Watch this robotic manta ray speed through the water

Building a robot is easy. Building a robot with soft, bendable parts is still doable. But building a soft robot “fish” that can swim as well as the real thing: a much trickier task. But now, a team of scientists in China has gotten much closer, creating a robotic fish that can swim twice as fast as the next best bot of its kind. The 9.3-centimeter-long fish (18.5 centimeters including its tail) can also swim for more than 3 hours on one battery charge, thanks to a clever propulsion system that acts like a muscle. The new system could lead to lifelike bots that can explore the ocean, monitor water quality, and discover new creatures.

The robotics labs of the world are already swimming in soft robots made of bendy polymers, like the octobot. Some, like a previously developed robotic stingray, are enhanced with living cells. But most swimming bots are powered by rigid motors. The motors turn propellers or flap fins, which also make it difficult for the robots to navigate in tight spaces, limiting where they can go. Some swimming bots use alternate methods of movement: soft actuators that flex like muscles. One particular kind of actuator—a dielectric elastomer actuator (DEA)—is especially fast and efficient. Using a DEA underwater was long thought to be a nonstarter, though, because the thick electrical insulation assumed to be required would hamper its flexing ability.

But engineer Tiefeng Li of Zhejiang University in Hangzhou, China, wanted to give it a go anyway. So he came up with a counterintuitive solution: Use the surrounding water itself as part of the bot's electrical circuitry—specifically as a "ground electrode" whose voltage stays constant while that on another electrode inside varies. People assumed that electricity leaking into the water would be dangerous for the researchers and anything near the bot. It turns out that water doesn’t conduct electricity well enough for that to be an issue. “The water is used as the ground end electrode, so it will not have the problem of current leakage and shocking people,” Li says.

The new robot, which was inspired by the manta ray, looks a bit like a jelly doughnut, with wings and a tail. Its outer layer of soft silicone is filled with a pocket of hydrogel connected to an electrode. When voltage is applied to the electrode, it attracts electrons, making the rest of the hydrogel positively charged. This gel is then attracted to negatively charged electrons in the water outside the robot. As the positive gel inside and the negative water outside try to get closer, they squeeze the body in between, flattening it and bringing the fins slightly upward. Remove the voltage from the inner electrode, and the body returns to its original configuration. Pulsing the electricity makes the fins flap up and down like wings.  

“For researchers, it is common sense that an actuator of this kind has to be paired with at least two electrodes,” says Jun Shintake, an engineer at École Polytechnique Fédérale de Lausanne in Switzerland, who has used such actuators in soft robotic fish but was not involved in this research. “Tiefeng Li had the bright idea of using the surrounding water as one side.”

Jennifer Lewis, a materials engineer at Harvard University who worked on the octobot, calls the new robot a “clever, multimaterial integration and design strategy.” She adds, “It’s a very elegant demonstration—one that will further propel the field of soft robotics—no pun intended!”

The robot is also wireless, and its remote-controlled tail makes it highly agile. It is strong enough to carry a camera, and it can function in temperatures from less than 0.4°C to at least 74.2°C. What’s more, most of its materials can be transparent, making it nearly invisible, the researchers report today in Science Advances.

Given that the fish is wireless, speedy, and temperature-resistant, Li sees many uses for its descendants. They could monitor the temperature and chemical components of oceans, rivers, and lakes, sensing when a toxic spill takes place, for example. They could explore shipwrecks and search for crashed planes and bodies. And Li suspects that the soft, stealthy robot wouldn’t startle ocean creatures in the same way that other underwater bots do, making it easy to sneak through coral reefs without damaging them.