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 167

( Plasmodium falciparum )[ 3 ]. On the other hand, lymphocytes, monocytes, and
granulocytes are white blood cells having similar electrical properties but are
different in size [ 4 ].
Dielectrophoresis field-flow fractionation (DEP-FFF) has been proposed for
separating cells in continuous flow [ 5 ]. DEP-FFF transports cells with pressure-
driven flow in microfluidic channels and separates them using an externally applied
DEP force perpendicular to the direction of flow. Under a nonuniform electric field,
polarizable cells in a buffer solution can be manipulated by the DEP force as a result
of polarization effects [ 6 ]. Since the magnitude and the direction of the DEP force is
dependent on the polarizability of cell relative to that of buffer solution and its
magnitude is also scaled to the cubic power of the cell radius, cells can be separated
based on their size or polarizability without any pretreatment [ 7 ]. These intriguing
benefits enable DEP to be a label-free, inexpensive and versatile separation tool,
superior to flow cytometry where modification of samples with expensive reagents
is often required [ 8 ].
To produce a DEP force, the conventional approach of generating a nonuniform
electric field in DEP devices is through planar metallic electrodes deposited on the
bottom or/and the top of the separation channel [ 9 - 11 ]. However, the rapid decay of
electric field strength from the electrode surface often causes inefficient manipula-
tion of cells at the center of the separation channel [ 12 , 13 ]. As a result, three
dimensional (3-D) or sidewall electrodes have been engineered to generate a 3-D
electric field across the channel height. Recently reported 3-D electrodes are made
from heavily doped silicon [ 14 ], pyrolytic SU-8 photoresist [ 15 ], electroplated gold
or titanium [ 16 , 17 ]. Nevertheless, the assembly of these devices is still rather
complicated to prevent liquid leakage as the devices are fabricated from glass
or silicon.
With the rapid advancement of soft lithography techniques, the use of
polydimethylsiloxane (PDMS) as the fabrication material is ubiquitous in
microfluidic devices due to its numerous advantages (e.g., ease of fabrication and
bonding, low cost, biocompatibility, and optical transparency) [ 18 ]. However, a
serious challenge is the infeasibility to embed 3-D metallic electrodes in PDMS-
based DEP devices, since the adhesion between metallic electrodes and PDMS is
extremely weak [ 19 ]. Manually embedding copper electrodes into PDMS channel
to a certain degree overcomes the problem [ 20 , 21 ], but this technique is not
practical for batch fabrication and also readily causes liquid leakage at high
flow rates due to the mismatch between the thickness of copper sheet and that of
channel height.
Here we introduce a complete polymer DEP device with sidewall conducting
PDMS composite electrodes for manipulating particles and biological cells
[ 22 , 23 ]. The proposed fabrication technique allows for very strong adhesion
between the conducting PDMS composite electrodes and PDMS microfluidic
channels, thus greatly facilitating the device assembly with only one-step oxygen
plasma treatment. This novel device with sidewall conducting PDMS composite
electrodes is so versatile that it can be employed for characterizing cells in stagnant
flow and separating particles or cells based on their size or their polarizability.