Asymmetric vibration pattern enables ultrafast locomotion of an insect-scale amphibious microrobot

Nature has long mastered the art of moving between worlds. Insects like water striders and whirligig beetles can sprint across the surface of a pond and then scurry up a muddy bank, all without changing their legs or their method of movement. They use the same limbs to push against water and to push against soil, relying on the physics of their environment to do the heavy lifting. For engineers trying to build tiny machines that can do the same, this biological trick has been a stubborn puzzle. Creating a robot small enough to fit in a pocket—specifically, one under five centimeters long—is difficult enough on its own. Making that same machine move quickly and nimbly on both dry land and the surface of water, while carrying its own battery and brain, has seemed nearly impossible. The challenge lies in the fact that land and water demand very different kinds of motion; what works to run on dirt often fails to swim, and vice versa. Usually, solving this requires complex machines with multiple sets of moving parts, which are too heavy and bulky to fit into an insect-sized frame.

A team of researchers has now built a solution that defies this trade-off. They have created an untethered, insect-scale amphibious robot that is just 2.1 centimeters long and weighs only 1.69 grams. This tiny machine can zip across the ground and skim across the water surface with remarkable speed, all while carrying its own power source and wireless control system. The secret to its success is a single, clever movement pattern that works in both environments. Instead of using different legs for swimming and running, the robot uses one set of actuators to drive a pair of limbs that act as both a running leg and a swimming paddle. By vibrating these limbs in a specific, asymmetric rhythm, the robot generates forward motion on land and water without needing to change its hardware or its gait.

The robot's design draws inspiration from the way whirligig beetles swim. These beetles have legs that spread out wide when they push backward to grab water, but fold up tight when they swing forward to reduce resistance. The researchers mimicked this behavior with a swimming paddle attached to the robot's front leg. This paddle is connected by a special hinge that allows it to swing freely in one direction but locks in place in the other. When the robot vibrates, the paddle swings wide during the power stroke to push against the water, creating a strong forward thrust. On the return stroke, the paddle folds up, slicing through the water with minimal drag. This creates a net push that drives the robot forward. On land, the same vibration pattern works differently. The robot's body is tilted slightly, so when the leg swings backward, it hits the ground hard to push the robot forward. When it swings forward, it lifts slightly into the air, avoiding friction. This simple, one-way motion allows the robot to run and swim using the exact same mechanical setup.

To prove this concept works in the real world, the team built a prototype and tested it in a controlled environment. They equipped the robot with a small lithium battery and a wireless control circuit, making it completely self-contained. In tests on a plastic surface, the robot achieved a top speed of 32.5 centimeters per second, which is about 15.5 times its own body length every second. On the water, it reached 11.3 centimeters per second, or 5.4 body lengths per second. These speeds are significantly faster than any other untethered robot of this size has ever managed. The robot also demonstrated impressive agility, capable of turning on the ground at a rate of 760 degrees per second and turning on the water at 450 degrees per second. It could navigate around floating obstacles, push aside light debris, and even climb a gentle slope to move from the water onto the land and back again.

The researchers did not just rely on trial and error; they used computer models to understand exactly how the robot moved. They found that the robot's speed depends heavily on the frequency of its vibrations. When the vibration frequency matches the natural resonance of the robot's structure, the movement becomes most efficient, generating the maximum possible speed. They also discovered that the size of the swimming paddle and the stiffness of its hinge were critical factors. If the paddle was too big or the hinge too stiff, the robot could not move as fast. By fine-tuning these details, they optimized the design to ensure it performed at its peak. The study confirms that it is possible to create a machine that is both incredibly small and incredibly fast, provided it uses a motion strategy that respects the physics of both air and water.

This achievement marks a significant step forward for the field of microrobotics. Until now, most tiny robots that could move on land and water were either too slow, too large, or required a wire to supply power. This new robot shows that a single, simple mechanism can handle the complex demands of two different worlds. It opens the door for future machines that could explore disaster zones, search for survivors in flooded areas, or monitor delicate ecosystems without disturbing them. The researchers noted that while their current robot works well on smooth surfaces, future versions will need to adapt to rougher terrain like sand or grass. They also plan to add visual sensors so the robot can navigate on its own without human control. For now, however, the demonstration of a 2.1-centimeter robot running and swimming at high speeds stands as a clear proof that nature's tricks can be successfully translated into engineering, creating machines that are as agile as the insects that inspired them.