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): zemanj2@fel.cvut.cz, hurak@fel.cvut.cz 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 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. 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 ( ) ( ) F = Q + 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 Fig.3: Principle of dielectrophoresis. q and d is displacement of both charges. The situation is Redrawn from [2]. depicted in Fig.3. If d is relatively small compared to the nonuniformity 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
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. ( q ) ( q) DEP TWD j ( ) { φ i q jωt i0 } E, t = R E e e, i = x, y, z, i F F m { ( ω) } ( + E ), x0 y0 z 0 { ( )}( + E φ 0 x 0 y 0 ) = πε r R K E E + 3 2 2 2 = 2πε r I K ω E φ + E φ 3 2 2 2 m x y z z 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 ε m. 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.
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. 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 Fig. 5: Interdigitated electrode array electrode with different and matrix electrode array. 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. 6: Model of the chamber. Individual layers of the chamber. From the bottom: glass substrate, electrodes, isolation layer and basin.
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 Fig. 7: Designed and fabricated interdigitated between electrode pads. electrode array with various width of electrodes The design of the chamber (experimental platform) and detail of the middle part of the interdigitated consists of multiple layers. The base of the chambers is a electrode array, width of electrodes varies from piece of glass. On the glass lays a layer of conductor 20 up to 250 microns. 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.
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. Fig. 8: Built-up designed chambers with connection. Electrode array is connected by 6 CONTROL STRATEGY FOR SELF-ASSEMBLY adjusted IC test clips. 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 Fig. 9: Concept of using switching matrix for destination that seduces us to consider randomness, or controlling phase on the electrodes. Desired pseudo-randomness. phase on every electrode is set by switching on We proposed use of a finite number of voltage channels. an appropriate switch in a row of matrix. 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.
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 Fig. 10: Force acting on particles above a 4 4 and thus induce motion of the particles. 2. Camera is matrix array during random perturbation of mounted above the chamber and captures images of the phase. Cones stand for direction of the total particles immersed in the medium above the electrodes. force and colour for the phase. 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. Fig. 11: Built-up experimental setup. Overview The influence of traveling of the electric field on 50- of the setup. From the left: computer, the main micron polystyrene beads was observed for various part with the electrode array and the camera, widths of the electrodes. Four-channel harmonic signal generator and oscilloscopes. Detailed view of was applied to one section of the intedigitated electrode the main part. array containing electrodes of the same width.
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. Fig. 12: Translation motion of 50-micron polystyrene beads in traveling electric field. 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.
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