# An Overview of Biopolymers for Drug Delivery Applications
## 5 Mechanisms Involved in Drug Delivery Systems
The equation that describes this process is [185]  
\[J=-D.\frac{dC}{dx} \tag{2}\]  
where \(J\)--the rate of diffusion per unit area on the section studied, \(C\)--the concentration of the molecule diffused, \(x\)--the distance between the two points of the section considered, the minus means that diffusion occurs in the direction of concentration decreasing, and \(D\)--the coefficient of diffusion that, in very diluted solutions, can be considered constant, but in the case of polymers, is highly dependent on concentration, and it is a measure of the molecule's mobility in the medium. Since large molecules diffuse more slowly than small ones, viscous liquids should slow this diffusion. Furthermore, Brownian motion is driven by the heat agitation intensity. Although diffusion is not effective over longer distances and is strictly related to the geometry of the carrier, this mechanism is a simple, efficient, and secure strategy to reach a sustained and controlled release of a drug and can usually happen when paired with dissolution and swelling. Moreover, it can produce a zero-order model of kinetic release, making it very advantageous [185]. Diffusion can occur in different phases of the release, as shown in the work by [PERSON] et al. where the authors analyzed the release from polyester blends. It was shown that there is a four-phase drug release mechanism where dissolution was observed in the first initial burst of release and again in the second burst release, although the release was mostly modulated by the degradation process of the carrier [187]. What can be considered a crucial mechanism for drug delivery in polymers is swelling. Swelling is a process where a material put in a liquid tends to increase in size and volume due to absorption phenomena. It is observed in various materials, including biopolymers, and can result in changes in the physical properties of the material (permeability, strength, flexibility, etc. ), leading to the release of the drug. Hydrogels are especially affected by this mechanism and can absorb many times their weight in water. In glassy hydrogels, the slow structural relaxations of the swelling polymer affect the kinetics of water uptake, which influences the release of the drug. The forces of osmotic, electrostatic, and entropy-favored dissolving of the polymer in water are typically balanced to drive the process of water intake and swelling [188]. These systems strictly refer to cases where water not only activates the release of the drug but is also the rate-controlling mechanism of drug release. Especially for hydrophilic polymers, swelling is coupled with diffusion, leading to the stabilization of an otherwise dissolution-controlled drug release rate as already seen in the work by [PERSON] et al. [186]. Swelling behavior has been largely studied for both biopolymers and hybrid-enhanced biopolymers, leading to many publications over the years. Osmosis is another important mechanism that is also part of the swelling mechanism. It is defined as the process in which a solvent is transferred through a membrane that is permeable to smaller solvent molecules and not permeable by larger ones in an attempt to equalize the concentrations of the not-permeable solute on both sides of the semipermeable membrane. Osmotic pressure is the energy that powers the process and directs the release, and it is regulated by the equation [185]  
\[\Delta\pi=\frac{dV}{dt}.\frac{h}{A}.\frac{1}{k} \tag{3}\]  
where \(h\) is the thickness of the membrane, \(A\) is the cross-sectional area for transport, \(\mathbf{k}\) is the effective permeability of the membrane, \(dV/dt\) is the volume flow of the solvent through the membrane, and \(\Delta\pi\) is the osmotic pressure. Osmotic systems are typically more complicated to build and less volume efficient than other drug delivery systems since they typically consist of a drug core surrounded by a membrane that allows water to permeate and create osmotic pressure, which causes the release of the medication.