Optimization of an EWOD Platform for 2D Droplet Translations utilizing Cross-Reference Electrodes

Digital micro fluidics is a key research field of many scientific groups which aims at realizing a lab on a chip. The focus of interest is set on the two dimensional manipulation of separated droplets which allows for performing an arbitrary sequence of reactions on the same microfluidic platform. Furthermore sample volume and measurement times can be controlled precisely utilizing single droplets as reaction and measurement volume. The following paper addresses the question which design rules are applicable when a covered electro-wetting on dielectrics (EWOD) platform is used for two dimensional manipulations of single droplets. The focus was laid on EWOD with a cross referenced electrodes scheme, where two identical electrode arrays rotated by 90° with respect to each other are patterned on the base and cover plate, which allows for droplet actuation in two dimensions. As parameters of interest the contact and hysteresis angles of a hydrophobic surface are defined. Their impact on minimally required electro-wetting forces, platform geometries and the dielectric layers are deduced. The extreme cases of large sample volumes or actuation by minimum voltages suitable for programmable electronics are discussed. Furthermore an experimental setup for a two dimensional actuation of large liquid volumes was designed and tested. A material system for inexpensive processing was utilized featuring a copper structure on a polymer film which is covered with an Al2O3 dielectric layer and a hydrophobic Teflon layer as base and cover plate. Thus, droplets can be addressed and directed to an arbitrary spot in a two dimensional array via a programmable sequence of translations. This concept allows for running different analytical reactions on the same platform just by changing the sequence of translations.