Sometimes big news in science is very small, and researchers in Georgia have created a new breed of robots so tiny that they're almost invisible to the naked eye. Measuring just 2 millimeters (0.08 inch) in length, the so-called microbristle bots are a fraction of the size of a grain of rice.

At this stage in their development, the 3D-printed robots aren't much to look at — think boxy mechanical fleas — and they can do little more than skitter about on their tiny legs. But the researchers have big dreams for the itty-bitty bots, which are described in a paper accepted for publication in the Journal of Micromechanics and Microengineering.

One day, swarms of the little bots — fitted with mechanical arms and tiny sensors to give them a semblance of intelligence — might be sent inside cramped or hazardous environments that are off limits to humans and larger robots to check on industrial processes or scout out hazards.

Even smaller versions of the bots might be sent inside human bodies, crawling and even swimming through our innards to look for signs of illness or injury and perhaps even deliver drugs, take tissue samples or make limited repairs to the body.

Bristle bots, or vibrots, of this size are too tiny to carry batteries. But neither batteries nor other power sources sometimes used to power small machines are required. These bots are powered by sound or ultrasound waves — acoustic vibrations directed at them from an external source.

The vibrations cause the bots' legs to move rapidly back and forth, fast enough to enable them to travel four times their own length per second. The vibrations also cause a so-called piezoelectric actuator aboard the bots to create a tiny electrical voltage to provide power for the sensors.

"We don't need a laser, we don't need a magnet, we just need a sound or ultrasound source," said Azadeh Ansari, an electrical engineer at the Georgia Institute of Technology in Atlanta and the leader of the team of researchers who designed the robots. The team took their inspiration, she added, from Hexbugs, small robotic toys whose vibrating appendages allow them to scoot across surfaces.

By changing the frequency and volume of the sound waves, the researchers are able to alter the robots' movements and vary their speed. "Right now, they're basically just moving," Ansari said. But she envisions the day, perhaps a decade in the future, when precisely controlled microbots find use in a variety of industrial and environmental settings.

Biomedical applications might take another decade, she said, adding that tiny medical robots might be swallowed or injected or even administered rectally for a less invasive alternative to colonoscopy.

This isn't the first time pint-sized machines have been designed with medical applications in mind.

In 2018, researchers from China's National Center for Nanoscience and Technology and Arizona State University created cancer-fighting nanobots designed to go inside a patient's body to target tumor cells. And Swiss researchers made headlines earlier this year when they announced that they'd developed a shape-shifting microrobot designed to swim through blood vessels and deliver drugs.

Simon Garnier, an assistant professor of biomedical sciences at the New Jersey Institute of Technology in Newark, praised the microbristle bots for their simple design and unique control mechanism. "I'm excited to see where these bots are going to go," he said. "What I'd like to see is the next step, where they work with each other. There's a lot of very exciting possibilities after that."

Garnier said one challenge could be making sure the tiny bots work well together. "As long as they all follow the right set of rules, things will go well," he said. "If you don't set the proper rules, then things can go completely wrong and do nothing or the opposite of what you're trying to achieve."

Ansari said her team is working on just that, adding that the next step will be making the bots steerable. "Once you have a fully steerable microrobot, you can imagine doing a lot of interesting things."

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