In a surprising breakthrough for the world of materials science, researchers have created some of the most powerful artificial muscles we've ever seen. And they did it with simple fishing line. These freakishly strong and cheap muscles could revolutionize robotics, and perhaps one day our own bodies.
Ray Baughman, director of the NanoTech Institute at the University of Texas at Dallas, has spent much of his career trying to build artificial muscles out expensive, cutting-edge materials like carbon nanotubes. But Baughman's team recently discovered that elegant solutions can come in cheap and easy packages: the answers to many of their research questions could be bought for $5 at a local tackle shop. Sometimes, scientific discoveries are just a matter of rethinking how we use something that's part of our everyday lives.
Above: A "breathing" textile, engineered from Baughman's team's new artificial musculature
A Clever Twist on Old Technology
How do you get muscle out of a fishing line? First, you have to create tension that can be released.
It's a simple process that goes by an equally simple moniker: "twist insertion." Researchers led by Baughman describe the technique in detail in this week's issue of Science, but the gist is as straightforward as it sounds. One end of a high-strength polymer fiber (like a 50 pound test-line, for example, available at pretty much any sporting goods store) is held fast, while the other is weighted and twisted. Twist a little and the line becomes an artificial "torsional" muscle that exerts energy by spinning. Twist a lot, however, and something interesting happens: the cord coils over on itself, creating an ordered series of stacking loops:
There's a decent chance you've seen this kind of looping before, maybe while twiddling your shoelace, a length of excess yarn, or – who knows? – a fishing line between your thumb and forefinger. Another good example, Baughman tells io9, is a rubber-band-powered plane. "If you finger-spin the propeller, initially what you see is that the rubber band just twists," he says, "but if you add more twist you get these nucleated coils."
First author Carter Haines, a PhD Candidate in Baughman's lab, demonstrates twist insertion | Credit: UT Dallas
And it turns out that in high-strength, low-cost polymer fibers like fishing line and sewing thread, the emergence of these coils signals a fundamental shift in the material's properties. It goes from being an artificial torsional muscle to a powerful, artificial tensile muscle. That means it becomes an actuator that contracts when activated, just like the muscles in our bodies do. What's more, these artificial muscles are really, really strong.
Study co-author Márcio D. Lima demonstrates the strength and energy density of his team's artificial muscles
"The energy per cycle that we obtain from these artificial muscles, and their weightlifting abilities, are extraordinary," says Baughman. "They can lift about 100 times heavier weight and generate about 100-times higher power than natural muscle of the same weight and length." When Baughman says power, he's referring to the the rate at which these artificial muscles perform (i.e. the work they carry out per unit time). It's a measurement that most people are accustomed to hearing expressed in units of horsepower. Buaghman's fishing line muscles can generate about seven horsepower of mechanical power per kilogram of polymer fiber. That's the kind of power-to-weight ratio you see with jet engines –about five-times that of your typical internal combustion engine.
The researchers' artificial muscles can be triggered by a range of stimuli, but the common denominator of activation is heat. In the video a couple of paragraphs up, a bundle of 4 artificial muscles made from fishing line contracts and relaxes when exposed to an intermittent bath of hot water, lifting and releasing a 30-pound stack of weights.
Another approach, illustrated by the animation on the left, is to create a coil of artificial muscle from silver-coated nylon sewing thread, which can be heated by passing electricity through it and passively cooled by immersing it in water. In this demonstration, a 180 micrometer diameter (about twice the width of an average human hair) piece of silver-coated nylon is used to lift and release a 100g weight at a rate of five times a second. A third option is to coat coiled threads with a material that absorbs photons and heating them by and exposing them to light.
Cheap, Strong and Versatile
Baughman and his colleagues report that their new artificial muscles are every bit as strong and effective as shape memory alloys (some of which have been around for close to half a century), and other artificial muscle materials like carbon nanotubes. Existing artificial muscle technologies are also more difficult to produce, and often less mechanically efficient than these simple coils of twisted fishing line. And here's the real kicker: the materials used in conventional artificial muscles can be orders of magnitude more expensive than high-strength polymer fiber. For example, the carbon nanotubes used to design the artificial muscle Baughman himself engineered in 2011 runs on the order of $5,000 per kilogram. The same quantity of fishing line costs just five bucks.
What's more, existing technologies are often plagued by something call hysteresis. Hysteresis, in the world of materials science, means that the activity of your artificial muscle depends not only on temperature, but on the history of the muscle's activity. The upshot is that shape memory alloys can't be used effectively if you want to control the position of your artificial muscle with any degree of sensitivity. Baughman's fishing-line muscles don't have that problem, and in fact demonstrate a wide range of precise, temperature-dependent control.
The team's artificial muscles also demonstrate impressive versatility. The performance of each coil can be varied based on the weight applied to the end of the cord during twist insertion, and, of course, by the total number of twists. A cord forced to coil in a direction opposite its twist-direction will expand when heated, rather than contract. Fibers can be twisted, coiled, pleated, plied and braided into all manner of configurations, some with the aid of a heat gun (what basically amounts to a fancy hair dryer, the heat gun is used to fix the cord into a desired configuration through a process known as "annealing"), and many without. The results are beautiful, and, in theory, limitless:
Artificial muscle configurations, courtesy AAAS
Arguably the most important property of these muscles, when it comes to versatility, is their scalability. "We can use 2 pound test line or we can use 700 pound test line," Baughman tells us, "and if we insert twist identically in those two different fibers we can get the same work per volume capabilities for both synthetic muscles." The difference, of course, is that the large diameter muscle lifts a lot of weight, while the small diameter muscle lifts a little. How you bundle the fibers matters, too: a single length of artificial muscles, made from a fishing line about ten times the width of a human hair, can lift about 16 pounds. Arrange 125 of them together and you could lift a ton.
So what can they do?
Versatility in strength and size translates to versatility in implementation – we're talking nano-scale, macro-scale, and everything in between. Here is a handful of the potential applications that Baughman listed when he talked to us:
- Musculature for humanoid robots (yes, he started with humanoid robots).
- Designing realistic face musculature for humanoid robots, to address issues surrounding the uncanny valley.
- Comfort adjusting textiles that change their porosity, and therefore their breathability, in response to their environment (the animation at the top of this post demonstrates the team's proof of concept for this application).
- Gas or liquid filters that open and close in a temperature-dependent fashion.
- Nanoscale architecture (in the development, for example, of more efficient labs-on-chips).
Here's an application that we found particularly interesting, which you can see in this animation. This is a window shutter, eletrothermally driven by a coiled stretch of silver-coated nylon, that reacts to ambient temperature to open and close, thereby regulating the inside temperature of the building in which it is installed.
"Another thing we've done is use these muscles for harvesting waste thermal energy," says Baughman. Many industrial processes release heat as a byproduct – heat that could be used to power coils of artificial muscle polymers. "Imagine you have a hot waste stream and available cold water," says Baughman. "We found you can pass hot water cold water over our muscles and generate about seven horsepower of mechanical energy per kilogram of polymer."
Baughman couldn't tell us about the "strangest" applications for his team's new artificial muscles, "for both publication and patent reasons" – but then, he says, that's the nature of a discovery as new as this. "We're improving what we have. Shape memory wire is more than fifty years old. Our technology is just a little over a year old. This is just the beginning."
Boris Yakobson – a professor of materials science and mechanical engineering at Rice University who was not involved in the study – praised the researchers' work for its "multiscale nature" in an email to io9. "With the clever micromechanical design of a twisted fiber, it connects the fundamentals of conformational entropy of the molecular chains in everyday fishing line right on through to the overall macro-contraction, which can be utilized in variety of tantalizing applications."
That Baughman's team's design is as versatile, affordable and easy to reproduce as it is suggests that we could see it popping up on real-world applications very soon. As for right now, Baughman says his team is focused on increasing the efficiency of its artificial muscles. Yakobson, for his part, agrees there could be room for improvement in this area.
"I wonder how the [cooling of the artificial muscles] can be accelerated for faster actuator performance," he writes. "Immersion into liquids, which the researchers suggest, can help with cooling, but on the other hand its viscosity may slow down the mechanics, so there should be room for optimization there."
The researchers' findings are published in the latest issue of Science.