Applications for mobile, centimeter-scale robots span various fields, including health care, exploration, communication, and agriculture. Imagine an array of tiny robots surveying hazardous or inaccessible environments, for instance. Robots at that scale have long relied on microactuator technologies—typically embodied by piezoelectric bimorphs or dielectric elastomer actuators—whose forces are limited by the low energy density of their power sources, such as batteries. Other microactuators used in robotics, such as electrostatic, thermal, and magnetic varieties, lack either the cycling speed, force, or displacement needed to perform more meaningful work. What’s more, as those actuators shrink in size, so do their power output and load-bearing capacity. (For an example of a paramecium-sized robot whose movement is powered by light, see Physics Today, December 2020, page 66.)
A team of electrical and mechanical engineers led by Cameron Aubin and his thesis adviser Robert Shepherd (both at Cornell University) have now addressed those problems by developing microactuators powered by chemical fuels. The energy density of combustibles such as methanol (22.7 MJ/kg), for example, far exceeds that of lithium batteries (1.0 MJ/kg). Aubin, Shepherd, and their colleagues developed a lightweight (325 mg) microactuator and demonstrated its integration into a working model, the quadrupedal robot shown here, which is 29 mm long and has actuators installed in two forefeet and two hindfeet. Each microactuator can in submillisecond impulses produce forces greater than 9 N—an order of magnitude higher than what existing actuators of similar size, weight, or composition can produce.
To power the microactuators, the researchers designed a 3D-printed combustion chamber, into which a mixture of gaseous methane and oxygen is injected through tubes. The methane is then ignited with a small spark between electrodes in the chamber. The exothermic reaction causes the product gases to expand and inflate—much like in a piston—an elastomer membrane atop the chamber. The explosion can be used to drive an actuator, launch objects, or do other work. The membrane deflates when the gases are vented from the chamber, and the cycle is ready to repeat.
The researchers demonstrated actuator displacements of 140%, which is greater than in contemporary microactuators, and they could tune the actuator’s performance by driving it at different frequencies and with different fuel concentrations. That tunability allowed them to have the robot crawl along or jump over a variety of surfaces. The robot was able to jump up to 59 cm high and 16 cm forward, which are respectively 20 and 5.5 times the robot’s body length. It was also able to carry a payload 22 times its body weight.
When the robot was operated at frequencies exceeding 50 Hz, its temperature ran high enough to constantly ignite the chamber. Below that frequency, the device proved quite durable. Indeed, in one of its tests, the team operated the quadrupedal robot for 750 000 cycles over 8.5 hours. (C. A. Aubin et al., Science 381, 1212, 2023.)