Chitosan is a linear polysaccharide composed of randomly distributed β-(1→4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit). It is made by treating the chitin shells of shrimp and other crustaceans with an alkaline substance, such as sodium hydroxide.
Chitosan has a number of commercial and possible biomedical uses. It can be used in agriculture as a seed treatment and biopesticide, helping plants to fight off fungal infections. In winemaking, it can be used as a fining agent, also helping to prevent spoilage. In industry, it can be used in a self-healing polyurethane paint coating. In medicine, it is useful in bandages (in the form of chitosan hydrocolloid dressing) to reduce bleeding and as an antibacterial agent; it can also be used to help deliver drugs through the skin.
History In 1799, British chemist Charles Hatchett experimented with decalcifying the shells of various crustaceans, finding that a soft, yellow and cartilage-like substance was left behind that is now known to be chitin. In 1859, French physiologist Charles Marie Benjamin Rouget found that boiling chitin in potassium hydroxide solution could deacetylate it to produce a substance that was soluble in dilute organic acids, that he called chitine modifiée. In 1894, German chemist Felix Hoppe-Seyler named the substance chitosan. From 1894 to 1930 there was a period of debate and confusion over the exact composition of chitin and particularly whether animal and fungal forms were the same chemicals. In 1930 the first chitosan films and fibres were patented but competition from petroleum-derived polymers limited their uptake. It was not until the 1970s that there was renewed interest in the compound, spurred partly by laws that prevented the dumping of untreated shellfish waste.
Manufacture
Chitosan is produced commercially by deacetylation of chitin, which is the structural element in the exoskeleton of crustaceans (such as crabs and shrimp) and cell walls of fungi. A common method for obtaining chitosan is the deacetylation of chitin using sodium hydroxide in excess as a reagent and water as a solvent. The reaction follows first-order kinetics though it occurs in two steps; the activation energy barrier for the first stage is estimated at 48.8 kJ·mol−1 at and is higher than the barrier to the second stage.
The degree of deacetylation (DD%) can be determined by NMR spectroscopy and the degree of deacetylation in commercially available chitosan ranges from 60 to 100%. On average, the molecular weight of commercially produced chitosan is 3800–20,000 daltons.
Nanofibrils have been made using chitin and chitosan.
Chemical modifications Chitosan contains the following three functional groups: C2-NH2, C3-OH, and C6-OH. C3-OH has a large spatial site resistance and therefore is relatively difficult to modify. C2-NH2 is highly reactive for fine modifications and is the most common modifying group in chitosan.Qin, Y.; Li, P.; Guo, Z. Cationic chitosan derivatives as potential antifungals: A review of structural optimization and applications. Carbohydr. Polym. 2020, 236, 116002. In chitosan, although amino groups are more prone to nucleophilic reactions than hydroxyl groups, both can react non-selectively with electrophilic reagents such as acids, chlorides, and haloalkanes to functionalize them.Sahariah, P.; Masson, M. Antimicrobial chitosan and chitosan derivatives: A review of the structure–activity relationship. Biomacromolecules 2017, 18, 3846–3868. Since chitosan contains a variety of functional groups, it can be functionalized in different ways such as phosphorylation, thiolation, and quaternization to adapt it to specific purposes.
Phosphorylated chitosan Water-soluble phosphorylated chitosan can be obtained by the reaction of phosphorus pentoxide and chitosan under low-temperature conditions using methane sulfonic acid as the catalyst; phosphorylated chitosan with good antibacterial activity and ionic properties can be prepared by graft copolymerization of chitosan monophosphate.Jayakumar, R.; Selvamurugan, N.; Nair, S.k.V.; Tokura, S.; Tamura, H. Preparative methods of phosphorylated chitin and chitosan—An overview. Int. J. Biol. Macromol. 2008, 43, 221–225.Ardean, C.; Davidescu, C.M.; Nemeş, N.S.; Negrea, A.; Ciopec, M.; Duteanu, N.; Negrea, P.; Duda-Seiman, D.; Musta, V. Factors influencing the antibacterial activity of chitosan and chitosan modified by functionalization. Int. J. Mol. Sci. 2021, 22, 7449.
The good water solubility and metal chelating properties of phosphorylated chitosan and its derivatives make them widely used in tissue engineering, drug delivery carriers, tissue regeneration, and the food industry.Liu, L.; Miao, Y.; Shi, X.; Gao, H.; Wang, Y. Phosphorylated Chitosan Hydrogels Inducing Osteogenic Differentiation of Osteoblasts via JNK and p38 Signaling Pathways. ACS Biomater. Sci. Eng. 2020, 6, 1500–1509.Wei, J.; Xue, W.; Yu, X.; Qiu, X.; Liu, Z. pH Sensitive phosphorylated chitosan hydrogel as vaccine delivery system for intramuscular immunization. J. Biomater. Appl. 2017, 31, 1358–1369.Han, G.; Liu, S.; Pan, Z.; Lin, Y.; Ding, S.; Li, L.; Luo, B.; Jiao, Y.; Zhou, C. Sulfonated chitosan and phosphorylated chitosan coated polylactide membrane by polydopamine-assisting for the growth and osteogenic differentiation of MC3T3-E1s. Carbohydr. Polym. 2020, 229, 115517.
In tissue engineering, phosphorylated chitosan exhibits improved swelling and ionic conductivity. Although its crystallinity is reduced, its tensile strength remains largely unchanged. These properties make it useful for creating scaffolds that can support bone tissue regeneration by binding growth factors and promoting stem cell differentiation into bone-forming cells.Muzzarelli, R.A. Chitosan composites with inorganics, morphogenetic proteins and stem cells, for bone regeneration. Carbohydr. Polym. 2011, 83, 1433–1445. Additionally, to enhance the solubility of chitosan-based hydrogels at neutral or alkaline pH, the derivative N-methylene phosphonic acid chitosan (NMPC-GLU) has been developed. This material maintains good mechanical strength and improve cell proliferation, making it valuable for biomedical applications.LogithKumar, R.; KeshavNarayan, A.; Dhivya, S.; Chawla, A.; Saravanan, S.; Selvamurugan, N. A review of chitosan and its derivatives in bone tissue engineering. Carbohydr. Polym. 2016, 151, 172–188.
Thiolated chitosan Thiolated chitosan is produced by attaching thiol groups to the amino groups of chitosan using a thiol-containing coupling agent.Bernkop-Schnürch, A.; Hornof, M.; Guggi, D. Thiolated chitosans. Eur. J. Pharm. Biopharm. 2004, 57, 9–17.Liu, X.; Li, X.; Zhang, R.; Wang, L.; Feng, Q. A novel dual microsphere based on water-soluble thiolated chitosan/mesoporous calcium carbonate for controlled dual drug delivery. Mater. Lett. 2021, 285, 129142. The primary site for this modification is the amino group at the 2nd position of chitosan's glucosamine units. During this process, thioglycolic acid and cysteine mediate the reaction, forming an amide bond between the thiol group and chitosan. At a pH below 5, thiol activity is reduced, which limits disulfide bond formation.
The modified chitosan exhibits improved adhesive properties and stability due to the covalent attachment of the thiol groups. Lower pH reduces oxidation, enhancing its adhesion properties.Mueller, C.; Verroken, A.; Iqbal, J.; Bernkop-Schnuerch, A. Thiolated chitosans: In vitro comparison of mucoadhesive properties. J. Appl. Polym. Sci. 2012, 124, 5046–5055.Laffleur, F. Evaluation of chemical modified hydrogel formulation for topical suitability. Int. J. Biol. Macromol. 2017, 105, 1310–1314.Federer, C.; Kurpiers, M.; Bernkop-Schnurch, A. Thiolated chitosans: A multi-talented class of polymers for various applications. Biomacromolecules 2020, 22, 24–56. Additionally, thiolated chitosan can interact with cell membrane receptors, improving membrane permeabilityZhang, Y.; Zhou, S.; Deng, F.; Chen, X.; Wang, X.; Wang, Y.; Zhang, H.; Dai, W.; He, B.; Zhang, Q. The function and mechanism of preactivated thiomers in triggering epithelial tight junctions opening. Eur. J. Pharm. Biopharm. 2018, 133, 188–199. and showing potential for applications in bacterial adhesion prevention, for example for coating stainless steel.Mirani, Z.A.; Fatima, A.; Urooj, S.; Aziz, M.; Khan, M.N.; Abbas, T. Relationship of cell surface hydrophobicity with biofilm formation and growth rate: A study on Pseudomonas aeruginosa, Staphylococcus aureus, and Escherichia coli. Iran. J. Basic Med. Sci. 2018, 21, 760.Xu, L.Q.; Pranantyo, D.; Neoh, K.-G.; Kang, E.-T.; Fu, G.D. Thiol reactive maleimido-containing tannic acid for the bioinspired surface anchoring and post-functionalization of antifouling coatings. ACS Sustain. Chem. Eng. 2016, 4, 4264–4272.
Ionic chitosan There are two main methods of chitosan quaternization: direct quaternization and indirect quaternization.
The direct quaternization of chitosan amino acids treats chitosan with haloalkanes under alkaline conditions. Another method is the reaction of chitosan with aldehydes first, followed by reduction, and finally with haloalkanes to obtain quaternized chitosan.Wei, L.; Tan, W.; Wang, G.; Li, Q.; Dong, F.; Guo, Z. The antioxidant and antifungal activity of chitosan derivatives bearing Schiff bases and quaternary ammonium salts. Carbohydr. Polym. 2019, 226, 115256.Liu, W.; Qin, Y.; Liu, S.; Xing, R.; Yu, H.; Chen, X.; Li, K.; Li, P. Synthesis, characterization and antifungal efficacy of chitosan derivatives with triple quaternary ammonium groups. Int. J. Biol. Macromol. 2018, 114, 942–949. The indirect quaternization method refers to introducing small molecules containing quaternary ammonium groups into chitosan, such as glycidyl trimethyl ammonium chloride, (5-bromopentyl) trimethyl ammonium bromide, etc.Shagdarova, B.; Lunkov, A.; Il′ina, A.; Varlamov, V. Investigation of the properties of N-[(2-hydroxy-3-trimethylammonium) propyl] chloride chitosan derivatives. Int. J. Biol. Macromol. 2019, 124, 994–1001.De Oliveira Pedro, R.; Schmitt, C.C.; Neumann, M.G. Syntheses and characterization of amphiphilic quaternary ammonium chitosan derivatives. Carbohydr. Polym. 2016, 147, 97–103. Quaternary ammonium groups can further be introduced into the chitosan backbone via azide-alkyne cycloaddition,Tan, W.; Zhang, J.; Mi, Y.; Dong, F.; Li, Q.; Guo, Z. Synthesis, characterization, and evaluation of antifungal and antioxidant properties of cationic chitosan derivative via azide-alkyne click reaction. Int. J. Biol. Macromol. 2018, 120, 318–324. or by dissolving chitosan in alkali and urea and then reacting it with 3-chloro-2-hydroxypropyl trimethylammonium chloride,Song, H.; Wu, H.; Li, S.; Tian, H.; Li, Y.; Wang, J. Homogeneous synthesis of cationic chitosan via new avenue. Molecules 2018, 23, 1921. which provides a simple and green solution to achieve chitosan functionalization.
Cationic derivatives of chitosan have important roles in bioadhesion, absorption enhancement, anti-inflammatory, antibacterial and anti-tumor applications. Chitosan modified with quaternary ammonium groups is one of the most common cationic chitosan derivatives. Quaternized chitosan with a permanent positive charge has increased antimicrobial activity and solubility compared to normal chitosan.Mi, Y.; Tan, W.; Zhang, J.; Wei, L.; Chen, Y.; Li, Q.; Dong, F.; Guo, Z. Synthesis, characterization, and antifungal property of hydroxypropyltrimethyl ammonium chitosan halogenated acetates. Mar. Drugs 2018, 16, 315.
Properties
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