Abstract:
The invention relates to a microdispenser ( 1 ) for dispensing a liquid sample in a dispensing device, with a sample container ( 2 ) for receiving the liquid sample, and with a nozzle ( 7 ) for dispensing the sample located in the sample container ( 2 ). The microdispenser ( 1 ) with the filled sample container ( 2 ) can in this case be stored independently of and fluidically separately from the dispensing device, without the sample escaping from the sample container ( 2 ) during storage.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a microdispenser for dispensing a liquid sample in a dispensing device and also to an associated operating method. 
     Microdispensers are known for example from WO 99/30168 and are used inter alia for biological or chemical purposes in order to dispense small volumes of liquid in a controlled manner in the form of microdroplets. One example of this is the production of miniaturized DNA arrays for genetic engineering uses, wherein a high clone density is achieved by means of a small droplet size, for which piezoelectrically operated microdispensers are particularly suitable. The known microdispensers consist of a glass capillary for holding the sample liquid to be dispensed, the glass capillary having a nozzle for dispensing the microdroplets and a flexible supply line for filling it with the sample liquid. The discharging of the microdroplets from the glass capillary in the known microdispensers is brought about by means of a piezo actuator, which surrounds the glass capillary in the manner of a sleeve in the region of a narrowing and expands or compresses the glass capillary in the radial and/or longitudinal direction in the event of electrical actuation, as a result of which a shock wave is generated in the glass capillary which, after reflection at the narrowing of the glass capillary, discharges microdroplets out of the glass capillary through the nozzle. 
     One disadvantage of the known microdispensers described above is the fact that filling and operation of the glass capillary via a flexible hose can lead to contamination since the plastic hoses which are typically used, may form biofilms. Sterile handling of the transition points between the plastic hose and the glass capillary is particularly problematic, since this area contains corners and edges in which it is possible for contaminations (e.g. biofilms) to be deposited, in which bacteria, viruses, fungi and yeasts may in turn settle. This problem of contamination when filling the glass capillaries is particularly disadvantageous when the microdispensers are used in the diagnostics sector, since the contamination may then lead to faulty products and incorrect diagnoses. 
     When handling the known microdispensers, the problem generally exists that the microdispensers are subjected to operating cycles which consist of uptake by suction (filling), dispensing and subsequent rinsing. These operating cycles are repeated with the same microdispenser for the next sample or for a number of samples and may lead to so-called cross-contamination, and take up a large amount of time. 
     Furthermore, it must be noted with regard to the prior art that, in the case of so-called high throughput screening, the substances to be tested are usually stored dissolved in DMSO (dimethyl sulphoxide) in microtiter plates, wherein the microtiter plates containing a plurality of samples are deep-frozen. In order to remove a single sample, the entire microtiter plate along with all the samples located therein then has to be thawed, whereby moisture from the air can condense into the samples, which leads to impairment of the samples. 
     Furthermore, U.S. Pat. No. 6,191,946 discloses a microdispenser which is filled, cleaned and emptied via the nozzle. Moreover, the microdispenser in this case has a sample channel which is accessible from outside and therefore cannot be stored or transported in the filled state. 
     WO 97/44 134 A1 also discloses a microdispenser, which is filled from the front via the nozzle and therefore has the same problems as the known microdispenser described above, i.e. the microdispenser cannot be stored or transported in the filled state. 
     Patent application EP 0 469 444 A1 discloses a disposable (single-use) nozzle unit which contains the analysis liquid in pre-filled form. However, here the analysis liquid is discharged according to the bubble-jet technique, which heats up the analysis liquid and therefore is not suitable for the microdispenser according to the invention since, with the small dimensions of the microdispenser according to the invention, the heating that is required in the bubble-jet method in order to produce vapor bubbles would damage, i.e. denaturize, the biological samples in the sample liquid. 
     U.S. Pat. No. 6,296,811 B1 and U.S. Pat. No. 6,866,825 disclose a microdispenser in which the filling of the microdispenser with the sample liquid to be dispensed takes place via lines, which is associated with the problems of contamination described above. 
     Furthermore, DE 26 53 051 A1 and DE 101 06 362 A1 disclose pipettes for taking blood, which also allow storage. However, these are not microdispensers within the meaning according to the invention. The same applies in respect of the pre-filled pipettes known from WO 02/42175 A1 and U.S. Pat. No. 6,457,612. 
     Moreover, U.S. Pat. No. 6,232,129 B1 discloses a conventional piezoelectric pipetting device for transferring droplets of samples between two containers. However, these pipetting devices do not allow long-term storage in the filled state. 
     Finally, U.S. Pat. No. 4,528,579, U.S. Pat. No. 4,418,354 and U.S. Pat. No. 4,528,578 disclose microdispensers for inkjet printers, which as such are not suitable for dispensing biological samples. 
     The task of the invention is therefore to improve the known microdispensers in such a way that as far as possible no contamination or impurities occur. 
     SUMMARY OF THE INVENTION 
     This task is solved by a microdispenser according to the invention and by an associated operating method. 
     The invention encompasses the general technical teaching of using a microdispenser not only for dispensing purposes but also for storing and transporting the sample that is to be dispensed, so that there is no need to fill the microdispenser at the start of a dispensing operation, as a result of which the problems of contamination which occur in the prior art can be avoided. Furthermore, no time-intensive rinsing operations are carried out. 
     The microdispenser according to the invention can therefore be stored and/or transported with the filled sample container independently of and fluidically separated from the actual dispensing device, without the sample liquid escaping from the sample container during storage or during transport. The known microdispensers described in the introduction are not suitable for this, as will be discussed below. In the known microdispensers with a flexible supply line for filling the sample container, the sample liquid could escape via the flexible supply line during storage or during transport. By contrast, in the other known microdispensers in which filling takes place from the front via the nozzle, the filling via the nozzles and the subsequent closing of the nozzles would take up a large amount of time. 
     The microdispenser according to the invention is preferably also characterized in that the filling of the sample container with the sample liquid takes place separately from the dispensing operation, whereas the filling operation in the case of the known microdispensers using a flexible supply line takes place immediately before or during the dispensing operation. In practice, the sample container of the microdispenser according to the invention is usually filled prior to the dispensing operation, whereas no refilling is provided during the subsequent dispensing operation. 
     In one variant of the invention, the microdispenser is configured for single use, wherein the sample container (e.g. a glass capillary) is not refillable. In this variant, the single-use microdispenser is filled with the desired sample liquid at the factory and then stored and delivered, i.e. transported to the customer, in the filled state, wherein no sample liquid can escape from the microdispenser. In this way, contamination of the sample liquid located in the microdispenser is completely prevented, since the sample liquid is enclosed in the microdispenser in a sterile manner. Preferably, the sample container is therefore fully closed apart from the nozzle and, apart from the nozzle which is closed during storage or transport, has no other opening through which sample liquid could escape during storage or transport. 
     On the other hand, in another variant of the invention, the microdispenser has a valve for venting the sample container during the dispensing operation and/or for filling the sample container, which allows reuse of the microdispenser according to the invention. The microdispenser can, e.g., be filled with the desired sample liquid through the valve at the factory, and then the valve is closed during storage and transport. For the actual dispensing operation, the valve is then opened in order to vent the sample container, so that the dispensing of the sample is not prevented due to the build-up of an excess vacuum within the sample container. The valve in this case is preferably structurally integrated in the microdispenser and forms a unit with the microdispenser, whereby the dispenser according to the invention differs from the known microdispensers described above in which a supply line is connected to a separate pressure control device. 
     Within the context of the invention, single use means that the microdispenser is filled with the sample substance once, i.e. usually at the factory. Once this filling has been dispensed, the dispenser is disposed of. Reusable in the present context means that the microdispenser can be used one or more times in the dispensing device. Between the individual dispensing operations, the microdispenser is stored along with the sample contained therein. 
     The microdispenser according to the invention preferably has a marking which contains information about the microdispenser and/or the sample located in the sample container. This is particularly useful when the microdispenser is filled at the factory and then is delivered in the filled state, since the end user can then identify the sample located in the sample container on the basis of the marking, as a result of which incorrect use is avoided. 
     The marking for identifying the microdispenser and/or the sample may for example consist of an optical code, such as e.g. a one-dimensional or two-dimensional barcode which is optically read. However, it is also possible as an alternative that the microdispenser according to the invention has an integrated transponder which can be read in a wireless manner, which allows a high degree of automation. In one particular embodiment of the microdispenser according to the invention, the latter is equipped with a transponder which has both a “read-only” area and a “read and write” area. In this embodiment, the tracking of the sample is solved for example in a particularly advantageous manner since each storage operation and each dispensing operation is stored directly on the microdispenser itself, for example along with a date stamp, the number of dispensed droplets or the batch number of the array produced. 
     In the microdispenser according to the invention, a sponge located in the sample container may for example be used to prevent the sample liquid from escaping from the sample container through the nozzle, in that the sponge exerts on the sample liquid a capillary force, which generates a holding vacuum in the sample container. The holding vacuum in the microdispenser according to the invention may be between 1 mbar and 100 mbar, but the invention is not restricted to this range of values with regard to the holding vacuum. 
     However, it is also possible as an alternative that the nozzle is closed for transport and/or storage, by means of a stopper, a film, by means of wax or by means of a closure cap. 
     Preferably, the sample container (e.g. a glass capillary) in the microdispenser according to the invention has a stepped cross section at which shock waves are reflected, said shock waves being generated by an actuator (e.g. a conventional piezo actuator). In one variant of the invention, the stepped cross section consists of a narrowing of the cross section, as is known from the prior art. By contrast, in another variant of the invention, the stepped cross section consists of a widening of the cross section, which also leads to the reflection of shock waves in the region of the stepped cross section. With regard to the configuration of the stepped cross section in order to achieve the desired reflection effect, reference should be made to Wolfgang Wehl: “Akustik und Fluidmechanik in Kanälen und Düsen von Tintenschreibwerken”, Dissertation TU Munich, 2nd edition 2002, the content of which is hereby fully incorporated in the present description, so that there is no need at this point for a detailed description of the configuration of the stepped cross section. 
     The sample container and/or the nozzle in the microdispenser according to the invention may be made for example of plastic, glass or ceramic, but the invention is not restricted to the aforementioned materials with regard to the materials for the sample container and/or the nozzle, but rather in principle it is also possible for these to be made of other materials. 
     Preferably, however, the sample container and/or the nozzle in the microdispenser according to the invention consist of an inert material, such as glass for example. 
     Furthermore, the microdispenser according to the invention preferably has an actuator for discharging the sample located in the sample container, wherein the actuator is preferably structurally integrated in the microdispenser. 
     However, it is also possible as an alternative that the actuator is structurally separate from the microdispenser and the latter is brought together with the actuator only at the time of the dispensing operation. The microdispenser according to the invention can therefore be stored and transported without the actuator. 
     By way of example, the actuator can be connected to the sample container by means of a material fit, friction fit and/or form fit. 
     Preferably, the actuator surrounds the sample container in the manner of a sleeve, as known per se from the prior art. Here, the sleeve-like actuator can be pushed onto the sample container, wherein the actuator can be fixed in the pushed-on state by means of a mounting sleeve. The mounting sleeve may for example be fixed on the sample container by screwing or by means of a securing ring. It is also advantageous if the mounting sleeve has a spring slot which runs in the axial direction, so that the mounting sleeve can expand slightly in the radial direction. 
     Furthermore, it is advantageous if the nozzle and/or the sample container is provided with a coating which acts in a protein-immobilizing or DNA-immobilizing manner. Such coatings are described for example in EP 0 880 535 B1, DE 44 470 15 A1, U.S. Pat. No. 6,852,851, U.S. Pat. No. 5,650,506, U.S. Pat. No. 5,705,628, U.S. Pat. No. 5,898,071, U.S. Pat. No. 5,447,864 and WO 97/21090, so that the content of these publications is hereby fully incorporated in the present description with regard to the structure and composition of the immobilizing coating. 
     It is also advantageous if the nozzle and/or the sample container is provided with a coating which permits specific adsorption of biomolecules, prevents coagulation of a sample contained in the microdispenser and/or is suitable for purifying and/or concentrating the sample. Suitable substances for such coatings are known per se from standard methods of biotechnology and biochemistry. 
     It should also be mentioned that the sample container preferably has a volume in the range from 1 nl to 1 ml, while the nozzle preferably has a nozzle diameter in the range from 10 μm to 100 μm. However, the invention is not restricted to the aforementioned ranges of values with regard to the volume of the sample container and the nozzle diameter, but rather can in principle also be implemented with other values. 
     Furthermore, the sample container preferably has both a sample reservoir and a working chamber. The working chamber in this case serves to discharge an individual sample from the microdispenser. To this end, the working chamber can be compressed radially and/or longitudinally for example by means of a piezo actuator, as known per se. The sample reservoir, on the other hand, serves to hold a sufficient quantity of sample to make it possible to dispense a number of samples one after the other without having to refill the microdispenser in a manner that may lead to contamination. The sample reservoir therefore preferably has a larger volume than the working chamber of the sample container. 
     In one variant of the invention, the nozzle consists only of a suitably shaped nozzle opening of the sample container. However, it is also possible as an alternative that the nozzle is formed by a separate nozzle film, a nozzle cap or a nozzle stopper, which close an opening in the sample container. 
     Furthermore, it is also possible within the scope of the invention that the sample reservoir permits a change in volume on account of the design or the material used. The sample reservoir may for example have a flexible and preferably elastic wall, so that the volume of the sample reservoir can be reduced by compressing the sample container in the region of the sample reservoir. 
     The sample reservoir may for example also be formed by an elastic membrane or a flexible hose. The change in volume in this case can take place for example by applying an external fluidic pressure to the membrane or the hose. Alternatively, it is possible that the volume of the sample reservoir can be changed by pressing a ram against the membrane or onto the hose. In the case of a hose, another possibility for reducing the volume consists in rolling up or folding the hose. 
     It is also possible that the sample reservoir consists of a tube in which a ram can be displaced, so that the position of the ram within the tube determines the volume of the sample reservoir. The ram may in this case be an integral part of the sample container or else can be moved by applying an external force (e.g. pressure, lever). 
     It should also be mentioned that the sample reservoir preferably has a flexible wall so that, when a sample is dispensed, the volume of the sample reservoir can be adapted according to the dispensed volume of sample. 
     Furthermore, the invention is not restricted to the above-described microdispenser according to the invention as an individual part, but rather also encompasses a complete dispensing device comprising at least one microdispenser according to the invention. Here, the dispensing device may be characterized in that no sample supply line for the microdispenser is provided, so that the microdispenser is fluidically separated from the dispensing device. The dispensing device in this case merely brings about actuation of the associated actuator for dispensing droplets, and in some circumstances also the spatial positioning and orientation of the microdispenser. 
     The microdispenser according to the invention is particularly advantageous for producing so-called working copies for screening processes in the pharmaceutical industry for example. This relates for example to the production of so-called microarrays or to the production of multiple assays. In the working methods of the prior art, aliquots are taken from the substances, which are generally located in microtiter plates or other containers, and these aliquots are distributed in microtiter plates. From these so-called grandmother plates, mother and then daughter plates (copies) are produced by means of two further aliquot-taking and multiplication steps. To this end, the microtiter plates which have been filled with the sample substances and deep-frozen firstly have to be thawed in order to allow removal of the liquid sample substances. During this thawing operation, moisture from the air may condense into the sample substances, which leads to impairment of the sample substances as has already been mentioned above. After thawing, the sample substances are then removed from the respective mother plate and transferred to a so-called daughter plate. In the same way, a further copy is then created from the daughter plate, this further copy being referred to as the screening plate or microarray. This procedure, which corresponds to the prior art, is difficult and complicated and consists of a large number of individual steps which may lead to contamination, cross-contamination, delays, dilution errors or dilution due to condensing water. Moreover, an individual sample can be taken only by carrying out the process on the entire microtiter plate. By contrast, the microdispenser according to the invention is particularly suitable for producing working copies in the screening process in the form of microarrays, multiple assays or for producing so-called diagnostic chips. Here, the sample substance is filled directly from the grandmother plate into the microdispenser and used to produce for example the arrays. There is no need for the mother and daughter plates which would otherwise be necessary. Individual samples can be dispensed without having to thaw the entire microtiter plate. It should also be mentioned that the invention is not restricted to the microdispenser described above and to a corresponding dispensing device, but rather also encompasses a corresponding operating method as will already be obvious from the above description. 
     The microdispenser according to the invention may be handled manually in the conventional manner by an operator. 
     However, it is also possible as an alternative that the microdispenser according to the invention may be handled by a robot. The microdispenser according to the invention is therefore preferably compatible, in terms of its dimensions and shape, with conventional robots for handling microdispensers, so that the microdispenser according to the invention can also be handled by conventional robots without any structural adaptations to the robots being necessary. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       Other advantageous further developments of the invention are characterized in more detail below with reference to the figures, together with the description of the preferred examples of embodiments. In the figures:— 
         FIG. 1  shows a cross-sectional view of a single-use microdispenser according to the invention, 
         FIG. 2  shows a cross-sectional view of an alternative example of embodiment of a reusable microdispenser according to the invention, with a valve for venting and filling the microdispenser, 
         FIG. 3  shows a cross-sectional view of a further example of embodiment of a microdispenser according to the invention, with a widening of the cross section in order to reflect shock waves in the sample container, 
         FIGS. 4   a ,  4   b  show different examples of embodiments of the valve for the microdispenser shown in  FIG. 2 , 
         FIG. 5  shows a cross-sectional view of a stopper for closing the microdispenser according to the invention during transport and storage, 
         FIG. 6  shows a cross-sectional view of a further example of embodiment of a microdispenser according to the invention, without a valve but with a widening of the cross section, 
         FIGS. 7   a ,  7   b  show a cross-sectional view and a side view, respectively, of a further example of embodiment of a microdispenser according to the invention, with an enlarged sample reservoir, 
         FIGS. 8   a - 8   d  show different views of a further example of embodiment of a microdispenser according to the invention, with a nozzle cap which forms the nozzle, 
         FIGS. 9   a - 9   c  show different views of a further example of embodiment of a microdispenser according to the invention, with a glued-on nozzle film which forms the nozzle, 
         FIGS. 10   a - 10   e  show different views of a further example of embodiment of a microdispenser according to the invention, with a glued-on closure film which closes the nozzle during storage and transport, and 
         FIGS. 11   a - 11   d  show different views of a further example of embodiment of a microdispenser according to the invention with a pushed-on piezo actuator, which is secured by a mounting sleeve and a securing ring. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The cross-sectional view in  FIG. 1  shows a microdispenser  1  according to the invention which can be used in a dispensing device (not shown) to dispense microdroplets. 
     In order to hold a sample liquid, the microdispenser  1  has a sample container  2  in the form of a glass capillary with a volume of 20 μl, wherein the sample container  1  has a narrowing  3  which divides the sample container  2  into a sample reservoir  4  and a working chamber  5 . 
     Arranged in the sample reservoir  4  is a sponge  6  which exerts on the sample liquid capillary forces which generate a holding vacuum in the sample reservoir  4 , as a result of which the sample liquid is prevented from running out of the sample container  2 . 
     Furthermore, the sample container  2  has a nozzle  7  with a nozzle diameter of 70 μm, through which microdroplets  8  can be dispensed. 
     The dispensing of the microdroplets  8  is brought about by a piezo actuator  9  which surrounds the sample container  2  in the manner of a sleeve in the region of the narrowing  3  and expands or compresses the sample container  2  in the radial and/or longitudinal direction upon electrical actuation. As a result, a shock wave is generated in the sample liquid located in the sample container  2 , which shock wave is reflected at the narrowing  3  and leads to the microdroplets  8  being discharged from the nozzle  7 . 
     Of particular importance in this example of embodiment is the fact that the sample container  2  is completely closed apart from the nozzle  7  and cannot be refilled, so that the microdispenser  1  is designed for single use. The microdispenser  1  is therefore filled with a predefined sample liquid at the factory and then delivered in the filled state, so that contamination as a result of refilling operations is completely ruled out. During storage and transport, the nozzle  7  may additionally be closed by means of a stopper, a film or by means of wax, in order to hermetically seal the sample container  2 . One example of embodiment of a stopper for closing the nozzle  7  is shown in  FIG. 5  and will be described in more detail below. 
     Moreover, the microdispenser  1  has a barcode  10  which contains information about the microdispenser  1  (e.g. nozzle diameter, container volume) and the sample liquid located in the sample container  2 , in coded form. 
     The alternative example of embodiment shown in  FIG. 2  largely corresponds to the example of embodiment shown in  FIG. 1  and described above, so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components and elements. 
     One special feature of this example of embodiment lies in the fact that the sample container  2  is not hermetically sealed but rather has a valve  11 . The valve  11  on the one hand makes it possible to vent the sample container  2  during the dispensing operation, so that the dispensing of the microdroplets  8  is not prevented by the build-up of a vacuum within the sample container  2 . The valve  11  also makes it possible to refill the sample container  2 , which permits reuse of the microdispenser  1 . However, the valve  11  is structurally integrated in the microdispenser and can be closed, so that no sample liquid can escape from the filled sample container  2  during transport and storage of the microdispenser  1 .  FIGS. 4   a  and  4   b  show different examples of embodiments of the valve  11 , which will be described in more detail below. 
       FIG. 3  shows a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the examples of embodiments described above, so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the sample container  2  has, instead of the narrowing  3 , a widening  12  of the cross section at which shock waves are likewise reflected, as explained in the aforementioned dissertation by Wolfgang Wehl: “Akustik und Fluidmechanik in Kanälen und Düsen von Tintenschreibwerken”. 
       FIG. 4   a  shows a cross-sectional view of an example of embodiment of the valve  11  with a valve housing  13  made of plastic, which is shrunk onto the sample container  2  and then securely connected to the sample container  2  in a gas-tight manner. To this end, the valve housing  13 , which has been slightly underdimensioned, is heated and then pushed onto the sample container  2  in the heated and therefore expanded state. During subsequent cooling to ambient temperature, the valve housing  13  then contracts again, which leads to a secure fit of the valve housing  13  on the sample container  2 . 
     However, it is also possible as an alternative that the valve housing  13  is connected to the sample container  2  by means of an adhesive join. 
     In the valve housing  13 , a valve ball  14  is pressed against a valve seat  16  by a coil spring  15 . The valve  11  therefore opens only in one direction when the pressure difference acting from outside is large enough to lift the valve ball  14  away from the valve seat  16  counter to the force of the coil spring  15 . Here, the prestress and the spring stiffness of the coil spring  15  are selected such that the valve  11  slowly opens when the piezo actuator  9  is actuated and as the sample container  2  becomes increasingly empty, so as to vent the sample container  2 . 
     The valve  11  in this example of embodiment also has a screen  17  (or a membrane) which prevents foreign bodies from entering the sample container  2  during filling of the sample container  2 . 
       FIG. 4   b  shows an alternative example of embodiment of the valve  11  which largely corresponds to the example of embodiment described above, so that, in order to avoid repetition, reference will largely be made to the above description in respect of  FIG. 4   a , wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the sample container  2  is shrunk onto the valve housing  13  during assembly. To this end, the sample container  2 , which has been slightly underdimensioned, is heated and then pushed onto the valve housing  13  in the heated and therefore expanded state. During subsequent cooling to ambient temperature, the sample container  2  then contracts again, which leads to a secure fit of the sample container  2  on the valve housing  13 . 
     Finally,  FIG. 5  shows a cross-sectional view of a stopper  18  which can be pressed into the nozzle  7  of the sample container  2  in order to prevent sample liquid from escaping from the sample container  2  during storage or transport. The stopper  18  is made of plastic and tapers slightly in a conical manner, which facilitates insertion of the stopper  18  into the nozzle  7 . On its conical outer surface, the stopper  18  has a plurality of sealing ribs  19  running annularly, as a result of which the surface pressure between the stopper  18  and the inside of the nozzle  7  is increased and thus the sealing effect is improved. 
     Furthermore, the stopper  18  is designed in a pot-shaped manner and is open towards the outside, so that the stopper  18  can be pressed into the nozzle  7  by means of a ram  20 . 
     The alternative example of embodiment shown in  FIG. 6  largely corresponds to the examples of embodiments described above which are shown in  FIG. 1  and  FIG. 3 , so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components and elements. 
     This example of embodiment combines the closed structure of the example of embodiment shown in  FIG. 1  with the widening of the cross section in the example of embodiment shown in  FIG. 3 . 
       FIGS. 7   a  and  7   b  show a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the examples of embodiments described above, so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the sample reservoir  4  has a much larger diameter than the working chamber  5  of the sample container  2 . Accordingly, the sample reservoir also has a larger volume than the working chamber  5 . This offers the advantage that the microdispenser  1  can dispense a plurality of more than one million microdroplets  8  one after the other, without intermediate refilling of the microdispenser being necessary. 
     Another special feature of this example of embodiment lies in the fact that the nozzle  7  is formed by a nozzle stopper which is pressed into a corresponding opening in the working chamber  5 . The nozzle geometry in this case is therefore not determined by the shape of the opening in the working chamber  5 , which opens up a greater number of design options with regard to the nozzle geometry. 
     Furthermore, the microdispenser  1  in this example of embodiment has a particular design of the valve  11  comprising the valve ball  14 , the valve seat  16  and a stopper  21 , wherein the coil spring  15  is supported against the inside of the stopper  21  and the coil spring  15  pushes into the valve seat  16 . 
     Furthermore, the piezo actuator  9  in this example of embodiment is entirely arranged in the region of the working chamber  5 , i.e. on the nozzle side of the narrowing  3 . 
       FIGS. 8   a  to  8   d  show a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the example of embodiment described above and shown in  FIGS. 7   a  and  7   b , so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the nozzle  7  is formed by a nozzle cap  22  which is pushed onto the front end of the working chamber  5  and has a nozzle opening in its end face. The nozzle geometry in this case is therefore not determined by the shape of the opening in the working chamber  5 , which opens up a greater number of design options with regard to the nozzle geometry. 
       FIGS. 9   a  to  9   c  show a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the example of embodiment described above and shown in  FIGS. 7   a  and  7   b , so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the nozzle  7  is formed by a nozzle film  23  which is glued or welded onto the front end of the working chamber  5  and has a nozzle opening in its centre. The nozzle geometry in this case too is therefore not determined by the shape of the opening in the working chamber  5 , which opens up a greater number of design options with regard to the nozzle geometry. 
       FIGS. 10   a  to  10   e  show a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the example of embodiment described above and shown in  FIGS. 7   a  and  7   b , so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the nozzle  7  is closed by a closure film  24 , which prevents sample liquid from being able to escape from the sample container  2  through the nozzle  7  during storage and transport of the microdispenser. Prior to the dispensing operation, the closure film  24  is then simply pulled away from the nozzle  7 , the removal of the closure film  24  from the nozzle  7  being facilitated by means of laterally protruding tabs. 
       FIGS. 11   a  and  11   c  show a further example of embodiment of a microdispenser  1  according to the invention, which once again largely corresponds to the examples of embodiments described above, so that, in order to avoid repetition, reference will largely be made to the above description, wherein the same reference numerals are used for corresponding components. 
     One special feature of this example of embodiment lies in the fact that the piezo actuator  9  is pushed axially onto the sample container  2 , wherein the piezo actuator  9  butts against an annular peripheral stop. In order to mechanically fix the piezo actuator  9  on the microdispenser  1 , a mounting sleeve  25  is provided which is pushed onto the piezo actuator  9  in the axial direction and has on each side an axially running slot which facilitates a slight radial expansion of the mounting sleeve  25 , as a result of which it is easier to push the mounting sleeve  25  on. Once the mounting sleeve  25  has been pushed on, the latter is mechanically fixed by means of a securing ring  26 . 
     The invention is not restricted to the preferred examples of embodiments described above. Rather, a plurality of variants and modifications are possible which likewise make use of the inventive concept and which therefore fall within the scope of protection. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  microdispenser 
           2  sample container 
           3  narrowing 
           4  sample reservoir 
           5  working chamber 
           6  sponge 
           7  nozzle 
           8  microdroplets 
           9  piezo actuator 
           10  barcode 
           11  valve 
           12  widening of cross section 
           13  valve housing 
           14  valve ball 
           15  coil spring 
           16  valve seat 
           17  screen 
           18  stopper 
           19  sealing ribs 
           20  ram 
           21  stopper 
           22  nozzle cap 
           23  nozzle film 
           24  closure film 
           25  mounting sleeve 
           26  securing ring