PIEZOELECTRIC MEMBRANE-MICROELECTRODE ARRAY

Abstract

The present disclosure relates to a piezoelectric membrane microelectrode array for spatially resolved electrical or mechanical stimulation and simultaneous spatially resolved measurement of electrical or mechanical activity of biological material. The array comprises at least two membrane microelectrode units, that are both arranged on a common substrate.

Claims

1. A piezoelectric membrane microelectrode array configured to spatially resolved electrical or mechanical stimulation and simultaneous spatially resolved measurement of electrical or mechanical activity of biological material, wherein the piezoelectric membrane microelectrode array comprises: at least two membrane microelectrode units, the membrane microelectrode units being arranged on a substrate; wherein the membrane microelectrode unit comprises at least one piezoelectric membrane adapted to mechanically stimulate or measure mechanical activity of biological material, wherein the at least one piezoelectric membrane comprises a piezoelectric film, the piezoelectric film being arranged on the substrate, wherein the piezoelectric film is deformable; and wherein the membrane microelectrode unit comprises at least a first microelectrode adapted to electrically stimulate or measure electrical activity of biological material.

2. The piezoelectric membrane microelectrode array according to claim 1, wherein the first microelectrode and the piezoelectric film are spaced apart.

3. The piezoelectric membrane microelectrode array according to claim 1, wherein the substrate comprises at least two regions, a first region having a first layer thickness and a second region having a second layer thickness, wherein the first layer thickness is greater than the second layer thickness, and wherein the piezoelectric film is disposed within the second region of the substrate.

4. The piezoelectric membrane microelectrode array according to claim 3, wherein the first microelectrode is arranged within the first or second region of the substrate.

5. The piezoelectric membrane microelectrode array according to claim 1, wherein the piezoelectric membrane comprises at least one first electrode, wherein the at least one first electrode is electrically conductively coupled to the piezoelectric film.

6. The piezoelectric membrane microelectrode array according to claim 5, wherein the first electrode is an interdigital electrode.

7. The piezoelectric membrane microelectrode array according to claim 1, wherein the piezoelectric film comprises a ferroelectric material.

8. The piezoelectric membrane microelectrode array according to claim 1, wherein the piezoelectric membrane is spaced apart from the first microelectrode at a distance (d2) of 0.5 to 500 μm, 0.5 to 50 μm, or 0.5 to 5 μm.

9. The piezoelectric membrane microelectrode array according to claim 1, wherein the first microelectrode is arranged within the piezoelectric membrane.

10. The piezoelectric membrane microelectrode array according to claim 1, wherein the piezoelectric membrane is configured as a piezoelectric cantilever or a piezoelectric nanoribbon.

11. The piezoelectric membrane microelectrode array according to claim 1, wherein the piezoelectric membrane microelectrode array comprises a receptacle and at least one counter electrode, wherein the receptacle forms a receiving space for the biological material and culture medium, and wherein the receptacle has a bottom, wherein the at least one membrane microelectrode unit forms the bottom of the receptacle, and wherein the counter electrode is adapted to detect electrical signals originating from the biological material.

12. A membrane microelectrode unit configured for electrical or mechanical stimulation and simultaneous measurement of electrical or mechanical activity of biological material, wherein the membrane microelectrode unit is arranged on a substrate; wherein the membrane microelectrode unit comprises: at least one piezoelectric membrane adapted to mechanically stimulate or measure mechanical activity of biological material, the at least one piezoelectric membrane comprising a piezoelectric film, wherein the piezoelectric film is arranged on the substrate, wherein the piezoelectric film is deformable; and wherein the membrane microelectrode unit comprises at least a first microelectrode adapted to electrically stimulate or measure electrical activity of biological material.

13. A multiwell plate configured for electrical and/or mechanical stimulation and simultaneous measurement of electrical or mechanical activity of biological material, said multiwell plate comprising: at least one receptacle and at least one membrane microelectrode unit according to claim 12, wherein said at least one receptacle forms a receiving space for said biological material and optionally culture medium, and wherein said at least one receptacle comprises a bottom, wherein said at least one membrane microelectrode unit forms the bottom of said receptacle.

14. A method of manufacturing a membrane microelectrode unit comprising: a) providing a substrate; b) fabricating a first microelectrode, the fabricating comprises: i) applying a first conductive layer; and ii) structuring the first microelectrode out of the first conductive layer; and c) depositing a piezoelectric film onto the substrate.

15. The method of manufacturing a membrane microelectrode unit according to claim 14, wherein step b) further comprises: b) fabricating the first electrode and a first microelectrode on the substrate, wherein the fabricating comprises: i) depositing the first conductive layer onto the substrate; and ii) structuring the first electrode and the first microelectrode out of the first conductive layer; wherein the piezoelectric film in step c) is deposited onto the first electrode.

16. The method of manufacturing a membrane microelectrode unit according to claim 14, wherein an insulator is at least partially applied onto one or more of the first microelectrode and the first electrode.

17. The method of manufacturing a membrane microelectrode unit according to claim 14, wherein the substrate is structured by a Bosch-process.

18. A method of manufacturing a piezoelectric membrane microelectrode array wherein the piezoelectric membrane microelectrode array comprises: at least two membrane microelectrode units, the membrane microelectrode units being arranged on a substrate; wherein the membrane microelectrode unit comprises at least one piezoelectric membrane adapted to mechanically stimulate or measure mechanical activity of biological material, wherein the at least one piezoelectric membrane comprises a piezoelectric film, the piezoelectric film being arranged on the substrate, wherein the piezoelectric film is deformable; and wherein the membrane microelectrode unit comprises at least a first microelectrode adapted to electrically stimulate or measure electrical activity of biological material, the method comprising: A) providing the at least two membrane microelectrode units in at least one receptacle, wherein the at least two membrane microelectrode units form the bottom of the receptacle; and B) providing at least one counter electrode, wherein the counter electrode is arranged within the receptacle.

19. The piezoelectric membrane microelectrode array according to claim 1, wherein the array is configured for electrical, mechanical, optical and/or biochemical spatially resolved stimulation of biological material.

20. The piezoelectric membrane microelectrode array according to claim 1, wherein the array is configured for spatially resolved measurement of electrical and/or mechanical activity of biological material triggered by electrical, mechanical, optical and/or biochemical stimulation.

21. The piezoelectric membrane microelectrode array according to claim 1, wherein the array is configured for spatially resolved measurement of electrical and/or mechanical activity of biological material and for spatially resolved stimulation of the biological material, wherein the measurement and the stimulation are simultaneous and spatially resolved.

22. The piezoelectric membrane microelectrode array according to claim 1, wherein the array is configured as an immunosensor, a gas sensor, or a nanogenerator.

23. The piezoelectric membrane microelectrode array according to claim 7, which is selected from lead-free oxides having a perovskite structure; CMOS-compatible ferroelectrics; and ferroelectric polymers or ferroelectrics with multiferroic properties.

24. The piezoelectric membrane microelectrode array according to claim 7, which is selected from 0.5(Ba0.7Ca0.3)TiO3-0.5Ba(Zr0.2 Ti0.8)O3; K0.5Na NbO0.53; Al1-xScxN with 0.2≤x≤0.5 or HfZrO0.50.52; polyvinylidene fluoride or BiFeO3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Exemplary embodiments of the disclosure are shown in the drawing and are explained in more detail in the following description. The figures show:

[0090] FIG. 1 a cross-section of a first embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0091] FIG. 2 a cross-section of a second embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0092] FIG. 3 a cross-section of a third embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0093] FIG. 4A a cross-section of a fourth embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0094] FIG. 4B a top view of the fourth embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0095] FIG. 4C a top view of a fourth embodiment of the piezoelectric membrane microelectrode array according to an aspect of the present disclosure,

[0096] FIG. 5 a cross-section of a fifth embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0097] FIG. 6A a cross-section of a sixth embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0098] FIG. 6B a cross-section of a seventh embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0099] FIG. 7 a cross-section of an eighth embodiment of the membrane microelectrode unit according to an aspect of the present disclosure,

[0100] FIG. 8A a top view of a ninth embodiment of the piezoelectric membrane microelectrode array according to an aspect of the present disclosure, and

[0101] FIG. 8B a top view of a tenth embodiment of the piezoelectric membrane microelectrode array according to an aspect of the present disclosure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0102] FIG. 1 shows a cross-sectional view of a first embodiment of the proposed membrane microelectrode unit 10, wherein the membrane microelectrode unit 10 is arranged on a substrate 12. The membrane microelectrode unit 10 comprises a piezoelectric membrane 14 having a diameter d1, wherein the piezoelectric membrane 14 comprises a piezoelectric film 16, wherein the piezoelectric film 16 is arranged on the substrate 12, and wherein the piezoelectric film 16 is deformable. The piezoelectric membrane 14 can be configured to be adapted for both mechanical stimulation and measurement of mechanical activity of biological material.

[0103] Spaced apart from the piezoelectric film 16, the membrane microelectrode unit 10 includes at least a first microelectrode 18. The microelectrode 18 is arranged on the substrate 12. The distance of the microelectrode 18 from the piezoelectric membrane 14 is indicated by d2. Preferably, the distance d2 is 0.5 to 500 μm. Alternatively, the distance d2 may be 0. The microelectrode 18 can be configured such that it is adapted for both electrical stimulation and measurement of electrical activity of biological material.

[0104] According to this embodiment, the substrate 12 comprises two regions 20, 22; a first region 20 having a first film thickness 24 and a second region 22 having a second film thickness 26, wherein the first film thickness 24 is greater than the second film thickness 26. As can be seen in FIG. 1, the piezoelectric film 16 is arranged within the second region 22 of the substrate 12. According to an aspect of the present disclosure the invention, the substrate 12 can be a pre-structured substrate or a silicon-on-insulator wafer, which can be (backside) structured, for example, during the manufacturing process using the Bosch process or another structuring process, such that the substrate 12 comprises the respective regions 20 and 22.

[0105] As can be seen in FIG. 1, the piezoelectric film 16 extends only partially across the piezoelectric membrane 14. In an embodiment not shown, the piezoelectric film 16 may extend across the entire piezoelectric membrane 14.

[0106] The piezoelectric film 16 can be consist of or comprise a ferroelectric material. This material is particularly advantageous for mechanical stimulation and mechanical measurement of biological material. According to an aspect of the present disclosure, those ferroelectric materials are preferred which do not cause toxicity to the biological material. Accordingly, lead-containing ferroelectrics such as lead zirconate titanate (PZT) are less preferred.

[0107] FIG. 2 shows a cross-sectional view of a second embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. It differs from the unit 10 shown in FIG. 1 just by an additional first electrode 28, which is arranged within the piezoelectric membrane 14. The first electrode 28 is electrically conductively coupled to the piezoelectric film 16.

[0108] In the shown embodiment, the first electrode 28 is arranged between the piezoelectric film 16 and the substrate 12. Accordingly, the first electrode 28 is not in direct contact with the biological material. According to an aspect of the present disclosure, the first electrode 28 is electrically conductive, so that it also consists of or comprises a conductive material. Preferably, the electrode layer thickness of the first electrode 28 is about 100 nm.

[0109] According to an aspect of the present disclosure, the first microelectrode 18 and the first electrode 28 can be connected to one or more measurement and control units via conductive paths (not shown). An electrically conductive culture medium/electrolyte solution can for example serve as the counter electrode. Alternatively, a dedicated counter electrode can be provided to provide a closed circuit. For example, the counter electrode immersed in the electrolyte solution can consist of or comprise AgCl.

[0110] As an alternative to the embodiment shown in FIG. 2, the first electrode 28 can also be arranged above the piezoelectric film 16 (see FIG. 6).

[0111] FIG. 3 shows a cross-sectional view of a third embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. The third embodiment differs from the second embodiment in that it includes an additional receptacle 30 and an additional counter electrode 32. Here, the receptacle 30 forms a receiving space for the biological material and the culture medium/electrolyte solution 34. As shown in FIG. 3, the receptacle 30 has a bottom 36, wherein the membrane microelectrode unit 10 forms the bottom 36 of the receptacle 30.

[0112] The counter electrode 32 or reference electrode 32 can be adapted such that it serves, for example, as a ground connection. In this case, the measurement is performed via potential differences of the counter electrode 32 and the first microelectrode 18 and via potential differences of the counter electrode 32 and the first electrode 28. In this embodiment, the counter electrode 32 is immersed in the culture medium or the electrolyte 34. According to an aspect of the present disclosure, the counter electrode 32 can also be arranged on the receptacle 30 or in the membrane microelectrode unit 10.

[0113] In FIG. 3, the receptacle 30 is configured as a cylindrical sample receptacle, whereby this is delimited at the bottom by the piezoelectric membrane microelectrode unit 10 according to an aspect of the present disclosure. Other configurations, such as a funnel-shaped sample receptacle, are also possible.

[0114] For connection to a measuring and/or stimulation device connections 51, 52, 53 can be provided. A common ground connection 51 can optionally be provided. For electrical stimulation, an electrical stimulation signal can be provided via connection 53. For measuring a mechanical response to the stimulation, a measurement amplifier, such as a differential amplifier or operational amplifier, can for example be connected to the connections 51, 52. Optionally, the measurement amplifier can be co-integrated on the substrate. This can reduce interference during further processing of the signals, since already amplified signals are routed away from the substrate.

[0115] FIG. 4A shows a cross-sectional view of a fourth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. This embodiment differs from the third embodiment in that a film electrode 38 is arranged above the piezoelectric film 16. In the shown embodiment, the film electrode 38 is smaller in diameter than the piezoelectric film 16 and accordingly only partially covers it. Accordingly, in the shown embodiment, the piezoelectric film 16 is at least partially and the film electrode 38 is completely in direct contact with the biological material in a measurement/stimulation. Whereby “direct contact” in this context means that the piezoelectric film 16 and the film electrode 38 may optionally also comprise an additional insulator layer, i.e., an insulator may be present between the piezoelectric film 16 or the film electrode 38 and the biological material (not shown). In this embodiment, the counter electrode 54 can be provided via the culture medium or electrolyte 34, for example by immersing a counter electrode 54 in the culture medium or electrolyte 34 (similar to FIG. 3). Alternatively, the film electrode 38 can serve as a counter electrode, e.g., a ground. In this case, no insulator is provided with respect to the biological material. An advantage is a simpler structure, since a common electrode can be provided for electrical and mechanical interaction.

[0116] For this fourth embodiment example shown in FIG. 4A, the manufacturing process of the membrane microelectrode unit according to an aspect of the present disclosure will be described here by way of an example. It is to be understood that the individual process steps also apply accordingly to the other embodiments of the other figures and generally for the invention, without departing from the scope of the present invention.

[0117] The method for fabricating a membrane microelectrode unit 10 according to an aspect of the present disclosure can be carried out using standard silicon and thin film technology processes. In a first step of the method, the substrate 12 is provided. The substrate 12 is preferably a planar substrate which is, for example, a silicon-on-insulator (SOI) wafer or a pre-structured substrate.

[0118] In a subsequent step, a first electrode 28 can be fabricated. Hereby, a conductive layer that will form the first electrode 28 is first applied to the substrate 12, and in a next step, the first electrode 28 can be structured out. For example, the first electrode 28 con consist of or comprise any of the following materials: Pt, TiN, SrRuO.sub.3. In this regard, the first electrode 28 can optionally be deposited with an adhesion promotion layer, for example of Ti or Ta, and/or a buffer layer, for example of SiO2. The adhesion promotion layer and buffer layer are not shown in FIG. 4A. Preferred layer thicknesses here are about 300 nm for the buffer layer, about 10 nm for the adhesion promotion layer and about 100 nm for the first electrode 28.

[0119] In a subsequent step, the piezoelectric film 16 is applied onto the substrate 12, or in this embodiment, to the first electrode 28. For example, the piezoelectric film 16 can be grown on the respective top/last layer by a thin film process. Preferably, a piezoelectric film 16 with a layer thickness of 500 to 1000 nm is used.

[0120] Subsequently, in a subsequent step, also in a thin-film process, a further conductive layer can be applied onto the substrate 12, or in this embodiment onto the piezoelectric film 16. A layer thickness of about 100 nm is preferred. The conductive layer preferably consists of or comprises the following materials: Au, Pt, TiN or conductive oxides such as SrRuO.sub.3 or SrRuO.sub.3 doped with Nb. In order to be able to form the film electrode 38, it is structured out of the conductive layer in a subsequent step. In addition, corresponding conductive tracks and, if necessary, contact pads can be simultaneously structured out of the conductive layer, for example by optical lithography. The conductive tracks serve a connection between the film electrode 38 and a measurement/control unit (not shown). The manufacturing process of the piezoelectric membrane is based on processes for manufacturing SOI wafers. Such processes are exemplified in M. D. Nguyen et. al, “Optimized electrode coverage of membrane actuators based on epitaxial PZT thin films,” Smart Mater. Struct. 22, 085013 (2013) or in C. T. Q. Nguyen et. al. “Process dependence of the piezoelectric response of membrane actuators based on Pb(Zr.sub.0.45 Ti.sub.0.55)O3 thin films,” Thin Solid Films 556, 509 (2014).

[0121] In a next step, an insulator, for example of Si.sub.3N.sub.4, can be applied onto the piezoelectric membrane 14, preferably structured in such a way that the conductive tracks and the electrodes 28 and 38 on the piezoelectric membrane 14 are insulated.

[0122] For producing the first microelectrode 18, a conductive layer is again applied onto the substrate 12; preferably with a layer thickness of approx. 100 nm. In a subsequent step, the first microelectrode 18 is structured out of this conductive layer; preferably, the associated conductive track and the corresponding contact pads are also structured out in this step. According to an aspect of the present disclosure, the first microelectrode 18 is manufactured in such a way that it is always provided at a distance from the piezoelectric film 16. This has the advantage that the first microelectrode 18 and the piezoelectric membrane 14 do not interfere with each other during measurement or stimulation. In a next step, an insulator can again be applied and structured such that the associated conductive path and the corresponding contact pads are insulated. The structuring out can be performed, for example, by reactive ion etching.

[0123] In a further step, the piezoelectric film 16 can, for example wet chemically, be structured in the piezoelectric membrane 14. This allows the ratio of piezoelectric film area to piezoelectric membrane area to be optimized.

[0124] If the substrate 12 is a non-pre-structured substrate, the backside—the side of the substrate 12 not facing the biological material—can be structured in a subsequent step so that the areas 22 in which the piezoelectric membrane 14 is arranged have a smaller layer thickness 26. This can be achieved, for example, by Bosch process methods. In a further step, the conductive tracks that are not yet insulated can be insulated.

[0125] Lastly, a receptacle 30 may be arranged around the membrane electrode unit 10 such that the membrane electrode unit 10 forms the bottom 36 of the receptacle 30. In case that more than one membrane electrode unit 10 are arranged within the receptacle, a piezoelectric membrane microelectrode array 100 according to an aspect of the present disclosure can be obtained.

[0126] FIG. 4B shows a top view of the fourth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. For the sake of clarity, the receptacle and the counter electrode are not shown in FIG. 4B, the embodiment otherwise corresponding to the embodiment in FIG. 4A.

[0127] According to the embodiment example in FIG. 4B, the microelectrode 18, the film electrode 38 and the piezoelectric membrane 14 have a circular shape; according to an aspect of the present disclosure, the electrodes 18 and 38 and membrane 14 can also have other shapes. Furthermore, conductive tracks 40, 42 are shown, which lead to the film electrode 38 on the one hand and to the microelectrode 18 on the other hand. The conductive tracks may be covered with an insulating layer. Alternatively, the conductive tracks can be guided on the back of the substrate, i.e. on the side of the substrate facing away from the biological material. In this way, there is no undesired electrical contacting of the biological material by the conductive track.

[0128] In FIG. 4B, the microelectrode 18 is arranged at a distance from the piezoelectric membrane 14. According to an aspect of the present disclosure, the distance between the microelectrode 18 and the piezoelectric membrane 14 is preferably selected to be as small as possible, with this distance preferably corresponding at most to the diameter of the biological material (for example biological cells) cultivated thereon. The advantage of this embodiment is that the microelectrode 18 and the piezoelectric membrane 14 can operate independently of each other. For example, if the microelectrode 18 were placed on top of the membrane 14 instead of next to it, this may affect the mechanical properties of the membrane 14.

[0129] According to an aspect of the present disclosure, a piezoelectric membrane microelectrode array 100 comprises at least two membrane microelectrode units 10, but a piezoelectric membrane microelectrode array 100 comprising more than two membrane microelectrode units 10 is preferred. Such an array is shown in FIG. 4C, wherein the individual membrane microelectrode units 10 may correspond to the units of FIG. 4B. In this embodiment, the piezoelectric membrane microelectrode array 100 has sixteen individual membrane microelectrode units 10. In particular, these are configured to be individually controllable. Thus, a plurality of corresponding terminals can be led out of the array, which can be controlled accordingly by a measuring and/or stimulation device. According to an aspect of the present disclosure it is also possible that, for example, multiple membrane microelectrode units 10, for example four, can also be respectively controlled together. Any other number of units 10 that can be controlled together is also possible.

[0130] According to an aspect of the present disclosure, the piezoelectric membrane microelectrode array 100 can be used for spatially resolved measurement of electrical and/or mechanical activity of biological material triggered by electrical, mechanical, optical and/or biochemical stimulation. Further, according to an aspect of the present disclosure, the piezoelectric membrane microelectrode array 100 can be used for electrical, mechanical, optical, and/or biochemical spatially resolved stimulation of biological material. Further, according to an aspect of the present disclosure, the piezoelectric membrane microelectrode array 100 can be used for spatially resolved measurement of electrical and/or mechanical activity of biological material and for spatially resolved stimulation of the biological material, wherein the measurement and the stimulation occur simultaneously (concurrently).

[0131] If the piezoelectric membranes 10 are deformed by a mechanical tension, an electrical voltage is generated due to the direct piezoelectric effect, which can be recorded, for example, by a (multi-channel) measuring amplifier (not shown). Conversely, the piezoelectric membranes 10 are mechanically deformed by an applied electrical voltage and can thus be used for mechanical stimulation. The microelectrodes 18 adjacent to the piezoelectric membranes 10 can be used simultaneously (concurrently or at the same time) for electrical recording and stimulation.

[0132] FIG. 5 shows a cross-section of a fifth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure, this embodiment corresponding to the first embodiment except for the difference that the microelectrode 18 is arranged within the piezoelectric membrane 14 on the substrate 12. According to this embodiment, particularly space-saving/compact membrane microelectrode units 10 can be manufactured, such that a single piezoelectric membrane microelectrode array 100 according to an aspect of the present disclosure can have more membrane microelectrode units 10 than an array 100 in which the first microelectrode 10 is not arranged within the piezoelectric membrane 14.

[0133] FIG. 6A shows a cross-section of a sixth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure, wherein this embodiment corresponds to the first embodiment except for the difference that the membrane microelectrode unit 10 has an interdigital electrode 44. Here, the interdigital electrode 44 is deposited on the piezoelectric film, and in this embodiment, there are two interdigital electrodes 44 spaced apart from each other. In this embodiment, the microelectrode 18 is present spaced apart from the piezoelectric membrane 14.

[0134] FIG. 6B shows a cross-section of a seventh embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure, wherein this embodiment corresponds to the sixth embodiment except for the difference that the microelectrode 18 is arranged within the piezoelectric membrane 14, namely on the piezoelectric film 16, in particular centrally on the piezoelectric film 16.

[0135] FIG. 7 shows a cross-section of an eighth embodiment of the membrane microelectrode unit 10 according to the invention, this embodiment corresponding to the first embodiment except for the difference that the piezoelectric membrane 14 is configured as a cantilever.

[0136] FIG. 8A shows a top view of a ninth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. This embodiment corresponds to the embodiment shown in FIG. 4B, except that the membrane microelectrode unit 10 has two microelectrodes 18 spaced apart from the piezoelectric membrane 14. In an embodiment not shown, the membrane microelectrode unit 10 according to an aspect of the present disclosure may also comprise more than two, for example three, four, five or more microelectrodes 18. Optionally, the microelectrodes may be arranged symmetrically about the membrane 14. For example, one on the right and one on the left. Multiple microelectrodes 18 may be arranged on a circle around one or more membranes 14.

[0137] FIG. 8B shows a top view of a tenth embodiment of the membrane microelectrode unit 10 according to an aspect of the present disclosure. This embodiment corresponds to the embodiment shown in FIG. 4B, except that the membrane microelectrode unit 10 comprises two piezoelectric membranes 14 spaced apart from the first microelectrode 18. In an embodiment not shown, the membrane microelectrode unit 10 according to an aspect of the present disclosure may also comprise more than two, for example three, four, five or more piezoelectric membranes 14. Optionally, the membranes 14 may be arranged symmetrically about the microelectrode 18. For example, one on the right and one on the left. Multiple membranes 14 may be arranged on a circle around one or more microelectrodes 18.