Electrospinning of Sustainable Materials

With increasing emphasis on reducing carbon footprint, there is a growing interest in using sustainable materials in electrospinning to produce nanofibrous products for various applications. Sustainable materials mostly come from plants, animals or bacteria. Although it is preferable that the electrospun product is made out purely of the sustainable material, it is generally challenging to electrospin natural polymers into fibers without a supporting polymer.

Polylactide (PLA) is a commonly used polymer for electrospinning into nanofibers for medical application. It is biodegradable and has already been used commercially in medical devices. The monomer of polylactide comes from fermented plant starch such as from corn, sugarcane and cassava. Industrial polymerization methods are then used to form high molecular weight PLA. There are many solvents such as chloroform, dichloromethane and 1,1,1,3,3,3-hexafluoro-2-propanol that can be used to dissolve PLA for electrospinning into nanofibers [Inai 2005].

Chitosan is a polysaccharide made from chitin shells of crab, shrimp and other crustaceans. It has several applications such as agriculture, natural antibacterial agents and medical scaffolds. Most reports on electrospinning chitosan were based on blending with other carrier polymers. However Ohkawa et al (2004) was able to electrospin pure chitosan from a solution with trifluoroacetic acid (TFA) and dichloromethane as the solvents. Dissolving chitosan in TFA may have facilitated electrospinning as the amino groups of the chitosan can form salt with TFA which blocks the intermolecular interactions between chitosan molecules [Sun and Li 2011]. Christ et al (2022) investigated the possibility of using fungal chitosan (CS) as the main component in a water-insoluble electrospun fibrous membrane. Prior to electrospinning, the fungal CS was modified to incorporate photo-reactive arylazide (Az) groups (CS-Az). Az group on the CS allows the fibers to be crosslinked using UV irradiation after electrospinning. A small amount of ultrahigh molecular weight poly(ethylene oxide) (UHMWPEO) (10 wt%) was mixed with the modified-CS in the solution to facilitate formation of fibers by electrospinning. The electrospun membrane was cross-linked by UV irradiation for 120s. For electrospun unmodified or modified CS without cross-linking, the fibrous structure was lost after immersing in water for 60s while the fibers dissolved in acidic media after 60s. Electrospun CS fibers treated in alkaline media were able to withstand contact in water but not in acidic media. UV cross-linked CS-Az electrospun fibers showed no fusion or swelling when incubated in water with pH from acidic to basic. Storage of the fibers in water for 30 days also showed no changes in morphology hence demonstrating the stability of the membrane in water.

Natural rubber (NR) is derived from the latex of rubber trees and is flexible, highly elastic and relatively low cost. Dognani et al (2020) demonstrated the electrospinning of pure NR by dissolving it in toluene and spiking the solution with ethanol. It is necessary to spike the solution with ethanol to introduce electrical conductivity to it for electrospinning. They were able to successfully electrospin NR fibers although the fiber diameter is quite thick at 5.5 µm.

Cyclodextrin (CD) is a small molecule that is prepared by enzymatic treatment of starch and it has an internal hydrophobic cavity and external hydrophilic surface. This allows it to form complexes with water insoluble molecules and be soluble in aqueous media. Hence CD is commonly used for drug encapsulation. For electrospinning, CD is often blended into a polymer solution before electrospinning to form fibers. However, under the right concentration and parameters, it is possible to electrospin modified CDs. Topuz et al (2020) demonstrated the ability to electrospin hydroxypropyl-β-CD (HP-β-CD) and HP-γ-CD by dissolving them in water containing quaternary ammonium salt, tetraethylammonium bromide (TEAB), to increase the solution conductivity. Unlike polymers, CDs are oligomers and their solution viscosity does not come from chain entanglements. Electrospinning is possible due to hydrogen bond interactions among CD aggregates at high concentrations. Topuz et al (2020) was able to get beads free fibers at solution concentration of 180 w/v % and diameters less than 400 nm with 1 wt% (with respect to CD) TEAB in the solution. However, such small molecules are sensitive to additives that disrupt the hydrogen bonding which will cause beads formation or breaking up of the spinning jet into spraying instead.

Zein, a prolamine protein found in maize, has been explored as a potential material for many applications in particular food packaging. Electrospun zein has been used to encapsulate Gallic acid, a naturally occurring phenolic acid which is known to exhibit anti-inflammatory and antimicrobial. Since zein is a naturally occurring protein, the electrospun packaging is fully made out of natural material. The electrospun mesh has been found to be effective against S. aureus and E. coli although it is only moderately effective against C. albicans with a log reduction of 1 to 2. The electrospun packaging material was found to be stable after 30 days of storage at 60 °C [Neo et al 2013].

Polyurethane has a wide range of mechanical properties which make it a popular material for many applications. Lee et al (2020) used castor oil (CO) in the production of biopolyurethane. CO can be used as polyol without any further functionalization due to the presence of hydroxyl groups. Addition reaction of CO and polycaprolactone-diol (PCL-diol) gave rise to biopolyurethane. To construct a bio-based antibacterial membrane, triclosan (TR) was used as the antibacterial agent. To solubilise TR, α, β and γ-cyclodextrin (CD) was tested as a TR carrier in the biopolyurethane matrix. Their test showed that only γ-cyclodextrin (CD) forms a full complex with TR for maximum solubility. Tests against S. aureus and K. pneumonia showed that biopolyurethane/TR/γ-CD cytostatic efficiency of more than 99.9%. Without γ-CD, the cytostatic efficiency was 97.0% and 98.1% against S. aureus and K. pneumonia respectively. Hence, the presence of γ-CD helps in the solubilising and release of TR in the media.

Electrospinning of BPU/TR-CD complexes [Lee et al 2020].

Starch is an abundant, low cost, biodegradable and renewable polymer. Zhu et al (2022) explored the potential of using starch as food packaging material. They were able to electrospin pure starch by dissolving the starch in dimethyl sulfoxide (DMSO) at 70 °C before cooling to room temperature. Electrospinning was carried out at an ambient temperature of 60 °C to facilitate fast vaporization of the solvent to form distinct fibers. Post spinning cross-linking using glutaraldehyde (GTA) vapor was necessary to render the membrane insoluble in water and to increase its tensile strength from ~0.66 MPa to ~9.65 MPa.

Poly(ethylene 2,5-furandicarboxylate) (PEF) is a bio-based alternative to petroleum-based polymers, especially poly(ethylene terephthalate) (PET). The components for synthesizing PEF, 2,5-furandicarboxylic acid (FDCA) may come from fructose or glucose, and bio-ethylene glycol can be obtained from bio-ethanol or hydrogenolysis of sugars. Electrospinning of PEF solutions using trifluoroacetic acid (TFA) as the solvent and at concentration of 8 wt% and lower was able to produce smooth, defect free fibers with average diameter below 240 nm. The resultant electrospun membrane has a water contact angle of 144° to 146°. Despite such high water contact angle the droplet adhered to the membrane surface instead of rolling off which is known as the rose petal effect.

Other than the use of sustainable polymers in electrospinning, researchers have also explored the use of "green" solvents. For electrospinning of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), Lapomarda et al (2022) dissolved the polymer in solvents derived from levulinic acid (LA, 4-oxopentanoic acid) which itself was produced from both hemicellulosic and cellulosic fractions of biomass. From the selection of solvents, it was found that a combination of 50% v/v formic acid and 50% v/v methyl ethyl ketone (MEK) was able to dissolve PHBV and produce smooth fibers through electrospinning. However, the mixture of solvents and PHBV needs to be heated to 60 °C for complete dissolution and during the electrospinning process, the solution was maintained at the same 60 °C to ensure sufficient evaporation of the solvents upon fiber deposition so that distinct fibers can be collected.

Apart from the selection of sustainable material, researchers have also explored the use of such materials in various applications. Rubio-Valleet al (2022) explored the use of lignin/cellulose acetate fiber-bead electrospun structures as vegetable lubricating oil. Eucalyptus Kraft lignin (EKL) was selected as it is not a food crop and there is an abundance of the tree and cellulose derived cellulose acetate (CA). When immersing EKL/CA in castor oil, it was found that only nanostructures in the form of fiber were able to form a uniform and stable gel-like dispersion. Electrosprayed particles only gave unstable dispersion and eventual separation of oil from the particles. Hence it is shown that a fiber form is essential to get a good uniform dispersion as the entangled fibers are unable to separate from each other. Between beaded and smooth fibers, When used as a lubricant, it was also found that oleogel with beaded fibers release more oil compared to smooth fibers and this has been attributed to the smaller fiber diameter between beads and softer texture which makes it easier for the oil to be released.