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Posted: July 9, 2007
Electrospinning technique lets nanofibers realize their potential
(Nanowerk News) Nano-scale polymer fibers—the thinner, the better—can potentially enable the manufacture of more effective chemical sensors, biochips, protective clothing and other innovations. But the standard technique for producing these fibers, electrospinning, produces a chaotic tangle rather than controllable patterns. A variant of the technique, near-field electrospinning, offers full control over the path of deposited nanofibers, allowing them to realize their engineering potential. "It could open up the field, taking it in completely different directions," says the technique's pioneer, Berkeley mechanical engineering professor Liwei Lin.
Polymer fibers such as polyester are traditionally made by ejecting a polymer-solvent solution from a small hole (the spinneret) and letting it solidify into a thread as the solvent evaporates. The process of electrospinning, first patented in 1934, makes these threads even thinner by using a polymer solution that's charged, then squirting it across a high-voltage gap. The liquid flows from a needle onto a grounded collection plate 10–50 cm below. Applying 10,000–30,000 volts across the needle and the collector accelerates and stretches the stream. But, as the stream approaches the plate, it whips around unpredictably, depositing the delicate fiber into a fine mess.
Lin and his student Chieh Chang revolutionized the technique by bringing the spinneret much closer to the collection plate, to between 500 microns and one millimeter above its surface, to steer the deposited fiber into a controllable shape. Their first success came by using a more highly concentrated polymer solution and, instead of squirting it from a syringe, using a 25-nanometer sharp tungsten point, which they dipped into the solution like an old-fashioned pen.
"The liquid leaves the tungsten tip and forms a stream that follows a fluid dynamics shape known as a Taylor cone," Lin explains, "That close, it's harder to get a thin fiber, so we use a lower voltage, usually around 600 volts."
Last year, Lin and Chang demonstrated the technique by writing a microscopic, cursive "Cal" in 100-nanometer thin polymer fiber. "This was very hard to do, because we did it manually," Lin recalls. "You're moving the stage, not the pen, so you have to write upside down and backwards. And the solution continues flowing whether the collector is moving or not, so if you slow down or stop, it bunches or beads up rather than making a steady line."
One limitation of this technique is that it can produce fibers no longer than several centimeters in length. Just like a fountain pen that runs out of ink, the fiber ends when the last of the liquid polymer has been drawn off of the tip. But for many applications, short fibers are all that's needed.
Lin and Chang have since tried near-field electrospinning with the more traditional hollow needle. Using a needle with an inside diameter of 70 microns, they can produce fibers ranging from between 500 nanometers and 3 microns, thicker than the ones produced using the tungsten tip, but with unlimited length.
For improved control, they now also put the collector onto a precision X-Y stage, a piece of lab and chip-fab equipment that makes small, controlled movements along two perpendicular axes. Lin's group is also testing ways of dynamically varying the voltage to even out the fiber's thickness as the speed of the collector changes.
Polymer nanofibers have several known applications and others yet to be discovered. With polymers formulated to change their electrical resistance in the presence of certain chemicals, nanofibers can be made to act as ultra-sensitive chemical sensors. String several strands together in a handheld device, and you've got a portable sniffer that detects a range of airborne compounds—explosives, for example.
If you make nanofibers out of polyethylene oxide, coat them with glass, then etch away the original fiber, you create nano-pipe fluidic connectors. These can move fluids around to different detection areas in "lab-on-a-chip" biochips for medical diagnosis.
Traditional, disordered electrospun fibers have already been used as "bio-scaffolding" for cell tissue cultures, and nanofiber scaffolding that's precisely arranged could potentially direct cell growth to produce blood vessels, bone, cartilage and other tissues following predetermined designs.
Other possible applications include filtration media, drug delivery, solar cells and even cosmetics.
"There are a lot of people working on electrospinning now; it's recently become a hot topic," observes Lin. "Part of this comes from nanotech's popularity, but it's also because electrospinning is very easy to do. For our work, all we need is a power supply and a needle."