U.S. patent application number 10/478612 was filed with the patent office on 2004-10-21 for microchip integrated multi-channel electroosmotic pumping system.
Invention is credited to Karger, Barry L, Lazar, Juliana M.
Application Number | 20040208751 10/478612 |
Document ID | / |
Family ID | 23126156 |
Filed Date | 2004-10-21 |
United States Patent
Application |
20040208751 |
Kind Code |
A1 |
Lazar, Juliana M ; et
al. |
October 21, 2004 |
Microchip integrated multi-channel electroosmotic pumping
system
Abstract
A microchip integrated microfabricated, microfluidic,
multichannel, preferably electroosmotic pump and pumping system is
disclosed. The electroosmotic pump of the invention comprises a
plurality of microchannels, which begin and end in common
compartments, complexed into an array. The microchannels within the
pump have substantially identical, optimal dimensions of
cross-section and length such that sufficient pressure for optimal
flow of fluid (e.g., liquid or gas) and pressure is generated by
the pump and flow rates are stable and reproducible. To effectuate
efficient flow of fluid without the hindrance of backpressure, an
electroosmotic pump of the invention is coupled to a single channel
of a larger cross-section. A similar structure is also used in an
electroosmotic valve of the invention, where samples are introduced
into an analytical device. The microfluidic electroosmotic pumping
system of the invention generates sufficient flow and pressures by
optimizing the dimensional parameters of cross-section and length
to the microchannels.
Inventors: |
Lazar, Juliana M; (Andover,
MA) ; Karger, Barry L; (Newton, MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN & LEBOVICI LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Family ID: |
23126156 |
Appl. No.: |
10/478612 |
Filed: |
November 24, 2003 |
PCT Filed: |
May 22, 2002 |
PCT NO: |
PCT/US02/16207 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60292780 |
May 22, 2001 |
|
|
|
Current U.S.
Class: |
417/48 |
Current CPC
Class: |
F16K 2099/0074 20130101;
F16K 2099/0094 20130101; F04B 17/00 20130101; B01J 2219/0086
20130101; B01L 2300/0861 20130101; B01L 2400/0415 20130101; B01L
3/50273 20130101; B01L 2400/0418 20130101; F16K 99/0042 20130101;
G01N 1/14 20130101; B01L 2300/0867 20130101; B01J 19/0093 20130101;
G01N 27/44704 20130101; F16K 99/0025 20130101; F16K 99/0001
20130101; F04B 19/006 20130101; B01J 2219/00783 20130101; F16K
99/0017 20130101; B01L 3/502738 20130101; F16K 99/0049 20130101;
B01L 2300/0864 20130101; G01N 27/44791 20130101; B01J 2219/00891
20130101; B01J 2219/00853 20130101; B01L 2300/0816 20130101 |
Class at
Publication: |
417/048 |
International
Class: |
F04F 011/00 |
Goverment Interests
[0002] Part of the work leading to this invention was carried out
with United States Government support provided under a grant from
the National Institutes of Health, Grant No. GM15847. Therefore,
the U.S. Government has certain rights in this invention.
Claims
What is claimed is:
1. A microfluidic pump or valve device comprising: a substrate,
said substrate comprising: a plurality of first microchannels, each
of said first microchannels having a first and a second end,
wherein said first ends of said first microchannels originate at a
common inlet compartment and wherein said second ends of said first
microchannels terminate at a common outlet compartment.
2. The microfluidic device of claim 1, wherein said first
microchannels are open-channeled.
3. The microfluidic device of claim 1, wherein said plurality of
first microchannels comprises at least 5 microchannels.
4. The microfluidic device of claim 1, wherein said plurality of
first microchannels comprises at least 50 microchannels.
5. The microfluidic device of claim 1, wherein said plurality of
first microchannels comprises at least 100 microchannels.
6. The microfluidic device of claim 1, wherein said plurality of
first microchannels comprises at least 1,000 microchannels.
7. The microfluidic device of claim 1, wherein said plurality of
first microchannels comprises at least 10,000 microchannels.
8. The microfluidic device of claim 1, wherein said plurality of
first microchannels are in a substantially parallel
configuration.
9. The microfluidic device of claim 1, wherein said plurality of
first microchannels are in a tortuous configuration.
10. The microfluidic device of claim 1, said device further
comprising an electrode embedded in said common outlet
compartment.
11. The microfluidic device of claim 1, said device further
comprising an electrode embedded in said common inlet
compartment.
12. The microfluidic device of claim 1, said device further
comprising an oriface connecting said inlet compartment to a
surface of said substrate.
13. The microfluidic device of claim 1, said device further
comprising an oriface connecting said outlet compartment to a
surface of said substrate.
14. The microfluidic device of claim 1, said device further
comprising a second microchannel coupled to said outlet
compartment, wherein said second microchannel is larger in
cross-section than an individual said first microchannel.
15. A microfluidic system configured in a substrate, said system
comprising: (a) an electroosmotic pump comprising: (i) a plurality
of first microchannels fabricated in said substrate, each of said
first microchannels having a first and a second end, wherein said
first ends of said first microchannels originate at a common inlet
compartment and wherein said second ends of said first
microchannels terminate at a common outlet compartment; and (ii) a
voltage source coupled to said inlet and outlet compartments for
said first microchannels; (b) a second microchannel, said second
microchannel having a larger cross-section than each of said
plurality of first microchannels, said second microchannel having
first and second ends, wherein said common outlet compartment of
said first microchannels is in fluid communication with said first
end of said second microchannel; and (c) an electroosmotic valve
comprising: (i) two sets of a plurality of third microchannels
fabricated on said substrate, said third microchannels each having
a smaller cross-section than said said second channel, each of said
third microchannels having a first and a second end, wherein said
first ends of said first set of a plurality of third microchannels
originate at a common inlet compartment, wherein said second ends
of said first set of a plurality of third microchannels is in fluid
communication with said second microchannel, wherein said second
ends of said second set of a plurality of third microchannels is
also in fluid communication with said second microchannel and
wherein said first ends of said second set of a plurality of third
microchannels terminate at a common outlet compartment; and (vi) a
voltage source coupled to said inlet and outlet compartments.
16. The microfluidic system system of claim 15, wherein the ratio
of the diameter of a said first microchannel to the diameter of
said second microchannel is 1:1.25-1:1000.
17. The microfluidic system system of claim 15, wherein said
substrate material is selected from the group consisting of glass,
quartz, silicon, polysilicon, polymeric materials (organic or
inorganic) and ceramic.
18. The microfluidic system system of claim 15, wherein said outlet
compartment associated with said plurality of firsts microchannels
comprises a semi-permeable gate.
19. The microfluidic system system of claim 15, wherein said
semi-permeable gate is selected from the group consisting of a
porous glass, graphite, and polymeric organic or inorganic
material.
20. A method of transporting a material for analysis through a
microchannel in a microchip, said method comprising the steps of:
providing in said microchip a plurality of first microchannels and
a second microchannel, wherein said second microchannel has a first
and a second end, wherein the cross-section of each of said first
microchannels is smaller than the cross-section of said second
microchannel, wherein said first end of said second microchannel is
in fluid communication with said plurality of first microchannels
at one end of each and wherein said second end of said second
microchannel is in communication with a detection system for
analysis of a sample; introducing a sample into said second
microchannel; and applying a potential difference across a length
of said plurality of microchannels to electroosmotically move said
sample in said second microchannel.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C.
.sctn.119(e) of U.S. Provisional Patent Application No. 60/292,780,
filed May 22, 2001, entitled MICROCHIP INTEGRATED OPEN-CHANNEL
ELECTROOSMOTIC PUMPING SYSTEM, the whole of which is hereby
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0003] Microfluidics and instrument miniaturization.sup.1 have
experienced significant growth in activity in recent years in
response to the significant increase in use of microchips as
bioanalytical tools. A major aspect of microfluidics refers to the
manipulation of fluid flows in the microchip channels. For
analytical processes, such as fluid valving,.sup.2,3 mixing,.sup.4
or electrically driven separations,.sup.2-5 flow streams are
typically generated using electrical forces (electroosmotic flow,
or EOF). However, EOF is dependent on the surface charge of the
channel walls and consequently is sensitive to the physicochemical
properties of the sample (pH, ionic strength, organic content). For
example, the EOF can be easily reduced or even suppressed if the
channel surface is altered by contact with specific sample types.
Alternatively, for some techniques such as micro-liquid
chromatography (.mu.LC) or flow injection analysis (FIA), or for
some samples, such as those containing cells that require zero
electric field conditions for their manipulation, fluid flows on
microchips may, or must, be generated by differential
pressure..sup.6-8 Typically, this is accomplished by connecting
external devices to the microchip, e.g., vacuum, gas pressure
generators or syringe pumps. However, the connection of external
devices to microchips, while effective, increases the complexity of
the system, and integration and multiplexing capabilities may be
compromised.
[0004] A number of micropumps have been described in the art.
Mechanical pumps that use a membrane actuated by various forces
(e.g., piezoelectric, electromagnetic, pneumatic).sup.9-15 are
capable of pumping fluids of various physicochemical properties;
however, the flow is pulsed, and the fabrication of the micropump
is relatively complex. Non-mechanical pumps, with no moving parts,
that operate on the basis of a large variety of principles (e.g.,
electrokinetic, centrifugal, electrohydrodynamic, etc.).sup.16-29
have been implemented. The flow produced by these pumps is
generally pulse free and varies from a few nL/min to hundreds of
.mu.L/min. However, the generated pressures are low, up to only a
few psi. Fabrication procedures are often complex, as well.
[0005] The use of electroosmosis for pressurized pumping was
advanced almost forty years ago.sup.30,31 and demonstrated later in
open.sup.32 and packed.sup.16,17 capillary columns, and for
open-channel microchip platforms..sup.18-22 While packed
electroosmotic pumps were shown to be capable of generating both
flow and pressure, having packed particles or porous materials
within the pump structure may, inter alia, impose limitations on
the pumping channel size, may necessitate additional steps in pump
fabrication and may affect reproducibility from one pump to
another. The open-channel configurations were capable of producing
flow but not sufficient pressure for the intended applications.
Accordingly, it would be useful to have a microchip-integrated
pumping device that simultaneously generates both sufficient flow
and pressure for most analytical applications on a microchip and
that is easy to fabricate, implement and use. Furthermore, an
optimum pump design would ensure that a generated EOF is delivered
to the analysis system and is not leaked out of the pump
itself.
BRIEF SUMMARY OF THE INVENTION
[0006] The invention is directed to a microchip integrated
microfabricated, microfluidic, multichannel, preferably
electroosmotic pump and pumping system. The electroosmotic pump of
the invention comprises a plurality of microchannels, which begin
and end in common compartments, completed into an array. The
microchannels within the pump have substantially identical, optimal
dimensions of cross-section and length such that sufficient
pressure for optimal flow of fluid (e.g., liquid or gas) and
pressure is generated by the pump and flow rates are. stable and
reproducible. In one aspect, the microchannels of the
electroosmotic pump of the invention are open channels, i.e., not
containing packed or porous materials. To effectuate efficient flow
of fluid without the hindrance of backpressure, an electroosmotic
pump of the invention is coupled to a single channel of a larger
cross-section. A similar structure is also used in an
electroosmotic valve of the invention, where samples are introduced
into an analytical device. The microfluidic electroosmotic pumping
system of the invention generates sufficient flow and pressures by
optimizing the dimensional parameters of cross-section and length
of the microchannels.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Other features and advantages of the invention will be
apparent from the following description of the preferred
embodiments thereof and from the claims, taken in conjunction with
the accompanying drawings, in which:
[0008] FIG. 1 shows a schematic diagram of the microfabricated
electroosmotic pumping system in accordance with the invention;
[0009] FIG. 2 shows a schematic diagram of a different embodiment
of the microfabricated electroosmotic pumping system in accordance
with the invention;
[0010] FIG. 3A shows representative flow, diameter and length in a
schematic representation of a micropump with one pumping
channel;
[0011] FIG. 3B shows a graph of the flow distribution, or the
F/F.sub.eof coefficients calculated for a micropump with one
microchannel at various d.sub.1/d.sub.2 ratios;
[0012] FIG. 4A shows a schematic representation of a micropump with
n pumping channels;
[0013] FIG. 4B shows a graph of the flow distribution, or the
F/F.sub.eof coefficients calculated for a micropump with n
microchannels at d.sub.1/d.sub.2=0.1;
[0014] FIG. 5A shows a graph of a total forward flow generated in a
pumping system with d.sub.1/d.sub.2=0.1 and L.sub.1/L.sub.2=0.1
with varying number of pumping channels;
[0015] FIG. 5B shows a graph of a total forward flow generated in a
pumping system with d.sub.1/d.sub.2=0.1 and L.sub.1/L.sub.2=0.001
with varying number of pumping channels;
[0016] FIG. 6A shows a graph of achievable pressure as a function
of the micropump channel diameter, for a restriction with
d.sub.2=30 .mu.m diameter (L.sub.1=0.03 m, L.sub.2=30 m, U=3000
V);
[0017] FIG. 6B shows a graph of achievable pressure as a function
of the micropump channel diameter, for a restriction with
d.sub.2=50 .mu.m diameter (L.sub.1=0.03 m, L.sub.2=30 m, U=3000
V);
[0018] FIG. 7A shows a schematic diagram of an injection of a well
delimited sample plug moving through the electroosmotic valve of
the invention;
[0019] FIG. 7B shows a schematic diagram of a sample plug being
transported down the channel in the electroosmotic valve of the
invention;
[0020] FIG. 8A shows a schematic diagram of an electroosmotic
pumping system functioning as an electroosmotic valve;
[0021] FIG. 8B shows a schematic diagram of an electroosmotic
pumping system functioning as an electroosmotic valve;
[0022] FIG. 9 shows an exemplary multiplexed pumping system in
accordance with the invention; and
[0023] FIG. 10A and FIG. 10B show a cross-sectional view and a plan
view, respectively, of a micropump according to the invention
having filtering and gating elements.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The microfabricated microfluidic pumping system of the
invention comprises one or more miniaturized pumping units, which
are capable of stable fluid delivery at flow rates and
backpressures compatible with common analytical applications
carried out on a microchip and sufficiently small to enable
multiplexing of individual pumps. A multiple channel micropump
based on electroosmotic pumping principles in accordance with the
invention comprises a plurality of parallel, small cross-section
microchannels (e.g., narrow or shallow) that deliver fluid to a
microchannel of larger cross-section. This design allows the
simultaneous generation of both flow and pressure and can be
implemented in both packed channel and open-channel configuration.
In the context of this invention, an open-channel means that no
packed particles or porous materials are added to or contained in
the microchannels. The pumping system of the invention offers
numerous advantages. First, it can easily be integrated on
microfluidic platforms and utilized for fluid propulsion on a
microchip. Secondly, its fabrication using standard
photolithographic and wet chemical etching technologies (or other
microelectromechanical systems (MEMS) technologies) ensures high
manufacturing and operating reproducibility. Thirdly, the
simplicity of the design ensures robustness, reliability and
trouble free operation.
[0025] To illustrate the utility of an electroosmotic pumping
system according to the invention, the micropump is designed to fit
into an integrated microfluidic analysis scheme for delivery of,
e.g., peptide samples for, e.g., electrospray ionization-mass
spectrometry (ESI-MS) analysis. In addition, the ability to
reproducibly control very low flow rates, at low and high pressure
drops, where conventional systems do not perform well, enables the
utilization of the micropump for many other micro-total analysis
systems (.mu.-TAS) applications. The principle of .mu.-TAS is based
on integrating all necessary parts and methods for analysis in
miniaturized devices. The benefits of miniaturizing an analysis
system include, but are not limited to, generation of multiple
units at low cost; reduction in sample and reagent consumption and
waste production; high throughput assays; small compact design;
simple and reliable operation; and integration of several units or
with existing systems.
[0026] Generally, the micropump of the invention can be used to
deliver fluid flows in electric field free regions and perform
sample transfer, gradient generation, or fraction
collection/deposition. A large variety of applications can be
envisioned for a micropump according to the invention, including,
inter alia, eluent flow and/or pressure generation for condensed
phase separation techniques (e.g., micro liquid chromatography in
open, packed, monolithic or microfabricated channels, pressure
assisted capillary electrochromatography or capillary
electrophoresis, isoelectric focusing, affinity chromatography,
immunoassays); single, parallel or sequential sample delivery for
electrospray ionization-mass spectrometry (ESI-MS), matrix assisted
laser desorption ionization (MALDI)-MS, optical detectors, or other
detection systems; single, parallel or sequential sample delivery
systems to microreactors, mixers, preconcentrators, filters, or
other functional elements integrated on the microchip; single,
parallel or sequential eluent or sample delivery systems to
off-chip applications; solvent gradient generation for on or
off-chip applications; solvent and reagent delivery for flow
injection analysis; sample or sample fraction
generation/dispensing/collection from microfluidic systems; sample
or sample fraction deposition on MALDI-MS targets; sample
transfer/delivery to, or from, combinatorial libraries; sample
delivery for medical applications (external or animal implanted
devices); generation of microreactors or sample alteration devices
by chemically or physically modifying the surface of the
micropuming channel walls, while simultaneously maintaining pumping
capabilities; generation of multiplexed devices for high throughput
screening by integration of a series of pumps on one single
microchip platform; and generation of pressure or vacuum in a
liquid or gas phase within a microfluidic device, or an attached
additional device.
[0027] FIG. 1 illustrates a simplified schematic drawing of an
exemplary microfabricated electroosmotic pumping system of the
invention. A pumping unit 10 can be incorporated on one microchip
device. However, multiple pumping units can also be incorporated,
e.g., in parallel as shown in FIG. 2 and FIG. 9. The microchip
device may be about a few square millimeters, e.g., 5-50 mm.sup.2.
A multiplexed device (see, e.g., FIG. 9) that may contain, for
example, eight, sixteen, or even 96 individual pumps may require a
larger size chip. Pumping channel system 11 of pumping unit 10
comprises a plurality of first shallow microchannels 12 (e.g.,
2-10,000 microchannels). All pumping channels 12 have first and
second ends, which are in fluid communication with a common inlet
14 and a common outlet compartment or reservoir 16, respectively.
Electrodes inserted in reservoirs 14 and 16 are connected to high
voltage power supply 30 and are used to apply a voltage drop across
the pump. The pumping channels are exposed only to buffer solutions
and do not come in contact with any sample that may be added to the
microchip. The pump size is quite variable and depends on the
application (e.g., 1.times.10, 2.times.10, 5.times.50, 1.times.50
mm). The length of each microchannel may range from, e.g., about
5-50 mm. The channel depth, width, equivalent diameter or
cross-section may range from about less than 1 .mu.m to a few
micrometers (e.g., 0.5-10 .mu.m). The number of microchannels in
one pump may range from two to more than a thousand (e.g., 2-10,000
channels). The spacing between the microchannels is in the low
micrometer range (e.g., 1-50 .mu.m). In one aspect, the pump unit
of the invention comprises 4 microchannels, 10 microchannels, 25
microchannels, 100 microchannels or 1000 microchannels.
[0028] The pumping (or valve) system of the invention is activated
by introducing into the channels a buffer or solvent, appropriate
for preserving or increasing the surface charge on the pumping
channel walls. The microchannels 12 can be filled by capillary
action or by action of a pressure differential. Through electrodes
placed in the reservoirs 14 and 16, a potential differential is
applied across the microchannels 12. Depending on the surface
properties of the channel (whether negatively or positively
charged), the larger voltage must be applied to the appropriate
reservoir, such that the electroosmotic flow will have the desired
direction. For example, for a glass micropump having uncoated
pumping channel walls, by applying a 2000 V on reservoir 14 and
connecting reservoir 16 to ground, electroosmotic flow will be
generated from reservoir 14 in the direction of reservoir 16.
Electroosmotic flow is created due to the following mechanism. The
functional groups on the pumping microchannel walls ionize in the
presence of the solvent/fluid that is filling the microchannels and
attract a layer of oppositely charged ions. When the potential
differential is applied, this layer of oppositely charged ions
start to move in the direction of the appropriate electrode
(negative electrode for positively charged ions), dragging along
bulk fluid flow. If this electroosmotic flow is restricted from
evolving freely by the structure of the system or by the rest of
the microfluidic channel network on the chip, pressure will be
created inside the microfluidic structure, and the electroosmotic
flow will be distributed through all the channels of the device
through a pressure controlled mechanism. By reversing the polarity
on the electrodes in reservoir 14 and 16, the direction of pumping
will be reversed, and the pump will draw fluid out from the rest of
the channels.
[0029] The pumping system delivers fluid via a common outlet
compartment, connected to reservoir 16, to larger diameter second
microchannel 13, which may be connected to a network of channels or
to other devices. As shown in FIGS. 1, 7 and 8, second microchannel
13 is connected to network channel 20 for, e.g., sample infusion or
separation. Channel 20 comprises electroosmotic valve 17 of the
invention, which is used, e.g., for sample introduction. The
electroosmotic valve is open to electroosmotic driven flows and
essentially closed to pressure driven flows. At the other end of
the network channel 20, an electrospray emitter 32, for example, or
a detection or a sample collection device using any of the systems
described above, can be integrated for analysis of a sample.
[0030] Sample injection in a microfabricated analysis systems is
accomplished electrokinetically.sup.2,3 or by pressure
gradients,.sup.44 and sample manipulations are performed with
electrokinetic forces. If sample manipulations are to be performed
using pressure driven flows, a fluid valving system, such as
described here, is necessary to prevent leakage of the fluid flow
into the sample or sample waste reservoirs. According to the
invention, electroosmotic valving is accomplished using very narrow
channels for the sample injection and waste lines, in a
configuration similar to that of the electooosmotic pump. In a
pressurized analysis system, narrow injection channels.sup.45 can
act as efficient valving components. As shown in FIG. 1, 7 and 8,
in a valve according to the invention, a sample can be infused
electroosmotically from reservoir 22 to reservoir 24 through the
sample and sample waste pluralities off channels, 26 and 28,
respectively, and a double-T injector 18..sup.46 The double-T
injector can be an open or packed channel, e.g., as part of a
preconcentration or .mu.LC system. After completion of the
injection, the pump can be started. For sample introduction and
waste channels with similar dimensions to the pumping
microchannels, additional pressure driven fluid loss through these
channels could be negligible. Theoretically, if the number of
injection and waste channels equals the number of pumping channels,
the coefficient F/F.sub.eof would be unaffected by the
injection/waste channels when d.sub.1/d.sub.2=0.01, and would drop
to only 50% of its original value when d.sub.1/d.sub.2=0.1
(L.sub.1:L.sub.2=1:1000 in both cases).
[0031] In operation of the electroosmotic valve (as shown in FIG.
7A), all the appropriate channels and reservoirs of valve 17 and
pump 10 are first filled with an appropriate eluent. Sample is
injected into the valve inlet reservoir 22, and a voltage drop is
established between reservoirs 22 and 24 to allow for a sample plug
to be loaded into microchannel 20. The sample is electroosmotically
carried from reservoir 22 in the direction of reservoir 24 by the
application of a potential difference, e.g., 2000 V, between
reservoir 22 and reservoir 24. A very small voltage drop (e.g., 100
V) between pump reservoir 14 and pump reservoir 16 may be needed on
the pump unit 12 as well to help focus the injected plug in the
microchannel 20.
[0032] As depicted in FIG. 7B, for injecting the sample plug down
the microchannel 20 for analysis, a large voltage drop, e.g., 2000
V, between reservoir 14 and reservoir 16 is applied to the pumping
unit 11. The electrodes in reservoirs 22 and 24 are maintained at
the same value as the voltage on reservoir 16, or they are removed
from these reservoirs, in order to suppress the electroosmotic flow
in the direction of the sample and sample waste reservoirs. The
voltage drop across the pump will generate electroosmotic flow in
the pump and pressurized flow in the rest of the system. The large
hydraulic resistance in the sample inlet and outlet channels will
prevent back pressure leakage, or will allow for only a small flow
leakage through these channels. Consequently, the sample plug will
be transported only down the microchannel 20.
[0033] As shown in FIG. 8A, an electroosmotic pumping system
composed of two parallel pumps can function, alternatively, as an
electroosmotic valve, as well. One of the pumps, e.g., pump 10, is
used for loading the sample, while the other pump, e.g., pump 100,
is used for loading the eluent. Small, well delimited sample plugs,
however, cannot be injected with this configuration.
[0034] In FIG. 8B, for injecting the plug for analysis, a large
voltage drop is applied across pump 100, for example, 2000 V
between reservoir 104 and reservoir 106. The voltage on reservoirs
14 and 16 is maintained at the same value as the voltage on
reservoir 106, or the electrodes are removed from these reservoirs.
The voltage drop across the pump 100 will generate electroosmotic
flow in this pump, and pressurized flow in the rest of the system.
For properly designed pumping channel diameters, the pressurized
leakage flow through pump 12 will be minimal.
[0035] The potential differential for electroosmotic flow (EOF)
generation is applied between the inlet and outlet reservoirs and
may be provided by a power supply. Depending on the length of the
pumping channels and the desired flow rate and pressure, the
necessary voltage drop for the operation of the electroosmotic
micropump may vary from a few tens to thousands of volts (e.g.,
50-2000 V/cm).
[0036] In some configurations, outlet compartment or reservoir 16
can be common for systems with multiple pumps. Referring now to
FIG. 10, a semi-permeable gate 34, that allows for an exchange of
ions but not of the bulk eluent flow (liquid or gas), is created at
the bottom of the outlet reservoir. The gate prevents EOF leakage
in the direction of the exit electrode 35' placed in the outlet
reservoir. For example, a porous glass disc (e.g., 5 mm in
diameter, 0.8-1 mm in width, and 40-50 .ANG. pore size, prepared by
Chang Associates (Worcester, Mass.)) can be used as the gate, but
other materials may be used, e.g., graphite, polymeric organic or
inorganic membrane materials. While most reservoirs on a microchip
are made of glass, reservoir 16 is fabricated from a PEEK external
nut. The porous glass disc is secured (e.g., between two gasket
elements) at the bottom of reservoir 16 with a corresponding
internal nut. This arrangement provides for robustness and
exchangeability of the porous glass disc. Since the pumping
microchannels act as a filter, clogging of some of the channels can
occur. To reduce this effect, a short filter element 36 (e.g., 100
.mu.m in length), of the same size and density as the collection of
pumping channels, can be introduced, preferably at each end of the
pump. Filter elements 36 are spaced from the ends of the pumping
channels by intermediate compartments 38 and terminate at end
compartments 14' and 16', respectively. In the embodiment shown,
end compartments 14', 16' are connected to orifaces 40 in the
substrate, which make connection with reservoirs 14 and 16 on the
exterior surface of the substrate.
[0037] The pump can be fabricated in a variety of substrate
materials such as, inter alia, glass, quartz, silicon, polysilicon,
polymeric materials (organic or inorganic), ceramic, or other
materials. The micropumps can be made using microfabrication
techniques, for example, photolithography and wet chemical etching,
or other microelectromechanical systems (MEMS) technologies (e.g.,
dry etching, laser ablation, injection molding, embossing,
stamping).
[0038] Microfabricated devices that contain an electroosmotic
micropump of the invention can be fabricated from one or two
substrates made of any of the materials described above. The
microfluidic channels can be made using one of the substrates
described above by any of the microfabrication techniques. For
example, micropump and sample handling channels defined on a
photomask by 2 and 20 .mu.m wide lines (other dimensions are
possible) are transferred to the substrate using photolithography.
After exposure, substrate etching is conducted to achieve channel
depths of 0.5-10 .mu.m for the micropump and 20-100 .mu.m for
sample handling. This substrate is placed into contact with the
second (or top) substrate such that the surface of the second
substrate is covering the channels of the first substrate. The two
substrates are then bonded together to seal the enclosed channels.
Alternatively, the micropump channels may be fabricated in the
bottom substrate, while the rest of the microfluidic network of
channels and chambers may be fabricated in the top substrate, made
of the same or a different material as the bottom substrate. They
come in contact with each other by proper alignment prior to
bonding. Alternatively, the micropump may be fabricated in a
substrate that is sandwiched between other two substrates.
[0039] The walls of the pumping channels may be the bare,
unmodified surface or the chemically/physically altered surface of
the substrate material that is used for the fabrication of the
device. In the case of certain substrate materials, chemical
treatment with, or physical adsorption of an appropriate surface
coating agent on the pumping channel walls, may be necessary in
order to provide for sufficient and adequate charged functional
groups on these walls.
[0040] The holes or the apertures necessary to access the pumping
channel and the other channels of the microfabricated device are
fabricated prior to bonding in one or both of the substrates. These
holes (typically of 1-2 mm in diameter) are used for the
introduction of fluids and reagents into the chip, and for the
placement of electrodes that ensure electrical contact with the
network of microfluidic channels. These holes can also function as
reservoirs for the solvent and reagents that are to be manipulated
on the chip. To increase the volume of solvent or reagents that may
be handled by the chip, additional reservoir structures may be
attached to the access holes. Cooling or heating devices may be
attached to the pump for specific applications.
[0041] Alternatively, in order to increase the resistance to back
flow leakage, the straight microchannels can be replace by a
tortuous arrangement, or even a more complicated microfabricated
monolithic structure.
[0042] Alternatively, the electrodes may be embedded in the
microfabricated device at appropriate positions. Embedded
electrodes are fabricated using MEMS technologies prior to bonding
of the two substrates.
[0043] Alternatively, the inlet and outlet reservoirs may be placed
not directly on the pump inlet or outlet, but at the terminus of
some larger channels, with small electrical resistance, that come
in direct contact with the inlet and outlet of the pumping
channels.
[0044] Alternatively, a structure (porous or multiple channel
structure, etc.) with similar properties to the semi-permeable
glass disc that is inserted in the outlet reservoir, can be created
by MEMS technologies directly in the microchip body, at appropriate
positions. This alternate gate structure can be used as fabricated,
or it may be filled before utilization with an electrically
conductive polymer that is replaceable. It will have the same
function, to allow the exchange of ions but prevent transport of
bulk flow.
[0045] Alternatively, in the case of configurations with two
pumping units, the walls of the pumping microchannels of one or
both pumps may be chemically or physically altered such that their
surface is positively charged in the case of one of the pumps and
negatively charged in the case of the second pump. Thus, by
applying a potential differential between the two inlet reservoirs,
14 and 104, both pumps will generate electroosmotic flow in the
same direction, but one will function under a positive voltage
gradient, while the other will function under a negative voltage
gradient. This configuration eliminates the need for the outlet
reservoir 16 and the semi-permeable gate.
[0046] In another embodiment, the electrospraying or sample
deposition capillary 32 may be replaced by a structure fabricated
by MEMS technologies.
[0047] As shown in FIG. 2, to apply an eluent gradient with
multiple pumps, e.g., two pumps 10 and 100, microchannels are
filled with solvent or buffer. The solvent or buffer (SA) in
reservoir 104 is replaced with a different solvent or buffer (SB).
A porous glass disc 34 is placed and secured in reservoir 16 and
106. An identical voltage is applied to both reservoir 16 and 106.
Voltages are then applied to reservoir 14 and 104 in a ratio that
will provide the desired solvent mixture. The ratio of the two
voltages is modulated (in steps or continuously) to generate a
desired solvent gradient that is delivered to the system. The
generated flow or pressure is monitored or measured.
[0048] In the present invention, a pump with a large number of
narrow or shallow open microchannels is designed to produce EOF.
Advantageously, the multiple microchannels ensure the generation of
sufficient flow rate, while the small dimensions of the
microchannels result in the necessary hydraulic resistance to
pressurized back flow leakage. In contrast, a single, large
diameter open-channel electroosmotic micropump could generate flow
only if the backpressure is small, and the flow would be highly
dependent on the backpressure..sup.37 Properly designed, multiple
channel pump configurations with small diameters according to the
invention overcome this shortcoming, being capable of generating
sufficient flow that is independent of backpressure. The pumping
principles are further explained below.
[0049] Electroosmotic flow generation in single channels. To
illustrate the principle of the open-channel electroosmotic
micropump, an ideal system composed of cylindrical capillaries is
considered and the conditions that must be met for proper pumping
are examined. For this discussion, the contributions of local
hydraulic resistances to the total pressure drop will be neglected.
The standard equations that describe fluid flow and velocity in
pressure driven (F.sub..DELTA.p and v.sub..DELTA.p) and
electroosmotic driven (F.sub.eof and V.sub.eof) open capillary
systems are as follows:.sup.38 1 F p = 128 p L d 4 ( 1 ) p = 1 32 p
L d 2 ( 2 ) F eof = o r 4 U L d 2 ( 3 ) eof = o r U L ( 4 )
[0050] where .DELTA.p =the pressure drop across a capillary of
diameter d and length L, .rho.=the viscosity of the fluid,
.epsilon..sub.o=the electrical permitivity of the vacuum,
.epsilon..sub.r=the relative permitivity of the medium (or
dielectric constant), .zeta.=the zeta potential at the capillary
wall, and U=the voltage applied across the capillary of length L.
From equations (1) and (3), it can be seen that F.sub..DELTA.p
(Poisseuille flow) and F.sub.eof (electroosmotic flow) are
dependent on d.sup.4 and on d.sup.2, respectively, due to the fact
that v.sub..DELTA.p is dependent on d.sup.2, while v.sub.eof is
independent of d. Therefore, if fluid flow generation and
distribution in a microfluidic structure occurs by both .DELTA.p
and EOF, a net fluid flow to be induced in preferential directions
is expected.
[0051] Consider a system comprised of a single narrow channel (I)
with dimensions d.sub.1 and L.sub.1 connected to a large channel
(II) with dimensions d.sub.2 and L.sub.2 (FIG. 3A). If a potential
drop is applied only to the narrow channel to generate flow through
an electroosmotic mechanism (i.e., .about.d.sub.1.sup.2), the flow
in both channels can be redistributed through a pressure-controlled
mechanism (i.e., .about.d.sub.1.sup.4 and d.sub.2.sup.4). Pressure
generation is due to the fact that the field free capillary (II)
acts as a restrictor for the F.sub.eof that is produced in
capillary (I) under the influence of the electric field. The
F.sub.eof will distribute into a forward flow through the large
channel (F) and a back flow through the narrow channel (F.sub.b).
Pressurized fluid flow will be proportional to
d.sub.1.sup.4/d.sub.2.sup.4 regardless of the method used to
produce it. Practically, for a ratio of d.sub.1/d.sub.2 of only
1:10 (L.sub.1 being equal to L.sub.2), the flow will be distributed
in a ratio F/F.sub.b of 1:10,000, i.e., essentially all the flow
will be directed through the large channel.
[0052] When both pressure and a potential gradient are applied to a
capillary, the resulting flow is a sum of the Poiseuille and
electroosmotic flows..sup.30 By balancing the electroosmotic input
flow (F.sub.eof) given by equation (3) with the pressurized output
flows (F and F.sub.b) given by equation (1), it can be shown that
the pressurized forward flow (F) in the large channel is dependent
on the EOF in the narrow channel (F.sub.eof), according to the
following equation: 2 F = F eof L 1 L 2 L 1 L 2 + ( d 1 d 2 ) 4 ( 5
)
[0053] The F/F.sub.eof ratio seems to be dependent on channel
dimensions, but independent of pressure or the actual EOF in the
system. F/F.sub.eof could be defined as an efficiency coefficient
for a given pump, since it will determine what fraction of the
originally produced EOF is actually pumped forward in the system.
F/F.sub.eof values can be calculated from equation (5) and
represented as a function of d.sub.1/d.sub.2 and L.sub.1/L.sub.2
(FIG. 3B). As seen, in this figure, small diameter and long pumping
channels (i.e., small d.sub.1/d.sub.2 and large L.sub.1/L.sub.2
ratios), result in large F/F.sub.eof values and a more efficient
electroosmotic pump. For example, for d.sub.1/d.sub.2=0.1, a
relatively effective pump is created, since the F/F.sub.eof
coefficient decreases from 1 to only 0.9 even when the large
channel is very long and imposes a considerable backpressure
(L.sub.1/L.sub.2=0.001).
[0054] Electroosmotic flow generation in multiple channels. For
very small d, values, the actual EOF in a channel and consequently
the overall flow in the system will be quite low. For a channel
diameter of 1 .mu.m, the EOF will be 0.17 nL/min [calculated from
equation (3) for a capillary with L.sub.1=0.03 m and U=3000 V, and
for parameters given in reference 45:
.epsilon..sub.o=8.85.times.10.sup.-12 C.sup.2 N.sup.-1 m.sup.-2,
.epsilon..sub.r=80, .zeta.=50.times.10.sup.-3 V, and n=0.001 N s
m.sup.-2]. In order to compensate for this low EOF, multiple small
diameter channels (n) can be connected to one large channel (FIG.
4A). However, in this case, by balancing again the flows, F will be
defined by equation (6), and F/F.sub.eof will be dependent on n: 3
F = F eof L 1 L 2 L 1 L 2 + n ( d 1 d 2 ) 4 ( 6 )
[0055] The larger n, the greater the total F.sub.eof, but
F/F.sub.eof becomes smaller. F/F.sub.eof f values are represented
as a function of n and L.sub.1/L.sub.2 for d.sub.1/d.sub.2=0.1 in
FIG. 4B. For example, for d.sub.1/d.sub.2=0.1 and
L.sub.1/L.sub.2=0.001, the F/F.sub.eof ratio drops from 0.9 to 0.09
when n is increased from 1 to 100.
[0056] The optimum number (n) and dimensions (d.sub.1 and L.sub.1)
of the micropump channels that can generate the desired flow in the
main channel can be calculated. If F/F.sub.eof is determined from
equation (6) and multiply this value with the total EOF value
calculated according to equation (3), diagrams that show the total
pressure driven flow for given system characteristics (in this case
d.sub.1/d.sub.2=0.1) can be constructed (FIG. 5). As expected, the
total flow rate increases with the increase of d, and n. However,
it is important to observe that when the backpressure increases
significantly (i.e., L.sub.2 becomes much larger than L.sub.1),
supplementing the pump with additional microchannels does not bring
any additional benefit in increasing the flow rate (compare FIG. 5B
and FIG. 5A). At high backpressures, the flow cannot be pumped, but
rather will leak backwards in proportion to the number of channels
n. The beneficial effect of increasing the number of microchannels
requires properly chosen dimensions. At progressively smaller
d.sub.1/d.sub.2 ratios, the back flow leakage becomes negligible.
For example, as shown in Table 1, the F/F.sub.eof coefficients
maintain a high and relatively constant value for a large range of
pumping microchannels when d.sub.1/d.sub.2 drops to a value of
0.01. Based on the above considerations, and depending on the
specific applications, micropumps with 100 to 1000 pumping channels
of 1-3 .mu.m in depth would perform as effective pumping systems on
microfluidic platforms.
1TABLE 1 Variation of F/F.sub.eof coefficient with d.sub.1/d.sub.2
and n (L.sub.1/L.sub.2 = 0.001) d.sub.1/d.sub.2 n = 1 n = 10 n =
100 n = 1000 n = 10000 1 0.001 1E-04 1E-5 1E-6 1E-7 0.1 0.909 0.5
0.091 0.009 0.001 0.01 0.999 0.999 0.999 0.990 0.910
[0057] Electroosmotic pressure generation. The pressure that can be
created in the system is calculated by balancing the input
electroosmotic flow in the n narrow channels with the output
pressure flow through all channels: 4 p = 32 n o r U d 1 2 L 1 n d
1 4 L 1 + d 2 4 L 2 ( 7 )
[0058] A plot of this pressure as a function of pumping channel
diameter, for given system characteristics, is shown in FIG. 6. It
is worth noting that the pressure depends on the number and
dimensions of the pumping microchannels, the voltage applied to the
pump, the zeta potential, and the relative permitivity of the
medium. The pressure is, however, not dependent on the fluid
viscosity. In a system with enhanced restriction, i.e., d.sub.2=30
.mu.m (FIG. 6A), the pressure will be higher than in a system with
a lower restriction, i.e., d.sub.2=50 .mu.m (FIG. 6B). The
condition of maximum pressure: 5 P d 1 = 0 ( 8 )
[0059] is accomplished when: 6 nd 1 4 L 1 = d 2 4 L 2 ( 9 )
[0060] The absolute maximum pressure that can be generated with
microchannels of a given dimension can be inferred from conditions
of zero flow out from the system [.e., the second term in the
denominator of equation (7) is set to zero], and is dependent only
on the channel diameter, applied voltage and channel surface
properties: 7 p max = 32 o U d 1 2 ( 10 )
[0061] For a pump that would be used for a LC separation, once the
parameters of the separation column (d.sub.2, L.sub.2) and the
optimum eluent flow rate are determined, the pressure necessary to
generate this flow can be calculated, and the appropriate pump
configuration (n, d.sub.1, and L.sub.1) can be chosen from diagrams
such as shown in FIGS. 5 or 6. For a packed column, d.sub.2 and
L.sub.2 are the dimensions of an equivalent open tubular LC column
that produces the same pressure drop. Alternatively, the
d.sub.2.sup.4/L.sub.2 ratio in the denominator of equation (7)
could be replaced by a term that takes into consideration the
porosity and the permeability of the column. The microchip
implementation of fully integrated micro-LC separations performed
on polymeric monolithic stationary phases that are commonly
performed under 100 psi.sup.39 thus becomes feasible with the
integration of such pumping systems. Theoretically, (see equation
10), if pressure generation and not flow were the main purpose of
these micropumps, and if microchip material and connecting elements
would be capable of withstanding the high pressures, hunrdreds of
bars could be produced with sub-micrometer sized channel EOF
pumps.
[0062] Use
[0063] A stand-alone microfabricated device with an integrated
multichannel EOF pump according to the invention has been
constructed for producing controllable pressure driven flows within
microfluidic channels. If desired, the pump can be constructed as a
stand-alone module and can be employed to deliver fluid flow for
on-chip or off-chip applications, as well. Micropumps with 4-100
pumping channels were constructed that produced flow rates in the
range of 10-400 nL/min and developed pressures up to 80 psi. The
flow rate and eluent gradients were adjusted by varying the
potential drop over the pump. The experimental results followed
closely the trends predicted by the theoretical calculations.
[0064] The new approach for microfluidic valving according to the
invention, e.g., "electroosmotic valving," in which sample plugs
can be injected in pressurized systems, was tested for the
investigation of peptide samples by MS analysis. The incorporation
of an adsorbing medium for sample preconcentration in the double-T
injector could prove useful in avoiding the electrophoretic bias
effect, inherently associated with the use of electrokinetic forces
for sample mobilization.
[0065] Two examples of system design to accomplish specific results
follow:
[0066] Consider conducting a liquid chromatography separation in an
open tubular capillary where the optimum separation parameters have
been determined to be: d.sub.2=20 .mu.m,. L.sub.2=0.5 m, and F=100
nL/min. The required pump design that can generate this flow rate
in a liquid chromatography column can be determined from plots such
as shown in FIG. 4A, which was constructed for d.sub.1/d.sub.2=0.1
and L.sub.1/L.sub.2=0.1. A pump with d.sub.1=2 .mu.m, L.sub.1=0.05
m, and n=200, would generate about 110 nL/min.
[0067] Consider conducting a liquid chromatography separation in a
column filled with monolith packing. The optimum separation
parameters are: d.sub.2=150 .mu.m, L.sub.2=0.05 m, F=700 nL/min,
and the necessary pressure to generate this flow rate through the
monolithic packing is about 3.4 bar. The equivalent column length
that would generate a 3.4 bar pressure drop, at 700 nL/min and
d.sub.2=150 .mu.m, is L.sub.2=362 m. The appropriate pump design
can be chosen, again from plots such as shown in FIG. 4A, or can be
directly calculated. For instance, a pump with d.sub.1=2 .mu.m,
L.sub.1=0.05 m, and n=2000, would generate a total F.sub.eof=1320
nL/min. For the selected parameters, one can determine
d.sub.1/d.sub.2=0.013, L.sub.1/L.sub.2=1.38, and F/F.sub.eof =0.70.
Consequently, F can be calculated. A flow of F=910 nL/min will be
produced, which is enough to drive the separation. Alternatively, a
more efficient pump (d.sub.1=1-1.5 .mu.m) that would guarantee a
larger F/F.sub.eof coefficient could be chosen, as well.
[0068] In other embodiments, the electroosmotic pump according to
the invention may be used with any other valving component, and the
electroosmotic valve of the invention may be used with any other
pump or pressure generating element (in particular, an
electroosmotic/electrokine- tic pump. The output of a pump and/or
valve of the invention may be monitored via connection to a flow or
pressure monitoring/controlling sensor and/or controller. A cluster
of pumps may be attached to an individual network channel for,
e.g., gradient generation or the introduction of a sequence of
solvents into the system. A microfluidic device with a multiple
channel structure constructed according to the invention allows for
material transport driven by any mechanism (electric, magnetic,
etc.) other than a pressure driven mechanism. Additionally, a
microfluidic pump according to the invention can serve as an
actuator for closing/opening of another valving component. The
above microfluidic devices and pumps operate with aqueous and/or
organic solutions, or other fluids (e.g., any liquid or gas).
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[0117] While the present invention has been described in
conjunction with a preferred embodiment, one of ordinary skill,
after reading the foregoing specification, will be able to effect
various changes, substitutions of equivalents, and other
alterations to the compositions and methods set forth herein. It is
therefore intended that the protection granted by Letters Patent
hereon be limited only by the definitions contained in the appended
claims and equivalents thereof.
* * * * *