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The water-soluble chitosan oligomers generated with chemical, physical or enzymatic hydrolysis were investigated in vitro and/or in vivo as antioxidants. They inhibited myeloperoxidase activity, and preserved DNA and proteins in mouse macrophages from oxidation [138, 139] in myeloid cells. In addition, chitosan has been shown to minimize levels of free fatty acids and malondialdehyde while increased levels of antioxidant enzymes such as glutathione peroxidase (GPX), catalase (CAT) in serum [140]. Microwave-synthesized chitosan oligomers showed scavenging capabilities on 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicals [141]. Chitosan’s biocompatibility makes it ideal for therapeutic use in vivo [142]. It is also used for subcutaneous, oral, ocular and transdermal drug delivery systems [79]. A chitosan–graphene oxide biocomposite was used as anti-cancer molecule, with superior loading characteristics [143]. Besides, it was also investigated as an agent for buccal, stomach, intestinal and colon-specific drugs being delivered in vivo [144]. In another study, folate conjugated chitosan–graphene oxide nanocomposites were also studied for drug and gene delivery [145]. Vaccines, which are drugs delivered directly to the affected site through injectable formulations, have also been delivered though chitosan based materials [146–148]. For example, Takeuchi et al. have developed chitosan coated, insulin-loaded liposomes to improve insulin absorption through enhanced muco-adhesion, making chitosan an ideal candidate for buccal delivery [149]. It can also be used for nasal vaccine delivery. For instance, a glutamate–chitosan biocomposite can increase transport of insulin in mammals through the nasal mucosa [150]. The presence of chitosan in the vascular system has been shown to improve Th1 immunity, downregulate Th2 immunity and enhance the release of cytokines by macrophages [151]. Modified chitosan inhibited the growth of tumor cells [152], while conjugated chitosan forms were effective against mice lymphocytic leukemia and inhibited the growth of Met-A fibrosarcoma and MH-134Y hepatoma cells [153]. Furthermore, γ-irradiated and cinnamon bark oil incorporated chitosan showed enhanced anti-oxidant activity [154]. A lesser known but equally important application of chitosan is related to its hydrolysis product, glucosamine. Along with chondroitin sulfate, glucosamine is used for the treatment of osteoporosis and arthritis [155].

The anti-tumor activity of chitosan is also molecular weight dependent; medium molecular weight chitosan was found effective against carcinoma cells. These nanoparticles were found to be acting against S180 and hepatoma 22 (H22) cell lines [156, 157]. In addition, chitosan based materials are investigated as anti-coagulants [158–160], anti-diabetic [161–164], anti-viral [165–167], thrombosis [168], hemostasis [169], and hepatoprotective [170–172] activities. It was interesting to note that, owing to its properties, the same chitosan was used for completely opposite applications: enhancing and dissolving blood clots. Besides, chitosan is also used as an effective flocculating agent for harvesting microalgal biomass [24, 173–176].