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This soft robot withstands crushing pressures at the ocean’s greatest depths

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Inspired by a strange fish that can withstand the punishing pressures of the deepest reaches of the ocean, scientists have devised a soft autonomous robot capable of keeping its fins flapping — even in the deepest part of the Mariana Trench.

The team, led by roboticist Guorui Li of Zhejiang University in Hangzhou, China, successfully field-tested the robot’s ability to swim at depths ranging from 70 meters to nearly 11,000 meters, it reports March 4 in Nature.

Challenger Deep is the lowest of the low, the deepest part of the Mariana Trench. It bottoms out at about 10,900 meters below sea level (SN: 12/11/12). The pressure from all that overlying water 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 about the equivalent of an elephant standing on top of your thumb,” says deep-sea physiologist and ecologist Mackenzie Gerringer of State University of New York at Geneseo, who was not involved in the new study.

The tremendous pressures at these hadal depths — the deepest ocean zone, between 6,000 and 11,000 meters — present a tough engineering challenge, Gerringer says. Traditional deep-sea robots or manned submersibles are heavily reinforced with rigid metal frames so as not to crumple — but these vessels are bulky and cumbersome, and the risk of structural failure remains high.

To design robots that can maneuver gracefully through shallower waters, scientists have previously looked to soft-bodied ocean creatures, such as the octopus, for inspiration (SN: 9/17/14). As it happens, such a deep-sea muse also exists: Pseudoliparis swirei, or the Mariana hadal snailfish, a mostly squishy, translucent fish that lives as much as 8,000 meters deep in the Mariana Trench.

a deep-sea snailfish
In 2018, researchers described three newly discovered species of deep-sea snailfish (one shown) found in the Pacific Ocean’s Atacama Trench, living at depths down to about 7,500 meters. Also found in the Mariana Trench, such fish are well adapted for living in high-pressure, deep-sea environments, with only partially hardened skulls and soft, streamlined, energy-efficient bodies.Newcastle University

Gerringer, one of the researchers who first described the deep-sea snailfish in 2014, constructed a 3-D printed soft robot version of it several years later to better understand how it swims. Her robot contained a synthesized version of the watery goo inside the fish’s body that most likely adds buoyancy and helps it swim more efficiently (SN: 1/3/18).

But devising a robot that can swim under extreme pressure to investigate the deep-sea environment is another matter. Autonomous exploration robots require electronics not only to power their movement, but also to perform various tasks, whether testing water chemistry, lighting up and filming the denizens of deep ocean trenches, or collecting samples to bring back to the surface. Under the squeeze of water pressure, these electronics can grind against one another.

So Li and his colleagues decided to borrow one of the snailfish’s adaptations to high-pressure life: Its skull is not completely fused together with hardened bone. That extra bit of malleability allows the pressure on the skull to equalize. In a similar vein, the scientists decided to distribute the electronics — the “brain” — of their robot fish farther apart than they normally would, and then encase them in soft silicone to keep them from touching.

robot and snailfish side by side
The design of the new soft robot (left) was inspired by the deep-sea snailfish (illustrated, right), which is adapted to live in the very high-pressure environments of the deepest parts of the ocean. The snailfish’s skull is incompletely ossified, or hardened, which allows external and internal pressures to equalize. Spreading apart the robot’s sensitive electronics and encasing them in silicone keeps the parts from squeezing together. The robots flapping fins are inspired by the thin pectoral fins of the fish (although the real fish doesn’t use its fins to swim).Li et al/ Nature 2021

The team also designed a soft body that slightly resembles the snailfish, with two fins that the robot can use to propel itself through the water. (Gerringer notes that the actual snailfish doesn’t flap its fins, but wriggles its body like a tadpole.) To flap the fins, the robot is equipped with batteries that power artificial muscles: electrodes sandwiched between two membranes that deform in response to the electrical 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 very bottom of the ocean. The robot was allowed to swim freely in the first two trials. For the Challenger Deep trial, however, the researchers kept a tight grip, using the extendable arm of a deep-sea lander to hold the robot while it flapped its fins.

This machine “pushes the boundaries of what can be achieved” with biologically inspired soft robots, write robotocists Cecilia Laschi of the National University of Singapore and Marcello Calisti of the University of Lincoln in England. The pair have a commentary on the research in the same issue of Nature. That said, the machine is still a long way from deployment, they note. It swims more slowly than other underwater robots, and doesn’t yet have the power to withstand powerful underwater currents. But it “lays the foundations” for future such robots to help answer lingering questions about these mysterious reaches of the ocean, they write.

Researchers successfully ran a soft autonomous robot through several field tests at different depths in the ocean. At 3,224 meters deep in the South China Sea, the tests demonstrated that the robot could swim autonomously (free swim test). The team also tested the robot’s ability to move under even the most extreme pressures in the ocean. A deep-sea lander’s extendable arm held the robot as it flapped its wings at a depth of 10,900 meters in the Challenger Deep, the lowest part of the Mariana Trench (extreme pressure test). These tests suggest that such robots may, in future, be able to aid in autonomous exploration of the deepest parts of the ocean, the researchers say.

Deep-sea trenches are known to be teeming with microbial life, which happily feed on the bonanza of organic material — from algae to animal carcasses — that finds its way to the bottom of the sea. That microbial activity hints that the trenches may play a significant role in Earth’s carbon cycle, which is in turned linked to the planet’s regulation of its climate.

The discovery of microplastics in Challenger Deep is also incontrovertible evidence that even the bottom of the ocean isn’t really that far away, Gerringer says (SN: 11/20/20). “We’re impacting these deep-water systems before we’ve even found 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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