**2.3. Chitosan**

Chitosan is a natural aminopolysaccharide produced from partial alkaline deacetylation of chitin. Chitin, the second largest natural polymer after cellulose, is the structural element found in the exoskeleton of crustaceans, insects, and fungi. Just like plants produce cellulose in their cell walls, insects and crustaceans produce chitin in their shells. Chitosan is composed of linear copolymer of β (1–4) linked 2‐acetamido‐2‐deoxy‐β‐D‐glucopyranose and 2‐amino‐2‐deoxy‐ β‐D‐glycopyranose. Different factors, such as alkali concentration, incubation time, ratio chitin to alkali, temperature, atmosphere, source of chitin, and particle size, play a role in affecting the properties of chitosan [10]. Chitin possesses poor solubility in aqueous solution and organic solvents mainly because of the highly extended hydrogen bonded semicrystalline structure of chitin, thus limiting its practical application in biomaterials [11]. Chitin has the degree of acetylation (DA) of 0.90 [12]. Whereas chitosan possess primary amino groups with pKa value of 6.3. These amines get protonated and form water‐soluble and bioadhesive chitosan which readily bind to negatively charged surfaces [13].

Unlike chitin, chitosan has highly sophisticated functionality and wide range of applications in biomedical and other industrial areas. The advantage of chitosan over other polysaccharides is because of its cationic character and primary amino group [14]. Although they exhibit similar structure, chitosan display different properties from that of cellulose. When the degree of deacetylation of chitin reaches about 50%, it becomes chitosan and soluble in aqueous acidic media [15]. Chitosan has been applied in agriculture, water and waste management, food and beverages, cosmetics and toiletries, and biopharmaceutics.
