I am sharing a very interesting feature article from the University of Michigan about “Stretchable gold conductor that grows its own wires.” How cool is that, right?
According
to the article, 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 medical implants 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 nanoparticles 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, liquid metals, nanowire networks and more. The team was
surprised that spherical gold nanoparticles embedded in polyurethane 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 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 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.
The study is titled "Stretchable Nanoparticle Conductors with
Self-Organized Conductive Pathways." The work is funded by the STX
foundation in Seoul, South Korea; U.S. Department of Energy's Office of
Science; Defense Advanced Research Projects Agency; Air Force Office of
Scientific Research; and National Science Foundation. U-M is pursuing patent
protection for the intellectual property and seeking commercialization partners
to help bring the technology to market.
Description: Networks of spherical nanoparticles embedded in elastic materials may make the best stretchy conductors yet, engineering researchers at the University of Michigan have discovered.
LEFT: an electron microscope image of the gold nanoparticles in a relaxed sample of the layer-by-layer material. The nanoparticles are dispersed. RIGHT: a similar sample stretched to a little over twice its original length, at the same magnification. The nanoparticles form a distinct network
Watch and embed the video at www.youtube.com/watch?v=KQ7_TPSSfys.
