Biomedical Engineering Reference
In-Depth Information
7.2.1 Synthesis based on Physical Constraints
The unified synthesis method described in Chapter 2 provides a powerful
tool for the automated design of digital microfluidic biochips. It combines
geometry-level synthesis with architectural synthesis and generates a com-
prehensive design with detailed resource binding, operation scheduling, and
module placement information. However, this synthesis approach is oblivi-
ous to constraints imposed by the manufacturing process. Resource-binding,
operation-scheduling and module-placement decisions are made without
any consideration of physical constraints such as transportation speed limit,
maximum switching frequency, and reservoir capacity. A chip design that
disregards these physical constraints can suffer from a severe “mismatch
problem.” An experimental example is shown in Figure 7.1, in which a dis-
pensing operation has been mapped onto a reservoir on a PCB chip. The
synthesis result requires four droplets to be dispensed from the reservoir.
However, the capacity of the reservoir is only 3.6 times the nominal volume
of a standard droplet. By carrying out four iterations of dispensing, we
obtained three droplets of normal volume and one droplet of smaller volume,
as shown in Figure 7.1. This “shrunk” droplet may be too small to overlap
with an adjacent electrode and therefore cannot be moved using electro-
wetting. This type of mismatch problem can be catastrophic for bioassay
execution, and must be avoided.
7.2.1.1 Mismatch Problems
Mismatch problems can appear in various forms. Here we discuss two of
the most common problems, namely, incorrect scheduling and undesir-
able electrode charging. Incorrect scheduling can result when the desired
Reservoir
capacity = 3.6 ×
nominal volume
Fluorescent photo
Left over volumes
in reservoir
Dispensed droplet
of abnormal volume
Dispensed droplet
of normal volume
Figure 7.1
An example of a mismatch problem—reservoir capacity overflow.
