Abstract:
A mixing and/or conveyance method and a mixing and/or conveyance device for a fluid substance, comprising a sample chamber, a cavity communicating with the sample chamber, said cavity being configured for accommodating a compressible medium, and a sound source that may be coupled to the compressible medium for the production of a fluctuation in pressure in the compressible medium. In the region of its mouth, the cavity is designed such that, in the case of an expansion in the volume of the compressible medium in the cavity, a directed fluid stream of the fluid substance escapes from the cavity through its mouth. A sample processing chip (‘lab on a chip’) comprising such a mixing and/or conveyance device.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention concerns a mixing and/or conveying method and a mixing and/or conveyance device for a fluid substance or liquid with a sample chamber, a cavity communicating with the sample chamber, designed to accommodate a compressible medium of given volume, and a sound source that may be coupled to the compressible medium for the production of a fluctuation in pressure in the compressible medium. The invention further relates to a sample processing chip (“lab on a chip”) with such a mixing and/or conveyance device. 
       BACKGROUND OF THE INVENTION 
       [0002]    Whereas various methods and devices such as magnetic agitators, vortex mixers and the like are used for the mixing of macroscopic quantities of liquids, these are usually unsuited for microscopic quantities of liquids. In particular, such mixing principles cannot be integrated into existing sample processing systems. 
         [0003]    In microfluidics, therefore, passive mixing methods and devices are mainly used. Such mixing methods and mixers used in chemical process technology, and also usually with continuous operating mode, are known for example from EP 1311341 B1 or EP 1390131 B1. These methods and mixers are usually unsuited for the mixing of small volumes of liquid. 
         [0004]    A new method is described in US 2003/0175947 A1 and in the article “Bubble-induced acoustic micromixing” by Robin H. Liu et al., LAB Chip, 2002, 2, 151-157. This presents a device for the mixing of very small volumes of liquid (22 μl), having a chamber, filled with the liquids being mixed, and cavities peripherally arranged and connected to the chamber. In the area of the cavities, air bubbles are captured and placed in resonant oscillation by acoustic excitation, so that the surrounding liquid is also placed in motion, leading to a faster blending as compared to diffusion mixing. In the corresponding model, the authors presume that the microflow created in this way is based on the frictional forces at the boundary surface between the liquid and the gas bubble. 
         [0005]    This same mixing principle is also the basis for publications WO 2006/105616 A1 and WO 2004/030800 A2. 
       SUMMARY OF THE INVENTION 
       [0006]    Accordingly, the basic problem of the invention is to achieve an improved mixing and pumping action. 
         [0007]    The problem is solved for the mixing and/or conveyance device described at the outset in that the cavity is configured in the mouth region so that, in the case of an expansion in the volume of the compressible medium in the cavity, a directed fluid stream of the fluid substance escapes from the cavity through its mouth. 
         [0008]    The method of the invention for mixing and/or conveying of a fluid substance in a sample chamber, especially for microfluidic mixing and/or conveying, accordingly calls for exciting a compressible medium in a cavity emerging into the sample chamber by means of sonic influence into a pressure oscillation, wherein a portion of the fluid substance penetrates into the mouth region of the cavity and as a result of an expansion in volume of the compressible medium it escapes from the cavity through its mouth in a directed fluid stream. 
         [0009]    By sample chamber or chamber in the sense of the invention is meant quite generally any given shaped volume with one or more openings, in which the fluid substance is enclosed, or through which the fluid substance flows—even during the mixing. 
         [0010]    The term cavity is understood here, unless otherwise specified, generally as a cavity of any given form. The term “opening” used hereafter describes the exit cross section of the cavity in a projection onto the surface of the body at the exit point. By mouth is meant the end segment or exit channel of the cavity near the opening. 
         [0011]    As described above, the “bubbles” of the compressible medium trapped in the cavities are subjected to pressure fluctuations by acoustic excitation. But the expansion in volume of the compressible medium resulting from the pressure drop is used by the invention to create a directed fluid stream of the fluid substance through the mouth and out from the cavity. The cavity must therefore be configured in the region of the mouth so that a directed stream (or jet) exits from it, ensuring a transfer of momentum to the fluid substance found in the chamber. The direction of flow of the emerging fluid jet can be adjusted, on the one hand, by the orientation of the cavity or its mouth, and on the other hand by the arrangement of shape of the opening or by a combination of both features. 
         [0012]    Preferably, the cavity has at least one channel-shaped mouth entering the sample chamber. 
         [0013]    In one simple form, the cavity has, at least in the region of the mouth, a tubular segment with constant cross section, or in more complicated configuration a special nozzle geometry, to boost the efficiency of the propulsion, for example. 
         [0014]    Thus, the invention makes use of a different principle, namely, at sufficiently large Reynold&#39;s number, Re≧around 50, a fluid emerging from a channel exits in the form of a directed jet, while a fluid sucked into the channel enters the channel equally from the entire available range of spatial angles. The principle is described in the article “The acoustic scallop: a bubble-powered actuator” by Dijkink et al., Journal of Micromechanics and Microengineering, 16, 2006, 1653-1659. Here, it is proposed to use the principle as propulsion for an “acoustic windmill”. A Teflon tube closed at one end is accordingly dipped into a water-filled container, so that an air bubble is enclosed in its capillary. Sound by means of a piezoactuator is delivered through the water into the air bubble, thereby exciting an oscillation. The alternating expansion and contraction of the volume alternately sucks liquid into the tube and expels it again. Thanks to the described asymmetry between the directed expulsion and the nondirected suction, an overall transfer of momentum to the tube occurs in the course of one full oscillation, which is used for its propulsion in the direction of its lengthwise axis. 
         [0015]    Unlike in the article, in the present case the principle is used not for propulsion, but to introduce a net momentum transfer into the standing or flowing liquid and thereby, in conjunction with the geometry of the sample chamber and the arrangement of the cavities, to achieve an increased swirling and thus an improved mixing of the liquids, on the one hand, and/or an (improved) transport of the liquids in a particular direction. 
         [0016]    Preferably the surface of the cavity is oriented to not cause wetting, at least in part. 
         [0017]    In this way, one makes sure that an air bubble enclosed in the cavity remains trapped there because of the surface tension of the fluid substance. 
         [0018]    Especially preferably, the surface of the cavity is oriented to cause wetting in the region of the mouth. 
         [0019]    This makes sure that a portion of the fluid substance or liquid, thanks to the capillary effect, enters at least into the region of the mouth and stands ready there as a liquid column to form the fluid stream. 
         [0020]    A local wetting or non-wetting of the surfaces can be achieved by modification of the surface 
         [0021]    When used for an aqueous solution, the mixing and/or conveyance device is either formed from a hydrophobic material, for example, and made hydrophilic in the region of the mount and as far as necessary in other surface areas outside the cavity, or it is formed from a hydrophilic material and made hydrophobic in the region of the cavity (or cavities), and possibly outside of the mouth(s). 
         [0022]    It can be made hydrophilic or hydrophobic in familiar fashion, by a dip method, as described in DE 100133111 C2, or by a coating. For example, polycarbonate, being a slightly hydrophobic material, can be made hydrophilic by an O2 plasma treatment on the surface. 
         [0023]    An alternative makeup has the cavity formed from a tube or hose segment or insert, for example, one made from polycarbonate or from polytetrafluorethylene, which is nonwettable (hydrophobic). 
         [0024]    When using the mixing and/or conveyance device for nonpolar organic liquids, accordingly, lipophilic surfaces can be used in the region of the mouth and as far as necessary in other surface regions, as well as lipophobic surfaces in the region of the cavity (or cavities), possibly outside the mouth(s). This is done preferably by forming the mixing and/or conveyance device from a lipophilic material and making it lipophobic in the region of the cavity, possibly outside of the mouth, or by forming it from a lipophobic material and making it lipophilic in the region of the mouth and as far as necessary in other surface regions, outside of the cavity. 
         [0025]    In one advantageous modification, the mixing and/or conveyance device has a pressure-increasing means in communication with the sample chamber, which is designed to apply a pressure to the fluid substance or liquid so that a portion of the fluid substance penetrates into the mouth region of the cavity under compression of the compressible medium. 
         [0026]    Alternatively or in addition to the wettable mouth region, this also ensures that enough fluid substance is present as a liquid column to form the fluid stream in the mouth region of the cavity. 
         [0027]    In one preferred embodiment of the method, the compressible medium emerges through the mouth or mouths relative to the sample chamber such that a net angular momentum is introduced into the fluid substance in the sample chamber. The corresponding mixing and/or conveyance device has cavities with mouths opening into the sample chamber, being arranged and oriented relative to the sample chamber so that the directed fluid streams exiting from the cavities through the mouths upon expansion in the volume of the compressible medium introduce a sum angular momentum into the fluid substance inside the sample chamber. 
         [0028]    In this way, the entire liquid present in the sample chamber is placed in a rotational movement. This ensures an overall greater swirling in the overall sample chamber. The risk of not enough mixing taking place in “dead corners” is lessened. 
         [0029]    In an alternative preferred embodiment of the method, the compressible medium emerges through the mouth or mouths relative to the sample chamber such that a net linear momentum is introduced into the fluid substance in the sample chamber. The corresponding mixing and/or conveyance device has cavities with mouths opening into the sample chamber, being arranged and oriented relative to the sample chamber so that the directed fluid streams exiting from the cavities through the mouths upon expansion in the volume of the compressible medium introduce a sum linear momentum into the fluid substance inside the sample chamber. 
         [0030]    In this way, a pump effect is achieved, which delivers the fluid in the sample chamber in one direction, for example, from one inlet opening toward one outlet opening. 
         [0031]    The cavity in the most simple and thus most cost-effective case is inserted in the form of a blind borehole or a channel closed at one end into one wall of the sample chamber. 
         [0032]    Preferably, the sound source is coupled directly to one wall of the sample chamber. 
         [0033]    If the sound source in one region of the sample chamber wall is connected to the mixing and/or conveyance device, which during operation is in contact with the fluid substance at the opposite side, the pressure oscillation is exported directly through the fluid substance and conveyed to the compressible medium. In this way, losses are avoided, such as those due to large impedance differences in the sound transmission, for example, because of an air/gas cushion next to the inside of the container wall. 
         [0034]    In another advantageous embodiment, the cavity is in the form of a connection channel, which is connected on the one hand via the mouth to the sample chamber and on the other hand to a pressure chamber filled with a coupling medium, to which the sound source is coupled. 
         [0035]    Thus, the pressure oscillation is delivered not via the substance being mixed but instead from the opposite side through the coupling medium located in the pressure chamber and into the compressible medium. Using an appropriate channel system for the coupling medium, one central pressure chamber can supply several or all cavities. It is advantageous for the cavities to be individually or groupwise actuated in the desired sequence by a valve control unit. 
         [0036]    The compressible medium is preferably a gas, especially air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Further problems, features and benefits of the invention will be explained more closely below by means of sample embodiments with the help of the drawings. There are shown: 
           [0038]      FIG. 1 , a cross section through the mixing device of the invention in a sample processing chip according to a first embodiment, viewed from the side; 
           [0039]      FIG. 2 , the mixing device of the invention according to a second embodiment, in cross section from above; 
           [0040]      FIG. 3 , a cross section through the conveyance device of the invention according to another embodiment; 
           [0041]      FIG. 4 , a sample processing chip with two mixing devices, one switched behind the other, and external pressure chamber, in cross section from above; 
           [0042]      FIG. 5 , a cross section through the mixing device of the invention in a sample processing chip according to another embodiment, viewed from above; and 
           [0043]      FIG. 6 , a cross section through the nozzlelike mouth region of a cavity. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]      FIG. 1  shows the most simple embodiment of the invented mixing device. A sample chamber  100  in the form of a channel with constant cross section or a sample chamber broadening out transversely to the cross sectional plane is placed in a sample processing chip  110 . A liquid drop  112  of the substance being mixed (hereinafter also known as the “plug”), recognizable from the boundary surfaces  113  and  114  with the surrounding air/gas, is delivered by a pump (not shown), for example, through the channel  100 . 
         [0045]    Transversely to the channel  100  is located a cavity  116 , in this case emerging into the channel  100  from below, and being designed in the shape of a small, unvented channel or blind borehole. If the liquid drop  112  reaches the region around the cavity  116 , as shown in  FIG. 1 , an air or gas bubble  118  becomes enclosed as compressible medium in the cavity  116 . 
         [0046]    It is critical to the mode of action of the invention that the liquid being mixed penetrates at least partly into the mouth region of the cavity  116  and provides here a liquid column, indicated by the boundary surface  120  between the air bubble  118  and the liquid plug  112 . Two different mechanisms can be used optionally or at the same time to form this liquid column. First, one can make sure that the cavity  116  is wetted in its mouth region. This can be achieved by an appropriate choice of the material of the cavity in the mouth region, i.e., possibly by a surface modification, for example, if the cavity is formed from a material not otherwise wettable. Alternatively or additionally, the liquid plug  112  can be subjected to pressure from the pump connected to the channel  100 , so that the air bubble  118  enclosed in the cavity is compressed. A suitable provision must make sure that the liquid plug  112  is located exactly above the cavity  116  when the nominal pressure is attained. This can be done by applying pressure to both sides of the liquid plug  112 . 
         [0047]    On the top side of the sample processing chip  110  is mounted a sound source in the form of a piezo-actuator  122 . The piezo-actuator  122  is hooked up to an alternating voltage source  124 , which excites it into oscillation. When using a piezo-actuator, one must make sure that it is prestressed. For this purpose, in the sample processing chip of  FIG. 1 , pressure can be exerted on a suitable wall  111  above the sample chamber  100 , which in practice is formed by a cover film on the top side. The oscillation is delivered across the wall  111  of the sample processing chip  110  opposite the cavity  116  into the liquid plug  112  and carried further by the latter. Because the input occurs in a region in direct contact with the liquid plug  112 , the sound is taken directly to the fluid substance and no large losses occur from impedance differences, for example, due to a layer of air lying in between. By adjusting an appropriate frequency, the input sound can be adapted to the resonance frequency of the air bubble  118  enclosed in the cavity  116 . This ensures a boosting of the amplitude of the oscillation in the cavity and, thus, an efficient utilization of the input sound. 
         [0048]    By oscillation of the air bubble  118 , the liquid column standing in the mouth region of the cavity  116  is sucked in and expelled out. The expelling of the liquid column, as shown by the arrow  126 , is directional, whereas the suctioning of the liquid from the sample chamber  100  occurs from all available directions with approximately the same speed. As a result, considering one complete oscillation, there is a net momentum introduced into the liquid plug  112  in the direction of the arrow. The liquid in the plug  112  is thereby swirled and the mixing is accelerated. 
         [0049]    The cross section of the cavity  116  is typically smaller than that of the channel or the sample processing chamber  100 , so that the volume of liquid sucked in and expelled does not already comprise a major portion of the liquid volume of the entire plug  112 . While the channel cross section or the height of the sample chamber  100  is typically several tens of millimeters to a few centimeters, the diameter of the cavity  116  is only several micrometers to a few millimeters, preferably a few dozen millimeters in size. The length of the cavity  116  can be 0.5 mm to 50 mm, which ensures that a sufficiently large volume of air is available for compression in the cavity  116 . 
         [0050]      FIG. 2  shows a sample chamber  200  in a sample processing chip  210  from above. The sample chamber  200  is bulging in the lateral dimension and of roughly circular cross section. The chamber can have a constant height, a broadening or a constriction in the vertical direction perpendicular to the plane of the drawing. 
         [0051]    The sample chamber  200  at the inlet end is connected to a feed  202  for the fluid substance being mixed and at the outlet end to a drain  204 . In the sample chamber  200  there is a liquid drop or plug  212 , recognizable from the boundary surfaces  213 ,  214 . Around the sample chamber  200  are arranged four cavities  216 , emptying tangentially into it. 
         [0052]    If sound is put into the liquid by means of a sound source (not shown), air bubbles enclosed in the cavities  216  are set in oscillation, as described in connection with the example from  FIG. 1 . The result is a tangential net momentum input from each cavity  216  into the liquid located in the chamber  200 . The momenta add up, due to the arrangement of the four cavities  216 , so that the liquid in the plug  212  is placed in rotation. This arrangement is advantageous for the mixing of rather large volumes of liquid. 
         [0053]    A more uniform momentum input from all four cavities  216  is only ensured if the air bubbles enclosed therein have the same dimensions, since only then will all four be placed equally in resonant oscillation by the same frequency. But one can also make deliberate use of resonance differences. For example, two of the four cavities can empty into the sample chamber in opposite tangential direction. If the cavities pointing in the same direction have the same dimensions, but the oppositely working ones have different dimensions, their resonance frequencies will also be different. Thus, by coordinating the sound frequency applied, one can excite one rotation in one direction and then in the other direction in alternating fashion. This switching ensures a chaotic flow pattern and thus an even better blending of the liquid. 
         [0054]      FIG. 3  shows a sample embodiment of the invented conveyance device in cross section, again comprising a sample chamber  300  in the form of a channel in a sample processing chip  310 . Moreover, the conveyance device has several cavities  316 , which empty at an acute angle to the lengthwise axis of the channel  300  into it. In the channel  300  there is a liquid, which penetrates in the above described manner into the mouths of the cavities  316  and then encloses an air bubble each time. If this is placed in pressure oscillation by a sound source, then the liquid column in the already described manner will be delivered out from the mouth region of each cavity (synchronously). The momentum inputs to the liquid located in the channel add up to a net linear momentum in the direction of the channel axis, indicated by the arrow  326 . In this way, a micropump device is created. 
         [0055]      FIG. 4  shows a cutout of a sample processing chip  410  with two sample chambers  400 ,  401 , into each of which two feeds  402 ,  403  and  404 ,  405  empty. The sample chambers are switched immediately one behind the other, so that the drain  406  of the first sample chamber  400  empties directly into one of the feeds  404  of the second sample chamber  401 . Thus, while in sample chamber  400  the substances A and B introduced by the feed  402  and  403  are mixed to form a mixed substance A+B, in the second sample chamber  401  the mixed substance A+B is mixed with a further substance C to form the mixed substance A+B+C. 
         [0056]    The mixing is done by the already described principle. One cavity each,  416  or  417 , empties into the two sample chambers. If the liquid plug from the two initially unmixed substances A and B is present in the first sample chamber  400 , an air or gas bubble  418  is enclosed in the cavity  416 . Contrary to the already described sample embodiments, however, the cavity  416  is not a blind borehole closed at one end, but instead a connection line joined at the back side to a pressure chamber  430 , which is filled with a coupling medium. The air bubble  418  is enclosed between the substance being mixed, on the one hand, and the coupling medium, on the other. A sonic field is introduced into the coupling medium in the already described manner, preferably by applying a sound source in the region of the pressure chamber  430 . This puts out the oscillation to the air or gas bubble, which pulsates thanks to its compressibility and causes the fluid substance to flow in the mount of the cavity. 
         [0057]    When the two substances A and B are sufficiently mixed, the liquid plug of the mixed substance A+B is transported toward the second sample chamber  401 , where it is combined with substance C. 
         [0058]    The pressure chamber can be connected by a branching system of lines both to the cavity  416  emptying into the first sample chamber  400  and to the cavity  417  emptying into the second sample chamber  401 . Therefore, one needs only one sound source connected to the common pressure chamber  430  for the mixing in the two sample chambers  400 ,  401 . 
         [0059]    A valve  432  installed in the line of the coupling medium is switched over after the mixing process in the first sample chamber  400 , so that now only the cavity  417  emptying into the second sample chamber  401  is connected to the pressure chamber  430 . An air bubble enclosed in the cavity  417  (not shown) can now be placed in oscillation by the coupling medium and the substances A, B and C will be mixed. 
         [0060]    The feeding and draining of the substances A, B and C occurs through a suitable valve system, not otherwise shown, to make sure that the substances take the proper path and remain in the respective mixing chambers during the mixing. 
         [0061]      FIG. 5  shows yet another simple embodiment of the mixing device of the invention in a sample processing chip  510 . A sample chamber  500  is formed in the shape of a volume bulging in the lateral dimension. The chamber  500  can have a constant height, a broadening or a constriction in the vertical direction perpendicular to the plane of the drawing. 
         [0062]    The sample chamber  500  is connected at the inlet end to a feed  502  for the fluid substance being mixed and at the outlet end to a drain  504 . Contrary to the sample chamber of  FIG. 2 , the feed and the drain each have bottlenecks or constrictions  506  and  508 . In the sample chamber  500  there is a liquid drop or plug  512 , recognizable from the boundary surfaces  513 ,  514 . The volume of the liquid drop  512  is attuned to the volume of the chamber so that the boundary surfaces  513 ,  514  come to lie in the region of the bottlenecks  506 ,  508 . This has two benefits: the liquid drop  512  can be held more easily in the sample chamber and the introduced sound can be utilized more efficiently thanks to the small surfaces  513 ,  514  bounding off from the surroundings—less sonic energy is dissipated. The liquid drop  512  is more easily held or positioned in that the surface of the chamber  500  and the feed and drain  502 ,  504  are preferably wettable in design. Thus, the fluid substance penetrates into the bottlenecks and is halted at their transition to the next segment of the drain  504  in the direction of flow. In order to move the liquid further out from the chamber  500 , energy must be expended, since an enlarging of the surface occurs. The liquid is therefore held securely in the chamber until enough energy is expended for the further transport. 
         [0063]    The cavity  516  in this case is located centrally and transversely to the axis formed by the feed and the drain. As in the example of  FIG. 1 , it is in the form of a small, unvented channel or a blind borehole. The principle of functioning is the same. The sample embodiments shown for the mixing device of the invention are especially suited for continuous or quasicontinuous operation. By quasicontinuous is meant in this context a sequentially working mixing device, in which individual volumes of liquid are taken through the sample chamber and mixed one after the other. 
         [0064]    The sample chamber—regardless of its shape otherwise—is preferably formed without sharp corners or edges and with the most streamlined possible form, so that no liquid residues remain at places of poor flow or in the corners and edges, and the chamber can be filled and emptied as completely as possible. 
         [0065]    This also preferably pertains to the cavity  616  of the invented mixing and/or conveyance device, which has a mouth  634  in the form of a nozzle with a streamlined tapering cross section of its mouth in the direction of the opening  636  and a sharp cutoff edge in the plane of the opening, as shown schematically in  FIG. 6 . 
       LIST OF REFERENCE SYMBOLS  
       [0000]    
       
           100  Sample chamber/channel 
           110  Sample processing chip 
           111  Wall 
           112  Liquid drop/plug 
           113  Boundary surface 
           114  Boundary surface 
           116  Cavity 
           118  Air bubble 
           120  Boundary surface 
           122  Piezoactuator 
           124  Alternating voltage source 
           126  Momentum 
           200  Sample chamber 
           202  Feed 
           204  Drain 
           210  Sample processing chip 
           212  Liquid drop/plug 
           213  Boundary surface 
           214  Boundary surface 
           216  Cavity 
           226  Angular momentum 
           300  Sample chamber 
           310  Sample processing chip 
           316  Cavity 
           326  Net momentum 
           400  Sample chamber 
           401  Sample chamber 
           402  Feed 
           403  Feed 
           404  Feed 
           405  Feed 
           406  Drain 
           407  Drain 
           410  Sample processing chip 
           416  Cavity 
           417  Cavity 
           418  Air bubble 
           430  Pressure chamber 
           432  Valve 
           500  Sample chamber 
           502  Feed 
           504  Drain 
           506  Bottleneck 
           508  Bottleneck 
           510  Sample processing chip 
           512  Liquid drop/plug 
           513  Boundary surface 
           514  Boundary surface 
           516  Cavity 
           616  Cavity 
           634  Mouth region 
           636  Opening