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This gentle robot withstands overwhelming pressures in the deepest depths of the ocean

Inspired by a strange fish that can withstand the painful pressures of the deepest confines of the ocean, scientists have designed a soft autonomous robot capable of keeping its fins flapping, even in the deepest part of the Marianas Trench.

The team, led by robotist Guorui Li from Zhejiang University in Hangzhou, China, has successfully tested the robot’s ability to swim to depths ranging from 70 meters to nearly 11,000 meters, reports on March 4 in Nature.

Challenger Deep is the lowest of the lowest, the deepest part of the Marianas Trench. Bottom at about 10,900 meters below sea level (SN: 12/11/12). The pressure of all that water on the surface is about a thousand times the atmospheric pressure at sea level, translating to about 103 million pascals (or 15,000 pounds per square inch). “It’s the equivalent of an elephant standing on its thumb,” says Mackenzie Gerringer, a physiologist and deep-water ecologist at New York State University in Genesis who did not participate in the new study.

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The tremendous pressures at these hadhal depths (the deepest oceanic zone, between 6,000 and 11,000 meters) present a tough engineering challenge, Gerringer says. Traditional deep-water robots or manned submarines are heavily reinforced with rigid metal frames so as not to crush, but these ships are bulky and heavy, and the risk of structural failure remains high.

To design robots that can maneuver gracefully through shallow water, scientists have already sought inspiration from soft-bodied oceanic creatures, such as the octopus (SN: 17/09/14). As it happens, there is also such a deep muse: Pseudoliparis swirei, or the mariana hadal snail, a mostly squishy translucent fish that lives up to 8,000 meters deep in Mariana’s grave.

a deep-water snailIn 2018, researchers described three newly discovered species of deep-water snails (one shown) found in the Atacama Trench of the Pacific Ocean, which live at depths of up to 7,500 meters. Also found in the Mariana Trench, these fish are well adapted to living in high-pressure offshore environments, with only partially hardened skulls and soft, streamlined and energy-efficient bodies.University of Newcastle

Gerringer, one of the researchers who first described the deep-water snail in 2014, built a 3D-printed version of the soft robot several years later to better understand it as nothing. His robot contained a synthesized version of the watery goo inside the fish’s body that most likely adds buoyancy and helps him swim more efficiently (SN: 1/3/18).

But devising a robot that can swim under extreme pressure to investigate the deepwater environment is another matter. Autonomous scanning robots require electronics not only to propel their movement, but also to perform various tasks, whether it’s water chemistry testing, lighting, and filming the inhabitants of deep ocean trenches, or collecting samples to bring them back to the surface. Under water pressure, these electronic devices can grind against each other.

So Li and his colleagues decided to borrow one of the snailfish’s adaptations to high-pressure life: its skull isn’t completely fused along with the hardened bone. That extra bit of malleability allows you to equalize the pressure on the skull. In a similar vein, the scientists decided to distribute the electronic devices – the “brain” – of their robot fish farther away than they normally would and then enclose them in soft silicone to prevent them from touching.

robot and snail side by sideThe design of the new soft robot (left) was inspired by the deep-water snail fish (pictured, right), which is adapted to live in very high-pressure environments in the deepest parts of the ocean. The skull of the snail is incompletely ossified or hardened, which allows to match the external and internal pressures. Separating sensitive electronics from the robot and wrapping them in silicone prevents the parts from tightening. Fin flapping robots are inspired by the thin pectoral fins of fish (although real fish do not use their fins to swim).Li et al / Nature 2021

The team also designed a soft body that slightly resembles the snail fish, with two fins that the robot can use to propel itself through the water. (Gerringer observes that the snail fish does not beat its fins, but twists its body like a tadpole.) To beat its fins, the robot is equipped with batteries that feed artificial muscles: electrodes interspersed between two membranes that deform in response to the electric charge.

The team tested the robot in several environments: 70 meters deep in a lake; about 3,200 meters deep in the South China Sea; and finally at the bottom of the ocean. The robot let it swim freely in the first two tests. However, for the Challenger Deep trial, the researchers maintained a strong grip, using the extendable arm of an offshore lander to hold the robot while flapping its fins.

This machine “pushes the limits of what can be achieved” with biologically inspired soft robots, write robotocysts Cecilia Laschi of Singapore National University and Marcello Calisti of Lincoln University in England. The couple has a comment on the research in the same issue of Nature. That said, the machine is still a long way from deployment, they note. Nothing slower than other underwater robots and still does not have the power to withstand strong underwater currents. They write “but lay the groundwork” for future robots to help answer persistent questions about these mysterious ocean rivers.

The researchers successfully performed a smooth autonomous robot using several field tests at different depths of the ocean. At 3,224 meters deep in the South China Sea, tests showed that the robot could swim autonomously (free swimming test). The team also tested the robot’s ability to move to the most extreme pressures of the ocean. The outstretched arm of a deep-water lander held the robot as it fluttered its wings to a depth of 10,900 meters on the Challenger Deep, the lowest part of the Mariana Trench (extreme pressure test). These tests suggest that these robots may, in the future, aid in autonomous exploration of the deepest parts of the ocean, the researchers say.

Deep-water trenches are known to be full of microbial life, happily feeding on the bonanza of organic matter — from algae to animal carcasses — found at the bottom of the sea. That microbial activity suggests that trenches may play a significant role in the Earth’s carbon cycle, which is linked to climate regulation by the planet.

The discovery of microplastics in Challenger Deep is also incontrovertible proof that even the ocean floor isn’t really that far away, says Gerringer (SN: 20/11/20). "We're impacting these deepwater systems before we find out what's down there. We have a responsibility to help connect these seemingly otherworldly systems, which are really part of our planet."

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