2, Harvard University, Cambridge, Massachusetts, United States
Micro-aerial vehicles (MAVs) are a rapidly growing area of interest due to potential applications in surveillance, exploration, and defense. Several research groups have taken inspiration from nature in creating bird and insect-sized MAVs that use flapping-wing motions for propulsion and control [1,2]. While lithium batteries are widely used in powering larger designs, they lack sufficient energy density at small scales to deliver meaningful flight times in smaller MAVs. Harvard’s RoboBee, the smallest sub-gram MAV developed to date, uses oscillating piezoelectric actuators to achieve flight, but this design requires an electric tether to deliver power to the robot [3,4]. Here we present an alternative method of high force and high frequency actuation that takes advantage of the large volumetric energy density of hydrocarbon fuels to sustain untethered flight.
This work details the creation of a small, soft, 3D-printed combustion actuator that integrates with the RoboBee. Weighing less than 30mg, these actuators possess thin elastomeric membranes that rapidly expand and contract to mechanically drive the wings of the RoboBee. Actuation occurs through the expansion of heated gases emitted from the combustion of heptane fuel, which takes place in an adjoining chamber. The vapor pressure of the heptane drives the fuel from a storage chamber to the combustion chamber, where autoignition is triggered through an embedded resistive heating element. This combustion actuator was fabricated from an elastomeric polyurethane resin using projection stereolithography 3D-printing. Actuation frequencies upwards of 100Hz were reported during tests with a compressed air setup. The advances in this work represent a unique method of achieving untethered locomotion in microrobotics, which is an essential milestone in realizing the envisioned applications of this class of devices.
 G. C. H. E. de Croon, K. M. E. de Clercq, R. Ruijsink, B. Remes, and C. de Wagter, “Design, Aerodynamics, and Vision-Based Control of the DelFly,” Int. J. Micro Air Veh., vol. 1, no. 2, pp. 71–97, 2009.
 M. Keennon, K. Klingebiel, and H. Won. “Development of the Nano Hummingbird: A Tailless Flapping Wing Micro Air Vehicle,” 50th AIAA Aerospace Sciences Meeting, 2012.
 K. Y. Ma, P. Chirarattananon, S. B. Fuller, and R. J. Wood, “Controlled Flight of a Biologically Inspired, Insect-Scale Robot,” Science, vol. 340, no. 6132, pp. 603-607, 2013.
 R. Wood, R. Nagpal, and G. Y. Wei, “Flight of the RoboBees,” Scientific American, 2013.