Marine flatworms are cool. Hear me out. They’ve mastered the art of smooth undulating motion and perfected it over millions of years of evolution. They can glide efficiently through water by undulating their thin, flat bodies in an almost ethereal way. Our best robots are nowhere near that performance — but they’re making progress.

Inspired by these natural swimmers, scientists at the École Polytechnique Fédérale de Lausanne (EPFL) have designed a highly agile, untethered robotic swimmer that mimics their movements. This centimeter-scale, soft-bodied robot could be used in environmental monitoring, aquaculture, and maybe even for exploring alien worlds.

Inspiration From Nature

Swimming robots aren’t a novelty. They’re already used to map pollution, monitor water quality, and study aquatic ecosystems like corals and lakes. But these devices typically use noisy propellers that are disturbing to wildlife, and they’re not very efficient either. Such robots are bulky and find it hard to maneuver around the natural chaos in these environments (like plants, animals, or debris).

That’s why robot researchers have long aimed to mimic the efficiency of nature, particularly in aquatic environments. The new robot, which basically looks like a PCB board with wings, ranges from 25 mm to 45 mm in length. And it’s a game-changer. It achieves high-speed movement both in tethered (12 cm/s) and untethered (5.1 cm/s) modes, demonstrating unmatched agility in its size category.

Unlike many previous designs, which relied on miniature DC electrical motors or hydrogels, this robot integrates soft electrohydraulic actuators. These are flexible, capacitive devices that use an applied electric field to create Maxwell stress, causing the electrodes to zip together and displace a liquid dielectric, resulting in controlled bending or undulating motion. These actuators provide a powerful yet lightweight propulsion system. They consume less than 35 mW of power and function for over 750,000 cycles before showing signs of wear.

“In 2020, our team demonstrated autonomous insect-scale crawling robots, but making untethered ultra-thin robots for aquatic environments is a whole new challenge,” says EPFL Soft Transducers Lab head Herbert Shea. We had to start from scratch, developing more powerful soft actuators, new undulating locomotion strategies, and compact high-voltage electronics.

Biomimicry and Beyond

Nature has already solved many of the engineering problems associated with aquatic locomotion. The EPFL researchers took inspiration from polyclads (marine flatworms), which use continuous undulations to glide effortlessly through water. Their robot replicates this movement by generating over 1.5 wavelengths along its fins, making it more stable and efficient than most artificial undulatory swimmers.

But the key innovation comes from geometry more than physics.

The robot’s flat structure is what makes all the difference. Measuring only 500 micrometers thick, it floats on the water surface due to surface tension, allowing it to carry additional weight with the aid of buoyant elements. The use of soft electrohydraulic actuators enables independent control of each fin, facilitating precise directional movement — a crucial advantage for applications requiring complex navigation.

The design doesn’t stop at imitating nature, however. The robot can flap its fins ten times faster than fish, achieving better directional control.

“Our design doesn’t simply replicate nature; it goes beyond what natural organisms can achieve,” explains former EPFL researcher Florian Hartmann, now a research group leader at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany.

Real-World Applications

These robots could essentially function like underwater drones, whether to detect pollutants, microplastics, or harmful algal blooms, or to study aquatic life with minimal disturbance.

As researchers refine this technology, we may soon see swarms of these miniature robots autonomously patrolling lakes, rivers, and oceans, collecting data, and assisting in environmental preservation efforts.

Although the researchers don’t mention this directly, we could even see this type of technology deployed on other worlds. Both Enceladus (a moon of Saturn) and Europa (a moon of Jupiter) are believed to harbor vast subsurface oceans beneath their thick ice crusts. These saltwater oceans, kept liquid by gravitational forces that generate heat, are among the most promising places in the solar system to look for extraterrestrial life, but we’d need some technology to explore them. Perhaps, something that moves like a worm.

The study was published in Science Robotics.