Hardware Reference
In-Depth Information
However, flow-based biochips have several inherent limitations and drawbacks.
First, flow-based chips are difficult to integrate and scale down, as the scale of each
component in the device may have a decisive impact on the performance of the
entire system. The second problem is that these chips cannot be reconfigured after
they are fabricated because the channels are etched in the substrate [
4
]. Thus, the
paths for liquid flow and all of the operations implemented by the chip are pre-
determined, and no software-based differentiation is possible. The third problem is
that these chips lack fault tolerance, hence the entire chip ceases to be functional if
any channel or other on-chip element is defective [
4
].
Digital (droplet-based) microfluidic biochips have been proposed as an alterna-
tive to flow-based microfluidics [
4
,
5
]. On the droplet-based microfluidic platform,
liquid droplets and all of the fluid-handling operations are manipulated using an
array of discrete electrodes [
1
,
3
]. Since each droplet is analogous to “a bit of
information” and operates under clock control, this device is referred to as a “digital
microfluidic biochip” [
3
].
In digital microfluidic biochips, all molecular processes and biochemical reac-
tions are conducted using discrete droplets (or “packets of biochemical payload”)
that have nanoliter/picoliter volumes, which allows a very significant reduction
in reagent/sample volume and reaction time. As microfluidic droplet platforms
have the capability of conducting fast and efficient mixing inside droplets, high
throughput can be achieved for experiments. The droplets on digital microfluidic
biochips have no contact with any of the solid walls of the flow channels because
they are surrounded by silicone oil and manipulated on the surface of the electrode
array. Thus, as a benefit of this structure, the risk of cross contamination in
experiments and the adsorption of reagents/samples by the walls of the channels
are minimized. Therefore, the quality of the product droplet and the reliability of
the biochip are improved.
Since discrete droplets are manipulated independently on the biochip, the
droplet-based microfluidic biochip is especially suitable for researchers to closely
monitor various reactions over time. For example, researchers have developed
a droplet-based process for the identification and enumeration of foodborne
pathogens [
9
]. In this process, each water-in-oil droplet acts as a microreactor for
the encapsulation of the cell. By observing the metabolic activity of a single bacteria
cell that is confined by a picoliter-scale droplet, the detection and enumeration of
bacteria can be performed quickly and precisely. The oil that surrounds each droplet
acts as a carrier of droplets and a barrier against cross contamination. Compared
with the conventional methods used to detect pathogens, in which researchers have
to incubate the specimens for several hours or even days before the pathogens can
be detected by conventional instruments, methods based on digital microfluidic
biochips can significantly reduce detection time and improve the reliability of the
measurement results.
Given all these advantages, digital microfluidics is now seeing increasing
acceptance in biotechnology applications that require high-precision control of
fluid flow during experiments [
1
-
3
]. The complexity of such systems and the
integration level of digital microfluidic biochips has increased markedly in the
