Biomedical Engineering Reference
In-Depth Information
Additional experimental findings showed that reducing the medium conduc-
tivity from 380 to 2
S/cm can result in a change from nDEP to pDEP at low
frequencies. According to ( 11.3 ), such switching of DEP behavior is expected
when the medium conductivity is lower than that of the cell (i.e., s m <s cell ).
Experiments using conducting AgPDMS electrodes clearly confirmed this DEP
switching. For all frequencies ranging from 10 kHz up to a cross-over frequency
of 40 MHz, yeast cells suspended in 2
m
S/cm DI water were observed to experi-
ence only a pDEP force, attracting them to the electrodes (images are not shown
here but the cell behavior under pDEP are similar to that shown in Fig. 11.4d ).
Above the cross-over frequency of 40 MHz, cells no longer moved under the
applied AC field because the DEP force became very weak (as shown by the red
dashed line at 40 MHz shown in Fig. 11.4a ). In addition, cells were also observed
to form chains in the same direction as the electric field lines as shown in
Fig. 11.3 . The mechanism of chain formations has been described in the literature
[ 25 ]. It is also important to note that in DI water, Baker's yeast cells still survive
though experiencing a high osmotic pressure[ 33 ].
DEP behavior of E. coli cells was also obtained through the use of conducting
AgPDMS electrodes. Under an AC voltage of 15.3 V at 400 kHz and 30 MHz,
E. coli cells in 380
m
S/cm NaCl solution exhibited weak pDEP but strong pDEP at
1 MHz (Fig. 11.4e ). These DEP behaviour regimes are reasonably consistent with
the CM factor prediction in Fig. 11.4a , although no DEP movement was experi-
mentally observed below 150 kHz and above 35 MHz. Moreover, two cross-over
frequencies were found at 150 kHz (from no DEP to pDEP) and 35 MHz (from
pDEP to no DEP). Similar in behavior to the yeast cells, E. coli cells suspended in
DI water only experienced pDEP at all frequencies from 10 kHz to 30 MHz, even
up to a cross-over frequency of 35 MHz. Formation of an E. coli chain was noted in
that cells aligned along the electric field lines.
In general, the DEP behavior and the cross-over frequencies of yeast and
bacterial cells are quite similar. They experienced nDEP in the low frequency
regime (e.g., 10-100 kHz) and pDEP in the medium-to-high frequency regime
(e.g., 500 kHz to 10 MHz). On one hand, at low frequencies, the CM factor is
mainly determined by the conductivity of the cell and the buffer solution. Since
their membrane conductivity is very low, biological cells behave like insulating
objects under a DC or low frequency electric field, thus experiencing nDEP [ 30 ].
On the other hand, at higher frequencies, an electric current can pass through the
capacitive membrane and the conductivity of the cells is dominated by the rela-
tively high conductivity of the internal parts of the cells. As a result, the cells tend to
exhibit pDEP in the medium-to-high frequency regime. Furthermore, our
experiments showed that E. coli cells are generally more conductive than yeast
cells. This finding is validated from the experimental facts that in the 380
m
S/cm
NaCl solution, E. coli cells experienced pDEP while yeast cells experienced nDEP
at 10 kHz.
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