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.
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.
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.