Functionalized Fabrics using Electrospun fibers

Fabric with multiple functional properties.

High porosity and small pore size of electrospun non-woven membrane meant that fabrics made from it are highly breathable. Apart from properties offered by its physical profile, ease of functionalizing electrospun fibers and fabrication of electrospun fibers with chemical functionality has also attracted the interest of using it to provide protections against chemical and biological threats. These properties may also be useful in civilian setting and industries where people may be exposed to harmful materials and bacteria. Other functionality may be incorporated to create fabrics with useful properties such as self-cleaning ability.

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. There are some compounds exhibit multiple properties and this reduces the variety of additives required to imbue the fiber with multiple functionality. Inorganic material such as ZnO exhibit photocatalytic property where it absorbs UV rays and demonstrates self-cleaning ability. ZnO is also known to exhibit antibacterial properties. With the multi-functional properties of ZnO in mind, Khan et al (2019) incorporated its nanoparticles into electrospun polyvinyl alcohol (PVA) fibrous membrane. 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.

While prevention of bacterial growth is important, just as important is the ability to detect any increase in bacteria count. Colorimetric means of alerting an increase in bacteria activity are particularly attractive due to the convenience of visual cues. Kinyua et al (2022) constructed a colorimetric sensor made of electrospun chitosan/polyethylene oxide nanofibers (CS/PEO NFs) grafted with chromogenic substrate 5-bromo-4-chloro-3-indolyl-β-D-glucuronide (X-Gluc) for the purpose of detecting bacteria, Escherichia coli, activities. In the presence of the enzyme β-glucuronidase (β-GUS) from Escherichia coli (E. coli), hydrolytic cleavage of the chromogenic substrate X-Gluc leading to the release of an indoxyl derivative which forms a blue colored dichlorodibromo indigo dye in air. Comparing the reaction rate of CS/PEO NFs and CS/PEO hydrogel of the same mass and enzyme concentration, the higher surface area of CS/PEO NFs gave a much higher reaction rate and greater amount of dye released. The limit of detection for β-glucuronidase by the nanofibers was 13 nM compared to 25 nM of CS/PEO hydrogel. The lower limit of detection for the CS/PEO NFs was 13 nM compared to 25 nM CS/PEO hydrogel sensors. For the lower limit of quantification for the enzyme, CS/PEO NFs was 45 nM while CS/PEO hydrogel was 60 nM.

Electrospun mat may be used as stand-alone fabric but a potential weakness is the low rate of production. To overcome this, applying the nanofibers as a coating over a base substrate may offer a more immediate solution. Dhineshbabu et al (2014) deposited MgO/Nylon 6 hybrid nanofibers on cotton fabric using electrospinning with the hybrid nanofibers providing anti-bacterial and fire retardancy properties. MgO/Nylon 6 hybrid nanofibers mat was found to be effective in inhibiting both E. coli and S. aureus. Addition of MgO/Nylon 6 hybrid nanofibers on the cotton also increases its burning time and ash count. In the development of functionalized electrospun fibers for sodium ion sensing, Jaffal et al (2021) investigated the mechanical and sensing properties of nylon-6/multi-walled carbon nanotube (MWCNT) nanocomposite as potential fabric coating. Jaffal et al (2021) first electrospun nylon-6 into nanofibrous mat. The mat is then dipped into a suspension of multi-walled carbon nanotube (MWCNT) with surfactant. Finally, the MWCNT coated matis dipped in a calixarene solution and dried to form MWCNT/calixarene-functionalized nylon-6 mat for sensing sodium ion concentration. The optimized sensor has a fiber diameter ranging between 250 to 320 nm and covered with MWCNT length less than 200 nm. A maximum sensor sensitivity of about 45 µA/mM to sodium ions was obtained. For integration of the composite sensor to a fabric material, it needs to have sufficient strength and elasticity to withstand stretching during usage while maintaining sensor reading consistency. The nylon-6/MWCNT nanocomposite material has a statistically higher stress at break with no significant difference on elastic modulus compared to neat nylon-6. Interestingly, it was found that the flow rate for electrospinning has a significant influence on the mechanical and sending properties of the nanocomposite mat. An optimum flow rate of 1.0 ml/h for electrospinning was found with either higher or lower flow rate resulting in stiffer mat and poorer sensing response after multiple stretches. Further, the ultimate strength of the nanocomposite mat was comparable to that of common commercially available fabrics which makes integrated use easier.

With the prevalence of mosquito borne diseases and the faster life cycle of mosquitoes due to global warming, some researchers are looking into the possibility of incorporating anti-mosquitoes agents into fabrics. Electrospinning has the benefits of easy active agent incorporation and large surface area of the resultant nanofibers. 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. More tests are needed to determine the duration of repelling efficacy and stability of the compound in storage.

There are many different ways of functionalizing electrospun fibers. Blending is one of the most common methods of introducing functional additives to the fiber. Depending on the functionalizing method and application, the outcome of the results can be very different. Xiang et al (2020) added permethrin to electrospun nylon 6 fibers using blending, dip coating and drip coating after plasma treatment. In terms of loading efficiency, having permethrin blended into a nylon solution before electrospinning is better than surface dip coating onto the spun fibers with or without surface plasma treatment. Plasma treated electrospun nylon fibers have a much higher permethrin loading compared to fibers without. Permethrin coated on plasma treated electrospun nylon fibers showed better permethrin retention compared to blended nylon fibers with permethrin loss of 30? and 78% respectively. There may be several factors that lead to the much lower retention of blended fibers. Firstly, blended fibers would already have higher loaded permethrin and there may be a limited capacity in the fiber matrix to hold them. Secondly, adhesion of permethrin molecules to plasma treated nylon surface may be much stronger than permethrin molecules just blended into the nylon fibers matrix.

A protective clothing should be light weight 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.

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Large surface area of nanofibers makes it a potential candidate for incorporating self-cleaning agents for nanofibrous stand-alone fabric. This is due to the greater exposure of the cleaning agents to the environment and its proximity to the stain. Bedford and Steckl (2010) fabricated core-shell nanofibers with cellulose acetate as the core and a dispersion of TiO₂ nanoparticles as the sheath. This method showed better distribution and coating of TiO₂ nanoparticles on the surface of the nanofibers compared to simultaneous electrospraying. Consequently, the photocatalytic property of nanofiber coated with TiO₂ nanoparticles by core-sheath electrospinning is better compared to dispersion of TiO₂ nanoparticles on nanofiber by electrospraying.

For consumer market, a functional fabrics may come in the form of comfortable thermo-regulated textiles. Yi et al (2019) constructed an electrospun core-shell polyvinyl butyral (PVB) fibers with a phase change material (PCM) in its core. Such PCM is able to provide latent heat during phase transition at a constant temperature. The material used by Yi et al (2019) is octadecane due to its high latent heat and low supercooling degree, The sheath material is therefore made of polyvinyl butyral (PVB) while the core is pure octadecane. To maintain a molten octadecane, the electrospinning ambient temperature was kept at 50°C. The resultant composite was able to achieve a high latent heat up to 118 J g^-1. To facilitate conversion of solar to thermal energy, hexagonal cesium tungsten bronze (Cs_xWO₃, a near infrared absorber) was introduced to PVB. Cs_xWO₃ is also able to absorb infrared light to enhance user comfort. The multi-components electrospun fibers retained a high latent heat up to 96.9 J g^-1. A 100 thermal cycle between 25 and 50 °C showed a slight reduction in latent heat capacity. Song et al (2021) carried out further studies of electrospinning derived SiO₂/Cs_xWO₃ and its near-infrared absorption performance. They found that the near-infrared absorption performance of the SiO₂/Cs_xWO₃ membrane is influenced by the crystallinity of Cs_0.33WO₃ crystal with the best crystallinity showing the highest near-infrared absorption. Cs/W atomic ratio of 0.5 and a calcination temperature of 700 °C has the highest crystallinity and the best infrared absorption performance. Beyond this temperature at 800 °C, the crystals are destroyed leading to poorer absorption. The absorbance of near-infrared light at 780-2500 nm by Cs_0.33WO₃ membrane calcinated at 700 °C was 5.56 times that of purer SiO₂ fiber membrane.

With the growing interest in wearable technology, researchers are looking into the development of smart fabrics where electronic functionality is built into it. Ranjith et al (2020) constructed a flexible asymmetric supercapacitor (ASC) with high electrochemical performance using reduced graphene oxide (rGO)-wrapped redox-active metal oxide-based negative and positive electrodes. The negative electrode was made of rGO-wrapped tubular FeMoO₄ nanofibers (NFs) via electrospinning followed by surface functionalization. The cathode material was made of hydrothermally synthesized binder-free rGO/MnO₂ nanorods on carbon cloth (rGO-MnO₂@CC). To prepare the FeMoO₄ nanotubes, co-polymeric precursors polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA) were blended for electrospinning with the Fe and Mo sources. The electrospun FeMoO₄-PAN/PMMA nanofibers (NF) were thermally decomposed to produce tubular FeMoO₄ NFs. The prepared FeMoO₄ nanotubes were dipped into a solution of ultra-thin rGO nanoflakes followed by thermal treatment to bind the ultra-thin rGO to the the hollow FeMoO₄ nanofibrous surface. The rGO-wrapped tubular FeMoO₄ NF showed a specific capacitance of 135.2 F g^-1. Such high capacitance can be attributed to the mixed oxide states with a high surface area and the ultrathin rGO layers providing surface-conductive channels for electron transport. When combined with rGO-MnO₂@CC, the resultant flexible asymmetric supercapacitor (ASC) showed a specific energy density of 38.8 W h kg^-1 with efficient cycling stability and excellent rate capability. The promising performance of this ASC demonstrated its potential use as an integrated hybrid smart textile.

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.

Yarn made from electrospinning has also been made conductive and in this form, it may be made into smart fabrics and clothing. Weerasinghe et al (2020) uses electrospinning on a rotating funnel and drawing the deposited fibers into a yarn via a take up roller. The collected, twisted yarn is subsequently coated with a layer of polyaniline (PANI) using in situ chemical oxidative polymerization of aniline on the surface of the yarn. The electrospun polycaprolactone (PCL) yarn coated with PANI showed an electrical resistance of 6 kΩcm^-1. The conductive PCL/PANI yarn was able to demonstrate repeatable changes in electrical resistance when subjected to strain up to 20%. When the yarn is stretched, the electrical resistance increases exponentially as the PANI molecules were pulled further apart. This measurable change in electrical resistance occurred almost instantaneously hence giving it the potential for use as strain sensors for human motion monitoring.

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