Scientists have used networks of spherical nanoparticles to create stretchable conductors in a development which could usher in a new era of elastic electrical materials.

According to a team of engineering researchers from the University of Michigan, the embedding of spherical nanoparticles in polyurethane permits the creation of materials which are bendable and flexible yet possess the conductive properties of metals.

"Essentially the new nanoparticle materials behave as elastic metals," said Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering at the University of Michigan.

Kotov and his team embedded spherical gold nanoparticles in plastic polyurethane in order to produce an elastic material which can be stretched and bent while still retaining its conductive properties.

"We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways," said Yoonseob Kim, a graduate student at the Kotov lab in chemical engineering and one of the authors of a study on the new articles which is scheduled for publication in Nature.

Using electron microscope imaging, the team discovered that when the material is stretched, the nanoparticles could create chains via gaps in the polyurethane, thus preserving their conductive properties.

gold nanoparticles

An electron microscope image of the gold nanoparticles in a relaxed sample of the layer-by-layer material. The nanoparticles are dispersed. BOTTOM: a similar sample stretched to a little over twice its original length, at the same magnification. The nanoparticles form a distinct network.

"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," said Kotov.

The team has produced two versions of the material. The first consists of a layer-by-layer accumulation of polyurethane and gold nanoparticles while the second involves the filtration of a liquid solution containing both polyurethane and gold nanoparticles in order to produce a composite mixture.

The layer-by-layer version was found to possess far greater conductivity, with conductance of 11,000 Siemens per centimetre when five layers were used - equivalent to the conductivity of mercury - as compared to 1,800 Siemens per centimetre for the filtered material with five layers.

The new material has the potential to be applied across a broad range of applications, including bendable display screens and flexible medical implants which are better capable of adapting to their contours of their human hosts.

"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," Kotov said.

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