SNAKE GAIT SCALED
Snakes have got to be some of the most creatively mobile animals ever. They can move fast and
stealthily. They’re good climbers and swimmers, and
can squeeze into very small holes. Some of them can
even fly (well, a little bit). Snakes can do all of this —
despite looking like a lizard that’s missing 100 percent
of its limbs.
Roboticists have been working on snake robots
for a long time, with a focus primarily on versatile
mobility in constrained spaces. We’ve seen a variety of
limbless robots that can mimic snake “gaits” fairly well,
however, it’s not just the lack of limbs that makes
snakes so special. It’s also their scales.
In a recent article in Science Robotics, researchers
from Katia Bertoldi’s group at Harvard show how mimicking snake scales with kirigami-inspired
deformable materials enabled them to make a limbless soft robot that can crawl by simply inflating
and deflating itself over and over.
A snake’s scales are all pointed the same direction, providing a substantial amount of favorable
friction that makes it easier for the snake to move forward rather than backwards. This makes
moving backwards inconvenient, but it also means that the snake is able to achieve forward motion
by generating a wave along its belly that first pulls its scales forward and then pushes them
The scales are effectively slippery when they move one way and sticky when they move the
other, so snakes are able to move forward as long as they can get a grip on a surface.
The Harvard researchers leveraged these “anisotropic frictional properties” of snake scales to
turn the repetitive pulsing motion of an inflatable soft robot into forward motion. This is in much the
same way that snakes can crawl forward on their bellies without using their trademark side-to-side
Interestingly, the team discovered that switching between different shapes of the scales — such
as triangular, circular, trapezoidal, or linear — changed the speed and the efficiency of the crawling
In order to make scaly skin, the researchers manufactured a variety of different stretchable
plastic sheets, each laser engraved with a unique pattern of flat scales. The pattern was structured so
that when the robot inflated itself and the sheets stretched, the flat scales would deform and pop up
away from the robot’s body. This aided in gripping the ground and turning that inflation and
expansion into forward motion. It’s simple, cheap, and effective.
“We believe that our kirigami-based strategy opens avenues for the design of a new class of soft
crawlers that can travel across complex environments for search and rescue, exploration and
inspection operations, environmental monitoring, and medical procedures,” Bertoldi commented.
She also stated that there are no current plans for commercialization of the technology,
although the team does plan to continue developing it. Future steps will involve applying the
principles to different types of soft actuators, such as those based on dielectric elastomers and shape
memory alloys, as well as using kirigami skins to explore and enhance other types of motions.
Robotic crawler with [from
left] linear, circular,
Images courtesy of Ahmad
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