Use of coaxial nozzle in electrospinning

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

Coaxial nozzle, where the nozzle comprises of an inner and outer orifice, is often associated with electrospinning core-shell fibers. However, the coaxial nozzle design has many other uses in electrospinning and the production of fibers. These include initiation and facilitation of electrospinning, controlling fiber quality, creating hollow fibers and reducing fiber diameter.

In an interesting demonstration on possible uses of coaxial nozzle in electrospinning, Panzavolta et al (2016) prepared calcium phosphate nanoparticles inside gelatine nanofibers through a triaxial needle. For this setup, the outermost channel is the gelatine solution, the intermediate channel contains calcium solution and the central channel contains phosphate solution. As all three solutions were being ejected, the phosphate and calcium solution mixed and form calcium phosphate nanoparticles while the gelatine solution encapsulate the resultant nanoparticles and electrospins from the needle tip. TEM analysis revealed the presence of nanoparticles at the core of the fibers but they were not uniformly distributed throughout the length of the fiber.

TEM of calcium phosphate nanoparticles encapsulated within gelatine nanofiber [Panzavolta et al. Journal of Nanomaterials 2016; 2016: Article ID 4235235. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Core-shell fibers

A coaxial nozzle design enables two different materials to be dispensed for electrospinning of nanofibers. Such fibers are known as core-shell fibers as the core and sheath materials that made up the fibers are made from two distinct materials. This is particularly useful in drug delivery application where the drug is typically loaded in the core such that the sheath provides a barrier to reduce the drug release rate. For this setup, only one of the materials needs to be electrospinninable to carry the drug. When the core-material is made out of low molecular material or oil, these can be easily extracted to form hollow fibers.

Coating individual fiber

For some applications, it is desirable for the electrospun fiber to be coated with another material. This is possible using a coaxial electrospinning setup. With injectable electrospun membrane, the membrane is usually dipped in a hydrogel material such that the whole membrane is encapsulated within the hydrogel. Amagat et al (2023) used coaxial electrospinning to construct nanofibers that were individually coated with hydrogel to form a core-shell fiber. The core of the fiber was made of mechanically stronger poly(lactide-co-ε-caprolactone) (PLCL) and methacrylated gelatin (GelMA)/alginate hydrogel as the shell. Active ingredients were added to the hydrogel shell as required. The membrane with triangle and square-shaped mesh sized 0.72 or 1 cm², respectively, was shown to support injection through inhalation and injection steps connected to a glass tube (inner diameter 0.9 mm). Similarly, the membrane can be ejected and inhalated back into a 20 G needle.

Hollow Fiber

Hollow fiber constructed using coaxial electrospinning typically with a temporary material as the core and the actual material as the shell. Depending on the post-spinning process, oil is often used as the temporary material as it is relatively easier to remove them than other higher molecular weight material. Hollow fibers may also be formed using a core solution in which the solvent used is a non-solvent for the shell material. This causes solidification of the shell material at the interface during electrospinning and subsequent solvent vaporization encourages the core material to migrate to the interface thus forming a hollow fiber [Na et al 2012]. In the construction of ceramic hollow fiber, other polymers may be used in the core as they can be removed during the sintering process. In a novel coaxial electrospinning technique, air, instead of solution was used as the core "material".

Using air as the core "material" means that post spinning process is not necessary. Yu et al (2014) examined the factors that allows for the fabrication of Fe₃O₄/Eu(BA) ₃phen/PVP hollow fibers. Their study showed that solvent selection plays an important role in the formation of hollow fibers by blowing air through its core. While Fe₃O₄/Eu(BA) ₃phen/PVP was able to form hollow fiber using chloroform, CHCl₃ as the solvent, solid fibers were formed using DMF. Yu et al (2014) hypothesized that low surface tension and higher evaporation rate of CHCl₃ is the reason for the formation of the hollow fibers. A high surface tension solvent will certainly overcome the supporting air blowing at the core and cause the collapsed or contraction of the wall leading to the formation of solid fibers. A higher evaporation rate facilitates the solidification and formation of an intact wall soon after leaving the nozzle tip.

Unique hollow ceramic composite nanofibers have also been constructed with clever selection of core and shell material. Chang et al (2008) used coaxial electrospinning to fabricate TiO₂ hollow nanofibers with silver nanoparticles littering the inner walls of the hollow fibers. PVP solution was used as the electrospinning agent for titanium n-butyloxide (Ti(OC₄H₉)₄ at the shell and silver nitrate AgNO₃ at the core. Following calcination of the electrospun composite fibers, the TiO₂ hollow nanofibers fibers with AgO₂ coated interior was found to exhibit greater photocatalytic activities toward decomposition of methylene blue compared to other nanostructured TiO₂ materials such as mesoporous AgTiO₂ blended fibers, TiO₂ hollow nanofibers, TiO₂ nanofibers and TiO₂ powders. A similar technique is to use mineral oil containing nanoparticles in the core and an electrospinnable polymer for the shell. Wang et al (2013) constructed a hollow carbon nanofiber with silicon nanoparticles along the inner wall using this concept. Polyacrylonitrile solution was used for electrospinning of the nanofiber shell and the core is a mixture of silicon nanoparticles and mineral oil. Using a co-axial electrospinning setup, Guo et al (2021) filled the inner core with cyclohexane and paraffin which will be removed during the calcination process to form the hollow core. NaYF₄/Yb/Tm/TiO₂ nanoparticles were added to the cyclohexane and paraffin core during electrospinning. The shell was formed by TiO₂/PVP precursor solution, Following calcination, NaYF₄/Yb/Tm nanoparticles were observed to line the inner wall of the hollow electrospun fibers. For calcinated pure TiO₂ hollow fibers, the inner wall was smooth.

Morphology characterization and diameter distribution of nanoparticle and nanofibers. (a, b) Cross-sectional SEM images of TiO2 and NaYF4/Yb/Tm/TiO2 core-shell hollow fiber (c, d) proving the existence of NaYF4/Yb/Tm nanoparticles on the inner surfaces. (e, f) Size statistic of nanoparticles and SEM image of core-shell fibers [Guo et al 2021].

A challenge to produce continuous hollow fibers is that the core material needs to run continuously through the length of the fiber without breakages. This has been a challenge in coaxial electrospinning. Research into the interaction between the core and shell fluid in a coaxial electrospinning setup has revealed some important requirements to facilitate smooth and continuous production of core-shell fibers. Vats et al (2021) did a comprehensive study in the effect of solvent miscibility between the core and shell solution on the production of core-shell fibers in a coaxial setup. Both extreme immiscibility and miscibility between the core and shell solution are detrimental to producing core-shell fibers. In complete immiscible solutions, the relatively high interfacial tension between the core and shell solution will cause the core to break up as the core solution tries to minimize its surface area in contact with the shell solution by forming a sphere. On the other extreme, completely miscible core and shell solution will remove the interface and cause premature gelation in the Taylor cone as the solutions mixed during the electrospinning process. Therefore, an ideal core and shell solution pairing would be partial miscibility between the two solutions. Although both solutions may be partially miscible, the concentration difference at the tip of the nozzle may cause local interfacial tension variation, creating a solutal Marangoni effect and resulting in beaded fibers. To minimize Marangoni flows and facilitate the production of continuous core-shell fiber, a small amount of sheath solvent was recommended to be added to the core prior to spinning. This will reduce the interfacial tension variation as the core and shell interface quickly reaches an equilibrium.

Electro-blowing

Instead of having an electrospinnable solution at the outer orifice, air jet may be introduced instead to initiate and facilitate electrospinning of the core material. It is particularly useful in solution which is highly viscous and application of high voltage only is unable to overcome the surface tension of the solution. This method has been shown to be non-material specific and has been used successfully to fabricate other materials such as poly(ether sulfone) [Lin Y et al 2008] and poly(methyl methacrylate)/tetraethoxysilane solution [Peng M et al 2008]. Terms such as electro-blowing and gas jet/electrospinning has been used to describe this method. The solution to be electrospun is dispensed through the inner nozzle while pressurized air is ejected from the outer ring. The air that is blown out exerts a shearing force on the inner solution that pulls and stretches the solution. As the air velocity rapidly declines, the stretching force from the charges on the solution took over and provides subsequent elongation force on the jet to bring the eventual fiber down to the nanometer scale. In an extension of electro-blowing, heated gas may be used instead to reduce the viscosity of the solution and encourage greater elongation of the fiber. This has been demonstrated to be more effective in reducing the fiber diameter than heating the solution [Ahmad et al 2012].

Beyond enhancing the quality of the fabricated nanofibers, electro-blowing has the potential to significantly increase the fiber production compared to conventional electrospinning. In conventional electrospinning, the production rate is limited by the speed at which the solution is ejected from the tip of the nozzle. Increasing the voltage may not lead to an increase in fiber production as there is a charge threshold which the solution can carry. With electro-blowing, the flow rate can be increased significantly as the blowing air is able to draw the solution from the nozzle while generating a strong initial stretching force.

Facilitating smooth fiber electrospinning

Using a coaxial nozzle, the solvent can be introduced through the outer orifice during electrospinning which prevents premature drying of the material at the nozzle tip. This is particularly useful when the solvent is highly volatile or when the solution concentration is very high. Yu et al (2011) demonstrated that at 30% (w/v) polyvinylpyrrolidone (PVP), the solution tends to solidify into a cylindrical column of semi-solid at the tip of the nozzle during electrospinning. However, introducing N,N-dimethylacetamide (DMAc) through the outer orifice allows for continuous electrospinning of PVP at the same concentration. The same method has been demonstrated on electrospinning of Eudragit L-100 in ethanol/ N,N-dimethylacetamide (DMAc) using the co-axial nozzle [Yu et al 2014]. The resultant fiber diameter is also much smaller than those from single orifice nozzle tip with a rounded cross-section. The solvent introduced to the surface of the electrospinning jet on the onset may have allowed a smoother mass transfer of the solvent from the inner core to the surface.

Left. Conventional electrospinning using single solution and single axial nozzle. Center and right. Electrospinning with a coaxial nozzle with a sheath solvent to stabilize the electrospinning jet. [Yu et al 2014 Journal of Nanomaterials, vol. 2014, Article ID 967295, 8 pages, 2014. doi.org/10.1155/2014/967295. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Reducing fiber diameter

Apart from facilitating continuous fiber electrospinning when solvent was ejected from the outer orifice of the coaxial nozzle, this setup also enables reduction in the fiber diameter. Yu et al (2012) demonstrated the effect of varying the flow rate of the sheath fluid comprising of a mixture of acetone, DMAc, and ethanol (4:1:1 by volume) for electrospinning of ketoprofen-loaded cellulose acetate. At optimal sheath fluid flow rate, the fibers demonstrated more homogeneous structure, smaller diameters and narrower diameter distribution. However, when the sheath fluid flow rate is too high, the fibers start to show beads and clumps. This is likely to be caused by excessive sheath fluid that is still wet after deposition. It is also suggested that the excess sheath fluid may form globules along the electrospinning jet and local mixing with the core solution will result in beads formation [Yu et al 2013]. A similar observation was made by Wu et al (2015) in the electrospinning of Shellac nanofibers using a coaxial nozzle. Compared to single nozzle electrospun Shellac fibers which have an average diameter of 1.2 µm, electrospun Shellac with coaxial nozzle and solvent dispensed through the outer orifice was able to produce fibers with diameter down to 0.58 µm. With higher flow ratio of the outer orifice solvent relative to the core material, the diameter of the Shellac nanofiber was reduced. However, when there were excessive sheath fluids, beads were formed on the fibers.

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