Flexible electronics are now one step closer to being a reality thanks to new research from the University of Michigan — the researchers there created an extremely stretchable electrical conductor out of networks of gold nano-particles embedded in elastic polyurethane.
“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 nanoparticles for a wide range of applications.”
Flexible/stretchable electronics are currently a “hot” area of research, with the possibility of flexible medical implants, flexible visual displays, and flexible batteries being the primary motivation for such research.
But — needless to say — there are still some barriers to be overcome before such devices could enter wide-scale use. One of these barriers, though, may have now finally been overcome, as a result of the new research — the creation of a good conductor which is still very flexible. Previous ideas (such as wires in zigzag or spring-like patterns, liquid metals, nanowire networks) have performed considerable worse than this new design, which outperforms the best of these with regard both to stretchability and to the concentration of electrons.
“We found that nanoparticles aligned into chain form when stretching. That can make excellent conducting pathways,” stated Yoonseob Kim, first author of the study, and a graduate student in the Kotov lab in chemical engineering. “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.”
The University of Michigan continues:
To find out what happened as the material 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.
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.
The researchers think that their new technology could function particularly well as stretchable electrodes — with brain implants being of particular interest to them.
“They can alleviate a lot of diseases — for instance, severe depression, Alzheimer’s disease and Parkinson’s disease,” Kotov stated. “They can also serve as a part of artificial limbs and other prosthetic devices controlled by the brain.”
The rigid sorts of electrodes currently in use cause tissue damage and usually end up encompassed in scar tissue, limiting their effectiveness. Flexible electrodes that move like brain tissue, though, could perhaps limit such tissue damage, and thus the build-up of scar tissue, according to the researchers.
“The stretchability is essential during implantation process and long-term operation of the implant when strain on the material can be particularly large,” Kotov continued.
The researchers also speculate if the gold were replaced with a semiconductor that the resulting material could prove very useful in batteries — potentially extending the operating lifespans of lithium-ion batteries.
U-M is currently pursuing patent protection for the new technology, and is also currently searching for economic partners to help them bring the technology to market.
The new research was just published July 18th in the journal Nature.
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