Nozzle Design

Most lab-based electrospinning setups comprises of a nozzle spinneret for dispensing of the solution. Despite the emergence of nozzle-less electrospinning techniques especially in mass production electrospinning, nozzle-based electrospinning still retains some advantages such as better charge concentration and nozzle variations for improving electrospinning or fabricating hybrid fibers. In most lab-based electrospinning setup, a hypodermic needle with a flat tip is used as the electrospinning nozzle. However, electrospinning may still proceed with other needle tip configurations. A flat tip is generally preferred as it provides a flat surface for the formation of a stable Taylor cone with little oscillation of the jet compared to chamfered tip needle [Ksapabutr et al 2005]. At higher feed-rate, it was found that a flat tip needle is more prone to generating multiple electrospinning jets compared to chamfered tip needle [Ksapabutr et al 2005]. Parameters such as nozzle diameter has been covered elsewhere and will not be covered here. Instead, other improvements and variations of nozzle design shall be discussed.

Coaxial Nozzle for electrospinning from ramé-hart instrument [Sponsored Info]

As a class, the coaxial nozzle is the most widely used design in electrospinning. While most commonly used for spinning core-shell nanofibers through injection of different solution through the core and shell of the nozzle, this design have also been used to inject solvent through the shell cavity to maintain spinnability of the solution (Read more). Injection of solvent through the shell of a co-axial electrospinning setup was also found to affect the cross-sectional profile of the resultant fibers. When electrospinning Eudragit L100 nanofibers using conventional single nozzle, the fibers produced were found to have a flat/ribbon cross-sectional profile. This is attributed to the formation of a thin wall/skin due to rapid evaporation of ethanol on the surface of the electrospinning jet. Subsequent evaporation from the core of the electrospinning jet causes the thin wall to collapse and form flat fibers. To produce round fibers, Yu et al (2014) injected solvent through the shell of a co-axial nozzle during electrospinning such that it delays the evaporation from the surface of the main electrospinning jet which allows smoother mass transfer of the solvents from the core to the atmosphere. This delay also enables more stretching of the electrospinning jet and thus smaller diameter fibers can be produced. Another use of the coaxial design is to allow incorporation of air jet surrounding the core spinning nozzle to facilitate and improve jet initiation (Read more).

Coaxial nozzle has also been used as a simple means to construct fiber-nanoparticles hybrid films where the fiber is produced by electrospinning and the nanoparticles by electrospraying using the same nozzle. Du et al (2023) demonstrated this by using a coaxial nozzle with the inner orifice made of metal and the outer sheath made of polyethylene. The core and sheath solutions were fed separately and charging of both solutions were via the inner metal orifice. The electrospinnable solution was to be fed through the core while the non-electrospinnable solution was to be fed through the sheath. For electrospinning of the inner core solution, only the inner core solution was fed and for electrospraying, only the outer sheath solution was fed. With this simple alternating of solution feeding either through the inner core or outer sheath solution, electrospinning and electrospraying respectively can be carried out. This setup also allows routine core-shell electrospinning by feeding both solutions simultaneously.

Electrospinning involves introducing charges to the solution under an electric field. In conventional electrospinning setup, a metallic nozzle is used for dispensing the solution and application of charge is through the nozzle. 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]. 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].

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.

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.]

Reduction in the power consumption of the electrospinning process can be achieved by modifying the nozzle design. For initiation of electrospinning jet, energy is expended to overcome the surface tension and capillary action of the solution. Kang et al (2020) showed that by inserting a solid Teflon needle in the nozzle, they were able to reduce the power needed for electrospinning. To further reduce energy loss to the environment, much of the metal nozzle was covered with epoxy resin except a short section at the tip for connecting to incoming power. Using polyvinylpyrrolidone (PVP) as the model solution for testing the nozzle design, they found that having a solid Teflon needle in the nozzle for electrospinning and insulation of the tip, only uses 54% of the power of conventional metal nozzles. This was calculated from the reduced voltage required for steady electrospinning and lower current. Such power reduction has been attributed to the hydrophobic Teflon inner rod that reduces the capillary counter force and the concentration of the applied charges to the solution at the tip. Evidence of more efficient charging of the electrospinning jet can be seen by a longer stable jet phase and larger spiraling radius of the electrospinning jet with the modified nozzle.

Conceptualization of the concentric spinneret with a solid Teflon-core rod and a traditional stainless steel needle utilized as the control. [Kang et al 2020)]

An important function of the nozzle is to maintain the shape of the Taylor cone and to reduce the capillary force that is pulling back the solution. McCarthy et al (2021) installed a 3D printed needle cap on the tip of the electrospinning nozzle to improve Taylor cone stability at higher solution feed rate. Due to the conical shape of the cap, it reduces excess solution from wicking up the outer surface of the nozzle due to capillary action. The shape of the cap also widens the meniscus diameter of the Taylor cone and a wider meniscus diameter has been shown to give a more stable jet. Without the cap, a pendant-shaped Taylor cone is formed which is less stable compared to a conical-shaped Taylor cone. With the cap, the Taylor cone maintained a conical-shape even at maximum flow rate.

Taylor cone stability while spinning 10% PCL + 0.1% F-127 with and without 3D printed needle caps. A, A schematic representation and description of the elements of a Taylor cone during electrospinning with and without a 3D printed cap. B, Taylor cone morphology at maximum spinnable flow rates on a traditional and capped set up. C, Qualitative scale used to categorize the stability of Taylor cones at increasing flow rates (stability increases numerically from least to most stable). D, Heat map contrasting ranges of Taylor Cone stability of capless and capped needles at 10 kV electrospinning [McCarthy et al 2021]

In nozzle-based electrospinning, the solution flows through a capillary or orifice. Using needle is just one form of it. There are other methods using this concept especially for mass production. A tube nozzle has been used by Fang et al (2017) for mass production electrospinning. The nozzle setup is made of a tube with an array of holes along its length. This setup was able to successfully increase the output of electrospun SU-8, photopatternable epoxy. For near field electrospinning, Pan et al (2017) came out with a circuit board design with solder balls as the nozzle. The orifice was created by a precision driller. A chamber body completes the setup for electrospinning polyvinylidene fluoride (PVDF) fiber.

Multi-spinneret structure: (a) multi-spinneret circuit design; (b) multi-spinneret circuit design with solder balls; (c) drilled holes in solder balls [Pan et al 2017].

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Content [Electrospinning Parameters]