Biomedical Engineering Reference
In-Depth Information
a square wave high voltage pulse generator. Because it is
DC, there may be an appreciable electrolysis and change
in pH near the electrodes. To keep the necessary voltage
low, the distance between the electrode plates is small.
It is possible to use a RF pulse instead of DC. The RF
causes mechanical vibrations in addition to the electrical
effects, and this may increase the poration or fusion
yield. As the effect is so dependent on the cell diameter,
it may be difficult to fuse or porate cells of different sizes
with DC pulses. The threshold level for the smallest cell
will kill the largest.
successfully used for other types of bioparticles like
DNA (Washizu and Kurosawa, 1990), proteins (Washizu
et al., 1994) and viruses (Schnelle et al., 1996).
More recently, dielectrophoretic studies have for in-
stance been reported on T-lymphocytes (Pethig et al.,
2002; Pethig and Talary 2007) and on how cell destruction
during dielectrophoresis can be minimized (or utilized) by
appropriate choice of AC frequency and amplitude
(Menachery and Pethig, 2005). Dielectrophoresis has also
been used for measurement of membrane electrical
properties such as capacitance and conductance for insulin
secreting pancreatic cells (Pethig et al., 2005).
Another interesting approach to particle separation is
called field-flow fractionation, and this technique can be
used in combination with dielectrophoresis (Davis and
Giddings, 1986). Particles are injected into a carrier flow
and another force (e.g. by means of dielectrophoresis) is
applied perpendicular to the flow. Dielectric and other
properties of the particle will then influence the
particle's distance from the chamber wall and hence its
position in the parabolic velocity profile of the flow.
Particles with different properties will consequently be
released from the chamber at different rates and
separation hence achieved. Washizu et al. (1994) used
this technique for separating different sizes of plasmid
DNA.
4.1.14.2 Cell sorting and characterization
by electrorotation and dielectrophoresis
The direction and rate of movement of bio-
particles and cells due to electrorotation, dielectropho-
resis and other electrokinetic effects depend on the
dielectric properties of, for example, the cell. These
dielectric properties may to some extent reflect the type
of cell or the condition of the cell and there is conse-
quently a significant potential in the utilization of these
techniques for cell sorting or characterization.
Electrorotation
was, for example, used to differentiate
between viable and non-viable biofilms of bacteria. Be-
cause of their small size, determination of the dielectric
properties of bacteria by means of electrorotation is
impractical. By forming bacterial biofilms on polystyrene
beads, however, Zhou et al. (1995) were able to in-
vestigate the effect of biocides on the biofilms.
Masuda et al. (1987) introduced the use of traveling
wave configuration for the manipulation of particles.
The frequency used was originally relatively low, so that
electrophoresis rather than dielectrophoresis was pre-
dominant. The technique was later improved by, among
others, Fuhr et al. (1991) and Talary et al. (1996), who
used higher frequencies where dielectrophoresis domi-
nates. Talary et al. (1996) used traveling wave dielec-
trophoresis to separate viable from non-viable yeast
cells and the same group have used the technique to
separate erythrocytes from white blood cells (Burt
et al., 1998).
Hydrodynamic forces in combination with stationary
electric fields have also been used for the separation of
particles. Particles in a fluid flowing over the electrodes
will to different extent be trapped to the electrodes by
gravitational or dielectrophoretic forces. Separation is
achieved by calibration of for example, the conductivity
of the suspending medium or the frequency of the ap-
plied field. This approach has been used for separation
between viable and non-viable yeast cells (Markx et al.,
1994), different types of bacteria (Markx et al., 1996),
leukemia and breast cancer cells from blood (Becker
et al., 1994, 1995). Dielectrophoresis has also been
4.1.14.3 Cell-surface attachment
and micromotion detection
Many types of mammalian cells are dependent on at-
tachment to a surface in order to grow and multiply.
Exceptions are the different cells of the blood and cancer
cells which may spread aggressively (metastases). To
study cell attachment a microelectrode is convenient: the
half-cell impedance is more dominated by electrode
polarization impedance the smaller the electrode surface
is.
Figure 4.1-22
shows the set up used by Giaever's
group (Giaever and Keese, 1993).
A monopolar electrode system with two gold elec-
trodes is used. A controlled current of 1
m
A, 4 kHz is
applied to a microelectrode
<
0.1 mm
2
, and the corre-
sponding voltage is measured by a lock-in amplifier. With
cell attachment and spreading, both the in-phase and
quadrature voltage increase as the result of cell-surface
coverage. It is possible to follow cell motion on the sur-
face, and the motion sensitivity is in the nm range. The
method is very sensitive to subtle changes in the cells
(e.g. by the addition of toxins, drugs and other chemical
compounds). It is also possible to study the effect of high
voltage shocks and electroporation.
Figure 4.1-23
shows an example of cell attach-
ment and motion as measured with the electric cell-
substrate impedance sensing (ECIS) instrument, which
