Consider the drug patch shown in Problem 26.4. The patch consists of a pure solid drug source mounted on top of a water-swollen polymer, which

Consider the €œdrug patch€ shown in Problem 26.4. The patch consists of a pure solid drug source mounted on top of a water-swollen polymer, which acts as a controlled diffusion barrier. The square patch is 4.0 cm × 4.0 cm. The bottom surface of the diffusion barrier is in direct contact with the skin. The desired amount of cumulative drug release is 0.02 µtmole after 10 hours. Independent experiments have shown that the drug is immediately taken up into the body after exiting the patch. The maximum solubility of the drug in the water-swollen polymer is 0.50 [µmole/cm³, which is dilute. The effective diffusion coefficient D_Aeof the drug in the water-swollen polymer is 2.08 × 10^-7cm²/s. What is the required thickness (L) of the diffusion barrier component of the drug patch needed to achieve the desired drug release profile?

Data From Problem 26.4

Consider the €œdrug patch€ shown in the figure on the next page. The drug patch consists of a pure drug source mounted on top of a gel diffusion barrier. The gel diffusion barrier has a thickness of 2.0 mm. The gel diffusion barrier is in direct contact with the skin. The cumulative drug release vs. time profile for a 3.0-cm × 3.0-cm square patch at 20°C is also shown below. Other experiments showed that the drug was immediately taken up into the body after exiting the patch. The drug is only slightly soluble in the gel material. The maximum solubility of the drug in the gel diffusion barrier is 0.50 µmole/ cm³, and the solubility of the drug in the gel diffusion barrier is not affected by temperature. 

Drug patch Drug reservoir Impermeable barrier -Gel diffusion barrier Skin surface Infected body tissue (sink for drug) 0.20 0.15 0.10 0.05 0.00 10 15 20 25 Time (hours) Drug released (u moles A)