Unlike motors, which are the mechanical devices we most often use to move or lift things, artificial muscles apply force and do work by expanding and contracting.
"That matches similarly to how natural muscles work," said Carter Haines, a PhD student at the University of Texas Dallas and lead author of a paper describing the new technology. "Natural muscles are just arrays of many fibres that, when given chemical energy, get shorter in length."
While electric motors need bulky gears to convert rotational motion into linear motion, artificial muscles don't.
"The advantage is that these can be made extremely compact," said John Madden, a University of British Columbia researcher on the team that came up with the new artificial muscles.
That means they could be used to make robots, prosthetic limbs or wearable exoskeletons that are smaller and lighter than current models based on motors and hydraulic systems, said Ray Baughman, director of the NanoTech Institute at the University of Texas Dallas, Haines's PhD supervisor, in a statement. Madden has also shown they can be used to open and close surgical forceps.
The artificial muscles described by the researchers Thursday in Science are made by twisting and coiling nylon fishing line. Applying heat to the coil causes it to contract up to 50 per cent, more than double the 20 per cent contraction for natural muscles.
"Per weight, they can generate 7.1 horsepower per kilogram, about the same mechanical power as a jet engine," said a university news release.
The researchers showed that heat can easily be applied by applying electricity to thread painted with a metal coating.
Automatic blinds, futuristic clothing
While a large contraction requires a 100 C to 200 C temperature change, they are also affected by smaller temperature changes, contracting about 1.2 per cent in length per degree Celsius, Haines said.
He and his colleagues have already demonstrated that they could be used to automatically open the windows of greenhouses or buildings when it is hot, and close them when it cools down, without the use of electricity.
He also envisions it being woven into clothing that would automatically become breezier in hot weather and tighten up again when it gets cold.
The researchers at the University of Texas Dallas had previously made similar artificial muscles with very special, expensive materials that are typically found only in science and engineering labs.
Some visiting scientists decided on a whim to try making similar devices with plastic thread, and were surprised to discover they performed better than the more expensive materials, Haines recalled. He did further testing with different kinds of nylon fishing line that optimized the artificial muscles.
"We found that when we heat it, we get these giant contractions, that they do amazing things, but we couldn't explain it," he said.
Phenomenon explained by UBC scientists
That's where Madden's team came in. A PhD student in his lab, Seyed Mohammad Mirvakili, derived an equation that helped explain what was happening. In the process, he discovered certain parameters that could boost performance, and his colleagues would test them out, he recalled in a phone interview from the Massachusetts Institute of Technology, where he is doing some of his PhD work.
Essentially what was happening, Madden said, was that the nylon thread was expanding in width when heated, as you might expect, but was also contracting in length. When it was twisted, the effect of those motions is amplified. When the twisted thread is coiled up, the motions are further amplified into a linear expansion and contraction.
"What's amazing about this discovery, is nylon is obviously a material that's very well characterized and very well known, so to find something like this that nobody else has discovered is a bit surprising," said Madden in a phone interview.
He suggested it was perhaps because there aren't a lot of things familiar to us that use the same twisting mechanism as the artificial muscles.
"One of the reasons this is going to be such an interesting discovery is that it's so easy for people to make," he added.
Both Madden and Haines said they had high school students making the artificial muscles in their labs.
Haines said you could even make them "at home in your living room."
"Every person I'm talking to about it always has their own ideas of what to use it for and they're all different, so I'm really excited to see what people will use it for in the future," he added.