Yoonseob Kim, a graduate student at the University of Michigan, shows the stretchy conductor made out of gold nanoparticles. (Joseph Xu, Michigan Engineering)
What looks like a shiny piece of gold foil is actually a new stretchy conductive material that could one day be fashioned into electrode implants for the brain or pacemakers for the heart. Crafted from gold nanoparticles and an elastic polymer, the material retains its conductivity even when stretched to four times its original length.
"It looks like elastic gold," said Nicholas Kotov, a chemical engineer at the University of Michigan. "But we can stretch it just like a rubber band." When it stretches, it retains all the properties of a metal, including the ability to transport electrons.
Normally, stretching a circuit disrupts the interatomic connections that keep electrons flowing from one end to the other. Most existing stretchable electronics overcome this difficulty by using accordion- or spring-like folding wires that can expand and contract. But in the new material, no folds or convolutions are needed.
Its secret? Self-organizing gold nanoparticles that have been embedded into an elastic polymer, polyurethane.
When the shiny material is stretched, the nanoparticles self-organize into conductive chains, scurrying to fill the gaps in the elongating material. It's the first material that relies on nanospheres to achieve intrinsic stretchable conductivity, Kotov and his colleagues report today in Nature.
Looking at the substance under electron microscopes revealed that the spheres snapped into chains under pressure, producing structures electrons could flow through. "And when you release the stress, they pretty much come back to their original position," Kotov said.
The process is repeatable. And although conductance at maximal stretch is decreased to less than 10 percent of the original, it's still enough to provide power to some devices, the team reports.
"The results suggest some very interesting, unexpected effects of nanoparticle-elastomer composites," said John Rogers, a materials scientist at the University of Illinois. Rogers and his lab have developed an array of super-cool flexible, silicon-based circuits that use serpentine wires and buckled folds for stretching and contraction. "These types of conducting materials could provide new options in engineering design," he said.
Someday, the gold-and-polyurethane material might live inside your head – in the form of implantable electrodes for treating movement disorders or other conditions. Or maybe, it will find its place on your heart, as part of a device that regulates cardiac activity. Scientists have been searching for ways to make pliant, biocompatible electronics that can bend and stretch and mold to the human body's many curved surfaces, whether in the form of temporary tattoos or circuits that hug the ridges on the brain's surface.
Kotov and his team are currently testing whether other nanoparticles can be used to create stretchy conductors. They're also evaluating how prototype implants made from the nanogold and elastic polymers perform in rat brains. Then, the key will be to move from a stretchy conductor to an functioning, stretchy electronic system.
Source : http://www.wired.com/wiredscience/2013/07/stretchy-gold-conductor/
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