noncontact parallel manipulation with meso- and...

20. - 22. 10. 2009, Rožnov pod Radhoštěm, Česká Republika

NONCONTACT PARALLEL MANIPULATION WITH MESO- AND MICROSCALE PARTICLES

USING DIELECTROPHORESIS

Jiří ZEMÁNEK, Zdeněk HURÁK

Department of Control Engineering,

Faculty of Electrical Engineering (FEL), Czech Technical University in Prague (ČVUT),

Karlovo náměstí 13/e, 12135 Praha 2, Česká republika

E-mail(s): [email protected], [email protected]

Abstract

The paper gives a short overview of the authors' recent research activities at the intersection of microfludics,

electrokinetics and self-assembly. The particular research introduced in the paper aims at developing an

electric-field-assisted self-assembly procedure for meso- and micro- (and possibly nano-) scale systems.

This research is carried within an EC-funded research project called "Golem: Bio-inspired self-assembly of

meso-scale components", see more at http://www.golem-project.eu/ for the overall motivation.

The particular goal for the group at Department of Control Engineering at FEE CTU in Prague is to steer tiny

particles such as LED chips, thick film resistors or microbeads submerged in a shallow liquid pool around by

controlling electric voltage applied to an electrode array at the bottom of the pool. The key phenomenon that

exhibits itself in this situation is dielectrophoresis, that is, a nonhomogeneous electric field exerts force on an

uncharged but polarizable particle. Dielectrophoresis as the major tool is introduced first, a brief report on

numerical simulations using a commercially available FEM solver is then given, design and fabrication of the

microfluidic chamber featuring microelectrode array is described in some detail, and finally laboratory

experiments are presented. As an outcome of the experiments, a series of videos demonstrating various

responses of dielectric particles to the external field as well as interaction among the particles were captured.

1 MOTIVATION FOR NONCONTACT PARALLEL MANIPULATION

Our research is motivated by Golem – European research initiative focused on self-assembly of small

components. The aim of the Golem project is to understand and investigate the use of bio-inspired bonds to

self-assemble small components. Let us imagine that we want to place components on specific positions on

a substrate. The usual way to do so would be to pick separate particles one by one with some high precision

microrobot and place them to their goal positions. However, the Golem project comes with a new approach.

The idea is to equip all particles with some kind of “smart glue”, thanks to which the particles would be able

to stick on their goal positions on the substrate once they find themselves in vicinity. The assembly strategy

would then consist in letting the particles move over the substrate and waiting until they reach their goal

positions and stick there. The idea is illustrated using the sequence in Fig.1.


Fig.1: Idea of self-assembly. Particles are moving randomly around the substrate and are fixed on the goal

positions (depicted as not filled shapes) by “smart glue”

Our role within this new approach is to

steer particles around over the

substrate, so that every particle would

have a chance to get close enough to

its goal position. The Golem project is

focused on micro- and meso-scale

objects such as LED chips or

microbeads or glass and polystyrene

beads of various diameters. Samples of

these objects can be seen in Fig.2.

2 SHORT INTRODUCTION INTO DIELECTROPHORESIS

If a polarizable particle is exposed to a non-uniform electric field,

a dipole model can be used to describe what is going on inside

the particle. This induced dipole in interaction with non-uniform

electric field gives rise to a net force. This force is called

dielectrophoretic force

( ) ( ) ( )Q Q+ −

= + − → = ⋅ ∇F E q d E q F p E ,

where Q+ and Q− is the charge accumulated on both sides of the

particles, E(q) describes electric field in position given by vector

q and d is displacement of both charges. The situation is

depicted in Fig.3. If d is relatively small compared to the non-

uniformity of the electric field, it is possible to approximate

E(q + d) by the first two terms of its Taylor series, p defines dipole moment Qd. The movement caused by

this force is called dielectrophoresis (DEP). The strength of dielectrophoretic force depends on electric

properties of the fluidic medium and the particle, on the shape and volume of the particle, and frequency and

magnitude (specifically the gradient of square of intensity) of the electric field [1, 2]. Since DEP is dependent

on the properties of particles, it is often used not only for transport of micro particles but also for separation

and characterization [3].

The advantage of DEP is that a particle does not need to be charged, it only has to be polarizable. It is not a

problem to move more particles at the same time, so methods based on DEP have a big throughput, but the

drawback is a small selectivity; possibilities of a parallel manipulation with a single particle are limited. To be

Fig.2: Particles considered for use within the Golem project: (a)

polystyrene microbeads with 50 microns in diameter, (b) thick-film

resistor, (c) chip of light-emitting diode (LED). (d) Knot on a

human hair for juxtaposition.

Fig.3: Principle of dielectrophoresis.

Redrawn from [2].


able to do so complex electrode array is necessary. Another effects related with DEP are traveling-wave

dielectrophoresis (TWD) and electrorotation – both caused by the phase lag between the dipole and the

electric field; the electric field has a spatial variation in its phase. In the case of TWD there is a traveling

electric field and in the case of electrorotation a rotating electric field.

Now let us have a look at specific case, in

which the electric field is harmonic and varies

in three dimensions. This variation can be in

magnitude of wave, phase or both. A so called

conventional dielectrophoresis occurs when

there is no phase gradient. It is the case when

on the electrodes we apply only one harmonic signal or two harmonic signals 180 ° out of phase, which are

equivalent. Conventional dielectrophoresis is caused by average force <F>DEP. When the phase gradient is

present we talk about traveling wave dielectrophoresis and force is given as <F>TWD.

Expression holds for spherical particle with radius r immersed in medium with permittivity εm. We can see

that conventional dielectrophoresis depends on gradient of square of the magnitude of the electric field. On

the other hand, for traveling wave dielectrophoresis a gradient of the phase is needed. Coefficient K

contained in the expressions is known as Clausius-Mossotti factor (CM factor) and it describes frequency

dependent behaviour of the particle. The real part influences conventional DEP and it can be either positive

or negative, which implies direction of the force.

3 SIMULATING DIELECTROPHORESIS USING FEM

To be able to design a suitable layout of electrodes and a suitable control strategy, it is necessary to

simulate dielectrophoresis. The first step is to simulate the electric field itself. As soon as the electric field is

known, it is possible to use the expressions in Section 2 for computation of the dielectrophoretic force. The

knowledge of dielectrophoretic force is the basis for simulation of motion of the particles.

The problem of simulation of electric field is formulated as finding the values of electrical potential of the field

at different places, while geometries of electrodes as well as voltages applied on the electrodes are known.

Because we are considering the applied voltage as harmonic single frequency we can use a phasor for

description of the potential. For both the real and the imaginary part of the potential Laplace’s equation holds

[5] and for solving we used finite element method (FEM).

Total force as a sum of conventional DEP force, TWD force, buoyancy and gravity was computed for various

ratio between width of electrodes and width of gaps, as it can be seen in Fig.4. For narrower electrodes there

is an area where force is almost zero and it seems that particles would have a tendency to stick in particular

places at the surface. For ratio 1:1 the field has more traversable character and there is a strong effect of

traveling wave. The situation is probably the best for the ratio 3:1. But it is hard to distinguish the contribution

of the change of the ratio and what is the consequence of growing gradients in the gap. Further increase in

this ratio makes the area with weak TWD force larger.

( ) ( ) ( ){ }( ){ } ( )

( ){ }( )

3 2 2 2

0 0DEP

3 2 2 2

0 0 0TWD

0

i0

2

, , ,

,

.

, ,i

m

m x y z z

x y z

x y

j j t

i

r K E E

r K E E

E

E

E t E e e i x y zφ ω

ω

ε ω φ φ

πε

π φ

=

= ℑ ∇ + ∇

+

∇

ℜ ∇ +

+

= ℜ =q

F

F

q q


4 ARRAY CONFIGURATION FOR PLANAR MANIPULATION

To induce dielectrophoresis it is necessary to shape the (gradient of

the) electric field and for that we need a set of electrodes. Our goal

is to move a set of particles in the plane and because of easy

construction, we use the electrodes placed in the plane. The layout

of electrodes significantly determines properties of the created

electric field and consequently the motion of the particles which is

possible to induce above the electrodes. We work with the following

proposals of the layout of the electrodes. They are seen in Fig. 5.

1. Interdigitated electrode array – Electrodes have a shape of long

strips. Every electrode is parallel to each other. This is a well known

and often used design. It is advantageous because of easy

fabrication; it does not need multilayered construction so for

example the laser ablation can be used for fabrication. This design

is most often used for travelling wave dielectrophoresis, when

harmonic signal is

connected to every

electrode with different

phase shift and thus the

created electric field has

space variation in phase. Such electrode array is capable to induce

motion of the particles just in one direction, perpendicular to the

electrodes.

2. Matrix electrode array – Electrodes are placed as cells of a

regular matrix. Matrix layout is also known and used for example for

individual cell manipulation. This electrode array is capable to

induce motion in two dimensions. It is suitable for our purposes

because our goal is to move with the particles so that each particle

Fig.4: Influence of the electrode/gap ratio on DEP and TWD force. Colour represents a magnitude of the

force (white places are out of scale) and arrows represent direction of the force.

Fig. 6: Model of the chamber.

Individual layers of the chamber.

From the bottom: glass substrate,

electrodes, isolation layer and basin.

Fig. 5: Interdigitated electrode array

and matrix electrode array.


travels above the whole substrate and this layout offers

sufficient freedom for that. Typically there is a great

number of the electrodes and this implies a lot of wires

needed for interconnection between electrodes and its

drivers. Multilayered planar construction is typically used.

For example, electrode pads are in one layer and

interconnection paths in second. Or they can be just in

one layer so, that interconnection paths are located

between electrode pads.

The design of the chamber (experimental platform)

consists of multiple layers. The base of the chambers is a

piece of glass. On the glass lays a layer of conductor

forming the electrodes, the contact pads and the

interconnection tracks. As a conductor film of gold is used and for better adhesion there is a thin layer of

chromium between the glass and the layer of gold. Electrodes could be covered with isolation layer from

PDMS to avoid connection between electrodes. Finally particles suspended in the medium are in the basin

made from PDMS, such as the isolation layer. The model of the whole chamber and individual layers are

depicted in Fig. 6.

5 FABRICATION OF THE ELECTRODE ARRAYS

Commonly used method is photolithography [6]. Photolithography is suitable for fabrication of multilayered

constructions, for example during fabrication of a chip for self-assembly [7]. An elegant way of fabrication is

integration of the electrodes themselves and their driving circuitries eventually also with a control logic.

Because drivers can be close to electrodes, there is no need to use external wires for connection between

each electrode and its driver [4, 8]. In our project we used laser ablation because of availability of an excimer

laser (in our project partner’s lab).

Fabrication starts with preparation of the substrate. In our case the substrate is a piece of glass. At first the

glass is cut to the certain dimensions. Then the substrate is cleaned. We are using an ultrasonic bath for

coarse cleaning, followed by immersing the piece of glass in hot acetone, rinsed with isopropyl alcohol (IPA)

and dried in a stream of nitrogen. Then substrate is placed to a plasma asher. Organic matter is removed by

oxygen plasma.

Fig. 7: Designed and fabricated interdigitated

electrode array with various width of electrodes

and detail of the middle part of the interdigitated

electrode array, width of electrodes varies from

20 up to 250 microns.


After the cleaning, a thin layer of chromium and gold is deposited on the top of the substrate. It is done in a

twinhead sputter coater. The layer of chromium is deposited first for good adhesion. The substrate fully

covered with chromium and gold is placed to the excimer laser. Laser ablation (also called excimer laser

fabrication} is used to selectively remove parts of the

conductive layer on the substrate. Material is locally

heated and consequently evaporated (ablated) by a

focused laser beam. The shape of the laser beam is

formed by lenses, an aperture and a mask. The mask

contains basic shapes such as circles, squares, triangles

etc. The final pattern is then created from these shapes

gradually. Laser ablation is fast compared to

photolithography, because there is no need to make the

mask and it is possible to fabricate the electrode array

directly. This method is well suitable for prototyping.

6 CONTROL STRATEGY FOR SELF-ASSEMBLY

The final aim of our work was to induce motion of the

particles for the purposes of self-assembly of micro- and

meso-scale components. A suitable motion of the

particles in this case is such that every particle has the

chance to get to its final position. Concerning the desired

particle motion pattern, some inspiration can be found in

Brownian motion, nonetheless, in this engineering work,

we do not stick to its strict mathematical definition. In

fact, we do not even need the property of randomness at

all. It is the control system’s unawareness of the particle

destination that seduces us to consider “randomness”, or

pseudo-randomness.

We proposed use of a finite number of voltage channels.

TWD needs the presence of a gradient of the phase of

the electric field. It is possible to create it using at least

three signals with difference in the phase. Our idea is to control DEP by the phase of the applied potential

instead of by the amplitude of the potential as it may seem more natural. The concept is based on seting a

phase from a finite set (three or four phases) on every cell, and the particles would move according to the

gradient of the phase caused by different phase of potential between the electrodes. The advantage is that in

this way it is possible to induce a long-distance movement for one configuration of phases on the cells.

Because phase is periodic, the gradient can be infinitely long. By contrast, the gradient of electric field is

limited by the maximum amplitude of the applied voltage. This concept allows the usage of small number of

generators together with switching matrix, as schematically depicted in Figure 9. The switching matrix can

simplify the design, because it needs only a small number of generators and switching matrix is available as

IC. We simulated the total force above the matrix array during the random perturbation of the phase.

Fig. 8: Built-up designed chambers with

connection. Electrode array is connected by

adjusted IC test clips.

Fig. 9: Concept of using switching matrix for

controlling phase on the electrodes. Desired

phase on every electrode is set by switching on

an appropriate switch in a row of matrix.


Directions of the total force in various positions in the

plane above the surface for a random setting of phases

are depicted in 10.

7 LABORATORY EXPERIMENTS

We constructed an experimental setup in order to

perform experiments with DEP and TWD. The

experimental setup consists of several parts: 1. Chamber

with electrode array represents the basic part of the

setup, because it is the place where dielectrophoresis

occurs. The medium with suspended particles is inside

the chamber above the electrode array. The electrodes

can create a suitable electric field inside the chamber

and thus induce motion of the particles. 2. Camera is

mounted above the chamber and captures images of the

particles immersed in the medium above the electrodes.

For

enlarging the image the camera needs to be equipped

with a suitable microscope objective (lens). The camera

is connected to a computer and it serves mainly for

observation and recording of the motion inside the

chamber during experiments. It can also be used as a

visual feedback for a control system. 3. Computer

controls the experiment. It receives data from the

camera, which means that it visualizes and records them.

The video sequences are then processed offline to obtain

trajectories of the particles. The computer controls

voltage on the electrodes through connected generator of

signals. 4. Generator generates individual signals for

each electrode. We used four-channel arbitrary

generator, so that one channel supplies several

electrodes. Dielectrophoresis needs voltage with

frequency up to MHz and generators with multiple

channels for high frequency are not commonly available,

therefore a special design is needed. The voltages for

every electrode are set by connected computer.

The influence of traveling of the electric field on 50-

micron polystyrene beads was observed for various

widths of the electrodes. Four-channel harmonic signal

was applied to one section of the intedigitated electrode

array containing electrodes of the same width.

Fig. 10: Force acting on particles above a 4×4

matrix array during random perturbation of

phase. Cones stand for direction of the total

force and colour for the phase.

Fig. 11: Built-up experimental setup. Overview

of the setup. From the left: computer, the main

part with the electrode array and the camera,

generator and oscilloscopes. Detailed view of

the main part.


Above 20-micron electrodes translation motion of the particles was observed for a harmonic potential with

frequency greater than 100 Hz and amplitude 6 Vpp. For the frequency 80 Hz a force repulsing the particles

from the area of the electrodes dominates. The translation motion was the fastest for frequency 200 Hz and

for higher frequency (1 kHz) it becomes insignificant. The particles on the edges of the array were always

repulsed away from the area independent on the direction of the traveling wave, probably because of

negative DEP. The translation motion was also observed above 50-micron electrodes. The motion of the

particles is in Figure 12.

We observed interesting phenomenon, as can be seen in Figure 13. The particles levitating above the

electrodes interact with each other and form chains perpendicular to the electrode (and electric field). This

phenomenon is known as pearl chaining and it can be explained by the fact that the dipole induced within the

particles deforms the electric field and when two particles are close to each other, mutual forces occur. The

dipoles are oriented in the same direction, so that the opposing charges face one another. The chains

repulse each other, since the same charges cause repulsion.

Fig. 13: Sequence of captured image with chaining of 50-micron polystyrene beads in traveling electric field.

8 CONCLUSIONS AND FUTURE WORK

The paper gives just glimpses of the current work of the authors within the attractive applied research

domain of dielectrophoresis. The goal of the paper was not to introduce a particular research result but

rather to give a scope of theoretical knowledge and practical skill owned by the authors with the major goal

of finding new research partners among the participants of the conference.

Fig. 12: Translation motion of 50-micron polystyrene beads in traveling electric field.


REFERENCES

a) Monographies, books

[1] T.B. Jones, Electromechanics of Particles, Cambridge University Press, 1995.

[2] M.P. Hughes, Nanoelectromechanics in Engineering and Biology, CRC Press, 2003.

b) Papers in conference proceedings

[3] D. Chang and S. Lorie, “Separation of bioparticles using the travelling wave dielectrophoresis with

multiple frequencies,” in Proceedings 42nd IEEE Conference on Decision and Control, 2003., pp.

6448-6453 Vol.6.

[4] K. Current et al., “A high-voltage integrated circuit engine for a dielectrophoresis-based programmable

micro-fluidic processor,” Proceedings in International Conference on MEMS, NANO and Smart

Systems, 2005, pp. 153-158.

c) Journal papers

[5] N.G. Green et al., “Numerical solution of the dielectrophoretic and travelling wave forces for

interdigitated electrode arrays using the finite element method,” 2002.

[6] C. Dalton and V. Kaier, “A cost effective, re-configurable electrokinetic microfluidic chip platform,”

Sensors and Actuators B: Chemical , vol. 123, Apr. 2007, pp. 628-635.

[7] A. O'Riordan et al., “Field Configured Assembly: Programmed Manipulation and Self-assembly at the

Mesoscale,” Nano Letters, vol. 4, May. 2004, pp. 761-765.

[8] N. Manaresi et al., “A CMOS chip for individual cell manipulation and detection,” IEEE Journal of

Solid-State Circuit , vol. 38, 2003, pp. 2297-2305.

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