Networks of spherical nanoparticles embedded in elastic materials may make the best stretchy conductors yet, engineering researchers at the University of Michigan and Korea Basic Science Institute have discovered.
Flexible electronics have a wide variety of possibilities, from bendable displays and batteries to medical implants that move with the body.
“Essentially the new nanoparticle materials behave as elastic metals,” said lead researcher Nicholas Kotov, the Joseph B. and Florence V. Cejka Professor of Engineering.
Finding good conductors that still work when pulled to twice their length is a tall order—researchers have tried wires in zigzag or spring-like patterns, liquid metals, and nanowire networks. The team was surprised that spherical gold nanoparticles embedded in polyurethane could outcompete the best of these in their stretchability and concentration of electrons.
“We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways,” said U-Mich. graduate student Yoonseob Kim, first author of the study published in the July 17 online edition of Nature (DOI: 10.1038/nature12401).
(a) An electron microscope image of the gold nanoparticles in a relaxed sample of the layer-by-layer material. The nanoparticles are dispersed. (b) A similar sample stretched to a little over twice its original length, at the same magnification. The nanoparticles form a distinct network. Reproduced with permission from Nature 500 (2013), DOI: 10.1038/nature12401. © 2013 Macmillan Publishers Ltd.
To find out what happened as the material was stretched, the team took state-of-the-art electron microscope images 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 electrical conductivity,” 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 leads to 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 1800 S/cm, which is more akin to good plastic conductors.
The 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 2400 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 implantable electrodes although other applications are also being developed. Rigid electrodes create scar tissue that prevents the electrode from working over time, but electrodes that move like brain tissue, for example, could avoid damaging cells, Kotov said. These electrodes 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.
