Chitosan

Chitosan is a natural and nontoxic linear biopolymer synthesized from alkaline N-deacetylation of chitin [143]. It can be extracted from exoskeleton of crustacean shells (i.e., crabs and shrimps) some microbes, yeast, and fungi [144]. It has different molecular weights and is soluble in various organic solutions at pH 6.5 and below. The shape of the chitosan nanoparticles is affected by the degree of deacetylation [145,146]. The presence of amine and hydroxyl group leads the use of these compounds in many research areas [147,148]. Chitosan has outstanding biochemical properties, making it very attractive for applications in many areas including tissue engineering and/or regenerative medicine (Figure 7) [149]. Chitosan nanoparticles carry well therapeutic agents and biomolecules because of their high biocompatibility and biodegradability. Because of their small size, they can pass through biological barriers in vivo and deliver the drugs at the targeted site [150].

**Figure 7.** Prospective applications of chitosan.

Applications of Chitosan: Scaffolds prepared from chitosan and ceramics, especially hydroxyapatite, may have superior osteoconductive properties [151]. Bone morphogenetic protein-2 (BMP-2)-loaded chitosan nanoparticles used for the coating of Ti implants were selected in order to examine bone regeneration in mice [55]. Chitosan and growth factor (BMP-7) were used to functionalize a thick electrospun poly(ε-caprolactone) nanofibrous implant (from 700 μm to 1 cm thick), which produced a fish scale-like chitosan/BMP-7 nano-reservoir. This nanofibrous implant mimicked the extracellular matrix and enabled in vitro colonization and bone regeneration [152]. There, the polycationic nature of chitosan entails an antimicrobial behavior at nanoscale [153]. Besides the orthodontic field, there are relevant applications of chitosan in skin healing, nerve regeneration, and oral mucosa [39]. Nanobioglass incorporated chitosan-gelatin scaffolds showed excellent cytocompatibility and ability to accelerate the crystallization of bone-like apatite in vitro [154,155]. The nanocomposite of chitosan/hydroxyapatite-zinc oxide (CTS/HAp-ZnO) supporting organically modified montmorillonite clay (OMMT) was synthesized and used for hard tissue engineering applications [156]. BMP-2 and BMP-7 loaded poly(3-hydroxybutyrate-co3-hydroxyvalerate) nanocapsules were used for the fabrication of chitosan-poly(ethylene oxide) scaffolds [157]. Mili et al. [158] used nerve growth factor (NGF) loaded chitosan nanoparticles for neural differentiation of canine mesenchymal stem cells. Freeze-dried nano-TiO2/chitosan scaffolds showed high biocompatibility and antibacterial effects [159]. Chitosan-poly(vinyl alcohol)-gum tragacanth (CS/PVA/GT) hybrid nanofibrous scaffolds showed 20 MPa ultimate tensile strength and supported L929 fibroblast cells growth [160]. Collagen–chitosan–calcium phosphate microsphere scaffolds fused with glycolic acid did not show relevant differences in their degradation, cytocompatibility, porosity, and Young's modulus [160,161].
