A new autonomous, untethered aquatic robot inspired by marine flatworms is setting performance benchmarks by navigating tight, cluttered spaces and pushing objects 16 times its weight.
Named SOMIRO (Soft Milli Robot), this miniature swimming robot, developed between 2021 and 2023, has potential applications in agriculture, pollution monitoring, and exploration. Unlike previous bulky, tethered models, SOMIRO’s streamlined design allows it to weave through plants and debris that would have once obstructed similar robotic systems.
A New Approach to Aquatic Robotics
SOMIRO measures just 45 millimeters in length and 55 millimeters in width, weighing only 6 grams when untethered and carrying its own power supply. Its soft, flexible fins—each just 1 millimeter thick—drive movement through undulation, creating powerful waves to propel the tiny robot.
Each fin is equipped with an independent actuator, allowing for precise maneuvering, including quick turns and multidirectional swimming—forward, backward, and sideways.
“The greatest challenge was to create propulsion with an undulating fin, using only a single artificial muscle,” lead author Florian Hartman of the École Polytechnique Fédérale de Lausanne explained to The Debrief.
“Our first approach, similar to many other larger swimming robots, was to use multiple artificial muscles (or actuators) to produce a traveling wave in a soft fin. However, we could not achieve our ambitious goal of developing a robot smaller than 5 centimeters by following this approach.”
“A larger number of artificial muscles requires space and more complex control, hence larger electronics circuit boards,” Hartman continued. “We wanted to simplify the system as much as possible but achieve complex motion, as seen in nature. We overcame this challenge by developing miniaturized artificial muscles and specifically tuning their resonance behavior. The rapid oscillations of the artificial muscle could now excite a traveling wave in a soft fin, which resulted in propulsion.”
Breaking Speed Records for Small Aquatic Robots
SOMIRO’s lightweight power source and integrated sensors enable untethered, autonomous operation, but it can also function with a tethered power source when necessary. While operating untethered allows greater flexibility, it comes with a trade-off in performance.
“The fastest version of the robot swims up to 12 centimeters per second,” Hartman said. “That might not sound like much at first, but the robot is only 4.5 centimeters long. It therefore reaches a speed of 2.6 body lengths per second relative to its size.”
“The fastest speed is achieved when the robot is tethered to thin cables through which energy is delivered,” Hartman added. “In an untethered version, where the robot carries its own power supply, it achieves a speed of 5 centimeters per second, or 1.1 body lengths per second.”
Testing SOMIRO’s Strength and Agility
During testing, researchers demonstrated SOMIRO’s ability to swim rapidly while avoiding grassy obstructions in a simulated rice field. One of its most impressive feats was pushing objects many times its own weight before continuing its path.
Additionally, the robot’s integrated sensors successfully guided it toward both stationary and moving light sources during field trials, showcasing its autonomous navigation capabilities.
“This agile robotic swimmer combines advances in propulsion performance, robustness, and functionality, and will help to inspire future miniaturized vehicles operating in natural aquatic environments,” Hartmann et al. write.
The Next Steps in Aquatic Robot Advancements
Hartman discussed with The Debrief where his team’s research is headed next, noting that while SOMIRO is currently designed to swim on the water’s surface, the team is currently developing a next-generation version capable of submarine operation.
To achieve this, his team is redesigning the power supply and actuators for longer operation times, with plans for deep-sea field testing in the near future. The robot’s compact size would allow it to explore coral reefs and other remote underwater ecosystems where human access is limited.
“Our current robot is designed to swim on the water’s surface, which is perfect for applications that benefit from communicating data with stationary infrastructure. Additionally, one could mount a solar cell to make it energy-autonomous,” Hartman said.
“However, for applications that require operation underwater, the robot would need improved artificial muscles and a reworked power supply,” Hartman added.
“When we tested the propulsion mechanism underwater, we realized that effective oscillation requires more complex control signals to drive the artificial muscles.
“In the next iteration, we will slightly increase the size of the robot to make space for more electronic components capable of generating arbitrary control signals,” Hartman said.
The paper “Highly Agile Flat Swimming Robot” appeared on February 19, 2025 in Science Robotics.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.