US 20050126912 A1
An electro-osmotic cell capable of substantially reducing zero-current transport is disclosed, wherein the cell includes a cell housing having a first half cell and a second half cell, with an ion selective membrane therebetween, a first electrode positioned within the first half cell, a second electrode positioned within the second half cell, an electrolyte in electrical communication with the first electrode and the second electrode, and a wiring apparatus electrically connecting the first electrode and the second electrode, wherein the wiring apparatus has one or more structures used for counteracting salt concentration increases within the electro-osmotic cell. Such a cell may be used within an electro-osmotic fluid delivery device, along with a fluid inlet, a piston member adjacent the electro osmotic cell, and a drug reservoir adjacent the piston member, wherein the drug reservoir includes a sealed compartment having an exit port. A method for using such a device is similarly disclosed.
1. An electro-osmotic cell, comprising:
a cell housing having a first half cell and a second half cell, with an ion selective membrane therebetween;
a first electrode positioned within the first half cell;
a second electrode positioned within the second half cell;
an electrolyte in electrical communication with the first electrode and the second electrode; and
a wiring apparatus electrically connecting the first electrode and the second electrode;
wherein the wiring apparatus comprises means for preventing zero-current transport within the electro-osmotic cell.
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12. The invention according to
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16. The electro-osmotic cell according 14, wherein the cathode comprises a material selected from the group consisting of carbon, platinum, zinc, magnesium, manganese, aluminum, silver, and silver/silver chloride.
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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.
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'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.
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
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
Fluid inlet 32 of fluid delivery device 30 is shown in
Electro-osmotic cell 34 is shown in
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
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
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
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
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
In the embodiment shown in
In operation, fluid delivery device 30 shown in
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
Similarly, in the embodiment of the invention shown in
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
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.