Oct 30 2017
Meshes produced from fibers with nanometer-scale diameters have a broad range of potential applications, including solar cells, water filtration, tissue engineering, and even body armor. However, their commercialization has been hindered by inefficient manufacturing methods.
In the latest issue of the Nanotechnology journal, MIT scientists describe an innovative device for generating nanofiber meshes, which match the power efficiency and production rate of its best-performing predecessor — but considerably reduces variation in the fibers’ diameters, a key consideration in most applications.
But whilst the predecessor device, from the same MIT team, was etched into silicon via a complex process that needed an airlocked “clean room,” the new device was created using a $3,500 commercial 3D printer. Thus, the work points towards the manufacturing of nanofibers that is both more reliable and much cheaper.
The new device is made up of an array of small nozzles through which a fluid that contains particles of a polymer are pumped. Due to to this, it is called a microfluidic device.
My personal opinion is that in the next few years, nobody is going to be doing microfluidics in the clean room, 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.
Luis Fernando Velásquez-García, principal research scientist, MIT’s Microsystems Technology Laboratories and senior author on the paper.
Velásquez-García is joined on the research paper by two postdocs in his team, Daniel Olvera-Trejo and Erika García-López. Both obtained their PhDs from Tecnológico de Monterrey in Mexico and colloaborated with Velásquez-García through the nanotech research partnership between Tecnológico de Monterrey and MIT.
Nanofibers are effective for any application that benefits from an extreme ratio of surface area to volume — such as fuel cell electrodes, which catalyze reactions at their surfaces, or solar cells, which try to increase exposure to sunlight. They can also yield materials that are penetrable only at very small scales, such as water filters, or that are extremely robust for their weight, such as body armor.
These types of applications depend on fibers with standard 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.”
Since the earlier device from the team was etched in silicon, it was “externally fed,” which means that an electric field drew a polymer solution up the sides of the individual emitters. The fluid flow was controlled by rectangular columns that were etched into the sides of the emitters, but it was still inconsistent enough to produce fibers of irregular diameter.
However, in the new device, emitters are “internally fed”, meaning that they have holes bored through them. The hydraulic pressure pushes fluid into the bores until they are completely filled and once it is done, an electric field draws the fluid out into tiny fibers.
The channels that feed the bores are wrapped into coils beneath the emitters, and they slowly taper along their length. That taper is important for controlling the nanofibers’ diameter, and it would be virtually impossible to attain with clean-room microfabrication techniques. “Microfabrication is really meant to make straight cuts,” Velásquez-García said.
The nozzles are arranged into two rows in the new device. The rows are slightly offset from each other, because the device was designed to show aligned nanofibers — nanofibers that maintain their relative position as they are gathered by a rotating drum. In particular, these aligned nanofibers are useful in some applications, such as tissue scaffolding. The nozzles could be arranged in a grid in order to increase output rate for applications where unaligned fibers are adequate.
Velásquez-García said that in addition to design flexibility and cost, another benefit of 3D printing is the ability to quickly test and revise designs. With his team’s microfabricated devices, he says, it will typically take two years to move from theoretical modeling to a published paper, and meanwhile, he and his colleagues could be able to test two or three variations on their basic design. He also added that the process took closer to a year with the new device, and they were able to analyze 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, 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.
Mark Allen, the Alfred Fitler Moore Professor at the University of Pennsylvania