Applications for water-soluble nanofibers

An obvious advantage of using water-soluble polymers in electrospinning is that the vaporized water is non-toxic to human and the environment when discharged. However, the disadvantage is that the resultant nanofibers are relatively weak and generally have shorter lifespan than solvent-based nanofibers. Despite this limitation, there are still many usage for nanofibers made from water-soluble polymers. In air filtration, electrospun polyvinyl alcohol (PVA) which is water soluble is commonly used for this application [Li et al 2013]. PVA may also be used as an oil clean up membrane such as in vehicle filters. PVA is a water soluble polymer with lipophilic properties which makes its processing environmentally friendly and having the property to adsorb oil. Since vehicle engine environments are mostly dry and filled with oil, water soluble polymer will not be adversely affected by the operating condition. Ge et al (2021) found that optimized electrospun PVA membrane was able to adsorb up to 12 g/g of engine oil. Small pore size was better able to trap oil droplets and smaller fiber diameter will have larger surface area to contain the trapped oil. The interconnected fibers also facilitate oil adhesion by spreading the oil droplet through capillary action while high porosity and interconnected pores allows oil penetration and storage within the pores.

Water-based polymers are great for applications where the nanofibers are used as a carrier for later release. Water soluble polymers such as polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) has been tested as quick release agent for oral drugs. High surface area of nanofibers encourages faster dissolution of the polymers compared to cast films. Domján et al (2020) used electrospun water soluble 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) fibers as a carrier for quick drying of infliximab antibodies. In this case, electrospinning is tested for use as a drying technology for encapsulating biological agents. Conventional technology such as freeze drying may affect the viability of proteins and other biological agents. While many of these biological agents are administered in a solution form, having them dried during storage can improve their shelf-life. High surface area of electrospun fibers and using water soluble polymers as encapsulation agents meant that the loaded fibers can be dissolved quickly for reconstituting the biological agent. Infliximab remained viable with no sign of degradation after electrospinning with HP-β-CD. The electrospun HP-β-CD fibers containing infliximab were able to completely dissolve in water after 120s without any vigorous mixing. This makes it convenient and fast to reconstitute infliximab prior to usage.

Water-soluble polymers are also attractive in the food industry. Sutjarittangtham et al (2014) successfully electrospun natural tapioca starch solution using deionized water in a modified electrospinning setup. In their setup, the metal target is placed in a -20 °C cooled ethanol solution to accelerate the dehydration process. The optimum concentration for producing fibers was found to be 4.5 wt%. Concentration below that produces mat with areas of completely fused fibers while higher concentration resulted in clogged spinneret due to high viscosity. Fuenmayor et al (2013) used pullulan, a polysaccharide, as the carrier for β-cyclodextrin and limonene for use in food packaging applications. Mixing the particles into pullulan solution formed water-in-water emulsion (cylcodextrin rich aqueous microdroplets dispersed within aqueous pullulan rich phase). The stability of the volatile limonene was confirmed when there was no further loss up to 45 days after the initial release of excess limonene. Similar method (using β-cyclodextrin) was used to encapsulate other aroma compound like perillaldehyde [Mascheroni et al 2013].

Wrapping of fresh food with electrospun membrane containing nutrient preservation or antimicrobial substances.

Another area where water soluble polymers may be used is as temporary templates. These typically use the nanofibers as a carrier, a base template for forming tubes or to form network features in a matrix. Non-water soluble beads or nanorods may be displayed into an aqueous polymer solution for electrospinning. The electrospinning process would arrange the beads and rods along the axis of the spinning jet [Lim et al 2007]. Sintering may be carried out to remove the matrix template and fuse the beads. Electrospun fibers are also useful for forming tubes. Coating or layering process may be used to build a layer of the desired material over the nanofibers. The inner nanofibers core may be removed to form tubes.

Continuous fibers of electrospun membrane make it an ideal candidate to create either discrete or interconnected channels within a polymer block that are in the nanometer and submicrometer range [Gualandi et al 2013]. To create this structure, the polymer matrix needs to be a sufficiently fluid such that it can fill the spaces between the fibers. A vacuum pump may be used to remove any air pockets in the electrospun membrane. After the polymer matrix has cured, the electrospun fibers may be removed by dissolving in suitable solvent to form interconnected channels and tubes within the polymer block. Another way of removing the template or sacrificial material is through thermal depolymerisation. This method involves using thermal treatment for vaporization of the sacrificial template such that little or no residues are left. Gergely et al (2015) used electrospun poly(lactic acid) treated with tin catalyst as the sacrificial material to form hollow channels in epoxy, a thermosetting polymer. The presence of tin catalyst helps to lower the depolymerisation temperature. The hollow channels created by sacrificial electrospun fibers in the epoxy matrix resembles natural vascular network which may be used in various engineering applications.

Perhaps the most commonly used application for water-based polymers is the fabrication of inorganic fibers. Inorganic precursors are usually salt that dissolves in water and ethanol. To facilitate electrospinning into fibers, water-soluble polymers are often added to form a precursor mixture. Electrospinning is carried out to form precursor fibers. Post-spinning sintering is then used to convert the precursor into inorganic oxide with the polymer burnt off in the process. Another use of the nanofibers is to form tubes. The desired material may be layered on the water soluble fibers through processes such as sputtering for inorganic material. The nanofibers template may be burnt off or washed away using water. Similar method may be employed where electrospun fibers mesh are used to form a network of channels in a matrix.

For applications which requires a certain level of structural stability. Water-based nanofibers may still be used by using post-spinning process to stabilize the structure and improve its mechanical strength. A common method is to use chemical cross-linking. Nada et al (2016) used a blend of hydroxyethyl cellulose (HEC) and polyvinyl alcohol (PVA) for loading of topical drug, nicrotinamide and both HEC and PVA are water soluble. Citric acid/sodium hypophosphite system was used to chemically cross-link HEC and PVA. The cross-linked electrospun HEC/PVA fibers were found to be more stable as demonstrated by much slower release rate of the drugs. PVA electrospun nanofibrous membrane may also be cross-linked for better stability in air filtration applications. Qin et al (2008) used maleic acid for the reaction and vitriolic acid was used as a catalyst activator during crosslinking. The filter media with PVA nanofibers were found to significantly improve filtration efficiency. Instead of using cross linking agent, another method is to blend two polymers with reactive groups that can be cross-linked under suitable stimulus. Liu et al (2018) used a combination of poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA) and carboxyl-functionalized graphene oxide (GO-COOH) for electrospinning into nanofibers. Both PVA and PAA are water soluble polymers and are regularly electrospun into nanofibers. The electrospun membrane was heated at 120 °C to induce cross-linking between carboxyl acid groups in PAA/GO-COOH components and hydroxyl groups in PVA molecules which renders the treated membrane insoluble in water. This membrane was further modified using AgNO₃ solution with ascorbic acid to immobilize silver nanoparticles on its surface. This membrane showed good photocatalytic capacity in the catalytic degradation of methylene blue dye solutions. Pullulan is a linear polysaccharide produced by strains of fungus Aureobasidium pullulans and exhibit advantages properties such as non-toxic and edible which makes it widely used in food or biomedical field. Pullulan is water soluble thus cross-linking may be used to stabilize the structure. Instead of cross-linking as a post-electrospinning process, Li et al (2019) blended cross-linking agent glutaraldehyde (GA) and sulfuric acid (H₂SO₄) as the catalyzer for in situ cross-linking electrospinning of pullulan solution. As cross-linking is initiated prior to electrospinning, an optimum amount of GA is necessary to ensure that the viscosity is not too high for electrospinning while sufficient cross-linking takes place such that the resultant membrane is insoluble in water. Although the combination of electrospinning and cross-linking reduces the need for an additional post-electrospinning step, the disadvantage of this technique is that the initiated cross-linking of the solution reservoir for electrospinning meant that the solution cannot be left for electrospinning over a sustained period of time. Photocrosslinking after adding cross-linking agents to the solution and electrospun into membranes is another method that may be utilized to produce water insoluble membranes. De Castro et al (2020) used maleic anhydride (MA) as a cross-linking agent in a solution of hyaluronic acid/polyvinyl alcohol (HA/PVA). All electrospun membranes were subsequently placed in a UV light reactor to activate the photocrosslinking process The amount of cross-linking agent is important to ensure water insoluble electrospun fibers are produced. With maleic anhydride (MA) concentration lower than 30%, the HA/PVA composite fibers were unstable with mass loss recorded when 15% MA and lower concentration were used. Above 30% of MA cross-linker, electrospinning cannot be carried out as the stretched solution jets break up into droplets. Only with 30% of MA cross-linker added to the polymer solution was the resultant electrospun fiber able to maintain their mass until 72 h. A freeze thawing process may be used for physical cross linking of certain polymers and it does not require the addition of any potentially harmful cross-linking agents. Nagakawa et al (2020) demonstrated the feasibility of using freeze thawing to improve the stability of electrospun polyvinyl alcohol (PVA) and glycerol (Gly) nanofibers in water. Freeze thawing improves stability of PVA by encouraging the formation of inter- and intramolecular hydrogen bonds and crystallites during the freezing process where ice crystal formation aggregated the PVA chains. Gly addition was also found to improve stability of electrospun PVA chains by forming more hydrogen bonds in the crystal structures. However, only freeze thawed electrospun PVA/Gly nanofibers were found to be stable in water. Although freeze thawing of electrospun pure PVA nanofibers showed approximately the same level of crystallinity improvements as PVA/Gly nanofibers, only PVA/Gly nanofibers were stable in water which showed the importance of hydrogen bonding between PVA and Gly molecules. Gelatin is a natural protein that exhibits good biocompatibility, biodegradability and is readily available commercially. This protein may be dissolved in water at elevated temperature for electrospinning but its water solubility limits its applications. Etxabide et al (2022) showed that it is possible to blend cross-linking agents into the gelatin solution to render the electrospun membrane insoluble in water. Using the Maillard reaction (MR), a condensation reaction between proteins and sugar, Etxabide et al (2022) blended ribose (a sugar) into the gelatin solution prior to electrospinning. With the addition of ribose, the resultant gelatin fibers were insoluble in water although the fibrous morphology was compromised when the electrospun membrane was soaked in water at 37 °C. Heat treatment of the ribose/gelatin fibers induces cross-linking and with heat treatment temperature of 110 °C and a ribose concentration of 20 wt% the electrospun ribose/gelatin membrane was able to retain the fibrous morphology after soaking in 37 °C water for 24 h although fusion of fibers still occurred.