Amino acids and their amino or carboxyl group blocked derivatives were selected to investigate the relative importance of the electroosmotic flow as comparing with the passive and electrical flows in transdermal iontophoresis. Specifically, the effects of the solute's hydrophobicity, the solute's charge, the buffer's salt concentration, and the electrical current density on the electroosmotic flow of transdermal were clearly demonstrated. In a summary, the electroosmotic flow direction enhanced the permeability of the positively charged solutes (anodic delivery), and the enhancements were diminished as the buffer's salt concentration increases; however, for anionic solutes and neutral solutes, the permeability reduced by increasing the buffer salt concentration was not as apparent as for cationic solutes since the negative effect of electroosmotic flow on anionic solutes was lessened as buffer concentration increases. Also, the increase in the electrical current density pushes up the permeability of cationic solutes by both increasing the electrical and electroosmotic flow; but for anionic solutes, the favor of the electrical flow in a higher electrical field was weakened by increasing negative electroosmotic flow. Furthermore, in the molecular weight range of the solutes under this investigation (89-300 g/mol), experimental results indicated that the hydrophobicity of solutes plays a more prominent role than molecular weight in passive diffusion, and the results allow to eliminate the argument of the permeability differences caused by the different molecular weights of the anionic solutes and the cationic solutes of this study.
