Chitosan fibers lack adequate stability under aqueous environments, and attempts have therefore been made to modify or cross-link the fibers using various approaches. Glyoxal was used to cross-link chitosan fibers, and the effect of cross-linking conditions on the structure and properties of the fibers were studied [05Yan1, 05Yan2]. Uncross-linked fibers had a tenacity of 1.2 g/den, and the tenacity improved to about 2.4 g/den after cross-linking. pH during cross-linking was found to affect the tenacity to a greater extent compared to concentration of glyoxal or cross-linking time. A pH of 3.8 provided the highest tenacity to the fibers. A relationship (S = 79.24 — 5.05C + 0.24C2) was developed to predict the swelling of the fiber (S) and the concentration of glyoxal (C) used. Cross-linking decreased the % crystallinity of the fibers to 27.2 % compared to 34.7 % and changed the crystal structure from a to the p form.

Epichlorohydrin was used to cross-link chitosan fibers, and the effect of the concentration of the cross-linking agent on the properties of the fibers was studied by Lee et. al. [04Lee]. A cross-linking agent was added into the coagulation bath, and the fibers were later drawn in a water bath at 99 °C. Increasing the concentra­tion of the cross-linking agent up to 0.05 M decreased water sorption and therefore swelling. Dry strength of the fibers decreased from 1.6 g/den to about 1.3 g/den when the concentration of epichlorohydrin was increased indicating over or excess cross-linking. However, the wet strength of the fibers showed an increasing trend from 0.9 to 1.2 g/den. Dry elongation decreased substantially from about 16 to 13 %, whereas wet elongation did not show any appreciable change. X-ray studies did not reveal any major changes in the crystal structure, and differential scanning calorimetry curves for the uncross-linked and cross-linked fibers were similar [04Lee]. In another study, the effect of the concentration of coagulation bath retardant sodium acetate on the cross-linking of chitosan fibers with epichlorohy — drin was studied [07Lee]. Both dry and wet tenacities improved with increasing concentration of sodium acetate. Dry tenacity of the fibers obtained was about

1.4 g/den, and wet tenacity was 1.3 g/den when the concentration of sodium acetate was 20 %, whereas the elongation did not show a significant difference. Thermal stability of the fibers increased substantially to 713 J/g compared to 359 J/g before cross-linking. A major advantage of this process was that the coagulation and cross-linking occurred in one step, and the wet stability of the fibers was considerably high.

Glutaraldehyde was also used as a cross-linking agent to cross-link chitosan fibers intended to be used for controlled drug release [00Den]. Chitosan dissolved using acetic acid was extruded into an ethanol coagulation bath containing glutar — aldehyde. The drug 5-fluorouracil (5-FU) was mixed with the chitosan and extruded with the fibers. Cross-linking substantially decreased the swelling ratio from 60 % to about 10 %. Drug loading on the fibers ranged from 0.2 to 7.6 mg/g, and there was an initial burst release and then a stable and uniform release. Fibers cross — linked to higher extents showed lower release because the fibers could not swell [00Den]. The effect of cross-linking conditions on the properties of chitosan fibers cross-linked with glyoxal and glutaraldehyde was studied by Knaul et al. [99Kna1]. Considerable changes in the tensile properties of the fibers were observed by varying the concentration of the cross-linking agent, time, and temperature of cross-linking. Fibers (18.6 den) with tenacities ranging from 1.4 to 1.8 g/den and elongation from 9 to 37 % were obtained by cross-linking with glyoxal. Glutaraldehyde provided higher strength (1.5-2.3) g/den but similar elongation (4.7-37 %) compared to cross-linking with glyoxal. Conventionally, the cross-linking between glutaralde — hyde and chitosan in acidic conditions was considered to be a Schiff’s base reaction. However, the reaction is also proposed when chitosan reacts with glutaraldehyde or glyoxal in neutral aqueous solutions. Knaul et al. confirmed that a Schiff’s base reaction occurs between glutaraldehyde and chitosan but suggested that the reaction is between the chitosan free amine and a hemiacetal [99Kna2]. The reaction between glyoxal and chitosan was considered to be between two polymer acetals and not between two carbon dialdehydes. Although the improvement in dry strength after glyoxal and glutaraldehyde cross-linking was evident, the changes in the wet strength or stability of the fibers in aqueous environments were not investigated.

Phosphate groups were incorporated onto chitosan fibers through ionotropic gelation to provide amphoteric characteristics and improve protein adhesion [11Pat]. Chitosan-tripolyphosphate fibers with varying phosphate content were prepared by extruding chitosan into coagulation bath containing 5 % sodium tripolyphosphate at different pHs. The TPP ions acted as coagulants and cross­linking agents, and a degree of cross-linking of up to 85 % was obtained which was highly dependent on the pH of the solution [11Pat]. Cross-linking with TPP was found to decrease crystallinity and thermal stability of the fibers due to freezing of the polymer network.

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