# An Overview of Biopolymers for Drug Delivery Applications
## 2 Types of Biopolymers Used in Drug Delivery Systems
### Polysaccharides
#### 2.1.1 Chitosan  
One of the most prevalent and non-toxic biopolymers that has been thoroughly studied for a variety of medicinal uses is chitosan [30; 31; 32]. This cationic polysaccharide, which is composed of N-acetyl and D-glucosamine, is produced from the naturally occurring polymer chitin. Naturally, chitosan is found in the exoskeletons of crustaceans, fungi [33], annelids, mollusks, and insects. Also, chitosan is biocompatible, adhesive, hemostatic, and mucoadhesive [31; 32; 34]. Chitosan is non-toxic, odorless, biocompatible, biodegradable, and has antibacterial properties. Frequently, this biopolymer is used for microencapsulation for cells that need a cationic medium. Chitosan has been used in DDSs in different forms, such as gels, films, beads, oral tablets, and microspheres for oral, nasal, ocular, and transdermal routes [35; 36]. Figure 3: The chemical structures of the most used polysaccharides for developing DDSs. Also, the anti-tumor and fungal properties of chitosan play an important role in the development of bio-dental materials and the treatment of periodontitis, as well as dental pulp regeneration [37]. Chitosan can be administered parenterally, intravenously, nasally, vaginally, or via injection. One major benefit of utilizing chitosan in DDSs is that drug absorption and stability can be achieved while controlling drug diffusion from the material to the target dosages [38]. By mixing chitosan with various natural and synthetic polymers, such as sodium alginate, polylactic acid, polycaprolactone, poly(ethylene oxide), poly(vinyl pyrrolidine), and graphene oxide, one can alter the material's characteristics [39; 40]. Chitosan's -NH\({}_{2}\) and -OH groups allow it to interact with biological molecules and other polymers. As a result, chitosan alone or combined with other materials represent an applicable substrate for obtaining different new nanocarriers (e.g., films, hydrogels, foams, beads, gels, nanoparticles, nanofibers, and sponges [41; 42]). These combinations improve the drug properties regarding the release process and improve their mechanical and physical properties [43]. [PERSON] and coworkers developed a hydrogel prepared from chitosan using sodium dialdehyde alginate and dopamine via grafting, crosslinking, and compounding for drug delivery and bladder cancer treatment. The obtained drug-delivery hydrogel demonstrated strong organ-wall adhesion and targeting capacity. Also, the developed hydrogel showed antibacterial and antimicrobial properties (98%) and biocompatibility (99%) [44]. To treat breast cancer, a hybrid nanoparticle of calcium phosphate and folate-functionalized carboxymethyl chitosan loaded with curcumin was created. The resulting materials showed good biocompatibility, stability, and pH-responsive drug release. The outcomes demonstrated that the materials based on carboxymethyl chitosan had an organelle-targeting cancer therapeutic method [45]. A recent study performed by [PERSON] et al. demonstrated that the molecular weight (20.6 and 57.5 kDa) of chitosan has an important role in the physicochemical, morphological, and biological properties of polyplex nanoparticles designed for gene delivery. Their research demonstrated that the low molecular weight of chitosan and the low nitrogen/phosphorus ratio were suitable for designing chitosan-based nonviral vectors for gene therapy because of their physicochemical and biological properties. Also, the stability of the obtained nanoparticles was greater than those formulated with chitosan of a higher molecular weight [46]. An important role of the nanoparticle assembly for gene therapy is the degree of deacetylation to form poly-D-glucosamine [47]. Thus, the amine groups of D-glucosamine make its conjugation possible with some crosslinkers to form covalent bonds with molecules, which permits directed gene therapy to specific cell types. In conclusion, these very important characteristics qualify chitosan as a suitable polymer for designing new nonviral vectors [48].