The invention relates to a method and a corresponding preparation apparatus for the enrichment of magnetic particles in a sample fluid.
The WO 2008/155716 discloses an optical biosensor in which an input light beam is totally internally reflected and the resulting output light beam is detected and evaluated with respect to the amount of target components at the reflection surface. The target components comprise magnetic particles as labels, which allows to affect the processes in the sample by magnetic forces.
Based on this background it was an object of the present invention to provide means that allow to detect low concentrations of target substances with a biosensor.
This object is achieved by a preparation apparatus according to claim 1 and a method according to claim 2. Preferred embodiments are disclosed in the dependent claims.
According to its first aspect, the invention relates to a preparation apparatus for the enrichment of magnetic particles in a sample fluid. In this context, the combination of a particular type of magnetic particles and a particular sample fluid shall be considered as being given and having predetermined characteristics, particularly in terms of magnetic properties of the magnetic particles and their migration velocity in the sample fluid under the influence of e.g. magnetic forces. The preparation apparatus has a design that is adapted to the given magnetic particles and sample fluid. It comprises an actuator magnet with a first and a second magnetic pole, wherein the following features shall be realized:
The invention further relates to a corresponding method for the enrichment of magnetic particles in a sample fluid having given characteristics, said method comprising the following steps:
The method comprises in general terms a procedure that can be executed with the preparation apparatus defined above. Consequently, the method is preferably executed with such an apparatus.
The preparation apparatus and the method described above have the advantage that they allow the enrichment of magnetic particles in a sample fluid with high efficiency, as both the magnetic flux and the magnetic field gradient in the sample fluid are determined with respect to the properties of the particular magnetic particles and sample fluid under consideration. It is possible to use this apparatus and method to enrich magnetically labeled target components of a sample to a level at which they can readily and reliably be detected by a biosensor, or can be further manipulated and processed, e.g. in an integrated lab-on-a-chip device or cartridge. The detection limit of the biosensor can hence be extended while still providing a procedure that is suited for a simple and rapid (e.g. outdoor) application. Compactness makes the apparatus particularly apt for an integration with further components (e.g. a biosensor), yielding a favorable near-patient (point-of-care) setting.
In the following, further developments of the invention will be described that relate to both the preparation apparatus and the method described above.
Concrete values for the magnetic flux that shall be established in the sample space preferably range above about 50 mT. Most preferred is a value of about 100 mT. With these values, the desired degree of magnetization can be achieved for a large class of magnetic particles that are often used in practice (e.g. superparamagnetic beads having a diameter of typically between about 3 nm and 5 μm).
A concrete value for the magnetic field gradient that shall be established during operation (everywhere) in the sample space is at least 0.2 T/m, preferably at least 0.6 T/m. These values prove to generate satisfactory migration velocities for a large class of practically important magnetic particles and a sample fluids. Typical average migration velocities that can be achieved by such gradient values range between about 10 μm/s and 300 μm/s.
The sample space preferably has a volume of about 0.1 ml to about 10 ml, most preferably of about 1 ml. As many known biosensors process small sample volumes of some μl, an enrichment factor of about 1000 can be achieved when an initial sample volume of about one ml is reduced to the μl size required by the biosensor. The detection limit of the bio sensor can hence be extended by several orders of magnitude.
The maximal distance of the surface points of the first pole from the second pole preferably ranges between about 5 mm and about 20 mm. The concrete values will be chosen according to the applied electrical excitation, i.e. the power input at given coil dimensions. Hence a quite typical value is about 10 mm.
The minimal distance of the surface points of the first pole from the second pole preferably ranges between about 2 mm and about 18 mm, preferably having a value of about 4.5 mm.
Furthermore, at least one of the poles of the actuator magnet preferably covers an area between about 100 mm2 and about 600 mm2, preferably of about 300 mm2. In this context, the “area of a pole” is defined by the cross-section perpendicular to the mean direction of the magnetic field between the poles. Preferably, the respective areas of the two poles are substantially of the same size.
The above mentioned specific values for the geometry of the poles prove to be suited for many typical boundary conditions occurring in practice.
By definition, the “tip region” of the first pole is the (connected) area where the distance of surface points of the first pole to the second pole is locally minimal. For this reason, the tip region (or, more precisely, the sample space volume adjacent to the tip region) will be the target zone to which magnetic particles in the sample space migrate under the influence of the applied magnetic fields. Depending on the particular design of the poles, the tip region may be a two-dimensional area, an (approximately) one-dimensional line, or (approximately) a point. The latter embodiment has the advantage to provide the highest spatial concentration of magnetic particles during the enrichment procedure.
In general, the surface of the first pole as well as the surface of the second pole may be arbitrarily shaped as long as the postulated features (e.g. the existence of a single tip region) are fulfilled. The surface shape of the tapered first pole can be optimized with respect to its intended effects, e.g. by implementing a parabolic shape that enables a stronger field gradient in the outer regions of the cartridge, which could accelerate the movement of single particles that are present in said region.
In a preferred embodiment, the surface of the first pole is composed of one or more planar facets. Such facets can readily be manufactured. Moreover, in combination with a similarly simple (e.g. planar) surface of the second pole, the extremes of the magnetic field gradient can readily be estimated for such a design as occurring along the edges of the facets.
According to another preferred embodiment of the invention, the actuator magnet comprises a yoke with two opposing ends that constitute the first and second pole with the intermediate sample space. As usual, a “yoke” denotes a (bended) bar of a material with high magnetic permeability that is used to concentrate magnetic field lines.
According to a further development of the aforementioned embodiment, the yoke extends through at least one electromagnetic coil. Supplying this coil with electrical currents can hence be used to controllably generate a magnetic field which is guided by the yoke to the sample space between the poles.
The aforementioned coil is preferably designed such that it has a number N≧1 of windings which can be supplied with current I (in a stable operation mode, i.e. observing given current-density limits etc.), wherein the product N·I ranges between about 500 A and about 2000 A. It is feasible to design an actuator magnet for these values that is suited for the integration into a compact enrichment apparatus and that provides an appropriate magnetic field in the sample space.
According to another embodiment, the yoke may comprise a permanent magnet for generating a magnetic field in the yoke and hence between the poles. The permanent magnet may be used alone or in combination with the aforementioned electromagnetic coil. The permanent magnet may optionally constitute an exchangeable component that can be inserted into the yoke if desired or that can be removed from the yoke (and e.g. be replaced by a neutral piece of yoke material).
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. These embodiments will be described by way of example with the help of the accompanying drawings in which:
Like reference numbers or numbers differing by integer multiples of 100 refer in the Figures to identical or similar components.
The detection of nucleic acids in a biological fluid requires a series of processing steps, such as sample enrichment, cell lysis, DNA isolation and amplification. Since the target analyte is often only available in trace amounts, large sample volumes are needed to collect a statistically sufficient amount of molecules. In such an environment, the detection is hampered by the background noise originating from other constituents of the sample, such as blood cells or cell debris. Hence, it is desirable to extract the available target molecules and to introduce them into a smaller volume, thus effectively enhancing their concentration. As a result, the requirements imposed by the detection limit of the subsequent sensing processes can be met.
Moreover, the processable sample volume of a bio sensor is ideally not larger than several microliters such that the typical characteristics of a microfluidic device, e.g. low consumption of reagents and rapid reaction kinetics, can be realized. However, lowly concentrated samples of this size might not contain enough target molecules to enable reliable detection results.
In a biosensor based on magnetic particles (beads), the target molecules (e.g. nucleic acids) may be caught from an initial volume by specific or non-specific attachment to the surface of said beads. In an enrichment step, an external magnetic field may then be used to collect the particles from the initial volume and transfer them to a confined region, thereby increasing their local concentration and preparing them for further processing.
In such a biosensor based on magnetic beads, the technological challenge arises from the typically large initial volume of the sample to be purified, which is here assumed to be at least 1 ml. Previous technological solutions towards the directed movement of magnetic beads commonly handle considerably smaller fluid volumes and cannot be easily adapted to the desirable sample size because the range of the generated magnetic forces is insufficient (cf. A. Rida, V. Fernandez, and M. A. M. Gijs, “Long-range transport of magnetic microbeads using simple planar coils placed in a uniform magnetostatic field”, Applied Physics Letters, vol. 83, no. 12, pp. 2396-2398, 2003; J. Joung, J. Shen, and P. Grodzinski, “Micropumps based on alternating high-gradient magnetic fields”, IEEE Transactions on Magnetics, vol. 40, no. 4, pp. 1944-1946, 2004). Other known designs for the purification of sample volumes by using magnetic beads feature numerous moving parts and are therefore not robust enough for hand-held solutions (EP 1 621 890 A1).
For the above reasons, efficient sample purification is considered a vital feature of future biosensor applications. It is therefore desirable to develop a magnetic actuator that fulfils as many as possible of the following requirements:
To meet the above requirements, a preparation apparatus is proposed here in which the actuation unit consists of a magnetic circuit comprising an air gap and at least one magnetic field generator, e.g. a field coil. At least one of the pole tips of the apparatus has a tapered shape such that a region of least distance exists between the pole tips. During operation of the apparatus, the magnetic flux density between the pole tips exhibits a maximum at the position of least distance. If a fluid sample containing magnetic beads in suspension is introduced into the air gap, the gradient of the magnetic field will elicit the migration of particles towards the maximum of the magnetic field.
While the second pole 112 has a flat surface that is perpendicular to the yoke axis in this branch (z-direction), the first pole 111 is tapered (wedge shaped) with a single tip T at one end. The distance between points on the surface of the first pole 111 and the second pole 112 hence decreases from a maximum value δmax to a minimal value δmin, which is assumed at the tip T (it should be noted that this distance is defined asymmetrically, i.e. considering single points on the surface of the first pole in relation to the whole second pole). The width of the first and second poles 111, 112 in x-direction is w. Assuming a square cross section, the same value w describes the depth of the poles in y-direction. From the values δmin, δmax, and w, the slope angle α of the first pole 111 can be calculated by
Analysis shows that this angle a of slope is directly proportional to the achievable force on a particle between the poles.
V=w
2δmin
(neglecting the wall thickness of the sample cartridge). This volume V preferably has a value of about 1 ml.
During operation of the preparation apparatus 100, the magnetic particles 1 are moved by the magnetic field gradient towards the point T of least distance between the poles 111, 112. Since it is desirable to integrate the sample enrichment with subsequent stages of the analytical process (e.g. a process according to WO 2008/155716), it has to be possible to readily remove beads from the sample cartridge 2. As shown in the Figure, it is therefore favorable to place the collection area at the outer border of the sample cartridge 2.
The shape of the poles 111, 112 is optimized with respect to the achievable traversal time of a single magnetic bead. To this end, the following boundary conditions can be assumed:
The maximum width δmax of the sample space 115 is then fixed to a value that guarantees the magnetic flux density Bmin at the given electrical excitation N·I.
Under these premises, the values for δmin and w may be varied under the condition that the available volume V for the box-shaped cartridge 2 remains constant, and that the total travel time Tbead a bead needs for the transversal migration through the whole sample space (i.e. across distance w) is minimal.
While a box-shaped, cuboid cartridge 2 has been assumed for the optimization, the implementation of a specifically shaped cartridge that exactly fits into the sample space 115 will allow for larger sample volumes V. The determined optimum values are expected to be approximately invariant to such a change of the shape of the cartridge.
By assigning a time constant to the enrichment process, the performance of the system with respect to changes of the parameters actuation current, particle concentration, pole tip geometry and bead type could be quantified. The results show that the enrichment of a typical sample consisting of an aqueous solution with 2.8 μm large magnetic beads at a concentration of 106 per ml could be enriched in less than 5 minutes at a power consumption of less than 5 W.
While the invention was described above with reference to particular embodiments, various modifications and extensions are possible, for example:
Finally it is pointed out that in the present application the term “comprising” does not exclude other elements or steps, that “a” or “an” does not exclude a plurality, and that a single processor or other unit may fulfill the functions of several means. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Moreover, reference signs in the claims shall not be construed as limiting their scope.
Number | Date | Country | Kind |
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09165750.2 | Jul 2009 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2010/053176 | 7/12/2010 | WO | 00 | 3/20/2012 |