Biomedical Engineering Reference
In-Depth Information
Diameter = 200 μm
Vias
Pitch: 500 μm
Figure 6.3
Illustration of wire routing limits on a PCB layer.
If direct addressing is used, that is, each cell of the patterned electrodes
is accessed directly and independently via a dedicated control pin, a total
of 1284 pins will need to be wired. However, the large number of electrodes
required leads to a cumbersome wiring problem for control pins, especially
when fabricated using PCB technology. In PCB technology, the diameter of
the via hole is usually comparable to the electrode pitch size. Therefore, there
is only a limited number of control lines that can be routed on one layer of
PCB. As shown in Figure 6.3, the via hole diameter is 40% of the electrode
pitch. Therefore, only four control pins can be wired in any row. To route a
large number of control pins, a multilayer PCB design is needed, which is
prohibitively expensive. Therefore, we adopt a “correlated” pin-assignment
method, which allows a control pin to be connected to multiple electrodes,
thereby reducing the total number of pins.
Next, we address the problem using the efficient and easy-to-implement
“Connect-5” algorithm presented in Chapter 3, Section 3.1. For simplicity, we
first look at the application of the algorithm for a two-dimensional array of
electrodes without wells. As shown in Section 3.1, five copies of Bagua rep-
etitions are sufficient to cover a biochip array of any size. This is because of
the following property of a Bagua repetition: vertices connected to the same
(shared) pin appear after exactly five cells in the same row or column of the
array, as shown in Chapter 3, Figure 3.8.
It has been shown in Chapter 3, Figure 3.9 that control pins assigned to
the electrodes using this method in a microfluidic array allow free move-
ment of a single droplet, that is, the “cross constraint” is met. We modify the
pin-assignment procedure described to make it applicable to our well-plate
design. Note that the well-plate design can be viewed as a special case of the
two-dimensional array where parts of the array are occupied by wells and
segregation walls. Unoccupied electrodes between wells can be used as trans-
portation pathways. Therefore, the pin assignment for these electrodes does
not need to be changed. The overall pin-assignment procedure is as follows.
1. Start with a two-dimensional electrode array of the same size as the
target well-plate design but with no cells reserved as wells or segre-
gation regions.
