WO2010123453A1

WO2010123453A1 - Device and method for manipulating particles utilizing surface acoustic waves

Info

Publication number: WO2010123453A1
Application number: PCT/SE2010/050455
Authority: WO
Inventors: Linda Johansson; Johannes Enlund; Ilia Katardjiev; Ventsislav Yantchev
Original assignee: Linda Johansson; Johannes Enlund; Ilia Katardjiev; Ventsislav Yantchev
Priority date: 2009-04-23
Filing date: 2010-04-23
Publication date: 2010-10-28

Abstract

The present invention provides a device and a method for particle manipulation utilizing interface acoustic waves. The device comprises a cavity (3) for a fluid formed at an interface between a piezoelectric substrate (1) and channel-forming substrate (2). An acoustic generator (4), such as one or more inter digital transducers, is arranged to excite surface acoustic waves which are converted to interface acoustic waves when entering the interface such that acoustic energy is efficiently transferred into the cavity (3) to create standing acoustic waves (6), whereby acoustic primary radiation forces associated with the standing acoustic waves (6) enable to focus or direct the particles (7) suspended in the fluid.

Description

Device and method for manipulating particles utilizing surface acoustic waves

Technical field of the invention

The present invention relates to methods and devices for particle manipulation and in particular to such methods and devices utilizing surface acoustic waves. Background of the invention

Manipulation of particles with dimensions in the nano- and microscale (5nm- 50 μm) suspended in a fluid is highly relevant for a large number of applications. Examples include separation, pre-concentration and/ or purification of cells, bacteria, viruses, target proteins, peptides, antibodies or nucleic material in fields such as biomedicine, biochemical analysis, medical diagnostics, genetics, microbiology, pharmaceutics, etc. By way of example, in flow cytometry and in point-of-care (POC) devices one common operation is alignment of particles in a fluid flow before detection. Another common operation is alignment of particles in a fluid flow to enable particle separation, concentration and/ or purification. Proteins (or other types of macromolecules) are commonly manipulated using chromatographic and electrophoretic methods where the target molecules are displaced relative to the surrounding medium. For contactless manipulation and trapping of individual particles, such as proteins, dielectrophoresis and optical manipulation methods has been used. However, these technologies are limited in that they process only small sample volumes (as low as picolitre) at a time and/ or they are not suited for flow-through processing and/ or they put certain requirements on the particles to be manipulated and/ or they required require costly equipment for driving, etc.

Acoustic particle manipulation, where acoustic waves are generated to provide a pressure gradient in the fluid that renders an acoustic primary radiation force on the particles, overcomes many of the deficiencies of the above mentioned methods since the acoustic primary radiation force is independent of particle properties such as charge, shape, structure, functional groups etc. Although acoustic particle manipulation was first demonstrated more than a century ago, most of the practical applications have only recently been identified. The technology first found use in the 1980s with kHz-frequency (cm-scale) levitators for manipulation of mm- sized objects in air, followed in the 90s by MHz-frequency (mm-scale) systems for manipulation of μm- and sub-μm sized objects in fluids. During the current decade, the implementation of the technology into microscale lab-on-chip systems has been initiated. As described in e.g. Wiklund et al, Lab Chip, 6, 1279- 1292 (2006) and Laurell et al., Chem. Soc. Rev., 36, 492-506 (2007), ultrasound standing wave (USW) particle manipulation can been used for this purpose. In current USW systems standing bulk acoustic waves (BAW) are commonly used for particle manipulation. The waves are generated by coupling an acoustic wave generated by a bulk transducer, such as a thick-film piezoceramic transducer, to a resonance cavity, i.e. a microchannel having sound reflecting walls.

The USW particle manipulation is an efficient and robust way to handle μm- sized bioparticles (such as cells, bacteria and bio-functionalized beads) in micro fluidic channels. In comparison to alternative contactless technologies, such as e.g. dielectrophoretic or optical manipulation, further advantages of USW manipulation are simple and cost-effective instrumentation, its biocompatibility and that it enables high-throughput processing with continuous fluid flow. However, the current USW systems utilizing bulk transducers are typically limited to frequencies lower than 15 MHz. Thus, since the acoustic primary radiation force in such systems is proportional to the acoustic frequency and to the particle volume, only particles above a certain size can be manipulated individually, for instance polystyrene particles with diameter of at least 1 μm. For high particle concentrations, secondary acoustic radiation force and particle agglomeration also influence the manipulation effect.

With surface acoustic waves (SAW), typically excited by means of interdigital transducers (IDTs), the frequency can be increased and varied over a broad range. In Shi et al., Lab on a Chip, 8, 221 -223 (2008), an on-chip microparticle-focusing technique using 40 MHz surface acoustic waves generated by a pair of IDTs symmetrically arranged on opposite sides of microfluidic channel formed in poly(dimetyl siloxane) (PDMS) on a piezoelectric substrate is disclosed. Constructive interference of the surface acoustic waves from the IDTs result in the formation of standing waves that couples into the channel and forms a standing wave in the fluid, whereby an acoustic radiation pressure associated with the standing waves in the fluid act to focus particles within the channel towards nodes or antinodes of the standing waves. Shi et al points out that the use of the surface acoustic waves eliminates the challenging need of channel materials with poor sound reflecting properties, i.e. PDMS. Particle alignment of 1.9μm polystyrene particles in continuous flow by using an IDT-excited SAW was demonstrated. Further, particle alignment of 0.5μm polystyrene particles in a surface fluid layer at stationary flow using an IDT-excited SAW has also been demonstrated by Wood et al. Appl. Phys. Lett., 92, 044104-3 (2008). Due to the acoustically "open" systems, i.e. the SAW is either transmitted through, or absorbed by, the medium surrounding the fluid containing the particles, the coupling of the acoustic energy to the particles is not expected to be efficient and the selection of channel materials is limited.

Summary of the invention In view of the foregoing one object of the invention is to improve acoustic particle manipulation.

Hence, the present invention provides a device and a method for manipulating particles suspended in a fluid as defined in the independent claims.

In a first aspect, the present invention relates to a method for manipulating particles in a fluid where interface acoustic waves are employed to efficiently transfer an acoustic energy into a fluid cavity, typically a microfluidic channel, formed at the interface between two substrates. The method is based on trapping of the acoustic energy towards the interface between the substrates and creating standing acoustic waves in the fluid cavity, whereby acoustic primary radiation force associated with the standing acoustic forces enable to focus or direct the particles, being in the fluid.

The acoustic energy is preferably generated as surface acoustic waves (SAWs) excited on an open surface of one of the substrates by means of an acoustic generator such that SAWs propagate towards the fluid cavity. When entering the interface between the substrates the SAWs are converted to interface acoustic waves (IAWs), whereby the acoustic energy is substantially trapped towards the interface between the substrates, and efficiently transferred along the interface towards the cavity.

Said method may be used to focus or direct, e.g. for sorting, separation or concentration purposes, particles in the microfluidic channel. Thanks to the invention it is possible to focus or direct the particles, being in the fluid in the microfluidic channel, in a very energy-efficient way. Strictly IAWs are known as Stoneley waves which propagate without losses along an interface between two solid media, however as explained in the following, the IAWs relating to some embodiments of the invention has a wider definition.

The IAWs may be selected from interface-guided acoustic waves and interface skimming acoustic waves, and may be generated by a method selected from direct piezoelectric excitation, direct optical excitation or intermode transformation. In intermode transformation, the interface acoustic waves are generated through acoustic coupling from a primary acoustic mode such as a surface acoustic wave (SAW) mode or a bulk acoustic wave (BAW) mode and where said SAW and BAW modes are in turn generated by a direct piezoelectric or direct optical excitation.

In a second aspect, the present invention relates to a device for manipulating particles suspended in a fluid where interface acoustic waves are employed to efficiently transfer acoustic energy into a fluid cavity, typically a microfluidic channel, formed at the interface between two substrates. The device is based on trapping of the acoustic energy towards the interface between the substrates and creation of standing waves propagating in the fluid cavity.

The device comprises a first substrate and a second substrate with a cavity for the fluid formed at an interface between the first substrate and the second substrate. Further an acoustic generator is arranged to generate acoustic energy such that standing acoustic waves are created in the cavity, whereby acoustic radiation forces associated with the standing acoustic waves enable to focus or direct the particles suspended in the fluid; characterised in that interface acoustic waves are employed to transfer the acoustic energy at least partly along the interface. Said device may further comprise an inlet for fluid flow into the cavity, and at least a first outlet for a first part of the fluid and a second outlet for a second part of the fluid to enable selective sorting of particles in the fluid. In operation, the fluid flow has a direction from the inlet and towards the first and second outlets. The acoustic generator may be arranged to apply a standing acoustic wave in a separation zone of the fluid within the cavity. Upon activating the acoustic generator, a particle in a fluid flow within the cavity is focused into the second outlet (s).

Said SAW, or IAW, may be generated by a standard IDT. The IDT may be unidirectional. Moreover, the use of two IDTs enables operation with a phase difference between the two IDTs, for shifting the position of nodes and/or antinodes of the standing acoustic waves.

Waves can be generated from one or both sides of the channel. Said particle manipulation is obtained by the primary acoustic radiation force generated as a result of the constructive interference of acoustic waves propagating in opposite directions, generated by two transducers on either side of the channel or one transducer and reflection at some or several interfacial walls at the cavity and/ or an outer surface of the second substrate.

Excitation by interdigital transducers (IDT) have several advantageous properties such as easy inspection since the transducers may be positioned outside a microfluidic channel, planar fabrication technique and a uniform excitation of the microfluidic channel. Additionally, IDTs may be operated at high frequencies which enable high acoustic forces for a certain acoustic pressure. The acoustic forces employed for particle manipulation have a strong dependence on the size of the particle and manipulation of smaller particles, such as bacteria, viruses and possibly also proteins requires proper design of the device.

The material of the fluidic channel and the material of the piezoelectric substrate, in addition to the direction of the generated wave relative to the crystal orientation of the piezoelectric substrate are selected such that the interface between the channel material and the piezoelectric substrate supports an IAW. For instance, in one embodiment, fused silica and X-cut Z-propagation LiTaCh are used.

The two substrates may be bonded by various methods, for instance anodic bonding or adhesive bonding. The use of adhesive bonding may give advantages such as low requirement on surface roughness, no conductivity requirement of either of the substrates, and enable the use of a low temperature process. Hence, adhesive bonding does not reduce the substrates that can be used for supporting an interface acoustic wave. It is also compatible with wafer-level batch fabrication. Anodic bonding may normally be used at the opposite conditions. Further, the adhesive layer may act as an interface layer that promotes wave guidance of surface acoustic waves, such as interface acoustic waves. This can be accomplished by coupling of the acoustic wave out from the piezoelectric substrate and into the interface layer. The two substrates do not a pήoή have to support said interface acoustic waves, but may be adhesively bonded by means of a bonding layer chosen specially to promote wave guidance of said interface acoustic waves.

The acoustic transfer mechanisms of the present invention offer great flexibility in the selection of channel materials, i.e. the second substrate. The channel material is preferably of high acoustic impedance as compared with that of the fluid, preferably larger than 10 MRayl, which enables a build-up of acoustic energy inside the fluid, for instance the channel material is glass, silica or fused silica. The material of the channel structure may also be of medium acoustic impedance, preferably 2-5 MRayl, for instance polymer material manufactured by injection moulding, moulding or casting such as PMMA, PC, COC (cycloolefin-copolymers). Use of these materials, instead of for example PDMS, which was used in the above mentioned standing surface acoustic wave focusing device, may give some energy build-up within the fluid and also give further beneficial advantages in terms of mechanical properties, chemistry of the surface, etc.

One advantage of the present invention is that the interface acoustic wave does not couple into the substrate comprising the cavity, i.e. the channel material, but propagates along the interface and hence a large fraction of the acoustic energy can reach the fluid. Further, the high (larger than 10 MRayl) acoustic impedance of the channel material is such that it enables confinement of the acoustic energy inside the fluid. As an additional effect the device may be operated with only one IDT. The large frequency range possible by the IDT enables the use of higher operation frequencies, preferably within 10MHz - IGHz, which enable higher acoustic forces for a certain acoustic pressure. The possibility to use a large aperture of the IDT enables a uniform excitation of the channel and low divergence of the excited wave. This is favourable for avoiding acoustic streaming and excitation of other acoustic cavity modes, which may deteriorate the manipulation performance. Additionally, a uniform excitation enables a contribution along the total channel to the total force on a particle passing through the channel. Further, in the present invention the distance between the particle alignment nodes may be determined by the IAW and yield large inter-node distances, despite using a high frequency, which is favourable for continuous-flow operation since it enables simpler fluid design. In addition, the material losses may be low for the piezoelectric material used for the acoustic excitation of a SAW and hence do not contribute much to temperature increase in the fluid, which is favourable for handling biological samples. A further advantage is that the device may be used as stand-alone or integrated with other operations.

Further advantages of the present invention technique includes the following. The acoustic manipulation is gentle for cell manipulation, as commonly known from several studies in the field.

Concentration of particles in a sample is one type of output signal amplification. The concentration of particles according to the present invention could thus for example be used as a substitute of biomolecular amplification. This would enable faster and simpler analysis methods.

Sheat flow can most often be omitted.

Efficient acoustic design enable sorting of smaller particles and higher flow rates to allow higher sample volumes at times or lower drive voltage.

Separation of several different particles into multiple outlets is beneficial for isolating several different types of particles in a sample in so called Field Flow Fractionation. For instance for separation of red blood cells, white blood cells and platelets in a blood sample.

The use of a single IDT enables a smaller footprint (smaller device). It may also enable less strict criteria for aligning the channel relative the transducers for the standing wave being formed by reflection at the interfaces of the channel structure rather than by counter-propagating waves from the two IDTs.

One further advantage with the presented devices is that they are able to be operated at high frequencies and are not strongly affected by other acoustic effects such as acoustic streaming and acoustic cavity (or channel) modes in addition to enable strong acoustic forces. The manipulation may be performed on a continuous flow of particles.

Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

Brief description of the drawings

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein FIG. 1 schematically illustrates a cross-sectional view of a device comprising two IDTs and a channel structure arranged on a piezoelectric substrate in accordance with the invention,

FIG. 2 schematically illustrates a top view of a particle-sorting device comprising one inlet and a plurality of outlets in accordance with the invention.

FIG. 3 schematically illustrates IDTs comprising reflector stripes (λ/4 spacing) and electrode stripes (λ/6 spacing) in accordance with the invention,

FIG. 4a schematically illustrate a device comprising an interface layer arranged in accordance with the invention, FIG 4b schematically classical and perturbed Stoneley waves in a device in accordance with FIG. 3 and FIG. 4a, respectively, compared in a COMSOL FEM model analysis with respect to their displacement amplitudes,

FIG. 5a schematically illustrates fluid flow through a non-active particle- sorting device comprising two IDTs and a channel structure arranged on a piezoelectric substrate in accordance with the invention, and

FIG. 5b schematically illustrates a particle-sorting device in accordance with FIG. 5a with an applied standing wave in accordance with the invention.

Detailed description of embodiments

The present invention relates to acoustic manipulation of objects in a fluid. The objects are preferably micro- or nanoparticles, hereinafter referred to as particles, that can be selected from beads, cells, bacteria, viruses, protein molecules, DNA molecules, organic molecules, clusters of organic molecules, clusters of inorganic molecules, antibodies, peptides, nucleic material or biological samples, however not limited to this. For the purpose of this application manipulation is intended comprise operations selected from sorting, concentration or separation of particles from other particles or particles from the fluid. Theses operations are typically performed in a microfluidic channel or cavity with a fluid flow therein. Due to the basic configuration of a device for manipulating particles suspended in a fluid in accordance with the invention acoustic forces act on the particles in a direction substantially perpendicular to the flow direction. Thus, when the fluid flows in the microfluidic channel or cavity, the manipulation generally does not render trapping of the particles in a fixed position, rather directs or forces the particles to a certain position within the fluid flow. FIG. 1 schematically illustrates one example of a device for manipulating particles suspended in a fluid in accordance with the invention. One of the two substrates, hereinafter referred to as a first substrate 1, comprises a piezoelectric material and the other substrate, hereinafter referred to as a second substrate 2, comprises the cavity 3, or the micro fludic channel, facing the first substrate 1 at an interface between the substrates 1 , 2. An acoustic generator 4, such as one or more interdigital transducers (IDTs) formed on the first substrate 1 , is arranged to generate acoustic energy. When the acoustic generator 4 is activated the acoustic energy propagates as surface acoustic waves from a position outside the cavity 3 towards the cavity 3. When entering the interface between the first substrate 1 and the second substrate 2 the surface acoustic wave is converted to a interface acoustic wave that propagates along the interface to the cavity 3 and creates standing acoustic waves in the cavity 3. Primary acoustic radiation forces associated with these standing acoustic waves enable to focus or direct the particles 7 suspended in the fluid. As illustrated in FIG. 1 the standing wave can be generated by interference of acoustic waves propagating from opposite sides of the cavity 3, for example by arranging one IDT on each side of the cavity in a double-IDT configuration, or by acoustic waves propagating from one source only, such as one IDT in a single-IDT configuration (not shown). With proper choice of substrate materials the transfer of acoustic energy along the interface is performed by Stoneley waves.

Almost a century ago R. Stoneley discovered the IAWs, also known as Stoneley waves. These propagate without leakage losses along an interface between two solid media. It is noted that IAWs only exist under specific conditions which imposes certain restrictions on the choice of materials. Nowadays, a number of specific combinations of materials and propagation directions supporting Stoneley waves are known. Thus the material of the fluidic channel and the material of the piezoelectric substrate, in addition to the direction of the generated wave relative the crystal orientation of the piezoelectric substrate may be selected such that the interface between the channel material and the piezoelectric substrate supports a Stoneley wave. For instance, as mentioned above, fused silica and X-cut Z- propagation LiTaCh may be used.

As mentioned above interface acoustic waves are commonly referred to as Stoneley waves. Strictly, Stoneley waves only exist under specific conditions which impose certain restrictions on the choice of materials. However, as disclosed in the following trapping of the acoustic energy towards the interface can be obtained under modified conditions as well. Thus, for the purpose of this application, interface acoustic waves are not strictly limited to classical Stoneley waves, if not explicitly referred to such.

FIG. 2 schematically illustrates a device comprising a cavity 3 configured as a micro fluidic channel, with at least one inlet 8, a first outlet 9a, a second outlet 9b and optionally a third outlet 9c arranged for a fluid flow through the cavity 3 from the inlet 8 to said outlets 9a, 9b, 9c in order to selectively sort out particles 7 into at least one of said outlets 9a, 9b, 9c due to the focusing or directing of the particles 7. How to generate the fluid flow is known by one skilled in the art, but typically a flow provider, such as a syringe pump is arranged for providing a continuous laminar fluid flow into the cavity. The selective sorting is enabled by the standing acoustic wave field forming pressure nodes or antinodes at different positions in the cross-section of the microfluidic channel 3, such that the particles 7 will move under the influence of the acoustic primary radiation forces, as explained in the following, in a plane extending in a x- and y-direction perpendicular to the fluid flow direction (z-direction). Their displacement in the x- and/or y-direction in the channel upon transducer activation will cause them to follow different fluid stream lines when transported by the fluid flow, which preferably is laminar, towards the outlets 9a, 9b, 9c. Hence, the particles are selectively directed or focused into a certain outlet(s). The alignment of the particles may be in the horizontal (x) and in the vertical (y) direction. The position of the pressure nodes and antinodes can be modulated by the frequency of the signal, the phase shift between two IDTs, the size, geometry and position of the inner channel relative the substrate 2 and the IDTs, the size, geometry and the position of the substrate 2 relative to the transducers, the material of the substrates 1 , 2, etc. Thus it is possible with the present invention, to sort particles after for example size, such that large and small particles or positive- and negative acoustic contrast factor particles exit through different outlets, respectively.

Alignment of the particles in the presented invention occurs due to the primary radiation force, F_P; which is generated in a standing wave field created by interference of acoustic waves. In an ideal standing wave the primary radiation force on a particle can be expressed as:

where r is the radius of the particle, p_a is the sound induced pressure amplitude, P₀ is the density of the fluid, C₀ is the speed of sound of the fluid and

Z₁ and /₂ are defined as:

2 /_{1 =} i _ Po£o_ _{(2)> and} pc

Λ = |^^ . (3)

2p + P₀ where p is the density of the particle and c is the speed of sound of the particle. The parenthesis containing the f_x and /₂ is referred to as the acoustic contrast factor. Small particles relative the acoustic wavelength is assumed. While the primary radiation force determines the strength of the acoustic manipulation effect, the time-averaged acoustic radiation potential field, U, displays the positions at which the particles will be gathered (i.e. minima in the acoustic potential). For the assumption of small particles relative the wavelength the acoustic potential U can for an arbitrary acoustic field be described as

4πrY , v 3

(U) = ^ — I I f /i₁( (EE_kj_m))- - //₂M- E U, ,4)

where the material dependent contrast factors fi and /2 are as above and the time-averaged potential and kinetic densities are

Compressible spherical particles are assumed and the expression is valid under the requirement that particles are small in relation to the wavelength of the acoustic waves. Provided that the pressure and velocity amplitudes are known, the primary radiation force is obtained as the spatial gradient of the acoustic potential at that point.

Referring to FIGs. 3, and 4 in one implementation of the above described device two IDTs are symmetrically placed on opposite sides of an IAW-supporting Z propagating X-cut LiTaOa/ fused silica microfluidic system. In other words, the first substrate 1 is a piezoelectric substrate made of LiTaO3 and the second substrate 2, which partly covers the first substrate 1 , is made of fused silica and comprises a cavity 3 at the interface to the first substrate 1. Optionally, as illustrated in Fig. 4, the substrates 1 , 2 are joined using an interface layer 10 functioning as an adhesive layer. The IDTs, which are not covered by the fused silica substrate 1 , are used to excite two surface acoustic waves (having a wavelength λ=96μm) propagating in opposite directions. The two transducers, each consisting of a non- reflective splitted electrode IDT in combination with a 20 fingers λ/4 reflecting grating are placed on the first substrate 1 on each side of the cavity 3. The proposed transducer design ensures that there is no energy trapping under the IDT. Reflector stripes positioned outside the IDT ensure a certain degree of unidirectionality of the SAW excitation. The energy distribution of the SAWs before reaching the channel-forming second substrate (fused silica) and of the IAW at the interface between the substrates 1 , 2 is schematically shown in FIG. 3. The energy distribution of the IAW is approximately one wavelength into the materials on each side of the interface. Accordingly, interface acoustic waves are employed to transfer the acoustic energy along at least part of the distance from the acoustic generator 4 to the cavity 3, in an interface region extending into both substrate 1 and substrate 2. A relatively small channel of 40 μm height and 100 μm in width can be considered an obstruction in the propagation path of the wave. Apart from coupling energy from the bottom channel wall the coupling of the energy into the fluid layer will be influenced by the boundary conditions along the channel inner wall which are dominated by the interface acoustic wave. The generated standing wave acoustic field inside the fluid causes particle alignment to specific positions inside the channel. Alignment can occur in both in vertical (y) and horizontal (x) direction.

For the fabrication of a particular device the IDTs were formed by resistive evaporation of Ti (10 nm adhesive layer) and Au (100 nm) through a lift-off lithography process. Each IDT consists of 120 fingers of λ/6 width and a 9 mm aperture. The reflectors represent a periodic grating consisting of 20 open circuited fingers with a λ/2 periodicity and a λ/4 finger width. The cavity, constituted as a semi-circular microfluidic channel, 40 μm high and 100 μm wide, micromachined in the fused silica, was micromachined in a 300μm thick fused silica wafer by wet etching using a poly-Si hard mask and subsequently bonded to the 500 μm thick X-cut LiTaCh wafer by an approximately 5 μm thick SU-8 adhesive layer. Inlet holes in the microfluidic glass structure were drilled with a diamond drill. Fluidic connections on the glass side were provided by bonding a PDMS part to the fused silica and using polyethylene tubing of 380 μm inner diameter outside a glass capillary (outer diameter 365 μm and inner diameter 95 μm). For a particle-sorting test with this particular device, the transducer was excited by a high-frequency function generator with maximum output power of 0.5 W or 20 Vpp for a 50 Ohm load. For this test a straight channel with one inlet and one outlet was used at a fluid flow of 0.05 μl/min ( 13mm/s) and 0.2 μl/min (50 mm/s). The ability to manipulate particles was evaluated by observing the (re- )distribution of particles upon transducer activation in an inverted fluorescence microscope with a high sensitive cooled CCD camera. A syringe pump was used to operate a syringe. Green fluorescing 0.5 μm polystyrene beads of 1.05 g/cm³ density in de-ionized water, with 0.5 % (v/v) Tween added to prevent particles from agglomeration, were used for the demonstration. Measurements were performed by exciting one as well as both IDTs connected in series. The transducer central frequency was 34 MHz corresponding to a SAW velocity of 3264 m/s. The transducer bandwidth was in the order of 1 MHz as defined by the number of strips in the IDT. Upon ultrasound activation the particles get aligned within a second. A large internode distance of about 50 μm between two pressure nodes was obtained. This relatively large inter-node distance is favourable for continuous flow operation with flow-split separation.

This implementation and particular device is a non-limiting example. The design can be further optimized to achieve even higher efficiency. For example, other IAW-supporting interfaces can be utilized in view of achieving stronger electromechanical couplings, such as ZnO /Glass, lead zirconate titanate (PZT) /glass, etc. Improvements are also foreseen for 50 ohm matching to the function generator, for more directional IDTs, for optimised channel inner wall geometry and surface roughness for optimised geometry and position of the second substrate relative the IDTs. As schematically illustrated in FIG. 4a, an interface layer 10 may be arranged between the first substrate 1 and the second substrate 2. The interface layer 10 may serve dual purposes. The two substrates may be bonded by various methods, for instance anodic bonding or adhesive bonding. In the latter the interface layer serves as an adhesive layer. The use of adhesive bonding may give advantages such as low requirement on surface roughness, no conductivity requirement of either of the substrates, and enables the use of a low temperature process. Hence, heat- sensitive substrate materials that are excluded due to high temperatures used in anodic bonding can now be used. The adhesive layer approach is also compatible with wafer-level batch fabrication.

Further, the adhesive layer may act favourably to promote wave guidance at the interface by having lower acoustic impedance than the surrounding substrates. With substrate materials supporting the formation of Stoneley waves, the interface layer has proven to act favourable to spatially confine the acoustic energy to the interface region in a perturbed Stoneley wave. In FIG. 4b, classical and perturbed Stoneley waves are compared in a COMSOL FEM model analysis with respect to their displacement amplitudes. Clearly the perturbed Stoneley wave demonstrates an improved energy trapping toward the interface. For the case of the two substrates that do not a pήoή support IAWs, the interface layer may act as a wave guide that confines the energy to the interface region by reflection due to the acoustic impedance mismatch relative the surrounding substrates. This enables use of channel-forming substrates comprising medium-acoustic impedance materials having an acoustic impedance lower than the materials normally used for forming Stoneley waves, i.e. less than 10 MRayl, such as polymers having an acoustic impedance of 2-5 MRayl.

A significant acoustic impedance mismatch between the fluid and the channel substrate promotes favourable energy build-up in the cavity. This is not the case with for example a PDMS channel material, which commonly is used to fabricate micro fluidic structures. Further, due to coupling of the wave into the PDMS channel material and the high acoustic absorption in the PDMS channel material, the coupling of acoustic energy into the fluid is not expected to be efficient. There will be a severe leakage of the acoustic energy along the interface between the PDMS and the first substrate 1.

The IAWs may be selected from interface-guided acoustic waves and interface skimming acoustic waves, and may be generated by a method selected from direct piezoelectric excitation, direct optical excitation or intermode transformation. The intermode transformation is such that said interface acoustic waves are generated through acoustic coupling from a primary acoustic mode such as a surface acoustic wave (SAW) mode or a bulk acoustic wave (BAW) mode and where said SAW and BAW modes are in turn generated by a direct piezoelectric or direct optical excitation.

FIG. 5a schematically illustrates a fluid flow through a non-active particle- sorting device in accordance with the device described with reference to FIG. 2. Particles 7 of different properties are suspended in the fluid. The fluid enters at the inlet 8 and travel through the microfluidic channel 3 towards the outlets 9a, 9b and 9c. Although the particles 7 may follow a straight flow path trough the microfluidic channel 3 due to a laminar flow the particles 7 will be distributed into the outlets 9a, 9b, 9c depending on their entrance position in the channel. FIG. 5b schematically illustrates the situation when the acoustic generator 4 is operating and, by way of example, IDTs on each side of the microfluidic channel 3 excites SAWs propagating towards the microfluidic channel 3. Selection of substrate materials having appropriate acoustic impedance and appropriate cavity reflection of the acoustic waves in interfacial cavity walls contributes to the formation of a standing acoustic wave in the microfluidic channel 3. The transportation of the acoustic energy across the substrate 2 into the fluid layer can occur by different mechanisms, for instance IAW. When the microfluidic channel is filled with a fluid containing particles 7, the surface acoustic waves generate acoustic primary radiation forces acting on the particles, which enable to focus or direct the particles 7 to different positions in the plane perpendicular to the fluid flow (x-, y-plane). The alignment positions depend on the appearance of the standing wave field in the fluid and on the acoustic contrast factor of the particles as illustrated for black and white particles. Hence, the particles may be selectively directed to different outlets in a controlled way and collected at the outlets. Basically a method for manipulating particles 7 suspended in a fluid in accordance with the invention comprises the step of generating acoustic energy by means of an acoustic generator 4 such that interface acoustic waves are employed to transfer the acoustic energy at least partly along the interface between a first substrate 1 and a second substrate 2 to create standing acoustic waves in a cavity 3 formed at said interface, whereby acoustic radiation forces associated with the standing acoustic waves enable to focus or direct particles 7 suspended in the fluid.

A method for sorting and /or concentrating and /or separating particles 7 suspended in a fluid in accordance with the invention comprises the steps of: -providing a cavity 3, such as a micro fluidic channel, formed at the interface between a first substrate 1 and a second substrate, the cavity 3 comprising at least one inlet 8, a first outlet 9a and a second outlet 9b arranged for a fluid flow through the cavity 3 from the inlet 8 to the first outlet 9a and the second outlet 9b; - generating acoustic energy by means of an acoustic generator 4 such that interface acoustic waves are employed to transfer the acoustic energy at least partly along the interface between a first substrate 1 and a second substrate to create standing acoustic waves in the cavity 3 formed at said interface, whereby acoustic radiation forces associated with the standing acoustic waves enable to focus or direct particles 7 suspended in the fluid; and

- particles 7 are selectively directed to one or more outlets 9a, 9b depending on the particle properties.

By way of example the above described interdigital transducers comprises Au/Ti or Au/Cr as electrode material, however not limited to this. Other metal or metal alloys can be used as well. For instance Pt, which better sustain high energy densities to avoid electrode migration or other degenerative effects when operated at high voltage levels can be used.

The first substrate may be made of a piezoelectric material selected from LiTaθ3, LiNbOβ, PZT, or other piezoelectric materials. Normally the piezoelectric substrate will be made of a homogenous material, but in some instances it may also be possible to mix materials, such that some parts comprise segments of a different suitable piezoelectric material or other material, for instance a piezoelectric thin film layer with high electromechanical coupling factor positioned onto a low loss piezoelectric material or a non-piezoelectric material. Suitable materials for the first substrate 1 , i.e. the piezoelectric substrate, are selected from LiTaCh, LiNbCh, PZT or any strong piezoelectric material or combinations thereof. Suitable materials for the second substrate 2, i.e. the channel material, is selected from high-acoustic impedance materials, preferably having an acoustic impedance larger than 10 MRayl, such as glass, silica or fused silica or medium-acoustic impedance materials having an acoustic impedance between 2-5 MRayl, such as poly(methyl methacrylate) , polycarbonate, cycloolefin- copolymers. One advantage with these polymer materials is that they can be formed with versatile methods such as moulding, injection moulding or casting. Although the present invention has been described in terms of two substrates with the microfluidic channel or the cavity formed in one of the substrates, i.e. in the second substrate, it should be appreciated that the microfluidic channel or cavity may extend into both substrates as well.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the appended claims.

Claims

1. A device for manipulating particles (7) suspended in a fluid, characterised in that the device comprises: a first substrate ( 1) and a second substrate (2); a cavity (3) for the fluid formed at an interface between the first substrate ( 1) and the second substrate (2); and an acoustic generator (4) arranged to generate acoustic energy such that standing acoustic waves are created in the cavity (3), whereby acoustic radiation forces associated with the standing acoustic waves enable to focus or direct the particles (7) suspended in the fluid; characterised in that interface acoustic waves are employed to transfer the acoustic energy at least partly along the interface.

2. The device of claim 1 , wherein the acoustic generator (4) is arranged to excite surface acoustic waves that propagate on the first substrate ( 1) towards the cavity (3) and are converted to said interface acoustic waves when entering the interface between the first substrate ( 1) and the second substrate (2).

3. The device of claim 1 or 2, wherein the first substrate ( 1) comprises a piezoelectric material.

4. The device of claim any of claims 1 -3, wherein the acoustic generator (4) comprises at least one interdigital transducer.

5. The device of claim 4, wherein the acoustic generator (4) comprises at least a first interdigital transducer and a second interdigital transducer arranged on opposite sides of the cavity (3).

6. The device of claim 5, wherein the standing acoustic waves are formed by constructive interference of acoustic waves from counter-propagating interface acoustic waves from said at least first and second interdigtial transducers.

7. The device of any of the preceding claims, wherein the standing acoustic waves are formed by reflection at interfacial walls at the cavity and/ or an outer surface of the second substrate (2).

8. The device of any of the preceding claims, wherein the second substrate (2) comprises a material of high acoustic impedance such that interface acoustic waves do not significantly couple into the second substrate (2) but propagate along the interface between the first substrate ( 1) and the second substrate (2) as a Stoneley wave.

9. The device of claim 1 , further comprising an interface layer ( 10) between the first substrate ( 1) and the second substrate (2).

10. The device of claim 9, wherein the interface layer ( 10) has lower acoustic impedance than the first substrate ( 1) and the second substrate (2) and is arranged to provide perturbed interface acoustic waves (5) with improved energy trapping towards the interface.

1 1. The device of claim 9, wherein the interface layer ( 10) has lower acoustic impedance than the first substrate ( 1) and the second substrate (2) and is arranged to promote wave guidance of said interface acoustic waves (5) in the interface layer ( 10).

12. The device of claim 1 1 or 12, wherein the interface layer ( 10) is an adhesive layer for joining said substrates ( 1 , 2).

13. The device of any of claims 8- 12, wherein the second substrate is made of a material having acoustic impedance larger than 2 MRayl.

14. The device of claim 13, wherein the second substrate is made of a material having acoustic impedance larger than 10 MRayl.

15. The device of any of claims 8- 14, wherein said material is selected from glass, silica or fused silica.

16. The device of any of the preceding claims, wherein the cavity (3) comprises at least one inlet (8), a first outlet (9a) and a second outlet (9b) arranged for a fluid flow through the cavity (3) from the inlet (8) to the first outlet (9a) and the second outlet (9b) in order to selectively sort out particles (7) into at least one of said outlets (9a, 9b) due to the focusing or directing of the particles (7).

17. A method for manipulating particles (7) suspended in a fluid in a cavity (3) formed at an interface between a first substrate ( 1) and a second substrate (2), the method comprising generating acoustic energy by means of an acoustic generator (4) such that interface acoustic waves are employed to transfer the acoustic energy at least partly along the interface and standing acoustic waves are created in the cavity (3), whereby acoustic radiation forces associated with the standing acoustic waves enable to focus or direct particles (7) suspended in the fluid.

18. The method of claim 17, wherein the generation of acoustic energy comprises exciting surface acoustic waves that propagate on the first substrate (1) towards the cavity (3) and are converted to said interface acoustic waves when entering the interface between the first substrate (1) and the second substrate (2).

19. The method of claim 17 or 18, wherein the surface acoustic waves are excited by at least one interdigital electrode.

20. The method of claim 19, wherein the surface acoustic waves have a frequency between 10 MHz to 1 GHz.

21. The method of any of claims 17-20, wherein the cavity (3) comprises at least one inlet (8), a first outlet (9a) and a second outlet (9b) arranged for a fluid flow through the cavity (3) from the inlet (8) to the first outlet (9a) and the second outlet (9b), and the method further comprises selectively sorting out particles

(7) into at least one of said outlets (9a, 9b) due to the focusing or directing of the particles (7).