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Theoretically, two layers are better than one for solar-cell efficiency

Solar cells have come a long way, but inexpensive, thin film solar cells are still far behind more expensive, crystalline solar cells in efficiency. Now, a team of researchers suggests that using two thin films of different materials may be the way to go to create affordable, thin film cells with about 34% efficiency.

“Ten years ago I knew very little about solar cells, but it became clear to me they were very important,” said Akhlesh Lakhtakia, Evan Pugh University Professor and Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State.

Investigating the field, he found that researchers approached solar cells from two sides, the optical side — looking on how the sun’s light is collected — and the electrical side — looking at how the collected sunlight is converted into electricity. Optical researchers strive to optimize light capture, while electrical researchers strive to optimize conversion to electricity, both sides simplifying the other.

“I decided to create a model in which both electrical and optical aspects will be treated equally,” said Lakhtakia. “We needed to increase actual efficiency, because if the efficiency of a cell is less than 30% it isn’t going to make a difference.” The researchers report their results in a recent issue of Applied Physics Letters.

Lakhtakia is a theoretician. He does not make thin films in a laboratory, but creates mathematical models to test the possibilities of configurations and materials so that others can test the results. The problem, he said, was that the mathematical structure of optimizing the optical and the electrical are very different.

Solar cells appear to be simple devices, he explained. A clear top layer allows sunlight to fall on an energy conversion layer. The material chosen to convert the energy, absorbs the light and produces streams of negatively charged electrons and positively charged holes moving in opposite directions. The differently charged particles get transferred to a top contact layer and a bottom contact layer that channel the electricity out of the cell for use. The amount of energy a cell can produce depends on the amount of sunlight collected and the ability of the conversion layer. Different materials react to and convert different wavelengths of light.

“I realized that to increase efficiency we had to absorb more light,” said Lakhtakia. “To do that we had to make the absorbent layer nonhomogeneous in a special way.”

That special way was to use two different absorbent materials in two different thin films. The researchers chose commercially available CIGS — copper indium gallium diselenide — and CZTSSe — copper zinc tin sulfur selenide — for the layers. By itself, CIGS’s efficiency is about 20% and CZTSSe’s is about 11%.

These two materials work in a solar cell because the structure of both materials is the same. They have roughly the same lattice structure, so they can be grown one on top of the other, and they absorb different frequencies of the spectrum so they should increase efficiency, according to Lakhtakia.

“It was amazing,” said Lakhtakia. “Together they produced a solar cell with 34% efficiency. This creates a new solar cell architecture — layer upon layer. Others who can actually make solar cells can find other formulations of layers and perhaps do better.”

According to the researchers, the next step is to create these experimentally and see what the options are to get the final, best answers.

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Materials provided by Penn State. Original written by A’ndrea Elyse Messer. Note: Content may be edited for style and length.

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Thin layer protects battery, allows cold charging

In the search for a reliable, quick-charging, cold-weather battery for automobiles, a self-assembling, thin layer of electrochemically active molecules may be the solution, according to a team or researchers.

“The lithium metal battery is the next generation of battery after the lithium ion battery,” said Donghai Wang, professor of mechanical engineering and a key researcher in the Battery and Energy Storage Technology Center, Penn State. “It uses a lithium anode and has higher energy density, but has problems with dendritic growth, low efficiency and low cycle life.”

The solution to these problems, according to the researchers, is a self-assembling monolayer that is electrochemically active so that it can decompose into its proper components and protect the surface of the lithium anode.

The battery is composed of the lithium anode, a lithium metal oxide cathode and an electrolyte which also has lithium-ion conducting materials and the protective, thin film layer. Without this layer, the battery would tend to grow lithium crystal spikes if charged rapidly or under cold conditions. These lithium spikes eventually short out the battery, greatly decreasing the usefulness and cycle life.

“The key is to tune the molecular chemistry to self-assemble on the surface,” said Wang. “The monolayer will provide a good solid electrolyte interface when charging, and protect the lithium anode.”

The researchers deposit the monolayer on a thin copper layer. When the battery charges, lithium hits the monolayer and decomposes to form a stable interfacial layer. Some lithium is deposited on the copper along with the remaining layer, and the decomposed portion of the original layer reforms on top of the lithium, protecting the lithium and preventing dendrites of lithium from forming.

According to the researchers, this technology can increase the amount of storage capacity of the battery and can increase the number of times the battery can be charged. However, at this point, the battery can only be charged a few hundred times. The researchers reported their work in a recent issue of Nature Energy.

“The key is that this technology shows an ability to form a layer when needed on time and decompose and spontaneously reform so it will stay on the copper and also cover the surface of the lithium,” said Wang. “Eventually it could be used for drones, cars, or some very small batteries used for underwater applications at low temperatures.

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Materials provided by Penn State. Original written by A’ndrea Elyse Messer. Note: Content may be edited for style and length.

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Robust high-performance data storage through magnetic anisotropy

The latest generation of magnetic hard drives is made of magnetic thin films, which are invar materials. They allow extremely robust and high data storage density by local heating of ultrasmall nano-domains with a laser, so called heat assisted magnetic recording or HAMR. The volume in such invar materials hardly expands despite heating. A technologically relevant material for such HAMR data memories are thin films of iron-platinum nanograins. An international team led by the joint research group of Prof. Dr. Matias Bargheer at HZB and the University of Potsdam has now observed experimentally for the first time how a special spin-lattice interaction in these iron-platinum thin films cancels out the thermal expansion of the crystal lattice.

In thermal equilibrium, iron-platinum (FePt) belongs to the class of invar materials, which hardly expand at all when heated. This phenomenon was observed as early as 1897 in the nickel-iron alloy “Invar,” but it is only in recent years that experts have been able to understand which mechanism are driving it: Normally, heating of solids leads to lattice vibrations which cause expansion because the vibrating atoms need more space. Surprisingly, however, heating the spins in FePt leads to the opposite effect: the warmer the spins are, the more the material contracts along the direction of magnetisation. The result is the property known from Invar: minimal expansion.

A team led by Prof. Matias Bargheer has now experimentally compared this fascinating phenomenon for the first time on different iron-platinum thin films. Bargheer heads a joint research group at Helmholtz-Zentrum Berlin and the University of Potsdam. Together with colleagues from Lyon, Brno and Chemnitz, he wanted to investigate how the behavior of perfectly crystalline FePt layers differs from the FePt thin films used for HAMR memories. These consist of crystalline nanograins of stacked monatomic layers of iron and platinum embedded in a carbon matrix.

The samples were locally heated and excited with two laser pulses in quick succession and then measured by X-ray diffraction to determine how strongly the crystal lattice expands or contracts locally.

“We were surprised to find that the continuous crystalline layers expand when heated briefly with laser light, while loosely arranged nano grains contract in the same crystal orientation,” explains Bargheer. “HAMR data memories, on the other hand, whose nano-grains are embedded in a carbon matrix and grown on a substrate react much weaker to laser excitation: They first contract slightly and then expand slightly.”

“Through these experiments with ultrashort X-ray pulses, we have been able to determine how important the morphology of such thin films is,” says Alexander von Reppert, first author of the study and PhD student in Bargheer’s group. The secret is transverse contraction, also known as the Poisson effect. “Everyone who has ever pressed firmly on an eraser knows this,” says Bargheer. “The rubber gets thicker in the middle.” And Reppert adds: “The nanoparticles can do that too, whereas in the perfect film there is no room for expansion in the plane, which would have to go along with the spin driven contraction perpendicular to the film.”

So FePt, embedded in a carbon matrix, is a very special material. It not only has exceptionally robust magnetic properties. Its thermomechanical properties also prevent excessive tension from being created when heated, which would destroy the material — and that is important for HAMR!

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Adhesive film turns smartwatch into biochemical health monitoring system

UCLA engineers have designed a thin adhesive film that could upgrade a consumer smartwatch into a powerful health-monitoring system. The system looks for chemical indicators found in sweat to give a real-time snapshot of what’s happening inside the body. A study detailing the technology was published in the journal of Science Advances.

Smartwatches can already help keep track of how far you’ve walked, how much you’ve slept and your heart rate. Newer models even promise to monitor blood pressure. Working with a tethered smartphone or other devices, someone can use a smartwatch to keep track of those health indicators over a long period of time.

What these watches can’t do, yet, is monitor your body chemistry. For that, they need to track biomarker molecules found in body fluids that are highly specific indicators of our health, such as glucose and lactate, which tell how well your body’s metabolism is working.

To address that need, the researchers engineered a disposable, double-sided film that attaches to the underside of a smartwatch. The film can detect molecules such as metabolites and certain nutrients that are present in body sweat in very tiny amounts. They also built a custom smartwatch and an accompanying app to record data.

“The inspiration for this work came from recognizing that we already have more than 100 million smartwatches and other wearable tech sold worldwide that have powerful data-collection, computation and transmission capabilities,” said study leader Sam Emaminejad, an assistant professor of electrical and computer engineering at the UCLA Samueli School of Engineering. “Now we have come up with a solution to upgrade these wearables into health-monitoring platforms, enabling them to measure molecular-level information so that they give us a much deeper understanding of what’s happening inside our body in real time.”

The skin-touching side of the adhesive film collects and analyzes the chemical makeup of droplets of sweat. The watch-facing side turns those chemical signals into electrical ones that can be read, processed and then displayed on the smartwatch.

The co-lead authors on the paper are graduate student Yichao Zhao and postdoctoral scholar Bo Wang. Both are members of Emaminejad’s Interconnected and Integrated Bioelectronics Lab at UCLA.

“By making our sensors on a double-sided adhesive and vertically conductive film, we eliminated the need for the external connectors,” Zhao said. “In this way, not only have we made it easier to integrate sensors with consumer electronics, but we’ve also eliminated the effect of a user’s motion that can interfere with the chemical data collection.”

“By incorporating appropriate enzymatic-sensing layers in the film, we specifically targeted glucose and lactate, which indicate body metabolism levels, and nutrients such as choline,” Wang said.

While the team designed a custom smartwatch and app to work with the system, Wang said the concept could someday be applied to popular models of smartwatches.

The researchers tested the film on someone who was sedentary, someone doing office work and people engaged in vigorous activity, such as boxing, and found the system was effective in a wide variety of scenarios. They also noted that the stickiness of the film was sufficient for it to stay on the skin and on the watch without the need for a wrist strap for an entire day.

Over the past few years, Emaminejad has led research on using wearable technology to detect indicator molecules through sweat. This latest study shows a new way that such technologies could be widely adopted.

“We are particularly excited about our technology because by transforming our smartwatches and wearable tech into biomonitoring platforms, we could capture multidimensional, longitudinal and physiologically relevant datasets at an unprecedented scale, basically across hundreds of millions of people,” Emaminejad said. “This thin sensing film that works with a watch shows such a path forward.”

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A close look at thin ice

On frigid days, water vapor in the air can transform directly into solid ice, depositing a thin layer on surfaces such as a windowpane or car windshield. Though commonplace, this process is one that has kept physicists and chemists busy figuring out the details for decades.

In a new Nature paper, an international team of scientists describe the first-ever visualization of the atomic structure of two-dimensional ice as it formed. Insights from the findings, which were driven by computer simulations that inspired experimental work, may one day inform the design of materials that make ice removal a simpler and less costly process.

“One of the things that I find very exciting is that this challenges the traditional view of how ice grows,” says Joseph S. Francisco, an atmospheric chemist at the University of Pennsylvania and an author on the paper.

“Knowing the structure is very important,” adds coauthor Chongqin Zhu, a postdoctoral fellow in Francisco’s group who led much of the computational work for the study. “Low-dimensional water is ubiquitous in nature and plays a critical role in an incredibly broad spectrum of sciences, including materials science, chemistry, biology, and atmospheric science.

“It also has practical significance. For example, removing ice is critical when it comes to things like wind turbines, which cannot function when they are covered in ice. If we understand the interaction between water and surfaces, then we might be able to develop new materials to make this ice removal easier.”

In recent years, Francisco’s lab has devoted considerable attention to studying the behavior of water, and specifically ice, at the interface of solid surfaces. What they’ve learned about ice’s growth mechanisms and structures in this context helps them understand how ice behaves in more complex scenarios, like when interacting with other chemicals and water vapor in the atmosphere.

“We’re interested in the chemistry of ice at the transition with the gas phase, as that’s relevant to the reactions that are happening in our atmosphere,” Francisco explains.

To understand basic principles of ice growth, researchers have entered this area of study by investigating two-dimensional structures: layers of ice that are only several water molecules thick.

In previous studies of two-dimensional ice, using computational methods and simulations, Francisco, Zhu, and colleagues showed that ice grows differently depending on whether a surface repels or attracts water, and the structure of that surface.

In the current work, they sought real-world verification of their simulations, reaching out to a team at Peking University to see if they could obtain images of two-dimensional ice.

The Peking team employed super-powerful atomic force microscopy, which uses a mechanical probe to “feel” the material being studied, translating the feedback into nanoscale-resolution images. Atomic force microscopy is capable of capturing structural information with a minimum of disruption to the material itself, allowing the scientists to identify even unstable intermediate structures that arose during the process of ice formation.

Virtually all naturally occurring ice on Earth is known as hexagonal ice for its six-sided structure. This is why snowflakes all have six-fold symmetry. One plane of hexagonal ice has a similar structure to that of two-dimensional ice and can terminate in two types of edges — “zigzag” or “armchair.” Usually this plane of natural ice terminates with a zigzag edges.

However, when ice is grown in two dimensions, researchers find that the pattern of growth is different. The current work, for the first time, shows that the armchair edges can be stabilized and that their growth follows a novel reaction pathway.

“This is a totally different mechanism from what was known,” Zhu says.

Although the zigzag growth patterns were previously believed to only have six-membered rings of water molecules, both Zhu’s calculations and the atomic force microscopy revealed an intermediate stage where five-membered rings were present.

This result, the researchers say, may help explain the experimental observations reported in their 2017 PNAS paper, which found that ice could grow in two different ways on a surface, depending on the properties of that surface.

In addition to lending insight into future design of materials conducive to ice removal, the techniques used in the work are also applicable to probe the growth of a large family of two-dimensional materials beyond two-dimensional ices, thus opening a new avenue of visualizing the structure and dynamics of low-dimensional matter.

For chemist Jeffrey Saven, a professor in Penn Arts & Sciences who was not directly involved in the current work, the collaboration between the theorists in Francisco’s group and their colleagues in China called to mind a parable he learned from a mentor during his training.

“An experimentalist is talking with theorists about data collected in the lab. The mediocre theorist says, ‘I can’t really explain your data.’ The good theorist says, ‘I have a theory that fits your data.’ The great theorist says, ‘That’s interesting, but here is the experiment you should be doing and why.'”

To build on this successful partnership, Zhu, Francisco, and their colleagues are embarking on theoretical and experimental work to begin to fill in the gaps related to how two-dimensional ice builds into three dimensions.

“The two-dimensional work is fundamental to laying the background,” says Francisco. “And having the calculations verified by experiments is so good, because that allows us to go back to the calculations and take the next bold step toward three dimensions.”

“Looking for features of three-dimensional ice will be the next step,” Zhu says, “and should be very important in looking for applications of this work.”

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Robots made from self-folding kirigami materials

Researchers have demonstrated how kirigami-inspired techniques allow them to design thin sheets of material that automatically reconfigure into new two-dimensional (2D) shapes and three-dimensional (3D) structures in response to environmental stimuli. The researchers created a variety of robotic devices as a proof of concept for the approach.

“This is the first case that we know of in which 2D kirigami patterns autonomously reshape themselves into distinct 3D structures without mechanical input,” says Jie Yin, an assistant professor of mechanical and aerospace engineering at North Carolina State University and corresponding author of a paper on the work. “Instead, we apply energy in the form of heat, and the material rearranges itself.”

Kirigami is an art form in which a single piece of paper is cut and folded to create new shapes and structures.

The new “active kirigami” concept relies on a three-layered material, consisting of two outer layers that are not responsive to heat, and a polymer layer in the middle that contracts in response to heat. The shape and structure of the material are controlled in two ways. Through-cuts, which penetrate all three layers, control the material’s range of motion. Etchings, which penetrate the outer layers and expose the heat-responsive polymer, control the angle and direction at which the material folds, as well as how far it folds. As the material folds, it opens the through-cuts, shifting the shape of the sheets into 2D or 3D designs.

“We can make a 2D template with the same pattern of through-cuts and use it to create many different 3D structures by making slight changes in the etching,” Yin says. “This effectively makes the active kirigami sheets programmable.”

As part of their proof of concept, researchers used their kirigami approach to create a suite of thermoresponsive kirigami machines, including simple gripping devices and self-folding boxes. The researchers also created a soft robot with a kirigami body and pneumatic legs. By switching the orientation of the body, the researchers could rapidly reposition the legs, changing the robot’s direction of movement.

“We used a temperature-responsive polymer for this work, but there’s no reason to think that other stimuli-responsive polymer materials — like photoactive liquid crystals — wouldn’t work as well,” Yin says. “We’re excited to explore the potential range of applications for these programmable, active kirigami materials.”

Video of the proof-of-concept machines can be seen at https://www.youtube.com/watch?v=4U2ARa83188&feature=youtu.be.

Video illustrating how active kirigami works can be seen at https://www.youtube.com/watch?v=KozSomLoKIg.

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Damaged hearts rewired with nanotube fibers

Thin, flexible fibers made of carbon nanotubes have now proven able to bridge damaged heart tissues and deliver the electrical signals needed to keep those hearts beating.

Scientists at Texas Heart Institute (THI) report they have used biocompatible fibers invented at Rice University in studies that showed sewing them directly into damaged tissue can restore electrical function to hearts.

“Instead of shocking and defibrillating, we are actually correcting diseased conduction of the largest major pumping chamber of the heart by creating a bridge to bypass and conduct over a scarred area of a damaged heart,” said Dr. Mehdi Razavi, a cardiologist and director of Electrophysiology Clinical Research and Innovations at THI, who co-led the study with Rice chemical and biomolecular engineer Matteo Pasquali.

“Today there is no technology that treats the underlying cause of the No. 1 cause of sudden death, ventricular arrhythmias,” Razavi said. “These arrhythmias are caused by the disorganized firing of impulses from the heart’s lower chambers and are challenging to treat in patients after a heart attack or with scarred heart tissue due to such other conditions as congestive heart failure or dilated cardiomyopathy.”

Results of the studies on preclinical models appear as an open-access Editor’s Pick in the American Heart Association’s Circulation: Arrhythmia and Electrophysiology. The association helped fund the research with a 2015 grant.

The research springs from the pioneering 2013 invention by Pasquali’s lab of a method to make conductive fibers out of carbon nanotubes. The lab’s first threadlike fibers were a quarter of the width of a human hair, but contained tens of millions of microscopic nanotubes. The fibers are also being studied for electrical interfaces with the brain, for use in cochlear implants, as flexible antennas and for automotive and aerospace applications.

The experiments showed the nontoxic, polymer-coated fibers, with their ends stripped to serve as electrodes, were effective in restoring function during monthlong tests in large preclinical models as well as rodents, whether the initial conduction was slowed, severed or blocked, according to the researchers. The fibers served their purpose with or without the presence of a pacemaker, they found.

In the rodents, they wrote, conduction disappeared when the fibers were removed.

“The reestablishment of cardiac conduction with carbon nanotube fibers has the potential to revolutionize therapy for cardiac electrical disturbances, one of the most common causes of death in the United States,” said co-lead author Mark McCauley, who carried out many of the experiments as a postdoctoral fellow at THI. He is now an assistant professor of clinical medicine at the University of Illinois College of Medicine.

“Our experiments provided the first scientific support for using a synthetic material-based treatment rather than a drug to treat the leading cause of sudden death in the U.S. and many developing countries around the world,” Razavi added.

Many questions remain before the procedure can move toward human testing, Pasquali said. The researchers must establish a way to sew the fibers in place using a minimally invasive catheter, and make sure the fibers are strong and flexible enough to serve a constantly beating heart over the long term. He said they must also determine how long and wide fibers should be, precisely how much electricity they need to carry and how they would perform in the growing hearts of young patients.

“Flexibility is important because the heart is continuously pulsating and moving, so anything that’s attached to the heart’s surface is going to be deformed and flexed,” said Pasquali, who has appointments at Rice’s Brown School of Engineering and Wiess School of Natural Sciences.

“Good interfacial contact is also critical to pick up and deliver the electrical signal,” he said. “In the past, multiple materials had to be combined to attain both electrical conductivity and effective contacts. These fibers have both properties built in by design, which greatly simplifies device construction and lowers risks of long-term failure due to delamination of multiple layers or coatings.”

Razavi noted that while there are many effective antiarrhythmic drugs available, they are often contraindicated in patients after a heart attack. “What is really needed therapeutically is to increase conduction,” he said. “Carbon nanotube fibers have the conductive properties of metal but are flexible enough to allow us to navigate and deliver energy to a very specific area of a delicate, damaged heart.”

Rice alumna Flavia Vitale, now an assistant professor of neurology and of physical medicine and rehabilitation at the University of Pennsylvania, and Stephen Yan, a graduate student at Rice, are co-lead authors of the paper.

Co-authors are Colin Young and Julia Coco of Rice; Brian Greet of THI and Baylor St. Luke’s Medical Center; Marco Orecchioni and Lucia Delogu of the Città della Speranza Pediatric Research Institute, Padua, Italy; Abdelmotagaly Elgalad, Mathews John, Doris Taylor and Luiz Sampaio, all of THI; and Srikanth Perike of the University of Illinois at Chicago. Pasquali is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering, a professor of materials science and nanoengineering and of chemistry.

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New retroreflective material could be used in nighttime color-changing road signs

A thin film that reflects light in intriguing ways could be used to make road signs that shine brightly and change color at night, according to a study that will be published on Aug. 9 in Science Advances.

The technology could help call attention to important traffic information when it’s dark, with potential benefits for both drivers and pedestrians, researchers say.

The film consists of polymer microspheres laid down on the sticky side of a transparent tape. The material’s physical structure leads to an interesting phenomenon: When white light shines on the film at night, some observers will see a single, stable color reflected back, while others will see changing colors. It all depends on the angle of observation and whether the light source is moving.

The research was led by Limin Wu, PhD, at Fudan University in China, whose group developed the material. Experts on optics at the University at Buffalo made significant contributions to the work, providing insight into potential applications for the film, such as employing it in nighttime road signs.

“You can use this material to make smart traffic signs,” says Qiaoqiang Gan, PhD, an associate professor of electrical engineering in the UB School of Engineering and Applied Sciences and a co-first author of the new study. “If a person is listening to loud music or isn’t paying attention while they’re walking or driving, a color-changing sign can help to better alert them to the traffic situation.”

Testing color-changing road signs at night

In one set of experiments, researchers created a speed limit sign with letters and numbers made from the new film. The scientists placed a white light nearby to illuminate the sign, and when a fast-moving car drove past, the color of the characters on the sign appeared to flicker from the perspective of the driver as the driver’s viewing angle changed.

In other tests, the team applied the new material to a series of markers lining the side of a road, denoting the boundary of the driving lane. As a car approached, the markers lit up in bright colors, reflecting light from the vehicle’s headlights.

From the driver’s perspective, the markers’ color remained stable. But to a pedestrian standing at the side of the road, the color of the markers appeared to flicker as the car and its headlights sped past.

“If the car goes faster, the pedestrian will see the color change more quickly, so the sign tells you a lot about what is going on,” says co-author Haomin Song, PhD, UB assistant professor of research in electrical engineering.

The experiments were performed in China, but Gan, Song and UB PhD graduate Dengxin Ji, all in the Department of Electrical Engineering, helped design the tests. In addition to his position at UB, Gan was a summer visiting professor at the University of Shanghai for Science and Technology during the period of the collaborative work.

The study was funded by the National Key Research and Development Program of China and the National Natural Science Foundation of China.

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