A technique used to produce a nanofibre prototype has increased the rate of production fourfold while cutting energy consumption by more than 90 per cent, according to researchers.
Nanofibres - polymer filaments only a couple of nanometres in diameter - have a huge range of potential applications, from solar cells to water filtration fuel cells. So far, their production cost has consigned them to a few niche industries. MIT researchers have devised a prototype that, they say, unlocks the nanofibre’s potential.
“We have demonstrated a systematic way to produce nanofibres through electro-spinning that surpasses the state of the art,” said Luis Fernando Velásquez-García, a principal research scientist in MIT's Microsystems Technology Laboratories, who led the new work.
“But the way that it's done opens a very interesting possibility. Our group and many other groups are working to push 3D printing further, to make it possible to print components that transduce, that actuate, that exchange energy between different domains, like solar to electrical or mechanical.
“We have something that naturally fits into that picture. We have an array of emitters that can be thought of as a dot-matrix printer, where you would be able to individually control each emitter to print deposits of nanofibres,” he said.
Nanofibres are useful for any application that benefits from a high ratio of surface area to volume – solar cells, for instance, which try to maximise exposure to sunlight, or fuel cell electrodes, which catalyse reactions at their surfaces. Nanofibres can also yield materials that are permeable only at very small scales, like water filters, or that are remarkably tough for their weight, like body armour.
Electro-spinning is the standard technique in manufacturing nanofibre and it comes in two varieties. In the first, a polymer solution is pumped through a small nozzle and then a strong electric field stretches it out. The process is slow, however, and the number of nozzles per unit area is limited by the size of the pump hydraulics.
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 100,000V.
Velásquez-García and his team used the second approach, but on a much smaller scale, using techniques from micro-electrical mechanical systems to produce dense arrays of tiny emitters. The emitters’ small size reduced the voltage necessary to drive them and allowed them to be packed together, increasing the production rate.
At the same time, a coarse texture carved into the emitters' sides regulated the rate at which fluid flowed 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,” Velásquez-García said.
The researchers were able to pack 225 emitters, several millimetres long, on a square chip about 35 millimetres on a side. At the relatively low voltage of 8,000V, that device yielded four times as much fibre per unit area as the best commercial electro-spinning devices.