As the pandemic due to COVID-19 takes its course around the globe, demand for protective gears such as face masks markedly increases. Evidently, in order to prevent virus transmission, health officials instruct citizens to wear masks when in public (Organization 2020, ChengWongChuangSoChenSridharToChanHung and Ho 2020). This has led to a significant surge in demand for efficient masks in many places around the world. In addition, the cost of using a mask per person per day can lead to a mounting financial burden, especially for low-income families and those living in the developing world. Consequently, various decontamination protocols for the purpose of reusing masks have been proposed (Fischer et al. 2020a, Grinshpun et al. 2020, O'Hearn et al. 2020, Probst et al. 2020, Rubio-Romero et al. 2020, Viscusi et al. 2007, Smith et al. 2020, Fischer et al. 2020b, Lore et al. 2012, Viscusi et al. 2009, Bergman et al. 2010, Viscusi et al. 2011, Gertsman et al. 2020, Woo et al. 2012, Bopp et al. 2020, Lowe et al. 2020, Juang and Tsai 2020, Yang et al. 2020). Nevertheless, concerns on filtration efficiency and mask integrity post decontamination treatment are still apparent.
While conventional masks fabricated by melt-blown technology (Sureka et al. 2020, Tsai 2020b) have been the target of several decontamination studies (Fischer et al. 2020a, Yang et al. 2020, Woo et al. 2012), in this manuscript, nanofiber-based masks are subject to chemical (ethanol, and bleaching), wet heat (boiling, steam, and autoclave), dry heat (oven, ironing), and irradiation (microwave and UVGI) treatment protocols and analyzed. More specifically, filtration efficiency and morphology of nanofibers before and post-treatment have been assessed.
In order to examine structural integrity of the samples, SEM and FESEM images of polyamide 6 (PA6) electrospun nanofibers were obtained. Figure 3.a shows a cross-sectional SEM image of the nanofibers on a nonwoven substrate. These structures are in fact what will be found inside nanofiber-based N95 masks. The average diameter of the nanofibers is 163±43nm (see Figure 3b and 3c). The illustrated nanofiber layer with submicron pore size enables efficient filtration of particles larger than 0.3µm (PM 0.3µm). The ultrafine nanometer with the thickness of about 20 nm, formed within the nanofibrous structures that helps trap up to 95% of PM 0.3µm with a pressure drop range of 110-330 Pa (at 85 L.min-1), which follows the standard NIOSH guidelines for N95 masks (Zhang et al. 2016).
Other than structural integrity, performance integrity of the samples is also of vital importance. These are measurements of i) pressure drops across the fabric, otherwise known as breathability of the fabric, and ii) filtration efficiency of the samples to capture specific size range of aerosols. In fact, these deciding parameters place a sample in the N95 category according to the NIOSH system. In order to compare the performance integrity of the samples before and after decontamination, all samples underwent PEE treatment. Table 2 tabulates the results of this investigation.
Table 2
Pressure Drop and Filtration efficiency before and after treatments.
|
Pressure Drop (Pa)
@ 85 L.min-1
|
Filtration Efficiency (%)
for PM = 0.3 𝛍m
|
Before Treatment
|
After Treatment
|
p-value
|
Before Treatment
|
After Treatment
|
p-value
|
Ethanol (70%)
|
158
|
162.33
|
0.831
|
98.96%
|
57.33%
|
0.0005
|
Bleaching
|
121.33
|
108.33
|
0.313
|
94.7%
|
89.2%
|
0.043
|
Boiling
|
134
|
151.33
|
0.103
|
98.66%
|
89%
|
0.005
|
Steam
|
147.66
|
150.66
|
0.815
|
98%
|
87%
|
0.080
|
Autoclave
|
171
|
158
|
0.463
|
99.6%
|
98%
|
0.006
|
Oven
|
147.33
|
127.66
|
0.007
|
99.9%
|
92.66%
|
0.489
|
Ironing
|
165.66
|
153.33
|
0.197
|
99.3%
|
98.33%
|
0.097
|
Microwave
|
147.33
|
141.33
|
0.818
|
97%
|
93.66%
|
0.523
|
UVGI
|
165
|
159
|
0.188
|
98.66%
|
98%
|
0.373
|
PM: Particulate Matters (PM)
UVGI: Ultraviolet Germicidal Irradiation
|
Ethanol
Expectedly, soaking samples in 70% ethanol has been shown effective for inactivation of viruses and bacteria situated on N95 masks (Fischer et al. 2020b, LinTangHungHua and Lai 2018). However, in contrast to the virucidal and bactericidal effectiveness, filtration efficiency of ethanol-soaked samples is considerably lower compared to the untreated (see Table 2). This is in agreement with a previous report by Ullah et al. where application of ethanol to melt-blown based masks was explored (UllahUllahLeeJeongHashmiZhuJooCha and Kim 2020). According to Table 2, the filtration efficiency of ethanol treated masks reduced by 41.63% (p: 0.0005). In addition, the pressure drop increased by 4.33 Pa (p: 0.831). This indicates that, once a nanofiber-based N95 mask is soaked in ethanol, its breathability will reduce and it will fail to efficiently halt hazardous PM aerosols of 0.3 µm in diameter. This may be due to a sudden change in the surface tension of the nanofibers when the fibers absorb ethanol and then dry out (NazeeriHilburnWuMohammedBadalChan and Kirschvink 2020a). As illustrated in Figure 4, nanofibers undergo a noticeable damage seen as large cavity formations due to laceration of nanofibers. This is also apparent in FESEM images shown in Figure 5a where fibers are disintegrated. This in fact explains lower filtration efficiency. On the other hand, ethanol can cause a swelling of the PA6 layer (HeffernanSemiãoDesmondCaoSafariHabimana and Casey 2013, GeensVan der Bruggen and Vandecasteele 2004) which forces pores to tighten in the microstructure and give rise to a pressure drop across the layer. However, it is likely that the main contributing factor for reduced breathability is the swelling of the nonwoven fabric by ethanol (NazeeriHilburnWuMohammedBadalChan and Kirschvink 2020b). This fabric is often made out of polypropylene (PP) and once swollen, a dense network with lower surface area is formed that limits air flow. Although, the swelling of fabrics is less evident in the case of polyethylene terephthalate/polyvinylidene difluoride (PET/PVDF) nanofibers, but it can nevertheless adversely affect filtration efficiency (Ullah et al. 2020).
Bleaching
Bleach is a 5-15% solution of sodium hypochlorite (NaOCl) which can act as an oxidizing agent against bacteria and viruses (Viscusi et al. 2009). According to Table 2, application of bleach on nanofiber-based masks caused a drop in filtration efficiency from approximately 95% to 89.2% (p: 0.043), but at the same time, it increased breathability, i.e. the pressure drop value decreased from 121 Pa to 108 Pa (p: 0.313). The sudden decrease in pressure can be an indication of damage to the consistency of the fibers along the substrate. It is suggested that when PA6 nanofibers are exposed to NaOCl, a reduction of amine groups (N-H) due to the presence of oxidative chlorine, leads to a cleavage of polyamide linkage (Simon and Nghiem 2014). As seen in Figure 5b, this causes thinning of nanofibers and therefore formation of large pores within the membrane. The white arrow in this image marks a PM that has been trapped by the fibers
According to Viscusi et al., bleaching N95 masks by NaOCl for 30 minutes result in no significant change in the permeability of PM through the samples (Viscusi et al. 2007). Other groups have also reported minimum adverse effect on filtration efficiency of melt-blown based N95 masks after bleaching, however, persisting undesirable bleach odor post treatment has been apparent (Viscusi et al. 2009, Bergman et al. 2010). Therefore, while bleaching has not significantly affected filtration efficiency, concerns about toxic chemical residue and carcinogenic remains of bleach on the surface of the samples challenge the safety of this mode of decontamination.
Boiling
Boiling offers a simple alternative decontamination method that is accessible to most people (GilbertsonQuintanar-SolaresLiland and Niermeyer 2020). Based on the findings, the filtration efficiency of PA6 nanofiber-based masks following boiling reduced from 98.66% to 89% (p: 0.005) and the pressure drop increased from 134 to 151 Pa (p: 0.103). This may be due to the thinning of the nanofibers when exposed to heated water thus forming large pores and cavities (Figure 5c). The white arrow in this figure marks an abnormal solidification of PA6 polymer in the midst of the fibers. It is speculated that the resulting morphological change is due to the absorbance of water molecules by hydrophilic groups (-COOH, -NH2 and -CO-NH-) available in PA6. This causes nanofibers to loosen their hydrogen bonds within their polymeric chains and dissolve (TomaraKarahaliouAnastassopoulosGeorgaKrontiras and Karger‐Kocsis 2019, WeversMathotPijpersGoderis and Groeninckx 2007, RazafimahefaChlebickiVroman and Devaux 2005). On the other hand, the hydrophobic part of PA6 (-(CH2)5-) results in partial aggregation of nanofibers (Razafimahefa et al. 2005). Similar studies suggest that while boiling does not alter the general appearance, filtration efficiency reduces and it is directly proportional to the number of heating cycles (Probst et al. 2020, Liao et al. 2020). Therefore, application of wet heat is generally not recommended to decontaminate N95 masks.
Steam
Application of heated steam is recommended by public health authorities to disinfect PPE against viruses (Yang and Wang 2020) and bacteria (OztoprakKizilates and Percin 2019). Interestingly, our findings indicate that while changes in pressure drop were not significant (p: 0.815), a meaningful reduction in filtration efficacy following steam exposure from 98% to 87% was apparent (p: 0.080). In the presence of water molecules, the electrical charges on the surface of nanofibers neutralizes thereby reducing filtration efficacy (Grinshpun et al. 2020). In addition, partial swelling due to the penetrating heated water molecules between nanofibers lead to an increase in diameter of nanofibers and a decrease in their surface area (see arrowhead in Figure 5d). This ultimately can result in a reduction of filtration efficiency (Wevers et al. 2007, Geens et al. 2004).
Autoclave
The effectiveness of autoclave has been previously demonstrated by other studies as a decontamination method in laboratories and hospitals (Lin et al. 2018). In this method, unlike boiling and steam, a significant reduction in filtration efficiency was not evident (99.6% to 98%, p: 0.006). In addition, reduction in pressure drop post treatment was negligible (p: 0.463). Also, other than the loss of nanonets, no apparent change in the microstructure of the nanofibers that would alter filtration efficiency was detected (see Figure 5e).
However, it is reported that autoclaving common N95 masks in particular, reduces filtration efficiency due to the loss of electrical charge and damaged integrity (Grinshpun et al. 2020). While other studies support using autoclave for decontamination of masks (Harskampvan StratenBoumanvan Maltha-van Santvoortvan den Dobbelsteenvan der Sijp and Horeman 2020). Our study supports the use of autoclaving for disinfecting PA6 nanofiber-based masks.
Dry Heat
Dry heat (oven) is known as an accessible decontamination method to inactivate viruses and bacteria (Tsai 2020a, Rogers 2012). It has been reported that treatment with dry heat does not have a significant negative impact on filtration efficiency of common N95 masks (Fischer et al. 2020a, Liao et al. 2020). In the following study, applying dry heat via an oven to decontaminate PA6 nanofibers-based masks led to a reduction in filtration efficiency (99.9% to 92.66%, p: 0.489) as well as a pressure drop (147 to 127 Pa, p: 0.007). In Figure 5f, a number of cavities with thick edges are observed across the membrane. Since the glass-transition temperature (Tg) of PA6 is 35–60°C (Maddah 2016, GuiboQingYahongYin and Yumin 2013) and the disinfection of N95 masks in the oven occurs at 70°C, macromolecular movement increases and nanofibers stick together leading to cavity formation. In addition, some nanofibers increase in diameter at this temperature. Thus, reduction of efficiency and pressure drop after dry heat treatment may be due to the presence of these cavities.
Ironing
Among all potential decontamination methods, ironing is one of the most rapid and available methods to be used by the public. The effectiveness of ironing on inactivation of microorganisms and viruses has been reported by previous studies (Lakdawala et al. 2011, Rodriguez-PalaciosCominelliBassonPizarro and Ilic 2020). In our study, ironing did not significantly alter the filtration efficiency (99.3% to 98.33%, p: 0.097) or pressure drop (167 to 153 Pa, p: 0.197) of PA6 nanofibers. As seen in Figure 5g, ironing did not greatly alter the PA6 nanofiber membranes of masks but resulted in the disintegration of nanonets. Since the temperature of ironing was higher than the Tg of PA6, molecular movements are expected to lead to morphological changes. However, this does not actually happen due to the very short contact time with the mask. Although ironing does not significantly affect the microstructure of nanofibers, it may melt the PP spun bond (because of its Tm is160-208 ◦C) (Maddah 2016) if the temperature is too high or ironed for too long. Therefore, this method largely depends on the individual using it.
Microwave
Microwave, which is presented as an electromagnetic wave in the frequency range of 300 MHz to 300 GHz, was presented as a technique for killing microorganisms in the mid-1980s. This technique relies on thermal energy to kill cells and microorganisms (Zhang et al. 2010). Microwave has been presented by different studies as a germicidal (Zhang et al. 2010) and virucidal (Woo et al. 2012) method to decontaminate masks for reuse when supply is short (for example, during the Covid-19 pandemic). We have shown that exposing PA6 nanofiber-based masks to microwave leads to a 3.33% reduction of filtration efficiency (from 97% to 93.66%, p: 0.523) along with a slight reduction in pressure drop (147 to 141 Pa, p: 0.818). In terms of macroscopic and microscopic features, no obvious changes are observed and nanofiber nanonets remain partly intact (Figure 5h).
Gertsman et al. (Gertsman et al. 2020) reported in a systematic review that microwave intervention in moist or dry conditions can decontaminate common N95 masks to be reused under NIOSH. However, Viscusi et al. have shown that decontaminating masks in dry microwave leads to melting (Viscusi et al. 2009, Viscusi et al. 2007). Others and we have shown that microwaving in a moist condition does not harm the mask structure and yields acceptable results in terms of filtration properties (Gertsman et al. 2020, Viscusi et al. 2011).
Ultraviolet
UVGI was previously confirmed to be an effective decontamination method against the influenza virus, H1N1 (HeimbuchWallaceKinneyLumleyWuWoo and Wander 2011, MillsHarnishLawrenceSandoval-Powers and Heimbuch 2018), H5N1 (Lore et al. 2012), Covid-19 (Fischer et al. 2020a), and bacteriophage MS2 (Fisher and Shaffer 2011) on masks (Yang et al. 2020, Anderson and Eng). Here, UVGI-treated PA6 nanofiber-based masks showed a 0.66% reduction in filtration efficiency (98.66% to 98.00%, p: 0.373) and a reduction in pressure drop (165 Pa to 159 Pa, p: 0.188) which are not significant. Microscopic features following UVGI treatment (Figure 5i) show thinner and partly broken up nanofibers as well as the absence of nanonets. However, the integrity of nanofiber membranes is preserved. The UVGI method was not destructive enough to reduce the filtration efficiency of PA6 nanofiber-based masks. However, previous work used FTIR characterization to show that longer exposure of PA6 nanofibers to UVGI can lead to an increase in the C=O peak of 1710 cm-1 which is related to oxidation and degradation of the nanofibers (PinpathomratYamada and Yokoyama 2020). Therefore, at longer exposure times and repeated disinfection cycles, UVGI may damage the nanofibers by physical degradation (TianWangQuWangZhuZhang and Liu 2018, O'Hearn et al. 2020). In agreement with this finding, we show that applying UVGI for one cycle (20 min) is not destructive in terms of filtration efficiency of PA6 nanofiber masks and can preserve the eligibility criteria of NIOSH. Similar results have been reported for N95 masks where UVGI does not affect the integrity, ability to filter aerosols, and ability to adapt to the face. In addition, it does not leave a smell or irritating/toxic residues. Finally, UVGI treatment of N95 masks does not create significant changes in appearance even when multiple disinfection cycles are performed (SalterKinneyWallaceLumleyHeimbuch and Wander 2010, Bergman et al. 2010, Fischer et al. 2020a, Liao et al. 2020, FisherRengasamyViscusiVo and Shaffer 2009, Fisher and Shaffer 2011, Heimbuch et al. 2011, Viscusi et al. 2011, LindsleyMartin JrThewlisSarkisianNwokoMead and Noti 2015, Lin et al. 2018).
Figures 6 and 7 illustrate changes in filtration efficiency and pressure drop before and after each decontamination method outlined above.
On the issue of PA6 nanofiber masks, investigations reveal that ethanol decontamination methods are not suitable because of a 42.66% reduction in filtration efficiency. In addition, concerns about odor and toxicity with bleaching make this method inappropriate. Boiling, steam, microwave and oven methods are associated with a reduction in filtration efficiency to 90%. Although ironing did not reduce filtration efficiency significantly, it is not recommended as it is dependent on the individual and can melt PP. UVGI and autoclave are the best methods to disinfect PA6 nanofiber masks without changing the microstructure and filtration efficiency (Figure 8). Changes in pressure drop for all methods is not a criterion for NIOSH standards for N95 respirators (Figure7).