Biomedical Engineering Reference
In-Depth Information
where biochemical reactions take place. his strategy for discrete luid manipulation is powerful
because the protocols are readily automated (since luid motion is electrically powered), but has
two important shortcomings: (a) the chips have thus far been expensive to produce, compared
with all-polymeric devices; and (b) the voltages and the enclosed environment are not friendly
to cells, so the chips are mostly limited to noncellular applications such as biochemical reac-
tions. (Recently, Aaron Wheeler's laboratory at the University of Toronto has demonstrated cell
culture and selective transfection of HeLa cells within a digital microluidic platform.)
3.7.2 “Oil Carrier” Microdroplet Platform
he second platform relies on conining the water droplets within the walls of microchannels; the
water droplets are surrounded by an oil luid that acts as the carrier. he shuttling speeds here
are orders of magnitude larger than with the electrowetting platform (typical droplet generation
rates are on the order of 10,000 droplets/second!), so the “oil carrier” microdroplet platform has
gradually displaced the electrowetting platform for throughput-intensive, biorelated applications.
Excluding digital microluidics, we may adopt a working deinition of droplet microluidics
as a family of microluidic techniques and devices based on the generation of a continuous train
of droplets on one luid suspended in a stream of a diferent, immiscible luids. Typically, a sur-
factant needs to be added to prevent colliding droplets from fusing. Although nonmicroluidic
formats had existed for decades, the irst droplet-generating microluidic device is credited to
Stephen Quake's group (then at Caltech) and was presented in January 2001 (
Figure 3.29a
). he
generation of the droplet was based on the lateral shear of a jet of water by an impinging low
of oil. Most present droplet generators, however, are based on nozzle designs, the irst of which
was presented in 2003 by Howard Stone's group at Harvard University (
Figure 3.29b
). Minoru
Seki's group at the University of Tokyo also pioneered a microluidic droplet generator design
(presented in March 2001) that has since been abandoned. In all cases, the droplet size is deter-
mined simply by the geometry of the junction and the ratio of the water low rate to the oil low
rate, and the generation frequency depends on the combined low rates. he monodispersity in
a
b
Water
Oil
W
o
W
o
= 278 µm
H
f
= 161 µm
H
f
D
Water
W
i
W
Oil
D = 43.5 µm
30 µm
60 µm
60 µm
W
i
= 197 µm
Oil
W
o
Q
i
/Q
o
= 1/4
13.1/12.4
(a)
5.6 × 10
-5
12.1/12.4
(b)
1.4 × 10
-4
11.1/12.4
(c)
60 µm
4.2 × 10
-4
FIGURE 3.29
Generation.of.droplets.in.microchannels..(a,.from.Todd.Thorsen,.Richard.W..Roberts,.
Frances.H..Arnold,.and.Stephen.R..Quake,.“Dynamic.pattern.formation.in.a.vesicle-generating.micro-
luidic. device,”.
Phys. Rev. Lett.
. 86,. 4163,. 2001.. Figure. contributed. by. Stephen. Quake;. b,. from.
Shelley.L..Anna,.Nathalie.Bontoux,.and.Howard.A..Stone,.“Formation.of.dispersions.using.''low.focus-
ing''.in.microchannels,”.
Appl. Phys. Lett.
.82,.364,.2003..Reprinted.with.permission.of.the.American.
Institute.of.Physics..Figure.contributed.by.Shelley.Anna,.Nathalie.Bontoux,.and.Howard.A..Stone.)
