5.2.1. Polymeric Membranes

Here, the drug formulation is encapsulated in the compartment of a drug reservoir, in which the surface of the drug-releasing layer is covered by a rate-controlled polymeric membrane. The drug reservoir could be solid, a solid dispersion, or a drug solution in liquid form (Figure 11). The encapsulation process for preparing the drug formulation inside the reservoir compartment includes fabrication by microencapsulation, coating, and molding techniques. The polymeric membrane is manufactured from a nonporous, microporousor semipermeable membrane (Figure 12) [130].

**Figure 11.** Polymeric membrane used for drug delivery system.

**Figure 12.** Classification of polymeric membranes for drug delivery.

Drug release from the polymeric membrane must be at a constant rate (Q/t) which is well-defined by the Equation (1),

$$\frac{\mathbf{Q}}{\mathbf{t}} = \frac{\mathbf{K\_{m/r}} \mathbf{K\_{a/m}} \mathbf{D\_d} \mathbf{D\_m}}{\mathbf{K\_{m/r}} \mathbf{D\_m} \mathbf{h\_d} + \mathbf{K\_{a/m}} \mathbf{D\_d} \mathbf{h\_m}} \times \mathbf{C\_{R'}} \tag{1}$$

where, K m/r and Ka/m are the partition coe fficient of the drug molecule from the reservoir to the rate-controlling membrane and from the membrane to the aqueous layer, respectively, Dd and D m are the di ffusion coe fficient of the rate-controlling membrane and the aqueous di ffusion layer, respectively, h m and hd are the thickness of the rate-controlling membrane and the aqueous di ffusion layer, respectively, and CR is the drug concentration in the reservoir compartment. The drug release from this system is controlled at a preprogramed rate by controlling the partition coe fficient, di ffusivity of the drug molecule, the rate-controlling membrane, and the thickness of the membrane. There are several controlled release polymeric membrane drug delivery systems which have been successfully marketed and some of the examples of these are outlined below [131].
