Dielectrophoretic Characterization and Continuous Separation of Cells in a PDMS Microfluidic Device with Sidewall Conducting PDMS Composite Electrodes - Bioanalysis Advanced Materials Methods and Devices - page 172

11.3.2 Separation of Samples by Size

Figure 11.2c illustrates the separation of particles having the same polarizability
(i.e., nDEP) by size . As larger particles experience a stronger repulsive nDEP
force, they are pushed laterally farther away from the electrodes in comparison to
the smaller particles. When the applied voltage is large enough, the larger particles
can be transported to the upper branch B separating them from the smaller ones,
which are transported to the lower branch A.

11.4 Results and Discussion

11.4.1 Numerical Simulation of Electric Fields and DEP Forces

In a microchannel with sidewall AgPDMS composite electrodes, the electric field
E

¼rf associated with the electric potential f is governed by Laplace's equation
ðr

2

. The boundary conditions are specified based on the channel configura-
tion shown in Fig. 11.2a . The component of the electric field normal to the noncon-
ducting PDMS walls as well as all inlets and outlets are set to be zero, the first and
third electrodes (from the left) are assigned with an electric potential

f¼ 0 Þ

ðf¼ V AC Þ , and

the potentials of the second and fourth electrodes are also set to be zero . The

contour electric fields (lines) and DEP forces (arrows) created by the sidewall
AgPDMS electrodes are numerically obtained using COMSOL Multiphysics (ver-
sion 3.4). As shown in Fig. 11.3a , when Re K ðoÞ

ðf¼ 0 Þ

0 , nDEP forces cause the
repulsion of cells away from the sidewall electrodes. The opposite scenario (cell
attraction to the electrodes) occurs when Re K ðoÞ

½ <

½ >

0, as shown in Fig. 11.3b . The
magnitude of the DEP forces shown by arrows are proportional to the length of
arrows. The simulation results also indicate that highly nonuniform electric fields are
induced near the AgPDMS electrode edges as can be seen from the high density of
accumulated electric field lines. However, electric field strength rapidly decreases at
further distance from the electrodes (along the y direction). Such an electric field
distribution results in markedly strong DEP forces near the electrodes. Furthermore,
along the horizontal direction, the DEP force is much weaker towards both the center
of the electrode and the center of the gap between any two adjacent electrodes.
Therefore, for all samples of experiencing dominant DEP force effects, the hydro-
dynamic focusing is incorporated in our two separation mechanisms to confine the
flowing particle stream near the edges of the sidewall electrodes.

11.4.2 DEP Characterization of Yeast and Bacterial Cells

DEP behaviors of yeast and bacterial cells were characterized under various AC
electric field frequencies ranging from 10 kHz to 80 MHz in stagnant buffer