. Lazar I 2010 Food And Envinronment Safety G6 [617701]

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

72
DIELECTROPHORETIC DEVICES FOR SE PARATING
FOODBORNE PATHOGENS

Iuliana Mihaela LAZAR1, Arcadie SOBETKII2, Ileana Denisa NISTOR1,
Neculai Doru MIRON1, Marius STAMATE1, Gabriel Octavian LAZAR1

1Vasile Alecsandri University of Bacau, Calea Marasesti 157, 600115 Bacau, Romania,
[anonimizat]
2MGM STAR CONSTRUCT SRL, Str. Pincota 7B, Bucuresti, Romania

Abstract. Today an increased interest in bioparticle separation research field is shown.
Microtechnology and nanotechnology opens new perspectives in food quality analysis such as
bioMEMS ( Micro -Electro -Mechanical -Systems ) devices for simultaneous detection of microorganism
in food products with improved sensitivity and velocity [1].
Diseases caused by different foodborne pathogens such as bacteria, viruses , fungi, parasites, etc. have
been a serious problem.
Classical microbiological methods are taking a long time to confirm results for a particular pathoge n
organism. At present numerous rapid methods are being studied, for example polymerase chain
reaction (PCR) [2], enzyme linked immunosorbent assay (ELISA) [3], dielectrophoresis (DEP) [4,5],
dielectrophoresis combined with ELISA, dielectrophoresis combined with electro rotation (ROT),
travelling wave dielectrophoresis, etc.
Dielectrophoresis is a method o f manipulation of a micro particle in an electric field gradient which
results from the interfacial polarization [6]. Using low voltages and temperatures below 39 degrees
allows us no permanent damage to the cells.
Theoretical modelling of behaviour in ele ctric field is very important for the applications we need.
Electrode arrangement from bioMEMS and channel geometries affects abilities to separate foodborne
pathogens.
This article describes the results obtained by our research team for manipulating bacte ria with a
specific electrode type of DEP.

Keywords: bioparticle , dielectrophoresis, separation

Introduction

The ability to accurately control and
handle micrometer and nanometer scale
particles is intensively studied in recent
years especially as a capture method in
different fields, including the food
industry. [7]
One of the methods used for this purpose is
dielectrophoresis – DEP. Dielectrophoresis
movement consists in a bioparticle capture
caused by an electric field applied. Dielectrophoretic force depends on the
electrical properties, geometrical and
morphological factors and environmental
bioparticle of suspension and the applied
electric field characteristics (frequency and
intensity).
It may be highlighted both in continuous
current (DC) [ 8] and in alternating current
(AC) [9 -14] because dielectrophoretic
force does not depend on the electric field
polarity [9]. In literature the most
commonly used is the alternating electric
field [10 -14].

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

73 Dielectrophoresis as electrokinetic
phenomenon th at utilizes an asymmetric
electric field to sort different bioparticle
like nano -pathogen agents was performed
using a micro fabricated device [11].
Separation was possible through operation
of the characteristically properties
differences between normal a nd infected
cells. Sorting experiments were placed
inside a microchip made from different
microelectrodes patterned on a glass
substrate.
In this study we establish the experimental
arrangement for capture lactic bacteria
with dielectrophoretic forces. Th is method
is not destructive, which is a great
advantage.

Theory
The basic law which describes the forces
acting on charged particles in electric field:
( ) ( ) F qE r r qE r       
where q = Q + = Q – is the dipole
electric charge and r= d is the distance
between charges.
The behavior of a dipole in a variable
electric field is described in figure 1 and
the movement of particles under the effect
of the resultant force in a uniform phase
(a,b) and variable phase (c) is descr ibed in
figure 2.

Fig.1 Dipole placed in irregular electric field [16]
In the nonlinear medium as biological
medium, the dependence between electric
field E and polarization P has the
following form:
P=E+ E+ E+… , 12
23
Size called molecule polarizability, is a
physical quantity numerically equal to the
induced dipole moment in the molecule by
an electric field intensity unit.

Fig. 2 (a) Uniform electric field resultant force
acting on the particle is zero.
(b) A particle is under a DEP force because of
gradient elec tric field intensity.
(c) A particle is under a DEP force because of a
gradient electric field phase [14]

Cases:
a. If α does not depend on the
direction of electric field
orientation, the molecule is
isotropic in terms of electric
polarization.
Polarizat ion vector is in this case: 
P=Np=NE=Nql , 
N is the number of molecule, q is
charge molecule and l is the
distance between positive center
charge distribution and negative
center charge distribution;
b. If α polarizability molecule varies
with electric field orientation, the
molecule behaves an anisotropic
induced dipole moment and the
electric field inducing a
relationship tensor form:
i ik k p=E cu (i, k=1,2,3) , 
ikis the components polarizability
tensor molecule;
c. In anisotropic media with different
properties after different directions:

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

74 k 0k ii P= E , cu (i, k=1,2,3=x,y,z) , 
is the electric susceptibility tensor
components of the environment:
11 12 13
21 22 23
31 23 33xx xy xz
ik yx yy yz
zx zy zz     
      
                     
For charged particles and electric field
polarized in the absence of magnetic field,
the force has two components, electrostatic
force and dielectrophoretic force.
( )tot es defF F F qE P E          

If there is a gradient ele ctric field, then it
will cause a net shift of the charged
particles. The dielectrophoretic force must
overcome Brownian motion.
This relationship establishes a requirement
on the minimum particle size that can be
manipulated, so high values of gradient
electric field are necessary to manipulate
the order of nano -size particles.
Next we express dielectrophoretic force
(DEP) in a three dimensional
configuration. In an AC electric field, E(t)
is a harmonic function of time and the
force can be write as:

where Γ is the geometric factor of the
particles, ε m is the dielectric constant of
suspending medium, and f cm is the
Clausius –Mossotti factor. The terms
Re(f cm) and Im(f cm) refer to the real and
imaginary parts of f cm [8].
The DEP force has two major term s: the
first term represents the “classical DEP
force” the second term represents the
“travelling -wave DEP force”.
For a spherical particle the Re[ fcm] is
determined by taking the real component
of the complex form of the Clausius –
Mossotti factor: * *
* *1
2 2 21p m
p m p m p m
CM sferic
p m p m p m
p mj
f
j  
                             

where *
p and *
m are the conjugate
complex dielectric permitivities of the
particle and medium, respectively and σp is
the conductivity of the particle, σm is the
conductivity of the medium and ω is the
angular frequency of the applied electric
field [15].

Results and discussion

We aimed to study the behaviour of lactic
acid bacteria, particularly lactobacillus and
streptococcus species.
For this purpose we used a starter culture
Yoghurt (YO -MIX 495 LYO 100 DCU,
Danisco, Sassenage, France)[12,13,16].
After the standard procedure , the
suspension of bacteria is put under the
microscope. We follow the behaviour of
bacteria under the influence of electric
field.
The DEP force allows particles to move
independently of their charge in an
inhomogeneous electric field applied to the
micro device (chamber electrode).
Materials constituting the dielectrophoretic
chamber electr ode are complex (for
example: Pt Pd Au Mo Cr Al). It is very
difficult to choose the material which the
electrodes are made from . Electrode
geometry, their number and distance
between them are important parameters in
choosing the experimental set -up.
To a void the complexity of the
manufacturing process, only one material
is usually used to manufacture electrodes.
Nowadays different combinations of
materials are used . The selection of
materials for manufacture of electrodes
depends on the desired destinatio n, ionic
species involved, environmental impact of

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

75 materials and their suitability to
manufacture
In our study we established to generate a
variable electrical field and to serve as
DEP electrodes for capturing bacterial
cells to the electrode surface.
We performed an experimental set -up
composed of alternating signal source, a
dielectrophoretic chamber made as a result
of collaboration between UMF Bucharest,
“Vasile Alecsandri" University of Bacau
and MGM STAR CONSTRUCT SRL.
Several electrodes images are s hown in
figure 3(a, b, c).

Fig. 3a Dielectrophoretic chamber with “teeth”
electrodes . This image is made at biophysics
department UMF Bucharest

Fig. 3b Dielectrophoretic chamber with linear
electrodes. This image is made at biophysics
department UMF Bucharest

Fig. 3c Dielectrophoretic chamber with
castellated electrodes. This image is made at
biophysics department UMF Bucharest

In this study the electrodes of
dielectrophoretic chamber were made from
chromium, with a thickness of about one
quarter micron. It is very difficult to
produce technologically the space between
electrodes, which must provide electrical
isolation on the one hand, and on the other
hand must have a good regular
dielectrophoretic effect. We made different
types of electrod es. Images are viewed
with optical microscopy at UMF Bucharest
and AFM microscope at “Vasile
Alecsandri" University of Bacau.
AFM applications in cell biology can be
classified into several broad categories:
imaging as shown in figure 4,
micromanipulation studies, material
property measurements and binding force
measurements. In this study we are using
AFM to view the lactic bacteria behaviour.

Fig.4 Scheme of an AFM coupled with
an inverted optical microscope [17]

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

76 Imaging of living cells has been couple d
with controlled culture systems, enabling
the possibility of continuous, long -term
imaging.
An important feature of AFM is the
possibility of studying dynamic processes
at high spatial resolution [17].
The AFM images of a micro
channel electrode which ha s handled the
bacteria are presented in figure 5 a, b:

Fig. 5a The image of a micro channel
with 0.29 µm thickness

Fig. 5b The image of a micro channel
with 0.25 µm thickness

Figure 6 shows the experimental set -up
accomplished for capture lactic b acteria
with DEP force and figure 7, the
dielectrophoretic chamber.

Fig.6 a Experimental set -up

Fig.6 b Dielectrophoretic chamber

Conclusions

The f irst steps in a DEP experimental set –
up were establish ed for lactic acid bacteria,
particularly lactob acillus and streptococcus
species.
We determined the optimal geometric
configuration for the elec trodes between
which a maximum electric field gradient is
applied .
We made an experimental arrangement
which allows the varia tion of frequency
and voltage applied to study the influence
of this pulse of electric current on bacteria.
A number of investigations by optical
microscopy lactic bacteria migration were
observed and the first results are
encouraging, but working with so many
variable parameters is a dif ficult task .
In the future we intend to establish the
experimental conditions in which lactic
bacteria capture is achieved by positive
dielectrophoresis (DEP); the method
allows concentration of lactic bacteria in a
sample.

Food and Environment Safety – Journal of Faculty of Food Engineering, Ștefan cel Mare University – Suceava
Year IX, No 3 – 2010

77 Acknowledgments

The author wo uld especially like to thank
the entire team of teachers from
“Biophysics and Cell Biotechnology”,
Master in Medicine, “Carol Davila”
University of Medicine and Pharmacy of
Bucharest for initiating and guiding into
cell biotechnology.

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