Introduction - Hardware/Software Co-Design and Optimization for Cyberphysical Integration in Digital Microfluidic Biochips - page 7

attracted by the hydrophilic surface [ 17 ]. Due to the effect of this unbalanced surface
tension, the droplet can be moved by applying a control voltage to the electrodes
[ 17 , 20 ].

1.1.2 Hardware Platform

A digital microfluidic biochip consists of a two-dimensional electrode array and
on-chip reservoirs [ 20 ]. By utilizing the effect of electrowetting, nanoliter droplets
containing biological/chemical samples and reagents can be manipulated on the chip
without curved channels or external pressure sources [ 4 , 17 ].
Figure 1.2 [ 17 ] shows the structure of a unit cell on a digital microfluidic biochip.
The upper plate is a large electrode which covers all cells on the array and serves as
the ground electrode for all unit cells [ 17 , 20 ]. When the biochip is used, the upper
plate is applied a common voltage, thus all the unit cells in the array have the same
voltage on their upper electrodes [ 17 , 20 ]. The lower plate of the unit cell consists
of an array of discrete control electrodes. During chip operation, the unit cells in
the array may have different voltages on their lower electrodes. The movements of
droplets are determined by signals applied on the discrete electrodes. In the literature
on microfluidic biochips, the term “control voltages applied to electrodes” usually
refers to the voltages applied to the lower electrodes of the unit cells on the chip.
As shown in Fig. 1.2 [ 17 ], droplets manipulated by the digital microfluidic
biochip are confined between the upper and lower electrodes [ 17 ]. To move a
droplet, a high voltage should be applied to a unit cell adjacent to the droplet, and
at the same time, a low voltage must be applied to the cell under the droplet [ 17 ].
The voltages applied to the electrodes can influence the surface characteristics of the
hydrophobic coating on the lower electrodes. In this way, different voltages applied
to the electrodes result in multiple levels of interfacial tension on the surface of the
biochip [ 17 ]. Due to this effect, the droplet is moved from the low-voltage electrode
to the high-voltage electrode[ 17 ]. This is the basic operating principle of digital
microfluidic biochips. All microfluidic functions, such as droplet merging, splitting,
mixing, and dispensing can be reduced to a set of basic operations [ 17 ]. Concurrent
manipulation of multiple discrete droplets can be coordinated by control software
and by voltages applied to the electrodes, without the use of mechanical devices,
such as tubes, pumps, and valves [ 4 ].

Ground electrode

Top plate

Hydrophobic
insulation

Droplet

Filler fluid

Bottom plate

Fig. 1.2 Schematic
cross-section of a unit cell on
the microfluidic biochip [ 17 ]

Control electrodes