To improve the solubility and obtain fibers with good properties, chitosan with Mw as high as 1,400 kDa and 5-25 % PEO was dissolved in a mixture of aqueous acetic and hydrochloric acids and electrospun into fibers [08Des]. Fibers with average diameters of 80 ± 35 nm were obtained with a 95:5 chitosan:PEO blend. Chromium binding as high as 18 mg/g of chitosan was obtained when high molecular weight chitosan and PEO were used, much higher than films with similar ratios of chitosan/PEO.

Bicomponent electrospun fibers were obtained using high molecular weight (1,600 kDa, 82.5 % degree of deacetylation) chitosan and PVA. Addition of PVA was thought to increase the molecular entanglement and lead to fiber formation with chitosan content as high as 50 %. Fibers with diameters ranging from 20 to 100 nm were obtained. It was found that hydrolysis of chitosan into lower molecu­lar weights substantially improved fiber formation. PVA component in the bicom­ponent fibers was removed by treating the electrospun structures with 1 M aqueous NaOH for 12 h. Removal of PVA resulted in porous chitosan fibers with pore diameters ranging from 10 to 100 nm [06Li]. A TEM image of the porous chitosan fiber is shown in Fig. 58.4.

Chitosan has been conventionally blended with PVA or PEO in weight ratios up to 50 % to improve spinnability and obtain fibers for medical and other applications. However attempts to reduce PEO/PVA component and obtain chitosan fibers had limited success. By using ultrahigh molecular weight PEO (UHMWPEO), Zhang et al. reported the production of chitosan fibers with PEO content as low as 5 % and ability to produce fibers with diameters from 100 nm to several micrometers. Aligned nanofibers that could be bundled and made into yarns shown in Fig. 58.5 were obtained [08Zha1].

In a similar study, electrospun fibers were developed from a blend of chitosan, collagen, and PEO and the membranes were cross-linked using glutaraldehyde. Fibers with diameters of 134 ± 42 nm were obtained. Cross-linking led to increase in fiber diameters and modulus, but substantial decrease in elongation, strength, and water absorption was observed [08Che2]. Matrices were found biocompatible to 3T3 fibroblasts in vitro and the in vivo studies indicated that the matrices were better than gauze and commercial collagen sponge wound dressing for wound healing. Addition of chitosan into collagen was found to improve cell attachment and proliferation and provide stable membranes after cross-linking [10Che]. The chitosan blend membranes developed had chitosan contents ranging from 0 to 100 % and strength from 1 to 10 MPa with elongations between 5 and 40 %.

The chelating properties of chitosan were used to study the effect of metal ions on the morphology and integrity of electrospun chitosan structures using a blend of chitosan/PEO solutions [11Su]. The influence of monovalent, bivalent, and triva­lent ions on electrospinnability and morphology of fibers was investigated. Calcium and iron ions reduced fiber diameters and number of beads in the fibers whereas sodium and potassium chloride ions recrystallized and were distributed homo­geneously in the fibers due to the inter — and intramolecular interactions between metal ions and the protonated chitosan [11Su].

Aligned or randomly oriented chitosan nanofibers were produced using chitosan: PEO in 9:1 ratio with acetic acid as the solvent. To improve biocompatibility, the chitosan surfaces were grafted with RGD containing surfaces using bifunctional polyethylene glycol (PEG) chains as the cross-linking agent. RGD containing scaffolds were found to have significantly higher cell compatibility. Although orientation of the nanofibers did not show much effect on cell proliferation, the cell morphology and guidance were influenced by the orientation of the fibers [10Wan].

In another study, carboxymethyl cellulose (CMC) was blended with PVA and made into nanofibers for tissue engineering applications. CMC and PVA were separately dissolved in water and mixed in various proportions and electrospun into fibers. Membranes containing CMC/PVA in 20/80 ratio were cross-linked using glutaraldehyde vapors and heat. Uncross-linked fibers dissolved in water after immersion for 1 h, whereas cross-linked membranes did not dissolve after being in water for 48 h. The membranes obtained could be mineralized using calcium phosphate and were also found to be suitable for culturing mesenchymal stem cells [09Sha]. A 50/50 blend of chitosan and PEO (600 kDa) was electrospun into fibers and cross-linked to various extents using glutaraldehyde [08Von]. Increasing cross-linking time from 10 min to 20 h increased tensile elastic modulus from 0.1 to 2.6 MPa, but the stability and morphological changes after exposure to water were not investigated. Instead of using NaOH in the coagulation bath, it was shown that using saturated sodium carbonate would enable the forma­tion of fibers that were stable in PBS or distilled water for up to 12 weeks [06San].

Antibacterial chitosan nanofibers were obtained by blending chitosan/PVA and adding silver nitrate and titanium dioxide. Fibers with diameters ranging from 270 to 360 nm and ability to inhibit 98-99 % of Escherichia coli and Staphylococ­cus aureus were obtained [09Son].

Biomimetic nanocomposites were prepared using chitosan and hydroxyapatite nanoparticles and then electrospinning the mixture into fibers with the addition of 10 % of ultrahigh molecular weight PEO [08Zha2]. Electrospun matrices containing 70/30 chitosan/hydroxyapatite were seeded with human fetal osteoblast and the ability of the matrices to support the attachment, growth and mineralization were studied. Hydroxyapatite nanoparticles that were synthesized had lengths of 100 nm and a diameter of 30 nm and the electrospun fibers had an average diameter of 214 ± 25 nm. TEM images of the electrospun nanofibers in Fig. 58.6 show that the spindle-shaped hydroxyapatite nanoparticles were distributed across the length of fiber with regions of aggregation. After 10-15 days of culture, the nanofibrous scaffolds were completely covered with layers of cells, secreted extracellular matrix, and mineral deposits. The extents of mineral deposits were found to be much higher on the hydroxyapatite/chitosan scaffolds compared to pure chitosan. Figure 58.7 shows an SEM image of the formation of the extracellular matrices and cluster of mineral deposits on the composite fibers [08Zha2].

Core-shell fibers with chitosan as core and PEO as sheath were developed with fiber diameters of about 250 nm using 3 % chitosan and 4 % PEO in water. Later, PEO was removed by washing with deionized water to obtain chitosan nanofibers

of approximately 100 nm in diameter [08Ojh]. It was also reported that bicompo­nent fibers with chitosan as sheath could be developed using a similar approach.

Blends of chitosan with PEO were made into nanofibers with the assistance of triton X-100 (0.3 %) and DMSO as a co-solvent. Fibers with diameters as fine as 40 nm were obtained and nanofibers developed from 90/10 blend of chitosan/PEO were stable in water and had good cell compatibility [05Bha]. Osteoblasts and chondrocytes cultured on the chitosan/PEO blend matrices showed excellent attachment, growth, and proliferation. Figure 58.8 shows an SEM image of the cells on the scaffolds after 5 days of culture. Scaffolds developed were considered to be suitable for tissue engineering applications.

Blends of collagen, chitosan, and thermoplastic polyurethane (TPU) (60/15/ 25 %) were made into random and aligned nanofibrous scaffolds. The scaffolds developed were cross-linked with glutaraldehyde vapors and characterized for structure and properties and evaluated for potential use as tubular grafts and nerve conduits [11Hua]. It was proposed that collagen and chitosan could mimic the protein and polysaccharide parts in extracellular matrices. Electrospun fibers had diameters in the range of 256-360 nm and the matrices were rolled into tubes and sutured for eventual use as nerve grafts. Figure 58.9 shows pictures of the vascular graft and nerve conduit developed. Addition of TPU increased the mechanical properties as seen from Table 58.6. Cell proliferation and orientation on the blend scaffolds was found to be considerably higher than that on TPU suggesting that the scaffolds could be used for tissue engineering [11Hua].

Chitosan was quaternized, blended with PVA, and electrospun into fibers with diameters ranging from 60 to 200 nm [06Ign]. The membranes were exposed to UV irradiation and cross-linked with TEGDA as the cross-linking agent to improve water stability. Membranes developed exhibited antimicrobial activity to both S. aureus and E. coli.

Formic acid/acetone solvent mixture was used to produce chitosan/poly (caprolactone) (PCL) nanofibers. Amount of chitosan in the solution was 1 % and PCL was 8 % to obtain fibers with diameters of about 116 nm. However, the stability of the fibers in aqueous systems was not studied [10Sha].

Blends of chitosan and poly(lactic acid) were prepared using trifluoroacetic acid as a co-solvent [09Xu1]. Fibers with diameters ranging from 300 to 1,100 nm were obtained and the diameter of the fibers increased almost linearly with increasing ratio of PLA in the blend. Weak interactions between PLA and chitosan were observed using FTIR.

Electrospun fibers for tissue regeneration were prepared using a blend of poly (lactide-co-glycolide) (PLGA), chitosan, and PVA [06Dua]. Chitosan used had a degree of deacetylation of 90 % and a molecular weight of 165 kDa and PVA had a degree of polymerization of 1,750. Fibers were electrospun from PLGA and from a mixture of PVA/Chitosan or PLGA/PVA and chitosan. PLGA was dissolved using tetrahydrofuran (THF) and N, N-dimethylformamide and PVA/chitosan was dissolved using aqueous acetic acid. One syringe containing PLGA and another with PVA/chitosan solution were co-electrospun onto a collection drum and later cross-linked using glutaraldehyde vapors. Table 58.7 shows properties of the electrospun membranes obtained and Fig. 58.10 shows the changes in the dimensions of the membranes before and after incubation in PBS. As seen in the table, the blend membranes had lower strength and modulus but higher elongation than PLGA. Also, cross-linking substantially decreased the swelling of the membranes. Electrospun chitosan/PVA membrane shrunk to 25 % of its original size, whereas the PLGA-chitosan-PVA composite membrane had a shrinkage rate of 47.4 % before cross-linking and 3.2 % after cross-linking [06Dua]. Fibroblasts cultured on the composite membranes showed good attachment and proliferation indicating that the fibers would be suitable for tissue engineering applications [06Dua].

Similarly, chitosan nanofibers implanted subcutaneously in mice did not show any significant changes in morphology after 7 days, but inflammatory cells such as

macrophages were observed on the surface of the fibers [06Noh]. After 28 days, degradation of connective tissue into short fragments was observed suggesting that the membranes were biocompatible.