MIT Research News' Journal
[Most Recent Entries]
[Calendar View]
Sunday, September 21st, 2014
| Time |
Event |
| 1:00p |
Magnetic fields make the excitons go ’round A major limitation in the performance of solar cells happens within the photovoltaic material itself: When photons strike the molecules of a solar cell, they transfer their energy, producing quasi-particles called excitons — an energized state of molecules. That energized state can hop from one molecule to the next until it’s transferred to electrons in a wire, which can light up a bulb or turn a motor.
But as the excitons hop through the material, they are prone to getting stuck in minuscule defects, or traps — causing them to release their energy as wasted light.
Now a team of researchers at MIT and Harvard University has found a way of rendering excitons immune to these traps, possibly improving photovoltaic devices’ efficiency. The work is described in a paper in the journal Nature Materials.
Their approach is based on recent research on exotic electronic states known as topological insulators, in which the bulk of a material is an electrical insulator — that is, it does not allow electrons to move freely — while its surface is a good conductor.
The MIT-Harvard team used this underlying principle, called topological protection, but applied it to excitons instead of electrons, explains lead author Joel Yuen, a postdoc in MIT’s Center for Excitonics, part of the Research Laboratory of Electronics. Topological protection, he says, “has been a very popular idea in the physics and materials communities in the last few years,” and has been successfully applied to both electronic and photonic materials.
Moving on the surface
Topological excitons would move only at the surface of a material, Yuen explains, with the direction of their motion determined by the direction of an applied magnetic field. In that respect, their behavior is similar to that of topological electrons or photons.
In its theoretical analysis, the team studied the behavior of excitons in an organic material, a porphyrin thin film, and determined that their motion through the material would be immune to the kind of defects that tend to trap excitons in conventional solar cells.
The choice of porphyrin for this analysis was based on the fact that it is a well-known and widely studied family of materials, says co-author Semion Saikin, a postdoc at Harvard and an affiliate of the Center for Excitonics. The next step, he says, will be to extend the analysis to other kinds of materials.
While the work so far has been theoretical, experimentalists are eager to pursue the concept. Ultimately, this approach could lead to novel circuits that are similar to electronic devices but based on controlling the flow of excitons rather that electrons, Yuen says. “If there are ever excitonic circuits,” he says, “this could be the mechanism” that governs their functioning. But the likely first application of the work would be in creating solar cells that are less vulnerable to the trapping of excitons.
Eric Bittner, a professor of chemistry at the University of Houston who was not associated with this work, says, “The work is interesting on both the fundamental and practical levels. On the fundamental side, it is intriguing that one may be able to create excitonic materials with topological properties. This opens a new avenue for both theoretical and experimental work. … On the practical side, the interesting properties of these materials and the fact that we’re talking about pretty simple starting components — porphyrin thin films — makes them novel materials for new devices.”
The work received support from the U.S. Department of Energy and the Defense Threat Reduction Agency. Norman Yao, a graduate student at Harvard, was also a co-author. | | 1:00p |
Engineered proteins stick like glue — even in water Shellfish such as mussels and barnacles secrete very sticky proteins that help them cling to rocks or ship hulls, even underwater. Inspired by these natural adhesives, a team of MIT engineers has designed new materials that could be used to repair ships or help heal wounds and surgical incisions.
To create their new waterproof adhesives, the MIT researchers engineered bacteria to produce a hybrid material that incorporates naturally sticky mussel proteins as well as a bacterial protein found in biofilms — slimy layers formed by bacteria growing on a surface. When combined, these proteins form even stronger underwater adhesives than those secreted by mussels.
This project, described in the Sept. 21 issue of the journal Nature Nanotechnology, represents a new type of approach that can be exploited to synthesize biological materials with multiple components, using bacteria as tiny factories.
“The ultimate goal for us is to set up a platform where we can start building materials that combine multiple different functional domains together and to see if that gives us better materials performance,” says Timothy Lu, an associate professor of biological engineering and electrical engineering and computer science (EECS) and the senior author of the paper.
The paper’s lead author is Chao Zhong, a former MIT postdoc who is now at ShanghaiTech University. Other authors are graduate student Thomas Gurry, graduate student Allen Cheng, senior Jordan Downey, postdoc Zhengtao Deng, and Collin Stultz, a professor in EECS.
Complex adhesives
The sticky substance that helps mussels attach to underwater surfaces is made of several proteins known as mussel foot proteins. “A lot of underwater organisms need to be able to stick to things, so they make all sorts of different types of adhesives that you might be able to borrow from,” Lu says.
Scientists have previously engineered E. coli bacteria to produce individual mussel foot proteins, but these materials do not capture the complexity of the natural adhesives, Lu says. In the new study, the MIT team wanted to engineer bacteria to produce two different foot proteins, combined with bacterial proteins called curli fibers — fibrous proteins that can clump together and assemble themselves into much larger and more complex meshes.
Lu’s team engineered bacteria so they would produce proteins consisting of curli fibers bonded to either mussel foot protein 3 or mussel foot protein 5. After purifying these proteins from the bacteria, the researchers let them incubate and form dense, fibrous meshes. The resulting material has a regular yet flexible structure that binds strongly to both dry and wet surfaces.
“The result is a powerful wet adhesive with independently functioning adsorptive and cohesive moieties,” says Herbert Waite, a professor of chemistry and biochemistry at the University of California at Santa Barbara who was not part of the research team. “The work is very creative, rigorous, and thorough.”
The researchers tested the adhesives using atomic force microscopy, a technique that probes the surface of a sample with a tiny tip. They found that the adhesives bound strongly to tips made of three different materials — silica, gold, and polystyrene. Adhesives assembled from equal amounts of mussel foot protein 3 and mussel foot protein 5 formed stronger adhesives than those with a different ratio, or only one of the two proteins on their own.
These adhesives were also stronger than naturally occurring mussel adhesives, and they are the strongest biologically inspired, protein-based underwater adhesives reported to date, the researchers say.
More adhesive strength
Using this technique, the researchers can produce only small amounts of the adhesive, so they are now trying to improve the process and generate larger quantities. They also plan to experiment with adding some of the other mussel foot proteins. “We’re trying to figure out if by adding other mussel foot proteins, we can increase the adhesive strength even more and improve the material’s robustness,” Lu says.
The team also plans to try to create “living glues” consisting of films of bacteria that could sense damage to a surface and then repair it by secreting an adhesive.
The research was funded by the Office of Naval Research, the National Science Foundation, and the National Institutes of Health. | | 1:00p |
Battling superbugs In recent years, new strains of bacteria have emerged that resist even the most powerful antibiotics. Each year, these superbugs, including drug-resistant forms of tuberculosis and staphylococcus, infect more than 2 million people nationwide, and kill at least 23,000. Despite the urgent need for new treatments, scientists have discovered very few new classes of antibiotics in the past decade.
MIT engineers have now turned a powerful new weapon on these superbugs. Using a gene-editing system that can disable any target gene, they have shown that they can selectively kill bacteria carrying harmful genes that confer antibiotic resistance or cause disease.
Led by Timothy Lu, an associate professor of biological engineering and electrical engineering and computer science, the researchers described their findings in the Sept. 21 issue of Nature Biotechnology. Last month, Lu’s lab reported a different approach to combating resistant bacteria by identifying combinations of genes that work together to make bacteria more susceptible to antibiotics.
Lu hopes that both technologies will lead to new drugs to help fight the growing crisis posed by drug-resistant bacteria.
“This is a pretty crucial moment when there are fewer and fewer new antibiotics available, but more and more antibiotic resistance evolving,” he says. “We’ve been interested in finding new ways to combat antibiotic resistance, and these papers offer two different strategies for doing that.”
Cutting out resistance
Most antibiotics work by interfering with crucial functions such as cell division or protein synthesis. However, some bacteria, including the formidable MRSA (methicillin-resistant Staphylococcus aureus) and CRE (carbapenem-resistant Enterobacteriaceae) organisms, have evolved to become virtually untreatable with existing drugs.
In the new Nature Biotechnology study, graduate students Robert Citorik and Mark Mimee worked with Lu to target specific genes that allow bacteria to survive antibiotic treatment. The CRISPR genome-editing system presented the perfect strategy to go after those genes.
CRISPR, originally discovered by biologists studying the bacterial immune system, involves a set of proteins that bacteria use to defend themselves against bacteriophages (viruses that infect bacteria). One of these proteins, a DNA-cutting enzyme called Cas9, binds to short RNA guide strands that target specific sequences, telling Cas9 where to make its cuts.
Lu and colleagues decided to turn bacteria’s own weapons against them. They designed their RNA guide strands to target genes for antibiotic resistance, including the enzyme NDM-1, which allows bacteria to resist a broad range of beta-lactam antibiotics, including carbapenems. The genes encoding NDM-1 and other antibiotic resistance factors are usually carried on plasmids — circular strands of DNA separate from the bacterial genome — making it easier for them to spread through populations.
When the researchers turned the CRISPR system against NDM-1, they were able to specifically kill more than 99 percent of NDM-1-carrying bacteria, while antibiotics to which the bacteria were resistant did not induce any significant killing. They also successfully targeted another antibiotic resistance gene encoding SHV-18, a mutation in the bacterial chromosome providing resistance to quinolone antibiotics, and a virulence factor in enterohemorrhagic E. coli.
In addition, the researchers showed that the CRISPR system could be used to selectively remove specific bacteria from diverse bacterial communities based on their genetic signatures, thus opening up the potential for “microbiome editing” beyond antimicrobial applications.
To get the CRISPR components into bacteria, the researchers created two delivery vehicles — engineered bacteria that carry CRISPR genes on plasmids, and bacteriophage particles that bind to the bacteria and inject the genes. Both of these carriers successfully spread the CRISPR genes through the population of drug-resistant bacteria. Delivery of the CRISPR system into waxworm larvae infected with a harmful form of E. coli resulted in increased survival of the larvae.
The researchers are now testing this approach in mice, and they envision that eventually the technology could be adapted to deliver the CRISPR components to treat infections or remove other unwanted bacteria in human patients.
“This work represents a very interesting genetic method for killing antibiotic-resistant bacteria in a directed fashion, which in principle could help to combat the spread of antibiotic resistance fueled by excessive broad-spectrum treatment,” says Ahmad Khalil, an assistant professor of biomedical engineering at Boston University who was not part of the research team.
High-speed genetic screens
Another tool Lu has developed to fight antibiotic resistance is a technology called CombiGEM. This system, described in the Proceedings of the National Academy of Sciences the week of Aug. 11, allows scientists to rapidly and systematically search for genetic combinations that sensitize bacteria to different antibiotics.
To test the system, Lu and his graduate student, Allen Cheng, created a library of 34,000 pairs of bacterial genes. All of these genes code for transcription factors, which are proteins that control the expression of other genes. Each gene pair is contained on a single piece of DNA that also includes a six-base-pair barcode for each gene. These barcodes allow the researchers to rapidly identify the genes in each pair without having to sequence the entire strand of DNA.
“You can take advantage of really high-throughput sequencing technologies that allow you, in a single shot, to assess millions of genetic combinations simultaneously and pick out the ones that are successful,” Lu says.
The researchers then delivered the gene pairs into drug-resistant bacteria and treated them with different antibiotics. For each antibiotic, they identified gene combinations that enhanced the killing of target bacteria by 10,000- to 1,000,000-fold. The researchers are now investigating how these genes exert their effects.
“This platform allows you to discover the combinations that are really interesting, but it doesn’t necessarily tell you why they work well,” Lu says. “This is a high-throughput technology for uncovering genetic combinations that look really interesting, and then you have to go downstream and figure out the mechanisms.”
Once scientists understand how these genes influence antibiotic resistance, they could try to design new drugs that mimic the effects, Lu says. It is also possible that the genes themselves could be used as a treatment, if researchers can find a safe and effective way to deliver them.
CombiGEM also enables the generation of combinations of three or four genes in a more powerful way than previously existing methods. “We’re excited about the application of CombiGEM to probe complex multifactorial phenotypes, such as stem cell differentiation, cancer biology, and synthetic circuits,” Lu says.
The research was funded by the National Institutes of Health, the Defense Threat Reduction Agency, the U.S. Army Research Laboratory, the U.S. Army Research Office, the Office of Naval Research, and the Ellison Foundation. | | 1:00p |
New formulation leads to improved liquid battery Researchers at MIT have improved a proposed liquid battery system that could enable renewable energy sources to compete with conventional power plants.
Donald Sadoway and colleagues have already started a company to produce electrical-grid-scale liquid batteries, whose layers of molten material automatically separate due to their differing densities. But the new formula — published in the journal Nature by Sadoway, former postdocs Kangli Wang and Kai Jiang, and seven others — substitutes different metals for the molten layers used in a battery previously developed by the team.
Sadoway, the John F. Elliott Professor of Materials Chemistry, says the new formula allows the battery to work at a temperature more than 200 degrees Celsius lower than the previous formulation. In addition to the lower operating temperature, which should simplify the battery’s design and extend its working life, the new formulation will be less expensive to make, he says.
The battery uses two layers of molten metal, separated by a layer of molten salt that acts as the battery’s electrolyte (the layer that charged particles pass through as the battery is charged or discharged). Because each of the three materials has a different density, they naturally separate into layers, like oil floating on water.
The original system, using magnesium for one of the battery’s electrodes and antimony for the other, required an operating temperature of 700 C. But with the new formulation, with one electrode made of lithium and the other a mixture of lead and antimony, the battery can operate at temperatures of 450 to 500 C.
Extensive testing has shown that even after 10 years of daily charging and discharging, the system should retain about 85 percent of its initial efficiency — a key factor in making such a technology an attractive investment for electric utilities.
Currently, the only widely used system for utility-scale storage of electricity is pumped hydro, in which water is pumped uphill to a storage reservoir when excess power is available, and then flows back down through a turbine to generate power when it is needed. Such systems can be used to match the intermittent production of power from irregular sources, such as wind and solar power, with variations in demand. Because of inevitable losses from the friction in pumps and turbines, such systems return about 70 percent of the power that is put into them (which is called the “round-trip efficiency”).
Sadoway says his team’s new liquid-battery system can already deliver the same 70 percent efficiency, and with further refinements may be able to do better. And unlike pumped hydro systems — which are only feasible in locations with sufficient water and an available hillside — the liquid batteries could be built virtually anywhere, and at virtually any size. “The fact that we don’t need a mountain, and we don’t need lots of water, could give us a decisive advantage,” Sadoway says.
The biggest surprise for the researchers was that the antimony-lead electrode performed so well. They found that while antimony could produce a high operating voltage, and lead gave a low melting point, a mixture of the two combined both advantages, with a voltage as high as antimony alone, and a melting point between that of the two constituents — contrary to expectations that lowering the melting point would come at the expense of also reducing the voltage.
“We hoped [the characteristics of the two metals] would be nonlinear,” Sadoway says — that is, that the operating voltage would not end up halfway between that of the two individual metals. “They proved to be [nonlinear], but beyond our imagination. There was no decline in the voltage. That was a stunner for us.”
Not only did that provide significantly improved materials for the group’s battery system, but it opens up whole new avenues of research, Sadoway says. Going forward, the team will continue to search for other combinations of metals that might provide even lower-temperature, lower-cost, and higher-performance systems. “Now we understand that liquid metals bond in ways that we didn’t understand before,” he says.
With this fortuitous finding, Sadoway says, “Nature tapped us on the shoulder and said, ‘You know, there’s a better way!’” And because there has been little commercial interest in exploring the properties and potential uses of liquid metals and alloys of the type that are most attractive as electrodes for liquid metal batteries, he says, “I think there’s still room for major discoveries in this field.”
Robert Metcalfe, professor of innovation at the University of Texas at Austin, who was not involved in this work, says, “The Internet gave us cheap and clean connectivity using many kinds of digital storage. Similarly, we will solve cheap and clean energy with many kinds of storage. Energy storage will absorb the increasing randomness of energy supply and demand, shaving peaks, increasing availability, improving efficiency, lowering costs.”
Metcalfe adds that Sadoway’s approach to storage using liquid metals “is very promising.”
The research was supported by the U.S. Department of Energy’s Advanced Research Projects Agency-Energy and by French energy company Total. |
|