Information on the ways in which attaching metal contacts affects electron transport in graphene will be important to scientists studying the material -- and to designers who may one day fabricate electronic devices from the carbon-lattice material.

"Graphene devices will have to communicate with the external world, and that means we will have to fabricate contacts to transport current and data," said Mei-Yin Chou, a professor and department chair in the School of Physics at the Georgia Institute of Technology. "When they put metal contacts onto graphene to measure transport properties, researchers and device designers need to know that they may not be measuring the instrinsic properties of pristine graphene. Coupling between the contacts and the material must be taken into account."

Information on the effects of metal contacts on graphene was reported in the journal Physical Review Letters on February 19th. The research was supported by the U.S. Department of Energy, and involved interactions with researchers at the National Science Foundation (NSF)-supported Materials Research Science and Engineering Center (MRSEC) at Georgia Tech.

Using large-scale, first-principles calculations done at two different NSF-supported supercomputer centers, the Georgia Tech research team -- which included postdoctoral fellows Salvador Barraza-Lopez and Mihajlo Vanevic, and assistant professor Markus Kindermann -- conducted detailed atomic-level calculations of aluminum contacts grown on graphene.

The calculations studied two contacts up to 14 nanometers apart, with graphene suspended between them. In their calculations, the researchers allowed the aluminum to grow as it would in the real world, then studied how electron transfer was induced in the area surrounding the contacts.

"People have been able to come up with phenomenological models that they use to find out what the effects are with metallic contacts," Chou explained. "Our calculations went a few steps farther because we built contacts atom-by-atom. We built atomistically-resolved contacts, and by doing that, we solved this problem at the atomic level and tried to do everything consistent with quantum mechanics."

Because metals typically have excess electrons, physically attaching the contacts to graphene causes a charge transfer from the metal. Charge begins to be transferred as soon as the contracts are constructed, but ultimately the two materials reach equilibrium, Chou said.

The study showed that charge transfer at the leads and into the freestanding section of the material creates an electron-hole asymmetry in the conductance. For leads that are sufficiently long, the effect creates two conductance minima at the energies of the Dirac points for the suspended and clamped regions of the graphene, according to Barraza-Lopez.

"These results could be important to the design of future graphene devices," he said. "Edge effects and the impact of nanoribbon width have been studied in significant detail, but the effects of charge transfer at the contacts may potentially be just as important."

The researchers modeled aluminum, but believe their results will apply to other metals such as copper and gold that do not form chemical bonds with graphene. However, other metals such as chromium and titanium do chemically alter the material, so the effects they have on electron transport may be different.

Beyond the new information provided by the calculations, the research further proposes quantitative models that can be used under certain circumstances to describe the impact of the contacts.

"Earlier models had been based on physical insights, but nobody really knew how faithfully they described the material," Kindermann said. "This is the first calculation to show that these earlier models apply under certain circumstances for the systems that we studied."

Data from the study may one day help device designers engineer graphene circuits by helping them understand the effects they are seeing.

"When we modify graphene, we need to understand what changes occur as a result of adding materials," added Chou. "This is really fundamental research to understand these effects and to have a numerical prediction for what is going on. We are helping to understand the basic physics of graphene."

This research was supported by Department of Energy grant DE-FG02-97ER45632. Comments and conclusions in this article are those of the researchers and do not necessarily reflect the views of the Department of Energy.

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Treatment of pressure ulcers costs an estimated $8 billion each year in the United States alone, but the painful skin injury can be prevented if detected early. The device, a hand-held multispectral imaging system that provides data in real-time, could be used by health care professionals to detect signs of pressure ulcers before they can be seen with conventional visual screening techniques -- especially in patients with darker skin.

“We have developed a novel multispectral imager that can be integrated onto a chip,” said Fengtao Wang, a Georgia Tech School of Electrical and Computer Engineering graduate student who explained the company’s plan to a judging panel. “We can deliver a compact, real-time and low-cost multispectral imager to detect erythema at an early stage.”

The device would be marketed to clinics, nursing homes, rehabilitation centers, hospitals and other facilities that treat patients whose mobility problems can result in development of pressure ulcers. In addition to the medical applications, Wang said the device may also have military, agricultural, manufacturing and environmental uses.

In addition to Wang, the company team includes Ali Adibi and Fuhan Liu in the Georgia Tech School of Electrical and Computer Engineering, and Linghua Kong and Stephen Sprigle of the Center for Assistive Technology and Environmental Access (CATEA) in the Georgia Tech College of Architecture. Adibi and Sprigle are both professors; Kong is a senior faculty engineer and Liu is research engineer.

The Georgia Tech Edison Prize was established to encourage formation of startup companies based on technology developed at Georgia Tech, and was made possible by a multi-year grant from the Charles A. Edison Fund, named for the inventor’s son. Awarding of the first Georgia Tech Edison Prize was part of the Georgia Tech Graduate Research and Innovation Conference held February 8.

“Thomas Edison often receives credit for inventing the electric light bulb, but his real accomplishment was in making that device -- along with the phonograph and motion picture camera -- commercially successful to create new companies and new industries,” said Stephen Fleming, Georgia Tech’s vice provost for economic development and technology ventures. “Through the Edison Prize, we want to advance this kind of company-producing technology commercialization at Georgia Tech.”

The Georgia Tech Edison Fund, which is managed by Fleming, also provides seed funding to startup companies that have a close association with Georgia Tech.

“The judges for the first Georgia Tech Edison Prize heard a number of excellent presentations,” Fleming explained. “The judges selected Fengtao Wang because he had successfully identified an un-served market for the product and had begun approaching potential partners to commercialize the technology. Innovation is ultimately about turning knowledge into money.”

Approximately 100 entries were received for the prize competition from among the 300 graduate students who submitted posters to the Graduate Research and Innovation Conference. Those entries were evaluated by a committee of entrepreneurs, venture capitalists and Georgia Tech faculty to create a list of 11 finalists. Those finalists were each invited to make presentations to a judging committee, which selected the winner announced at a reception on the evening of February 8.

The judging committee included:

• Jamie Bardin, former CEO of EZ-Prints
• Nelson Chu, general partner of Kinetic Ventures
• Merrick Furst, distinguished professor in the Georgia Tech College of Computing
• Gary Lee, former CEO of Flexlight Systems
• Keith McGreggor, manager of technology evaluation in the ATDC
• Nina Sawczuk, assistant director for biosciences in the ATDC
• Jim Stratigos, president of Broadband Strategies

About the Enterprise Innovation Institute:
The Georgia Tech Enterprise Innovation Institute helps companies, entrepreneurs, economic developers and communities improve their competitiveness through the application of science, technology and innovation. It is one of the most comprehensive university-based programs of business and industry assistance, technology commercialization and economic development in the nation.

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By applying a commercially-available spin-on-glass (SOG) material to graphene and then exposing it to electron-beam radiation, researchers at the Georgia Institute of Technology created both types of doping by simply varying the exposure time. Higher levels of e-beam energy produced p-type areas, while lower levels produced n-type areas.

The technique was used to fabricate high-resolution p-n junctions. When properly passivated, the doping created by the SOG is expected to remain indefinitely in the graphene sheets studied by the researchers.

"This is an enabling step toward making possible complementary metal oxide graphene transistors," said Raghunath Murali, a senior research engineer in Georgia Tech's Nanotechnology Research Center.

A paper describing the technique appeared February 10, 2010 in the journal Applied Physics Letters. The research was supported by the Semiconductor Research Corporation and the Defense Advanced Research Projects Agency (DARPA) through the Interconnect Focus Center.

In the new doping process, Murali and graduate student Kevin Brenner begin by removing flakes of graphene one to four layers thick from a block of graphite. They place the material onto a surface of oxidized silicon, then fabricate a four-point contact device.

Next, they spin on films of hydrogen silsesquoxane (HSQ), then cure certain portions of the resulting thin film using electron beam radiation. The technique provides precise control over the amount of radiation and where it is applied to the graphene, with higher levels of energy corresponding to more cross-linking of the HSQ.

"We gave varying doses of electron-beam radiation and then studied how it influenced the properties of carriers in the graphene lattice," Murali said. "The e-beam gave us a fine range of control that could be valuable for fabricating nanoscale devices. We can use an electron beam with a diameter of four or five nanometers that allows very precise doping patterns."

Electronic measurements showed that a graphene p-n junction created by the new technique had large energy separations, indicating strong doping effects, he added.

Researchers elsewhere have demonstrated graphene doping using a variety of processes including soaking the material in various solutions and exposing it to a variety of gases. The Georgia Tech process is believed to be the first to provide both electron and hole doping from a single dopant material.

Doping processes used for graphene are likely to be significantly different from those established for silicon use, Murali said. In silicon, the doping step substitutes atoms of a different material for silicon atoms in the material’s lattice.

In the new single-step process for graphene, the doping is believed to introduce atoms of hydrogen and oxygen in the vicinity of the carbon lattice. The oxygen and hydrogen don't replace carbon atoms, but instead occupy locations atop the lattice structure.

"Energy applied to the SOG breaks chemical bonds and releases hydrogen and oxygen which bond with the carbon lattice," Murali said. "A high e-beam energy converts the whole SOG structure to more of a network, and then you have more oxygen than hydrogen, resulting in a p-type doping."

In volume manufacturing, the electron beam radiation would likely be replaced by a conventional lithography process, Murali said. Varying the reflectance or transmission of the mask set would control the amount of radiation reaching the SOG, and that would determine whether n-type or p-type areas are created.

"Making everything in a single step would avoid some of the expensive lithography steps," he said. "Gray-scale lithography would allow fine control of doping across the entire surface of the wafer."

For doping bulk areas such as interconnects that do not require patterning, the researchers simply coat the area with HSQ and expose it to a plasma source. The technique can make the nanoribbons up to 10 times more conductive than untreated graphene.

Because HSQ is already familiar to the microelectronics industry, the one-step approach to doping could help integrate graphene into existing processes, avoiding a disruption of the massive semiconductor design and fabrication system, Murali noted.

Over the past two years, researchers in the Nanotechnology Research Center had observed changes caused by application of HSQ during electrical testing. Only recently did they take a closer look at what was happening to understand how to take advantage of the phenomenon.

For the future, they'd like to better understand how the process works and whether other polymers might provide better results.

"We need to have a better understanding of how to control this process because variability is one of the issues that must be controlled to make manufacturing feasible," Murali explained. "We are trying to identify other polymers that may provide better control or stronger doping levels."

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MOF membranes offer an alternative to more energy intensive processes for separating gases such as carbon dioxide, methane, nitrogen and hydrogen. The technology has generated significant interest because of the broad range of crystalline structures that can be synthesized, but development of new MOF membranes is still at an early stage.

“Metal-organic framework membranes will be useful for doing large-scale energy-related separations in an efficient way. We are trying to accelerate their development to help move the world’s energy economy toward a more sustainable path,” said David Sholl, a professor in the Georgia Tech School of Chemical and Biomolecular Engineering. “A lot of chemists are interested in developing these metal-organic frameworks, and we hope to provide a new approach to designing the membranes.”

A publication on the use of atomically detailed calculations for designing metal-organic framework membranes was recently cited by ScienceWatch as its “fast-breaking paper in engineering” for February 2010. Details of the work were published in the journal Industrial Engineering Chemical Research in January 2009. The research was funded in part by the National Science Foundation (NSF).

Metal-organic framework materials are nanoporous crystals that combine metal-organic complexes with organic linkers to create highly porous frameworks. They offer advantages such as high surface area, porosity, low density and both thermal and mechanical stability – all important for separation membranes.

There are many possible material combinations that could be used in the membranes. By comparing such properties as binding strength and flow rates, the computational modeling could give researchers a way to rapidly identify the materials that will work best in high-volume industrial applications.

“The extra challenge with using metal-organic frameworks is that there are literally thousands of different materials that could be considered for use,” said Sholl, who is a Georgia Research Alliance eminent scholar in energy sustainability. “This is where computational modeling really helps. We are doing the materials screening problem computationally to guide us in attacking the actual fabrication problem experimentally.”

Sholl hopes the technique will narrow the list of candidate materials from thousands down to as few as 10. Researchers would then fabricate the membranes and test them in real-world conditions.

“If we were testing all of these in the lab without the computational guidance, it’s unlikely that we would ever choose the right material,” he said. “The biggest challenge for making a new membrane is that it really requires a lot of work to make a functioning device. Even if we know exactly what material to use, there is a very long development path.”

At Georgia Tech, Sholl’s modeling group is working with experimentalists such as Sankar Nair and Christopher Jones – both professors in the School of Chemical and Biomolecular Engineering – to produce prototype membranes for evaluation.

“The big push right now is to make some devices and get test data,” Sholl said. “In particular, we want to do this within a technology framework that we know can scale up to real-world industrial levels quickly.”

In addition to colleagues at Georgia Tech, the group is also working with industrial partners to help ensure that the membranes work in industrial conditions.

“If we can go from the idea in the academic lab to a serious field test within five years, that would be a real success,” said Sholl, who holds the Michael Tennenbaum Family Chair in the School of Chemical and Biomolecular Engineering. “We can’t afford for this to take 25 years because there is a need for this technology now.”

The new membrane technology could be used to address environmental issues such as removal of carbon dioxide from stack gases of coal-burning facilities in a cost-effective way. The technology could also make it economically attractive to use natural gas supplies that are contaminated with carbon dioxide, potentially expanding supplies of that fuel.

The researchers, including graduate student Seda Keskin, have modeled how the membrane technology would operate in separating methane from carbon dioxide, hydrogen from carbon dioxide, nitrogen from carbon dioxide, hydrogen from methane, nitrogen from hydrogen and methane from nitrogen.

“The common thread of this work is that we are interested in very large scale, large volume applications that can only be economical with very low energy input,” Sholl added.

This research was supported in part by the National Science Foundation (NSF) under grants CTS-0413027 and CTS-0556831. The content of this article is solely the responsibility of the principal investigator and does not necessarily represent the official view of the NSF.

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GTRI’s latest product is a pair of arthritis simulation gloves, which reproduce the reduction in functional capacity experienced by persons with arthritis. The gloves help those responsible for consumer products better understand how arthritis affects a person’s ability to grasp, pinch, turn, lift and twist objects.

“A product manager or designer can put these gloves on and attempt to open their company’s products or packaging,” explained GTRI principal research scientist Brad Fain. “If they are unable to open a product or package, then chances are high that people with moderate to severe symptoms of arthritis will also have difficulty opening it.”

The gloves can be used with a variety of consumer products, including medicine bottles, beverage containers, office supplies, medical devices, vehicles, cell phones and many other consumer products. They can also be used with many different types of packaging, including clamshell packages, cardboard boxes, cereal containers and foil packages.

Three companies, including Kraft Foods, are currently using the gloves in-house.

“Maxwell House always keeps our consumers’ needs in mind when designing packaging,” said Linda Roman, senior group leader for packaging strategic research at Kraft Foods. “For example, we used the gloves created by the Georgia Tech Research Institute to verify that the lid on our new instant coffee jar is accessible for those who have difficulty opening jars with regular caps. The gloves helped us evaluate the EZ Grip lid to be sure that our lid is, in fact, easy for our consumers to use.”

The gloves were designed to reduce a wearer’s functional ability to grasp something and either pull or rotate it by 33-50 percent. They also stiffen an individual’s finger joints and restrict the range of motion of his or her fingers. To create the finger stiffness and reduced finger strength experienced by individuals with arthritis, the gloves were designed with metal wires between layers of neoprene and other fabrics.

In addition to identifying ease of use issues with products, the gloves are also intended to raise awareness about issues faced by people with disabilities and to support programs focused on ease of use in design. Currently, the Arthritis Foundation in the United States and Arthritis Australia are using the gloves for such educational purposes.

The gloves can be purchased alone, or as part of GTRI’s disability awareness kit, which also includes a low-vision simulation kit, a finger strength simulation kit and a CD training program. The finger strength simulation kit consists of finger exercises that are calibrated to certain amounts of force recommended for packaging and the training program teaches individuals how to use the gloves.

The low-vision simulation kit contains a pair of glasses that simulate common visual disabilities, including various degrees of cataracts, visual acuity problems, contrast sensitivity issues and age-related macular degeneration.

“A product manager can put the glasses on and observe products to see if he or she can read important things written in small print, like instructions or an expiration date,” added Fain.

In the future, many baby boomers will likely demand the same access to products that they currently have -- even as their functional abilities decline.

“These older individuals will attribute any inability to open or use a product with deficiencies in the product itself,” added Fain. “That message or perception can be detrimental to companies because they want to avoid being associated with a product that’s difficult to use. The arthritis simulation gloves and the rest of the items in the disability awareness kit can help companies avoid these design mistakes.”

The gloves were created through funding by GTRI’s independent research and development program. To purchase the arthritis simulation gloves or the disability awareness kit, please visit: http://www.gtri.gatech.edu/facilities/aef.

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This “circulation control” aerodynamic technology could allow the wind turbines to produce significantly more power than current devices at the same wind speed.

Research aimed at adapting circulation control technology to wind turbine blades will be conducted by a California company, PAX Streamline, in collaboration with the Georgia Institute of Technology. The two-year project, which will lead to construction of a demonstration pneumatic wind turbine, will be supported by a $3 million grant from the Advanced Research Projects Agency-Energy -- the federal energy research and development organization also known as ARPA-E.

“Our goal will be to make generation of electricity from wind turbines less expensive by eliminating the need for the complex blade shapes and mechanical control systems used in current turbines,” said Robert J. Englar, principal research engineer at the Georgia Tech Research Institute (GTRI). “Because these new blades would operate effectively at lower wind speeds, we could potentially open up new geographic areas to wind turbine use. Together, these advances could significantly expand the generation of electricity from wind power in the United States.”

Circulation control techniques use compressed air blown from slots on the trailing edges of wings or hollow blades to change the aerodynamic properties of those wings or blades. In aircraft, circulation control wings improve lift, allowing aircraft to fly at much lower speeds – and take off and land in much shorter distances. In helicopter rotor blades, the technique -- also known as the “circulation control rotor” -- both simplifies the rotor and its control system and produces more lift.

The ARPA-E project will apply the technique to controlling the aerodynamic properties of wind turbine blades, which now must be made in complicated shapes and controlled by complex control mechanisms to extract optimal power from the wind.

“The speed at which these turbines would begin to operate will be much lower than with existing blades,” said Englar. “Places that wind maps have previously indicated would not be suitable locations for wind turbines may now be useful. The blown technology should also allow safe operation at higher wind speeds and in wind gusts that would cause existing turbines to be shut down to prevent damage.”

Because they would produce more aerodynamic force, torque and power than comparable blades, these blown structures being developed by Georgia Tech and PAX could also allow a reduction in the size of the wind turbines.

“If you need a specific amount of wind force and torque generated by the wind turbine to generate electricity, we could get that force and torque from a smaller blade area because we’d have more powerful lifting surfaces,” Englar explained.

A major question awaiting study is how much energy will be required to produce the compressed air the blown blades need to operate. Preliminary studies done by Professor Lakshmi Sankar in Georgia Tech’s School of Aerospace Engineering suggest that wind turbines with the blown blades could produce 30 to 40 percent more power than conventional turbines at the same wind speed -- even when the energy required to produce the compressed air is subtracted from the total energy production.

The new turbine blades will be developed at GTRI’s low-speed wind tunnel research facility located in Cobb County, north of Atlanta.

Officials of PAX Streamline see the circulation control technology as key to the development of a new generation of turbines that could significantly lower the cost of producing electricity from the wind.

“This is a significant validation of our established turbine R&D,” said PAX CEO John Webley. “With this grant, we can rapidly accelerate our research program and, within the next two years, deploy a prototype wind turbine which demonstrates our game-changing technology.”

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The three compounds are potentially important because they absorb infrared energy in the so-called “atmospheric window” region – at wavelengths where other major greenhouse gases such as carbon dioxide allow radiation to pass freely out into space. Though these long-lived compounds now exist in relatively low concentrations, their ability to absorb energy at these wavelengths means their contributions to global warming could increase if their levels continue to rise.

Because the compounds are relatively inert chemically, information on how they react with electronically excited atomic oxygen – known as O(1D) – will help improve the accuracy of global climate models by providing a better estimate of how long these absorbers remain in the atmosphere. The information could also inform public policy debate about whether the chemicals, now used in industrial applications, should be replaced with compounds that have less climate change impact.

“This study will contribute to an understanding of the long-term effect of these compounds on climate,” said Paul Wine, a professor in the Schools of Chemistry and Biochemistry and Earth and Atmospheric Sciences at the Georgia Institute of Technology. “There is significant interest in trying to establish the role of these heavy absorbers of infrared radiation, especially the compounds that absorb in the window region where other greenhouses gases are not factors.”

Information on the reaction rates of sulfuryl fluoride (SO2F2), nitrogen trifluoride (NF3) and trifluoromethyl sulfur pentafluoride (SF5CF3) was published Jan. 25, 2010, in the early edition of the PNAS, and will be part of a special issue on atmospheric chemistry. The research was funded by the National Aeronautics and Space Administration (NASA).

Sulfuryl fluoride is a fumigant widely used as a replacement for the ozone-depleting compound methyl bromide (CH3Br). Nitrogen trifluoride is used in the electronics industry for plasma etching and equipment cleaning. Trifluoromethyl sulfur pentafluoride – the most powerful known greenhouse gas on a per-molecule basis – is believed to be a breakdown product of an insulating compound used in high-voltage equipment.

The three compounds have some of the highest global warming potentials (GWP) of any compounds in the atmosphere. Trifluoromethyl sulfur pentafluoride has a global warming potential approximately 18,000 times greater – on a per unit mass basis – than carbon dioxide when evaluated over a 100-year time period. Nitrogen trifluoride has a GWP of approximately 17,000, while sulfuryl fluoride is approximately 4,000 times more effective than carbon dioxide at trapping infrared radiation.

The presence of these compounds in the atmosphere and their potential contributions to climate change were only recently recognized. Reaction with electronically-excited oxygen atoms is the only known pathway by which these compounds are destroyed at atmospheric altitudes below the ionosphere. Though present at relatively low levels today, studies show that their concentrations are increasing – with atmospheric levels of NF3 growing at more than 10 percent per year.

“These chemicals are relatively inert, which makes them useful for specific applications,” Wine said. “But because of their chemical inertness, they tend to have long lifetimes in the atmosphere and are available to trap radiation for a long time. That contributes to their high global warming potential.”

To study the rate at which the compounds react with and deactivate the atomic oxygen species, Wine and Georgia Tech collaborators Zhijun Zhao, Patrick Laine and J. Michael Nicovich used laser flash photolysis in the laboratory to create O(1D) and expose it to the three compounds in controlled environments at temperatures ranging from about 200 to 350 degrees Kelvin.

O(1D) is produced in the atmosphere by the interaction of ozone (O3) and molecular oxygen (O2) with ultraviolet light. This electronically-excited oxygen interacts quickly with other molecules around it – such as N2 and O2 – to form ground-state atomic oxygen. Hence, its levels are higher in the upper atmosphere than in the lower atmosphere.

The researchers found that O(1D) interaction with trifluoromethyl sulfur pentafluoride destroys this compound in – at most – one out of a thousand interactions. That means amounts of that compound released into the atmosphere will remain there for long periods of time, probably around a thousand years.

For NF3, the researchers found a reaction rate more than double one that had been reported in a previous study, meaning the material may have less warming impact than previously thought. For SO2F2, which also may be taken up by the ocean, the Georgia Tech findings agreed with one earlier study.

Wine said the new data on these compounds will be factored into the next major report of the Intergovernmental Panel on Climate Change. Knowing how long the compounds will likely remain in the atmosphere permits more accurate accounting for what could be a significant infrared trapping effect.

“If you put new molecules into the atmosphere that absorb infrared radiation where carbon dioxide and methane already absorb, they would have to be present in very large quantities to have any effect at all,” Wine noted. “But because these molecules absorb in the window region at wavelengths between 8 and 12 microns, they don’t have to be present at high levels to have an effect.”

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The study shows that delivering stem cells on a polymer scaffold to treat large areas of missing bone leads to improved bone formation and better mechanical properties compared to treatment with the scaffold alone. This type of therapeutic treatment could be a potential alternative to bone grafting operations.

"Massive bone injuries are among the most challenging problems that orthopedic surgeons face, and they are commonly seen as a result of accidents as well as in soldiers returning from war," said the study's lead author Robert Guldberg, a professor in Georgia Tech's Woodruff School of Mechanical Engineering. "This study shows that there is promise in treating these injuries by delivering stem cells to the injury site. These are injuries that would not heal without significant medical intervention."

Details of the research were published in the early edition of the journal Proceedings of the National Academy of Sciences on January 11, 2010. This work was funded by the National Institutes of Health and the National Science Foundation.

The study was conducted in rats in which two bone gaps eight millimeters in length were created to simulate massive injuries. One gap was treated with a polymer scaffold seeded with stem cells and the other with scaffold only. The results showed that injuries treated with the stem cell scaffolds showed significantly more bone growth than injuries treated with scaffolds only.

Guldberg and mechanical engineering graduate student Kenneth Dupont experimented with scaffolds containing two different types of human stem cells -- bone marrow-derived mesenchymal adult stem cells and amniotic fluid fetal stem cells.

"We were able to directly evaluate the therapeutic potential of human stem cells to repair large bone defects by implanting them into rats with a reduced immune system," explained Guldberg, who is also the director of the Petit Institute for Bioengineering and Bioscience at Georgia Tech.

Micro-CT measurements showed no significant differences in bone regeneration between the two stem cell groups. However, combining the two types of stem cells produced significantly higher bone volume and strength compared to scaffolds without cellular augmentation.

Although stem cell delivery significantly enhanced bone growth and biomechanical properties, it was not able to consistently repair the injury. Eight weeks after the treatment, new bone bridged the gaps in four of nine defects treated with scaffolds seeded with adult stem cells, one of nine defects treated with scaffolds seeded with fetal stem cells, and none of the defects treated with the scaffold alone.

"We thought that the functional regeneration of the bone defects may have been limited by stem cells migrating away from the injury site, so we decided to investigate the fate and distribution of the delivered cells," said Guldberg.

To do this, Guldberg labeled stem cells with fluorescent quantum dots -- nanometer-scale particles that emit light when excited by near-infrared radiation -- to track the distribution of stem cells after delivery on the scaffolds and completed the same experiments as previously described.

Throughout the entire study, the researchers observed significant fluorescence at the stem cell scaffold sites. However, beginning seven to 10 days after treatment, signals appeared at the scaffold-only sites. Additional analysis with immunostaining revealed that the quantum dots present at the scaffold-only sites were contained in inflammatory cells called macrophages that had taken up quantum dots released from dead stem cells.

"While our overall study shows that stem cell therapy has a lot of promise for treating massive bone defects, this experiment shows that we still need to develop an improved way of delivering the stem cells so that they stay alive longer and thus remain at the injury site longer," explained Guldberg.

The researchers also found that the quantum dots diminished the function of the transplanted stem cells and thus their therapeutic effect. When the stem cells were labeled with quantum dots, the results showed a failure to enhance bone formation or bridge defects. However, the same low concentration of quantum dots did not affect cell viability or the ability of the stem cells to become bone cells in laboratory studies.

"Although in vitro laboratory studies remain important, this work provides further evidence that well-characterized in vivo models are necessary to test the ability of regenerative tissue strategies to effectively integrate and restore function in complex living organisms," added Guldberg. "Improved methods of non-invasive cell tracking that do not alter cell function in vivo are needed to optimize stem cell delivery strategies and compare the effectiveness of different stem cell sources for tissue regeneration."

Guldberg is currently exploring alternative cell tracking methods, such as genetically modifying the stem cells to express green fluorescent protein and/or other luminescent enzymes such as luciferase. He is also investigating the addition of programming cues to the scaffold that will direct the stem cells to differentiate into bone cells. These signals may be particularly effective for fetal stem cells, which are believed to be more primitive than adult stem cells, according to Guldberg.

Lessons learned from the current work are also being applied to develop effective stem cell therapies for severe composite injuries to multiple tissues including bone, nerve, vasculature and muscle. This follow-on work is being conducted in the Georgia Tech Center for Advanced Bioengineering for Soldier Survivability in collaboration with Ravi Bellamkonda and Barbara Boyan, professors in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University.

Other authors on the paper include Andres Garcia, professor and Woodruff Faculty Fellow in Georgia Tech's Woodruff School of Mechanical Engineering and the Petit Institute for Bioengineering and Bioscience; Georgia Tech research scientist Hazel Stevens, Georgia Tech graduate student Joel Boerckel; and National University of Ireland medical student Kapil Sharma.

This work was funded by grant number R01-AR051336 from the National Institutes of Health (NIH) and by grant number EEC-9731643 from the National Science Foundation (NSF). The content is solely the responsibility of the principal investigator and does not necessarily represent the official views of the NIH or NSF.

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Media Relations Contacts: Abby Vogel (avogel@gatech.edu; 404-385-3364) or John Toon (jtoon@gatech.edu; 404-894-6986).

Writer: Abby Vogel

All-optical switching could allow dramatic speed increases in telecommunications by eliminating the need to convert photonic signals to electronic signals – and back – for switching. All-optical processing could also facilitate photonic computers with similar speed advances.

Details of these materials – and the design approach behind them – were reported February 18th in Science Express, the rapid online publication of the journal Science. Conducted at the Georgia Institute of Technology, the research was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research (ONR).

“This work provides proof that at least from a molecular point of view, we can identify and produce materials that have the right properties for all-optical processing,” said Seth Marder, a professor in the Georgia Tech School of Chemistry and Biochemistry and co-author of the paper. “This opens the door for looking at this issue in an entirely different way.”

The polymethine organic dye materials developed by the Georgia Tech team combine large nonlinear properties, low nonlinear optical losses, and low linear losses. Materials with these properties are essential if optical engineers are to develop a new generation of devices for low-power and high-contrast optical switching of signals at telecommunications wavelengths. Keeping data all-optical would greatly facilitate the rapid transmission of detailed medical images, development of new telepresence applications, high-speed image recognition – and even the fast download of high-definition movies.

But favorable optical properties these new materials developed at Georgia Tech have only been demonstrated in solution. For their materials to have practical value, the researchers will have to incorporate them in a solid phase for use in optical waveguides – and address a long list of other challenges.

“We have developed high-performing materials by starting with optimized molecules and getting the molecular properties right,” said co-author Joseph Perry, also a professor in the Georgia Tech School of Chemistry and Biochemistry. “Now we have to figure out how to pack them together so they have a high density and useful physical forms that would be stable under operation.”

Marder, Perry and collaborators in Georgia Tech’s Center for Organic Photonics and Electronics (COPE) have been working on the molecules for several years, refining their properties and adding atoms to maximize their length without inducing symmetry breaking, a phenomenon in which unequal charges build up within molecules. This molecular design effort, which builds on earlier research with smaller molecules, included both experimental work – and theoretical studies done in collaboration with Jean-Luc Bredas, a also a professor in the School of Chemistry and Biochemistry.

The design strategies identified by the research team – which also included Joel Hales, Jonathan Matichak, Stephen Barlow, Shino Ohira, and Kada Yesudas – could be applied to development of even more active molecules, though Marder believes the existing materials could be modified to meet the needs of all-optical processing

“For this class of molecules, we can with a high-degree of reliability predict where the molecules will have both large optical nonlinearities and low two-photon absorption,” said Marder. “Not only can we predict that, but using well-established chemical principles, we can tune where that will occur such that if people want to work at telecommunications wavelengths, we can move to where the molecules absorb to optimize its properties.”

Switching of optical signals carried in telecommunications networks currently requires conversion to electrical signals, which must be switched and then converted back to optical format. Existing electro-optical technology may ultimately be able to provide transmission speeds of up to 100 gigabits-per-second. However, all-optical processing could theoretically transmit data at speeds as high as 2,000 gigabits-per-second, allowing download of high-definition movies in minutes rather than hours.

“Even if the frequency of signals coming and going is high, there is a latency that causes a bottleneck for the signals until the modulation and switching are done,” Perry explained. “If we can do that all optically, then that delay can be reduced. We need to get electronics out of the system.”

Perry and Marder emphasize that many years of research remain ahead before their new materials will be practical. But they believe the approach they’ve developed charts a path toward all-optical systems.

“While we have not made all-optical switches, what we have done is provide a fundamental understanding of what the systems are that could have the combined set of properties that would make this possible,” Marder said. “Conceptually, we have probably made it over the hump with this class of molecules. The next part of this work will be difficult, but it will not require a fundamental new understanding of the molecular structure.”

This article is based on work supported in part by the STC program of the National Science Foundation under agreement DMR-0120967, the DARPA MORPH Program and ONR (N00014-04-0095 and N00014-06-1-0897) and the DARPA ZOE Program (W31P4Q-09-1-0012). The comments and opinions expressed are those of the researchers and do not necessarily represent the views of the NSF, DARPA or ONR.

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Media Relations Contacts: John Toon (404-894-6986)(jtoon@gatech.edu) or Abby Vogel (404-385-3364)(avogel@gatech.edu).

Writer: John Toon