Insect-Scale Robots Emit Light When Flying

A team of researchers at Massachusetts Institute of Technology were inspired by fireflies to create soft actuators that can emit light in different colors or patterns when flying. The artificial muscles control the wings of the flying robots, and they light up when in flight mode. This new approach provides an innovative way to track the flying robots and could help them communicate. 

The new research was published in IEEE Robotics and Automation Letters. 

The electroluminescent soft artificial muscles could be used for a variety of applications. For example, the robots could play a role in a search-and-rescue mission, where they could find survivors and signal to other robots for help. 

Tracking and Enabling Communication

The microscale robots only weigh a little more than a paperclip, and their ability to emit light could help them fly on their own outside of the lab environment. Because of their weight, the microbots can’t carry any sensors, meaning researchers had to track them with infrared cameras that struggle outdoors. However, the team has come up with a new method to track them by using the light they emit and three smartphone cameras. 

Kevin Chen is the D. Reid Weedon, Jr. Assistant Professor in the Department of Electrical Engineering and Computer Science (EECS), the head of the Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE), and the senior author of the paper. 

“If you think of large-scale robots, they can communicate using a lot of different tools — Bluetooth, wireless, all those sorts of things. But for a tiny, power-constrained robot, we are forced to think about new modes of communication. This is a major step toward flying these robots in outdoor environments where we don't have a well-tuned, state-of-the-art motion tracking system,” says Chen.

Making Microrobots Glow

The team embedded miniscule electroluminescent particles into the artificial muscles, which only adds 2.5 percent more weight without impacting the flight performance of the robot. 

The group of researchers previously developed a new fabrication technique to build soft actuators that flap the wings of the robot. They are created by alternating ultrathin layers of elastomer and carbon nanotube electrodes in a stack before rolling it into a squishy cylinder. After a voltage is applied to the cylinder, the electrodes squeeze the elastomer, and this strain causes the wings to flap. 

To create the glowing actuator, the team placed electroluminescent zinc sulfate particles into the elastomer, but this required some work. 

The researchers first had to create an electrode that would not block light. They did this by using highly transparent carbon nanotubes, which allow light to pass through. Even with these nanotubes, the zinc particles still required a very strong and high-frequency electric field to light up. The electrofield excites the electrons in the zinc particles, causing the latter to emit subatomic particles of light, or photons. A strong electric field was then created with a high voltage in the soft actuator, and it was used to drive the robot at high frequency. This process allowed the particles to light up. 

“Traditionally, electroluminescent materials are very energetically costly, but in a sense, we get that electroluminescence for free because we just use the electric field at the frequency we need for flying. We don't need new actuation, new wires, or anything. It only takes about 3 percent more energy to shine out light,” Chen says.

The team learned that the addition of zinc particles reduced the quality of the actuator, so they were only mixed into the top elastomer layer. This caused the actuator to be about 2.5 percent heavier, but it could emit light without impacting flight performance. 

“We put a lot of care into maintaining the quality of the elastomer layers between the electrodes. Adding these particles was almost like adding dust to our elastomer layer. It took many different approaches and a lot of testing, but we came up with a way to ensure the quality of the actuator,” Kim says.

By adjusting the chemical combination of the zinc particles, the light color can be changed. The team created orange, green, and blue particles, with each actuator shining one solid color. 

The team also enabled the actuators to emit multicolored and patterned light by placing a mask over the top layer, adding zinc particles, then curing the actuator. 

Motion Tracking System

The next step was to test the mechanical properties of the actuators and to measure the intensity of the light. They ran flight tests with a specially designed motion-tracking system, with iPhone cameras being used to track each electroluminescent actuator, which served as an active marker. After the cameras detect each light color, a computer program tracks the position of the robots.

“We are very proud of how good the tracking result is, compared to the state-of-the-art. We were using cheap hardware, compared to the tens of thousands of dollars these large motion-tracking systems cost, and the tracking results were very close,” Chen says.

The team will now look to enhance the motion tracking system to enable real-time tracking of the robots, as well as try to enable the microbots to turn their light on and off during flight.