A transdermal patch for drug delivery contains a 0.06M solution of the drug in a capsule 2 mm thick and 3.0 cm in diameter. Just before application, a 250-μm polymer film is placed between the capsule and the skin. The drug diffusion coefficient in solution is 5.4 x 10^-6 cm²/s; in the polymer it is 1.8 x 10^-7 cm²/s. The equilibrium concentration in the polymer phase is 0.6 times that in the solution. The drug israpidly absorbed by the skin, so the concentration at the polymer-skin interface is assumed to be zero. 

(a) What fraction of the drug in the capsule is absorbed in the first 2 hours? 

(b) At what time will the rate of drug delivery be only half the initial rate? How many moles will have been delivered at this time? 

(c) Can you suggest a method of getting a more nearly constant rate of drug delivery?