# Electrocharged facepiece respirator fabrics using common materials

M. M. Bandi  Nonlinear and Non-equilibrium Physics Unit, OIST Graduate
University, Onna, Okinawa, 904 0495 Japan<EMAIL_ADDRESS>
###### Abstract

Face masks in general, and N95 filtering facepiece respirators (FRs) that
protect against SARS-Cov-2 virion in particular, have become scarce during the
ongoing COVID-19 global pandemic. This work presents practical design
principles for the fabrication of electrocharged filtration layers employed in
N95 FRs using commonly available materials and easily replicable methods. The
input polymer is polypropylene or polystyrene, and can include discarded
plastic containers of these materials, and the fabrication setup is based on
the cotton candy (CC) principle. The primary parameters underlying the CC
principle are translated to simple design rules that allow anyone to construct
their own fabrication system from common parts, or employ a commercial cotton
candy machine with minimal modifications. Finally, basic characterization
results for structural and filtration properties of electrocharged fabrics
made using the CC principle are detailed.

##  I Introduction

The ongoing COVID-19 pandemic has witnessed multi-fold increase in face mask
use for protection against viral infection, with many countries now mandating
face masks in public areas CDC . This sudden demand surge has created a
scarcity of face masks, necessitating homemade cloth mask fabrication CDC .
But neither homemade cloth masks nor surgical masks are designed to protect
against the SARS-Cov-2 virion, only respirators conforming to N95 or higher
standard are rated for such protection. This capability of N95 filtering
facepiece respirators (FRs) is owed primarily to an electrocharged filtration
layer among other notable design features, deemed the most efficient among
various particle filtration methods employed in face masks.

Taken together, these features permit filtration of $\geq 95\%$ of particles
of size $\geq 0.3\mu$m under test standards designated by the US National
Institute for Occupational Safety and Health (NIOSH) NIO  for N95 (US:
NIOSH-42C-FR84) and its counterparts, including FFP2 (Europe: EN149-2001),
KN95 (China: GB2626-2006), P2 (Australia and New Zealand: AS/NZ1716:2012),
Korea 1st class (South Korea: KMOEL-2017-64), and DS2 (Japan: JMHLW-
Notification214 from year 2018) facepiece respirators 3M . Unfortunately, the
electrocharged polymer filtration layers used in these FRs are manufactured
through industrially sophisticated processes that are hard to duplicate using
commonly available materials or methods.

This article details a process to fabricate electrocharged polymer based
fabric using commonly available materials and easily replicable methods. The
fabrication setup is based on the Cotton Candy (CC) principle, also known as
rotary jet spinning or centrifugal spinning method Rogalski et al. (2017). The
primary control parameters for the CC method Mellado et al. (2011) are
translated to practical design rules for either the construction of a fabric
manufacturing setup from common parts, or through minimal modification of
commercial cotton candy machines. Practical solutions to tune the control
parameters for fabrication of electrocharged polymer fabrics are also
specified. Finally, characterisation of electrocharged fabrics made using the
CC method for structural as well as filtration properties using two mask
designs are presented.

##  II Filtration Principles

###  II.1 Basic Mechanisms

Face mask filtration mechanisms must optimize between two competing
requirements. On one hand the mask’s filtration layers must possess an average
pore diameter small enough to trap and filter particles from being inhaled,
but at the same time too small a pore diameter prevents the user from
breathing comfortably Revoir and Bien (1995). For this reason, face mask
filtration layers cannot be fabricated below a certain pore diameter.
Furthermore, mechanisms for filtration of large particles differ from those
for small particles, thereby requiring a range of filtration strategies to be
adopted. These strategies commonly involve three physical mechanisms, viz.
inertial impaction, diffusion, and electrostatic attraction NAS (2006).

Large particles with diameters $\geq 1\mu$m possess inertia to deviate from
aerodynamic streamlines and collide with filtration fibers and get caught in
the filter mesh. Small particles of typical diameters $\leq 0.1\mu$m which
follow streamlines while undergoing diffusion, execute a complex, meandering
trajectory through the tortuous porous matrix and get trapped in the
filtration layers. These two mechanisms are easily achieved in any cloth-based
or commercial (surgical or PM2.5) masks, but they do not incorporate the third
mechanism of electrostatic filtration Konda et al. (2020); Davies et al.
(2013), which traps particles of intermediate sizes in the range $0.1-1\mu$m
and is considered most effective of the three mechanisms. When an
electrostatically charged layer is embedded among standard mask filtration
layers, oppositely charged particles (both small and large in diameter) are
attracted by the long-range electrostatic Coloumb force towards the
electrocharged layer. Once caught, the particles are held in place through van
der Waals forces.

###  II.2 Electrocharged Filtration

It is known from common experience, especially in cold climates with low
ambient humidity, that when two dissimilar fabrics rub against each other they
gain static electricity, a phenomenon known as triboelectric charging. Fabrics
woven from natural fibers like wool or cotton, which possess high roughness,
and even synthetic fabrics like Nylon are common examples of fabrics with high
triboelectric charging ability. The idea of exploiting charged fabrics to aid
in filtration goes back nearly five decades Frederick (1974), and indeed some
early face mask designs incorporating electrocharged filtration employed wool
or felt fibers, with resin additives to enhance filtration efficiency many
times over that achieved with basic fabric materials alone NAS (2006).
However, resin additives degrade upon exposure to airborne oil aerosol
droplets, which can shield electrostatic charges. Consequently over time,
synthetic electret fabrics such as plastic fibers (e.g. polypropylene and
polystyrene) with high electrostatic charge characteristies were found to
resist the shielding effect of oil aerosols quite effectively and came to be
adopted as the material of choice for electrocharged filtration fabrics. An
additional advantage of plastic electret fabrics was that they were non-woven,
thus saving fabrication time. Finally, the disordered fabric pattern in non-
woven electret fabrics, as opposed to knitted fabric with interleaving fiber
strands in a grid, assures a highly irregular porous medium ensuring particles
follow tortuous streamlines through the porous matrix.

##  III Electrocharged material fabrication methods

All plastic electrets or electrocharged fabrics are manufactured by bonding
polymer fibers in a porous mesh. In order to generate polymer fibers, one
starts with liquid polymer either from solution (polymer dissolved in solvent)
or polymer melt. The bulk liquid must be forced through a tiny orifice to
overcome capillary forces, and quickly accelerated to stretch or extend the
viscoelastic solution into fibers of diameter ranging from 100s of nanometers
to 10s of $\mu$m. This dispersity in fabric diameters helps provide large
surface area exposure (relative to bulk volume) to attract particles.
Additionally, the non-woven fibers bonding together with the dispersity helps
increase fabric disorder and results in a tortuous porous matrix. These fibers
either evaporate their solvent or cool if derived from melt, as they traverse
from the orifice to be collected on a surface where they solidify into an
enmeshed fabric. The various fabrication methods therefore vary in the forcing
mechanisms employed to overcome capillary force and thence accelerate the
polymer. The three primary methods, viz. Electrospinning, Melt Blowing, and
Rotary Jet Spinning, are briefly reviewed as they inform the design rules to
follow subsequently.

Figure 1: (Color online) Schematic of (a) a generic electrospinning setup and
(b) cross-sectional view of a Melt Blowing setup.

Electrospinning: Electrospinning is a widely used platform for generating
polymer fibers Teo and Ramakrishna (2006); Pham et al. (2006); Huang et al.
(2003); Schiffman and Schauer (2008); Chronakis (2005). In this method,
polymer solution or polymer melt in fewer settings Hutmacher and Dalton
(2011); Dalton et al. (2007) is forced out of a container (emitter) with tiny
orifice using a piston, such as a syringe pump. This emitter is connected to
the positive terminal of a high voltage DC source ($\sim$ few to 10s of kV)
and a flat plate or drum placed at a distance (collector) is connected to the
voltage source’s negative terminal, thereby setting up a high voltage DC
electric field between the emitter and collector. The piston pressure competes
with surface tension to generate a polymer jet, whereas the DC electric field
accelerates the jet from emitter towards the collector and stretches them into
fibers. The fibers are deposited on the collector where they evaporate their
solvent to result in the electret fabric. A schematic representation of the
electrospinning principle is shown in fig. 1a. Whereas polymers used in
electrocharged fabrics possess embedded charges, the electric field between
the emitter and collector aids in orienting dipoles of the polymer melt during
droplet stretching, thereby further enhancing the material’s electrocharging
properties.

The electrospinning method is agnostic to the type of polymer material, with
material dependent parameters, e.g. melting temperature and DC field voltage,
being easy to adjust for each material once the primary setup is in place.
However, the electrospinning process suffers from two disadvantages. Firstly,
electrospun polymer throughput scales linearly with the number of orifices or
syringes, requiring several syringes in parallel for increased throughput.
Secondly, the high voltage DC electric field is expensive and requires
additional operational safety features, hence not suitable outside laboratory
and industrial settings where the method outlined below is intended to find
its primary use.

Melt Blown Process: The most common method for manufacture of electrocharged
filtration fabrics is Melt Blowing Pu et al. (2018); Zhang et al. (2018a);
Drabek and Zatloukal (2019). In this method, jets of molten polymer are
generated by injecting it from a conical die, wedged in a gap within an air
knife that converge at the die tip; Fig. 1b shows a cross-sectional view of
the cylindrical geometry for a generic Melt Blowing setup. The hot air
accelerates and stretches the polymer jet into fiber. Cold air sprayed at the
polymer strands as they depart the die tip solidifies them as they land on a
conveyor belt or drum Kellie (2016); Zhang (2014). Like electrospinning, the
melt blown process too can employ a variety of polymers by controlling
temperatures of the die (for polymer melting) and hot air jets. Melt blowing
offers a higher throughput relative to electrospinning and does not require
high DC fields. However, melt blowing setups are inherently suited for large
volume manufacturing, requiring a dedicated source of high pressure air at
large flow rates, and thus not easily amenable to construction from commonly
available parts. The quality of fabric too is ever so sensitive to die
geometry and shaping of the high velocity hot air currents, thus requiring
time-consuming fine tuning of the process.

Figure 2: (Color online) (a) Top and (b) Side view schematic of Rotary Jet
Spinning (CC method).

Rotary Jet Spinning: In recent times, Rotary Jet or Centrifugal Spinning
process has emerged as an attractive alternative platform Badrossamay et al.
(2010); Mellado et al. (2011). In this method (see schematic in Fig. 2), a
central cylindrical container (emitter) holds the polymer and has several
orifices along its wall. The emitter is heated to melt the polymer, but high
viscosity prevents it from flowing out of the orifices under static
conditions. However, when the emitter is spun at several 1000s of revolutions
per minute (rpm), the molten polymer is forced out of the orifices in jets,
which are stretched into fibers by centrifugal force generated from fast
spinning. A cylindrical drum enclosure (collector) surrounds the emitter and
as the stretched polymer jets traverse the distance from emitter to collector
in a spiral trajectory as shown in Fig. 2a, they are cooled and deposited on
the collector surface. Rotary jet spinning employs the same forcing scheme to
generate as well as to accelerate and stretch polymer jets. It neither
requires a high voltage electric field as employed in electrospinning Rogalski
et al. (2018) nor hot air jets used in melt blowing. Furthermore, since the
emitter wall has several orifices, rotary jet spinning offers higher
throughput relative to electrospinning Rogalski et al. (2017). In fact, rotary
jet spinning has been long known in regular life through the cotton candy
machine and for this reason, it is also called the Cotton Candy (CC) method
Wang et al. (2011); Wongpajan et al. (2018). Replacing sugar with polymer and
tuning the temperature and emitter rpm offers an easy design one can construct
from commonly available parts.

##  IV Fabrication Setup

The ability to construct a low-cost fabrication setup from commonly available
parts and materials forms the primary consideration behind the design strategy
detailed in this article. For this reason, some design choices were made at
the very outset to keep the design rules accessible to the layperson. Firstly,
non-woven electret polymer fabrics can also be manufactured by dissolving
polymers in suitable organic solvents, as opposed to melting them at high
temperatures. However, organic solvents are not commonly available whereas
heat sources are universally accessible. For this reason, methods involving
liquid polymers dissolved in solvents are not explored here and will form part
of a future study. Secondly, as already discussed, CC method offers easy
construction from common materials as opposed to electrospinning and melt
blowing. The design strategy therefore relies heavily on the CC method (Fig.
2), with hybrid characteristics that adopt some aspects of electrospinning and
melt blowing wherever practicable.

###  IV.1 Control Parameters

As discussed earlier, electret fabrication involves forcing in two stages,
whose order of magnitude analysis presented here is borrowed from Ref. Mellado
et al. (2011) and the terms defined below are also shown in the schematic in
Fig. 2. The first stage concerns droplet generation in order to initiate a jet
by overcoming the capillary force $F_{\sigma}=\sigma r_{O}$, where $\sigma$ is
the surface tension of the polymer melt and $r_{O}$ is the orifice radius.
Hydrostatic pressure being much smaller in magnitude than the centrifugal
force $F_{\omega}\sim\rho\omega^{2}R_{E}r_{O}$ ($\rho$ being polymer melt
density, $\omega$ the emitter’s angular speed, and $R_{E}$ emitter radius),
balancing the inertial force $\rho\omega^{2}R_{E}r_{O}^{3}$ with $F_{\sigma}$
provides the threshold angular speed $\omega_{th}$ for droplet generation and
jet initiation:

$\omega_{th}\sim\sqrt{\frac{\sigma}{r_{O}^{2}R_{E}\rho}}$ (1)

The second stage of droplet acceleration or jet elongation concerns a
competition between the centrifugal force $F_{\omega}$ and the viscous force
$F_{\mu}\sim\mu v/x$ where $v$ is the jet velocity at a distance $x$ from the
orifice and $\mu=\sqrt{\sigma R_{E}\rho}$ is its extensional viscosity since
the polymer melt is a viscoelastic fluid and the droplet stretching represents
a case of extensional rheology. Applying mass conservation between matter
ejected at the orifice with speed $u$ and the elongated jet of radius $r$
arriving at the collector at a distance $R_{C}$ from emitter, provides the
mean fiber radius:

$r\sim\frac{r_{O}u^{1/2}\nu^{1/2}}{R_{C}^{3/2}\omega},$ (2)

where $\nu=\mu/\rho$ is the extensional kinematic viscosity; it is assumed
$R_{C}\gg R_{E}$.

Whereas equations 1 and 2 resulting from scaling analysis Mellado et al.
(2011) provide appropriate control parameters for the CC method, some of them
are invariant. For instance, surface tension $\sigma$ does not vary
significantly with temperature $T$ and maybe assumed constant, and since
polymers dissolved in solvents by concentration are not explored here, the
extensional viscosity $\mu$ varies only with temperature through density
$\rho$. Therefore the dominant parameters that control our design are orifice
radius $r_{O}$, emitter radius $R_{E}$, collector radius $R_{C}$, angular
speed $\omega$ and heating temperature $T$ through which $\rho$ (and $\mu$) is
varied. It may then be surmised that the material dependence enters only
through the temperature. Armed with these parameters, practical design rules
may now be developed.

###  IV.2 Design Rules

####  IV.2.1 Materials

The material of choice in manufacture of electrocharged filtration fabrics is
usually polypropylene (PP) of high molecular weight, but polystyrene (PS) and
poly(4-methylpent-1-ene) also possess high electrocharging characteristics.
Commonly available materials being the primary goal, PP and PS become the
natural materials of choice as they can be easily sourced as raw material, or
are commonly available through plastic containers. This has implications for
the manufacturing process in that PS being glassy, requires better temperature
control, but cooling is less difficult. On the other hand, PP viscosity is
nearly insensitive to temperature above its melting point, but does require
cooling well below its melting point in order to crystallise.

In using PP, it is known that low molecular weight ($M_{W}$) isotactic PP
($M_{W}<12000$) tends to form brittle fibers that easily break up Choi et al.
(1988). If working with PP pellets ordered from regular suppliers, high
molecular weight isotactic PP is preferred. In the following, fabrics made
from isotactic PP were fabricated with material characteristics:
$M_{W}=250000$, melting temperature $T_{M}=160-165^{\circ}$C and density
$\rho=900$kg/m3; this is in fact the preferred material for manufacture of N95
FR electrocharged layers. Another commonly available polymer used in
electrocharged filtration fabrics is polystyrene owing to its high charge
retention property, as is known from common experience with styrofoam
packaging material which easily sticks to surfaces due to static charge. PS
material was employed with material properties molecular weight $M_{W}=35000$,
glass transition temperature $T_{G}=100^{\circ}$C but with a reasonable
minimum temperature for processing in the range $T=123-128^{\circ}$C and
density $\rho\sim 1060$ kg/m3.

One can also use discarded PP and PS plastic containers, but care should be
taken not to use expanded PS that is generally available in the form of
packaging material, but rather regular PS known as General Purpose Polystyrene
in industry and used in fabrication of plastic containers through injection
moulding. When using discarded PP and PS containers, they were crushed into
powder using a commonly available blender. However, PP containers are often
manufactured from low crystallinity PP, which results in very dense fabric
mesh due to high cohesive properties, and is uncomfortable when breathing Lee
and Wadsworth (1990). Additionally, low crystallinity PP was found not to
possess high charge relative to isotactic PP. However, when mixed with PS from
discarded containers, the resulting fabric is more compliant, less dense
allowing easier respiration, and retains excellent electrocharging
characteristics. A mixture of 80% low crystallinity PP with 20% PS (PP-PS
blend) gave very good results. It is noted that PP and PS are not miscible,
and that by a blend it is merely implied that both powders were molten
together.

The melting and extrusion process of PP or PP-PS would likely inactivate most
biological material (in particular bacteria, fungi and viruses). As a
comparison, autoclaves are typically run at comparable temperatures (160-
190∘C) on a dry cycle for 15-120 minutes. For compatible materials,
sterilization is usually done in a wet cycle at 120∘C and fairly high pressure
($\sim$100 kPa) as the steam helps to break open cells and irreversibly
denature protein and nucleic acids. Quickly melting polymer powder of
contaminated bottles passed through a blender may not result in destruction of
100% of the infectious properties of potential contaminants. This is of
particular concern when using the resulting material for face protection. It
is therefore advisable that plastic containers be cleaned in a domestic
pressure cooker at high steam for 20 minutes before crushing them in a food
processor to turn into powdery material.

####  IV.2.2 Jet Generation

Temperature: Tests on the appropriate range of temperatures were heavily
informed by Ref. Wongpajan et al. (2018). Although the transition temperature
for PS is $T_{T}=123-128^{\circ}$C and that of PP is $T_{T}=160-165^{\circ}$C,
they do not readily flow at these temperatures but merely soften into highly
viscous fluids. Whether working with pure PP or PP-PS, heating the emitter to
higher temperatures in the range $T=175-200^{\circ}$C reduces the viscosity,
but the jets tend to break up during extension and do not result in fibers. We
found that fibers were indeed generated in the temperature range
$T=200-250^{\circ}$ but resulted in beaded structures. Increasing the emitter
angular speed $\omega$ did result in continuous fibers in accord with
published literature Mellado et al. (2011). However, temperatures
$T>280^{\circ}$ reduced viscosity sufficiently to give continuous fibers even
at lower angular velocities. Note that Eq. 2 shows mean fiber radius
$r\sim\nu^{1/2}$ and inversely proportional to $\omega$. It is therefore
desirable to tune the heating temperature $T$ to match the maximum rpm
achievable by the motor employed to spin the emitter as discussed below.

Small commercial cotton candy machines usually employ electrical heating
elements whereas larger ones are gas fired. Irrespective of the heating
method, most cotton candy machines operate at temperatures around
$T=160-175^{\circ}$C, which falls below the temperature range desirable for
generating jets of pure PP or PP-PS ($T=280-340^{\circ}$). If working with
commercial machines, tweaking the heating elements to achieve the desirable
temperature range is suggested. If building one’s own machine, developing
one’s own electrical heating element is desirable if one has working
knowledge, since electrical heating elements provide precise control. A
simpler alternative is to use a gas torch and tweak the torch flame and its
distance from emitter by checking emitter temperature with an ordinary
thermometer used for home baking.

Emitter Motor: The choice of emitter motor for CC method is dictated by Eq. 1
as it sets the lower bound on the revolutions per minute (rpm) to overcome
$\omega_{th}$. Taking surface tension of PP to be $\sigma_{PP}\sim 20\times
10^{-3}$ N/m, $\rho=900$ kg/m3, $r_{O}$ of order 100s of microns ($\sim
10^{-4}$ m), and $R_{E}$ of order few cm ($\sim 10^{-2}$ m) yields
$\omega_{th}\sim$ 470 s-1 or 4500 rpm. In practice however, fibers emerge at
around 2500 rpm, which is easily achieved with most commercial cotton candy
machines as they operate between 3000 - 4500 rpm. If constructing one’s own
machine, high rpm DC motors capable of going up to 15000 rpm are suggested. A
simple alternative is to repurpose an electrical drill to spin the emitter as
they commonly achieve up to 4500 rpm or a Dremel drill with variable
adjustable speeds in the range 3000 - 37000 rpm. Drills can be easily
connected to an emitter container with a suitably threaded steel rod, nut, and
washer. Two simple alternatives exist in the event a high rpm motor is
unavailable, as presented below when discussing emitter geometry.

Figure 3: (Color online) Commercial candy machine emitters usually come with
((a) vertical or diagonal slats or (b) wire mesh for orifices.

Emitter: The emitter follows a standard cylindrical geometry of radius
$R_{E}\sim 5$ cm (0.05 m) and height of roughly 0.1 m. The side wall of the
cylindrical emitter is dotted with several tiny orifices through which the
polymer jets are forced out when the emitter is spun. Since orifice radius
$r_{O}$ and emitter radius $R_{E}$ are the system design parameters entering
Eq. 1 ($r_{O}$ also enters Eq. 2), they should be chosen to match the material
parameters $\sigma$ and $\rho$ in order to obtain a comfortable $\omega_{th}$
within the operational rpm range of one’s chosen emitter motor. Smaller the
orifice radius $r_{O}$, higher is the required $\omega_{th}$ as they’re
inversely proportional, thus setting the upper bound on choice of $r_{O}$.
However, the mean fiber radius $r$ scales linearly with $r_{O}$, and
constrains the upper bound through Eq. 2. If one has access to high rpm motor,
then it is advisable to go to lower $r_{O}$ of order few 100 microns.

Commerical cotton candy machines either use an emitter mesh with large orifice
dimensions of order 1-2 mm or vertical/diagonal slats of width $\sim 1$ mm,
see fig. 3 for an exemplar. Replacing such emitters with home-built emitter
cylinders of lower orifice radii is suggested. Alternatively, if constructing
one’s own fabrication system, an easy way to construct the emitter is using a
soda or beer can as they come readily manufactured to the right radial
dimension and aluminum is an excellent thermal conductor. Cutting or shearing
a soda or beer can in half and using the bottom half gives a readymade
emitter. Folding the top open edge along the wall perimeter and drilling holes
of few 100 micron radius along the emitter wall provides satisfactory results.
For instance, the present study used drill bits of gauge 87 to obtain
$r_{O}=0.254$ mm with a Dremel drill motor connected to bottom half of a soda
can with a nut and washer to drive the emitter, which gave very satisfactory
results.

In the event, one does not have a high rpm motor, a hybrid design combining
rotary jet spinning (CC method) with melt blowing works just as well.
Recognizing that Eq. 1 results from a balance between surface tension and
inertial forces because hydrostatic pressure is low, one could consider
sealing the top of the emitter and pump compressed air at roughly 0.2 - 0.3
MPa, while spinning the emitter at lower rpm to overcome the surface tension
force to generate polymer jets. This is simply the melt blowing principle in
disguise, and it shifts the inertial force by a DC offset value proportional
to the pumping pressure divided by emitter wall’s surface area $\pi
R_{E}^{2}h$ ($h$ is emitter height), thus bringing down the $\omega_{th}$
value. Of course, as a consequence, the lower $\omega$ would lead to larger
mean fiber radius since $r\sim 1/\omega$ but this can be overcome by either
decreasing $r_{O}$ and/or increasing $R_{C}$, the latter usually being easier
to accomplish.

####  IV.2.3 Fluid Acceleration and Stretching

Fluid acceleration and stretching is the stage at which the fibers are formed.
Once the jets are generated, they’re flung radially outwards and execute a
spiral trajectory as shown in Fig. 2b before they’re deposited on the
collector surface. From Eq. 2, the mean fiber radius $r\sim R_{C}^{-3/2}$
scales linearly with $r_{O}$ and inversely with $\omega$, which together form
the system design parameters controlling fiber radius. It is therefore
advantageous to have as large a collector radius to achieve fibers of ever
smaller radii. These system design parameters are complemented by the
polymer’s kinematic extensional viscosity $\nu=\mu/\rho$ which scales as
$r\sim\nu^{1/2}$. For this reason, emitter temperatures in the range
$T=280-340^{\circ}$C are suggested so that the viscosity is low enough for the
polymer melt to flow, but still high enough to result in continuous, fibers of
lower radii. The droplet ejection velocity $u$ is not a controllable parameter
since it is a resultant of the competing (viscous, surface tension, and
centrifugal) forces.

Typical range of collector radii for commercial candy machines fall in the
range $R_{C}\sim 0.1-0.3$ m. Naturally, larger the $R_{C}$, the finer the
fiber radii generated, by virtue of Eq. 2. If constructing one’s own
fabrication system, one has the freedom to set $R_{C}$ to larger radii. We
have found $R_{C}\sim 0.25-0.4$ m worked best as they gave sufficient distance
for fibers to stretch and cool as they traverse from emitter to collector.
Furthermore, if one’s emitter motor is unable to achieve high rpm, we suggest
a simple hybrid solution borrowed from electrospinning to circumvent the
problem. It was observed that at low rpm $\sim 3000-4000$ rpm, and $R_{C}\sim
0.3$ m, connecting a car battery or laboratory power supply at 12 - 24V DC
(negative terminal to emitter and positive terminal to collector with
insulation between the two) generates a DC electric field to add sufficient
droplet acceleration to stretch the fibers to desired radii. Irrespective of
acceleration needs, the electric field also plays an indirect role in
enhancing the fabric’s surface charge characteristics, as discussed later.

####  IV.2.4 Electrocharging

Electrets – dielectric materials which exhibit an external electric field in
the absence of an applied field – can be broadly classified in two varieties,
viz. space-charge or real-charge electrets and dipolar or oriented-dipole
electrets Sessler (1998). Real-charge electrets possess an injected or
embedded excess charge (of one or both polarities) within the dielectric
volume or at the surface; they are usually manufactured via direct charge
injection into the dielectric material. Dipolar electrets are formed by dipole
orientation (polar groups) within the dielectric material and are usually
formed, or rather polarized, by applying an electric field to the material
either at an ambient temperature or by heating the material above its
transition temperature $T_{T}$ where dipoles become mobile. An alternative
method is to employ charge injection techniques on dipolar electrets where the
embedded charge causes dipole reorientation. The fundamental limitation to all
electret-charging methods is dielectric breakdown of the polymer material or
external medium, which depends on the dielectric strength of a given material
(typically of order few MV/cm for polymers with charge densities of a few
tenths of $\mu$C.cm-2). The primary methods for electric charging of electrets
include, Triboelectric charging, Thermal charging, Isothermal charging,
Electron and ion beam charging and Photoelectret charging.

Figure 4: (Color online) Fabrication flow process: Fibers generated in a
modified cotton candy machine (see movie M1.mov in supplementary material for
brief movie of the process) could be scooped out as a concentric fiber fabric,
which was sandwiched and cut into individual sheets. The sheets were then
subjected to isothermal charging with an air ionizer 1 cm from the fabric for
a 10 minute duration to improve the fabric’s static charge characteristics.

As the name suggests, triboelectric charging occurs from charge transfer due
to frictional contact between dissimilar dielectric materials. Not only is
this method very unreliable as it requires intimate contact between the two
surfaces being charged, the present study does not employ two different
materials, hence triboelectric charging does not apply here. Thermal charging
involves application of an external electric field to the polymer at elevated
temperatures, as occurs with the electrospinning process. Even though PP and
PP-PS materials used here fall under the real-charge electret classification,
application of a weak (12-24 V) DC electric field as described in the previous
subsection did enhance surface charge characteristics as shown later. Electron
and ion beam charging involves low energy secondary electron cascade resulting
from scattering of the primary beam within the dielectric bulk and is not
generally efficient for non-woven polymer fabrics of the kind explored here.
On the other hand high energy electron beams cause chemical damage to most
dielectric materials, hence not a suitable process for present needs. The
photoelectret process only applies to charging of photoactive polymers and not
relevant to the current study.

The best results were obtained by Isothermal charging Giacometti and Oliveira
(1992); Kumara et al. (2009). In this method the polymer fabric is placed
between two electrodes maintained at high electric fields (typically kilovolts
and 100s of $\mu$A current). Although this method may seem complicated due to
requirement of high voltages, it is in fact the easiest to achieve Molinié
(1999); Kravtsov et al. (2000). Ionizing air purifiers used in homes and
offices operate on the isothermal charging principle, where they apply high
voltage to ionize or electrically charge air molecules to attract charged dust
particles, bacteria, and viruses. Air ionizers come in two varieties – those
generating negative ions (anions) and electrostatic discharge (ESD) ionizers
(balanced ion generators). ESD ionizers should not be used for iosthermal
charging because not only do they not impart charge, they in fact neutralize
existing charges on surfaces. The relevant ionizers suitable for
electrocharging fabrics are anion generators. The present study used an
ionizing air purifier for home settings (Model NIP-6E from Mystic Marvels LLC)
operating at 9kV and 160 $\mu$A. Exposing the fabricated polymer fabric to
isothermal charging for 10 minutes at a distance of 1 cm (0.1 m) substantially
enhanced their surface charge characteristics. Details of the charge
characterization are presented in the next section.

###  IV.3 Fabrication Process

####  IV.3.1 Setup and Material

Having outlined the primary control parameters for the CC method and various
ways to optimize them in the design process, the basic fabrication process is
now explained. The dimensions and operating ranges naturally vary by user; the
fabrication setup employed in this study had emitter radius $R_{E}=0.055$ m,
collector radius $R_{C}=0.35$ m, and orifice radius $r_{O}=254~{}\mu$m
($2.54\times 10^{-4}$ m). The emitter heating element could be adjusted to
achieve temperatures up to $T=400^{\circ}$C, but the operating temperature in
this study was $T=285^{\circ}$C for PP-PS and $T=300^{\circ}$C for PP. The
emitter motor could achieve a maximum rpm of 35000, but the emitter was run at
10000 - 12000 rpm, capable of yielding a throughput of approximately 0.65 kg
of fibers per hour.

The basic process steps are outlined in Fig. 4, also see movie M1.mov in
supplementary material for a brief video of fiber generation process. Each
fabrication run used 12 grams of polymer as input, and the resulting fibers
were available in less than a minute. The fibers scooped out from the
collector wall had a concentric circular profile, which were then sandwiched
between two flat and clean surfaces at high pressure, this study employed 1 cm
thick float glass plates, resulting in thin fabric sheets of 0.2 - 0.3 mm
thickness. The values vary with the collector dimensions, quantity of material
used per fabrication run, pressure applied to generate the final fabric sheet
and are best worked out through trial-and-error by each end user.
Characterisation details for fabrics are provided in the next section.

####  IV.3.2 Face Masks

Two approaches were followed to turn the fabricated material into face masks.
Lacking in-house capability to stitch fabric layers into masks, in the first
approach a sheet of the electrocharged fabric manufactured by the CC method
was added to the inner surface of surgical and PM2.5 face masks available
commercially as shown in Fig. 5 (a-c). Three different sets of tests were
performed on these masks. The first, a control test, was performed on the
surgical masks in the condition they were procured to measure their baseline
filtration quality. In the second test, a sheet of the manufactured fabric was
added after intentionally bleeding the sheet of its electrostatic charge with
a static eliminator. The reason for this second test was to discriminate
between filtration quality arising purely from the presence of an additional
porous layer without electrocharged filtration capability. The final test was
performed on a surgical mask with an electrocharged fabric layer added to its
inner surface. In all three cases, the static electricity on the masks before
and after filtration tests were measured with a non-contact electrostatic
potentiometer (KSS-2000 Digital Electrostatic Potentiometer, Manufacturer:
Kasuga Denki Inc.). Whereas these filtration tests did show improvement in
filtration quality due to addition of the electrocharged fabric layer, as is
well known, the lack of tight facial fit left gaps through which particles
could easily pass unfiltered.

Figure 5: (Color online) (a) Standard surgical masks (typical dimensions 9 cm
$\times$ 16 cm) were (b) reinforced with an electrocharged filtration layer (5
cm $\times$ 10 cm) fabricated in-house and attached to mask’s inner surface to
improve filtration efficiency upon (c) wearing the mask. Montana Mask holders
with tight facial fit with (d) placement design for Object 500 3D printer and
the (e) front and (f) back view of finished Montana Mask, to hold square (5 cm
$\times$ 5 cm) patches of electrocharged filtration fabric (3 layers per mask)
to achieve N95 filtration quality.

In the second approach, face mask holders were 3D printed from an open-source
design known as the Montana Mask MMa  as shown in Fig. 5(d-f)) to overcome the
deficiency of standard surgical masks in providing a tight facial fit. The
Stereolithography (STL) format design files for the Montana Mask are publicly
available for download from Ref.MMa . One of the design features that allows
N95 FRs achieve superior filtration efficiency of $\geq 95\%$ is their ability
to provide tight facial fit and prevent air from leaking through the
interstitial gap between the mask and skin during respiration. Mask holders
(fig. 5d) were manufactured on an Objet 500 3D printer from the open source
STL files MMa . The filter holder shown in fig. 5e-f (front and back views,
respectively) with a square grid could hold a square patch roughly 5 cm
$\times$ 5 cm in dimensions. Adding up to three electrocharged filtration
layers resulted in the desired N95 filtration quality, but respiration became
difficult with 5 layers because the fabricated layers were denser than the
layers present in commercial N95 FRs. These 3D printed Montana masks were
therefore limited to 4 electrocharged filtration layers for optimal
respiratory comfort without sacrificing filtration quality. Table 1 lists the
mean densities (g.cm-3) obtained by dividing the measured weight by the
dimensions of the samples for commercial fabrics and ones manufactured in-
house. The standard deviation is quoted over measurements for ten samples of
each material, except for the N95 FR for which only one sample was used. All
values are rounded off to second decimal place, and the raw data is available
at Ref. Bandi (2020). The variability is higher in fabrics manufactured in-
house, and that is to be expected strict quality control processes possible in
industrial methods were not permissible in the in-house fabrication method.

Table 1: Electrocharged fabric densities, data available at Ref. Bandi (2020) Material | Density (g.cm-3): Mean $\pm$ Stdev.
---|---
N95 FR | 0.45
Surgical mask | 0.38 $\pm$ 0.03
Isotactic PP | 0.73 $\pm$ 0.09
PP-PS | 0.63 $\pm$ 0.09
Figure 6: (Color online) Scanning electron micrographs for electrocharged
fabrics from (a) a commercial N95 FR, (b) isotactic Polypropylene (PP)
fabricated in-house, and (c) Low crystallinity Polypropylene - Polystyrene
(PP-PS) blend fabricated in-house. High resolution images available at Ref.
Bandi (2020)

##  V Characterisation

Following details of the design rules and fabrication process,
characterisation studies performed on the fabricated electrocharged material
and resulting face masks are now presented for the structural, charge
retention, and filtration properties of fabrics and masks developed in-house.
Recognizing that knowledge of their properties were not meaningful by
themselves, the same tests were performed on a NIOSH-certified commercial N95
FR to serve as the benchmark against which to compare the quality of our
fabrics and masks.

###  V.1 Structure

Structural properties of the electrocharged filtration fabrics were studied
using scanning electron microscopy (SEM). As a preparatory step, platinum-
palladium sputter coating deposition was performed on the fabric sample
surfaces for SEM visualization, followed by interrogation under a scanning
electron microscope (Quanta 250 FEG, Manufacturer: FEI Thermo Fisher) at 2 kV
acceleration voltage.

Figure 6 presents scanning electron micrographs for electrocharged fabrics. In
qualitative terms, the N95 electrocharged fabric layer (fig. 6a) seems
structurally similar to PP (fig. 6b) and PP-PS (fig. 6c) fabrics in terms of
obtaining a heterogeneous, non-woven fabric of enmeshed fibers. This
heterogeneous structure results from fluctuations in fiber trajectories
arising from the individual forcing conditions, viz. acceleration under DC
electric field in electrospinning, hot air jet in case of melt blowing, and
centrifugal forcing for the CC method. The qualitative similarity in fabric
heterogeneity obtained by CC method (fig. 6b and c) relative to N95
electrocharged fabric (fig. 6a) presumably manufactured via melt blowing was
therefore very encouraging.

A cursory inspection suggests the commercial N95 FR fabric’s fibers were
slightly more tortuous relative to PP fabric, and less tortuous than PP-PS
fibers. To understand the qualitative difference in fiber tortuosity, it is
recalled that PP melt forms crystalline fibers whereas PS is a glass. PP
fibers are therefore expected to result in linear, crystalline fibers relative
to PS fibers which freeze into tortuous structures as the molten fluid
undergoes glass transition under cooling and viscosity abruptly shoots up.
Within PP, isotactic PP is more crystalline than low crystallinity PP obtained
from discarded plastic containers. It is therefore important to ascertain how
degree of crystallinity affects tortuosity or other structural
characteristics. Fiber tortuosity is expected to impact the fabric’s porosity.
Though porosity could not be measured directly, filtration tests did present a
small measurable difference between isotactic PP and PP-PS fabrics, which may
be attributed to charge characteristics rather than porosity.

Figure 7 presents scanning electron micrographs for low crystallinity PP (fig.
7a) and PS (fig. 7b) fabrics. Firstly, a comparison of isotactic PP (fig. 6b)
and low crystallinity PP (fig. 7a) fabrics manufactured under similar
conditions shows both fibers are relatively linear. However, low crystallinity
PP fibers possess a more uniform and thicker radius and its fibers are more
linear than isotactic PP fibers. When manufactured under similar conditions,
mean fiber radii for low crystalline PP were almost twice that of isotactic PP
(see Table 2). This implies, the presence of crystalline order, be it low or
high, is sufficient to obtain relatively linear fibers whereas the degree of
crystallinity determines the average fiber radius under identical fabrication
conditions (temperature and rpm).

A comparison of scanning electron micrographs for low crystallinity PP (fig.
7a) and PS (fig. 7b) shows PS fibers are far more tortuous owing to their
glassy nature, albeit with less variability in fiber radius (see Table 2).
Combining the two materials in different proportions helps control the fiber
tortuosity, and therefore the porosity, as done in this study which used 80%
low crystalline PP with 20% PS by weight (fig 6c). Commercial N95 FR fabrics
most likely use a proprietary mix of polymers to control porosity and charge
retention, but the specific materials and their percentages are not available
in the public domain. The mean and standard deviation of the fiber radii
obtained from image analysis using ImageJ open source software of the scanning
electron micrographs for all material combinations investigated are listed in
Table 2. High resolution images for scanning electron micrographs presented in
fig. 7 and fig. 8 are available at Ref. Bandi (2020).

Figure 7: (Color online) Scanning electron micrographs for electrocharged fabrics manufactured in-house with (a) Low crystallinity Polypropylene and (b) Polystyrene. High resolution images available at Ref. Bandi (2020) Table 2: Electrocharged fabric fiber diameters Material | Radius: Mean $\pm$ Stdev ($\mu$m)
---|---
N95 FR | 4.1 $\pm$ 4.7
Isotactic PP | 4.54 $\pm$ 6.2
PP-PS | 4.9 $\pm$ 5.1
Low crystallinity PP | 9.63 $\pm$ 2.4
PS | 18.5 $\pm$ 2.8

###  V.2 Filtration

Setup: Filtration tests for face mask certification are usually performed on
specialized equipment such as the Portacount Respirator Fit-Tester and MITA
8120, both from TSI Inc. or AccuFIT 9000 from Accutec-IHS Inc. Lacking access
to such special testing system and its non-affordability, a filtration testing
system was designed in-house as shown in fig. 8. A manikin head used in retail
store fronts was drilled with a hole from its mouth to the back of its head.
The face mask under test was then mounted onto the manikin head’s face and
placed in a confining box. An inexpensive piezeoelectric atomizer (APGTEK
Aluminum Mist Maker) usually employed in home decoration was submerged in
Sodium Chloride solution (5% by weight NaCl in de-ionized water) to generate
aerosol particles. The generated mist was exposed to negative ion air purifier
to charge the aerosol particles for some of the tests. The mist could pass
through a pipe with a second connecting pipe open to ambient air as shown in
fig. 8 and both pipes had valves to help control the total aerosol
concentration in the air entering the confining box.

A portable PM2.5 air quality monitor (Manufacturer: Dienmern) used normally
for home and office air quality monitoring was placed in the confining box
(Monitor A in fig. 8) to measure the particle concentration within the box. By
reading this PM2.5 monitor, the two inlet valves were adjusted for aerosol
mist and ambient air to control aerosol concentration in the confining box.
The back of the manikin head was connected to a pipe which exited the
confining box and terminated in a box containing a second PM2.5 air quality
monitor (Monitor B in fig. 8). This monitor gave reading of particles that had
passed through the fabric and manikin mouth and allowed measurement of
filtration quality. This box containing the second PM2.5 monitor was, in turn
connected to a vacuum pump as shown in fig. 8. When the vacuum pump was turned
on, a suction pressure was felt in the confining box and aerosol particles
mixed with ambient air were sucked into the confining box and passed through
the face mask to enter the drilled hole in the manikin head and exited the
confining box. By controlling the vacuum pump valve, one was able to simulate
flow rates for normal (30 liters per minute) and high (85 liters per minute)
respiration rates.

Figure 8: (Color online) Filtration test setup schematic.

The general ambient conditions under which all filtration tests were conducted
are now specified before explaining some of the design shortcomings of the
home-designed filtration test setup. The lab temperature during all tests was
maintained at 23 ∘C $\pm$ 2∘C and relative humidity of 43%. The relative
humidity within the filtration test setup’s confining box was however higher
due to aerosol presence at 58%. The area of fabric samples used for surgical
masks (SM) was 9 cm $\times$ 16 cm with the electrocharged filtration layer
occupying an area of 5 cm $\times$ 10 cm, where as the fabric area in the 3D
printed montana mask was 5 cm $\times$ 5 cm. The aerosol number concentration
at 30 L/min flow rate was $1.7\times 10^{8}$ particles per cm3 and at 85 L/min
flow rate was $1.25\times 10^{8}$ particles per cm3, providing high enough
aerosol concentration. Two tests were performed to study time to failure, i.e.
the total duration from test commencement after which filtration started to
deteriorate and varied between 14.5 hours (for PP fabrics) to 17 hours (for
PP-PS blend). As a comparison commercial N95 FRs failed after 22 hours. NIOSH
certification standard for N95 FRs requires use of neutrally charged sodium
chloride solution (10% NaCl in water). However the filtration tests in the
current study had a dual role, firstly to demonstrate the efficacy of
electrocharged filtration, and secondly to test the filtration efficiency
itself. To demonstrate the efficacy of electrocharged filtration, independent
tests were conducted with aerosol particles that were charged and charge
neutralized for N95 FR and the 3D printed Montana Mask design.

Design shortcomings: It is emphasized that this filtration test setup does not
conform to some of the stringent testing specifications employed in face piece
respirator certification. For instance, NIOSH 42 CFR Part 84 standard for N95
FRs requires filter performance of $\geq 95\%$ with NaCl test agent at 85
liters per minute flow rate and inhalation resistance (maximum pressure drop
across mask) of $\leq 343$ Pa and exhalation resistance of $\leq 245$ Pa 3M .
The filtration test setup developed in-house had no means to measure the
pressure drop nor could one simulate the oscillatory respiratory air flow from
inhalation and exhalation and the scheme could only generate steady suction
flow.

An important quantity in face mask filtration quality testing is the Most
Penetrating Particle Size (MPPS); MPPS for N95 FRs is 300 nm or 0.3 $\mu$m. An
important shortcoming of the filtration test setup detailed here is that it
cannot provide a size distribution of aerosol particles generated by the
relatively inexpensive piezoelectric atomizer. Secondly, commercial respirator
testing systems use laser-based particle counter sensors that are sensitive to
detection of particles below the MPPS value down to about 100 nm. The PM2.5
air quality monitors, as their name suggests, are rated for measuring
particles as small as 2.5 $\mu$m. Be that as it may, PM2.5 monitors also
employ the same laser-based particle counter sensors and do hold the
capability to detect particles down to 0.3 $\mu$. Although a proper
verification was not possible, it is reasonable to assume the PM2.5 monitors
could detect particles at least down to 0.3 $\mu$m diameter.

Figure 9: Filtration test results showing penetration P(t) (%) versus time
(hours) over 12 hour duration for (a) N95 FR with charged (open black symbols)
and neutral (filled black symbols) aerosols at 30 L/min (circles) and 85 L/min
(squares) flow rate and (b) commercial surgical mask (SM) taped (filled black
symbols) and not taped (open black symbols) around manikin head at 30 L/min
(upright triangles) and 85 L/min (inverted triangles).

Test Results: FR filtration efficiency is usually measured in terms of the
penetration percentage (P), defined as percentage of particles present in the
environment that pass through the FRs and is quoted against the particle
diameters. Since particle sizes could not be measured by the PM2.5 monitors,
the penetration is instead defined as:

$P(t)=\frac{C_{B}(t)}{C_{A}(t)}\times 100\%$ (3)

where $C_{A}(t)$ and $C_{B}(t)$ are the particle concentrations of PM2.5
monitors A and B respectively at time $t$. The penetration as a function of
time was followed to study any deterioration in filtration properties. The
PM2.5 monitors A and B were connected to a laptop and programmed to record
concentration values at 15 minute intervals over a duration of 12 hours. Raw
time series data for all filtration tests presented in fig. 9 and fig. 10 and
Table 3 are available at Ref. Bandi (2020).

The first tests were performed on commercial N95 FR and surgical masks (SMs)
to obtain baseline calibration readings on the filtration test setup designed
in-house, which would then form the comparative standard for tests performed
on filtration fabrics manufactured with the CC method. Since a tight facial
fit for N95 FR is often emphasized, the test N95 FR was taped with a mask tape
on to the manikin face and performed separate tests using aerosol particles
that were both charged and charge neutral. On the other hand, baseline SM
calibration tests were performed only with neutrally charged aerosol
particles, but with and without mask tape applied for tight facial fit. Figure
9 shows the results for the baseline commercial N95 FR and SM tests. A few
details that are known among filtration research community are immediately
apparent. Firstly, N95 FR filters out more than 95% of the particles at both
30 and 85 L/min flow rates (see fig. 9a). Secondly, a small but measurable
difference in penetration percentage is clearly observable between charged (1
- 2% penetration) and neutral ($\sim 3\%$ penetration) aerosol particles.
Finally, no measurable dependence on flow rate could be observed for the N95
FR.

The SM results were in stark contrast with the N95 FR. Firstly, emphasis on
the importance of tight facial fit becomes immediately apparent from fig. 9b.
When no tape was applied to close the interstitial gap between the face and
mask, the penetration was nearly 50% at 85 L/min and around 40% at 35 L/min
implying substantial leakage. Taping the mask around the manikin face
drastically brought down the penetration percentage values to around 12% at
both flow rates, thereby underscoring the importance of tight facial fit in
filtration masks and respirators. Secondly, even after applying tape to close
the gap between face and mask, the penetration value of around 12% was still
higher than the N95 standard of $\leq 5\%$. The reason for this discrepancy
lies in the fact that SM fabrics do not employ electrocharged layers and rely
entirely on inertial impaction and diffusion to achieve filtration, thus
underscoring the importance of the electrocharged filtration mechanism.

Figure 10: Penetration P(t) (%) versus time (hours) for (a) PP fabric mounted
on a taped SM (b) PP fabric mounted on 3D printed Montana mask holder, (c) PP-
PS fabric mounted on a taped SM, and (d) PP-PS fabric mounted on a 3D printed
Montana mask holder.

Following these baseline tests, results are presented for tests performed on
the two mask designs using isotactic PP fabrics. Figure 10a presents results
for the first design where a filtration fabric layer manufactured in-house was
placed on the inner side of a commercial surgical mask (SM). Tests were
performed using an electrocharged layer as well as a fabric layer whose charge
was depleted with a static eliminator as described earlier. All tests were
performed by taping the SM to the manikin head. From fig. 10a it is seen that
commercial SM with non-electrocharged layer exhibits the same penetration
percentage of around 12% at both flow rates (30 and 85 L/min) and the results
are statistically identical to ones shown in fig. 10b for taped masks.
However, adding the electrocharged filtration layer leads to a marked decrease
in penetration of aerosol particles down to around 7% at both flow rates.
Unfortunately though, at 7% penetration, it still falls short of the N95 FR
requirement of $\leq 5\%$. Be that as it may, at just 7% penetration the
proposed solution of an electrocharged fabric layer in a commercial SM
provides ample protection for people using face masks in non-critical
settings, but falls short of protection deemed necessary for say, emergency
response and healthcare services.

Figure 10b presents filtration test results for the 3D printed Montana mask
design meant to achieve the N95 FR tight facial fit. Four electrocharged
filtration fabric layers of 0.2 - 0.3 mm thickness each were employed here.
Charged aerosol particles exhibited slightly less penetration at 3.6% (30
L/min) and 4.35% (85 L/min) relative to neutral aerosols with 4.6% (30 L/min)
and 5.2% (85 L/min). This trend is similar to one observed for commercial N95
FR in fig. 10a, but the penetration values for fabrics manufactured using the
CC method were unfortunately slightly higher than commerical N95 FR. The
penetration values do fall within the 5% requirement for the N95 standard for
all cases except neutral aerosols at 85 L/min, which misses the target by a
meagre 0.16%. However, in natural work settings, one expects to be exposed to
both charged and neutral aerosols. Therefore, when taken together, the mean
penetration for all aerosols at 30 L/min flow rate is 4.12% and at 85 L/min is
4.86%, i.e. just below the 5% penetration requirement. Ergo, the 3D printed
Montana mask design fit with electrocharged filtration fabrics barely manages
to meet the N95 standard, but it is emphasized once again that the test for
pressure drop across the filtration fabrics could not be measured nor could
the oscillatory inhalation-exhalation cycles present in normal respiration be
simulated with the in-house filtration test setup.

Filtration tests were also performed under identical conditions using PP-PS
fabrics. Figure 10c shows results for SMs with and without PP-PS
electrocharged fabric layer and should be compared against PP results in fig.
10a. The penetration percentages were identical at both flow rates for the
non-electrocharged layer, suggesting the slightly higher tortuosity in PP-PS
fabric (fig. 7c) relative to PP fabric (fig. 7b) had no measurable impact on
filtration quality. However, SM with PP-PS electrocharged filtration layer
performed marginally better by $\sim 1\%$ (fig. 10c compared to identical test
with PP fabric (fig. 10a). A similar improvement in penetration percentage was
also observed for the 3D printed Montana mask design (see fig. 10d). Tests for
both charged and neutral aerosols at 30 as well as 85 L/min flow rate yielded
penetration values in the same range of 3 - 4%, modulo the variability one
observes in the data (fig. 10c). Ergo, the PP-PS fabrics surpassed the N95
requirement to within the testing limitations. Since the minor structural
differences (tortuosity) between PP and PP-PS tests did not show any
difference, it can only be surmised that the higher performance of PP-PS
fabrics (relative to PP) comes from electrocharging as is confirmed in the
following subsection. The mean penetration and standard deviation are listed
for all our tests in Table 3, with all values rounded off to the second
decimal place.

Table 3: Penetration percentage values measured from filtration tests. Raw time series for all data are available at Ref. Bandi (2020) Material | Penetration (%): Mean $\pm$ Stdev
---|---
N95 FR Charged (30 L/min) [Fig. 9a] | 1.43 $\pm$ 0.03
N95 FR Neutral (30 L/min) [Fig. 9a] | 3.29 $\pm$ 0.02
N95 FR Charged (85 L/min) [Fig. 9a] | 1.54 $\pm$ 0.03
N95 FR Neutral (85 L/min [Fig. 9a] | 3.37 $\pm$ 0.03
SM No Tape (30 L/min) [Fig. 9b] | 40.72 $\pm$ 0.85
SM Taped (30 L/min) [Fig. 9b] | 11.98 $\pm$ 0.73
SM No Tape (85 L/min) [Fig. 9b] | 47.18 $\pm$ 0.45
SM Taped (85 L/min) [Fig. 9b] | 13.13 $\pm$ 0.57
PP: SM + Non-Electrocharged Layer (30 L/min) [Fig. 10a] | 12.04 $\pm$ 0.26
PP: SM + Non-Electrocharged Layer (85 L/min) [Fig. 10a] | 12.26 $\pm$ 0.31
PP: SM + Electrocharged Layer (30 L/min) [Fig. 10a] | 7.19 $\pm$ 0.32
PP: SM + Electrocharged Layer (85 L/min) [Fig. 10a] | 7.3 $\pm$ 0.37
PP: 3D Charged (30 L/min) [Fig. 10b] | 3.62 $\pm$ 0.13
PP: 3D Neutral (30 L/min) [Fig. 10b] | 4.65 $\pm$ 0.2
PP: 3D Charged (85 L/min) [Fig. 10b] | 4.34 $\pm$ 0.17
PP: 3D Neutral (85 L/min) [Fig. 10b] | 5.14 $\pm$ 0.27
PP-PS: SM + Non-Electrocharged Layer (30 L/min) [Fig. 10c] | 12.31 $\pm$ 0.31
PP-PS: SM + Non-Electrocharged Layer (85 L/min) [Fig. 10c] | 12.1 $\pm$ 0.24
PP-PS: SM + Electrocharged Layer (30 L/min) [Fig. 10c] | 6.67 $\pm$ 0.19
PP-PS: SM + Electrocharged Layer (85 L/min) [Fig. 10c] | 6.22 $\pm$ 0.22
PP-PS: 3D Charged (30 L/min) [Fig. 10d] | 2.97 $\pm$ 0.22
PP-PS: 3D Neutral (30 L/min) [Fig. 10d] | 3.53 $\pm$ 0.25
PP-PS: 3D Charged (85 L/min) [Fig. 10d] | 3.17 $\pm$ 0.29
PP-PS: 3D Neutral (85 L/min) [Fig. 10d] | 3.48 $\pm$ 0.25

Nanoparticle filtration test: An important question that arose was if the
masks were capable of trapping individual viral particles, such as the SARS-
Cov-2 virion. The SARS-Cov-2 virion is estimated to have a diameter between 50
- 200 nm. More precisely, Ref. Kim et al. (2020) places the SARS-Cov-2 virion
of order 70 - 90 nm and Ref. Chen et al. (2020) places it at 50 - 200 nm.
Although MPPS for N95 FRs is set at 300 nm (0.3 $\mu$m), some studies have
suggested MPPS for N95 FRs occurs in the range of 40 - 60 nm Balazy et al.
(2006); Rengasamy et al. (2007). In particular, experiments by Balazy et al
Balazy et al. (2006) using MS2 virion – bacteriophage with single-stranded RNA
comprising 3569 nucleotides that infects male E. Coli bacteria – with an
approximate diameter of 27.5 nm shows that N95 FRs can achieve superlative
filtration for particle diameters smaller than its rated MPPS. This indicates
that N95 FRs would perform equally well at trapping an individual SARS-Cov-2
virion with lower bound on diameter roughly 1.5 times that of MS2.

Having placed the approximate range for the SARS-Cov-2 virion, the next
question is if the SARS-Cov-2 virion possesses a surface electric charge that
permits it to get attracted to an electrocharged filtration layer. The net
charge would be determined by the sum of the charges exposed on the viron
surface and can be calculated from the protein structure(s) at the surface.
However, in order for the virus to be trapped by the electrocharged filters,
there are two small issues to contend. Firstly, individual SARS-Cov-2 virions
are not likely to be airborne. Instead, like most enveloped viruses, it is
hydrated or in solution and is transmitted by aerosol droplets. When
dehydrated, the virion’s lipid membrane collapses and its proteins become
denatured within a certain time, depending on temperature etc., i.e., it is
rendered inactive. Despite the low probability of encountering individual
SARS-Cov-2 virions, it was a question still worthy of an investigative test.

Secondly, the net charge of a protein in solution (water, typically) depends
on the pH. Every amino acid has a pKa (the log of the acid dissociation
constant), which is an equilibrium constant indicating the pH at which the
charges are balanced (net neutral). There are pKa values for each chemical
group. If the pH is lower than the pKa, that amino acid becomes protonated, if
it can. At low pH, there is an abundance of positively charged protons. Even
some acids can become protonated at low pH. One can approximate the net charge
of a linear peptide from its sequence and by providing a pH for the solvent
using tools on the web (e.g. see Ref. pro ) one can arrive at an estimate for
surface charge. However, this does not take into account that structures are
folded in 3D and some charges are hidden inside. Although it is only a rough
value, it does still provide an estimate because residues buried inside are
often hydrophobic, i.e. not charged. There are several online tools available
to calculate net charges of folded protein structures as well as their surface
charge distribution, e.g. DELPHI del . The isoelectric point pI is the pH at
which the net charge is neutral. Below the pI, the net charge is positive,
above it is negative. Ref. Michen and Graule (2010) lists some examples of
viruses, most of their pIs being $<7$, suggesting that most viruses would be
net negatively charged at neutral pH, thus suggesting that the COVID-19 virion
is also net negative. The main surface protein for SARS-Cov-2 is the spike
protein spi . It is glycosylated, thus has some amino acids, mostly arginines,
chemically attached to sugars that can be charged, like sialic acid, which
would also contribute to an overall negative charge. When the sequence of the
spike 6VXX is input into Ref. iso , a pI=5.8 is obtained, thus net negative.

Commercial filtration test systems permit detection of individual virion
penetration as these systems can identify particle diameters. In the absence
of such a system, and lacking knowledge of handling even inactive virions, a
surrogate technique using fluorescent tagged nanoparticles was employed.
Polystyrene nanospheres fluorescently tagged with Dragon Green (480 nm
Absorption wavelength, 520 nm Emission wavelength) with mean diameter 50 nm
$\pm$ 10 nm (Manufacturer Bangs Labs, Catalog No. FSDG001, Lot No. 14092) were
used as a substitute for the SARS-Cov-2 virion. Since the polystyrene
nanospheres were to mimic virions, the aqueous solution was first dried and
the nanoparticle dust was exposed to a brief burst of coronal discharge.
Unsure whether coronal discharge exposure would dissociate fluorescent markers
off the surface of the nanoparticles, a quick fluorescene test was performed
and confirmed that the fluorescent nanoparticles performed to specification.

The filtration test was performed on the 3D mask design with nanoparticle dust
replacing the mist generator in fig. 9. Measurement of penetration was
meaningless as the PM2.5 monitors could detect dust particles from ambient air
entering the confining box. Instead, the test was conducted at 30 L/min flow
rate for one hour duration and then the electrocharged filtration layers were
interrogated under a confocal microscope (Nikon Andor Revolution WD spinning
disk confocal, Laser wavelength 455 nm) for fluorescence signals from the
trapped nanoparticles, if any. Figure 11 presents the fluorescence emission
signal from polystyrene particles trapped along fiber surfaces of the
electrocharged layer as measured under the confocal at a depth of 500 nm from
the outer surface (exposed to the environment). It is observed that the
fluorescence signal occurs in clusters because the nanoparticles were not
dispersed in aqueous solution as usually done in experiments. Instead the
solution was dried to obtain nanodust that comprised clumps of particles, or
alternatively it is possible the charged particles were preferentially
attracted to charge centers that acted as traps and begs further future
investigation. This was a proof of principle test to check whether the
electrocharged fabrics were capable of trapping charged particles of
dimensions comparable to the SARS-Cov-2 virion, and further quantitative
analysis was not undertaken for the purposes of the current study.

Figure 11: (Color online) Confocal image of fluorescence emission signal from
polystyrene nanospheres trapped along fiber surfaces at a depth of 500 nm from
the outer surface of the electrocharged filtration layer exposed to the
environment.

###  V.3 Charge

Electrostatic charge measurements were made with a non-contact electrostatic
potentiometer (KSS-2000 Digital Electrostatic Potentiometer, Manufacturer:
Kasuga Denki Inc.) with a piezoelectric transducer capable of measuring
electrostatic voltage at a distance of 50 -100 nm from the sample. The
measurement readings for various fabric samples are listed in Table 4. The
mean and standard deviation are taken over a set of 100 measurements conducted
at various locations on several samples of each fabric material.

Table 4: Electrostatic potential of various fabric samples. Material | Electrostatic potential (kV)
---|---
N95 FR | 11.1 $\pm$ 0.9
SM | 0.4 $\pm$ 0.3
PP | 5.3 $\pm$ 0.44
Isothermal charged PP | 7.4 $\pm$ 0.23
PS | 8.1 $\pm$ 0.28
Isothermal charged PS | 10.8 $\pm$ 0.41
PP-PS | 8.4 $\pm$ 0.36
Isothermal charged PP-PS | 10.1 $\pm$ 0.3

The commercial N95 FR fabric material which serves as the comparative standard
for this study clearly holds the highest electrostatic potential at 11.1 $\pm$
0.9 kV (see Table 4. The electrostatic potential quoted here for the
commercial N95 FR was measured after extracting the electrocharged fabric
layer sandwiched between the neutral outer layers. For this reason, the value
quoted here will be higher than the actual potential an aerosol or dust
particle experiences because of the outer neutral layer forming a dielectric
barrier between the electrocharged layer and ambient environment. Be that as
it may, for purposes of this study, the electrostatic potential of the
commercial N95 FR’s bare electrocharged layer forms the right comparative
standard for fabrics manufactured with the CC method. SM fabrics had the
lowest electrostatic potential at 0.4 $\pm$ 0.3 kV (see Table 4) and were
practically neutral given the standard deviation (0.3 kV) nearly equals the
mean (0.4 kV) electrostatic potential.

With the commercial N95 FR’s electrocharged layer and SM fabrics as the two
bounds of the comparative standard, it becomes possible to establish a
correspondence between the electrostatic potential and filtration efficiency
for all the fabrics studied here. Firstly, the measured electrostatic
potential listed in Table 4 shows PS fabrics possess more charge than
isotactic PP fabrics, as is established by prior independent measurements
Dodbiba et al. (2003). Although both PP and PS are poor conductors in pure
form, PS molecules have benzene rings which improve charge concentration of PS
molecules Wypych (2016), this is not so for PP molecules which do not possess
the benzene ring. However, several extraneous factors such as purity, thermal
treatment (recrystallization in PP versus physical aging in PS) etc. also
influence charge retention. Indeed, it is known that N95 FR electrocharged
layers embed dopants Zhang et al. (2018b); Kilic et al. (2015) for charge
enhancement, particularly in polypropylene. Fabrication with simple materials
and methods being the design strategy, such charge enhancement using dopants
was not explored in the present work.

Secondly, isothermal charging of the manufactured fabric certainly increases
the electrostatic potential of all samples by roughly 25% (for PP-PS) to 40%
(for isotactic PP) thereby establishing the efficacy of the secondary
isothermal charging step employed in the present study. Finally, a one-to-one
correspondence is observed between electrostatic potential and filtration
efficiency obtained from the penetration tests. The difference in filtration
efficiency we observed between the PP and PP-PS samples could not be
attributed to slight structural difference in tortuosity of the fiber porous
matrix, but PP-PS fabrics hold higher electrostatic potential than PP fibers.
This explains why PP-PS 3D printed Montana mask respirators consistently met
the N95 requirement of $\leq 5\%$ penetration, whereas PP 3D printed Montana
masks could barely meet the requirement, with especially neutral particles
falling slightly above the requirement.

##  VI Conclusions

In summary, a set of general design principles have been presented to
construct one’s own fabrication setup and manufacture of electrocharged
filtration fabrics using the CC principle, together with some potential
designs to use these filtration fabrics in face masks. In doing so, attention
has been paid to utilising commonly available materials and easily replicable
methods using two commonly available candidate materials, viz. PP and PS and
their blends.

Electrocharged filtration fabrics must meet two requirements, structural
heterogeneity for filtration through inertial impaction and diffusion, and
high charge retention for electrocharged filtration. Fabric heterogeneity
comes in several forms and includes a non-woven fabric comprised of fibers
enmeshed in a random manner, fiber tortuosity which affects the fabric
porosity, and variability of fiber radii which control the surface to volume
ratio. The CC method can easily achieve a disordered non-woven fabric of
enmeshed fibers, but tortuosity and variability of fiber radii is a material
dependent parameter that enters via operating temperature. Whereas isotactic
PP fabrics exhibited all three forms of heterogeneity (fiber radius 4.54 $\pm$
6.2 $\mu$m), they possessed less charge (5.3 $\pm$ 0.44 kV without and 7.4
$\pm$ 0.23 kV with isothermal charging) relative to PS fabrics (8.1 $\pm$ 0.28
kV without and 10.8 $\pm$ 0.41 kV with isothermal charging). On the other hand
PS fabrics resulted in thicker fiber radii of lower variability (18.5 $\pm$
2.8 $\mu$m), albeit with higher tortuosity than PP fabrics. Achieving an
optimal mix of heterogeneity and charge retention by blending PP and PS (fiber
radii 4.9 $\pm$ 5.1 $\mu$m and charge of 8.4 $\pm$ 0.36 kV without and 10.1
$\pm$ 0.3 kV with isothermal charging) resulted in fabrics that were able to
meet the N95 FR standard (fiber radii 4.1 $\pm$ 4.7 $\mu$m and electrostatic
potential 11.1 $\pm$ 0.9 kV), at least within the limitations of the
filtration test setup designed and constructed in-house.

The current work maintained exclusive focus towards providing a proof of
principle for the process and underlying mechanics of the CC principle as a
viable method for fabrication of electrocharged filtration fabrics. Further
work to be explored in future includes researching more commonly available
fabric materials, such as Poly Ethylene Terepthalate, universally available as
PET bottles for beverages, and Biaxially-oriented Poly Ethylene Terepthalate
(BoPET), better known as Mylar and available as plastic bags and food
wrapping. A more detailed characterisation of the structural and
electrocharging properties of fabrics obtained via the CC method is also
needed. Another avenue that begs exploration is the introduction of metallic
nanoparticles at the fabrication stage to embed them into fabrics for charge
enhancement, and possibly even decontamination, without endangering users as
these fabrics are meant to be worn around mouth and nose for safe respiration.

Although the COVID-19 pandemic provided the impetus for the current effort,
face masks may well become a mainstay of human social interactions going
forward. On order of two viruses jump across from animals to humans per year
Woolhouse et al. (2008) with most animals exhibiting viral richness showing
propensity for close human contact Olival et al. (2017). If even a small
fraction of those viruses result in asymptomatic viral shedding in human
exhalation (breathing, coughing, or sneezing) Fraser et al. (2004), face mask
protection at the population level becomes a necessary means of protection.
Decentralized local manufacture of face masks with high filtration efficiency
from commonly available materials and simple designs could potentially
alleviate global supply chain disruptions during such times, as witnessed
during the COVID-19 pandemic Molina (2020). It is hoped that this effort will
help communities with face mask protection during such pandemics.

###### Acknowledgements.

This work was supported by the Nonlinear and Non-equilibrium Physics Unit,
OIST Graduate University. MMB first learned about the cotton candy method from
a passing remark by Dr. L. Mahadevan in 2010. MMB acknowledges advice from Dr.
M. Wolf on COVID-19 surface charge characteristics, Dr. S. Ghosh on using
PM2.5 air quality monitors for filtration testing, and help from Dr. K. Deasy
with 3D printing, N. Ishizu with Scanning Electron Microscopy and Dr. H. B.
Kang for characterisation, and OIST Imaging section with Confocal Microscopy.
Dr. S. Velankar is gratefully acknowledged for critical reading of the
manuscript and several helpful suggestions for its improvement.

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