Consider the "drug patch" shown in the figure. 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 $0\mathrm{.}45mm.$ The gel diffusion barrier is in direct

contact with the skin. The cumulative drug release vs$.$ time

profile for a $1c{m}^2$ square patch at $20C$ is also shown.

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 $1$

$$ mole $\frac{?}{c{m}^3},$ and the solubility of the drug in the gel

diffusion barrier is not affected by temperature.

From the data shown, estimate the effective diffusion

coefficient of the drug in the gel diffusion barrier.