**1. Introduction**

Biopolymers are generated by living organisms [1–5] and are defined as biologically degradable polymers [6]. They represent possible materials for the replacement of synthetic plastics due to an increased interest in developing environmental sustainability [7]. Biopolymers have a structural backbone with carbon, oxygen, and nitrogen atoms which makes them easily biodegradable. Biodegradation breaks them down into carbon dioxide, water, humic matter (organic macromolecular material), biomass, and other natural substances; thus, these materials are naturally recycled through biological processes [3].

A classification system based on their origin, synthesis and processing of different biodegradable polymers [8–12] is presented in Figure 1 in the form of a social network analysis. It divides the biopolymers in four major categories: extracted from biomass products (agrobiopolymers), from microorganisms, and from biotechnological and petrochemical products. Biopolymers from biomass products have diverse compounds such as polysaccharides (starches, celluloses, alginates, pectins, gums, and chitosan) [13]; proteins of animal origin (whey, collagen, and gelatin); proteins of vegetal origin (zein, soya, and wheat gluten) [14,15]; and lipids (bees wax, carnauba wax, and free fatty acids) [16,17]. Most biopolymers can be extracted from natural sources such as plants, animals, and microorganisms including algae and agro-wastes [18]. Agro-sources of biopolymers include bananas, maize, potatoes, tapioca, yams, rice, corn, wheat, cotton, sorghum, and barley [19,20], while animal sources are derived from cattle, pigs, and other products. Agrowaste-based sources include apple pomace [21], tomato pomace, pineapple [22], orange

**Citation:** Gheorghita, R.; Anchidin-Norocel, L.; Filip, R.; Dimian, M.; Covasa, M. Applications of Biopolymers for Drugs and Probiotics Delivery. *Polymers* **2021**, *13*, 2729. https://doi.org/10.3390/ polym13162729

Academic Editors: José Miguel Ferri, Vicent Fombuena Borràs and Miguel Fernando Aldás Carrasco

Received: 30 June 2021 Accepted: 11 August 2021 Published: 15 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and lemon peels, wheat straw, rice husks [23,24], paper wastes, crops, wood, and green wastes, while the marine sources are sponges [25], corals, lobsters, fishes, and shrimps [26]. Biomaterials manufactured from these products are described as stretchy, soft, and gel-like, with many characteristics of both solids and fluids. It is known that biopolymers can be smart and flexible materials even in living organisms [6] because they have a structure that is constantly manipulated either in response to environmental changes or by enzymes throughout different stages of the organism's lifecycle [27]. can be smart and flexible materials even in living organisms [6] because they have a structure that is constantly manipulated either in response to environmental changes or by enzymes throughout different stages of the organism's lifecycle [27]. The biopolymer composites can be prepared by several methods such as extrusion, electrospinning, grafting, different types of molding [28], solvent casting, melt blending, intercalation, filament winding, phase separation [29], laser printing, and film stacking [5].

waste-based sources include apple pomace [21], tomato pomace, pineapple [22], orange and lemon peels, wheat straw, rice husks [23,24], paper wastes, crops, wood, and green wastes, while the marine sources are sponges [25], corals, lobsters, fishes, and shrimps [26]. Biomaterials manufactured from these products are described as stretchy, soft, and gel-like, with many characteristics of both solids and fluids. It is known that biopolymers

*Polymers* **2021**, *13*, x FOR PEER REVIEW 2 of 33

**Figure 1.** A social network graphical illustration of biopolymers classification**. Figure 1.** A social network graphical illustration of biopolymers classification.

Currently, the manufactured design and optimization of biopolymers through mathematical models are very advantageous because they improve their physical, chemical, electrical, and mechanical properties in order to increase resistance in humid, warm, or The biopolymer composites can be prepared by several methods such as extrusion, electrospinning, grafting, different types of molding [28], solvent casting, melt blending, intercalation, filament winding, phase separation [29], laser printing, and film stacking [5].

cold storage conditions and for applications that require specific features [30]. Although significant work has been done on the use of a wide range of biopolymers, most of them have been based on polysaccharides due to their improved properties compared to other categories such as proteins or lipids. Thus, this review focuses on the ability Currently, the manufactured design and optimization of biopolymers through mathematical models are very advantageous because they improve their physical, chemical, electrical, and mechanical properties in order to increase resistance in humid, warm, or cold storage conditions and for applications that require specific features [30].

of biopolymers to be used successfully in the pharmaceutical industry as encapsulating agents, particularly for delivery of drugs and probiotics. Specifically, the paper describes the use of alginate, chitosan, agar, starch, and cellulose by focusing on the properties and characteristics that make them suitable candidates for product delivery, offering advantages over the chemically derived polymers. The following will also be presented: the development of encapsulated substances based on biopolymers; challenges and limitations such as the encapsulation process, shelf life, controlled release of embedded drugs, protection, and viability of live strains; and the rate of release at different pH mediums of the gastrointestinal fluids. Although significant work has been done on the use of a wide range of biopolymers, most of them have been based on polysaccharides due to their improved properties compared to other categories such as proteins or lipids. Thus, this review focuses on the ability of biopolymers to be used successfully in the pharmaceutical industry as encapsulating agents, particularly for delivery of drugs and probiotics. Specifically, the paper describes the use of alginate, chitosan, agar, starch, and cellulose by focusing on the properties and characteristics that make them suitable candidates for product delivery, offering advantages over the chemically derived polymers. The following will also be presented: the development of encapsulated substances based on biopolymers; challenges and limitations such as the encapsulation process, shelf life, controlled release of embedded drugs, protection, and viability of live strains; and the rate of release at different pH mediums of the gastrointestinal fluids.
