ARTIFICIAL ‘MUSCLES’
ACHIEVE POWERFUL ROBOTIC PULLING FORCE
New MIT system of contracting fibres could be a boon for biomedical devices and robotics.
As a cucumber plant
grows, it sprouts tightly
coiled tendrils that
seek out supports in
order to pull the plant upward. This
ensures the plant receives as much
sunlight exposure as possible. Now,
researchers at MIT have found a way
to imitate this coiling-and-pulling
mechanism to produce contracting
fibres that could be used as artificial
muscles for robots, prosthetic limbs,
or other mechanical and biomedical
applications.
While many different approaches
have been used for creating artificial
muscles, including hydraulic
systems, servo motors, shapememory
metals, and polymers
that respond to stimuli, they all
have limitations, including high
weight or slow response times.
The new fibre-based system, by
contrast, is extremely lightweight
and can respond very quickly, the
26 August 2019
researchers say. The findings are
being reported today in the journal
Science.
The new fibres were developed
by MIT postdoc Mehmet Kanik
and MIT graduate student Sirma
Örgüç, working with professors
Polina Anikeeva, Yoel Fink, Anantha
Chandrakasan, and C. Cem Taşan,
and five others, using a fibredrawing
technique to combine two
dissimilar polymers into a single
strand of fibre.
The key to the process is mating
together two materials that have
very different thermal expansion
coefficients - meaning they have
different rates of expansion when
they are heated. This is the same
principle used in many thermostats,
for example, using a bimetallic strip
as a way of measuring temperature.
As the joined material heats up, the
side that wants to expand faster is
held back by the other material. As
a result, the bonded material curls
up, bending toward the side that is
expanding more slowly.
Using two different polymers
bonded together, a very stretchable
cyclic copolymer elastomer and
a much stiffer thermoplastic
polyethylene, Kanik, Örgüç and
colleagues produced a fibre that,
when stretched out to several times
its original length, naturally forms
itself into a tight coil, very similar
to the tendrils that cucumbers
produce. But what happened next
actually came as a surprise when
the researchers first experienced
it. “There was a lot of serendipity in
this,” Anikeeva recalls.
As soon as Kanik picked up the
coiled fibre for the first time, the
warmth of his hand alone caused
the fibre to curl up more tightly.
Following up on that observation,
he found that even a small increase
in temperature could make the coil
tighten up, producing a surprisingly
strong pulling force. Then, as soon
as the temperature went back
down, the fibre returned to its
original length. In later testing, the
team showed that this process of
contracting and expanding could be
repeated 10,000 times “and it was
still going strong.”
One of the reasons for that
longevity, she says, is that
“everything is operating under very
moderate conditions,” including
low activation temperatures.
Just a 1-degree Celsius increase
can be enough to start the fibre
contraction.
The fibres can span a wide range
of sizes, from a few micrometres
(millionths of a metre) to a few
millimetres (thousandths of a
metre) in width, and can easily be
ROBOTICS AND AUTOMATION