Patent Publication Number: US-6663757-B1

Title: Method and device for the convective movement of liquids in microsystems

Description:
The invention concerns processes for convective movement of static or flowing fluids in Microsystems, particularly for electroconvective or thermoconvective mixing of the fluids, and devices for implementation of the processes, particularly electrode arrangements in Microsystems for triggering convective fluid movements. 
     In numerous technical fields, particularly in chemical technology, the task of circulating or stirring a fluid or mingling or mixing several fluids frequently arises. For this purpose, fluid currents are generated which are, for example, mechanically circulated by means of mechanical barriers and/or actively movable elements. During the turbulent swirling, the fluid(s) is/are mutually interspersed. The Reynolds number of a fluid is important for the effectiveness of its circulation in a channel or container structure. For the mechanical mixing of fluids in a container structure, this Reynolds number must have a value over 1000. These values can only achieved in macroscopic systems, as the following estimation shows. 
     The Reynolds number of a channel can be estimated according to Re =(ρ·U·L)/η, with ρ being the density of the fluid, η being the dynamic viscosity of the fluid, U being the flow speed, and L being a characteristic channel dimension (e.g. radius of the channel cross-section). An aqueous solution with kinematic viscosity ν=η/ρ=1.6·10 −2  cm 2 /s which flows through a channel with a radius r=25 μm at a speed U=500 μm/s, would, for example, result in a Reynolds number R e ≈0.008, which is far below the required value of 1000 mentioned above. The mingling of fluids due to obstacles in the flow by means of flow mechanics is therefore restricted to macroscopic systems. The use of actively moving elements for the circulation of fluid is also restricted to macroscopic systems, because movable elements are subject to breakdown and can easily cause blockages or flow impediments in miniaturized systems. 
     The measurement and/or analysis systems of many biological, medical, and chemical technology applications have been miniaturized in the last decade for reasons of cost and resources and to achieve highly specific analyses. The problem of fluid circulation in Microsystems has, however, not yet been solved. Due to the low Reynolds number, even with flowing around, e.g., barriers which cross in a meander shape or sharp-edged flow obstructions, no turbulent flow can result. Therefore, if two fluids are introduced into a miniaturized channel (typical cross-section: 1 mm 2 ), no mixing of the fluids will result except through diffusion, even during flow through a channel length of several millimeters. 
     A generally known attempt to cause the circulation of flowing fluids in Microsystems consists of splitting a channel into a number of narrower channels and rejoining them with a changed relative arrangement. It is true that no movable parts are used hereby. However, the narrowed channels have a characteristic diameter which is smaller than the initial channel by a factor of 10 to 40. The flow resistance thereby increases and an acute danger of blockage arises. Usage for suspensions which contain particles such as biological cells or microbeads is excluded. In addition, only a quasi-blending corresponding to the number and rearrangement of the narrowed channels results. 
     Pumping fluids on the basis of electrohydrodynamic effects is also known. Traveling electric fields are generated in fluid channels with electrode systems which are affixed to opposing channel walls over the entire length of the channel. In combination with a temperature gradient which is directed from one of the electrode systems to the opposite electrode system, so-called electroconvection occurs, which effects a stationary fluid transport in the channel. These types of systems are, for example, described as traveling wave pumps or electrohydrodynamic pumps by J. R. Melcher et al. in “The Physics of Fluids”, vol. 10, 1967, p. 1178 et seq. The mechanical fluid propulsion is caused in such a way that, due to the temperature gradients in the fluid, conductivity and/or dielectric constant gradients arise. Volume charges are thereby generated which exercise a propulsive force on the fluid by interacting with the traveling electric field. 
     The system described by J. R. Melcher et al. is a macroscopic system with a channel length of approximately 1 m and a typical channel cross-section of approximately 3 cm. It serves exclusively for the investigation of electrode convection and, due to the expensive measures for the production of the temperature gradients and for driving the electrodes over the entire length of the channel, does not allow for practical use. 
     Miniaturized traveling wave pumps are described by Fuhr et al. in “MEMS 92”, 1992, p. 25. The implementation of the traveling wave principle in Microsystems has, however, not yet found practical application, because there are significantly simpler possibilities for fluid transport in microchannels and a contribution to the problems described above of fluid circulation in Microsystems has not been provided. Fluid circulation would specifically mean that the sum of the fluids circulating in a region of the Microsystems is zero. The conventional traveling wave pumps, however, always provide a net flow of solution. Directed pumping along the channel direction occurs in the microsystem. Mixing of fluids is not possible with the conventional traveling wave pumps. 
     It is the object of the invention to provide improved processes for convective movement of fluids in Microsystems, with which circulation or blending of fluids in microchannels is made possible without moving parts and without narrowing the channels, and with any desired channel cross-section. In particular, the object is to provide a process for effective fluid mixing in Microsystems which can also be used with suspensions containing microparticles. It is also the object of the invention to indicate devices for implementation of the process mentioned, particularly miniaturized fluid mixers. 
     According to a first aspect of the invention in particular, a new process for convective fluid movement in microsystems is created in which one or more fluids in the microsystem are subjected to traveling electric fields, alternating fields, or electrical field gradients having an alignment which deviates from a flow direction of the fluid in the microsystem and/or a preferred lengthwise alignment of a section of the microsystem (e.g. channel section). The alignment of the alternating fields (preferred direction of the field-generating electrodes), of the traveling electrical fields (direction of travel), or of the field gradients will be generally referred to in the following as the field direction. According to the invention, the field direction runs, e.g., perpendicular to the flow direction of the fluid and/or perpendicular to the channel alignment. 
     The convective fluid movement can be generated both in flowing fluids (transverse to the flow direction) and in static fluid volumes (e.g. in a closed part of the microsystem). The convective fluid movement is characterized by a closed fluid circulation. The sum of the flows caused in the region of the field gradients implemented according to the invention is zero. Thus, for example, flow loops are generated transverse to the direction of the channel which cause a swirling and mixing of the fluids involved. This is a surprising result, after free mixing of fluids in microsystems was thought to be impossible due to the reasons of flow mechanics described above. 
     The convective fluid movement is triggered according to the following principles. At the interface between two fluids with different dielectric constants (or conductivities), the field gradients lead to the appearance of polarization and force effects which lead to blending at the interface and at each new interface. In fluids or fluid mixtures with sufficient anisotropy of the dielectric properties or polarization properties, blending is achieved by the electrical field gradients alone. If the fluid is isotropic, electrical anisotropy must be artificially achieved by formation of a thermal gradient. The action of the thermal gradients will be explained with the following image. As the temperature changes in a fluid which is initially isotropic, gradients of the dielectric properties or polarization properties corresponding to the temperature gradients are also formed. The fluid can be viewed as a lamination of many dielectrically different fluids. The effects mentioned for the anisotropic fluids arise at the interfaces between the layers. The appearance of electrical polarization leads to mingling of the fluid. 
     According to a preferred embodiment of the invention, the formation of a thermal field gradient parallel to the field direction thereby occurs simultaneously with the generation of the electric fields. The thermal gradient is required in order to generate the anisotropy in the fluid which leads, together with the electrical field, to the fluid propulsion. In contrast to the conventional traveling wave pumps, a thermal gradient with a temperature difference between opposing channel walls of 0.5° to 1° is sufficient to generate the fluid circulation or cross-current according to the invention. A particular advantage of the invention is that this type of temperature difference can be achieved just by the application of electrical voltages for generating the electric fields to the electrode arrangements, so that separate generation of an external thermal gradient is not urgently necessary. 
     If the thermal gradient is externally generated, this is preferably done with optical irradiation. The region of the microsystem of interest, in which the electrical field gradients are implemented, is radiated with light of a suitable wavelength which is well absorbed in the respective fluid. The irradiation is preferably performed with a focused laser beam which, depending on the application, can be coupled from any desired side of the microsystem through transparent wall regions or using light guides. So-called “hot spots” are formed by the optically induced increase in temperature, which work together particularly effectively with the electrical field gradients to generate the convective fluid movement. 
     According to the invention, there is a predetermined difference in angle between the field direction and the direction of the current flow direction of the fluid and/or the flow direction before or after realization of the process. In the following, the concept of flow direction is generally used for the alignment of the fluid flow or for the alignment of the microsystem region in which the fluid flows. The angle between the field direction and the flow direction is preferably in the range from 60° to 120°. For values above 90°, this means that the field direction has a component which is opposite to the flow direction. 
     According to a further aspect of the invention, a fluid microsystem is provided having structures which are set up for fluid conduction or fluid accomodation and, in at least one predetermined section (swirling section), have an electrode arrangement for the formation of the traveling electric fields, electrical field gradients, or AC voltages corresponding to the desired field direction. The structures in the microsystem preferably have a characteristic cross-sectional dimension of less than 150 μm. Typically, a structure is implemented as a microchannel with a cross-sectional area of approximately 1 mm 2  (or less), e.g. cross-sectional dimensions of 100 μm·100 μm or less. The provision of swirling sections is possible in all types of Microsystems known per se. The application of electrode arrangements according to the invention to straight channels is preferred. 
     The subject of the invention is also an electrode arrangement affixed to at least one wall of a microchannel for the implementation of the field effects mentioned in a field direction deviating from the channel alignment. Because the thermal gradients are generated in the field direction simultaneously with the electrical driving, the electrode arrangement consists of electrode elements which have an asymmetrical or irregular shape relative to the field direction. This applies at least for the embodiment in which the electrical fields comprise electrical field gradients or AC voltages. If traveling electrical fields are used, asymmetry of the electrode elements is not necessary, because then the thermal field gradient can also be generated through delayed driving of the electrode elements. 
     The invention has the following advantages. For the first time, the convective fluid movement for the generation of fluid cross-currents and/or swirls in microchannels is realized. The electrode arrangements according to the invention have a simple and compact design. It is therefore sufficient if the swirling sections in the microsystem have a relatively small extension in the lengthwise direction of the channel, in the range approximately between a fifth of and one channel cross-sectional dimension. The fluid swirling according to the invention can be realized in both still and flowing fluids. An effective temperature gradient can be easily generated electrically with the electrode arrangements. The application of an additional, external temperature gradient is possible, but not urgently required. The invention is easily compatible with other microstructure technologies. Thus, the electrode arrangements can consist of electrodes which are essentially designed like electrodes for the generation of field barriers for dielectrophoretic manipulation. According to the invention, no moving parts are required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and characteristics of the invention will become apparent from the following description of the attached drawings. These show: 
     FIGS. 1 to  7  various embodiments of electrode arrangements according to the invention in microchannels shown in sections in schematic perspective view, and 
     FIG. 8 an illustration for the use of the invention in fluid blending in DNA chips. 
    
    
     For reasons of clarity, the invention will be described in the following with reference to exemplary embodiments in which the angle between the field direction and the flow direction is 90°. Implementation with different angle values is possible by appropriate adjustment of the electrode arrangements. For this purpose, the electrode arrangements are each aligned appropriately for the desired field effects. 
     An enlarged perspective view of a section of a channel  13  in a microsystem is shown in FIG.  1 . The channel  13  has a rectangular cross-section with dimensions a and b, which lie in the range from a few to a few hundred micrometers or even less. The upper limit for the dimensions a and b is approximately 1 mm. The walls of the channel  13  are referred to in the following as the bottom, cover, and side surfaces according to their location in the operating position. The channel  13  is part of a microsystem which, for example, essentially consists of plastic or a semiconductor material. The microsystem is preferably processed with methods from semiconductor technology on a substrate for formation of a microsystem chip. 
     The channel  13  is set up to have a fluid (solution or suspension) flow through it in the direction of the arrow  14 . The flow direction  14  corresponds to the lengthwise extension of the channel  13 . On the entrance side, the channel  13  is connected with other parts of the microsystem (not shown). In the implementation as a fluid mixer, several partial channels discharge into the channel  13  upstream relative to the swirling section  10 , which is described in the following. 
     The swirling section  10  is formed by an electrode arrangement  11 ,  12  affixed to the channel walls. The electrode arrangement  11 ,  12  consists of two electrode groups which are affixed to opposing channel walls. For a rectangular channel cross-section (as shown), the electrode groups are, in order to achieve higher mixing effectiveness, preferably provided on the channel walls with the larger transverse width, i.e. in the present case on the bottom and cover surfaces. Alternatively, one or more electrode groups can also be affixed to the side surfaces or, depending on the application, to one or more of the bottom, cover, or side surfaces. 
     The electrode groups extend along the respective channel wall over the entire width of the channel and, in the flow direction  14 , over the length of the swirling section, which is selected according to the application. The length can, for example, correspond to the channel width or be shorter than this (down to a fifth of the channel width). The electrode groups preferably have an equal dimension in the lengthwise direction of the channel (corresponding to the flow direction  14 ). Different dimensions can also, however, be provided, as is described below. The electrode groups are positioned, relative to the flow direction  14 , opposite to one another or offset. 
     In the embodiment according to FIG. 1, each electrode groups consists of a number of lower electrode strips  11  on the bottom surface and/or upper electrode strips  12  on the cover surface of the channel  13 . The electrode strips each have separate control lines. For reasons of clarity, only the control lines  11   a  of the lower electrode strips  11  are shown. The electrode strips can be driven individually or in groups (e.g. joint driving of every third electrode strip). 
     The electrodes strips have a planar shape, i.e. they are applied as layers on the respective channel wall, with a thickness which is significantly smaller than the channel height a. The channel cross-section is thus practically not narrowed by the electrodes. The electrode strips have a length corresponding to the length of the swirling section and a predetermined width and/or predetermined strip intervals. The strip width and the strip interval selected are in the range from approximately {fraction (1/20)} to ⅕ of the channel height a or less. Depending on the application, it can be provided that the electrode strips have differing widths and differing strip intervals, as well as having differing shapes, because these characteristics influence the effectiveness of the fluid swirling. The electrode strips run lengthwise along the channel and are set up to generate a field effect transverse to the lengthwise channel direction (see below). 
     The electrodes preferably consist of an inert metal (e.g. gold, platinum, titanium) in all embodiments of the invention. The electrode strips and the associated control lines are advisably produced using the methods of semiconductor technology on the respective substrate surface. 
     The electrode groups are driven according to the invention with a control device (not shown) according to one or more of the following alternatives. 
     According to a first design, electrical traveling waves, as they are known from the traveling wave pumps mentioned above, are formed at the electrode strips. To generate a traveling wave, the electrode strips are driven in sequence in such a way that a field maximum moving transverse to the flow direction results. For this purpose, high frequency signals with a specific phase shift are applied to the electrode strips. The frequency of the high frequency signals approximately corresponds to the reciprocal value of the relaxation time of the charge carriers in the fluid and is in the range from kHz to MHz. According to a preferred embodiment, a traveling wave with at least three signals phase shifted to one another is generated. For example, four signals with an amplitude in the range of volts are provided, which are each phase-shifted by 90°. 
     According to a second design, electrical field gradients are constructed in the field direction slanted or transverse to the flow direction  14 . The electrode strips have high frequency signals applied to them in phase, which nonetheless have an amplitude (e.g. in the range of 0.1 V to 100 V) varying from strip to strip (typically &lt;20 V). 
     Finally, it is provided according to a further design that a high frequency AC voltage (amplitude in the range of volts) is applied to one or both of the electrode groups, as parts or in a unit, in order to achieve fluid crosscurrents or a fluid swirling in the swirling section. In this embodiment, all partial electrodes of the electrode groups are driven jointly, or the electrode groups each consist of only one joint electrode, which is nonetheless structured to generate the thermal gradients (cf. FIG.  5 ). 
     An electroconvective circulation of the fluid passing through the channel  13  occurs under the effect of electric fields according to the invention. A particular advantage of the invention is that the circulation of the fluid (e.g. mixing of several fluids) can be realized in flow operation at flow speeds of up to 1000 μm/s. 
     The generation of the swirling or the transverse and/or ring flows transverse or slanted to the alignment of the channel can be influenced by an additional temperature adjustment of the channel. If a temperature gradient is applied in the region of the swirling section transverse to the channel alignment, particularly by heating the cover surface or cooling the bottom surface of the channel  13 , the swirling can be intensified. This is advantageous because, simultaneously to the temperature adjustment, reduction of the amplitude of the control signal is made possible. 
     Although FIG. 1 only shows one pair of electrode groups, multiple swirling sections with, correspondingly, multiple electrode groups can be provided in the lengthwise direction of the channel. 
     FIG. 2 shows further embodiments of electrode arrangements according to the invention, which again each consist of two electrode groups affixed to opposite channel walls. Each electrode group consists of triangular or arrow-shaped electrode elements arranged in a straight line. Lining up the elements forms a strip with an alignment corresponding to the desired field direction and slanted or transverse to the flow direction. The electrode elements are lined up in such a way that each triangle tip points toward a triangle side of the neighboring electrode element. In the channel  23 , three pairs of electrode groups are shown. The electrode groups  21   a ,  22   a  are symmetrically designed, i.e. both electrode groups consists of electrode elements of the same size and orientation. The electrode groups  21   b ,  22   b  form an asymmetrical design, in which the electrode group  21   b  on the bottom surface has a smaller number of larger electrode elements than the electrode group  22   b on the cover surface. The pair of electrode groups  21   c ,  22   c , which each consist of electrode elements of equal size, but with a reversed orientation in regard to the triangle direction, show a further asymmetrical design. 
     In FIG. 2, the control lines of the individual electrode elements are not shown. The electrode elements are arranged electrically insulated from one another and thus can be driven separately or in groups. The driving of the electrode elements can be performed analogous to the driving of the strip electrodes according to FIG.  1 . 
     Further embodiments with irregular electrode shapes are depicted in FIG.  3 . Again, an electrode arrangement according to the invention consists of two electrode groups which are affixed to opposing channel walls. Each electrode group consists of electrode elements arranged in a row which have flat, triangular, or rectangular shapes of various sizes. In the electrode groups  31   a ,  32   a , the rectangular electrode elements of each electrode group each form a strip which is aligned in the desired field direction (in this case, e.g., perpendicular to the flow direction). In the electrode groups  31   b ,  32   b , alternating rectangles and triangles are provided as the electrode elements, which are again arranged in a line to form a strip. 
     Both electrode arrangements according to FIG. 3 again represent asymmetrical arrangements. The arrangement of larger or smaller rectangular electrode elements and/or rectangular or triangular electrode elements provides an orientation of the respective strips. The orientations of the electrode groups  31   a ,  32   a  and/or  31   b ,  32   b  lying opposite to one another are each reversed relative to one another. 
     The strips formed by the electrode elements extend essentially over the entire width of the channel and typically have dimensions in the lengthwise direction of the channel like those of the electrode strips shown in FIG.  1 . 
     To achieve specific field gradients, the shapes of the electrode elements can be altered depending on the application. Again, the electrode elements can be driven individually or in groups. 
     A further design of an electrode arrangement according to the invention is shown in FIG.  4 . In channel  43 , a meander-shaped electrode arrangement  41  is affixed to the bottom surface and a flat electrode  42  (indicated with dots) is affixed to the cover surface. The meander-shaped electrode group consists in the example depicted of four electrodes which are positioned, separated from one another, in a spiral shape around one another in the plane of the bottom surface. The flat electrode  42  forms a counterelectrode. Again, the electrode groups  41  are driven according to the principles explained with reference to FIG.  1 . Applying four phase-shifted signals to the four electrodes is preferred. The flat electrode  42  can be replaced by a corresponding meander arrangement. 
     According to a further embodiment of the invention (cf. FIG.  5 ), an electrode arrangement is provided, in the microchannel  53  with fluid flowing through it, consisting of two structured individual electrodes  51 ,  52 . The individual electrodes  51 ,  52  are affixed to opposing channel walls analogous to the positioning of the electrode groups according to the embodiments described above. Each of the individual electrodes has a structuring, e.g., in the form of a line of triangular electrode elements (as shown), which, however, are connected electrically to one another, in contrast to the design according to FIG.  2 . The electrode elements can also have other geometric shapes. 
     The production of the individual electrodes  51 ,  52  is performed either by processing the desired electrode surfaces on the respective bottom or cover surface by applying a coating corresponding to the desired shape of the electrode elements or by the covering technique described in the following. According to this, each individual electrode  51 ,  52  consists of a flat, rectangular electrode which extends over the entire width of the channel (shown with dashed lines). The electrode carries an insulation layer with cutouts corresponding to the desired shapes of the electrode elements. Only at these cutouts or openings is the electrode in direct contact with the fluid and it is thereby also only active according to this cutout pattern. The shape has the advantage that the electrode elements of the individual electrodes  51 ,  52  do not have to touch, because electrical contact is ensured via the electrode surface under the insulation layer. 
     FIG. 5 again shows an asymmetrical shape, in which the electrode elements of the lower individual electrode  51  are arranged in a row with fewer, but therefore larger, triangles than the electrode elements of the upper individual electrode  52 . 
     In the embodiment according to FIG. 6, the electrode arrangement according to the invention consists of two electrode groups  61   a ,  61   b  and/or  62   a ,  62   b  affixed to opposing channel walls, each consisting of two electrode strips meshing in one another like combs. The fluid flows through the channel  63  corresponding to the direction of the arrow  64  (or in the reverse direction). If the fluid is subjected to high frequency electric fields in the region of the electrode arrangement, then the desired electroconvective circulation transverse to the direction of the channel again results. The embodiment depicted comprises a total of four electrode strips, which are preferably driven in four phases with a high frequency alternating field. The electrode strips are positioned asymmetrically in regard to the strip width and strip intervals. 
     An electrode arrangement according to the invention can also comprise an octopole electrode arrangement according to FIG.  7 . Two electrode groups are provided on opposing channel walls. The electrode groups on the bottom surface consist of four individually drivable rectangular electrode elements  71   a  to  71   d . Opposite to these, the electrode groups on the cover surface consist of four individually drivable, rectangular electrode elements  72   a  to  72   d . The fluid flowing through the channel  73  in the direction of the arrow  74  is preferably subjected to a rotating four-phase alternating field. An example of how this is generated is indicated in the following table: 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 elec- 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 trode/ 
               
               
                 var- 
               
               
                 iant 
                 71a 
                 71b 
                 71c 
                 71d 
                 72a 
                 72b 
                 72c 
                 72d 
               
               
                   
               
             
            
               
                 1 
                 0° 
                 90° 
                 180° 
                 270° 
                 180° 
                 270° 
                  0° 
                  90° 
               
               
                 2 
                 0° 
                 90° 
                 180° 
                 270° 
                  0° 
                  90° 
                 180° 
                 270° 
               
               
                 3 
                 0° 
                 90° 
                 180° 
                 270° 
                 float- 
                 float- 
                 float- 
                 float- 
               
               
                   
                   
                   
                   
                   
                 ing 
                 ing 
                 ing 
                 ing 
               
               
                 4 
                 0° 
                 float- 
                  90° 
                 float- 
                 270° 
                 float- 
                 180° 
                 float- 
               
               
                   
                   
                 ing 
                   
                 ing 
                   
                 ing 
                   
                 ing 
               
               
                 5 
                 0° 
                  0° 
                 270° 
                 270° 
                  90° 
                  90° 
                 180° 
                 180° 
               
               
                 6 
                 0° 
                 float- 
                 float- 
                 270° 
                  90° 
                 float- 
                 float- 
                 180° 
               
               
                   
                   
                 ing 
                 ing 
                   
                   
                 ing 
                 ing 
               
               
                   
               
            
           
         
       
     
     The octopole arrangement can be modified in such a way that only four electrodes are provided, with the floating control then being left out. 
     The invention was described above to illustrate various forms of the electrode arrangement, with a field direction perpendicular to the flow direction always being assumed. Alignments deviating from this in the angular range mentioned initially can be realized with corresponding adjustment of the electrode elements and their arrangement. In each case, the individual electrode groups can be arranged offset to one another in the channel direction. The realization of the invention in channels with rectangular cross-section by application of the electrode arrangement to the wider channel walls is preferred, with altered geometrical designs also, however, possible. Instead of driving the electrodes with continuous, high frequency alternating current voltages as described, pulse-shaped driving is also possible. The electrodes can also comprise electrode elements which can be structured in relation to the flow direction and can be driven separately. The field direction can thereby be changed during the fluid circulation, e.g. to respond to the result of the circulation or to specific fluid properties. 
     Preferred applications of the invention are in all fields of the use of microsystems for biotechnological, medical, diagnostic, chemical technology, or pharmacological functions. An advantageous application of the invention in so-called DNA chips will be described in the following with reference to FIG.  8 . 
     A DNA chip is generally a sample chamber with at least one modified surface. The modified wall surface has a predetermined molecular coating for formation of the substrate for DNA reactions. To construct specific DNA configurations, nucleotides are introduced into the sample chamber and caused to react with the substrate and/or DNA strands already grown. The reaction is accelerated by circulation of the fluid. However, the DNA strands which have already grown must also be prevented from separating from the modified wall surface. The process according to the invention for convective fluid movement can be advantageously used for this purpose. 
     FIG. 8 shows a schematic cross-sectional view of the DNA chip  80 , on whose inner wall electrode arrangements  81  and/or  82  are provided. The DNA chip has an inlet  83  and an outlet  84 . The inner chip wall  85 , which is on the bottom in the illustration, forms the surface-modified substrate for DNA growth. The DNA strands  86  (shown schematically) grow in the nucleotide solution introduced through the inlet  83  (direction of arrow). According to the principles explained above, electrical field gradients with an alignment deviating from the flow direction are generated with the electrode arrangements  81 ,  82 . A blending of the nucleotide solution thereby results in the DNA chip  80 . This blending can be locally delimited by producing optically induced thermal gradients in predetermined focal positions  87  of the laser irradiation  88 , so that blending only occurs at the free ends of the DNA strands  86 . 
     Blending in the entire DNA chip  80  can also, however, be provided. In each case, the circulation of the nucleotide solution supplied has the advantage that the speed of DNA synthesis is significantly increased. 
     The invention was described here with reference to flowing suspension fluids, but can also be correspondingly used in still fluids or swirled fluids. The invention was further described above with reference to embodiments, in each of which electrode arrangements were provided on opposing channel walls. According to an alteration, it is also possible to provide an electrode arrangement for generation of the field gradient(s) on only one channel wall.