With a diameter just twice that of a human hair, they look more like short snips of fishing line than advanced robotic appendages. But these micro-tentacles can curl and grip. They can lasso an ant or scoop up a tiny fish egg. And they could give a robot of any size an astonishingly gentle but precise grasp.

A team of three material scientists at Iowa State University have just invented this new way for robots to softly handle delicate and diminutive objects. As they describe today in a paper in the science journal Scientific Reports, their clever micro-tentacles are hundreds of times smaller than the next smallest self-spiraling, lifelike tentacle, making them a unique tool for everything from microsurgery to microbiology. Better still, they hug with less than 1 micro-newton of force. That's thousands of times softer than your blinking eye, and it makes mechanical pinching (the traditional approach for robot's tiny grip) look absolutely medieval.

"Two of the biggest trends in robotics right now are soft-robotics—utilizing soft materials for purposes like gentler human interaction—and micro-robotics, making robots smaller," says Jaeyoun Kim, the material scientist who lead the team. "These micro-tentacles fuse those together."

Jaeyoun Kim / Iowa State University

Kim and his colleagues built their micro-tentacles out of a cheap, naturally soft, and commercially available material called PDMS. They used the PDMS to form hollow tubes which curl up when the air is sucked out of them. One side of the tube is corked, while the other is connected to a pneumatic controller. The micro-tentacles (which are less than 8 millimeters long) curl in a specific direction because one side of the tube is thinner than the other.

The biggest challenge was having the tubes respond like real tentacles

The process wasn't easy. PDMS is quite liquid, almost like olive oil, which makes casting it with precision over a hair-thin, rod-like template almost impossible—it will bead up in drops. But the trio of researchers discovered a way to heat-treat the material to slightly gelatinize it, smoothing out the material and the problem. Another issue was finding a way to remove the tubes from their cylindrical template without destroying them. To do this, the scientists used a tool that looks much like a tiny wire-stripper.

But the biggest challenge was having the tubes respond like real tentacles. The scientists could easily get the tubes to bend backwards with a vacuum, but it proved much harder to make them fully curl up on themselves like a party horn. Finding the solution took a lot of trial and error "and a little serendipity," says Kim, and even today the scientists still aren't entirely sure why what they did works. Kim explains that adding a tight sturdy ring of silicone rubber at the base of each tube somehow squeezes the material and makes it curl far more than it otherwise would—giving the tubes a real tentacle grip.

Kim has shown that the micro-tentacles can easily pick up objects like ants or fish eggs without damaging them. The gentleness of the tentacles comes from not only the softness of the PDMS material but also the fact that the tentacles spread their pinching force across the entire tentacle—unlike, say, mechanical tweezers, which grab an object with force at two points.

"There's a benefit here for microsurgery too... there's a great benefit to utilizing tools you know can't accidentally damage the tissue around where you're working," he says.

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