Chitin, Chitosan, and Alginate Fibers

Keywords

Alginate • Seaweed • Alginate-chitosan • Gel properties • Wound healing

Alginates are polysaccharides found as the cell wall constituents in brown algae (Phaeophyceae) which is considered a seaweed. Although some quantities of alginate are found in most species of brown algae, certain species (Laminaria, Lessonia, Macrocystis, Sargassum) contain 30-45 % alginate by dry weight, and these species are used for extraction. In China, about 2 million tons of seaweeds are artificially cultured for alginate production. In 2009, about 27,000 tons of alginate with an estimated value of $318 million were reportedly produced. Alginates are extracted from raw seaweeds by treating with sodium hydroxide when the alginates in salt form are converted into the water-soluble sodium alginate [08Qin]. Chemically, alginates are linear polymers composed of 1,4,-P-d — mannuronic acid (M) and a-L-guluronic acid (G) residues. The amount of M and G residues and the proportion of the blocks of M and G residues vary between different species and are responsible for the variations in properties between the different alginates [08Qin]. Table 28.1 lists the percentage of M and G residues and the block structures found in commercially available alginates extracted from different types of Brown seaweeds.

Alginate is extensively used in the medical industry mainly as a wound dressing due to the excellent ability to sorb moisture and keep wounds dry. In addition, alginates are also used in food industries as stabilizer and as emulsifier in the textile industry for fiber production and other applications. The naturally occurring sodium alginate is water soluble but is made water insoluble by converting the sodium alginate into other forms. Some of the derivatives of sodium alginates that have been used for fibrous applications are calcium alginate, zinc alginate, copper alginate, barium alginate, aluminum alginate, beryllium alginate, and chromium alginate by changing the type of metal ion in the coagulation bath. Calcium chloride is the most common metal used since it is inexpensive, readily dissolves in water,

and is nontoxic, and therefore calcium alginate fibers are more extensively studied. In addition, alginates have been combined with other natural or synthetic polymers, and several additives have been included to improve the properties and make alginate fibers suitable for various applications.

Fibers have been made from alginates using various approaches and are reported to have unique ion-exchange, gel-forming, and medical properties. Alginate fibers have been widely used for wound dressing and other medical applications because alginates become gels by absorbing wound exudates which avoids the trauma/ discomfort when removing wound dressings [04Kni]. Gelling of alginate also keeps the wound moist and assists in better wound healing.

Sodium alginate fibers were formed by dissolving 5 % alginate in distilled water and aging the solution overnight and later extruding the solution into coagulation baths containing various non-solvents for alginate. Tensile properties of the fibers were dependent on the type of coagulation bath and the drawing speeds. Fibers with fineness of 0.7 den and tenacity of 1.1 g/den have been reported [95Kob].

Due to the relatively low tensile properties of sodium alginate fibers, several methods have been used to improve the properties of alginate fibers. Sodium alginate was mixed with graphene oxide in various ratios (0-8 %) and extruded into fibers by the wet spinning method at different draw ratios [12He]. The addition of graphene oxide substantially increased the tensile strength but decreased elon­gation as seen in Fig. 28.1. The increase in strength and decrease in elongation was also observed with increasing draw ratios from 0 to 100 %. Morphologically, outer surfaces of the fibers were considerably striated and rough as seen in Fig. 28.2 when higher concentrations of graphene oxide were used.

0.70

„ 0.65 —

0 60 — 0.50 — 0.40 —

— 12

0 25

Weight of Graphene Oxide W (%)

Fig. 28.1 Influence of addition of various levels of graphene oxide (GO) on the tensile properties of alginate fibers [12He]

Sodium alginate (1-6 %) was dissolved in water and extruded into coagulation bath containing hydrochloric acid to produce alginic acid fibers or into a calcium chloride (1-3 %) bath to produce calcium alginate fibers [04Kni]. Fibers produced were later coated with hydrolyzed and unhydrolyzed chitosans by passing through chitosan solutions (0-5 %) to enhance their suitability for medical applications. The addition of unhydrolyzed chitosan did not improve the strength but decreased the elongation considerably (Table 28.2) suggesting that chitosan acted as a coating and not as filler. Up to 25 % hydrolyzed chitosan could be added onto the fibers, and an increase in strength was also observed. The addition of chitosan, especially hydrolyzed chitosan, also provided better antibacterial activity [04Kni]. Fiber tenacities obtained were in the range of 1.4-2.8 g/den, and elongation was up to 30 %.

References

[95Kob] Kobayashi, Y., Kamishima, H., Fukuoka, S., Obika, H., Asaoka, T., Tenma, K.: US Patent 5474781 (1995)

[04Kni] Knill, C. J., Kennedy, J. F., Mistry, J., Miraftab, M., Smart, G., Groocock, M. R., Williams, H. J.: Carbohydr. Polymer. 55, 65 (2004)

[08Qin] Qin, Y.: Polym. Int. 57, 171 (2008)

[12He] He, Y., Zhang, N., Gong, Q., Qiu, H., Wang, W., Liu, Y., Gao, J.: Carbohydr. Polymer. 88, 1100 (2012)