Improving electrospun fiber diameter uniformity

Electrospinning fibers with low variance in diameter

Electrospinning is a relatively easy process of producing sub-micron to nanometer diameter fibers. However, large deviation in the fibers diameter can be quite large and this may affect the performance and consistency of the electrospun product. For commercial application of electrospun fibers, consistency in the fiber diameter may be necessary to ensure predictable and reliable output.

Improving charge distribution on the electrospinning jet may be achieved by using a more conductive solvent or adding salt to the solution. At optimum salt content, the charges may be more uniformly distributed along the spinning jet and this may potentially facilitate uniform stretching of the fiber [Lee et al 2005, Ding et al 2010]. In mass production electrospinning setup, maintaining fiber diameter uniformity is a greater challenge as there are numerous jets spinning concurrently. Cengiz et al (2009) showed that adding tetraethylammoniumbromide (TEAB) salt in roller electrospinning of polyurethane helps to improve fiber diameter uniformity. Although higher salt content may increase production rate, it reduces fiber diameter consistency. When there are excessive charges, chaotic and random splitting of electrospinning jets will produce fibers with different diameters. Such random splitting may be attributed to localized charge concentration when charge migration is slower than the rapid thinning of the jet [Garg et al 2011]. If the localized charge concentration is sufficiently strong, a secondary jet may split off from the main electrospinning jet. Another way to improve fiber diameter consistency is to use surfactants in the solution. Surfactants reduces the surface tension and may increase the conductivity of the solution which facilitates stretching of the electrospinning jet. Zheng et al (2014) tested the effect of surfactants, sodium dodecyl sulfate (SDS), Triton X-100, and hexadecyl trimethyl ammonium bromide (HTAB) for electrospinning of polyvinylidene fluoride (PVDF). In general, the addition of these surfactants reduces fiber diameter distributions compared to neat PVDF. Tarus et al (2016) compared the effect of mixed solvent systems for electrospinning of cellulose acetate (CA). Between acetone/N,N-dimethylacetamide (DMAc) and acetone/N,N-dimethylformamide (DMF) as solvents for CA, the latter gives lower coefficient of variance for the resultant fibers of similar fiber diameter. In general, higher concentration resulted in larger fiber diameter and lower coefficient of variance. DMF is often used to increase conductivity in the electrospinning solution and this may have helped improved fiber diameter uniformity.

There are some studies that showed that applied voltage polarity has an effect on electrospun fiber diameter distribution although it may be solution dependent. Experiment by Yang et al (2006) showed with increasing positive voltage, the fiber diameter and distribution of diameter starts to increase for polyethylene oxide. With negative voltage, the mean fiber diameter starts to reduce and the increment in the scatter of the fiber diameter is less. Tong et al (2012) also reported increasing fiber diameter with increasing applied positive voltage (Gelatin, chitosan, PLGA, PBT) but there is no significant changes in the fiber diameter when negative voltage was used. Using aqueous polyvinyl acetate solution, Wu et al (2012) found that fiber diameter reduces with increasing voltage to a minimum before the trend reverses with further increase in voltage for both positive and negative high voltages. However, there is no observable trend in the coefficient of variation in the fiber diameter when the positive voltage is increased although a higher coefficient of variation is observed when higher negative voltage is applied which is contrary to the results from Yang et al (2006).

Apart from using conductivity and voltage to control fiber diameter variance, another method is to use physically constrain the diameter using a shealth working fluids. Li et al (2017) demonstrated this method for electrospinning Eudragit E100 with a mixture of DMF and ethanol as its solvent and as the sheath fluid in a co-axial electrospinning setup. With an optimum sheath DMF and ethanol ratio, they were able to produce Eudragit E100 with minimal fiber diameter distribution. However, further tests would be required to determine whether reduction in fiber diameter distribution is due to the effect of having a sheath solvent system or it is just a matter of the solvent used in making the solution.