Electrokinetic infusions into hydrogels and brain tissue: Control of direction and magnitude of solute delivery

Abstract

Background: Delivering solutes to a particular region of the brain is currently achieved by iontophoresis for very small volumes and by diffusion from a microdialysis probe for larger volumes. There is a need to deliver solutes to particular areas with more control than is possible with existing methods.

New method: Electrokinetic infusions of solutes were performed into hydrogels and organotypic hippocampal slice cultures. Application of an electrical current creates electroosmotic flow and electrophoresis of a dicationic fluorescent solute through organotypic hippocampal tissue cultures or larger hydrogels. Transport was recorded with fluorescence microscopy imaging in real-time.

Results: Electrokinetic transport in brain tissue slice cultures and hydrogels occurs along an electrical current path and allows for anisotropic delivery over distances from several hundred micrometers to millimeters. Directional transport may be controlled by altering the current path. The applied electrical current linearly affects the measured solute fluorescence in our model system following infusions.

Comparison with existing methods: Localized drug delivery involves iontophoresis, with diffusion primarily occurring beyond infusion capillaries under current protocols. Pressure-driven infusions for intraparenchymal targets have also been conducted. Superfusion across a tissue surface provides modest penetration, however is unable to impact deeper targets. In general, control over intraparenchymal drug delivery has been difficult to achieve. Electrokinetic transport provides an alternative to deliver solutes along an electrical current path in tissue.

Conclusions: Electrokinetic transport may be applied to living systems for molecular transport. It may be used to improve upon the control of solute delivery over that of pressure-driven transport.

Keywords: Drug delivery; Electrokinetic transport; Electroosmosis; Iontophoresis.

Figure 1.

Example of an electrokinetic infusion of Ru(bpy)₃²⁺ in a hydrogel with a ζ-potential similar to rat OHSCs with 75 μA applied current for 60 minutes. (A) Experimental set-up. (B) Fluorescence in a hydrogel with an applied current of 75 μA. Left images are the control gel with no applied current and right images are after an applied current. (C) Infusion profiles were obtained at varying time points in a hydrogel, with an applied current of 75 μA. Top panel is at time 0 and bottom panel is at time 50 minutes. Scale bar in C is 100 μm. Corresponding surface intensity (right column) plots were also examined with fluorescence images as shown in C. The color bar to the right represents relative intensity with white (255) and black (0). White vertical dotted line shows the position of dye and infusion capillary and white/red arrow represents the position of the counter capillary/electrode (left). An Olympus objective lens of 1.25× with a numerical aperture of 0.04 was used for imaging.

Figure 2.

Delivery of Ru(bpy)₃²⁺ in OHSCs at 1.5μA current. Left column is at initial timepoint; Center column is after 25 minutes; Right column is after 25 minutes with points showing 25% (yellow) and 15% (blue) of the maximum intensity at the infusion capillary. Top row: counter electrode in the GF-HBSS bath below the tissue. Bottom row: counter electrode at lower left in tissue, denoted with white outline. The scale bar is 500 μm.

Figure 3.

Electrokinetic transport of Ru(bpy)₃²⁺ in a hydrogel model of brain tissue using 3 different currents: 25 μA, 75 μA and 150 μA for 1 hour (N = 3 each). (A) Percentage of pixels in viewing area at a magnification of 1.25x demonstrating fluorescence intensity above background. Significant difference exists between 25 μA and 75 μA and between 25 μA and 150 μA. One-way ANOVA: F (2,33) = 41.36, p = 0.000001; and Tukey post-hoc test: p<0.05. (B) Aggregate sum of fluorescence intensity above background within viewing area at a magnification of 1.25× with numerical aperture 0.04. Significant differences exist between the three currents with error bars representing standard error of the mean (SEM).