Abstract:
A device for transporting magnetic or magnetizable microbeads ( 25 ) in a capillary chamber ( 14 ) comprises a permanent magnet ( 10 ) or an electromagnet ( 11 ) for subjecting the capillary chamber to a substantially uniform magnetic field, to apply a permanent magnetic moment to the microbeads ( 25 ). At least one planar coil ( 22 ) and preferably an array of overlapping coils are located adjacent to the capillary chamber ( 14 ) for applying a complementary magnetic field on the microbeads parallel or antiparallel to said substantially uniform magnetic field, to drive the microbeads. An arrangement is provided for switching the current applied to the coil(s) ( 22 ) to invert the field produced thereby, to selectively apply an attractive or repulsive driving force on the microbeads ( 25 ). The device is usable to transport microbeads for performing chemical and biochemical reactions or assay, as is done for instance in clinical chemistry assays for medical diagnostic purposes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation application of PCT/IB03/000956 filed Mar. 8, 2003, which is included in its entirety by reference made hereto. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to devices for manipulation and transport of magnetic beads. The invention concerns in particular an apparatus of the above-mentioned kinds wherein the magnetic beads are used for performing chemical and biochemical reactions or assays, as is done for instance in clinical chemistry for medical diagnostic purposes. 
     BACKGROUND OF THE INVENTION 
     It is known that magnetic particles (‘beads’) embedded in a liquid can be used to carry a probe molecule on their surface that specifically interacts with a complementary target molecule (for example single stranded probe DNA interacting with complementary target DNA). Upon reaction with a molecule to be probed and, for example, using optical or electrochemical measurements, one can determine the amount of target molecules on a bead or within a certain volume containing beads (see for example Hsueh et al., Techn. Digest Transducers &#39;97, p. 175 (1997)). The interest in using magnetic microbeads, is that they can be manipulated using magnetic fields irrespective of fluid motion. In this way one can create an important relative motion of the beads with respect to the fluid and, hence, a large probability of binding a target molecule to a probe molecule fixed on the bead surface. One can then magnetically extract the beads to a place of detection/collection. Historically, beads have been locally fixed by using external magnets or have been transported using mechanically moving external magnets. The latter procedure may be for example used to fabricate mixing devices (Sugarman et al., U.S. Pat. No. 5,222,808) and in immuno-assay methods (Kamada et al., U.S. Pat. No. 4,916,081). 
     “Separation” of magnetic microbeads means that a liquid flow, containing the beads, passes a zone with a large magnetic field (gradient) and that the magnetic microbeads are filtered out (separated) by the field. U.S. Pat. No. 5,779,892 describes the use of a permanent magnet to separate (filter) the magnetic microbeads from a passing liquid solution. U.S. Pat. No. 6,013,188 describes a ferromagnetic capture structure, made of a Ni grid and placed in the field of a permanent magnet to select magnetic microbeads from a liquid solution that passes through the grid. Other patents on separation of magnetic beads are U.S. Pat. No. 6,132,607 and the US patents mentioned therein. Finally, U.S. Pat. No. 6,193,892 describes how a rack that is to hold containers with magnetically responsive solutions is configured with permanent magnets to extract the magnetic microbeads from the solution. U.S. Pat. No. 5,541,072 concerns the creation of magnetic clusters (ferrophases) that are transported by a permanent magnet. Ahn et al. [C. H. Ahn, M. G. Allen, W. Trimmer, Y. J. Yun, and S. Erramilli,  J. Microelectromechanical Syst.  5, 151-158, 1996] have reported magnetic bead separation device using integrated inductive components; in follow-up work, electroplated spiral coils in Cu were combined with an electroplated permalloy yoke structure to separate microbeads from a liquid solution passing over an array of coils [J.-W. Choi, T. M. Liakopoulos, and C. H. Ahn,  Biosens . &amp;  Bioelectronics  16, 409-416, 2001]. The coils were arranged spaced apart from one another side-by-side. As the magnetic field is localised over an area of the order of the coil width, the described simple juxtaposition of the coils will not enable microbead transport, but merely allow separation of the microbeads. With this proposal, the microbeads were retained and separated by action of a magnetic field generated by the coil, but it was not possible to transport the beads by the action of the magnetic field. Transporting the beads required using a liquid flow. 
     Magnetic transport of beads is essential for bringing the beads to a well-defined position within a microfluidic circuit, for example near to a magnetic bead detection device. “Transport” means that the microbeads are effectively moved by a magnetic force, i.e. using a magnetic field and not just retained by a magnetic field from a liquid solution passing by (=separation). Nevertheless, manipulation of these beads in general and transport in particular, is a difficult task, as the effective relative magnetic susceptibility χ eff  of the (super)paramagnetic beads is rather weak (typically χ eff &lt;&lt;1, due to demagnetization effects of the mostly spherical particles) and the magnetic volume of the particles is small. This explains why mostly the large field of (mechanically moving) permanent magnets or large electromagnets have been used for the separation, transport, and positioning of magnetic microbeads [See webpage of Miltenyi Biotec Inc., Auburn, Calif.: http://www.miltenyibiotec.com.; S. Østergaard, G. Blankenstein, H. Dirac, and O. Leistiko,  J. Magn. Magn. Mat.  194, 156-162, 1999 and WO 99/49319]. In other work, micropatterned conductors, actuated by large currents, have been demonstrated to present a useful solution for magnetic microbeads capture and transport. These devices allow precise positioning and transport over 10-100 μm distances in a single actuation event [T. Deng, G. M. Whitesides, M. Radhakrishnan, G. Zabow, and M. Prentiss,  Appl. Phys. Lett.  78, 1775-1777, 2001; C. S. Lee, H. Lee, and R. M. Westervelt,  Appl. Phys. Lett.  79, 3308-3310, 2001]. 
     In the work of Deng et al., the field of a permanent magnet placed at some distance beneath the device has been combined with the field generated by the current through an electrical conductor. Here, the electrical conductor was made of two side-by-side serpentine wires shifted linearly in phase by π/3, that generated a magnetic field having local field maxima in every turn and with opposite directions in neighbouring turns. However, the generated magnetic field gradient (several 0.1 T/mm) is localized over a small distance (˜100 μm) which leads to the consequence that many actuation steps are necessary to transport beads through a large surface area rapidly. This disadvantage is particularly serious for application in biotechnology, where it is desirable to rapidly transport beads over distances of several millimetres which requires several hundreds of actuation steps with this serpentine wire arrangement. Also the magnetic field generated by a single wire is weak, so that large currents (of the order of 10 6  A/mm 2 ) are required to transport the microbeads over these small distances. 
     WO 02/31505 describes the use of an electromagnetic chip to transport and detect the presence of magnetic beads. 
     In previous work on magnetic bead transport, the moving magnetic field is obtained by mechanically moving a permanent magnet (magnetic induction of the order of 0.1-1.5 Tesla), which is a very large value that can induce an important magnetic moment in the microbead (the magnetic moment is given by μ=Vχ eff B 0 , with B 0  the magnetic field generated by the permanent magnet, χ eff  the magnetic permeability and V the magnetic microbead volume). One should realize that a very small microbead has no effective magnetization when there is no external field, ie it is superparamagnetic. The magnetic force on such moment in a total magnetic induction field B is given by: 
                   F   =       μ     μ   0       ⁢     ∇   B               (   1   )               
making it clear that a strong magnetic force is obtained when having a large moment AND a large gradient of the magnetic induction. To have appreciable magnetic forces, relatively important magnetic fields (about 10 −2  T) and large magnetic field gradients (from 10 to 100 T/m) must be generated locally [G. P. Hatch, and R. E. Stelter, “Magnetic design considerations for devices and particles used for biological high-gradient magnetic separation (HGMS)”,  J. Magnetism Magn. Materials  225, pp. 262-276, 2001]. A permanent magnet hence delivers a large force, but the problem is that it is cumbersome in generating a ‘moving’ field.
 
     On the other hand, the magnetic field generated by a coil fed with a current can be varied in time easily but is very small. Typically fields of just a few milliTesla are generated by a simple coil using currents of the order of 0.1-1 Amp. When looking at equation (1), it is clear that the magnetic moment of the microbead will be typically a factor 1000 smaller and that also the magnetic gradient will be a factor 10 smaller. The consequence is that magnetic forces of coils can be easily varied in space and time but that the forces are too small to effectively transport the microbeads. An improvement would be to fabricate a magnetic yoke structure made of a soft magnetic material around the coil, which amplifies somewhat the magnetic field that is generated by the coil (typically a factor 10). 
     However, the prior art does not disclose any effective way of using simple coils to displace magnetic beads. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a device for manipulating and/or transporting microbeads employing simple planar coils which generate low magnetic fields to displace microbeads through longer distances at higher speeds than were heretofore possible. 
     A further object of the invention is provide such a device employing simple technology, notably Printed Circuit Board (PCB) technology for the manufacture of the coils, together with simple permanent magnets or electromagnets. 
     The invention proposes a novel approach for magnetic microbead transport in a capillary chamber over long-range distances using at least one coil, and, preferably, using an array of simple planar coils. The coil(s) is/are placed in a uniform static magnetic field, the role of which is to impose a permanent magnetic moment to the microbeads. The very small magnetic field gradient of a simple planar coil is then sufficient to displace the microbeads. 
     The invention thus provides a simple planar coil array-based magnetic microbead transport system, in which an individual coil is capable of displacing beads over millimeter distances in a liquid-containing capillary. A drastic increase of the magnetic energy and magnetic forces acting on the beads is obtained by placing the coil array in a uniform static magnetic field that imposes a permanent magnetic moment to the microbeads. The very small magnetic field (gradient) of a simple planar coil is then sufficient to displace the microbeads over a distance of the order of the coil size. Arranging adjacent coils with spatial overlap and actuating them in a specific phase (for example a three-phase scheme) assures the long-range displacement of the microbeads 
     A preferred embodiment of the invention concerns a two-dimensional array of coils that can be operated collectively to induce microbead transport, which can be used for the manipulation and/or transport of microbeads. 
     The inventive microbead transport system is based on the use of a coil, preferably a set of planar coils obtainable by simple Printed Circuit Board technology, that is/are placed inside the large static magnetic field. This is done in such a way that the magnetic induction is uniform, i.e. it contributes to the formation of the magnetic moment μ, but not to the formation of a gradient ∇B. Thanks to the formation of large magnetic moments (typically a factor 10-100 larger than when using a coil only), the very small field gradients of simple planar coils are sufficient to transport these magnetic microbeads. This becomes especially attractive for very small magnetic particles (nano-beads) that have a very small volume V and are, otherwise, very difficult or impossible to magnetise. 
     With the inventive device, the magnetic field generated by the coils has a magnetic field gradient localized over a distance equal to the coil width. Consequently, the magnetic field gradient generated by the coils is typically several mT/mm (˜10 Gauss/mm), localized in distances measured in a scale of several millimetres. This localisation of the magnetic field gradient is several magnitudes larger than the field gradient localization scale (100 μm) of the above-discussed serpentine wire arrangement. As a further consequence, with the inventive device it is possible to perform a long-range displacement (10-100 mm) in a few actuation steps (2-20, for instance) whereas several hundreds of steps would be necessary to perform the same displacement range using the above-discussed serpentine wire arrangement. 
     Switching of the coils at a desired frequency can be computer controlled. The maximum switching frequency for any particular device can be determined as a function of the time necessary for a microbead to go from the centre to the border of a coil, which depends in particular on the characteristics of the microbeads and the fluid. For example, where this time is about 0.2 sec, the maximum switching frequency is 5 Hz. 
     As a result the device according to the invention is applicable in areas requiring rapid microbead displacement, such as compact bio-analysis systems, where magnetic beads are the ‘carriers’ for the biochemical reactions or play a role in optical, electrical or electrochemical detection of biochemical reactions. This invention can be used in a diagnostic system to detect very low concentration biomolecules. Microbeads can be transported by the magnetic field to a sensor region in the microfluidic circuit or can be used to mix different solutions or enhance the cross section for chemical interaction between the bead and the activated surface (i.e. the chemically activated surface of the plane of the coils). Also, selection and transport of specially marked beads is possible in a two-dimensional coil array structure. 
     The device can for example comprise a Hall sensor, the coil(s) being arranged to transport the microbeads to the Hall sensor. 
     Moreover, the widely used and simple PCB technology can be used to integrate the coil system in the device, making manufacture simple and inexpensive. In PCB technology, coils are distributed over at least two functional layers separated by an insulating layer in such a way that electrical short-circuiting between neighbouring coils is avoided. 
     The invention also relates to a method of transporting microbeads in a fluid in a capillary chamber, which comprises: subjecting the capillary chamber to a substantially uniform magnetic field, to induce a permanent magnetic moment to the microbeads; applying a complementary magnetic field on the microbeads parallel or antiparallel to said substantially uniform magnetic field by means of at least one coil adjacent to the capillary chamber; and switching the current applied to the coils to invert the field produced thereby, to selectively apply an attractive or repulsive driving force on the microbeads. Preferably, the coils are generally planar and the sustantially uniform magnetic field is perpendicular to the planar coils. 
     Further aspects of the invention are set out in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying schematic drawings, which are given by way of example: 
         FIG. 1  is a lateral view of a first embodiment of a bead transport device according to the invention, employing two permanent magnets; 
         FIG. 2  is a lateral view of a second embodiment of a bead transport device according to the invention employing a single permanent magnet; 
         FIG. 3  is a perspective view of the centre part of the device of  FIG. 2 ; 
         FIG. 4  is a lateral view of a third embodiment of a bead transport device according to the invention employing an electromagnet generating a large static magnetic field; 
         FIG. 5  is a graph showing the magnetic field generated by a coil; 
         FIGS. 6 and 7  are sectional views through a coil illustrating the effect of the generated magnetic field respectively to attract or to repel beads; 
         FIG. 8  is a diagram illustrating a three-phase connection of a series of overlapping coils; 
         FIG. 9  is a diagram illustrating the displacement of a bead by the successive switching of a series of overlapping coils; 
         FIG. 10  is a diagrammatic plan view of a series of overlapping coils with a single capillary tube; 
         FIG. 11  is a similar view showing an array of side-by-side series of overlapping coils with respective capillary tubes; and 
         FIG. 12  is a similar view showing another array of overlapping coils with a single capillary chamber extending over the array. 
     
    
    
     DETAILED DESCRIPTION 
     A first example of a bead transport structure according to the invention is shown in schematic lateral view in  FIG. 1 . Two bar-shaped NdFeB permanent magnets  10 , for example measuring 40 mm×15 mm×8 mm, are placed on top of a soft magnetic sheet  12 , and generate a uniform field B 0  (for instance 50 mT) over the total length of a microfluidic glass capillary  14 , for instance 1 mm outer diameter, 0.5 mm inner diameter. A coil array  20  is positioned on the magnetic sheet  12  directly underneath the capillary  14 . 
     The capillary  14  contains microbeads in a suitable fluid, for example water. The microbeads typically have dimensions from 0.01 to 10 μm and can be suspended in water and injected in the capillary  14 . They can for example be made of Fe 3 O 4 . Different types of suitable particles and coatings are listed in WO99/49319. An example of suitable magnetic microbeads are Streptavidine MagneSphere® Paramagnetic Particles available from Promega Corporation, Madison, USA. Such particles have a 1 μm diameter, and χ eff  approximately 0.8. 
     The coil array  20  for example has the layout shown in  FIG. 3  and  FIGS. 8-12 . It comprises a series of overlapping coils  22  produced using standard Printed Circuit Board (PCB) technology. The coils  22  are for instance made of copper, 100 μm winding width, 35 μm winding height, 200 μm winding pitch and can have a small number of windings (typically, N=4-10). A single coil  22  of the given dimensions typically generates a magnetic field gradient of about 5 mT/mm for a maximum allowed current density of 400 A/mm 2 . 
     One should note that the coil  22  at its centre has no windings, indicating the need of having a feed through to another functional layer of the Printed Circuit Board. However, in practice, one will fill the coil  22  as much as possible with electrical windings. 
       FIGS. 2 and 3  show a second example of a bead transport structure according to the invention in schematic lateral view and in perspective, respectively. In this example the coil array  20  and capillary  14  are placed centrally on a single permanent magnet  10  generating at its centre the uniform magnetic field B 0  (for instance 50 mT) over the total length of capillary  14 . 
     For illustrative purposes,  FIG. 3  shows the overlapping coils  22  on a support  10 . In practice, however, the overlapping coils are arranged over two (or more) functional layers separated by an insulating layer or support  10 . 
       FIG. 4  shows a third example of a bead transport structure according to the invention in schematic lateral view, wherein the coil array  20  and capillary  14  are placed centrally in an electromagnet  11  generating along its central axis the uniform magnetic field B 0  (for instance 50 mT) over the total length of capillary  14 . 
     A special feature of the inventive device is the partial overlap of adjacent coils  22  (as shown in  FIG. 3  and  FIGS. 8-12 ), so that there is never a local magnetic energy minimum in between two coils  22 . This is to be contrasted with the simple juxtaposition of the prior art that cannot provide microbead transport, but merely allows separation of microbeads transported by a moving fluid. 
       FIGS. 6 and 7  respectively show how the field produced by a coil  22  can be used to attract or to repel microbeads  25 . As shown in  FIG. 6 , when the field produced by coil  22  is parallel to the uniform field B 0 , the microbeads  25  above the coil  22  are attracted towards the open centre of the coil  22  formed by its inner turn  23 . The distribution of this magnetic field produced by the coil  22  is illustrated in  FIG. 5 . As shown in  FIG. 7 , when the field produced by coil  22  is antiparallel to the uniform field B 0 , the microbeads  25  above the coil  25  are repelled towards the exterior part of the coil  22  formed by its outer turn  24 . 
     By switching the direction of the current in the coil  2 , microbeads  25  in a fluid in a capillary chamber above the coil  22  can be made to move between the equilibrium positions at the periphery and the centre of the coil  22 . 
     The current actuation scheme of these coils  22  constitutes another innovative aspect. One should note that, due to finite size of the cluster, not all microbeads will be subjected to the same force. Therefore care needs to be taken to transport effectively all microbeads in a given direction. Consider a system consisting of at least three neighbouring coils  22 , as for example illustrated in  FIGS. 8 and 9 : a first one repulsive, a middle one repulsive/attractive and a third one attractive. When the middle coil is switched from the attractive ( FIG. 6 ) to the repulsive ( FIG. 7 ) mode, part of the beads will go to the left and part to the right of the coil center. When thereafter, the center coil is again in the attractive mode ( FIG. 6 ), the microbeads  25  , which first have moved to the left, are now displaced to the right. By repeating the attractive and repulsive sequences, thereby creating an ‘oscillatory’ field, one can effectively transport all microbeads of the cluster from the left to the right. 
       FIG. 8  illustrates an arrangement wherein the coils  22  are connected in at least two series such that the magnetic field of adjacent and overlapping coils  22  can be varied independently of one another to provide a coordinated driving force on the microbeads  25 , namely in this example by using a three-phase supply. 
     As shown in  FIG. 8  series  20  of thirteen coils (numbered  1 A to  1 E) is realised by PCB technology on an insulating support  30  with two sets of integrated current supply terminals  1 , 2 , 3  for a three-phase supply. Starting from the left of  FIG. 8 , terminal  1  is connected to the outer winding of coil  1 A whose inner winding is connected to the outer winding of coil  1 B whose inner winding is connected in turn to the outer winding of coil  1 C. The latter&#39;s inner winding is then connected in series to the outer winding of coil  1 D and so on to the end coil  1 E whose inner winding is connected to the corresponding terminal  1 . In like manner, coil  2 A is connected via coils  2 B,  2 C and  2 D between the terminals  2 , and coil  3 A is connected via coils  3 B,  3 C and  3 D between the terminals  3 . 
       FIG. 9  shows how, using for example FIG.  8 &#39;s  3 -phase arrangement, one can combine the magnetic fields from adjacent coils  22  properly in time and create a magnetic field maximum, which propels the microbeads  25  in the capillary  14 . Here we can benefit from the advantage that the permanent magnetic field imposes the magnetic moment always in the same direction of the microbeads, so that we can apply the coil-generated magnetic field (B z ) from up (parallel to the uniform field component B 0 ) to down (antiparallel to the uniform field component B 0 ), allowing to generate both attractive and repulsive magnetic forces. This enables the combined use and actuation of neighbouring coils  22  to generate the time- and position-dependent magnetic forces. 
     In  FIG. 9 , a succession of coils numbered  1 A to  2 C, as in  FIG. 8 , are illustrated as being located on opposite s;des of an insulating support  30 . Other arrangements are of course possible, in which the coils  22  are distributed over at least two functional layers, separated by an insulating layer. In  FIG. 9(   a ) a microbead  25  is shown at one end of the array, over the centre of coil  2 A which is illustrated in the attractive mode, i.e. its field directed upwards as indicated by the arrow, parallel to the uniform field Boz. In  FIG. 9(   b ), the current in the coils has been reversed, so the coil  2 A is in repulsive mode, whereas the adjacent overlapping coil  3 A is in the attractive mode. The microbead  25  is hence attracted to the centre of coil  3 A, so it is displaced to the new position shown in  FIG. 8(   b ). When the current in the coils is reversed again as shown in  FIG. 8(   c ), the coil  3 A is now repulsive and coil  1 B is attractive, so the microbead  25  is displaced to the new position shown in  FIG. 8(   c ), over the centre of coil  1 B. Likewise for  FIG. 9(   c ), where only the left hand part of coil  3 B is visible. In this Figure, a further reversal of the current in the coils has brought the microbead  25  over the centre of coil  2 B, in the attractive mode and between the peripheries of coils  1 B and  3 B, in the repulsive mode. 
     Arranging adjacent coils  22  with spatial overlap and actuating them in a specific three-phase sequence, as described above, allows transporting single microbeads (specifically the MagneSphere® Paramagnetic Particles) with characteristic velocities of 0.1 mm/s, and complete clusters of beads with an effective velocity of the order of several 0.1 mm/s. For these beads in water, it was found that the switching time necessary for a microbead to go from the centre to the border of a coil was about 0.2 sec, corresponding to a maximum switching frequency of 5 Hz. 
       FIGS. 10 ,  11  and  12  schematically show several arrangements for transporting microbeads in one direction ( FIGS. 10 and 11 ) or in several directions in a two-dimensional arrangement ( FIG. 12 ). 
       FIG. 10  illustrates a row  20  of coils  22  associated with a rectilinear capillary channel  14  extending along the row, over the centres of the overlapping coils, enabling transport of microbeads along the capillary channel  14 , as explained with reference to  FIGS. 8 and 9 . 
       FIG. 11  illustrates an arrangement that consists of a juxtaposition of several rows as illustrated in  FIG. 10 , side-by-side. In this arrangement, each row of coils  22  is associated with its own capillary channel  14 . 
       FIG. 12  illustrates an array of overlapping coils  22  arranged along alternate rows, in quincunx in the illustrated example. In this case, a capillary chamber  14  extends over the open centres of the array of coils  22 , i.e over several rows of coils. As indicated at  40 , the centres of the coils in this arrangement form a hexagon. The walls of the chamber  14  can, but do not need to, be grooved or channeled in correspondence with the hexagons  40  to guide the microbeads. When the currents in the overlapping coils are reversed, microbeads can be displaced in the capillary chamber  14  along the lines of this hexagon  40 , from the centre of one coil  22  to the next. If desired, with this arrangement, individual coils  22  can be selectively addressed so that by selective actuation of the coils the microbeads  25  can be guided along given paths to facilitate their separation. 
     Hence, two-dimensional magnetic circuits are possible by placing a two-dimensional coil array  20  over a large surface area where there is a constant field generated by permanent magnets. The microbeads will not have a preferential position if the magnetic induction is constant, but the actuation of a two-dimensional array of coils will allow transport in two dimensions inside a microfluidic structure. Or beads can be transported in a complex microfluidic system, containing numerous channels, reactors, etc. 
     In one application, a bottom surface of the capillary chamber  14  is bio-chemically activated for the retention of molecules or substances to be detected, and the microbeads  25  are coated with a biological active layer selectively retainable by said molecules or substances such that when the microbeads are transported in the capillary chamber  14  by the coil(s)  22  the transported microbeads  25  can be retained by the molecules or substances to be detected and thus be separated from the transported microbeads. 
     In another application, the capillary chamber  14  is part of a microfluid circuit comprising a Hall sensor, the coil(s)  22  being arranged to transport the microbeads to the Hall sensor which recognises the presence of a microbead, specifically bound on top of it. 
     Many modifications of the described embodiments of the device are possible and the device can be used for many applications, other than those described, e.g. magnetic filtration.