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Mid-term presentation of Nano-electromechanical Systems
Multiphase electrodes for micro-bead control applications:
Integration of DEP and electro-kinetics
for bio-particle positioning
Professor: Yi-Chu Hsu,
Student: 武文仁 (Eric)
Stu. ID: MA01Y204
Date: Nov. 04 2011
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Outline
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Introduction
Theory
Experiment setup
Results and discussion
Conclusion
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Introduction
 Dielectrophoresis (DEP) has been used for years in
biotechnological applications to isolate particles in solution
using non-uniform electric fields.
 Achieving a priori knowledge of the location of or microbeads
in solution is essential for coordinating advanced analysis
protocols like immunoassays.The aim of this paper is to:
-Introduce a novel electrode array architecture that
provides particles’ separation and particles’ movement
functionalities controlled by selective activation of electrodes.
-Demonstrate joint use of electrokinetics and DEP for
bioparticle positioning.
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Theory
Brief introduction about:
 Dielectrophoresis (DEP)
 Electrode and Partical scaling
 AC electrokinetics
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Dielectrophoresis (DEP)
 DEP is a phenomenon first described by Pohl in
1951 and can be used to move particles in nonuniform AC or DC electric fields.
 Depending on the material properties of the particle,
electric dipoles are generated on opposing ends of
the particle in response to an electric field.
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Electrode and Particle scaling
 The force of DEP is a function of the voltage between the
electrodes, and electrode dimensions play a significant
role in the resultant field effects.
 The DEP force is proportional to the volume of the
particle (1). For small particles like microbeads which are
on the order of microns, relatively small electric fields (1–
5 V/mm) are sufficient for organizational effects.
However, for proteins or DNA which are on the order of
nanometers, the electric fields required increase
considerably (~1 kV/mm) to overcome the effects of
Brownian motion and thermal agitation (Zheng et al.,
2003).
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AC electro-kinetics
 The electric fields used for DEP also generate AC
electro-kinetic currents that can transport particles in
suspension.
 The nature of AC electro-kinetics strongly depends
on the frequency and the electrical properties of the
fluid and small changes can have significant effects
on flow pattern and magnitude.
 Low frequencies (below 1 MHz) generate electroosmotic fluidic currents which are caused by the
interaction of the tangential electric field with the
diffuse double layer above the electrode surface.
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Experiment setup
 The electrodes were fabricated using standard
microphotolithography techniques.
 Electrode activation was achieved using a function
generator and the signal was verified by a Yokogawa
DL9140L oscilloscope.
 “Constellation” polystyrene microbeads 15 µm in
diameter were suspended in a 2 mM sodium azide
solution acquired from Molecular Probes were used for
the analysis of electric field effects.
 Imaging was carried out using a Nikon EclipseE600FN
fluorescent microscope and captured with a Nikon DS-U1
camera and control unit.
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Experiment setup
 The solution with suspended beads was applied to the
electrode surface while the function generator created a
sinusoidal electric field between two of the four
electrodes A–D (Fig. 1).
 The non-uniform electric fields generated between the
electrodes enabled microbead positioning. Each
activation scheme consists of connecting two electrodes
(AB, BC or AD) to the function generator while keeping
the other electrodes electrically isolated giving them a
floating voltage state.
 Experiments were performed with an applied voltage
ranging from 1 to 10 V and frequencies ranging from 10
kHz to 5 MHz.
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Results and discussion
Table 1
Experiment conditions
Frequency
Voltage
Observations
= 10 KHz
5
Bubble formation on the edges of the
electrodes indicating electrolysis
0.1–0.5 MHz
5
Beads collected into a less defined
grouping
0.5–5 MHz
5
Beads migrated to characteristic
location with a high degree of
definition
1 MHz
1-3
Tightly packed clusters of beads
which collected at a slower rate
(<3 min)
1 MHz
3-7
Tightly packed clusters of beads
which collected at a moderate rate
(<1 min)
1 MHz
7-10
Marangoni effect very prominent
beads clustered into tight bands very
quickly (<30 s)
Fig. 1. Experimental results showing the formation of beads in response to
activation of electrodes A and D with a schematic of the four-electrode
setup.
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Frequency and voltage dependencies
Table 1 shows that:
 The 1–5 MHz frequency range is optimal for DEP
effects (fast response and tightly packed beads).
 Higher voltage leads to greater dissipation of energy
creating powerful AC thermal gradients.
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Multi-electro DEP
 Although DEP is effective at reproducibly delivering
particles to specific locations, the range of the DEP force
is limited to the geometric area between the electrodes.
 Using two electrodes, the only way to increase the range
of DEP action is to increase the inter-electrode gap.
 Multi-electrodes have been used to circumvent the
distance limitations of two-electrode DEP by extending
the range over which the DEP forces are exerted, by
introducing a traveling wave DEP (TWD) effect.
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Multi-electro DEP
Fig. 2. Logarithmic plot of x component of DEP force when electrodes B and C are activated while the electrodes A and
D are electrically floating. Arrows indicate the direction of force and illustrate the stable field traps created above and
around electrodes A and D (D not shown).
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First activation
 In the first activation scheme, the interlocked
electrodes A and D are activated and electrodes B
and C are kept electrically floating.
 The low field regions over B and C create an
attractive force on the beads that brings them within
range of the even stronger field trap between B and
C where the beads were ultimately confined (Fig.1)
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Second activation
 The second activation scheme has only one electrically
oating electrode in between the activated electrodes
(either A and D, or B and C). In this case, a stable field
trap exists over electrically floating electrode (Fig. 2).
 Any bead located electrode B and C will experience a
negative DEP force inducing movement towards the
low field region at the center of electrode A(Fig. 3).
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The first activation
Fig. 3. Experimental results showing beads collecting near the edges of A and
D when B and C are activated.
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Marangoni effect and AC electroosmosis
 AC electrokinetics can provide a second mechanism for
particle concentration over a wide range. The same
inhomogeneous fields that give rise to the DEP effect
also cause fluidic currents in the solution. These effects
occur at the electrode surface-solution boundary, and as
a result create differences in surface tension which cause
the Marangoni effect.
 In a symmetrical setup, the Marangoni convection
currents produce a deposition of particles in solution at
the central point. The hydrodynamic currents constitute a
mechanism for particle positioning at the center of a
microchamber (Fig. 4)
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Marangoni effect and AC electroosmosis
Fig. 4. Four-electrode geometry with phases A and D activated with 10 V and 1 MHz. The discrete bands
between phases B and C are a result of DEP, while the movement of beads to the center is a result of the
Marangoni effect: (a) after 30 s, (b) after 60 s, (c) after 2 min, (d) after 3 min.
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Control application
 The ability to manipulate beads into specific
conformations through electrode activation enables
controllable interfaces for raster scanning laser
applications or CCD arrays suitable for fluorescence
analysis.
 By activating electrodes A–D and then switching to B
and C, bands of beads are formed which can be
moved from the space between electrodes B and C
to the space between electrodes A and B or C and D
(Fig.5)
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Control application
Fig. 5. (a) A and D activated, B and C floating, (b) B and C activated, A
and D floating, (c) beads moving to stable trap, (d) final stable position.
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Conclusion
 Predictably clustering and positioning microbeads is
central to devices dedicated to automated
biomedical analysis.
 A novel four-electrode design made from single
fabrication layer was used to manipulate microbeads
into one of several predictable on-chip locations by
altering which electrodes were activated.
 Future biomedical devices will rely on predictable
actions like these to perform sophisticated
biomedical analysis in an unsupervised fashion.
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for your attention
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