The Astounding Physics of N95 Masks

The ongoing COVID-19 pandemic has necessitated use of face masks for protection. Many countries now mandate individuals to wear face masks when in public. However face masks have disappeared from store fronts, necessitating homemade cloth masks.

But neither homemade cloth masks nor most commercial masks can filter the COVID-19 virion. The N95 mask presents the only available design capable of stopping the COVID-19 virion, because it contains an electrocharged layer capable of attracting and trapping aerosol droplets down to micrometer dimensions, virions, and bacteria in the 100s of nanometer dimensions.

Unfortunately, this electrocharged polymer layer is manufactured through an industrially sophisticated electrospinning process that is a bit hard (but not impossible) to duplicate using commonly available materials.

This webpage details a process we have developed in-house at OIST to manufacture this electrocharged layer using commonly available parts and knowledge of basic electrical circuits to construct facemasks.

 

N95 Filtration Principle

Face mask filtration mechanisms have to optimize between two competing requirements. The mask filters must have a pore size small enough to trap particles, but a pore size too small also prevents the individual from breathing, so mask filters cannot go below a certain average pore diameter. To meet this optimization requirement, N95 masks essentially operate on three principles (please see Figure 1 below for cartoon representation):

1) Inertial Impaction: Aerosol or dust particles typically 1 micron or larger in size with enough inertia to prevent them from flowing around the fibers in the filtration layers slam into the mask material and get filtered.

2) Diffusion: Particles smaller than 1 micron, usually 0.1 microns and smaller that are not subject to inertia undergo diffusion and get stuck to fibrous layers of the filter while undergoing brownian motion around the tortuous porous matrix of the filter fiber.

3) Electrostatic attraction: This mechanism employs electrocharged polymer or resin fibers that attract both large and small oppositely charged particles and trap them.

Figure 1: Cartoon representation of the three principles that N95 masks utilize. (a) The National Academies Press report from 2006 details three stages (Image credit: Institute of Medicine 2006. Reusability of Facemasks During an Influenza Pandemic: Facing the Flu. Washington, DC: The National Academies Press. https://doi.org/10.17226/11637.) whereas (b) the CDC webpage lists four principles, but we have subsumed interception principle listed by CDC within inertial impaction principle .

Any cloth or commercially available face mask can achieve the first and second principle with little effort. The third stage of electrostatic attraction is one of several design features that sets the N95 mask apart from other face masks. The ability to make one’s own electrocharged filtration layer is not so straightforward.

 

Electrocharged Filtration

We know from common experience, especially in cold climates with low ambient humidity, that when two fabrics rub against each other they gain static electricity — a phenomenon commonly known as triboelectric charging. Fabrics woven from natural fibers like wool or cotton, which possess higher roughness, and even synthetic fabrics like Nylon gain static charge from rubbing.

The idea of exploiting charged fabrics to aid in filtration goes back a few decades (e.g. please see a brief review by Edward R. Frederick (1974) Some Effects of Electrostatic Charges In Fabric Filtration, Journal of Air Pollution Control Association, 23. 1164. DOI: 10.1080/00022470.1974.10470030), and indeed some early face mask designs incorporating electrocharged filtration did use wool or felt fibers. Over time though, electrospun polymer fabrics have become the mainstay material for the electrocharged layers in face mask designs.

The electrospun polymer material is manufactured using a method called electrospinning — a widely used platform for generating polymer nanofibers. In this method, molten polymer of high viscosity is forced out of tiny orifices. The metallic container (we will call it the emitter) with the orifices holding the polymer melt is positively charged and a flat plate or drum placed at a distance is negatively charged (we will call this the collector).

As the molten polymer is forced out of the emitter’s orifice, the positively charged polymer melt shoots out and is attracted towards the negatively charged collector, and rapidly cools down over the distance covered between the emitter and collector, thereby resulting in polymer nanofibers that are electrically charged. This industrial electrospinning process is not generally found in everyday use.

There are many youtube videos out there that demonstrate the electrospinning process in both lab-based and industrial settings. We include a few representative links to motivate your intuition here. They were the first hits we got on youtube, otherwise there’s no specific reason why we chose these links over any other(s).

Figure 2: Cartoon illustration of the electrspun polymer nanofiber manufacturing mechanism. Molten polymer in a viscous fluid form is forced through an orifice, usually a motorized syringe pump, shown as the EMITTER.

A distance apart from the orifice is a metallic plate, we denote as the COLLECTOR. The syringe pump’s metallic needle is connected to the positive, and collector is connected the negative terminal of a high voltage source, usually in the order of 10s of kilovolts. As the viscous molten polymer is forced out of the syringe needle, the DC electric field between the emitter and collector forces the droplet to stretch, undergoing extension and cooling simultaneously, and gets deposited as polymer nanofiber (shown as a red spiral in the cartoon illustration) on the collector plate. This fast extending and cooling polymer gets spit out as shown bounded within the blue dashed line in the illustration.

Manufacturing Electrospun polymer fabric with Cotton Candy Machine

A simple way to generate electrospun polymer fabric with commonly available parts is by slightly modifying a cotton candy machine. We all know the basic principle on which a cotton candy machine operates:

1) There is a drum, at the center of which sits a fast spinning container holding 3 parts sugar and 1 part water. This container, which we call the EMITTER, is heated to about 160 – 175 degrees centigrade to caramelize the sugar into a viscous fluid.

2) The fast spinning container has small holes through which the viscous caramelized sugar flows out and the centrifugal force extends the viscous sugar droplet into a thin nanocrystalline fiber of sugar, which we collect with a stick in the form of cotton candy.

3) Now, let’s modify this slightly. The electrospinning system requires a high voltage DC electric field to accelerate the polymer droplet from collector to emitter. The cotton candy machine instead provides mechanical acceleration from the centrifugal spinning of the emitter, so it does not require such a high voltage. But it does still require a DC electric field to induce charges in the polymer nano fabric. Let’s say we setup a low voltage DC electric field (we worked in the range of 12 – 24 volts) using a lab voltage source– you can also use a car battery.

Figure 3: Cartoon illustration of a cotton candy machine comprised of a large cylindrical drum (called the COLLECTOR), at the center of which is a heated container (which we call EMITTER) in which we typically pour moist sugar (3 parts of sugar with 1 part of water + food coloring if you wish). The container spins at seveal 1000 RPM and as the viscous caramelized sugar fluid oozes out of the holes in the EMITTER, and the centrifugal force splashes the caramelized sugar fluid droplets, and in the process, extending them into thin crystalline nanofibers. Applying a low electric field (12-24 V) between the emitter and collector, and replacing sugar with molten polymer, provides a high throughput electrospinning system one can construct at one’s home with simple parts.

4) And instead of sugar, we pour powdered polypropylene polymer — you can take any of your polypropylene plastic bottles, cut them up in pieces and put them in a high power blender to get powdered polypropylene — and start spinning at several 1000 RPM. You now start generating the electrospun polymer which starts collecting as a fabric comprised of electrocharged nanofibers on the collector surface.

5) We used polypropylene for a simple reason. The standard cotton candy machine available commercially has a heating temperature between 160 – 175 degrees centigrade. Polypropylene has a melting point temperature of 160 degrees centigrade. So the cotton candy machine needs no modification for the heating element. Otherwise, one is free to use other plastic long-chain polymeric materials like Polystyrene or Polyethylene (PET bottles, for instance). Some of the parameters such as melting point and applied electric field will certainly vary with the material used, but its easily figured out by quick trial-and-error, it ain’t rocket science.

6) An easier way would be to dissolve the polyproylene in an organic solvent (e.g. Acetone), but we figured acetone is not a commonly available material.

7) Additionally, we were fortunate to have a plastic cover that we could place on our cotton candy machine. We simply drilled a hole in it and stuck a pipe through it connected to an ULVAC vacuum pump to evacuate the chamber — we also placed silly putty or bread dough to seal the lid and container during evacuation. We did this simply to keep the environment clean and free of dust particulates during the electrospinning process in our rotary jet electrospinning system — that’s the fancy name we decided to give our in-house electrospinning system rigged from common parts. We realize, most people don’t have access to a vacuum pump, at least not in residential environments, but if you’re at a university lab that’s still open (our’s is), you can use this simple hack to make sure your electrospun polymer nanofibers are manufactured in semi-clean room conditions. In any event, the vaccum pump is not necessary, but helps. Please see the figure 4 below for a simple cartoon illustration.

8) Also, if you don’t have a cotton candy machine, there are several DIY videos to rig your own setup with minimal, commonly available parts. We include a few DIY youtube video links here. Once again, we included links that were immediate hits on youtube and there was no specific reason to choose these over others. And once again,

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