Biomedical Engineering Reference
In-Depth Information
+100 V
a
b
+450 V
GND
+500 V
40 µm
GND
+450 V
FIGURE 5.11
Switching. cells. and. biochemical. reagents. using. electrokinetic. low.. (From. Paul. C..
H..Li.and.D..Jed.Harrison,.“Transport,.manipulation,.and.reaction.of.biological.cells.on-chip.using.
electrokinetic.effects,”.
Anal. Chem.
.69,.1564,.1997..Reprinted.with.permission.of.the.American.
Chemical.Society.)
low was used—requiring voltages on the order of 400 to 500 V (
Figure 5.11
). (Note: the term
“
electrokinetic
” refers to all varieties of electrohydrodynamic mechanisms for moving luids
and particles; in this work, the luids were likely moving by electro-osmosis and, in addition, the
cells might also have moved by dielectrophoresis.) On-chip chemical treatment (cell lysis) was
also demonstrated.
Because of the required large voltages, electrokinetic low turns out to be impractical, rais-
ing concerns about cell viability (the electrodes can generate pH gradients and electrolysis),
researcher safety, and equipment size and cost (high-voltage ampliiers/switches are bulky and
expensive). herefore, a variety of alternatives has been proposed.
5.2.3 Dean Flow in Spiral Microchannels
In 1997, Ashutosh Kole's team at the Palo Alto Research Center introduced a novel concept
for particle separation: subject the particles to a curvilinear laminar low by forcing them
through a spiral microchannel at high speeds—similar to passing them through a centrifuga-
tion machine. Since then, the designs have been improved by many groups, using spiral as
well as serpentine designs. Interestingly, the separation efect is ampliied not only by the cen-
trifugal force but also by the so-called Dean's vortex. he vortex generates the recirculation of
luid close to the walls to satisfy luidic continuum when the fast-moving core moves outward
at the center of the channel due to centrifugal force. Using a ive-loop spiral microchannel
100 μm wide and 50 μm high, Ian Papautsky's group succeeded in separating 7.3 μm beads
from 1.9 μm beads at Dean number
De
= (ρVD
h
/μ)(D
h
/2
R
)
1/2
=
Re
(D
h
/2
R
)
1/2
= 0.47, where
ρ
is
the density of the luid medium,
V
is the average luid velocity,
D
h
is the channel's
hydraulic
diameter
[
D
h
≡ 2
HW
/(
H
+
W
)],
μ
is the luid viscosity, and
R
is the radius of curvature of the
path of the channel (
Figure 5.12
).
5.2.4 Pinched-Flow Fractionation
In 2004, Minoru Seki and colleagues from Osaka Prefecture University realized that, as a
liquid containing particles is “pinched” against a wall of a microchannel by another particle-
free liquid, the particles exit the microchannel sorted by sizes (
Figure 5.13a
). his efect,
dubbed “
pinched-low fractionation
,” is generated because the particle-free liquid forces all
the particles against the wall, thus homogenizing their speed (in the edge of the parabolic
low proile) as a function of their size; as they exit the microchannel into the broadened
segment, the particles follow lines of equal low velocity, so they are sorted by size with sub-
micron resolution (
Figure 5.13b
).
