#electrospun

The polymer concentration in the solution has great effects on the structure of electrospun nanofibers owing to the resulting significant variation in the viscosity, which have been widely studied by researchers. Bead-on-string fibers are usually formed from solutions with low polymer concentration, which could be ascribed to the low viscosity. With increasing solution concentration, the spherical beads change to spindlelike beads and then form uniform nanofibers, and the fiber diameter increases accordingly. Similarly, changes in solution concentration result in different structures of nanonets during the electronetting process. Ding et al. first fabricated PAA nanonet membranes from solutions with various concentrations and then investigated the structures of the nanonets systematically. The nanonet membranes obtained from 6 wt% PAA solution had the largest pore size and nanowire diameter deviation, while the pore size of the nanonets formed from 8 wt% solution decreased, and their shapes became uniform. Moreover, with further increasing PAA concentration, the resultant membranes showed the smallest average pore size compared with the two other membranes. Then, Zhang et al. found that PA-6 nanofibers obtained from 5 and 7.5 wt% solutions showed microsized beads along the fiber axis, due to the low solution viscosity. As shown in Fig. 8.9AeD, when the PA-6 concentration was higher than 10 wt%, the beads disappeared and a handful of nanonets appeared in the membranes, as indicated in the dashed circle. In particular, the coverage rate of nanonets in PA-6 membranes obtained from a 12.5 wt% solution increased sharply, which could be ascribed to the increased conductivity and reduced surface tension of the solution (Fig. 8.9).

In 2009, Thorsen’s group fabricated side-by-side nanofibers by microfluidic electrospinning (Srivastava et al., 2009). There are many benefits to making use of the microfluidic devices, such as rapid prototyping, ease of fabrication, and the ability to spin multiple Janus fibers in parallel through arrays of individual microchannels. There are electrospun conductive and nonconductive side-by-side nanofibers of PVP and polypyrrole (PPy)/PVP. The schematic of the electrospinning setup is shown in Fig. 5.15A. The TEM image of the nanofibers is shown in Fig. 5.15B. There is an evident boundary in the middle of the fibers, indicating the formation of biphasic Janus nanofibers.

TiO2, as a typical species of semiconductor oxide, has been widely applied in photocatalysis, mesoporous membranes, solar cells, etc. Considering their large specific surface area as well as surface/ interface effects, electrospun TiO2 nanofibers possess superior performance in various fields. In 2003, Li and Xia successfully prepared electrospun TiO2 nanofibers by employing PVP/ethanol/titanium tetraisopropoxide as a precursor solution system. The diameter and the porous structure of the obtained TiO2 nanofibers can be controlled by varying diverse parameters during the electrospinning process (Fig. 3.9). Since then, various solvent systems have been used to produce TiO2 nanofibers with different kinds of structure. Furthermore, numerous works have been reported with regard to the applications of electrospun TiO2 nanofibers. For instance, Chuangchote et al. synthesized diverse TiO2 nanofibers under different annealing temperature from 300C to 700C, and employed them as photocatalysts against hydrogen evolution. The results showed that the sample calcined at 450C exhibited the optimum catalytic activity.

Gelatin is a kind of protein derived from the hydrolysis of collagen, which possesses composition as well as properties similar to those of collagen. Considering that gelatin aqueous solution easily forms a gel, water was generally not used as a solvent for electrospinning in the early years. Huang et al. utilized 2,2,2-trifluoroethanol as a solvent to prepare pristine gelatin nanofibers with good morphology. Gelatin/PCL nanofibers have also been prepared via a similar electrospinning process. In 2006, Li et al.  successfully prepared uniform electrospun gelatin nanofibers from aqueous solution by controlling the temperature of the solvent and environment (Fig. 3.4C), which not only realized the blending of gelatin with other water-soluble biomolecules, but also dramatically increased the potential application of electrospun gelatin nano-fibers in biomedicine. Crosslinked electrospun gelatin nanofibers have been prepared to enhance their mechanical properties for practical applications (Fig. 3.4D). Similarly, diverse functionalized electrospun gelatin nanocomposites have been developed as well.

 A solution containing only casein could not be electrospun due to its strong intermolecular force and 3D structure. However composite dispersions blending PEO or PVA with casein could be electrospun successfully as indicated in Figure. The 10% (w/w) casein solutions containing 80% PEO or 50% PVA are used to electrospin with average fiber diameters ranging from 100 to 500 nm.

 Thickening and stabilizing are the main applications in the food of gelatin as well as electrospun gelatin nanofibers, of which the amounts are smaller and the effects are stronger. The concentration of type B gelatin powder from bovine skin and the applied voltage have been investigated in research by Okutan et al. It is demonstrated that electrospun gelatin nanofibers are nanoscale only at the concentration of 20% (w/v) gelatin. Furthermore, the lower the applied voltage is, the higher the z potential and diffusion coefficient values for electrospun gelatin nanofiber dispersions are. According to a study, gelatin from cold-water fish skin is electrospun and then crosslinked. Water or concentrated acetic acid served as electrospinning solvents and both of them are electrospun successfully. There are three different techniques, namely GTA vapor, genipin, and dehydrothermal treatment, applied in the crosslinking process. The results of three studies show that fish gelatin modified with GTA is the most suitable substrate.

 Many efforts have been made to study how to electrospun collagen because of its wide biomedical and food applications. The solvent system is one of the toughest problems to solve before electrospinning. The spinnability of lyophilized type I collagen sponges derived from cow tendon, as the raw material, has been investigated by altering the solvent systems. To increase the rate of evaporation, the proportion of ethanol in the water/ethanol solution is increased. However, the collagen is denatured by the increase in ethanol concentration. Moreover, collagen solutions at a wide variety of concentrations are prepared in 90% (v/v) aqueous acetic acid solution. The same result is obtained because of the low viscosity. Zein from corn and collagen from bovine Achilles tendon are dissolved in aqueous acetic acid (70% v/v) and then co-electrospun. The figure shows that the average diameter of the fibers becomes bulkier with an increased concentration of zein. Furthermore, the tensile strength and the water contact angles of the collagen/zein nanofiber membranes also increase, while the elongation displays the opposite trend. It has been confirmed that the spinnability of collagen is improved with the addition of zein.