Note: Descriptions are shown in the official language in which they were submitted.
CA 02502671 2010-09-07
ELECTROKINETIC DEVICES
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 that by using capacitive electrodes in electrokinetic
devices, it is
possible to convert electronic current into ionic current without chemical
change of the
electrolyte.
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In accordance with one illustrative embodiment, there is provided an
electrokinetic
device. The device includes: first and second electrodes; a conduit which
comprises a first end
which is adjacent to the first electrode, and a second end which is adjacent
to the second
electrode; and a porous dielectric material in the conduit between the first
and second
electrodes. 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 are comprised of a material having a capacitance of at least 10"4
farads/cm2.
The first and second electrodes may have a capacitance of at least 10"2
farads/cm2.
The first and second electrodes may have a capacitance of at least 1
farad/cm2.
At least one of the first and second electrodes may have an electroactive
surface every
point on which may be separated from the adjacent end of the conduit by a
distance which
may be not more than 1.2 times the minimum distance between any point on the
electroactive
surface and the adjacent end of the conduit.
Each of the first and second electrodes may have an electroactive surface
which may
be annular, or may be at least part of the interior concave surface of a
spherical shell, or may
be at least part of the interior concave surface of a cylindrical shell having
a circular or
elliptical cross-section.
Each of the first and second electrodes may include 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, or nanoporous gold.
Each of the first and second electrodes may contribute at least 30% of the
capacitance
between them.
The first and second electrodes may be the only electrodes in the device and
may be
substantially identical to each other.
The device may include sensors for measuring the voltage drop across the
conduit.
The device may be an electrokinetic pump for pumping a working fluid that is
not the
electrolyte.
The device may include a second conduit which has an open or openable end,
whereby
the device, when in use, may be operated to draw the working fluid into the
second conduit
through the open end of the second conduit, or to dispense the electrolyte or
the working fluid
through the open end of the second conduit.
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The device may include a chamber which includes a deformable barrier, and
which,
when the device is in use, may contain the electrolyte, whereby electroosmotic
flow of the
electrolyte may cause deformation of the deformable barrier and dispensing of
the working
fluid.
The deformable barrier may include a piston and a cylinder around the piston.
In accordance with another illustrative embodiment, there is provided an
apparatus
including: the device; and 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 the
electrolyte, causes electroosmotic flow of the electrolyte through the
conduit.
In accordance with another illustrative embodiment, there is provided an
electrical
circuit which includes: the device, wherein each of the first and second
electrodes has a
respective electroactive surface; the electrolyte which fills the
electrokinetic device; and a
power source which is connected to the first and second electrodes and which
causes
electroosmotic flow of the electrolyte through the conduit.
The current flux at all points on the respective electroactive surfaces of the
first and
second electrodes may be less than 20 microamps per cm2.
After the power source has been connected to the electrodes, a maximum of the
current flux at all points on the respective electroactive surfaces may be
sufficiently low such
that the device will operate for a period of at least one day without
significant chemical
change of the electrolyte.
After the power source has been connected to the electrodes, a maximum of the
current flux at all points on the respective electroactive surfaces may be
sufficiently low such
that the device will operate for a period of at least six days without
significant chemical
change of the electrolyte.
The voltage drop across the conduit may be at least 85% of the voltage drop
between
the electrodes.
In accordance with another illustrative embodiment, there is provided a method
of
operating the circuit. The method involves 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 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.
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In accordance with another illustrative embodiment, there is provided 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, 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.
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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 (H2S04).
iv) 0.5 molar lithium perchlorate. (LiC1O4) 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
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E. The voltage across the electrodes, immediately after the power supply has
been
connected, V1, 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 V1 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.
10' 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 V3 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. 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 (V4) is recorded.
L. The magnitude of the difference between V2 and V4 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 V2 and V4 is at least 10 microvolts, the magnitude of the
capacitance(C)
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of the electrode pair is calculated by taking the magnitude of the formula
I x T2
C = --------------
V4 - V2
After calculating the capacitance, the capacitance per cm2 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 accordance with another illustrative embodiment, there is provided an
apparatus
comprising
(A) the electrokinetic device, 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 within the
conduit.
In accordance with another illustrative embodiment, there is provided an
electrical
circuit comprising
(A) the electrokinetic device,
(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 may be operated so that there is no chemical change of the
electrolyte,
and embodiments will generally be described with reference to such operation.
However,
there may alternatively be acceptable chemical change of the electrolyte at
one or both of the
electrodes.
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In accordance with another illustrative embodiment, there is provided a method
of
operating the circuit, 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 accordance with another illustrative embodiment, there is provided an
electrode
which is suitable for use in the electrokinetic device 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
Illustrative embodiments are illustrated in the accompanying drawings, which
are
not to scale and which are schematic cross-sections of devices and systems.
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 III-IV 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.
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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
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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
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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 nun.
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
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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.
In 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
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cm2, less than 20 microamps per cm2, or less than 2 microamps per cm2, 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.V.
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.
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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 (ne0) to the electroosmotic
mobility
(neO), 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 3V 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 gl/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
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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. In 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
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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
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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 which 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
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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.
In 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
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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
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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 21 a and 2lb 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 51 a and 5
lb 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 3a and 3b are
centered on
the ends of vias 12a and 12b, respectively, and are powered by power supply 6.
Chambers 2a
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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 disc-
shaped 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 811 a and 81
lb fitted
with check valves 812a and 812b to control the flow of liquid through chamber
81. Chamber
82 has ports 821 a and 821b fitted with check valves 821 a 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 now (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.
In 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 11 a,
porous spacer 12b, porous capacitive electrode 3b, porous spacer 12c., second
PDM 1lb,
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
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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. In 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 PVDF
membrane,
about 84 micron thick and having a size of about 5 x 30 mm. The PVDF 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
cm2, 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
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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
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was-used to pump a 0.5 millimolar lithium chloride solution, 34 millimolar
acetic acid, and
34 millimolar carbonic acid.
22
