Charged PVDF multilayer nanofiber filter in filtering simulated airborne novel coronavirus (COVID-19) using ambient nano-aerosols

Abstract

The novel coronavirus (COVID-19), average size 100 nm, can be aerosolized by cough, sneeze, speech and breath of infected persons. The airborne carrier for the COVID-19 can be tiny droplets and particulates from infected person, fine suspended mists (humidity) in air, or ambient aerosols in air. To-date, unfortunately there are no test standards for nano-aerosols (≤100 nm). A goal in our study is to develop air filters (e.g. respirator, facemask, ventilator, medical breathing filter/system) with 90% capture on 100-nm airborne COVID-19 with pressure drop of less than 30 Pa (3.1 mm water). There are two challenges. First, this airborne bio-nanoaerosol (combined virus and carrier) is amorphous unlike cubic NaCl crystals. Second, unlike standard laboratory tests on NaCl and test oil (DOP) droplets, these polydispersed aerosols all challenge the filter simultaneously and they are of different sizes and can interact among themselves complicating the filtration process. For the first time, we have studied these two effects using ambient aerosols (simulating the bio-nanoaerosols of coronavirus plus carrier of different shapes and sizes) to challenge electrostatically charged multilayer/multimodule nanofiber filters. This problem is fundamentally complicated due to mechanical and electrostatic interactions among aerosols of different sizes with induced charges of different magnitudes. The test filters were arranged in 2, 4, and 6 multiple-modules stack-up with each module having 0.765 g/m² of charged PVDF nanofibers (mean diameter 525 ± 191 nm). This configuration minimized electrical interference among neighboring charged nanofibers and reduced flow resistance in the filter. For ambient aerosol size>80 nm (applicable to the smallest COVID-19), the electrostatic effect contributes 100-180% more efficiency to the existing mechanical efficiency (due to diffusion and interception) depending on the number of modules in the filter. By stacking-up modules to increase fiber basis weight in the filter, a 6-layer charged nanofiber filter achieved 88%, 88% and 96% filtration efficiency for, respectively, 55-nm, 100-nm and 300-nm ambient aerosol. This is very close to attaining our set goal of 90%-efficiency on the 100-nm ambient aerosol. The pressure drop for the 6-layer nanofiber filter was only 26 Pa (2.65 mm water column) which was below our limit of 30 Pa (3.1 mm water). For the test multi-module filters, a high 'quality factor' (efficiency-to-pressure-drop ratio) of about 0.1 to 0.13 Pa^-1 can be consistently maintained, which was far better than conventional filters. Using the same PVDF 6-layer charged nanofiber filter, laboratory tests results using monodispersed NaCl aerosols of 50, 100, and 300 nm yielded filtration efficiency, respectively, 92%, 94% and 98% (qualified for 'N98 standard') with same pressure drop of 26 Pa. The 2-6% discrepancy in efficiency for the NaCl aerosols was primarily attributed to the absence of interaction among aerosols of different sizes using monodispersed NaCl aerosols in the laboratory. This discrepancy can be further reduced with increasing number of modules in the filter and for larger 300-nm aerosol. The 6-layer charged nanofiber filter was qualified as a 'N98 respirator' (98% capture efficiency for 300-nm NaCl aerosols) but with pressure drop of only 2.65-mm water which was 1/10 below conventional N95 with 25-mm (exhaling) to 35-mm (inhaling) water column! The 6-layer charged PVDF nanofiber filter provides good personal protection against airborne COVID-19 virus and nano-aerosols from pollution based on the N98 standard, yet it is at least 10X more breathable than a conventional N95 respirator.

Keywords: COVID-19; Electrostatically charged PVDF nanofiber filter; Facemask/Respirator; Multimodule/multilayer; Nano-aerosol; Novel coronavirus.

Schematic 1a

Coronavirus attached to a carrier of similar size or smaller and become airborne. The resulting aerosol is 60 nm.

Schematic 1b

Coronavirus (60–140 nm) attached to a carrier of larger size and become airborne. The resulting aerosol may be 100–300 nm.

Fig. 1a

Electrospinning nanofibers using a syringe connected to a high voltage supply.

Fig. 1b

Corona discharge.

Fig. 1c

Portable test rig for measuring unfiltered and filtered air, respectively, from the ambient.

Fig. 2a

PVDF nanofibers.

Fig. 2b

Distribution of nanofiber diameter with mean fiber diameter 525 nm ± 191 nm.

Fig. 3

Typical size distribution of aerosols detected by inner condensation particle counter (CPC) of PAMS of ambient aerosols with median size at 75 nm.

Fig. 4

Efficiency of 2L PVDF nanofiber filter versus aerosol size, 10–400 nm. Measurement of 2L PVDF filter.

Fig. 5

Empirical relationship of Z and single-fiber efficiency due to dielectrophoretic effect.

Fig. 6

Efficiency versus aerosol size for 4L filter.

Fig. 7

Efficiency versus aerosol size for 6L filter.

Fig. 8

Pressure drop comparison between single and multilayering.

Fig. 9

Electrostatic to mechanical capture efficiency.

Fig. 10

Efficiency (100 nm) and pressure drop versus increasing layers and comparing with iso-QF condition with QF = 0.1 Pa⁻¹.

Fig. 11a

Efficiency (25 nm) and pressure drop vs. increasing layers and comparing with iso-QF condition with QF = 0.13 Pa⁻¹.

Fig. 11b

Efficiency (55 nm) and pressure drop vs. increasing layers and comparing with iso-QF condition with QF = 0.1 Pa⁻¹.

Fig. 11c

Efficiency (300 nm) and pressure drop vs. increasing layers and comparing with iso-QF condition with QF = 0.11 and 0.13 Pa⁻¹.

Fig. 12

Multi vs. single layer, and uncharged vs. charged on filtration efficiency of 100-nm aerosol.

Fig. 13

Multi vs. single layer, and uncharged vs. charged on filtration efficiency of 55-nm aerosol.

Fig. 14a

100-nm ambient aerosol with polydispersed size distribution versus 100-nm monodispersed NaCl aerosol challenging the filter both at 5.3 cm/s.

Fig. 14b

55-nm ambient aerosol with polydispersed size distribution versus 50-nm monodispersed NaCl aerosol challenging the filter both at 5.3 cm/s.

Fig. 14c

300-nm ambient aerosol with polydispersed size distribution versus 300-nm monodispersed NaCl aerosol challenging the filter both at 5.3 cm/s.