Electropipettor and compensation means for electrophoretic bias

What is claimed is:

1. A sampling system comprising:

an electropipettor for introducing subject materials into a microfluidic system, said electropipettor fluidly connected to said microfluidic system, said electropipettor comprising:

a first capillary channel having a first end for contacting at least one source of said subject materials and a second end terminating in said microfluidic system;

a second capillary channel having a first end terminating near said first end of said first capillary channel and a second end terminating in a source of first spacer material;

a third capillary channel having a first end terminating near said first end of said first capillary channel and a second end terminating in a source of second spacer material; and

a voltage source for applying voltages between said at least one subject material source and said microfluidic system, between said first spacer material source and said microfluidic system such that subject material from said one subject material source and spacer material from said first spacer material source are electrokinetically introduced into said electropipettor toward said microfluidic system, and between said second spacer material source and said microfluidic system such that second spacer material from said second spacer material source is electrokinetically introduced into said electropipettor toward said microfluidic system,

a sample substrate, said sample substrate having a plurality of different samples immobilized thereon; and

a translation system for moving said electropipettor relative to said sample substrate.

2. The sampling system of claim 1, wherein said first end of said first capillary channel and said first end of said second capillary channel terminate in a fluid retention well at a tip of said electropipettor.

3. The sampling system of claim 1, wherein said plurality of different samples are dried onto a surface of said sample substrate, and wherein said electropipettor is capable of expelling an amount of a fluid to resolubilize said sample on said sample substrate.

4. The sampling system of claim 3, wherein said samples are applied to said sample substrate surface in a fluid form, and said substrate surface comprises a plurality of fluid localization regions.

5. The sampling system of claim 4, wherein said fluid localization regions comprise relatively hydrophilic regions surrounded by relatively hydrophobic regions.

6. The sampling system of claim 4, wherein said fluid localization regions comprise relatively hydrophobic regions surrounded by relatively hydrophilic regions.

7. The sampling system of claim 4, wherein said fluid localization regions comprise depressions on said surface of said sample substrate.

8. A sampling system comprising:

a microfluidic system comprising at least first and second intersecting channels disposed within a substrate;

an electropipettor for introducing materials into at least one of the at least first and second intersecting channels in the microfluidic system, the electropipettor comprising:

a body having at least a third capillary channel therein, the third capillary channel having a first end for contacting at least one source of sample materials and a second end fluidly connected to at least one of the first and second intersecting channels in the microfluidic system; and

a voltage source for applying a voltage gradient between the one source of sample materials and a first electrode in the microfluidic system when the first end of the third channel contacts the source of sample materials such that material from the source of sample materials is electrokinetically introduced into the electropipettor toward the microfluidic system;

a sample substrate having a plurality of different samples localized thereon; and

a translation system for moving the electropipettor relative to the sample substrate.

9. The sampling system of claim 8, wherein the sample substrate comprises at least one multiwell plate, and each of the plurality of different sample materials is located in a separate well of the at least one multiwell plate.

10. The sampling system of claim 8, wherein the plurality of different sample materials are immobilized in separate locations on the sample substrate.

11. The sampling system of claim 10, wherein the plurality of different sample materials are dried onto separate locations of the sample matrix.

12. The sampling system of claim 11, wherein the electropipettor is capable of expelling an amount of a fluid to resolubilize said sample on said sample substrate.

13. The sampling system of claim 12, wherein the plurality of different sample materials are applied to the sample substrate surface in a fluid form, and the substrate surface comprises a plurality of fluid localization regions.

14. The sampling system of claim 13, wherein said fluid localization regions comprise first regions surrounded by second regions, the second regions being relatively more hydrophobic than the first regions.

15. The sampling system of claim 13, wherein the fluid localization regions comprise first regions surrounded by second regions, the first regions being relatively more hydrophobic than the second regions.

16. The sampling system of claim 13, wherein the fluid localization regions comprise depressions on the surface of the sample substrate.

17. The sampling system of claim 8, wherein the electropipettor further comprises at least a fourth microscale channel disposed in the body of the electropipettor, the fourth channel having a first end terminating in a source of a first fluid and a second end terminating in the third channel at a point adjacent to the first end of the third channel, and wherein the voltage source applies a voltage gradient between a second electrode disposed in electrical contact with the source of first fluid and the at least one source of sample materials.

18. The sampling system of claim 8, wherein the first end of the third capillary channel and the first end of the fourth capillary channel terminate in a fluid retention well at a tip of the electropipettor.

19. A method of sampling a plurality of different sample materials, comprising:

providing a microfluidic system comprising:

at least first and second intersecting microscale channels disposed within a substrate;

an electropipettor comprising a body having at least a third capillary channel therein, the third channel having a first end for contacting at least one source of sample materials and a second end fluidly connected to at least one of the at least first and second intersecting channels in the microfluidic system; and

a voltage source for applying a voltage gradient between the source of sample materials and a first electrode in electrical contact with the microfluidic system when the first end of the third channel contacts the source of sample materials such that material from the source of sample materials is electrokinetically introduced into the electropipettor toward the microfluidic system;

a sample matrix having a plurality of different sample materials separately localized thereon;

a translation system for moving the electropipettor relative to the sample matrix;

contacting the first end of the third channel to a source of first sample material on the sample matrix;

applying a voltage gradient between the source of first sample material and the microfluidic system to draw a volume of first sample material into the third channel toward the microfluidic system;

contacting the first end of the third channel to a source of second sample material; and

applying a voltage gradient between the source of second sample material and the microfluidic system to draw a volume of second sample material into the third channel toward the microfluidic system.

20. The method of claim 19, wherein:

in the providing step, the plurality of different sample materials are provided dried on the sample matrix, and the electropipettor further comprises a source of a fluid material, and at least a fourth microscale channel and a source of a first fluid disposed in the body, the fourth channel fluidly connecting the source of first fluid with the third channel at the first end; and

in the applying step, a voltage gradient is applied between the source of first fluid and the first end of the third channel to expel a volume of the first fluid from the first end of the third channel.

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BACKGROUND OF THE INVENTION

There has been a growing interest in the manufacture and use of microfluidic systems for the acquisition of chemical and biochemical information. Techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, etc., are being used in the fabrication of these microfluidic systems. The term, "microfluidic", refers to a system or device having channels and chambers which are generally fabricated at the micron or submicron scale, e.g., having at least one cross-sectional dimension in the range of from about 0.1 .mu.m to about 500 .mu.m. Early discussions of the use of planar chip technology for the fabrication of microfluidic systems are provided in Manz et al., Trends in Anal. Chem. (1990) 10(5):144-149 and Manz et al., Avd. in Chromatog. (1993) 33:1-66, which describe the fabrication of such fluidic devices and particularly microcapillary devices, in silicon and glass substrates.

Applications of microfluidic systems are myriad. For example, International Patent Appln. WO 96/04547, published Feb. 15, 1996, describes the use of microfluidic systems for capillary electrophoresis, liquid chromatography, flow injection analysis, and chemical reaction and synthesis. U.S. patent application Ser. No. 08/671,987, filed Jun. 28, 1996, and incorporated herein by reference, discloses wide ranging applications of microfluidic systems in rapidly assaying large number of compounds for their effects on chemical, and preferably, biochemical systems. The phrase, "biochemical system," generally refers to a chemical interaction which involves molecules of the type generally found within living organisms. Such interactions include the full range of catabolic and anabolic reactions which occur in living systems including enzymatic, binding, signaling and other reactions. Biochemical systems of particular interest include, e.g., receptor-ligand interactions, enzyme-substrate interactions, cellular signaling pathways, transport reactions involving model barrier systems (e.g., cells or membrane fractions) for bioavailability screening, and a variety of other general systems.

Many methods have been described for the transport and direction of fluids, e.g., samples, analytes, buffers and reagents, within these microfluidic systems or devices. One method moves fluids within microfabricated devices by mechanical micropumps and valves within the device. See, Published U.K. Patent Application No. 2 248 891 (Oct. 18, 1990), Published European Patent Application No. 568 902 (May 2, 1992), U.S. Pat. Nos. 5,271,724 (Aug. 21, 1991) and 5,277,556 (Jul. 3, 1991). See also, U.S. Pat. No. 5,171,132 (Dec. 21, 1990) to Miyazaki et al. Another method uses acoustic energy to move fluid samples within devices by the effects of acoustic streaming. See, Published PCT Application No. 94/05414 to Northrup and White. A straightforward method applies external pressure to move fluids within the device. See, e.g., the discussion in U.S. Pat. No. 5,304,487 to Wilding et al.

Still another method uses electric fields to move fluid materials through the channels of the microfluidic system. See, e.g., Published European Patent Application No. 376 611 (Dec. 30, 1988) to Kovacs, Harrison et al., Anal. Chem. (1992) 64:1926-1932 and Manz et al. J. Chromatog. (1992) 593:253-258, U.S. Pat. No. 5,126,022 to Soane. Electrokinetic forces have the advantages of direct control, fast response and simplicity. However, there are still some disadvantages. For maximum efficiency, it is desirable that the subject materials be transported as closely together as possible. Nonetheless, the materials should be transported without cross-contamination from other transported materials. Further, the materials in one state at one location in a microfluidic system should remain in the same state after being moved to another location in the microfluidic system. These conditions permit the testing, analysis and reaction of the compound materials to be controlled, when and where as desired.

In a microfluidic system in which the materials are moved by electrokinetic forces, the charged molecules and ions in the subject material regions and in the regions separating these subject material regions are subjected to various electric fields to effect fluid flow.

Upon application of these electric fields, however; differently charged species within the subject material will exhibit different electrophoretic mobilities, i.e., positively charged species will move at a different rate than negatively charged species. In the past, the separation of different species within a sample that was subjected to an electric field was not considered a problem, but was, in fact, the desired result, e.g., in capillary electrophoresis. However, where simple fluid transport is desired, these varied mobilities can result in an undesirable alteration or "electrophoretic bias" in the subject material.

Without consideration and measures to avoid cross-contamination, the microfluidic system must either widely separate the subject materials, or, in the worst case, move the materials one at a time through the system. In either case, efficiency of the microfluidic system is markedly reduced. Furthermore, if the state of the transported materials cannot be maintained in transport, then many applications which require the materials to arrive at a location unchanged must be avoided.

The present invention solves or substantially mitigates these problems of electrokinetic transport. With the present invention, microfluidic systems can move materials efficiently and without undesired change in the transported materials. The present invention presents a high throughput microfluidic system having direct, fast and straightforward control over the movement of materials through the channels of the microfluidic system with a wide range of applications, such as in the fields of chemistry, biochemistry, biotechnology, molecular biology and numerous other fields.

SUMMARY OF THE INVENTION

The present invention provides for a microfluidic system which electroosmotically moves subject material along channels in fluid slugs, also termed "subject material regions," from a first point to a second point in the microfluidic system. A first spacer region of high ionic concentration contacts each subject material region on at least one side and second spacer regions of low ionic concentration are arranged with the subject material regions of subject material and first or high ionic concentration spacer regions so that at least one low ionic concentration region is always between the first and second points to ensure that most of the voltage drop and resulting electric field between the two points is across the low ionic concentration region.

The present invention also provides for a electropipettor which is compatible with a microfluidic system which moves subject materials with electroosmotic forces. The electropipettor has a capillary having a channel. An electrode is attached along the outside length of the capillary and terminates in a electrode ring at the end of the capillary. By manipulating the voltages on the electrode and the electrode at a target reservoir to which the channel is fluidly connected when the end of the capillary is placed into a material source, materials are electrokinetically introduced into the channel. A train of subject material regions, high and low ionic concentration buffer or spacer regions can be created in the channel for easy introduction into the microfluidic system.

The present invention further compensates for electrophoretic bias as the subject materials are electrokinetically transported along the channels of a microfluidic system. In one embodiment a channel between two points of the microfluidic system has two portions with sidewalls of opposite surface charges. An electrode is placed between the two portions. With the voltages at the two points substantially equal and the middle electrode between the two portions set differently, electrophoretic forces are in opposite directions in the two portions, while electroosmotic forces are in the same direction. As subject material is transported from one point to the other, electrophoretic bias is compensated for, while electroosmotic forces move the fluid materials through the channel.

In another embodiment a chamber is formed at the intersection of channels of a microfluidic system. The chamber has sidewalls connecting the sidewalls of the intersecting channels. When a subject material region is diverted from one channel into another channel at the intersection, the chamber sidewalls funnel the subject material region into the second channel. The width of the second channel is such that diffusion mixes any subject material which had been electrophoretically biased in the subject material region as it traveled along the first channel.

In still a further embodiment, the present invention provides a microfluidic system and method of using that system for controllably delivering a fluid stream within a microfluidic device having at least two intersecting channels. The system includes a substrate having the at least two intersecting channels disposed therein. In this aspect, the one of the channels is deeper than the other channel. The system also includes an electroosmotic fluid direction system.

The system is particularly useful where the fluid stream comprises at least two fluid regions having different ionic strengths.

The present invention also provides a sampling system using the electropipettor of the invention. The sampling system includes a sample substrate, which has a plurality of different samples immobilized thereon. Also included is a translation system for moving the electropipettor relative to said sample substrate.

The invention as hereinbefore described may be put into a plurality of different uses, which are themselves inventive, for example, as follows:

The use of a substrate having a channel, in transporting at least a first subject material from at least a first location to a second location along the channel, utilizing at least one region of low ionic concentration which is transported along the channel due to an applied voltage.

A use of the aforementioned invention, in which the ionic concentration of the one region is substantially lower than that of the subject material.

A use of the aforementioned invention, wherein a plurality of subject materials are transported, separated by high ionic concentration spacer regions.

The use of a substrate having a channel along which at least a first subject material may be transported, in electrophoretic bias compensation, the channel being divided into a first and a second portion, in which the wall or walls of the channel are oppositely charged, such that electrophoretic bias on the at least first subject material due to transportation in the first portion is substantially compensated for by electrophoretic bias due to transport in the second portion.

A use of the aforementioned invention in which a first electrode is located at a remote end of the first portion, a second electrode is located at the intersection between the portions and a third electrode is located at a remote end of the second portion.

A use of the aforementioned invention, in which the substrate is a microfluidic system.

A use of the aforementioned invention in which the substrate is an electropipettor.

A use of the aforementioned invention, in which the electropipettor has a main channel for transportation of the subject material and at least one further channel fluidly connected to the main channel from which a further material to be transported along the main channel is obtained.

A use of the aforementioned invention, in which the further material is drawn into the main channel as a buffer region between each of a plurality of separate subject materials.

The use of a microfluidic system having at least a first and a second fluid channel which intersect, in optimizing flow conditions, the channels having different depths.

A use of the aforementioned invention in which one channel is between 2 to 10 times deeper than the other channel.

The use of a microfluidic system having a first channel and a second channel intersecting the first channel, in electrophoretic compensation, the intersection between the channels being shaped such that a fluid being transported along the first channel towards the second channel is mixed at the intersection and any electrophoretic bias in the fluid is dissipated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of one embodiment of a microfluidic system;

FIG. 2A illustrates an arrangement of fluid regions traveling in a channel of the microfluidic system of FIG. 1, according to one embodiment of the