Researchers Create New Conductive Material Capable of Stretching

Using gold nanoparticles and elastic polymers, researchers have developed a new stretchy yet conductive material that they believe could soon make its way into everything from bendable displays and batteries to medical implants capable of moving with the body.

The material, which looks like gold foil, contains networks of spherical nanoparticles embedded in elastic materials

"Essentially the new nanoparticle materials behave as elastic metals," Nicholas Kotov, a professor of engineering at the University of Michigan and an author of the study, said in a press release.

Finding good conductors that still work once expanded has been seen as something of a holy grail by scientists, prompting researchers to try wires in tortuous zigzag or spring-like patterns, liquid metals, nanowire networks and more.

So when the team of scientists found that spherical gold nanoparticles embedded in polyurethane could outcompete the best of these in stretchability and concentration of electrons, they were surprised.

Key to this, they found, were the nanoparticles' self-organizing behavior.

"We found that nanoparticles aligned into chain form when stretching," said Yoonseob Kim, the first author of the study and a graduate student in chemical engineering. "That can make excellent conducting pathways."

In order to determine exactly what happened as the material is stretched, the team took state-of-the-art electron microscope images of the materials at various tensions. As they did this, they observed that the nanoparticles started out dispersed but could filter through the minuscule gaps in the polyurethane under strain, 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 electrical conductivity," Kotov explained.

The researchers made two versions of the materials, finding that by building it in alternating layers the material became more conductive. Filtering a liquid containing polyurethane and nanoparticle clumps in order to create a mixed layer, on the other hand, created a more supple result.

In either case, however, the blood-vessel-like web of nanoparticles emerged the moment it was stretched, disappearing as it was relaxed. As a result, even when it was stretched close to its breaking point at twice its original length, the layer-by-layer material still conducted at 2,400 Siemens per centimeters. Furthermore, when pulled to an unprecedented 5.8 times its original length, the filtered material maintained electrical conductance of 35 S/cm, which is enough for some devices.

In terms of application, Kotov is especially excited about what this could mean for brain implants.

"They can alleviate a lot of diseases -- for instance, severe depression, Alzheimer's disease and Parkinson's disease," he said of implants, adding that while rigid electrodes create scar tissue that prevents them from working over time, one that could move like brain tissue would avoid damaging cells.

Going forward, Kotov said his team is exploring various nanoparticle fillers for stretchable electronics, including less expensive metals and semiconductors.

"It's just the start of a new family of materials that can be made from a large variety of nanoparticles for a wide range of applications," he said.