Abstract:
The present invention provides a device and a method for separating particles from fluids using ultrasound, laminar flow, and stationary wave effects comprising a micro-technology channel system with an integrated branching point or branching fork, and a single ultrasound source. One of the characteristics of the invention is that the single ultrasound source, which generates the standing waves, excites the complete structure including the channel system. No special reflectors or the like are needed. Extremely thin dividers can separate the flow, thereby enhancing the effectiveness of the device. The device could be manufactured in silicon and the ultrasound energy could preferably be delivered by a piezoelectric element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device and a method for separating a fluid containing suspended particles into fractions of higher and lower concentration of said suspended particles using ultrasonic standing waves and micro-technology. 
     STATE OF THE ART 
     It is known that when particles in a fluid are subjected to an acoustic standing wave field, the particles are displaced to locations at, or in relation to the standing wave nodes. A number of attempts to use ultrasound standing wave field for the manipulation or separation are known. 
     In WO 00/04978 is described a device for performing the manipulation of particles suspended in a fluid. It comprises a duct for the flow of a fluid in which particles are suspended, and an acoustic transducer and a reflector for establishing an acoustic standing wave field across the width of said duct, the spacing between the transducer and reflector being 300 μm or less. 
     In an abstract to the 4 th  annual European conference on micro &amp; nanoscale technologies for the biosciences (NanoTech 2000), Hawkes and Coakley describes a “force field particle filter, combining laminar flow and ultrasound standing waves” with an acoustic path length at right angles to the flow of 0.25 mm. 
     In WO 98/50133 is described a device for performing the manipulation of particles suspended in a fluid. It comprises a duct for the flow of a fluid, in which particles are suspended, said duct having means for establishing an acoustic standing wave field so that the particles are displaced to form parallel bands. The duct includes an expansion in width. 
     In IBM, technical disclosure bulletin vol. 25, No. 1, June 1982, page 192/193 is disclosed an ultrasonic continuous flow plasmapheresis separator comprising two orthogonally mounted ultrasound transducers with one reflector each and a volume between where a dilute suspension is subjected to an acoustic standing wave field. 
     In JP 06241977 A is described a fine particle measuring instrument that uses a standing ultrasonic wave with a node at the centre of a flow cell to centre and concentrate fine particles. 
     In EP 0 773 055 A2 and A3 is described a method and an apparatus for handling particles by an acoustic radiation force. The apparatus comprises a chamber for accommodating a fluid containing the particles to be concentrated, filtered or arranged, and a plurality of ultrasound sources disposed to make direct or indirect contact with the fluid. The apparatus also comprises a control device for controlling said ultrasound sources to generate an ultrasound beam obtained by superimposing ultrasound beams from said ultrasound sources on one another, said beams each having a specific intensity, a specific frequency and a specific phase. 
     In WO 93/19367 A2 and A3 is described a method and an apparatus for particle aggregation, said apparatus comprising a tube for containing of a sample of a liquid, and an ultrasonic transducer arranged to generate a standing wave ultrasound field transverse to the tube. The standing wave exhibiting a progressive change in pressure amplitude transverse to the tube, so that, in use of the apparatus, particles in suspension are displaced transversely of the tube to one or more predetermined regions. After termination of exposure to the standing wave particles are allowed to settle and can then be inspected. Appreciated use of the apparatus includes carrying out immuno-agglutination assays. The document is based on U.S. Pat. Nos. 5,665,605 and 5,912,182. 
     In JP 07 047259 A is described an apparatus for transporting fine particles in fluid with ultrasonic waves. The apparatus comprises a multitude of ultrasonic wave generating elements two-dimensionally arranged on two flat plates. Between the plates a solution can be deposited. 
     SUMMARY OF THE INVENTION 
     The present invention provides a device and a method for separating particles from fluids using ultrasound, laminar flow, and stationary wave effects comprising micro-technology channels formed in the surface portion of a plate, having integrated branching points or branching forks, and an ultrasound source arranged in close contact to an opposing surface of said plate. 
     Standing waves are generated in the channels so that particles suspended in the fluid are brought into certain lamina of said fluid, and that one or more lamina are formed devoid of particles, or are formed carrying particles of different properties than the first mentioned ones. Said laminae are thus arranged perpendicular to said plate, this is important because the branching of a channel must take place within the plate, so that a connection with another channel can take place also within the same plate. The advantages of this will be obvious below. 
     One of the characteristics of the invention is that the ultrasound source is arranged in perpendicular contact with the plate, conveying ultrasound energy in a direction that is perpendicular the plate. The inventors have tested and proved that in the present invention, as a result of the dimensions of the channels and the properties of the plate and the ultrasound transmitter, a standing wave is generated that reaches from one side wall of a channel to the opposing side wall of the same channel. It would normally be expected that such an arrangement would generate (only) a standing wave reaching from a bottom wall to a top wall of said channel, continuing in a direction of the original energy flow. 
     The inventors have also realised the great importance of this idea. Because, according to the invention, the ultrasound source now do not have to be a part of the plane or layer where the channels reside, and space becomes available for packing more channels into a limited space, greatly enhancing the possibilities of manufacturing devices with a multitude of parallel channels providing high capacity particle separation. A high degree of particle separation could also easily be provided by a serial arrangement of separation units, as will be further explained below. The capability of high yield parallel and serial processing of a fluid using ultrasound is thus a central part and consequence of the inventive concept. 
     The above is possible because the channels and branching points are formed in a plate comprising one piece of material or in a few pieces of material closely bonded together. No special reflectors or the like are used. It may also be possible to use more than one ultrasound source. Thin dividers are arranged to separate the laminar flows after the branching points, thereby enhancing the effectiveness of the device. The device is preferably manufactured using silicon technology benefiting from the possibility of small precise dimensions, and the ultrasound energy could preferably be delivered by a piezoelectric element, which in turn could be driven from a control unit capable of delivering electrical energy of certain shape, frequency and power. 
     The invention is defined in the accompanying claim  1 , while preferred embodiments are set forth in the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described below with reference to the accompanying drawings, in which: 
         FIG. 1  shows a top view of a cross channel system arrangement; 
         FIG. 2  shows a perspective view of the object in  FIG. 1 ; 
         FIG. 3  shows a bottom view of the object in  FIG. 1 ; 
         FIG. 4  shows a side view of the object in  FIG. 1 ; 
         FIG. 5  shows a top view of a repeated arrangement; 
         FIG. 6  shows a detail top view of a parallel arrangement branching point; 
         FIG. 7  shows standing waves in the space between two walls; 
         FIG. 8  shows a cross section view of the device; 
         FIG. 9  shows schematically separation using one-node standing wave; 
         FIG. 10  shows schematically separation using two-node standing wave; 
         FIG. 11  shows schematically a one-node three-step serial wash; 
         FIG. 12  shows schematically a one-node three-step concentrator; 
         FIG. 13  shows schematically a one-node four-step integrated wash and concentrator; 
         FIG. 14  shows a top view of an embodiment with labelled branching angles; 
         FIG. 15  shows a parallel arrangement of eight channel units; 
         FIG. 16  shows the parallel arrangement of  FIG. 15  in perspective; 
         FIG. 17  shows schematically a serial arrangement of two channel units; 
         FIG. 18  illustrates a separation of two different kinds of particles with different density; 
         FIG. 19  illustrates a channel unit with the inlets and the outlets; 
         FIG. 20  illustrates the channel unit of  FIG. 19  including particles; 
         FIG. 21  shows schematically a radial arrangement of the channel units; 
         FIG. 22  shows the embodiment of  FIG. 21  in perspective. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 ,  2 , and  8 , one embodiment of the invention comprises a plate  51 , 851 , with an integrated channel system, with a base stem  110  and a left arm  120 , a right arm  130  and a central arm  140 . The walls of the base stem  810 ,  820  are essentially perpendicular to the plate  51  and parallel or near parallel to each other, and to the flow, which is a prerequisite for the establishment of standing waves across the channel along its entire depth and length, see below. 
     At the back of the plate  51 , means for delivering ultrasound energy to the plate is arranged in the form of a piezoelectric element  150 ,  853 . The device will function as follows: 
     A fluid with suspended particles entering the base stem  110  at the inlet  160  will flow towards the branching point  175  because of an arranged pressure gradient, which gradient could be created by e.g. a suction pump, a pressure pump, a syringe or by gravity. By controlling the frequency of the ultrasound and use of certain frequencies suitable to the dimensions of the base stem  110 , particularly the width  185  of said stem  110 , a stationary wave pattern is formed in the fluid inside said stem  110 . Especially there will form a stationary wave pattern orthogonal to the direction of the flow between a left  810  and a right  820  side wall of the base stem  110 . Pressure nodes will form in greater numbers in the middle part of the channel than at the walls, where pressure antinodes will form. During said flow, particles in the fluid will tend to accumulate in nodes of said stationary wave-pattern, or in certain layers in relation to the nodes depending on the particles&#39; density/densities/acoustic impedance relative to the surrounding fluid. Particles with a higher density than said surrounding fluid will tend to accumulate in the nodes, whereas particles with a density lower than the surrounding fluid will tend to accumulate in the antinodes. The layers of fluid discussed in the following are the layers parallel to the sidewalls  810 ,  820  of the base stem  110 . 
     Depending on the density/acoustic impedance, size and weight of the particles, certain patterns of accumulations of particles will be formed. This is an advantage when separating out particles of a certain weight and/or size from a medium containing a spectrum of particles of different density/acoustic impedance. Generally, particles having a density higher than the density of the surrounding fluid, accumulates in the nodes, and particles having a density lower than the fluid without particles, accumulate in the antinodes. By providing a branching fork with two side branches or arms and one central branch or arm  140  as shown in  FIGS. 1 ,  6  or  8 , it is possible to separate out said particles. The post-branch point arms or channels are preferably arranged with spacing adapted to the wavelength, i.e., a centre to centre distance of approximately ⅜ of a wavelength. 
     Depending on the resonance conditions, confer  FIG. 7 , different results of the above will be obtained. For a single node condition, the result of the above is that the layers of fluid near the walls of the base stem  110  will contain a decreasing concentration of high density particles as the fluid flows along said stem  110  towards the branching point  175 . At said branching point  175 , fluid that mainly originates from the central parts of the fluid-stream in the stem  110  will, due to laminar flow, continue its movement straight ahead and enter the central arm  140 . Fluid originating from the fluid-stream appearing near the walls of the stem  110 , will deflect into the left arm  120  (from the left wall) and into the right arm (from the right wall). Fractions of fluid containing a low concentration of high-density particles can then be collected at the left outlet  170  and the right outlet  180 . The fraction of fluid containing a high concentration of high-density particles can be collected at the top outlet  190 . In  FIG. 9  is shown how a number of high density particles (higher density than surrounding fluid) accumulates in a central division or channel with a central outlet  91 , whereas fluid with a low or zero concentration of said particles flows out at the lateral divisions and outlets  92 . As a comparison,  FIG. 10  shows one way of using a two-node standing wave pattern to move the particles so that they can be collected at two lateral divisions provided with outlets  102 . Fluid with a low or zero concentration of said particles flows out at the central division and outlet  101 . A similar effect could also be achieved using five divisions or channels, where the most lateral channels and the central channel collect fluid with low or zero concentration of high density particles, and the other two channels collect fluid with high concentration of said particles, i.e. n=3 below. 
     By controling the frequency of the uitrasound that creates the standing wave field it is possible to generate a standing wave between the side walls of the base stem  110  with a standing wave length 0.5, 1.5, 2.5 etc. wavelengths, i.e., n times 0.5 wavelengths, n-1, 3. 5, 7 . . . cf.  FIG. 7. A  device according to the invention making use of the invention&#39;s ability to separate particles into the nodes and entinories could therefore have a number of channels after the branching point corresponding to the number of nodes plus the number of aritinodes in the standing wave field. For example, frequencies having 0.5, 1.5 and 2.5 wavelengths across the base stem  110  could have 3, 5 and 7 channels correspondingly. 
     Preferred embodiments of the invention therefore include means for controlling the frequency of the ultrasound generating means. In  FIG. 8  is shown how a control unit  863  (shown in a different scale) can be connected to the piezoelectric element  853 . Said control unit  863  is capable of delivering electrical energy to said element  853 . Said electrical energy is controllable with regard to waveform, frequency and power, where said waveform is controllable to be one of, but not limited to sinus wave, triangular wave or square wave. 
     Other embodiments of the invention include bifurcation and “trifurcation” of different shape, integrated on the same piece of material, and with the overall purpose to divide the laminar flow of fluid. 
     In  FIG. 6  is shown a detail of another embodiment where the branching point comprises the branching of the base stem  110  directly into three parallel arms  610 ,  620 ,  630  divided by thin dividing walls. By the use of the techniques described below it is possible to arrange these thin walls with a thickness of down to 1 μm and even lower. Thin walls will give better performance due to better preservation of the laminar flow profile across the full channel width. 
       FIG. 14  shows an embodiment with a left branching angle α 1  between a left arm  143  and a central arm  144  and a right branching angle α 2  between said central arm  144  and a right arm  145 . By varying the angles α 1  and α 2  it is possible to optimise certain factors such as e.g. the degree of particle concentration. However, certain angles can be difficult to manufacture with certain manufacturing processes. Angles between 0 and 90 degrees show good ability to separate flow. 
     In  FIG. 3 , which shows the device from beneath, are shown the connections  31 - 34  to the inlet  160  and to the outlets  170 ,  180 ,  190  from FIG.  1 . The piezoelectric element is not shown for the sake of clarity. 
     In  FIG. 4  the device is shown from the side. The device preferably comprises two layers, one layer  51  including the channel system, made e.g. of silicon, and one sealing layer  52  made of e.g. glass which makes it possible to visually inspect the process. The sealing glass layer could preferably be bonded with known techniques to the base layer  51 . The piezoelectric element  53  is arranged in acoustic contact with the base layer  51 . 
     In  FIGS. 5 ,  11 ,  12 , and  13  arrangements are shown where certain effects can be achieved through a consecutive or serial use of repeated structures. For example, high and low density particles can be separated using the arrangement in FIG.  5 . (high and low density indicate merely the density relatively to the surrounding fluid). Here, fluid is entered at a main inlet  60 . If a one-node resonance condition is present, fluid with high concentration of high-density particles will accumulate at outlet  61 . Fluid with low concentration of high-density particles together with high concentration of low-density particles will accumulate at outlet  62 , and fluid with intermediate concentration of high-density particles will accumulate at outlet  63 . A piezoelectric element  65  is arranged in acoustic contact with the common supporting structure, giving rise to standing wave fields in channels with appropriate dimensions, i.e. the channel parts  66  and  68 . To compensate for fluid loss, inlets  69  are provided for adding pure fluid without particles. The inlets could also be used for cleaning of the system. 
     Parallel arrangements of single or serial structures according to  FIGS. 5 ,  11 ,  12 , and  13  can easily be achieved. Channel systems according to embodiments of the invention could e.g. repeatedly and inter-connectedly be arranged, filling the area of the plate, which plate can comprise e.g., a silicon wafer or other area sheets or discs of other materials such as e.g. plastics. Parallel arrangements will add capacity, i.e. more fluid volume can be processed per time interval. 
       FIG. 11  shows schematically a one-node three-step serial washer. Contaminated fluid with particles of interest to save (e.g. red blood cells) enters at inlet  111 . Contaminated fluid with low or zero concentration of particles leaves at outlets  112 . Particles continue to flow, passing inlet  113  which adds clean fluid to the particles and some still remaining contaminants will become more diluted. Separation will be repeated in a second step where contaminated fluid with low or zero concentration of particles leaves at outlets  114 . Particles continue to flow, passing inlet  115 , which adds clean fluid to the particles and if still some remaining contaminants, these will become even more diluted. Separation will then be repeated in a third step, and particles suspended in now very clean fluid will leave at outlet  117 . 
       FIG. 12  shows schematically a one-node three-step serial concentrator. Contaminated fluid with particles of interest to save (e.g. red blood cells) enters at inlet  121 . Particles are concentrated at outlets  122 ,  124  and  128 . Contaminated fluid is removed at outlets  126 . 
       FIG. 13  shows schematically a one-node four-step integrated washer and concentrator. Contaminated fluid with particles of interest to save (e.g. red blood cells) enters at inlet  131 . Contaminated fluid with low or zero concentration of said particles leaves at outlets  132 . Clean fluid is added at inlet  134 . In a second step, (less) contaminated fluid with low or zero concentration of particles leaves at outlets  133 . Clean fluid is added at inlet  136 . In steps  3  and  4  particles are concentrated and removed through outlets  137  and  138 . Excess fluid is removed through outlets  139 . 
     Returning now to  FIG. 1 , the channel system, including the base stem  110  and the branching point, is preferably integrated on a single piece of homogenous material  51  in FIG.  4 . This entails the advantage of ease to repeat a number of channel systems thereby easily increasing the capacity of a separation apparatus making use of the invention. 
     Preferred embodiments include embodiments with channel systems integrated with a single substrate or deposited on a substrate by a continuous series of compatible processes. 
     The device according to the present invention can be manufactured for example in silicon. The requirement to make the walls of the base stem ( 810 ,  820 ) essentially perpendicular to the plate and parallel or near parallel to each other is easily fulfilled by using silicon of a &lt;110&gt; crystal structure and well known etching techniques. The desired channel wall structure described may also be realised by deep reactive ion etching, DRIE. 
     It is also possible to form the layers in plastic materials, for instance by using a silicon matrix. Many plastics have good chemical properties. The silicon layer structure can be produced by means of well-known technologies. Channels and cavities can be produced by means of anisotropic etching or plasma etching techniques. The silicon layer may be protected against etching by an oxide layer that is by forming a SiO 2  layer. Patterns may be arranged in the SiO 2  layer by means of lithographic technologies. Also, etching may be selectively stopped by doping the silicon and using p.n. etch stop or other etch stop techniques. Since all these process steps are well known in the art they are not described in detail here. 
     The above described technology is also suitable for producing a matrix or mould for moulding or casting devices of the invention in e.g. plastic. 
     The piezoelectric element providing the mechanical oscillations is preferably of the so-called multi-layer type, but a bimorph piezoceramic element may also be used as well as any other kind of ultrasound generating element with suitable dimensions. 
     An appreciated application of an embodiment of the invention is in the field of cleaning a patient&#39;s blood during surgical operations. The object in this field is to sort out the red blood cells from the contaminated plasma. Contamination could include air bubbles, fat particles, coagulation products and other not desirable biological material. The red cells will thereafter be brought back to the patient&#39;s circulation. One disadvantage with prior art in the form of centrifuges is that the red blood cells can become deformed, a disadvantage that is not present with a device according to the present invention. 
     Depending on the application, the shape and dimensions of the channel, the length of the stem  110  and the arms  120 ,  130 ,  140 , and the frequency of the ultrasound may vary. In an application for separating out red blood cells from diluted blood recovered from a patient during a surgical operation, the channel is preferably rectangular in cross-section and the stem part of the channel has a width of 700 μm for a one-node standing wave ultrasound field. Greater widths will be appropriate for standing wave ultrasound fields with more nodes. 
     The mechanical tolerance of the width of the channel is important. The difference should preferably be less than a few percent of half the wavelength of the frequency used in the material/the fluid concerned. 
       FIG. 15  shows a separation unit comprising eight channel units  1501 - 1508 , which units are supplied with fluid from a distribution cavity  1510  having one inlet  1512  and eight outlets  1521 - 1528 . Each channel unit  1501 - 1508  is provided with three outlets, one central outlet  1541  and two lateral outlets. Said lateral outlets are connected in pairs, except for the two most lateral outlets of the separation unit  1500 , forming nine intermediate outlets  1531 - 1539 . Said intermediate outlet are connected to a fast collecting cavity (not shown) alternatively to a first collecting manifold (not shown). The central outlets  1541 - 1548  are connected to a second collecting cavity alternatively to a second collecting manifold (neither shown). 
       FIG. 16  shows the separation unit  1500  of  FIG. 15  in a perspective view. The plate  1602  in which the separation unit  1500  is formed is arranged on top of an ultrasound source  1620 , preferably a piezoelectric element  1620  and a support structure  1612 . An inlet tube  1610  is connected to the distribution cavity inlet  1542  to provide an inlet for the fluid connectable to outside tubing. 
     A first outlet tube  1631  is providing a connection from the nine intermediate outlets  1531 - 1539  via a first collecting manifold to a free end  1641  of said first outlet tube  1631 . A second outlet tube  1632  is providing a connection from the eight central outlets  1541 - 1548  via a second collecting manifold to a free end  1642  of said second outlet tube  1632 . 
       FIG. 17  shows a serial arrangement in a plate  1701  of two channel units, devised to increase particle separation from a fluid. A first channel unit  1710  is formed in the plate  1702  having a central branch  1712 , which branch is connected to a base channel  1721  of a second channel unit  1720 . Each channel unit is provided with ultrasound energy from piezoelectric elements arranged under the plate  1701  at positions approximately under a portion of the base channel of each channel unit as indicated by rectangles  1716 ,  1726 . 
       FIG. 18  show a channel unit  1800  used to separate a fluid containing two types of particles, indicated as black and white, respectively. 
     When fluid flows in the direction of the arrow  1804 , ultrasound-standing waves are separating the particles in the channel unit into three fluid layers  1801 - 1803 . The position of the ultrasound source is indicated by the rectangle  1810 . 
     The described process separating two types of particles is illustrating a solution to the need within the field of medical technology to separate blood components from each other, i.e. red and white blood cells and platelets (erythrocytes, leukocytes and thrombocytes), also called the formed elements of the blood. 
     Known art in the field comprises mainly or solely solutions based on centrifugation. A disadvantage is that it is very difficult to obtain a complete separation of the formed elements, instead a so-called “buffy coat” is obtained. This buffy coat comprises a high concentration of thrombocytes, leukocytes and a low concentration of erythrocytes. In this context one should bear in mind that the sensitive thrombocytes have been centrifugated and subjected to high g-forces, which probably have induced an impaired function within said erythrocytes. 
     An embodiment of the present invention can be used to separate thrombocytes and leukocytes from erythrocytes, because they possess different densities as can be seen in table 1. Blood consists of plasma and formed elements. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Relative density 
                 Standard deviation 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Particles 
                   
                   
               
               
                   
                 Erythrocytes 
                 1.09645 
                 0.0018 
               
               
                   
                 Leukocytes 
                 1.07-1.08 
                 N/A 
               
               
                   
                 Thrombocytes 
                 1.0645 
                 0.0015 
               
               
                   
                 Fluids 
               
               
                   
                 Plasma 
                 1.0269 
                 0.0009 
               
               
                   
                 Glucose 30% 
                 1.10 
                 0 
               
               
                   
                 Glucose 50% 
                 1.17 
                 0 
               
               
                   
                 Addex electrolyte 
                 1.18 
                 0 
               
               
                   
                   
               
               
                   
                 Relative density. Source: Geigy Scientific Tables  
               
             
          
         
       
     
     Other solutions possible, iodine control agents. 
     As can be seen in table 1, different components have different density. The variation in density is very small for the table entries. When ordinary blood is separated, a channel unit will separate all formed elements in the same way, because their density is higher than the medium they are suspended in, i.e. the plasma. 
     As an alternative embodiment, the medium is modified, i.e. the plasma is modified so that its density is altered, giving the possibility to separate the different blood cells. This is achieved by adding an amount of denser liquid to the plasma and thereby dilute the plasma to a lower concentration, but with a higher density. 
     EXAMPLES 
     Take 100 ml blood with a haematocrit of 40%. This entails that 60% (=60 ml) of the blood is plasma. The plasma has a density of 1.0269. By adding 30 ml of 50% glucose solution we get according to the formula: 
         d   tot     =           v   1     *     d   1       +       v   2     *     d   2             v   1     +     v   2             
 
where
 
     v 1  is the volume of the first fluid 
     d 1  is the density of the first fluid 
     v 2  is the volume of the second fluid 
     d 2  is the density of the second fluid 
     d tot  is the density of the mix 
     The density of the mix medium becomes 1.0746. 
     When this mixture is entered in an embodiment, a separation is achieved where thrombocytes and erythrocytes are directed into separate branches, because now the thrombocytes are lighter than the medium. 
     This is of course just an example. It is also possible to separate out leukocytes because they have a specific weight, different from the one of erythrocytes and thrombocytes. It should also be possible to separate out bacteria and virus with this method. The method can be used on all solutions except those solutions where it is impossible or otherwise inappropriate to manipulate the density of the solution. It is also possible to separate out bacteria and stem cells from cultures of the same, having them suspended in a suitable solution. 
     FIG.  19  and  FIG. 20  shows a channel unit with three inlets A,B,A and three outlets C,D,C. A first fluid is fed to the channel unit at both A-inlets and a second fluid is fed to the B inlet. At this microscale, the fluids will not blend. 
       FIG. 20  shows how particles from the fluid entered at the A-inlets are forced by the ultrasound standing wave field to migrate over to the fluid entered at the B-inlet. This type of “separation” is especially useful when the objective is to keep formed elements of the blood and discard the plasma, as in e.g. plasmapheresis, and in blood wash applications where blood cells in contaminated plasma (A) are moved to a clean solution (B) and finally blood cells in a clean medium is produced (D). The waste plasma (C) is discarded. This method will enable a highly efficient blood wash with very low amounts of washing substance needed. 
       FIGS. 21 and 22  show a radial arrangement of the channel units, said arrangement being particularly advantageous when base material of the plate are circular discs or the like. 
     It will be appreciated by persons skilled in the art that the structure of the device according to the present invention has several advantages including ease of manufacture and solving of the problem of separating particles liable to disintegration in filtering and centrifugation processes.