Chemical cross-linking of electrospun fibers

For applications which requires a certain level of structural stability. Water-based nanofibers may still be used by using a 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.

High performance electrospun polymer mesh with high thermal stability may be produced using cross-linking. 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.

The ideal green electrospun fibers would be one where the material and the reagent used in cross-linking is from a natural source. Cecone et al (2022) demonstrated the feasibility of electrospinning maltodextrin-based microfibers. Maltodextrins are water-soluble, high molecular weight polysaccharide molecules derived from starch hydrolysis. Using maltodextrin of molecular weight ranging from 20 kDa to 272 kDa, fibers can be produced from electrospinning at concentrations of 50 wt% to 62.5 wt% with higher concentrations needed for lower molecular weight maltodextrins. Since the resultant microfibers are water soluble, citric acid was added as a cross-linking agent. Citric acid was added to the maltodextrin solution for electrospinning. The carboxylic groups on citric acid molecules were able to bind with the hydroxyl groups on maltodextrins through condensation reaction resulting in ester formation. This cross-linking process is initiated by heat treatment of the electrospun fibers at 180 °C. It was found that higher molecular weight maltodextrins were less affected by the cross-linking process compared to low molecular weight maltodextrins and were better able to maintain their fibrous morphology. The resultant cross-linked maltodextrin fibrous membrane still contains some water soluble low molecular weight fractions which are removed by washing of the cross-linked fibers. Cross-linked higher molecular weight maltodextrin fibers were able to retain more mass after the washing process with less than 13 wt% loss and with 33.3 wt% citric acid added prior to heat treatment. 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.

Reactive electrospinning differs from conventional electrospinning in that the polymer molecules in the solution are reactive and capable of further reaction during electrospinning under appropriate condition. A common form is to pass the electrospinning jet through ultra-violet (UV) rays such that the polymer molecules in the solution undergo further polymerisation before deposition on the collector. Some advantages of reactive electrospinning are the speed of cross-linking and possibly lower toxicity compared to conventional chemical cross-linking. Ismail et al (2018) demonstrated this technique using acrylated poly(decanediol-co-tricarballylate) polymer (APDET) as the cross-linking polymer, polyvinyl pyrrolidone (PVP) as the carrier polymer and photoinitiator, 2,2-dimethoxy-2-phenylacetophenone with UV photo-radiation. An optimum concentration of APDET was needed to ensure proper fiber formation and beyond which the solution starts to solidify at the needle tip without electrospinning jet formation. Cell viability study using H9C2 cardiomyoblasts showed that the resultant poly(decanediol-co-tricarballylate)-based electrospun fibers mesh is biocompatible.

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