The foundation for the artificial skin is a flexible organic transistor, made with flexible polymers and carbon-based materials. Credit: L.A. Cicero. |
“Super skin”
is what Stanford researcher Zhenan Bao wants to create. She’s already developed
a flexible sensor that is so sensitive to pressure it can feel a fly touch
down. Now she’s working to add the ability to detect chemicals and sense
various kinds of biological molecules. She’s also making the skin
self-powering, using polymer solar cells to generate electricity. And the new
solar cells are not just flexible, but stretchable—they can be stretched up to
30% beyond their original length and snap back without any damage or loss of
power.
Super skin, indeed.
“With artificial
skin, we can basically incorporate any function we desire,” said Bao, a
professor of chemical engineering. “That is why I call our skin ‘super
skin.’ It is much more than what we think of as normal skin.”
The foundation for the
artificial skin is a flexible organic transistor, made with flexible polymers
and carbon-based materials. To allow touch sensing, the transistor contains a
thin, highly elastic rubber layer, molded into a grid of tiny inverted
pyramids. When pressed, this layer changes thickness, which changes the current
flow through the transistor. The sensors have from several hundred thousand to
25 million pyramids per square centimeter, corresponding to the desired level
of sensitivity.
To sense a particular
biological molecule, the surface of the transistor has to be coated with
another molecule to which the first one will bind when it comes into contact.
The coating layer only needs to be a nanometer or two thick.
“Depending on what
kind of material we put on the sensors and how we modify the semiconducting
material in the transistor, we can adjust the sensors to sense chemicals or
biological material,” she said.
Bao’s team has
successfully demonstrated the concept by detecting a certain kind of DNA. The
researchers are now working on extending the technique to detect proteins,
which could prove useful for medical diagnostics purposes.
“For any
particular disease, there are usually one or more specific proteins associated
with it—called biomarkers—that are akin to a ‘smoking gun,’ and detecting those
protein biomarkers will allow us to diagnose the disease,” Bao said.
Chemical engineering Professor Zhenan Bao presented her work on Feb. 20 at the AAAS annual meeting in Washington, D.C. Credit: L.A. Cicero. |
The same approach would
allow the sensors to detect chemicals, she said. By adjusting aspects of the
transistor structure, the super skin can detect chemical substances in either
vapor or liquid environments.
Regardless of what the
sensors are detecting, they have to transmit electronic signals to get their
data to the processing center, whether it is a human brain or a computer.
Having the sensors run
on the sun’s energy makes generating the needed power simpler than using
batteries or hooking up to the electrical grid, allowing the sensors to be
lighter and more mobile. And having solar cells that are stretchable opens up
other applications.
A recent research paper
by Bao, describing the stretchable solar cells, will appear in Advanced Materials. The paper details the ability of the cells
to be stretched in one direction, but she said her group has since demonstrated
that the cells can be designed to stretch along two axes.
The cells have a wavy
microstructure that extends like an accordion when stretched. A liquid metal
electrode conforms to the wavy surface of the device in both its relaxed and
stretched states.
“One of the
applications where stretchable solar cells would be useful is in fabrics for
uniforms and other clothes,” said Darren Lipomi, a postdoctoral fellow in
Bao’s lab and lead author of the paper.
“There are parts
of the body, at the elbow for example, where movement stretches the skin and
clothes,” he said. “A device that was only flexible, not stretchable,
would crack if bonded to parts of machines or of the body that extend when moved.”
Stretchability would be useful in bonding solar cells to curved surfaces
without cracking or wrinkling, such as the exteriors of cars, lenses and
architectural elements.
The solar cells
continue to generate electricity while they are stretched out, producing a continuous
flow of electricity for data transmission from the sensors.
Bao said she sees the
super skin as much more than a super mimic of human skin; it could allow robots
or other devices to perform functions beyond what human skin can do.
“You can imagine a
robot hand that can be used to touch some liquid and detect certain markers or
a certain protein that is associated with some kind of disease and the robot
will be able to effectively say, ‘Oh, this person has that disease,'” she
said. “Or the robot might touch the sweat from somebody and be able to
say, ‘Oh, this person is drunk.'”
Finally, Bao has
figured out how to replace the materials used in earlier versions of the
transistor with biodegradable materials. Now, not only will the super skin be
more versatile and powerful, it will also be more eco-friendly.
