Electrospinning fibers for Facemask

In the recent decades, with greater awareness of air pollution, PM 2.5 airborne particles and the Covid-19 epidemic, there is greater demand for facemask beyond industrial usage. There is also a shift in the facemasks requirements. In the past, most facemasks were for one-time use. Now, there are greater demands for reusable and washable facemasks. Conventional production methods for facemask may not be able to meet these new requirements.

An electrospinning setup by Bioinicia [Video of FLUIDNATEK LE-50].

Air pollution in a city is often gauged by measuring the amount of PM2.5 (Particulate matter, 2.5 µm) in the environment as particles of this size are believed to pose the greatest health risks. Such particles come from various sources such as car exhaust, burning, cooking and power plants. 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. Typical air pressure drop in commercially available face masks 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.

Permeability and intercept rate comprehensive comparison [Li and Gong. Journal of Chemistry, vol. 2015, Article ID 460392, 2015.]

The Covid-19 epidemic in 2020 has brought much attention to the importance of facemasks for both health care workers and the general public. In particular, the sudden surge in demand for facemasks have and shortage has brought to attention other considerations in facemask design such as reusability. As of 2020, almost all N95 certified facemasks are made using melt-blown filtrate. Greater research on electrospun nanofibers for use as facemasks has shown some interesting results. Ullah et al (2020) did a comparison study on the reusability of filters made from melt-blown (MB) polypropylene (PP) and polyvinylidene difluoride (PVDF) bonded on spun bond PET (Polyethylene terephthalate). Filtration efficiency of MB filter dropped significantly to around 65% or less after 10 cycles of ethanol spraying or 24 h ethanol dipping. With the nanofiber filter, the filtration efficiency is maintained at 98% regardless of ethanol cleaning method. The difference in the filtration efficiency after cleaning may be attributed to the filtering mechanism. For MB filters, static charge plays an important role in its filtering capability. Therefore, repeated washing would substantially reduce its static charges. For nanofiber membranes, its main filtration mechanism comes from particles impacting the fiber and size exclusion. Therefore, repeated washing does not adversely impact its performance. Other advantages of nanofiber filter membrane are its quick drying time of 10 min compared to 3 h for MB filters and greater user comfort as it is thinner which reduces heat retention within the mask. In terms of reusability, nanofiber filter membrane has demonstrated several significant advantages over conventional single use MB filters. In the construction of reusable facemasks, one option is to introduce functional groups with permanent charges that retains the ability to adsorb ultrafine PM when moist. Choi et al (2021) used electrospinning to construct chitosan nanowhiskers (CsWs) coated poly(butylene succinate) (PBS) microfiber/nanofiber-integrated filters. The chitosan nanowhiskers (CsWs) chitosan with its cationic sites and polar amide groups 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. 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.

In an epidemic, facemasks may be worn for a long duration through the day. The Covid-19 pandemic has shown that people have had skin issues on the face where the mask is in constant contact with the skin after wearing it for long hours. One contributing factor may be propagation of microbes in the warm and moist interface between the mask and the skin. Therefore, development of facemasks with antibacterial properties may improve the hygiene of mask wearing. Qiu et al (2021) selected polyvinyl butyral (PVB) as the material to be electrospun into nanofibers. An advantage of PVB is that the solvent used is ethanol which is commonly used for antibacterial purposes and is less toxic to humans. Berberine hydrochloride (BH), a plant-based chemical was used as the antibacterial agent that is loaded into PVB solution and electrospun to give antibacterial nanofibers. The electrospun PVB/BH nanofibers were collected on an hydrophilic polypropylene spunbond nonwoven base. The surface with the electrospun PVB/BH nanofibers was shown to be hydrophobic with a water contact angle of about 140° while the uncoated spunbond surface remains hydrophilic. The membrane with PVB/BH exhibited antibacterial properties with zone of inhibitions on staphylococcus aureus agar plate. The composite filter membrane also showed good air permeability with air filtration efficiency of 96.4% for PM 0.3 and 100% for PM2.5 and a pressure drop of 108 Pa. Blosi et al (2021) constructed a facemask with electrospun membrane which has antibacterial properties and has been shown to be effective in filtering out virus-size particles. The anti-bacterial effect comes from silver nanoparticles produced by the reduction of AgNO₃ using hydroxyethyl cellulose (HEC) as a reducing and capping agent. Electrospinning was carried out on a suspension of AgHEC in polyvinyl alcohol (PVA) solution to give PVA/AgHEC nanofiber membrane. Thermal treatment was carried out on the PVA/AgHEC nanofiber membrane to stabilize the membrane. The heat-treated membrane has been shown to maintain its shape and porous structure in water while non-treated membranes lose their porous structure. The heat-treated AgHEC membranes were shown to be biocidal against Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus. To determine its air filtration efficiency, a sample with 5 layers of the membrane was prepared. NaCl aerosol particles comparable to a virus size distribution with median diameter around 70 nm and mass median diameter of 500 nm were tested on the filter sample. A high filtration efficiency of more than 95% was recorded for all particle sizes at filtration velocity of 5.5 cm/s and this was reduced to a minimum of 80% for filtration velocity of 16.7 cm/s. Pressure drop and filtration efficiency values are within range of FFP1 and FFP2 masks, even at the highest filtration velocity of 16.7 cm/s which makes the five layered sample suitable for use as facemasks. Lou et al (2022) used a blend of polyvinyl alcohol and water-soluble chitosan (WS-CS) for electrospinning into antibacterial mask-filter media. The WS-CS provides the antibacterial property to the membrane and its inhibition against Staphylococcus aureus (S. aureus) increases with increasing WS-CS loaded into the electrospun membrane. In terms of filtration performance, the porosity of the electrospun membrane is greater than commercial facemask but the pore size and water vapor transmission rate (WVTR) was lower. Filtration efficiency of the electrospun membrane is greater than 90% across all blend ratios while commercial facemask is only 73%. The pressure drop of the PVA/WS-CS electrospun membranes were between 41 and 44 Pa which is slightly higher than commercial facemask at 40 Pa. However, this is less than a N95 mask with pressure drops of 48 Pa. Taking into account the filtration efficiency and pressure drop, the electrospun fibers quality factor is still better than commercial facemask.

High utilization of facemasks due to the pandemic has led to researchers searching for ways to reduce waste such as using recycled materials in the production of facemasks. Baselga-Lahoz et al (2022) investigated the use of recycled PET for the production of PET nanofiber filter media in facemask. Diameters of the electrospun PET fibers can be varied easily by changing the concentration of the solution. Tests showed that smaller fiber diameters increases filter performance but also increases pressure drop which is undesirable. Higher diameter fibers have lower pressure drop but the filter performance is not as good. Therefore, Baselga-Lahoz et al (2022) used a combination of smaller and larger fiber diameters in the construction of the filter media. With electrospinning, they are able to stack layers of fibers with different diameters using the same process but different solution concentration. A facemask was constructed with the middle filter layer having an average fiber diameter of 1.24 µm and thickness greater than 500 µm, sandwiched between thicker fiber layers with average fiber diameter of 3.18 µm. The constructed facemask has a retention efficiency of more than 98.2% against particles between 0.5 and 10 µm, and 100% against particles of 3 µm with a pressure drop of 0.36 mbar. This facemask showed better retention ability for fine and coarse particles compared to commercial surgical masks. A concern of using recycled PET in the construction of filter media is its degradation over time especially with the higher surface area of fine electrospun fibers. Storage of the electrospun masks in room condition for 4 months showed a drop in filtration efficiency of less than 2.2% with the lowest efficiency at 97.8%.

Potential long duration of facemask usage has prompted research into introducing skincare property to facemask. Balogh-Weiser et al (2023) constructed a 3-ply facemask containing two different lipases and antibacterial drug Nadifloxacin in it. Lipase is an enzyme that could help to reduce fatty or oily layers on the skin surface thus leading to better skin conditions. The materials tested for encapsulation of the active ingredients using electrospinning were hydrophobic polylactic acid (PLA) and hydrophilic polyvinylpyrrolidone (PVP). For the electrospun layers, the bottom layer contained lipase from Candida rugosa (CrL), the middle layer with lipase from Rizomucor miehei (RmL) and the top layer contained nadifloxacin (NF). The catalytic activities from the lipases were found to be higher from the electrospun PLA fibers although the diameter of electrospun PVP nanofibers were smaller. The lower lipase activities from PVP nanofibers may be due to dissolution of the fibers in the aqueous reaction medium while PLA is insoluble in water. It is hypothesized that the polymer matrix protects and stabilizes the lipase hence with the dissolution of PVP, the lipase is exposed to the environment and decomposes unlike the water insoluble PLA which maintains its coverage over lipase. Similarly, it was found that nadifloxacin was readily released from the dissolved PVP nanofibers but at a much slower rate with PLA nanofibers.