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Wednesday, November 23rd, 2016
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Nylon fibers made to flex like muscles 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.
Long-lived material
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.”
Seyed Mirvakili was supported by the Natural Sciences and Engineering Research Council of Canada. | | 1:00p |
The science of friction on graphene Graphene, a two-dimensional form of carbon in sheets just one atom thick, has been the subject of widespread research, in large part because of its unique combination of strength, electrical conductivity, and chemical stability. But despite many years of study, some of graphene’s fundamental properties are still not well-understood, including the way it behaves when something slides along its surface.
Now, using powerful computer simulations, researchers at MIT and elsewhere have made significant strides in understanding that process, including why the friction varies as the object sliding on it moves forward, instead of remaining constant as it does with most other known materials.
The findings are presented this week in the journal Nature, in a paper by Ju Li, professor of nuclear science and engineering and of materials science and engineering at MIT, and seven others at MIT, the University of Pennsylvania, and universities in China and Germany.
Graphite, a bulk material composed of many layers of graphene, is a well-known solid lubricant. (In other words, like oil, it can be added in between contacting materials to reduce friction.) Recent research suggests that even one or a few layers of graphene can also provide effective lubrication. This may be used in small-scale thermal and electrical contacts and other nanoscale devices. In such cases, an understanding of the friction between two pieces of graphene, or between graphene and another material, is important for maintaining a good electrical, thermal, and mechanical connection. Researchers had previously found that while one layer of graphene on a surface reduces friction, having a few more was even better. However, the reason for this was not well-explained before, Li says.
“There is this broad notion in tribology that friction depends on the true contact area,” Li says — that is, the area where two materials are really in contact, down to the atomic level. The “true” contact area is often substantially smaller than it would otherwise appear to be if observed at larger size scales. Determining the true contact area is important for understanding not only the degree of friction between the pieces, but also other characteristics such as the electrical conduction or heat transfer.
For example, explains co-author Robert Carpick of the University of Pennsylvania, “When two parts in a machine make contact, like two teeth of steel gears, the actual amount of steel in contact is much smaller than it appears, because the gear teeth are rough, and contact only occurs at the topmost protruding points on the surfaces. If the surfaces were polished to be flatter so that twice as much area was in contact, the friction would then be twice as high. In other words, the friction force doubles if the true area of direct contact doubles.”
But it turns out that the situation is even more complex than scientists had thought. Li and his colleagues found that there are also other aspects of the contact that influence how friction force gets transferred across it. “We call this the quality of contact, as opposed to the quantity of contact measured by the ‘true contact’ area,” Li explains.
Experimental observations had shown that when a nanoscale object slides along a single layer of graphene, the friction force actually increases at first, before eventually leveling off. This effect lessens and the leveled-off friction force decreases when sliding on more and more graphene sheets. This phenomenon was also seen in other layered materials including molybdenum disulfide. Previous attempts to explain this variation in friction, not seen in anything other than these two-dimensional materials, had fallen short.
To determine the quality of contact, it is necessary to know the exact position of each atom on each of the two surfaces. The quality of contact depends on how well-aligned the atomic configurations are in the two surfaces in contact, and on the synchrony of these alignments. According to the computer simulations, these factors turned out to be more important than the traditional measure in explaining the materials’ frictional behavior, according to Li.
“You cannot explain the increase in friction” as the material begins to slide “by just the contact area,” Li says. “Most of the change in friction is actually due to change in the quality of contact, not the true contact area.” The researchers found that the act of sliding causes graphene atoms to make better contact with the object sliding along it; this increase in the quality of contact leads to the increase in friction as sliding proceeds and eventually levels off. The effect is strong for a single layer of graphene because the graphene is so flexible that the atoms can move to locations of better contact with the tip.
A number of factors can affect the quality of contact, including rigidity of the surfaces, slight curvatures, and gas molecules that get in between the two solid layers, Li says. But by understanding the way the process works, engineers can now take specific steps to alter that frictional behavior to match a particular intended use of the material. For example, “prewrinkling” of the graphene material can give it more flexibility and improve the quality of contact. “We can use that to vary the friction by a factor of three, while the true contact area barely changes,” he says.
“In other words, it’s not just the material itself” that determines how it slides, but also its boundary condition — including whether it is loose and wrinkled or flat and stretched tight, he says. And these principles apply not just to graphene but also to other two-dimensional materials, such as molybdenum disulfide, boron nitride, or other single-atom or single-molecule-thick materials.
“Potentially, a moving mechanical contact could be used as a way to make very good power switches in small electronic devices,” Li says. But that is still some ways off; while graphene is a promising material being widely studied, “we’re still waiting to see graphene electronics and 2-D electronics take off. It’s an emerging field.”
“Researchers have studied the unique frictional behavior of graphene for many years, but the complex mechanisms underlying these observations are still not fully understood,” says Ashlie Martini, an associate professor of engineering at the University of California at Merced, who was not involved in this work. “This paper tackles the challenge head on and provides new insights into the origins of friction on graphene that I anticipate will be applicable to two-dimensional materials in general.”
Martini adds: “The authors of the paper correctly suggest that their work could be used as a foundation for ‘tuning’ friction on graphene. Actually implementing this tuning has the potential for significant impact, and an exciting next step based on this research would be to implement the proposed tuning as a first step toward controllable friction in scientific and engineering applications.”
Besides Li and Carpick, the research team included former MIT and University of Pennsylvania visiting student Suzhi Li, now a Humboldt Research Fellow in Germany; Qunyang Li at Tsinghua University in China; Xin Liu at the University of Pennsylvania and now at Intel; Peter Gumbsch at Karlsruhe Institute of Technology in Germany; and Xiangdong Ding and Jun Sun at Xi’an Jiaotong University in China.
The work was supported by the National Science Foundation. | | 2:00p |
Saharan dust in the wind Every year, trade winds over the Sahara Desert sweep up huge plumes of mineral dust, transporting hundreds of teragrams — enough to fill 10 million dump trucks — across North Africa and over the Atlantic Ocean. This dust can be blown for thousands of kilometers and settle in places as far away as Florida and the Bahamas.
The Sahara is the largest source of windblown dust to the Earth’s atmosphere. But researchers from MIT, Yale University, and elsewhere now report that the African plume was far less dusty between 5,000 and 11,000 years ago, containing only half the amount of dust that is transported today.
In a paper published today in Science Advances, the researchers have reconstructed the African dust plume over the last 23,000 years and observed a dramatic reduction in dust beginning around 11,000 years ago. They say this weakened plume may have allowed more sunlight to reach the ocean, increasing its temperature by 0.15 degrees Celsius — a small but significant spike that likely helped whip up monsoons over North Africa, where climate at the time was far more temperate and hospitable than it is today.
“In the tropical ocean, fractions of a degree can cause big differences in precipitation patterns and winds,” says co-author David McGee, the Kerr-McGee Career Development Assistant Professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “It does seem like dust variations may have large enough effects that it’s important to know how big those impacts were in past and future climates.”
McGee’s co-authors include lead author Ross Williams, a former graduate student at MIT; along with Christopher Kinsley, Irit Tal, and David Ridley from MIT; Shineng Hu and Alexey Fedorov from Yale University; Richard Murray from Boston University; and Peter deMenocal from Columbia University.
A wet Sahara
Around 11,000 years ago, the Earth had just emerged from the last ice age and was beginning a new, interglacial epoch known as the Holocene. Geologists and archaeologists have found evidence that during this period the Sahara was much greener, wetter, and more livable than it is today.
“There was also extensive human settlement throughout the Sahara, with lifestyles that would never be possible today,” McGee says. “Researchers at archaeological sites have found fish hooks and spears in the middle of the Sahara, in places that would be completely uninhabitable today. So there was clearly much more water and precipitation over the Sahara.”
This evidence of wet conditions shows that the region experienced regular monsoon rains during the early Holocene. This was primarily due to the slow wobbling of Earth’s axis, which exposed the Northern Hemisphere to more sunlight during summer; this, in turn, warmed the land and ocean and drew more water vapor — and precipitation — over North Africa. Increased vegetation in the Sahara may have also played a role, absorbing sunlight and heating the surface, drawing more moisture over the land.
“The mysterious thing is, if you try to simulate all these changes in these early and mid-Holocene climates, the models intensify the monsoons, but nowhere near the amounts suggested by the paleodata,” McGee says. “One of the things not factored into these simulations is changes in windblown dust.”
Tracking a dust plume
In their results published today, McGee and colleagues propose a reduction in African dust may indeed have contributed to increasing monsoon rains in the region. The researchers came to their conclusion after estimating the amount of long-range windblown dust emitted from Africa over the last 23,000 years, from the end of the last ice age to today.
They focused on dust transported long distances, as these particles are small and light enough to be lifted and carried through the atmosphere for days before settling thousands of kilometers away from their source. This fine-grained dust scatters incoming solar radiation, cooling the ocean’s surface and potentially affecting precipitation patterns, depending on how much dust is in the air.
To estimate how the African dust plume has changed over thousands of years, the team looked for places where dust should accumulate rapidly. Dust can sink to the floor of open ocean, but there layers of sediment build up very slowly, at a rate of 1 centimeter every 1,000 years.
Places like the Bahamas, by contrast, accumulate sediment much more quickly, making it easier for scientists to determine the ages of particular sediment layers. What’s more, it’s been shown that most of the windblown dust that has accumulated in the Bahamas originated not from local regions such as the U.S., but from the Sahara.
Dust’s climate role
McGee and his colleagues obtained sediment core samples from the Bahamas that were collected in the 1980s by scientists from the Woods Hole Oceanographic Institution. They brought the samples back to the lab and analyzed their chemical composition, including isotopes of thorium — an element that exists in windblown dust worldwide, at known concentrations.
They determined how much dust was in each sediment layer by measuring the primary isotope of thorium, and determined how fast it was accumulating by measuring the amount of a rare thorium isotope in each layer.
In this way, the team analyzed sediment layers from the last 23,000 years, and showed that around 16,000 years ago, toward the end of the last ice age, the dust plume was at its highest, lofting at least twice the amount of dust over the Atlantic, compared to today. However, between 5,000 and 11,000 years ago, this plume weakened significantly, with just half the amount of today’s windblown dust.
Colleagues at Yale University then plugged their estimates into a climate model to see how such changes in the African dust plume would affect both ocean temperatures in the North Atlantic and overall climate in North Africa. The simulations showed that a drop in long-range windblown dust would raise sea surface temperatures by 0.15 degrees Celsius, drawing more water vapor over the Sahara, which would have helped to drive more intense monsoon rains in the region.
“The modeling showed that if dust had even relatively small impacts on sea surface temperatures, this could have pronounced impacts on precipitation and winds both in the north Atlantic and over North Africa,” McGee says. Noting that the next key step is to reduce uncertainties in the modeling of dust’s climate impacts, he adds: “We’re not saying, the expansion of monsoon rains into the Sahara was caused solely by dust impacts. We’re saying we need to figure out how big those dust impacts are, to understand both past and future climates.”
Ina Tegen, a professor at the Leibniz Institute for Tropospheric Research in Germany, says the group’s results suggest that “dust effects today may be considerable as well.”
“Dust loads vary with changing climate, and due to the effects of dust on [solar] radiation, ice formation in clouds, and the carbon cycle, this may cause important climate feedbacks,” says Tegen, who was not involved in the research. “The changing climate since the last ice age can be considered a ‘natural laboratory’ to study such effects. Understanding the past is the basis for predicting future changes with any confidence.”
This research was supported, in part, by the National Science Foundation. | | 2:00p |
Enhancing education from pre-K to MIT and beyond To improve education — whether pK-12, college, professional training, or online courses — one must first gain an understanding of how people learn. Applying that learning on a large scale requires a forward-thinking focus on expanding the reach of high-quality education for learners of all ages, all across the globe.
These are the challenges that drive two Institute-wide initiatives announced by President L. Rafael Reif earlier this year: the MIT Integrated Learning Initiative (MITili) and the pK-12 Action Group.
The integrated sciences of learning, now emerging as a significant field of research, is at the core of MITili (pronounced “mightily”). By applying scientific rigor to investigate the methods that lead to effective learning, MITili aims to enhance the educational experience at all perspectives — from improving education at MIT to inspiring lifelong learning online to advancing the Institute’s campaign to promote STEM understanding within elementary, middle, and high schools.
Fueled by MIT’s residential education and global online efforts, MITili pulls together resources from across campus to integrate faculty insights and foster rigorous quantitative and qualitative research in education. The initiative leverages expertise in cognitive psychology, neuroscience, economics, engineering, public policy, and other fields.
It is this cross-discipline thinking that led to the recent appointment of Parag Pathak, professor of economics and a founder of the School Effectiveness and Inequality Initiative (SEII), as MITili deputy director. Pathak, who has worked extensively with the Boston school system to make it easier to navigate school assignment systems and level the playing field for city families, will join MITili Director John Gabrieli, a professor in the Department of Brain and Cognitive Sciences, in guiding the group’s vision. Based on Pathak’s background, the new position is a natural fit.
“MIT is known for solving problems, so if we can improve how people learn then we can improve how much education they get,” Pathak explains. “Individuals who have more access to education not only learn more but live longer and are better citizens.”
Supported by two new staff members, Associate Director Jeff Dieffenbach and Program Coordinator Steve Nelson, Pathak and MITili are off and running on several projects, including continued exploration into Boston’s school assignments, an in-depth analysis of charter schools and their effectiveness for special education students, and an upcoming study on the impact of affirmative action policies in education. Says Pathak: “A lot of our work is very fresh and new. By taking a scientific perspective to solve problems, we are breaking free of the old way of thinking.”
The Office of Digital Learning has also established a separate, though related, initiative called the pK-12 Action Group, which enables a diverse MIT community to collaborate on STEM projects for pre-kindergarten through 12th grade students and teachers. By working together, MIT faculty, staff, and students amplify their impact on existing efforts — studies, classroom technologies, curriculum, teacher professional development — while driving new work and outreach, all with the goal of understanding how learning happens and transforming how students learn.
Professor Eric Klopfer, director of both the MIT Scheller Teacher Education Program and MIT Education Arcade, has been involved with the pK-12 Action Group since its early stages. Recently named co-chair of the pK-12 advisory group, Klopfer joins Professor Angela Belcher and provides breadth to the leadership team. Associate Director Claudia Urrea brings over 20 years of experience in the field of education and technology. She works together with the faculty to coordinate direction and vision and to engage the larger pK-12 community at MIT.
“We come at this from different perspectives,” Klopfer says. “Angie is passionate about science and engineering and making them accessible to all, while I come from a more established learning and education focus. Both angles are important to tackle these global challenges and make a significant impact on pk-12 education. We’re thinking big.”
Collaboration with the community is key. For this reason, the effort is led by practicing educators, not administrators. And it’s why the work is already making a big difference, with the following initiatives:
- Connected Learning Initiative (CLIx), a cross-unit project with MIT’s Office of Digital Learning, gives thousands of young people from under-served communities in India an opportunity for quality education through the meaningful integration of technology;
- Teaching Systems Lab (TSL), working in partnership with the Woodrow Wilson National Fellowship Foundation, examines what it takes to prepare new teachers for today’s classrooms and the systems needed to help these teachers transform learning through tomorrow’s learning environments; and
- on-campus workshops, which leverage many existing pK-12 efforts at MIT, are designed to provide professional teacher development, advance STEM curricula, and explore new ways to enhance educational experiences.
The goal of influencing how people around the world get educated is big, bold — and shared by both MITili and the pK-12 Action Group. But that doesn’t mean the goal is out of reach. As Pathak says: “It all starts with the science of learning.” |
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