Biomedical Engineering Reference
In-Depth Information
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Figure 1.1
Fabricated digital microfluidic arrays: (a) glass substrate [23]; (b) PCB substrate [21].
well-defined and simple applications, but are unsuited for more complex
tasks requiring a high degree of flexibility or complicated fluid manipula-
tions. Continuous-flow systems are inherently difficult to integrate because
the parameters that govern the flow field (e.g., pressure, fluid resistance, elec-
tric field strength) vary along the flow path, making the flow at any one loca-
tion dependent on the properties of the entire system. As liquids mix and
react in the system, their electrical and hydrodynamic properties change,
resulting in even more complicated behavior. Consequently, the design
and analysis of even moderately complex systems can be very challenging.
Furthermore, since structure and function are so tightly coupled, each system
is only appropriate for a narrow class of applications.
A digital microfluidic biochip utilizes the phenomenon of electrowetting
to manipulate and move microliter or nanoliter droplets containing biologi-
cal samples on a two-dimensional (2-D) electrode array [22]. A unit cell in the
array includes a pair of electrodes that acts as two parallel plates. The bottom
plate contains a patterned array of individually controlled electrodes, and the
top plate is coated with a continuous ground electrode. A droplet rests on a
hydrophobic surface over an electrode, as shown in Figure 1.1. It is moved by
applying a control voltage to an electrode adjacent to the droplet and, at the
same time, deactivating the electrode just under the droplet. This electronic
method of wettability control creates interfacial tension gradients that move
the droplets to the charged electrode. Using the electrowetting phenomenon,
droplets can be moved to any location on a 2-D array.
The division of a volume of fluid into discrete, independently controllable
packets or droplets for manipulation provides several important advan-
tages over continuous flow. The reduction of microfluidics to a set of basic
repeated operations (i.e., “move one unit of fluid one distance unit”) allows
a hierarchical and cell-based design approach to be utilized. Large systems
may be constructed out of repeated instances of a single well-characterized
device in the same way that complex microelectronic circuits may be built
upon a single well-characterized transistor. Thus, the design and analysis of
