That enviable elasticity is one of several new features built into a
new transparent skin-like pressure sensor that is the latest sensor
developed by Stanford's Zhenan Bao, associate professor of chemical
engineering, in her quest to create an artificial "super skin." The
sensor uses a transparent film of single-walled carbon nanotubes that
act as tiny springs, enabling the sensor to accurately measure the force
on it, whether it's being pulled like taffy or squeezed like a sponge.
The sensor is stretchy in all directions and then rebounds to the original shape.
"This sensor can register pressure ranging from a firm pinch between
your thumb and forefinger to twice the pressure exerted by an elephant
standing on one foot," said Darren Lipomi, a postdoctoral researcher in
Bao's lab, who is part of the research team.
"None of it causes any permanent deformation," he said.
Lipomi and Michael Vosgueritchian, graduate student in chemical
engineering, and Benjamin Tee, graduate student in electrical
engineering, are the lead authors of a paper describing the sensor
published online Oct. 23 by Nature Nanotechnology. Bao is a coauthor of the paper.
The sensors could be used in making touch-sensitive prosthetic limbs
or robots, for various medical applications such as pressure-sensitive
bandages or in touch screens on computers.
The key element of the new sensor is the transparent film of carbon
"nano-springs," which is created by spraying nanotubes in a liquid
suspension onto a thin layer of silicone, which is then stretched.
When the nanotubes are airbrushed onto the silicone, they tend to
land in randomly oriented little clumps. When the silicone is stretched,
some of the "nano-bundles" get pulled into alignment in the direction
of the stretching.
When the silicone is released, it rebounds back to its original
dimensions, but the nanotubes buckle and form little nanostructures that
look like springs.
"After we have done this kind of pre-stretching to the nanotubes,
they behave like springs and can be stretched again and again, without
any permanent change in shape," Bao said.
Stretching the nanotube-coated silicone a second time, in the
direction perpendicular to the first direction, causes some of the other
nanotube bundles to align in the second direction. That makes the
sensor completely stretchable in all directions, with total rebounding
afterward.
Additionally, after the initial stretching to produce the
"nano-springs," repeated stretching below the length of the initial
stretch does not change the electrical conductivity significantly, Bao
said. Maintaining the same conductivity in both the stretched and
unstretched forms is important because the sensors detect and measure
the force being applied to them through these spring-like
nanostructures, which serve as electrodes.
The sensors consist of two layers of the nanotube-coated silicone,
oriented so that the coatings are face-to-face, with a layer of a more
easily deformed type of silicone between them.
The middle layer of silicone stores electrical charge, much like a
battery. When pressure is exerted on the sensor, the middle layer of
silicone compresses, which alters the amount of electrical charge it can
store. That change is detected by the two films of carbon nanotubes,
which act like the positive and negative terminals on a typical
automobile or flashlight battery.
The change sensed by the nanotube films is what enables the sensor to transmit what it is "feeling."
Whether the sensor is being compressed or extended, the two nanofilms
are brought closer together, which seems like it might make it
difficult to detect which type of deformation is happening. But Lipomi
said it should be possible to detect the difference by the pattern of
pressure.
With compression, you would expect to see sort of a bull's-eye
pattern, with the greatest deformation at the center and decreasing
deformation as you go farther from the center.
"If the device was gripped by two opposing pincers and stretched, the
greatest deformation would be along the straight line between the two
pincers," Lipomi said. Deformation would decrease as you moved farther
away from the line.
Bao's research group previously created a sensor so sensitive to
pressure that it could detect pressures "well below the pressure exerted
by a 20 milligram bluebottle fly carcass" that the researchers tested
it with. This latest sensor is not quite that sensitive, she said, but
that is because the researchers were focused on making it stretchable
and transparent.
"We did not spend very much time trying to optimize the sensitivity aspect on this sensor," Bao said.
"But the previous concept can be applied here. We just need to make
some modifications to the surface of the electrode so that we can have
that same sensitivity."
Lipomi, Vosgueritchian and Tee contributed equally to the research and are co-primary authors of the Nature Nanotechnology
paper. Sondra Hellstrom, a graduate student in applied physics;
Jennifer Lee, an undergraduate in chemical engineering; and Courtney
Fox, a graduate student in chemical engineering, also contributed to the
research and are co-authors of the paper.
From sciencedaily
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