Chitin and chitosan polymers: Chemistry, solubility and fiber formation
Introduction
Chitin and chitosan (CS) polymers are natural aminopolysaccharides having unique structures, multidimensional properties, highly sophisticated functions and wide ranging applications in biomedical and other industrial areas [1], [2], [3]. Being considered to be materials of great futuristic potential with immense possibilities for structural modifications to impart desired properties and functions, research and development work on chitin and CS have reached a status of intense activities in many parts of the world [4], [5], [6]. The positive attributes of excellent biocompatibility and admirable biodegradability with ecological safety and low toxicity with versatile biological activities such as antimicrobial activity and low immunogenicity have provided ample opportunities for further development [7], [8], [9], [10], [11], [12]. It has become of great interest not only as an under-utilized resource but also as a new functional biomaterial of high potential in various fields [13], [14], [15].
With data emerging from not less than 20 books, over 300 reviews, over 12,000 publications and innumerable patents, the science and technology of these biopolymers are at a turning point where one needs a very critical look on its potential to deliver the goods [16], [17]. Prior to doing so, it is necessary to overview the data emerged on one of the serious problems faced in the utilization of chitin and CS. Despite its huge annual production and easy availability, chitin still remains an under utilized resource primarily because of its intractable molecular structure [10], [16]. The non-solubility of chitin in almost all common solvents has been a stumbling block in its appropriate utilization [4], [5], [6], [13]. This review proposes to consolidate and discuss the available data on the work on the chemistry related to the solubilization of chitin and CS and the attempts at fiber formation.
There have been a number of earlier attempts at reviewing the area on chitin and CS fibers covering certain aspects of their importance, properties and applications [18], [19], [20], [21], [22], [23], [24], [25]. Rathke and Hudson [18] pointed out that chitin's microfibrillar structure indicated its potential as fiber- and film-former, but as chitin was found to be insoluble in common organic solvents, the N-deacetylated derivative of chitin, CS, was developed. After Rinaudo and coworkers [24] who described the production of chitin and CS fibers by wet spinning method in 2001 and Rajendran and Anand [25] who discussed briefly the properties of chitin and chitin fibers in 2002, there have been no serious attempts at reviewing the production, properties and applications of chitin and CS fibers. Considering the potential applications of chitin and CS fibers, it appears that a consolidation of the data relating the chemistry, solubility and fiber formation of chitin and CS polymers is required. Chitin fibers stand apart from all the other biodegradable natural fibers in many inherent properties such as biocompatibility, non-toxicity, biodegradability, low immunogenicity, non-toxicity, etc. [5], [10], [11], [18]. These properties in combination with good mechanical properties make them good candidate materials for sutures that form the largest groups of material implants used in human body [5], [8], [26]. It was reported that the chitin suture was absorbed in about 4 months in rat muscles [26]. Application in 132 patients proved satisfactory in terms of tissue reaction and good healing indicating satisfactory biocompatibility. Toxicity tests, including acute toxicity, pyrogenicity, and mutagenicity were negative in all respects. The persistence of the tensile strength of the chitin was better than Dexon™ or catgut in bile, urine and pancreatic juice but weakening occurred early in the presence of gastric juice [26]. Apart from sutures, chitin and CS fibers have been found to be useful in other medical textiles [27], [28], wound dressing [2], [29], [30], [31], [32], [33], [34] and haemostatic materials [35], [36], [37], [38], [39] and several other prosthetic devices such as haemostatic clips, vascular and joint prostheses, mesh and knit abdominal thoracic wall replacements and as antimicrobial agents [39], [40], [41].
Section snippets
General remarks
It is now well established that the difficulty in solubilization of chitin results mainly from the highly extended hydrogen bonded semi-crystalline structure of chitin [6], [14], [42], [43], [44]. Chitin is a structural biopolymer, which has a role analogous to that of collagen in the higher animals and cellulose in terrestrial plants [43], [44], [45]. Plants produce cellulose in their cell walls and insects and crustaceans produce chitin in their shells [42]. Cellulose and chitin are, thus,
Criteria for polymer solubility
Owing to the semi-crystalline structure of chitin with extensive hydrogen bonding, the cohesive energy density and hence the solubility parameter will be very high and so it will be insoluble in all the usual solvents [6], [44], [50], [94], [95], [96], [97], [98]. The solubility parameter of chitin and CS was determined by group contribution methods (GCM) and the values were compared with the values determined from maximum intrinsic viscosity, surface tension, the Flory–Huggins interaction
General remarks
The general properties of chitin and CS are provided in Table 1.
While chitin is insoluble in most organic solvents, CS is readily soluble in dilute acidic solutions below pH 6.0. This is because CS can be considered a strong base as it possesses primary amino groups with a pKa value of 6.3. The presence of the amino groups indicates that pH substantially alters the charged state and properties of CS [12]. At low pH, these amines get protonated and become positively charged and that makes CS a
Chitin fiber formation and uses
Sutures are probably the largest groups of material implants used in human body and the suture market is very huge with a total tally exceeding $1.3 billions annually [329]. Physicians have used sutures for the past at least 4000 years [330]. Archaeological records from ancient Egypt and India show use of linen, animal sinew, flax, hair, grass, cotton, silk, pig bristles, and animal gut to close wounds [330], [331]. The famed Susruta is reported to have used suture materials of bark, tendon,
Fiber formation
Development of fibers from CS was comparatively easy as it was soluble in dilute acids such as acetic acid. Formation of the fiber was reported as early as 1926 [150]. But CS fibers were found to be expensive due to high production cost [330]. This induced researchers to look into blends or composites with other existing yarns. Production of fibers with chemical modification such as grafting has also been reported. Table 4 provides the attempts at production of CS fiber. CS fibers having
Chitosan fibers and blends by electrospinning technique
Electrospinning is emerging as a promising and highly versatile method to process solutions or melts, mainly of polymers, into continuous fibers with diameters ranging from a few micrometers to a few nanometers [372], [441]. Application of this method has provided CS nanofibers and CS fiber blends with nanofibers with improved properties [442]. Parameters such as type of solvent (fluorinated solvents such as trifluoracetic acid, fluoroalcohols, etc. are also being used for electrospinning [442]
Comparative evaluation of the merits of various processes
It is appropriate at this stage to discuss the comparative merits of various methods that have been developed for fiber formation and spinning of chitin and CS polymers. One of the major hurdles was the necessity to use strong acids and polar solvents to induce chitin solubility [6], [18], [112], [113], [114]. Chlorohydrocarbons used in some processes [159], [160], [161], [162], [163], [164], [165] are well known as environmentally unacceptable solvents and HFP and HFAS [169], [170], [171] are
Novel applications
Porous CS fibers have been shown to be useful as reinforcement in CS based nerve conduits fabricated from CS yarns and a CS solution by combining an industrial braiding method with a mold casting/lyophilization technique [478]. The compressive load of the reinforced conduits was significantly higher than that of a non-reinforced control conduit at equal levels of strain. The tensile strength of the reinforced conduits was also increased from 0.41 ± 0.17 to 3.69 ± 0.64 MPa. An in vitro cytotoxicity
Conclusion
Chitin and CS are biopolymers having immense structural possibilities for chemical and mechanical modifications to generate novel properties, functions and applications especially in the biomedical area. Despite its huge availability, the utilization of chitin has been restricted by its intractability and insolubility. Several attempts have been reported on solving these problems, which have been reviewed. However, there are several drawbacks that need to be addressed. The corrosive and
Acknowledgements
We are grateful to the Prof. K. Mohandas, Director, and Dr. G.S. Bhuvaneshwar, Head BMT Wing of Sree Chitra Tirunal Institute for Medical Sciences & Technology, for providing facilities for the completion of this review. We are thankful to the laboratory staff and library staff for their assistance. We also acknowledge the partial assistance under FADDS by Department of Science & Technology New Delhi.
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