**1. Introduction**

Chitin and chitosan belong to the polymeric materials of natural origin from the polysaccharides group. The widely used polysaccharides of natural origin also include cellulose and derivatives of hyaluronic and alginic acid. Use for the production of medical devices (contact with the patient's body), makes them meet several requirements: they should maintain their physicochemical properties after

treatment at elevated temperature during sterilization, after exposure to X-ray, detergents and aseptic. Polysaccharide biopolymers, like most polymeric materials, degrade after some time of use, so it is also important that their decomposition products do not cause inflammation, allergic or immune reactions or any other interactions with patients' bodies.

Chitin is a polysaccharide composed of N-acetylglucosamine residues linked by β-1,4-glycosidic bonds. Chitin is the main component of the fungal walls and the shells of arthropods (crustaceans, insects, and arachnids), but is also present in sponges, corals, and mollusks. However, for laboratory and industrial purposes, it is obtained mainly from marine invertebrates such as: crabs, shrimps, lobsters and krill. The properties of chitin depend on its origin. Chitin used in the production of medical devices must come from certified, controlled fisheries and must be properly purified. The methods of isolating chitin from natural sources are strictly dependent on the choice of the organism from which it is isolated. This polysaccharide is similar in structure to cellulose. It differs in the presence of an acetyl amide group (-NHCOCH3) in place of one of the hydroxyl groups (**Figure 1**). The presence of this group means that there are much stronger intermolecular hydrogen bonds in chitin, which results in its greater mechanical strength compared to cellulose [1, 2].

Depending on the origin source, chitin can occur in three amorphous forms: α, β and γ [2, 3]. The most widespread is α chitin found in fungi, shells of crustaceans and krill, and the skeletons of insects. The β form, which can mainly be isolated from squids, is much less common. The differences in the crystal structure of both amorphous forms of chitin affect their processing capabilities. The ordered crystal structure of chitin limits its solubility in commonly used solvents, and thus, reduces its use in industry. α-Chitin is moderately soluble in aqueous thiourea solution, aqueous alkaline urea solution, 5% LiCl/DMAC, some ionic liquids, hexafluoroacetone, hexafluoro-2-propanol, methanesulfonic acid [4, 5]. The form of β-chitin, on the other hand, swells in water (forms a suspension) and is soluble in formic acid. Chitin has no cytotoxic effect *in vitro*, is physiologically inert, biodegradable, has antibacterial properties and is characterized by a high affinity to proteins. During its biodegradation in the wound environment, its oligomers and units are released. Most often, it is used in the form of gel, membranes, fibers, polymer films or is a component of polymer blends. Chitin activates macrophages, stimulates the proliferation of fibroblasts and influences vascularization [6–11].

Despite the very good biological properties of chitin, its practical use is moderate, which is related to its low solubility causing difficulties in its processing. Therefore, chitin is used as a raw material to obtain new derivatives with better performance parameters. In terms of practical use, the most important chitin derivatives are its esters and chitosan, which is a product of chitin deacetylation, which can be classified into the group of amino-polysaccharides.

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*Modulating the Physicochemical Properties of Chitin and Chitosan as a Method of Obtaining…*

The esterification of chitin hydroxyl groups allows to increase the utility potential of the polysaccharide by introducing various substituents, and thus, influencing the physical, chemical and biological properties of materials. The best known are chitin esters, in which the hydroxyl groups are esterified with one type of acylating reagent (presence of the same ester groups on both hydroxyl groups of chitin). Acetylated chitin derivatives (CH3CO- substituent) are prepared with acetic anhydride in the presence of an acid catalyst. However, the physicochemical properties conditioning the processing of chitin acetate turned out to be unsatisfactory [12]. The use of a mixture of orthophosphoric acid and trifluoroacetic acid anhydride as a catalyst allowed to obtain a variety of chitin esters derived from: butyric acid, cyclopropanecarboxylic acid, cyclobutanecarboxylic acid, cyclopentanecarboxylic acid, cyclohexanecarboxylic acid and substituted benzoic acids. In the case of chitin butyrate, the process efficiency (DS (degree of substitution) included in the range 1.9–2.38) was dependent on the excess of butyric acid anhydride use [13–15]. Di-butyrylchitin (chitin di-butyrate, DBC) is an example of a chitin derivative soluble in typical organic solvents [16]. DBC is obtained by chitin esterification with butyric anhydride. Typically, it is a two-stage process. In the first step, chitin is purified from calcium salts with 2 M hydrochloric acid. The next stage is the process of proper esterification of purified chitin. The substrates of the reaction, apart from chitin, are butyric anhydride and the catalyst, which is most often chloric (VII) acid. The reaction is carried out in a heterogeneous system by adding powdered chitin in appropriate proportions to the reaction mixture consisting of butyric anhydride and chloric (VII) acid. The classical esterification process requires the use of substrates in a molar ratio of acid anhydride to N-acetylaminoglucose unit of 10: 1. It is also crucial to carry out the reaction at a temperature of 20°C. Increasing the reaction temperature to 40°C causes a rapid lowering of the molecular weight of the modified polymer. The catalyst concentration has a direct influence on the efficiency of the esterification reaction. The yield of the reaction is the higher when more concentrated chloric (VII) acid is used. However, it should be remembered that the use of too much concentrated chloric (VII) acid results in the macromolecule degradation. The esterification process is completed by adding diethyl ether to the reaction mixture. The isolated product is then heated with water to remove residual chloric (VII) acid. The product obtained in this way is treated for 24 hours with acetone, in which only di-butyrylchitin is dissolved. Then, the solution is concentrated to 5–6%. After the desired concentration is reached, the solution is poured into deionized water to precipitate the polymer, then the product is dried to obtain solid di-butyryl chitin. The above-described process of chitin esterification allows the conversion of free hydroxyl groups on the C3 and C6 carbon of the chitin into ester groups (CH3(CH2)2CO- substituent). Di-butyrylchitin is composed of dibutyl-N-acetyl-amino-glucose units linked by 1,4-β-glycosidic bonds. The polymer is stabilized by hydrogen bonds between the polymer chains. Hydrogen bonds are formed with the participation of the hydrogen atom from the acetylamino group and the oxygen atom from the ester group. This kind of intermolecular interaction determines its good mechanical properties [12–15]. Di-butyrylchitin does not dissolve and does not swell in water, but it dissolves in many organic solvents such as: acetone, methanol, ethanol, tetrahydrofuran (THF), dimethylformamide (DMF), chloroform, methylene chloride and others. Di-butyryl chitin is not easily degraded, it is resistant to γ-radiation (possibility of radiation sterilization), while enzymatic degradation under the influence of lysozyme and CE econase occurs at a

**2. Chitin esters - materials with tailored functional properties**

*DOI: http://dx.doi.org/10.5772/intechopen.95815*

**2.1 Chitin esters in dressing materials**

**Figure 1.** *Structural resemblance of cellulose, chitin and chitosan.* *Modulating the Physicochemical Properties of Chitin and Chitosan as a Method of Obtaining… DOI: http://dx.doi.org/10.5772/intechopen.95815*
