hi chala
sorry for using medical jargon's as i was not knowing your back ground.

iontophoresis is a physiotherapy treatment technique using electrical current.the current is used to drive ions from a solution into the body.this is an effective way to drive medicines to target area without loss in intensity of medicine.
for eg if we are taking a medicine orally it will pass thru various metabolic process in the body,so only a minimal amount of it only reach the site as in your case foot .
hope i made it simple