Nanopower. Yarn made of intertwined carbon nanofibers and wax can expand and contract like muscle.
Credit: Images Courtesy of Science/AAAS
Here's a twist: Scientists have designed a flexible, yarnlike
artificial muscle that can also pack a punch. It can contract in 25
milliseconds—a
fraction of the time it takes to blink an eye—and can generate power
85 times as great as a similarly sized human muscle. The new muscles
are made of
carbon nanotubes filled with paraffin wax that can twist or stretch
in response to heat or electricity. When the temperature rises, the wax
melts and
forces the nanotubes to contract. Such artificial muscles, the
researchers say, could power smart materials, sensors, robots, and even
devices inside the
human body.
It's hard to make a smart muscle: an artificial muscle that is
simultaneously efficient, fast, powerful, and able to twist and turn.
But such muscles would
be a great boon to numerous industries, including robotics and smart
sensors, because they can turn power into movement on a tiny scale.
Seeking a strong,
flexible material, scientists have turned to carbon nanotubes: long,
hollow cylinders of graphene with unusually strong bonds holding them
together. But
previous carbon nanotube muscles have been electrochemically based:
The muscles were immersed in an electrolyte solution that would conduct
signals to
force the nanotubes to contract.
"The problem with that is you end up needing an electrolyte and an
electrode and a container for all this, and the total volume of the
device ends up being
much larger than the muscle," says materials scientist Ray Baughman
of the University of Texas, Dallas. Moreover, he says, the electrolyte
solution would
degrade over time and the required bags of liquid could leak.
Heave-ho. A miniature Greco-Roman style catapult driven by a wax-filled carbon nanotube yarn.
Video courtesy of The Alan G. MacDiarmid NanoTech Institute, The University of Texas at Dallas
But, Baughman's team realized, if they could instead infuse a
material into carbon nanotubes to control the contraction, they could do
away with the
electrolyte solution. The researchers came up with a simple design:
They soaked nanofibers in wax and then twisted them into yarns. The
arrangement of the
carbon nanofibers in the yarns is similar to the fibers in a finger
trap child's game in which attempting to pull your fingers out of a tube
only tightens
it more. In the case of the carbon nanofibers, the expansion of the
integrated wax shortens the fibers. And the wax's volume can be changed
by altering the
temperature, either using external power sources or in response to the
surrounding environment. The new muscles, the team reports online today
in
Science,
can lift about 100,000 times their own weight—many times more than a
natural human muscle fiber.
"Compared to their size and weight, the performance of these muscles
is spectacular," Baughman says. "And we can do all sorts of things with
them: We can
weave them; we can braid them; we can knit them; we can cut them in
different lengths."
Baughman suggests that the muscles could be useful for providing
power for microfluidics chips, generating precise facial expressions in
robots, and
providing movement in small toys such as robotic fish in an
aquarium. For many other applications—such as those inside the human
body and "smart
fabrics" that could become more porous when the temperature heats up
or contract around an open wound—the muscles will need to be improved
and scaled up
in size.
"The new muscles fill an area that we haven't been able to fill
before," says mechanical engineer Mark Schulz of the University of
Cincinnati in Ohio who
was not involved in the new work. However, Schulz notes, "I think
this is definitely still in a stage of progression. I think we'll start
to see different
geometries and new materials being integrated in. There's a lot of
potential to make it stronger."
The muscles currently work most efficiently at high temperatures, he
points out, which limit their current use in everyday application.
"Designers are
going to have to understand all the properties and then do some
careful analysis to see if it matches the application they're interested
in."
