WO2004036041A2

WO2004036041A2 - Electrokinetic devices

Info

Publication number: WO2004036041A2
Application number: PCT/US2003/032895
Authority: WO
Inventors: Deon S. Anex; Phillip H. Paul; David W. Neyer
Original assignee: Eksigent Technologies, Llc
Priority date: 2002-10-18
Filing date: 2003-10-17
Publication date: 2004-04-29
Also published as: CA2502671C; AU2003277420C1; EP1556612A2; AU2003277420B2; WO2004036041A3; EP1556612B1; AU2003277420A1; CA2502671A1

Abstract

Electrokinetic devices, including electrokinetic pumps, include a conduit and first and second electrodes adjacent to respective ends of the conduit. The electrodes together constitute a capacitive electrode pair. The electrodes can be composed of double layer capacitive materials, for example carbon-based materials, or pseudocapacitive materials. The devices can operate without producing chemical change in the electrolyte solution.

Description

ELECTROKINETIC DEVICES

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application serial No. 10/322,083, filed December 17, 2002. U.S. application serial No. 10/322,083 is a continuation-in-part of U.S. patent application No. 10/273,723, filed October 18, 2002, The entire disclosure of each of those US applications is incorporated by reference herein for all purposes.

BACKGROUND

, The invention relates to electrokinetic devices. The term electrokinetic device is used herein to denote a device which comprises

(1) first and second electrodes, and

(2) a conduit having

(a) a first end which is adjacent to (i.e. is in contact with or separated from) the first electrode, and (b) a second end which is adjacent to (i.e. is in contact with or separated from) the second electrode, whereby, when the device is filled with a suitable electrolyte solution, the application of a suitable electrical potential to the electrodes will cause electroosmotic flow of the electrolyte solution through the conduit. The net flow rate of the electrolyte solution will be the electroosmotic flow modified by any other factors, e.g. hydrostatic pressure, affecting the flow rate.

In conventional electrokinetic devices, the electrodes are simple wire or wire mesh electrodes, and the electrolyte solution undergoes chemical change at the interface between the electrolyte and the electrodes. In this specification, in the interests of brevity, the term "electrolyte" is used to denote the electrolyte itself (for example a compound such as an ionic salt) and the solvent in which the compound is dissolved; and the term "chemical change" is used to denote any chemical reaction involving the compound or the solvent or both. The reaction products produced by the chemical change of the electrolyte are undesirable, because they can be gases which must be vented and/or electrochemical products which dissolve in the electrolyte and change its composition, for example change its pH. SUMMARY OF THE INVENTION

We have discovered, in accordance with the present invention, that by using capacitive electrodes in electrokinetic devices, it is possible to convert electronic current into ionic current without chemical change of the electrolyte.

The present invention provides an electrokinetic device which comprises (1) first and second electrodes, and

(2) a conduit which comprises

(a) a first end which is adjacent to the first electrode, and

(b) a second end which is adjacent to the second electrode, whereby, when the device is filled with electrolyte, electrolyte within the conduit provides an electrical connection between the first and second electrodes; the first and second electrodes constituting a capacitive electrode pair as hereinafter defined. In many devices of the invention, the conduit contains a porous dielectric medium, abbreviated in this specification to PDM.

The first and second electrodes are defined herein as constituting a capacitive electrode pair if the device, when tested by the test routine described below, is found to have a capacitance of at least 10^"4 farads/cm², preferably at least 10^"2 farads/cm², particularly at least 1 farad per cm², based on the total area of the electroactive surfaces of both electrodes. If the device to be tested already has an electrolyte in it, the electrolyte is removed, and the device flushed, before the device is tested by the test routine.

The term "electroactive surface" is used herein to denote the surface of the electrode through which, when the device is in operation, substantial current flows to or from the adjacent end of the conduit. In all devices, if a straight line can be drawn from the end of the conduit to any part of the surface of the electrode without passing through any electrically insulating material or through the electrode itself, then that part of the electrode is part of the electroactive surface. In some devices, such straight lines can be drawn from all points on the electroactive surface. In other devices, such straight lines can be drawn from some points on the electroactive surface but not from others. In yet other devices, no such straight lines can be drawn. Those skilled in the art will have no difficulty in determining the electroactive surfaces in any particular device. The area referred to in the definition is the geometric area and does not include any surface features having a length scale less than about 0.5 mm (e.g. small pores, pits, scratches and ridges). The test routine is made up of the following steps A-L.

A. The total electroactive area (A) of both electrodes is determined by inspection and measurement.

B. The device is filled with one of the following electrolytes. i) 1 Normal aqueous potassium chloride (KC1) at pH 7. ii) 1 Normal aqueous sodium acetate. iii) 1 Normal aqueous sulfuric acid (H₂SO₄). iv) 0.5 molar lithium perchlorate. (LiClO₄) in dry propylene carbonate.

In a device having Li-intercalation pseudo capacitive electrodes, only the lithium perchlorate solution is used. In all other devices, only the aqueous solutions are used. C. A voltmeter having an input impedance of at least 10⁷ ohm is connected to the electrode leads of the device, and the voltage drop between the electrode leads (Vo) is recorded.

D. Leaving the voltmeter connected across the electrode leads, a constant current DC power supply is connected to the electrode leads. The current supplied by the power supply is equal to A x J microamps, where J is 25 microamps/cm² E. The voltage across the electrodes, immediately after the power supply has been connected, VI, is measured.

F. The power supply remains connected to the electrode leads until the first of the following two conditions (a) and (b) is satisfied: (a) the charge in coulombs acquired by the device is equal to

A x 1 coulomb per cm² (i.e. 1 coulomb per cm² of the electroactive area), and

(b) the magnitude of the difference between VI and the observed voltage across the electrodes reaches 0.5 volts.

As soon as one of these conditions is satisfied, the power supply is disconnected. G. 20 seconds after disconnecting the power supply, the voltage across the electrode leads (V2) is recorded.

H. 60 seconds after disconnecting the power supply, the power supply is reconnected to the electrode leads.

I. The voltage across the electrode leads, immediately after the power supply has been reconnected, V3, is measured, and the current through the device (I) is measured.

J. The power supply remains reconnected to the electrode leads until the first of the following two conditions (c) and (d) is satisfied:

(c) the charge in coulombs acquired by the device after the reconnection is equal to A x 1 coulomb per cm² (i.e. 1 coulomb per cm² of the electroactive area), and (d) the magnitude of the difference between N3 and the observed voltage across the electrodes reaches 0.5 volts.

As soon as one of these conditions is satisfied, the power supply is discoimected. The time, T2, from the reconnection to the disconnection is recorded.

K. 20 seconds after disconnecting the power supply, the voltage across the electrode leads (N4) is recorded.

L. The magnitude of the difference between N2 and N4 is then calculated. If it is less than 10 microvolts, the electrode pair is regarded as not capacitive. If the magnitude of the difference between N2 and N4 is at least 10 microvolts, the magnitude of the capacitance(C) of the electrode pair is calculated by taking the magnitude of the formula _

I x T2

N4-N2 After calculating the capacitance, the capacitance per cm of the electrode pair is obtained by dividing the calculated capacitance by the measured value of the electroactive area (A).

It will be noted that for all electrodes except Li-intercalation electrodes, there are three possible test routines, one for each of the aqueous electrolytes. A device is defined as containing a capacitive electrode pair if it has the defined capacitance of at least 10 ^"4 farads/cm² when measured by any one of the test routines, even if it has less than the defined capacitance when measured by one or both of the other test routines.

If the device contains more than two electrodes, each pair of electrodes should be examined in turn, without connecting the other electrode(s) to an electrical source, to determine whether it is a capacitive electrode pair as defined. If the device is in practical use, it should be disconnected from the power source, and the electrolyte removed from it, before it is examined in the test circuit.

In a second aspect, this invention provides apparatus comprising

(A) an electrokinetic device according to the first aspect of the invention, and

(B) a power source which can be connected to the first and second electrodes and winch, when it is connected to the electrodes and the device is filled with a suitable electrolyte, causes electroosmotic flow of the electrolyte within the conduit.

In a third aspect, this invention provides an electrical circuit comprising

(A) an electrokinetic device according to the first aspect of the invention,

(B) an electrolyte which fills the electrokinetic device, and (C) a power source which is connected to the first and second electrodes and which causes electroosmotic flow of the electrolyte through the conduit.

The circuits of the third aspect of the invention are preferably operated so that there is no chemical change of the electrolyte, and the invention will generally be described with reference to such operation. However, the invention includes methods in which there is acceptable chemical change of the electrolyte at one or both of the electrodes. In a fourth aspect, this invention provides a method of operating a circuit of the third aspect of the invention, the method comprising

(A) operating the circuit so that the electrolyte flows in a first direction through the conduit for a time such that there is no significant chemical change of the electrolyte, and

(B) reversing the polarity of the power supply so that the electrolyte flows in the opposite direction through the conduit for a time such that there is no significant chemical change of the electrolyte.

In a fifth aspect, this invention provides an electrode which is suitable for use in an electrokinetic device according to the first aspect of the invention and which has an inner surface, i.e. a concave surface, which is at least part of the interior surface of a spherical shell or of a cylindrical shell having a circular or elliptical cross-section, or of a partial cylindrical shell having a parabolic or hyperbolic cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is illustrated in the accompanying drawings, which are not to scale and which are schematic cross-sections of devices and systems of the invention.

Figure 1 shows a first pump having a relatively long, narrow conduit and hemispherical shell electrodes.

Figures 2-4 shows second and third pumps having relatively long, narrow conduits, and hemispherical shell or cylindrical shell electrodes, Figures 3 and 4 being alternative cross-sections on line Ifl-1N of Figure 2.

Figure 5 shows a fourth pump having a relatively long, narrow conduit and including flexible membranes such that the device can be used to pump a working fluid which is not the electrolyte. Figure 6 shows a fifth pump having a short wide conduit.

Figure 7 shows a sixth pump having a short wide conduit and including flexible membranes such that the device can be used to pump a working fluid which is not the electrolyte.

Figure 8 shows a seventh pump which has three electrodes and a short wide conduit containing two different PDM's, and which is part of a heat exchange system. DETAILED DESCRIPTION OF THE INVENTION

In the Summary of the Invention above and in the Detailed Description of the Invention, the Examples, and the claims below, and in the accompanying drawings, reference is made to particular features of the invention. It is to be understood that the disclosure of the invention in this specification includes all appropriate combinations of such particular features. For example, where a particular feature is disclosed in the context of a particular aspect or embodiment of the invention, or a particular Figure, or a particular claim, that feature can also be used, to the extent appropriate, in combination with and/or in the context of other particular aspects and embodiments of the invention, and in the invention generally. The term "comprises", and grammatical equivalents thereof, are used herein to mean that other components, ingredients, steps etc. are optionally present in addition to the component(s), ingredient(s), step(s) specifically listed after the term "comprises". The term "at least" followed by a number is used herein to denote the start of a range beginning with that number (which may be a range having an upper limit or no upper limit, depending on the variable being defined). For example "at least 1" means 1 or more than 1, and "at least 80%" means 80% or more than 80%. The term "at most" followed by a number is used herein to denote the end of a range ending with that number (which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined). For example, "at most 4" means 4 or less than 4, and "at most 40%" means 40% or less than 40 %. When, in this specification, a range is given as "(a first number) to (a second number)" or "(a first number) - (a second number)", this means a range whose lower limit is the first number and whose upper limit is the second number. Where reference is made herein to "first" and "second" components, e.g. first and second conduits, this is generally done for identification purposes; unless the context requires otherwise, the first and second components can be the same or different, and reference to a first component does not mean that a second component is necessarily present (though it may be present).

Number of Electrodes

The devices of the invention often contain only two electrodes, and the invention will generally be described by reference to such devices. However, the devices can contain three or more electrodes, for example three electrodes, one pair of which are active in one period of operation and another pair of which are active in another period of operation. For example, the device can contain three or more electrodes with PDM's having zeta potentials of opposite signs alternating between the electrodes. The electrodes in a device can be the same or different. When one of the electrodes in a capacitive electrode pair is composed of non- capacitive material, there is chemical change of the electrolyte at the non-capacitive electrode but not at the capacitive electrode. Materials for Capacitive Electrodes

At least one of the electrodes in a capacitive electrode pair must be composed of a capacitive material, i.e. a material which exhibits double-layer capacitance or pseudo- capacitance. Preferably each of the electrodes comprises a capacitive material. Preferably each of the electrodes in a capacitive electrode pair contributes at least 30% of the capacitance between them.

The capacitance of conventional double-layer capacitive materials results from the ability to store electrical energy in an electrochemical double layer at the electrode-electrolyte interface. Pseudocapacitive materials are materials which can also store electrical energy, but through a different mechanism. An electrode or pair of electrodes can comprise both double- layer materials and pseudocapacitive materials.

A preferred double-layer capacitive material for the electrodes is carbon having a very large ratio of microscopic surface area to geometric surface area. Carbon paper impregnated with carbon aerogel is particularly preferred. Other carbon materials that can be used include carbon aerogel, e.g. monolithic carbon aerogel foam, woven carbon cloth, carbon fibers (e.g. pyrolized polyacrylonitrile fibers and pyrolized cellulose fibers), carbon nanotubes, carbon black, a polymer having carbon particles dispersed therein, carbon nanotubes, and frits of carbon particles.

It is also possible to use other conductive materials having a high microscopic surface area, for example sintered metals, nanoporous metals, for example nanaporous gold, perforated plates, porous frits, porous membranes, deLevi brushes, and metals that have been treated to increase their surface area, for example by surface roughening, surface etching or platinization.

Some pseudocapacitive materials are metal oxides which are relatively insoluble in water and many other solvents, and in which the metal can adopt different oxidation states, for example cobalt, manganese, iridium, vanadium and ruthenium oxides. In operation of electrodes comprising such materials, a redox reaction takes place in the solid phase of the electrode, with uptake or release of a specific ion, eg. H⁺ for ruthenium oxide. Other pseudocapacitive materials are solid materials into which a soluble ion, e.g. Li⁺, can be inserted ("intercalation") or from which a soluble ion can be dispensed ("de-intercalation"), for example manganese nitrides, titanium molybdenum disulfides, carbon, and conducting polymers and such as polyaniline, polythiophene and polyacetylene. Some pseudocapacitive materials react with water, and should, therefore, be used with non-aqueous electrolytes. In operation of electrodes comprising such materials, a redox reaction takes place in the solid phase of the electrolyte, and results in release or uptake of ions. When the electrode is composed of a pseudocapacitive material, care is needed a) to correlate the electrolyte and the electrode, in order to provide the ions needed for the particular pseudocapacitive material and to prevent unwanted chemical reactions, and b) to preserve a balance between increasing ionic concentration (to support the reversible electrode reactions) and decreasing ionic concentration (to draw less current to increase the run time). The electrode material is preferably insoluble in the electrolyte and has an electrical conductivity substantially greater than, preferably at least 100 times, the conductivity of the electrolyte. For example, the conductivity of a carbon aerogel foam is about 100 mho/cm and a conductivity of a typical electrolyte, 5 mM NaCl, is about 0.5 x 10^"3 mho/cm.

The electrodes are preferably washed, and, if necessary, leached in the electrolyte before use. Porous electrodes are preferably degassed after such treatment.

In some devices, the electrolyte must flow through the electrodes when the devices are operating. In those devices, preferably at least 25%, more preferably at least 50%, of the geometric area of the electrode is open and/or the flow permeability of the electrode material is at least 10 times, particularly at least 100 times, the flow permeability of the PDM in the conduit. Such electrodes can also be used when the electrolyte does not need to flow through the electrode.

Often, so that the electrode has sufficient strength, it has a thickness of at least 0.5 mm, preferably at least 1 mm, particularly at least 2 mm.

Shape. Size and Positioning of Electrodes, and Current Flux on Electrodes The capacitance of an electrode depends on its composition and on the size and shape of its active electrochemical surface. When the conduit is relatively short and wide, for example has an equivalent diameter which is 1 to 30 times, e.g. 5 to 20 times, its length, the area of the active electrochemical surface of the electrode is preferably 0.6 to 1.1 times, e.g. 0.8 to 1.0 times, the cross-sectional area of the conduit. The term "equivalent diameter" is used herein to mean the diameter of a circle having the same area as the cross-sectional area of the conduit. When the conduit is relatively long and narrow, for example has an equivalent diameter which is 0.01 to 0.3 times, e.g. 0.05 to 0.1 times, its length, the area of the active electrochemical surface of the electrode is preferably at least 2 times, particularly at least 10 times, especially at least 100 times, the cross-sectional area of the conduit.

During operation of the device, the rate at which charge is transferred to a particular area on the electrode is proportional to the current flux at that area, and as soon as any area of the electrode reaches the liquid electrolysis potential, chemical change of the electrolyte will commence at that area. [The electrolysis potential is generally less than a few volts; for example for water it is about 1.2V, and for propylene carbonate it is about 3.4V.] As a result, the run time of the device (i.e. the time for which the device will operate without chemical change of the electrolyte) depends on the highest current flux at any point on the electrode. Therefore, the smaller the maximum current flux on the electrode, the longer the run time. Furthermore, the smaller the variation in current flux over the electrode, the greater the ;total amount of charge that can be transferred to an electrode having a particular geometric size, order to reduce the variation in current flux, the electrodes are preferably shaped and positioned so that the maximum current flux at any point on the electroactive surface of the electrode is at most 2 times, preferably at most 1.2 times, the minimum current flux at any point on the active surface. Those skilled in the art will have no difficulty in calculating the current flux at any point on the electroactive surface through the application of Laplace's equation. In some devices, the conduit is a short tube which is filled by a transverse disc of

PDM. In such devices, the electrodes are preferably substantially planar discs which lie on either side of the conduit and are parallel to each other and to the disc of PDM. The electrodes preferably cover at least 60%, particularly at least 80%, of the disc of PDM. The current flux on the electrodes in such devices can be relatively high, for example at least 0.05, e.g. 0.2 to 1, milliamps per cm².

In other devices, the conduit is a relatively long narrow tube, for example of round or rectangular (including square) cross-section, filled by PDM. The current flux on the electrodes in such devices can be relatively low, for example less than 0.05 milliamps per 9 9 0 cm , less than 20 microamps per cm , or less than 2 microamps per cm , e.g. 1 to 20 microamps per cm². In such devices, the electrode can for example be a) an annular member placed concentrically around the end of a conduit of circular cross section or around the end of a via of circular cross section through which the current flows after leaving the conduit; b) a pair of strips placed on either side of a via in the form of a slot through which current flows after leaving the conduit; c) at least part of the interior concave surface of a spherical shell positioned so that its center is at the end of a conduit of circular cross section or at the end of a via of circular cross section through which the current flows after leaving the conduit; the inner diameter of the spherical shell can for example be 4 to 6 times, e.g. about 5 times, the diameter of the conduit; or d) at least part of the interior concave surface of a cylindrical shell positioned so that its axis is at the end of a conduit of generally rectangular cross-section or at the end of a via of generally rectangular cross-section through which the current flows after leaving the conduit, and so that its axis coincides with the long axis of that cross- section; the inner diameter of the cylindrical shell can for example be 4 to 6 times, e.g. about 5 times, the short axis of the rectangular cross-section; the ends of the cylindrical shell can be open or each end can be closed by at least part of the inner concave surface of a hemispherical shell which extends away from the conduit and is positioned so that its center is at one end of the rectangular cross-section of the conduit or via.

For further information about electrode shapes which will produce the desired substantially uniform field, reference may be made for example to Classical Electrodynamics (1975) by J.D. Jackson, and Complex Variables and Applications (1990) by R.N. Churchill and J.W. Brown.

Planar electrodes can be divided from sheet materials, for example sheet materials obtained by impregnating carbon aerogel into a carbon-fiber paper or by coating ruthenium oxide onto a metal sheet, screen or porous metal frit. Three-dimensional electrodes can be directly cast into the desired shape or machined out of a block, e.g. a carbon aerogel foam.

The leads to the electrodes are preferably placed and/or insulated so that they do not influence the electrical field in the electrolyte. Run Times

In some embodiments, the run time is relatively short, for example 1 to 60 seconds. In other embodiments, the time is relatively long, for example at least 24 hours, e.g. 24 to 240 hours, or even more, for example at least 144 hours, e.g. 144 to 480 hours. Electrolytes

The electrolytes used in the present invention are often aqueous, but can be non- aqueous. One suitable non-aqueous electrolyte is a solution of tetra (alkyl) ammonium tetrafluoroborate in propylene carbonate. The ionic strength of the electrolyte should be sufficiently high that the reduction in the ionic strength which results from operation of the device does not cause the ionic strength to fall below a preferred minimum.

The lower the ratio of the counter-ion mobility (n_co) to the electroosmotic mobility (ii_eo), the greater the flowrate and/or run time can be. This ratio is preferably less than 5, particularly less than 1, e.g. 0.3 to 1.

The ionic species in the electrolyte are preferably univalent. Flow Rates

The rate at which the electrolyte flows through the conduit may be constant or variable. In some embodiments, in which the conduit is relatively wide and short, the rate is relatively high, for example greater than 1 mL/min. For example, a large diameter flat pump of the kind shown in Figure 6 running at about 3N might have an open-load flow rate of about 1.2 mL/min.cm², and, therefore, a flow rate of about 10 mL per minute if the area is about 8.8 cm². In other embodiments, in which the conduit is relatively narrow and long, the rate is relatively low, for example 5 or 25 nL/min to 10 μl/min.

Power Supplies

The power applied to the electrodes in the devices can be controlled with respect to voltage or current, or at some times one and at other times the other. The flow rate depends upon the potential drop over the conduit, which will decrease as the capacitive electrodes are charged, particularly when the applied potential is comparable to the electrolysis potential. If desired, the power applied to the electrodes can be increased to compensate for this decrease, for example by using a constant current source, or by monitoring the potential drop across the conduit by means of sensors placed near the ends of the PDM in the conduit (but preferably outside the direct field path between the electrode and the PDM), and adjusting the power source appropriately. The power can alternatively or additionally be adjusted in response to temperature or another variable, for example to produce a desired heat transfer rate, temperature, flow rate, pressure, or actuator displacement, for example in response to a signal from a measurement device, e.g. through a feedback loop. When the device is operated in the cyclic mode described below (in which the polarity of the power supply is changed from time to time), the cycle duration and the power supply may be controlled so that the total charge supplied in each cycle is the same, in order to ensure that the electrodes do not acquire a time-average positive or negative potential. When using a constant current power supply, the product of current and duration of each of the cycles is preferably the same. When using a constant voltage supply, the time-integrated current of each of the cycles is preferably the same, h some cases, the devices are powered by batteries, for example one or more 3 volt lithium batteries, optionally with an up-converter to obtain higher voltages, e.g. 18-30 volts.

Voltage Drops

The greater the proportion of the applied voltage which is dropped across the conduit, the lower the applied voltage needed to obtain a given flow rate. Therefore, the device is preferably designed so that the voltage drop across the conduit is at least 10%, more preferably at least 50%, particularly at least 85%, of the voltage drop between the electrodes.

The device can include sensors for measuring the voltage drop across the conduit, and control means connected to the power supply to control the voltage supplied to the electrodes, in order to ensure that the electrolyte flows at a desired, e.g. constant, rate.

Conduits and PDMs

The conduit between the electrodes can be of any shape. In some embodiments, the conduit is relatively long and narrow. In other embodiments, it is relatively short and wide. The conduit preferably contains a PDM, and the invention will generally be described with reference to conduits containing PDMs. The PDM can extend out from the conduit, be flush with the end of the conduit, or terminate within the conduit. However, it is also possible for the conduit to be an "open" conduit, i.e. a conduit which does not contain any packing material, or to be composed of a plurality of fine parallel channels. There may be two or more PDM's within a conduit. In one embodiment, the conduit is divided into two sections, e.g. two relatively long and narrow sections, containing PDM's having different zeta potentials (and preferably a zeta potentials of opposite sign), each of the two sections having one end adjacent to an electrode and an opposite end communicating with a central chamber which does not contain an electrode. Application of a suitable power source to the electrodes of such a device can cause the electrokinetic fluid in both sections to be pumped towards, or away from, the central chamber.

Suitable PDM's are well-known to those skilled in the art, and may be organic, e.g. a porous polymer membrane or a phase-separated organic material, or inorganic, e.g. a porous sintered ceramic, a porous inorganic oxide (e.g. silica, alumina or titania) membrane or aerogel, packed silica beads, micromachined, stamped or embossed arrays, phase- separated porous glasses (e.g. Vycor), and phase-separated ceramics. Preferably the pores in the PDM have a diameter of 50 to 500 nm, for example about 200 nm, so that the conduit has a high stall pressure (for which small pores are desirable) but does not have substantial double-layer overlap (which can result if the pores are too small). Other preferred features for the PDM are a high zeta potential and a narrow pore size distribution. Particular examples of PDM's are the high purity alumina membranes sold under the tradename Anopore, and porous polyvinylidene fluoride (PVDF) membranes, for example those sold under the tradename Durapore, which may have a pore size of 100-200 nanometers, and which may be modified to be hydrophilic and have a zeta potential of- 30 to - 60 millivolts,. The ionic strength of the electrolyte is preferably sufficient to provide a Debye length that is less than 0.1 times the diameter of the pores in the PDM. The mobilities of the ions in the electrolyte are preferably less than 20 times, more preferably less than 3 times, and most preferably less than 1 time, the electroosmotic mobility of the PDM.

The PDM may have either a positive or a negative zeta potential. Electrolytes containing polyvalent ions having a charge of opposite sign to the zeta potential of the PDM are preferably avoided. For example, phosphates, borates and citrates are preferably avoided when the PDM has a positive zeta potential, and barium and calcium ions are preferably avoided when the PDM has a negative zeta potential.

Spacers. Supports, Electrical Leads, and Assembly

The devices can contain one or more electrolyte-permeable internal spacers to separate components of the device. Such spacers are particularly desirable in flat, large diameter devices of the kind shown in Figure 6, for example to reduce undesirable effects resulting from irregularities in the electrode; such spacers may for example have 5-10 micron pores, a formation factor of 1.7, and a thickness of 50 micron. The electrical and flow resistances of such internal spacers are preferably much smaller than the electrical and flow resistances of the conduit. The spacers are generally composed of a large pore dielectric material, e.g. foamed polypropylene or acrylic polymer.

The devices can also contain one or more external supports to prevent the device from flexing during use and generally to maintain the components in a desired configuration. In operation, power must be supplied to the electrodes through leads, and these leads are often integral parts of the device. The leads preferably do not contact the electrolyte, and if they do, they are preferably composed of platinum or another electrochemically stable metal.

The components of the device can be secured together in any way. For example, they can be laminated together to form a chip-like assembly, e.g. as described in copending, commonly assigned US Application Serial No. 10/198,223 filed July 17, 2002, by Paul, Neyer and Rehm (Docket 14138).

Types of Device, and Uses of the Devices

The electrokinetic devices of the present invention can be of any kind. Preferred devices are electrokinetic pumps, and the invention will generally be described with reference to electrokinetic pumps. The pump can be a direct pump, in which the only liquid is the electrolyte. A direct pump can for example simply dispense the electrolyte or pump the electrolyte along a flow path in which the electrolyte performs a useful function, e.g. heat exchange. Alternatively, the pump can be an indirect pump, in which pumping of the electrolyte causes the flow of a different fluid in a part of the device which is not subject to the electric field of the electrodes. The different fluid is referred to herein as a "working fluid".

The working fluid need not be, and generally will not be, an electrokinetic liquid (i.e. a liquid which will support electroosmotic flow). For example, the working fluid can be a liquid wliich cannot or should not flow through the conduit, e.g. a hydrocarbon fuel, a propellant, a pure solvent, a liquid of high salt content, a liquid which does not support a zeta potential, a liquid having particles dispersed therein, or a liquid which contains a compound which cannot or should not flow through the pump, e.g. a protein or a drug.

In one form of indirect pump, the device includes a second conduit which is not subject to the electrical field of the electrodes and which has an open or openable end. In use, the second conduit is filled with electrolyte, the open end of the second conduit is placed in contact with a working fluid, and the device is operated so that a sample of the working fluid is drawn into the second conduit. The sample, e.g. a sample of a subcutaneous fluid, can be examined in the second conduit, or after it has been expelled from the second conduit by reversing the flow direction of the electrolyte. One device of this kind is the device of the type described above containing a first conduit having two sections which communicate with a central chamber and are filled with PDM's are different zeta potentials. The second conduit is attached to the central chamber of such a device. The central chamber can be made much smaller than a chamber containing an electrode, so that the device has a very rapid response. hi another form of indirect pump, the pumping of the electrolyte changes the volume of a chamber containing the electrolyte, and thus changes the volume of an adjacent chamber so that a working fluid is drawn into or expelled from the adjacent chamber. For example, the chambers can share an intermediate deformable member which changes shape as a result of flexure (e.g. a bellows) and/or stretching (e.g. a flexible diaphragm) and/or which comprises a piston/cylinder combination. The intermediate member can for example be composed of a multilayer polymeric film, which may be metallized. The chamber which contains the electrolyte, and whose volume changes, can be a chamber containing an electrode or a separate chamber, for example the central chamber in a device as described above in which two sections of the conduit communicate with a central chamber.

In some cases, the adjacent chamber comprises a port to which a delivery device, e.g. a syringe, can be fitted. The delivery device can be loaded with a liquid to be delivered before or after it is fitted to the port. In other cases, the adjacent chamber comprises a receptacle into'which can be placed a separable capsule containing a liquid which is to be dispensed through a delivery device e.g. a syringe, connected permanently or a movably to the chamber. In both cases, they electrolyte applies pressure to a component of the delivery device, either directly or indirectly through a working fluid. When a separable capsule is used, it may initially be sealed and be opened before, or after, or while, the capsule is placed in the receptacle. These devices are useful for example when the working fluid must be stored under controlled conditions and/or poses dangers to those handling it, e.g. is a biohazard, a toxic hazard or a radioisotope. After the working fluid has been dispensed, the capsule can be removed so that the device can be reused. Alternatively, and commonly in medical uses, the device is a single use device which is discarded after use.

In some indirect pumps, each of the electrodes is in a chamber which shares an intermediate deformable member with an adjacent chamber for the working fluid. The adjacent chambers optionally communicate with each other, for example through a loop in which the working fluid performs some useful function, e.g. is used for heat exchange, or picks up a sample for examination.

The devices can be designed so that they can be implanted in a human or animal body, for example to deliver a drug at a desired continuous rate. . When the devices are substantially free of metal, they do not cause interference in the operation of systems making use of high electromagnetic fields, for example medical imaging systems.

Particular uses of the devices of the invention include drug delivery, medical diagnostics, sample extraction, fuel cells, actuators, and liquid dispensers. Two or more pumps can be connected in parallel for increased flow rates, or in series for increased pressures, e.g. as described in copending commonly assigned US Application Number 10/066,528 filed January 31, 2002, by Rakestraw et al. (Docket 14131). Because operation of the device does not produce gases which must be vented or result in changes in the composition of the electrolyte, the device can be part of a sealed system. The device can also be part of a system in which the electroosmotic flow causes a liquid to be dispensed from the system in a controlled fashion or to be withdrawn in a controlled fashion from a liquid source.

In some embodiments of the invention, the device is operated in a cyclic mode. In the cyclic mode, the device is first operated for a first period of time during which the electrolyte flows in one direction through the conduit; and thereafter the polarity of the power supply is reversed and the device is operated for a second period of time during which the electrolyte flows in the opposite direction. Each period of time is sufficiently short that there is no substantial chemical change of the electrolyte. The duration of each period can be quite short, e.g. 4 to 10 seconds, or much longer, e.g. 5 to 30 minutes, or 10 to 40 hours, depending upon the device. In this way, if necessary with the aid of check valves, the system can be operated continuously or intermittently. For example, a system containing two check valves can give unidirectional flow, but only during alternate cycles; whereas a system containing four check valves can give unidirectional flow during both cycles.

Particular systems employing electrokinetic pumps of the invention include for example heat transfer systems, liquid-dispensing systems, liquid- withdrawing systems, drug delivery systems, medical monitoring systems, fuel cells, and actuators. Some of the systems employ direct pumping, e.g. heat transfer systems in which the electrolyte is a heat transfer fluid. Other systems employ indirect pumping, e.g. a medical monitoring system that handles whole blood or a drug delivery system which handles a protein therapy drug. In one example, 10-80 microliters of a fluid are dispensed at regular intervals, preferably without contact between the nozzle of the dispenser and the receptacle. In another example, the system dispenses a liquid in a medical monitor at a rate of 100 nL/min, using a pump of the type shown in Figure 5 having a cross-sectional area of about 0.4 mm². At this rate, the pump can run for about seven days before chemical change of the electrolyte commences.

If a device of the invention, after it has been in use, is disconnected from the power supply and the leads to the electrodes are connected to each other or to other elements forming a circuit (or if the polarity of the power source is reversed), the charges stored in the electrodes will discharge, thus causing (or assisting) electroosmotic flow in the reverse direction.

The Drawings

In Figure 1, conduit 1 contains a PDM 11 whose ends extend out of the conduit into vias 12a and 12b, each of which has a circular cross-section. The vias 12a and 12b communicate with reservoirs 2a and 2b having ports 21a and 21b respectively. Hemispherical porous electrodes 3a and 3b are centered on the ends of vias 12a and 12b, respectively, and are powered by power supply 6. Sensor electrodes 51a and 51b are placed in the vias 12a and 12b respectively so that they are outside the direct field between the PDM and the electrodes. The sensors communicate with device 5, which in turn communicates with power supply 6 and, if desired, changes the power output, for example to maintain a desired potential difference across, and, therefore, flowrate through, the PDM 11.

In Figures 2-4, conduit 1 contains a PDM 11 whose ends extend out of the conduit into reservoirs 2a and 2b having ports 21a and 21b respectively. Porous electrodes 3a and 3b are centered on the extending ends of PDM 11, and are hemispherical shells, as shown in Figure 3, when the cross-section of the conduit is circular or square, and hemi-cylindrical shells, as shown in Figure 4, when the cross-section of the conduit is rectangular. Leads 61 and 62 connect a power supply 6 to the electrodes 3a and 3b.

In Figure 5, conduit 1 contains a PDM 11 whose ends extend out of the conduit into vias 12a and 12b, each of which has a circular cross-section. The vias 12a and 12b communicate with reservoirs 2a and 2b. Annular porous electrodes 3 a and 3b are centered on the ends of vias 12a and 12b, respectively, and are powered by power supply 6. Chambers 2a i and 2b include flexible membranes 7a and 7b, which are also part of adjacent chambers 81 and 82 respectively. Chamber 81 has port 21a and chamber 82 has port 21b. In use, electroosmotic flow of an electrolyte between the chambers 2a and 2b changes the volumes of the chambers 2a and 2b and, therefore, the volumes of the chambers 81 and 82. In this way, a working fluid (or air or other fluid) can be drawn into the chamber 81 and the same or a different fluid can be expelled from the chamber 82, or vice versa.

In Figure 6, conduit 1, which is a short tube of circular cross-section, contains a discshaped PDM 11 and porous capacitive electrodes 3a and 3b having inner surfaces contacting the PDM and outer surfaces communicating with chambers 2a and 2b respectively. Housing 110 adds structural stability.

In Figure 7, conduit 1, which is a short tube of circular cross-section, contains a PDM 11 supported by porous spacers 12a and 12b. Porous capacitive electrodes 3a and 3b contact the support members, and have outer surfaces communicating with chambers 2a and 2b respectively. Chambers 2a and 2b include flexible membranes 7a and 7b, which are also part of adjacent chambers 81 and 82 respectively. Chamber 81 has ports 811a and 81 lb fitted with check valves 812a and 812b to control the flow of liquid through chamber 81. Chamber 82 has ports 821a and 821b fitted with check valves 821a and 821b to control the flow of liquid through chamber 82. Leads 61 and 62 connect a power supply 6 to the electrodes 3a and 3b. In use, electroosmotic flow of an electrolyte between the chambers 2a and 2b changes the volumes of the chambers 2a and 2b and, therefore, the volume or the chambers 81 and 82, thus making it possible, with appropriate operation of the check valves, to expel a working fluid from one chamber and draw the same or a different working fluid into the other chamber. By periodically changing the direction of the electroosmotic flow (by reversing the polarity of the power supply) and operation of the check valves, working fluid flows from one or other of the chambers 81 and 82, and if the working fluids in the chambers are the same, their outputs can be combined to provide a continuous flow. hi Figure 8, conduit 1, which is a tube of circular cross-section, contains successive layers which are porous capacitive electrode 3 a, porous support member 12b, first PDM 11a, porous spacer 12b, porous capacitive electrode 3b, porous spacer 12c, second PDM lib, porous spacer 12d, and porous capacitive electrode 3c. The first and second PDM's have opposite zeta potentials. Porous capacitive electrodes 3a and 3c have outer surfaces communicating with chambers 2a and 2b respectively. Leads 61, 62 and 63 connect a power supply 66 to the electrodes 3a, 3b and 3c. The power supply 66 drives electrode 3b with respect to electrodes 3a and 3c that are commonly connected. Ports 12a and 12b of chambers 2a and 2b communicate with a heat exchange loop comprising secondary heat exchangers 506a and 506b in which heat is radiated and primary heat exchanger 508 in which heat is absorbed, hi use, the power supply cyclically reverses the potential between electrode 3b and commonly connected electrodes 3a and 3c, and causes electroosmotic flow in one or other direction around the loop for a time less than that which causes the chemical change of the electrolyte.

Examples

Example 1 A pump as illustrated in Figure 5 was constructed. The vias were about 4 mm in diameter and were separated by about 30 mm. The PDM was a porous PNDF membrane, about 84 micron thick and having a size of about 5 x 30 mm. The PNDF had been modified to be hydrophilic and to have a zeta potential of- 50 millivolts. The annular electrodes have a thickness of about 2 mm, an internal diameter of 10 mm and an outer diameter of about 14 mm. The electrodes were divided from sheets of porous carbon aerogel which had been washed and leached in deionized water. The flexible members. 81 and 82 were about 20 mm in diameter, and were thermoformed from a multilayer polymeric sheet about 0.075 mm (3 mil) thick. The layers in the sheet included a scratch-resistant layer, two gas diffusion barriers, a liquid diffusion barrier and a thermal adhesion layer. The flexible members could accommodate a change of about 2 mL in the volume of the chambers 2a and 2b. The device contained about 3 mL of the electrolyte, which was TRIS/acetate whose concentration was initially about 5 mM and dropped to 2.5 mM after the pump had been in operation for a week. Using a power supply which resulted in an initial current of about 1.6 microamp, and a calculated maximum current flux on the electrodes of about 2.5 microamps per cm , the pump was used to deliver a working fluid at a flow rate of about 100 nL/min for about one week.

Example 2

A pump substantially as illustrated in Figures 2 and 3 was constructed, except that annular electrodes, centered on the respective ends of the conduit, were used instead of the hemispherical electrodes illustrate. The electrodes were punched from a sheet of aerogel foam impregnated carbon fiber, were 0.76 mm (0.03 in.) thick., and had an inner diameter of about 2 mm and an outer diameter of about 4 mm. The conduit was a silica capillary tube which protruded from the support housing about 0.25 mm into the chambers 2a and 2b and which had a length of 10 mm, an inner diameter of 0.15 mm and an outer diameter of 0.36 mm. The PDM was 0.7 micron silica particles packed into the conduit.

Example 3 A pump as illustrated in Figure 6 was constructed. The PDM was a 25 mm diameter

"Anopore" membrane. The electrodes were 19 mm in diameter and were carbon paper impregnated with carbon aerogel. The pump was used to pump a 1 millimolar sodium acetate buffer solution having a pH of about 5. At a driving current of 40 milliamps, the flowrate was up to 170 microliters per second. Example 4.

A pump as illustrated in Figure 6 was constructed. The PDM was a 13 mm diameter Durapore-Z membrane. The electrodes were 11 mm in diameter and were carbon paper impregnated with carbon aerogel. The diameter of the conduit was 8 mm. The pump was used to pump 0.5 millimolar lithium chloride solution using a power supply which delivered a square wave alternating current of +/- 0.5 milliamps with a 10 second period. The solution was pumped at a rate of 0.8 microliters per second, first in one direction for 10 seconds and then in the other direction for 10 seconds. The pump was operated for 35 hours without degradation of the solution.

Example 5 The pump used in Example 4 was connected to a power supply which delivered 0.2 milliamps for 9.5 seconds and then -3.8 milliamps for 0.5 seconds. When the current was 0.2 milliamps, the liquid was pumped slowly in one direction. When the current was -3.8 milliamps, the liquid was pumped in the other direction, delivering a total of 3 microliters.

Example 6 A 1 volt power supply was connected to the pump used in Example 4 to charge the double layer capacitance of the electrodes. The power supply was then disconnected and the leads to the electrodes where shorted together. This resulted in electroosmotic flow of the liquid through the device.

Example 7 A pump as illustrated in Figure 6 was constructed. The PDM was an organic amine- derivatized membrane. The electrodes were carbon mesh. In separate operations, the pump was used to pump a 0.5 millimolar lithium chloride solution, 34 millimolar acetic acid, and 34 millimolar carbonic acid.

Claims

1. An electrokinetic device which comprises

(1) first and second electrodes, and

(2) a conduit which comprises (a) a first end which is adjacent to the first electrode, and

(b) a second end which is adjacent to the second electrode, whereby, when the device is filled with electrolyte, electrolyte within the conduit provides an electrical connection between the first and second electrodes; the first and second electrodes constituting a capacitive electrode pair as hereinbefore defined.

2. A device according to claim 1 wherein the capacitive electrode pair has a capacitance of at least 10^"2 farads/cm².

3. A device according to claim 2 wherein the capacitive electrode pair has a capacitance of at least 1 farads/cm .

4. A device according to any one of the preceding claims wherein at least one of the first and second electrodes has an electroactive surface every point on which is separated from the adjacent end of the conduit by a distance which is not more than 1.2 times the minimum distance between any point on the electroactive surface and the adjacent end of the conduit. 5. A device according to any one of the preceding claims wherein each of the first and second electrodes has an electroactive surface which is annular, or is at least part of the interior concave surface of a spherical shell, or is at least part of the interior concave surface of a cylindrical shell having a circular or elliptical cross-section.

6. A device according to any one of the preceding claims wherein each of the first and second electrodes is composed of a material selected from the group consisting of carbon paper impregnated with carbon aerogel, woven carbon cloth, monolithic carbon foam, a polymer having carbon particles dispersed therein, carbon nanotubes, a frit of carbon particles, carbon aerogel, a deLevie brush, and nanoporous gold.

7. A device according to any one of claims 1 to 5 wherein each of the first and second electrodes is a pseudocapacitive electrode.

8. A device according to any one of preceding claims wherein each of the first and second electrodes contributes at least 30% of the_. capacitance between them. 9. A device according to any one of the preceding claims wherein the first and second electrodes are the only electrodes in the device and are substantially identical to each other.

10. The device according to any one of the preceding claims wherein the conduit contains a porous dielectric material.

11. A device according to any one of the preceding claims which comprises sensors for measuring the voltage drop across the conduit.

12. A device according to any one of the preceding claims which is an electrokinetic pump.

13. A device according to claim 12 which comprises a second conduit which has an open or openable end, whereby the device, when in use, can be operated to draw a working fluid into the second conduit through the open end of the second conduit, or to dispense an electrolyte or a working fluid through the open end of the second conduit. i4. A device according to claim 12 or 13 which comprises a chamber which comprises a deformable barrier, and which, when the devices in use, contains an electrolyte, whereby, when the device is in use, electroosmotic flow of an electrolyte causes deformation of the deformable barrier and dispensing of a working fluid.

15. A device according to claim 14 wherein the deformable barrier comprises a piston and a cylinder around the piston.

16. A device according to any one of the preceding claims which comprises

(3) a first enclosure which (i) surrounds the first electrode and the first end of the conduit and (ii) comprises a first deformable barrier,

(4) a second enclosure which (i) surrounds the second electrode and the second end of the conduit and (ii) comprises a second deformable barrier,

(5) a third enclosure, one wall of which is the first deformable barrier, and

(6) a fourth enclosure, one wall of which is the second deformable barrier, whereby, when the first and second enclosure and the conduit contain an electrolyte, and there is electroosmotic flow of the electrolyte between the first and second enclosures, the volumes of the third and fourth enclosures change.

17. Apparatus comprising (A) an electrokinetic device as claimed in any one of claims 1 to 16, and

(B) a power source which can be connected to the first and second electrodes and which, when it is connected to the electrodes and the device is filled with a suitable electrolyte, causes electroosmotic flow of the electrolyte through the conduit.

18. An electrical circuit which comprises (A) an electrokinetic device as claimed in any one of claims 1 to 16,

(B) an electrolyte which fills the electrokinetic device, and

(C) a power source which is connected to the first and second electrodes and which causes electroosmotic flow of the electrolyte through the conduit.

19. A circuit according to claim 18 wherein the current flux at all points on the electroactive surface of each of the first and second electrodes is less than 20 microamps per cm².

20. A circuit according to claim 18 or 19 wherein, after the power source has been connected to the electrodes, the device will operate for a period of at least one day without significant chemical change of the electrolyte.

21. A circuit according to claim 20 wherein, after the power source has been connected to the electrodes, the device will operate for a period of at least six days without significant chemical change of the electrolyte.

22. A circuit according to any one of claims 18 to 21 wherein the voltage drop across the conduit is at least 85% of the voltage drop between the electrodes.

23. A method of operating a circuit as claimed in any one of claims 18 to 22 which comprises

(A) operating the circuit so that the electrolyte flows in a first direction through the conduit for a time such that there is no significant chemical change of the electrolyte, and (B) reversing the polarity of the power supply so that the electrolyte flows in the opposite direction through the conduit for a time such that there is no significant chemical change of the electrolyte.

24. An electrode which is suitable for use in an electrokinetic device as claimed in any one of claims 1 to 16 and which has an inner surface which is at least part of the interior surface of a spherical shell or of a. cylindrical shell having a circular or elliptical cross- section, or of a partial cylindrical shell having a parabolic or hyperbolic cross-section.

25. An electrode according to claim 24 which is composed of a material selected from the group consisting of carbon paper impregnated with carbon aerogel, woven carbon cloth, monolithic carbon foam, a polymer having carbon particles dispersed therein, carbon nanotubes, a frit of carbon particles, carbon aerogel, a deLevie brush, and nanoporous gold.