Biomedical Engineering Reference
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mapping of bioassay protocols to the microfluidic array. We next propose
a new method to incorporate droplet routing in the PRSA-based synthesis
flow for defect-tolerant microfluidic biochips developed in [50].
2.2.1 Droplet-routability estimation
For a synthesized biochip, the droplet routability of a route between two
modules is quantified in terms of the length, measured by the number of
electrodes, of the droplet transportation path. Droplet routability is evalu-
ated in terms of the average length of all the droplet pathways for the com-
plete chip. Also, we have to control the maximum length of droplet paths.
Large values for the maximum path length lead to long routing times; for
example, more than 5% of the module operation time can have the undesir-
able consequence of having to halt an assay temporarily until the droplets
are routed to their destinations. Moreover, long routing pathways are likely
to be blocked by obstacles, that is, intermediate modules. For example, in
Figure 2.1, all routing pathways from M 1 to M 4 are blocked by M 2 and M 3 ;
therefore, droplet routing is not feasible for this design. Note that guard-
ring cells are used to avoid inadvertent mixing, and they cannot be used
for routing. Synthesized designs with large values for the maximum droplet
path length suffer from a high probability of being nonroutable. Based on the
preceding considerations, we adopt the maximum droplet path length as a
parameter for evaluating routability of a synthesized biochip.
A straightforward technique to derive the routability information is to
carry out postsynthesis routing to generate an actual routing plan. However,
this approach adds to the computational burden of the synthesis tool. In
particular, if a routing plan involving all the droplets on the array is gen-
erated for each chromosome in the PRSA-based unified synthesis method,
M 1
Route
start
M 2
Route
destination
M 3
M 4
Guard ring
Figure 2.1
An example of a nonroutable interdependent pair.
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