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A new fabrication technique produces low-voltage, power-dense artificial muscles that improve the performance of flying micro robots.

Bigger isn’t necessarily better when it comes to robots.
A swarm of insect-sized robots could one day fertilize a wheat field or hunt for survivors among the ruins of a fallen building.

Drones that can zip around with bug-like agility and durability, as demonstrated by MIT researchers, could potentially do similar functions.
These micro robots’ soft actuators are extremely robust, but they require significantly greater voltages than stiff actuators of similar size.
The power electronics that would allow the robots to fly on their own are too heavy for them to carry.

These researchers have now developed a fabrication approach that allows them to create soft actuators that function at 75 percent lower voltage while carrying 80 percent more weight than existing models.
These soft actuators operate as artificial muscles, flapping the robot’s wings rapidly.

This novel fabrication approach generates artificial muscles with fewer faults, increasing the robot’s performance and payload while also extending the component’s lifespan.

“This opens up a lot of opportunity in the future for us to transition to putting power electronics on the microrobot. People tend to think that soft robots are not as capable as rigid robots. We demonstrate that this robot, weighing less than a gram, flies for the longest time with the smallest error during a hovering flight. The take-home message is that soft robots can exceed the performance of rigid robots,” says Kevin Chen, who is the D. Reid Weedon, Jr. ’41 assistant professor in the Department of Electrical Engineering and Computer Science, the head of the Soft and Micro Robotics Laboratory in the Research Laboratory of Electronics (RLE), and the senior author of the paper.

Chen’s coauthors include Zhijian Ren and Suhan Kim, co-lead authors and EECS graduate students; Xiang Ji, a research scientist in EECS; Weikun Zhu, a chemical engineering graduate student; Farnaz Niroui, an assistant professor in EECS; and Jing Kong, a professor in EECS and principal investigator in RLE. The research has been accepted for publication in Advanced Materials and is included in the jounal’s Rising Stars series, which recognizes outstanding works from early-career researchers.

Making Muscles

Each of the four sets of wings on the rectangular microrobot, which weighs less than a quarter of a penny, is controlled by a soft actuator. Layers of elastomer are layered between two very thin electrodes and then rolled into a squishy cylinder to create these muscle-like actuators. The electrodes squeeze the elastomer when voltage is provided to the actuator, and the mechanical strain is employed to flap the wing.

The less voltage required, the larger the surface area of the actuator. As a result, Chen and his team alternate between as many ultrathin layers of elastomer and electrode as possible to create these artificial muscles. Elastomer layers grow more unstable as they become thinner.

The researchers were able to develop an actuator with 20 layers, each of which is 10 micrometers thick for the first time (about the diameter of a red blood cell). To get there, though, they had to redesign aspects of the fabrication process.

The spin coating technique was a big stumbling hurdle. During spin coating, an elastomer is poured over a flat surface and rapidly rotated, causing the film to thin out due to centrifugal force.

“In this process, air comes back into the elastomer and creates a lot of microscopic air bubbles. The diameter of these air bubbles is barely 1 micrometer, so previously we just sort of ignored them. But when you get thinner and thinner layers, the effect of the air bubbles becomes stronger and stronger. That is traditionally why people haven’t been able to make these very thin layers,” Chen explains.

He and his collaborators found that if they perform a vacuuming process immediately after spin coating, while the elastomer was still wet, it removes the air bubbles. Then, they bake the elastomer to dry it.

Best Performance on its own:

They compared it to their earlier six-layer version and state-of-the-art, stiff actuators after utilizing this technology to produce a 20-layer artificial muscle.

During liftoff tests, the 20-layer actuator, which runs on less than 500 volts, provided enough power to give the robot a 3.7-to-1 lift-to-weight ratio, allowing it to move materials nearly three times its own weight.

They also showed out a 20-second hovering flight, which Chen claims is the longest by a sub-gram robot. Their hovering robot remained in place longer than any of the others. After more than 2 million cycles of operation, the 20-layer actuator was still running well, significantly outlasting other actuators.

“Two years ago, we created the most power-dense actuator and it could barely fly. We started to wonder, can soft robots ever compete with rigid robots? We observed one defect after another, so we kept working and we solved one fabrication problem after another, and now the soft actuator’s performance is catching up. They are even a little bit better than the state-of-the-art rigid ones. And there are still a number of fabrication processes in material science that we don’t understand. So, I am very excited to continue to reduce actuation voltage,” he says.

Source: MIT News