# An Overview of Biopolymers for Drug Delivery Applications
## 4 Preparation Methods for Obtaining Biomaterials Used in DDSs  
Multiple methods are used for the preparation of the biomaterials that are further used in DDSs. Among them, important to be mentioned, are supercritical fluid extraction [130], electrospraying [131], desolvation [132], spray freeze-drying [133; 134], layer-by-layer self-assembly [135], and microemulsion [136]. These multiple types of preparation methods allow for the obtaining of important carriers for different drugs (e.g., different molecule sizes, protein, and gene drugs). Properties such as swelling and crosslinking can be tailored by changing the ratio between biomaterials and the modifying materials [29]. Generally, every preparation method has advantages and disadvantages. Thus, desolvation using solvents (e.g., ethanol, acetone) is the easiest method that can be used for the preparation of protein-based nanoparticles. The amount of additional desolvation chemicals supplied and the flow rate can regulate the size of the nanoparticles [137; 138]. Electrospray is an electrohydrodynamic single step technique that is adaptable and has very good reproducibility. Through this technique, small-sized particles are formed from a macroscopic mass, a process controlled by electrostatic forces that break up a dielectric liquid [139]. This technique is an emerging technology in fabricating drug carriers that can produce highly homogeneous materials at room temperature [140]. Electrospraying demonstrated benefits for preparing particles. Most of the time, a regulated and tight particle size distribution is seen together with a high drug-loading efficiency [141]. Another advantage of the electrospray technique is that biopharmaceuticals are not degraded during the encapsulation process [142]. Table 2 outlines various DDSs, containing different biopolymers loaded with various active drugs. For compounds that are very sensitive to temperature and pressure, the spray freeze-drying technique is used [160]. This technique involves a two-step process, involving the spray freezing step and freeze-drying step. This technique is highly used in the food and pharmaceutical industries [161; 162]. In the spray freezing step, the pharmaceutical dissolved or suspended in the liquid is further atomized to obtain fine droplets that are directly frozen, generally in liquid nitrogen. The resulting frozen particles are then put through a process called freeze-drying, which produces dried and porous particles by sublimating the solvents at low pressure and temperature [160]. This technique enhances drying efficiency and increases the production yield and capacity. The solutions used for spray freeze-drying are non-toxic aqueous-based suspensions with content of dispersed nano- or micro-sized hydrophobic active pharmaceutical ingredients. Liquid nitrogen has been the most popular refrigerant for spray freeze-drying up until now [163; 164]. Multilayer sequential film formation is performed with a novel layer-by-layer self-assembly method. This is achieved through electrostatic, hydrophobic interactions, and hydrogen bonding between the layers. Through these kinds of interactions, a deposition of alternate layers of oppositely charged biomaterials can be performed, which offers precise control of nanoscopic features (e.g., thickness, composition, and surface characteristics) of the film by using additional buffers, such as acid, base, and salt buffers [165; 166]. This technology is frequently used for fabricating hollow microcapsules with certain sizes, compositions, and properties. Among the advantages of this technique are simple preparation conditions, various loading methods, and favorable surface functionalization [5]. Obtainable capsules with particular structures and small wall-to-diameter ratios have an exceptional capacity for encapsulation, good stability, fluidity, and deformability [6]. Additionally, their semipermeable nature is also a decisive trait that allows them to communicate with external enclosing structures [167].