(196b) Electrodes for Microfluidic Control and Sensing

This presentation will cover microfabricated electrodes for control and sensing in microfluidic systems. The electrodes here are chemically inert and are used to drive dielectrophoretic flows or to sense impedance changes due to flowing particles. For some of these applications, submicron thickness “thin-film” electrodes are good enough. Thin-film platinum electrodes work for impedance sensing, and are easily integrated into planar fabrication during construction of polymer microfluidics or glass-based microfluidic systems. Besides sensing, these electrodes can also perform electrolysis to inject bubbles and ions into long-lived microfluidic devices if the materials are chosen carefully. For more diverse applications, thick electrodes (>1 micron) are important. A very common motivation for thick electrodes is to create a uniform electric field on opposite sides of a channel for sensing impedance changes caused by particles.  Another motivation is to control the direction of and increase the volume occupied by fields in electrically-driven microfluidic flows. However, thick-film fabrication is more costly than thin-film, typically requiring a slow electroplating step that happens outside the planar fabrication sequence. This problem has driven the development of many workarounds to integrate thick electrodes in the microfluidic chip fabrication process. Conductive liquid electrode channels work well when the liquid electrode need not make direct contact with the fluid, for example when driving AC dielectrophoretic flows or doing capacitive sensing. For resistive sensing, ion injection, or performing induced-charge electroosmosis on metal surfaces with different spatial orientations, though, direct contact with thick electrodes is necessary, creating a fabrication challenge. A useful question is whether the thick electrodes really need to be solid in a given application, or whether they can be a thin film over a three-dimensional insulating shape. The presentation will include three alternate methods for patterning thin film over topography or for creating three-dimensional objects from released thin films. These techniques do not require electroplating and can be applied when it is only the three-dimensional electrode shape that matters. Examples will be presented from induced-charge electroosmosis.