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Nanofibre prod'n made easy: Realising cost, power savings

09 Jun 2015

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The other approach is to apply a voltage between a rotating drum covered by metal cones and a collector electrode. The cones are dipped in a polymer solution, and the electric field causes the solution to travel to the top of the cones, where it's emitted toward the electrode as a fibre. That approach is erratic, however, and produces fibres of uneven lengths; it also requires voltages as high as 100kV.

Thinking small

Velasquez-Garcia and his co-authors: Philip Ponce de Leon, a former master's student in mechanical engineering; Frances Hill, a former postdoc in Velasquez-Garcia's group who's now at KLA-Tencor; and Eric Heubel, a current postdoc, adapt the second approach, but on a much smaller scale, using techniques common in the manufacture of microelectromechanical systems to produce dense arrays of tiny emitters. The emitters' small size reduces the voltage necessary to drive them and allows more of them to be packed together, increasing production rate.

At the same time, a nubbly texture etched into the emitters' sides regulates the rate at which fluid flows toward their tips, yielding uniform fibres even at high manufacturing rates. "We did all kinds of experiments, and all of them show that the emission is uniform," Velasquez-Garcia said.

Scanning electron micrograph of the new microfiber emitters

A scanning electron micrograph of the new microfiber emitters, showing the arrays of rectangular columns etched into their sides. Courtesy of the researchers

To build their emitters, Velasquez-Garcia and his colleagues use a technique called deep reactive-ion etching. On either face of a silicon wafer, they etch dense arrays of tiny rectangular columns, tens of micrometres across, which will regulate the flow of fluid up the sides of the emitters. Then they cut sawtooth patterns out of the wafer. The sawteeth are mounted vertically, and their bases are immersed in a solution of deionised water, ethanol and a dissolved polymer.

When an electrode is mounted opposite the sawteeth and a voltage applied between them, the water-ethanol mixture streams upward, dragging chains of polymer with it. The water and ethanol quickly dissolve, leaving a tangle of polymer filaments opposite each emitter, on the electrode.

The researchers were able to pack 225 emitters, several millimetres long, on a square chip about 35mm on a side. At the relatively low voltage of 8kV, that device yielded four times as much fibre per unit area as the best commercial electrospinning devices.

The work is "an elegant and creative way of demonstrating the strong capability of traditional MEMS fabrication processes toward parallel nanomanufacturing," said Reza Ghodssi, a professor of electrical engineering at the University of Maryland. Relative to other approaches, he added, there is "an increased potential to scale it up while maintaining the integrity and accuracy by which the processing method is applied."

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