Characterization of solute distribution following iontophoresis from a micropipet

Iontophoresis uses a current to eject solution from the tip of a barrel formed from a pulled glass capillary and has been employed as a method of drug delivery for neurochemical investigations. Much attention has been devoted to resolving perhaps the greatest limitation of iontophoresis, the inability to determine the concentration of substances delivered by ejections. To further address this issue, we evaluate the properties of typical ejections such as barrel solution velocity and its relation to the ejection current using an amperometric and liquid chromatographic approach. These properties were used to predict the concentration distribution of ejected solute that was then confirmed by fluorescence microscopy. Additionally, incorporation of oppositely charged fluorophores into the barrel investigated the role of migration on the mass transport of an ejected species. Results indicate that location relative to the barrel tip is the primary influence on the distribution of ejected species. At short distances (<100 μm), advection from electroosmotic transport of the barrel solution may significantly contribute to the distribution, but this effect can be minimized through the use of low to moderate ejection currents. However, as the distance from the source increases (>100 μm), even solute ejected using high currents exhibits diffusion-limited behavior. Lastly a time-dependent theoretical model was constructed and is used with experimental fluorescent profiles to demonstrate how iontophoresis can generate near-uniform concentration distributions near the ejection source.