Chitosan of different molecular weights and degree of deacetylation has been electrospun into fibers for medical, filtration, and other applications. Typically, chitosan with relatively low molecular weight is dissolved in acidic solution or using toxic solvents such as trifluoroacetic acid. Table 58.1 provides a comparison of the type of chitosan, solvents, and properties of fibers obtained. Although several

solvents and mixtures of acids and organic solvents were used to dissolve and produce electrospun fibers from pure chitosan, Ohkawa et al. claim that only trifluoroacetic acid was able to produce fibers. It was also found that addition of dichloromethane assisted in electrospinning and fibers with diameters of 380 nm were obtained [04Ohk]. Similarly, inclusion of surfactants either promoted or did not affect formation of chitosan-PEO nanofibers depending on the type of surfac­tant used. Nonionic surfactants with chitosan or ionic surfactants with neutral polymers such as polyethylene oxide (PEO) assisted in fiber formation whereas ionic surfactants and charged polymers led to formation of beads [09Kri].

Although chitosan can dissolve in dilute aqueous acetic acid, it was necessary to add other solvents to obtain electrospun fibers. Gong et al. were able to obtain electrospun chitosan fibers using acetic acid as solvent in concentrations from 10 to 100 %. Uniform fibers were obtained with increase in acetic acid concentration to 90 % and using chitosan with molecular weight of 106,000 mol/g and a solution concentration of 7 % [05Gen]. Other researchers have also showed that hydrolyzed chitosan with lower molecular weights could be electrospun into fibers using 70­90 % acetic acid [09Hom]. For instance, electrospun fibers with diameters of 140 nm were obtained using chitosan with molecular weight of 2.94 x 105 g/mol compared to fiber with diameters of 250-284 nm obtained when lower molecular weight chitosan was used [09Hom]. Similar results were also obtained by Vrieze et al. who produced chitosan fibers with diameters of 70 nm using 90 % acetic acid [07Vri].

Chitosan was dissolved in trifluoroacetic acid and methylene chloride and electrospun into oriented and non-oriented nanofibrous tubes with inner diameter of 1.2 mm and outer diameter of 2 mm with lengths of 15 mm [09Wan]. Images of the tubes containing oriented, unoriented, and a mixture of the two types of fibers are shown in Fig. 58.2a-c, respectively. The tubes were used to culture Schwann cells and also implanted into rats for nerve regeneration [09Wan]. Cells were found to align unidirectionally in the case of the oriented fiber mats, but such arrangement was not seen in the unoriented fiber mats as seen from Fig. 58.3. Scaffolds developed were considered to be suitable for autogenous nerve grafts.

A modified wet spinning approach was used to produce ultrafine fibers from chitosan. Chitosan (4 %) was dissolved in acetic acid and extruded through fine silicone rubber tubing into a coagulation bath consisting of either sodium tripolyphosphate/ethanol or 1 M NaOH/ethanol. Fibers obtained were washed

thoroughly with distilled water until the fibers were neutral in pH [11Pat]. Formation of the fibers in NaOH solution led to ionic cross-linking and fibers with good properties. In the dry state, the fibers had tensile strength in the range of 1-2.5 MPa but decreased to 100-300 kPa in the wet state. Chitosan fibers had a swelling of about 500 % in PBS compared to about 300 % for the chitosan- tripolyphosphate fibers.

Chitosan of various molecular weights was electrospun and then cross-linked with glutaraldehyde to improve water stability [07Sch]. Table 58.2 shows some of the properties of the chitosan fibers produced. As seen from the table, chitosan with medium molecular weights (190-310 kDa) had the highest viscosity and produced fibers with diameters of 172 nm. However, high molecular weight chitosan had better water stability. Cross-linking considerably decreased elongation and strength (from 1.4 to 1.2 MPa) but did not affect the modulus of the fibers. Cross-linked fibers were stable in acetic acid, water, and NaOH solution whereas the uncross — linked fibers disintegrated in water. A one-step cross-linking and electrospinning of chitosan fibers was done using glutaraldehyde as the cross-linking agent. The cross­linker (50 % water/50 % GA) solution was added to the spinning solution and the cross-linking occurred during electrospinning [07Sch]. Average diameter of the fibers obtained was about 128 nm, considerably lower than the average diameters of fibers (178 nm) obtained using a two-step cross-linking process. The fibers obtained were stable in acetic acid, water, and sodium hydroxide for up to 72 h.

A new set of cross-linkers were developed to improve the properties of electrospun chitosan mats. Genipin, hexamethylene-1,6-diaminocarboxysulfonate, and epichlorohydrin were added into the chitosan solution in various ratios and electrospun into matrices with fiber diameters of 267, 644, and 896 nm, respec­tively. Cross-linked mats showed good stability to dissolution at pHs 3, 7, and 12 after posttreatment with heat and alkali [12Aus]. In a similar approach, glycerol phosphate, tripolyphosphate, and tannic acid were identified and used as non-covalent cross-linkers for electrospun chitosan fibers. Glycerol phosphate and tannic acid cross-linked fibers had average diameters in the range of 145-334 and 145-554 nm, respectively, whereas tripolyphosphate cross-linking produced branched fibers with diameters between 117 and 462 nm. A two-step cross-linking was necessary for tannic acid to obtain fibers that were stable in 1 M acetic acid after immersion for 72 h [13Kie].

Quaternized chitosan was mixed with poly(vinylpyrrolidine) (PVP) and electrospun into fibers with antibacterial property. Fibers with diameters between

1.5 and 2.8 pm were obtained. However, the fibers were unstable and dissolved in water even after UV treatment. To improve stability, triethylene glycol diacrylate (TEGDA), 4,4-diazidostilbene-2,2-disulfonic acid disodium salt (DAS), and 2,2-dimethoxy-2-phenylacetophenone (DMPA) were used to improve cross-linking efficiency. After addition of these cross-linking enhancers, the fibers were found to be stable in water for up to 6 h. Fiber morphology, especially diameter, was found to decrease with increase in the chitosan content and was attributed to better solubility [07Ign].

Chitosan was PEGylated to improve solubility and enable fiber formation through electrospinning. Fibers with diameters between 40 and 360 nm were produced using tetrahydrofluoride and dimethylformamide (DMF) as a cosolvent system with the addition of triton X as surfactant [07Du]. PEGylated chitosan with a degree of substitution higher than 1.5 was completely soluble in electrospinnable solvents such as CHCl3DMF, DMSO, and THF.