HFIP
Cyclodextrin, a toroid‐shaped oligosaccharide (~1000 g/mol) has also been successfully electrospun into uniform fibers (Celebioglu and Uyar 2012; Manasco et al. 2012). For example, modified cyclodextrins such as hydroxypropyl‐β‐cyclodextrin and methyl‐β‐cyclodextrin have been used for electrospinning at high concentrations, e.g. 70 wt%. Fiber formation has been attributed due to hydrogen‐bonding‐induced aggregation behavior indicated by the increase in solution viscosity. Such fibers are promising for drug delivery applications (Celebioglu and Uyar 2011; Celebioglu et al. 2014; Vigh et al. 2013).
Other small molecules have also been successfully electrospun into fibers. These small molecules require high concentrations and a solvent that is highly volatile and electrophilic such as hexafluoro‐2‐propanol (HFIP) or trifluoroacetic acid (TFA) (Nuansing et al. 2013). Peptides such as diphenylalanine have been electrospun into 400–850 nm fibers due to π‐interaction of the phenyl groups entangling the molecules (Singh et al. 2008). Fmoc‐FG (Fmoc‐Phe‐Gly) is another peptide with aromatic groups that would allow for π‐stacking and hydrogen bonding to form continuous fibers. At 18 wt% Fmoc‐FG produced circular fibers, whereas at higher concentrations, it produced ribbons with a needle‐like topography (Nuansing et al. 2013). Tetraphenylporphyrin, a porphyrin derivate used in photovoltaic devices, has been electrospun at a concentration over 9 wt% to produce a beads‐on‐a‐string morphology. The entanglement of these molecules was enhanced by addition of diphenylalanine which increased π‐stacking due to the presence of phenyl groups (Nuansing et al. 2014).
Recently, colloid electrospinning has been explored as means to create nanofibers with unique properties. Suspensions containing colloids (inorganic or organic, i.e. polymeric) and a small amount (~few wt%) of a fiber‐forming polymer has been used as a template. Inorganic nanoparticles, e.g. metal, metalloid oxide, aluminosilicates, hydroxyapatite, and nonoxide ceramics, are the most common. Silica and polymer particle electrospinning dispersed in a solution of a second incompatible polymer has also been considered. As the jet thins, the particles are in close contact leading to high‐packing densities in the resulting fibers. Due to the timescale of particle arrangement (milliseconds), the resulting structure is less ordered than classical hexagonal packing observed in films (Crespy et al. 2012). Bead formation during electrospinning may lead to colloid aggregation. Adding salt to increase the net charge density can reduce bead formation. The resulting hierarchical structures are especially promising for superhydrophobic materials.
Similarly, cells such as bacteria, viruses, yeast, and mammalian cells have also been electrospun from suspension (Crespy et al. 2012; Zussman 2011; Canbolat et al. 2011, 2013). Mammalian cells dehydrate during fiber formation (Canbolat et al. 2011). Yeast and viruses are more robust and show some biological activity after electrospinning (Crespy et al. 2012; Canbolat et al. 2013). However, maintaining biological function after direct electrospinning remains challenging (Crespy et al. 2012).
1.6 Advanced Fiber Characteristics
1.6.1 Ribbons, Wrinkles, Branching, and Netting
Electrospinning generally produces fibers with circular cross sections. However, other morphologies have been observed, e.g. flat ribbons, wrinkled surfaces, and branched fibers (Figure 1.4) (Koombhongse et al. 2001; Huang et al. 2000). The morphology affects the specific surface area as well as the mechanical properties of the resulting fiber mats (Camposeo et al. 2013).
Ribbon structures can have two forms, flat ribbon and a ribbon with two tubular ends (Figure 1.4) (Koombhongse et al. 2001). With the use of a high‐speed camera, Reneker and coworkers determined that this morphology formed prior to deposition. The fibers emerged from the tailor cone with a circular cross section. A thin skin formed on the fiber as the outer layer of the fiber dried and thinned. The solvent inside the skin diffused out rapidly due to the high surface area to volume of the fiber during thinning, and with atmosphere pressures, the skin deforms. The fibers changed from circular to elliptical to either flat ribbons or ribbon with two tubular ends when the fiber skin collapses (Koombhongse et al. 2001). The final shape, i.e. flat ribbon or ribbon with two tubular ends, is dictated by the stiffness of the skin and self‐repulsion charges. Ribbon structures were observed with fibers made with 30 wt% PS in DMF, 10% poly(ether imide) in hexafluoro‐2‐propanol (Koombhongse et al. 2001), and 81 kDa recombinant protein (Huang et al. 2000).
Figure 1.4 Overview of interesting electrospun structures: (A (a–d)) ribbons
(Source: Reprinted from Koombhongse et al. (2001). Copyright (2001). Wiley.),
(B) wrinkled fiber
(Source: Reprinted (adapted) with permission from Pai et al. (2009). Copyright (2009). American Chemical Society.),
(C (a, b)) branched fibers
(Source: Reprinted from Koombhongse et al. (2001). Copyright (2001). Wiley.),
and (D) fiber nets
(Source: Reprinted from Wang et al. (2017). Copyright (2017), with permission from Elsevier.).
Alternatively, when the skin collapses, the fiber surface can wrinkle due to the buckling instability associated with the skin pulling inward as the solvent evaporates. The final shape, i.e. wrinkled fibers or a flat fiber, depends on the ratio of the Young's modulus of the core to that of the shell as well as the ratio of the radius of the fiber to thickness of the shell. The wrinkles are less deep when there are more wrinkles and the cross section is closer to circular. The morphology can also be affected by solvent volatility. Higher volatility tends to result in ribbons, whereas lower volatility tends to lead to wrinkling. For example, polystyrene/tetrahydrofuran (high volatility) formed ribbons, whereas polystyrene/DMF (low volatility) produced wrinkled fibers at the same polymer concentration (Koombhongse et al. 2001; Wang et al. 2009).
Another morphology that can occur in electrospun fibers is branching. Branched fibers are a small fiber splitting from the main fiber, and these splits are often times formed at the bends of the fibers. Similarly, split fibers occur when a primary fiber splits into smaller fibers. This phenomenon is due to jet instability. Nanofiber/net structures have also been reported. Din et al. electrospun a polyvinyl alcohol/H2O/formic acid mixture and observed typical electrospun nanofibers ~200 nm and a 2D‐nanonet of interconnected nanofibrils with average fiber diameter ~25 nm. This morphology was attributed to the formic acid. The authors posit that the ionized formic acid causes excess charge and a number of charged droplets. Charged droplets form thin films in contact with the liquid jet. Upon solvent evaporation and thermal‐induced phase separation (TIPS) in the thin film, the solidification of the polymer‐rich phase results in a Steiner network of 2D nanonet/nanofibrils. The structure affected transport properties through the electrospun membrane as well as the membrane wettability. Similar structures have been observed with polyamide 6, polyacrylic acid, polyurethane, chitosan, and poly(methyl
