Transdermal Drug Delivery. See photo on page 781. The principles of steady-state diffusion have been used in a number of drug-delivery systems. Specifically, medicated patches are commonly attached to the skin to deliver drugs for nicotine withdrawal, birth control, and motion sickness, to name a few. The U.S. transdermal drug-delivery is a multibillion dollar market. The figure shown in the Additional Material and on page 781 shows a drug-delivery vehicle (patch) along with the concentration gradient in the epidermis and dermis skin layers.
a. Use a shell balance to show
dWAzdz=0
b. Show the concentration profile in the epidermis layer
CA0−CACA0−CA1=zδ1
c. Show the concentration profile in the dermis layer
CACA1=δ2−zδ2−δ1
d. Equate the fluxes using WA1=−DA1dCAdz and WA2=−DA2dCAdz and to show
CA1=DA1CA0δ1DA2δ2−δ1+DA1δ1
e. What are the concentration profiles in the dermis and epidermis layers?
f. Show the flux in the dermis layer is 

WAz=DA2δ2−δ1[DA1CA0δ1DA2δ2−δ1+DA1δ1]
g. What is the flux in the epidermis layer?