Monday, August 31, 2009

ORGANIC ELECTRONICS A TWO-WAY STREET, THANKS TO NEW PLASTIC SEMICONDUCTOR

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Plastic that conducts electricity holds promise for cheaper, thinner and more flexible electronics. This technology is already available in some gadgets -- the new Sony walkman that was introduced earlier this summer and the Microsoft Zune HD music player released recently both incorporate organic light-emitting electronic displays. Until now, however, circuits built with organic materials have allowed only one type of charge to move through them. New research from the University of Washington makes charges flow both ways.

The cover article in an upcoming issue of the journal Advanced Materials describes an approach to organic electronics that allows transport of both positive and negative charges.

"The organic semiconductors developed over the past 20 years have one important drawback. It's very difficult to get electrons to move through," said lead author Samson Jenekhe, a UW professor of chemical engineering. "By now having polymer semiconductors that can transmit both positive and negative charges, it broadens the available approaches. This would certainly change the way we do things."

Co-authors are Felix Kim, a doctoral student working with Jenekhe, and graduate student Xugang Guo and assistant professor Mark Watson at the University of Kentucky. The research was funded by the National Science Foundation, the Department of Energy and the Ford Foundation.

Silicon Valley got its name for a reason: Silicon is the "workhorse" of today's electronics industry, Jenekhe said. Silicon is fairly expensive and requires costly manufacturing, however, and because its rigid crystal form does not allow flexible devices.

About 30 years ago it was discovered that some plastics, or polymers, can conduct electricity. Since then researchers have been working to make them more efficient. Organic, or carbon-based, electronics are now used in such things as laptop computers, car audio systems and mp3 players.

A major drawback with existing organic semiconductors is most transmit only positive charges (called "holes" because the moving areas of positive charge are actually places where an electron is missing). In the last decade a few organic materials have been developed that can transport only electrons. But making a working organic circuit has meant carefully layering two complicated patterns on top of one another, one that transports electrons and another one that transports holes.

"Because current organic semiconductors have this limitation, the way they're currently used has to compensate for that, which has led to all kinds of complex processes and complications," Jenekhe said.

For more than a decade Jenekhe's lab has been a leader in developing organic semiconductors that can transmit electrons. Over the past few years the group has created polymers with a donor and an acceptor part, and carefully adjusted the strength of each one. In collaboration with Watson's lab, they have now developed an organic molecule that works to transport both positive and negative charges.

"What we have shown in this paper is that you don't have to use two separate organic semiconductors," Jenekhe said. "You can use one material to create electronic circuits."

The material would allow organic transistors and other information-processing devices to be built more simply, in a way that is more similar to how inorganic circuits are now made.

The group used the new material to build a transistor designed in the same way as a silicon model and the results show that both electrons and holes move through the device quickly.

The results represent the best performance ever seen in a single-component organic polymer semiconductor, Jenekhe said. Electrons moved five to eight times faster through the UW device than in any other such polymer transistor. A circuit, which consists of two or more integrated devices, generated a voltage gain two to five times greater than previously seen in a polymer circuit.

"We expect people to use this approach," Jenekhe said. "We've opened the way for people to know how to do it."

(Photo: University of Washington)

ANTI-AGING GENE LINKED TO HIGH BLOOD PRESSURE

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Researchers at the University of Oklahoma Health Sciences Center have shown the first link between a newly discovered anti-aging gene and high blood pressure. The results, which appear this month in the journal Hypertension, offer new clues on how we age and how we might live longer.

Persistent hypertension, or high blood pressure, is a risk factor for stroke, heart attack, heart failure, arterial aneurysm and is the leading cause of chronic kidney failure. Even a modest elevation of arterial blood pressure leads to shortened life expectancy.

Researchers, led by principal investigator Zhongjie Sun, tested the effect of an anti-aging gene called klotho on reducing hypertension. They found that by increasing the expression of the gene in laboratory models, they not only stopped blood pressure from continuing to rise, but succeeded in lowering it. Perhaps most impressive was the complete reversal of kidney damage, which is associated with prolonged high blood pressure and often leads to kidney failure.

“One single injection of the klotho gene can reduce hypertension for at least 12 weeks and possibly longer. Klotho is also available as a protein and, conceivably, we could ingest it as a powder much like we do with protein drinks,” said Sun, M.D., Ph.D., a cardiovascular expert at the OU College of Medicine.

Scientists have been working with the klotho gene and its link to aging since 1997 when it was discovered by Japanese scientists. This is the first study showing that a decline in klotho protein level may be involved in the progression of hypertension and kidney damage, Sun said. With age, the klotho level decreases while the prevalence of hypertension increases.

Researchers used one injection of the klotho gene in hypertensive research models and were able to markedly reduce blood pressure by the second week. It continued to decline steadily for the length of the project – 12 weeks. The klotho gene was delivered with a safe viral vector that is currently used for gene therapy. The virus is already approved by the U.S. Food and Drug Administration for use in humans.

Researchers are studying the gene’s effect for longer periods to test its ability to return blood pressure levels to normal. They also are looking at whether klotho can prevent hypertension.

University of Oklahoma Health Sciences Center

FLYING BY THE SKIN OF OUR TEETH

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It's been a mystery: how can our teeth withstand such an enormous amount of pressure, over many years, when tooth enamel is only about as strong as glass? A new study by Prof. Herzl Chai of Tel Aviv University's School of Mechanical Engineering and his colleagues at the National Institute of Standards and Technology and George Washington University gives the answer.

The researchers applied varying degrees of mechanical pressure to hundreds of extracted teeth, and studied what occurred on the surface and deep inside them. The study, published in the May 5, 2009, issue of the Proceedings of the National Academy of Science, shows that it is the highly-sophisticated structure of our teeth that keeps them in one piece — and that structure holds promising clues for aerospace engineers as they build the aircraft and space vehicles of the future.

"Teeth are made from an extremely sophisticated composite material which reacts in an extraordinary way under pressure," says Prof. Chai. "Teeth exhibit graded mechanical properties and a cathedral-like geometry, and over time they develop a network of micro-cracks which help diffuse stress. This, and the tooth's built-in ability to heal the micro-cracks over time, prevents it from fracturing into large pieces when we eat hard food, like nuts."

The automotive and aviation industries already use sophisticated materials to prevent break-up on impact. For example, airplane bodies are made from composite materials — layers of glass or carbon fibers — held together by a brittle matrix.

In teeth, though, fibers aren't arranged in a grid, but are "wavy" in structure. There are hierarchies of fibers and matrices arranged in several layers, unlike the single-thickness layers used in aircrafts. Under mechanical pressure, this architecture presents no clear path for the release of stress. Therefore, "tufts" — built-in micro cracks — absorb pressure in unison to prevent splits and major fractures. As Prof. Chai puts it, tooth fractures "have a hard time deciding which way to go," making the tooth more resistant to cracking apart. Harnessing this property could lead to a new generation of much stronger composites for planes.

Prof. Chai, himself an aerospace engineer, suggests that if engineers can incorporate tooth enamel's wavy hierarchy, micro-cracking mechanism, and capacity to heal, lighter and stronger aircraft and space vehicles can be developed. And while creating a self-healing airplane is far in the future, this significant research on the composite structure of teeth can already begin to inspire aerospace engineers — and, of course, dentists.

Dental specialists looking for new ways to engineer that picture-perfect Hollywood smile can use Dr. Chai's basic research to help invent stronger crowns, better able to withstand oral wear-and-tear. "They can create smart materials that mimic the properties found in real teeth," he says.

In natural teeth, there may not be any way to speed up the self-healing ability of tooth enamel, which the Tel Aviv University research found is accomplished by a glue-like substance that fills in micro-cracks over time. But fluoride treatments and healthy brushing habits can help to fill in the tiny cracks and keep teeth strong.

(Photo: TAU)

Tel Aviv University

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