The physics of iontophoretic pipettes.

A theoretical study has been made of the release of drugs from iontophoretic pipettes. The chief predictions are: (1) in the steady state, release becomes linear with current and independent of pipette geometry when the ejecting voltage exceeds approximately 100 mV; (2) release rises relatively slowly during an ejecting pulse to approach its steady state value. At the end of the pulse, release falls abruptly. This asymmetry of the release curve is greatly accentuated by the prior application of a retaining current. It provides an explanation for the characteristic time course of response commonly seen when drugs are applied iontophoretically to central neurones: (3) the above effects occur on a time scale which depends strongly on the geometry of the pipette. Release from pipettes of large tip diameter and nearly cylindrical bore, such as those generally used in the central nervous system, should be thousands of times slower than from pepettes of small tip diameter and larger taper angle. Experimental observations of the release of an ionized fluorescent compound quantitatively confirm prediction (1) and qualitatively confirm predictions (2) and (3).