Insect-Sized Robots Get an Explosive Boost from Carbon and EPX 86FR

Introduction

Many roboticists take inspiration from biology when designing their creations. You might have seen videos of humanoid robots performing acrobatic feats, or quadrupedal, canine-like robots assisting technicians in industrial settings. However, a growing group of scientists and researchers are turning their focus to much smaller length scales.

Insect-sized robots represent a rapidly-expanding area of interest among roboticists. In the past two decades miniature robots, from a few inches in length to just a single millimeter, have been made to jump like grasshoppers, fly like bumblebees, and crawl like cockroaches. Their intended functions: everything from environmental monitoring and exploration, to search and rescue and digital agriculture. The future looks bright for these biologically-inspired bots, but a number of challenges still remain.

Research Challenge

Physical scaling laws make conventional actuators (things like motors, pumps, or engines) difficult (if not impossible) to implement, let alone manufacture, at these reduced scales. As a result, state-of-the-art insect-scale robots are primarily actuated by bespoke mechanisms. These mechanisms are usually electrically driven, often by high voltages, but they are typically lacking in important metrics like force output, displacement, and power density. Insects are famously capable of feats of strength or speed that belie their small size, but these micro actuators often fall short of truly mimicking their biological counterparts.

Early in my graduate career my advisor, Dr. Robert Shepherd (read about his work here), approached me with the idea of creating a new type of actuator. Several years prior, while working as a postdoc at Harvard, Rob had published a paper in which he used the combustion of hydrocarbon fuels to power a soft jumping robot, roughly the size of a human hand. The principle was fairly straightforward: in a combustion reaction, heat and energy are rapidly released, which results in an expansion of both the ambient and product gases. If you contain this combustion to a soft, flexible volume, the expanding gases will cause the volume to quickly and forcefully inflate, rather like a balloon. Our idea was to create a much, much smaller actuator that operated under these same principles.

Figure 1. Diagram of the combustion actuation process
SOURCE: Figure 1B in: Cameron A. Aubin, et al. Powerful, soft combustion actuators for insect-scale robots. Science 381, 1212-1217 (2023). DOI:10.1126/science.adg5067

Utilizing the Carbon Printer

My immediate inclination was to turn to our group’s Carbon M1 printer, which had rapidly established itself as the most powerful prototyping tool in the lab. Most micro actuators that I had come across in the literature were created through a combination of Micro-Electromechanical System (MEMs)-based fabrication techniques and manual assembly. The high resolution and extensive material library of the M1 meant that I could automate much of this process. 3D printing our actuator materials maximized our ability to intelligently iterate. Additionally, the knowledge that our eventual designs could be cheaply and rapidly produced in parallel gave us a sense of scalability that felt unique.

Our early designs looked a bit like drums (the musical instruments), consisting of a rigid, hollow cylindrical body roughly 8 mm in diameter, capped by a thin, flexible elastomer membrane. A combination of methane and oxygen would be externally pumped into the actuators through thin lengths of tubing. We could then ignite these combustible gases using sparking electrodes embedded within the actuator body. Our hope was that the resulting tiny explosion would lead to the inflation of the soft membrane. As luck would have it, after a great deal of trial, error, experimentation, and iteration, that’s exactly what we were able to achieve.