Biomedical Engineering Reference
In-Depth Information
11.4.3 Continuous Separation of Samples by Polarizability
The separation of samples of similar sizes based on their different polarizabilities is
demonstrated by subjecting one type of samples by nDEP and the others by pDEP.
This separation mechanism, previously described in Sect. 11.3, is also shown in
Fig.
11.2b
. Three binary mixtures were prepared: (1) yeast cells and 5
m fluores-
m
cent latex particles, (2) bacterial cells and 3
m latex particles, and (3) live and dead
yeast cells. These latex microspheres (of similar sizes to yeast and bacterial cells)
were selected to eliminate the size-dependent effect.
As shown in Fig.
11.4a
, the CM factor predicts that yeast cells and latex particles
will exhibit distinct DEP polarizabilities at the medium range of the AC electric
field frequency (i.e., from 300 kHz to 1 MHz) in 380
m
S/cm NaCl solution. The
latex particles are predicted to experience nDEP for all frequencies since they are
electrically insulating, and the resulting CM factor of latex particles approaches
m
0.5. Figure
11.5a
presents a snapshot image of the continuous-flow separation of
yeast cells (black dots) by pDEP and 5
m fluorescent latex particles (white dots) by
nDEP under 31.2 V at 300 kHz (cross-over frequency of yeast cells) after 4 min.
Both yeast cells and particles were hydrodynamically focused near the sidewall
AgPDMS composite electrodes so as to have a strong DEP force as shown by the
distribution of the simulated DEP force (Fig.
11.3
). The latex particles were
repulsed from the electrodes by the nDEP force and separated to outlet D. In
contrast, as the yeast cells underwent a weak pDEP force, most of them ended up
preferentially in outlet C, though the others were observed to be attracted to the
electrodes. The rationale that yeast cells exhibited the weak pDEP force rather than
zero DEP force at 300 kHz results from their DEP polarizability which is highly
sensitive to frequency in this frequency regime. In these experiments, the time
window defined by the velocity of the fluid flow (306
m
m/s) and the separation
m
channel length (1,400
m) was about 4.6 s, giving rise to an nDEP drift velocity of
m
about 11
m
latex particles (with a throughput ~1,200/min) were reliable for more than 5 min at
the flow rate of 0.15
m/s. The separation of yeast cells (with a throughput ~780/min) and 5
m
m
l/min, and the separation efficiency was up to 97%.
Compared to yeast cells, separation of smaller samples like
E. coli
cells and
2.9
m
m latex particles becomes more challenging as the DEP force—scaled to the
cubic power of cell size—on these small cells is much weaker. Therefore, strong
pDEP and nDEP forces are preferred for separating these two samples. Based on the
CM factor shown in Fig.
11.4a
, the most suitable AC electric field frequency to
separate
E. coli
from the particles is 10 MHz, since it provides a strong pDEP force
for
E. coli
and a strong nDEP force for the particles. However, the maximum
frequency achievable by our customized amplifier is limited to 1 MHz to generate a
higher voltage (40.4 Vac). Under 40.4 Vac at 1 MHz, clear separation of
E. coli
and
2.9
m
S/cm NaCl solution still can be successfully achieved
as illustrated in Fig.
11.5b
. This separation of
E. coli
(with a throughput
~1,440/min) and 2.9
m latex particles in 380
m
m
m latex particles (with a throughput ~780/min) were reliable
at the flow rate of 0.06
m
l/min for more than 6.18 min with a separation efficiency
m
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