Walking on water has long been the stuff of biblical lore. Now, researchers say they’ve built machines that can do just that.
Scientists at the University of Virginia have developed a process called HydroSpread, which lets them create tiny, flexible robots directly on water. These paper-thin devices don’t just float — they can move, steer and potentially work alongside humans to test water quality or assist in aquatic search-and-rescue efforts.
“Engineering on-liquid locomotion devices holds immense potential for widespread applications in liquid quality monitoring and aquatic search and rescue,” the team wrote in a paper recently published in Science Advances.
The machines resemble thin, flexible sheets but can flap or scuttle across a pond like mechanical water striders.
Traditionally, making soft robotics has been like cooking crepes: thin films — the flexible material that forms the “skin” of robots — are produced on solid surfaces, then transferred to water. That delicate handoff often damages the films before they can be used. HydroSpread bypasses that problem by starting on the liquid surface itself.
The method involves dropping a special ink onto water, where it naturally spreads into a uniform sheet. That sheet can then be engraved with lasers to carve out shapes and features.
“Fabricating soft, thin films directly on liquid surface provides a straightforward way that allows seamless creation of on-liquid locomotion devices,” the researchers explained. In this approach, the water itself serves as the factory floor.
Once the films are formed, heat is used to trigger controlled bending and buckling, transforming the material into functional robots. Two prototypes emerged: the HydroFlexor and the HydroBuckler.
The HydroFlexor moves like a paddling fish, using finlike flaps that bend and twist when heated. As the fins dip in and out of the water, they generate propulsion and nudge the device forward. The HydroBuckler walks. Its thin “legs” buckle and straighten in cycles.
“By applying a heating stimulus, the free end of fins could bend and penetrate water,” the team wrote. “Once immersed, the heating energy will be rapidly dissipated, leading to bending deformation recovery of fins back to their original configurations at the water surface.”
In tests, both devices moved in straight lines, turned on command and adjusted speed based on the temperature stimulus.
The sight of miniature robots flapping across a pond may seem like nothing more than a simple toy, but the implications are serious. Fleets of these low-cost devices could skim across lakes, rivers or reservoirs carrying sensors to monitor pollutants or detect algal blooms. Because the HydroSpread process is inexpensive and scalable, many could be deployed simultaneously.
Search-and-rescue is another potential application. After floods or hurricanes, water-walking robots could scout conditions, relay information or help guide rescuers. Unlike drones, they don’t need batteries to stay airborne. Unlike boats, they don’t require motors or propellers.
This effort reflects a broader trend in robotics: borrowing from biology. Just as nature has evolved insects that skim across water, engineers are replicating those tricks through materials science.
Biomimicry, especially when paired with artificial intelligence, is becoming an influential design tool. For example, when researchers used AI to study termite mounds, they discovered that air inside the intricate network of tunnels and chimneys reverses direction twice a day because of thermodynamic properties. Those findings have already been applied to architecture. In Harare, Zimbabwe, a commercial building was designed with passive cooling properties modeled on termite mounds, reducing the need for air-conditioning.
The HydroSpread method is not limited to water strider impersonations. Because it can work with different inks and patterns, researchers see it as a platform for a range of soft devices, from floating sensors to electronic skins.
“This cost-effective, scalable approach is compatible with a wide range of polymeric and composite inks, paving the way for broad applications in fabricating soft electronics, sensors, and actuators,” the paper noted. It is, in essence, 3D printing for water-based robotics — with the water itself serving as the printer bed.
The prototypes remain limited. They are tiny, powered by external heating, and not yet ready to patrol coastlines or rescue flood victims. Future versions could integrate materials that respond to electricity, magnetism or light for greater precision.
Researchers say the breakthrough is drawing attention in the scientific community.
“This work eliminates fragile post-fabrication transfers in soft device manufacturing, bridging the gap between soft films and structure fabrication, and establishes a streamlined pathway for designing and deploying functional soft devices directly in liquid environments,” the team wrote.
