Biomedical Engineering Reference
In-Depth Information
Figure 4.23
View of an EWOD system. The light surfaces are the electrodes (Photo courtesy of
LETI).
4.2.3.1 Open and Covered EWOD Systems
Two types of EWOD devices are currently used: open and covered systems. In the
following we present these two types of systems and then we analyze their architec-
ture; finally we investigate how fundamental operations on droplets—like motion,
coalescence (addition), division, dispensing—are performed.
In open EWOD systems, sessile droplets are moved directly on the paving of
electrodes. The zero potential electrodes can be a catena parallel to the surface
or special electrodes embedded in the substrate. In covered EWOD systems, the
catena is replaced by a top plate covering the droplet [Figure 4.23 and 4.24(b)].
In such system the free surface [i.e., the surface area between liquid and surround-
ing fluid (usually air, sometimes silicone oil)] is reduced. The substrate at the bot-
tom is similar to that of open EWOD systems. The top plate is constituted by
a plate electrode usually made of ITO coated with a thin layer of Teflon. Thus,
without electric actuation, the contact is hydrophobic with both plates, as shown in
Figure 4.24.
In reality, in a covered EWOD system, the droplet closely follows the shape of
the electrodes (Figure 4.25). In such a configuration, the vertical gap is very small
compared to the horizontal dimension of the droplet, and the surface area of the
free interface (liquid-air) is very small compared to that of the solid-liquid interface.
Figure 4.24
Schematic view of open and covered EWOD microsystems; (a) sessile droplet on elec-
trodes; (b) covered EWOD microsystem fabricated by the LETI.
