Functionalized Electrospun Air Filters

Electrospun fibers have been used commercially as a high performance air filter media. The ability of electrospun fibers to filter out particles is mainly due to the small diameter of the fibers forming a network. Therefore, the material selection for its use as air filter media is mainly restricted by its electrospinnability to the required diameter and its stability under given conditions. Beyond looking at the filtration performance of different electrospun membrane, it is also useful to consider potential added functions to the filter media.

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

Conventional filter media is typically opaque due to the necessary nonwoven membrane thickness to achieve satisfactory filtration performance. High filtration efficiency of electrospun membrane meant that the membrane may be made so thin that it is virtually transparent and yet having superior filtration performance. However at low electrospun fiber membrane thickness, preliminary investigation suggested that its performance is dependent on the material used. Polyvinylpyrrolidone and polyvinyl alcohol exhibited deteriorating removal efficiency with reduced electrospun fiber membrane thickness while polyacrylonitrile (PAN) maintains its particle removal efficiency. Thickness of the membrane has an effect on its transparency and a thin electrospun PAN filter membrane was able to show removal efficiency of more than 95% while allowing 90% transparency [Liu et al 2015]. The reason for the material, thickness and removal efficiency is not apparent although it may be due to the strength and rigidity of the fibers. Comparison with commercial filtration membranes showed much better quality factor in transparent PAN electrospun membrane [Liu et al 2015]

Air filter media especially those used in ventilation system may harbour bacteria or fungus growth as the air carrying their spores pass through the membrane. Having a membrane with anti-bacterial property and anti-fungal property may prevent proliferation of microbes and fungus on the surface of the membrane. Electrospun air filtration membrane has been loaded with anti-bacterial substances and shown to be effective in inhibiting bacteria growth while maintaining good filtration performance. 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 Wang et al (2016) loaded poly(lactic acid) with titania nanoparticles and this combination exhibits high antibacterial activity of 99.5 against Staphylococcus aureus with high filtration efficiency of 99.996% and relatively low pressure drop of 128.7 Pa. Victor et al (2021) conducted a bacterial filtration efficiency of antibacterial membrane through an Andersen sampler. The prepared filtration membrane was electrospun polyvinylidene fluoride (PVDF) blended with titanium nanotubes (TNT) and sandwiched between polypropylene (PP) nonwoven sheets. The filter media was first shown to exhibit antibacterial properties through zone of inhibition on agar gel cultivated with Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Bacterial filtration efficiency (BFE) was tested using S. aureus aerosol where aerosols of particle size in the range of 0.1-10 µm were passed through the prepared filter membrane. Optimized filter membrane showed a bacterial filtration efficiency of 99.88% with good antibacterial properties against both gram-positive and negative organisms. Using electrospun polylactide/polyhydroxybutyrate (PLA/PHB) nanofibers loaded with ammonium-based ionic liquid (IL) which is a quaternary ammonium compounds (QACs), the resultant fibrous mat was able to show inhibition against fungus, Aspergillus niger and Chaetomium globosum.

SEM images of the PLA/TiO2 fibrous membranes loaded with 1.75 wt% TiO2 Nanoparticles at 60% relative humidity [Wang et al. Journal of Nanomaterials, vol. 2016, Article ID 6272983, 17 pages, 2016. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

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 100Pa. 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.