Emulsion Electrospinning

The concept of emulsion electrospinning is based on the solution to create electrospun fibers of different composition and structure. An emulsion is made of minute droplets of one solution which is insoluble in another. In general, this is usually between a polar and non-polar solution. Such a system is useful for loading a substance that is only soluble in an organic solvent into a aqueous media or vis versa. In electrospinning, using emulsion has also been shown to form fibers with different structures.

Most polymers used in electrospinning are soluble only in organic solvent. However, there are many drugs and additives that exist in the form of salt that are soluble only in aqueous media. Emulsion electrospinning is designed to enable loading of these drugs into polymer dissolved organic solvent for spinning into fibers. Typical preparation of the solution for emulsion electrospinning is by dissolving the drug and polymer in aqueous and organic medium separately followed by mixing of the two solutions with an emulsifier. Vortexing is carried out to form the emulsion. The critical quality attributes identified for emulsion electrospinning are stability, viscosity and conductivity [Badawi et al 2014]. Emulsion electrospinning has been used to fabricate membranes containing volatile fragrance. Poly(vinyl alcohol) (PVA) fibrous matrix has been demonstrated to release encapsulated (R)-(+)-limonene over 15 days under ambient conditions [Camerlo et al 2013]. Papadaki et al (2016) used coconut oil as solvent for recovery of multifunctional extracts from microalga Haematococcus pluvialis (HP) and the diatom Phaeodactylum tricornutum (PT). The coconut oil enriched with β-carotene and polyunsaturated fatty acids extracts was emulsified with an aqueous solution of ulvan and pullulan blend. The aqueous solution contains 15 mM H₃BO₃ and 7 mM CaCl₂ and Tween 20 surfactant was added to form the oil in water emulsion. Electrospinning of the emulsion was able to produce smooth nanofibers with diameters of about 65 nm. The resultant nanofibers provide a potential delivery vehicle as well as a protective barrier for the encapsulated extracts.

Some biological agents may be sensitive to organic solvents used in the preparation of polymer solution for electrospinning. Therefore, emulsion electrospinning offers a way to encapsulate the biological agent and shield it from the organic solvents for electrospinning. Koplányi et al (2021) was able to prepare an emulsion containing Petroselinum crispum phenylalanine ammonia lyase (PcPAL) enzyme and biodegradable polylactic acid (PLA) for electrospinning. The addition of emulsifiers was demonstrated to generate more uniform, and mainly spherical droplets in the emulsion as compared to polymorphic droplets in emulsifier-free precursor mixtures. Electrospun fibers from emulsifiers with lower HLB values (< 9.7) resulted in thinner and more uniform nanofibers. However, the lowest HLB values and the corresponding smallest diameter fiber did not result in the best enzymatic activity. Electrospun fibers from emulsifier-free precursor mixtures showed low enzymatic activity while significantly higher enzymatic activity were found in emulsion formed with PLA, PcPAL and Brij 30 emulsifier (HLB 9.7). PLA/Brij 30 with 0.15% PcPAL loading has a specific enzyme activity (U_E) of 117 Ug^-1 compared to non-immobilized PcPAL with U_E of 96 Ug^-1. Lee et al (2014) electrospun a core-shell fiber comprising of poly(acrylonitrile) (PAN) as the shell and curing agent, dimethyl methylhydrogen siloxane as the core. Fabrication of this core-shell fiber was through emulsion electrospinning which negates the need for a co-axial nozzle. The ability for self-organisation of the molecules during emulsion electrospinning makes it possible to modify the structure of the resultant fibers.

Electrospinning of emulsions have been shown to generate hollow tubes within the fiber. This may be due to the elongation and merging of droplets in the solution during electrospinning. Zhang et al (2017) was able to construct TiO₂ nanofibers with axially aligned channels by first electrospinning tetrabutyl titanate (TBT) with paraffin oil followed by calcination. The emulsion is formed by butoxyl groups in TBT serving as additional surfactant to cetyltrimethylammonium bromide surfactant for encapsulation of paraffin oil. Polyvinyl pyrrolidone (PVP) was added as a carrier to facilitate formation of fibers. Observation of the calcinated nanofiber under Transmission electron microscope (TEM) and scanning electron microscope (SEM) showed presence of multiple channels inside the nanofibers. Without paraffin oil, solid fibers were formed. In some cases, linear arrangement of immiscible droplets along the core of the fiber was able to form core-shell fibers [Wang et al 2014].

Fabrication procedure of porous TiO2 nanofibers via emulsion electrospinning [Zhang et al 2017].

Surface SEM images of sample TBT/paraffin oil ratios of 2.25 (a), sample TBT/paraffin oil ratios of 1.9 (c), and sample TBT/paraffin oil ratios of 1.55 (e); cross-sectional SEM images of sample after calcination with TBT/paraffin oil ratios of 2.25 (b), sample after calcination with TBT/paraffin oil ratios of 1.9 (d), sample after calcination with TBT/paraffin oil ratios of 1.55 (f) and solid TiO2 nanofibers (g), inset SEM images were the corresponding images with higher magnification; and representative TEM images of sample after calcination with TBT/paraffin oil ratios of 2.25 (h) [Zhang et al 2017].