The principles of steady state diffusion have been used in a number of drug delivery systems. Specifically medicated patches are commonly used to attach to the skin to deliver drugs for nicotine withdrawal, birth control, and motion sickness to name a few. The U.S. transdermal drug delivery market was $1.2 billion in 2001. Equations similar to Equation 11-26 have been used the model the release, diffusion and absorption of the drug from the patch into the body. Figure SN11.1 shows a drug delivery vehicle (patch) along with the concentration gradient in the epidermis and dermis skin layers.
Figure SN 11.1 Transdermal Drug Delivery Schematic
As a first approximations, the delivery rate can be written as
where
Where Ap is the area of the patch, CAp the concentration of drug in the patch, R the overall resistance and Rp the resistance to release form the patch. There are a number of situations one can consider, such as the patch resistance limits the delivery or diffusion through the epidermis limits delivery or the concentration of the drug is kept constant in the patch by using solid hydrogels. When diffusion through the epidermis layer limits, the rate of drug delivery rate is
Other models include the use a quasi-steady analysis to couple the diffusion equation with a balance on the drug in the patch or the zero order dissolution of the hydrogel in the patch.† Problem 11-2(f) explores these situations.
†For further information see