Biomedical Engineering Reference
In-Depth Information
Wa ste-handling
reservoir
Solution
storage
reservoirs
Control-signal input ports
Figure 5.1
A fabricated digital microfluidic biochip [24].
provides the maximum freedom for test-droplet manipulation, but it requires
a large number of control pins.
To reduce production cost, unused electrodes are often removed from
the rectangular array, resulting in an irregular chip layout
(see Figure 5.1)
.
To further reduce the number of control pins, pin-constrained design tech-
niques are used in practice as discussed in Chapter 3, whereby multiple elec-
trodes are connected to a single control pin. These design methods achieve a
significant reduction in the number of input pins needed for controlling the
electrodes. However, as a trade-off, droplet manipulation steps must satisfy
additional constraints. These constraints can result in test procedures being
either completely ineffective or effective only for a small part of the chip.
To evaluate the effectiveness of a test procedure, we define a parameter
referred to as
testability
. Given a specific test method, the testability of a chip
design is defined as the ratio of testable electrodes/functional units to the
total number of electrodes/functional units on the chip, where a functional
unit is defined as a cluster of adjacent electrodes that can carry out a specific
type of fluidic operation. Depending on the test method and chip functional-
ity, chip testability can be classified into two categories, namely, structural
testability and
functional testability.
Structural testability
is defined as the percentage of testable electrodes on
the chip during a structural test. An electrode is considered “testable” if
it can be traversed by the test droplet. Note that for most biochips, includ-
ing pin-constrained chips, any on-chip electrode has to be traversed by at
least one droplet in order to carry out the fluidic operations mapped on it.
This means it can also be traversed by the test droplet. Therefore, most chip
designs can achieve a structural testability of 100%.
Functional testability
is defined as the percentage of testable functional units
on a chip in a functional test procedure. High testability indicates that the
test method can probe the functionality of the chip thoroughly and identify a
