Characterization of Solute Distribution Following Iontophoresis from a Micropipet

Abstract

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.

Quantitative analysis of iontophoretic drug delivery from micropipettes

“…This was also the case in our prior studies for ejections with anodic current for positive and neutral species, owing to diffusion as the dominant mass transport mechanism. 15 Likewise, the fluorescein profiles follow a 1/r distribution at distances beyond 100 µm from the barrel tip for currents up to 500 nA, indicating diffusion is the dominant mass transport mechanism beyond this distance regardless of ejection current magnitude.…”

“…15 Fast scan cyclic voltammetry (FSCV) with a carbon-fiber electrode adjacent to the iontophoretic barrel allows determination of the time course of ejections for electroactive species (Figure 1A, green). Additionally, the total amount ejected over a time period can be quantitated by ejections into small volumes (~100 µL) that are analyzed by liquid chromatography with electrochemical detection.…”

“…15 A chloridized Ag wire served as a pseudo-reference electrode. A liquid chromatograph (HP Series 1050, Hewlett Packard, Palo Alto, CA) was used to separate analytes within each aliquot.…”

Electrochemical Analysis of Neurotransmitters

“…With both drug delivery methods, dopamine D1 receptor antagonist SCH23390 blocks lever pressing during ICSS. However, unlike microinfusion, the smaller drug volumes introduced by iontophoresis (119) do not affect animal performance during the task, thereby allowing the neurochemical basis of the behavior to be investigated without influencing behavior.…”

Evaluation of Drug Concentrations Delivered by Microiontophoresis

“…A Matlab script (Mathworks, Natick, MA) was used to determine the average radial Ru(bpy) 3 2+ distribution of each image by averaging the fluorescence intensity along 11 background subtracted cross-sectional lines spanning from the barrel tip. 32 …”