Friday, February 15, 2008

Power from Fabrics

Nanowires that convert motion into current could lead to textiles that can generate power.

Power suit: Gold-plated zinc oxide nanowires (yellow), each about 3.5 micrometers tall, are grown on a flexible polymer fiber. The gold-plated nanowires brush against untreated nanowires (green), which flex and generate current. Yarn spun from the fibers could lead to fabrics that convert body movements into electric current.
Credit: Z. L. Wang and X. D. Wang, Georgia Tech

Georgia Tech researchers have taken an important step toward creating fabrics that could generate power from the wearer's walking, breathing, and heartbeats. The researchers, led by materials-science professor Zhong Lin Wang, have made a flexible fiber coated with zinc oxide nanowires that can convert mechanical energy into electricity. The fibers, the researchers say, should be able to harvest any kind of vibration or motion for electric current.

The zinc oxide nanowires grow vertically from the surface of the polymer fiber. When one fiber brushes against another, the nanowires flex and generate electric current. The researchers described a proof-of-concept yarn in a paper published this week in the journal Nature. They show that the output current increases by entwining multiple fibers to make the yarn.

By the researchers' calculations, a square meter of fabric made from the fibers could put out as much as 80 milliwatts--enough to power portable electronics. The development could make shirts and shoes that power iPods and medical implants, curtains that generate power when they flap in the wind, and tents that power portable electronics devices.

In 2007, Wang and his colleague the 2007 TR 35 winner Xudong Wang (no relation) built a zinc oxide nanowire array that generated direct current when exposed to ultrasonic vibrations. The piezoelectric nanowires stood on an electrically conducting substrate that acted as an electrode. The other electrode was a platinum-coated silicon plate with parallel peaks and trenches carved on its surface. (See "Nanogenerator Fueled by Vibrations.") When the ultrasonic waves pushed the electrodes together, the nanowires bent and produced current.

In the new work, the researchers have substituted the rigid, zigzag electrode with a flexible one. They convert some of the bendable fibers into electrodes by applying a thin layer of gold to them. These gold-plated fibers act as flexible electrodes.

The researchers entangle a gold-coated fiber with an uncoated fiber. When the fibers are pulled back and forth with respect to each other, the individual gold-plated nanowires push and bend the uncoated nanowires, generating current.

The flexibility of the fibers brings the idea of wearable, foldable energy sources closer to fruition, says Charles Lieber, a chemistry professor at Harvard University. The flexibility is also crucial for harvesting energy from extremely small ambient motion, says Thomas Thundat, who studies nanoscale biological sensors at Oak Ridge National Laboratory. Entwining the flexible fibers, he explains, leads to very close contact between the gold-coated and the uncoated nanowires. As a result, small motions, such as a light wind or walking movements, make the coated and uncoated nanowires brush against each other and generate current.

"The idea is ingenious," says Min-Feng Yu, a mechanical-science and engineering professor at the University of Illinois at Urbana-Champaign. "It's like you have millions of nanogenerators outputting electricity simultaneously, each at maximum performance.".

The generator's ability to capture small movements makes it especially useful for powering biological sensors, Thundat says. Microscale sensors can be implanted in the body to measure such things as cancer biomarkers and glucose. But chemical batteries are bulky compared with the tiny sensors, and they have a limited lifetime. "Implanted sensors based on [the fiber nanogenerator] concept could use blood pressure or muscle movement for operation," Thundat says.

The Georgia Tech advance would not be possible without the simple but highly innovative process the researchers have used to make the fibers, Lieber points out. Zhong Lin Wang and his colleagues first cover a polymer fiber with a 100-nanometer-thick zinc oxide layer. They immerse the fiber in a reactant solution at 80 °C, which results in nanowires growing vertically from the surface. Then the researchers use a final trick to keep the nanowires firmly attached to the fibers while keeping the fibers flexible. They dip the fibers in tetraethoxysilane, a liquid used in weatherproofing and protective coatings. The tetraethoxysilane forms two coatings: one between the fiber and the zinc oxide layer, and another on top of the zinc oxide layer.

This tetraethoxysilane coating makes the fiber robust. The zinc oxide layer did not crack or peel off even when the fiber was twisted. The nanowires also stayed put after the researchers continuously brushed two fibers against each other for 30 minutes. The fibers will have to last even longer and have higher output power in order to be used practically, Wang says.

Power-generating shirts might still be out of reach for most. At this point, the fabric might be affordable for the military for use in tents and shoes, says Wang, but "it is probably too expensive for you and me to buy."

Plucking Cells out of the Bloodstream

A new implantable device can extract stem cells for therapeutic transplant or program cancer cells to die.

Cell catcher: University of Rochester bioengineer Michael King holds up a section of plastic microtubing lined with proteins that trap cancer and stem cells.
Credit: Richard Baker, University of Rochester
Watch Michael King's new device in action.

Bioengineers have developed an implantable device that captures very pure samples of stem cells circulating in the blood. The device, a length of plastic tubing coated with proteins, could lead to better bone-marrow transplants and stem-cell therapies, and it also shows promise as a way to capture and reprogram cancer cells roaming the bloodstream. The company CellTraffix is commercializing the technology.

When patients get bone-marrow transplants, what they're really receiving are infusions of a type of adult stem cell. Bone-marrow-derived stem cells play a crucial role in renewing the blood throughout adulthood, creating new cells to carry oxygen and fight infections. These adult stem cells can be sampled using the new device.

The new device mimics a small blood vessel: it's a plastic tube a few hundred micrometers in diameter that's coated with proteins called selectins. The purpose of selectins in the body seems to be to slow down a few types of cells so that they can receive other chemical signals. A white blood cell, for instance, might be instructed to leave the circulation and enter a wound, where it would protect against infection. "Selectins cause [some] cells to stick and slow down," says Michael King, a chemical engineer at the University of Rochester who's developing the cell-capture devices. Different types of selectins associate with different kinds of cells, including platelets, bone-marrow-derived stem cells, and immune cells such as white cells.

In an upcoming publication in the British Journal of Hematology, King reports that selectin-coated microtubes implanted in rats can capture very pure samples of active stem cells from circulating blood. He gave a similar demonstration of stem-cell purification with samples taken from human bone marrow last year. Cancer patients often require bone-marrow transplants following harsh chemotherapy and radiation treatments that kill adult stem cells in the blood.

The purity of these transplants can be a matter of life or death. When the transplant is derived from the patient's own bone marrow--extracted before treatment--it's critical that it not contain any cancer cells. When it comes from another person, there's a chance that the donor's immune cells will attack the recipient if they're not filtered out. But current purification methods are slow and inefficient, King says. Those that rely on antibody recognition or cell size and shape typically extract only a small fraction of the stem cells in a blood sample; the rest go to waste.

Twenty-eight percent of the cells captured by King's implants were stem cells. "This is astounding given how rare they are in the bloodstream," says King. Implants would probably not be able to capture enough stem cells for transplant. But King believes that filtering a donor's blood through a long stretch of selectin-coated tubing outside the body, in a process similar to dialysis, would be very efficient. "This technique will clearly be useful outside the body" as a means of purifying bone-marrow-derived stem cells, says Daniel Hammer, chair of bioengineering at the University of Pennsylvania.

Hammer believes that King's devices will also have broader applications as implants that serve to mobilize a person's own stem cells to regenerate damaged tissues. By slowing down cells with selectins and then exposing them to other kinds of signals, says Hammer, King's devices "could capture stem cells, concentrate them, and differentiate them, without ever having to take the cells out of the body." There might be a way to use selectins to extract neural stem cells, too.

"This is a very broad-reaching discovery," says Hammer. Indeed, King says that he has already had some success using selectin coatings to reprogram cancer cells.

Cancer cells appear to highjack selectin pathways in order to spread to other parts of the body, the process known as metastasis. Tumors shed cells into the bloodstream. Some of those cells seem to exit with the help of selectins; ensconced in new tissue, they then establish new tumors. These secondary tumors cause more cancer deaths than initial tumors do.

King says he has unpublished work demonstrating that leukemia cells that roll along a coating of selectins and a cancer-specific signaling molecule will go through a process called programmed cell death. Healthy stem cells also roll across the device because they're attracted to the selectins, but the death signal doesn't affect them. Leukemia is a blood cancer, but King expects that the anticancer coating would work for solid tumors as well. Devices lined with these coatings might be implanted into cancer patients to prevent or slow metastasis.

King hopes to test antimetastasis implants in animals this year. He's collaborating with Jeffrey Karp, a bioengineer at the Harvard-MIT Division of Health Sciences and Technology, and Robert Langer, an MIT Institute Professor, to develop selectin coatings that are stable over months rather than days.

CellTraffix CEO Tom Fitzgerald says that the company's first product, a kit that will enable researchers to capture large numbers of stem and cancer cells in the lab, will likely reach the market early next year. The company hopes to begin clinical testing of the anticancer coatings by early 2010.

Wiring Up DNA

Measuring the conductivity of DNA could provide a way to detect mutations.

Hot-wired: By placing a double-stranded DNA segment in a gap in a single-walled carbon nanotube, researchers have measured the electrical properties of the biological molecule. Since even a single mismatch in the DNA letters affects the conductivity of the segment, the system could eventually be the basis of chemical sensors to detect mutations in DNA.
Credit: Colin Nuckolls

By wiring up DNA between two carbon nanotubes, researchers have measured the molecule's ability to conduct electricity. Introducing just a single letter change can drastically alter the DNA's resistance, the researchers found, a phenomenon that they plan to exploit with a device that can rapidly screen DNA for disease-linked mutations.

Measuring the electrical properties of DNA has proved tricky because the molecule and its attachments to electrodes tend to be very fragile. But in the new study, Colin Nuckolls, a professor of chemistry at Columbia University, in New York, teamed up with Jacqueline Barton, a professor of chemistry at Caltech, in Pasadena, CA, who's an expert in DNA charge transport. Nuckolls's group had previously developed a method for securely hooking up biological molecules to single-walled carbon nanotubes, which act as the electrodes in a miniscule circuit.

The researchers used an etching process to slice a gap in a carbon nanotube; they created a carboxylic acid group on the nanotube at each end of the gap. They then reacted these groups with DNA strands whose ends had been tagged with amine groups, creating tough chemical amide links that bond together the nanotubes and DNA. The amide linkages are robust enough to withstand enormous electrical fields.

The team estimated that DNA strands of around 15 base pairs (around 6 nanometers) in length had a resistance roughly equivalent to that of a similar-sized piece of graphite. This is a finding that the researchers might have expected since the chemical base pairs that constitute DNA create a stack of aromatic rings similar to those in graphite.

"In my opinion, the results of this work will survive, in contrast to many other publications on this topic," says chemist Bernd Giese, of the University of Basel, Switzerland. Previous estimates of DNA's conductivity have varied dramatically, Giese says, partly because it was unclear if the delicate DNA or its connection to electrodes had become damaged by the high voltages used. "One thinks one has burned the DNA to charcoal," Giese says. "It's extremely complicated experimentally."

Barton and Nuckolls performed two tricks with their wired-up DNA. For their first, they introduced a restriction enzyme that bound and cut the DNA at a specific sequence. When severed, the current running through the DNA vanished. "It's a way of biochemically blowing a fuse," Nuckolls says. It also demonstratesthat the DNA keeps its native structure in the circuit; if it had not, the enzyme would not recognize and cut the molecule.

For their second trick, the researchers introduced a single base-pair mismatch into the DNA so that, for example, a C was paired up with an A (rather than its normal partner, G). This tweak boosted the molecule's resistance some 300-fold, probably because it distorts the double helical structure. They could do this easily by connecting only one of DNA's two strands into the circuit. The second strand - which can either be a perfect match to the first or contain a mismatch - can lift on or off.

Showing the electrical effect of such sequence mismatch and enzyme cutting is the real strength of the experiments, says Danny Porath, of Hebrew University, in Jerusalem, Israel, who has also measured current through DNA. "They play with the parameters and show that conductivity of DNA clearly depends on them, and that's beautiful," he says.

Nuckolls is now working to exploit this discovery to detect single nucleotide polymorphisms (SNPs), the one-letter variations in DNA that are linked to, for example, susceptibility to Alzheimer's, diabetes, and many other major diseases. Nuckolls hopes that his method can be used to identify SNPs more rapidly and with greater sensitivity than existing methods. In such a device, a reference strand of DNA is wired into the circuit and other strands allowed to pair up with it. If the second strand carries a different base at the position of the SNP, this would be enough to trigger a change in the current through a nanoscale circuit, just as the base-pair mismatch did. Nuckolls says that he is already working with electrical engineers to create a sensor that can slot into existing semiconductor chips, making it cheap and readily available. "It's one of our big focuses, and we're pretty close," he says.

The team is likely to have competition. Late last year, a group led by Wonbong Choi, of Florida International University, in Miami, reported that it had strung 80 base pairs of DNA between two carbon nanotubes and sent current through the DNA. Choi says that he is working to create a sensor that can rapidly reveal the presence of specific genetic sequences--such as the avian influenza virus--by looking at changes in current through the tiny circuit.

Barton, meanwhile, is intent on finding out whether the conductivity of DNA serves any biological purpose in the cell. She has evidence that proteins bound to DNA may detect DNA damage by changes in its electrical properties, perhaps triggering repair of the damage. "We think it's something nature takes advantage of," she says. "It's a radical idea, but I think as we get more and more evidence, the case will be built."

A Better Way to Capture Carbon

New materials provide a potentially cheaper way to reduce carbon dioxide emissions from power plants.

Carbon-capturing crystals: This is an optical micrograph of a new material that can pull carbon dioxide from a stream of gases, making it possible to sequester the greenhouse gas.
Credit: Omar Yaghi

Researchers have developed porous materials that can soak up 80 times their volume of carbon dioxide, offering the tantalizing possibility that the greenhouse gas could be cheaply scrubbed from power-plant smokestacks. After the carbon dioxide has been absorbed by the new materials, it could be released through pressure changes, compressed, and, finally, pumped underground for long-term storage.

Such carbon dioxide capture and sequestration could be essential to reducing greenhouse-gas emissions, especially in countries such as the United States that depend heavily on coal for electricity. The first stage, capturing the carbon, is particularly important, since it can account for 75 percent of the total costs, according to the Department of Energy.

The new materials, described this week in Science, were created by researchers at UCLA led by Omar Yaghi, a chemist known for producing materials with intricate microscopic structures. They absorb large amounts of carbon dioxide but do not absorb other gases.

Techniques already exist for capturing carbon dioxide from smokestacks, but they use large amounts of energy--15 to 20 percent of the total electricity output of a power plant, according to one estimate, Yaghi says. That is because existing materials, known as amines, need to be heated to release the carbon dioxide they've absorbed. Indeed, capturing and compressing carbon dioxide through these existing methods can add 80 to 90 percent to the cost of producing electricity from coal, says Thomas Feeley, a project manager at the National Energy Technology Laboratory.

Feeley says that Yaghi's materials "compare favorably" with other experimental materials that absorb carbon dioxide that are being developed to help bring down these costs. Yaghi says that his materials could lower costs considerably since they use less energy, although exactly how much will require testing the materials at power plants.

Beyond being potentially useful in smokestacks, the materials could be employed in coal gasification plants. In these plants, coal is first processed to produce a mixture of carbon dioxide and hydrogen gas. The hydrogen is then used to generate electricity. The carbon dioxide could be captured using a solvent that increases energy consumption. But as in the smokestack-based process, the new UCLA materials could require less energy.

The materials belong to a class called zeolitic imidazolate frameworks (ZIFs). They're made of metal atoms bridged by one of a number of ring-shaped organic molecules called imidazolates. Prior to Yaghi's research, 24 types of ZIFs had been developed over the course of 12 years. Yaghi made 25 new versions in just three months. These materials can be extremely versatile, since the metal atoms can act as powerful catalysts, and the organic molecules can serve as anchors for a number of functional molecules.

ZIF proliferation: New automated techniques allow researchers to quickly synthesize dozens of new materials called zeolitic imidazolate frameworks (ZIFs). Credit: Omar Yaghi

The new materials absorb carbon dioxide in part because they're extremely porous, which gives them a high surface area that can come into contact with carbon dioxide molecules. The most porous of the materials that Yaghi reports in Science contain nearly 2,000 square meters of surface area packed into one gram of material. One liter of one of Yaghi's materials can store all of the molecules of carbon dioxide that, at zero °C and at ambient pressure, would take up a volume of 82.6 liters.

While the exact mechanisms are not fully understood, Yaghi thinks that the slightly negative charge of organic molecules in his material attracts carbon dioxide molecules, which have a slightly positive charge. As a result, carbon dioxide is held in place, while other gases move through the material. This method of trapping carbon dioxide is better than some other methods because it does not involve strong covalent bonds, so it doesn't take much energy to release the gas.

The next step for the materials is commercialization. This means scaling up production and incorporating the materials into a system at a power plant, such as by packing the materials into canisters that can be filled with pressurized exhaust gases--something that the UCLA group says could be possible in two to three years. Yaghi estimates that the materials could easily be made in large quantities, since they are similar to other materials he has developed that can now be made by the ton by BASF, the giant chemical company. "Now it's in the hands of industry," Yaghi says. And he has developed automated techniques that could lead to more materials that could have even better properties.