Smallest liquid-fuelled robot
RoBeetle , University of Southern California
0.088 gram(s)
United States (Los Angeles)

The smallest liquid fuelled robot is RoBeetle, developed by roboticists at the University of Southern California in 2020. RoBeetle has four legs, weighs 88 milligrams, and measures just over one centimeter in length. Its body is a fuel tank that can store methanol, which gets released through a vent at the top. A shape-memory alloy wire that runs across the vent reacts with the methanol to move the robot’s legs through a mechanical transmission. RoBeetle’s top speed is 0.76 mm/s, and it can walk for two and a half hours without stopping. The details of RoBeetle's design were published in Science Robotics on 19 August 2020.

For very small robots, the weight of a battery and the associated power conversion electronics can substantially outweigh the robot itself. Liquid fuel has a much higher energy density than batteries, but converting it into useful work in a controlled manner also generally requires an associated system that isn’t suitable for very small robots. RoBeetle uses a shape memory alloy actuator that heats up and expands in the presence of methanol vapor, moving its legs through a linkage. The robot is useful in that it can carry a substantial (230mg) payload.