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Wax-Filled Nanotubes Flex Their Muscles


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Here’s a twist: Scientists have designed a flexible, yarnlike artificial muscle that can also pack a punch. It can contract in 25 milliseconds—a fraction of the time it takes to blink an eye—and can generate power 85 times as great as a similarly sized human muscle. The new muscles are made of carbon nanotubes filled with paraffin wax that can twist or stretch in response to heat or electricity. When the temperature rises, the wax melts and forces the nanotubes to contract. Such artificial muscles, the researchers say, could power smart materials, sensors, robots, and even devices inside the human body.

It’s hard to make a smart muscle: an artificial muscle that is simultaneously efficient, fast, powerful, and able to twist and turn. But such muscles would be a great boon to numerous industries, including robotics and smart sensors, because they can turn power into movement on a tiny scale. Seeking a strong, flexible material, scientists have turned to carbon nanotubes: long, hollow cylinders of graphene with unusually strong bonds holding them together. But previous carbon nanotube muscles have been electrochemically based: The muscles were immersed in an electrolyte solution that would conduct signals to force the nanotubes to contract.

“The problem with that is you end up needing an electrolyte and an electrode and a container for all this, and the total volume of the device ends up being much larger than the muscle,” says materials scientist Ray Baughman of the University of Texas, Dallas. Moreover, he says, the electrolyte solution would degrade over time and the required bags of liquid could leak.

But, Baughman’s team realized, if they could instead infuse a material into carbon nanotubes to control the contraction, they could do away with the electrolyte solution. The researchers came up with a simple design: They soaked nanofibers in wax and then twisted them into yarns. The arrangement of the carbon nanofibers in the yarns is similar to the fibers in a finger trap child’s game in which attempting to pull your fingers out of a tube only tightens it more. In the case of the carbon nanofibers, the expansion of the integrated wax shortens the fibers. And the wax’s volume can be changed by altering the temperature, either using external power sources or in response to the surrounding environment. The new muscles, the team reports online today in Science, can lift about 100,000 times their own weight—many times more than a natural human muscle fiber.

“Compared to their size and weight, the performance of these muscles is spectacular,” Baughman says. “And we can do all sorts of things with them: We can weave them; we can braid them; we can knit them; we can cut them in different lengths.”

Baughman suggests that the muscles could be useful for providing power for microfluidics chips, generating precise facial expressions in robots, and providing movement in small toys such as robotic fish in an aquarium. For many other applications—such as those inside the human body and “smart fabrics” that could become more porous when the temperature heats up or contract around an open wound—the muscles will need to be improved and scaled up in size.

“The new muscles fill an area that we haven’t been able to fill before,” says mechanical engineer Mark Schulz of the University of Cincinnati in Ohio who was not involved in the new work. However, Schulz notes, “I think this is definitely still in a stage of progression. I think we’ll start to see different geometries and new materials being integrated in. There’s a lot of potential to make it stronger.”

The muscles currently work most efficiently at high temperatures, he points out, which limit their current use in everyday application. “Designers are going to have to understand all the properties and then do some careful analysis to see if it matches the application they’re interested in.”