applications. For example, coaxial electrospinning has been used for encapsulating biological components, e.g. proteins and cells (Crespy et al. 2012). Coaxial electrospinning has also been extended to tricomponent systems with triaxial spinnerets (Yu et al. 2015; Han and Steckl 2013). Coaxial electrospinning allows for control over the location of the therapeutic agent (or any other desired core or shell material) through the use of multiple concentric spinnerets. Utilizing triaxial electrospinning, gradient release of a drug agent over the period of 20 hours or dual drug release (Yu et al. 2015). Use of more complex multichannel spinnerets and hierarchical multichannel nanotubes has been achieved by removing the inner fluid solution after spinning (Li et al. 2010). Advanced hierarchical structure with advanced spatial distribution of nanoparticles within nanofibers has also been achieved by combining triaxial electrospinning and block copolymer self‐assembly (Kalra et al. 2009). Although coaxial electrospinning allows tuning the internal hierarchical structure of multicomponent fibers, the main disadvantage is the complex setup required, i.e. multiple pumps, complex spinneret configurations. Scalability has also been a concern, although, free surface electrospinning of multiple fluid layers to achieve coaxial fibers has been reported (Forward et al. 2013).
1.6.3.2 Emulsion Electrospinning
Coelectrospinning multiple fluids from a single spinneret to achieve core–shell fibers via phase separation is an alternative to coaxial electrospinning (Crespy et al. 2012; Yarin 2011). For example, when a polymer solution is emulsified with an immiscible solution (stabilized by surfactants) is electrospun, coaxial fibers can be achieved (Bannwarth et al. 2015; Bazilevsky et al. 2007; Xiaoqiang et al. 2017; Xu et al. 2006; Yang et al. 2008; Cai et al. 2016; Camerlo et al. 2013; Hu et al. 2014; Hu and Huang 2007; Jagur‐Grodzinski 2006; Li et al. 2010; Maretschek et al. 2008; Qi et al. 2006). The continuous phase of the emulsion is an electrospinnable polymer solution. During electrospinning, the dispersed droplets are stretched and elongated. Since the solvent evaporates from the outer shell solution faster than the core solution, the viscosity of the shell increases relative to the core. The viscosity gradient from the outer to inner layer forces the stretched droplets inward so that the dispersed phase is encapsulated within the fiber (Xu et al. 2006). Partial de‐emulsification, i.e. merging of the droplets during fiber formation is thought to result in a continuous core of dispersed phase (Hu et al. 2014). Mathematic modeling of the two‐phase flow indicates that annular flow, i.e. a continuous core, is expected when the Weber number is less than 0.5 and greater than 26 (Hu and Huang 2007). Note the Weber number is
(1.11)
where ρ is the density of the core solution, v is the velocity of the core solution at the exit of the nozzle, douter is the diameter of the outer needle, and σ is the surface tension coefficient (Hu and Huang 2007).
Oil‐in‐water (O/W) emulsions in which a hydrophobic component is encapsulated within a hydrophilic polymer (Xu et al. 2006; Maretschek et al. 2008; Qi et al. 2006) as well as water‐in‐oil (W/O) in which hydrophilic components are encapsulated within a water insoluble polymer (Bannwarth et al. 2015; Camerlo et al. 2013) have been reported. For O/W emulsions, typical core loadings are ~20% or less (weight ratio of core material to shell polymer) (Choi et al. 2011). The loading of dispersed phase in the nanofiber is typically 5% or less (mass of core:mass of shell polymer) for W/O systems (Yang et al. 2008). Thus, using emulsions, the loading of core materials is typically less than can be achieved with coaxial electrospinning. Surfactants are generally used to stabilize the emulsion. Surfactants with low HLB (hydrophile–lipophile balance) have been used to stabilize oil‐in‐water emulsions for electrospinning (e.g. sorbitan monooleate) (Cai et al. 2016). Surfactants with high HLB, e.g. sodium dodecyl sulfate, have been used to stabilize both oil‐in‐water and water‐in‐oil emulsions for electrospinning (Bannwarth et al. 2015; Xu et al. 2006; Yang et al. 2008). The emulsion is usually electrospun immediately after preparation to prevent premature phase separation (Cai et al. 2016; Camerlo et al. 2013; Garcia‐Moreno et al. 2016). Stabilizing the emulsion is a critical practical consideration.
1.7 Process Scalability
One key limitation to implementing electrospinning on a commercial production scale is the low production rate. Conventionally, production rates used with electrospinning are less than 0.3 g/h. To increase productivity, it is possible to increase the number of needles. However, the effect of additional needles/liquid jets on the electric field profile presents a significant challenge. Electrostatic repulsion limits the areal density of jets that can be achieved. There is strong repulsion among the jets which lead to poor fiber quality and reduced fiber production (Persano et al. 2013).
1.7.1 Melt Electrospinning
To avoid the use of hazardous solvents or to process polymers that are difficult to dissolve, e.g. polypropylene and polyethylene, melt electrospinning in which fibers are spun from molten polymer rather than a polymer solution has been considered. Eliminating the solvent increases the mass throughput of electrospinning ~5‐ to 10‐fold. The melt electrospinning process was first described in a patent approved in 1936 for Charles Norton at the Massachusetts Institute of Technology (Norton 1936). Larrondo and Manley published a melt electrospinning method in 1981, demonstrating that micron‐scale filaments could be spun from molten polypropylene and high‐density polyethylene (Larrondo and St. John Manley 1981).
In addition to the high‐voltage power supply and collector, additional elements key to the melt electrospinning apparatus are heating elements to melt the polymer and a temperature control system. Commonly used polymer heating methods may include electrical, heated air, circulating fluid, laser heating, and microwave heating (Zhang et al. 2016). Electrical heating is most widely used due to ease of use, but may result in electrical interference and safety concerns. Electrical interference can be mitigated by reversing the electrodes so that positive voltage is applied to the collector and the spinneret is grounded (Deng et al. 2009; Lyons et al. 2004). The heated air method also offers simplicity, but the polymer melt temperature can be difficult to control using this method (Dalton et al. 2007; Qin et al. 2015). Heated gas has also been used to maintain the polymer temperature in the spinning region and achieve thinner fibers (Zhmayev et al. 2010). Polymers have also been heated with circulating water or oil systems, but the range of temperatures is limited by properties of the heating fluid (Dalton et al. 2007; Detta et al. 2010). Solid polymer rods may also be melted using a collection of lasers, preventing undesirable contact between the voltage and heat sources (Ogata et al. 2007).
Polymer properties (molecular weight and tacticity) play a substantial role in determining the feasibility of electrospinning fibers from a melt and the resulting fiber diameter. Lyons et al. found that the highest molecular weights of polypropylene used in a study produced the largest diameter fibers. Higher polymer tacticity also corresponds to higher crystallinity and larger fiber diameters (Lyons et al. 2004).
When electrospinning from the melt, the conductivity is much lower and the viscosity is much greater than solutions (Hutmacher and Dalton 2011). Semiconducting polymer melts (conductivity of 10−6 to 10−8 S/m) are ideal for forming a stable Taylor cone and jet (Lyons et al. 2004; Brown et al. 2015). Jets formed from highly conductive polymers will break when the voltage is increased and nonconductive polymer melts will not sustain a sufficient surface change to be electrospun. The viscosity is affected by the polymer molecular weight and temperature. Achieving optimal melt viscosity is necessary for polymer flow through the spinneret and for the formation of a stable Taylor cone (Lyons et al. 2004; Brown et al. 2015). Typically, melts with viscosities ~40 to 200 Pa S can be electrospun (Hutmacher and Dalton 2011). As the spinneret temperature increases, polymer viscosity and resulting fiber diameter decrease (Zhou et al. 2006). This phenomenon is due to thermal and mechanical degradation of the polymer (Zhang et al. 2016; Sukigara et al. 2003).
Analogous to solution electrospinning,
