WO2008033009A1

WO2008033009A1 - Effective use of dielectrophoresis in serpentine micro-channels

Info

Publication number: WO2008033009A1
Application number: PCT/NL2007/000217
Authority: WO
Inventors: Adrianus Bossche; Florin Tatar; Lujun Zhang
Original assignee: Stichting Voor De Technische Wetenschappen
Priority date: 2006-09-14
Filing date: 2007-09-06
Publication date: 2008-03-20

Abstract

The invention relates to a micro channel system comprising a micro channel, the micro channel comprising at least first and second ends and at least a first and a second curved portion and a straight intermediate portion between the first and the second curved portion, wherein in the straight intermediate portion positioning means are provided arranged to position, during operation, particles having different size or different DEP-sign at substantially the same position in front of the second curved portion. By positioning the particles in front of the next curve, a maximum result of the dielectrophoresis forces in the curves can be used for separation in a serpentine micro channel.

Description

Effective use of dielectrophoresis in serpentine micro-channels

TECHNICAL FIELD

The present invention relates to the use of dielectrophoresis (DEP) for compound separation in curved micro-channels.

STATE OF THE ART

Capillary electrophoresis (CE) is a powerful technique used to separate a variety of compounds. CE analysis is performed by applying appropriate electrical fields to exits of narrow tubes and can result in the rapid separation of many hundreds of different compounds. The separation principle is based on the phenomena that ionized molecules of different mass and charge will travel with different velocities through a capillary under the influence of an electric field. So when a small sample plug is injected at one end of a capillary, the molecules are separated in different zones that can be detected with optical or electrical detectors at the other end. Several attempts have been made to integrate the capillaries in a chip.

Although CE is widely used in laboratory situations for off-line measurement, it has still not found its way to in-line applications in industrial processes, not even after the recent developments of CE microchips. The drawbacks to be overcome for CE micro system integration are: the high separation voltages required (1500-3000V), fluidic interfacing between process and CE microchip, sample pre-treatment (filtering, enrichment, fluorescence labelling etc.). Furthermore, the channel length required (several em's) is too large for efficient micro-chip integration when using straight separation channels, since chip cost grows with chip area. To make more efficient use of the chip area, serpentine channels have been introduced. However, this is at the cost of a good separation quality. The electrical field, and so the electro-osmotically generated flow in the outer curves is smaller than in the inner curves which results in an enhanced sample plug dispersion and thus in a less accurate separation of the compounds. In publication US 5842787 Al the plug dispersion in curves is controlled by modifying the geometry of the channel in the curves, so as to reduce the difference in electrical field between an inner and an outer wall of the curve(s). Publication "Continuous electrodeless dielectophoretic separation in a circular channel", L. Zhang et al, Journal of Physics/conference series 34 (2006), no. 34, April 2006 (2006-040, pages 527-532, International MEMS conference 9-12 May 2006 Singapore, presents a continuous separation structure based on dielectrophoresis (DEP). A non-uniform electric field is generated by applying a voltage over a circular channel. Driven by the electro-osmotic flow, the particles with different dielectric properties move continuously to a different location across the channel as they flow due to the different DEP forces. At the end of the channel, different outlets are present through which particles having different properties are separated. A disadvantage of the circular channel is that, in a single channel layer, only a single circle can be effectively used for separation. Additional circles can be added to form a spiral, however, because of the increasing diameters the effective contribution to the dielectrophoretic separation quickly diminishes.

SHORT DESCRIPTION

It is an object of the invention to provide a system and method for separating components of a sample using dielectrophoretic forces wherein the separation quality is improved.

Therefore, according to an aspect of the claimed invention, there is provided a micro channel system comprising a micro channel, wherein the micro channel comprises at least first and second ends and at least two electrodes, one on either end of said micro channel, and at least a first and a second curved portion and a straight intermediate portion between the first and said second curved portion, wherein during operation, a sample mixture of particles can be driven along the micro channel by electro-osmotic flow by means of applying a voltage across said two electrodes, wherein due to a difference in DEP force, said particles with different DEP responses move to different locations across the first and second curved portions as they flow, wherein in the straight intermediate portion positioning means are provided arranged to position, during operation, particles having different size or different DEP-sign at substantially the same position in front of the second curved portion.

In an embodiment, the positioning means comprise electrodes arranged on a inner wall of said intermediate portion. The electrodes may for example be triangular shaped. This will produce a suitable electric field for positioning the particles. According to another embodiment, the positioning means comprise topographic arranged inside the intermediate portion. These topographic static structures are nonelectrical and can be produced together with the micro-channel fabrication. In an embodiment, the topographic structures are protruding from one or more walls of the channel.

The topographic structures may comprise an array of posts arranged between a top wall and a bottom wall of said intermediate portion. In another embodiment, the topographic structures comprise a plurality of parallel hurdles arranged on a bottom wall of said intermediate portion. Preferably, a main direction of each of said plurality of parallel hurdles forms an angle with a side wall of said intermediate portion, said angle being unequal to 90 DEG.

In an aspect of the invention there is provided a method of separating a substance using dielectrophoresis, according to claim 9.

SHORT DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

- Figure 1 shows the principle of dielectrophoresis for positive DEP particles; - Figure 2 shows the principle of dielectrophoresis for negative DEP particles;

- Figure 3 illustrates an example of dielectrophoretic separation of particles in a curved channel;

- Figure 4 shows a serpentine channel according to an embodiment in which negative DEP particles having different sizes are separated; - Figure 5 shows an intermediate portion comprising squared posts;

- Figure 6 shows a cross sectional view of the intermediate portion of Figure 5;

- Figure 7 shows an embodiment with rectangular posts;

- Figure 8 shows a cross sectional view of the channel of Figure 7;

- Figure 9 shows a three dimensional view of an embodiment of the intermediate portion having parallel hurdles;

- Figure 10 shows a top view of the embodiment of FigurelO;

- Figure 11 shows a Matlab simulation of particle trajectories in a serpentine channel. DETAILED DESCRIPTION

This invention uses non-uniform electric fields in channel curves for dielectrophoretic (DEP) separation of particles. Dielectrophoresis is the movement of particles induced by polarization effects in non-uniform electric fields. The dielectrophoretic force acting on a spherical particle can be described by:

F_DEP = 2πεy Re[K(ώ)]VE² (1)

where V£² is the gradient of electric field squared, ^ε- is the permittivity of the suspending medium, r is the radius of the particle, and

(2) ε_p + 2ε_n

K{ω) is the frequency dependent Claussius-Mosotti (CM) factor, ε" and ε_m' represent the frequency dependent complex permittivities of the particle and medium, respectively, The complex permittivity is defined as ε_p = ε_p ~j{σ_plai) and ε_m' = ε_m ~ j(σ_m I ω) , where j - V^T , ε is the permittivity, and σ is the conductivity o f the dielectric. Positive DEP occurs whenicT(cϋ) > 0 , the force is toward the high electric field. The converse of this is negative DEP, which occurs wheniT(ω) < 0 , the force is in the direction of decreasing field intensity.

Figures 1 and 2 show the principle of DEP force. The direction of the particle movement depends on the specific DEP response. Figure 1 shows that positive DEP particles move in the direction of increasing E-field and Figure 2 shows that negative

DEP particles move in the direction of decreasing E-field.

Figure 3 illustrates an example of dielectrophoretic separation of particles in a curved channel 10. By applying a DC voltage on two electrodes 11, 12 over the curved channel 10, a non-uniform electric field will be generated whose gradient directs towards a center 13 of the curved channel 10. The curved channel comprises an inlet 14 through which a sample mixture is applied. The sample mixture is driven along the curved channel 10 by the electro-osmotic flow. Due to the different DEP force magnitudes and directions, particles with different DEP responses move continuously to the different location across the curved channel 10 as they flow. For example, particles 16 with a positive DEP feature will follow a trajectory like trajectory 17 and particles 18 with a negative DEP feature will follow a trajectory like trajectory 19. Since the flow at the inner side is faster than that at the outer side of the channel, the particle running at the outer side path will be left behind.

From the above equation (1) it can be seen that the DEP force is proportional to r³. Thus, for the particles having the same DEP polarity, the larger particles will receive more DEP force.

Figure 4 shows an embodiment of the invention in which the above described separation mechanism is used to separate negative DEP particles having different sizes. A serpentine shaped separation channel 20 comprises at least two curved portions 21, 22 (also referred to as curves 21, 22) and a straight intermediate portion 23. Fluid, such as water, enters the channel 20 by way of an inlet 24 and leaves the channel 20 through an outlet 25. Across the channel 20, a voltage is applied by means of two electrodes 26, 27 so that a sample mixture is driven along the channel by the electro-osmotic flow. The sample mixture is applied in the channel 20 through a sample inlet 28. In this example, the sample mixture comprises negative DEP particles having different sizes. At the curved portions 21, 22 of the channel 20, there exits a non-uniform electric field whose gradient directs towards the centre of the curve. Particles (macro molecules) having a different electric permeability then the carrier fluid will experience a Dielectrophoretic force. For negative DEP particles (most biological macromolecules in water based fluids), the force acts along the radius of the curve to the outside. Since the force increases with the volume of the particles, a relatively larger particle 40 will be driven more to the outside of the curve than a smaller particles 41. Since the electro- osmotic flow at the inner side of the curve 21 is faster than that at the outer side of the curve 22, the large particles 40 following the outer side will leave behind because they have a lower flow speed and a longer path length through the curve. However, since the next curve 22 bends in the opposite direction, the effect would be partly cancelled in subsequent curves. To avoid this, according to an embodiment, the straight intermediate portion 23 comprises positioning means used to compel all the particles to the same point of assembly 30 before the next curve 22, see Figure 4. For negative DEP particles, the point of assembly is a point close to an inner wall of the next curve 22.

The positioning means may comprise non-electrical topographic structures causing non-uniform electrical fields that drive all particles to one side of the channel 20 so that all particles will enter the next curve 22 on the inside. Such topographic structures for particle concentration have been described in literature [1, 2]. However, the topographic structure were not used in conjunction with serpentine micro channels used for dielectrophoretic separation.

According to an embodiment, a portion 44 of the micro channel 20 upstream the first curved portion 21 also comprises positioning means for bringing specific particles to an assembly point, e.g. the inner side of the micro channel 20, see Figure 4.

Examples of non-electrical topographic structures which could be used in the straight intermediate portions 23, 44 of the channel 20 are illustrated with reference to Figures 5-10. In Figure 5, the straight intermediate portion 23 comprises squared posts 50 standing from bottom to top in the channel 20. The DEP force between the posts 50 build up a barrier by which the particles are forced to go through narrow channels, see dotted lines, and end up at the other side of the channel 20. This embodiment is only for positive DEP particles, since a barrier between the rows of posts 70 are formed by a low electric field region. The positive DEP particles will be blocked from passing through the low field regions and can only follow the small channels between the columns of the posts. Figure 6 shows a cross sectional view of the straight intermediate portion 23 across the line VI-VI in Figure 5.

Figure 7 shows an embodiment wherein rectangular posts 70 are utilized to repel the particles to the other side of the channel 20. Figure 8 shows a cross sectional view of the channel of Figure 7.

Figure 9 shows a three dimensional view of yet another embodiment of a straight intermediate portion 23 of the micro channel 20 wherein the topographic structures comprise parallel hurdles 90 standing on a bottom wall but that do not touch a top wall of the channel 20. This embodiment can be used for both positive DEP and negative DEP particles. Figure 10 shows a top view of the embodiment of FigurelO. From Figure 10, one can see that the particles all flow to one side of the micro channel 20. In another embodiment, the straight intermediate portion 23 comprises electrodes on both top and bottom side of the micro channel 20 to bring particles from one side to the other side of the straight portion of the channel. Figure 11 shows a simulation of particle trajectories in a serpentine channel using Matlab. Due to an electric field produced between top and bottom electrodes, particles will follow the trajectories shown in Figure 11. As can be seen from Figure 11, the particles all start at the same position before going in the third curved part 110 of the channel 20.

According to an embodiment, a plurality of S-shaped channels are cascaded together to yield higher separation resolution. After a number of curves, the smaller negative DEP particles leave behind the larger negative DEP particles thereby separated. Furthermore, topographic dielectrophoretic structures can also be used for sample filtering and concentration in the sample injection channel. When the topographic structures are placed at the sample inlet port, only the target particles could enter the separation channel while the larger particles are blocked outside due to the larger DEP forces. When the topographic structures are placed at the sample outlet port, the target particles will stay in the separation channel while the smaller particles are filtered out.

It should be noted that on top of the dielectrophoretic separation (caused by field gradients in the curves) as described above, also electrophoretic separation due to the electric field itself will take place in the micro channel 20. In the embodiment described above, the separating of neutral particles are considered for which the electrophoresis does not take effect. However, it should be noted that for charged particles, the separation in the spatial domain will be also visible at least. The relative influence of DEP separation over CE separation can be increased by applying a voltage with a large AC component (for DEP separation) and a small DC component for electro osmotic flow.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Literature:

[1] Louise M. Barrett, et. al. , "Insulator-based ridges for the manipulation of particles and cells in micro channels", uTAS2005, pp. 1404-1406. [2] Eric B. Cummings, "Streaming dielectrophoresis for continuous-flow microfluidic devices", IEEE Engineering in medicine and biology magazine, 2003, pp. 75-84.

Claims

1. A micro channel system comprising a micro channel, said micro channel comprising at least first and second ends and at least two electrodes, one on either end of said micro channel, and at least a first and a second curved portion and a straight intermediate portion between said first and said second curved portion, wherein during operation, a sample mixture of particles can be driven along the micro channel by electro-osmotic flow by means of applying a voltage across said two electrodes, wherein due to a difference in DEP force, said particles with different DEP responses move to different locations across the first and second curved portions as they flow, wherein in said intermediate straight portion positioning means are provided arranged to position, during operation, particles having different size or different DEP-sign at substantially the same position in front of said second curved portion.

2. The micro channel system of claim 1, wherein said positioning means comprise electrodes arranged on a inner wall of said intermediate portion.

3. The micro channel system of claim 2, wherein said electrodes are triangular shaped.

4. The micro channel system of claim 1, wherein said positioning means comprise topographic structures arranged inside said straight intermediate portion.

5. The micro channel system of claim 4, wherein said topographic structures comprise an array of posts arranged between a top wall and a bottom wall of said straight intermediate portion.

6. The micro channel system of claim 4, wherein said topographic structured comprise a plurality of parallel hurdles arranged on a bottom wall of said straight intermediate portion.

7. The micro channel system of claim 6, wherein a main direction of each of said plurality of parallel hurdles forms an angle with a side wall of said straight intermediate portion, said angle being unequal to 90 DEG.

8. The micro channel according to any of the previous claims, said micro channel being formed as a serpentine channel comprising a plurality of S-shaped channels cascaded together.

9. Method of separating a substance using dielectrophoresis, said method comprising:

- providing a micro channel system comprising a micro channel, said micro channel comprising at least first and second ends and at least two electrodes, one on either end of said micro channel, and at least a first and a second curved portion and a straight intermediate portion between said first and said second curved portion, wherein in said straight intermediate portion positioning means are provided arranged to position, during operation, particles having different size or different DEP-sign at substantially the same position in front of said second curved portion;

- introducing a sample of said substance into the micro channel;

- drive said sample along the micro channel by electro-osmotic flow by means of applying a voltage across said two electrodes.