A newly developed artificial skin is made touch-sensitive by
depositing germanium/silicon nanowires on a polyimide film substrate.
"The idea is to have a material that functions like the
human skin, which means incorporating the ability to feel and touch
objects," says Ali Javey, associate professor of electrical engineering
and computer sciences, and head of the UC Berkeley research team developing the
The goal is to overcome a significant problem in robotics:
adapting the amount of force needed to hold and manipulate a wide range of
"Humans generally know how to hold a fragile egg
without breaking it," says Javey, who is also a member of the Berkeley
Sensor and Actuator
Center and a faculty
scientist at the Lawrence Berkeley National Lab Materials Sciences Division.
"If we ever wanted a robot that could unload the dishwasher, for instance,
we'd want to make sure it doesn't break the wine glasses in the process. But
we'd also want the robot to be able to grip a stock pot without dropping it.
The UC Berkeley researchers say it is the first such
material made out of inorganic single crystalline semiconductors. Earlier
efforts to develop an artificial skin relied upon organic materials because
they are flexible and easier to process.
"The problem is that organic materials are poor
semiconductors, which means electronic devices made out of them would often
require high voltages to operate the circuitry," says Javey.
"Inorganic materials, such as crystalline silicon, on the other hand, have
excellent electrical properties and can operate on low power. They are also
more chemically stable. But historically, they have been inflexible and easy to
crack. In this regard, work by various groups, including ours, have shown that
miniaturized strips or wires of inorganics can be made highly flexible - ideal
for high-performance, mechanically bendable electronics and sensors."
In a major innovation by the UC Berkeley engineers, nanowire
"hairs" are deposited in a process they describe as a kind of lint
roller in reverse.
They started by growing germanium/silicon nanowires on a
cylindrical drum, which was then rolled onto a sticky substrate. The substrate
they used was polyimide film, but the researchers say the technique can work
with a variety of materials, including other plastics, paper or glass. As the
drum rolled, the nanowires were deposited, or "printed," onto the substrate in
an orderly fashion, forming the basis from which thin, flexible sheets of
electronic materials could be built.
For the e-skin, the engineers printed the nanowires onto an
18-by-19 pixel square matrix measuring 7 cm on each side. Each pixel contained
a transistor made up of hundreds of semiconductor nanowires. Nanowire
transistors were then integrated with a pressure-sensitive rubber on top to
provide sensing functionality. According to the researchers, the matrix
required less than 5V of power to operate and maintained its robustness after
being subjected to more than 2,000 bending cycles.
The e-skin detects pressure from 0 to 15 kPA, a range
comparable to the force used for such daily activities as typing on a keyboard
or holding an object.
"This is the first truly macroscale integration of
ordered nanowire materials for a functional system - in this case, an
electronic skin," says study lead author Kuniharu Takei, post-doctoral
fellow in electrical engineering and computer sciences. "It's a technique
that can be potentially scaled up. The limit now to the size of the e-skin we
developed is the size of the processing tools we are using."
The technology, dubbed "e-skin," could be extended to
prosthetics in the future. That, however, would require significant advances in
the integration of electronic sensors with the human nervous system.