&DIGITAL-MICROFLUIDIC LAB-ON-A-CHIP (LoC) technology offers a platform for developing diagnostic applications with the advantages of portability, sample and reagent volume reduction, faster analysis, increased automation, low power consumption, compatibility with mass manufacturing, and high throughput. In addition to diagnostics, digital microfluidics is finding use in airborne chemical detection, DNA sequencing by synthesis, and tissue engineering. In this article, we review efforts to develop various LoC applications using electrowetting-based digital microfluidics. We describe these applications, their implementation, and associated design issues. The ''Related work'' sidebar gives a brief overview of microfluidics technology.

A low voltage, two-level-metal, and multi-layer insulator electrowetting-on-dielectric (EWD) platform is presented. Dispensing 300pl droplets from 140nl closed on-chip reservoirs was accomplished with as little as 11.4V solely through EWD forces, and the actuation threshold voltage was 7.2V with a 1Hz voltage switching rate between electrodes. EWD devices were fabricated with a multilayer insulator consisting of 135nm sputtered tantalum pentoxide (Ta 2 O 5 ) and 180nm parylene C coated with 70nm of CYTOP. Furthermore, the minimum actuation threshold voltage followed a previously published scaling model for the threshold voltage, V T , which is proportional to (t/ε r ) 1/2 , where t and ε r are the insulator thickness and dielectric constant respectively. Device threshold voltages are compared for several insulator thicknesses (200nm, 500nm, and 1µm), different dielectric materials (parylene C and tantalum pentoxide), and homogeneous versus heterogeneous compositions. Additionally, we used a two-level-metal fabrication process, which enables the fabrication of smaller and denser electrodes with high interconnect routing flexibility. We also have achieved low dispensing and actuation voltages for scaled devices with 30pl droplets.

A hydrodynamic scaling model of droplet actuation in an electrowetting-on-dielectric (EWD) actuator is presented that takes into account the effects of contact angle hysteresis, drag from the filler fluid, drag from the solid walls, and change in the actuation force while a droplet traverses a neighboring electrode. Based on this model, the threshold voltage, V T , for droplet actuation is estimated as a function of the filler medium of a scaled device. It is shown that scaling models of droplet splitting and liquid dispensing all show a similar scaling dependence on [t/e r (d/L)] 1/2 , where t is insulator thickness and d/L is the aspect ratio of the device. It is also determined that reliable operation of a EWD actuator is possible as long as the device is operated within the limits of the Lippmann-Young equation. The upper limit on applied voltage, V sat , corresponds to contact-angle saturation. The minimum 3-electrode splitting voltages as a function of aspect ratio d/L \ 1 for an oil medium are less than V sat . However, for an air medium the minimum voltage for 3-electrode droplet splitting exceeds V sat for d/L C 0.4. EWD actuators were fabricated to operate with droplets down to 35pl. Reasonable scaling results were achieved.

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