Electrowetting platform

Figure 16. Electrowetting platform (EWOD). Implementation of a colorimetric glucose assay in a single chip. Four reservoirs with injection elements are connected to an electrode circuitry, where the droplets are mixed, split and transported to detection sites for readout.

Digital microfluidic architecture is under software-driven electronic control, eliminating the need for mechanical tubes, pumps, and valves that are required for continuous-flow systems. The compatibility of each chemical substance with the electro-wetting platform must be determined initially. Compatibility issues include the following (1) Does the liquid s viscosity and surface tension allow for droplet dispensing and transport by electrowetting ... 

Some of the platforms can also be considered as multi-application platforms, which is of special interest in the field of research instrumentation. Here, portability is of less importance, and the number of multiple parameters per sample as well as programmability (potentially also during an assay run) gains impact. The microfluidic large scale integration and the droplet based electrowetting and surface acoustic waves platforms are such versatile examples. 

The droplet-based microfluidic platforms for Lab-on-a-Chip applications can be fundamentally divided into two basic setups, the channel-based and the planar surface approach [2]. The channel-based systems are mostly pressure driven with droplet generation and manipulation relying on actuation via liquid flows within closed microchaimels. For the planar surface-based platforms, droplets can be arbitrarily moved in two dimensions representing planar programmable Lab-on-Chips. They are actuated by electrowetting (EWOD) or surface acoustic waves (SAW). 

Droplet reactors are basic components of digital microfluidics. There are still a number of opportunities in droplet reactor research. The future directions can be categorized into fundamentals and applications. Fundamental research could result in other platforms for droplet reactors. While electrowetting has been widely reported in the past, microfluidic platforms based on thermocapillary and other forces are still underrepresented. More research on the systematic design of droplet-based reactors is needed to secure industrial adaptation and commercial... 

Electrowetting-based platforms have also been proposed as a convenient means for driving micro-mixing of discrete drops of different chemicals. The simplest form of such mixing is to adopt a passive scheme in which two drops are translated together and merged by electrowetting... 

A more complex and integrated prototype miniaturized medical diagnostic platform based on electrowetting, as shown in Fig. 5, was engineered by Srinivasan et al. [15], in which it was confirmed that electrowetting actuation can... 

Real-world structured surfaces for superhydrophobic electrowetting range from geometrically uniform like those in Figs Id and 2a, to randomly oriented fiberlike structures. Most demonstrated works include a composite dielectric approach. In this composite approach the structured electrode is first insulated with a conventional dielectric such as a metal or semiconductor oxide. The composite dielectric is then completed with a plasma deposited fluorocarbon or solution deposited flu-oropolymer in order to provide adequate hydrophobicity for a stable Cassie state. A brief review of techniques to create superhydrophobic electrowetting surfaces is provided below. The demonstrated structures vary substantially in geometry and materials, however, electrowetting results are somewhat similar across all platforms. 

Disadvantages of conventional microchips include resistance to hydrodynamic flow (backpressure), adsorption of some biomolecules on walls, and limited robustness (e.g., clogging narrow channels). Digital microfluidic systems overcome the problems with mixing reagents, which are normally associated with conventional microfluidic devices that use laminar hydrodynamic flow. Such microscale platforms are normally based on the electrowetting-on-dielectric (EWOD) principle [66]. In these digital microchips. 

Microcalorimetry has been performed on several sample biochemical reactions using electrowetting to initiate the reactions that are then monitored by on-chip embedded thermistors [13]. The differential temperature rise between adjacent sample-control pairs was used and 96 of these pairs were microfabricated on 9 mm spacing consistent with SBS 96-well format for compatibility with laboratory automation. Examples of reactions that were used to validate this platform were the binding of 2 -CMP to RNase A and the binding of biotin to streptavidin. Phosphorylation of glucose by hexokinase and the power output of mitochondrial respiration were also monitored. 

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