Device for Dielectrophoretic Capture of Particles

Abstract

The disclosure relates to a device for dielectrophoretic capture of particles. The device includes at least one electrical contact and at least one layer. The at least one layer includes a top layer side, a bottom layer side, and a barrier structure. The barrier structure is configured such that a fluid comprising the particles can flow through the barrier structure which is disposed on the top layer side. The barrier structure spaces the top layer side apart from the bottom layer side of at least one of the same layer and a second of the at least one layer.

Claims

1. An apparatus for dielectrophoretic trapping of particles, comprising; at least one electrical contact; at least one layer, the at least one layer including a top layer side, a bottom layer side, and a barrier structure, wherein the barrier structure is configured such that a fluid comprising the particles can flow through the barrier structure, is disposed on the top layer side, and spaces the top layer side apart from the bottom layer side of at least one of the same layer, or of and a second of the at least one layer.

2. The apparatus as claimed in claim 1, wherein the at least one layer is coiled.

3. The apparatus as claimed in claim 2, wherein the at least one layer is coiled so as to form a fluid channel with the barrier structure disposed therein between the top layer side and the bottom layer side of the at least one layer.

4. The apparatus as claimed in claim 1, wherein the at least one layer comprises two or more stacked layers.

5. The apparatus as claimed in claim 4, wherein the two or more stacked layers are stacked so as to form a fluid channel with the barrier structure disposed therein between the top layer side of a first of the two or more stacked layers and the bottom layer side of an adjacent layer of the two or more stacked layers.

6. The apparatus as claimed in claim 1, wherein the barrier structure is an insulator structure.

7. The apparatus as claimed in claim 1, wherein the barrier structure is an electrode structure.

8. The apparatus as claimed in claim 1, further comprising: at least one of an electrical passivation, and an electrical insulation.

9. The apparatus as claimed in claim 1, further comprising at least one electrode that extends at least partly over the at least one layer.

10. A process for producing an apparatus for dielectrophoretic trapping of particles, comprising: providing at least one electrical contact providing at least one layer, the at least one layer including a top layer side, a bottom layer side, and a barrier structure, wherein the barrier structure is configured such that a fluid comprising the particles can flow through the barrier structure, is disposed on the top layer side, and spaces the top layer side apart from the bottom layer side of at least one of the same layer and a second of the at least one layer; and at least one of coiling the at least one layer, and stacking at least a first and a second of the at least one layer.

Description

[0037] The solution presented here and the technical field thereof are elucidated in detail hereinafter with reference to the figures. It should be pointed out that the working examples shown are not intended to restrict the invention. More particularly, unless explicitly stated otherwise, it is also possible to extract some aspects of the matter elucidated in the figures and combine them with other constituents and/or findings from other figures and/or the present description. The figures show, in schematic form:

[0038] FIG. 1: a layer of an apparatus proposed here in a section view,

[0039] FIG. 2: an apparatus proposed here in a section view,

[0040] FIG. 3: a layer according to FIG. 1 or from FIG. 2 in a perspective view,

[0041] FIG. 4: a further layer for the apparatus proposed here in a perspective view,

[0042] FIG. 5: a detail view of the working example according to FIG. 4,

[0043] FIG. 6: a sequence of a process proposed here,

[0044] FIG. 7: an illustration of a step of the process proposed here,

[0045] FIG. 8: an illustration of a further step of the process proposed here,

[0046] FIG. 9: a further apparatus proposed here in a perspective view, and

[0047] FIG. 10: a further apparatus proposed here in a perspective view.

[0048] With regard to the technical field of the solution presented here, which also relates to an apparatus for dielectrophoretic trapping of particles, the following may be stated:

[0049] The underlying mechanism, called dielectrophoresis (DEP), refers to the movement of (even uncharged) polarizable particles in a nonhomogeneous electrical field. A dipole induced in the particle as a result of an electrical alternating field applied from the outside interacts here with that same external field and leads to a dielectrophoretic force acting on the particle.

[0050] If only the first-order dipole moment is taken into account, and all other higher-order terms and the force acting on charged particles in the form of the coulombic term (electrophoresis) are neglected, the time-averaged dielectrophoretic force on a particle can be formulated in the most general case for a spatially stationary electrical field as

[{right arrow over (F)}.sub.DEP]=Γ.Math.ε.sub.m.Math.Re({tilde over (f)}.sub.CM).Math.{right arrow over (Δ)}|{right arrow over (E)}.sub.RMS|.sup.2

[0051] Γ here denotes the geometry factor of the particle, ε.sub.m the (absolute) real electrical permittivity of the surrounding medium, {right arrow over (E)}.sub.RMS the effective value of the electrical field vector applied (root mean square, RMS), and Re({tilde over (f)}.sub.CM) the real part of the “Claudius-Mosotti factor” (CM factor).

[0052] In the simplest case of a spherical particle, representative of a tumor cell by way of example, given that

[00001]${\tilde{f}}_{\mathrm{CM}}=\frac{{\widetilde{\mathrm{.Math.}}}_p-{\widetilde{\mathrm{.Math.}}}_m}{{\widetilde{\mathrm{.Math.}}}_p+2{\widetilde{\mathrm{.Math.}}}_m}$$\mathrm{and}$$\Gamma =2\pi R^3$

[0053] this expression can be rewritten as

{right arrow over (F)}.sub.DEP=2πε.sub.m.Math.Re({tilde over (f)}.sub.CM).Math.R.sup.3.Math.{right arrow over (Δ)}|{right arrow over (E)}.sub.RMS|.sup.2.

[0054] R here represents the radius of the cell in question, and {tilde over (ε)}.sub.p and {tilde over (ε)}.sub.m represent the (absolute) complex electrical permittivity of particle and surrounding medium, where, moreover,

[00002]$\widetilde{\mathrm{.Math.}}=\mathrm{.Math.}-j\frac{\sigma }{\omega }$

[0055] with j=√{square root over (−1)} as the complex unit, σ as the electrical conductivity, and ω as the angular frequency of the electrical field applied.

[0056] According to the sign of Re({tilde over (f)}.sub.CM) (depending on the operating point of the electrical field and the relative match between frequency-dependent (absolute) real electrical permittivity ε and electrical conductivity σ between medium and material), for manipulation, either an attractive (positive dielectrophoresis, pDEP) or repulsive (negative dielectrophoresis, nDEP) force may be caused to act on particles.

[0057] This is of interest particularly when, for example by virtue of external limitations such as undefined flow conditions in microfluidic channels or lack of space in the DEP system, no continuous separation (equilibrium approaches) is achieved in a flowing field, and instead all that can be monitored is the trapping (inequilibrium approaches) of target particles therein either by metal electrodes (mDEP) or insulating posts (iDEP). The latter approach is based on the fundamental principle that particles to be isolated by pDEP are addressed and held by electrodes, in the face of the flow forces of the flowing medium, whereas unwanted particles are simultaneously repelled by nDEP.

[0058] For the mode of function of DEP manipulation, in particular in the design of the concept, the last factor {right arrow over (Δ)}=|{right arrow over (E)}.sub.RMS|.sup.2 in particular in the above expression for ({right arrow over (F)}.sub.DEP) is of significance, which occurs independently of the material, shape and size of the target particle. As well as the amplitude and the distribution of the electrical field in time, it expresses the spatial inhomogeneity thereof. This spatial inhomogeneity may be generated in a microfluidic channel, for example, via suitable structuring of microelectrodes in the channel and direct application of a corresponding electrical signal thereto (metal-based dielectrophoresis, mDEP), or (alternatively) via appropriately designed insulator structures in the channel and an externally applied electrical field (insulator-based dielectrophoresis, iDEP). In the case of mDEP, deformation of the electrical field to the approximately planar electrode edges would be observed, and, in the case of iDEP, around the insulating extruded structures as a result of deformation.

[0059] If the DEP system is not (or not only) designed for continuous separation in a flowing fluid but (as here) for trapping of target particles (assuming that the operating point via the electrical field is more particularly set in such a way that a sufficiently high pDEP can act on all target particles), both variants especially have the aim of using appropriate dimensions of {right arrow over (Δ)}=|{right arrow over (E)}.sub.RMS|.sup.2 to configure the spatial energy landscape for particles in such a way that energy minima caused (energy minimum for particles, since pDEP) retain only target particles in the face of any other energies that exist in the system within the scope of fixed boundary conditions (throughput, destruction of the cell, recovery and purity rates of the separation, etc.), while all other species that occur in the medium remain very substantially unaffected by this action (force acting through DEP either positive and very small or even negative).

[0060] The meaning of the term “energy minima” is elucidated in detail hereinafter: in the case of pDEP (attractive force), particles are generally moved in the direction of the maxima of the electrical field strength. But these regions correspond to minima in an energy landscape, called “potential wells”. Another way in which this can be described is that, in the case of pDEP, the particles move in the direction of higher field strength, but fall into a “potential well” or into “potential wells”. “Energy minima” are understood here more particularly to mean the minima described in the energy landscape, or the potential wells described. What this means more particularly, in other words, is that the energy minima are minima in the energy landscape and/or potential wells.

[0061] The (above-described) principle of particle trapping by means of pDEP is an attractive manipulation approach which is preferably pursued in the context of the solution presented here.

[0062] In conventional implementations of mDEP or iDEP trapping separators, it was possible to observe the following:

[0063] On account of the generally very small range of the dielectrophoretic force, in particle trapping, however, maximum distances between the electrodes and the particles should generally be observed between the electrodes and the particles (up to 100 μm), but this can conversely result in limited channel dimensions, at least in vertical direction, and hence comparatively low throughputs. These can be increased only to a limited degree by increasing the flow rate, since the resulting flow forces should never be dominant over the dielectrophoretic forces (in the pN range) or damage the cells (corresponds as an equivalent to maximum flow rates in conventional DEP systems up to about 100 μm/s). The result would be that the particles trapped would be rinsed away immediately, and a severe drop in separation efficiency would be expected.

[0064] Alternatively, it would be possible to increase the cross section and hence the throughput in conventional designs also by significantly broadening the channel in horizontal direction, but the extent of such a channel here too is highly limited by the typically limited size of the DEP system.

[0065] Against this background, a brief calculation example is to be presented hereinafter for a conventional DEP trapping filtration (which is compared further down with a calculation example for an embodiment of the solution presented here):

[0066] If it were necessary, for liquid biopsy applications, for example, to process a blood sample of size 10 ml within one hour in a channel of height 50 μm at a fluid velocity of 100 μm/s, it would be necessary to accept an effective channel width of more than 55 cm (the cross-sectional area for flow without barriers is then about 28 mm.sup.2), but this would be much too unwieldy from a microfluidic point of view.

[0067] Proceeding from this, it is a particular aim of the invention to redistribute a channel which is forced to be very flat and broad by the small range of the dielectrophoretic force, which would conventionally take up too great a footprint for filter operation in the form of dielectrophoretic particle trapping at sufficiently high throughput, to a volume that can be managed with maximum efficiency. In this case, for example, the extent of a quasi-planar DEP structure may be extended by a third spatial dimension in such a way that compression to a maximum cross-sectional flow area can be provided. The arrangement is especially capable of giving the target cells a minimum interaction distance (high gradients of the electrical field or high DEP forces), but a sufficiently long interaction distance with the DEP electrodes, coupled with otherwise advantageous low flow rates, but advantageously sufficiently high throughput.

[0068] FIG. 1 shows a schematic of a layer 4 for an apparatus proposed here in a section view. The layer 4 has a top layer side 6, a bottom layer side 7 and a barrier structure 8. A fluid comprising the particles 2, 3 (not shown here) can flow through the barrier structure 8. In addition, the barrier structure 8 is disposed on the top layer side 6.

[0069] In the execution variant according to FIG. 1, the layer 4, by way of example, is formed in the manner of a DEP film. The layer 4 here is formed by a sandwich arrangement composed of two insulator strata 13 and a metal electrode 12 embedded therein over the whole area. The insulator strata 13 constitute an example of how a layer 4 can have an electrical insulation 11. The counterelectrodes form extruded metal posts that are applied to one of the two insulator strata 13 and are connected to one another at the base by flat conductor tracks 14 (not shown here; cf. FIG. 3) (an exchange of polarities takes place, for example, in the chessboard pattern between the posts). The extruded metal posts constitute an example of how the barrier structure 8 can be executed as electrode structure.

[0070] Furthermore, the layer 4 in the diagram according to FIG. 1, by way of example, has electrical passivation 10 over the whole area of the strip. This electrical passivation 10 may be formed, for example, by a chemically inert material of, however, maximum electrical transparency. In addition, the layer 4 in FIG. 1, by way of example, has a thin adhesive stratum 15 on the reverse side of the strip or on the bottom layer side 7. This adhesive stratum 15 could ensure ultimate strength in the case of stacking, and integrity of the microchannels in later operation.

[0071] FIG. 2 shows a schematic of an apparatus 1 proposed here in a section view. The reference numerals are used uniformly, and so reference may be made in full to the above details relating to FIG. 1.

[0072] The apparatus 1 is set up for dielectrophoretic trapping of particles 2, 3 (not shown here). The apparatus 1 comprises one or more layers 4 and electrical contacts 5 (not shown here). The layers 4 each have a top layer side 6, a bottom layer side 7, and a barrier structure 8. A fluid comprising the particles 2, 3 (not shown here) can flow through the barrier structure 8. The barrier structure 8 is disposed on the top layer side 6. In addition, the barrier structure 8 spaces the top layer side 6 apart from the bottom layer side 7 of the same layer 4 or one of the further layers 4.

[0073] The arrangement according to FIG. 2 may be formed, for example, by stacking multiple layers 4 according to FIG. 1 or alternatively by coiling a layer 4 according to FIG. 1. In this connection, the arrangement may be similar to a coplanar conduit. The effective cross-sectional area of a theoretical microfluidic channel can be defined via the distances and heights of the individual posts. Once rolled up, the barrier structure 8 or the stratum having the metal posts is surrounded on both sides, insulated by two layers 4 of a planar electrode 12, and hence fully shielded from adjacent barrier structures 8 or (barrier structure) strata—winding or stacking is thus enabled in a particularly advantageous manner. This is especially manifested in a symmetrical and uniform electrical field 16 that can be discovered in each of the “cages” (component microfluidic channels 17).

[0074] However, if crosstalk between two layers 4 of the DEP strip were negligible in operation, it would also be possible to omit the shielding for simplified production and operation. All that could remain by way of example could be just one insulator stratum 13 to which the metal posts (barrier structure 8) together with conductor tracks 14 (not shown here; cf. FIG. 3) and contacts 5 (not shown here; cf. FIG. 3) could be applied.

[0075] In all cases, it would be possible for incoming particles (target cells) to pass through the stacked barrier structures 8 and advantageously to interact as strongly as possible with the inhomogeneous electrical field 16. The summation of many theoretical small component microfluidic channels 17 across the width of an overall strip, in the stacked state, results in a parallel connection to form a channel having acceptable effective cross-sectional area.

[0076] It is also apparent in FIG. 2 that a fluid channel 9 with the barrier structure 8 disposed therein has been formed between the top layer side 6 of a bottom layer side 7 facing it. The sum total of these fluid channels 9 or of the component microfluidic channels 17 results in a (total) flow cross-sectional area of the apparatus, which has also been referred to as effective cross-sectional area above.

[0077] FIG. 3 shows, in schematic form, a layer 4 according to FIG. 1 or from FIG. 2 in a perspective view. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the previous FIGS. 1 and 2.

[0078] FIG. 4 shows, in schematic form, a further layer 4 for an apparatus proposed here in a perspective view. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0079] The configuration according to FIG. 4 differs from that according to FIGS. 1 to 3 especially in that the barrier structure 8 here is not an electrode structure but an insulator structure. In this case, instead of the metal posts, insulating spacers (posts) are used as barrier structure 8 with planar metal electrodes 12 applied (on one or both sides). It is particularly advantageous here when (owing to possible crosstalk of fields of adjacent layers 4) exact adjustment is additionally ensured on stacking or winding.

[0080] FIG. 5 shows, in schematic form, a detailed view of the working example according to FIG. 4. The corresponding detail section is marked by IV in FIG. 4. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0081] FIG. 6 shows, in schematic form, a sequence of a process proposed here. The process serves for production of an apparatus proposed here. The sequence of process steps shown with blocks 110 and 120 is established in a regular operating sequence. In block 110, one or more of the layers are provided. In block 120, at least one of the layers is coiled or two or more of the layers are stacked.

[0082] FIG. 7 shows, in schematic form, an illustration of a step of the process proposed here. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0083] FIG. 7 illustrates, in this connection, provision of a layer 4. The layer 4 is held by way of example by a securing means 18 on a carrier roll 19. The securing means 18, for this purpose, is at the same time formed by way of example in the manner of a spacer.

[0084] FIG. 8 shows a schematic of an illustration of a further step of the process proposed here. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0085] FIG. 8, in this connection, illustrates coiling of the layer 4 provided in FIG. 7. In this case, by way of example, a layer 4 (suitably a structured film arrangement) is wound onto a carrier roll 19 and electrically contacted at the end. The effective flow cross-sectional area of such a “coiled DEP cylinder” can be calculated from the outer areas of the rolled-up strip and of the carrier present therein, and varies according to the layout. The diameter of the carrier 19 may be minimized in favor of a maximum throughput. Such a cylinder would be integratable without any great difficulty into a likewise cylindrical channel (cf. FIG. 9).

[0086] FIG. 9 shows, in schematic form, a further apparatus 1 proposed here in a perspective view. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0087] The apparatus 1 could have been produced, for example, by the process steps illustrated in FIG. 7 and FIG. 8. What this means more particularly, in other words, is that the apparatus 1 as illustrated in FIG. 9 takes the form of a “coiled cylinder”.

[0088] The advantages of this embodiment are to be discussed hereinafter with reference to a calculation example for a coiled cylinder:

[0089] A layer 4 (DEP strip), for example according to FIG. 1, of thickness 100 μm (substrate thickness 50 μm and post height 50 μm, at a ratio of post width to post separation of 1:1) and length of approximately 1 m, could be rolled up through 35 windings on a roll 19 of diameter 6 mm to form a cylinder having a total diameter of less than 13 mm. The length of such a coiled cylinder could be chosen individually (e.g. 1 cm). A 10 ml blood sample could then likewise be processed within approximately one hour at a maximum flow rate of 100 μm/s, with the major difference (from the calculation example set out above for a conventional DEP trapping filtration) that such a filter could then be incorporated relatively simply into lab-on-a-chip systems.

[0090] Such a filter could be produced by microscale fabrication technology: the insulator strata 13 could be produced with insulator films. The insulator films could be thin, spin-coated polyimide films having a thickness of up to 25 μm, for example. A carrier roll 19 of plastic, for example, could have a bending radius of down to one millimeter or less. The film and roll could be joined to one another by an adhesive tape of suitable height, which could simultaneously also serve as spacer and protection for the posts (barrier structure 8) in a first wrap. A suitable electrode material could be a metal, for example copper or gold, which has been structured and applied (beforehand), for example, by photolithography, sputtering methods and/or electroplating, especially in different heights for conductor tracks 14 and metal posts (of an illustrative barrier structure 8). Conductor tracks 14 here could have thicknesses between a few nanometers up to a few micrometers, and posts of an illustrative barrier structure 8 could have heights of possibly up to 100 μm. For electrical contacting 5 of the layer 4, solder contacts would be conceivable. If the intention were to passivate the metal electrodes electrically with chemically inert material (as an illustrative passivation 10), it would be possible for this purpose to apply aluminum oxide by vapor deposition, for example.

[0091] A similar effect would also be achievable by stacking of multiple layers 4 to form a “DEP stack”, especially with defined trap cross section.

[0092] FIG. 10 shows a schematic of a further apparatus 1 proposed here in a perspective view. The reference numerals are used uniformly, and so reference may be made in full to the above remarks relating to the preceding figures.

[0093] FIG. 10 illustrates, by way of example, the aforementioned execution as a “DEP stack”. In other words, this relates more particularly to an apparatus 1 in which two or more of the layers 4 are stacked.

[0094] It would be possible in principle to produce layers or “DEP strips”, as presented above, in any widths and lengths and, as a result, individually to form cylinders and stacks of any diameters, lengths, widths and heights.

[0095] The solution proposed here especially has one or more of the following advantages: [0096] High degree of parallelization of individual microfluidic channels with precisely adjustable dimensions and field strengths, which enables efficient exploitation of the dielectrophoretic trap volume: increase in the effective cross-sectional flow area or reduction in the relative flow rate in the fluidic channel in compact form achievable, with particles keeping a maximum distance from the electrodes [0097] Various layout options, since a large selection of design parameters is available (particularly with regard to length and width of the DEP strips used, which would be easily adjustable) [0098] Process potentially very inexpensive, since mass production conceivable [0099] Relatively simple in principle, and good integratability into MEMS or microfluidic technologies [0100] Optional operation with passivated metal electrodes (possibly on both sides and extruded) achievable in a simple manner: generation of high field strength gradients even at high frequencies with comparatively low operating voltages without bubble formation, for example, resulting from chemical reactions, etc.