Abstract:
A method and a device are used for carrying out measurements on cells located in a liquid environment. Each cell is positioned with an underside of its membrane on a surface having a channel running through it. A negative pressure is established to aspirate the cells. Each cell is electrically scanned via at least one electrode which is spaced apart from the cell. The negative pressure is preferably established in a pulse-like manner to rupture the membrane in such a way that the cell interior enclosed by the membrane is connected to the channel.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of copending international patent application PCT/EP00/08895 filed on Sep. 12, 2000 designating the U.S., which claims priority of German patent application DE 199 48 473.2 filed on Oct. 8, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The invention relates to a method for carrying out measurements on cells located in a liquid environment wherein each cell is positioned with an underside of its membrane on a surface, the surface having at least one channel running through it, in which, to aspirate the cell to the surface, a pressure differential is established and the cell, in addition, is electrically scanned via at least one electrode.  
           [0003]    The invention further relates to a device for carrying out electrical measurements on cells located in a liquid environment, comprising a substrate which has a channel running through it, above which a cell can be positioned with an underside of its membrane on a surface of the substrate, means being provided to generate a pressure differential along the channel, and a first electrode being provided for electrical scanning of the cell.  
           [0004]    It is known to employ so-called microelectrode arrays for the study of biological cells, the microelectrode arrays being used e.g. to stimulate the cells or to tap potentials. The studies can be carried out in a biological environment or alternatively in an artificial environment. The arrays to this end comprise, in a matrix, a plurality of microelectrodes whose dimensions are of about the order of magnitude of the cells, i.e. in a range from a few μm up to a few tens of μm. A microelectrode array of this general type is disclosed e.g. by WO 97/05922.  
           [0005]    In conventional microelectrode arrays it is more or less a matter of having to rely on chance as to whether one cell or some other cell will or will not settle on a specific electrode. In practice, the cells will generally only partially cover an electrode when settling on it, so that stimulation of the cell or tapping of a cell potential is limited to this sub-area. Moreover, the cells are only loosely seated on the electrodes. This may lead to problems with regard to the sealing resistance with respect to the reference electrode. Alternatively, cells may come to lie outside the range of an electrode, so that the measurement will not pick them up.  
           [0006]    In the case of the microelectrode array disclosed by DE 197 12 309 A1 these drawbacks are avoided by the cells being collected in micro-cuvettes at whose bottom an electrode is located. The electrode is provided with a central channel in which a negative pressure can be produced via suitable connecting channels which run below the electrodes. Thus it is possible for individual cells to be drawn to the electrodes in a controlled manner and to be affixed to the electrodes with a certain contact pressure. Measurements can then be carried out on the electrodes, but only from outside them.  
           [0007]    From another art, the so-called Patch Clamp technique, it is known to aspirate cells at a pipette using negative pressure (cf. US-Z “NATURE”, vol. 260, pp. 799-801, 1976). The Patch Clamp technique, however, requires a controlled approach of the pipette to an individual cell. With the Patch Clamp technique the cells to be contacted are not moved, since as a rule they are adhering to a substrate. Conventional contacting of cells using Patch Clamp pipettes has a drawback, however, that the number of cells that can be contacted simultaneously is extremely limited, since for reasons of space it is not possible to introduce arbitrarily many pipettes into the culture chamber.  
           [0008]    On the other hand, the Patch Clamp technique has the advantage, compared with the above-described technique which only permits measurements from the outside of the cell, that the cell interior can be included in the measurement.  
           [0009]    In the Patch Clamp technique as employed conventionally, using individual pipettes, this is achieved, under observation by microscope, by a fragile glass pipette being moved, by means of a micromanipulator, to a single cell adhering to a substrate and the membrane being cautiously aspirated to the pipette mouth. There is therefore direct contact between glass surface and membrane surface. By this means, a membrane patch is sealed and electrically insulated from the surrounding fluid. This insulation is also referred to as a gigaseal. The transition from this “cell-attached configuration” to the so-called “whole cell configuration” is achieved by further aspiration of the sealed membrane. This is done in such a way that the membrane section below the pipette is perforated. This results in hydraulically and electrically sealed access via the pipette mouth to the cell interior. The remaining cell membrane is thus electrically accessible in its entirety (so-called “whole cell patch”). Using this conventional technique does, however, require a considerable level of experience and very sensitive touch. A plurality of cells can only be processed sequentially. This method is therefore unsuitable for large-scale studies as would be required e.g. in the field of pharmaceutical screening, substance screening and the like.  
         SUMMARY OF THE INVENTION  
         [0010]    It is an object of the invention to refine a method and a device of the type mentioned at the outset so as to avoid the above-mentioned drawbacks. In particular, the invention is to permit measurements as consistent as possible, preferably in parallel on a plurality of cells, and particularly as desired in the field of experimental or application-oriented screening of the action of pharmaceutical active ingredients on a cellular level.  
           [0011]    With a method of the type mentioned at the outset this object might be achieved by the underside of the membrane being ruptured by means of an increase in the pressure differential and/or the membrane being rendered microporous and electrically low-resistant or being ruptured by means of an addition of pore-forming substances or by means of an electric current pulse.  
           [0012]    With a device of the type mentioned at the outset, the object of the invention is achieved by at least one second electrode being arranged so as to be spaced apart, towards the channel, from the first electrode.  
           [0013]    The object of the invention is thus completely achieved.  
           [0014]    The invention makes it possible, compared with conventional Patch Clamp techniques, to dispense with the awkward handling of a fragile glass pipette, since the function of the conventional glass pipette is performed by the channel in a substrate, a negative pressure being applied to the channel to establish a cell-attached configuration. If a gigaseal has thus been established between the cell wall and the surface to which the cell is aspirated, according to the invention either the underside of the membrane is ruptured by an increase in the negative pressure, thus then permitting measurements to be carried out via the channel throughout the cell interior enclosed by the membrane. Alternatively, or in addition, the membrane, by means of an addition of pore-forming substances can be rendered microporous and electrically low-resistant. As a result of the addition of such pore-forming substances such as e.g. nystatine or amphotericine B, pores are formed in the membrane, thus enabling low-resistance access to the cell interior without, however, larger molecules being able to diffuse through said access. The resulting membrane currents can thus be measured without the underside of the membrane having to be destroyed for this purpose. Alternatively, the membrane in this region can also be rendered permeable or ruptured by a brief electric pulse.  
           [0015]    A particular advantage of the invention is that the measuring electrode can be arranged spaced apart from the membrane, so that the cell does not, with its membrane, lie directly on the electrode, but the measurement is instead effected via the intracellular medium.  
           [0016]    Whereas the known array according to DE 197 12 309 A1 only permits extracellular measurements, i.e. measurements of changes in potential, caused by membrane currents, in the immediate vicinity of the cell, the invention allows intracellular and extracellular measurements to be carried out, i.e. the voltage obtaining across the membrane can be measured and checked. This purpose is preferentially achieved by a current being injected into the membrane.  
           [0017]    The invention is therefore especially suitable for large-scale studies in the field of pharmaceutical screening and substance screening, the identification of clones (GMOs; genetically modified organisms) and within the context of substance optimizations, where the cytoplasm of biological cells is measured electrically, either simultaneously or directly sequentially for a plurality of such cells. The invention therefore provides for the first time the option of employing a technique having the same advantages as conventional Patch Clamp techniques in a fully automated mode. It is therefore possible to study many such cells in parallel, in a manner lending itself to automation and with high throughput.  
           [0018]    In a preferred refinement of the invention, the pressure differential to rupture the membrane is increased in a pulse-like manner.  
           [0019]    This measure permits a controlled transition from the cell-attached configuration to the whole cell configuration.  
           [0020]    In principle it is possible for a plurality of channels to be provided in a common substrate and, with the aid of the negative pressure, to aspirate cells to the surface of the substrate at the mouths of the channels.  
           [0021]    According to a preferred refinement of the invention, however, the cell is positioned on the bottom of a micro-cuvette.  
           [0022]    This measure has the advantage that a liquid medium containing the cells to be studied can be introduced in a controlled manner above each channel by means of a pipette or the like. Thus a large number of micro-cuvettes located in a common mount can be measured simultaneously via one channel each associated with a micro-cuvette and at least one measuring electrode each.  
           [0023]    A method is preferred which involves the use, as a measurand of the electric signal, of the current (I st ) through the cell interior of the cell and/or a measurement of the electrical potential at the electrodes.  
           [0024]    In a preferred embodiment of the method according to the invention, the cell is electrically scanned via an electrode which is spaced apart from the underside of the membrane in the direction of the channel. A current can be injected into the cell interior for this purpose.  
           [0025]    This measure has the advantage that direct electrical access is established only into the interior of the cell. As a result of the cell with its outer membrane being seated tightly on the bottom of the micro-cuvette and even being fixed in position there by means of negative pressure, the gigaseal, i.e. an extremely high leakage resistance, which consequently does not significantly affect the measurement, between intracellular and extracellular medium, known from the conventional Patch Clamp technique is established in an automated manner. Having the electrode spaced apart from the gigaseal further ensures that the cell itself will only come into contact with electrically insulating materials, thus ensuring that the gigaseal is maintained.  
           [0026]    A further preferred embodiment of the method according to the invention involves, in an arrangement comprising a plurality of channels, the pressure pulse or the electric current pulse being generated simultaneously at all the channels. Alternatively, however, the pressure pulses or current pulses can be applied successively to individual selected channels, either in a sequence chosen at will or in a strictly sequential procedure.  
           [0027]    These measures have the advantage that almost any experiments can be carried out in an automated manner on a plurality of cells, thereby producing very many measurement results per unit time.  
           [0028]    In a preferred refinement of the method according to the invention, the composition of the intracellular liquid medium is altered by the addition of substances or the intracellular liquid medium is replaced after the membrane has been opened up or micropores have been generated. To this end, the channel can be connected to two or more connecting channels, one of which is charged with electrolyte which has a composition similar to that of the cytoplasm (intracellular fluid) or is a special fluid with added active ingredients.  
           [0029]    This allows specific and controlled modification of the intracellular medium while at the same time considerably broadening the possible spectrum of feasible measurements.  
           [0030]    In the device according to the invention at least one second electrode is arranged so as to be spaced apart, towards the channel, from the first electrode.  
           [0031]    In a preferred refinement, means for controlling the pressure differential are provided, both for the purpose of generating a static pressure differential to establish a cell-attached configuration and for the purpose of a pulse-type increase in the pressure differential to rupture the underside of the membrane.  
           [0032]    Thus it is possible, firstly, to establish the gigaseal in a reliable and controlled manner and, secondly, to maintain the gigaseal while the underside of the membrane is ruptured by a brief pressure pulse and comes to lie against the channel wall.  
           [0033]    The control system in this context is preferably designed so as to continuously maintain the static negative pressure (offset pressure), thus maintaining the gigaseal even in the whole-attached configuration.  
           [0034]    According to a further embodiment of the device according to the invention, the electrode is disposed on that side of the channel which faces away from the first electrode.  
           [0035]    In this arrangement, the electrode can annularly surround the far end of the channel.  
           [0036]    These measures have the advantage that the electrode can be integrated into a microstructure in a simple manner by being fashioned on the underside of the layer which has the channel running through it.  
           [0037]    In a further variant of the invention, however, the electrode is spaced apart from the far end of the channel.  
           [0038]    This provision has the advantage that the electrode, as will be explained later, can also be disposed so as to be movable relative to the channel, so that the same electrode can be used to perform measurements successively on a plurality of cells.  
           [0039]    According to a further feature of the invention, the channel, at its end facing away from the first electrode, is connected via valves to a plurality of channels via which the fluid can be supplied or discharged.  
           [0040]    This provision has the advantage that as a result of the pressure conditions in the connecting channels being suitably controlled, the cell interior, once the whole cell configuration has been established, i.e. once the membrane has been ruptured, will come into contact with intracellular fluid.  
           [0041]    In an additional refinement of the invention, above the substrate a micro-cuvette is disposed in whose bottom an opening is provided.  
           [0042]    This allows the fluid to be stored above the substrate in which in a manner particularly suitable for large-scale screening the channel or channels are provided. In such an arrangement the cells, by virtue of a suitable funnel-like design of the microcuvettes, can be guided into the immediate vicinity of a channel mouth of a substrate surface. Alternatively, however, the opening in the bottom of the micro-cuvette may have a distinctly larger diameter, thus essentially ensuring that the applied negative pressure will guide the cell as far as the mouth of the channel. This facilitates fabrication of the structure according to the invention.  
           [0043]    In addition it is preferable for a plurality of channels to be arranged in a common substrate.  
           [0044]    This allows compact design to be achieved in conjunction with simple fabrication.  
           [0045]    In addition it is preferable in this context for a plurality of micro-cuvettes to be arranged in a plate.  
           [0046]    This provides the advantage of allowing preparations to be made in a simple manner for parallel or sequential measurements on many cells, since all the micro-cuvettes are located in a common plate.  
           [0047]    In the case of embodiments of the invention which use a common plate for the micro-cuvettes it is further preferable for the plate to have a multilayer structure.  
           [0048]    This measure has the advantage that the different requirements with respect to the various elements of the plate can be taken into account by a suitable choice of materials.  
           [0049]    This is particularly the case where, in a refinement of this variant, the plate comprises a top layer, a middle layer and a bottom layer, the micro-cuvettes being disposed in the top layer, the middle layer forming the substrate with the channels, and the bottom layer having disposed therein connecting channels which lead to the channels and, in a preferred embodiment, electric leads running to the channels, and microelectrodes.  
           [0050]    This tripartite division of the plate has the advantage that the three essential functions can each be served by individual layers of different thickness and different materials being used.  
           [0051]    According to a further embodiment of the invention, the substrate is bonded to a bottom layer which comprises one or more layers of photopatternable materials which permit spatial routing of connecting channels which run to the channels.  
           [0052]    In such an arrangement, the bottom layer can be applied to a glass mount.  
           [0053]    These measures allow a particularly compact design and simple fabrication. Known photopatternable materials include certain polymers, but also certain glasses.  
           [0054]    It is preferable for the connecting channels to have a width of between 10 μm and 40 μm, preferably about 20 μm.  
           [0055]    The channels themselves preferably have an inner width of less than 10 μm, preferably less than 5 μm.  
           [0056]    These dimensions have proved optimal in the present context. In particular, an inner width of the channels which is smaller than the cell diameter promotes positioning of one cell at a time on a channel.  
           [0057]    As indicated previously, it is particularly preferable in these embodiments of the invention for the electrodes to be disposed on the underside of the middle layer or the topside of the bottom layer.  
           [0058]    This measure has the advantage that the electrodes together with their leads can be formed by simple printing, deposition, vapour deposition and subsequent mircropatterning by known methods (photolithography, etching procedures, lift-off etc.).  
           [0059]    In this case it is further preferable for the electrodes, as seen in a plan view from above, to be fashioned as a square area having an edge length of between 20 μm and 60 μm, preferably about 40 μm.  
           [0060]    As mentioned above, provision can be made in this case for the conductor tracks leading to the electrodes to be disposed between the middle and the bottom layer. This can be effected either by applying them to the bottom of the middle layer or to the top of the bottom layer. Application to the underside of the middle layer has the advantage that the conductor tracks can be formed together with the electrodes, especially also using the same material, particularly precious metal, preferably gold.  
           [0061]    In this arrangement the conductor tracks preferably have a width of between 5 μm and 30 μm, particularly about 10 μm.  
           [0062]    As mentioned above it is possible, in the case of a multilayer construction of the plate, to use different materials for the individual layers.  
           [0063]    The various layers can be produced e.g., independently of one another from plastic, polymethylmethacrylate (PMMA), silicone, PTFE, polyimide or of an inorganic material, particularly of glass, ceramic material or silicon.  
           [0064]    The preferred material used for the substrate is polyimide, which for this purpose is employed as a sheet in which the channels are fashioned as drilled holes. The substrate (the sheet) then preferably has a thickness of between 2 μm and 40 μm, preferably about 5 μm.  
           [0065]    The preferred material used for the bottom layer is glass. In the glass, which can be provided in virtually any thickness, so as also to ensure the mechanical stability, the necessary connecting channels and the like can be produced in a conventional manner.  
           [0066]    In an additional refinement of the invention it is preferable for the substrate to be disposed at the underside of a plate in which a plurality of drilled holes are fashioned as microcuvettes in whose bottom holes are provided by means of which the channels of the substrate are centred.  
           [0067]    Thus a combined body having a plurality of micro-cuvettes to which individual channels and electrodes are allocated can be prepared in a relatively simple manner.  
           [0068]    According to a further embodiment of the invention, a hydraulic and measuring unit is provided which has a chamber which is open towards the underside of the substrate and can be positioned at the underside of the substrate in such a way that the chamber communicates with a selected channel and is sealed with respect to the outside, the chamber containing at least one electrode and being connectable to at least one terminal channel which is connected to a negative-pressure source.  
           [0069]    In addition, a traversing unit for traversing and positioning the plate and the negative-pressure and measuring unit relative to one another can be provided.  
           [0070]    Thus a single measuring unit can be used for sequential measurements on a plurality of cells, which can result in considerable cost savings.  
           [0071]    For this purpose, by preference, commercial, conventional standard grid plates (so-called “96-well plates”, “384-well plates” or the like) can be used. These merely require sealing at the bottom by applying the sheet provided with channels (holes). The measurements on the many individual cells in the drilled holes of the well plate are then carried out sequentially by traversing the negative-pressure and measuring unit or conversely by traversing the standard grid plates relative to a stationary negative-pressure and measuring unit. The latter generates the negative-pressure pulse to open up the cell and also contains the electrode to carry out the subsequent measurement through the cell interior.  
           [0072]    In so doing it is again possible, as already mentioned above, for the chamber to be connected to a plurality of terminal channels via valves, so that the cell interior is brought into contact with intracellular fluid after the whole-cell configuration has been established or that the composition of the intracellular fluid can be altered.  
           [0073]    Further advantages can be deduced from the description and the accompanying drawing.  
           [0074]    It goes without saying that the above mentioned features and those still to be explained hereinafter can be used not only in the combination specified in each instance, but also in other combinations or on their own without going beyond this scope of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0075]    Exemplary embodiments of the invention are shown in the drawing and are explained in more detail in the following description. In the drawing:  
         [0076]    [0076]FIG. 1 shows an extremely schematic perspective view of a specific embodiment of a device according to the invention;  
         [0077]    [0077]FIG. 2 shows a section through a micro-cuvette of the arrangement according to FIG. 1, likewise highly schematic;  
         [0078]    [0078]FIG. 3 shows a plan view from above of the micro-cuvette according to FIG. 2, on a slightly reduced scale;  
         [0079]    [0079]FIG. 4 shows a detail from FIG. 2 on an even larger scale, to illustrate the method according to the invention;  
         [0080]    [0080]FIG. 5 shows a depiction, similar to FIG. 3, depicting the prior art;  
         [0081]    [0081]FIG. 6 shows a perspective view and an enlarged detail therefrom of a further specific embodiment of a device according to the invention; and  
         [0082]    [0082]FIG. 7 a ) to  7   c ) show various phases as a cell is aspirated, a cell-attached configuration and a whole-cell configuration are established when two connecting channels to the channel are used, in a simplified, schematic view. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
     EXAMPLE 1  
       [0083]    In FIGS.  1  to  4 ,  10  denotes a plate. Established in a surface  11  of the plate  10  by means of moulding is a grid of micro-cuvettes  12 . The micro-cuvettes  12  are three-dimensional and have suitable dimensions for culturing cells. A plate  10  may e.g. contain 8×12=96 micro-cuvettes  12  disposed therein.  
         [0084]    An arrow  14  indicates that the micro-cuvettes  12  can be charged from above, specifically with a fluid which contains cells to be studied. In so doing it is possible to charge each of the micro-cuvettes  12  individually with different fluids and cells.  
         [0085]    For the purpose of performing the measurements, an electric terminal module  16  is provided which can be docked laterally onto the plate  10 , a sufficient plurality of plugs  18  being provided for this purpose. The plugs  18  communicate with a network of conductor tracks. These conductor tracks run to electrodes which are disposed in the vicinity of the micro-cuvettes  12 , as will be explained later. Running from the electric terminal module is a data line  20  to a controller  22 .  
         [0086]    Also provided is a hydraulic terminal module  24  which can like-wise be docked laterally onto the plate  10 , by means of a corresponding plurality of hydraulic plugs  26 . Using the hydraulic terminal module  24  it is possible, in a predetermined manner, particularly individually, to generate a negative pressure below the micro-cuvettes  12 , especially as a pulse-shaped function of time, as will be explained below in detail.  
         [0087]    To this end, the hydraulic plugs  26  are connected to the micro-cuvettes  12  via a network of connecting channels and openings  49  at the bottom of the micro-cuvettes. If the same negative pressure is to be applied to all the micro-cuvettes  12 , all the connecting channels are connected in parallel and are directly linked, in the hydraulic terminal module  24 , to a central, controlled negative-pressure source. If, however, the individual micro-cuvettes  12  are each to be driven with an individual negative pressure, it is likewise possible to employ a central negative-pressure source which is connected to the network of connecting channels, said connecting channels then containing individually controllable valves. Alternatively, however, it is also possible for the connecting channels to be fitted with miniaturized pumps, especially miniature membrane pumps, which are driven individually. Electric actuation of the valves and/or miniature pumps can be effected either via the electric terminal module  16  or the hydraulic terminal module  24 . In each case, a line  28  for driving the abovementioned elements runs from the hydraulic terminal module  24  to the controller  22 .  
         [0088]    The controller  22  in turn communicates with a multiplexer  30  to be able to carry out, in a predetermined manner, a plurality of measurements simultaneously or optionally sequentially.  
         [0089]    As can be seen from FIG. 2, the plate  10  essentially consists of three layers. Located on the bottom layer  32  is a middle layer or a substrate  34  which is in the form of a sheet. A top layer  36  is fashioned as a micropatterned layer. The bottom layer  32  in this arrangement is preferably made of glass. The substrate  34  is preferably a polyimide sheet. The micropatterned layer  36  in contrast is preferably made of polymethyl methacrylate (PMMA).  
         [0090]    Formed in the top side of the bottom layer  32  is a connecting channel  38 . The connecting channel  38  is used for individually driving the micro-cuvette  12  shown in FIG. 2. The connecting channel  38  communicates via a vertical channel  40  in the substrate  34  with an opening  49  at the bottom  48  of the micro-cuvette  12 . At its top, the micro-cuvette  12  is provided with a cylindrical section  42  which is lined with a reference electrode  44 . The reference electrode  44  is connected to a first electric terminal  46 . The latter is preferably connected to earth.  
         [0091]    Adjoining the cylindrical section  42  at the bottom is a funnel-shaped section which forms the bottom  48  in which the opening  49  is provided.  
         [0092]    An electrode  50  is arranged approximately annularly around the bottom end of the vertical channel  40 . To this end it is applied to the underside of the substrate  34 . The electrode  50  is connected to a lead  52  which runs between bottom layer  32  and the substrate  34 . The lead  52  can e.g. together with the electrode  50  be printed, vapour-deposited, deposited or the like onto the underside of the substrate  34 . The lead  52  is connected to a second electric terminal  54 .  
         [0093]    The electrodes  46  and  50  consist of silver/silver chloride (Ag/AgCl). Electrodes of this type are referred to in the art as “reversible” or as “non-polarizable”. They offer the advantage of permitting not only a.c. voltage measurements on the cells, i.e. measurements of the potential spikes, but also d.c. voltage measurements. They can also be used for current injection.  
         [0094]    Between the electrode  46  and  50  the measured voltage U amp  is measured. In addition, a stimulation current I st  can be fed in via the second terminal  54  parallel to the voltage measurement. This will be explained below in more detail with reference to FIG. 4.  
         [0095]    As can be seen in the plan view from above according to FIG. 3, a plurality of electrodes  12  are arranged in the plate  10  in the form of a grid, the grid spacing d being between 0.1 and 10 mm, preferably about 9 mm.  
         [0096]    The micro-cuvettes  12  in the region of their cylindrical section  42  have an internal radius r of between about 1 and 9 mm, preferably about 7 mm, thus facilitating charging. The inner width x of the vertical channel  40  is less than 10 μm, preferably less than 5 μm.  
         [0097]    The connector tracks  52  have a width b 1  of between 5 μm and 30 μm, preferably about 10 μm. The electrodes  50  are preferably of square design, in a plan view from above, and have an edge length a of between 20 μm and 60 μm, preferably about 40 μm.  
         [0098]    The connecting channels  38  have a width b 2  of between 10 μm and 40 μm, preferably about 20 μm.  
         [0099]    The substrate  34  or the film has a thickness of between 2 μm and 40 μm, preferably about 5 μm.  
         [0100]    The distance  1  of the micro-cuvettes  12  from the edge of the plate  10  is preferably at least 2 cm, thus achieving a high shunt resistance, i.e. electrical decoupling of the individual electrodes.  
         [0101]    The electrode  50  and the connector tracks  52  are preferably made of gold.  
         [0102]    [0102]FIG. 2 also illustrates that a micropump  56  can be integrated in the connecting channel  38 , said micropump being drivable by means of a third terminal  58 . Using the micropump  56  or using a central negative-pressure source, optionally with the incorporation of valves in the connecting channels  38 , it is possible to establish a negative pressure, whose variation of the time can be controlled, in the connecting channels  38 .  
         [0103]    [0103]FIG. 4 shows, on an even larger scale, the situation where a cell  60  has sunk to the bottom  48  of the micro-cuvette  12 . This is achieved either by gravity or controllably by the cell  60  being aspirated in a controlled manner by a preferably constant negative pressure being applied in the channel  40 . The funnel-like shape of the section at the bottom  48  of the micro-cuvette  12  additionally causes the cells to be singled, so that as a rule only a single cell  60  will come to lie on the opening  49  at the bottom  48  of the micro-cuvette  12  above the channel  40 .  
         [0104]    In FIG. 4, the outer skin or membrane of the cell  60  is denoted by  62  and the cell interior by  64 . The cell  60  is located in an ambient fluid  66  which, in the specific example shown, can also fill the connecting channels  38  and the channels  40  and can be replaced there. It lies on the bottom  49 , on its underside  68 .  
         [0105]    As soon as this position is reached, a negative-pressure pulse is generated in the connecting channel  38 , as indicated in FIG. 4 by an arrow  70 . This negative-pressure pulse  70  is of such magnitude that the underside  68  is ruptured and, in the manner of a collar  72 , is drawn into the channel  40 . The cell interior  64  then communicates directly with the channel  40  and the liquid present therein. Alternatively, a low-pressure pulse can be dispensed with and the underside of the membrane can be rendered permeable by the addition of pore-forming substances, e.g. nyastatin or amphotericin B, resulting in low-resistance access to the cell interior without larger molecules being able to diffuse therethrough.  
         [0106]    [0106]FIG. 4 additionally shows the electric equivalent circuit diagram of the cell  60 .  
         [0107]    R M  and C M  denote the resistance and the capacitance of the membrane  62 . R S  is the seal resistance, i.e. the insulation resistance between cell interior  64  and extracellular medium  66  outside the cell  60 . R P  is the resistance between the cell interior  64  and the electrode  50 , while R k  is the resistance between electrode  50  and reference electrode  44 .  
         [0108]    As a result of the collar  72  being drawn into the channel  40  and lying against the wall  48 , without however reaching the relatively remote electrode  50 , the value of R S  is very high (“gigaseal”)  
         [0109]    In the zero-current state, i.e. when no current flows via the terminal  54  and through the connecting channel  38 , the voltage U amp  corresponds to the membrane voltage U M  obtaining at the cell membrane. If, on the other hand, a finite current is able to flow via the connecting channel, the following relationship applies:  
           U   amp     =         R   k         R   k     +     R   p              U   M         ,                         
 
         [0110]    i.e. the membrane voltage U M  is proportional to the voltage U amp .  
         [0111]    If a stimulation current I ST  is fed to the second terminal  54 , the following relationship holds for the membrane voltage U M  in the steady state:  
           U   M     =           R   S     ·     R   M           R   M     +     R   S              I     S                 T           ,                         
 
         [0112]    if R k &gt;&gt;R p +R S R m /(R S +R m ). This condition is met, according to the invention, by sufficiently long connecting channels having a small channel cross section.  
         [0113]    Both hydraulic and electric contact is therefore made in the above-described manner with the cell  60  at the bottom  48  of the micro-cuvette  12 , by causing the required negative pressure via the electrolyte solution and—by the additional generation of a low-pressure pulse  70 —bringing about the opening-up operation on the cell  60 .  
       COMPARATIVE EXAMPLE  
       [0114]    To illustrate the difference between the procedure according to the invention and the prior art according to DE 197 12 309 A1, FIG. 5 shows a depiction which is similar to FIG. 4 and corresponds to the known arrangement, identical elements being indicated by identical reference symbols, with the addition of an “a” in each case.  
         [0115]    As can be clearly gathered from FIG. 5, in this prior art the cell  60   a  lies directly on the electrode  50   a . Electrical input to the cell  60   a  therefore takes place from outside, i.e. from the exterior of the membrane  62   a . The channel  40   a  in this prior art is used solely to attract the cell  60   a  to the bottom  48   a , by application of a specific, slight negative pressure, and to locate it fixedly there, the bottom  48   a  in this prior art, in contrast to the present invention, being formed by the electrode  50   a.    
         [0116]    Even if, in this prior art, a negative-pressure pulse were to be applied to the cell  60   a  via the channel  40   a  and the membrane  62   a  were to be opened up (of which there is no mention in said prior art), a collar  72   a  on the underside  68   a  of the cell  60   a  would form which covers the electrode  50   a  in the region of the channel  40   a . Despite the cell  60   a  having been opened up, the electrode  50   a  would therefore still not permit a direct measurement through the cell interior  64   a , but continue to lie only against the outside of the membrane  62   a . With this prior-art device, the measurements, even with the underside  68   a  of the cell  60   a  having been opened up, would take a course no different from that described for the case where the negative pressure does not result in the cell  60   a  being opened up.  
       EXAMPLE  2   
       [0117]    Finally, FIG. 6 shows yet another specific embodiment of a device according to the invention.  
         [0118]    In FIG. 6, 80 denotes a plate of a design which is conventional per se. Plates  80  of this type are referred to as a “96-well plate”. They contain  96  vertical cylindrical drilled holes  84  arranged in a grid. These drilled holes  84  can be used as micro-cuvettes. As indicated in FIG. 6 top right by a dashed line, the plate  80  can alternatively be of multilayer, especially double-layer, design.  
         [0119]    A bottom  82  of the cylindrical drilled holes or micro-cuvettes  84  is formed by a substrate  86  in the form of a sheet which is glued, welded or bonded in some other way to the underside of plate  80 . The substrate  86  in each instance in the centre of the bottom  82  has a channel  88  in the form of a hole.  
         [0120]    In contrast to the inventive exemplary embodiment according to FIGS.  2  to  4 , no support having a system of connecting channels is present underneath the substrate  86 . Instead a traversable hydraulic and measuring unit  90  is provided which can be individually moved up from below to the underside  91  of the sheet  86 .  
         [0121]    The unit  90  comprises a barrel-shaped chamber  92  which at its upper end face is provided with a ring seal  94 . Thus the chamber  92  can be docked tightly to the underside  91  in such a way that the vertical axis of the chamber  92  is aligned with the axis of one hole  88  at a time.  
         [0122]    Leading from the chamber  92  is a conduit  96  to a negative-pressure unit (not shown). Thus a negative-pressure pulse can be generated in the chamber  92 , as indicated by an arrow  98 .  
         [0123]    Disposed on the bottom of the chamber  92  is an electrode  100  which is connected to an external terminal  102 .  
         [0124]    [0124] 104  indicates a multiaxial traversing unit. The traversing unit  104  enables the hydraulic and measuring unit  90  to be guided along the underside  91 , specifically from micro-cuvette  84  to micro-cuvette  84 , and then to press the unit  90  tightly from below against the underside  91  around the respective hole  88  in question. By applying a negative-pressure pulse  98  to the conduit  96  it is then possible to carry out the same experiment as was explained hereinabove with reference to FIG. 4 relating to the first specific embodiment of the invention.  
         [0125]    The first exemplary embodiment of the invention according to FIGS.  2  to  4  has the advantage that a compact plate comprising all the connecting channels for driving the micro-cuvettes  12  is available so that a plurality of measurements can be carried out sequentially or in multiplex mode without any further actuation devices, solely by driving valves, contacts and the like.  
         [0126]    The second exemplary embodiment according to FIG. 6, in contrast, has the advantage that a commercially available plate can be used and that the expense for a bottom layer with a plurality of connecting channels, connector tracks and individual electrodes can be dispensed with.  
       EXAMPLE 3  
       [0127]    [0127]FIGS. 7 a ) to  c ) provide an extremely schematic depiction of a further exemplary embodiment of the invention, which is explained below in more detail.  
         [0128]    Again, a substrate  110  is provided which, for example, can consist of a polyimide sheet. Formed in the substrate  110  is a plurality of channels, one of which, denoted by the numeral  122 , is shown. Above the substrate  110  there is fluid in which cells  112  are present. Below the channel  122  a chamber  124  is formed which communicates with the channel  122 , and at the bottom of which, in an arrangement similar to that of the embodiment according to FIG. 6, an electrode  126  is provided.  
         [0129]    In contrast to the above-described embodiments, this chamber  124 , however, communicates not with just one connecting channel but with two connecting channels  130 ,  132 . These connecting channels  130 ,  132  can be connected, via valves  118 ,  120 , to fluid reservoirs F 1  and F 2 .  
         [0130]    It goes without saying that the depiction is purely schematic and that the connecting channels  130 ,  132  can be formed e.g. in a photopolymerizable layer and that the valves are preferably configured at the outer ends of the channels.  
         [0131]    If, as shown in FIG. 7 a ), the valve  120  is in a closed position and the valve  118  is in an open position, the application of a pressure P 1  to the channel  130  which is lower than the pressure P 0  in the fluid  114  results in a flow being established in the direction of the arrow  133  through the channel  122  and the feeder channel  130 . This results in the cell  112  being aspirated and settling onto the surface  128  of the substrate  110  above the orifice of the channel  122  and forming a gigaseal if the negative pressure is maintained, so that the cell-attached configuration is established.  
         [0132]    If then, in accordance with FIG. 7 b ), the valve  120  is opened, while the relationship P 1 &lt;P 2 &lt;P 0  is adhered to, the chamber  124  fills with intracellular medium from the fluid reservoir F 2 , whilst a flow is established in the direction of the arrow  134  from the connecting channel  132  through the chamber  124  into the feeder channel  130 . In this situation the pressure P 2  must be greater than the pressure P 1  for the flow to be directed in the direction of the arrow  134  from the connecting channel  132  to the connecting channel  130 ; in addition, the two pressures P 1  and P 2  must be lower than the pressure P 0  in the extracellular medium  114  by which the cell  112  is surrounded.  
         [0133]    Then, in the following phase in accordance with FIG. 7 c ), the valve  118  is closed and a pulse-type negative pressure is applied to the connecting channel  132 , so that P 2  becomes very much smaller than P 0  (P 2 &lt;&lt;P 0 ). This causes the underside of the membrane of the cell  112 , the membrane patch, to be aspirated and, owing to the negative-pressure surges, to be ruptured, thereby establishing a whole-cell patch clamp. The flow during this phase is in the direction of the arrow  135  through the channel  122  and the connecting channel  132  towards the fluid reservoir F 2 . A state is then reached in which the chamber  124  is filled solely with intracellular medium  116 . Subsequently it is possible, via the valve  118 , to work again with some other medium if this is desirable for the measurements to be carried out.  
         [0134]    The advantage of this arrangement and this method is that it is possible to work with an accurately controlled intracellular medium, whose composition can be modified, or that it is even possible to use a different intracellular medium.  
         [0135]    This embodiment comprising two or more connecting channels which can be controlled via valves can be combined, as a matter of principle, both with the embodiment previously explained with reference to FIGS.  1  to  4 , and with the embodiment according to FIG. 6.