## 2 Biopolymeric Microparticles--Carrier System Composed of Alginate and Chitosan  
Nowadays, research is more focused on developing natural products like, for example, natural carriers for different active agents. The most used biodegradable polymers in the process of producing microparticles are next-generation biopolymers sodium alginate and chitosan. Biodegradable polymers are used to prepare microparticles and are of main interest because they are safe for the environment and are generally non-toxic [35]. Encapsulation in biopolymeric material offers stability to the encapsulated material and can offer a controlled and targeted release of the latter. In the literature, encapsulated material is often labeled as filler, fill, core material, or internal phase, while the material used for encapsulation is called matrix, coating, shell, or external phase. Biopolymericmicroparticles are usually spherical, but may have some deviations. The appearance of microparticles varies in size, shape, and composition mainly because of the influence of encapsulated material, biopolymer, and the method used for its preparation [36]. Many microparticle classifications can be found in the literature [37; 38; 39; 40; 41; 42], while here in Figure 1, a representation of microparticles relevant to this review is presented wherein matrix microsphere, an encapsulated material is homogenized through the biopolymeric matrix (including surface), while shell-matrix particles combine the features of both the matrix and shell materials, where encapsulated material is not found on the particle surface, respectively. Biopolymeric microparticles have an advantage, due to their controlled and targeted release, but some disadvantages exist, like (i) particle-particle aggregation, (ii) limitation of the storage stability, (iii) difficulty to encapsulate molecules with different degrees of hydrophilicity at the same time, (iv) challenging precise control of the dispersity of the particles. The above-mentioned disadvantages can negatively influence the efficiency of active agent delivery [43]. Even though, biopolymers are considered as great materials to use in the production of microparticles, mainly due to their abundance and affordability alongside their stability and durability throughout the process of encapsulation. Biopolymers can be easily extracted from natural sources or may be prepared with the use of microorganisms. Furthermore, biopolymers can be prepared synthetically with precision and predetermined properties as specific molecular weight, solubility, and permeability [44]. Biopolymers may vary in composition and physicochemical properties, and because of that, their utilization is often dependent exclusively on them. To achieve successful encapsulation, it is necessary to understand the structure of used biopolymers. Sodium alginate is negatively charged, due to the presence of carboxyl groups from the uronic acid residues [45]. It is composed of two repeating carboxylated monosaccharide units (mannuronic and gularonic acids), and the ratio between them influences the properties of the biopolymer. Chitosan is a partially deacetylated polymer of N-acetylglucosamine obtained after alkaline deacetylation of chitin. The N-deacetylation is seldom complete, and the fraction of N-acetylglucosamine determines the degree of acetylation, which serves as a base to classify the biopolymer either as chitin or chitosan. When the degree of N-acetylation (defined as the average number of N-acetylglucosamine units per 100 monomers expressed as a percentage) is below 50%, chitin becomes soluble in aqueous acidic solutions (pH < 6.0) and is called chitosan [46]. The electrostatic attraction between  
Figure 1: Representation of microparticles relevant to this review paper, a matrix microsphere, and a coated shell-matrix microcapsule. the cationic amino groups of chitosan and the carboxylic groups of the alginate leads to the formation of the polyelectrolyte complexes of various structures.