UPPER AND LOWER LIMITS OF HUMAN SKIN ELECTRICAL-RESISTANCE IN IONTOPHORESIS

The time-dependent electrical resistance of human cadaver skin was shown to be bound by an upper limit at equilibrium and a lower limit at steady state. The time to reach steady state from any initial condition by applying a current was much shorter than the recovery to equilibrium, due to different driving forces. The ratio of the equilibrium to the solution resistances increased with the salt concentration in free solution. This equilibrium relative resistance was sensitive to skin variability, primarily due to its dependence on porosity and pore geometry. A model was developed relating the equilibrium relative resistance to the structural properties of skin. The estimated pore radius of 25 x 10(-10) m from the equilibrium resistance data is consistent with the average pore radius of human skin reported in the literature. In contrast, the steady-state resistance could be several orders of magnitude lower than the equilibrium resistance and depended on current density and salt concentration but not on the history of prior exposures of a skin sample to a current. This difference can greatly reduce the power requirement for iontophoresis and can make this technology more practical. A linear relationship was found between the logarithm of the steady-state relative resistance (skin/solution resistances) and the logarithm of the current density divided by the salt concentration. The slope of - 0. 65 was in excellent agreement with the theoretical slope of - 2/3 that was derived for the convective transport of ions in a charged pore under a constant current.