Abstract: Separation media for electrophoresis, and methods of filling and flushing of electrophoretic devices such as capillaries are described. By preparing submicron to above-micron sized cross-linked gel particles and using gel swelling equilibrium concepts, such devices can be easily filled and flushed. Gel particles can be prepared by inverse suspension, precipitation and suspension polymerization. These particles can be swollen and collapsed by small changes in temperature, pH, and ionic strength of solvent. Other approaches involve the formation of reversible cross-links by use of polyelectrolyte complexes, chelating agents or copolymers of hydrophobic and hydrophilic repeat units. Finally, reversibly solubilized systems may be used to change the viscosity of the media.