Minimizing tissue damage in electroosmotic sampling.

Electroosmotic sampling is a potentially powerful method for pulling extracellular fluid into a fused-silica capillary in contact with the surface of tissue. An electric field is created in tissue by passing current through an electrolyte-filled capillary and then through the tissue. The resulting field acts on the counterions to the surface charges in the extracellular space to create electroosmotic fluid flow within the extracellular space of a tissue. Part of the development of this approach is to define conditions under which electroosmotic sampling minimizes damage to the tissue, in this case organotypic hippocampal slice cultures (OHSCs). We have assessed tissue damage by measuring fluorescence resulting from exposing sampled tissue to propidium iodide solution 16-24 h after sampling. Sampling has been carried out with a variety of capillary diameters, capillary tip-tissue distances, and applied voltages. Tissue damage is negligible when the power (current x potential drop) created in the tissue is less than 120 microW. In practical terms, smaller capillary i.d.s, lower voltages, and greater tissue to capillary distances lead to lower power.