MIXING APPARATUS AND METHOD

Abstract: The invention concerns an apparatus for performing mixing in thin liquid layers containing a suspension of a multiplicity of movable particles of magnetic material. The apparatus comprises at least two magnets or magnet sys­ tems, of which at least one is an electromagnet. The magnets or magnet systems are arranged in order to provide at least one slit for receiving at least one support means containing the thin liquid layer, wherein the magnetic particles are pre­ sent. When the liquid layer in the support means is inserted in the slit the thin layer will be subjected to the combined mag­ netic field originating from the two magnets or magnet sys­ tems. The apparatus also comprises driving means for the electromagnet(s), timing means and a current source. The support means, which fixedly supports the thin liquid layer containing a multiplicity of magnetic particles, is arranged between the magnets in such a manner that the thin layer is subjected to the combined magnetic field of the magnets, which magnetic field alternatingly concentrates and fades out. The invention also comprises a method of performing mixing in thin liquid layers.

Description: Field of invention The present invention concerns an apparatus and a method for treating liquids. Especially the invention concerns an apparatus and a method for mixing one or more liquids using magnetic particles which, subsequent to the mixing, may be transported to selected areas. Prior Art SE-C-221 918 discloses an apparatus and a method for mixing liquids using magnetic particles. More specifically, the patent discloses an apparatus achieving a magnetic field that varies as regards the intensity and the direction in order to keep the magnet particles at a distance from each other and give them a rotation and/or translation movement. The magnetic field is obtained by using a solenoid. Optionally the apparatus can include a collar of magnetic material. The magnetic particles used are permanent magnets. Furthermore, it is disclosed (page 3, right column, 4 last lines) that a separate permanent magnet can be arranged close to the mixing zone in order to obtain a stronger mixing within predetermined parts of the fluid. An essential difference between this previously known apparatus and mixing method and the present invention, which also uses small magnetic particles in order to effect mixing, concerns the mixing process. According to the present invention the mixing process comprises one component that can be characterized as a reciprocating transporting motion or movement of the magnetic particles. Optionally this component can be combined with another component, which consists of the rotation of each individual particle around its own centre of gravity. The transporting function that can be a reciprocating radial or lateral motion is used for retaining particles in preselected areas after completed mixing. This feature constitutes an important part of the present invention, which is not disclosed in the Swedish patent. The mixing process according to the present invention is achieved by using the combined magnetic field effect originating from at least two different magnets. Another mixing apparatus is disclosed in US-A-3,752,443. According to this patent the magnetic particles are subjected to a centrifugal force generated by a rotating permanent magnet. The centrifugal force is balanced by the influence of a second permanent magnet in order to obtain a substantially uniformity of distribution of the magnetic particles. The apparatus known from this patent differs from the apparatus according to the present invention, i.a. in that it comprises movable parts and in that it cannot be used for retaining the magnetic particles in preselected areas. EP-A-0 014 109 discloses another apparatus involving magnetic fields and particles of magnetic material dispersed in a fluid medium. However, according to this invention the magnetic particles are not inert but take part in the reactions occuring in the fluid. Further, the fluid medium in this prior art reference is not contained in a thin liquid layer defined by a microcuvette. EP-A-0 014 109 neither discloses the inventive step of repeatedly changing the direction of a first magnetic field generated by a first electromagnet while maintaining a second magnetic field from a second magnet unchanged. Nor does the document disclose the step of retaining the magnetic particles in preselected areas in order to subject the liquid to optical analysis within areas depleted of particles. Object of the invention One object of the invention is to provide an apparatus and a method for mixing liquids using magnetic particles, which can be transported to and retained at preselected areas after completed mixing. A second object is to provide an apparatus and a method for mixing small volumes for e.g. analytical purposes. A third object is to provide a small mixing apparatus or mixing unit without any movable parts. A forth object is to provide a small mixing unit that can be built-in in a portable instrument. A fifth object of the invention is to provide a flexible system for mixing liquids using magnetic particles. The present invention concerns an apparatus for performing mixing in a thin liquid layer. The liquid layer includes a suspension of a multiplicity of moveable particles of magnetic material and is contained in a microcuvette for optical analysis. The apparatus comprises at least two magnets at least one of which is an electromagnet. The magnets are arranged to provide at least one slit for receiving said microcuvette between said magnets in such a way that said layer is subjected to a combined magnetic field originating from said at least two magnets. The slit is arranged in a way that the microcuvette is received and arranged between at least two opposite poles of at least two different magnets. The remaining poles of said magnets are arranged essentially in the plane of said microcuvette and adjacent to the circumference of said microcuvette. The apparatus further comprises driving means for said at least one electromagnet, said driving means comprising timing means and a current source. The invention also comprises a method of performing mixing and optical analysis of a thin liquid layer contained in a microcuvette, by activation of a multiplicity of magnetic particles in suspension in said liquid layer. The method comprises the following steps: a. generating a first magnetic field by activation of at least one first electromagnet; b. generating one or more second magnetic fields by one or more permanent magnets and/or by activating one or more second electromagnets; c. subjecting said thin liquid layer to a combined magnetic field originating from said first and second magnetic fields generated in step a and b, respectively; d. repeatedly changing the direction of said first magnetic field generated by the first electromagnet; e. interrupting said changing of the direction of said first magnetic field of the first electromagnet to retain the magnetic particles in first preselected areas within the microcuvette, a second preselected area within the microcuvette thereby being depleted of particles; and f. subjecting the liquid within said second preselected area depleted of particles to optical analysis. Brief description of the drawings Figure 1A and 1B illustrate the principle of the invention. Figure 2A and 2C are sectional views illustrating the principle of the invention applied on a liquid volume containing magnetic particles. Figure 2B and 2D are top plan views illustrating a magnetic particle distribution pattern. Figure 3A and 3C illustrate a further embodiment of the invention. Figure 3B and 3D are top plan views illustrating another magnet distribution pattern. Figure 4 is a sectional view illustrating a further arrangement of the magnets of the apparatus according to the invention. Figure 5 is a block scheme of the apparatus according to the invention. Detailed description of the invention The principle of the present invention is disclosed in figure 1A and 1B, wherein 1 and 2 are magnets having their poles facing each other. At least one of the magnets is an electromagnet which is connected to a polarity shifting DC source (not shown). The combined magnetic field generated when both of the magnets interact is marked out by the dashed lines. If, as is assumed in this embodiment, the magnets are of equal strength, there will be alternatingly a concentration and fading out of the combined magnetic field in an area in a plane between and parallel to the magnetic poles and at equal distance from each pair of poles, the area being centrally located with respect to each pair of poles. The influence of the magnets on a multiplicity of magnetic particles 4 in a liquid layer of a support 3 is disclosed in figure 2A and 2C. When both magnets are driven by AC, each of the magnetic particles is imparted a rotational movement around its centre of gravity and a reciprocating lateral movement obtained when the magnets repeatedly and alternatingly are driven in phase and in antiphase to each other to and away from the area centrally located around an axis through the centre of the container 3 and perpendicular to its extension, in which area the magnetic field alternatingly concentrates (figure 2A) and fades out (figure 2C). The figure 2B illustrates the top view of the pattern formed by the multiplicity of magnetic particles 4 in the support when the opposite poles have a square or rectangular form and are of the same kind, i.e. north poles and south poles respectively. Figure 2D illustrates a top view of pattern formed when the opposite poles are of different kind. In this connection it should be pointed out that also the distance between the magnets influences the form and appearance of the areas with magnetic particles. The closer the magnets 1, 2 are, the more marked the profiles of the magnetic poles in the particle area become. Figure 3A and 3C disclose another arrangement of the magnets 6, 10 in the apparatus according to the present invention. In this embodiment two identical magnets 6, 10 are facing each other. Each magnet 6, 10 comprises a cylindrical wall 7, 11, a circular bottom plate 8, 12 and an inner cylinder 9, 13, the wall, bottom and cylinder being in one piece. The cylinder extends perpendicular from the centre of the bottom plate 8, 12. An elongated support 5 is arranged in a slit centrally between the magnets 6, 10. The patterns formed by the magnetic particles, when the magnets are activated and the magnetic fields generated, are alternatingly working to reinforce each other and to fade each other out are disclosed as 14, 15, 16 and 17 in figure 3B and 3D, respectively. The coils 18 are connected to current sources (not shown), which can be a DC source or an AC source as in figure 5. Not specifically shown but within the scope of the invention is also an embodiment according to figure 3A and 3C, wherein only one coil 18 is provided and the remaining magnet 6 or 10 is a permanent magnet. Figure 4 discloses a further embodiment of the invention. In this embodiment the magnets 19, 20 are arranged as in figure 3A, C and each magnet 19, 20 comprises a cylindrical wall 21, 25, a circular bottom plate 22, 26 and an inner cylinder 23, 27, the top of which has the form of a cone. Furthermore, each magnet 19, 20 has a collar 24, 28 on the cylindrical wall 21, 25 extending towards the support or container 33, which is arranged centrally between the cones of the inner cylinders 23, 27 and the annular collars 24, 28. When the support 33 is inserted in or taken out from the slit of the apparatus the magnets are taken apart. Alternatively a grove can be provided in the collars 24, 28. Furthermore, there is provided a hole 29, 30 through the inner cylinder 23, 27 of each magnet 19, 20. This embodiment of the invention is especially adapted for using in optical assays of liquids/reagents in the support 33, which has the form of a micro-cuvette having plane-parallel walls of transparent material. The volume of the cuvette may vary between 0.1 ul-1 ml. The thin liquid layer within the support, the cuvette, may vary between 0.01 and 2.00 mm, preferably 0.1 and 1.0 mm. The change of colour, intensity, turbidity etc during or subsequent to a mixing operation when the magnets 19, 20 are activated as previously described is measured by a detector arranged at one opening of the hole 29, 30 and opposite to a light emitting device arranged on the opposite side of the container or support. The assay is performed when the mixing action is completed, the phase shifting of the magnet(s) is interrupted and the centre of the cuvette in the path of the light is depleted of magnetic particles, which are actively locked in predetermined positions by the combined magnetic field. It is obvious to the man skilled in the art that the poles can be designed and arranged in a wide variety of different ways, which makes it possible to solve a great variety of mixing and transporting problems in thin liquids. It is also obvious that by arranging more than two magnets the flexibility of the mixing system is highly increased. According to one embodiment of the invention the thin liquid layer inserted in the slit is arranged between at least two opposing poles of at least two different magnets, the poles of which are opposing each other, within a spacial angle of at most 160 o , preferably 0-80 o , and especially 0-20 o , with respect to the centre of each pole. The remaining poles of the magnets may be arranged essentially in the plane of the thin layer and adjacent to the circumference of the layer. Each magnet can have the shape of a cylinder with a coaxial annular recess at one end. This recess is intended for receiving the activating coil of the magnet. The recess defines the core of the magnet. Furthermore, the slit may be arranged in such a way that the thin liquid layer when inserted into the slit will be arranged between at least two opposing poles of at least two different magnets around a common central axis or plane through the poles. The core of each magnet could have a through hole extending along its central axis. This through hole makes it possible to perform the optical analysis discussed above. An important advantage that can be obtained according to the present invention concerns the possibility of transporting the magnetic particles to one or more different areas within the support depending on the arrangement of the magnets or magnet systems, their number, the design of the poles and the driving function (regime). Consequently, it is possible to transport the magnetic particles from one end of an elongated support to the other by sequentially activating and deactivating different magnets along the support. In the same way as it is possible to transport the magnetic particles to preselected areas it is also possible to transport the particles from preselected areas by timely interrupting the activation or phase shifting of the magnet(s). This inherent property of the apparatus according to the invention is important for e.g. optical assays when the area subjected to the light beam must be free from magnetic particles (c.f. the arrangment according to figure 4). The geometrical form of the magnets determines where in the liquid layer the particles will be locked by the magnetic field(s). The magnets used according to the present invention can be electromagnets or a combination of permanent magnets and electromagnets. When driven by AC it is preferred that most of the magnets are electromagnets. When DC is used preferably half of the number of the magnets are permanent magnets. If the apparatus according to the present invention comprises a mixture of electromagnets and permanent magnets, the electromagnets can be driven by polarity shifting DC having a shifting frequency varying between 0.001 and 10 Hz. Alternatively all the magnets of the apparatus are electromagnets driven by polarity shifting DC or phase shifting AC, whereby the AC frequency can vary between 0.01 hz and 100 kHz and polarity or phase shifting frequency between 0.001 and 10 Hz. When a magnet combination including an electromagnet and a permanent magnet is used, the electromagnet can be superposed by either an alternating DC voltage or a constant DC voltage. In the first case the electromagnet and the permanent magnet cooperate in order to generate a magnetic field across the thin liquid layer in the support, whereby the field provides an essentially linear or lateral movement of the magnetic particles and a mixing action is obtained. When the electromagnet is superposed by a constant DC voltage, a locking of each separate magnetic particle in a predetermined position in the layer will be obtained. If, on the other hand, a combination including two electromagnets is used, each of the electromagnets can be superposed by a DC voltage, the reciprocal phase shift of which could be varied between 0 o and 180 o . When, in this case, the voltages from the two electromagnets cooperate the magnetic field across the thin liquid layer will provide an essential linear or lateral movement of the magnetic particles. When, on the other hand, the voltages from the two electromagnets counteract, a magnetic field across the thin liquid layer will lock each separate magnetic particle in a predetermined position in the liquid layer. For most applications where few magnets are used it is advantageous to use magnets having a central and a peripheral pole (cf. figure 3 and 4). In applications using a larger number of magnets, each pole of the magnet can be arranged so as to face a pole of another magnet and a sequence of poles can thus be arranged on opposite sides of a support means including one or more thin liquid layers along its extension. By using this arrangement in combination with a preprogrammed activation/deactivation of the magnets, the magnetic particles can be transported from one end of the support to another. The field strength of the magnets are chosen depending on the distance of the poles of the magnets from the liquid layer(s) in the support, on the distance and the strength of the pole of the facing magnet and of the desired function. The apparatus according to the invention consists of several functional units as illustrated in figure 5. The two main parts, the driving unit and the working unit, can be placed physically apart from each other. The driving unit involves a current source capable of delivering suitable DC and/or AC voltages for the other parts of the apparatus. It also contains means for polarity or phase shifting the current to one or some of the electromagnets in the working unit. Also there might be contained means for activating or deactivating the electromagnets. These controlled switches are not always needed when the apparatus contains few electromagnets but is advantageous with a larger system. These means could also involve a voltage controlling circuit to provide a selected voltage for the individual electromagnet. A timing unit provides means for timingly control of the polarity or phase shifting unit and the activating/deactivating means. The timing unit is preferably programable but for simple operation regimes this is not needed. For a more complex system this unit also could provide control of different voltages and computing power. It is obvious to the man skilled in the art that the driving unit can be designed in a wide variety of different ways with the tools of modern electronics. In the following the invention is explained in further details with reference to figure 3A, C, where the magnet 6 is a permanent magnet. The mixing effect is obtained by driving the coil 18 of the electromagnet 10 with polarity shifting DC with a current giving a magnetic field strength in about the same magnitude as the field from the permanent magnet. The shifting period depends on the field strength, the magnetic particles, the design of the support, the viscosity of the liquid and the desired mixing effect and can vary from 0.001 s to 60 s. The arresting of the movement of the magnetic particles is achieved by simply stopping the polarity shifting in the desired mode. When AC is used the permanent magnet 6 of the above example is exchanged by a constantly AC driven electromagnet and the other magnet 10 is driven by phase shifting AC instead of polarity shifting DC. The frequency of the AC is preferably the same as the line voltage, e.g. 50/60 Hz, but practically any frequence can be used. The support for the liquid volume can have any shape and should consist of non-magnetic material such as, e.g. glass, plastic, ceramic or non-magnetic metals. According to one preferred embodiment of the invention the container has the form of a cuvette such as described in the US-A-4,088,448. The expression "magnetic particles" referred to in this text is meant to include particles that are influenced by a magnetic field. They may consist of purely ferro-magnetic material or a ferro-magnetic material coated or mixed with another material such as a polymer, a protein, a detergent, a lipid or a non-corroding material. The size of the particles can vary from 0.001 um to 1 mm. The size as well as the composition of the particles depends on the intended use and the design of the container. The magnetic material is preferably not permanent magnetic but permanent magnetic particles can be used. Preferably the particles are essentially inert to the surrounding liquid and reactions occuring therein and suspended in the liquid volume subjected to the mixing processes. Example A Hemocue® microcuvette for optical measurement is prepared with sodium hydroxide, sodium carbonate and nitrobluetetrazoliumchloride as in the Fructosamine Test (Roche). The exact amount of the reagent depends on the volume of the microcuvette. 0.1 mg ferrite particles (2 µm) is also included inside the microcuvette. The amount of magnetic particles depends on the volume of the microcuvette, the magnetic material and the size of the particles and can easily be determined by a person skilled in the art. The microcuvette is filled with blood serum and inserted into an apparatus according to figure 4 and the working unit in figure 5. The two essentially identical electro magnets are connected to the driving unit according to figure 5. The optical unit of a photometer is arranged so that the light path can traverse the central holes of the two electromagnets and the microcuvette, and the optical changes of the reaction mixture can be registered. The electromagnets are activated and the polarity unit is set to shift each fifth second. The magnetic particles are forced to alternate from one position to the other as roughly indicated in figure 3B and 3D each fifth second. After two minutes the polarity shifting unit is locked in the polarity giving the pattern of magnetic particles that is indicated in figure 3D and the optical measurement takes place in the central area that is now depleted of magnetic particles, which are actively held or locked by the magnetic field in the peripheral of the cuvette cavity.

Claims: Apparatus for performing mixing in a thin liquid layer, which includes a suspension of a multiplicity of moveable particles of magnetic material and which liquid layer is contained in a microcuvette (33) for optical analysis, which forms part of the apparatus, said apparatus further comprising at least two magnets (19, 20) at least one of which is an electromagnet, said magnets (19, 20) being arranged to provide at least one slit for receiving said microcuvette (33) between said magnets in such a way that said layer is subjected to a combined magnetic field originating from said at least two magnets (19, 20); wherein said slit is arranged in a way that said microcuvette (33) is received and arranged between at least two opposite poles (23, 27) of at least two different magnets (19, 20) and wherein the remaining poles of said magnets (19, 20) are arranged essentially in the plane of said microcuvette (33) and adjacent to the circumference of said microcuvette (33); and driving means for said at least electromagnet, said driving means comprising timing means and a current source. Apparatus according to claim 1, wherein each of said magnets (19, 20) has the shape of a cylinder with a coaxial annular recess at one end for receiving an activating coil (18) of said magnet, whererby the recess defines a core of each magnet. Apparatus according to claim 2, wherein said core of each magnet (19, 20) has a through hole (29, 30) extending along a central axis thereof. Apparatus according to any of the claims 1-3, wherein said slit is arranged in such a way that said thin liquid layer when received therein will be centrally arranged between at least two opposing poles (23, 27) of at least two different magnets (19, 20) around a common central axis or plane through said poles. Apparatus according to claim 1, wherein said at least two magnets (19, 20) comprise a mixture of electromagnets driven by polarity shifting DC, having a polarity shifting frequency varying between 0.001 and 10 Hz, and permanent magnets. Apparatus according to any of the claims 1-4, wherein all the magnets are electromagnets driven by polarity shifting DC or phase shifting AC, whereby the AC frequence can vary between 0.01 Hz and 100 kHz and polarity of phase shifting frequency between 0.001 and 10 Hz. Apparatus according to any of claims 1-6, wherein said driving means for said at least one electromagnet is arranged to repeatedly change the direction of the magnetic field generated by said one electromagnet in order to accomplish an alternatively concentration and fading out of said combined magnetic field originating from said at least two magnets (19, 20). Apparatus according to claim 7, wherein said driving means for said at least one electromagnet is arranged to interrupt said changing of the magnetic field generated by said at least one electromagnet to retain the magnetic particles in first preselected areas within the microcuvette, a second preselected area within the microcuvette thereby being depleted of said magnetic particles. Apparatus according to claim 8, further comprising means for performing optical analysis on said second preselected area depleted of magnetic particles. A method of performing mixing and optical analysis of a thin liquid layer contained in a microcuvette, by activation of a multiplicity of magnetic particles in suspension in said liquid layer, comprising the following steps: a. generating a first magnetic field by activation of at least one first electromagnet: b. generating one or more second magnetic fields by one or more permanent magnets and/or by activating one or more second electromagnets; c. subjecting said thin liquid layer to a combined magnetic field originating from said first and second magnetic fields generated in step a and b, respectively; d. repeatedly changing the direction of said first magnetic field generated by the first electromagnet; e. interrupting said changing of the direction of said first magnetic field of the first electromagnet to retain the magnetic particles in first preselected areas within the microcuvette, a second preselected area within the microcuvette thereby being depleted of particles; and f. subjecting the liquid within said second preselected area depleted of particles to optical analysis.

Inventors: LILJA, JAN EVERT, NILSSON, SVEN ERIK LENNART
Applicants: MIGRATA UK LTD
Application year: 1987
Main classification: B01F13
IPC codes: B01F13