Patent Publication Number: US-10767641-B2

Title: Micropump with electrostatic actuation

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a U.S. National Phase of International Patent Application PCT/IB2016/053985, filed on Jul. 1, 2016, which claims priority to Italian Application No. 102015000030182, filed on Jul. 2, 2015, each of which is incorporated by reference as if expressly set forth in their respective entireties herein. 
     TECHNICAL FIELD 
     The present invention relates to a micropump with electrostatic actuation. 
     BACKGROUND ART 
     As is known, manufacturing techniques for semiconductor devices have been successfully exploited also outside the field closely related to microelectronics and for instance they have been used to develop microelectromechanical and microfluidic systems for a number of applications. 
     The field of fluidics, in particular, has benefited from the possibility of manufacturing miniaturized components, such as micropumps and valves, which allow volumes of liquids in the order of microlitres or smaller to be processed with a very high degree of precision. Thus, devices for ink-jet printing, biomedical devices (for example, insulin pumps), and devices for biochemical analyses (for example, microreactors for amplification and detection of nucleic acids), among others, have been improved. 
     However, the basic components and microfluidic devices available (micropumps and valves) are still relatively complex and their structure is a limitation to miniaturization, besides entailing non-negligible manufacturing costs. For example, micropumps and valves must be equipped with movable members and electromechanical actuators which, by acting on the movable members, control the movement of the fluids in accordance with the required functions. Generally, the integration of the actuators is rather difficult and complicates the manufacturing processes. In fact, the actuators normally require dedicated structures, which must often be made from specially designated structural layers. Furthermore, the actuators employ special materials, such as piezoelectric or magnetic materials, which require changes to the most common manufacturing processes and additional processing steps (for example, deposition, masking, and photolithographic definition of layers of special materials). 
     It must also be considered that in many cases the tightness of the valves, especially when integrated into membrane micropumps, is not optimal and can be the cause of leakage and backflow, which affect the working of the device. 
     DISCLOSURE OF INVENTION 
     The object of the present invention is to provide a micropump that makes it possible to overcome or at least mitigate the limitations described above. 
     According to the present invention, there is provided a micropump comprising: 
     a pumping chamber, between a first semiconductor substrate and a second semiconductor substrate bonded to each other; 
     an inlet valve, having an inlet shutter element between an inlet passage and the pumping chamber; 
     an outlet valve, having an outlet shutter element between the pumping chamber and an outlet passage; 
     a first recess for housing the inlet shutter element when the inlet valve is in the open configuration, the first recess and the pumping chamber being fluidly coupled; 
     a second recess for housing the outlet shutter element when the outlet valve is in the open configuration, the second recess and the pumping chamber being fluidly decoupled. 
     The configuration of the inlet and outlet valves, with the first recess communicating with the pumping chamber and the second recess decoupled from it, allows the direction of the processed flow to be controlled in a completely passive way. More precisely, the inlet and outlet valves do not require dedicated actuators and so the structure is generally simplified, for the benefit of both the overall dimensions and the manufacturing costs. For instance, the micropump as just defined may be made from just two semiconductor wafers joined together. Moreover, the micropump control is simplified because it does not have to take into account the synchronization of the valves. Dedicated actuators for the valves, in particular for the output valve, may optionally be provided if specific circumstances make this advisable. However, the micropump is still fully operative even with purely passive valves. 
     According to a further aspect of the invention, the inlet valve and the outlet valve are of the orthoplanar type. 
     Valves of this type are effective, they have a good seal and they lend themselves to be integrated into the manufacturing processes for semiconductor microelectromechanical systems. 
     According to another aspect of the invention, the micropump comprises: 
     a first pumping membrane made of semiconductor material and delimiting the pumping chamber on a first side; 
     a first electrode structure, capacitively coupled to the first pumping membrane and configured to apply a first electrostatic force to the first pumping membrane in the presence of a first actuating voltage between the first electrode structure and the first pumping membrane; and 
     a control unit configured to apply the first actuating voltage in the form of a frequency-controlled periodic wave. 
     The use of a pumping membrane made of semiconductor material and of a capacitively coupled electrode structure makes it possible to efficiently exploit an actuation mechanism based on electrostatic forces. In particular, it is advantageous that the electrode structure may be made for example of polysilicon, and thus be easily integrated into the manufacturing processes for semiconductor microelectromechanical devices without the need to use special materials, such as magnetic or piezoelectric materials. 
     According to a further aspect of the invention, the micropump comprises a third recess delimited on one side by the first pumping membrane and fluidly decoupled from the pumping chamber, the first electrode structure being arranged on a wall of the third recess opposite to the first pumping membrane and configured to retract the first pumping membrane within the third recess. 
     In this way, the space occupied by the first electrode structure is really negligible and its provision has no appreciable effect on the manufacturing processes. 
     According to another aspect of the invention, the micropump comprises: 
     a second pumping membrane made of semiconductor material and delimiting the pumping chamber on a second side opposite to the first side; and 
     a second electrode structure capacitively coupled to the second pumping membrane and configured to apply a second electrostatic force to the second pumping membrane in response to a second actuating voltage between the second electrode structure and the second pumping membrane; 
     the control unit being configured to supply the second actuating voltage in the form of a periodic wave with a controlled frequency equal to the frequency of the first actuating voltage. 
     The use of two opposing membranes advantageously makes it possible to increase the volume of fluid that can be processed in each pumping cycle and, therefore, to increase the maximum flow rate of the micropump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention, an embodiment thereof will now be described, purely by way of non limiting example and with reference to the accompanying drawings, wherein: 
         FIG. 1  is a simplified block diagram of a microfluidic system incorporating a micropump according to one embodiment of the present invention; 
         FIG. 2  is a bottom plan view, with parts removed for clarity, of the micropump of  FIG. 1 ; 
         FIG. 3  is a cross section, taken along the plane III-III of  FIG. 2 , of the micropump of  FIG. 2  in a resting configuration; 
         FIG. 4  shows the same view as  FIG. 3 , with the micropump in a first operating configuration; 
         FIG. 5  shows the same view as  FIG. 3 , with the micropump in a second operating configuration; 
         FIG. 6  is a graph illustrating electrical quantities related to the micropump of  FIG. 2 ; 
         FIG. 7  is a cross section of a micropump according to another embodiment of the present invention; 
         FIG. 8  is a cross section of a micropump according to a further embodiment of the present invention; 
         FIG. 9  is a cross section of a micropump according to a further embodiment of the present invention; 
         FIG. 10  is a graph illustrating electrical quantities related to a micropump according to another embodiment of the present invention; and 
         FIG. 11  is a graph illustrating electrical quantities related to a micropump according to a further embodiment of the present invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to  FIG. 1 , a microfluidic system is indicated as a whole with number  1  and comprises a microfluidic device  2 , a micropump  3  coupled to the microfluidic device  2  through fluid connection lines  4 , and a control unit  5 . 
     The microfluidic device  2  may be any device that processes and/or dispenses a controlled volume of fluid, typically in the order of microlitres or nanolitres. To mention a few non-limiting examples, the microfluidic device  2  may include an ink-jet print head, an infusion pump dispenser for the continuous administration of drugs, or a device for the amplification and detection of nucleic acids in a biological sample. The components of the microfluidic system  1  may be provided on respective separate carriers or be integrated, all or in part, into a single carrier, including for example a semiconductor substrate. 
     The control unit  5  controls the micropump  3  by means of one or more pumping control signals S CK  and auxiliary control signals S AUX  so that the micropump  3  transfers to the microfluidic device  2  a controlled fluid flow rate through the fluid connection lines  4 , as required by the functions of said microfluidic device  2 . In one embodiment, the pumping control signals S CK  may be in the form of periodic voltages, for example a square wave voltage, with a frequency controlled as a function of the fluid flow rate to be supplied to the microfluidic device  2 . 
     According to one embodiment of the present invention, referred to in  FIGS. 2-5 , the micropump  3  comprises a first semiconductor substrate  7  and a second semiconductor substrate  8  joined together by a bonding layer  9 . Here and hereinafter, “semiconductor substrate” is intended to mean a structure obtained by the processing of a wafer of semiconductor material essentially by manufacturing techniques for electronic and semiconductor microelectromechanical devices. In particular, it is to be understood that each semiconductor substrate may comprise several layers and/or structures of semiconductor material, with respective doping types and levels, and, in addition, layers and/or structures of materials different from semiconductors, including dielectric materials. 
     For example, in the embodiment of  FIGS. 2-5 , the first semiconductor substrate  7  comprises a first carrier layer  10  made of monocrystalline silicon and a first structural layer  11  made of polycrystalline silicon, which are mechanically connected to each other and electrically isolated from one another by a first dielectric layer  12 , for example of silicon oxide. 
     Similarly, the second semiconductor substrate  8  comprises a second carrier layer  15  made of monocrystalline silicon and a second structural layer  16  made of polycrystalline silicon, which are mechanically connected to each other and electrically isolated from one another by a second silicon oxide dielectric layer  17 . 
     The micropump  3  further comprises an inlet passage  18 , an outlet passage  19 , a pumping chamber  20 , an inlet valve  21 , an outlet valve  22 , a main actuator  25 , and an auxiliary actuator  26 . 
     In one embodiment, the inlet passage  18  and the outlet passage  19  are both formed through the second semiconductor substrate  8  for connecting the pumping chamber  20  with the fluid connection lines  4 , not shown here. In one embodiment, the inlet passage  18  and the outlet passage  19  extend perpendicularly to a main surface of the second semiconductor substrate  8  and to the pumping chamber  20 . 
     The pumping chamber  20  is defined between the first semiconductor substrate  7  and the second semiconductor substrate  8 , and the inlet valve  21  and outlet valve  22  allow the pumping chamber  20  to be fluidly coupled with the inlet passage  18  and the outlet passage  19 , respectively. 
     In more detail, the inlet valve  21  is of the orthoplanar type and has an inlet shutter element  27  between the inlet passage  18  and the pumping chamber  20 . A first recess  28  in the first substrate  7  houses at least one portion of the inlet shutter element  27  when the inlet valve  21  is in the open configuration. In one embodiment, the first recess  28  is defined by an interruption in the first dielectric layer  12 . 
     The inlet shutter element  27  is connected to the first structural layer  11  of the first substrate  7  by elastic suspension elements  30 , also made of polycrystalline silicon, which extend in a transverse direction with respect to a direction of movement of the inlet shutter element  27 . Fluid passages  29  are defined between the elastic suspension elements  30  and fluidly couple the first recess  28  with the pumping chamber  20 . Therefore, the first recess  28  and the pumping chamber are substantially at the same pressure. 
     The inlet shutter element  27  is maintained against the second substrate  8  by the elastic suspension elements  30 , closing the inlet passage  18 , with a preload force. The inlet shutter element  27  is provided with a spacer  32 , whose thickness determines the state of tension of the elastic suspension elements  30  and, consequently, the preload force with which the inlet shutter element  27  is maintained for the closure of the inlet passage  18 . As long as the pressure difference between the inlet passage  18  and the pumping chamber  20  is lower than a first pressure threshold, the preload force prevails and the inlet valve  21  remains closed. When the first pressure threshold is exceeded, the inlet shutter element  27  retracts into the first recess  30  and the inlet valve  21  opens. In one embodiment, the inlet shutter element  27  is movable along a longitudinal axis of the inlet passage  18 . 
     The outlet valve  22 , also of the orthoplanar type, has an outlet shutter element  33  between the outlet passage  19  and the pumping chamber  20 . A second recess  35  houses at least one portion of the outlet shutter element  33  when the outlet valve  22  is in the open configuration, the second recess and the pumping chamber being fluidly decoupled. 
     The outlet shutter element  33  is connected to the second structural layer  16  of the second substrate  8  by means of an elastic valve membrane  36 , which delimits the second recess  35  on one side and is continuous. The second recess  35  is therefore fluidly decoupled from the pumping chamber  20  by means of the valve membrane  36 . In one embodiment, in particular, the second recess  35  is sealed. 
     The outlet shutter element  33  is maintained against the second substrate  8  by the valve membrane  36 , closing the outlet passage  19 , with a preload force. The outlet shutter element  33  is provided with a spacer  37 , whose thickness determines the state of tension of the valve membrane  36  and, consequently, the preload force with which the outlet shutter element  33  is maintained for the closure of the outlet passage  19 . As long as the pressure difference between the pumping chamber  20  and the outlet passage  19  is lower than a second pressure threshold, the preload force prevails and the outlet valve  22  remains closed. When the second pressure threshold is exceeded, the outlet shutter element  33  retracts into the second recess  35  and the outlet valve  22  opens. In one embodiment, the second pressure threshold is greater than the first pressure threshold. Thanks to the preload force, any unwanted backflows toward the pumping chamber from the fluid connection line  4  connected to the outlet passage  19  may be eliminated or at least reduced. In one embodiment, the outlet shutter element  33  is movable along a longitudinal axis of the outlet passage  19 . 
     The main actuator  25  comprises a first pumping membrane  40 , a second pumping membrane  41 , a first electrode structure  42 , and a second electrode structure  43 . 
     The first pumping membrane  40  and the second pumping membrane  41 , made of polycrystalline silicon and substantially circular, are respectively connected to the first structural layer  11  of the first substrate  7  and to the second structural layer  16  of the second substrate  8 , and they delimit the pumping chamber  20 , each on a respective side. 
     A third recess  45  is formed in the first substrate  7  and is delimited on one side by the first pumping membrane  40 . A fourth recess  46  is formed in the second substrate  8  and is delimited on one side by the second pumping membrane  41 . 
     The first pumping membrane  40  and the second pumping membrane  41  are continuous and therefore fluidly decouple the pumping chamber  20  from the third recess  45  and from the fourth recess  46 . 
     The first electrode structure  42  is located on a wall of the third recess  45  opposite to the first pumping membrane  40  and, in one embodiment, it comprises a plurality of concentric annular first electrodes  48  (see particularly  FIG. 2 ). A dielectric layer  49  isolates the first electrode structure  42  from the first structural layer  11  of the first substrate  7 , which defines the wall of the third recess  45 . The first electrode structure  42  is capacitively coupled to the first pumping membrane  40  and it applies a first electrostatic force F 1  ( FIG. 3 ) to the first pumping membrane  40  in the presence of a first actuating voltage V A1  ( FIG. 6 ) between the first electrode structure  42  and the first pumping membrane  40 . The first electrostatic force F 1  retracts the first pumping membrane  40  towards the third recess  45 , helping to create a negative pressure inside the pumping chamber  20 . When the first electrostatic force F 1  is removed, the first pumping membrane  40  returns to its resting configuration and determines a compression in the pumping chamber  20 . 
     The first actuating voltage V A1  is determined by one or more of the pumping control signals S CK  provided by the control unit  5  and it may be in the form of periodic voltages, for example a square wave voltage, with a frequency controlled as a function of the fluid flow rate to be supplied to the microfluidic device  2 . In one embodiment, the first electrodes  48  are all biased to the first actuating voltage V A1 . In a different embodiment, however, the first electrodes  48  may receive actuating voltages of the same frequency, but different for example in amplitude and duty-cycle, so as to obtain a different distribution of the first actuating force along the first pumping membrane  40 . 
     The second electrode structure  43  is located on a wall of the fourth recess  46  opposite to the second pumping membrane  41  and, in one embodiment, it comprises a plurality of concentric annular second electrodes  50 , substantially formed symmetrically to the first electrodes  48 . A dielectric layer  51  isolates the second electrode structure  43  from the second carrier layer  15  of the second substrate  8 , which defines the wall of the fourth recess  46 . The second electrode structure  43  is capacitively coupled to the second pumping membrane  41  and it applies a second electrostatic force F 2  ( FIG. 3 ) to the second pumping membrane  41  in the presence of a second actuating voltage V A2  ( FIG. 6 ) between the second electrode structure  43  and the second pumping membrane  41 . The second electrostatic force F 2  retracts the second pumping membrane  41  towards the fourth recess  46 , creating a negative pressure inside the pumping chamber  20 . When the second electrostatic force F 2  is removed, the second pumping membrane  41  returns to its resting configuration and determines a compression in the pumping chamber  20 . 
     The second actuating voltage V A2  is determined by one or more of the pumping control signals S CK  provided by the control unit  5  and it may be in the form of periodic voltages, for example a square wave voltage, with a frequency controlled as a function of the fluid flow rate to be supplied to the microfluidic device  2 . Like the first electrodes  48 , the second electrodes  50  may all be biased to the second actuating voltage V A2  or they may receive respective actuating voltages of the same frequency, but different for example in amplitude and duty-cycle, so as to obtain a different distribution of the second actuating force along the second pumping membrane  41 . 
     The actuating voltages applied to the first pumping membrane  40  and to the second pumping membrane  41  still have the same frequency and are synchronized so as to optimize the pumping effect, coordinating the deflection of the first pumping membrane  40  and of the second pumping membrane  41 . The frequency may be varied depending on the desired flow rate. 
     The auxiliary actuator  26  comprises an auxiliary electrode structure  55 , arranged on a wall of the second recess  35  opposite to the outlet shutter element  33  and to the valve membrane  36 . The auxiliary electrode structure  55  is capacitively coupled to the outlet shutter element  33  and to the valve membrane  36 . In the presence of an auxiliary actuating voltage between the auxiliary electrode structure  55  on one side and the outlet shutter element  33  and the valve membrane  36  on the other side, the auxiliary electrode structure  55  applies an auxiliary electrostatic force that helps the opening of the outlet valve. The auxiliary actuating voltage may be determined by the auxiliary control signals S AUX  supplied by the control unit  5 . 
     The micropump  3  is operated by the control unit  5  through the actuating control signals S CK , following which the actuating voltages V A1 , V A2  are produced, and, optionally, through the auxiliary control signals S AUX . In the active phase of each period of the actuating voltages V A1 , V A2 , the first pumping membrane  40  and the second pumping membrane  41  will deform due to the effect of the electrostatic forces F 1 , F 2  ( FIG. 4 ) and they retract inside the third recess  45  and the fourth recess  46 , respectively, causing a negative pressure within the pumping chamber  20 . The pressure difference between the inlet passage  18  and the pumping chamber  20  prevails over the preload force on the inlet shutter element  27  and the inlet valve  21  opens, allowing for the loading of the pumping chamber  20 . The inlet valve  21  closes again when the pressure difference between the inlet passage  18  and the pumping chamber  20  drops below the first pressure threshold. 
     Instead, the outlet valve  22  remains closed, both because of the higher preload force due to the action of the valve membrane  36 , also by reason of the thickness of the spacer  37 , and because of the back pressure of the gaseous fluid present in the second recess  35 , which is sealed (or at least fluidly decoupled from the pumping chamber  20 ). 
     When the electrostatic forces F 1 , F 2  are removed (inactive phase of the period of the actuating voltages V A1 , V A2 ), the first pumping membrane  40  and the second pumping membrane  41  return to their respective resting configurations ( FIG. 5 ), compressing the fluid in the pumping chamber  20 . The increase in the pressure has no influence on the inlet valve  21 , since the first recess  28  is fluidly coupled to the pumping chamber  20  through the fluid passages  29  between the elastic suspension elements  30 . 
     Instead, the second recess  35  is decoupled from the pumping chamber  20  by means of the valve membrane  36 . The compression produced by the return movement of the first pumping membrane  40  and of the second pumping membrane  41  then causes an imbalance between the faces of the valve membrane  36 , which tends to open the outlet valve  22 . When the pressure difference between the pumping chamber  20  and the outlet passage  19  exceeds the second pressure threshold, the outlet shutter element  33  detaches from the second substrate  8  and the outlet valve  22  is actually open. 
     Like the inlet valve  21 , also the outlet valve  22  may therefore operate in a completely passive way, without the need for external controls. However, in an initial working phase, it may be useful to control the opening of the outlet valve  22  by the auxiliary actuator  26  and the auxiliary control signals S AUX  to facilitate the filling of the pumping chamber  20 . In particular, during the initial loading (priming) of the working fluid, the outlet valve  22  may be kept open by the auxiliary actuator  26  to favour the evacuation of the air initially present and to avoid the formation of gas bubbles that may affect the functionality of the micropump  3 . The possibility of controlling the opening of the outlet valve  22  is thus particularly advantageous to facilitate the initial filling of the microfluidic device  2 . 
     The above-described micropump advantageously has a simplified structure, which in particular benefits from inlet and outlet valves that can be used in a completely passive way. Therefore, no specific control is required. An auxiliary electrostatic actuator for the outlet valve can be provided if necessary to facilitate functioning under particular transient conditions, but as a rule it is unnecessary under normal operating conditions. 
     The structure is simplified to the point that the micropump can be manufactured from just two semiconductor wafers, from which the first substrate and the second substrate are derived. 
     The presence of membrane electrostatic actuators also contributes to this, both through the pumping chamber, and, possibly, through the outlet valve. In fact, the electrode structures of the actuators are housed in the recesses between the carrier layers and the respective membranes. Moreover, the manufacture thereof is perfectly compatible with the techniques normally used in the production of microelectromechanical devices. Techniques for making membranes are, in fact, known and may comprise, for example, growing a structural layer from the seed layer before forming a sacrificial layer on a semiconductor substrate and thus, after depositing a seed layer on the sacrificial layer. The structural layer may be selectively etched by a photolithographic process for opening trenches through regions dedicated to the formation of the membranes. The sacrificial layer may then be removed by etching through the trenches, which may then be closed, for example, by an annealing process (i.e. a high temperature processing in the presence of hydrogen which allows the semiconductor material to be redistributed, making the structure more homogeneous). The annealing process restores the continuity of the semiconductor material in the regions corresponding to the membranes. The electrode structures of the actuators can be easily incorporated into the sacrificial layer during the initial steps of the process. After forming an insulating layer, for example silicon oxide, the electrode structures may be made by photolithographically defining a polysilicon layer deposited on the insulating layer. The sacrificial layer, also of silicon oxide, may then be deposited so as to incorporate the electrode structures. During the removal of the sacrificial layer, the electrode structures themselves protect the underlying portions of the insulating layer, which are spared and subsequently serve as anchors. The use of covering sheets of the dry film type may be contemplated for membrane impermeabilization. 
     A further advantage of the above-described membrane actuators is given by the fact that, thanks to the arrangement of the electrode structures with respect to the membranes, the pumping chamber is not affected by the electric fields that determine the pumping effect. For this reason, the micropump according to the invention may be used with no drawbacks even when the fluid to be circulated is an ionic solution. 
     The micropump has an essentially planar structure and may have inlet and outlet passages on the same face. This is generally considered to be advantageous because the structure of the fluidic circuit connected to the micropump can be simplified. 
     However, this structure is not mandatory. For example, in the embodiment of  FIG. 7 , an outlet passage, designated here by  119 , is made through the first substrate  107 . In this case, as previously described, the moving parts of the inlet valve  121  are integrated into the first substrate  107 , while the inlet passage  118  is formed in the second substrate  108 . A first recess  128 , defined in the first substrate  107 , receives the inlet shutter element  127  when the inlet valve  121  is open and fluidly coupled to the pumping chamber  120 . Instead, the outlet valve  122  and the auxiliary actuator  126  are incorporated into the second substrate  108 . In particular, the outlet valve  122  comprises an outlet shutter element  133 , which closes the outlet passage  119  and is connected to the second structural layer  116  of the second substrate  108  through a valve membrane  136 . A second recess  135 , defined in the second substrate  108  and fluidly decoupled from the pumping chamber  120  by the valve membrane  136 , receives the outlet shutter element  133  when the outlet valve  122  is open. The auxiliary electrode structure  155  of the auxiliary actuator  126  is located on a wall of the second recess  135  opposite to the valve membrane  136  and capacitively coupled thereto. 
     The presence of two opposing pumping membranes is also generally advantageous, although not strictly necessary. 
     In the embodiment shown in  FIG. 8 , for example, there is a single pumping membrane  240  in the same first substrate  207  into which the moving parts of the inlet valve  221  and of the outlet valve  222  are integrated. In this case, the second substrate  208  may purely serve as a carrier and a delimitation of the pumping chamber  220 , in addition to being the site of the inlet passage  218  and of the outlet passage  219 . 
     In the embodiment of  FIG. 9 , the moving parts of the inlet valve  321  and of the outlet valve  322  are integrated into the first substrate  307 , while the single pumping membrane  341  present is integrated into the second substrate  308 . On the opposite side, the pumping chamber  320  is bounded by the first substrate  307 . In this case, a recess  346  is formed in the second substrate  308  and is bounded on one side by the pumping membrane  341 . The electrode structure  343  is located on a wall of the recess  346  opposite to the second pumping membrane  341 . 
     As already mentioned with reference to  FIGS. 2-5 , in one embodiment, the electrodes of each actuating structure may receive actuating voltages of the same frequency, but different, for example, in amplitude and/or duty-cycle, so as to optimize the working of the micropump  1  by controlling the distribution of the actuating forces along the pumping membranes.  FIG. 10  illustrates an example, also related to the structure of  FIGS. 2-5 , wherein the first electrodes  48  of the first electrode structure  42  receive respective actuating voltages V A11 , . . . , V A1K  different in amplitude (in the example, K first electrodes  48  are deemed to be present; index 1 refers to the first most external electrode  48  and index K refers to the first central electrode  48 ). The second electrodes  50  of the second electrode structure  43  receive respective actuating voltages V A21 , . . . , V A2K  equal to the corresponding actuating voltages V A11 , . . . , V A1K . 
     In the example of  FIG. 11 , the actuating voltages V A11 , . . . , V A1K  and the actuating voltages V A21 , . . . , V A2K  differ in duty-cycle. Obviously, it is possible to envisage the use of actuating voltages different both in amplitude and in duty-cycle. 
     Lastly, it is evident that the micropump described can be subject to modifications and variations without departing from the scope of the present invention, as defined in the appended claims.