ReviewA review of chitin and chitosan applications☆
Introduction
Chitin, a naturally abundant mucopolysaccharide, and the supporting material of crustaceans, insects, etc., is well known to consist of 2-acetamido-2-deoxy-β-d-glucose through a β (1→4) linkage. Chitin can be degraded by chitinase. Its immunogenicity is exceptionally low, in spite of the presence of nitrogen. It is a highly insoluble material resembling cellulose in its solubility and low chemical reactivity. It may be regarded as cellulose with hydroxyl at position C-2 replaced by an acetamido group. Like cellulose, it functions naturally as a structural polysaccharide. Chitin is a white, hard, inelastic, nitrogenous polysaccharide and the major source of surface pollution in coastal areas. Chitosan is the N-deacetylated derivative of chitin, although this N-deacetylation is almost never complete. A sharp nomenclature with respect to the degree of N-deacetylation has not been defined between chitin and chitosan [1], [2]. The structures of cellulose, chitin and chitosan are shown in Fig. 1. Chitin and chitosan are of commercial interest due to their high percentage of nitrogen (6.89%) compared to synthetically substituted cellulose (1.25%). This makes chitin a useful chelating agent [1]. As most of the present-day polymers are synthetic materials, their biocompatibility and biodegradability are much more limited than those of natural polymers such as cellulose, chitin, chitosan and their derivatives. However, these naturally abundant materials also exhibit a limitation in their reactivity and processability [3], [4]. In this respect, chitin and chitosan are recommended as suitable functional materials, because these natural polymers have excellent properties such as biocompatibility, biodegradability, non-toxicity, adsorption properties, etc.
Recently, much attention has been paid to chitosan as a potential polysaccharide resource [5]. Although several efforts have been reported to prepare functional derivatives of chitosan by chemical modifications [6], [7], [8], very few attained solubility in general organic solvents [9], [10] and some binary solvent systems [11], [12], [13]. Chemically modified chitin and chitosan structures resulting in improved solubility in general organic solvents have been reported by many workers [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The present review is an attempt to discuss the current applications and future prospects of chitin and chitosan.
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Processing of chitin and chitosan
Chitin is easily obtained from crab or shrimp shells and fungal mycelia. In the first case, chitin production is associated with food industries such as shrimp canning. In the second case, the production of chitosan–glucan complexes is associated with fermentation processes, similar to those for the production of citric acid from Aspergillus niger, Mucor rouxii, and Streptomyces, which involves alkali treatment yielding chitosan–glucan complexes. The alkali removes the protein and deacetylates
Economic aspects
The production of chitin and chitosan is currently based on crab and shrimp shells discarded by the canning industries in Oregon, Washington, Virginia and Japan and by various finishing fleets in the Antarctic. Several countries possess large unexploited crustacean resources, e.g. Norway, Mexico and Chile [25]. The production of chitosan from crustacean shells obtained as a food industry waste is economically feasible, especially if it includes the recovery of carotenoids. The shells contain
Properties of chitin and chitosan
Most of the naturally occurring polysaccharides, e.g. cellulose, dextran, pectin, alginic acid, agar, agarose and carragenans, are neutral or acidic in nature, whereas chitin and chitosan are examples of highly basic polysaccharides. Their unique properties include polyoxysalt formation, ability to form films, chelate metal ions and optical structural characteristics [29].
Like cellulose, chitin functions naturally as a structural polysaccharide, but differs from cellulose in its properties.
Natural microfibriller arrangement
Chitin has been known to form microfibrillar arrangements in living organisms. These fibrils are usually embedded in a protein matrix and have diameters from 2.5 to 2.8 nm. Crustacean cuticles possess chitin microfibrils with diameters as large as 25 nm. The presence of microfibrils suggests that chitin has characteristics which make it a good candidate for fibre spinning. To spin chitin or chitosan fibres, the raw polymer must be suitably redissolved after removal of extraneous material such
Applications
The interest in chitin originates from the study of the behaviour and chemical characteristics of lysozyme, an enzyme present in human body fluids [70]. A wide variety of medical applications for chitin and chitin derivatives have been reported over the last three decades [71], [72], [73]. It has been suggested that chitosan may be used to inhibit fibroplasia in wound healing and to promote tissue growth and differentiation in tissue culture [74].
The poor solubility of chitin is the major
Conclusion
Chitin and chitosan have a wide range of applications. They may be employed, for example, to solve numerous problems in environmental and biomedical engineering. Chitin derivatives including partially deacetylated chitosan can be easily molded to various forms and their derivatives are digested in vivo by lysozomal enzymes. Thus, it appears that this material can be a most interesting candidate for use as a carrier of a variety of drugs for controlled-release applications. Lately, the
Acknowledgements
The author thanks Dr K.G. Ramachandran Nair, Central Institute of Fisheries Technology, Kochi, India, for providing the sample of chitosan. The author is grateful to the Council of Scientific and Industrial Research (CSIR), Ministry of Human Resource Development Groups, Govt. of India, New Delhi, for financial assistance to carry out this research. The author is indebted to the referees for a thorough revision of the manuscript.
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This paper is dedicated to Professor M.N.V. Prasad, Ph.D., FNIE (New Delhi), DSc. (hc Colombo), School of Life Sciences, University of Hyderabad, Hyderabad, India, who inspired me with his scientific approach, honesty and human warmth.