How to End Plastic Food Wrap

Aiming to produce environmentally friendly alternatives to plastic food wrap and containers, a Rutgers scientist has developed a biodegradable, plant-based coating that can be sprayed on foods, guarding against pathogenic and spoilage microorganisms and transportation damageThe scalable process could potentially reduce the adverse environmental impact of plastic food packaging as well as protect human health.

We knew we needed to get rid of the petroleum-based food packaging that is out there and replace it with something more sustainable, biodegradable and nontoxic,” said Philip Demokritou, director of the Nanoscience and Advanced Materials Research Center, and the at the Rutgers School of Public Health and Environmental and Occupational Health Sciences Institute.And we asked ourselves at the same time, ‘Can we design food packaging with a functionality to extend shelf life and reduce food waste while enhancing food safety?’’’

Demokritou added, “And what we have come up with is a scalable technology, which enables us to turn biopolymers, which can be derived as part of a circular economy from food waste, into smart fibers that can wrap food directly. This is part of new generation, ‘smart’ and ‘green’ food packaging.”

The research was conducted in concert with scientists at Harvard University and funded by the Harvard-Nanyang Technological University/Singapore Sustainable Nanotechnology Initiative.

Their article, published in the science journal Nature Food, describes the new kind of packaging technology using the polysaccharide/biopolymer-based fibers. Like the webs cast by the Marvel comic book character Spider-Man, the stringy material can be spun from a heating device that resembles a hair dryer and “shrink-wrapped” over foods of various shapes and sizes, such as an avocado or a sirloin steak. The resulting material that encases food products is sturdy enough to protect bruising and contains antimicrobial agents to fight spoilage and pathogenic microorganisms such as E. coli and listeria.

The research paper includes a description of the technology called focused rotary jet spinning, a process by which the biopolymer is produced, and quantitative assessments showing the coating extended the shelf life of avocados by 50 percent. The coating can be rinsed off with water and degrades in soil within three days, according to the study.


How To Boost Batteries Conductivity And Improve Safety

Building a better lithium-ion battery involves addressing a myriad of factors simultaneously, from keeping the battery’s cathode electrically and ionically conductive to making sure that the battery stays safe after many cycles.

In a new discovery, scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have developed a new cathode coating by using an oxidative chemical vapor deposition technique that can help solve these and several other potential issues with lithium-ion batteries all in one stroke.

The coating we’ve discovered really hits five or six birds with one stone.” Khalil Amine, Argonne distinguished fellow and battery scientist. In the research, Amine and his fellow researchers took particles of Argonne’s pioneering nickel-manganese-cobalt (NMC) cathode material and encapsulated them with a sulfur-containing polymer called PEDOT. This polymer provides the cathode a layer of protection from the battery’s electrolyte as the battery charges and discharges.

Unlike conventional coatings, which only protect the exterior surface of the micron-sized cathode particles and leave the interior vulnerable to cracking, the PEDOT coating had the ability to penetrate to the cathode particle’s interior, adding an additional layer of shielding. In addition, although PEDOT prevents the chemical interaction between the battery and the electrolyte, it does allow for the necessary transport of lithium ions and electrons that the battery requires in order to function.

This coating is essentially friendly to all of the processes and chemistry that makes the battery work and unfriendly to all of the potential reactions that would cause the battery to degrade or malfunction,” said Argonne chemist Guiliang Xu, the first author of the research.


Pixels A Million Times Smaller

The smallest pixels yet created – a million times smaller than those in smartphones, made by trapping particles of light under tiny rocks of gold – could be used for new types of large-scale flexible displays, big enough to cover entire buildings. The colour pixels, developed by a team of scientists led by the University of Cambridge, are compatible with roll-to-roll fabrication on flexible plastic films, dramatically reducing their production cost.
It has been a long-held dream to mimic the colour-changing skin of octopus or squid, allowing people or objects to disappear into the natural background, but making large-area flexible display screens is still prohibitively expensive because they are constructed from highly precise multiple layers. At the centre of the pixels developed by the Cambridge scientists is a tiny particle of gold a few billionths of a metre across. The grain sits on top of a reflective surface, trapping light in the gap in between. Surrounding each grain is a thin sticky coating which changes chemically when electrically switched, causing the pixel to change colour across the spectrum.

The team of scientists, from different disciplines including physics, chemistry and manufacturing, made the pixels by coating vats of golden grains with an active polymer called polyaniline and then spraying them onto flexible mirror-coated plastic, to dramatically drive down production cost. The pixels are the smallest yet created, a million times smaller than typical smartphone pixels. They can be seen in bright sunlight and because they do not need constant power to keep their set colour, have an energy performance that makes large areas feasible and sustainable. “We started by washing them over aluminized food packets, but then found aerosol spraying is faster,” said co-lead author Hyeon-Ho Jeong from Cambridge’s Cavendish Laboratory.

These are not the normal tools of nanotechnology, but this sort of radical approach is needed to make sustainable technologies feasible,” said Professor Jeremy J Baumberg of the NanoPhotonics Centre at Cambridge’s Cavendish Laboratory, who led the research. “The strange physics of light on the nanoscale allows it to be switched, even if less than a tenth of the film is coated with our active pixels. That’s because the apparent size of each pixel for light is many times larger than their physical area when using these resonant gold architectures.”

The pixels could enable a host of new application possibilities such as building-sized display screens, architecture which can switch off solar heat load, active camouflage clothing and coatings, as well as tiny indicators for coming internet-of-things devices.

The results are reported in the journal Science Advances.


Self-Healing Coating Protects Metals From Corrosion

It’s hard to believe that a tiny crack could take down a gigantic metal structure. But sometimes bridges collapse, pipelines rupture and fuselages detach from airplanes due to hard-to-detect corrosion in tiny cracks, scratches and dents. A Northwestern University team has developed a new coating strategy for metal that self-heals within seconds when scratched, scraped or cracked. The novel material could prevent these tiny defects from turning into localized corrosion, which can cause major structures to fail.


Localized corrosion is extremely dangerous,” said Jiaxing Huang, who led the research. “It is hard to prevent, hard to predict and hard to detect, but it can lead to catastrophic failure.” Huang is a professor of materials science and engineering in Northwestern’s McCormick School of Engineering.

When damaged by scratches and cracks, Huang’s patent-pending system readily flows and reconnects to rapidly heal right before the eyes. The researchers demonstrated that the material can heal repeatedly — even after scratching the exact same spot nearly 200 times in a row.While a few self-healing coatings already exist, those systems typically work for nanometer- to micron-sized damages. To develop a coating that can heal larger scratches in the millimeter-scale, Huang and his team looked to fluid. “When a boat cuts through water, the water goes right back together,” Huang said. “The ‘cut’ quickly heals because water flows readily. We were inspired to realize that fluids, such as oils, are the ultimate self-healing system.” But common oils flows too readily, Huang noted. So he and his team needed to develop a system with contradicting properties: fluidic enough to flow automatically but not so fluidic that it drips off the metal’s surface.

The team met the challenge by creating a network of lightweight particles — in this case graphene capsules — to thicken the oil. The network fixes the oil coating, keeping it from dripping. But when the network is damaged by a crack or scratch, it releases the oil to flow readily and reconnect. Huang said the material can be made with any hollow, lightweight particlenot just graphene. “The particles essentially immobilize the oil film,” Huang said. “So it stays in place.”

The study was published  in Research, the first Science Partner Journal recently launched by the American Association for the Advancement of Science (AAAS) in collaboration with the China Association for Science and Technology (CAST).


Invisible Plastic For Super Efficient Solar Panels

Antireflection (AR) coatings on plastics have a multitude of practical applications, including glare reduction on eyeglasses, computer monitors and the display on your smart-phone when outdoors. Now, researchers at Penn State have developed an AR coating that improves on existing coatings to the extent that it can make transparent plastics, such as Plexiglas, virtually invisible.

Plastic dome coated with a new antireflection coating (right), and uncoated dome (left)

This discovery came about as we were trying to make higher-efficiency solar panels,” said Chris Giebink, associate professor of electrical engineering, Penn State. “Our approach involved concentrating light onto small, high-efficiency solar cells using plastic lenses, and we needed to minimize their reflection loss.”

They needed an antireflection coating that worked well over the entire solar spectrum and at multiple angles as the sun crossed the sky. They also needed a coating that could stand up to weather over long periods of time outdoors. “We would have liked to find an off-the-shelf solution, but there wasn’t one that met our performance requirements,” he said. “So, we started looking for our own solution.”

That was a tall order. Although it is comparatively easy to make a coating that will eliminate reflection at a particular wavelength or in a particular direction, one that could fit all their criteria did not exist. For instance, eyeglass AR coatings are targeted to the narrow visible portion of the spectrum. But the solar spectrum is about five times as broad as the visible spectrum, so such a coating would not perform well for a concentrating solar cell system.

Reflections occur when light travels from one medium, such as air, into a second medium, in this case plastic. If the difference in their refractive index, which specifies how fast light travels in a particular material, is large — air has a refractive index of 1 and plastic 1.5 — then there will be a lot of reflection. The lowest index for a natural coating material such as magnesium fluoride or Teflon is about 1.3. The refractive index can be graded — slowly varied — between 1.3 and 1.5 by blending different materials, but the gap between 1.3 and 1 remains.

In a paper recently posted online ahead of print in the journal Nano Letters, Giebink and coauthors describe a new process to bridge the gap between Teflon and air. They used a sacrificial molecule to create nanoscale pores in evaporated Teflon, thereby creating a graded index Teflon-air film that fools light into seeing a smooth transition from 1 to 1.5, eliminating essentially all reflections.

The interesting thing about Teflon, which is a polymer, is when you heat it up in a crucible, the large polymer chains cleave into smaller fragments that are small enough to volatize and send up a vapor flux. When these land on a substrate they can repolymerize and form Teflon,” Giebink explained.


We’ve been interacting with a number of companies that are looking for improved antireflection coatings for plastic, and some of the applications have been surprising,” he said. “They range from eliminating glare from the plastic domes that protect security cameras to eliminating stray reflections inside virtual/augmented -reality headsets.”


Nanorobots Clear Bacteria From Blood

Engineers at the University of California San Diego have developed tiny ultrasound-powered robots that can swim through blood, removing harmful bacteria along with the toxins they produce. These proof-of-concept nanorobots could one day offer a safe and efficient way to detoxify and decontaminate biological fluids.

Researchers built the nanorobots by coating gold nanowires with a hybrid of platelet and red blood cell membranes. This hybrid cell membrane coating allows the nanorobots to perform the tasks of two different cells at once—platelets, which bind pathogens like MRSA bacteria (an antibiotic-resistant strain of Staphylococcus aureus), and red blood cells, which absorb and neutralize the toxins produced by these bacteria. The gold body of the nanorobots responds to ultrasound, which gives them the ability to swim around rapidly without chemical fuel. This mobility helps the nanorobots efficiently mix with their targets (bacteria and toxins) in blood and speed up detoxification.

The work, published May 30 in Science Robotics, combines technologies pioneered by Joseph Wang and Liangfang Zhang, professors in the Department of NanoEngineering at the UC San Diego Jacobs School of Engineering. Wang’s team developed the ultrasound-powered nanorobots, and Zhang’s team invented the technology to coat nanoparticles in natural cell membranes.

SEM image of a MRSA bacterium attached to a hybrid cell membrane coated nanorobot

By integrating natural cell coatings onto synthetic nanomachines, we can impart new capabilities on tiny robots such as removal of pathogens and toxins from the body and from other matrices,” said Wang. “This is a proof-of-concept platform for diverse therapeutic and biodetoxification applications.”

The idea is to create multifunctional nanorobots that can perform as many different tasks at once,” adds co-first author Berta Esteban-Fernández de Ávila, a postdoctoral scholar in Wang’s research group at UC San Diego. “Combining platelet and red blood cell membranes into each nanorobot coating is synergistic—platelets target bacteria, while red blood cells target and neutralize the toxins those bacteria produce.


How Solar Cells Absorb 20 % More Sunlight

Trapping light with an optical version of a whispering gallery, researchers at the National Institute of Standards and Technology (NIST) have developed a nanoscale coating for solar cells that enables them to absorb about 20 percent more sunlight than uncoated devices. The coating, applied with a technique that could be incorporated into manufacturing, opens a new path for developing low-cost, high-efficiency solar cells with abundant, renewable and environmentally friendly materials.

Illustration shows the nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells

The coating consists of thousands of tiny glass beads, only about one-hundredth the width of a human hair. When sunlight hits the coating, the light waves are steered around the nanoscale bead, similar to the way sound waves travel around a curved wall such as the dome in St. Paul’s Cathedral in London. At such curved structures, known as acoustic whispering galleries, a person standing near one part of the wall easily hears a faint sound originating at any other part of the wall.

Using a laser as a light source to excite individual nanoresonators in the coating, the team found that the coated solar cells absorbed, on average, 20 percent more visible light than bare cells. The measurements also revealed that the coated cells produced about 20 percent more current.