Electrospinning using insulated nozzle

Electrospinning is commonly carried out using a metallic needle as the spinneret due to the convenience of charge application and transfer of a high voltage to the solution flowing through it. However, there are several studies that suggest that having an insulated spinneret tip may yield better quality fibers. This is especially important in industrial scale electrospinning which uses nozzle spinnerets. There are a few reasons why having an insulated spinneret tip is better. First is that the charge transfer may be more efficient. Second, the electric field profile between the spinneret tip and the collector may be more uniform due to the absence of a point source. Third, the adhesion between the solution and the coated surface may be weaker and this reduces the force needed to eject and stretch the solution. For electrospinning at industrial level using nozzles, having an electrical insulating covering over metallic nozzles may reduce the power consumption by the process. In a preliminary study by Wu et al (2016), a metal nozzle consumes 3 times more power than a polypropylene coated nozzle. Higher power consumption by uncoated nozzle may be due to power wastage from ionising air surrounding the nozzle instead of transferring to the solution. Unfortunately, further studies will be necessary to verify this result as it is unclear from the publication how the values for calculating power usage were obtained.

Applying a high voltage on a metal needle may lead to unnecessary loss of charges. The sharp edge of the exposed nozzle results in point discharge where the air around the nozzle tip gets ionized. Experiments have shown that even without any solution present, an electrical current can be picked up at the collector [Collins et al 2012, Lien 2013]. Li et al (2014) hypothesized that having a material that lowers the interfacial tension between the solution and the nozzle and retarding loss of electrical energy to the atmosphere, coated on the outer surface of the nozzle will lead to more efficient electrospinning. Using polyvinyl chloride (an antistatic polymer) as a coating over the stainless steel nozzle, their experiment showed that for the coated nozzle, the deflection angle of the electrospinning jet is larger, Taylor cone is smaller and the stable section of the jet shorter. These observations suggest better transfer of electrical charges to the solution from the coated nozzle compared to the purely stainless steel nozzle [Li et al 2014]. Xiang et al (2013) investigated the use of a Teflon tip spinneret for electrospinning and compared its performance over stainless steel spinneret. The diameter of polyvinyl pyrrolidone fibers electrospun from Teflon tip spinneret was much smaller at 540 nm compared to 870 nm from stainless steel spinneret. Xiang et al (2013) hypothesized that this is due to lower interfacial tension between the solution and the Teflon tip compared to the stainless steel tip. Teflon makes it easier for the solution to be ejected from the tip as the solution does not spread over the spinneret tip surface. Hohman et al (2001) suggested that bending instability will be greater when the charges on the jet contribute more to the local electric field than the tangential electric field from the spinneret. With the insulated coating, the tangential electric field from the spinneret will be reduced and a shorter stable jet section can be expected. Since thinning of the electrospinning jet to the nanometer dimension comes from the longer stretching distance as a result of the bending instability, an earlier transformation to the unstable phase by the electrospinning jet is likely to result in a thinner fiber [Zhou et al 2013]. Using an epoxy coating over the electrospinning spinneret, Li et al (2013) showed that the diameter of electrospun ethyl cellulose (EC)/ketoprofen (KET) blended fibers was smaller than the fibers from uncoated metal spinneret. Having an outer coating would also reduce loss of charges through edge emission and ionization of air [Collins et al 2012].

Investigation of how the PVC-coated spinneret affects electrospinning. (a) The experimental setup, (b) electrospinning with two PVC-coated spinnerets (inner diameter 1.0 mm), (c) spinning with two stainless steel spinnerets (inner diameter 1.0 mm), and (d) a schematic diagram illustrating the interfacial tensions between the sheath fluid and the spinneret. The sheath fluid is shown on the left and the core fluid on the right in (b) and (c). [Li et al. Nanoscale Research Letters 2014; 9: 258. This work is licensed under a Creative Commons Attribution 4.0 International.]

An alternative method to achieve this is to use a non-conducting nozzle and charging of the solution using an electrode inserted into the solution. Without any solution present, no electrical current was detected at the collector providing the evidence that there was none or very few ionization of the air [Lien 2013]. Lien (2013) tested the influence of having an external insulating cover over a Teflon nozzle and charging of the solution by inserting an electrode into the solution on the current flowing through the collector. The study showed negligible difference in current with and without the insulating cover. A simple insulated electrospinning tip setup will be to use a slip-on nozzle tip plastic syringe for electrospinning without the hypodermic needle attached to it. Shaid et al (2018) used this plastic syringe with a thin wire forming a curved loop immersed in the solution within the syringe for electrospinning and electrospraying. The wire is responsible for introducing high voltage necessary for electrospinning and electrospraying. Thermoplastic polyurethane (TPU) fibers were electrospun using this setup.