric matrix tends to stiffen the polymer and make it less susceptible to stretching.

Kotov and colleagues decided to try putting gold nanoparticles into the polymer in an effort to solve this problem. “Previous attempts to use nanoparticles for conducting films did not lead to high-performance materials because there was often an insulating shell around the nanoparticles that prevented efficient charge transport between them,” Kotov says. “Instead of nanoparticles with thick organic shells, we used nanoparticles with very thin shells made from citrate.”

The gold nanoparticles made the polyurethane conductive while retaining high elasticity. Seeking the source of this phenomenon, the researchers trained their scanning electron microscope and transmission electron microscope on the material. What they found surprised them. When stretched, the nanoparticles reversibly assemble into conducting structures, such as chains. The observation upends the school of thought that this kind of organization could occur only in liquids.

“The ability to bend, flex, and stretch electronic components has been a challenge,” notes Christopher B. Murray, a materials science professor at the University of Pennsylvania. “This work opens the door to thinking about many more designs that are built on flexible substrates or will be subject to bending and flexing.”



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