Biomedical Engineering Reference
In-Depth Information
DsS
DsB
1
Sample dilution:
C
DsB
3
DsB
2
Dlt
1
(
Dlt
2-3)
C
/2
(
DsB
4-7)
C
/4
(
Dlt
4-7)
(
DsB
8-15)
C
/8
(
Dlt
8-15)
(
DsB
16-23)
C
/16
(
Dlt
16-23)
(
DsB
24-31)
C
/32
(
Dlt
24-31)
(
DsB
32-39)
C
/64
(
Dlt
32-39)
(
DsR
1-8)
(
Mix
1-8)
C
/128
(
Opt
1-8)
Figure 2.5
Sequencing graph for a protein assay.
dilution factor of
DF
= 2
N
can be obtained in
N
steps. After dilution, droplets
of reagents, such as Coomassie brilliant blue G-250 dye, are dispensed into
the chip, and they mix with the diluted sample droplets. Next, the mixed
droplet is transported to a transparent electrode, where an optical detector
(e.g., a LED-photodiode setup) is integrated. The protein concentration can
be measured from the absorbance of the products of this colorimetric reac-
tion using a rate kinetic method [38]. Finally, after the assay is completed, all
droplets are transported from the array to the waste reservoir. A sequenc-
ing graph model can be developed from the preceding protocol for a pro-
tein assay (
DF
= 128), as shown in Figure 2.5. There are a total of 103 nodes
in one-to-one correspondence with the set of operations in a protein assay,
where
DsS
,
DsB
i
(
i
= 1, …, 39), and
DsR
i
(
i
= 1, …, 8) represent the generation
and dispensing of sample, buffer, and reagent droplets, respectively.
In addition,
Dlt
i
(
i
= 1, …, 39) denotes the binary dilution (including mix-
ing/splitting) operations;
Mix
i
(
i
= 1, …, 8) represents the mixing of diluted
sample droplets and reagent droplets; and
Opt
i
(
i
= 1, …, 8) denotes the optical
detection of the droplets. Until the fourth step of a serial dilution, all diluted
sample droplets are retained in the microfluidic array. After that stage, for
each binary dilution step, only one diluted sample droplet is retained after
splitting, while the other droplet is moved to the waste reservoir.
The basic operations for the protein assay have been implemented on a dig-
ital microfluidic biochip [9]. Experiments indicate that the dispensing opera-
tion takes 7 s [9]. The operation times of various mixers have been found to
