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April 13, 2006

Larger nanoscale: Improved electrospinning

Techniques improved could be useful on smaller scales.

Daoheng Sun and Liwei Lin's at the university of Berkeley have improved electrospinning. They enable fibers ranging from 50 to 500 nanometers in diameter to be deposited onto a collector plate in a directed, controlled manner. In reference to the shortened distance between the ejector and collection points that it used, the team named the new process "near-field electrospinning."

First, instead of applying the polymer solution into the electric field with a syringe, they used a fine-tipped tungsten electrode, which they dipped into the solution like a pen into ink. Then, positioning the electrode above a collection plate, they applied electrical voltage to it, creating the electric field and initiating the process of electrospinning with the tiny drop of polymer on the electrode's tip. This allowed the team to reduce the initial diameter of the polymer stream as it leaves the electrode far below the diameter of the stream produced by the conventional syringe.

Second, the researchers shortened the distance the polymer travels in the electric field from the conventional 10 to 30 centimeters to between one-half millimeter and three millimeters. This allowed them to take advantage of the brief period of stability that polymer fibers exhibit when the electrospinning process begins. Just like the exhaust of a jet engine that shoots out in a straight line before billowing into random patterns, the fibers move in a relatively straight line for a fleeting moment when they enter the electric field. In Sun and Lin's near-field technique, the fibers are captured before their billowing begins.

The shortened distance also meant that Lin and Sun could dramatically reduce the voltage required from 30,000 volts to as low as 600 volts. Finally, rather than using a screen fixed in place to capture the fibers, Sun and Lin let the fibers land on a plate that could be moved in various patterns at various speeds. This allowed the researchers to pattern the fibers onto the plate the way a quilter creates a design by maneuvering fabric under her sewing machine's needle.

Lin said he foresees the possibility of two immediate directions for the new process. One is for device applications that require precise deposition of the nanofibers, such as making nanosensors for biological measurements – a glucose monitor, for instance. The other will be to make non-woven fabrics with organized patterns that can have many applications, such as scaffolds for living cells. Near-field electrospinning may also be useful in nanolithography for making next-generation microchips, Lin predicted. But, he said, this will require more effort to develop.

Lin is currently working on two improvements to the near-field process: an electrode that can provide a continuous supply of polymer and a movable stage with good planar control to capture the fibers.

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