EP1129345A1 - Practical device for controlling ultrasmall volume flow - Google Patents

Abstract

A device for control of ultrasmall volume fluid flow used in the fields of electrophoretic separation, chemical analysis, and microchemical reactions has a substrate defining a capillary channel and integrated external electrodes to control electroosmotic flow. The channel geometry and integrated external electrode proximity reduce the voltage required for control of flow. Longitudinal electrodes provide electrophoretic separation of components. High dielectric material between the integrated external electrode and capillary reduces the voltage required for the control of flow. Real-time flow monitoring and capillary channel surface coating enhance the control of flow.

Description

PRACTICAL DEVICE FOR CONTROLLING

ULTRASMALL VOLUME FLOW

SPECIFICATION

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority of United States Provisional

Application No. 60/108,086, filed November 12, 1998, which is hereby incorporated

by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the fields of electroosmosis and

electrophoresis, and in particular to a device for controlling the movement of fluids in

a capillary channel used in chemical systems for separations, reactions, or analysis.

BACKGROUND ART

Microdevices for fluids. Movement of fluids on microchips has been

accomplished by a number of methods. Most notably by pneumatic pressure and

electroosmosis, as described in Seiler, et al., Analytical Chemistry 1994, 66, 3485-

3491, which is hereby incorporated by reference in its entirety. Pressure-induced flow

generally requires physical valves to be fabricated and placed in the flow stream. This

must be done to control the variety of fluid movements needed on complex microdevices. However, these valves are difficult to design and fabricate, and exhibit

poor back-pressure and leakage performance, as described in Manz, et al., Sensors and

Actuators 1990, Bl, 244-248, which is hereby incorporated by reference in its

entirety. Also, the valves have not been fabricated on the micron to sub-micron scale

that are required for future generations of microdevices. Even if these physical

structures could be fabricated reliably with good performance characteristics,

pressure-induced flow does not scale down very well to narrow passages and

ultrasmall volumes. The back pressure generated by these minute passages is

immense and the size of the valve structures lead to dead volume and time delays in

flow between volume elements.

Electroosmosis is the most important flow-generating mechanism,

which originates at the solution/wall interfacial region. Immediately adjacent to the

solid-solution interface, the so-called double layer is formed, as described in Davies,

et al., Interfacial Phenomena. 2nd ed., Academic Press, New York, 1963, which is

hereby incorporated by reference in its entirety. Under most normal aqueous buffer

conditions, a silica wall surface has an excess negative charge. This results from

chemical ionization of surface functional groups. This negatively charged surface

attracts buffer counter ions which collect near the surface in a complex layered

system. This action creates a potential across these layers, where the potential

dropped across the diffuse layer is termed the ζ(zeta)-potential. The ζ-potential is

dependent upon the viscosity of the fluid, the dielectric constant of the solution and

the charge on the inner surface of the wall of the capillary. The cationic counter ions (H₃O⁺, Na⁺ typically) entrained in the diffuse layer are free to migrate towards the

anode, and because these ions are solvated, they drag solvent with them. The ζ-

potential and the longitudinal electric field strength governs the rate of flow, as

described in Rice, et al., Phys. Chem. 1965, 69, 4017, which is hereby incorporated by

reference in its entirety.

In the field of microfabricated devices, remarkable progress has been

made in miniaturization of separation-based systems. In 1992, Manz, et al. and

Harrison, et al., first established the use of a separation system on a microfabricated

device, as described in Manz, et al., Journal of Chromatography 1992, 593, 253-258,

and Harrison, et al., Analytical Chemistry 1992, 64, 1926-1932, which are hereby

incorporated by reference in their entirety. Efforts have continued to optimize and

miniaturize a wide variety of analytical based separation systems, as described in

Effenhauser et al., "Integrated chip-based capillary electrophoresis," Electrophoresis

1997, 18, 2203-2213, which is hereby incorporated by reference in its entirety. The

development of such devices has drawn emphasis to the field of small volume fluid

manipulation. Electroosmosis (also termed electroosmotic flow) provides an efficient

means of fluid flow control in a network of interconnecting channels. This flow

generates a flat-flow profile regardless of shape and dimension of the channel, thus

minimizing dispersion within the system. Since electroosmosis is directly

proportional to the applied longitudinal voltage field, the control of flow in each

channel is effected by varying its potential gradient. Flow in interconnecting channels

can be controlled by applying voltages in accordance to a model based on Kirchoff s law where the various channels are treated as homogeneous resistors in an electrical

network, as described in Seiler, et al., Analytical Chemistry 1994, 66, 3485-3491.

While electroosmosis provides a near-ideal flow propulsion

mechanism on microdevices, in practice it has proven difficult to apply reliably and,

as practiced in present systems, has some inherent limitations, as described in

Effenhauser, et al., Electrophoresis 1997, 18, 2203-2213. The mechanisms which

generate electroosmosis are complex, involving an interplay between surface

composition and buffer characteristics. Since it is an interfacial phenomenon, minute

amounts of materials depositing (or leaving) the surface can create dramatic changes

in this flow. This has resulted in poor reproducibility in standard separation

techniques and made microfluidic flow control problematic. For instance, to apply

the flow control model according to KirchofP s law, the ζ-potential, ionic strength and

the buffer pH in all channels must be kept constant. Another limitation of using

electroosmosis as a propulsion mechanism is that both the flow rate and the

electrophoretic migration rates of charged species are directly coupled to the voltage

field strength. In standard systems the flow rate cannot be independently varied

without unduly influencing the movement of charged species.

Electroosmosis is also an important component of capillary zone

electrophoresis, which is a powerful separation technique characterized by high-

efficiency, low volume separations, as described in Beale, Analytical Chemistry

1998, 70, 279R-300R and P. Camilleri, Capillary Electrophoresis Theory and

Practice, 2nd ed.; CRC Press: New York, 1998, which are hereby incorporated by reference in their entirety. This technique can be used to separate both charged and

neutral analytes in a wide variety of applications, including amino acids, proteins, and

nucleic acids. The flow generated is usually large enough to force all species present

(cations, anions, and neutrals) to migrate in one direction allowing the analysis of all

species at a single detector. Electroosmosis directly influences the efficiency,

resolution and reproducibility of electrokinetic separation techniques. Capillary

electrophoresis and its ancillary techniques have also been demonstrated for a number

of different applications on microdevice formats, as described in Effenhauser et al.,

Electrophoresis 1997, 18, 2203-2213.

Electroosmosis can be altered in a variety of ways. Examples of

purposefully altering electroosmotic flow (EOF) include buffer additives, as described

in Jorgenson, et al., Science 1983, 222, 266-272; Hjerten, Chromatogr. 1985, 347,

191-198 and Bruin, et al., Chromatogr. 1989, 471, 429-436 altering buffer pH, as

described in Lukacs, et al., J. High Res. Chrom. & Chrom. Comm. 1985, 8, 407-411 ;

Lambert, et al., Analytical Chemistry 1990, 62, 1585-1587 and McCormick,

Analytical Chemistry 1988, 50, 2322-2328 altering buffer concentration, as described

in Lukacs, et al., J. High Res. Chrom. & Chrom. Comm. 1985, 8, 407-411; Issaq, et

al., Chromatographia 1991, 32, 155-161; Atamna, et al., J. Liq. Chromatogr. 1990,

13(16); 3201-3210 and Atamna, et al.. J. Liq. Chromatogr. 1990, 13, 2517-2527

coating the inner wall of the capillary, as described in Jorgenson, et al., Science 1983,

222, 266-272; Hjerten, Chromatogr. 1985, 347, 191-198 and Moseley, et al.,

Analytical Chemistry 1991, 63, 109-114 and organic modifiers, as described in VanOrman, et al., J. Microcol. Sep. 1990, 2, 176-180 and Schwer, et al., Analytical

Chemistry 1991, 63, 1801-1807, which are hereby incorporated by reference in their

entirety. These techniques either (1) permanently alter the surface structure, or (2)

alter the buffer composition. They result in a static, new rate of electroosmotic flow

(EOF) which cannot actively be altered in response to changing conditions with the

channel or tube. However, dynamic control of electroosmosis has been predicted and

demonstrated by applying an additional radial voltage field across the wall of the

capillary (for fused silica capillaries used in conventional capillary electrophoresis), as

described in Lee, et al., Analytical Chemistry 1990, 62, 1550-1552; Lee, et al.,

Analytical Chemistry 1991, 63, 1519-1523; Huang, et al . , "Mechanistic Studies of

Electroosmotic Control at the Capillary-Solution Interface," Analytical Chemistry

1993, 65, 2887-2893; Hayes, et al., "Electroosmotic Flow Control and Monitoring

with an Applied Radial Voltage for Capillary Zone Electrophoresis," Analytical

Chemistry 1992, 64, 512-516, and Hayes, et al., "Effects of Buffer pH on

Electroosmotic Flow Control by an Applied Radial Voltage for Capillary Zone

Electrophoresis," Analytical Chemistry 1993, 65, 27-31, which are hereby

incorporated by reference in their entirety. The radial voltage flow control technique

does not require permanent changes in surface structure, or altered buffers. This

control effectively decouples the electrophoretic migration of charged species and the

bulk flow rate. Thus, it would be beneficial to provide an apparatus for controlling the

flow of ultrasmall fluid volumes of the kind used in microchips and microdevices for

the chemical, biochemical, and analytical sciences.

Radial voltage flow control. Radial voltage flow control, a method to

control electroosmotic flow, was first demonstrated using resistive solutions or

materials covering the majority of the outer surface of the capillary, as described in

Lee, et al., Analytical Chemistry 1990, 62, 1550-1552. This design required resistive

materials so that radial potential matched the potential gradient of the buffer on the

interior of the capillary (offset by the radial voltage experimental value). Later work

demonstrated that the effect could be generated by conductive materials or ionized gas

and that the matching of the interior potential gradient was unimportant in obtaining

the effect, as described in Hayes, et al., Analytical Chemistry 1993, 65, 27-31 and

Wu, et al., "Dispersion Studies of Capillary Electrophoresis with Direct Control of

Electroosmosis," Analytical Chemistry 1993, 65, 568-571, which is hereby

incorporated by reference in its entirety. In fact, control was demonstrated while

covering only very small portions (4%) of the outer surface with a conductor, as

described in Hayes, et al., "Electroosmotic Flow Control and Surface Conductance in

Capillary Zone Electrophoresis," Analytical Chemistry 1993, 65, 2010-2013, which is

hereby incorporated by reference in its entirety. Surface conductance within the

electric double layer was attributed for the effective control where the induced charge

from the radial voltage spread along the inner surface. This charge affected the ζ- potential over the entire capillary length effectively inducing the change in

electroosmosis.

Investigations of this effect have demonstrated some limitations to this

technique. The radial voltage cannot manipulate flow in standard fused silica

capillaries with buffer pH above approximately 5, as described in Hayes, et al.,

Analytical Chemistry 1993, 65, 27-31. High ionic strength buffers have also been

predicted to limit its effectiveness. Additional dispersion is predicted from the

heterogeneous (-potential caused by radial fields in the partially covered capillaries.

This has been the subject of several theoretical discussions, but has yet to be

experimentally confirmed, as described in Potocek. et al.. Journal of Chromatography

1995, 709, 51-62; Keely, et al., J. Chromatogr. A 1993, 652, 283-289; Cortes, et al.,

J. Microcol. Sep. 1989, 1, 278-288; Keely, et al., Analytical Chemistry 1994, 66,

4236-4242; Kasicka, et al., Journal of Chromatography 1997, 772, 221-230;

Anderson, et al., Chem. Engin. Commun. 1985, 38, 93-106 and Chien, et al.,

Analytical Chemistry 1991, 63, 1354-1361, which are hereby incorporated by

reference in their entirety. Radial voltage flow control also requires very large

voltages, at least several to many kilovolts, to generate the radial fields in fused silica

capillaries. These large electrical potentials have presented severe design limitations,

and safety and expense problems for the application of this technique.

For example, Ghowsi disclosed in U.S. Patent No. 5,092,972 that

radial voltage flow control could be done at lower voltages in a silica capillary of up

to 100 micrometer wall thickness, in which radial voltage differences could be applied uniformly across the entire length of the capillary. However, no capillary channel

cross section was disclosed.

In another example, Blanchard et al. disclosed in U.S. Patent No.

5,151,164 a radial voltage flow control device using a fused silica capillary of 530

micrometer inside diameter having an inner capillary of 75 micrometer outside

diameter (channel cross section 216 x 10^"9 square meters of the annular region

between the capillaries) and 630 micrometer outside diameter in which radial voltage

differences of 5 to 6 kilovolts were applied across the annular region between the

capillaries to halt electroosmotic flow. The distance between the radial voltage

electrode and the annular channel inner wall surface was 100 micrometers.

In another example, Young et al. disclosed in U.S. Patent No.

5,180,475 a radial voltage flow control device using a fused silica capillary of 50

micrometer inside diameter (channel cross section 2 x 10^"° square meters) and from

140 to 360 micrometer outside diameter, in which radial voltage differences of 5

kilovolts were applied across the capillary to control electroosmotic flow. The

distance between the radial voltage electrode and the channel inner wall surface was

from 45 to 155 micrometers.

In a further example, Ewing et al. disclosed in U.S. Patent No.

5,320,730 a radial voltage flow control device using a fused silica capillary of either

20 micrometer inside diameter (channel cross section 0.3 x 10^~9 square meters) and

144 micrometer outside diameter, or 50 micrometer inside diameter (channel cross

section 2 x 10^"9 square meters) and 370 micrometer outside diameter, in which radial voltages of up to 30 kilovolts were applied across the capillary to control

electroosmotic flow. The distances between the radial voltage electrode and the

channel inner wall surfaces were 62 and 160 micrometers, respectively.

In a recent example, Ewing et al. disclosed in U.S. Patent No.

5, 358, 618 a radial voltage flow control device using a fused silica capillary of 20

micrometer inside diameter (channel cross section 0.3 x 10^"9 square meters) and 144

micrometer outside diameter, in which radial voltages of up to 30 kilovolts were

applied across the capillary to control electroosmotic flow. The distance between the

radial voltage electrode and the channel inner wall surface was 62 micrometers.

However, the above-mentioned patents fail to disclose devices or

methods that allow efficient control of electroosmotic flow for ultrasmall fluid

volumes with reduced perpendicular voltage fields.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide a device

and method for producing reliable electroosmotic flow of a fluid in a capillary channel

with dynamic control using an external voltage field that is applied in an orientation

perpendicular to the capillary channel. It is another object of the present invention to

provide a device and method for efficient control of electroosmotic flow of a fluid in a

capillary channel with a reduced perpendicular voltage applied. A further object of the

invention is to provide a device and method for monitoring uniform electroosmotic

flow of a fluid. These objectives have been substantially satisfied and the

shortcomings of the prior art have been substantially overcome by the present

invention, which in one embodiment is directed to a capillary channel device having

an integrated external electrode positioned microscopically close to a capillary

channel of ultrasmall cross section, wherein the overall geometry increases the

channel inner wall surface charge density produced by a particular strength of the

perpendicular voltage field. The microscopic distance between the integrated external

electrode, which provides the perpendicular voltage field, combined with the

ultrasmall cross section of the capillary channel reduces the voltage required for

electroosmotic flow control.

In another embodiment, the present invention is directed to a capillary

channel device having an integrated external electrode positioned microscopically

close to a capillary channel of ultrasmall cross section, wherein longitudinal

electrodes are positioned at the immediate ends of the channel to apply a longitudinal

voltage field selectively within the channel to induce electrophoretic migration of

substances within the channel. This embodiment permits independent control of the

bulk fluid flow and the electrophoretic migration, and permits selective control of the

flow in each channel when a plurality of channels are combined in a device.

In another embodiment, the present invention is directed to a capillary

channel device having an integrated external electrode positioned microscopically

close to a capillary channel of ultrasmall cross section, wherein a material of high

dielectric constant is positioned between the integrated external electrode and the capillary wall to inject charge to the capillary channel inner wall surface when voltage

is applied to the integrated external electrode. This embodiment further reduces the

voltage required for electroosmotic flow control, and reduces the effect that the

perpendicular voltage field applied to one channel has on other nearby channels when

a plurality of channels are combined in a device.

In a further embodiments, the present invention is directed to a

capillary channel device as described above, further comprising a means to monitor

the flow of fluids in the capillary channel, and optionally having an inner channel wall

surface coating to enhance control of fluid flow.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features, and advantages of the present invention will

be more fully appreciated from a reading of the detailed description when considered

in conjunction with the accompanying drawings, wherein:

Fig. 1 illustrates an example of a device for ultrasmall volume flow

control according to a preferred embodiment of the present invention;

Fig. 2 illustrates the physical parameters of geometry of a device for

ultrasmall volume flow control according to a preferred embodiment of the present

invention;

Fig. 3 illustrates a plot of model capillary inner wall surface charge

density versus internal and external diameter from an applied external perpendicular

voltage; Fig. 4 illustrates an example of a microchip device for ultrasmall

volume fluid flow control according to a preferred embodiment of the present

invention;

Fig. 5. illustrates a plot of fluorescent intensity of a dye migrating

through a microchip channel versus time at various applied perpendicular voltages;

Fig. 6. illustrates an example of a device for ultrasmall volume flow

control according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

By "integrated external electrode" we mean an electrical conductor

positioned with respect to the capillary channel so that a potential applied to the

integrated external electrode will produce a perpendicular voltage field with respect to

the capillary channel.

By "perpendiclar voltage field" we mean the component of the electric

field emanating from the integrated external electrode which is in a direction

perpendicular to the capillary channel, and the electric charges produced anywhere

within the device by the application of an electric potential to the integrated external

electrode, whether produced directly by the field or indirectly by conduction or other

movement of charge. The perpendiclar voltage field provides control of

electroosmotic flow in the capillary channel.

The present invention provides a practical device for controlling

ultrasmall volume fluid movements using reduced voltages to control electroosmotic flow, as shown in Fig. 1. Fluid flow is provided in a capillary channel 170 defined by

a substrate 160, wherein the channel has two ends 180. The voltage field used to

control electroosmotic flow is applied perpendicularly across a capillary channel

having an ultrasmall cross section 200, wherein the distance 140 between the

perpendicular field-generating integrated external electrode 120 and the capillary

channel inner wall surface 190 is microscopically small. A means 150 for applying a

voltage to the integrated external electrode is provided. In one embodiment, a

material of high dielectric constant 130 is positioned between the integrated external

electrode and the capillary channel, and optionally the material of high dielectric

constant may form a portion of the capillary channel inner wall surface. Longitudinal

electrodes 110 are provided at the immediate ends 180 of the capillary channel to

effect electrophoretic migration and electroosmosis of fluids within the channel. The

longitudinal electrodes can be electrically connected to nodes 100 for connecting to a

means for applying a voltage difference between the two longitudinal electrodes, or

optionally the longitudinal electrodes 110 can be adjacent to, and in electrical contact

with an object outside the device that provides a voltage difference between the two

longitudinal electrodes.

This device will find application, for example, with capillary zone

electrophoresis. Another example is any fluid movement within microinstrumentation

driven by electrokinetic effects, including instrumentation and methods for separation

science. Both of these applications involve the transport and/or storage of fluids for chemical reactions or analysis. All of these examples will benefit from the processes

described in this disclosure.

The device can be made as a microchip, as shown in Fig. 2. The

capillary channel 170 is again defined by the substrate 160, and can have ultrasmall

cross sectional dimensions 200. Integrated external electrodes 120 can be positioned

a microscopically small distance 140 from the capillary channel. The substrate of the

device can be a ceramic, silica, fused silica, quartz, a silicate, a titanate, a metal oxide,

a nitride, silicon, titanium dioxide, and the like, or a polymer, a plastic, a

polydimethylsiloxane, or a polymethylmethacrylate.

Microscopic distances between integrated external electrodes and the

channel. Without intending to be bound by any one particular theory, the equation

describing the physical and electrical properties of the capillary channel predicts that

reduced distances between the integrated external electrodes and the channel wall will

enhance the efficiency of electroosmotic flow control, where control is done with the

perpendicular voltage effect, by a factor of l/ln(r₀/ri), as shown in Fig. 3, and as

described in Hayes, et al., Analytical Chemistry 1992, 64, 512-516. The reduced

distances will have an additional benefit: the applied voltages may be lower in

absolute magnitude, thus reducing technological requirements for insulation and

safety. The two limitations for applying this concept are: the structural integrity of the

wall and the electrical breakdown of the insulating (or wall) material. Through

careful design, these limitations are minimized. The first issue is structural integrity. The electric double layer surface

conductance, mentioned briefly above, aids in the design of this system, as described

in Wu, et al., Analytical Chemistry 1993, 65, 568-571, and Hayes, et al., Analytical

Chemistry 1993, 65, 2010-2013. Surface conductance provides a mechanism for the

charge to spread out along the length of the capillary channel on the inner wall

surface. Earlier studies have shown that the charge created by a perpendicular voltage

in one limited section of the capillary channel can affect the double layer throughout

the entire length of the capillary channel, and is able to effectively control flow. The

electrode (or conductor, more accurately) could be placed very near (nanometers to

microns) to the capillary channel inner wall surface of this segment.

Electrical breakdown is not an issue. As the initial surface charge

density increases, the induced surface charge becomes less and less effective in

changing the flow in this system, as described in Huang, et al., Analytical Chemistry

1993, 65, 2887-2893 and Hayes, et al, Analytical Chemistry 1993, 65, 27-31. The

maximum effective surface charge on the inner surface will be obtained at voltages

well below the electrical breakdown voltage limit. This is especially true for the high

dielectric constant materials. The additional induced charge becomes ineffective in

changing the flow at high positive or negative values. This limit is obtained at

approximately 2.1 x 10¹³ charges/cm². Experimentally, this charge may be induced

across a 10 micrometer silicate wall with 350 volts with only 17% of the calculated

electrical breakdown voltage for this thickness. It is even more favorable with a titanium dioxide wall, where only 8.8 V is required, which is only 0.4% of the

breakdown voltage.

Channels fabricated with reduced distances between the integrated

external electrodes and the channel wall dramatically improve control of

electroosmosis. A small portion of the device used to demonstrate this is shown in

Fig. 4, wherein a microchip capillary channel device is illustrated. The microchip

substrate 160 defines a capillary channel 170. Two integrated external electrodes 120

are positioned at a reduced distance 140 of 50 micrometers from the capillary channel

and its inner wall surface 190. A material of high dielectric constant 130 can be

positioned between the integrated external electrodes and the channel wall. Injection

can be done with a standard offset cross-tee design and detection can be done with

laser induced fluorescence. With this device the control of electroosmosis is

accomplished at perpendicular voltages that are ten to one hundred times less than

conventional systems, as shown in Fig. 5, wherein the elution data for the device

indicate that using reduced distances between the integrated external electrodes and

the channel wall for a capillary channel of ultrasmall cross section results in

dramatically improved control of electroosmosis with less demanding power supplies.

We have also demonstrated that electrical breakdown is not a problem with this

design. To construct the device as a microchip or microdevice, other fabrication

techniques are used, including chemical vapor deposition and alternative materials.

Ultrasmall capillary channel cross section. A fused silica capillary

tube may be modeled as a cylindrical capacitor, as described in Keely, et al., Chromatogr. A 1993, 652, 283-289. Without intending to be bound by any one

particular theory, in this model the surface-charge density (q) created on the inner

surface by an applied radial voltage has a relationship with the physical properties as

follows:

q = e_q V i/r_i)(l/ln(r₀/r_i)) (2)

where e_q is the permittivity of the fused silica, V_r is the applied radial voltage, is the

inner radius, and r₀ is the outer radius. This equation indicates that at very small inner

diameters the efficiency of the applied radial voltage is maximized.

The quantitative improvements predicted are illustrated by graphing

inner diameter versus radial voltage-induced inner surface charge density, as shown

by the x-axis versus y-axis, respectively, in Fig. 3. Compared to previously used

experimental radii of about 10 to 75 micrometers inside diameter (i.d.) and 150 to 360

micrometers outside diameter (o.d.), the charge density, as shown in Fig. 3, increases

by several orders of magnitude, as described in Hayes, et al., Analytical Chemistry

1993, 65, 27-31 and Wu, et al., Analytical Chemistry 1993, 65, 568-571. This change

of the geometry of the capillary channel provides for improved control of

electroosmosis.

Materials with high dielectric constant. Materials which make up the

capillary channel wall can also improve the control of flow. They can increase the

effectiveness of flow control by inducing a greater amount of charge on the inner

surface of the channel for a given applied radial voltage. This higher induced charge,

in turn, induces greater affect on the (-potential, thereby improving the control of flow. A material with high dielectric constant (the e_q term in eqn. 2), such as titanium

dioxide, can be positioned between the integrated external electrode and the channel

to serve this purpose. The typical substrate material, quartz (or fused silica) has a

dielectric constant of 3.8, whereas that of titanium dioxide has been reported to be as

much as 170. The amount of charge transferred to the channel inner wall surface is

linear with the permittivity (as indicated by the dielectric constant). Thus, by using a

material of high dielectric constant, up to 40 times more electric charge will be

injected to the channel inner wall surface for a given applied perpendicular voltage

field, all other factors being equal. Other materials useful for the high dielectric

material are ceramics, a silicate, a titanate, a metal oxide, a nitride, titanium dioxide,

and the like, or a high dielectric polymer or plastic.

The direction in which the electric charge is transferred can be

controlled by using high dielectric constant materials because charge is more

effectively transferred across high dielectric constant materials, than the substrate.

The position of the integrated external electrode with respect to the high dielectric

material can be used to inject charge in a particular direction in the device. By

placing a high dielectric material between an integrated external electrode and the

channel, the charge will be preferentially injected towards the channel. In one

embodiment, wherein the device is a combination of capillary channels each with

perpendicular voltage flow control, as shown in Fig. 6, controlling the direction of

electric charge from the integrated external electrodes is particularly important. As

shown in Fig. 6, it prevents the perpendicular voltage field from one integrated external electrode 120 from influencing the flow in nearby channels 170. As devices

become smaller and more complex, both the channels and the flow control

mechanisms will necessarily be in closer proximity. Thus, it will become increasingly

important to reduce the influence of one integrated external electrode on the flow in

adjacent channels. By inducing charge preferentially in one direction, the effects on

nearby channels are minimized. The amount of charge injected to the desired channel

versus that injected to a nearby channel is the result of two properties: (1) the ratio of

the distance between neighboring channels and the distance between the integrated

external electrode and the channel wall, and (2) the dielectric constant of the material

positioned between the integrated external electrode and its channel wall (for

example, TiO₂), and the dielectric constant of the substrate material between the

integrated external electrode and the nearby channel (for example, SiO₂). The ratio of

the distances can be 100 or more, and the ratio of the dielectric constants can be as

high as 40. Thus, the discrimination between changes in flow produced in the desired

channel by the integrated external electrode and changes of flow unintentionally

produced in nearby channels can be as high as 4000, even for complex microchip

devices.

Channel inner wall surface coating. An optimal inner-surface coating

for the perpendicular voltage flow control of electroosmosis requires three properties.

First, the surface must retain low surface charge density in the presence of the

aqueous buffers typically used in capillary electrophoresis, as described in Poppe, et

al., Analytical Chemistry 1996, 68, 888-893, and Hayes, "Extension of External Voltage Control of Electroosmosis to High-pH Buffers," Analytical Chemistry 1999,

71, 3793-3798. Second, the surface charge density should be insensitive to pH

changes of the buffer, thus remaining consistent over a large range of normally

encountered pH (for example, pH from 1 to 11 ) and buffer types, as described in

Hayes, et al., Analytical Chemistry 1993, 65, 27-31. Finally, the surface must not

increase the solution viscosity near the surface, as described in Huang, et al., J.

Microcol. Sep. 1992, 4, 135-143, and Huang, et al., J. Chromatogr. A 1994, 685, 313-

320. The viscosity within the electric double layer defines the frictional forces

retarding the entrained ions movement in the longitudinal voltage gradient and has a

direct effect on electroosmotic mobility. High-viscosity surface layers produce low

electroosmosis altogether, as described in Manz, et al., Sensors and Actuators 1990,

Bl, 244-248, and Jorgenson, et al., Science 1983, 222, 266-272. Avoiding the use of

polymers or polymer-forming reactants, or the use of monolayer surface coverage

minimizes increases in local viscosity.

A variety of coatings fulfill these criteria. Notably, silicate surfaces

treated with hindered organosilanes and ceramic oxide surfaces (TiO₂, for example)

with organosilane treatments. The silicate surface is labile to acid and base

degradation reactions, but with the hindered organosilane treatment this surface

remains stable up to eight weeks, as described in Hayes, "Extension of External

Voltage Control of Electroosmosis to High-pH Buffers," Analytical Chemistry 1999,

71, 3793-3798. Coating silicate with titanium dioxide and then reacting that surface

with organosilanes forms an uncharged, stable surface, as described in Pesek, et al., Chromatographia 1997, 44, 538-544, which is hereby incorporated by reference in its

entirety. The organosilane coating on the titanium dioxide does not require hindered

reagents since the underlying material is not liable to the acid and base degradation

reactions. Any additional coatings which meet the criteria listed above will function

to aid the flow-control system.

The surface charge generated by the chemical equilibrium of

buffer/wall interface must be minimized to extend radial voltage flow control to

higher buffer pH, as described in Hayes, et al., Analytical Chemistry 1993, 65, 27-31,

and Poppe, et al., "Theoretical Description of the Influence of External Radial Fields

on the Electroosmotic Flow in Capillary Electrophoresis," Analytical Chemistry 1996,

68, 888-893, which is hereby incorporated by reference in its entirety. This has

generally been accomplished with surface coatings, which are described here.

Coatings constructed with polymers eliminate the chemical

equilibrium-based surface charge and increase local viscosity, as described in

Srinivasan, et al., Analytical Chemistry 1997, 69, 2798-2805; Huang, et al.,

Microcol. Sep. 1992, 4, 135-143; and Huang, et al., J. Chromatogr. A 1994, 685, 313-

320, which are hereby incorporated by reference in their entirety. They are designed

to minimize protein adsorption and eliminate or permanently change electroosmosis.

Polymers have been covalently bound and physically adsorbed to the inner wall

surface of the capillary channel or used as dynamic coatings (where buffer additives

with surface-active properties adhere to the wall in a adsorbed/free-solution

equilibrium), as described in Srinivasan et al., Analytical Chemistry 1997, 69, 2798- 280, and Iki et al., J. Chromatogr. A 1996, 731, 273-282, which is hereby

incorporated by reference in its entirety. These polymers suppress electroosmosis by

reduced surface charge density and increased viscosity within the electric double

layer. This local viscosity is unaffected by the perpendicular voltage potential

gradients which alter electroosmosis, and therefore polymer coatings are unacceptable

for dynamic flow control by an applied perpendicular field, as described in Huang et

al., Analytical Chemistry 1993, 65, 2887-2893.

Fused-silica capillaries coated with organosilane treatments have been

reported, most notably for application to capillary gas chromatography. However, due

to the labile silicon/oxygen/carbon bond system, previous organosilane treatments

were not stable at buffer pH extremes, either high or low, as described in Srinivasan,

et al., Analytical Chemistry 1997, 69, 2798-2805; Hjerten, et al., Electrophoresis

1993, 14, 390-395; and Kirkland, et al., Analytical Chemistry 1989, 61, 2-11, which

are hereby incorporated by reference in their entirety.

Organosilane treatments have also been explored for perpendicular

voltage flow control for capillary electrophoresis. One example was the use of

commercially 'deactivated' tubing (the surface treatment was proprietary, but was

known to be organosilane based), where the authors merely mention that it "... yields

effective EOF [electroosmotic flow] control by applied radial voltage," without

further explanation, as described in Hayes, et al., Analytical Chemistry 1992, 64, 512-

516. A butylsilane surface was also used to improve the effectiveness of flow control,

but the surface was unstable above pH 5, as described in Huang, et al., Analytical Chemistrv 1993, 65, 2887-2893, and Towns, et al., J. Chromatogr. 1990, 516, 69-78,

which is hereby incorporated by reference in its entirety. Sterically hindered

triorganosilane treatments have demonstrated stability to acidic and basic buffers and

provided for perpendicular voltage flow control from pH 2 to pH 10, as described in

Hayes, "Extension of External Voltage Control of Electroosmosis to High-pH

Buffers," Analytical Chemistry 1999, 71, 3793-3798, which is hereby incorporated by

reference in its entirety.

Flow monitoring. The flow rate and direction of flow for each channel

can be monitored. This information is used as a feedback mechanism to confirm or to

appropriately adjust the flow control mechanisms. The rate of flow will be adjusted

according to the information provided by the monitor. One requirement of this

monitoring device is that the materials and fluid within the channel must remain

unchanged by the monitoring system. The monitoring system must be non-invasive

because any disturbance of the condition or make-up of fluid contained within the

channel may preclude its use in subsequent operations. Any flow monitoring system

which can detect flow rates non-invasively in microns-wide channels will function for

the technology described here.

A summary of methods for real-time monitoring of electroosmosis

prior to 1989 is given in Goor, et al., J. Chromatogr. 1989, 470, 95-104, which is

hereby incorporated by reference in its entirety. The first and most commonly applied

of these methods is the use of a neutral marker, as described in Lukacs, et al., J. High

Res. Chrom. & Chrom. Comm. 1985, 8, 407-411; Lauer, et al., Analytical Chemistry 1986, 58, 166-170; and Stevens, Analytical Chemistry 1983, 55, 1365-1370, which

are hereby incorporated by reference in their entirety. In capillary electrophoresis,

neutral species are swept along at the electroosmotic flow rate (in the absence of

surface interactions). Therefore, if the length from the injector to the detector is

known, the flow may be calculated from the elution time. This technique is limited to

monitoring only the average flow during the analysis.

Streaming potential has been used to determine the (-potential where

the flow is calculated from this value, as described in Rutgers, et al., in Physical

Chemistry: enriching topics from colloid and surface science, edited by Olphen and

Mysels; Theorex, La Jolla, California, 1975; Hunter, Zeta Potential in Colloid

Science. Principles and Applications. Academic Press, London, 1981; Wegenen, et

al., J. Colloid Interface Sci. 1980, 76, 305; Wegenen, et al., J. Electrochem. Soc. 1976,

123, 1438; and Reijenga, et al., J. Chromatogr. 1983, 260, 241, which are hereby

incorporated by reference in their entirety. This system requires pressure driven

buffer reservoirs and highly sensitive voltage sensing devices. This also requires off¬

line analysis, from which the flow is back calculated.

One method to directly measure EOF is to weigh the mass transferred

from the injection or the mass delivered to the detection reservoir, as described in

Goor, et al., J. Chromatogr. 1989, 470, 95-104; Altria, et al., Anal. Proc. 1986, 23,

453-454; and Altria, et al., Chromatographia 1987, 24, 527-532, which are hereby

incorporated by reference in their entirety. This of course requires calibration for each buffer system and a high accuracy mass balance system. In addition, to calculate the

linear velocity, the capillary internal diameter must be accurately known.

Monitoring the current flow in a capillary has been used to examine the

rate of electroosmosis when a buffer of differing concentration is introduced into the

injection end of the capillary, as described in Lee, et al., Analytical Chemistry 1990,

62, 1550-1552: and Huang, et al.. Analytical Chemistry 1988. 60. 1837-1838, which

is hereby incorporated by reference in its entirety. Under these conditions the total

conductivity across the capillary is proportional to a weighted average of the

conductivity of each buffer solution. Therefore, the rate of change in the current is a

function of the flow rate. Buffers must be changed for each analysis and flow will

slightly vary as the capillary fills with a different buffer.

An example of a flow monitoring system that can be used in a

preferred embodiment in the present invention is described in Ewing et al., U.S.

Patent No. 5,624,539, which is hereby incorporated by reference in its entirety.

Longitudinal electrode positioning. In existing designs, the electrodes

which generate the electrokinetic effects are typically placed in buffer or sample

reservoirs. In the present invention, electrodes placed at the immediate ends of all

channels, or selected channels, allow introduction of an electric field selectively

within the channel. Because of their positioning, these longitudinal electrodes provide

the option of limiting the electrokinetic effects to the materials and fluids contained

only within the channel. Thus, either the electrophoretic migration or the bulk flow

may be independently adjusted. Bulk flow can be directly changed by the applied longitudinal voltage field, or by changes in the (-potential caused by perpendicular

voltage fields. Electrophoretic migration may be changed by varying the longitudinal

voltage field. The manipulations provided for with this device and procedures will

allow for precise liquid injection and handling within a microdevice.

The following examples are given to illustrate important features of the

present invention and are not intended to limit the invention in any way. It should be

understood that the present invention is not limited to the above-mentioned

embodiments. Numerous modifications can be made by one skilled in the art having

the benefits of the teachings of the present invention. Such modifications should be

taken as being encompassed within the scope of the present invention as set forth in

the appended claims.

EXAMPLE 1

Reagents. Sodium dihydrogen phosphate (NaH₂PO₄) was obtained

from Aldrich Chemical Company, Inc. (Milwaukee, Wisconsin) and was used as

received. N-(2-aminoethyl)-4-amino-3,6-disulfo-l,8-naphthalimide, dipotassium salt

(lucifer yellow) was obtained from Molecular Probes (Eugene, Oregon) and was used

as received. All NaH₂PO₄ buffers were prepared to a 20 mM (millimolar)

concentration and adjusted to pH 3.0 using phosphoric acid (EM Science, Gibbstown,

New Jersey). Lucifer yellow was prepared (1 mg/mL) using NaH₂PO₄ buffer. All

buffers and samples were degassed under vacuum for 5 minutes and were filtered with

a Millex-LCR Filter Unit, 0.5 micrometer pore size (Bedford, Massachusetts). Lucifer yellow solutions were filtered with a Nalgene filter (0.2 micrometer pore size,

Fisher Scientific, Pittsburgh, Pennsylvania). All buffers and samples were prepared

with 18 megohm purified water drawn from a NANOpure UV ultrapure water

filtration system (Barnstead, Dubuque, Iowa).

Planar Microdevice. A capillary channel microdevice was designed in-

house and manufactured by the Alberta Microelectronic Centre (Edmonton, Alberta).

This device consisted of a long capillary channel, used for electrophoretic separation,

intersected by two off-set side channels. The substrate was Corning 0211 glass

(Precision Glass and Optics, Santa Ana, California). The overall device measured

2.54 cm x 7.62 cm. The channel dimensions were 30 micrometers wide and 10

micrometers deep. The side channels were off-set by 500 micrometers. The

separation channel (injection zone to the buffer waste reservoir) was 5.0 cm long.

Integrated external electrodes were positioned parallel to the main channel, separated

by 50 micrometers of glass substrate, as shown in Fig. 4. The integrated external

electrodes extended 6 mm total, centered at 9 mm from the end of the separation

channel. The effective perpendicular voltage field strength was determined by first

calculating the potential of the buffer (assuming a linear potential gradient)

immediately adjacent to the center of the integrated external electrode, derived from

the longitudinal voltage gradient. The effective perpendicular voltage field was the

difference between the calculated buffer potential at that point and the potential

applied to the integrated external electrode by the power supply means. Apparatus. Two Series 225 high voltage power supplies were used to

apply potential to the longitudinal and integrated external electrodes (Bertran,

Hickesville, New York). An Olympus Vanox microscope (Tokyo, Japan) and an

Olympus 1X70 Inverted Research microscope (Tokyo, Japan) were used for imaging.

An Omnichrome Model 100 HeCd laser was used as the fluorescence excitation

source (442 nm). Image acquisition was performed with an RSI 70 CCD camera (CSI

Electronics, East Hartford, Connecticut) integrated with National Instruments Lab

VIEW IMAQ image acquisition software and hardware (National Instruments, Austin,

Texas) where imaging programs were developed in-house. Data analysis was

performed using MathCAD 7.0 (MathSoft, Inc., Cambridge, Massachusetts) and

Excel (Microsoft Corporation, Seattle, Washington) programs that also were

developed in-house.

Results. Dramatically improved efficiency was demonstrated for

control of electroosmosis with small applied potentials to the integrated external

electrodes of less than about 120 V. Two separate quantitative data sets (normalized

and simulated capillary zone electrophoresis) indicated that the system was stable and

consistent while providing efficient control. The device was approximately 40 times

more efficient than conventional fused silica capillary systems described in the

literature to control fluid flow.

Representative digital images were acquired of the flow of a

fluorescent sample bolus moving through the capillary channel five seconds after the

injection of the sample. Different velocities of the injected bolus were observed under various effective voltage fields of the integrated external electrodes, as shown in

Table I. The change observed in the electroosmotic mobility of the sample for the

experiments in which the value of the effective voltage field of the integrated external

electrodes was changed from +4 V to +124 V was 8.0 x 10^"5 cm²/Vs. In theory, the

maximum change in mobility that could be achieved for this device under these

conditions was about 8 x 10^"4 cm²/Vs. Thus, by changing the effective voltage field of

the integrated external electrodes by only 120 V, about 10% of the maximum

allowable change in sample mobility was obtained. Furthermore, the results in Table

I exhibited a linear correlation between the normalized change in observed mobility

and the effective voltage field of the integrated external electrodes according to the

equation y = {-2.13 x 10^"3}x + 1.07 (where y is the normalized change in observed

mobility, x is the effective voltage, and R² = 0.84 for the correlation).

Further studies were performed by simulating a capillary zone

electrophoresis experiment. The fluorescence intensity was monitored at a pseudo-

detection window located approximately 5 mm away from the injector with the

CCD camera and software manipulation. Changes in electroosmosis caused by the

effective voltage field of the integrated external electrodes were observed in a more

conventional manner with this method. A higher voltage gradient between the

longitudinal electrodes was used for the electrophoretic separation (-123.9 V/cm), and

the injection rate was increased for these experiments to generate shorter run times.

The elution time for the fluorescent sample dye varied dramatically with the change in

the effective voltage field of the integrated external electrodes. Peak elution times varied by as much as 16 ± 3 seconds over a 5 mm separation distance, as shown in

Table II. In Table II, the observed mobility changed by 7.9 x 10^"5 cm²/Ns for a

change in the effective voltage field of the integrated external electrodes of 120 V

(from 52 V to 172 V). Thus, the results of the experiments shown in Tables I and II

were in agreement, even when the centers of the ranges of effective voltages of the

integrated external electrodes that were used were somewhat different (+4 V to +124

V versus +52 V to +172 V). A linear correlation also existed between the observed

mobility and the effective voltage fields of the integrated external electrodes for the

results in Table II, according to the equation y = {6.6 x 10^"7}x-{2.1 x 10^"4} (where y is

the observed mobility, x is the effective voltage, and R² = 0.94 for the correlation).

The values for observed mobility in Table II were not normalized to initial mobility,

as were the values in Table I, thus the slope of the correlation and the magnitude of

the values were different in Tables I and II.

The effectiveness of flow control for the microdevice tested here

versus standard capillary electrophoresis systems which use fused silica substrates

was calculated by comparing the experimental results obtained here to the published

literature. This analysis was limited to studies using buffers consistent with those

used in this study (pH 3, 1 to 50 mM phosphate). To quantitate the effectiveness of a

voltage field of the integrated external electrodes with respect to flow velocity, the

following analysis was undertaken, as shown in Table III. First, the total positive

range of the applied voltage used to control electroosmosis in the literature reference

was listed in Table III. Since the absolute value of the inner and outer radii of the capillary tubes used in these literature references influenced the effectiveness of the

applied radial voltage (see equation 2, above), the applied radial voltage was

multiplied by a cylindrical capacitor factor of l/(riln(r₀/ri)) to obtain the values of

effective radial voltage of 109 to 450 V/micrometer in Table III. The corresponding

change in the electroosmotic mobility (Δμ_eof) from the literature references

ranged from 8.0 x 10^"5 cm²/Ns to 3.2 x 10^"4 cm²/Ns in Table III. An efficiency factor,

r, was calculated, where Δ _eof was divided by the applied capacitor field strength, as

given by the equation, V - Δ,α_eof/[V/(r_iln(r₀/r_i))]. The efficiency factor in Table III

varied from 3.7 x 10^"7 (cm²/Vs)/(V/micrometer) to

1.5 x 10^"6 (cm²/Ns)/(V/micrometer).

For the experiments shown in both Tables I and II, the range of applied

potential of the integrated external electrodes was 120 V. Since the voltage was

applied across a distance of 50 micrometers between the integrated external electrodes

and the capillary channel wall, the range of the field gradient that was applied was

2.4 V/micrometer. The corresponding change in the observed electroosmotic mobility

was 8 x 10^"5 cm²/Vs. Thus, for the experiments of Tables I and II, the efficiency

factor was Y = 3.3 x 10^~5, which was 22 times greater than the next highest literature

reference value shown in Table III, 43 times greater than the average value of the

literature references, and 90 times greater than the lowest value of the literature

references. Thus, the efficiency of the microdevice shown here in controlling

electroosmotic flow was far greater than for standard capillary electrophoresis

systems.

Linear correlation: y = {-2.13 x 10^"3}x + 1.07 (R² = 0.84). Note: 0 V applied voltage to the integrated external electrodes results in a 64 V effective field (data were normalized to this value).

Linear correlation y = {6.6 x 10^"7}x-{2.1 x 10^'4} (R² = 0.94). Data taken 5 mm away from injection zone.

Data taken directly from reference, or calculated from experimental description.

All buffers were pH 3.

According to V_I/(r_iln(r₀/r_i); see text above and reference for explanation/⁵'

Capillary coated with an organic phase containing butyl functional groups.

Capillary coated with an organic phase containing amino functional groups.

(1) Wu, et al., Analytical Chemistry 1993, 65, 568-571. (2) Wu, et al., Analytical Chemistry 1992, 64, 2310-2311. (3) Huang, et al., Analytical Chemistry 1993, 65, 2887-2893. (4) Hayes, et al., Analytical Chemistry 1993, 65, 27-31. (5) Hayes, et al., Analytical Chemistry 1992, 64, 512-516.

EXAMPLE 2

Reagents. Sodium dihydrogen phosphate (NaH₂PO₄), sodium

hydroxide and anhydrous ethyl alcohol (Aldrich); t-butyldiphenylchlorosilane (United

Chemical Technologies, Inc., Bristol, Pennsylvania); and 200 nm carboxylate

modified yellow-green fluorescent (505 nm excitation/515 nm emission) latex microspheres (Molecular Probes, Eugene, Oregon) were used as received. All

NaH₂PO₄ buffers were prepared to 100 mM concentration and adjusted with 100 mM

sodium hydroxide to pH 5.1. Capillaries were coated by combining 30 micro liters

of t-butyldiphenylchlorosilane with 1 mL anhydrous ethyl alcohol and pressure

rinsing the capillary.

Instrumentation. Capillaries were fused silica (35 and 45 cm in length,

20 micrometers i.d. and 150 micrometers o.d.; Polymicro Technologies, Phoenix,

Arizona), where the one tip was sputter-coated with chromium, and then gold, after

removing the polyimide coating (Desk II Sputtering Unit, Denton Vacuum Inc.,

Cherry Hill, New Jersey). Thus, a longitudinal electrode was placed exactly at the

end of the capillary channel, and in this example did not occupy any portion of the

capillary channel. These tips were physically connected to a platinum electrode

which was formed about the circumference of the solution reservoir that was external

to the device. The capillary electrophoresis system to which the device was interfaced

was built in-house and used a CZEIOOOR high voltage power supply (Spellman High

Voltage Electronics Corporation, Hauppauge, New York); a vacuum pump system

(CENCO Hyvac, Fort Wayne, Indiana); a 100 mW He-Cd dual wavelength laser

(442 nm/325 nm) (Omnichrome laser, Chino, California); a CC-5E CCD camera

(HutchNET, East Hartford, Connecticut); and an Olympus VANOX stereo

microscope (Tokyo, Japan). Data collection and analysis programs were developed

in-house using LabVIEW software and an IMAQ PCI- 1408 image acquisition board from National Instruments (Austin, Texas). Modeling was accomplished using

programs developed in-house using MathCAD 7.0.

The device was interfaced by placing the cathodic buffer reservoir in a

sealed plexiglas container where vacuum or pressure could be applied. The anodic

buffer reservoir was fashioned from plexiglas material to form a container where the

gold-coated capillary tip of the device and the reservoir buffer were maintained at the

same potential. This allowed the longitudinal potential field to be initiated at the

immediate end of the capillary channel. Data analysis was performed by recording

fluorescence intensity near the end of the capillary channel (quantitation was 10 pixels

by 500 pixels for 2.5 micrometers by 120 micrometers). The fluorescence was

monitored from the carboxylate-modified latex spheres over time as voltage fields

were adjusted to balance electrophoretic migration of the microspheres against the

bulk inward flow. A cross-sectional 10 pixels were then averaged and analyzed using

programs developed in-house using MathCAD 7.0 and Excel (Microsoft) on an

Optiplex GXI Pentium 233 (DELL Computer Corporation, Round Rock, Texas).

EXAMPLE 3

A substrate of Corning 0211 glass is fabricated defining a capillary

channel 30 micrometers wide by 10 micrometers deep, and 5 cm long, as in Example

1. An integrated external electrode is positioned parallel to the channel separated by

50 micrometers from the channel, and extending longitudinally 1 cm in both

directions from the longitudinal center of the channel. A layer of titanium dioxide, a high dielectric material, is positioned between the integrated external electrode and

the channel, extending longitudinally 0.2 cm in both directions from the longitudinal

center of the channel. A voltage is applied to the integrated external electrode to

directionally inject charge density to the channel wall.

Claims

THE INVENTION CLAIMED IS:

1. A device for performing fluid flow comprising:

a substrate defining a capillary channel, wherein the capillary

channel comprises an inner wall surface, two ends, and a cross section of less than

about 200 x 10^"9 square meters; and

at least one integrated external electrode spaced apart from the

inner wall surface of the capillary channel by a distance d of less than about 160 x 10^"6

meters, wherein the integrated external electrode is positioned to provide a

perpendicular voltage field to the capillary channel; and

two longitudinal electrodes, one of said longitudinal electrodes

being positioned at one end of the capillary channel and the other of said longitudinal

electrodes being positioned at the other end of the capillary channel, wherein the

longitudinal electrodes are positioned at the immediate ends of the capillary channel

and are positioned to provide a longitudinal voltage field selectively through the

capillary channel.

2. The device of claim 1, wherein the distance d is less than about

50 x 10^'6 meters.

3. The device of claim 1, wherein the capillary channel cross

section is less than about 50 x 10^"9 square meters and the distance d is less than about

50 x lO^'6 meters.

4. The device of claim 1, wherein the capillary channel cross

section is less than about 2 x 10^"9 square meters and the distance d is less than about

50 x 10^"6 meters.

5. The device of claim 1, further comprising a high dielectric

material being positioned between at least one of the integrated external electrodes

and the capillary channel.

6. The device of claim 1 , further comprising a means for real-time

flow measurement with feedback for monitoring and controlling the flow of fluids in

the capillary channel.

7. The device of claim 1 , further comprising a coating on said

inner wall surface.

8. A combination device for performing fluid flow comprising a

plurality of devices according to claim 1.

9. A microchip comprising the device according to claim 1.

10. The device of claim 1 , wherein said substrate comprises a

material selected from the group consisting of ceramics, silica, fused silica, quartz,

silicates, titanates, metal oxides, nitrides, silicon, titanium dioxide, polymers, plastics,

polydimethylsiloxanes, polymethylmethacrylates, and mixtures thereof.

11. The device of claim 5, wherein said high dielectric material

comprises a material selected from the group consisting of ceramics, silicates,

titanates, metal oxides, nitrides, polymers, plastics, polydimethylsiloxanes,

polymethylmethacrylates, and mixtures thereof.

12. The device of claim 5, wherein the high dielectric material

comprises titanium dioxide.

13. An electrophoretic separation process using the device of claim

1 , comprising the steps of:

(1) introducing a fluid comprising the species to be separated into the

capillary channel;

(2) applying a voltage of less than about 2000 volts to the integrated

external electrodes to control fluid flow; and

(3) applying a voltage difference to the longitudinal electrodes,

thereby causing electrophoretic migration of the species to occur.

14. The process of claim 13, wherein the voltage applied to the

integrated external electrodes is less than about 200 volts.

15. A fluid flow process using the device of claim 1 , comprising

the steps of:

(1) introducing a fluid into the capillary channel;

(2) applying a voltage of less than about 2000 volts to the integrated

external electrodes to control fluid flow; and

(3) applying a voltage difference to the longitudinal electrodes,

thereby causing fluid flow to occur.

16. The process of claim 15 , wherein the voltage applied to the

integrated external electrodes is less than about 200 volts.