Photo cross-linking of electrospun fibers

Cross-linking is commonly used in electrospun fibers to improve mechanical properties and stability in an environment. Chemicals used in cross-linking are often toxic and there is always concern about residual cross-linking agents. Photo cross-linking provides an alternative option to create molecular bonding between the fibers or substrates through the use of light hence avoiding the use of potentially toxic cross-linking agents. To enable photo cross-linking, the material to be photo-reactive or contains photo-reactive moieties.

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 its mass until 72 h.

Polymer for electrospinning may be modified such that in situ process or post electrospinning process may be used to transform the fibers into insoluble form. One concept is to incorporate photo-crosslinkable moieties into the base polymer molecule. UV treatment may be carried out during electrospinning or post electrospinning to cross-link the molecular chain thereby making the fibers insoluble. Zhang et al (2017) bonded methacrylic groups on zein and the modified zein remains soluble in ethanol. An electrospinnable solution of the methacrylated-zein (m-zein) was prepared using ethanol aqueous solution concentration of 50 - 80%. During electrospinning, the jet was irradiated by UV light. Post-electrospinning irradiation was carried out for a further 5 minutes to ensure sufficient cross-linking of the fibers. The treated fibers were found to be stable in water and ethanol solution and therefore have the potential to be used in aqueous environment. Yerkinbekova et al (2022) mixed polyethylene glycol diacrylate (PEGDA) and hydrolyzed 3-(Trimethoxysilyl)propyl methacrylate (HMEMO) as cross-linking agents into a solution of maleated lignin (ML) and poly(acrylonitrile) (PAN) for electrospinning into PAN/ML/HMEMO/PEGDA (PMHP) nanofibrous membrane to be used as lithium ion battery separator (LIB). During electrospinning, a UV source was used to initiate crosslinking between photosensitive acrylic groups of HMEMO and PEGDA and maleic groups of ML. Thermal treatment of the resultant electrospun membrane also encouraged further cross-linking by condensing the silanol groups of HMEMO to form siloxane bonds (Si-O-Si). The produced PAN/ML/HMEMO/PEGDA (PMHP) membrane with an average thickness of 25 µm showed high porosity and wettability, greater heat resistance, mechanical and electrochemical properties compared to commercial separators. 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.

A) Image of fully (width of 170 mm) coated collector after 6 h spinning process of fungal CS and UHMWPEO in a ratio of 90/10 at standard conditions (I), B) SEM image of uniform fiber mesh suspended on window screen (50x magnification, inset = 100 µm), C) SEM image of representative spot with uniform, linear and defect free nanofibers (5000x magnification, inset = 1 µm), and D) histogram of fiber diameter with overlayed normal distribution [Christ et al 2022].

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 conditions. 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.