Abstract:
A valve system for an ocular implant device includes a boss structure. The boss structure includes an inlet for fluid inflow, and a number of outlets for fluid outflow. The valve system further includes a deformable membrane positioned over and spaced apart from the inlet, the membrane comprising a piezo-based material, an actuating element to apply pressure to the membrane to deform the membrane into a position that obstructs fluid flow from the inlet, and a control system to detect a position of the membrane based on measured electrical properties of the membrane.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to apparatuses and methods incorporating a valve position detection system, and more particularly, to apparatuses and methods including the position detection system in an implantable device. 
     BACKGROUND 
     Glaucoma, a group of eye diseases affecting the retina and optic nerve, is one of the leading causes of blindness worldwide. Most forms of glaucoma result when the intraocular pressure (IOP) increases to pressures above normal for prolonged periods of time. IOP can increase due to high resistance to the drainage of the aqueous humor relative to its production. Left untreated, an elevated IOP causes irreversible damage to the optic nerve and retinal fibers resulting in a progressive, permanent loss of vision. 
       FIG. 1  is a diagram of the front portion of an eye that helps to explain the processes of glaucoma. In  FIG. 1 , representations of the lens  110 , cornea  120 , iris  130 , ciliary body  140 , trabecular meshwork  150 , and Schlemm&#39;s canal  160  are pictured. Anatomically, the anterior segment of the eye includes the structures that cause elevated IOP which may lead to glaucoma. 
     Aqueous humor fluid is produced by the ciliary body  140  that lies beneath the iris  130  and adjacent to the lens  110  in the anterior segment of the eye. This aqueous humor washes over the lens  110  and iris  130  and flows to the drainage system located in the angle of the anterior chamber. The angle of the anterior chamber, which extends circumferentially around the eye, contains structures that allow the aqueous humor to drain. 
     The trabecular meshwork  150  is commonly implicated in glaucoma. The trabecular meshwork  150  extends circumferentially around the anterior chamber. The trabecular meshwork  150  seems to act as a filter, limiting the outflow of aqueous humor and providing a back pressure that directly relates to IOP. 
     Schlemm&#39;s canal  160  is located beyond the trabecular meshwork  150 . The Schlemm&#39;s canal  160  is fluidically coupled to collector channels (not shown) allowing aqueous humor to flow out of the anterior chamber. The two arrows in the anterior segment of  FIG. 1  show the flow of aqueous humor from the ciliary bodies  140 , over the lens  110 , over the iris  130 , through the trabecular meshwork  150 , and into the Schlemm&#39;s canal  160  and its collector channels. 
     One method of treating glaucoma includes implanting a drainage device in a patient&#39;s eye. The drainage device allows fluid to flow from the interior chamber of the eye to a drainage site, relieving pressure in the eye and thus lowering IOP. In some cases, a valve is used to control the flow through the drainage device. 
     In order to provide consistency and accuracy in fluid flow through the drainage device, it may be important to limit changes and degradation that may occur in the drainage device over time. To do this, it may be important to know the position of the valve such as the amount or percentage that the valve is open or closed at a given moment in time. The system and methods disclosed herein overcome one or more of the deficiencies of the prior art. 
     SUMMARY 
     This disclosure relates generally to, and encompasses, apparatuses and methods for removing ocular tissue and/or fluid from the eye, and more specifically to ophthalmic surgical systems and methods of using the systems to remove ocular tissue and/or fluid from the eye. 
     According to some embodiments, a valve system for an ocular implant device includes a boss structure. The boss structure includes an inlet for fluid inflow, and a number of outlets for fluid outflow. The valve system further includes a deformable membrane positioned over and spaced apart from the inlet, the membrane comprising a piezo-based material, an actuating element to apply pressure to the membrane to deform the membrane into a position that obstructs fluid flow from the inlet, and a control system to detect a position of the membrane based on measured electrical properties of the membrane. 
     According to some embodiments, a valve system for an ocular implant device includes a boss structure. The boss structure includes an inlet for fluid inflow, and a number of outlets for fluid outflow. The valve system further includes a deformable membrane positioned over and spaced apart from the inlet. The membrane includes a support layer and a piezo-resistive layer. The valve system further includes an actuating element to apply pressure to the membrane to adjust a position of the membrane to adjust fluid flow between the inlet and the outlets, a sensor to detect an electrical characteristic of the membrane, and a control system to determine the position of the membrane based on a measured electrical property of the membrane. 
     According to some embodiments, a method for determining a position of a valve in an ocular implant includes applying a voltage to a piezoresistive membrane, the membrane being deformable between an open position, that allows fluid flow between an inlet and a number of outlets, and a closed position that obstructs flow between the inlet and the outlets. The method further includes measuring an electric current flowing through the membrane to determine a measured electric current value, and determining a position of the membrane based on the measured electric current value. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory in nature and are intended to provide an understanding of the present disclosure without limiting the scope of the present disclosure. In that regard, additional aspects, features, and advantages of the present disclosure will be apparent to one skilled in the art from the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate embodiments of the devices and methods disclosed herein and together with the description, serve to explain the principles of the present disclosure. 
         FIG. 1  is a diagram of the front portion of an eye. 
         FIG. 2  is a block diagram of an exemplary IOP control system according to one example incorporating the principles described herein. 
         FIG. 3  is a schematic diagram of an exemplary IOP control system according to one example incorporating the principles described herein. 
         FIGS. 4A and 4B  are diagrams showing a piezo-based membrane valve in an open and closed position according to one example incorporating the principles described herein. 
         FIG. 5  is a diagram showing a piezoresistive membrane valve according to one example incorporating the principles described herein. 
         FIG. 6  is a diagram showing a piezoelectric membrane valve according to one example incorporating the principles described herein. 
         FIG. 7  is a diagram showing an illustrative feedback control system for a membrane valve according to one example incorporating the principles described herein. 
         FIGS. 8A and 8B  are flowcharts illustrating methods for determining the position of a piezo-based membrane valve according to examples incorporating the principles described herein. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is intended. Any alterations and further modifications to the described devices, instruments, methods, and any further application of the principles of the present disclosure are fully contemplated as would normally occur to one skilled in the art to which the disclosure relates. In particular, it is fully contemplated that the features, components, and/or steps described with respect to one embodiment may be combined with the features, components, and/or steps described with respect to other embodiments of the present disclosure. For simplicity, in some instances the same reference numbers are used throughout the drawings to refer to the same or like parts. 
     This disclosure is directed to devices, systems, and methods that determine the position, as an amount or degree to which the valve is open or closed. The exemplary aspects described herein include a membrane valve can be used to control the flow of aqueous humor through a drainage device implant. Because the membrane valve is very small, it can be difficult to determine the actual position of the membrane valve. In addition, damage can occur if too much pressure is applied to the membrane valve. By knowing the position of the membrane valve, a control system can determine how much pressure should be applied to move the membrane to the desired position. 
     According to certain embodiments, the devices, systems, and methods described herein monitor electrical characteristics of the valve to determine its position. In some aspects, the membrane valve includes at least one layer that is made of a piezo-based material, such as a piezoresistive material or a piezoelectric material. Piezo-based materials exhibit specific electrical characteristics that have a direct relationship with applied mechanical forces. 
     For example, a piezoresistive material changes its resistance in a predictable manner based upon its shape. By applying a voltage to the membrane, and then measuring the electric current flowing through the membrane, the resistance of the piezoresistive membrane can be determined. By knowing the resistance, the position of the membrane can be determined. 
     In the case of a piezoelectric material, electric current is produced during movement of the membrane. This current can be measured and correlated with a change in position of the membrane. By knowing a starting position and a change in position of the membrane, it is possible to determine the present position of the membrane. 
       FIG. 2  is a block diagram of an exemplary IOP control system  200  implantable in an eye of a patient for the treatment of glaucoma or other conditions. The IOP control system  200  is configured in a manner that provides IOP pressure control, but also regulates and controls bleb pressures, reducing complications arising from surgical implant glaucoma treatments. In  FIG. 2 , the IOP control system  200  includes a power source  205 , an IOP sensor system  210 , a processor  215 , a memory  220 , a data transmission module  225 , a valve system  230 , and a valve sensor system  240 . 
     The power source  205 , which provides power to the IOP control system  200 , is typically a rechargeable battery, such as a lithium ion or lithium polymer battery, although other types of batteries may be employed. The power source can be recharged via inductive coupling such as an RFID link or other type of magnetic coupling. 
     The processor  215  is typically an integrated circuit with power, input, and output pins capable of performing logic functions. In various embodiments, the processor  215  may be a targeted device controller or a microprocessor configured to control more than one component of the device. 
     The memory  220 , which is typically a semiconductor memory such as RAM, FRAM, or flash memory, interfaces with the processor  215 . As such, the processor  215  can write to and read from the memory  220 , and perform other common functions associated with managing semiconductor memory. In this manner, a series of IOP readings can be stored in the memory  220 . 
     The data transmission module  225  may employ any of a number of different types of data transmission. For example, in various embodiments, the data transmission module  225  may be an active device such as a radio or a passive device with an antenna on an RFID tag. Alternatively, the data transmission module  225  may be activated to communicate an elevated IOP condition to a secondary device such as a PDA, cell phone, computer, wrist watch, custom device exclusively for this purpose, remote accessible data storage site (e.g. an internet server, email server, text message server), or other electronic device or service. 
     The valve system  230  includes one or more valves, pumps or other flow regulators to pride at least some regulation of fluid flow through the IOP control system  200 . In the embodiments described herein, and as described further below with reference to  FIGS. 4A, 4B, 5, and 6 , the valve system  230  may include a membrane valve with a flexible membrane that is displaceable to regulate flow through the membrane valve. 
     The valve sensor system  240  may form a part of the valve system and may measure the electrical properties of the membrane valve. For example, the valve sensor system  240  includes a voltage supply to apply an excitation voltage to the membrane. The valve sensor system  240  also includes an electric current measuring device such as an ammeter to measure the electric current flowing through the membrane. Additionally, the valve sensor system  240  may include various pressure sensors that are used to determine how much fluid flow should be allowed through the valve system  230 . 
       FIG. 3  is a diagram of at least a portion of the IOP control system  200  disposed along elements of an eye. Here, the IOP control system  200  includes the exemplary IOP sensor system  210 , a drainage tube  330 , the valve system  230 , and a divider  340 . In  FIG. 3 , the IOP sensor system  210  includes four pressure sensors, P 1 , P 2 , P 3 , and P 4 . The pressure sensor P 1  is located in or is in fluidic communication with an anterior chamber  350  of an eye, the pressure sensor P 2  is located to measure intermediate pressures found within the valve system  230 , the pressure sensor P 3  is located remotely from P 1  and P 2  in manner to measure atmospheric pressure, and the pressure sensor P 4  is located at a drainage site  360  in the subconjunctival space and is arranged to measure bleb pressure. 
     In some embodiments, the pressure sensor P 1  is located in a lumen or tube that is in fluid communication with the anterior chamber of an eye. The pressure sensor P 4  may be located in a pocket, such as a bleb, that generally contains aqueous humor or in communication with such a pocket, via a tube for example, and is in the wet site  360 . The drainage site  360  may be, by way of non-limiting example, in a subconjunctival space, a suprachoroidal space, a subscleral space, a supraciliary space, Schlemm&#39;s canal, a collector channel, an episcleral vein, and a uveo-scleral pathway, among other locations in the eye. 
     The drainage tube  330  drains aqueous humor from the anterior chamber  350  of the eye. The valve system  230  controls the flow of aqueous humor through the tube  330 . In the embodiment shown, the pressure sensor P 1  measures the pressure in the tube  330  upstream from the valve system  230  and downstream from the anterior chamber  350 . In this manner, pressure sensor P 1  measures the pressure in the anterior chamber  350 . The expected measurement discrepancy between the true anterior chamber pressure and that measured by P 1  when located in a tube downstream of the anterior chamber (even when located between the sclera and the conjunctiva) is very minimal. For example, Poiseuille&#39;s law for pipe flow predicts a pressure drop of 0.01 mmHg across a 5-millimeter long tube with a 0.300 millimeter inner diameter for a flow rate of 3 microliters per minute of water. 
     In some embodiments, the system includes barriers that separate the sensors P 1 , P 2 , P 3 , and P 4 . These barriers may be elements of the system itself. For example, in  FIG. 3 , the pressure sensor P 3  is physically separated from the pressure sensor P 4  by the divider  340 . The divider  340  is a physical structure that separates the wet site  360  of P 4  from the dry site  365  of P 3 . In one example, the barrier separating the anterior chamber pressure sensor P 1  and the drainage site pressure sensor P 4  is the valve system  230 . 
     Generally, IOP is a gauge pressure reading—the difference between the absolute pressure in the eye (as measured by P 1 ) and atmospheric pressure (as measured by P 3 ). In one embodiment of the present disclosure, pressure readings are taken by the pressure sensors P 1  and P 3  simultaneously or nearly simultaneously over time so that the actual IOP can be calculated (as P 1 −P 3  or P 1 −f(P 3 ), where f(P 3 ) indicates a function of P 3 ). The pressure readings of P 1  and P 3  can be stored in memory  220  by the processor  215 . They can later be read from memory so that actual IOP over time can be interpreted by a physician. 
     The pressure sensors P 1 , P 2 , P 3 , and P 4  can be any type of pressure sensors suitable for implantation in the eye. They each may be the same type of pressure sensor, or they may be different types of pressure sensors. 
     Since the pressure sensor P 1  measures the pressure in the anterior chamber  350  and pressure sensor P 4  measures pressure at the drainage site  360 , the difference between the readings taken by these two pressure sensors (P 1 −P 4 ) provides an indication of the pressure differential between the anterior chamber  350  and the drainage site  360 . In one embodiment, this pressure differential dictates the rate of aqueous humor flow from the anterior chamber  350  to the drainage site  360 . 
     Readings from the pressure sensors P 1 , P 2 , P 3 , and P 4  can be used to control the flow rate through the tube  330  by controlling the valve system  230 . For example, the valve system  230  may be controlled based on the pressure readings from pressure sensors P 1 , P 2 , P 3 , and P 4 . The valve system  230  may be controlled by the processor  215  based on input data received from the sensors. A desired pressure differential (that corresponds to a desired flow rate) can be maintained by controlling the operation of the valve system  230 . Likewise, various intraocular pressure parameters, such as, by way of non-limiting example, the desired IOP, the IOP change rate, and/or the bleb pressure may be controlled by controlling the operation of valve system  230 . Note that in some embodiments, the physician is able to set the high/low IOP thresholds wirelessly to meet each patient&#39;s specific requirements. 
       FIGS. 4A and 4B  are diagrams showing a valve system  400  with a piezo-based membrane valve  401  in an open and in a closed position, respectively. The valve system  400  may be part of the valve system  230  described above.  FIG. 4A  illustrates a stylized cross-sectional view of the membrane valve  401  in the open position. According to the exemplary embodiment, the valve system  400  includes a housing  405  and a boss structure  406  having an inlet  402  extending therethrough. The valve system  400  also includes a number of outlets  404 , a membrane  408 , an actuating element  418 , and a valve sensor system  230 . 
     The fluid inlet  402  is in fluid communication with the anterior chamber (e.g.  350 ,  FIG. 3 ) of the patient&#39;s eye. The inlet  402  is positioned in the center of the boss structure  406 . 
     The boss structure  406  is a support structure through which fluid flows. The boss structure  406  supports the fluid inlet  402 . In one example, the boss  406  structure may be circular from a top perspective. In some embodiments, the boss structure  406  may be elliptical, or rectangular from the top perspective. Other shapes for the boss structure are contemplated. 
     The outlets  404  lead, directly or indirectly, to a drainage site (e.g.  360 ,  FIG. 3 ). The outlets are formed into a housing  405 . In the illustrated embodiment, the outlets  404 - 1 ,  404 - 2  are positioned at opposite sides from each other. Thus, the inlet  402  is positioned between the two different outlets  404 - 1 ,  404 - 2 . While only two outlets  404  are shown, in some embodiments, there may be one or more additional outlets  404 . For example, one exemplary embodiment includes five outlets  404  forming a circle around the inlet  402 , when viewed from the top perspective. 
     The membrane  408  is a thin layer of material that is flexible and can thus be displaced under pressure. The membrane  408  may be supported at the edges by the housing  405 . The membrane  408  is disposed to extend across the boss structure  406 , the opening of the inlet  402 , and the openings of the outlets  404 . The membrane  408  is configured to deflect toward and away from the interfacing surface of the boss structure  406  to inhibit or regulate flow from the inlet in varying degrees. 
     In some embodiments, the membrane  408  may include multiple layers. In the illustrated embodiment, the membrane  408  includes a support layer  407 , and a piezo-based layer  409 . In some examples, the support layer  407  may be made of a thin piece of glass or silica. In some examples, the support layer  407  may be formed of a biocompatible elastomeric material such as, by way of non-limiting example, Parylene, silicone, silicone nitride, silicone elastomeric, and polyimide. 
     The piezo-based layer  409  may be made of a variety of materials. For example, the piezo-based layer  409  may be a thin film grown through an epitaxial process. The thin film may be a semiconductor material such as silicon and may be doped with a variety of dopants including boron, arsenic, aluminum nitride, zinc oxide, lead zirconate titanate and/or others. In some examples, the piezo-based layer  409  may be a ceramic metal compound such as a combination of silicon oxide and chromium grown over a thin membrane, or coated by, a crystalline substrate such as sapphire. In some embodiments, a piezoresistive layer may be made of polysilicon or other type of semiconductor material. Other piezo-based materials are contemplated. 
     The actuating element  418  applies pressure to the membrane  408  to cause deflection of the membrane to or away from the boss structure  406  and inlet  402 . The actuating element  418  may be formed of any variety of mechanisms to actuate the membrane  408 . In one example, the actuating element  418  may utilize electrolysis to apply pressure to the membrane  408 . Particularly, the actuating element  418  may include a chamber containing electrodes. The chamber may be filled with an electrolytic fluid. Some of the electrolytic fluid will change from a liquid state to a gas state under application of a voltage by the electrodes. This will cause a pressure build up in the chamber, thereby pressing against the membrane  408 . Other methods of actuation are also contemplated. 
     The valve sensor system  240  includes the tools used to measure the electrical properties of the membrane  408 . The valve sensor system  240  may be part of the control system  200  illustrated in  FIG. 2 . The valve sensor system  240  includes a voltage supply to apply an excitation voltage to the membrane  408 . The control system  416  also includes an electric current measuring device such as an ammeter to measure the electric current flowing through the membrane  408 . 
     When in an open position, the valve system  400  allows fluid flow  412  through the inlet  402  toward the membrane  408 . The valve system  400  also allows fluid flow  414  between the inlet and the outlets  404 . The fluid flow  410  then continues through the outlets  404  to the drainage site. During normal operation, the  10 P control system  200  of a device implant utilizing the valve system  400  may determine that the pressure in the anterior chamber is too low. Thus, the valve system  400  should be closed so that fluid no longer flows from the anterior chamber to the drainage site. 
       FIG. 4B  illustrates the valve system  400  in a closed position. According to one embodiment, pressure  420  applied to the membrane  408  increases the deflection of the membrane  408 . The pressure  420  can be controlled to press the membrane  408  against the opening of the inlet  402  with enough force to prevent fluid flow  412  through the inlet  402 . 
     Being aware of the position of the membrane  408  may be important when determining how much pressure  420  should be applied to regulate or block fluid flow through the valve structure. Specifically, if the membrane  408  is already pressed against the boss structure  406 , then applying additional pressure may cause damage. 
       FIG. 5  is a diagram showing a valve system  400  with a piezoresistive membrane valve  501 . The piezoresistive membrane valve  501  may form part of the valve system  400  described above with references to  FIGS. 2 and 3 . Some of the features of the piezoresistive membrane valve  501  are the same as the features of the piezo-based membrane valve  401 . Accordingly, not all the features will be discussed again as the above description applies. Like the pizezo-based membrane valve  401  discussed above, the piezoresistive membrane valve  501  includes a membrane  408  and a control system  240 . In this embodiment, however, the membrane  408  comprises a layer of a piezoresistive material  509 . A piezoresistive material  509  is a material that changes its electrical resistance based on mechanical stress. Thus, the resistance of the membrane  408  is a function of the deflection position of the membrane  408 . 
     The control system  240  includes a voltage supply  504  and a current measuring device such as an ammeter  502 . The voltage supply  504  applies a small voltage to the membrane  408 . This will cause a current  506  to flow through the membrane  408 . The electric current  506  passing through the membrane is then measured by the ammeter  502 . The resistance can then be calculated based on the applied voltage and the measured current  506 . Specifically, the resistance is equal to the voltage divided by the measured current. The resistance can then be used to determine the deflection position of the membrane  408 . 
     The position of the membrane  408  in  FIG. 5  may be determined based on the resistance in a number of manners. In one example, before use of the valve system, the resistance of the membrane  408  at a set of known positions is measured. For example, the resistance of the membrane  408  may be measured at  10  different points between fully open and fully closed positions. This set of data may be used to determine the position of the membrane during normal operation. Particularly, if the determined resistance is a value close to a value corresponding to one of the points that was previously measured, then it is known that the present position of the membrane  408  corresponds to that position. 
     In some embodiments, a function can be used to determine the position of the membrane  408  based on its resistance. This function may be extracted from a set of data points as described above. Or, the function may be derived based on known piezoresistive and mechanical properties of the membrane. Thus, to determine the position of the membrane  408 , the valve sensor system  240  in  FIG. 5  may plug in the calculated resistance value into the function to determine the position of the membrane  408 . 
       FIG. 6  is a diagram showing a piezoelectric membrane valve  601 . Again, some of the features of the piezoelectric membrane valve  601  are the same as the features of the piezo-based membrane valve  401 . Accordingly, not all the features will be discussed again as the above description applies. Like the piezo-based membrane valve  401  discussed above, the piezoelectric membrane valve  601  includes a membrane  408  and a control system  240 . In this embodiment, however, the membrane  408  comprises a layer of a piezoelectric material  609 . A piezoelectric material  609  will generate an electric current while moving from one position to another. Thus, the present position of the piezoelectric membrane  408  is a function of a previously known position and a current measurement taken during movement between the present position and the previous position. 
     The valve sensor system  240  for the piezoelectric membrane  408  includes a current measuring device such as an ammeter  602 . The ammeter  602  measures an electric current  604  flowing from the membrane  408  in response to a change in deflection position of the membrane  408 . 
     The position of the membrane  408  may be determined based on the measured current in a number of manners. In one example, a function is derived that correlates electric current with change in position. This function is then used to determine the change in position based on a measured current value. By knowing the previous position and the change in position, it is possible to determine the current position. Other methods for determining the position based on the measured electric current are contemplated. 
       FIG. 7  is a diagram showing an illustrative feedback control system  700 , which may form a part of the IOP control system  200 . Feedback control systems can be used to achieve a desired output based on an input. For example, it may be desirable that the membrane (e.g.  408 ,  FIG. 4 ) be at a specific position. The specific position may be either fully closed or fully open. The specific position may also be somewhere in between. 
     The feedback control system  700  includes an input  702 , an error  704 , the IOP sensor system  210 , a system input  708 , the valve system  400 , an output  712 , the valve sensor system, and a measured output  716 . For convenience, the feedback control system  700  will be discussed with reference to the valve system  400 , recognizing that it may be used with all valve systems described herein. 
     The operation of the feedback control system  700  will become more apparent in view of the following example. In one example, the valve is 20% closed and it is desired to move the valve to a position that is 80% closed. Thus, the input  702  of the system is set to 80% closed. At that time, the output  712  of the valve system  400  is still at 20% closed. The measured error  704  is the difference between the input  702  and the measured output  716 . In this case, the measured error  704  is 60%. Thus, the  10 P sensor system  210 , which operates as the feedback controller in this case, knows to apply more pressure to the membrane to make the measured error  704  zero. 
     By using the methods and principles described herein, the measured output  716  more closely resembles the actual system output  712 . Thus, the IOP sensor system  210  can more accurately adjust the valve system  400  to the desired position. Moreover, the IOP sensor system  210  will regulate pressure on the membrane when the membrane is already in the fully closed position to avoid over-pressurization conditions that may cause damage to the membrane. 
       FIGS. 8A and 8B  are flowcharts illustrating methods for determining the position of a piezo-based membrane valve.  FIG. 8A  is a flowchart showing an illustrative method for determining the position of a piezoresistive membrane valve, while  FIG. 8B  describes a method for determining the position of piezoelectric membrane valve. Referring first to  FIG. 8A , and according to the present example, the method  800  includes applying a voltage to a piezoresistive membrane at step  802 . This voltage will cause an electric current to flow through the piezoresistive membrane. 
     The method  800  further includes measuring an electric current flowing through the membrane at step  804 . This may be done with a current measuring device such as an ammeter. Resistance is equal to the voltage applied divided by the electric current flowing through the membrane. Thus, the resistance is determined by applying a known voltage and measuring the current. 
     The method further includes determining a position of the membrane at step  806 . This is done based on the calculated resistance value. As described above, the position can be determined from the resistance based on either a predefined data set or by applying the resistance value to a function that relates resistance to valve position. 
       FIG. 8B  illustrates a method  810  for determining the position of a membrane valve made of a piezoelectric material. According to one embodiment, the method  810  includes applying pressure with an actuating element to move a piezoelectric membrane at step  812 . As described above, the actuating element may use electrolysis to move the membrane. 
     Because of the piezoelectric properties of the membrane, movement of the membrane will cause an electric current to flow through the membrane. The method  810  includes measuring an electric current flowing through the membrane at step  814 . This may be done with a current measuring device such as an ammeter. 
     The method  810  includes determining a change in position of the membrane at step  816 . The change in position is based off of the measured electrical current during the change. As described above, this may be done using a predefined data set or a function that relates measured current during change to the change in position. Knowing the original position and the change in position of the membrane allows for determination of the present position of the membrane. 
     Persons of ordinary skill in the art will appreciate that the embodiments encompassed by the present disclosure are not limited to the particular exemplary embodiments described above. In that regard, although illustrative embodiments have been shown and described, a wide range of modification, change, and substitution is contemplated in the foregoing disclosure. It is understood that such variations may be made to the foregoing without departing from the scope of the present disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the present disclosure.