Elastic electronics: Stretchable gold conductor grows its own wires

Jul 17, 2013

Networks of spherical nanoparticles embedded in elastic materials may make the best stretchy conductors yet, engineering researchers at the University of Michigan have discovered.

Flexible electronics have a wide variety of possibilities, from bendable displays and batteries to that move with the body.

"Essentially the new nanoparticle materials behave as elastic metals," said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering. "It's just the start of a new family of materials that can be made from a large variety of for a wide range of applications."

Finding good conductors that still work when pulled to twice their length is a tall order— researchers have tried wires in tortuous zigzag or spring-like patterns, , nanowire networks and more. The team was surprised that spherical gold nanoparticles embedded in could outcompete the best of these in stretchability and concentration of electrons.

"We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways," said Yoonseob Kim, first author of the study to be published in Nature on July 18 and a graduate student in the Kotov lab in chemical engineering.

To find out what happened as the material stretched, the team took state-of-the-art of the materials at various tensions. The nanoparticles started out dispersed, but under strain, they could filter through the minuscule gaps in the polyurethane, connecting in chains as they would in a solution.

"As we stretch, they rearrange themselves to maintain the conductivity, and this is the reason why we got the amazing combination of stretchability and ," Kotov said.

The team made two versions of their material—by building it in alternating layers or filtering a liquid containing polyurethane and nanoparticle clumps to leave behind a mixed layer. Overall, the layer-by-layer material design is more conductive while the filtered method makes for extremely supple materials. Without stretching, the layer-by-layer material with five gold layers has a conductance of 11,000 Siemens per centimeter (S/cm), on par with mercury, while five layers of the filtered material came in at 1,800 S/cm, more akin to good plastic conductors.

The eerie, blood-vessel-like web of nanoparticles emerged in both materials upon stretching and disappeared when the materials relaxed. Even when close to its breaking point, at a little more than twice its original length, the layer-by-layer material still conducted at 2,400 S/cm. Pulled to an unprecedented 5.8 times its original length, the filtered material had an electrical conductance of 35 S/cm—enough for some devices.

Kotov and Kim chiefly see their stretchable conductors as electrodes. Brain implants are of particular interest to Kotov.

"They can alleviate a lot of diseases—for instance, severe depression, Alzheimer's disease and Parkinson's disease," he said. "They can also serve as a part of artificial limbs and other prosthetic devices controlled by the brain."

Rigid electrodes create scar tissue that prevents the electrode from working over time, but electrodes that move like brain tissue could avoid damaging cells, Kotov said.

"The stretchability is essential during implantation process and long-term operation of the implant when strain on the material can be particularly large," he said.

Whether in the brain, heart or other organs—or used for measurements on the skin—these electrodes could be as pliable as the surrounding tissue. They could also be used in displays that can roll up or in the joints of lifelike "soft" robots.

Because the chain-forming tendency of nanoparticles is so universal many other materials could stretch, such as semiconductors. In addition to serving as flexible transistors for computing, elastic semiconductors may extend the lives of lithium-ion batteries. Kotov's team is exploring various nanoparticle fillers for stretchable electronics, including less expensive metals and semiconductors.

Kotov is a professor of chemical engineering, biomedical engineering, materials science and engineering and macromolecular science and engineering.

Explore further: Scientists fabricate defect-free graphene, set record reversible capacity for Co3O4 anode in Li-ion batteries

More information: DOI: 10.1038/nature12401

Related Stories

Checkerboard surface put to flexible electronics test

Dec 13, 2012

(Phys.org)—Interest mounts in stretchable electronics, seen as the future direction in mobile electronics. How long before manufacturing giants load retail shelves with devices that have stretchable electronics ...

Nanoparticles, made to order—inside and out

Jul 02, 2013

A new coating technology developed at MIT, combined with a novel nanoparticle-manufacturing technology developed at the University of North Carolina at Chapel Hill, may offer scientists a way to quickly mass-produce ...

Researchers create 'rubber-band electronics'

Jul 02, 2012

For people with heart conditions and other ailments that require monitoring, life can be complicated by constant hospital visits and time-consuming tests. But what if much of the testing done at hospitals could be conducted ...

Recommended for you

Copper shines as flexible conductor

19 hours ago

Bend them, stretch them, twist them, fold them: modern materials that are light, flexible and highly conductive have extraordinary technological potential, whether as artificial skin or electronic paper.

Nanoparticles may aid oil recovery, frack fluid tracking

21 hours ago

Two Colorado State University researchers are examining how nanoparticles move underground, knowledge that could eventually help improve recovery in oil fields and discover where hydraulic fracking chemicals ...

Nanostructure enlightening dendrite-free metal anode

Aug 19, 2014

Graphite anodes have been widely used for lithium ion batteries (LIBs) during the past two decades. The replacement of metallic lithium with graphite enables safe and highly efficient operation of LIBs, however, ...

User comments : 0