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Robotic Fleas Spring into Action

Tiny rubber bands can power microrobots that could serve as ultrasmall sensors.

An autonomous robotic flea has been developed that is capable of jumping nearly 30 times its height, thanks to what is arguably the world’s smallest rubber band.

Heave: Tiny micro electromechanical systems (MEMS) motors stretch a diminutive nine-micron-thick, two-millimeter-long rubber band in order to allow a microbot to catapult itself through the air like a flea.

Swarms of such robots could eventually be used to create networks of distributed sensors for detecting chemicals or for military-surveillance purposes, says Sarah Bergbreiter, an electrical engineer at University of California, Berkeley, who developed the robots.

The idea is that stretching a silicone rubber band just nine microns thick can enable these microrobotic devices to move by catapulting themselves into the air. Early tests show that the solar-powered bots can store enough energy to make a 7-millimeter robot jump 200 millimeters high.

This flealike ballistic jumping would enable these sensors to be mobile, covering relatively large distances and overcoming obstacles that would normally be a major problem for micrometer-sized bots, says Bergbreiter.

Such sensors could be scattered from a plane but may not land in the most ideal positions, so making them mobile could allow them to be repositioned, if somewhat haphazardly. “Distributed sensors in general give you the large picture,” Bergbreiter says. This is because they can provide a more detailed resolution over a larger area compared with more-traditional nondistributed approaches to sensing.

“With miniature robots, hopping is a good option if you’re trying to move over uneven terrains,” says Metin Sitti, an assistant professor at the nanorobotics lab at the Robotics Institute at Carnegie Mellon University, in Pittsburgh. “At that size, the critical issue is power, so it is a good choice to store energy,” he says.

The impressive jumping skills of insects such as fleas come from their ability to store energy in an elastomeric protein called resilin. This allows them to store a large amount of energy and then release it very suddenly as movement. But while insects store the energy through compressing an elastomer, Bergbreiter opted for a system that stretches one.

Working with Kris Pister as part of the Berkeley Smart Dust Project, which was set up to build distributed-sensor networks that can communicate over long distances using mesh networks, Bergbreiter aimed to give these kinds of sensors useful mobility. She created a tiny solar-cell array to power the device, a microcontroller to govern its behavior, and a series of micro electromechanical systems (MEMS) motors on a silicon substrate. The last were used as part of a ratcheting mechanism called inchworm motors, which draw two hooks apart as a means of stretching the rubber band.

Bergbreiter, in collaboration with the Smart Dust Project, created the rubber band by cutting a circular strip measuring just nine microns thick and two millimeters long out of a thin sheet of silicone using a very fine infrared laser. It was then hooked onto the robot’s stretching mechanism using nothing more than a pair of ultraprecision tweezers, a stereoscopic microscope, and a steady hand. This was a bit like playing the children’s game Operation, only harder, says Bergbreiter.

To test the robot prototype, Bergbreiter hooked it up so that rather than the bot actually jumping, its leg was positioned to kick an object. This allowed her to calculate the energy being released. So far Bergbreiter has only tried partially stretching the rubber band, which would achieve a jump of about 12 millimeters for the 10-milligram robot. However, she says that based on the results of this test, a full stretch would be capable of producing jumps as high as 200 millimeters, and they would cover roughly twice as much ground horizontally. The results will be presented next week at the International Conference on Robotics and Automation, in Rome, Italy.

The current seven-millimeter-long prototype is still much larger than a flea. But Bergbreiter is keen to shrink the robot down to about one millimeter, or flea size. Also, she still needs to add the tiny photovoltaic solar cell that has been fabricated separately. “The next step is to put it all together,” she says.

One of the benefits of making robots on the insect scale is that it is possible to generate very high takeoff velocities. This is why insects can achieve such relatively huge jumps. As an object’s volume is reduced, its mass diminishes at a much greater rate, which in turn allows for great accelerations.

However, there is a trade-off. “Drag increases as you get smaller,” says Bergbreiter. So the trick is to ensure that the bots’ size offers enough benefits in terms of acceleration to outweigh the cost of any additional drag.

But generating this movement still requires more energy than the robot is capable of scavenging from its environment through its solar cells. This is often the case with autonomous robots, which is why storing the energy is necessary, says Chris Melhuish, a professor of robotics and director of the Bristol Robotics Laboratory at the University of Bristol and the University of the West of England, U.K.

It’s probable that the only other way to cover such relatively large distances is through flight. “But flying adds a whole new set of challenges,” says Bergbreiter. It requires very high-powered motors to flap wings or drive a propeller, and given the effect that wind can have on such small objects, there are major control issues. Jumping, on the other hand, would allow robots to move much greater distances without huge power requirements.

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