Introduction - Digital Microfluidic Biochips - page 10

direct addressing scheme. For large arrays, direct addressing schemes lead
to a large number of control pins, and the associated interconnect-routing
problem significantly adds to the product cost. Thus, the design of pin-
constrained digital microfluidic arrays is of great practical importance for
the emerging marketplace.
Pin-constrained design of digital microfluidic biochips was recently pro-
posed in [37]. This method uses array partitioning and careful pin assign-
ment to reduce the number of control pins. However, it requires detailed
information about the scheduling of assay operations, microfluidic module
placement, and droplet-routing pathways. Thus, the array design in such
cases is specific to a target biofluidic application. In another method proposed
in [38], the number of control pins for a fabricated electrowetting-based bio-
chip is minimized by using a multiphase bus for the fluidic pathways. Every
n th electrode in an n -phase bus is electrically connected, where n is a small
number (typically n = 4). Thus, only n control pins are needed for a transport
bus, irrespective of the number of electrodes that it contains. Although the
multiphase bus method is useful for reducing the number of control pins,
it is only applicable to a one-dimensional (linear) array.
An alternative method based on a cross-reference driving scheme is pre-
sented in [39]. This method allows control of an N × M grid array with
only N + M control pins. The electrode rows are patterned on both the top
and bottom plates, and placed orthogonally. In order to drive a droplet
along the X-direction, electrode rows on the bottom plate serve as driving
electrodes, while electrode rows on the top serve as reference-ground
electrodes. The roles are reversed for movement along the Y-direction,
as shown in Figure 1.4. This cross-reference method facilitates the reduc-
tion of control pins. However, due to electrode interference, this design
cannot handle the simultaneous movement of more than two droplets.
The resulting serialization of droplet movement is a serious drawback for
high-throughput applications.
The minimization of the assay completion time, that is, the maximization
of throughput, is essential for environmental-monitoring applications in

V

Bottom
electrode

To p
electrode

Y

X

X

Y

Figure 1.4
A cross-referencing microfluidic device that uses single-layer driving electrodes on both
top and bottom plates. (Adapted from Fan, S.-K. et al., Proceeding of IEEE MEMS Conference ,
pp. 694-697, 2003.)