Soft robots do a lot of things well but they're not exactly known for their speed. The artificial muscles that move soft robots, called actuators, tend to rely on hydraulics or pneumatics, which are slow to respond and difficult to store.
Dielectric elastomers, soft materials
that have good insulating properties, could offer an alternative to pneumatic
actuators but they currently require complex and inefficient circuitry to
deliver high
voltage as well as rigid components to maintain their form—both of
which defeat the purpose of a soft robot.
Now, researchers at the Harvard John A.
Paulson School of Engineering and Applied Sciences (SEAS) have developed a dielectric
elastomer with a broad range of motion that requires relatively low voltage and
no rigid components. They published their work recently in Advanced
Materials.
"We think this has the potential
to be the holy grail of soft robotics," said Mishu Duduta, a
graduate student at SEAS and first author of the paper. "Electricity is
easy to store and deliver but until now, the electric fields required to power
actuators in soft robots has been too high. This research solves a lot of the
challenges in soft actuation by reducing actuation voltage and increasing
energy density, while eliminating rigid components."
Duduta co-authored the paper with
Robert Wood, the Charles River Professor of Engineering and Applied Sciences
and David Clarke, the Extended Tarr Family Professor of Materials.
In building a new dielectric elastomer,
the team combined two known materials that worked well individually—an
elastomer based on one developed at UCLA that eliminated the need for rigid
components and an electrode of carbon nanotubes developed in the Clarke Lab.
The complementary properties of these two materials enabled the new device to
outperform standard dielectric elastomer actuators.
Most dielectric elastomers have limited
range of motion and need to be pre-stretched and attached to a rigid frame.
Starting with an elastomer that doesn't need to be pre-stretched, developed by
researchers at UCLA, the modified materials begin as liquids and can be cured
rapidly under UV light to produce paper-thin sheets. They are sticky—like
double-sided tape—so they can adhere well to each other, and to the electrodes.
For the electrodes, the team replaced
carbon grease, which is typically used as an electrode in dielectric
elastomers, with a mat of thin carbon nanotubes. The nanotubes neither increase
the stiffness of the elastomer nor decrease the energy density—meaning the
elastomer can still stretch and provide significant force. The team fabricated the
elastomers one on top of the other, creating a multilayer sandwich of
elastomer, electrode, elastomer, electrode and so on. In this way, each
electrode gets double usage, powering the elastomer above and below.
"The voltage required to
actuate dielectric elastomers is directly related to the thickness of the
material, so you have to make your dielectric elastomer as thin as
possible," said Duduta. "But really thin elastomers are flimsy and
can't produce force. A multilayer elastomer is much more robust and can
actually provide significant force." "The significance of this work
is that the combination of materials and processing enables two of the current
technical limitations of dielectric elastomers—the need for high voltage and
pre-stretch—to be overcome," said Clarke.
This type of actuator could be used
in everything from wearable devices to soft grippers, laparoscopic surgical
tools, entirely soft robots or artificial
muscles in more complex robotics.
"Actuation is one of the most
difficult challenges in robotics," said Wood. "The vast majority of
existing robots rely on conventional electromagnetic rotary motors. In cases
where we cannot use such motors, for example in soft robots, there are few
alternatives for high performance actuation. This breakthrough in
electrically-controlled soft actuators brings us much closer to muscle-like
performance in an engineered system and opens the door for countless
applications in soft robotics."
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