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    This ‘thermal cloak’ keeps spaces from getting either too hot or cold

    If you’ve ever burned your hands on a car steering wheel, you know how hot the inside of a car can get on a summer day. But a new fabric could one day help cars and other objects stay cool in the summer and warm in the winter.

    Researchers created a prototype of the fabric, which acts as a “thermal cloak” that keeps the space underneath it from getting too hot or too cold. The cloak, described in the July 11 Device, doesn’t require an external power source, which could reduce energy consumption associated with heating and cooling (SN: 9/28/18).

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    Globally, heating and cooling make up 38 percent of energy use in buildings and 12 percent of total energy consumption. Materials like this thermal cloak could help keep us comfortable during heat waves while reducing carbon dioxide emissions associated with electricity used in temperature control, says Aaswath Raman, an applied physicist at UCLA who was not involved in the study.

    In the new study, Kehang Cui, an engineer at Shanghai Jiao Tong University, and colleagues built the cloak using two layers. The outer layer is made of white silica fibers that reflect visible light, coated with hexagonal boron nitride, a ceramic material that reflects ultraviolet light and helps dissipate heat. Together, the silica fibers and boron nitride reflect 96 percent of the sunlight that hits the fabric. At the same time, the outer layer absorbs heat from the surrounding area and emits that energy as infrared light, which also lowers the temperature under the cloak through a process called radiative cooling.

    Though the outer layer keeps the space under the cloak cooler for longer than an uncovered area, the cloaked space slowly warms up throughout the day. The inner layer, made of aluminum foil, keeps the space warm at night by trapping some of that heat inside, similar to an insulating survival blanket.

    The researchers tested the cloak material’s durability under several extreme conditions. They baked the fabric at 800° Celsius, just about hot enough to melt table salt. They also exposed it to extreme cold by dunking it in liquid nitrogen, subjected it to the same amount of vibration as a rocket launch, doused it in acid and blasted it with fire from a butane torch — all with virtually no changes to the material’s structure or performance. This extreme durability might lend itself to use in spacecraft or extraterrestrial environments, the team says.

    To see the fabric in action, Cui and colleagues built a full-size prototype cloak and tested it on an electric car. On a summer day in Shanghai, the cloak kept the car at about 23° C — up to 8 degrees C lower than the outside temperature and 28 degrees C lower than the inside of an uncloaked car. The cloak also kept the car about 5 degrees C warmer than the outside air on a winter night.

    The cloak “definitely shows the capability of saving energy, but the next step is that we want to demonstrate it in even larger-scale field tests [such as rooftops] to see the impact on our daily lives,” Cui says. More

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    A vegan leather made of dormant fungi can repair itself

    Imagine if a ripped leather jacket could repair itself instead of needing to be replaced.

    This could one day be a reality, if the jacket is fashioned from fungus, researchers report April 11 in Advanced Functional Materials. The team made a self-healing leather from mushrooms’ threadlike structures called mycelium, building on past iterations of the material to allow it to fix itself.

    Mycelium leather is already an emerging product, but it’s produced in a way that extinguishes fungal growth. Elise Elsacker and colleagues speculated that if the production conditions were tweaked, the mycelium could retain its ability to regrow if damaged.

    That novel approach could offer inspiration to other researchers trying to get into the mycelium leather market, says Valeria La Saponara, a mechanical and aerospace engineer at the University of California, Davis.

    Elsacker, a bioengineer now at the Vrije Universiteit Brussel, and her colleagues first grew mycelium in a soup rich in proteins, carbohydrates and other nutrients. A skin formed on the surface of the liquid, which the scientists scooped off, cleaned and dried to make a thin, somewhat fragile leather material. They used temperatures and chemicals mild enough to form the leather but leave parts of the fungus functional. Left dormant were chlamydospores, little nodules on the mycelium that can spring back to life and grow more mycelium when conditions are prime.

    After punching holes in the leather, the researchers doused the area in the same broth used to grow it to revive the chlamydospores. The mycelium eventually regrew over the punctures. Once healed, the hole-punched areas were just as strong as undamaged areas — however, the repairs were visible from one side of the leather.

    Chlamydospores are little nodules on fungi’s threadlike mycelium that can spring back to life. They’re dormant in the punctured leather (left). With the right nutrients, the chlamydospores reanimated and the leather healed itself (middle), but the tiny patches are still slightly visible in the repaired leather (right).E. Elsacker et al/Advanced Functional Materials, 2023

    The technique could potentially go beyond a proof-of-concept and into commercialization in the next decade, says study coauthor Martyn Dade-Robertson, codirector of the Hub for Biotechnology in the Built Environment in Newcastle upon Tyne. But first, the team will need to make the leather stronger and determine how to control the chlamydospores’ growth. Otherwise, he says, someone could “walk out in the rain, and then all of a sudden find that [their] jacket is growing, or perhaps [has] mushrooms popping out of it.”  More

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    These shape-shifting devices melt and re-form thanks to magnetic fields

    Shape-shifting liquid metal robots might not be limited to science fiction anymore.

    Miniature machines can switch from solid to liquid and back again to squeeze into tight spaces and perform tasks like soldering a circuit board, researchers report January 25 in Matter.

    This phase-shifting property, which can be controlled remotely with a magnetic field, is thanks to the metal gallium. Researchers embedded the metal with magnetic particles to direct the metal’s movements with magnets. This new material could help scientists develop soft, flexible robots that can shimmy through narrow passages and be guided externally.  

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    Scientists have been developing magnetically controlled soft robots for years. Most existing materials for these bots are made of either stretchy but solid materials, which can’t pass through the narrowest of spaces, or magnetic liquids, which are fluid but unable to carry heavy objects (SN: 7/18/19).

    In the new study, researchers blended both approaches after finding inspiration from nature (SN: 3/3/21). Sea cucumbers, for instance, “can very rapidly and reversibly change their stiffness,” says mechanical engineer Carmel Majidi of Carnegie Mellon University in Pittsburgh. “The challenge for us as engineers is to mimic that in the soft materials systems.”

    So the team turned to gallium, a metal that melts at about 30° Celsius — slightly above room temperature. Rather than connecting a heater to a chunk of the metal to change its state, the researchers expose it to a rapidly changing magnetic field to liquefy it. The alternating magnetic field generates electricity within the gallium, causing it to heat up and melt. The material resolidifies when left to cool to room temperature.

    Since magnetic particles are sprinkled throughout the gallium, a permanent magnet can drag it around. In solid form, a magnet can move the material at a speed of about 1.5 meters per second. The upgraded gallium can also carry about 10,000 times its weight.

    External magnets can still manipulate the liquid form, making it stretch, split and merge. But controlling the fluid’s movement is more challenging, because the particles in the gallium can freely rotate and have unaligned magnetic poles as a result of melting. Because of their various orientations, the particles move in different directions in response to a magnet.

    Majidi and colleagues tested their strategy in tiny machines that performed different tasks. In a demonstration straight out of the movie Terminator 2, a toy person escaped a jail cell by melting through the bars and resolidifying in its original form using a mold placed just outside the bars.

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    On the more practical side, one machine removed a small ball from a model human stomach by melting slightly to wrap itself around the foreign object before exiting the organ. But gallium on its own would turn to goo inside a real human body, since the metal is a liquid at body temperature, about 37° C. A few more metals, such as bismuth and tin, would be added to the gallium in biomedical applications to raise the material’s melting point, the authors say. In another demonstration, the material liquefied and rehardened to solder a circuit board.

    [embedded content]
    With the help of variable and permanent magnets, researchers turned chunks of gallium into shape-shifting devices. In the first clip, a toy figure escapes its jail cell by liquefying, gliding through the bars and resolidifying using a mold placed just outside the bars. In the second clip, one device removes a ball from a model human stomach by melting slightly to wrap itself around the foreign object and exiting the organ.

    Although this phase-shifting material is a big step in the field, questions remain about its biomedical applications, says biomedical engineer Amir Jafari of the University of North Texas in Denton, who was not involved in the work. One big challenge, he says, is precisely controlling magnetic forces inside the human body that are generated from an external device.

    “It’s a compelling tool,” says robotics engineer Nicholas Bira of Harvard University, who was also not involved in the study. But, he adds, scientists who study soft robotics are constantly creating new materials.

    “The true innovation to come lies in combining these different innovative materials.” More

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    Want a ‘Shrinky Dinks’ approach to nano-sized devices? Try hydrogels

    High-tech shrink art may be the key to making tiny electronics, 3-D nanostructures or even holograms for hiding secret messages.

    A new approach to making tiny structures relies on shrinking them down after building them, rather than making them small to begin with, researchers report in the Dec. 23 Science.

    The key is spongelike hydrogel materials that expand or contract in response to surrounding chemicals (SN: 1/20/10). By inscribing patterns in hydrogels with a laser and then shrinking the gels down to about one-thirteenth their original size, the researchers created patterns with details as small as 25 billionths of a meter across.

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    At that level of precision, the researchers could create letters small enough to easily write this entire article along the circumference of a typical human hair.

    Biological scientist Yongxin Zhao and colleagues deposited a variety of materials in the patterns to create nanoscopic images of Chinese zodiac animals. By shrinking the hydrogels after laser etching, several of the images ended up roughly the size of a red blood cell. They included a monkey made of silver, a gold-silver alloy pig, a titanium dioxide snake, an iron oxide dog and a rabbit made of luminescent nanoparticles.

    These two dragons, each roughly 40 micrometers long, were made by depositing cadmium selenide quantum dots onto a laser-etched hydrogel. The red stripes on the left dragon are each just 200 nanometers thick.The Chinese University of Hong Kong, Carnegie Mellon University

    Because the hydrogels can be repeatedly shrunk and expanded with chemical baths, the researchers were also able to create holograms in layers inside a chunk of hydrogel to encode secret information. Shrinking a hydrogel hologram makes it unreadable. “If you want to read it, you have to expand the sample,” says Zhao, of Carnegie Mellon University in Pittsburgh. “But you need to expand it to exactly the same extent” as the original. In effect, knowing how much to expand the hydrogel serves as a key to unlock the information hidden inside.  

    But the most exciting aspect of the research, Zhao says, is the wide range of materials that researchers can use on such minute scales. “We will be able to combine different types of materials together and make truly functional nanodevices.” More

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    This fabric can hear your heartbeat

    Someday our clothing may eavesdrop on the soundtrack of our lives, capturing the noises around and inside us.

    A new fiber acts as a microphone — picking up speech, rustling leaves and chirping birds — and turns those acoustic signals into electrical ones. Woven into a fabric, the material can even hear handclaps and faint sounds, such as its wearer’s heartbeat, researchers report March 16 in Nature. Such fabrics could provide a comfortable, nonintrusive — even fashionable — way to monitor body functions or aid with hearing.

    Acoustic fabrics have existed for perhaps hundreds of years, but they’re used to dampen sound, says Wei Yan, a materials scientist at Nanyang Technological University in Singapore. Fabric as a microphone is “totally a different concept,” says Yan, who worked on the fabric while at MIT.

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    Yan and his colleagues were inspired by the human eardrum. Sound waves cause vibrations in the eardrum, which are converted to electrical signals by the cochlea. “It turns out that this eardrum is made of fibers,” says Yoel Fink, a materials scientist at MIT. In the eardrum’s inner layers, collagen fibers radiate from the center, while others form concentric rings. The crisscrossing fibers play a role in hearing and look similar to the fabrics people weave, Fink says.

    Analogous to what’s happening in an eardrum, sound vibrates fabric at the nanoscale. In the new fabric, cotton fibers and others of a somewhat stiff material called Twaron efficiently convert incoming sound to vibrations. Woven together with these threads is a single fiber that contains a blend of piezoelectric materials, which produce a voltage when pressed or bent (SN: 8/22/17). The buckling and bending of the piezoelectric-containing fiber create electrical signals that can be sent through a tiny circuit board to a device that reads and records the voltage.

    The fabric microphone is sensitive to a range of noise levels, from a quiet library to heavy traffic, the team reports, although it is continuing to investigate what signal processing is needed to detangle target sounds from ambient noise. Integrated into clothing, this sound-sensing fabric feels like regular fabric, Yan says. And it continued to work as a microphone after washing it 10 times.

    Woven into fabric, a specialized fiber (pictured, center) creates electrical signals when bent or buckled, turning the entire material into a microphone.Fink Lab/MIT, Elizabeth Meiklejohn/RISD, Greg Hren

    Piezoelectric materials have “huge potential” for applications from observing the function of bodies to monitoring the integrity of aircraft materials, says Vijay Thakur, a materials scientist at Scotland’s Rural College in Edinburgh who was not part of this work. They’ve even been proposed for energy generation, but, he says, many uses have been limited by the tiny voltages they produce (SN: 10/1/15). The way the fibers are made in this fabric — sandwiching a blend of piezoelectric materials between other components, including a flexible, stretchy outer material — concentrates the energy from the vibrations into the piezoelectric layer, enhancing the signal it produces.

    As a proof of concept, the team incorporated the fabric into a shirt, which could hear its wearer’s heart like a stethoscope does. Used this way, the fabric microphone could listen for murmurs and may someday be able to provide information similar to an echocardiogram, an ultrasound of the heart, Thakur says. If it proves effective as a monitoring and diagnostic tool, placing such microphones into clothing may someday make it easier for doctors to track heart conditions in young children, who have trouble keeping still, he says.

    The team also anticipates that fabric microphones could aid hearing and communication. Another shirt the team created had two piezoelectric fibers spaced apart on the shirt’s back. Based on when each fiber picked up the sound, this shirt can be used to detect the direction a clap came from. And when hooked up to a power source, the fabric microphones can project sound as a speaker.

    “For the past 20 years, we’ve been trying to introduce a new way of thinking about fabrics,” Fink says. Beyond providing beauty and warmth, fabrics may help solve technological problems. And perhaps, Fink says, they can beautify technology too.  More

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    Researchers have unlocked the secret to pearls’ incredible symmetry

    For centuries, researchers have puzzled over how oysters grow stunningly symmetrical, perfectly round pearls around irregularly shaped grains of sand or bits of debris. Now a team has shown that oysters, mussels and other mollusks use a complex process to grow the gems that follows mathematical rules seen throughout nature.

    Pearls are formed when an irritant gets trapped inside a mollusk, and the animal protects itself by building smooth layers of mineral and protein — together called nacre — around it. Each new layer of nacre built over this asymmetrical center adapts precisely to the ones preceding it, smoothing out irregularities to result in a round pearl, according to an analysis published October 19 in the Proceedings of the National Academy of Sciences.

    “Nacre is this incredibly beautiful, iridescent, shiny material that we see in the insides of some seashells or on the outside of pearls,” says Laura Otter, a biogeochemist at the Australian National University in Canberra.

    A pearl’s symmetrical growth as it lays down layers of nacre relies on the mollusk balancing two basic capabilities, Otter and her colleagues discovered. It corrects growth aberrations that appear as the pearl forms, preventing those variations from propagating over the pearl’s many layers. Otherwise, the resulting gem would be lopsided.

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    Additionally, the mollusk modulates the thickness of nacre layers, so that if one layer is especially thick, subsequent layers will be thinner in response (SN: 3/24/14). This helps the pearl maintain a similar average thickness over its thousands of layers so that it looks perfectly round and uniform. Without that constant adjustment, a pearl might resemble stratified sedimentary rock, amplifying small imperfections that detract from its spherical shape.

    The researchers studied keshi pearls collected from Akoya pearl oysters (Pinctada imbricata fucata) at an eastern Australia coastal pearl farm. They used a diamond wire saw to cut the pearls into cross sections, then polished and examined the gems using Raman spectroscopy, a nondestructive technique that allowed them to characterize the pearls’ structure. For one of the pearls showcased in the paper, they counted 2,615 layers, which were deposited over 548 days.

    This cross section of a keshi pearl shows that the round gem grows around a misshapen lump of debris.Jiseok Gim

    The analysis revealed that fluctuations in the thicknesses of the pearls’ layers of nacre exhibit a phenomenon called 1/f noise, or pink noise, in which events that appear to be random are actually connected. In this case, the formation of nacre layers of different thicknesses may appear random, but is actually dependent on the thickness of previous layers. The same phenomenon is at work in seismic activity: The rumbling of the ground seems random, but is actually connected to previous recent seismic activity. Pink noise also crops up in classical music and even when monitoring heartbeats and brain activity, says coauthor Robert Hovden, a materials scientist and engineer at the University of Michigan in Ann Arbor.  These phenomena “belong to a universal class of behavior and physics,” Hovden says.

    This is the first time that researchers have reported “that nacre self-heals and when a defect arises, it heals itself within a few [layers], without using an external scaffolding or template,” says Pupa Gilbert, a physicist studying biomineralization at the University of Wisconsin–Madison who wasn’t involved with the study. “Nacre is an even more remarkable material than we had previously appreciated.”

    Notes Otter: “These humble creatures are making a super light and super tough material so much more easily and better than we do with all our technology.” Made of just calcium, carbonate and protein, nacre is “3,000 times tougher than the materials from which it’s made of.”

    This new understanding of pearls, Hovden adds, could inspire “the next generation of super materials,” such as more energy-efficient solar panels or tough and heat-resistant materials optimized for use in spacecraft. More

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    Lithium-ion batteries made with recycled materials can outlast newer counterparts

    Lithium-ion batteries with recycled cathodes can outperform batteries with cathodes made from pristine materials, lasting for thousands of additional charging cycles, a study finds. Growing demand for these batteries — which power devices from smartphones to electric vehicles — may outstrip the world’s supply of some crucial ingredients, such as cobalt (SN: 5/7/19). Ramping up recycling could help avert a potential shortage. But some manufacturers worry that impurities in recycled materials may cause battery performance to falter.

    “Based on our study, recycled materials can perform as well as, or even better than, virgin materials,” says materials scientist Yan Wang of Worcester Polytechnic Institute in Massachusetts.

    Using shredded spent batteries, Wang and colleagues extracted the electrodes and dissolved the metals from those battery bits in an acidic solution. By tweaking the solution’s pH, the team removed impurities such as iron and copper and recovered over 90 percent of three key metals: nickel, manganese and cobalt. The recovered metals formed the basis for the team’s cathode material.

    In tests of how well batteries maintain their capacity to store energy after repeated use and recharging, batteries with recycled cathodes outperformed ones made with brand-new commercial materials of the same composition. It took 11,600 charging cycles for the batteries with recycled cathodes to lose 30 percent of their initial capacity. That’s about 50 percent better than the respectable 7,600 cycles for the batteries with new cathodes, the team reports October 15 in Joule. Those thousands of extra cycles could translate into years of better battery performance, Wang says. More

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    These colorful butterflies were created using transparent ink

    You’ve heard of disappearing ink. Now get ready for suddenly appearing ink. Using a clear liquid, researchers can print a full rainbow of colors on transparent surfaces. The trick is printing the liquid in precise, microscale patterns that create structural color.

    Structural colors arise from the way different wavelengths of light bounce off microscopic imperfections on surfaces (SN: 8/17/21; SN: 6/1/16). “In nature, there are many beautiful structure colors, such as the wings of butterflies, the feathers of peacocks, the skin of chameleons and so on,” says Yanlin Song, a materials chemist at the Chinese Academy of Sciences in Beijing.

    Song and colleagues printed structural colors on transparent silicone sheets using an ordinary ink-jet printer and clear polymer ink. The printer studded the silicone sheets with millions of microscopic ink domes, each of which served as a single pixel in the resulting image. Adjusting the size of a microdome changed the wavelengths of light that the dome reflected and therefore its color (SN: 3/8/19). Increasing the width of a single dome from 6.6 to 11 micrometers shifted its hue along the spectrum from blue to red and back again, the researchers report online September 22 in Science Advances.

    The denser the domes were packed, the brighter the image. And printing a medley of differently colored ink pixels across a single area created blended shades, such as brown and gray. Using the technique, Song’s team printed multicolor, photorealistic portraits of Isaac Newton, Marilyn Monroe and other famous figures.

    By printing tiny dollops of clear ink on transparent surfaces, researchers created structural color portraits of famous figures, such as Isaac Newton, Audrey Hepburn and Marilyn Monroe.K. Li et al/Science Advances 2021

    By printing tiny dollops of clear ink on transparent surfaces, researchers created structural color portraits of famous figures, such as Isaac Newton, Audrey Hepburn and Marilyn Monroe.K. Li et al/Science Advances 2021

    “I was excited to see that somebody had used [structural color] for this purpose,” says Lauren Zarzar, a materials chemist at Penn State who has studied similar structural colors cast by water and oil droplets. “They had some nice examples that I think illustrated the versatility of this mechanism.”

    Zarzar imagines using structural colors to create complex optical signatures for anti-counterfeiting features on ID cards or currency. Such shimmery, colorfast hues could also make useful materials for cosmetics, clothing or architecture, she says. More