Counterbalanced Valve Metal Oxide as a Reliable Dielectric Layer for Electrowetting-on-dielectric Devices

Valve metal oxide (anodized oxide) is used in a mirrored configuration to serve as self-limiting dielectric layers of electrowetting-on-dielectric (EWOD) devices. Resembling nonpolar electrolytic capacitors, the two opposing metal-dielectric layers always counterbalance each other so that voltages of any polarity can be applied without current flow. By using tantalum pentoxide as the valve metal oxide for experimental evaluation, the mirrored configuration is compared with the usual configuration and further compared with silicon oxide and silicon nitride, which are two dielectric materials commonly used for EWOD. Experiments with a range of applied biases confirm that in a mirrored configuration, one of the two metal-dielectric-electrolyte combinations is always reverse-biased to prevent the current leakage, which is the most common mode of failure for the EWOD devices. The utility is demonstrated by manipulating droplets of KCl solution on both the parallel-plate and one-plate EWOD devices.