Electrospinning charge influence on molecular organization

Electrospinning has been shown to be capable of generating fibers with different form and their assembly into various structures. Beyond physical form and structure and deliberate adjustment of material composition during solution preparation, some studies have shown that selection of electrospinning parameters contribute to subtle changes especially at the molecular level.

Influence of electrospinning jet surface charges on the orientation of the charged groups on the molecules.

The ability of applied charges to draw solution into fibers is a well understood fact in electrospinning. However, a lesser known characteristic is the effect of applied charges on molecular organization. Using positive voltage and negative voltage for electrospinning nylon 6, a difference in the total surface free energy and the distribution of oxygen was detected. Surface free energy of electrospun nylon 6 nanofibers using negative high voltage was almost 20% lower than nanofibers spun from positive high voltage. The percentage of electronegative oxygen atom on the surface (measured at grazing angles) was also much lower with fibers spun from negative polarity [Stachewicz et al 2012]. Using a blend of alginate and polyethylene oxide, XPS characterization showed that alginate with its negatively charged carboxylic acid group were found near the surface of the fibers when a positive high voltage was used [Bonino et al 2012].

In vitro release profile of naproxen from cellulose acetatae electrospun nanofibers [Li et al 2014. Journal of Nanomaterials, vol. 2014, Article ID 360658, 8 pages, 2012. doi:10.1155/2014/360658. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Migration of ions due to the external charges can be used to influence applications outcome. In drug release, drawing the loaded drug to the surface of the nanofibers or driving the drug towards the core will certainly affect its release profile. Li et al (2014) demonstrated this effect using cellulose acetate loaded with naproxen. Instead of using positive and negative high voltage on the spinneret, the test was carried out using charge application either on the spinneret or at the collector. Application of high positive charge on the spinneret will give the electrospinning jet a positive surface charge and this may draw negatively charged ions to its surface. Application of high positive charge to the collector will induce negative surface charge on the electrospinning jet and this may drive negative ions to its core. Energy dispersive X-ray spectrometer (EDX) analysis showed more oxygen atom (negatively charged) on the surface of fibers generated by having positive charge on the spinneret. Conversely, less oxygen atoms was found on the surface of fibers generated by having positive charge on the collector. Since cellulose acetate contains more oxygen atoms than naproxen, this translated to the concentration of cellulose acetate molecules along the cross-section of the fiber. With more cellulose acetate on the surface, naproxen may be forced to the fiber core and vice versa. Drug release study of naproxen does demonstrate greater burst release from fibers electrospun by having positive charge on the collector despite having a larger fiber diameter [Li et al 2014]. Thus the influence of electrospinning jet surface charges need to be taken into consideration when fabricating drug loaded fibers. Although technically, the presence of surface charges should influence the distribution of charged molecules for any electrospun fibers, there are other factors that may prevent such charge-related distributions.

Distribution and orientation of molecules and ions are influenced by several factors such as crystallinity, interaction between material mixture and molecular mobility. Tsaroom et al (2011) observed the formation of core-shell polymer-metal salt fibers with the positively charged metal salt concentrated at the core of the fibers after electrospinning with high positive charge. However, this was only seen when the metal salt is mixed with polyethylene oxide polymer but not with polyacrylic acid. It was hypothesized that the interaction between the negative ions of polyacrylic acid with the positive metal salt restricted any metal-salt ions distribution under the influence of the positive external charge. It is with polyethylene oxide, which is neutral, that core-shell structure was formed with metal salt rich core. However, application of negative high voltage does not see a concentration shift of the positive metal towards the shell. This has been attributed to crystallization of polyethylene oxide from the surface which prevented the aggregation of metal-salt at the surface.