**2. Properties of chitosan**

The molecular structure of chitosan, which is obtained by partial deacetylation of chitin and contains β-(1,4)-D-glucosamine and N-acetyl-D-glucosamine units, is shown in **Figure 1** [5].

#### **2.1 Physical and chemical properties of chitosan**

There are some parameters that affect the characteristic properties of chitosan, such as molecular weight, degree of deacetylation, and solubility. These parameters vary according to the conditions applied during the production of chitosan [6]. The process of removing acetyl groups in the chitin polysaccharide chain is called deacetylation. Chitin, which usually has a degree of deacetylation greater than 50%, is considered to be chitosan [7, 8]. The molecular weight of chitosan varies depending on the conditions applied during the deacetylation process and the source from which it is obtained. For example, exposure of chitosan to oxygen and high temperature causes its molecular weight to decrease [9].

Some parameters affecting the physical and chemical properties of chitosan are as follows:

## *2.1.1 Resolution*

Thanks to its cationic structure, chitosan can be easily dissolved in some solutions in pH <6 environments. Organic acids such as acetic acid, formic acid, and lactic acid are generally used to dissolve chitosan. On the other hand, the solubility of chitosan in inorganic acids is quite low [9]. Some mineral acids such as hydrochloric acid and nitric acid can also be dissolved, but it is not suitable to use phosphoric acid and sulfuric acid as solvents [10, 11]. Among them, the most widely used is acetic acid. The solubility of chitosan mostly depends on the degree of N-acetylation (or degree of deacetylation), distribution of acetyl groups, deacetylation time, pH, ionic strength [12], temperature, particle size, pre-treatments before isolation, base concentration, and chitin/base solution ratio [13].

The solubility problem of chitosan is a disadvantage and must be used by dissolving in acid. In order to improve this, water-soluble chitosan derivatives can be synthesized, thanks to the reactivity of the primary amine and primary and secondary hydroxyl groups in its structure. Modification of chitosan increases its solubility and stability, making it versatile as a biopolymer [14].

### *2.1.2 Deacetylation degree (DD)*

The degree of deacetylation of chitosan plays an important role in its solubility and solution properties [15]. DD and molecular weight have the effect of changing the physicochemical and biological properties of chitosan. Studies have been carried out with different chitosan samples with similar molecular weights but varying DD between 70% and 95%, and these parameters were found to be related to physicochemical properties. Chitosan samples with high DD have more crystalline regions than those with low DD. As the DD increases, the elasticity and tensile strength also increase [16]. In terms of biological properties, chitosan samples with DD exceeding 90% were examined. It has been observed that chitosan samples with high DD play a role in the regeneration of nerve cells and resemble cells in the peripheral nervous system. As a result, the DD of chitosan is a very important parameter in terms of physicochemical and biological properties [17].

#### *2.1.3 Molecular weight (Mw)*

The Mw of chitosan varies depending on its source and deacetylation conditions (temperature, time, and base concentration). The dissolved oxygen in the solution medium reduces the stability of chitosan, causing it to decompose, and the Mw of chitosan decreases. Also, high temperature (≥280o C) breaks the polymer chains of chitosan and lowers its Mw [9]. In the literature, it is stated that high molecular weight polymers slow down the release rate of drugs [18, 19]. The Mw, viscosity, polarity, solubility, and thermal properties of the drug carrier material matrix significantly affect the release mechanism [19].

#### *2.1.4 Viscosity*

Many factors affect the viscosity of chitosan, such as the degree of deacetylation, molecular weight, ionic strength, pH, and temperature [9]. Viscosity increases with an increasing degree of deacetylation. Chitosan with a high and low degree of deacetylation has different conformations in an aqueous solution. Chitosan with a high degree of deacetylation has an expanded conformation with more flexible chains due to charge repulsion in the molecule. However, chitosan with low DD is rod-like or coilshaped due to the low charge density in the polymer chains. The viscosity of chitosan is also affected by concentration and temperature. If the concentration of the medium increases or the temperature decreases, the viscosity of chitosan also increases [13].

The viscosity of chitosan is also highly related to its Mw. The viscosity of chitosan with a high Mw is higher than that of a low Mw one. Many studies show that physical and chemical processes affect viscosity. Processes such as increasing the grinding time, heating, autoclaving, ultrasonic, and ozonation reduce the viscosity [20]. It was stated that the viscosity decreased with increasing deproteinization and demineralization time [9, 20].

#### *2.1.5 Color*

The pigment in the shellfish structure forms a complex with chitin. The color of powdered chitosan can vary from light yellow to white. When obtaining chitosan from chitin, decolorization can be done by extraction with acetone followed by interaction with 0.3% NaOCl. Chemicals such as KMnO4, NaHSO3, Na2S2O4, and H2O2 are also used for color removal [9].

The amino and hydroxyl functional groups in chitosan allow it to form stable covalent bonds with other materials. It can carry out esterification and etherification reactions with alcohol groups, while amino groups on D-glucosamine can be quaternized or react with aldehyde functions [21]. Thanks to its amino groups, chitosan can form complexes with metal ions, and thus it can be used in the treatment of wastewater and recovery of heavy metals [22, 23]. The complexing feature of chitosan also enables it to be used in the purification of beverages (juices, wine, etc.) [22].

Chitin has a stable structure and is insoluble in water, alcohol, dilute acid, and base solutions, and its chemical reactivity is quite low. Due to these features, it is not widely used in industrial applications [23, 24]. Today, chitosan is used in numerous fields such as food, agriculture, cosmetics, textile, medicine, and pharmacy sectors. Thanks to the mentioned properties of chitosan, its field of study can be further increased by its modifications with various functional groups in the future.

### **3. Biomedical applications chitosan-based nanocomposites**

#### **3.1 Controlled drug delivery systems**

Before controlled release systems were developed, there were many systems with long-acting, different drug releases, and different names. Modified release systems, which differ from basic drug delivery methods, fall into two groups:

#### 1.Delayed-release systems

2.Extended-release systems:


The main systems defined in the modified release systems classification and differing from each other in active substance releases are as follows [25, 26]:

*Repeat Action Systems:* There is more than one dose of the active substance in the applied dosage, and these doses are released at certain time intervals.

*Delayed-Release Systems:* The release of the active substance from the system takes place in a certain region. It is used for enteric-coated tablets, generally.

*Sustained Release Systems:* These systems can maintain the plasma and tissue levels of the active substance for a longer period of time than conventional release systems. However, since the system is affected by environmental conditions, it is difficult to determine the release mechanism in advance. In general, its velocity is compatible with first-order kinetics.

*Controlled Release Systems:* These systems exhibit different behavior from sustained-release systems in that release rates can be planned in advance and can realize drug release with zero-order kinetics [25, 27].

Controlled release is a constantly evolving topic with applications in many different fields such as medicine, pharmacy [28], chemistry [29], environment, agriculture [30], and veterinary medicine [31]. Thanks to the controlled release systems, the pollution caused by the traditional application methods in the soil can be prevented by using low amounts in agriculture (agricultural pesticides such as fertilizers and insecticides) and applications related to environmental protection [30]. It is used in controlled release applications of parasitic drugs, hormones, vaccines, antibiotics, substances that increase milk yield, and birth control drugs in veterinary medicine [31]. In chemical applications, it is also used for the controlled release of expensive and waste-producing materials, thus ensuring economic and continuous production. Controlled release practices in medicine and pharmacy have emerged in order to better control drug administration, facilitate the treatment of the patient, and increase the quality of life of the patient [32].

Controlled drug release is a method in which the active substance is designed to be released from a system at the desired time, at a determined rate, and in the required quantities [33]. The interest in controlled drug delivery systems is increasing day by day because developing a new drug takes a long time and is economically burdensome. Thanks to controlled drug release systems, the drug dose used decreases, the dosage range increases, and the side effects of the drug are largely eliminated [32]. After the active substance mixes into the blood, its level blood should remain within the plasma range where it is effective. In classical release systems, the drug level fluctuates between the maximum and minimum values, and when the drug life is over, it is necessary to take the drug in high doses again. In controlled drug release systems, the drug can be released at the optimum plasma concentration, in a stable manner, and over a long period of time [25, 34].

In order for the drug to be effective, it must first form the dosage form that carries the active substance, then mix it into the blood safely and effectively, disperse into the tissues, and show its effect in the target area. The dosage should be maintained in a range above the effective amount and below the toxic amount after mixing with the blood. Each dose of the drug taken reaches a peak according to its specific half-life in the blood, then decreases below the effective amount, and is finally eliminated from the body. In conventional drug systems, it is not possible for the drug to select its place of

action or to mix into the blood in a controlled manner [25, 35]. Controlled drug release application, it is aimed to show the effectiveness of the drug according to a pre-planned process in the body and to perform treatment at longer intervals, with low doses, without side effects. Thus, the life of the drug in its circulation in the body is prolonged, the absorption is accelerated and its target ability to the effect site is ensured [36].

The conventional and controlled exchange of the drug in the blood is shown schematically in **Figure 2**.

Drug delivery systems provide predetermined and reproducible controlled drug release for long-term treatment locally or systemically at specified time intervals. In the traditional method, the drug is given at once and in high doses, and the dose is repeated after a few hours or a day. This method is not economical and has side effects [37, 38]. The purpose of controlled drug release systems is to improve the performance of drug therapy. This mechanism enhances the therapeutic activity and reduces side effects by reducing the toxicity caused by overdose during treatment. If the drug concentration level is not stable, the drug falls below the normal level or rises above the toxic level. This may cause undesirable side effects in the patient. The controlled drug release system maintains a constant level of drug concentration in the blood plasma [39, 40].

Controlled drug delivery systems are produced by combining a polymer with a drug or other active agent. Polymers used in drug delivery systems should be nontoxic and non-allergenic, high purity and reusable, biodegradable in vivo, and structures formed after the degradation process should be usable in metabolism [38]. These polymers used as biomaterials can be natural or synthetic. Despite the advantage that synthetic polymers can be synthesized at any time, biopolymers have advantages such as being easily obtained from nature, cheap, and chemically modified [1, 5]. Chitosan has a very important role in medical applications due to its nontoxicity, biocompatibility, biodegradability, mucoadhesion, and low allergenic. Its high biocompatibility and biodegradable properties are its most important advantages for drug delivery systems [38, 39].

#### **3.2 Advantages and disadvantages of controlled drug delivery systems**

Controlled release systems are used successfully in the treatment of many diseases today. In the coming years, with the use of controlled-release drugs (especially tablets and capsules), the effectiveness and safety of the treatment will increase. The

**Figure 2.** *A schematic drawing of drug release [12].*
