Biomedical Engineering Reference
In-Depth Information
also discussed. We evaluate the proposed functional test methods using
simulations as well as experiments for a fabricated biochip.
To avoid ambiguity, we refer to the test method proposed in Section 4.1 as
a structural test, since it routes test droplets to all the electrodes in the array
in order to ensure structural integrity. The structural test targets at physical
defects, which are defined as the underlying cause of erroneous chip opera-
tion, where the defect affects either a unit cell (electrode) or the electrical
connection to the unit cell. Functional testing, on the other hand, involves
test procedures to check whether groups of cells can be used to perform
certain operations, for example, droplet mixing and splitting. For the test
of a specific operation, the corresponding patterns of droplet movement are
carried out on the target cluster of cells. If a target cell cluster fails the test,
for example, the mixing test, we label it as a malfunctioning cluster, which
implies that the synthesis tool cannot place the corresponding module—a
mixer in this case—in this region.
As in the case of structural testing, we first develop a fault model for func-
tional testing. Since functional testing is an extension of structural testing,
all the defect-oriented fault models are also included here. Therefore, we
start from the fault models proposed in Section 4.1. Malfunctions in fluidic
operations are then identified as shown in Table 4.2.
TAble 4.2
Fault Models for Digital Microfluidic Biochips
Number
of Cells
Involved
in the
Defect
Cause of
Malfunction
Observable
Error
Malfunction Type
Fault Model
Electrode
actuation for
excessive
duration
Irreversible charge
concentration on
the dispensing
electrode
3
Dispensing stuck
on (droplet is
dispensed by not
fully cutting off
from the reservoir)
No droplet can
be dispensed
from the
reservoir
Electrode shape
variation in
fabrication
Deformity of
electrodes
3
No overlap
between droplets
to be mixed and
center electrode
Mixing failure
Electrode
electrostatic
property
variation in
fabrication
Unequal actuation
voltages
3
Pressure gradient
(net static
pressure in some
direction)
Unbalanced
volumes of
split droplets
Bad soldering
Parasitic
capacitance in the
capacitive sensing
circuit
1
Oversensitive or
insensitive
capacitive sensing
False positive/
negative in
detection
 
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