Method and Arrangement for Focusing Objects in a Flow

FIELD OF THE INVENTION

The invention relates to the field of focusing structures for focusing small objects, such as particles or cells, in a flow. More specifically it relates to focusing arrangements which are based on acoustic radiation forces.

BACKGROUND OF THE INVENTION

For many microfluidic applications, flow focusing of particles or cells is an essential step for sample enrichment or separation. This is traditionally accomplished by hydrodynamic focusing using a sheath fluid to guide particles into the center of a channel. To generate a sufficient focusing effect on the particles, sheath fluid flow must, however, be sometimes orders of magnitude greater than the flow of the sample fluid.

In prior art standing wave acoustophoresis systems acoustic radiation forces are used to separate particles or cells from a liquid. In such systems it is possible to separate particles or cells based on their size and density. The migration velocity of particles in standing wave acoustophoresis is proportional to the square of the particle radius, the frequency of the acoustic field and the square of the pressure amplitude.

In reality, however, the frequency and the driving power cannot be infinitely increased to increase the speed of focusing.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide an arrangement for focusing particles or cells in a flow.

The above objective is accomplished by a method and arrangement according to the claims.

In a first aspect, embodiments of the present invention relate to a focusing arrangement for focusing particles or cells in a flow. The arrangement comprises at least one channel, and at least one acoustic confinement structure comprising acoustic field boundaries adapted for confining acoustic fields, at least partially, in the channel.

The channel comprises at least one particle confinement structure comprising particle flow boundaries. The movement space of the particles is limited by these particle flow boundaries. The particle flow boundaries may for example be the walls of the channel or they may for example comprise one or more pillar arrays within the channel which allow to limit the free flow of the particles.

In embodiments of the present invention the acoustic field boundaries are different from the particle flow boundaries.

It is an advantage of embodiments of the present invention that the efficiency of focusing particles can be improved. This is achieved by a focusing arrangement wherein the acoustic field boundaries are different from the particle flow boundaries.

Design of the acoustic confinement structures may for example be such that resonance conditions are satisfied, and/or such that specific pressure fields in the acoustic confinement structures can be obtained. The particles may for example be focused towards the center of the flow. Thus, a better and/or more efficient focusing of the particles in the flow in the channel may be obtained. For these designs it is advantageous that the acoustic field boundaries are different from the particle flow boundaries.

It is an advantage of embodiments of the present invention that focusing of particles or cells in parallel channels is enabled. It is moreover an advantage that focusing arrangements according to the present invention are scalable with regard to the number of channels.

In case particles or cells are flowing in parallel channels, the focusing of the particles in the flow is improved by appropriate design of the acoustic confinement structures. Thereby the resonance conditions of the acoustic confinement structures are tuned depending on the position on the chip. It is an advantage of embodiments of the present invention that the acoustic confinement structures are designed such that it is possible to align particles synchronously in different channels. It may for example be possible to have a similar or even the same acoustic field distribution in the different channels. In embodiments of the present invention it may also be avoided that the acoustic field of an acoustic confinement structure of a first channel is disturbing the focusing of the particles in the second channel. It is an advantage of multi-channel focusing arrangements according to the present invention that a higher throughput can be achieved than when using only one channel.

In case particles are flowing in one channel through consecutive acoustic fields generated by consecutive acoustic confinement structures, the focusing of the particles in the flow is improved by designing the acoustic confinement structures depending on their position in the sequence. The first acoustic confinement structure may for example be designed such that in operation the acoustic field in this structure forces the particles away from the side walls of the channel. The second acoustic confinement structure may for example be designed such that in operation the acoustic field in this structure forces the particles to the center of the channel.

In embodiments of the present invention the focusing arrangement comprises two channels wherein each channel comprises an acoustic confinement structure adapted for confining acoustic fields, at least partially, in that channel.

It is an advantage of embodiments of the present invention that each channel comprises an acoustic confinement structure and that the acoustic confinement structures are adapted for confining the acoustic field in the channel. This allows to design the acoustic confinement structures depending on their position in the focusing arrangement. The design of the acoustic confinement structure may for example define the resonance condition for the acoustic wave within the acoustic confinement structure.

In embodiments of the present invention the acoustic confinement structure may be designed to obtain a specific amplitude and phase of the generated acoustic field. In embodiments of the present invention cross-channel interference is compensated for when designing the acoustic confinement structures. In embodiments of the present invention the acoustic confinement structures are comprising a generator for generating the acoustic field. In these embodiments the focusing arrangement may be adapted for controlling the generators to control the acoustic fields with regard to each other.

In embodiments according to the present invention the focusing arrangement comprises an acoustic barrier wherein the acoustic barrier is present as the acoustic confinement structure.

It is an advantage of embodiments of the present invention that the focusing arrangement comprises an acoustic barrier to avoid that the acoustic field of one of the acoustic confinement structures interferes with the acoustic field of the other acoustic confinement structure. The acoustic barrier may prevent that the acoustic field within the first acoustic confinement structure is disturbing the focusing of the particles in the second channel and vice versa. Thereby the cross-channel interference is reduced.

In embodiments according to the present invention at least one acoustic confinement structure comprises acoustic barriers adapted to reflect a travelling acoustic wave.

In embodiments of the present invention the acoustic barrier of an acoustic confinement structure allows to define the resonance condition for the acoustic waves within the acoustic confinement structure. In embodiments of the present invention the uniformity of the acoustic field may be improved by strengthening the resonance of one individual channel as a result of acoustic barriers which will reduce or even make the coupling with the remaining part of the chip negligible.

In embodiments of the present invention at least one channel comprises two consecutive acoustic confinement structures.

It is an advantage of embodiments of the present invention that the focusing of the particles in the flow can be improved by controlling the acoustic confinement structures with regard to each other. The first acoustic field may for example have control on the inlet of the focusing channel.

In embodiments of the present invention the focusing arrangement comprises a particle confinement structure adapted for restricting the flow of the particles in the channel. The particle confinement structure comprises particle flow boundaries which may or may not be different from the channel walls. In embodiments of the present invention the acoustic field boundaries are different from the particle flow boundaries.

It is an advantage of embodiments of the present invention that the flow of the particles in the channel is restricted. This may for example cause the particles to be moved away from the side wall of the channel. By moving away the particles from the side wall they can be better focused by an acoustic confinement structure in a later stage in the channel. It is an advantage of embodiments of the present invention that the focusing of particles can be accelerated by shortening the migration path and increasing the force applied on the particles.

In embodiments according to the present invention the particle confinement structure comprises a pillar array.

It is an advantage of embodiments of the present invention that the pillar array is not disturbing the acoustic field or the liquid flow and that the pillar array can restrict the flow of the particles.

In embodiments of the present invention the particle confinement structure comprises an acoustic transparent layer.

It is an advantage of embodiments of the present invention that the particle flow is restricted and that the acoustic field is not disturbed by the acoustic transparent layer. In embodiments according to the present invention the channel walls may be polymer walls and the acoustic confinement structures may comprise acoustic field boundaries to modify the resonance condition of the acoustic wave and improve the focusing effect on the particles.

In a second aspect embodiments of the present invention relate to a diagnostic device which comprises:

- a focusing arrangement according embodiments of the present invention, for focusing cells or bioparticles in a flow,

The diagnostic device may for example comprise a module for determining a quality and/or quantity of the focused bioparticles or cells and for providing an output based thereon on which a diagnosis can be based.

In a third aspect embodiments of the present invention relate to an industrial inspection device for monitoring a liquid flow comprising cells or particles. The industrial inspection device comprises:

- a focusing arrangement according to embodiments of the present invention, for focusing cells or particles in a flow.

The industrial inspection device may for example comprise a module for determining a quality and/or quantity of the focused particles or cells and for providing an output based thereon for characterizing the liquid flow.

In a fourth aspect embodiments of the present invention relate to a method for focusing particles or cells in a flow. The method comprises limiting movement of the particles using particle flow boundaries resulting in a confined flow of particles, focusing the particles in the confined flow of particles using an acoustic confinement structure comprising acoustic field boundaries wherein the acoustic field boundaries are different from the particle flow boundaries.

In embodiments according to the present invention the particles in a first channel are focused by applying a first acoustic field and the particles in a second channel are focused using a second acoustic field.

It is an advantage of embodiments of the present invention that it is possible to focus particles in parallel channels.

In embodiments according to the present invention the method comprises two steps for focusing particles consecutively in a channel by consecutive acoustic fields wherein in a first step the particles are moved away from the channel wall by a first acoustic field and wherein in a second step the particles are focused in the channel by a second acoustic field. In embodiments according to the present invention the particles may for example be moved towards the center of the channel.

In embodiments according to the present invention the method comprises a step for limiting the flow of the particles using particle flow boundaries different from the channel boundaries before focusing the particles using an acoustic field.

In embodiments according to the present invention the flow of the particles is limited using a pillar array or using an acoustic transparent layer.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of a focusing arrangement comprising parallel channels in accordance with embodiments of the present invention.

FIG. 2 shows a schematic drawing of one acoustic confinement structure of a focusing arrangement comprising matched walls and a barrier in accordance with embodiments of the present invention.

FIG. 3 shows the pressure field in function of the location in an acoustic confinement structure of a focusing arrangement in accordance with embodiments of the present invention.

FIG. 4 shows a schematic cross-section of a focusing arrangement comprising two parallel channels in accordance with embodiments of the present invention.

FIG. 5 shows a schematic drawing of a focusing arrangement comprising three consecutive local acoustic confinement structures in accordance with embodiments of the present invention.

FIG. 6 shows a channel comprising a pillar array in accordance with embodiments of the present invention.

FIG. 7 shows the cross-section of a channel comprising an acoustic transparent layer in accordance with embodiments of the present invention.

FIG. 8 shows the top view of a channel comprising an acoustic transparent layer in accordance with embodiments of the present invention.

FIG. 9 shows the velocity of a laminar flow in a channel.

FIG. 10 shows the acoustic pressure field in a channel.

FIG. 11 shows the acoustic radiation force in a channel.

FIG. 12 shows the velocity of a laminar flow in a channel comprising micro pillars in accordance with embodiments of the present invention.

FIG. 13 shows the acoustic pressure field in a channel comprising micro pillars in accordance with embodiments of the present invention.

FIG. 14 shows the acoustic radiation force in a channel comprising micro pillars in accordance with embodiments of the present invention.

FIG. 15 shows an analytical simulation of transient particle trajectories in a channel with or without micro pillars in accordance with embodiments of the present invention.

FIG. 16 shows a conventional channel wherein the wall of the channel serves as an acoustic confinement structure and as a particle confinement structure.

FIG. 17 shows the pressure field and acoustic radiation force in the channel of FIG. 16.

FIG. 18 shows a finite element analysis (COMSOL simulation) of the pressure field in a conventional five channel chip.

FIG. 19 shows a cross-section of a micromachined ultrasonic transducer designed to launch a transversal acoustic wave through a focusing arrangement in accordance with embodiments of the present invention.

Any reference signs in the claims shall not be construed as limiting the scope. In the different drawings, the same reference signs refer to the same or analogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device or arrangement comprising means A and B” should not be limited to devices or arrangements consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device or arrangement are A and B.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

Where in embodiments of the present invention reference is made to flow focusing, reference is made to the 2D or 3D confinement of the flowing particles or cells in a channel or micro-channel. Flow focusing is an essential element for many microfluidic techniques, for example cell separation, enrichment or sorting.

In a first aspect, embodiments of the present invention relate to a focusing arrangement for focusing particles or cells in a flow, also referred to as flow focusing. Therefore the arrangement comprises at least one channel, and at least one acoustic confinement structure comprising acoustic field boundaries adapted for confining acoustic fields, at least partially, in the channel. The channel comprises at least one particle confinement structure comprising particle flow boundaries wherein the particle flow boundaries are different from the acoustic field boundaries. In embodiments according to the present invention the channel is a microfluidic channel.

The focusing arrangement may be a microfluidic chip and the at least one channel may be a microchannel. The channel may be a channel in a chip. The channel may have any kind of cross-section (e.g. a rectangular cross section).

In embodiments of the present invention the acoustic confinement structure is adapted for confining an acoustic field in a certain location. Focusing arrangements according to the present invention may comprise a transducer for generating an acoustic field. This generated acoustic field may be confined in the acoustic confinement structures. In embodiments of the present invention, one transducer, driven by one frequency, is sufficient for generating the acoustic fields within the acoustic confinement structures of the focusing arrangement. Thus it is possible to obtain a desired standing wave distribution in different channels. The reason therefore being that the channels have an individual acoustic confinement structure for confining an acoustic field resulting in an individual acoustic field distribution per channel. The standing wave distribution may for example be the same in different channels. In embodiments of the present invention the acoustic fields of the acoustic confinement structures are controlled with regard to each other. The transducer may be a piezoelectric chip which is used as an acoustic source for the focusing arrangement.

In embodiments of the present invention the focusing arrangement may have a planar structure wherein the channel is arranged in the planar structure. Hence, the primary direction of the focusing effect is the transversal direction.

In embodiments of the present invention the transducer may be positioned against the surface of the planar focusing arrangement. Acoustic waves will in that case mainly be launched orthogonal to the surface of the focusing arrangement. Only those waves which are propagated in other directions may result in acoustic waves in the acoustic confinement structure corresponding with the eigenmode of the acoustic confinement structure.

In embodiments of the present invention the transducer may be positioned at the side of the planar focusing arrangement. Thereby transversal acoustic waves can be generated by the transducer; however, the side area is much smaller than the bottom area and therefore it is less efficient to transfer acoustic wave energy through the side.

Therefore, in embodiments of the present invention the transducer may be a micromachined ultrasonic transducer (e.g., capacitive micromachined ultrasonic transducer or piezoelectric micromachined ultrasonic transducer) which is located inside the microchannel. In this case, a microchannel is formed on top of the manufactured micromachined ultrasonic transducer. This configuration should enable a higher acoustic energy delivery efficiency to the microchannel compared to the conventional configuration in which a transducer is placed outside the microchannel. Micromachined ultrasonic transducer is designed to launch a transversal acoustic wave in a more controlled fashion through the focusing arrangement. FIG. 19 shows a cross-section of a possible configuration of such a capacitive micromachined ultrasonic transducer or piezoelectric micromachined ultrasonic transducer 1910 wherein a microchannel 1930 is formed on top of the transducer 1910 and wherein an acoustic hard boundary 1920 (e.g., glass) is present as a wall of the microchannel 1930. Using such a transducer a transversal standing wave acoustic field can be generated in the microchannel. The micromachined transducer may be manufactured on the surface of a silicon or glass substrate 1940, a microchannel 1930 is placed on top of it.

The generated field may be a bulk acoustic wave (BAW). Once the frequency of the acoustic vibration matches the resonance condition for the at least one channel, a standing wave can be generated transversally in each microfluidic channel.

The acoustic confinement structure may be global or local. In the case of local acoustic confinement structure, more than one of such a structure may be present for the same channel and/or in case of more than one channel each channel may have a different acoustic confinement structure.

Focusing arrangements according to the present invention may comprise multiple channels. The focusing arrangement is thereby constructed such that a focusing effect can be realized in every channel.

It is thereby an advantage of embodiments of the present invention that even though the channels are on different position of the chip it is possible to obtain sufficient standing waves for focusing in each channel by one operating frequency. This is possible because of the plurality of acoustic confinement structures. It is an advantage of embodiments of the present invention that it is possible to get the same standing wave and hence the same focusing effect in every channel.

FIG. 1 shows a focusing arrangement 100 in accordance with embodiments of the present invention. Particles or cells 140 are flowing through the channel 110 in a liquid flow 150. In this exemplary embodiment of the present invention the focusing arrangement 100 comprises three parallel channels 110.

In this example the particle flow boundaries 112 are corresponding the walls of the channel 110. As can be seen from this figure the acoustic field boundaries 122 are different from the particle flow boundaries 112.

In such an embodiment with parallel channels the acoustic field of one acoustic confinement structure 120 may be interfering with the acoustic field of the other acoustic confinement structures 120. It is thereby an advantage of embodiments of the present invention that it is possible to cancel or at least suppress the acoustic wave going out of the acoustic confinement structure. Hence, it is possible to reduce coupling effects in a multichannel system with a plurality of acoustic confinement structures. Thereby consistent focusing effect acting on the particles in all of the three channels is possible.

In embodiments according to the present invention acoustic barriers 130 are present between the channels. Such an acoustic barrier can be a low acoustic impedance barrier, correspondingly a soft acoustic field boundary, or a high acoustic impedance barrier (a hard acoustic field boundary). These acoustic barriers result in crosstalk between different channels being suppressed.

FIG. 16 shows a conventional channel 110 wherein the walls 230 of the channel 110 serve as acoustic field boundaries and as particle flow boundaries. In this prior art example the particle flow boundaries and the acoustic field boundaries are the same. The wall of the channel can for example be made of silicon or glass which has a large impedance mismatch with water.

The pressure field 1730 and acoustic radiation force 1740 in a conventional channel are shown in FIG. 17.

Whereas in the conventional channel illustrated in FIG. 16 the particle confinement structure and the acoustic confinement structure are the same, being delimited by the channel 110, in embodiments according to the present invention the particle flow boundaries are different from the acoustic field boundaries.

The particle confinement structure may for example be transparent for the acoustic field. Such a transparent layer 210 is used as material of the walls in the embodiment illustrated in FIG. 2. The transparent layer 210 forms the particle flow boundaries 112. Such a layer may for example be made of polydimethylsiloxane (PDMS) or photo-patternable adhesive (PA material). In that case most of the acoustic energy will be dissipated to the walls since they have an acoustic impedance close to the one of water. In embodiments according to the present invention an acoustic barrier 130 may be added to form the acoustic field boundaries 112. An exemplary embodiment of such a focusing arrangement is illustrated in FIG. 2. FIG. 2 schematically show one acoustic confinement structure 120 of a focusing arrangement in accordance with embodiments of the present invention. In the embodiment illustrated in FIG. 2 the acoustic barriers 130 are used to confine the acoustic wave and thus are comprised in the acoustic confinement structure 120. In embodiments according to the present invention the acoustic confinement structures comprise acoustic field reflectors, absorbers or isolators.

Such a reflector may be an acoustic barrier. This acoustic barrier may be placed adjacent to the channel or in the neighborhood of the channel or may be a cavity used to reflect the traveling acoustic wave.

An acoustic channel or cavity 110 may for example be formed by etching a piece of silicon wafer or glass 230, and then bonding a covering 220 such as a thin glass layer on top of it. The channel or cavity can be made by a thin layer of bonding polymers such as PDMS or PA material. These polymers have a similar acoustic impedance as water.

The acoustic barrier 130 can be either a soft barrier (in that case the acoustic impedance is much smaller than the matched walls) or a hard barrier (in that case the acoustic impedance is much larger than the matched walls). For instance, PDMS can be considered as the matched walls 210 and an air gap can be used as soft barrier 130. Due to the significant acoustic impedance mismatch between air and PDMS, most of the acoustic energy will be reflected by the air/PDMS interface 122 and a pressure node is formed. A standing wave can be generated between two air gaps at a certain frequency, correspondingly, a partial standing wave is obtained in the cavity.

It is an advantage of embodiments of the present invention that the acoustic energy is reflected on the barrier 130 at the acoustic field boundary 122. Thereby the acoustic energy remains in the channel 110 as the waves do not continue into the chip where they would be dissipated and attenuated. Hence it is an advantage of embodiments of the present invention that resonance power within the cavity is increased by adding the barriers 130.

This is illustrated in FIG. 3. FIG. 3 shows the pressure field in function of the location in an acoustic confinement structure of a focusing arrangement in accordance with embodiments of the present invention. The results are based on an Eigen frequency study in COMSOL (which is a finite element analysis solver and simulation software package). The unit of the Y-axis is in Pascale, however, more important is the distribution of the pressure field than the absolute value thereof. In embodiments according to the present invention the waveform of the standing wave can be adjusted according to the location of air gaps and the width of PDMS layers (i.e. by the position of the acoustic field boundaries 122). As will be explained later, such a structure may be used when using consecutive acoustic confinement structures in a channel and thereby controlling the acoustic fields of the acoustic confinement structures with regard to each other, in accordance with embodiments of the present invention.

FIG. 18 shows a finite element analysis (COMSOL simulation) of the pressure field in a conventional five channel chip. It indicates that when the chip takes use of the transversal resonance mode, the pressure field is determined by the resonance of the whole chip. This is extremely evident when the wall of microchannel is made of polymers. The pressure field distribution in an individual channel highly relies on the location of the channel in the chip, which makes it is difficult to drive them together by using one frequency while also focusing the particles in the center of every channel. Therefore it is difficult to achieve a same standing wave distribution by just one eigenmode in a conventional five channel chip. If the center channel works at its resonance mode, the side channel is operated at a frequency (the driving frequency) shifted from its own resonance frequency. In reality, since each mode has a certain bandwidth, it is possible to achieve focusing in all of the channels by one frequency. But it also means that, in different channels, the acoustic radiation force and the corresponding focusing effect will be different.

In embodiments according to the present invention this difference is reduced by increasing the resonance of an individual channel. This is done by providing acoustic confinement structures which are designed to tune the resonance for the different channels and hence make the coupling of the whole chip negligible.

It is therefore an advantage of embodiments of the present invention that the focusing arrangement comprises acoustic confinement structures of which the acoustic field boundaries are different from the particle flow boundaries. FIG. 4 shows a schematic cross-section of a focusing arrangement 100 comprising two parallel channels 110 in accordance with embodiments of the present invention. FIG. 4 shows that the acoustic barriers 130 can be used to suppress the acoustic coupling between different channels 110. The acoustic barriers 130 cut off the acoustic wave travelling in the thin layer of walls 210. Each channel 110 has a local resonance, and the resonance frequency is mainly determined by the space between the acoustic field boundaries 122 of the barriers 130. In this exemplary embodiment of the present invention only a slight frequency variation for different channels 110 at different locations is expected. This allows to have multiple channel focusing by one operating frequency. In embodiments according to the present invention this frequency variation may for example be smaller than 5%. Also in this example the acoustic field boundaries 122 are different from the particle flow boundaries 112.

In embodiments of the present invention the required operating frequency is dominated by the dimension of the channel or by the acoustic confinement structures. This resonance frequency is depending on the position within the channel. If the wall is made by polymers, the resonance in the transversal direction is very weak. Later on, when the channel couples with the whole chip, the resonance frequency may shift significantly. The resonance frequency shift depends on the location of the channel in the chip. However, in embodiments of the present invention where the acoustic confinement structures comprise barriers to strengthen the resonance, the resonance frequency is dominated by the space between barriers. The reason therefore being that the barriers have a larger impedance mismatch with the fluid (i.e., water) than the matched material of the walls of the channel. Therefore there will be a stronger resonance for an individual channel.

In a focusing arrangement with a plurality of channels a non-uniformity may exist between the resonance of the different channels which can be resolved using acoustic confinement structures in accordance with embodiments of the present invention.

This can be illustrated using the following example.

Consider a focusing arrangement in which the actual resonance frequency of a first channel is 10-41, wherein 10 is the nominal frequency which is determined by the acoustic confinement structure at that position, and wherein f1 is the shifted frequency which is determined by the location of the channel in the chip (this shift originates from the coupling between the region within the acoustic confinement structure and the remaining part of the whole chip, which is expected from the coupled mode theory). The actual resonance frequency of the second channel is f0′+f2, the actual resonance frequency of the third channel is f0″+f3, and so on. Wherein the frequency shift f1, f2, f3 is depending on the location of the channel. Therefore in embodiments of the present invention the acoustic confinement structures are dimensioned such that the actual resonance frequencies are adapted. In practice, f0, f0′ and f0″ are approximately equal for an identical space between acoustic field boundaries of an acoustic confinement structure; in embodiments of the present invention the resonance of the channel can be strengthened, thereby reducing the coupling effect compared to the resonance effect (i.e., f1, f2 and f3 are negligible compared to f0, f0′ and f0″).

It is therefore an advantage of embodiments of the present invention that the uniformity of the acoustic field in each channel can be improved. This can be achieved by design of the acoustic field boundaries and is possible because the acoustic field boundaries are different from the particle flow boundaries.

In embodiments according to the present invention the frequency of the standing wave on a certain location is determined by the barriers around the channel and hence also the required operating frequency is determined by the barriers around the channel (i.e. by the position of the acoustic field boundaries). By increasing the barrier distance the required operating frequency will be lowered.

Focusing arrangements for which the acoustic field boundaries coincide with the particle flow boundaries may be confronted with the problem that the force by the acoustic field on the particles nearby the walls of the channel is limited. This is caused by a weak resonance of the acoustic field near the channel wall. Close to the channel walls the force on the particle may even approximate a zero force. This is the case in prior art focusing arrangements as illustrated in FIG. 17 which shows the pressure field 1730 and the acoustic radiation force 1740.

FIG. 5 illustrates a channel 110 comprising consecutive acoustic confinement structures in accordance with embodiments of the present invention. Initially, the particles are randomly distributed in the channel. In zone 1 the acoustic confinement structure 120 comprising the barriers 130 (defining the acoustic field boundaries 122) is designed such that the acoustic radiation force is maximal at the wall of the channel. In zone 2, the acoustic confinement structure 120 is designed such that the acoustic radiation force forces the already centered particle closer to the center. Therefore the barriers 130 may be positioned closer to the center than the barriers in zone 1, such that the maximum acoustic radiation force is located closer to the center. In zone 3, the acoustic confinement structure 120 is configured such that the acoustic confinement field focuses the particles even closer to the center. Therefore a set of barriers 130 can be positioned closer to the center than the barriers in zone 2. Hence the acoustic fields in the three different zones are controlled with regard to each other such that the particles are forced to the center of the channel. In this example the particle flow boundaries 112 of the particle confinement structure correspond with the walls of the channel 110.

In embodiments according to the present invention the position of the pressure node, this is the minimum (zero) amplitude point in the standing wave, of an acoustic confinement structure is not necessarily located at the center of the channel which may be advantageous for the local trapping, observation and analysis of particles and cells.

Embodiments according to the present invention may comprise a particle confinement structure adapted for restricting the particle flow in the channel. In these embodiments the particle confinement structure comprises particle flow boundaries 112 different from the channel walls. The particle confinement structure may for example move the particles away from the channel wall. In embodiments of the present invention the particle confinement structure may comprise the wall or part of the wall of the channel. In embodiments of the present invention there may be more than one channel and in embodiments of the present invention micro structures may be present wherein the fluid can be guided. These micro structures may also comprise particle confinement structures adapted for restricting the flow of particles. The particle confinement structure may comprise a wall. It may also comprise an array of pillars that confine the particle flow. Such an array of pillars may confine the particle flow but not the liquid flow. In that case the fluid can still flow along the wall but the particle cannot.

FIG. 6 shows a channel 110 comprising such a pillar array 610 in accordance with embodiments of the present invention. FIG. 6 shows the acoustic pressure field 630 (i.e. the standing wave) and the acoustic radiation force 640. In FIG. 6 the acoustic radiation force 640 on the particles is zero at the side walls but is already different from zero when away from the side walls. It is thereby an advantage that by introducing the micro pillars it is possible to speed up the acoustic focusing. When the dimension of micro pillars is much smaller than the acoustic wavelength, micro pillars are almost invisible to the acoustic field, and the desired standing wave field can be maintained. Therefore the pillars should be significantly smaller than the wavelength of the acoustic field. The pillars are preferably as small as possible. This is limited by the minimum dimensions for the pillars in manufacturing. When, for example, working with PA material, the radius of the PA pillars can be as small as 5-10 μm. In embodiments according to the present invention the space (or gap) between two pillars is between 2 and 100 μm, or preferably between 5 and 50 μm.

In embodiments of the present invention the pillar array is designed such that particles will not be able to pass through the micro pillar array. These micro pillars can be used to limit and guide the movement region of particles.

In embodiments of the present invention the channel may be a conventional hard wall channel (e.g. made of silicon or glass). In these embodiments the channel may be partially filled with an acoustic transparent layer which serves as particle confinement structure. FIG. 7 and FIG. 8 show a channel comprising such an acoustic transparent layer 810. By introducing the acoustic transparent layer 810 the acoustic field boundaries 122 are different from the particle flow boundaries 112. The acoustic transparent layer thereby serves as a particle confinement structure. The region inside the dashed line in FIG. 7 is the conventional channel. The hard walls of the channel are the acoustic confinement structure.

In embodiments according to the present invention an acoustic confinement structure is combined with a particle confinement structure. It is an advantage of embodiments of the present invention that by combining a separate acoustic confinement structure and a particle confinement structure the particle focusing can be enhanced.

FIG. 8 shows the pressure field 830 and the acoustic radiation force 840. In FIG. 8 the acoustic radiation force on the particles is zero at the acoustic field boundaries 122 but is already different from zero at the particle flow boundaries 112. It is thereby an advantage that by introducing the acoustic transparent layer 810 it is possible to speed up the acoustic focusing. In this exemplary embodiment of the present invention the focusing arrangement 100 is configured such that the width between the acoustic field boundaries 122 is half of the wavelength of the acoustic field and such that the acoustic transparent layers against the side wall have as width λ/8 and the remaining center of the channel 110, between the particle flow boundaries 112, has as width a quarter of the wavelength.

Focusing arrangements according to the present invention may comprise acoustic confinement structures in combination with particle confinement structures. It is thereby an advantage that the fluidic and acoustic conditions can be adjusted independently for improving the particle focusing performance. The acoustic confinement structure may comprise materials to define the acoustic field. These may be acoustically reflecting materials. In embodiments of the present invention different resonators are present at different places of the channel as acoustic confinement elements. In these embodiments the particle confinement structure may be the fluidic channel wall. In that case the fluidic channel wall defines the liquid and the particle flow. Additionally pillars may be added to confine the particle flow.

It is an advantage of embodiments of the present invention that in a first section the particles are pushed away from the channel walls towards the center of the channel.

FIG. 9 shows the velocity of a laminar flow in a channel (in m/s). This and the following figures are to illustrate the distribution of the flow speed, the acoustic pressure, and the acoustic radiation force in a conventional channel compared to those in a channel in accordance with embodiments of the present invention (the absolute values are not important). The highest speed is indicated with +++ and is achieved at the inlet of the channel. Lower speeds (++, +) are obtained later on in the channel and a zero speed (0) is obtained at the side walls of the channel. Particles will be randomly distributed across the width of the channel as a result of the laminar flow. According to the acoustic radiation force theory, those particles very close to the wall will receive zero radiation force, which will greatly lower the focusing speed.

FIG. 10 shows the acoustic pressure field (indicated by ++, 0, −−) in a channel (in Pa).

FIG. 11 shows the radiation force (in N) which is zero add the side walls of the channel.

FIG. 12 shows the velocity (in m/s) of a laminar flow in a channel 110 comprising micro pillars 610 in accordance with embodiments of the present invention.

FIG. 13 shows the acoustic pressure field (in Pa) in a channel comprising micro pillars in accordance with embodiments of the present invention. As can be seen, the acoustic pressure fields with or without pillars are the same (FIG. 10 compared to FIG. 13).

FIG. 14 shows the radiation force (in N) in a channel comprising micro pillars in accordance with embodiments of the present invention. As can be seen, the distribution of radiation forces with or without pillars is similar (FIG. 11 compared to FIG. 14).

FIG. 15 shows an analytical simulation of transient particle trajectories in a channel with or without micro pillars 610. The dashed horizontal line 1510 corresponds with the central line of the channel. Curve 1520 corresponds with the particle trajectory in function of time when starting at the micro pillars. The position of the micro pillars is indicated by line 1540. Curve 1530 corresponds with the particle trajectory in function of time when starting at a side wall of the channel. The x-axis is the time in seconds and the y-axis is the distance from one side wall of the channel. The transient trajectories of one 10 μm diameter polystyrene particle is shown. The start location is at the boundary of the channel 110 and at the boundary of micro pillars respectively. The simulation shows that in the presence of a pillar array particles take much shorter time to migrate to the center of the channel. In this exemplary embodiment it takes about 0.02 s for a particle to migrate to the center when staring from the pillar array. When starting from the side wall this would be about 0.05 s.

Method and Arrangement for Focusing Objects in a Flow

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