Biomedical Engineering Reference
In-Depth Information
droplet sizes can be as low as 1% with accurate control of the low rates. Silicone oils, perluo-
rocarbons, oleic acid, Fluid 200 from Dow Corning, and mineral oil, are among the commonly
used oils (viscosities on the order of 5 to 50 cst are recommended as a starting point).
Droplet microluidics, in particular in its “oily microluidic” platform, has four powerful
characteristics:
he reaction vessel—the droplet—has no surfaces, hence, there is no contaminant
adsorption nor any loss of reactants to the walls, so it is ideal for low molecule-count
reactions such as single-copy PCR ampliication, single-molecule enzymology, etc.
he reaction vessel is very small, so the reactants difuse quickly to react with one
another (
Figure 3.30
), and there is less cost in reagents and waste.
he format provides for multiple (on the order of thousands) repeats, naturally deliv-
ering rich statistics.
he repetition of the assay is inexpensive because it is provided in a “train” format.
A rich variety of biochemical assays have already been implemented in droplet format, and the
list only grows by the day: PCR (of single copies of cDNA), protein crystallization screening (1300
trials in 20 minutes, see
Figure 4.27
), cell encapsulation (see Section 5.3.9), and encapsulation of
C. Elegans
. We note that not all the droplets have to be an aqueous suspension—the generation
of alginate gel droplets of various shapes (plugs, disks, spheres, rods, and threads) by the fusion
of alginate droplets with CaCl
2
droplets, as recently pioneered by Xing-Zhong Zhao's group at
Wuhan University in China, is a promising advancement in cellular engineering. Importantly,
Bincheng Lin's group from the Dalian Institute of Chemical Physics has presented a microvalve-
actuated droplet generator that can produce droplets on demand (
Figure 3.31
)—all previous
droplet generators only function continuously. he size of the droplets is controlled by the amount
of time that the microvalve is open. Each microvalve gates an inlet with a diferent solution (a total
of four inlets are featured), and by alternating which inlets are “on” with computer control it is
possible to establish sequences and mixtures of droplets. Generation of droplet sizes between
1.3 and 13.3 nL (with volume variations of 7.2% and 1.6%, respectively) at 100 droplets/s was
demonstrated. Pei-Yu Chiou's group at UCLA has shown that it is possible to generate droplets
a
Water
Water
Water
Oil
Out
50 µm
b
Stretch and fold
Reorient
FIGURE 3.30
High-throughput.droplet.microluidics..(From.Helen.Song,.Michelle.R..Bringer,.Joshua.
D. Tice,. Cory. J.. Gerdts,. and. Rustem. F.. Ismagilov,. “Experimental. test. of. scaling. of. mixing. by. cha-
otic.advection.in.droplets.moving.through.microluidic.channels,”.
Appl. Phys. Lett.
.83,.4664,.2003..
Reprinted.with.permission.of.the.American.Institute.of.Physics..Figure.contributed.by.Rustem.Ismagilov.)
