The sensor is stretchy in all directions and then rebounds to the original shape. Photo: Steve Fyffe
skin so supple you could stretch it out to more than twice its normal length in
any direction—repeatedly—yet it would always snap back completely wrinkle-free
when you let go of it. You would certainly never need Botox.
elasticity is one of several new features built into a new transparent
skin-like pressure sensor that is the latest sensor developed by Stanford University’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
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,” says Darren Lipomi, a postdoctoral researcher in Bao’s laboratory, who is part of the research
it causes any permanent deformation,” he says.
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 by Nature Nanotechnology. Bao is a coauthor of the paper.
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
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.
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
silicone is released, it rebounds back to its original dimensions, but the
nanotubes buckle and form little nanostructures that look like springs.
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 says.
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.
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 says. 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.
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.
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.
sensed by the nanotube films is what enables the sensor to transmit what it is
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 says it should be possible to detect the
difference by the pattern of pressure.
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.
device was gripped by two opposing pincers and stretched, the greatest
deformation would be along the straight line between the two pincers,”
Lipomi says. Deformation would decrease as you moved farther away from the
group previously created a sensor so sensitive to pressure that it could detect
pressures “well below the pressure exerted by a 20 mg bluebottle fly
carcass” that the researchers tested it with. This latest sensor is not
quite that sensitive, she says, but that is because the researchers were
focused on making it stretchable and transparent.
not spend very much time trying to optimize the sensitivity aspect on this
sensor,” Bao says.
“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