# An Overview of Biopolymers for Drug Delivery Applications
## 1 Introduction  
Nowadays, technology is continuously developing, and the highest priority of researchers is the standard of medical care. Medication distribution and therapeutics are an important concern for scientists because the effectiveness of many drug delivery systems (DDSs) is a constant issue. The major problems experienced in drug delivery include different and many side effects, toxicity, non-specificity, low bioavailability, brief drug delivery, and rapid degradation [1; 2; 3]. To date, promising studies have been performed for the development of DDSs based on biopolymers due to their multiple characteristics and advantages.  
Natural or plant-based resources, such as various bio-wastes from horticulture and crops, are used to make biopolymers, which are then produced as byproducts [4]. These materials are readily biodegradable because they contain atoms of carbon, oxygen, and nitrogen that compose their structural backbone. They are broken down into carbon dioxide, water, biomass, organic macromolecular material (humic matter), and other natural substances during the biodegradation process. Therefore, materials that are naturally recycled through biological processes are known as biopolymers [5]. Among the advantages of biopolymers are renewability, biocompatibility, affordability, and the release of less carbon (Figure 1). Also, a very important advantage of biopolymers is that they are environmentally friendly [4]. Biopolymers are proven to be non-toxic, non-carcinogenic, non-thrombogenic, and easy to extract [6; 7; 8].  
Numerous factors, like the kind of material employed as a structural matrix (conformation and distribution), the preparation circumstances (pH, concentration, temperature, solvent, etc.), and the type and concentration of additives (antimicrobials, antioxidants, plasticizers, crosslinking agents, etc.), all have a precise impact on these qualities [9].  
The presence of hydroxyl, amino, or carboxyl functional groups in the structure of natural biopolymers provides good chemical reactivity and adaptability, which are comparable with synthetic biopolymers. For example, it was demonstrated that these reactive groups allow for changes that lead to an increase in the stability of biopolymers in different biological media. Also, cross-linking procedures with aldehydes and polyethylene glycol were studied for stability enhancement in order to avoid the rapid degradation of various biopolymeric coatings [10; 11; 12].  
Even though biopolymers have many benefits, there are still a number of processing-related restrictions, beginning with the extraction and concluding with the isolation of the finished biomaterial. Because it is a completely natural material, the final properties of the biopolymer significantly depend on the source of the primary starting material [13]. To obtain and reach comparable results with DDSs in which conventional materials are used, research on DDSs based on biopolymers is still developing [14]. In controlled drug delivery, biopolymers are used as hydrogels, microcapsules, micro/nanospheres, and liposomes (Figure 2).  
Researchers are interested in nanotechnology as a potential remedy for the issues with traditional DDSs. The interest is therefore shifted towards nanotechnology drug delivery systems (NDDSs). The main advantages of these systems are their flexibility in composition; the fact that, due to their design at the nano-level, they present free movement; biocompatibility; and non-toxicity, which is also very important [15; 16].  
Figure 1: The main characteristics of biopolymers.  
Figure 2: The application form of biopolymers in the pharmaceutical field.  
This review aims to summarize the most commonly used biopolymers in the biomedical field, especially in DDSs. It also includes the progress so far in this field and the potential applications of biopolymer nanocomposites. Emphasis was placed on the mechanisms of action of DDSs developed so far--a less detailed aspect addressed in previous reviews.