Iontophoretic permeability of polyethylene glycols through hairless rat skin: application of hydrodynamic theory for hindered transport through liquid-filled pores.

The electrically-assisted transport of a homologous series of polyethylene glycols (PEGs) through hairless rat skin (HRS) was examined in order to determine the relationship between the iontophoretic permeability of flexible macromolecules and their size and conformation in free solution. Using fully hydrated HRS, the anodic transport of 26 low molecular weight oligomers of PEG was studied over a 14 hour period at a current density of 0.50 mA/cm2. Analysis of the data revealed a distinct sigmoidal relationship between iontophoretic permeability and molecular size. In addition, the reflection of a relatively small molecular weight species was significant. Accordingly, a hydrodynamic model relating the convective transport of flexible macrosolutes through cylindrical pores was fit to the data generated from the iontophoretic transport of PEGs ranging in weight from 282 to 1382 daltons. A nonlinear least squares analysis of the data demonstrated that the 'effective' mean diameter of aqueous channels within HRS is approximately 36 A.