Introduction
Chitin is a long-chain polymer of N-acetylglucosamine derived from glucose. It is a natural mucopolysaccharide with the chemical formula (C8H13NO5) that is abundantly found in the exoskeletons of arthropods, such as shrimp, crabs, insect cuticles, and lower plants, including yeasts. Chitin is a nitrogen-containing polysaccharide that is white, hard, inelastic and insoluble in water, concentrated solutions, and alkaline solutions. It also has proven therapeutic properties and industrial applications. Chitosan is of great interest due to its non-toxicity, biodegradability, environmental friendliness, economic viability, ability to remove a wide range of dyes and metals, rapid manufacturing, and the potential to prepare a variety of derivatives [1]. It is a natural biopolymer that serves as a scaffold in bone tissue engineering [2]. Chitosan is the deacetylated form of chitin [3-6], a structural element found in the exoskeletons of crustaceans, such as shrimp and crabs, as well as in insect cuticles and fungal cell walls. Chitosan possesses low to moderate mechanical properties, which limits its use in various applications. However, the addition of various polymers or nanoparticles enhances its mechanical properties [3, 7]. Chitosan is a suitable material for the preparation of oral, nasal, vaginal, and subcutaneous delivery forms due to its mucosal adhesive nature and increased penetration. It is also used as a vaccine additive to enhance the bioavailability and immunogenicity of antigens. Additionally, chitosan-based gels possess the viscosity required for injection in periodontal applications. Most importantly, they can serve as carriers for the release of active drugs at the site of disease [9]. In the present study, the applications of chitin and chitosan in the biomedical field are discussed, with an emphasis on tissue engineering.

Methods
Articles on chitosan and its extensive applications were searched in the Google Scholar, Scopus and PubMed databases using the keywords “chitin,” “chitosan,” AND “tissue engineering.” Medical subject headings and similar phrases were created and searched by two researchers proficient in scientific search methods appropriate for this topic. For books, the selection criteria included thematic relevance and availability.

Results
Applications of chitin and chitosan
Chitin is an amino polysaccharide and the second most widely used natural polymer. Extensive studies have highlighted its non-toxic, biocompatible and biodegradable properties [10]. These advantages make it an ideal material for preparing functional hydrogels that can serve as wound dressings for various injuries. Additionally, due to the presence of amino and hydroxyl groups, chitosan possesses important intrinsic properties, such as mucoadhesiveness, antimicrobial and antifungal properties, controlled drug release capabilities, excellent mechanical and thermal properties, hydrophilicity, ease of surface modification, high affinity for metals, dyes, and proteins, as well as the ability to form polycations in acidic environments and to create films. These properties have made chitin and chitosan suitable for use in medicine and pharmacy, as well as in various industries focused on environmental protection, water purification processes, and different types of separation processes [11, 12]. The insolubility of chitosan in water and most chemical solvents has limited its use in many fields. However, chemical modification of chitosan has enhanced its physicochemical properties and expanded its diverse applications [13, 14]. The aforementioned properties depend on the molecular weight and degree of acetylation of chitosan. Considering these remarkable properties, chitosan appears to be a suitable material for the preparation of biomaterials that can replace lost or damaged tissues and organs while allowing for cell attachment and proliferation [22, 23]. Chitosan, as a hydrogel, is an interesting biomaterial because it can retain a high water content. This high water content makes it compatible with most living tissues; additionally, it is soft and flexible, which minimizes damage to the surrounding tissue during and after transplantation in patients. Furthermore, the mechanical properties of chitosan hydrogels are similar to those of soft tissues in the body, enabling them to provide the functional and morphological characteristics required for tissue repair [24]. For these reasons, chitosan is often used as a biomedical scaffold for tissue replacement, as well as for other biomedical applications, such as drug and growth factor delivery [25-27]. Chitosan applications have also been observed in cosmetics, skin, hair, oral and dental applications, antimicrobial and antitumor activities, and tissue engineering.
Chitin and chitosan have numerous applications in new treatments within the medical field. Recently, the use of chitosan in tissue engineering has yielded effective and impressive results. Chitosan plays a beneficial role in the treatment of diseases, including cancer, in the form of nanoparticles and within drug delivery systems. It significantly impacts skin repair systems and has also been recognized in the medical field as an antioxidant, antimicrobial and antibacterial agent. Many studies are still being conducted in the scientific community to explore new applications for this material. As previously mentioned, chitin and chitosan, the most abundant amino polysaccharides in nature, possess unique properties (biocompatibility, biodegradability, non-toxicity, antimicrobial, antitumor, antioxidant, and hemostatic properties) [80, 81]. These characteristics have attracted considerable attention to chitin and chitosan, not only due to their abundance in natural resources but also because of their high potential for the preparation of functional materials. Significant progress has been made in enhancing their properties for use in tissue engineering, wound healing and drug delivery systems, as well as for effective treatments in cancer research. 

Conclusion
The remarkable properties of chitosan present unique opportunities for the development of biomedical applications. Elucidating their mechanisms will lead to a better understanding of chitosan in the medical and pharmaceutical fields.

Ethical Considerations
Compliance with ethical guidelines
There were no ethical considerations to be considered in this research. 

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Conceptualization, study design, data collection, analysis, interpretation and drafting of the manuscript: Leila Rezakhani and Morteza Alizadeh; Critical review: Leila Rezakhani, Morteza Alizadeh and Taybeh Sadat Tabatabaei.

Conflicts of interest
The authors declared no conflicts of interest.



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