Promising New Nanotechnology for Spinal Cord Injury

A spinal cord injury often leads to permanent paralysis and loss of sensation below the site of the injury because the damaged nerve fibers can't regenerate. The nerve fibers or axons have the capacity to grow again, but don’t because they're blocked by scar tissue that develops around the injury.

Northwestern University researchers have shown that a nano-engineered gel inhibits the formation of scar tissue at the injury site and enables the severed spinal cord fibers to regenerate and grow. The gel is injected as a liquid into the spinal cord and self -assembles into a scaffold that supports the new nerve fibers as they grow up and down the spinal cord, penetrating the site of the injury.

When the gel was injected into mice with a spinal cord injury, after six weeks the animals had a greatly enhanced ability to use their hind legs and walk.

The research is published today in the April 2 issue of the Journal of Neuroscience.

"We are very excited about this," said lead author John Kessler, M.D., Davee Professor of Stem Cell Biology at Northwestern University's Feinberg School of Medicine. "We can inject this without damaging the tissue. It has great potential for treating human beings."

Kessler stressed caution, however, in interpreting the results. "It's important to understand that something that works in mice will not necessarily work in human beings. At this point in time we have no information about whether this would work in human beings."

"There is no magic bullet or one single thing that solves the spinal cord injury, but this gives us a brand new technology to be able to think about treating this disorder," said Kessler, also the chair of the Davee Department of Neurology at the Feinberg School. "It could be used in combination with other technologies including stem cells, drugs or other kinds of interventions."

“We designed our self-assembling nanostructures -- the building blocks of the gel -- to promote neuron growth,” said co-author Samuel I. Stupp, Board of Trustees Professor of Materials Science and Engineering, Chemistry, and Medicine and director of Northwestern’s Institute for BioNanotechnology in Medicine. “To actually see the regeneration of axons in the spinal cord after injury is a fascinating outcome.”

The nano-engineered gel works in several ways to support the regeneration of spinal cord nerve fibers. In addition to reducing the formation of scar tissue, it also instructs the stem cells --which would normally form scar tissue -- to instead to produce a helpful new cell that makes myelin. Myelin is a substance that sheaths the axons of the spinal cord to permit the rapid transmission of nerve impulses.

The gel's scaffolding also supports the growth of the axons in two critical directions -- up the spinal cord to the brain (the sensory axons) and down to the legs (the motor axons.) "Not everybody realizes you have to grow the fibers up the spinal cord so you can feel where the floor is. If you can't feel where the floor is with your feet, you can't walk," Kessler said.

Now Northwestern researchers are working on developing the nano-engineered gel to be acceptable as a pharmaceutical for the Food and Drug Administration.

If the gel is approved for humans, a clinical trial could begin in several years.

"It's a long way from helping a rodent to walk again and helping a human being walk again," Kessler stressed again. "People should never lose sight of that. But this is still exciting because it gives us a new technology for treating spinal cord injury."
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Strength Is But Skin Deep at the Nanoscale

For centuries, engineers have bent and torn metals to test their strength and ductility. Now, materials scientists at the University of Pennsylvania School of Engineering and Applied Science are studying the same metals but at nanoscale sizes in the form of wires a thousand times thinner than a human hair. This work has enable Penn engineers to construct a theoretical model to predict the strength of metals at the nanoscale. Using this model, they have found that, while metals tend to be stronger at nanoscale volumes, their strengths saturate at around 10-50 nanometers diameter, at which point they also become more sensitive to temperature and strain rate. Such prediction of different strength regimes of nano-solids is important for future application and engineering design of nanotechnology.

Such small-volume materials with relatively large surface areas are now routinely employed in microchips and nanoscience and technology, and their mechanical properties can differ vastly from their macroscale counterparts. Typically, smaller is stronger. A gold wire 200 nanometers in diameter can be 50 times stronger per area than centimeter-sized single-crystal gold. Engineers investigated the "smaller is stronger" trend.

Ju Li, an associate professor in the Department of Materials Science and Engineering at Penn, and his collaborators at the Georgia Institute of Technology have combined transition state theory, explicit atomistic energy landscape calculation and computer simulation to establish a theoretical framework to predict the strengths of small-volume materials. Unlike previous models, their prediction can be directly compared with experiments performed at realistic temperature and loading rates. This research appeared as a cover article in Volume 100 of Physical Review Letters.

Their study demonstrated that the free, exterior surface of nanosized materials can be fertile breeding grounds of dislocations at high stresses. Dislocations are string-like defects whose movements give rise to plastic flow, or shape change, of solids. In large-volume materials, it is easy for dislocations to multiply and entangle and to maintain a decent population inside; however, in small-volume materials, dislocations could show up and then exit the sample, one at a time. To initiate and sustain plastic flow in this case, dislocations need to be frequently nucleated fresh from the surface.

Since surface is itself a defect, researchers asked to what degree the measured strength of a small-volume material reflects surface properties and surface-mediated processes, particularly when the sample size is in the range of tens of nanometers. Li and his team modeled tiny bits of gold and copper to investigate the probabilistic nature of surface dislocation nucleation. The study showed that the activation volume associated with surface dislocation nucleation is characteristically in the range of 1–10 times the atomic volume, much smaller than that of many conventional dislocation processes. Small activation volumes will lead to sensitive temperature and strain-rate dependence of the critical stress, providing an upper bound to the size-strength relation.

From this, the team predicted that the "smaller is stronger" trend will saturate at wire diameters 10-50 nanometers for most metals. For comparison, computers now contain microchips with 45 nanometer strained silicon features. Associated with this saturation in strength is a transition in the rate-controlling mechanism, from collective dislocation dynamics to single dislocation nucleation.

The National Science Foundation-funded study was performed by Li and Amit Samanta of Penn and, from Georgia Tech, Ting Zhu and Ken Gall of the Woodruff School of Mechanical Engineering and Austin Leach of the School of Materials Science and Engineering.
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Nanotube wires shown to operate at speed of commercial chips

Integrated circuits, such as the silicon chips inside all modern electronics, are only as good as their wiring, but copper conduits are approaching physical performance limitations as they get thinner. Chipmakers have hoped that carbon "nanotubes" would allow them to continue using thinner wiring as they pack more devices into chips, but no one had demonstrated nanotube wires working on a conventional silicon chip. In a paper published online today by the journal Nano Letters, electrical engineers at Stanford University and Toshiba report using nanotubes to wire a silicon chip operating at speeds comparable to those of commercially available processors and memory.

"This is the first time anyone has been able to show digital signals going through nanotubes at 1 gigahertz [a billion times a second]," said H.-S. Philip Wong, a professor of electrical engineering at Stanford and a co-author of the report. "There had been a lot of expectations that nanotubes could do this, but no experimental proof so far."

At stake is the continuation of the famous Moore's Law, which calls for doubling the number of transistors on a chip every two years. The increase in transistors correlates strongly with greater computing power but also requires thinner and thinner wiring. The advance reported by the Stanford and Toshiba team shows that nanotubes are capable not only of connecting transistors at industrially relevant speed but of doing so in real circuits that use materials, designs and manufacturing processes compatible with those that chipmakers use today, added Gael Close, an electrical engineering doctoral student and the paper's lead author.

Joining Close and Wong in the research were Shinichi Yasuda and Shinobu Fujita of Toshiba's Advanced Semiconductor Laboratory in Japan and Bipul Paul of Toshiba America Research in San Jose.

The silicon chip Close and his collaborators built is an array of 256 circuits called "ring oscillators," which are industry-standard circuits for testing the speed of chips. Including other control circuitry that allowed for selectively operating each of the 256 oscillators, the chip comprised a total of 11,000 transistors in an area one hundredth of a square inch.

When designing the chip, Close, Wong and the Toshiba researchers purposely left one wire of each oscillator unconnected so the circuit is not completely wired up. After the semiconductor foundry TSMC made the chip, Close then engaged in a few more fabrication steps at the Stanford Nanofabrication Facility to complete the missing connections with the nanotubes. Each nanotube measured between 50 and 100 nanometers (billionths of a meter) in diameter and about 5 millionths of a meter in length.

The nanotubes, purchased from a commercial vendor, were "metallic" in that they were synthesized for maximum electrical conductivity.

The quality of the nanotubes and their connections varied widely, but in the end 19 of the ring oscillators were successfully connected. The nanotubes rested directly above the transistors they were connecting, minimizing electrical capacitance and allowing for the transmission of zeroes and ones at 1.02 gigahertz, or billions of times a second, in the best case. In 16 of the 19 good connections, the oscillators ran at speeds better than 800 megahertz, or millions of cycles a second.

The processors in personal computers currently on the market run at speeds between 2 and 3 gigahertz. The processor in an iPhone reportedly runs at about 700 megahertz.

Consumers should not expect the research to mean that they'll be putting nanotubes in their pockets next year, the researchers cautioned. Many improvements are needed for nanotube wiring to enter commercial use, Wong said, including more consistent nanotube purity and size, and more reliably made connections. The nanotubes in Close's chip were about the same size as the copper wires used today. Transmission of even higher-frequency signals in even thinner nanotubes will require improvements in both nanotube quality and circuit design.

But Wong and Close both said the research provides the most definitive confirmation to date that nanotubes can be the heir apparent to copper that the industry needs.

"This is a significant step but it is still very much at the proof of concept level," Close said. "The industry has been waiting for this kind of a demonstration to really move forward."

In addition to Toshiba, support for the research came from the semiconductor industry's Interconnect Focus Center, one of five research centers funded under the Focus Center Research Program, which is a Semiconductor Research Corporation program; and Close's Intel Graduate Fellowship.

Source

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Scientists propose test of string theory

Ancient light absorbed by neutral hydrogen atoms could be used to test certain predictions of string theory, say cosmologists at the University of Illinois. Making the measurements, however, would require a gigantic array of radio telescopes to be built on Earth, in space or on the moon.

String theory – a theory whose fundamental building blocks are tiny one-dimensional filaments called strings – is the leading contender for a “theory of everything.” Such a theory would unify all four fundamental forces of nature (the strong and weak nuclear forces, electromagnetism, and gravity). But finding ways to test string theory has been difficult.

Now, cosmologists at the U. of I. say absorption features in the 21-centimeter spectrum of neutral hydrogen atoms could be used for such a test.

“High-redshift, 21-centimeter observations provide a rare observational window in which to test string theory, constrain its parameters and show whether or not it makes sense to embed a type of inflation – called brane inflation – into string theory,” said Benjamin Wandelt, a professor of physics and of astronomy at the U. of I.

“If we embed brane inflation into string theory, a network of cosmic strings is predicted to form,” Wandelt said. “We can test this prediction by looking for the impact this cosmic string network would have on the density of neutral hydrogen in the universe.”

Wandelt and graduate student Rishi Khatri describe their proposed test in a paper accepted for publication in the journal Physical Review Letters.

About 400,000 years after the Big Bang, the universe consisted of a thick shell of neutral hydrogen atoms (each composed of a single proton orbited by a single electron) illuminated by what became known as the cosmic microwave background.

Because neutral hydrogen atoms readily absorb electromagnetic radiation with a wavelength of 21 centimeters, the cosmic microwave background carries a signature of density perturbations in the hydrogen shell, which should be observable today, Wandelt said.

Cosmic strings are filaments of infinite length. Their composition can be loosely compared to the boundaries of ice crystals in frozen water.

When water in a bowl begins to freeze, ice crystals will grow at different points in the bowl, with random orientations. When the ice crystals meet, they usually will not be aligned to one another. The boundary between two such misaligned crystals is called a discontinuity or a defect.

Cosmic strings are defects in space. A network of strings is predicted by string theory (and also by other supersymmetric theories known as Grand Unified Theories, which aspire to unify all known forces of nature except gravity) to have been produced in the early universe, but has not been detected so far. Cosmic strings produce characteristic fluctuations in the gas density through which they move, a signature of which will be imprinted on the 21-centimeter radiation.

The cosmic string network predicted to occur with brane inflation could be tested by looking for the corresponding fluctuations in the 21-centimeter radiation.

Like the cosmic microwave background, the cosmological 21-centimeter radiation has been stretched as the universe has expanded. Today, this relic radiation has a wavelength closer to 21 meters, putting it in the long-wavelength radio portion of the electromagnetic spectrum.

To precisely measure perturbations in the spectra would require an array of radio telescopes with a collective area of more than 1,000 square kilometers. Such an array could be built using current technology, Wandelt said, but would be prohibitively expensive.

If such an enormous array were eventually constructed, measurements of perturbations in the density of neutral hydrogen atoms could also reveal the value of string tension, a fundamental parameter in string theory, Wandelt said. “And that would tell us about the energy scale at which quantum gravity begins to become important.”

SOURCE

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Light could trap and release bacteria

Sensors that use light to trap, analyse and then release tiny objects such as bacteria or DNA are one step closer thanks to new computer simulations done by researchers in the US.

The team modelled the optofluidic interactions that occur when a liquid flows through a tiny channel next to an optical waveguide. The work could lead to new “lab-on-a-chip” sensors that could be used in a wide range of areas including medicine and security.

In such optofluidics sensors, the tiny objects of interest would be mixed with a fluid that would flow through tiny channels next to solid waveguides. When light is passed through the waveguides, it would create a short-range evanescent electric field in the channel that would trap tiny particles. While being held the objects could be studied using analytical probes, before being released.

However, it has proved very difficult to predict exactly how objects will behave in practical optofluidic systems. Now, researchers at Cornell University have devised a "stability number" that describes the conditions under which it is possible to transport an object optofluidically (Nanotechnology 19 045704).

Swept away

"If the stability number is greater than one then the particle will be confined to the waveguide and can therefore be transported along it using optical forces,said Cornell's David Erickson. "If the stability number is less than one then the particle will diffuse or be swept away."

The team's numerical results consider two classes of optical waveguide – a silicon waveguide operating at 1550 nm and a polymer waveguide operating at 1064 nm – located at the bottom of a simple microfluidic channel.

"We believe that the silicon-based system, with its high refractive index contrast, is more suited to trapping nanoscale objects, such as DNA or quantum dots," said Erickson. "The polymer-based system, as we have already demonstrated experimentally in another paper, is very appealing as a cheap platform for guiding micron-sized objects, such as biological cells."

Stability maps

For both systems, the group has generated "stability maps" that cover particles measuring from 600 to 300 nm in diameter. The charts provide a range of flow velocities where particle-waveguide trapping is likely to be successful based on three-dimensional finite element simulations. Additionally, the data highlights critically unstable regions where the drag force on the particle is stronger than the calculated trapping force.

So far so good, but what about moving to smaller particle diameters? "As the particle approaches tens of nanometers in diameter, its relative surface area to volume ratio increases and surface-particle interactions, such as adhesion, will become stronger," explained Erickson. "For small molecules, the approximation of a spherical continuous particle will not be valid and it may become necessary to account for non-uniformity in the particle structure and makeup."

As Erickson mentioned, the group has already demonstrated the optofluidic trapping and transport of polystyrene beads using polymer waveguides. Now the team is planning to work with more advanced photonic devices, such as ring resonators, and will complement its research with additional simulations.

SOURCE

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Ampere could be defined one electron at a time

There could soon be a new and more accurate method of defining the standard unit of current, the ampere, thanks to a tiny electronic device built by physicists in Finland and the US. The team, led by Jukka Pekola of the Helsinki University of Technology, has made a single-electron transistor that converts an oscillating voltage into a very precise electrical current.

The ampere, volt and ohm are the three fundamental units of electricity. While physicists have devised modern microscopic definitions of the volt and ohm – through measurements of the Josephson voltage and quantum Hall resistance respectively – the most accurate measurements of the ampere are made using a refined version of a technique first developed in the 19th century.

Today, the ampere is defined as the current which, when flowing through two parallel conductors one metre apart, exerts a certain force between the conductors. This is a macroscopic measurement involving a specific geometrical configuration of conductors – which limits the accuracy of the measurement.

Instead, physicists would like to define the ampere by creating an extremely precise source of electric current capable of delivering one electron at a time. Although researchers have already tried to make such single-electron devices in order to redefine the ampere, none have been successful because detecting such tiny electron currents has proved very difficult.

Now, Pekola and colleagues have made a single-electron transistor that could be used to overcome this problem (Nature Physics doi: 10.1038/nphys808). Their device consists of a small conducting island that is connected to two tunnel junctions. Electons can flow into the island via one junction and out via the other. The device also includes a gate electrode, which can be used to control the flow of electrons through the island by applying a voltage.

Each tunnel junction contains a very thin insulating layer, through which the electrons can quantum mechanically tunnel. The junctions are so tiny that the electric repulsion between electrons prevents more than one electron tunnelling at a time – creating a single-electron device.

The device is cooled to 0.1 K to reduce thermal noise and the team applied a constant voltage across island and junctions. An oscillating voltage is applied to the gate electrode. The precise number of electrons that pass through the device during one cycle of the oscillation is determined by the amplitude and mean value of the gate voltage.

The current flowing through the device is simply the number of electrons that tunnel per gate cycle multiplied by the charge of the electron and the frequency of the gate voltage. The gate frequency and number of electrons per cycle can be determined and the charge on the electron is fixed – which means that the device is a very precise source of current.

Although the researchers still need to improve the accuracy of their device, Pekola believes that the transistor is one of the best candidates to create a "metrological current pump" for defining the ampere. He told physicsworld.com that his could be done by placing about ten of the devices in parallel, which would deliver a current of about 100 pA, which is large enough to measure.

"Our simple one-gate device is easy to operate and it is straightforward to put many devices in parallel to make the output current larger," said Pekola. "The small current level has been the bottleneck in making single electron current pumps in the past."

The device might also help close the so-called “quantum metrological triangle” that relates current, voltage and frequency. Voltage and frequency can be related through the AC Josephson effect, while current and voltage can be related through the quantum Hall effect. Both these relationships include the same two fundamental constants -- the Planck constant and the charge on the electron. A metrological current pump would allow physicists to relate current directly to frequency.

"The result looks very interesting and may be important if it achieves its promise of providing a reliable way of making accurate devices that can both pump more than one electron per cycle and be placed in parallel," said Ian Robinson of the National Physical Lab in the UK. Robinson works on the "watt balance" that can already measure current with an uncertainty of much less than 1 x 10^-8. "The technique described here has around a factor of 1 million to go before it approaches the 1 x 10^-8 level but it shows promise," he added.

Source:nanotechweb.org

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Nanoscience: Weak Force. Strong Effect.

he van der Waals force, a weak attractive force, is solely responsible for binding certain organic molecules to metallic surfaces. In a model for organic devices, it is this force alone that binds an organic film to a metallic substrate. This data, recently published in Physical Review Letters, represents the latest findings from a National Research Network (NRN) supported by the Austrian Science Fund FWF. These findings mean that numerous calculation models for the physical interactions between thin films and their carrier materials will need to be revised.

Although they fulfil complex functions when used, for example, as computer chips, inorganic semiconductors have a simple construction that greatly limits their application. The same does not apply to semiconductors made of organic materials. Because organic molecules are extremely flexible, they can be used in a whole new range of applications. However, before this advantage can be exploited to the full, scientists need to have a better understanding of the far greater complexity of these materials over their inorganic counterparts.

Up & Down
Organic semiconductors are manufactured by applying thin films of an electrically conductive organic material to a carrier surface. When carrying out this process, it is important to understand the interactions that occur at the interfaces between the carrier material and the organic material. A team from the "Interface controlled and functionalised organic thin films" National Research Network (NRN) at the University of Leoben has made an important contribution to scientific understanding in precisely this field. Using complex calculations, the team has been able to show that a thin film of organic thiophene is held on to a copper surface solely by the van der Waals force. The team calculated that the adsorption energy involved is -0.50 eV.

The spokesperson for the NRN, Prof. Helmut Sitter from the Institute of Semiconductor and Solid State Physics at Johannes Kepler University (JKU) in Linz, explains: "The van der Waals force is a weakly interacting force between atoms that occurs as a result of asymmetric charge distribution in atoms. We now know that this exerts a highly significant influence on the kinds of extremely thin material films used to manufacture organic semiconductors. Indeed, this force can successfully bind the materials entirely on its own. However, due to its weakness, several previous methods used to calculate the interactions between different materials have attached only minor importance to this force, or have ignored it altogether." This would also seem to provide some explanation for why the generalized gradient approximation (GGA) often used in such instances has been unable to satisfactorily explain the bonding behaviour in thin layers. In fact, these newly published results could explain the discrepancies that have long been found between various experimental data and models for calculating the interaction between thin layers.

Publications, Prizes, Products
The new data adds to our fundamental understanding of the interactions that take place at interfaces. The influence of the van der Waals force also indicates that no charge is transferred between the atoms of the organic materials and their substrates in the calculated system. This finding is of key significance to the production and functionality of organic semiconductors.

Several articles in the Advanced Materials journal this year demonstrate how research carried out by members of the NRN maintains a steady focus on practical applications. As a result of one such article, the Institute of Experimental Physics at JKU won the official Innovation Prize of the Province of Upper Austria. It is no surprise that three spin-off companies - run almost exclusively by graduates from the Institutes involved in the NRN - have already been established as a direct result of the findings. One of these companies, Nanoident, was declared "Entrepreneur of the Year 2007" by Ernst & Young Austria.

Prof. Sitter believes that all of these achievements, together with an article by the NRN published in SCIENCE in the summer of this year, prove how this National Research Network has successfully combined fundamental research, applied research and technology transfer - with the support of the FWF.

Original publication: Importance of Van Der Waals Interaction for Organic Molecule-Metal Junctions: Adsorption of Thiophene on Cu(110) as a Prototype, P. Sony, P. Puschnig, D. Nabok & C. Ambrosch-Draxl. Phys. Rev. Lett. 99, 176401 (2007).

Source:http://www.fwf.ac.at

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