26 THE SCIENTIST | the-scientist.com
JACOBS SCHOOL OF ENGINEERING, UC SANDIEGO; ©WIKIMEDIA COMMONS, KILSNOTEBOOKwater structures, constructing pipelines,
and drilling. Scientists, meanwhile, are
increasingly using the technology for envi-
ronmental sensing, biological research,
and finding ways to protect marine
resources.
But most ocean-going robots come
with a catch: they are noisy. The decibel
levels and vibrations of the propellers and
jets scare away fish and other animals.
What’s more, most robots are built with
metallic frames, which can bump into
and damage fragile ocean life. Christian-
son decided to take a different approach,
with a design incorporating “muscles that
don’t have any rigid materials to them,” he
says. “They’re just completely soft.”
In May 2016, Christianson and his
colleagues at UCSD set out to build this
soft robot. They weren’t the first to look to
the natural world for this purpose—other
research groups have used fish and manta
rays as the basis for marine-robot design.
But Christianson decided to focus on eel
larvae, finding the wavelike motion of the
fish to be an ideal model for the technology
they were working on in the lab. What’s
more, replicating the larva’s transparency
would help the team’s creation blend into
the ocean environment.
To create their eel-inspired robot, the
researchers used transparent dielectric elas-
tomer actuators (DEAs)—“pieces of rubber
that respond to an electrical stimulation,”
Christianson explains. While the rubber
itself doesn’t conduct electricity, a voltage
applied to the upper and lower surfaces of
the elastomer causes it to flatten, making the
material both thinner and longer.
To achieve movement, each individual
DEA, or artificial muscle, required two
electrodes: one, a small pouch of water
inside the rubber, and the other, the water
surrounding the robot in a tank in the lab.
Initially, the researchers started with
two actuators, which, placed back-to-back,
had the ability to bend to the right or to
the left. To achieve more-fluid motion and
improve propulsion, the team next placed
three of these actuator pairs end-to-end,
creating a 22-cm-long, 5-cm-high, and
1.5-mm-thick robot. “We arranged several
of these in such a way that we can get anundulating motion to mimic the motion
of an eel swimming,” says Christianson.
By applying voltage to the six actuators in
diagonal pairs from the front to the back
of the eelbot via external cables, Christian-
son’s group was able to prod the robot into
eel-like motion around the tank.
It was at this point in the project, around
June 2017, that Dimitri Deheyn, a marine
biologist at UCSD’s Scripps Institution of
Oceanography, became involved in this
research. To see how closely the robot resem-
bled its inspiration, Deheyn used a hyper-
spectral imaging system that gives every
single pixel of the sample its own spectral
identity. The more light going through the
robot, the less conspicuous it would be in the
open ocean. “We were able to have a highpercentage of light going through, close to
85 to 95 percent or more, indicating that it
is truly a transparent robot as opposed to
a translucent robot,” says Deheyn. In areas
with a reduced passage of light, the trans-
parency matched that of the bony bits of an
eel larva (Sci Rob, 3:eaat1893, 2018).
The researchers also found that they
could reproduce another aspect of eel biol-
ogy in their robot—fluorescence—by adding
a dye to the fluid pouch electrode. Certain
species of eel naturally fluoresce, possiblySOFT AND SLEEK: Researchers have
designed a marine robot that blends in with its
environment (above) and moves by contracting
artificial muscles to perform an undulating
motion resembling that of an eel larva (below).