Electronic-microfluidic system for sorting particles and whole blood using gel electrodes

Dielectrophoresis (DEP) is a MEMS technique widely used for sorting particles ranging from nanometer-sized latex beads to rare mammalian cells based on variations in their dielectric properties. Negative dielectrophoresis (nDEP), a type of DEP in which particles migrate down an electric field gradient, has been used to continuously sort particles into flowstreams for high-throughput sorting. Presented is an nDEP device utilizing vertical electrodes spanning the height of a microfluidic channel which generate clean sorting across flow lines and can be positioned along both channel sidewall, increasing control of the gradient shape compared to a traditional planar system. This method bypasses various microfabrication difficulties associated with vertical electrode fabrication by using channel geometry and a conductive gel formed via laminar flow to create nonuniform electric fields in the flow channel. The device consists of straight channel broken into three regions: a focusing inlet, a central sorting section, and a final trifurcation extending into three outlets for retrieval of sorted particles. Extending orthogonally from this channel on either side of the sorting region are hundreds of 30 micron wide side channels that lead from the main channel to two planarized Au/Ti electrodes on the glass substrate, between which the main channel lies. Channels are cast in PDMS using soft lithographic technique. All side channels are filled with a conductive gel of 0.5% agarose gel in PBS that provides a semisolid conductive medium beginning at the electrodes and terminating flush with the sidewalls of the main channel; these gel structures thus act as liquid vertical electrodes, focusing electric field lines from the planar electrodes and injecting them across the main channel. By strategically molding these vertical electrodes asymmetrically along the walls of the main flow channel, resultant electric field lines are dispersed as they travel from vertical electrodes of on- wall to those of the opposing wall, yielding an electric field non-uniform in the x-y plane but uniform in the z-direction. To ensure even starting positions for entering particles and thus fair exposure to DEP force, attached to the entrance of the flow channel is a novel focusing system: several hundred 15μm-wide pillars spaced 10μm apart are arranged in rows tiled ±1.5° relative to laminar flow. Buffer passes through this focuser unobstructed but particles cannot and are redirected into flowlines progressively closer to either the center or one edge of the main channel. Particles thus enter the nDEP sorting region single-file along a strategically-chosen flowline, and sorting is a function only of the particles´ DEP response.