Biomedical Engineering Reference
In-Depth Information
Source
Source
Sink
Sink
Figure 4.1
Illustration of a single-droplet scan-like test using a single droplet.
detect an electrode-short defect, a test droplet needs to traverse the two adja-
cent electrodes that are involved in the short. The test droplet will reside
in the middle of the two shorted electrodes, which are activated simultane-
ously; there will not be sufficient overlap area with the next electrode for
further transportation.
Most prototype digital microfluidic devices consist of a two-dimensional
(2-D) a r ray of elect rodes w it h one or more sou rces a nd si n k is, t hat i is, reser voi rs,
on the boundary, as shown in Figure 4.1 [25]. In this regular structure, elec-
trodes are carefully aligned in columns and rows. We next describe the par-
allel scan-like test method, named thus because it manipulates multiple test
droplets in parallel to traverse the target microfluidic array, just as test stimuli
can be loaded in parallel to multiple scan chains in integrated circuits.
We first describe the special case of a single test droplet. We determine the
pathway for the test droplet, irrespective of the bioassay operation, as shown
in Figure 4.1. Starting from the droplet source, the test droplet follows the
pathway to traverse every cell in the array, and it finally reaches the sink.
During concurrent testing, a test droplet is guided to visit the available cells
in accordance with a predetermined path. If the target cell is temporarily
unavailable for testing, that is, it is occupied by a droplet or it is adjacent to
active microfluidic modules, the test droplet waits in the current position
until the target cell becomes available. The test outcome is read out using a
capacitive sensing circuit connected to the electrode for the sink reservoir,
as shown in Figure 4.2. The figure shows details about the setup and how it
was validated. This single-droplet scan-like algorithm is easy to implement.
Moreover, the test plan is general, in the sense that it can be applied to any
microfluidic array and for various bioassay operations.
However, in this simple test procedure method,
N
×
M
steps (clock cycles for
droplet actuation) are needed for the test droplet to traverse an
N
×
M
micro-
fluidic array. As a result, the test time may be excessive for large arrays. For
example, a 600,000-electrode array manufactured by Silicon Biosystems (based
on dielectrophoresis) will require 600,000 clock cycles [68]. At a typical actua-
tion clock frequency of 1 Hz, this amounts to 7 days of test application time!
Moreover, in online testing, the test droplet may have to be stalled several
times, and each time a long waiting period may be necessary. Finally, the test
