U.S. patent application number 13/279426 was filed with the patent office on 2012-04-05 for electroosmotic devices.
This patent application is currently assigned to Old Dominion University Research Foundation. Invention is credited to Tarek Abdel-Fattah, Helmut BAUMGART, Ali Beskok, Diefeng Gu, Seungkyung Park.
Application Number | 20120080313 13/279426 |
Document ID | / |
Family ID | 43011782 |
Filed Date | 2012-04-05 |
United States Patent
Application |
20120080313 |
Kind Code |
A1 |
BAUMGART; Helmut ; et
al. |
April 5, 2012 |
ELECTROOSMOTIC DEVICES
Abstract
Electroosmotic (EO) devices are provided which are not subject
to mechanical wear and tear and with no moving parts, and having
improved flow rates and electrical properties. Atomic layer
deposition can be used to prepare three electrical terminal active
zeta potential modulated EO devices from porous membranes. First,
second, and further thin layers of materials can be formed with the
pores. Thus, embedded electrodes can be formed along the length of
the pores. The zeta potential in the pores can be modified by use
of a voltage potential applied the embedded electrode, thereby
achieving active control of surface zeta potential within the pores
and active control of flow through the pores.
Inventors: |
BAUMGART; Helmut; (Yorktown,
VA) ; Gu; Diefeng; (Newport News, VA) ;
Abdel-Fattah; Tarek; (Yorktown, VA) ; Beskok;
Ali; (Chesapeake, VA) ; Park; Seungkyung;
(Norfolk, VA) |
Assignee: |
Old Dominion University Research
Foundation
Norfolk
VA
|
Family ID: |
43011782 |
Appl. No.: |
13/279426 |
Filed: |
October 24, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/US2010/032316 |
Apr 23, 2010 |
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13279426 |
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61172632 |
Apr 24, 2009 |
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Current U.S.
Class: |
204/451 ;
156/278; 204/601; 204/604; 977/781; 977/962 |
Current CPC
Class: |
B01D 61/427 20130101;
B01D 69/02 20130101; B01D 2325/08 20130101; C23C 16/01 20130101;
B01D 71/022 20130101; B01D 71/024 20130101; F04B 19/006 20130101;
B01D 2313/345 20130101; B01D 67/0062 20130101; C23C 16/403
20130101; B01D 71/02 20130101; B01D 67/0072 20130101 |
Class at
Publication: |
204/451 ;
156/278; 204/601; 204/604; 977/781; 977/962 |
International
Class: |
C25B 7/00 20060101
C25B007/00; B32B 38/08 20060101 B32B038/08 |
Claims
1. An electroosmotic device comprising: a substrate comprising at
least one pore extending from a first major surface of the
substrate to a second major surface of the substrate, the at least
one pore comprising one of a macropore, a micropore, or a nanopore;
at least one first material disposed on at least the inner surface
of the at least one pore, wherein the at least one first material
is electrically conductive, at least one second material disposed
at least on the at least one first material in the at least one
pore, wherein the at least one second material is an electrical
insulator, at least one anode electrode adjacent to the first major
surface; and at least one cathode electrode adjacent to the second
major surface.
2. The device according to claim 1, wherein the at least one first
material is in electrical communication with a first voltage
source, independently biased from a second voltage source applying
a voltage difference between the at least one cathode electrode and
the at least one anode electrode.
3. The device according to claim 1, the device further comprising a
pump housing and at least two fluid chambers to provide inlet flow
and outlet flow.
4. The device according to claim 1, wherein the substrate comprises
aluminum oxide or silicon.
5. The device according to claim 1, wherein the at least one first
material comprises at least one of metal, a metal alloy, a
semiconductor, a conducting metal nitride, or a conducting
oxide.
6. The device according to claim 1, wherein the first material
comprises at least one of Ti, Au, Pt, Al, Cu, Ag, W, nitride
thereof, or ZnO
7. The device according to claim 1, wherein the at least one second
material comprises an oxide or metal oxide.
8. The device according to claim 1, wherein the at least one second
material comprises an oxide selected from the group consisting of
HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, TiO.sub.2, or SiO.sub.2.
9. The device according to claim 1, wherein the at least one first
material has a thickness of at least 10 nm.
10. The device according to claim 1, wherein an aspect ratio of the
at least one pore is about 5 to about 1,200.
11. The device according to claim 1, wherein the substrate is about
10 microns to about 200 microns thick.
12. The device according to claim 1, wherein the at least one pore
has a pore size that is between about 50 nm and about ten
microns.
13. The device according to claim 1, further comprising at least
one electrically conductive mesh adjacent to the substrate and
defining at least one of the at least one cathode electrode or the
at least one anode electrode.
14. A method of preparing an electroosmotic device comprising:
obtaining a substrate comprising a first major surface, a second
major surface, and at least one pore extending from the first major
surface to the second major surface, the pore comprising one of a
macropore, a micropore, or a nanopore; forming at least one first
material on at least the inner surface of the at least one pore,
wherein the at least one first material is electrically conductive;
depositing at least one second material on at least one first
material, wherein the second material is an electrical insulator;
and providing at least one anode electrode adjacent to the first
major surface and at least one cathode electrode adjacent to the
second major surface.
15. The method of claim 14, wherein the step of forming comprises
providing a thickness for the at least one first material of at
least about 10 nm.
16. The method of claim 14, wherein the step of forming comprises
depositing at least one electrically conductive material on the at
least the inner surface of the pores to define the at least one
first material.
17. The method of claim 16, wherein the at least one first material
is deposited by atomic layer deposition (ALD).
18. The method of claim 17, wherein the ALD is carried out at a
deposition temperature of about 100.degree. C. to about 350.degree.
C.
19. The method of claim 17, wherein ALD is carried out for at least
300 ALD cycles.
20. The method of claim 17, wherein the ALD is carried out with a
dwell time adapted to avoid blocking the pores but provide
conformal coverage.
21. The method of claim 14, wherein the at least one second
material is deposited by atomic layer deposition (ALD).
22. The method of claim 14, wherein the step of providing the
substrate further comprising selecting the substrate such that the
at least one pore has an aspect ratio of about 300 to about
1,200.
23. The method of claim 14, wherein the step of forming comprises
implanting at least one dopant into the at least the inner surface
of the pores to define the at least one first material.
24. The method of claim 14, wherein the step of providing further
comprises positioning at least one electrically conductive mesh
adjacent to the substrate to define at least one of the at least
one cathode electrode or the at least one anode electrode.
25. A method for using an electroosmotic device comprising:
providing an electroosmotic device comprising a substrate having at
least one pore that is a macropore, a micropore, or a nanopore, at
least one first material formed on the inner surface of the at
least one pore, at least one second material disposed on the at
least one first material, at least one anode electrode adjacent to
a first major surface of the substrate, and at least one cathode
electrode adjacent to a second major surface of the substrate,
wherein the at least one first material is electrically conductive
and the at least one second material is an electrical insulator;
applying a first voltage across the anode and cathode to generate
electroosmotic flow through the at least one pore; and applying a
second voltage independently biased from the first voltage to the
at least one first material to modify the electroosmotic flow.
26. The method of claim 25, wherein the modification of
electroosmotic flow is an increase in flow rate, a decrease in flow
rate, or a reversal of flow.
27. The method of claim 25, wherein the step of applying the first
voltage comprises selecting the first voltage to be five volts or
less.
28. An electroosmotic device comprising: a substrate comprising at
least one pore extending from a first major surface of the
substrate to a second major surface of the substrate, the at least
one pore comprising one of a macropore, a micropore, or a nanopore;
a plurality of dopant atoms embedded into at least the inner
surface of the at least one pore to define at least one first
material at the at least the inner surface of the at least one
pore, the at least one first material being electrically
conductive; at least one second material disposed at least on the
at least one first material in the at least one pore, wherein the
at least one second material is an electrical insulator, at least
one anode electrode adjacent to the first major surface; and at
least one cathode electrode adjacent to the second major
surface.
29. The device according to claim 28, wherein the at least one
first material is in electrical communication with a first voltage
source, independently biased from a second voltage source applying
a voltage difference between the at least one cathode electrode and
the at least one anode electrode.
30. The device according to claim 28, the device further comprising
a pump housing and at least two fluid chambers to provide inlet
flow and outlet flow.
31. The device according to claim 28, wherein the substrate
comprises aluminum oxide or silicon.
32. The device according to claim 28, wherein the plurality of
dopants comprise p-type or n-type dopants.
33. The device according to claim 28, wherein the at least one
second material comprises an oxide or metal oxide.
34. The device according to claim 28, wherein the at least one
second material comprises an oxide selected from the group
consisting of HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, TiO.sub.2, or
SiO.sub.2.
35. The device according to claim 28, wherein the at least one
first material has a thickness of at least 10 nm.
36. The device according to claim 28, wherein an aspect ratio of
the at least one pore is about 5 to about 1,200.
37. The device according to claim 28, wherein the substrate is
about 10 microns to about 200 microns thick.
38. The device according to claim 28, wherein the at least one pore
has a pore size that is between about 50 nm and about ten
microns.
39. The device according to claim 28, further comprising at least
one electrically conductive mesh adjacent to the substrate and
defining at least one of the at least one cathode electrode or the
at least one anode electrode.
Description
RELATED APPLICATIONS
[0001] This application claims priority to PCT patent application
No. PCT/US10/32316 for "ELECTROOSMOTIC PUMP", filed Apr. 23, 2010,
which claims priority to U.S. provisional application Ser. No.
61/172,632, filed Apr. 24, 2009, both of which are hereby
incorporated by reference in their entirety.
FIELD OF THE INVENTION
[0002] The present invention is directed to non-mechanical
micropump devices, and more particularly to electroosmotic
devices.
BACKGROUND
[0003] Over the past decade, increasingly more research and
development has gone into the development of lab-on-a-chip (LOC)
systems. LOC systems are minute chemical processing plants,
typically including a complex network of micro/nanoscale channels.
In general, such LOC systems are configured for automatically
performing common laboratory procedures, such as filtration,
mixing, separation, and detection. LOC systems are of great
interest in biomedical, pharmaceutical, environmental, and security
applications, as they can potentially provide a fast, inexpensive,
and portable means of handling and analyzing materials.
[0004] Of particular interest for LOC and other microfluidic
systems is how to provide a robust and reliable means of
transferring materials within the system. Although a variety of
micropumps have been developed for LOC applications, electroosmotic
(EO) pumps have garnered a significant amount of interest. EO pump
devices are typically fabricated using macroporous or nanoporous
materials to form an EO pump membranes. In general, EO pump devices
are preferred in microfluidic systems, since they enable fluid
pumping and flow control without the need for mechanical pumps or
valves. Further, EO pump devices typically can minimize sample
dispersion effects.
[0005] However, the relatively small sizes required for the pores
in such EO pump devices generally result in a non-trivial
fabrication process. Further, the performance of most conventional
EO pump devices is generally limited. For example, some of the
drawbacks of such conventional EO pump devices typically include
high operating voltage requirements (e.g., on the order of 1 kV to
10 kV), electrolysis of water, oxidation of electrode surfaces, and
Joule heating. Such drawbacks therefore limit the usefulness of
conventional EO pump devices for certain applications. For example,
high operating voltages typically required for conventional EO pump
devices generally prevent EO pump devices from being successfully
integrated into lab-on-a-chip (LoC) type portable devices.
SUMMARY
[0006] Embodiments of the invention concern electroosmotic pumps
and methods for manufacture and use thereof. In a first embodiment
of the invention, an electroosmotic device is provided. The device
includes a substrate comprising at least one pore extending from a
first major surface of the substrate to a second major surface of
the substrate, where the at least one pore is one of a macropore, a
micropore, or a nanopore. The device also includes at least one
first material disposed on at least the inner surface of the at
least one pore, where the at least one first material is
electrically conductive. The device further includes at least one
second material disposed at least on the at least one first
material in the at least one pore, where the at least one second
material is an electrical insulator. The device also includes at
least one anode electrode adjacent to the first major surface and
at least one cathode electrode adjacent to the second major
surface.
[0007] In a second embodiment of the invention, a method of
preparing an electroosmotic device is provided. The method includes
obtaining a substrate comprising a first major surface, a second
major surface, and at least one pore extending from the first major
surface to the second major surface, where the pore is one of a
macropore, a micropore, or a nanopore. The method also includes
forming at least one first material on at least the inner surface
of the at least one pore, where the at least one first material is
electrically conductive. The method further includes depositing at
least one second material on at least one first material, where the
at least one second material is an electrical insulator. The method
additionally includes providing at least one anode electrode
adjacent to the first major surface and at least one cathode
electrode adjacent to the second major surface.
[0008] In a third embodiment of the invention, a method for using
an electroosmotic device is provided. The method includes providing
an electroosmotic device comprising a substrate having at least one
pore that is a macropore, a micropore, or a nanopore, at least one
first material formed on the inner surface of the at least one
pore, at least one second material disposed on the at least one
first material, at least one anode electrode adjacent to a first
major surface of the substrate, and at least one cathode electrode
adjacent to a second major surface of the substrate, where the
first material is electrically conductive and the second material
is an electrical insulator. The method also includes applying a
first voltage across the anode and cathode to generate
electroosmotic flow through the at least one pore and applying a
second voltage independently biased from the first voltage to the
at least one first material to modify the electroosmotic flow.
[0009] In a fourth embodiment of the invention, an electroosmotic
device is provided. The device includes a substrate comprising at
least one pore extending from a first major surface of the
substrate to a second major surface of the substrate, the at least
one pore comprising one of a macropore, a micropore, or a nanopore.
The device also includes plurality of dopant atoms embedded into at
least the inner surface of the at least one pore to define at least
one first material at the at least the inner surface of the at
least one pore, the at least one first material being electrically
conductive. The device further includes at least one second
material disposed at least on the at least one first material in
the at least one pore, where the at least one second material is an
electrical insulator. The device additionally includes at least one
anode electrode adjacent to the first major surface and at least
one cathode electrode adjacent to the second major surface.
[0010] In the various embodiments, the cathode and anode electrodes
can be formed using electrically conductive materials deposited on
the substrate or an electrically conductive mesh adjacent to the
substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a schematic of a section of an electroosmotic
device including a pore for directing fluid from a first reservoir
or inlet to a second reservoir or inlet that is useful for
describing the various embodiments of the invention;
[0012] FIG. 2 is a schematic of an exemplary microfluidic system
including an EO micropump configured in the accordance with an
embodiment of the invention;
[0013] FIG. 3 is a schematic of a portion of section of an
electroosmotic device configured in accordance with the various
embodiments of the invention;
[0014] FIG. 4 is a schematic illustration of an exemplary process
flow for manufacturing EO pump devices in accordance with the
various embodiments of the invention;
[0015] FIGS. 5A and 5B show top-down and cross-section SEM images,
respectively, of a porous anodic aluminum oxide (AAO) substrate
after ion milling;
[0016] FIGS. 6A and 6A are SEM images of the front side and back
side, respectively, of a silicon substrate-based porous
membrane;
[0017] FIG. 7 is an X-Y plot of sheet conductance as a function of
ALD growth temperature;
[0018] FIG. 8A is an X-Y plot of sheet conductance as a function of
the number of ALD cycles for as-deposited and annealed platinum
thin films deposited using an ALD process at 300.degree. C.;
[0019] FIG. 8B is an X-Y plot of resistivity of the films in FIG.
8A as a function of thickness;
[0020] FIGS. 9A, 9B, 9C, 9D, and 9E show deposition of Pt by ALD at
300.degree. C. for 50 cycles, 100 cycles, 200 cycles, 400 cycles
and 1000 cycles, respectively;
[0021] FIGS. 10A, 10B, and 10C show SEM images of Pt thin films
deposited on AAO membranes with using the conformal ALD deposition
processes described above and for exposure times of 0 s, 10 s and
30 s;
[0022] FIG. 11 is an SEM image of platinum tubes fabricated in AAO
membranes;
[0023] FIG. 12 is a top-down SEM image of a silicon substrate with
ALD deposition of Pt;
[0024] FIG. 13A shows a cross-sectional SEM image of ALD (atomic
layer deposited) zirconia coated AAO substrate;
[0025] FIG. 13B shows a corresponding EDS Zr mapping showing
uniform distribution of zirconia throughout the entire thickness of
the 60 .mu.m AAO substrate in FIG. 13A.
[0026] FIGS. 14A, 14B, and 14C show a top-down SEM images of an AAO
substrate, the AAO substrate with a thin film ALD coating of
ZrO.sub.2, and the coated AAO substrate after the AAO walls have
been removed to show single ZrO.sub.2 nanotubes, respectively;
and
[0027] FIGS. 15A, 15B, 15C, and 15D show zeta potential
measurements of ALD-deposited alumina, titania, zirconia, and
silica films, respectively, as a function of solution pH for
different aqueous solutions (1 mM, 10 mM, 100 mM, and 1M) of
potassium chloride (KCl).
DETAILED DESCRIPTION
[0028] The present invention is described with reference to the
attached figures, wherein like reference numerals are used
throughout the figures to designate similar or equivalent elements.
The figures are not drawn to scale and they are provided merely to
illustrate the instant invention. Several aspects of the invention
are described below with reference to example applications for
illustration. It should be understood that numerous specific
details, relationships, and methods are set forth to provide a full
understanding of the invention. One having ordinary skill in the
relevant art, however, will readily recognize that the invention
can be practiced without one or more of the specific details or
with other methods. In other instances, well-known structures or
operations are not shown in detail to avoid obscuring the
invention. The present invention is not limited by the illustrated
ordering of acts or events, as some acts may occur in different
orders and/or concurrently with other acts or events. Furthermore,
not all illustrated acts or events are required to implement a
methodology in accordance with the present invention.
[0029] Embodiments of the invention provide methods for fabricating
electroosmotic (EO) pumps and devices therefrom. In particular, EO
pump devices are provided consisting of a macroporous, microporous,
or nanoporous membrane. Such EO pump devices are configured with
electrodes on the surfaces of the membrane in order to provide a
relatively high. electric field for inducing electroosmotic flow
through the pores with relatively low voltages. Further, such EO
pump devices are configured to include embedded electrodes in the
pores, where the embedded electrodes can be independently biased
relative to the surface electrodes. These embedded electrodes can
then be used to adjust a surface zeta potential in the pores, and
thus adjust the amount of flow and/or the direction of the flow in
the pores.
[0030] Prior to describing the various exemplary embodiments of the
invention, it is useful to provide a description of the role of the
surface zeta potential in electroosmotic devices. This is
illustrated with respect to FIG. 1. FIG. 1 is a schematic of a
section of an electroosmotic device 100 including a pore 102 for
directing fluid from a first reservoir or inlet 104 to a second
reservoir or inlet 106. The device 100 can include anode electrodes
108 and cathode electrodes 110. The pore 102 consists of an opening
in a membrane 112 with dielectric surfaces 114.
[0031] In general, the chemical equilibrium between a solid surface
(e.g., surfaces 114) and an electrolyte solution typically leads to
an interface region 116 acquiring a net fixed electrical charge due
to the formation of a layer of mobile ions known as an electrical
double layer (EDL) or Debye layer. In general, the formation of the
EDL is a function of the surface zeta potential along the interior
surfaces of the pore. Surface zeta potential refers to the
electrical potential at an electrokinetic plane of shear at a solid
surface in contact with a liquid and is generally a property of the
materials forming the solid surface. During EDL formation, the
charge due to surface zeta potential causes the local free ions in
the liquid to rearrange and provide the region 116 with a nonzero
net charge density near the solid-liquid interface. The interface
region can consist of a stern region 116a adjacent to surfaces 114
and having a large concentration of ions from the electrolyte and
oppositely charged with respect to the surface zeta potential and a
diffuse region 116b with a lower concentration of such ions.
[0032] When an electric field is applied (e.g., via electrodes 108
and 110) to the fluid and across the length of the pore 102, the
ions associated with the net charge in the interface region 116 are
induced to move by the resulting Coulomb force. The direction of
flow is dictated by the polarity of the surface zeta potential, as
the ions accumulated in the interface region will have a polarity
to that of the surface zeta potential. The viscous drag caused by
movement of these ions localized in the interface region 116
transfers momentum to the rest of the fluid, including the fluid in
a bulk region 118 (i.e., a region with no net charge). Since there
is no other fixed surface inside the pore 102 against which the
fluid in the bulk region 118 can dissipate this momentum, the fluid
in the bulk region 118 also begins to move with the same velocity
as the ions in the interface region 116. The resulting flow is
termed electroosmotic flow.
[0033] Depending on the ionic concentration in the electrolyte, the
EDL thickness can vary. For example, the EDL can be as small as 3
nm for a 1.times.10.sup.-2 M electrolyte and up to 300 nm for
deionized water (1.times.10.sup.-6 M). As a result, the channel
hydraulic diameter and EDL thickness may become comparable in some
pore configurations, depending on the pore size and the EDL
thickness. If large ionic concentration buffers result in EDL
thicknesses on the order of a few nanometers and the pore diameter
is the order of 200 nm and above, the EDL constitutes a very small
portion of the flow domain. This simplification is convenient for
mathematically describing fluid flow in the pore 102. In
particular, rather than modeling the flow using a Poisson-Boltzmann
equation that governs the ion distribution near the surfaces and
the corresponding EDL effects on momentum transport, the fluid
velocity in vicinity of the charged surface can instead be modeled
by the Helmholtz-Smoluchowski electroosmotic velocity:
U HS = - .zeta. E x .mu. ( 1 ) ##EQU00001##
where .zeta. is the zeta potential, .di-elect cons. is permittivity
of the liquid, .mu. is the dynamic viscosity, and E.sub.x is the
electric field applied in the stream wise direction. For steady
electric field and constant channel cross-section, this equation
also models the EO-flow velocity in the channel/pore, which happens
to be a plug type flow. Without considering the EDL effects and/or
upstream/downstream pressure head, the flow rate (Q) in a pore then
be approximated by:
Q=U.sub.HS.times.A, (2)
where A is the pore area. Thus, Equation 2 shows that the flow rate
within a pore of a selected size can be increased either by
increasing the zeta potential or the applied electric field.
[0034] As described above, the surface zeta potential is generally
a function of the materials at the surfaces of the pore. However,
the zeta potential will also vary as a function of the fluid,
specifically its ionic strength and pH. Thus, it is possible to
provide "passive" zeta potential control to improve flow through
the pore by selection of an appropriate surface material for the
pore based on the types of buffers to be used, or vice versa.
Unfortunately, the issue with providing such "passive" zeta
potential control is that once the pores are manufactured, it is
not possible to provide further adjustment of the surface zeta
potential other than by adjustment of the buffer. In many cases,
this is undesirable or not possible. As a result, the flow can only
be actively adjusted via adjustment of the applied electric field.
Thus, to achieve some levels of flow, a relatively high electric
field needs to be applied. However, such electric fields levels can
result in electrolysis of water, oxidation of electrode surfaces,
and Joule heating, as described above. As a result, many
conventional EO pump are configured to provide relatively high
flows for a limited combination of buffer types and voltages
ranges.
[0035] Accordingly, the various embodiments of the invention
overcome the limitations of such EO pump configurations by
providing "active" control of the zeta surface potential. As
described above, an EO pump in accordance with the various
embodiments of the invention is configured to include embedded
electrode portions adjacent to the inner surfaces of the pores. In
operation, a voltage can be applied to the embedded electrode in
addition to the voltages applied to the electrodes in the
reservoirs. As a result, the applied voltage at the embedded
electrode alters the effective electrical potential at the solid
surface in contact with the liquid, altering the surface zeta
potential with respect to the fluid. This therefore allows "active"
adjustment of the surface zeta potential and provides an additional
means for adjusting flow through the pore. Consequently, since the
embedded electrode can be used to alter the surface zeta potential,
the adverse effects of applying a high electric field can be
reduced or eliminated by applying lower electric fields, but
providing an increased flow by altering the surface zeta potential
via the "active" control of the surface zeta potential.
[0036] Referring now to FIG. 2, there is shown a schematic of an
exemplary microfluidic system 200 including an EO micropump 202
configured in the accordance with an embodiment of the invention.
As shown in FIG. 2, the system 200 includes an EO micropump 202
consisting of a porous substrate or membrane 204 with first and
second outer pore surfaces 205, 206 and electrodes 208 disposed on
these surfaces. Further, the EO pump 202 is configured to include
embedded electrodes (not shown), as described above. The
configuration and fabrication of EO pump 202 will be described in
further detail below with respect to FIG. 3.
[0037] In system 200, the electrodes 208 can be coupled to a direct
current (DC) power supply 210 to provide a voltage across
electrodes 208 and thus between the surfaces 205, 206 of micropump
202 to induce electroosmotic flow. For example, a voltage between 0
and 10V, such as 1-5V, can be provided. In some configurations, a
multi-meter 212 can be provided to monitor the output of supply
210. Additionally, the system 200 can include a second DC power
supply 211 to generate a voltage for the embedded electrodes (not
shown) in EO pump 202. However. The various embodiments are not
limited in this regard and a single voltage supply or voltage
supply system can be used to provide voltages for electrodes 208
and the embedded electrodes in EO pump 202.
[0038] The EO micropump 202 can disposed in a membrane holder 214
coupled to an inlet 216 and an outlet 218. The EO micropump 202 can
be arranged in membrane holder so as to provide a fluid connection
between inlet 216 and outlet 218. Inlet 216 can be coupled to a
first reservoir 220 and outlet 218 can be coupled to a second
reservoir 222. Additionally, reservoir 220 can be associated with a
first scale 224 and second reservoir 222 can be associated with a
second scale 226. Thus the flow rate between reservoirs 220 and 222
can be monitored by the changes at scales 224 and 226,
respectively.
[0039] In some configurations, pressure transducers 228 can be
provided at the inlet 216 and the outlet 218 for measuring the
pressure across the pump 202. Additionally, an outlet valve 230 can
be provided to prevent flow from reservoir 220 to 222.
Alternatively, valve 230 can be used with transducers 228 to
measure the maximum pressure build-up across the pump 202 by
closing the outlet valve 230.
[0040] Referring now to FIG. 3, there is shown a schematic of a
portion of section of an electroosmotic device 300 configured in
accordance with the various embodiments of the invention. For
example, device 300 can be used to provide the EO pump 202 in FIG.
2. As shown in FIG. 3, device 300 includes a membrane 312 having
one or more pores 301 for defining one or more channels 302 for
directing fluid from a first reservoir or inlet portion 304 to a
second reservoir or inlet portion 306. The device 300 can also
include anode electrodes 308 and cathode electrodes 310 for
inducing electroosmotic flow through the channels 302 by applying a
voltage across electrodes 308 and 310. The membrane 312 can also
have dielectric surfaces 314 formed thereon, similar to the
configuration in FIG. 1, defining the inner surface of the channels
302. Thus, similar to the configuration illustrated in FIG. 1, the
resulting zeta potential at the dielectric surfaces 314 can result
in the formation of an interface layer 316 with a stern region 316a
and a diffuse region 316b.
[0041] In contrast to the configuration in FIG. 1, the device 300
can be configured to include embedded electrode portion 320 that is
coupled to a separate voltage supply (not shown). As described
above, by applying a voltage at the embedded electrode 320, the
effective surface zeta potential can be modified. In particular,
the net charge or effective zeta potential provided at the surface
314 is interface provided by the combination of the intrinsic zeta
potential for the electrolyte/surface material combination in use
and the potential at the embedded electrodes. Thus, by varying this
effective zeta potential, the total number or concentration of ions
accumulating in the stern region 316a and the diffuse region 316b
of the interface layer 316 can be adjusted. Further, the thickness
of the interface layer 316 can also be adjusted. Accordingly, as
the ion concentration and interface layer thickness is adjusted in
these regions, the impact of the Columbic forces is modulated.
Accordingly, the flow through the channels 302 can be adjusted
upwards or downwards. In some instances, by providing a voltage
opposite in polarity to the polarity of the intrinsic zeta
potential of the dielectric surfaces 314, the flow can be halted or
the direction of flow can be reserved.
[0042] The device 300 can therefore be operated in two modes. In a
first "passive" mode, the device 300 can rely on the intrinsic zeta
potential by allowing the embedded electrode 320 to remain floating
or to be coupled to ground. In a second "active" mode, the
electrode 320 can be biased to modulate the surface zeta potential
and thus modulate the flow of liquid through the channels 302 to
increase, decrease, or reverse flow.
[0043] Manufacture of EO Pump Devices
[0044] Referring now to FIG. 4, there is shown a schematic
illustration of an exemplary process flow 400 for manufacturing EO
pump devices in accordance with the various embodiments of the
invention. The process 400 begins at step 402, where the membrane
312 with one or more pores 301 is provided. In the various
embodiments of the invention, the membrane or porous substrate can
be macroporous, microporous, or nanoporous. These porous membranes
can be fabricated using substrates made of inorganic materials such
as, for example, silicon or alumina. Further details regarding the
fabrication of some exemplary membrane configurations will be
described in greater detail below.
[0045] In the membranes, the average pore size can be between
approximately 200 nm and 10 .mu.m. For example the average pore
size can be 10 .mu.m, 5 .mu.m, or 1 .mu.m. Further the range of
pore sizes can vary. For example, in some embodiments, the pore
size can be, for example, about 200 nm to about 10 .mu.m, about 200
nm to about 5 .mu.m, or about 200 nm to about 3 .mu.m. Further, the
aspect ratio of the pores 301 can vary. For example, in some
embodiments the aspect ratio can vary from 5 to about 1,200, such
as from approximately 300 to about 1200. However, the various
embodiments of the invention are not limited in this regard and
aspect ratios of 50, 100, 200, 300, 400, 500, 600, 700, 800, 900,
or 1000 can also be provided. These pore sizes and aspect ratios
are provided for illustrative purposes only. Accordingly, a
membrane in accordance with the various embodiments of the
invention can be manufactured with other pore sizes or aspect
ratios.
[0046] Once the membrane 312 is provided or fabricated at step 402,
the method 400 can then proceed to step 404. At step 404, the
embedded electrodes 320 can be formed.
[0047] In some embodiments, the embedded electrodes 320 can be
formed by depositing a layer of electrically conductive material on
at least inside the inner surfaces of the pores 301 in the membrane
312. In such embodiments, the embedded electrodes 320 can be formed
in a variety of ways using a variety of electrically conductive
materials so that the embedded electrodes are formed 320, but that
the resulting channels 302 are not significantly reduced in size or
closed off.
[0048] In these embodiments, the electrically conductive material
can consist of a metal, a conducting metal nitride, a
semiconductor, or a conducting metal oxide. Non-limiting examples
of metals and metal nitrides include Ti, Au, Pt, Al, Cu, Ag, and
metal nitrides, such as TiN, thereof or any other stable noble
metal. A non-limiting example of a semiconductor includes ZnO. Such
materials can be deposited using atomic layer deposition (ALD),
chemical vapor deposition (CVD), or any other methods suitable for
conformal deposition of electrode materials over all surfaces,
including pore surfaces, of the microporous or nanoporous membrane
geometries described above. Some exemplary methods for such
deposition will be described below in greater detail.
[0049] In other embodiments, the embedded electrode 320 can be
formed by increasing the electrical conductivity of the membrane
312 along at least the inner surfaces of the pores 301. For
example, the membrane 312 can be formed from a semiconducting
material and at least the inner surfaces of the membrane 312 can be
exposed to dopants to increase the electrical conductivity of at
least these inner surfaces of the membrane to form the embedded
electrode 320. In such embodiments, the dopants can be provided via
diffusion or ion implantation processes. For example, in the case
of membrane 312 comprising silicon or another column IV
semiconductor material, an N+ or P+ layer can be formed to define
the embedded electrode 320. However, these embodiments are not
limited solely to column IV materials. Rather, membrane 312 can be
formed from any type of semiconductor material. Further, the
various embodiments are not limited to any particular diffusion or
implantation techniques. Rather, any other types of doping
techniques can be used in the various embodiments. Some exemplary
methods for such deposition will be described below in greater
detail.
[0050] After the embedded electrodes are formed at step 404, these
materials can be electrically isolated from the channel 302 at step
406. In particular, the dielectric material defining the surface
dielectric layer 314 can be deposited over substantially all the
surfaces of the membrane in which an embedded electrode 320 was
formed at step 404. As a result, an embedded electrode 320 that is
electrically isolated from the interior of pore 302 is formed.
[0051] In some embodiments, the second insulating material
comprises a semiconductor oxide or an oxide or insulating metal
oxide. Non-limiting examples of semiconductor oxides or metal
oxides include HfO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, TiO.sub.2,
and SiO.sub.2 or any other electrically insulating material.
[0052] Once the surface dielectric materials are formed at step
406, the method can proceed to step 408 to form electrodes 308 and
310.
[0053] In some embodiments, an electrically conductive material can
be deposited at step 408 on the outer surfaces of the membrane 312.
The deposition can be configured to avoid deposition of such
materials in the channels 302. For example, sputtering or e-beam
techniques can be used to deposit such materials. However, any
other methods that result in preferential deposition of materials
on the outer surfaces of the membrane can be used without
limitation. Accordingly, layers of materials forming electrodes 308
and 310 are primarily confined to regions outside the pores. In
some embodiments, the electrodes 308 and 310 comprise a metal.
Examples of useful metals, include, but are not limited to, Au, Pt,
and W, and any other stable noble metals or alloys thereof. In
other embodiments, other metals and alloys, metal nitrides,
semiconductors, semiconductor oxides, or metal oxides can also be
used without limitation. Processes for forming such materials will
be described below in greater detail.
[0054] In other embodiments, the electrodes 308 and 310 can be
formed by disposing an electrically conductive mesh on or above
each of the outer surfaces of the membrane 312. In such
configurations, the mesh can be an electrically conductive
substrate or film with perforations or other openings to allow
fluids to traverse through the mesh without significantly impacting
flow through the channels 302. In other configurations, the
perforated substrate or film can be electrically non-conductive
with a layer of electrically conductive material formed thereon.
Processes for forming such a mesh will be described below in
greater detail.
[0055] Additionally, the process flow described above can include
additional steps not shown in FIG. 4. For example, additional
photolithography and etching steps can be provided for forming an
electrical contact region for any of electrodes 308, 310, or 320.
Further, additional photolithography or wirebonding techniques can
also be included in such methods to provide structures for
electrically coupling any of electrodes 308, 310, and 320 to a DC
voltage supply. In another example, following step 408, a
passivation layer can be deposited over the electrodes 308 and 310
or over any contact regions formed for device 300. For example, a
layer of dielectric material can be deposited. Such passivation
layers can be used to passivate the exposed surfaces of electrodes
308 and 310 or any other reactive materials exposed to the liquid
in device 200. Further, a process flow in accordance with the
embodiment of the invention can include any additional steps
required for configuring device 300 to operate in the system
illustrated in FIG. 2 or any other electroosmotic system requiring
an EO pump.
[0056] The coaxial nanostructures described above (i.e., the
embedded electrodes 320 and surface dielectric layer 314) can be
prepared, as noted above, according to various methods.
[0057] A. Porous Membrane
[0058] In some embodiments of the invention, the porous membrane
can be a nanoporous anodized aluminum oxide (AAO) film. A
nanoporous AAO substrate can be prepared by a two-step anodization
procedure. First, high purity aluminum sheets (.about.0.5 mm thick)
can be degreased using, for example, acetone or any other suitable
solvent. These sheets can then be electropolished in a solution of
HClO.sub.4 and ethanol. For example, an exemplary process can be
performed using a 1:4, v/v solution at 20 V for 5-10 min or until a
mirror-like surface is achieved.
[0059] Thereafter, a first anodization step can carried out to form
a porous alumina layer on the surface of the sheet. For example, an
exemplary process can include anodization in a 0.3 M oxalic acid
solution electrolyte under a constant direct current (DC) voltage
of 80 V at 17.degree. C. for 24 h. After the first anodization
step, the porous alumina layer can be stripped away from the Al
substrate. For example, the stripping can be performed by treating
the anodized sheet using a solution containing 6 wt % phosphoric
acid and 1.8 wt % chromic acid at 60.degree. C. for 12 h. A second
anodization step can then be carried out. For example, the Al sheet
can be treated using a 0.3 M oxalic acid solution under a constant
direct current (DC) voltage of 80 V at 17.degree. C. for 24 h.
Finally, AAO substrates with highly ordered arrays of pores can be
obtained by selectively etching away the unreacted Al and, if
necessary performing planarization of the AAO substrate. For
example, the unreacted Al can be etched away using a saturated
HgCl.sub.2 solution. The planarization can then be performed using
ion milling.
[0060] A result of the above-mentioned processes is illustrated in
FIGS. 5A and 5B. FIG. 5A shows a top-down scanning electron
microscope (SEM) image of the pore structure of a 60 .mu.m thick,
planarized AAO substrate. As shown in FIG. 5, the resulting pore
size is in the range of 200-300 nm and the wall width between pores
is approximately 50 nm. As can be observed in FIG. 5, some of the
pores are connected via thinning of the wall. FIG. 5B is a
cross-sectional SEM image of the AAO substrate shown in FIG. 5A. As
shown in FIG. 5B, the pores are substantially parallel to each
other. The inset in FIG. 5B shows the formation of branches in some
of the pores. Such branches can be eliminated by using shorter
anodization times, although this results in a shorter pore
length.
[0061] In some AAO formation processes, one end of the pores can
have a smaller pore size than another end of the pores, to a depth
of a few micrometers. Accordingly, in some processes, this thin
layer can be removed by etching.
[0062] High magnification FE-SEM of such samples also shows a
smooth morphology of the inside walls of the AAO pores can be
achieved. Such smooth wall formation is generally desirable, as the
process using for forming subsequent layers in the pores replicate
the surface of the pores in the membrane on an Angstrom scale.
[0063] However, the various embodiments of the invention are not
limited solely to AAO-based porous membranes. In other embodiments,
as described above, the porous membranes can be formed using
silicon or other types of semiconductor substrates. In such a
process flow, openings can formed in the semiconductor substrate
via photolithography processes followed by substrate etching
processes. Accordingly, this process flow can include deposition
and removal of photoresist, deposition and etching of masking
layers, and cleaning or degreasing steps. The advantage of such a
photolithography-based process is that the size and spacing of the
pores can be directly controlled. Accordingly, the ordered porous
membranes can be more reliably and reproducibly manufactured. For
example, the result of such a process is shown in FIGS. 6A and
6B.
[0064] FIGS. 6A and 6A are SEM images of the front side and back
side, respectively, of a silicon substrate-based porous membrane.
As shown in FIGS. 6A and 6B, the pores are substantially identical
and arranged in a regular array. Further, since more control over
size and spacing of the pores is provided, issues regarding
branching or connected pores are eliminated. Additional, the etch
process or post-etch processes can be configured to provided
relatively smooth surface in the pores.
[0065] Although various exemplary processes have been described
above, the various embodiments of the invention are not limited in
this regard. Rather, one of ordinary skill in the art will readily
recognize that other process can be used to form AAO or
silicon-based porous membranes in accordance with the various
embodiments of the invention.
[0066] B. Embedded Electrode
[0067] In the various embodiments of the invention, a principal
concern is the deposition process for the electrically conductive
materials forming the embedded electrodes in the pores. In
particular, to provide superior performance, the electrically
conductive materials should be substantially uniformly deposited
along the length of the pores, having a relatively smooth surface
morphology, and high sheet conductance (i.e., low sheet
resistance). As used herein with respect to any comparison, the
term "substantially" refers to a difference of less that 20%.
[0068] In some embodiments of the invention, such embedded
electrodes can be formed using platinum thin films deposited by
atomic layer deposition (ALD). However, the various embodiments are
not limited solely to ALD. For example, methods can also include
chemical vapor deposition (CVD) techniques or any other methods
available to deposit layers (also referred to as films herein) of
the types of materials described above on the inner surface of the
pores of a nanoporous substrate.
[0069] Briefly, ALD technology deposits thin films using pulses of
chemical precursor gases to adsorb at the target surface one atomic
layer at a time. ALD is based on the sequential deposition of
individual monolayers or fractions of a monolayer in a controlled
fashion. More specifically, in ALD the growth substrate surface is
alternately exposed to the vapors of one of two chemical reactants
(complementary chemical precursors), which are supplied to the
reaction chamber one at a time. The exposure steps are separated by
inert gas purge or pump-down steps in order to remove any residual
chemical precursor or its by-product before the next chemical
precursor can be introduced into the reaction chamber. Thus, ALD
involves a repetition of individual growth cycles. Since a film
deposited by ALD is grown in a layer-by-layer fashion and the total
film thickness is given by the sum of the number of ALD cycles, it
is possible to calculate the number of cycles necessary to reach a
desired final film thickness. Conversely, the thickness of a film
can be set digitally by counting the number of reaction cycles. In
general, ALD achieves deposition rates on the order of 0.1-1.0
.ANG. per cycle and with cycle times ranging from one to ten
seconds. Due to the self-limiting nature of the surface reactions,
accidental overdosing with precursors does not typically result in
increased film deposition. Thus, ALD is able to achieve conformal
films with good across-wafer film thickness uniformity and good
step coverage. Because of the nature of ALD, film thickness is
immune to variations caused by non-uniform distribution of reactant
vapor or temperature in the reaction chamber. Further, a variety of
chemical precursors may be used with ALD, depending upon the
desired film. Specific chemical precursors are provided in the
Examples below. For example, platinum thin films can be formed
using an ALD process with (methylcyclopentadienyl)
trimethylplatinum (MeCpPtMe.sub.3) and oxygen (O.sub.2) as the
precursors. The deposition temperature for such a process can be in
the range of 270-320.degree. C. to provide a predictable sheet
conductance, as shown in FIG. 7.
[0070] FIG. 7 is an X-Y plot of sheet conductance as a function of
ALD growth temperature. As shown in FIG. 7, the conductance (and
thus resistivity) of a ALD platinum thin film approaches a constant
value when stable ALD Pt film growth is achieved. As noted above,
this constant value is achieved for the above-mention process in
the temperature range of 270-320.degree. C. The increase in growth
rate after 320.degree. C. is indicative of Pt precursor
decomposing. Further, a temperature in such a range can also be
selected based on other criteria. For example, based on other
properties such as surface roughness and impurity species in the
film. Based on such properties and the data of FIG. 7, in at least
one embodiment, an ALD deposition temperature of 300.degree. C. can
be used to provide stable growth of platinum thin films with
acceptable surface roughness and impurity species.
[0071] As the sheet conductance, G.sub.sh=t/.rho. (where t is the
film thickness and .rho. is the resistivity) is proportional to
film thickness, sheet conductance with have an approximately linear
relationship to the number of ALD cycles. Accordingly, a number of
ALD cycles to be used (and thus the film thickness) can be selected
to provide a desired sheet conductance. This is illustrated in FIG.
8A.
[0072] FIG. 8A is an X-Y plot of sheet conductance as a function of
the number of ALD cycles for as-deposited and annealed platinum
thin films deposited using an ALD process at 300.degree. C. As
shown in FIG. 8A, good linearity is demonstrated between sheet
conductance and ALD cycles, specifically after 300 cycles. This
shows that the above-mentioned ALD processes provide a
substantially constant growth rate for Pt thin films. As noted
above, the Pt thin films were also annealed. Specifically, the
films were annealed in forming gas (95% N2 and 5% H2) at
450.degree. C. for 30 min. The forming gas anneal (FGA) was
effective to improve the adhesion and conductivity of the Pt thin
films by passivating dangling bonds in the grain boundaries with
hydrogen. As also shown in FIG. 8A, a higher sheet conductance was
observed after FGA.
[0073] However, as shown in FIG. 8A, there is a drop in the sheet
conductance for both as-deposited and annealed Pt films with ALD
cycles less than 300. It is believed that the decrease in the sheet
conductance or the increase in the sheet resistance is either due
to the surface scattering or the structure of Pt film when the film
is thinner than a certain thickness. For example, as shown in FIG.
8A, for the first 150 cycles, no sheet conductance was observed.
This may be because no continuous Pt film was yet formed.
[0074] The resistivity of these Pt thin films is shown in FIG. 8B.
FIG. 8B is an X-Y plot of resistivity of the films in FIG. 8A as a
function of thickness. For FIG. 8B, the resistivity values were
calculated using the film thickness as determined by
cross-sectional TEM. As shown in FIG. 8B, the resistivity of these
films was shown not to be strongly dependent on thickness for films
thicker than 15 nm. The 20 nm thick Pt film after anneal showed a
resistivity of 12 .mu..OMEGA.-cm, which is very close to the bulk
resistivity of Pt (10.8 .mu..OMEGA.-cm). Thus an ALD-deposited Pt
film of 15 nm is thick enough to achieve good electrical
conductivity.
[0075] The growth of the Pt thin films grown using the
above-mention ALD processes were analyzed by cross-sectional TEM
(XTEM) to determine surface morphology and structure over time.
This is shown in FIGS. 9A-9E. FIGS. 9A, 9B, 9C, 9D, and 9E show
deposition of Pt by ALD at 300.degree. C. for 50 cycles, 100
cycles, 200 cycles, 400 cycles and 1000 cycles, respectively. FIG.
9A shows the very early stage of Pt deposition on a Si substrate
with native oxide. As shown in FIG. 9A, no continuous Pt thin film
was formed. Rather, isolated Pt nanoparticles (size less than 5 nm)
are randomly distributed on the surface.
[0076] As shown in FIG. 9B, with more deposition cycles, the Pt
nanoparticles continue to grow. In certain areas, the Pt islands
coalesce into larger, but still isolated islands, as shown in FIG.
9B. With addition cycle, as shown in FIG. 9C, the larger Pt islands
coalesce, thereby forming a continuous Pt network. However, at this
stage, the Pt film may not have covered the whole surface and may
be a porous surface with low sheet conductance. Thus, this may
explain the relatively low sheet conductance corresponding to 200
ALD cycles observed in FIG. 8A.
[0077] As additional ALD cycles are performed (>300), the Pt
film becomes thicker and continuous Pt, with a smooth surface, as
shown in FIGS. 9D and 9E. At this point, the ALD growth follows the
classical model of monolayer by monolayer growth per ALD deposition
cycle. Based on the TEM images, it is determined that astable
growth rate 0.5 .ANG./ALD cycle can achieved for the precursors
described above at or around 300.degree. C.
[0078] FIGS. 10A, 10B, and 10C show SEM images of Pt thin films
deposited on AAO membranes with using the conformal ALD deposition
processes described above and for exposure times of Os, 10 s and 30
s. The pore size and thickness of the AAO membranes were 250-300 nm
and 60 .mu.m, respectively, which correspond to an aspect ratio of
more than 200. FIG. 10A shows that the Pt can be deposited 10 .mu.m
deep into the AAO pore without exposure time, with possible
thickness gradient from the surface. The bright area shown the
regions coated with Pt deposition. The presence of Pt was confirmed
by energy dispersive spectroscopy (EDS) mapping. FIG. 10B shows
that the penetration of Pt is deeper with a 10 sec exposure.
However, FIG. 10C shows that for further exposure times, the depth
penetration into the AAO membranes saturates at around 20 .mu.m
from the surface, even with exposure times of 30 sec. This is
attributed to the combined effects of AAO pore size and the Pt
precursor diffusion rate, which is inversely proportional to the
square root of the molecular weight for this Pt precursor.
[0079] The quality of such platinum films is shown in FIG. 11. FIG.
11 is an SEM image of platinum tubes fabricated in AAO membranes.
In FIG. 11, the AAO membrane has been etched away using a NaOH
solution. The SEM image reveals that the Pt tubes are about 15
.mu.m in length. This'length is less than the Pt penetration depth
shown in FIG. 9C with 30 sec exposure time. However, this can be
explained by the TEM inset images (a) and (b) in FIG. 11. Inset (a)
is a TEM image of the portion of a Pt tube formed near a surface of
the AAO membrane. Inset (b) is a TEM image of the portion of the Pt
tube formed away from the surface of the AAO membrane. As show in
inset (a), at the end close to the surface of the AAO membrane, the
Pt film is continuous and forms dense tubes. In contrast, away from
the surface of the AAO membrane, the Pt film is discontinuous.
Accordingly, the processes described above will be suitable for
fabrication EO pump devices with AAO membrane thicknesses on the
order of 15-20 nm. However, other membrane thicknesses can be used
by adjusting the above-mentioned processes.
[0080] To provide EO pump devices with longer pore lengths and
using the above mentioned ALD processes, the pore size can be
increased. For example, although control of pore size and
distribution is somewhat constrained with respect to AAO membranes,
using silicon or semiconductor substrates allows greater control of
pore size and distribution, as described above.
[0081] For example, as shown in FIGS. 6A and 6B, a silicon
substrate can be subjected to provide an array of openings defining
pores, where the openings are .about.1 .mu.m wide and .about.1
.mu.m apart. In such a configuration, the significantly larger
openings allow the precursor for the ALD Pt deposition processes
described above to reach the entire length of the openings and thus
provide uniform deposition along the entire length to form Pt tubes
extending along the entire length of the pores. This is illustrated
in FIG. 12. FIG. 12 is a top-down SEM image of a silicon substrate
with ALD deposition of Pt. For purposes of FIG. 12, a portion of
the silicon substrate has been etched away to show the Pt tubes.
These tubes extend along the entire length of each of the openings
in the substrate.
[0082] In other embodiments, as described above, the embedded
electrode layer can be formed by incorporating dopants into the
surface of the membrane, at least along the inner surfaces of the
pores, to provide a surface layer with high electrical
conductivity. For example, in one exemplary process, the embedded
electrodes can be formed by diffusion doping a porous silicon
membrane with n-type doping so that the surface of each pore is
rendered n+ and thereby rendered highly electrically conductive.
Such a method can use gas phase doping of the surface layer of the
porous silicon membrane, using phosphine or arsine gas. Gas phase
diffusion is advantageous for such a process since a gas can easily
penetrate the pores in the Si membrane and diffuse into the exposed
silicon surface. Further, the depth of the diffused layer and the
diffusion dose can be precisely controlled by adjusting diffusion
time and temperatures during and after exposing the silicon
membrane to the doping gas. However, as described above, the
various embodiments are not limited to silicon membranes and n+
doping. Rather, any type of semiconductor membrane can be used and
any appropriate types of dopants can be used.
[0083] Although various exemplary processes have been described
above, the various embodiments of the invention are not limited in
this regard. Rather, one of ordinary skill in the art will readily
recognize that other processes and materials can be used to form
the embedded electrodes in accordance with the various embodiments
of the invention.
[0084] C. Dielectric Surface Layer
[0085] As described above, the inherent zeta potential is based on
the dielectric surface layer disposed over the embedded electrodes
and the chemical properties of the liquid. Accordingly, selection
of the materials of the dielectric surface layer can be selected to
provide a desired level of "passive" control, as described above
with respect to FIG. 1. Thereafter, the embedded electrode can be
used to adjust the zeta potential and provide "active" control,
also as described above.
[0086] As described above with respect to the embedded electrodes,
a smooth surface morphology and low impurity levels are also
desirable for these films. Further, a dielectric film that extends
along an entire length of the pores is also desirable. Accordingly,
in some embodiments of the invention, such films can also be
provided using ALD processes.
[0087] For example, ALD transition metal oxide film ZrO.sub.2 can
be deposited at 250.degree. C. using tetrakis (dimethylamido)
zirconia and H.sub.2O as precursors. FIG. 13A shows a
cross-sectional SEM image of ALD (atomic layer deposited) zirconia
coated AAO substrate. FIG. 13B shows a corresponding EDS Zr mapping
showing uniform distribution of zirconia throughout the entire
thickness of the 60 .mu.m AAO substrate. Further, FIGS. 14A, 14B,
and 14C show top-down SEM images of an AAO substrate, the AAO
substrate with a thin film ALD coating of ZrO.sub.2, and the coated
AAO substrate after the AAO walls have been removed to show single
ZrO.sub.2 nanotubes, respectively. As shown in these figures, the
ZrO.sub.2 is capable of forming dense, smooth ZrO.sub.2 dielectric
layers.
[0088] Another example of a suitable dielectric film is ALD
Al.sub.2O.sub.3 deposited at 300.degree. C. using trimethylaluminum
(TMA) as precursor and water vapor as oxidizing agent. In such a
process, a precursor pulse time can be 0.1 sec for both precursors
with a purging pump time was set at 5 sec. Another example is ALD
TiO.sub.2 deposited at 250.degree. C. using titanium isopropoxide
(Ti(iPrO).sub.4) and H.sub.2O vapor as precursors. Still another
example is silica formed using any available methods. However, the
various embodiments of the invention are not limited to these
dielectric materials and the described processes and other
dielectric materials and/or processes can also be used.
[0089] FIGS. 15A, 15B, 15C, and 15D show zeta potential
measurements of ALD-deposited alumina, titania, zirconia, and
silica films, respectively, as a function of solution pH for
different aqueous solutions (1 mM, 10 mM, 100 mM, and 1M) of
potassium chloride (KCl). This data was collected using
electrophoretic light scattering measurements. As shown in FIGS.
15A-15D, the molarity of the solution has a significant impact on
surface zeta potential. In particular, the molarity and the zeta
potential are inversely related. Further, as shown in FIGS.
15A-15D, the zeta potential is reduced with increasing PH.
[0090] The pH at which the transition from positive to negative
occurs is called the isoelectric point. In other words the
isoelectric point can be defined as the pH value of the aqueous
solution, where the suspended particles or surfaces carry no net
electric charge. It is commonly observed that solid surfaces charge
to form a double layer when in contact with a liquid. For example,
in the case of the aqueous KCl solution, the surface charge
determining ions are H+ and OH-, from the water, and the added K+
and Cl- ions and the oxide films are assumed to be covered with
OH.sup.- surface hydroxyl groups. FIGS. 15A-15D also shown that the
zero point charge (ZPC) remains approximately the same even though
the KCl concentration is different. In a direct comparison the
isoelectric point of ALD alumina (Al.sub.2O.sub.3) occurred at a pH
value exceeding 8. Next is the isoelectric point of ALD zirconia
(ZrO.sub.2) at a pH value of around 7.5, while the zero point
charge of titania (TiO.sub.2) occurred at a pH value of about 6. In
the case of silica, the isoelectric point occurs at a pH of about
3.
[0091] In all these cases, the maximum zeta potential values vary
depending on the type of dielectric surface. Accordingly, depending
on the level of control and the range of zeta potential values
required, an appropriate dielectric material can be selected.
Additionally, this selected can be further based on the molarity
and pH values that will be observed. For example, the data in FIGS.
15A-15D suggests that an ALD alumina surface film is the better
candidate for microfluidic applications that require high, positive
polarity zeta potentials. In contrast, silica surfaces would be
better for microfluidic applications that require high, negative
polarity zeta potentials.
[0092] Although various exemplary processes have been described
above, the various embodiments of the invention are not limited in
this regard. Rather, one of ordinary skill in the art will readily
recognize that other processes and materials can be used to form
the dielectric layers in accordance with the various embodiments of
the invention.
[0093] A description of methods and calculations for evaluating the
performance of the disclosed electroosmotic pumps is provided
below. In providing an EO pump, materials can be selected to be
biocompatible and satisfy other commercialization requirements.
Several embodiments can be described further for which other
materials can be used and adapted for a particular application.
[0094] For example, in some embodiments of the invention, flow
chambers can be built by sandwiching a single or multiple porous
substrates within a PDMS microchannel and a millimeter-scale
plexiglass (PMMA) channel. After curing the PDMS, the porous
substrate can be installed. The integrated system can be sealed
using an adhesive. In one design, space can be created for each
nanoporous substrate within the two pieces of the PMMA channels.
Thin PDMS spacers or plastic o-rings can be used as a bushing
material for bolting the nanoporous substrates between the two
sides of the PMMA channels.
[0095] The performance of such EO pumps can be evaluated based on
the flow rate and thermodynamic efficiency, as outlined in Chen, et
al., "Low-Voltage Electroosmotic Pumping Using Porous Anodic
Alumina Membranes," Microfluid Nanofluid, 5(2); 235-244, 2008. The
thermodynamic efficiency (.eta..sub.eff) is given by:
.eta. eff = .DELTA. P .times. Q V .times. I ( 3 ) ##EQU00002##
where V is the applied voltage, and I is the electric current,
.DELTA.P is the pressure change across the membrane and Q is the
volumetric flow rate. The numerator of this equation is the power
delivered by the pump, while the denominator is the electrical
power input to drive the EO pump. For a fixed pressure load, it is
desirable to have the maximum flow rate per given electrical
power.
[0096] In order to obtain the total mass flow rate from the
nanoporous substrate, the formula derived by Yao, et al., "Porous
Glass Electroosmotic Pumps: Design and Experiments," Journal of
Colloid and Interface Science, 268(1); 143-153, 2003 and Yao; S,
and Santiago, J. G., "Porous Glass Electroosmotic Pumps: Theory,"
Journal of Colloid and Interface Science, 268(1); 133-142, 2003 may
be used:
Q max = - .psi. A .delta..zeta. E x .mu. = .psi. A .delta. .times.
U HS , ( 4 ) ##EQU00003##
where .psi. is the porosity, A is the membrane area, and .delta. is
a correction parameter to account for the EDL displacement
thickness. As noted above with respect to Equation 1, the maximum
flow rate is related to the Helmholtz-Smoluchowski EO velocity UHS
and .psi..times.A is the net flow area, while 6 is the correction
for finite EDL effects on the flow rate, which is expected to be
.delta..apprxeq.0.9 for a 1.times.10.sup.-2M electrolyte. The axial
electric field E.sub.x can be predicted by dividing the potential
drop across the membrane (Veff) with the membrane thickness, L.
Using a porosity value of .psi.=0.3 (a typical value), zeta
potential of 25 mV, nanoporous substrate area of 0.78 cm.sup.2
(A=.pi..times.0.5.sup.2) and a target potential difference of 1 V
across the L=50 .mu.m thick nanoporous substrate, a maximum flow
rate of 50 .mu.L/min is calculated. Thereafter, using Equation 4,
the maximum flow rate increases linearly with membrane porosity,
zeta potential and the applied electric field. Porosities of about
.psi.=0.7 and zeta potential of .zeta.=80 mV using ALD covered
silica nanoporous substrates will provide a 10 fold increase in the
flow rate, resulting in Qmax=0.5 ml/min. Thus, the maximum value of
Qmax/A/V for conventional EO pump devices (with electrodes in the
reservoirs away from the pore openings) is generally about 0.15
ml/(minV cm.sup.2). The EO-pump shown in FIG. 1 (with electrodes at
or near the pore openings) provides a Qmax/A/V of 0.3-0.6 ml/(min V
cm.sup.2).
[0097] Calculations show that the performance of the three-terminal
EO pump shown in FIG. 3 can provide an order of magnitude increase
in the maximum Qmax/A/V value achievable with conventional EO pump
devices (i.e., .about.1-1.5 ml/(min V cm.sup.2)). At the same time,
the disclosed EO pump devices use a minimal applied electric
potential to reduce the electrolysis, electrode oxidation and Joule
heating effects, and increase the thermodynamic efficiency.
APPLICATIONS
[0098] There are a variety of applications for EO pump devices. The
core structure for the membrane and electrodes can be adapted to
function with other pump components such as, for example, fluid
chambers, inlet port(s), and outlet port(s), as known in the
art.
[0099] These applications include, for example, lab-on-a-chip
devices and applications, inkjet printing, ink delivery, drug
delivery, liquid drug delivery, chemical analysis, chemical
synthesis, proteomics, healthcare related applications, defense and
public safety applications; medical applications, pharmaceutical or
biotech research applications, environmental monitoring and defense
applications, in vitro diagnostic and point-of-care applications,
and medical devices. EO pump devices of the embodiments disclosed
herein may also be incorporated into an inkjet printing device.
Other application include PCR (DNA amplification, including real
time PCR on a chip), electronic cooling (e.g., for
microelectronics), using EO pump devices as valves by opposing
pressure driven flow, using EO pump devices to fill and empty
flexible reservoirs to induce functionality via shape change. Still
other applications include, for example, pumping ionized fluids and
colloidal particles, heat transfer and electronic cooling, adaptive
microfluidic mirror arrays, EO-actuators, and EO valves. In another
application, pores can be coated with certain chemicals, enabling
chemical reactions and synthesis of new materials. A benefit for at
least one of the embodiments is high throughput screening and
compound profiling.
[0100] Multiple EO pump devices can be used together in series or
parallel. The EO pump devices can be also integrated within
micro-meter and millimeter scale fluidic systems, by, for example,
stacking them together, for example, to increase the pressure
build-up, or to maintain flow rate to overcome the viscous losses
and pressure loads in long channels.
[0101] The devices described herein can be run on small watch
batteries, and can thus enable a variety of hand held devices.
[0102] Electroosmotic pumps and their applications are known and
can be adapted for the EO pump devices provided herein. See, for
example, U.S. Pat. Nos. 7,667,319; 7,645,368; 7,274,106; 7,231,839,
7,185,697, 7,149,085, 7,134,486, 7,131,486, 7,105,382, 7,086,839,
7,084,495, 7,037,416, 6,992,381, 6,991,024, 6,942,018, 6,934,154,
6,861,274, 6,805,841, 6,747,285, 6,726,920, 6,639,712, 6,613,211,
6,595,208, 6,541,021, 6,395,106, 6,323,042, 6,315,940,
5,573,651.
[0103] While various embodiments of the present invention have been
described above, it should be understood that they have been
presented by way of example only, and not limitation. Numerous
changes to the disclosed embodiments can be made in accordance with
the disclosure herein without departing from the spirit or scope of
the invention. Thus, the breadth and scope of the present invention
should not be limited by any of the above described embodiments.
Rather, the scope of the invention should be defined in accordance
with the following claims and their equivalents.
[0104] Although the invention has been illustrated and described
with respect to one or more implementations, equivalent alterations
and modifications will occur to others skilled in the art upon the
reading and understanding of this specification and the annexed
drawings. In addition, while a particular feature of the invention
may have been disclosed with respect to only one of several
implementations, such feature may be combined with one or more
other features of the other implementations as may be desired and
advantageous for any given or particular application.
[0105] The terminology used herein is for the purpose of describing
particular embodiments only and is not intended to be limiting of
the invention. As used herein, the singular forms "a", "an" and
"the" are intended to include the plural forms as well, unless the
context clearly indicates otherwise. Furthermore, to the extent
that the terms "including", "includes", "having", "has", "with", or
variants thereof are used in either the detailed description and/or
the claims, such terms are intended to be inclusive in a manner
similar to the term "comprising."
[0106] Unless otherwise defined, all terms (including technical and
scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this
invention belongs. It will be further understood that terms, such
as those defined in commonly used dictionaries, should be
interpreted as having a meaning that is consistent with their
meaning in the context of the relevant art and will not be
interpreted in an idealized or overly formal sense unless expressly
so defined herein.
* * * * *