Soft Robotic Fish with 3D Printed Components Ready for Marine Life Observation

I am always excited on the days I get to learn about the latest remarkable 3D printing innovation coming out of MIT’S Computer Science and Artificial Intelligence Laboratory (CSAIL). This time, the researchers are taking on soft robotics again, and have introduced SoFi, a soft robotic fish made of silicone rubber with 3D printed components, which can swim independently next to real fish in the ocean in order to study marine life up close.

Earlier this month, scientists published some rare footage of the ancient and elusive Greenland shark, which lives in the Arctic. Their findings show that it’s still not an easy task to document life underwater, regardless of numerous technological advances. But CSAIL’s project could provide the answer, as SoFi was successfully tested in Fiji’s Rainbow Reef at depths of over 50 feet for up to 40 minutes at a time.

Using its undulating tail and a unique ability to control its own buoyancy, SoFi can swim in a straight line, turn, or dive up or down.

The soft robotic fish was able to take high-resolution photos and videos while handling currents at the same time, without disturbing the real fish it was observing.

CSAIL PhD candidate Robert Katzschmann said, “To our knowledge, this is the first robotic fish that can swim untethered in three dimensions for extended periods of time. We are excited about the possibility of being able to use a system like this to get closer to marine life than humans can get on their own.”

Typically, autonomous underwater vehicles (AUVs) are powered by costly, large propellers or tethered to a boat. But SoFi, which is controlled by a waterproofed Super Nintendo controller, is much more lightweight, with only a motor, a camera, and a lithium polymer battery, like you’d find in any smartphone.

The researchers developed a custom acoustic communications system in order to change the robot’s speed and perform turns. The motor pumps water into two balloon-like chambers in the tail, which act like a pair of pistons in an engine to make SoFi swim. While one chamber expands, it flexes and bends in one direction, and when water is pushed to the other chamber with actuators, that channel then moves in the opposite direction, creating a side-to-side motion similar to how a real fish moves.

SoFi System Overview.

The back half of SoFi was made out of flexible plastic and silicone rubber. The head, which holds all of the robot’s electronics, and several other components were 3D printed. The head also contains a small amount of baby oil, which won’t compress from pressure changes, in order to lower the chance of any water leaking in.

The team published a paper on their work, titled “Exploration of Underwater Life with an Acoustically Controlled Soft Robotic Fish,” in the journal Science Robotics. Katzschmann was the lead author, and co-authors include graduate student Joseph DelPreto, former postdoc Robert MacCurdy, now an assistant professor at the University of Colorado at Boulder, and CSAIL director Daniela Rus.

“The authors show a number of technical achievements in fabrication, powering, and water resistance that allow the robot to move underwater without a tether. A robot like this can help explore the reef more closely than current robots, both because it can get closer more safely for the reef and because it can be better accepted by the marine species,” explained Cecilia Laschi, a professor of biorobotics at the Sant’Anna School of Advanced Studies in Italy.

SoFi’s hydraulic system can change its flow patterns in order to attain different tail maneuvers, resulting in a range of swimming speeds; however, the robot’s average speed is roughly half a body length per second. But the real challenge was getting SoFi to swim at different depths.

The robot’s two side fins take care of this, adjusting the pitch up or down for diving. An adjustable weight compartment and buoyancy control unit, which compresses and decompresses air to change density, allow SoFi to vertically adjust its position.

According to the paper, “It is the first instance of a soft fluidic actuator successfully used as a propulsive mechanism for prolonged untethered underwater exploration at multiple depths.”

The soft robotic fish is actually part of a larger body of CSAIL work centered around soft robots, which can be sturdier, safer, and easier to control than rigid ones.

“Collision avoidance often leads to inefficient motion, since the robot has to settle for a collision-free trajectory. In contrast, a soft robot is not just more likely to survive a collision, but could use it as information to inform a more efficient motion plan next time around,” explained Rus, the Andrew and Erna Viterbi Professor of Electrical Engineering and Computer Science at MIT.

(​L): On-board view filming several fish passing by the lens of SoFi. (R): Photo taken by a diver from further behind, showing both the robotic fish and the observed biological fish.

From the communication system’s ultrasonic emissions to the motor’s minimal noise, SoFi was developed to be as non-disruptive as possible to the fish in its surrounding environment.

Rus said, “The robot is capable of close observations and interactions with marine life and appears to not be disturbing to real fish.”

The team, which received support from the National Science Foundation, will now begin several improvements to SoFi, including changes to the tail and body design and improving the pump system to increase its speed. Additionally, they will soon adjust the on-board camera so SoFi can automatically follow real fish.

“We view SoFi as a first step toward developing almost an underwater observatory of sorts. It has the potential to be a new type of tool for ocean exploration and to open up new avenues for uncovering the mysteries of marine life,” Rus said.

Additional SoFis will also be built, so biologists can observe how real fish respond to environmental changes.

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