Biomedical Engineering Reference
In-Depth Information
Connected to a capacitive
sensing circuit shown in Figure 4.2
Reservoir
Figure 4.26
Illustration of the electrode to which a capacitive sensing circuit is connected.
Output from
capacitive
sensing
circuit
(a)
(b)
Figure 4.27
Test readouts for (a) normal dispensing and (b) dispensing failure.
some fluid left at the third electrode, which is indicated by a positive pulse,
with a smaller amplitude, in the test readout. Therefore, we can easily detect
a dispensing failure by reading the output of the capacitive sensing circuit,
as shown in Figure 4.27.
To identify abnormal droplets, two threshold values for the pulse ampli-
tude are used. These thresholds are determined through calibration of the
sensing circuit. First, we fix a nominal value µ and a maximum allowable
droplet volume variance σ. Then, two droplets with volumes of µ + σ and
µ − σ are routed to the sensing circuit. Signal levels are recorded and used as
the upper and lower threshold values, respectively.
4.5.2 routing Test and Capacitive Sensing Test
The routing test focuses on evaluating a single electrode's ability to transport
droplets. This procedure is similar to that proposed earlier for the structural
test [28,34]. In the structural test, a test droplet is dispensed and routed to
cross the target electrode from two orthogonal directions, that is, along the
row and the column directions. The routing problem can be solved by map-
ping the array to an undirected graph and applying the Euler-path-based
method [36], as shown in Figure 4.28. On the other hand, a test droplet must
be routed along all four directions relative to the target electrode. We can
solve the route-planning problem in this case by mapping the target array
 
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