MIT researchers have been working on devices that can produce nanofiber meshes for a while, and they have developed a new one that performs as well as its best-performing predecessor in terms of production rate and power efficiency – with a few key differences. The new device significantly reduces variation in the diameters of the fibers, and it was 3D printed, whereas the previous one was etched into silicon in a complicated process that required a clean room. The latest device required no such precautions.
The new device is a microfluidic one, made up of an array of small nozzles through which a fluid containing particles of polymer is pumped. It was manufactured using a $3,500 commercial 3D printer in a method that’s both cheaper and more reliable than previous methods of manufacture.
“My personal opinion is that in the next few years, nobody is going to be doing microfluidics in the clean room,” said Luis Fernando Velásquez-García, a principal research scientist in MIT’s Microsystems Technology Laboratoriesr. “There’s no reason to do so. 3-D printing is a technology that can do it so much better — with better choice of materials, with the possibility to really make the structure that you would like to make. When you go to the clean room, many times you sacrifice the geometry you want to make. And the second problem is that it is incredibly expensive.”
Nanofiber materials are frequently used for their high ratio of surface area to volume. They’re useful in solar cells, for example, because solar cells need to maximize their exposure to sunlight, while fuel cell electrodes need to catalyze reactions at their surfaces. Nanofibers can also be used to create materials that are permeable only at very small scales, like water filters, or that are tough yet lightweight, such as body armor. Those types of applications require nanofibers with regular diameters.
“The performance of the fibers strongly depends on their diameter,” Velásquez-García said. “If you have a significant spread, what that really means is that only a few percent are really working. Example: You have a filter, and the filter has pores between 50 nanometers and 1 micron. That’s really a 1-micron filter.”
The previous device, the one etched in silicon, was externally fed, meaning that an electric field drew a polymer solution up the sides of the individual emitters. Rectangular columns etched into the sides of the emitters regulated the fluid flow, but not enough to prevent fibers of irregular diameter from being generated. The emitters of the new 3D printed device, however, are internally fed. Holes are bored through them, and hydraulic pressure pushes fluid into the holes until they’re filled. An electric field then draws the fluid out into tiny fibers.
Under the emitters, the channels that feed them are wrapped into coils and gradually taper along their length, which regulates the diameter of the nanofibers. That taper would be almost impossible to create using traditional techniques, which is why 3D printing has proved so valuable. The nozzles of the device are arranged into two rows, which are slightly offset from each other. The device was designed to demonstrate aligned nanofibers, which preserve their relative position as they’re collected by a rotating drum. Aligned nanofibers are useful for applications such as tissue scaffolding.
To create a device such as this, the process typically takes about two years to go from theoretical concept to published paper. With 3D printing, the process took closer to a year because the team was able to produce and test designs so quickly. Over the course of the project, the researchers were able to test 70 iterations of the design.
“A way to deterministically engineer the position and size of electrospun fibers allows you to start to think about being able to control mechanical properties of materials that are made from these fibers. It allows you to think about preferential cell growth along particular directions in the fibers — lots of good potential opportunities there,” said Mark Allen, the Alfred Fitler Moore Professor at the University of Pennsylvania, with joint appointments in electrical and systems engineering and mechanical engineering and applied mechanics. “I anticipate that somebody’s going to take this technology and use it in very creative ways. If you have the need for this type of deterministically engineered fiber network, I think it’s a very elegant way to achieve that goal.”
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[Source: MIT / Images: Luis Fernando Velásquez-García]