An eco-friendly aquatic robot made from fish food holds promise for environmental monitoring

An edible robot made by EPFL scientists leverages a combination of biodegradable fuel and surface tension to zip around the water’s surface, creating a safe and nutritious alternative to environmental monitoring devices made from artificial polymers and electronics.

A paper describing this work is published in the journal Nature Communications.

The boat-shaped robot takes advantage of the same phenomenon—the Marangoni effect—used by some aquatic insects to propel themselves across the surface of water. A chemical reaction within a tiny detachable chamber produces carbon dioxide gas, which in turn enters a fuel channel, forcing the fuel out. The sudden reduction in water surface tension caused by the ejected fuel then propels the robot forward.

This clever design is not only effective, allowing the robots to move freely around the surface of the water for several minutes, but also entirely non-toxic and biodegradable. Indeed, some may recognize the components of the triggering chemical reaction, citric acid and sodium bicarbonate, as the same ones used in a typical school science experiment involving a volcano. The fuel is propylene glycol—a liquid commonly found in skin care products.

“While the development of miniature swimming robots for natural environments has progressed rapidly, these typically rely on plastics, batteries, and other electronics, which pose challenges for mass deployment in sensitive ecosystems,” says EPFL Ph.D. student Shuhang Zhang. “In this work, we show how those materials can be replaced by completely biodegradable and edible components.”

Zhang and a team from the Laboratory of Intelligent Systems were led by Dario Floreano in the School of Engineering.

Bioinspired locomotion

The robot is designed to be not only harmless to aquatic fauna, but even beneficial. To add strength and rigidity to the outer structure, which is about 5 cm long, the researchers used fish food with a 30% higher protein content and 8% lower fat content than commercial pellets. The device can therefore act as nourishment for aquatic wildlife at the end of its lifetime, just as animals do.

The EPFL team envisions the robots being deployed in large numbers. Each device would be equipped with biodegradable sensors for collecting environmental data like water pH, temperature, pollutants, and the presence of microorganisms, which could be read out after collection or by remote sensing.

Rather than controlling precisely the directional movement of the robots, the team fabricated left-turning and right-turning variants by altering the fuel channel’s asymmetric design. This level of control is all that is needed to disperse the robots across the water’s surface, and their pseudo-random movements mimic those of insects, making them ideal vessels for delivering nutrients or medication to fish.

The researchers even speculate that the robots could stimulate the cognitive development of aquatic pets, but future research is needed to explore this, as no animal experiments were conducted as part of the EPFL study.

A robotic food frontier

This work is the latest innovation in the burgeoning field of edible robotics. The Laboratory of Intelligent Systems has already published several papers on edible devices, including edible soft actuators as food manipulators and pet food, fluidic circuits for edible computation, and edible conductive ink for monitoring crop growth. Floreano has also published a perspective on robotic food with colleagues from the RoboFood consortium: a project he coordinates that was launched in 2021 to explore the potential of these devices.

“The replacement of electronic waste with biodegradable materials is the subject of intensive study, but edible materials with targeted nutritional profiles and function have barely been considered, and open up a world of opportunities for human and animal health,” Floreano says.