MIT researchers have pioneered a new fabrication technique that enables them to produce low-voltage,
power-dense, high endurance soft actuators for an aerial microrobot.
The artificial muscles vastly improve the robot’s payload and allow it to achieve
best-in-class hovering performance.
When it comes to robots, bigger isn’t always better.
Someday, a swarm of insect-sized robots might pollinate a field of crops or search for survivors
amid the rubble of a collapsed building.
MIT researchers have demonstrated diminutive drones that can zip around with bug-like agility
and resilience, which could eventually perform these tasks.
The soft actuators that propel these microrobots are very durable, but they require much higher
voltages than similarly-sized rigid actuators.
The featherweight robots can’t carry the necessary power electronics that would allow
them fly on their own.
Now, these researchers have pioneered a fabrication technique that enables them to build soft
actuators that operate with 75 percent lower voltage than current versions while carrying
80 percent more payload.
These soft actuators are like artificial muscles that rapidly flap the robot’s wings.
This new fabrication technique produces artificial muscles with fewer defects, which dramatically
extends the lifespan of the components and increases the robot’s performance and payload.
People tend to think that soft robots are not as capable as rigid robots.
The researchers demonstrate that this robot, weighing less than a gram, flies for the longest
time with the smallest error during a hovering flight.
The researcher says that the take-home message is that soft robots can exceed the performance
of rigid robots.
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.
The rectangular microrobot, which weighs less than one-fourth of a penny, has four sets
of wings that are each driven by a soft actuator.
These muscle-like actuators are made from layers of elastomer that are sandwiched between
two very thin electrodes and then rolled into a squishy cylinder.
When voltage is applied to the actuator, the electrodes squeeze the elastomer, and that
mechanical strain is used to flap the wing.
The more surface area the actuator has, the less voltage is required.
So, the researchers build these artificial muscles by alternating between as many ultrathin
layers of elastomer and electrode as they can.
As elastomer layers get thinner, they become more unstable.
For the first time, the researchers were able to create an actuator with 20 layers, each
of which is 10 micrometers in thickness (about the diameter of a red blood cell).
But they had to reinvent parts of the fabrication process to get there.
One major roadblock came from the spin coating process.
During spin coating, an elastomer is poured onto a flat surface and rapidly rotated, and
the centrifugal force pulls the film outward to make it thinner.
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 the researchers
just sort of ignored them.
But when layers become thinner, 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.
The researchers 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.
Removing these defects increases the power output of the actuator by more than 300 percent
and significantly improves its lifespan.
The researchers also optimized the thin electrodes, which are composed of carbon nanotubes, super-strong
rolls of carbon that are about 1/50,000 the diameter of human hair.
Higher concentrations of carbon nanotubes increase the actuator’s power output and
reduce voltage, but dense layers also contain more defects.
For instance, the carbon nanotubes have sharp ends and can pierce the elastomer, which causes
the device to short out.
After much trial and error, the researchers found the optimal concentration.
Another problem comes from the curing stage — as more layers are added, the actuator
takes longer and longer to dry.
They found that baking each layer for a few minutes immediately after the carbon nanotubes
are transferred to the elastomer cuts down the curing time as more layers are added.
After using this technique to create a 20-layer artificial muscle, they tested it against
their previous six-layer version and state-of-the-art, rigid actuators.
During liftoff experiments, the 20-layer actuator, which requires less than 500 volts to operate,
exerted enough power to give the robot a lift-to-weight ratio of 3.7 to 1, so it could carry items
that are nearly three times its weight.
They also demonstrated a 20-second hovering flight, which is the longest ever recorded
by a sub-gram robot.
Their hovering robot held its position more stably than any of the others.
The 20-layer actuator was still working smoothly after being driven for more than 2 million
cycles, far outpacing the lifespan of other actuators.