UCLA's 11-Centimeter Frog Robot Jumps Using Twisted Elastic Rods

The research comes from teams at UCLA and the University of Michigan and was published in Science Advances. At its core, the robot uses bent elastic rods: by twisting these rods, it stores elastic energy and then releases it in a sudden jump, driving the small machine forward.

The robot measures 11 centimeters long and weighs 98.2 grams. It is driven by a pair of spiral-shaped elastic rods at its rear. A low-power motor slowly tightens the rods; once they reach a critical point, the stored energy is released abruptly, pushing the robot into a forward leap.

The mechanism is notable because the robot does not rely on a high-power actuator for the jump itself. The motor gradually loads the elastic rods, and the rods provide the rapid burst of motion.

The researchers tested the robot on six surfaces: wood, fabric, acrylic, leather, grass, and sand. Across these surfaces, it achieved an average speed of 2.46 body lengths per second. On wood, the robot reached a peak speed of 3.21 body lengths per second.

For comparison, a version of the same robot with rigid legs averaged only 0.79 body lengths per second. The difference was especially clear on fabric and grass. On those surfaces, the rigid-leg version nearly stopped, while the elastic-rod robot was still able to keep jumping.

The team also attached a pair of flexible fins to the robot. With the fins, it could swim at about 0.5 body lengths per second. It could also turn and avoid obstacles. This swimming speed is in the same order of magnitude as that of a real frog.

A key design principle from the study is that shape, not size, determines whether an elastic rod deforms slowly or snaps suddenly. This finding means the same design rules can be scaled down to millimeter-scale robots. The researchers emphasize that miniaturization should not change the fundamental jumping behavior.

Traditional small jumping robots often depend on rigid legs and complex mechanical structures. Those designs can easily fail on soft or uneven ground. The elastic-rod approach works differently: it stores and releases energy through bending and twisting of the material, which makes the robot more adaptable to different surfaces.

The research team believes this design approach could be used to develop small robots that move through complex terrain. Possible applications include post-disaster search and rescue and environmental monitoring. In such settings, small robots may need to traverse rubble, vegetation, sand, and other unpredictable surfaces.

The study was published in Science Advances, and the collaboration involved UCLA and the University of Michigan. The robot's ability to jump and swim suggests that elastic instabilities can be a practical alternative to conventional rigid mechanisms in small-scale robotics.

The robot's jumping performance depends on the interaction between the elastic rods and the surface. On hard surfaces like wood, the robot can push off efficiently and reach higher speeds. On soft surfaces like fabric and grass, the rigid-leg version loses traction, but the elastic-rod version can still release energy rapidly enough to jump repeatedly.

According to the researchers, the shape of the elastic rod can be tuned to control whether it bends gradually or snaps through. This tunability allows the same basic mechanism to work across different scales and surfaces. It also reduces the need for complex gears or joints.