Engineers at Rutgers have recently created a way to spray extremely thin wires made of a plant-based material that could be used in N95 mask filters, devices that harvest energy for electricity and potentially the creation of human organs.
Jonathan Singer, assistant professor in the Department of Mechanical and Aerospace Engineering, said his team altered a standard nanofabrication process to form nanowires of a certain material.
“We practice a technique called electrospray deposition, which is a method of taking fluids, charging them and then using the repulsion that the fluids have to break the fluids up into very tiny droplets,” he said. “Once you do that, if you have an electric field between your fluid and your target, it draws the droplets to that target. That’s a technique that’s been practiced for a very long time."
Singer said many previous electrospray results, including their own, have been spraying these tiny particulars, but that their lab was now trying to spray nanowires instead.
He said the team wanted to find a way to take 3D surfaces and coat them with nanowires of materials that could then be used for grabbing particles. They focused on figuring out why these nanowires do not form in electrospray and experimented with different materials to see what could cause them to form.
“The electrospray experiments essentially involve taking a target, and the target we usually use is a silicon wafer, which is very flat and easy to observe, and then we have our needle, and we put our high voltage on the needle and pass different solutions through it,” he said.
The study found one cellulose-derived family of materials whose unique properties allowed them to form nanowires when they were sprayed, Singer said. The cellulose materials acted like inverse gelatin.
“The thing that we were interested in was having a material that would be liquid at room temperature and then we could heat the target, and when it reached the target, it would separate from the water, and be solid. We thought that this would be useful because it would allow us to use a lot less material and to use water,” Singer said.
He said the team started looking at their results and found that the experiment had yielded nanowires. The next step was to figure out why this was happening, he said.
“We partnered with Professor Xin Yong from Binghamton (University), who was an expert in theoretical models of fluids and polymers,” Singer said. “Working together with him, we essentially made these computer models and figured out why it is that a droplet would form a particle versus forming a wire, and what had to happen inside that droplet to give it the properties to form a wire.”
He said they found that these tiny droplets evaporate from the outside in, which in turn creates a particle. Singer said that droplets that are more viscous evolve homogeneously and can result in elongation into nanowires. He said the team also experimented with the properties that controlled the size of the nanowires.
He said they took very short sprays and used an electron microscope to see what the nanowires looked like.
“Students measured wires one at a time, eventually we figured out how to get software to do the wire measurement and sort of compare large numbers of wires and their properties and find the best spray conditions to make the best wires,” Singer said.
They found that in order to make the best wires, they could add some liquid material that would make the wires softer and allow them to stretch further, he said. The lab is also currently testing the durability of these nanowires.
“One of the things that we’re really interested in right now is that because the methyl cellulose is dissolvable in water, it can be washed away very easily, so we’re trying to figure out ways to make it more durable chemically to water and ambient humidity,” he said.
Singer said that through testing the mechanical properties of the wires, they found that they are somewhat brittle.
“We’ve been looking at adding different materials to make them less brittle, we’ve been looking at coating them with different materials to make them water resistant or cross linking them — making chemical bonds that will prevent them from being dissolved,” he said.
The goal was to prevent the nanowires from being dissolvable in water, so then they would be able to operate in air, Singer said.
“So if we really wanted to make something like the cilia in the lung, or put it onto a filter or an N95 mask, it can't be dissolved by water droplets” he said. “Our big focus right now ... is how can we make it (nanowires) water resistant.”
Singer said the team hoped to add the nanowires to the surfaces of N95 masks and cloth masks to increase the probability of trapping particles. In terms of the creation of human organs, the nanowires could be used to add texture to bioprinted materials.
He also said these nanowires could be used to harvest energy by means of triboelectric surfaces, which have a lot of hairs and are able to store a lot of charge.
“We have all of these examples in nature of where these nanowires could be useful, and it’s all about tuning the properties so they can fit into those application areas,” he said.
Whether those properties include water resistance, mechanical compliance for adhesion or better connection with the nanowire interface, Singer said they are working on adding functionality to the nanowires.