Reverse Iontopherisis
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Iontophoresis
When an electric field is established across a membrane, ions on either side will migrate in the direction dictated by their charge. This is done by electromigration and electoosmosis.
Electromigration
Conventionally, in iontophoresis, a constant current is applied, such that the flow of electrons is translated into an ion flux across the skin. A power supply establishes the electric field that causes electrons to migrate in the �electrical’ portion of the circuit and ions to flow in the �ionic’ part (figure 1). It follows that the number of electrons flowing through the �electrical’ portion of the circuit is exactly balanced by the amount of ionic charge flowing through the skin.
The sum of the individual ionic charges flowing across the skin must equal the number of electrons �delivered’ by the power supply; in other words, there is �competition’ among all the ions present to carry the charge.
Furthermore, with respect to electromigration, only the ionized fraction of the analyte is extractable and this will depend on the relevant pKa. Similarly, for analytes that are bound to proteins, it is clear that only the free fraction can significantly contribute to charge transport across the skin.
In summary, therefore, it can be concluded that an ion can function as a major charge carrier if it is small, fully charged, at high concentration, and not significantly protein-bound.
In reverse iontophoresis, the major charge carriers are Na+ and Clв?. Na+ is the major charge carrier in iontophoresis in the outward direction towards the cathode