5.2.4. Progestasert

In this type of system, a drug reservoir layer is used which consists of BaSO4 and progesterone dissolved in silicone fluid and encapsulated into a T-shaped device using an ethylene-vinyl acetate copolymer (nonporous membrane) (Figure 15). This system is particularly designed to continuously deliver progesterone to the uterus with a dose of 65 μg/day to attain contraception for a year [137]. Another example of this system is the Mirenaplastic T-shaped device(steroid reservoir), which contains 52 mg of levonorgesterol for achieving contraception for 5 years [138].

**Figure 15.** Progestasert IUD with structural components shown (adapted from [137]).

### 5.2.5. Polymer Matrix Diffusion-Controlled Drug Delivery

In this type of pre-programmed delivery system, the drug reservoir is prepared by homogenous dispersion of the drug in a rate-controlling polymer matrix fabricated by a hydrophilic or hydrophobic polymer. Secondly, the drug dispersion in the matrix is fabricated by blending the drug particles with a polymer or highly viscous base polymer followed by the cross-linking of polymer chains. Next, the drug solids are mixed in a rubbery polymer at an elevated temperature. The resulting drug-polymer dispersion is then extruded or molded to form a drug delivery device of varying shapes/sizes (Figure 16). It can also be prepared by dissolving the drug and the polymer in a solvent, followed by solvent evaporation, or under vacuum conditions [139].

Release of drug molecules from this type of controlled release drug delivery system is controlled at a pre-programmed rate by controlling the:


An example of this polymer matrix diffusion-controlled drug delivery system is given in the subsection on the Nitro-Dur system.

**Figure 16.** A polymer matrix diffusion-controlled system.

The drug release rate from the polymer matrix is not constant and can be expressed by the equation

$$\frac{\text{Q}}{\text{t}^{1/2} = \left(2\text{AC}\_{\text{R}}\text{D}\_{\text{P}}\right)^{1/2}},\tag{2}$$

where, Q/t1/<sup>2</sup> is the rate of release of the drug, A is the initial drug loading dose in the polymer matrix, CR is the drug solubility in the polymer and Dp is the diffusivity of the drug in the polymer matrix.
