Hardware Reference
In-Depth Information
6.5
Manipulation of Large Droplets
Bioassay benchmarks used to evaluate biochip design automation methods in the
literature make the assumption that all input and output droplets of mixing/dilution
operations are 1
droplets [
13
,
14
]. However, with recent advances in fabrication
technology, digital microfluidic biochips can now transport 2
and even larger
droplets; the transportation of larger droplets requires higher actuation voltages [
15
].
Some newly developed bioassay protocols for lab-on-chip include the manipulation
of 2
droplets. For example, to capture antibodies, a 1
blood droplet needs to be
mixed with a 2
droplet that contains magnetic beads in the “on-magnet incubation”
protocol [
2
]. In order to execute this bioassay, the biochip needs to be able to
transport 2
and even larger droplets.
To move droplets with large volumes, more electrodes need to be activated at
each clock cycle [
16
]. However, the manipulation of droplets with different volumes
introduces new constraints for the design of pin-limited biochips. We discuss these
details below.
6.5.1
Transportation of
2
Droplets
To avoid pin-sharing conflicts, the layout of a biochip that can manipulate 2
droplets must satisfy Constraint
1
and Constraint
2
derived in Sect.
6.1
. A difference
here is that more pins may be actuated to manipulate large droplets, i.e., the CPG of
a large droplet may contain more pins than the CPG of a 1
droplet.
As shown in Fig.
6.9
a, a 2
droplet will elongate in the direction of move-
ment [
16
]. Here, the CPG of the droplet movement includes Pins A, B, C, D, E,
and F. To move the 2
droplet in the direction of the arrow, the signals applied to
pins B and C must be set as “High”, and the signals applied to Pins A, D, E, and F
must be set as “Low”. It is important to note that when the 2
droplet is moved to
a different direction, the elements in the CPG of the 2
droplet are changed to Pins
A, B, D, E, F, and G, as shown in Fig.
6.9
b.
When converting the CPG of a 2
droplet to its graph model, we must consider
all the possible directions in which the droplet can move. An example is shown in
Fig.
6.10
a. The 2
droplet has two possible movement directions, i.e., Direction 1
and Direction 2. If the droplet is moved in Direction 1, the elements in its CPG are
Pins A, B, C, and F. The corresponding graph model for this CPG (G
D
1
)isshown
in Fig.
6.10
b; if the droplet is moved in Direction 2, the elements in its CPG are
Pins A, B, F, and G. Fig.
6.10
c shows the corresponding graph model for this CPG
(G
D
2
). The graph model for the CPG of the droplet can be derived by calculating
the
union
of graphs G
D
1
and G
D
2
, as shown in Fig.
6.10
d.
For the given pin-assignment configuration discussed in Sect.
6.4
, let the set
of electrodes be defined as E
layout
. When considering the transportation of the 2
droplet, we can construct a graph model for the layout E
layout
by virtually placing a
