Introduction to Electrospun Fibers for Defence Technology

Fabric with electrospun membrane layer.

High surface area of electrospun nanofiber and its ease of functionalizing have made it an ideal material for use in defence technology especially in the area of decontamination and protection against chemical and biological agents. Almost any equipment or apparels that give protection to the soldier will benefit from the use of nanofibers. The most common form of personal protective equipment (PPE) is the protective jumpsuit and the facemask. Currently, activated charcoal in the form of cloth or pellets is often used for the removal of chemical toxicants. For facemask, a separate filtering layer is needed for removing particulate matters.

Electrospun membrane offers several advantages over conventional activated charcoal based PPE. Electrospun membrane is already used commercially as an air filter media. When functionalized with detoxification property, this membrane is able to perform two functions simultaneously. This potentially cuts down on the weight of the facemask by eliminating the need for two separate materials. High porosity and small interfiber pore size also makes electrospun membrane a possible candidate for making breathable fabric. Electrospinning has already been used for making clothing and apparels from a US startup company called Electroloom in 2015 [White et al 2015].

Protection against chemical agents

During war, there is often the threat of chemical agents used in the battlefield. Protective jumpsuit using activated charcoal is usually heavy and bulky. Functionalized fabrics using electrospun nanofibers or electrospun nanofibers as one of the components may give rise to a new class of protective clothing that is lightweight, waterproof and breathable while offer the same or better protection against chemical agents. 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 mimicks 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.

Protection against biological agents

Electrospun membrane has been functionalized with anti-bacterial and anti-viral properties for various applications including healthcare and in military use. Different types of active compounds have been added to electrospun nanofibers and are shown to be effective against bacteria. Inorganic additives such as silver nanoparticles, MgO and CuO are known for their antibacterial property. Electrospun nanofiber membrane containing silver nanoparticles [Yuan et al 2010], MgO [Dhineshbabu et al 2014] and CuO [Haider et al 2015] have been shown to be effective in inhibiting bacteria growth. Incorporation of these additives is usually achieved through direct blending into the electrospinning solution [Yuan et al 2010; Haider et al 2015].

Organic compounds with anti-bacterial properties have also been used for loading into electrospun fibers. Kim et al [2007] selected quaternary ammonium salt (benzyl triethylammonium chloride, BTEAC) to be blended with polycarbonate solution for electrospinning. The resultant fiber diameter was reduced significantly from more than 8 µm to about 1 µm with improved fiber uniformity. The electrospun membrane exhibited antibacterial property with good filtration performance.

Electrospun fibers containing drugs such as chlorhexidine have also shown good anti-bacterial functionality. For drugs loaded into the fibers by blending, a zone of inhibition is seen on the bacteria culture plates where the drugs have leached out and kill the bacteria [Chen 2009]. It is important to note that once the drug level in the fiber has fallen below a certain level, its anti-bacterial property will also diminish. Therefore, it may be advisable to chemically bond the anti-bacterial agent to the fiber polymer matrix.

Apart from the ability to neutralize biological agents, another important consideration is on detection. Color changing sensors in the presence of biological agents will be very useful in detecting and raising countermeasures. Yapor et al (2017) showed that polydiacetylenes (PDAs) coupled with either electrospun poly(ethylene oxide) (PEO) or polyurethane (PU) was able to demonstrated colorimetric changes to the presence of Escherichia coli ATCC25922 bacterial cells. The composite fibers where constructed by blending 10,12-pentacosadiynoic acid (PCDA) with a supporting polymer, poly(ethylene oxide) (PEO) and polyurethane (PU), and the blended solution was electrospun to produce fiber composites. The electrospun fibers were subsequently photopolymerized using UV irradiation to form PEO/PDA and PU/PDA. When the nanofibers membrane was exposed directly to E. coli ATCC25922 grown on Luria-Bertani agar, there was a rapid colorimetric response. The mechanism for the color change has been attributed to endotoxins released by Gram-negative bacterial strain which disrupts the hydrogen bonding within the PDA molecules and causes color change.

Novel Application

The relative ease of generating fibers from different materials creates novel application for electrospinning and its fibers. Hussein et al (2022) used electrospinning to increase the safety of highly sensitive explosives by converting the material into fibers. The explosive substance, Cis-1, 3, 4, 6-tetranitrooctahydroimidazo-[4,5-d] imidazole (BCHMX) was mixed with polystyrene (PS) to form a solution followed by electrospinning. The resultant PS/BCHMX (40:60) fibers showed significant reduction in its impact sensitivity and friction sensitivity compared to pure BCHMX. Such reduction has been attributed to coverage of BCHMX explosive crystals with PS molecules in the electrospun fibers. The electrospun PS/BCHMX fibers was insensitive to external mechanical stimuli thus electrospinning may be used to increase the safety of explosive materials which would otherwise be highly sensitive. As with the results from encapsulation of Cis-1, 3, 4, 6-tetranitrooctahydroimidazo-[4,5-d] imidazole (BCHMX) in polystyrene (PS) through electrospinning, Abdelhafiz et al (2022) used polystyrene (PS) for encapsulating 1,3,5-trinitro-1,3,5-triazinane (RDX) by blending to form a solution and electrospun into nanofibers. RDX is a solid high explosive and the resultant PS/RDX nanofibers showed low impact and friction sensitivities. Examination by FTIR suggested that RDX crystals were fully encapsulated by PS fibers in agreement to the results from electrospun PS/BCHMX fibers. Thermal analysis showed that the PS was able to absorb heat and increased the decomposition temperature of RDX in the PS nanofibers by 19.2 °C which facilitated the reduction in impact and friction sensitivities of the composite fibers compared to pure RDX. This makes the PS/RDX nanofibers safer for handling compared to pure RDX.

Summary

There are a few ways of fabricating a fabric or filter media using electrospun nanofibers that offers protection against both chemical and biological agents. Since electrospun nanofibers can be easily functionalized by blending, having both additives in the same nanofiber will be the most direct way to imbue the fiber with both properties. Alternatively, nanofibers with specific functionality may be electrospun alternatively to form a layer by layer composite structure [Chen 2009]. This way, there will not be any interference in the reaction of both additives.

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