DISPERSION AND ACCUMULATION OF MAGNETIC PARTICLES IN A MICROFLUIDIC SYSTEM

Abstract

The invention relates to a microfluidic system comprising a magnetic source (150) and two chambers (110) that are connected by a channel. According to a preferred embodiment, the chambers and the channel are filled with different fluids such that a non-zero surface tension is created at the associated fluidic interfaces. Moreover, the magnetic source (150) is arranged to provide at least two separate magnetic gradient regions (GR) and to allow for the attraction of magnetic particles (MP) present in one of the chambers into these different regions, wherein furthermore the magnetic attraction forces (F) generated by at least one of the gradient regions (GR) is strong enough to allow for pushing or pulling magnetic particles through said fluidic interfaces. In a preferred embodiment, the magnetic source may be realized by a permanent magnet (150) of hexahedral shape. The invention further relates to a method for achieving dispersion and a method for achieving accumulation of an ensemble of magnetic particles in said microfluidic system.

Claims

1. A microfluidic system for processing fluids containing magnetic particles (MP), comprising: a) at least two chambers arranged to include first fluids; b) at least one channel communicating with the two chambers and arranged to comprise a second fluid, wherein a non-zero surface tension is created at the two fluidic interfaces between the first fluids and the second fluid; c) a magnetic source, wherein: the magnetic source is arranged to provide at least two separate magnetic gradient regions to attract into these regions magnetic particles present in the fluid of one of the chambers; at least a portion of one of those gradient regions can apply a magnetic attraction force on at least a part of said magnetic particles which is sufficiently high to allow for pushing and/or pulling them through said fluidic interfaces.

2. The microfluidic system according to claim 1, wherein the magnetic source is a permanent magnet (150).

3. The microfluidic system according to claim 2, wherein the permanent magnet has a hexahedral shape, particular a cubic or a parallelepiped shape.

4. The microfluidic system according to claim 1, wherein the magnetic source is an electromagnet.

5. The microfluidic system according to claim 1, wherein the magnetic source is arranged such that the relative position of the gradient regions with respect to the chamber containing the magnetic particles can be changed.

6. The microfluidic system according to claim 1, wherein the magnetic source is movable with respect to the chambers and/or the channel.

7. The microfluidic system according to claim 1, wherein the first fluids are hydrophilic and the second fluid is hydrophobic, or vice versa.

8. A method to achieve dispersion of an ensemble of magnetic particles in a chamber of a microfluidic system according to claim 1, comprising the positioning of the magnetic source adjacent to said chamber such that different parts of the ensemble are subjected to magnetic attraction forces generated by at least two gradient regions, thereby effectuating a splitting of the ensemble.

9. The method according to claim 8, wherein the ensemble of magnetic particles is located on at least one connecting line between two gradient regions of the magnetic source.

10. The method according to claim 8, wherein the distance between said at least two gradient regions corresponds to about one to about five times the diameter of the ensemble of magnetic particles.

11. The method according to claim 8, wherein the magnetic source is moved during the process of dispersion.

12. A method to accumulate an ensemble of magnetic particles in a chamber of the microfluidic system according to claim 1, comprising the positioning of the magnetic source adjacent to said chamber such that all magnetic particles of the ensemble are subjected to magnetic attraction forces generated by only one gradient region.

13. The method according to claim 12, wherein the magnetic source is moved during the process of accumulation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0037] In the drawings:

[0038] FIG. 1 shows a side view of a microfluidic system according to an embodiment of the present invention during the stages of dispersion and transportation of an ensemble of magnetic particles;

[0039] FIG. 2 is a three-dimensional illustration of a magnetic source of the system of FIG. 1;

[0040] FIG. 3 illustrates a top view onto the magnetic source at the beginning of the dispersion of an ensemble of magnetic particles by four gradient regions;

[0041] FIG. 4 illustrates a top view onto the magnetic source during the transportation of an ensemble of magnetic particles through a fluidic interface.

[0042] Like reference numbers refer in the Figures to identical or similar components.

DETAILED DESCRIPTION OF EMBODIMENTS

[0043] A microfluidic device with a magneto-capillary valve (MCV) for liquids has been disclosed in the WO 2010/070461 A1. During sample preparation using such a MCV technology, magnetic particles interact with an external magnetic field and are thereby displaced through several stationary and separate volumes of different buffer solutions. In this process, the particles are washed as the original sample matrix is progressively diluted by the washing buffers.

[0044] The MCV requires transportation of magnetic particles (from buffer to buffer) as well as mixing (in a new buffer), and both functions need a different magnetic configuration. This can be achieved by an MCV instrument including two magnets, a transport magnet and a washing magnet, which must be separated by a distance of several centimeters to avoid cross-talk. The necessity to provide two magnets limits however the possibility to miniaturize the MCV instrument.

[0045] To address the aforementioned needs, it is proposed here to design a single magnet that can do both transportation and washing. In particular, an embodiment of a microfluidic system for processing fluids may comprise: [0046] A microfluidic device comprising at least two chambers arranged to include (first) fluids, and at least one channel communicating with the two chambers and arranged to comprise another (second) fluid, the microfluidic device being further arranged such that a non-zero surface tension is created at the two fluidic interfaces (i.e. meniscus) between said fluids. Due to the surface tension conditions, the aforementioned channel is an MCV between the two chambers. [0047] A magnetic source arranged to provide at least two separate magnetic gradient regions to attract into these regions some magnetic particles present in the fluid of one chamber, wherein at least a portion of one of those regions applies a magnetic attraction force on at least part of said particles sufficiently high to overcome the resistance of the fluidic interfaces between the chambers and the channel in case the particles are magnetically pushed on or pulled out this interface. The effect of the aforementioned pushing or pulling is to drive the particles through the MCV. Preferably the ensemble of particles may be located between two gradient regions such that at least two particles are drawn to different attraction zones. Optionally, the magnetic source may further be arranged such that the relative position of the gradient regions with respect to the chamber can be changed, allowing for the mixing of magnetic particles.

[0048] FIG. 1 schematically illustrates a microfluidic system 100 according to an embodiment of the above principles in four different stages of its usage.

[0049] In FIG. 1a, the microfluidic device of the microfluidic system 100 is shown. A first fluid (e.g. a biological sample) with magnetic particles MP is comprised by a first chamber 110, another (first) fluid (e.g. a buffer) is comprised by a second chamber 120, and a channel 130 connecting the first chamber 110 to the second chamber 120 is filled with a second fluid that is immiscible with the first fluids in the first and second chambers, respectively. The second fluid may for example be air or some other gas. Two fluidic interfaces 131 and 132 are constituted between the first fluids and the second fluid at which there is a non-zero surface tension. Moreover, at least a part of the walls of the chambers 110, 120 and the channel 130 may have differently functionalized surfaces, particularly hydrophilic surfaces in the chambers and a hydrophobic surface in the channel, as an optional refinement of the microfluidic configuration.

[0050] Magnetic particles MP are comprised by the first fluid in the first chamber 110. They tend to form an ensemble (or cloud, cluster) due to mutual magnetic attraction forces.

[0051] In FIG. 1b, the microfluidic system 100 is completed by a magnetic source 150 which subjects the magnetic particles MP to magnetic forces.

[0052] A possible embodiment of the magnetic source 150 is illustrated in FIG. 2. This source 150 is a permanent magnet of cubic shape exhibiting a magnetic north pole N at its top side and a magnetic south pole S at its bottom side. Due to the cubic shape, four gradient regions GR are formed at the four corners of the magnetic source 150 where magnetic gradients are particularly high. One such gradient region GR is schematically indicated in the drawing by dotted lines. The gradient regions GR extend from the surface of the magnetic source 150 somewhat into the adjacent space. The magnetic gradients within these regions substantially lie within the xy-plane of the associated coordinate system. Hence forces exerted on magnetic particles within and by the gradient regions GR will lie in this plane, too, and substantially point towards the corners of the magnetic source 150.

[0053] Returning to FIG. 1b, it can be seen that the ensemble of magnetic particles MP is simultaneously subjected to the influence of several gradient regions GR (two can be seen in the Figure). The ensemble of magnetic particles MP is therefore split into several parts which collect within the respective gradient regions. As indicated by arrows, the magnetic source 150 may additionally be moved with respect to the first chamber 110 to assist the effect of splitting and to achieve a washing effect by moving the magnetic particles through the surrounding fluid.

[0054] Accordingly, the microfluidic system 100 provides for a method to achieve dispersion of an ensemble of accumulated magnetic particles by positioning the magnetic source adjacent to the microfluidic device at some given velocity such that, when projected into the plane of the microfluidic device, the ensemble of particles is located on at least one connecting line between at least two of the magnetic field gradient regions such that the field of magnetic forces exerted on different parts of the ensemble of particles exhibits at least two attraction zones, thereby effectuating a splitting of the particle ensemble.

[0055] In FIGS. 1c and 1d, the transfer of the ensemble of magnetic particles MP through the magneto-capillary valve formed by the channel 130 is illustrated. In this procedure all magnetic particles MP of the sample in the first chamber 110 are attracted to one gradient region of the magnetic source 150. When the source 150 is moved to the right, the ensemble of magnetic particles MP is first pulled through the first interface 131, then moved within the channel 130, and finally pulled through the second interface 132 to be released into the fluid of the second chamber 120.

[0056] Accordingly, the microfluidic system 100 provides for a method to accumulate an ensemble of magnetic particles by positioning the magnetic source adjacent to the microfluidic device at some given velocity such that, when projected into the plane of the microfluidic device, the ensemble of particles is located such that the field of magnetic forces exerted on different parts of the ensemble of particles exhibits only one attraction zone, i.e. in the vicinity of one of the magnetic gradient regions.

[0057] The magnetic source 150 can thus be used for both particle transport and mixing which allows for a reduction of the size and speed requirements of the magnetic actuator in the system 100.

[0058] The magnetic source 150 may be attached to an actuator that allows displacement of the magnet in two dimensions (x and y in the Figures) while keeping a constant distance to the bottom side of the MCV microfluidic device. By using only one magnet to both transport and mix the particles inside the cartridge, the travel range of the actuator does not have to be larger than the maximum extents of the relevant fluidic structures of the cartridge.

[0059] In general, the magnetic source 150 may be an electromagnet and/or a permanent magnet. In a particular embodiment, the magnetic source 150 may be realized as a single permanent magnet with a hexahedral shape. The shape may especially be cubic (as shown in FIG. 2) or parallelepiped. The magnetic field of such a magnet exhibits the strongest gradients at the four corners (tips) of a face that comprises a pole of the magnet (i.e there are four magnetic gradient regions in this particular embodiment). When displacing the magnet, the particle ensemble will be drawn towards one of the tips.

[0060] As is illustrated in FIG. 3 in a top view onto the magnetic source 150, the magnet 150 can be positioned such that the particle ensemble is surrounded by the tips of the magnet. The resulting magnetic forces will draw the particles towards different points and thereby cause splitting of the magnetic particle ensemble. Hence mixing may be achieved when an ensemble of magnetic particles MP is located above the center of the top face of the magnet. If the projected area of the particle ensemble is not substantially larger than the top face of the magnet, the magnetic particle cloud will experience approximately equal forces F originating from the four corners of the magnet 150. Provided that the cohesive forces between the particles, e.g. by entanglement with long macromolecules, are not larger than the applied magnetic forces, the particle ensemble will be dispersed. Importantly, the dispersal of particles does not require rapid movements of the magnet.

[0061] As is illustrated in FIG. 4 in another top view onto the magnetic source 150, magnetic particle transport is achieved by concentrating particles MP above any of the four upper corners of the magnet 150. When transporting the magnetic particle ensemble through a fluid meniscus (e.g. 131 or 132), the resulting force exerted by the fluid meniscus and the magnet will draw the particles MP towards the trailing tip of the magnet. If for example the diagonal of the upper face of the magnet is aligned with the main transport direction of the MCV cartridge (i.e. the long axis of the cartridge, corresponding to the x-axis in the Figure) the magnetic particles MP will be drawn towards the rearmost corner of the magnet (left corner in the Figure) during transport between chambers. This effect can be explained by the balance between the capillary force at the fluid meniscus 131 and the magnetic gradient force.

EXAMPLE

[0062] To prove the effectiveness of a single cubic magnet 150 in achieving equal or better transport and mixing performance, the inventor has determined the extraction yield of radioactively labelled RNA, i.e. the percentage of input RNA that could be transported through the microfluidic channel 130 and made available for downstream processing. In order to establish this evidence, the inventor compared a single cubic magnetic source 150 according to the invention with a magnetic system comprising the cylindrical magnet as disclosed in FIG. 5 of WO 2010/070461: the edge of the cubic magnetic source 150 was of 5 mm and the diameter of the cylindrical magnet was of 4 mm for 10 mm long, both applied on the same magnetic particles (i.e. having the same properties and the same number) to transport them from a chamber 110 to a chamber 120 via the channel 130, chambers 110 and 120 having 220 micrometers height and a volume of about 20 microliters each, and a channel 130 of about 5 mm width. Further to the cylindrical magnet, and in order to find an equivalent extraction yield as the one found with the single cubic magnetic source 150, the inventor had to further add in said magnetic system an array of magnets having polarities successively opposed one to the other arranged to mix the magnetic particles in chamber 110 and/or chamber 120 by moving this magnetic array above the chamber(s).

[0063] Using an actuation protocol of the same length and the same sample matrix, the inventor has therefore shown that the compact square magnet system 150 can perform equally well as said dual-magnet assembly. In particular, the function of transportation of the cylindrical magnet and the function of mixing of the magnetic array are both exerted by the single cubic magnetic source 150, and with the same efficiency, although the magnetic source 150 of the invention is a single magnetic element, and so clearly more simple and less cumbersome than the dual-magnet assembly.

[0064] Furthermore, the integration of said dual-magnet assembly would lead to separate said cylindrical magnet from said magnetic array by a gap sufficiently large to prevent the cross-talk between the two types of magnets. Typically this would lead to separate the cylindrical magnet from the magnetic array by about 30 mm, which increases considerably the size of this magnetic assembly.

[0065] As already indicated, the usage of a single (e.g. cubic, permanent) magnet to operate an MCV leads to a further miniaturization of the surrounding instrument or sub-assembly of an instrument, which is essential for integration with detection technologies and for operation in compact instruments. Furthermore, the velocity requirements for the magnet actuator can be reduced which enables the use of low-cost actuators, e.g. such as the ones found in standard CD drives.

[0066] In summary, an approach has been disclosed in which the shape of a magnet is used as an actuator in particle-based sample preparation for in-vitro diagnostics. By choosing a magnet with multiple tips and a size comparable to the particle ensemble to be actuated, one magnet can be used for both particle transport and mixing which reduces the size and speed requirements of the magnetic actuator. A microfluidic system according to an embodiment of the invention includes a magnetic source with [0067] (i) at least one tip (i.e. a region of increased magnetic field gradient that attracts magnetic particles in three dimensions) with bulk dimensions and tip sharpness sufficient to effectuate transport of an ensemble of magnetic particles, and [0068] (ii) more than one tip preferably spaced between one and five times the diameter of the particle ensemble such as to generate magnetic forces that draw particles inside the particle clouds towards different tips.

[0069] Embodiments of the invention can for example be used as part of the magnet actuator assembly of a MCV sample preparation system.

[0070] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.