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reduces the scope of flexibility as well as scalability for multiple bioassay execution within
the same device. Hence a new class of device known as digital microfluidic biochip
(DMFB) has been emerged in last decade. In DMFBs samples are digitized as nanolitre
volumes of discrete fluidic droplets. Such droplets are manipulated using different
actuation methods within a 2D planar array of electrodes. The process of manipulation of
nanolitre samples are carried out through different mechanisms, viz. electrowetting [1-2],
dielectrophoresis [3], thermo capillary transport [4], and surface acoustic wave transport
[5]. Compared to continuous flow-based techniques, digital microfluidics offers the
advantage of individual sample addressing, reagent isolation, and compatibility with array-
based techniques used for biochemical and biomedical application [1, 6].
Electro Wetting
on Dielectric (EWOD) is currently used as one of the best actuation methods for droplet
manipulation in digital micro
fluidic systems [7].
The mechanism of electro wetting
phenomenon is based on the control of interfacial tension between
liquid and an electrode
coated with a dielectric layer.
An
imbalance of interfacial tension is created on application
of an electric field to the driving electrode. Due to the difference in contact angle between
two sides of the droplet, it is moved towards the energized electrode [8]. Advantages of
EWOD devices include ease of fabrication, simple control signal, digital operation,
programming flexibility, and minimal fluid consumption.
The basic microfluidic operations performed in DMFBs include droplet generation
and storage, droplet transportation, droplet mixing and droplet splitting .The store
operation is performed by applying an insulating voltage around the droplet. The mix
operation is performed by routing two droplets to the same location, to merge them
into one droplet. Since the size of a droplet is kept small, effective mixing can be
achieved by fluid diffusion after merging. Finally, the split operation is performed by
creating opposite surface tension at the two ends of a fluid droplet and tearing it into
two smaller droplets [10].
The microfluidic array contains a set of basic cells that is made up of two parallel
glass plates (Figure 1).The bottom plate contains a patterned array of individually
controllable electrodes, and the top plate is coated with a continuous ground electrode.
The filler medium, such as silicone oil and the sample droplet is sandwiched between
the two plates. By assigning time-varying voltage values to turn on/off the electrodes
on the digital microfluidic biochip, the interfacial tension of the droplets are
modulated- resulting in their transportation around the entire 2-D array and execution
of fundamental microfluidic operations for different bioassays. The operations
performed by actuating control voltages at the electrodes are also called
reconfigurable
operations because of their flexibility in location and in execution time.
Fig. 1.
Schematic for a unit cell used in DMFB
