Solution Pre-Treatment for Electrospinning

Electrospinning has been shown to be a highly versatile method for producing nano and micro-fibers from a wide variety of polymers. Given sufficient solution viscosity and conductivity, almost any solution can be electrospun. Despite its ease of producing fibers, achieving good fiber diameter uniformity or to lower diameter may be a challenge for some polymer solution. Research have shown that solution pre-treatment may be useful in enhancing electrospinnability of the solution.

A common method for solution pre-treatment before electrospinning is plasma irradiation. Since electrospinning is strongly affected by molecular chain mobility and the conductivity of the solution, plasma treatment has the advantage of increasing solution conductivity as it ionizes both the polymer chain and solvent molecules. Viscosity of the solution may also increase due to greater interaction between the charged solvent molecules and polymer chains. Shi et al (2010) used atmospheric plasma treatment of polyethylene oxide (PEO) solution to demonstrate the influence on its electrospinnability. Their result showed enhanced conductivity, increased viscosity and surface tension following plasma treatment. The treated solution maintained a higher conductivity over untreated solutions over 240 min although it dropped significantly during the first 120 mins. In their experiment, electrospinning of the solution was carried out immediately after treatment. Electrospinning of treated solution was found to exhibit fewer beads and better quality fibers. Crystallinity of electrospun PEO fibers were also greater in treated solution. This has been attributed to arrangement of charged polymer chain during electrospinning. Enhancement in crystallinity was more apparent in lower concentration solution, probably due to lower density of polymer chain which allows greater freedom of polymer chain movement. Alessandri et al (2013) reported that electrospinning poly-L-lactic acid (PLLA) in dichloromethane (DCM) produces only micro-diameter fibers. Exposing the solution to plasma prior to electrospinning was able to bring the resultant electrospun fiber diameter down to about 400 nm. While plasma solution pre-treatment for electrospinning has also been reported by Asadian et al (2017) to reduce beads formation in the poly-ε-caprolactone (PCL) fibers, they reported an increase of fiber diameter over untreated solution. However, it is important to note that generally, beaded fibers showed smaller fiber diameter than smooth fibers. More in-depth study of the effect of plasma treatment on polylactic acid (PLA) solution for electrospinning was carried out by Rezaei et al (2018). Similar to the findings as Shi et al (2010), they found that plasma treatment increases solution conductivity and viscosity. Further, they demonstrated that the use of argon gas bubbling during plasma treatment to improve treatment outcome.

3D image of the physical properties of selected pristine and plasma-treated PLA solutions and SEM images of PLA nanofibers produced from these solutions (starting PLA concentration: 6% w/v) [Rezaei et al 2018].

Plasma treatment is not restricted to improving electrospinnability, it has also other usage in modifying the solution characteristic prior to electrospinning. Shi et al (2011) used atmospheric plasma treatment for conversion of silver nitrate (AgNO₃) in polyacrylonitrile (PAN) solution to silver nanoparticles prior to electrospinning. This may generate a more uniform Ag nanoparticles size and distribution in the resultant electrospun PAN fibers compared to conversion of silver ions into nanoparticles after fiber formation. The resultant composite fibers demonstrated good antibacterial properties against both Gram-positive B. cereus and Gram-negative E. coli.