# An Overview of Biopolymers for Drug Delivery Applications
## 5 Mechanisms Involved in Drug Delivery Systems
Non-covalent interactions are the driving force for the formation of stable micelles that can ensure, due to the presence of both a hydrophilic and hydrophobic part, kinetic stability, increased drug loading efficiency, and release capacity [181]. A large number of micelles made with biopolymers have already been studied with protein, peptides, and polysaccharides [71]. Dendrimers are equally important and have similar characteristics to micelles, which makes them very useful for the controlled and targeted release of a drug. However, dendrimers are synthetic polymers, and their combination with biopolymers to create hybrid materials is still being explored [182; 183]. It is important to wisely choose which type of loading is needed by keeping in mind the type of release--immediate, non-immediate, sustained, and in situ-specific--and the dosage wanted [184]. Once the drug is loaded on the most compatible and efficient system, the last step is the release of it. Many release mechanisms can occur in a solution, and usually, there are more happening at the same time, but the final classification and kinetic study of controlled drug delivery systems depends on the primary mechanism that occurs. The simplest is dissolution, a process in which the molecules of a solute are dissolved in a solvent. In the case of drug delivery systems, this means that the drug molecules pass from solute to solvent until the solution reaches the solubility limit at the established pressure and temperature conditions. The rate of dissolution is directly proportional to the solubility of the drug when no chemical reaction occurs, and the solubility coefficient of the therapeutic molecule decreases with its increasing melting point and increases with increasing chemical compatibility with the solution. Consequently, the dissolving process may be expressed numerically as [185]  
\[\frac{dC}{dt}=D\cdot A\cdot(C_{s}-C) \tag{1}\]  
where the surface area of solid \(A\), the diffusivity coefficient of solute \(D\), and the difference between solute concentration \(Cs\) and solid solubility \(C\) determine the dissolving rate \(dC/dt\). In the work of [PERSON] et al., it is evidenced that the dissolution rate depends on the characteristics of the biopolymer, such as its molecular weight and chemical structure. The authors have done a comparative study on the delivery mechanism by dissolution, using DDSs with xanthan gum and tragacanth gum, which are both biopolymers but with different properties. They observed different drug release profiles that are dependent on the biopolymer used. In the case of tragacanth gum, due to its molecular weight of 840 kDa and polarity, the polymer is poorly soluble in aqueous media, resulting in a release profile of the drug that was similar to the behavior of classical tablets, which ensured an immediate release of the therapeutics and was not compatible with a prolonged and sustained release. However, the opposite results were obtained with xanthan gum, which was characterized by the presence of anions in its chains and has a molecular weight of 2000-16,000 kDa. Due to these characteristics, in the case of xanthan gum, the release of the drug was sustained, prolonged, and pH-dependent because, at neutral pH values, the repulsion of the carboxyl groups in the chain causes the polymer's backbone to extend, which decreases interaction with the drug, favoring the release. However, at that pH, the interaction with the medium is more beneficial, leading to an apparent gelation process that further modulates the release; at acidic pH values, on the other hand, this process is unfavorable, decreasing swelling and increasing matrix erosion [186]. Another mechanism usually involved in the release of therapeutic molecules is diffusion. This process is defined as the mass transfer of molecules from one part of the system to another carried by random molecular motions such as a concentration gradient.