MIT engineers find a simple and inexpensive new approach to creating bending artificial muscle fibers
Artificial muscles — materials that contract and expand somewhat like muscle fibers do — can have many applications, from robotics to components in the automobile and aviation industries. Now, MIT researchers have come up with one of the simplest and lowest-cost systems yet for developing such “muscles,” in which a material reproduces some of the bending motions that natural muscle tissues perform.
The key ingredient, cheap and ubiquitous, is ordinary nylon fiber.
The new approach to harnessing this basic synthetic fiber material lies in shaping and heating the fibers in a particular way, which is described in a new paper in the journal Advanced Materials by Seyed Mirvakili, a doctoral candidate, and Ian Hunter, the George N. Hatsopoulos Professor in the Department of Mechanical Engineering.
Previously, researchers had come up with the basic principle of using twisted coils of nylon filament to mimic basic linear muscle activity. They showed that for a given size and weight, such devices could extend and retract further, and store and release more energy, than natural muscles. But bending motions, such as those of human fingers and limbs, proved more challenging and had not yet been achieved in a simple and inexpensive system until the new work at MIT.
There are some existing materials that can be used to produce these kinds of bending motions, which could be useful for some biomedical devices or tactile displays. However, those tend to use “exotic materials to do the job, and they are very expensive and very difficult to make,” Mirvakili says. For example, carbon nanotube yarns can provide great longevity (more than a million linear contraction cycles) but are still too expensive for widespread use, and shape-memory alloys provide a strong contracting pull but have a poor cycle life (fewer than 1,000 cycles).
Cheap and simple
The new nylon-based system, by contrast, uses cheap material and a simple manufacturing process, and demonstrates very good cycling longevity. It all comes down to how the nylon fibers are shaped.
Some polymer fiber materials, including highly oriented nylon, have an unusual property: When heated, “they shrink in length but expand in diameter,” Mirvakili says, and this property has been harnessed to make some linear actuator devices. But to turn that linear shrinking motion into bending typically requires a mechanism such as a pulley and a takeup reel, adding extra size, complexity, and expense. The MIT team’s advance was to directly harness the motion without requiring extra mechanical parts.
One of the limitations on linear actuators made from such materials is that after being heated to trigger the contraction, they take some time to cool back down. “The cooling rate can be a limiting factor,” Mirvakili says. “But I realized it could be used to an advantage.” Selectively heating one side of the fiber, he says, causes that side to begin contracting faster than the heat can penetrate to the other side, and thus can produce a bending motion in the fiber. “You need a combination of these properties,” he says: “high strain [the pull of the shrinking motion] and low thermal conductivity.”
To make this system work effectively as an artificial muscle, the fiber’s cross-section needs to be carefully shaped. The team used ordinary nylon fishing line to start with, and compressed it to change its cross-section from round to rectangular or square. Then, selectively heating one side caused the fiber to bend in that direction. Changing the direction of the heating could also produce more complex motions; in their lab tests, the team used this heating technique to get the fibers to move in circles and figure-eights, and much more complex patterns of movement could easily be achieved, they say.
Various heat sources can be used on the fibers, including electric resistance heating, chemical reactions, or a laser beam that shines on the filament. For some of their tests, the researchers used a special conductive paint applied to the fibers and held in place by a resin binder; when a voltage was applied to the material, it selectively heated the portion of the fiber directly below the paint, causing the fiber to bend that way.
The researchers have demonstrated that the material can maintain its performance after at least 100,000 bending cycles, and can bend and retract at a speed of at least 17 cycles per second.
Hunter suggests that ultimately, applications for such fibers might include clothes that contract to adjust snugly to the contours of an individual body, drastically reducing the number of different sizes a manufacturer would need to produce, while improving the comfort and fit. Or, the fibers might be used in shoes that would tighten themselves when put on or adjust their stiffness and shape during each stride.
The system may also allow for self-adjusting catheters or other biomedical devices. And in the longer run, it could even lead to mechanical systems such as vehicle exterior panels that adjust their aerodynamic shape to adapt to changes in speed and wind conditions, or automatic tracking systems for solar panels that would use excess heat generated by the panels themselves to keep the panels aimed at the sun.
This method “is novel and elegant, with very good experimental data supported by appropriate physics-based models,” says Geoffrey Spinks, a professor at the University of Wollongong in Australia, who was not connected with this research. “This is a simple idea that works really well. The materials are inexpensive. The manufacturing method is simple and versatile. The method of actuation is by simple electrical input. The bending actuation performance is impressive in terms of bending angle, force generated, and speed.”
Spinks adds, “Bending-type actuators are needed for robotic grippers, microscopic tools, and various machine components. These new bending actuators could have immediate application.”
These are “exciting and game-changing findings,” adds Andrew Taberner, an associate professor of bioengineering at the University of Auckland in New Zealand, who also was not involved in this research. “One can imagine many applications for this type of actuator in the medical and instrumentation fields,” he says. “I expect that this work will become highly cited.”
Learn more: Nylon fibers made to flex like muscles
In recent years, researchers at The University of Texas at Dallas and colleagues at the University of Wollongong in Australia have put a high-tech twist on the ancient art of fiber spinning, using modern materials to create ultra-strong, powerful, shape-shifting yarns.
In a perspective article published Sept. 26 online in the Proceedings of the National Academy of Sciences, a team of scientists at UT Dallas’ Alan G. MacDiarmid NanoTech Institute describes the path to developing a new class of artificial muscles made from highly twisted fibers of various materials, ranging from exotic carbon nanotubes to ordinary nylon thread and polymer fishing line.
Because the artificial muscles can be made in different sizes and configurations, potential applications range from robotics and prosthetics to consumer products such as smart textiles that change porosity and shape in response to temperature.
“We call these actuating fibers ‘artificial muscles’ because they mimic the fiber-like form-factor of natural muscles,” said Dr. Carter Haines BS’11 PhD’15, associate research professor in the NanoTech Institute and co-lead author of the PNAS article, with research associate Dr. Na Li. “While the name evokes the idea of humanoid robots, we are very excited about their potential use for other practical applications, such as in next-generation intelligent textiles.”
Science Based on Ancient Art
Spinning animal fur and plant fibers to make thread and yarn goes back thousands of years. Aligning the fibers and then twisting them into yarn gives the yarn strength.
By exploiting this concept, and adding 21st-century science, the UT Dallas researchers have produced actuating muscle yarns that, like their wooly counterparts, can be woven, sewn and knitted into textiles.
For example, carbon nanotubes are essentially tendrils of tiny, hollow tubes that are super-strong and electrically conductive. In 2004, led by Dr. Ray Baughman, director of the NanoTech Institute and the Robert A. Welch Distinguished Chair in Chemistry at UT Dallas, the team developed a method to draw “forests” of nanotubes out into sheets of aligned fibers — much like carded wool — and then twist the sheets into yarns.
When heated and cooled, spiral-shaped artificial muscles expand and contract back and forth.
Next, the group turned to polymer fibers such as nylon sewing thread and fishing line, which consist of many individual molecules aligned along the fiber’s length. Twisting the thread or fishing line orients these molecules into helices, producing torsional — or rotational — artificial muscles that can spin a heavy rotor more than 100,000 revolutions per minute.
When these muscles are so highly twisted that they coil like an over-twisted rubber band, they can produce tensile actuation, where the muscle dramatically contracts along its length when heated, and returns to its initial length when cooled. That research, published in 2014, showed that simple, low-cost muscles made from fishing line can lift 100 times more weight and generate 100 times higher mechanical power than a human skeletal muscle of the same length and weight.
“The success of our muscles derives from their special geometry and the fact that we start with materials that are anisotropic — when they are heated, the materials expand in diameter much more than they expand along their length,” said Baughman, senior author of the PNAS perspective. This anisotropy is an intrinsic property of high-strength polymer fibers, and is the same principle that drives powerful artificial muscles the researchers discovered in 2012, which they made by adding a thermally responsive “guest” material within a carbon nanotube yarn.
“When these fibers are then twisted and coiled, their internal geometry changes so that when they are heated, that diameter expansion results in a change in length,” Baughman said. “The fiber’s diameter only has to expand by about 5 percent to drive giant changes in length.”
The Latest Twist
In their most recent experiments, described for the first time in the PNAS article, Haines and Li added a new twist to their artificial muscles.
“The coiled artificial muscles we initially made from fishing line and nylon sewing thread were limited in the amount they could expand and contract along their length,” Haines said. “Because of their geometry — like a phone cord — they could only contract so far before the coils began to collide with one another.”
The solution: Form the coiled actuators into spirals.
“The advantage to the spiral shape is that now our muscle can contract into a flat state, expand out in the other direction, and return to its original length, all without getting stuck on itself,” Li said. “Our experiments to date have been proof-of-concept, but have already shown that we can use heating and cooling to drive this back-and-forth motion across a giant range. This type of telescoping actuator can produce over an 8,600 percent change in length, compared to around 70 percent for our previous coils.”
Li said one potential application for the spiral-shaped coil might be thermally responsive clothing. Instead of a down-filled jacket, a coat that incorporates many small coils could change the loft and insulating power of the garment in response to temperature.
In the laboratory, Haines and Li have produced spools of coiled polymer muscle threads suitable for sewing. “We have shown that these thermally responsive fibers can be used in conventional machines, such as looms, knitting machines and sewing machines,” Li said. “As we move forward with our research, and scale it up, we hope to incorporate our ideas into functional fabrics and textiles for a variety of purposes, from clothing to environmentally responsive architecture to dynamic art sculptures.”
Designed specifically for tasks that require heavy lifting, such as commerce or nursing
Researchers from the Tokyo University of Science in Japan are now developing a light exoskeleton concept that can carry items as heavy as 40 kilograms with little to no difficulty. Created by professor Hiroshi Kobayashiand his team of experts, the exoskeleton is affixed to the hips and shoulders by straps and a padded waistband, while its A-shaped frame is equipped with four pneumatic artificial muscles (lightweight rubber blades encased in mesh) that contract when pressurized air is pumped in and can exert up to 30 kilograms of instant support for extra strenuous tasks. The frame is specially designed to augment the functions of the arms and back specifically in tasks that require heavy lifting, such as commerce or nursing.