Biomedical Engineering Reference
In-Depth Information
Figure 4.35  (a) typical electrode shape; (b) the interface is not sufficiently elastic to adjust to the
electrode boundaries.
microfabrication, but are very efficient for droplet motion provided the dimension-
ing of the dents is correctly done.
Centering Electrodes
Solid substrates of EWOD microsystems are microfabricated using extra care to
make them as smooth as possible. Surface defects can lead to unwanted pinning,
resulting in the malfunctioning of the microchip. A consequence of the smooth-
ness of the surface is that microdrops, if not anchored by a boundary line, may not
always be positioned at the same location on the surface. They show an unstable
positioning and tend to drift until they find an anchored position by pinning to a
singular point or to a boundary line. To maintain a microdrop at a given location,
star-shaped electrodes are used (Figure 4.36).
Dispensing Electrodes
In order to maintain the liquid close to the inlet port of the system, the reservoir
liquid is maintained at the system entrance by a special electrode. The principle is
similar to that of the star-shaped electrode (Figure 4.37).
4.2.4  Conclusion
EWOD designs, especially covered, can complete the fluidic operations required
to build a lab-on-a-chip. The have proved to be able to perform PCR (polymerase
chain reactions) as well as conventional systems, but in a much smaller volume.
Figure 4.36  Principle of star-shaped electrodes (Surface Evolver calculation).
 
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