Alternative methods for reducing electrospun fiber diameter

Electrospun fiber diameter is typically controlled by optimizing the electrospinning parameters. However, there is a limit which the fiber diameter can be reduced solely through electrospinning parameters adjustment before beads start to form. Depending on the polymer used, this lower limit may be in the tens or hundreds of nanometers. Beyond parameter optimization, there are some alternative methods for further fiber diameter reduction.

Conventional fiber spinning involves mechanical drawing of fiber from a spinneret. In electrospinning, it is the repulsion of the electrical charges carried by the electrospinning jet in an electric field gradient that caused the thinning of the solution. Using a rotating drum collector, an additional mechanical drawing force can be imparted on the electrified jet. The fiber diameter may further be reduced with this additional drawing. Haider et al (2013) showed that when the rotation speed of collector was increased from 2.09 to 21.98 ms^-1, the diameter of the electrospun chitosan fibers collected were reduced from 164 nm to 137 nm. However, there is a limit at which the fibers can be mechanically drawn before it starts to break. Zussman et al (2003) showed that electrospun polyethylene oxide fibers exhibited necking when the fibers were collected with a linear velocity of 5.3 m^-1 on their disk collector. Using near field electrospinning, Liu et al (2014) also found that fiber diameter of electrospun polyvinylidene fluoride (PVDF) fiber decrease when the rotating collector speed increases. However, beyond rotation speed of 1900 rpm (tube collector diameter of 20 mm), broken fibers may be seen.

An electrospinning disk collector may be used to adjust the fiber diameter by varying its rotation speed. txt-rotating-disc-collector.

Solidification of the spinning jet due to vaporization of the solvent will prevent further stretching of the electrospinning jet. Thus if the onset of solidification can be delayed, the duration for fiber stretching may be lengthened leading to a reduction in fiber diameter. Yu et al (2014) used a co-axial nozzle to introduce additional solvent through the outer orifice to the solution during electrospinning. Comparing the diameters of the resultant fibers from the co-axial nozzle and the single orifice nozzle, the diameter of the co-axial electrospun Eudragit L-100 was found to be about 650 nm compared to 1280 nm diameter fiber from the single orifice nozzle. Yu et al (2014) hypothesize that the presence of a solvent layer on the surface of the electrospinning jet at initiation gives the jet more time to undergo bending instability and fiber stretching before it solidifies. This would have facilitated the reduction of the fiber diameter. Ding et al (2020) constructed a core-shell Eudragit S100 (ES100) fibers using triaxial nozzle electrospinning with the outermost layer extruding only pure solvent and both the core and second layer extruding ES100 polymer solution. Uniaxial nozzle electrospinning of ES100 polymer solution gave an average diameter of 870 nm. However, the triaxial nozzle electrospun ES100 polymer solution had an average diameter of 740 nm. Although the cumulative fluid flow rate from the triaxial nozzle was larger than the uniaxial electrospinning solution flow rate, the former produced fiber with smaller diameter. Hence, the presence of the outer solvent mixture was able to delay the solidification of the electrospinning jet while allowing the polymer solution to stretch and thin further.

To impart additional stretching force on the electrospinning jet, gas may be blown at high speed around the nozzle tip towards the collector. This may facilitate initial stretching and thinning of the electrospinning jet when the solution volume and jet radius is at its greatest and the resistance to extension is high. Electroblowing has been shown to be more effective in reducing fiber diameter than heating the solution [Ahmad et al 2012].

With optimised conventional electrospinning parameters, the resultant fiber diameter may go down to about 200 nm. By combining electroblowing with parameter adjustments, Zhang et al (2018) was able to bring the fiber diameter to below 100 nm. In their electrospinning of polyvinyl alcohol/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PVA/PEDOT:PSS), conventional electrospinning with applied voltage of 20 kV and tip to collector distance of 15 cm, the resultant fiber has a diameter of 263 nm. By using high pressure airflow assisted electrospinning or electroblowing, modifying the setup with negative 35 kV voltage applied to the nozzle and a positive 35 kV voltage applied to the collector (a potential difference of 70 kV), and increasing the tip to collector distance to 120 cm, they were able to reduce the average fiber diameter to 68 mm. Without the airflow directing the flight of the electrospinning jet, the fibers may not reach the collector given the large distance between the tip and collector.

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