Abstract:
The present invention relates to a device for controlling a liquid flow in a liquid channel, comprising: an elongate liquid holder in which a liquid channel is provided in longitudinal direction; first voltage means for applying a first voltage difference over substantially the longitudinal direction of the liquid channel; a conductor member arranged in at least a part of the liquid channel against the liquid holder; an insulator member arranged in the liquid channel against at least the conductor member; second voltage means for providing a second voltage difference between the conductor member and the liquid in the liquid channel; wherein the thickness of the insulator member is a maximum of 1 μm and preferably in the order of magnitude of some tens of nanometres.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for controlling a liquid flow in a liquid channel. The present invention also relates to an assembly and integrated circuit in which this device is placed, and to a method for manufacture thereof. 
     2. Description of the Prior Art 
     Charged particles in a solution or suspension of liquid channel con be transported by applying an electric field substantially parallel to the liquid channel. Under the influence of the electric field positively and negatively charged particles will move in opposing directions. This transport is also referred to as electrophoresis. 
     Another mechanism for generating a liquid flow in a liquid channel is formed by so-called electro-osmosis. The liquid channel is in this case enclosed by an electric insulator. At the location of the transition between the insulator and the liquid are situated charged insulator particles which are chemically bound to the insulator. As a consequence of the charge of these insulator particles, particles with an opposing charge are formed close to the insulator wall in the liquid channel. The layer consisting of the chemically bound insulator particles and the liquid particles charged in opposing directions is also referred to as the electric double layer. As a result of the presence of these particles with opposing charge, which are not chemically bound to the insulator, and the above mentioned electric field applied parallel to the direction of the liquid channel, a liquid flow will be generated along the walls of the liquid channel. The liquid flow along the walls brings about a liquid flow across the entire diameter of the liquid channel as a result of the friction between the liquid particles. 
     The moving charged particles define a shear plane at some distance of the insulator wall. The electrical potential at the location of this shear plane is called the ζ-potential (Zeta potential). The magnitude of the ζ-potential depends inter alia on factors such as the type of liquid or insulator, the concentrations of the different particles in the liquid, the pH value and the like. The direction and the degree of liquid flow resulting from electro-osmosis can be controlled by changing these factors. 
     It can be deemed known to vary the potential of the outer surface of the insulator with a voltage source, as a result of which the above stated ζ-potential in the liquid channel can be varied. Since the direction and speed of the liquid flow in the liquid channel depends on the magnitude of the ζ-potential, the movement of the particles in the liquid can be controlled with the voltage source, i.e. the movement of particles resulting from electrophoresis can be enhanced or decreased. While there are indeed devices known which enable such a control of the liquid flow in a liquid channel, they have large dimensions and require very high control voltages in the order of magnitude of several kVs, so that in practice they cannot be integrated with standard electronic components such as transistors, integrated circuits and so on. 
     The object of the present invention is to obviate these drawbacks. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, a device is provided for this purpose for controlling a liquid flow in a liquid channel, comprising: 
     an elongate liquid holder in which a liquid channel is provided in longitudinal direction; 
     first voltage means for applying a first voltage difference over substantially the longitudinal direction of the liquid channel; 
     a conductor member arranged in at least a part of the liquid channel against the liquid holder; 
     an insulator member arranged in the liquid channel against at least the conductor member; 
     second voltage means for providing a second voltage difference between the conductor member and the liquid in the liquid channel; wherein the thickness of the insulator member is a maximum of 1 μm and preferably in the order of magnitude of some tens of nanometres. In accordance with this aspect of the invention a device is therefore provided for controlling a liquid flow, wherein the functions of liquid container or liquid holder on the one hand and of insulator or ζ-potential control layer on the other are separated, so that a great flexibility can be achieved in choice of material and method of manufacture. 
     According to a preferred embodiment of the invention the insulator member is formed from a thin layer or coating of insulator material, the conductor member and the liquid holder are combined and formed from a mechanically stable conductor material. The mechanically stable material provides in this case the required sturdiness of the device. 
     According to a further preferred embodiment of the invention the insulator member and the conductor member are formed from thin layers of respectively insulator material and conductor material, wherein the liquid holder is preferably formed from a mechanically stable material. 
     According to another aspect of the invention, a device is provided for controlling the liquid flow in a liquid channel, comprising: 
     an insulator member which defines an elongate liquid channel; 
     first voltage means for applying a first voltage difference over substantially the longitudinal direction of the liquid channel; 
     a conductor member arranged over at least a part of the outer surface of the insulator member; 
     second voltage means for providing a second voltage difference between the conductor member and the liquid in the liquid channel; wherein the distance between the outer surface and the inner surface of the insulator member is a maximum of 1 μm and preferably in the order of magnitude of some tens of nanometres. By making the wall thickness of the insulator member so small, the control of the liquid flow can advantageously be performed with a small second voltage difference, for instance with a voltage difference of less than 20 Volt. At such small wall thicknesses there moreover occurs a reduced loss of power and an improved heat discharge is possible. 
     According to a further preferred embodiment of the invention the device can be directly connected to standard electronic elements or integrated circuits or can even be integrated therewith. This preferred embodiment can therefore be advantageously connected directly onto and controlled by the output of the standard electronic elements such as integrated circuits, without additional provisions being required therein. 
     According to a further preferred embodiment of the invention the insulator member and the conductor member are manufactured from optically transparent materials. This has the advantage that the content and/or composition of the content of the liquid channel can be optically detected in simple manner. 
     According to a further preferred embodiment of the invention the insulator member is constructed from two or more insulator part-members manufactured with materials of different ζ-potential. This has the advantage that various flows with differing speeds and directions can be generated in the liquid channel without applying an external potential difference. 
     According to a further embodiment of the invention the insulator member is provided with two or more conductor members, to which mutually differing voltages can be applied. By applying different potentials to the conductor members, the associated ζ-potentials in the liquid channel will accordingly differ from each other. This has the advantage that different flows with differing speeds and/or directions of movement can be generated within the same liquid channel. 
     According to another preferred embodiment the voltage means comprise two electrodes which are arranged in the liquid channel. Because the electrodes can be arranged in the liquid the distance between the electrodes can be reduced, at least relative to the distance in the case of external electrodes, to an order of magnitude of a few-micrometers. Lower voltages are hereby sufficient to obtain the desired field strength of the electric field. 
     According to a further aspect of the invention, a system is provided for analysis and/or synthesis of chemical solutions or suspensions, wherein one or more of the above stated devices is used. According to a preferred embodiment of this system, this system comprises control means for controlling one or more liquids through a network of said devices. 
     According to a further preferred embodiment the network of said devices comprises one or more feed channels, two intermediate channels branched from the feed channels and one or more drain channels connected to the intermediate channels, wherein the intermediate channels are provided with gates which are supplied with voltage such that in the intermediate channels a substantially loop-like liquid flow results. Using such loop-like liquid flows the liquid in the intermediate channels can be circulated so that the liquid is mixed or enters into a chemical reaction. 
     According to a further aspect of the present invention, a pump system is provided for circulating liquid, in which preferably one or more of the above stated devices are used, comprising: 
     a liquid holder in which are provided a liquid feed channel and a liquid drain channel branched from the liquid feed channel; 
     first voltage means for applying an electric field in the longitudinal direction of the liquid feed channel; 
     a first and a second gate electrode which are placed on either side of the connection of the liquid drain channel to the liquid feed channel; 
     second voltage means for providing the first and second gate electrode with voltage; 
     control means for adjusting the first and second voltage means such that a pressure build-up occurs at the location of said connection and the liquid is drained via the liquid drain channel. 
     With the above stated system a pump can be realized on micro-scale, wherein the drain channel of the pump is voltage-free, this being advantageous since the drain channel can thereby be connected more easily to peripheral equipment and the like. 
     According to further embodiments of thee above stated pump system, the first voltage means generate an electric field alternately in a first and in a second, opposing direction in the liquid feed channel and the second voltage means, substantially synchronously with the first voltage means, switch the first gate electrode into enhancement mode and the second gate electrode into a reversement mode and vice versa. By circulating the liquid in such a manner polarization effects, such as the creation of air bubbles as a result of electrolysis, do not occur, which polarization effects could have an adverse effect on the operation of the pump. 
     According to a further aspect of the present invention, an electronic circuit is provided into which the above stated device is integrated. 
     According to a further aspect of the invention a method is provided for manufacturing the above stated device, comprising of: 
     etching a channel in a wafer; 
     depositing insulating material on the wafer; 
     manufacturing a glass plate; 
     anodic binding of the wafer on the glass plate; 
     etching the wafer; and 
     fixing the conductor member. 
     By manufacturing the device in this manner liquid channels with the correct properties and very small wall thicknesses of a few tens of nanometres can be realized. 
     According to a further aspect of the present invention a method is provided for mixing two or more liquids, comprising of: 
     supplying the liquids via one or more liquid channels; 
     mixing the liquids by circulating the supplied liquid, preferably in the above stated system; 
     draining the liquids via one or more liquid drain channels. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further embodiments, advantages, features and details of the present invention will be elucidated in the following description with reference to the annexed figures, in which: 
     FIG. 1 shows a longitudinal section of a preferred embodiment of the invention; 
     FIGS. 2 a  and  2   b  show schematic sketches of the operation of the electro-osmosis and electrophoresis mechanisms; 
     FIG. 3 is a schematic perspective view of another preferred embodiment of the invention, in which a network of liquid channels forms an electrical switch; 
     FIG. 4 shows a schematic perspective view of another preferred embodiment of the invention, depicting another electrical switch; 
     FIG. 5 shows a longitudinal section of another preferred embodiment of the invention; 
     FIG. 6 shows a schematic view of a further preferred embodiment of the invention for applications in a bioreactor; 
     FIG. 7 shows a schematic view of a further preferred embodiment for applications in a bioreactor; 
     FIG. 8 is a schematic view of a further preferred embodiment for applications as pump; and 
     FIGS. 9 a  and  9   b  are schematic views of another preferred embodiment of a pump. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Shown in the longitudinal section of FIG. 1 is a channel of rectangular cross-section, with an upper wall  1  and a lower wail  2  between which flows a liquid  3 . Placed on the left-hand side of the channel is an anode  4  which is set to a voltage of V 1 . Placed on the right-hand side of the liquid channel is a cathode  5  which is connected to the earth potential. As a result of the voltage difference between anode  4  and cathode  5  an electric field E is generated which transports positively charged particles in the direction of the arrow shown in the figure and negatively charged particles in the opposite direction (electrophoresis). The presence of positively or negatively charged particles in the liquid channel can be controlled by the choice of the insulator material of the insulator, the pH value of liquid  3 , the concentration of the particles in the solution or suspension and so on. The presence of positively or negatively charged particles in the electrical double layer can however also be controlled using a conductor  6  which is arranged on the outside of insulator wall  1  and set to a voltage V 2 . 
     FIG. 2 a  shows the progression of the positive voltage applied to the insulator wall by conductor  6 . On the interface  7  between conductor  6  and wall  1  the voltage equals V 2 , while the voltage decreases as the interface between wall  1  and liquid  3  is approached. At the location of interface  8  the voltage has a value ψ 0 , which voltage is designated as the wall potential. The wall potential is the consequence of charged particles  9 , in this case positively charged particles, chemically bound to wall  1 . Negatively charged particles  10  will occur in the liquid to compensate herefor. The voltage in the liquid channel decreases further as the distance from interface  8  increases. As a result of the applied field E the non-bound negatively charged particles  10  will be subjected to a force in the direction of the arrow a. This electric force decreases as the distance from the wall surface increases, since the number of negatively charged particles in this direction decreases. As a result of this electric force the part of the liquid in the electric double layer to the left of the shear plane or inner Helmholtz plane  11  will therefore start to move parallel to the wall surface, while the remaining part of the liquid is co-displaced by friction. 
     Shown in FIG. 2 b  is the situation where the voltage V 2  on conductor  6  is negative, so that the wall potential ψ 0  is negative. There therefore results in the electric double layer, in addition to the chemically bound negative charges  12 , a quantity of positively charged particles  13  which are transported in the direction of arrow b as a result of the electric field E which is present. 
     The liquid channels can be manufactured according to a method as described in the article “Glass channels and capillary injectors for capillary zone electrophoresis”, pages 77-84, Y. Fintschenko et al, in: A van de Berg en P. Bergveld, “Sensor Technology in the Netherlands: State of the Art”, Kluwer Academic Publishers, Dordrecht, 1998, pages 77-84. As alternative to this method of manufacture the devices according to the invention can be manufactured with a so-called “Self-Assembled Mono-layer” (SAM layer) on gold, silver or Si. A monolayer of thioalkanes for instance (which form a very good and thin insulator) is 
     herein coated from the inside on a hollow Au pipe. A sulphur group S is herein bound on the inside of the Au pipe in chemical manner, which group is connected via hydrophobic hydrocarbon chains to a functional end group, which end group influences the ζ-potential. The total thickness of the SAM layer is about 0.5-10 nm. Alternative manufacturing methods can also be envisaged in addition to the above described methods of manufacture. 
     By in any case embodying the liquid channels in this manner very thin wall thicknesses of less than 1 μm, preferably in the order of magnitude of a few (tens of) nanometres can be realized. As a result of these small dimensions the required magnitude of the control voltage V 2  is very low, for instance a few mV or V, and generally a maximum of 20 Volt. It is hereby possible to influence the liquid flow with relatively low voltages, wherein use can therefore be made in practice of the voltages occurring in standard electronic components such as transistors, integrated circuits and so on. An improved heat discharge can also be realized due to the small wall thickness. Relative to known liquid channels, which have a wall thickness of about 100 μm, the heat discharge is for instance up to four times faster. 
     FIG. 3 shows a view of a preferred embodiment of the invention, in which an electrical switch is formed in a network of liquid channels. A liquid channel  14  branches at a given position into two liquid channels  15  and  16 . Between the beginning of liquid channel  14  and the ends of liquid channels  15  and  16  a potential difference is applied by means of an anode  20  and two cathodes  21  and  22 . As a result of this voltage difference a flow occurs in liquid channel  14  in the direction of the arrow shown in FIG.  3 . In order to create an electrical switch with which the flow can be divided over the two liquid channels  15  and  16  at the branching, a conductor  17  is arranged on liquid channel  14 , a conductor  18  on liquid channel  15  and a conductor  19  on a liquid channel  16 . By supplying conductors  17 ,  18  and  19  with suitable voltages, the associated potentials are adjusted and the liquid flow in the network of channels can be controlled. Shown in table I is an overview of the voltage values required to control the direction of the liquid flow. This shows that for flow from channel  14  to channel  15  the voltage V 17  on conductor  14  must be positive, the voltage V 18  on conductor  15  must be positive and the voltage V 19  on conductor  16  must be negative. For flow from channel  14  to channel  16  the voltage V 17  on conductor  14  must be positive, the voltage V 18  on conductor  15  must be negative and the voltage V 19  on conductor  16  must be positive. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 14 → 15 
                 14 → 16 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 V 17   
                 + 
                 + 
               
               
                   
                 V 18   
                 + 
                 − 
               
               
                   
                 V 19   
                 − 
                 + 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 4 shows an alternative electrical switch wherein the infeed consists of a channel  23  and a channel  24  and the outfeed consists of a channel  25 , which either transports the liquid out of channel  23  or the liquid out of channel  24 . Anodes  29  and  30  are placed at the beginning of liquid channels  23  and  24 , while a cathode  31  is placed at the end of liquid channel  25 . By applying a voltage difference hereover an electric field is created in the liquid. Conductors  26  and  27  are moreover arranged on respectively liquid channel  23  and liquid channel  24  and a conductor  28  is arranged on liquid channel  25 . Table II shows the voltage values required to control the direction of the liquid flows. This shows that when the liquid flow of channel  23  has to be drained via channel  25 , the voltage V 26  on conductor  26  must be positive, the voltage V 27  on conductor  27  must be negative and the voltage V 25  on conductor  25  must be positive. If on the other hand the liquid from liquid channel  24  must be drained through liquid channel  25 , the voltage V 26  on conductor  26  must be negative, the voltage V 27  on conductor  27  must be positive and the voltage V 28  on conductor  28  must be positive. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 23 → 25 
                 24 → 25 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 V 26   
                 + 
                 − 
               
               
                   
                 V 27   
                 − 
                 + 
               
               
                   
                 V 28   
                 + 
                 + 
               
               
                   
                   
               
             
          
         
       
     
     FIGS. 3 and 4 show that conductors  17 ,  18 ,  19 ,  26 ,  27 ,  28  are preferably connected to a central control  40  in order to control the direction and speed of the liquid flows in the network of liquid channels. 
     In an embodiment of the invention which is not shown, a large number of electrical switches according to FIGS. 3 and 4 connected in parallel or in series can be combined to an extensive network of liquid channels in which the flow of the liquid can be regulated by a central control. 
     FIG. 5 shows a liquid channel wherein on the upper side of the channel the insulator  32  is manufactured from a first material, while on the underside the insulator  33  is manufactured from a second material, wherein the first and second materials have different ζ-potentials. It is also possible to arrange a conductor  34  on the top side of insulator  32 , while a second conductor  35  is arranged against the underside of the bottom insulator  31 . If different voltage values are applied to conductors  34  and  35 , different ζ-potentials occur in the liquid. When the voltage on conductor  34  is for instance positive, while the voltage on conductor  35  is negative, the liquid in the vicinity of the upper wall will move in the direction of arrow C and the liquid in the vicinity of the bottom wall will move in the direction of arrow D. It is hereby possible to bring about different directions of movement of the liquid in one channel, which may for instance be important when separating compositions. 
     The most important field of application of the present invention is in the development of new medicines and bio-analysis. Large numbers of substances must herein be analyzed very quickly, at a speed of for instance more than 10,000 analyses per hour. Another important field of application of the present invention is so-called “fluid-chemical computing” or “DNA-computing”, as for instance described in “computing with DNA” by L. M. Adleman, Scientific American, August 1998, page 34-41. A determined liquid volume in the liquid channel according to the invention, for instance with dimensions of 10 μm*1 μm*1 μm, can easily contain 10 3  DNA molecules, or 10 12  bits or information. Control of the liquid flow in the liquid channel as set out above with a switching time of 1 μs yields a data transfer speed of 10 18  bits/s, which is much faster than the data transfer speed in the present electronics. 
     FIG. 6 shows a schematic view of an application of the invention on a pump for circulating and mixing liquids in bioreactor applications. Use is made herein of a circuit of two flow channels in a so-called twin channel network. FIG. 6 shows that a liquid is fed via channel  41 , which channel  41  subsequently branches into a channel  42  and a channel  43 . Channels  42  and  43  join together again a little further along in drain channel  44 . Using anode  45  and cathode  46  an electric field, which in the example of FIG. 6 is directed substantially from left to right, is generated in the channels. Liquid channels  41 ,  42  and  43  are respectively provided with conductors  47 ,  48  and  49 . Using conductor  47  the liquid is fed in a manner already described with reference to FIG.  3 . By then providing conductors  48  and  49  with suitable voltages, i.e. conductor  48  such that an enhancement mode is generated and conductor  49  such that a reversement mode is generated, the liquid in liquid channels  42  and  43  is circulated in a clockwise direction, which is indicated in the figure with an arrow. By providing conductors (gates)  48  and  49  with voltage such that in channel  42  the reversement mode and in channel  43  the enhancement mode prevails, the rotation direction of the liquid flow can be reversed. With above stated (twin channel) network liquids can be fed in simple manner and subsequently mixed during circulation. It is also possible to have different liquids react with each other during circulation. Control of gates  48  and  49  (and  47 ) preferably takes place by means of a central control  40  so that the direction and speed of the liquid flows in the network of liquid channels is easy to control. After the liquid has been circulated sufficiently, gates  48  and  49  are both switched into the enhancement mode whereby the liquid can be drained via liquid channel  44 . It is noted that the above stated circulation can also be implemented in other ways. Conductor  47  for instance may thus be omitted as the case requires, or an extra conductor may be added in drain channel  44 . It is also possible to place anode and cathode  45  and  47  at other positions or to provide each conductor (gate)  47 ,  48 ,  49  with its own anode-cathode pair, for instance in a manner as occurs in a preferred embodiment discussed herein below. 
     FIG. 7 shows another advantageous embodiment, in which liquid is fed via two different liquid channels  51  and  55  into a twin channel network consisting of an upper liquid channel  52  and a lower liquid channel  53 , and the liquid is drained in drain channel  54  in a manner corresponding wholly with the embodiments of FIG.  6 . Using anodes  56  and  57  and cathode  58  an electric field is generated in the channel system. Through a suitable switching of gates  59 ,  60 ,  61  and  62  associated with respective liquid channels  51 ,  55 ,  52  and  53 , the different liquids can be fed via the associated feed channels  51  and  55  in adjustable ratios and can be mixed with each other through being pumped round in channels  52  and  53 , wherein a chemical reaction may occur. When for instance a first component is fed via liquid channels  51  and a second component via liquid channel  55 , a reaction between the two components can take place during pumping of the two liquids round liquid channels  52  and  53 . Depending on the set voltages, the mixing ratio of substances fed via channel  51  and channel  55  can be adapted as desired. At a desired moment, for instance when a reaction between -the two liquids has ended, the liquids which have reacted with each other are drained via drain channel  54  also referred to as drain. The above stated mixing ration depends on the feeding speeds in liquid channels  51  and  55  and the volumes in the channels. In addition to being used for a continuous supply of different liquids, the network can also be used in applications in which processes have to be performed batchwise. 
     It is noted that additional branches of the twin channel network can be connected as desired in order to allow further different components into the circuit. 
     It is important to adjust the voltages on the gates such that the maximum circulation takes place while the hold-up, i.e. the mixing ratio between the liquids, is optimal. 
     FIGS. 9 a  and  9   b  show another embodiment of a pump. FIG. 9 a  shows in schematic manner a channel  80  which is provided with the branch  83 . Channel  80  is provided with a gate  81  and a gate  82 . In a manner as described in the foregoing embodiments, an electric field is generated in channel  80  in the direction of the double arrow. By switching gate  81  into the enhancement mode E and gate  82  into the reversement mode R, a pressure build-up is created in channel  80  such that the liquid is carried into the side channel  83  and is drained via this side channel. The advantage of this manner of pumping is that no electric field is hereby present in channel  83 , or channel  83  is hereby voltage-free. As a result hereof the drain of such a pump can be connected more easily onto external equipment. 
     FIG. 9 b  shows a situation in which a similar pumping action is brought about in side branch  83 , with the difference that the electric field is now directed from bottom to top and gates  81  and  82  are switched in opposing directions, i.e. gate  81  is switched into the reversement mode R and gate  82  into the enhancement mode E. In this configuration the liquid from above is also urged via tube  80  into side tube  83  whereby the channel system functions as pump. By now alternating the situations shown in FIGS. 9 a  and  9   b  with a suitable frequency, i.e. reversing the electric field and reversing the switching mode of gates  81  and,  82 , no polarization effects will occur on the electrodes in the case of a substantially continuous pumping action. The term “polarization effects” refers to the adverse effects which can for instance cause electrolysis in water, whereby gas bubbles occur in the liquid channels and the pumping action is greatly reduced. 
     FIG. 8 shows a further preferred embodiment of a pump. Channel  70  is provided with a gate electrode  73 . Arranged on either side of gate electrodes  73  are metal electrodes  71  and  72  with which an electric field can be generated. By applying the electric field between electrodes  71  and  72  an electro-osmotic flow can be created which is influenced by the voltage of gate electrode  73 . 
     By providing electrodes  71  and  72  with alternating voltage there would indeed be no occurrence of polarization effects such as formation of gas bubbles if the voltage of gate electrodes  73  remained constant, but the liquid in the channel is not displaced either. By also switching gate electrode  73  substantially synchronously with alternating of the voltage of electrodes  71  and  72 , a liquid flow can still be generated in channel  70  without polarization effects occurring. 
     In this embodiment the gate electrodes  71  and  72  are integrated in the tube and. (external) electrodes outside the channel can be omitted. This not only has the advantage that such a channel  70  can be connected directly onto external peripherals, but also has the advantage that much lower voltages can be used since the distance d between electrodes  71  and  72  can be much smaller than in the case where the electrodes are arranged externally. Since the distance d is in the order of magnitude of a few micrometers, a pump of extremely small dimensions can be realized. 
     The above stated invention can be applied not only on aqueous media but also on non-aqueous media such as for instance alcohol, methanol, THF, DMSO or any other random solvents. It may be necessary herein to dissolve organic salts in the medium to ensure a sufficient degree of conductivity. 
     The present invention is not limited to the above described preferred embodiment thereof; the rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.