The rate of diffusion of a chemical across the skin is re-lated to these, among other features:
· Its concentration when applied
· The surface area to which it is applied
· Its movement through the epidermis (the dif-fusion constant)
· The relative tenacity with which it binds to its vehicle compared with epidermis (the partition coefficient)
· The thickness of the stratum corneum (barrier)
The amount of drug absorbed can be enhanced by increasing its applied concentration, increasing the size of the area to which it is applied, decreasing the barrier to its mobility through the layers (generally by hydrat-ing the skin), and increasing its affinity for the skin (usu-ally by increasing its hydrophobic component). Drug absorption is also greater in regions in which the skin is thinner.