Transparent eel-like soft robot can swim silently underwater

Scientists have developed an innovative, transparent eel-like robot that can swim silently in salt water without an electric motor. The robot created by engineers and marine biologists at the University of California in the US uses artificial muscles filled with water to propel itself. The foot-long robot, which is connected to an electronics board that remains on the surface, is also virtually transparent.

The bot, described in the journal Science Robotics, is an important step towards a future when soft robots can swim in the ocean alongside fish and invertebrates without disturbing or harming them. Today, most underwater vehicles designed to observe marine life are rigid and submarine-like and powered by electric motors with noisy propellers, researchers said.

"Instead of propellers, our robot uses soft artificial muscles to move like an eel underwater without making any sound," said Caleb Christianson, a PhD student at the University of California San Diego. One key innovation was using the salt water in which the robot swims to help generate the electrical forces that propel it. The bot is equipped with cables that apply voltage to both the salt water surrounding it and to pouches of water inside of its artificial muscles.

The robot's electronics then deliver negative charges in the water just outside of the robot and positive charges inside of the robot that activate the muscles. The electrical charges cause the muscles to bend, generating the robot's undulating swimming motion. The charges are located just outside the robot's surface and carry very little current so they are safe for nearby marine life. "Our biggest breakthrough was the idea of using the environment as part of our design," said Michael T Tolley, a professor UC San Diego.

"There will be more steps to creating an efficient, practical, untethered eel robot, but at this point we have proven that it is possible," said Tolley. Previously, other research groups had developed robots with similar technology. 

However, to power these robots, engineers were using materials that need to be held in constant tension inside semi-rigid frames. The new study shows that the frames are not necessary. "This is in a way the softest robot to be developed for underwater exploration," Tolley said.