Patent Publication Number: US-2005126912-A1

Title: Voltage modulation of advanced electrochemical delivery system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention is drawn generally to electrochemical fluid delivery devices, and specifically to an improved electro-osmotic fluid delivery system.  
      2. State of the Prior Art  
      Fluid delivery devices are well known in the art, ranging from pressurized fluid delivery, to mechanical fluid delivery, to electrochemical fluid delivery devices and beyond. One particularly interesting fluid delivery system is an electro-osmotic cell coupled with a delivery pump, forming an electro-osmotic pump. These simple pumps operate through the combination of an electrochemical cell and an ion-selective membrane to create a driving force for fluid delivery.  
      Conventional electro-osmotic pumps, however, have a number of problems that have not, as of yet, been addressed in the prior art. One particular problem has occurred in constant fluid delivery applications. As the operation of the device is continued over a period of time, it has been observed that the delivery rate is inconsistent, even though the current rate between the anode and the cathode is maintained at a constant rate. Generally, two types of osmosis are occurring with an electro-osmotic cell simultaneously. The primary and most prevalent type of osmosis is electro-osmosis, whereby charged ions (salts) are driven across an ion exchange membrane as the cell is operated, thereby dragging water molecules along its path. The secondary, and less prevalent form of transport is osmosis due to environmental conditions. Osmosis is the transfer of a solvent across a barrier, generally from an area of lesser solute concentration to an area of greater concentration. Given normal cell operating conditions, the environmentally-driven osmosis is negligible in comparison to the electro-osmosis.  
      As the relative concentrations of salts within the half cells of an electro-osmotic delivery device change, however, significant changes in the amount of fluid delivered have been observed. It has been postulated that as operation of the device is continued, the passage of ions (salts) across the membrane of the device causes an increase in the salt concentration within one of the half-cells resulting in an increased osmotic flow of a solvent across the membrane. Thus, environmental osmosis becomes more prevalent, and affects the predictability and reliability of the cell operations. The fluid transfer causes an increase in the overall fluid amount contained in the one half-cell, increasing the rate of delivery of fluid.  
      The above-described effect can continue even after the operation of current within the cell has stopped. Even though the anode and the cathode are removed from electrical communication with one another, the concentration difference between the half-cells remains. Thus, additional electrolyte/solvent will continue to be transported across the membrane, causing the fluid delivery device to continue delivering fluid even after the cell has ceased operation. This additional fluid delivery is termed “zero-current transport,” and is deemed unacceptable—especially for long term use of a constant-rate fluid delivery device.  
      It is a thus an object of the present invention to eliminate, or substantially reduce, unwanted zero-current transfer.  
      It is another object of the present invention to provide an improved cell design wherein the concentration differences between the half-cells within the device are mitigated or avoided.  
      It is another object of the present invention to increase the reliability and consistency of the delivery rate of the device.  
      These and other objects will become apparent to one of ordinary skill in the art in light of the present specification, claims and drawings appended hereto.  
     SUMMARY OF THE INVENTION  
      The present invention, disclosed herein, teaches an electro-osmotic cell having an improved mechanism for the cessation of cell operations after removal of operational current. The electro-osmotic cell includes a cell housing with a first half cell and a second half cell, which are separated by an ion-exchange membrane. Within each half cell is an electrode; a first electrode within the first half cell, and a second electrode within the second half cell. The electro-osmotic cell also includes an electrolyte in electrical communication with the first electrode and the second electrode, a wiring apparatus electrically connecting the first electrode and the second electrode. All of these elements ensure the normal operation of the electro-osmotic cell. Additionally, however, the electro-osmotic cell includes means for counteracting at least some of the effects of salt concentration increases within the electro-osmotic cell associated with the wiring apparatus. The counteracting means ensures that, after operation of the cell has been halted, the zero-current transport seen in conventional electro-osmotic cells can be minimized.  
      Preferably, the electrolyte used within the electro-osmotic cell can be any solution containing Na +  and/or K +  and Cl −  ions, such as fluid from a body (where the solvent is water and the electrolytes are naturally-occurring salt ions such as sodium and chloride ions) that can be delivered from the surrounding tissues to an implanted fluid delivery device. Alternatively, a number of other electrochemically compatible bodily fluids could similarly be used (e.g., Ringer&#39;s solution, renal dialysis solution, PBS etc).  
      In a preferred embodiment of the invention, the wiring apparatus of the electro-osmotic cell includes a forward wiring loop and a reverse wiring loop. In this embodiment, both of the forward wiring loop and the reverse wiring loop have a switch for enabling the electrical connection the loops. Specifically, the switch associated with the forward wiring loop can be closed so as to allow current to flow from the first electrode to the second electrode. Alternatively, a switch associated with the reverse wiring loop can be closed so as to allow current to flow from the second electrode to the first electrode. In order to further facilitate current flow in the reverse wiring loop, the counteracting means of the electro-osmotic cell comprises a power source associated with the reverse wiring loop, helping to drive current within that wiring loop. Additionally, it is preferred that the reverse wiring loop includes a controlling element capable of controlling the magnitude and/or time course of current flow across that loop.  
      In another preferred embodiment, the first and second electrodes of the electro-osmotic cell include both a forward and a reverse electrode. The forward electrodes of both the first electrode and the second electrode are connected through the forward wiring loop. Similarly, the reverse electrodes of the first and second electrodes are connected through the reverse wiring loop. Thus, in this embodiment, the forward loop and reverse loop can comprise separate wiring structures.  
      Further, the electro-osmotic cell of this embodiment may include a sensing means for detecting a parameter such as the concentration of at least one ionic species within the first and/or second half cells. The sensing means can detect the concentration by, for example, detecting the conductivity within the first and/or second half cells or detecting the electrode potential of the second electrode. The sensing means may comprise a separate sensor or may comprise a sensing circuit connecting the forward second electrode and the reverse second electrode, between the forward first electrode and the reverse first electrode, or between the forward first electrode and the forward second electrode, as may be needed.  
      Preferably, the first electrode is an anode, the second electrode is a cathode, and the membrane is cationic selective membrane. Alternatively, the first electrode could be a cathode, the second electrode an anode, and the membrane is anionic selective membrane. Anode materials may be of any suitable material to which a cation will migrate in a given electrolytic reaction, and may include materials such as carbon, platinum, zinc, magnesium, manganese, aluminum, silver, and silver/silver chloride. Cathode materials can include carbon, platinum, zinc, magnesium, manganese, aluminum, silver, and silver/silver chloride, among others. As with the dual-electrode embodiment, a single first electrode and a single second electrode preferably include a sensing means for detecting ionic concentration within the cell.  
      Such an electro-osmotic cell can beneficially be utilized within an electro-osmotic fluid delivery device. The above-described cell, along with all of the preferred embodiments of that cell, can deliver fluid by combining the cell with a fluid inlet, a movable barrier such as a piston member adjacent the electro osmotic cell, and a drug reservoir adjacent the piston member/movable barrier, the drug reservoir comprising a sealed compartment having an exit port. Preferably, the fluid inlet comprises a membrane (such as a permeable membrane or osmotic membrane), or a fluid conduit. Also, the piston member/movable barrier preferably comprises a slideable piston, or a flexible diaphragm.  
      Such a device can be beneficially used in a method for controlling the unwanted fluid flow out of the electro-osmotic delivery device. The method preferably includes the steps of (1) delivering a fluid using an electro-osmotic fluid delivery device having an electro-osmotic cell therein, wherein the step of delivering the fluid causes an increase in a salt concentration within the electro-osmotic cell, (2) sensing the salt concentration within the electro-osmotic cell, (3) halting the step of delivering, and (4) counteracting the increased salt concentration by reversing the direction of current proportionally (either as a linearly or some more complex proportional relationship) to the sensed salt concentration so as to reduce unwanted zero current transport, and, in turn, unwanted fluid delivery out of the fluid delivery device. Preferably, the step of sensing comprises the step of sensing the conductivity differences between at least two of a forward first electrode, a forward second electrode, a reverse first electrode and a reverse second electrode, wherein the forward and reverse first electrodes are located within a first half cell of the electro-osmotic cell, and the forward and reverse second electrodes are located within a second half cell of the electro-osmotic cell.  
      In yet another embodiment of this invention, the electro-osmotic cell contains forward first and second electrodes and controlling circuitry connecting the electrodes. A coulometric circuit element is contained within the controlling circuitry. The salt increase in the half cell will be a function of the charge passed. To halt delivery, the current is reversed for a time and magnitude based on the charge passed, as sensed by the coulometric element. This embodiment may include reverse first and second electrodes. The coulometric element may be complex and able to sense both forward and reverse currents, with the controlling element using these data in a complex manner to apply the reverse current. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  of the drawings depicts a cut-away side view of prior-art electrochemical fluid delivery device;  
       FIG. 2  of the drawings shows a cut-away side view of the fluid delivery device of the present invention; and  
       FIG. 3  of the drawings shows a cut-away side view of an alternative embodiment of the fluid delivery device of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will be described in detail, several specific embodiments with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the embodiments illustrated.  
      A prior art electrochemical device  100  comprising an electro-osmotic pump is shown in  FIG. 1  as including fluid inlet  101 , electro-osmotic (or electrochemical) cell  102 , and fluid outlet  104 . The fluid inlet  101  is shown as a pair of fluid conduits  108 , which provide and then return a fresh supply of saline to cell  102 , as needed. The electro-osmotic cell  102  is typically made up of a first half-cell  112 , membrane  114 , and second half-cell  116 . Conventional devices such as the one shown in  FIG. 1  typically operate by introducing a fluid into first half-cell  112 , and pumping a portion of the fluid through membrane  114  by the electrochemical transfer of an ion across membrane  114 . The fluid introduced into second half-cell  116  accumulates, delivering a fluid contained within second half cell  116  out of cell  102  via fluid outlet  104 .  
      Typically, such prior art electro-osmotic devices, which may be used in fluid delivery systems, have a number of drawbacks. Although such devices are effective in delivering fluid through electro-osmotic transport, the consistency and predictability of fluid delivery can be affected through osmotic transport during and after cessation of the operation of the device. During operation of the cell, conventional cells see an increase in the salt concentration within the cell itself. The salt concentration increase can affect cell operations, and, in particular, acts as a driving force for unwanted osmotic transport within the cell. This effect can even extend beyond cessation of electro-osmotic transport, causing osmotic transport even after cell current has been cut off. This type of post-operational osmotic transport is termed zero-current transport.  
      Fluid delivery device  30  of the present invention helps to overcome these problems, among others. Fluid delivery device  30  of the present invention is shown in  FIG. 2  as generally comprising an elongated cylindrical shaped device. The teachings of the present invention may be utilized with a wide variety of fluid-delivery devices. One particularly useful application for the present pump technology is in implantable medical pumps. These pumps are implanted within patients for the delivery of medicament to a patient over a long period of time. The teachings of the present invention will be discussed relative to such an implantable device, and will be shown within such an environment. Although device  30  is shown in conjunction with the implantable devices, it should be noted that the teachings contained within this specification and the appended claims may be translated to other devices and applications without straying from the intended scope of this disclosure.  
      Fluid inlet  32  of fluid delivery device  30  is shown in  FIG. 2  as generally comprising a porous, permeable or semipermiable membrane associated with an end portion  33  of fluid delivery device  30 . Preferably, fluid delivery device  30  is associated with a water-rich environment, such as being implanted in a human body, so that, during operation, water may be allowed into cell  34  through inlet  32 . Alternatively, and as shown in  FIG. 1 , fluid inlet  32  could comprise one or more conduits for the delivery of a fluid into cell  34 . In any case, the fluid inlet of the particular device should provide a constant and ready supply of fluid for the electro-osmotic cell so as to ensure ongoing and consistent operation.  
      Electro-osmotic cell  34  is shown in  FIG. 2  as comprising first half-cell  38  and second half-cell  46 , with ion-selective membrane  54  in-between. Fluid inlet  32  is shown associated with first half-cell  38 , allowing fluid from the surrounding environment of fluid delivery device  30  into cell  34 . Within first half-cell  38  and second half-cell  46  are electrodes, shown in  FIG. 3  with first electrode  40  in first half-cell  38 , and second electrode  48  in second half-cell  46 . As will be described further in the operation section below, first electrode  40  and second electrode  48  comprise an anode and a cathode electrode, respectively. Alternatively, first electrode  40  could comprise a cathode, and second electrode  48  could comprise an anode, depending upon the materials selected for the electrodes and membrane  54 , and the operation of the fluid delivery device  30 . Thus, these electrodes are interchangeable within first half-cell  38  and second half-cell  46  of cell  34 , depending upon the particular materials used for first electrode  40  and second electrode  48  and for membrane  54 .  
      Numerous materials can be used for both first electrode  40  and second electrode  48 , but they must be electrochemically compatible with one another so as to allow for the flow of ions and electrons during cell operation. Typical electrode material pairings could include, among others, Zn/Ag/Ag/Cl, Pt/Pt, Ag/Ag/Cl/Pt, Zn/Pt, Pt/Ag/Ag/Cl, Ag/Ag/Cl/Ag/Ag/Cl, and Zn/Ag/Cl. In one preferred embodiment, first electrode  40  comprises a zinc electrode, and second electrode  48  comprises an Ag/Ag/Cl electrode.  
      Membrane  54  of cell  34  generally comprises an ion-selective or ion-exchange membrane that allows the passage of the ions, while substantially maintaining the integrity between first half-cell  38  and second half-cell  46 . The particular material selected for membrane  54  is dictated by the electrode materials selected and the desired pumping rate of fluid delivery device  30 . Typical materials, however, include NAFION, CMI 7000, Membranes International C/R, CMB and CCG-F from Ameridia, AM-1, AM-3 and AM-X from Ameridia and PC-200D from PCA GmBH.  
      The teachings associated with the electro-osmotic cell  30  of the present device do not necessarily need to be limited to fluid-delivery devices. Applications for the extended use and consistent operation of the cell  34  of the present invention can extend beyond the fluid delivery art, to and including controlled release of any substance in manner that is minimally affected by temperature or pressure. Thus, although the present disclosure is shown in conjunction with a fluid-delivery device, it may be possible to transplant the teachings of the electrochemical cell into another device, as mentioned above, without departing from the scope of this disclosure.  
      Piston  78  is associated with the distal end  37  of second half-cell  46 , sealing off that portion of fluid delivery device  30  from drug reservoir  80 . Piston  78  is slideably associated within fluid delivery device  30  so that, as the volume of fluid contained within second half-cell  46  increases or decreases, piston  78  is correspondingly maneuvered into and out of drug reservoir  80 . From this process, fluid contained within drug reservoir  80  can be pushed out for delivery, or drawn in if fluid delivery device  30  operations so dictate. Other structures could similarly be utilized to perform the same functional task with an alternative structure. For example, piston  78  could comprise a diaphragm, movable partition, or another similar structure that is capable of conveying an increase in pressure from one compartment to another, while maintaining the integrity of each compartment.  
      Drug reservoir  80  is shown generally in  FIG. 2  as an enclosed portion of fluid delivery device  30  associated with exit port  84  leading out of reservoir  80 . Drug reservoir  80  preferably includes a sealed compartment  82 . Sealed compartment  82  provides a secure containment system for a drug or other fluid to be enclosed within reservoir  80 . Sealed compartment  82  must have at least one opening  83 , so as to allow fluid flow out of sealed compartment  82 , through exit port  84 , and into the surrounding environment.  
      Exit port  84  preferably comprises an open aperture between the reservoir  80  and the surrounding environment. Although not shown, exit port  84  may additionally include any number of fluid-delivery control devices such as nozzles, valves, or other control devices for regulating flow rate of fluid out of fluid delivery device  30 . In its simplest and most preferred form, however, exit port  84  is merely a static aperture, and delivery rate of fluid out of reservoir  80  is dictated entirely by operation of electro-osmotic cell  34 .  
      Wiring apparatus  56  is shown in  FIG. 2  as comprising forward wiring loop  60 , reverse wiring loop  68 , and zero-current transport control means  58 , which together help to control the direction and rate of ion flow across membrane  54  to, in turn, control the flow of fluid out of fluid delivery device  30 . Forward wiring loop  60  and reverse wiring loop  68  provide an electrical connection between first electrode  40  and second electrode  48  that, in combination with the electrolyte contained in first half cell and second half cell, completes an electrical loop between the electrodes, and across the membrane  54 , enabling operation of the electro-osmotic cell  34 . Thus, conventional wiring materials can be utilized for both loops. Forward wiring loop  60  and reverse wiring loop  68  can comprise a single electrical wiring connection, or, as shown in  FIG. 2 , single connections to both the first electrode  40  and second electrode  48 , with two separate wiring connections, one comprising the forward wiring loop  60  and one comprising the reverse wiring loop  68 .  
      In order to facilitate normal cell operation, forward wiring loop  60  additionally comprises switch  62 , and may additionally include power source  64 . When switch  62  is closed, power source  64  provides the potential energy necessary to drive the ions produced by first electrode  40  out of first half-cell  38 , through membrane  54 , and into second half-cell  46  and second electrode  48 . Alternatively, power source  64  could be omitted, and the electrochemical potential of the cell itself would then drive the operation of the system. Further, forward wiring loop  60  may additionally comprise a control element  66 , wherein control element  66  provides a regulating mechanism for the flow of current across forward wiring loop  60  that is able to control at least one of the magnitude and time course of current flow. Control element  66  can comprise any number of conventional current controls, including simple mechanisms such as resistors, and more complicated devices such as microprocessor-controlled current controls. Forward wiring loop  60  is utilized during the normal operations of the cell  34 .  
      Reverse wiring loop  68  comprises a similar structure as forward wiring loop  60 . Reverse wiring loop  68  is shown in  FIG. 2  as having switch  70 , and control element  72  (shown as a battery). Similar to the forward wiring loop  60 , when switch  70  is closed, reverse wiring loop  68  allows the flow of current from the second electrode  48  out of second half-cell  46 , across membrane  54  and into first half-cell  38  and to first electrode  40 . Control element  72  again provides regulation for the magnitude and/or the time course of current flow. Additionally, as may be needed, reverse wiring loop  68  can additionally include a power source (if control element  72  isn&#39;t itself a power source) for contribution to the potential driving force of current across reverse wiring loop  68 . Reverse wiring loop  68 , as will be described below, is utilized during the concentration counteracting operation of the cell  34 .  
      Control member  66  or control member  72  may additionally include sensing means  74 . Sensing means  74  helps to monitor the build-up of ions within second half-cell  46  as operation of the fluid delivery device  30  commences. Sensing means  74  preferably comprises a sensing circuit  76  connecting first electrode  40  and second electrode  48 , which helps to measure the ionic buildup within the second half-cell  46 , and could additionally comprise a stand-alone sensor. For example, sensing circuit  76  could measure the difference in concentration measurements of a particular species, or could measure the difference in ionic conductivity between first half cell  38  and second half-cell  46 . Similarly, sensing circuit  76  could measure the concentration of a particular species, or the ionic conductivity of a particular half-cell, and utilize a computing element (not shown) and known variables to calculate the differences in ionic conductivity concentration between the half-cells. In order to do so, sensing circuit  76  comprises any number of conventional ionic sensors or species-specific analyte sensors, such as sodium ion sensor, Ag/Ag/Cl chloride ion sensor, electric conductivity meter etc. Once determined, control member  66  or control member  72  can utilize this information to alter the magnitude/time course of current to properly operate fluid delivery device  30 .  
      In another embodiment of the present invention, shown in  FIG. 3 , first electrode  40  comprises two electrodes, forward first electrode  42  and reverse first electrode  44 . Further, it is preferred that second electrode  48  could comprise forward second electrode  50  and reverse second electrode  52 . Forward first electrode  42  and forward second electrode  50  can be connected electrically through forward wiring loop  60 , and reverse first electrode  44  and reverse second electrode  52  through reverse wiring loop  68 . As will be described in the operation section below, such an embodiment allows for a wider selection of electrode materials, as the first electrode  40  and second electrode  46  do not have to be manufactured from electrochemically reversible materials. Although the range of electrode material pairings highlighted above are available for use, in a preferred embodiment of  FIG. 3 , forward first electrode  42  comprises a zinc electrode, reverse first electrode  44  comprises a platinum electrode, forward second electrode  50  comprises an Ag/Ag/Cl electrode, and reverse second electrode  52  comprises an Ag/Ag/Cl electrode.  
      Alternatively, and in another preferred embodiment, it may be possible for a single electrode to act as both the forward and reverse electrodes in a single half cell. For example, in the embodiment shown in  FIG. 3 , there are two separate electrodes within second half cell, namely forward second electrode  50  and reverse second electrode  52 . Both the forward and reverse second electrodes could, however, comprise a single second electrode  48  made from electrochemically reversible materials, such as Ag/Ag/Cl. In such an embodiment, forward wiring loop  60  and reverse wiring loop  68  connect forward first electrode  42  and reverse first electrode  44  to a single second electrode. Of course, the entire system could be reversed also, with the first electrode comprising a single electrode, and the second electrode comprising a forward and reverse electrode, as would be known by one of ordinary skill in the art. In any case, a number of different wiring and electrode configurations could provide the present invention with the functionality herein described.  
      In the embodiment shown in  FIG. 3 , the sensing means  74  comprises sensing circuits  76  connecting two or more of the electrodes of fluid delivery device  30 . In a preferred embodiment, sensing circuit  76  connects forward second electrode  50  and reverse second electrode  52 . Alternatively, sensing circuit  76  connects forward first electrode  42  and reverse second electrode  52 , forward first electrode  42  and forward second electrode  50 , or any other similar combination. Additionally, three or more of the electrodes can be interconnected through sensing means  74 . Alternatively, a separate sensing circuit can be inserted into chambers  46  and  38 .  
      In operation, fluid delivery device  30  shown in  FIG. 2  is first placed into a fluid-rich environment, such as a human being, so that fluid from the surrounding environment can be absorbed into cell  34  through fluid inlet  32 , filling first half-cell  38 . Alternatively, the first half-cell  38  could be pre-filled just prior to use or even during manufacture. Once implanted, or just prior to implantation, switch  62  on forward wiring loop  60  is closed, causing the production of ions at the first electrode  40 , which ions are then driven across membrane  54  toward and into second half-cell  46 . As the ions pass across membrane  54 , they take with them small amounts of the water in which they are dissolved, increasing the overall volume of fluid within second half-cell  46 .  
      As volume within the second half-cell  46  is increased, piston  78  is pushed into the sealed compartment  82  of drug reservoir  80 , collapsing the volume of that portion inward and towards exit port  84  of fluid delivery device  30 . As the sealed compartment  82  is collapsed, it pushes fluid contained therein towards and out of exit port  84 , delivering the fluid to the surrounding environment in a consistent and steady manner. Normal operation of the device will continue in the same manner until the first electrode  40  is spent, all fluid is delivered, or an operator makes a decision to halt normal operation of the device.  
      As normal operation of the device continues, the ionic concentration within the first half-cell  38  and/or second half-cell  46  continues to change. For example, as in one preferred cell  34 , namely the Zn/Ag/Ag/Cl embodiment described above, upon closing switch  62 , zinc ions are created in the first half cell  38 , and sodium ions are then passed across membrane  54  and into second half-cell  46 . These ions begin to build up over time, creating an increase in ionic concentration within second half-cell  46 , creating an ionic concentration differential between first half cell  38 , and second half cell  46 . This differential, in part, is the cause for zero-current transport within conventional electro-osmotic cells.  
      In order to counteract the problem of zero-current transport, an operator of the present invention must counteract the salt concentration increase within the electro-osmotic cell by using zero-current transport control means  58 . Control means  58  preferably comprises a combination of wiring apparatus  56 , and sensing means  74 , which are utilized beneficially together upon halting the operation of the cell. Once a decision has been made to halt the normal operation of the cell  34 , sensing means  74  is used to determine the extent of ionic concentration differential between first half-cell  38  and second half-cell  46 . In the embodiment of the present invention shown in  FIG. 2 , sensing circuit  76  detects the ionic concentration differential between the first electrode  40  and the second electrode  46 . This information is then transmitted to the control element  72  of the reverse wiring loop  68 , where it is evaluated. Control element  72  determines the proper magnitude and time course of current flow necessary to counteract the effects of the ionic concentration differential. Once determined, switch  62  is opened, and switch  70  is closed, and control element  72  directs a reverse flow of current sufficient to counteract the salt concentration increase within the cell, and, in turn, to halt cell  34  operations, and cease the delivery of fluid from fluid delivery device  30  altogether.  
      Similarly, in the embodiment of the invention shown in  FIG. 3 , operation of the cell  34  during normal cell operations is commenced by closing switch  62  on forward wiring loop  60 , electrically connecting forward first electrode  42  and forward second electrode  50 . Once connected, forward first electrode  42  produces an ion within first half-cell  38 , which is then transferred across membrane  54 , and into second half-cell  46 . As with the embodiment discussed above, the transfer of the ions across membrane  54  causes an ionic concentration differential between first half cell  38  and second half cell  46 , which in turn causes zero current transport upon the cessation of normal cell operations.  
      In order to prevent the zero current transport, upon cessation of the normal cell operations, sensing means  74  detects the ionic concentration differential between first half-cell  38  and second half-cell  46 . Sensing means  74  uses sensing circuit  76  to detect concentration differences between one of the pairings of forward second electrode  50  and reverse second electrode  52 , forward first electrode  42  and reverse first electrode  44 , and forward first electrode  42  and forward second electrode  50  (or other electrode combinations). The concentration differences are transferred to control element  72  of reverse wiring loop  68  so that, upon opening switch  62 , and closing switch  70 , the normal fluid delivery operations of cell  34  can be halted without unwanted zero current transport.  
      An alternative embodiment is shown in  FIG. 4 . In such an embodiment, fluid delivery device  30  includes most of the same elements as embodiments shown in  FIGS. 2 and 3 , and discussed above. Device  30  includes first half cell  38 , and second half cell  46 , with forward wiring loop  60  and reverse wiring loop  68  connecting one or more first electrodes  40 , and second electrodes  48 . In this embodiment, however, no sensing means  74  is present. The device in  FIG. 4  is intended for a single operative session, wherein it is activated once, and then, at the end of its operation, halts function totally. Since the operational life of the device  30  in  FIG. 4  is known or limited, control element  72  can implement a known, predetermined current program, reversing the current operation of cell  34  upon cessation of operations. This known current program enables the substantially immediate cessation of fluid delivery from device  30  for the one-time use embodiment shown in  FIG. 4 .  
      The foregoing description merely explains and illustrates the invention and the invention is not limited thereto except insofar as the appended claims are so limited, as those skilled in the art that have the disclosure before them will be able to make modifications without departing from the scope of the invention.