Protective Clothing using Electrospun Fibers

Protective clothing is often a necessity for specific work environment. This may be battlefield, hospital or other occupational environment where there is a risk of exposure to chemicals, nano-particles or pathogens. Electrospun fibers with its high surface area and ease of functionalizing makes it an ideal candidate for the construction of protective clothings. Protective clothing made from electrospun fibers can be light-weight while exhibiting a wide range of functionality.

For people who are often exposed to the sun, an important function of the clothing is its ability to block off UV rays. UV rays may penetrate clothing although it shades the skin against visible light rays. Koozekonan et al (2021) tested the UV protection ability of electrospun polyacrylonitrile (PAN) loaded with multi-walled carbon nanotubes (MWCNT) and TiO₂ nanoparticles and both. Electrospun PAN fibers alone do not offer UV protection and 1% loading of either MWCNT or TiO₂ nanoparticles would offer good protection. However, to achieve excellent protection, more than 10% loading of either MWCNT or TiO₂ nanoparticles needs to be added. Higher concentration of TiO₂ needed to offer excellent UV protection is probably due to recombination of electrons and hole pairs. When MWCNT and TiO₂ are added together for electrospinning in PAN, the resultant composite nanofibers require only 1% of the additives to achieve excellent UV protection. MWCNT, being conductive, was able to transfer the photo-generated electron from the TiO₂ away and prevented immediate recombination of the electron-hole pair. This leaves behind the hole in the valence band of the TiO₂ for reaction.

Electrospun membrane has been shown to be effective in filtering out PM2.5 particles. While this is important to prevent inhalation of 2.5 µm particles, nanoparticles present a potentially higher risk as it may get directly absorbed through the skin.Faccini et al (2012) investigated the performance of electrospun polyamide 6 (PA6) in blocking nanoparticles for use as protective clothing. In their studies, the electrospun fibers were adhered to a viscose nonwoven textile by hot-melt lamination using a thermoplastic adhesive powder. For the thinnest membrane coating (coating duration of 5 mins), less than 50% of 200 nm nanoparticle penetrate through the membrane while only 20% of 20 nm particles penetrated through. At 60 mins of coating, less than 1% of the nanoparticles from 20 nm to 200 nm were able to penetrate through but this is at the expense of high pressure drop.

Figure 1: Schematic representation of the nanofiber-based protective textile. [Faccini et al. Journal of Nanomaterials, vol. 2012, Article ID 892894, 9 pages, 2012. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Increased traveling and flying between countries have also increased the likelihood of spreading life-threatening diseases. Having fabric incorporated with anti-bacterial materials will help protect the wearer against contracting diseases spread by physical contact. Cotton is one of the most widely used materials in making fabrics. Thus, the ability to fabricate cotton based nanofibers with anti-bacterial substances incorporated will raise its potential use in the future. Kampeerapappun (2012) was able to electrospin pre-treated cotton waste using trifluoroacetic acid as the solvent. Antimicrobial agent solution, 3-(trimethoxysilyl) propyldimethyl octadecyl ammonium chloride (AEM) was sprayed onto the electrospun mat to impart anti-microbial property to it. AEM is a quaternary ammonium salt which is amphiphilic and is able to chemically bond to the surfaces of the nanofiber mats. For the nanofibrous mat to exhibit anti-bacterial property, a minimum of 2% w/w AEM is needed to be added to the nanofibrous mat. It is important that the mechanical properties of the electrospun fibers are not weakened due to the anti-bacterial additives. Having anti-bacterial additives that are natural will also reduce the environmental impact when the fabric is discarded. Andersson et al (2014) used Lanasol, a brominated cyclic compound extracted from red sea algae, as anti-bacterial agent additive in poly(methyl methacrylate) (PMMA) and polyethylene oxide (PEO) nanofibers. A potential benefit of using Lanasol as additive is that it is a solution rather than a solid. Thus with high loading of up to 25 wt% of PMMA/PEO (75/25 wt %), there is no significant change in the fibrous mat strength. However, there is an increase in modulus and drop in toughness. With a loading as low as 4 wt%, the anti-bacterial effect against Staphylococcus aureus subsp. aureus is significant with 99.99% reduction in bacterial viability.

A protective clothing should be lightweight while still have the ability to protect the user against chemical and biological warfare agents. Agarwal et al (2012) investigated the detoxicification performance of zeolite embedded electrospun fibers against paraxon, a nerve agent stimulant. The zeolite was incorporated onto electrospun cellulose/polyethylene terephthalate (PET) blend nanofibers by simultaneous electrospraying of dispersed zeolite solution. The resultant hybrid nanofibrous mat with zeolite particles embedded on the surface of the nanofibers was shown to be effective in detoxifying the nerve agent stimulant. Another way of incorporating detoxification functionality in protective clothing is to coat the fabric with functional, inorganic nanofibers using electrospinning. Agarwal et al (2012) demonstrated a one-step synthesis of hollow strontium titanate (STO) and barium titanate (BTO) for detoxification of nerve agents. Coaxial electrospinning was used to generate the hollow fibers following sintering. Comparing the detoxification rate of nerve agent simulant, paraoxon, between hollow and solid fibers, it was found that hollow fibers showed better detoxification results despite larger diameter of the hollow fibers (1.5 µm diameter) compared to solid fibers (80 - 100 nm diameter). The better results may be attributed to higher surface area of hollow fibers which was 1.2 times higher than that of solid fibers.

In many countries, mosquitoes are vectors for dangerous diseases including malaria, dengue fever and Zika virus. Clothing offering protection against mosquitoes is important for any person venturing into known mosquito infested area or high risk region. In a demonstration of the potential use of electrospun fibers for protection against mosquitoes, Teli et al (2017) encapsulated citronella into electrospun polyvinyl alcohol (PVA)/starch nanofibers. The resultant fabric was tested to be effective in reducing mosquito bites. Ciera et al (2019) examined the effectiveness of electrospun polyvinyl alcohol (PVA) nanofibers loaded with mosquito repellent, permethrin, chilli and catnip oil respectively. Tests using A. gambiae s.s. mosquitoes showed that all the repellents significantly reduce the number of mosquito landings compared to the control (PVA nanofibers without repellent) with chili and catnip oil in PVA nanofibers showing mosquito landing reduction of 51% and permethrin with 89% reduction. This shows the effectiveness of electrospun mosquito repellent loaded nanofibers in repelling mosquitoes. Xiang et al (2020) conducted several tests on electrospun nylon 6 fibers functionalized with permethrin for potential use as mosquito repelling fabric and comparing its performance against commercial nylon fabrics. In terms of loading efficiency, having permethrin blended into a nylon solution before electrospinning is better than surface dip coating onto the spun fibers or fabrics with or without surface plasma treatment. Permethrin coated on plasma treated electrospun nylon fibers showed the best permethrin retention compared to permethrin coated on plasma treated nylon fabrics. The former lost 30% of permethrin while the latter two lost 63% to 76%. Electrospun nylon fibers with permethrin blended into its matrix showed the highest loss at 78% although this may partly be due to much higher permethrin loading into it compared to plasma treated and dip coated fibers. All permethrin treated fibers and fabrics showed increased mosquito repellency compared to untreated counterparts although it is interesting to note that electrospun fibers had the lowest repellency compared to the fabrics. This is despite plasma treated electrospun fibers having the highest loading of permethrin compared to plasma treated fabrics. It seems that good retention of permethrin on electrospun fibers as demonstrated by the washing test may have a slower permethrin release rate and this resulted in a lower mosquito repellency. On the other hand, the mosquito repellent usage life of electrospun fibers may be longer although this has yet to be tested.

In healthcare services, medical workers are at a high risk of contracting infections especially in times of emergency where proper hygiene and infection control services are hard to come by. Therefore, a multifunctional surgical gown will offer an excellent first line of protection. Khan et al (2019) aims to develop a surgical gown that is antibacterial, self-cleaning and offer UV protection. To achieve, they incorporated ZnO nanoparticles into electrospun polyvinyl alcohol (PVA) fibrous membrane. ZnO exhibit photocatalytic property where it absorbs UV rays and demonstrates self-cleaning ability. ZnO is also known to exhibit antibacterial properties. The PVA/ZnO electrospun fibers demonstrated a 98% self-cleaning efficiency within 3 h for methylene blue dye under strong UV ray. Since ZnO strongly absorb UV radiation, this property also enables it to provide UV shielding. PVA/ZnO nanofibers showed nearly zero UV transmission compared to 20% transmission in neat PVA nanofibers. Antibacterial property of PVA/ZnO nanofibers were tested with staphylococcus aurous and Escherichia coli were used as model bacteria. Neat PVA nanofibers membrane does not exhibit any zone of inhibition whereas PVA/ZnO nanofibers membrane showed distinct zone of inhibition for both bacteria strains with the highest loading of ZnO (9 wt%) showing the largest zone of inhibition.

A function of military uniform is to provide protection and comfort for the soldier from the environment. In deserts and regions where the daytime temperature can be very high, it is very beneficial if the uniform can have a cooling effect. Silk fibers are known to exhibit strong light scattering characteristics which helps to protect pupae from overheating from direct sunlight. Parks et al (2021) showed that with electrospun silk fibroin membrane the temperature of the underlying substrate was 7.5 °C lower than nonwoven raw silk fabric during daytime under solar radiation. Smaller diameter electrospun silk fibroin fiber (250 nm) was found to have a greater cooling effect than larger diameter electrospun silk fibroin fiber. The mid-infrared emissivity of electrospun nanosilk was found to be greater than raw silk. Electrospun silk fibers may be used in clothes to give a passive cooling effect on the wearer.

Due to the small diameters of electrospun fibers, the membrane is typically very thin compared to conventional fabrics. Electrospun membranes have been functionalized for different purposes and having multiple layers of membranes with different properties may be advantages. Zhang et al (2023) constructed a multilayered membrane with electrospun silk fibroin nanofiber (SFNF) as the base layer, an electrospun polyurethane nanofiber with silver nanoparticles (PUNFs-AgNPs) middle layer and a polyurethane nanospheres top layer. Silk fibroin is hydrophilic, having it as the base layer gives the membrane a more comfortable skin contact touch. The middle layer containing AgNPs gave the membrane antibacterial functionality and has been shown to inhibit gram negative (E. coli) and positive (S. aureus) bacteria.The top or outer layer was built with polyurethane nanospheres to give the membrane antifouling and self-cleaning property. Without the polyurethane nanospheres, the middle PUNFs-AgNPs layer had a water contact angle of 136.7° which is insufficient to achieve superhydrophobicity. With the polyurethane nanospheres, the water contact angle was increased to 152.9°. Carbon powders covering the surface of the membrane can be rinsed off using water hence demonstrating the self-cleaning property of the membrane.

Protective clothes are designed according to the environment and situation which they are used in. In an extreme environment where the user is exposed to high heat radiation, the fabric or material used to construct the protective clothing needs to be flexible while tolerating high temperature. Most polymers used in electrospinning have relatively low melting points. High performance polymers that can withstand high temperatures are generally more challenging to electrospin into fibers. Commarieu et al (2022) found that polynorbornenes (PNBE) produced by catalytic polymerization of functionalized norbornenes can be made into ultra-high Tg thermosetting materials. The PNBE can be dissolved in common solvents and easily electrospun into fibrous mesh. Cross-linking agent has been added into the solution for electrospinning for fiber processing. PNBE with an epoxy pendant functional group can be easily cross-linked using isophorone diamine (IPDA) and curing at 200 °C to form a thermoset with decomposition temperature above 340 °C. PNBE bearing pendant carboxylic acid groups can be cross-linked by curing at 200 °C using butanediol diglycidyl ether (BDE) as the cross-linker to form a thermoset with decomposition temperature above 350 °C. Following the curing and cross-linking process, the resultant fibrous mesh is impervious to swelling by water or any organic solvent. Dyes may also be added into the electospinning solution and the processed fibers were shown to be stable with no leakage of the dye.