Hardware Reference
In-Depth Information
Detector
Incoming
flow
Cooled region:
65°C
Heated region:
94
°
C
Fig. 5.1
Schematic of a DMFB that can perform DNA amplification [
3
,
7
]
An example of a DMFB performing DNA amplification (i.e., the first stage of the
PCR procedure) is shown in Fig.
5.1
[
3
,
7
]. A “heated” region and a “cooled” region
are created by a heater. Typically, the temperatures of the heated region and cooled
region are set as 94
ı
and 65
ı
, respectively.
First, “PCR mix”, which contains Taq DNA polymerase, dNTPs, MgCl
2
,and
reaction buffers in the solution, is prepared on the biochip and stored in a reservoir.
Next, the “PCR mix” droplets are blended with the biological sample that contains
the target DNA strands. Then, the well-mixed droplet is allowed to pass alternately
through several heating and cooling steps [
3
,
7
]. Each thermal cycle contains two
steps, i.e., (1) heating the droplet, which splits the double-stranded DNA inside the
droplet into two strands, called “DNA melting”; and (2) cooling the droplet, with
each strand of the DNA working as a template for synthesizing new DNA strands
called “primer annealing”. In this way, the initial DNA strands can be amplified
into millions of copies [
3
]. As the amplified DNA strands in the droplet are labeled
with fluorophores, the generation rate of the target DNA strands can be measured at
each PCR cycle by measuring their intensity of the fluorescence with the sensors
integrated in the microfluidic system [
4
-
6
].
In order to enable the on-chip hybridization of amplified DNA, some biochips
have detection electrodes (DEs) integrated into them [
11
-
14
]. Gold detection spots
with different immobilized DNA probes are fabricated on the surface of these DEs,
as shown in Fig.
5.2
[
11
,
13
]. After the PCR procedure, the droplets that contain the
amplified DNA strands are left stationary on the DEs. DNA hybridization occurs
if the amplified DNA strands and the probes immobilized on the detection spot are
complementary strands. In this method, the DNA sequences in the amplified DNA
strands can be analyzed.
