November 1, 2004
Originally Published MDDI November 2004
Nanotube Fibers Emerge from a Tangled Skein
A length of fiber, no thicker than a human hair, is the result of untangling and aligning millions of single-walled nanotubes.
A combined team of chemists and chemical engineers at Rice University (Houston) and the University of Pennsylvania (Philadelphia) has found a way to weave single-walled carbon nanotubes (SWNTs) into continuous macroscopic fibers. Applications could emerge in a number of sectors, including medical devices.
A paper describing the group's research appeared in the September 3, 2004, issue of the journal Science. Nanotubes are hollow carbon cylinders that are only one atom thick. Producing fibers for practical use is difficult because nanotubes are chemically complicated. They are strongly attracted to each other and tend to clump in tangled balls. To detangle them, until now, scientists have used a detergent-and-water dispersion processed with polymer solutions. But the dispersion is less than 1% nanotubes by volume, a concentration too low for industrial use. Moreover, the soap-and-polymer residue is difficult to remove and the nanotubes cannot easily be converted back into a pure form.
The researchers tackled detangling the nanotubes in much the same way they detangled other strong fibers, like Kevlar and Zylon. Using a strong solution of sulfuric acid, the researchers were able to disperse up to 10% by weight of pure carbon nanotubes. The acid interacts with the carbon and reassembles the tubes into aligned and mobile fibers. The resulting form is pure, with no additives or detergents. Each strand of the fiber is approximately 100 µm in diameter and contains a million closely packed and aligned nanotubes.
Commercially wrought SWNT fibers could have 10 times the tensile strength of Zylon, the strongest fiber currently on the market. Zylon is used by the military and has demonstrated twice the strength of Kevlar. Richard Smalley, who leads the group of chemists at Rice University, estimates that the density his team can achieve is about 77% of what's theoretically possible. And, he says, even that will improve with elements of production such as heat treatment and spinning of the fibers.
Besides unprecedented strength, the SWNTs are also conductors of electricity and heat. In addition, they can act either as metals or semiconductors. Jade Boyd, senior science editor for Rice University, said studies are being done right now that indicate nanotube fibers may be just as conductive as copper wiring. Using nanowires rather than traditional copper wiring, he says, can theoretically result in lowering the overall weight of an electronic device. The researchers estimate that nanotube fibers are roughly six times lighter than copper.
The nanotubes condense into a solid structure known as alewives when water is added to the untangling solution.
The tubes could also be woven into high-tensile fabrics, such as bulletproof vests that are currently made with Kevlar, or creating smart cable tethers for elevators. Medical device applications, however, may be trickier. As Boyd explains, “Nanoparticles act differently than their macroscopic counterparts, in ways that cannot be predicted easily. There is some evidence that nanotubes can be attached to DNA, but we just aren't sure about toxicity concerns at this point.” However, he also says that superstrong materials that have conductive properties may be appropriate for devices such as artificial hearts, as long as the material is biocompatible.
The high strength and stiffness of the nanofibers may also be appropriate for use in probes, says Wade Adams, director of the Center for Nanoscale Science and Technology at Rice. Some nanotubes may have a capacity for fluorescents. Others may have drugs or radioactive isotopes implanted inside the tube for precise distribution in the body.
“The size and shape of SWNTs make them desirable for medical use, since they can pass through cell membranes,” says Adams.
The study was funded by the Office of Naval Research and the Department of Energy and practical use may come sooner in these areas, according to demand. “The need [in these sectors] is high,” says Adams, “so development of the SWNTs may be as quick as three to four years.”
Researchers still have to solve the problem of how to separate the conductive and semiconductive nanotubes from other types, which Adams estimates could take 10 years or more. Ultimately, he says, the development of practical applications depends on who will fund the various projects. “Anything that is macroscopic in one dimension, such as length, and nanoscopic in another can yield endless possibilities,” Adams says.
Copyright ©2004 Medical Device & Diagnostic Industry
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