MIT researchers build microrobot with soft actuators
When it comes to robots, bigger is not always better. Someday, a swarm of insect-sized robots could pollinate crops or search for survivors among the rubble of a collapsed building.
Researchers at the Massachusetts Institute of Technology have demonstrated tiny drones that can move with bug-like agility and resilience, and that could ultimately perform these tasks.
The soft actuators that power these microrobots are very durable, but they require much higher voltage than rigid actuators of similar size. Lightweight robots cannot carry the power electronics needed to let them fly on their own.
The researchers were the first to develop a fabrication technique that allows them to create soft actuators that operate at 75% lower voltage than current versions and carry 80% more payload. These soft actuators resemble artificial muscles that rapidly flap the robot's wings.
The new fabrication technique makes it possible to create artificial muscles with fewer defects, which significantly extends component lifespan and increases the robot's performance and payload capacity.
“This opens up many possibilities in the future for moving toward using power electronics in the microrobot. People tend to think that soft robots are not as capable as rigid ones. We show that this robot, weighing less than a gram, flies longest with the smallest error during hovering flight. The main idea is that soft robots can outperform rigid robots in performance”
The rectangular microrobot has four sets of wings, each driven by a soft actuator. These muscular actuators are made of layers of elastomer sandwiched between two very thin electrodes and then rolled into a soft cylinder. When voltage is applied to the actuator, the electrodes compress the elastomer, and this mechanical stress is used to flap the wing.
The larger the surface area of the actuator, the less voltage is required. Chen and his team create these artificial muscles by alternating as many ultrathin layers of elastomer and electrode as possible. As the elastomer layers become thinner, they become less stable.
For the first time, researchers were able to create an actuator with 20 layers, each 10 micrometers thick (roughly the diameter of a red blood cell). But to do this, they had to reinvent parts of the manufacturing process.
One of the main obstacles was the spin-coating process. During spin coating, the elastomer is poured onto a flat surface and rapidly rotated, and centrifugal force pulls the film outward, making it thinner.
“In this process, air gets trapped in the elastomer and creates many microscopic air bubbles. These air bubbles are only one micrometer in diameter, so previously we simply ignored them. But as the layers become thinner and thinner, the effect of the air bubbles becomes stronger and stronger. That is why people cannot make very thin layers”
The researchers found that if they apply vacuum degassing right after spin coating, while the elastomer is still wet, it removes the air bubbles. They then bake the elastomer to let it dry.
According to Chen, eliminating these defects increases the actuator's output power by more than 300% and significantly extends its lifespan.
The researchers also optimized the thin electrodes, which are made of carbon nanotubes, ultrastrong bundles of carbon that are roughly 1/50,000 the diameter of a human hair. Higher concentrations of carbon nanotubes increase the actuator's output power and reduce voltage, but dense layers also contain more defects.
For example, carbon nanotubes have sharp ends and can puncture the elastomer, causing the device to short-circuit. After much trial and error, the researchers found the optimal concentration. Another problem arises during the curing stage: the more layers are added, the longer the actuator takes to dry.
“The first time I asked my student to make a multilayer actuator, he got 12 layers, but he had to wait two days for it to set. That is completely unacceptable, especially if you want to increase the number of layers”
They found that baking each layer for several minutes right after transferring the carbon nanotubes onto the elastomer reduces curing time as additional layers are added.
After using this technique to create a 20-layer artificial muscle, the researchers tested it against their previous six-layer version and state-of-the-art rigid actuators.
During takeoff experiments, the 20-layer actuator, which requires less than 500 volts to operate, produced enough energy to give the robot a lift-to-weight ratio of 3.7:1, allowing it to carry objects almost three times its own weight.
They also demonstrated a 20-second hovering flight. The hovering robot remained more stable than any other. The 20-layer actuator still operated without failure after more than two million cycles, far exceeding the lifespan of other actuators.
“Two years ago, we built an actuator with maximum power, and it could barely fly. We began to wonder whether soft robots could ever compete with rigid robots. We observed one defect after another, so we kept working and solved one manufacturing problem after another, and now the performance of the soft actuator is on par. They are even slightly better than modern rigid ones. And in materials science there are still a number of manufacturing processes that we do not understand. That is why I am very excited to keep reducing the actuation voltage”
Chen hopes to collaborate with Niroui on fabricating actuators in the MIT.nano cleanroom and using nanofabrication techniques. Right now, his team is limited by how thin they can make the layers because of dust in the air and the maximum coating speed. Working in a cleanroom eliminates this problem and allows them to use methods that are more precise than spin coating.
Although Chen is focused on producing 10-micrometer actuator layers, he hopes to reduce the thickness to one micrometer, which would open the door to many applications for these insect-sized robots.
