Biomedical Engineering Reference
In-Depth Information
while beyond 50 MHz,
ReŒCM
Š
"
m
"
p
"
m
"
p
C
2"
m
:
(3.32)
Relation (
3.31
) is justified by the fact that at low frequencies, the DEP force is
determined mainly by the conductive properties of the surrounding medium and the
dielectric particles, and thus the conductivity is the prevailing mechanism of DEP.
On the contrary, at high frequencies, the dielectric properties dominate, which is
suggested also by (
3.32
), whereas at intermediate frequency values, between 50 kHz
and 50 MHz, both conductive and dielectric properties are important.
Most biological entities such as bacteria, viruses, or cells are not homogeneous
dielectrics, as considered in the model presented above. This unfavorable situation
is alleviated by using a multishell model. The multishell model can be simplified
by assuming that the concepts of effective permittivity and conductivity still apply
and by modeling cells as particles with a spherical shape. In particular, some cells,
for example, the discoidal-shaped human erythrocyte, can be treated as single-shell
spherical dielectric particles since they take a spherical form in cytoplasm. In this
single-shell model, the permittivity of the dielectric particle becomes (
Pethig 2010
)
2
4
3
5
C
2
"
cyt
"
mem
"
cyt
C
Rd
3
2"
mem
"
p
D
"
m
"
cyt
"
mem
"
cyt
C2"
mem
;
(3.33)
Rd
3
where "
cyt
and "
mem
are the complex permittivities of the cell interior and plasma
membrane, respectively, R denotes the outer radius of the cell and d is the thickness
of the membrane.
The main application of DEP is to manipulate cells and to characterize biological
entities, referred to also as soft matter, in suspensions. The implementation of
DEP is made by micro-nanofabrication techniques, which consist mainly in the
fabrication of various electrode shapes able to trap and manipulate cells. There is
a multitude of substrates and electrode shapes for an efficient strong nonuniform
electric field generation, such as interdigitated, interdigitated and castellated, spiral,
sawtooth, etc. (
H olzel 2009
). We present some examples of electrodes below.
A planar interdigitated ring electrode (PIRE), which constitutes a cell in a 6
6
array, is displayed in Fig.
3.8
(
Hsiung et al. 2008
).
The PIRE array is microfabricated on a glass substrate using standard micro-
nanotechnologies and is integrated in a microfluidic device. The input gap has a
width of 10m, the output gap is 15
m wide, and the outer diameter is of 125m.
The 6
6 array is able to trap a number of 48
˙
6 cells when an ac signal of 5 V
and a frequency of 5 MHz is applied. The electric field is nonuniform and spatially
localized, being stronger in the central region of the PIRE. Adhesion of cells on the
PIRE is promoted by collagen coating.
Search WWH ::
Custom Search