Military Protective Face Mask

The use of offensive chemicals in military conflicts or on civilians by terrorists is an ever present danger. Most of these chemical threats are in the form of gas due to its rapid dispersal and reach to maximize casualties. While our skin provides a natural barrier to the chemicals, poisoning from inhalation can only be prevented by filtering the air such as through the use of gas mask. Conventional gas mask uses activated charcoal to adsorb any harmful chemicals from the air but the cartridges that contain them are heavy and bulky. Since the charcoal does not neutralize the chemical and the contaminated gas cartridges need to be treated and disposed properly. A separate filter may also be required for filtering out dust particles since the gas cartridge is not for particle exclusions. Electrospun membrane provides an alternative and more effective substrate for use in gas mask. Electrospun fibrous membrane have been used in air filter for decades and it can be used as a particle exclusion filtration unit and functionalized to neutralize toxic gas.

As an air filtration substrate, electrospun membrane has been shown to be very effective in removing small particles with low pressure drop. Li and Gong (2015) did a study comparing the filtration efficiency and pressure drop of electrospun polysulfone fibers against several commercially available face masks for PM2.5 protection (See figure 1). Typical air pressure drop in commercially available face mask is between Δ0.5 to Δ0.7. Pressure drop for electrospun fiber deposition at 15 minutes is about Δ0.41 and at 30 minutes is about Δ0.87. Rejection rate is 91% for the former and 97% for the latter. Thus it can be anticipated that a fiber deposition duration of between 15 to 30 minutes will provide a nanofiber coating that gives a balance between pressure drop and rejection.

Fig. 1 Permeability and intercept rate comprehensive comparison [Li and Gong. Journal of Chemistry, vol. 2015, Article ID 460392, 2015. This work is licensed under a Creative Commons Attribution 3.0 Unported License.].

Apart from particle exclusion, electrospun membrane may also be used as protection against biological agents, in particular, bacteria. Due to the small pore size of electrospun membrane, it is able to effectively screen out bacteria. Chaudhary et al (2014) used an electrospun polyacrylonitrile-silver composite filter media to cover a nutrient media in room condition and passes ambient air through the filter media. When compared to the negative control which is without the protective filter media, the nutrient media protected by the nanofibrous filter remains free of bacteria growth after two months while the unprotected nutrient media show microorganism growth. Huang et al (2019) constructed a protective face mask by incorporating biocidal, 1-chloro-2, 2, 5, 5-tetramethyl-4-imidazolidinone (MC) which is a kind of N-halamine, into polyacrylonitrile (PAN) nanofibers by electrospinning a blend of the solution. N-halamines are known to be effective against a broad spectrum of organisms including bacteria, yeast, fungus and virus. The resultant electrospun PAN/MC 5% membrane showed an air permeability of 27 mm.s^-1 at differential pressure of 100?Pa. This is 20% slower than pure PAN electrospun membrane but higher than commercial nano-surgical mask (about 20 mm.s^-1). Antibacterial tests showed that the PAN/MC 5% membrane was able to inactivate all inoculated S. aeurus (6.04 log reduction) and E. coli (6.60 log reduction) within 1 and 10 mins contact time respectively. Tests with S. aureus bioaerosols also showed more than 6 log reductions after 3 h.

For a military facemask, it is likely that it will be in use for several days without replacement. Therefore, the facemask would need to maintain its performance even if it is moist from the user's breath. To retain adsorption of ultrafine particulate matters (PM) when moist, Choi et al (2021) introduced functional groups with permanent charges to the electrospun membrane. The electrospun poly(butylene succinate) (PBS) membrane was coated with chitosan nanowhiskers (CsWs) chitosan which contains cationic sites and polar amide groups that is able to attract polar ultrafine PM (e.g., SO₄^2- and NO^3-) even under humid conditions. By varying the concentration of PBS solution, Choi et al (2021) was able to electrospun both PBS micro and nanofibers. electrospin individual membranes made of microfiber and nanofiber respectively. CsWs were loaded onto the membrane by dip coating in a CsWs suspension. The presence of CsWs significantly increases the filtration performance of nanofiber membrane and microfiber membrane with little increase in pressure drop. By combining a layer of nanofiber membrane and microfiber membrane with CsWs coating, they were able to achieve a PM_1.0 removal efficiency of 97.5% and PM_2.5 removal efficiency of 98.3%. The quality factor of the integrated membrane is also greater than that of individual CsWs coated nanofiber and microfiber membranes. The maximum pressure drop was found to be 59 Pa (comfortable for human breathing) with negligible loss of PM removal efficiency when completely wet. The superior removal efficiency can be attributed to a combination of physical size exclusion and electrostatic adsorption coming from the permanent CsW charges.

There are several ways which electrospun fibers may be used to neutralize toxic gas. Ramaseshan (2011) used electrospinning to produce ZnTiO₃ nanofibers from its precursor. The annealed fibers have diameters mainly in the range of 50 to 300 nm. ZnTiO₃ containing α-Zn₂TiO₄ demonstrate the best efficiency with Paraoxon, simulant for the organophosphorus compounds, decomposition of 91% in the first 50 minutes and CEES (2-chloroethyl ethyl sulfide), simulant for mustard gas, decomposition of 69% in the first 10 minutes. Instead of using pure inorganic fibers which can be brittle, another method is to incorporate active agents into polymeric fibers. Ramaseshan (2011) fabricated (3-carboxy-4- iodosobenzyl) oxy-β-Cyclodextrin from o-iodosobenzoic acid(IBA) and β-cyclodextrin (β-CD) for incorporation into polyvinyl chloride nanofibers. With a mass of (3-carboxy-4- iodosobenzyl) oxy-β-Cyclodextrin to PVC ratio of 0.5:1, the rate of hydrolysis of the nanofibrous membrane was found to be 11.5 times faster than that of activated carbon. Chen (2009) used a blend of reactive polyacrylamidoxime (PAAO) and polyacrylonitrile (PAN) as the carrier to form electrospun fibers. The reactive fibers were shown to hydrolyse of p-nitrophenyl acetate (PNPA) which mimics nerve agents. However, the study also showed that the presence of the PAN matrix also affects the accessibility of the reactive sites.

A drawback of using blending to incorporate functional additives into nanofibers is that the polymer matrix covering the additives may reduce its effectiveness. Han et al (2011) used co-axial electrospinning to construct a composite nanofiber with a polymeric core and an active sheath comprising of diisopropylfluorophosphatase (DFPase). DFPase is an enzyme that is able to breakdown organophosphates. The fiber coated with the enzyme was found to be effective in hydrolysing diiopropylfluorophosphate (DFP). However, generation of acid by-products reduces the pH and this reduces the DFPase-DFP reaction rate. Instead of co-axial electrospinning, Chen (2009) chemically modify the surface of PAN by a one-step oximation reaction with excess hydroxylamine to form reactive amidoxime groups on the surface of the fiber. The amidoxime was able to decompose DFP in the presence of water similar to the use of DFPase.