FOCUSING OF MICROPARTICLES IN MICROFLUIDIC SYSTEMS

Abstract: Methods and systems for particle focusing to increase assay throughput in microscale systems are provided. The invention includes methods for providing substantially uniform flow velocity to flowing particles in microfluidic devices. Methods of sorting members of particle populations, such as cells and various subcellular components are also provided. Integrated systems in which particles are focused and/or sorted are additionally included.

Description: FOCUSING OF MICROPARTICLES IN MICROFLUIDIC SYSTEMS COPYRIGHT NOTIFICATION Pursuant to 37 C F.R. 1 71(e), Applicants note that a portion of this disclosure contains mateπal which is subject to copyπght protection The copyπght owner has no objection to the facsimile reproduction by anyone of the patent document or patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyπght πghts whatsoever CROSS REFERENCE TO RELATED APPLICATIONS This application is related to and claims pπoπty to and the benefit of provisional application 60/134,472, filed 5/17/99, Wada et al., "Focusing of Microparticles in Microfluidic Systems," pursuant to 35 U.S.C. § 119(e), as well as any other applicable statute or rule. This pπoπty application is incorporated herein in its entirety for all purposes. BACKGROUND OF THE INVENTION A vaπety of cell-based assays are of considerable commercial relevance in screening for modulators of cell-based activity. For example, compounds which affect cell death can have profound biological activities and are desirably screened for in cell-based assays Cell death has become recognized as a physiological process important in normal development, hormonal regulation of vaπous tissues, and, e.g., in regulation of the receptor repertoires of both T and B lymphocytes. The finding that a pattern of morphological changes is common to many examples of programmed cell death (or PCD) led to the suggestion of a common mechanism, and the term "apoptosis" was defined to include both the morphological features and the mechanism common to such programmed cell death (Kerr et al., Br J. Cancer 26.239) This concept was extended by the finding that nuclear DNA fragmentation correlates well with apoptotic morphology (Arends et al , Am J. Pathol. 136 593 (1990);. and the scientific literature contains many examples of PCD accompanied by these features There are also clear examples of PCD in the absence of apoptotic morphology or DNA fragmentation (Clarke, Anat. Embryl 181 : 195 (1990), Martin et al. J Cell Biol 106.829 (1988). and Ishigami et al., I Immunol 148.360 (1992)) Cell-based assay systems model relevant biological phenomena, and have generally been widely adopted as screening assays, e.g , when screening for a compound's effect(s) on apoptosis or other biological phenomena Pioneeπng technology providing cell- and other particle-based microscale assays are set forth in Parce et al "High Throughput Screening Assay Systems in Microscale Fluidic Devices" WO 98/00231 , in PCT/USOO/04522, filed February 22, 2000, entitled "Manipulation of Microparticles In Microfluidic Systems," by Mehta et al.; and in PCT/US00/04486, filed February 22, 2000. entitled "Devices and Systems for Sequencing by Synthesis," by Mehta et al. Other cell-based assays include vaπous methods for the preparative or analytic sorting of different types of cells. For example, cell panning generally involves attaching an appropπate antibody or other cell-specific reagent to a solid support and then exposing the solid support to a heterogeneous cell sample. Cells possessing, e.g., the corresponding membrane-bound antigen will bind to the support, leaving those lacking the appropπate antigenic determinant to be washed away. Other well-known sorting methods include those using fluorescence-activated cell sorters ("FACSs"). FACSs for use in sorting cells and certain subcellular components such as molecules of DNA have been proposed in, e.g., Fu, AN. et al. (1999) "A Microfabπcated Fluorescence-Activated Cell Sorter," Nat Biotechnol 17: 1109-1111; Unger, M., et al. (1999) "Single Molecule Fluorescence Observed with Mercury Lamp Illumination," Biotechniques 27:1008-1013; and Chou, H.P. et al. (1999) "A Microfabπcated Device for Sizing and Sorting DNA Molecules," Proc. Nat'l. Acad Sci 96 11-13 These sorting techniques utilizing generally involve focusing cells or other particles by flow channel geometry While cell-based assays are generally preferred in certain microscale screening applications, certain of these assays are difficult to adapt to conventional notions of high-throughput or ultra high-throughput screening assay systems. For example, one difficult in flowing assay systems is that, duπng pressure-based flow of fluids in channels non-uniform flow -velocities are expenenced Faster fluid and mateπal flow is observed in the center of a moving fluid stream than on the edge of a moving fluid stream This non-uniform flow velocity reduces throughput for flowing assays, because assa\ runs have to be spaced well apart in the fluid stream to prevent overlap of mateπals moving at different velocities Accordingly, it would be advantageous to provide mechanisms for facilitating cell-based assays, including cell sorting techniques, especially in microscale systems Additional microscale assays directed at subcellular components, such as nucleic acids would also be desirable The present invention provides these and other features which will become clear upon consideration of the following SUMMARY OF THE INVENTION The present invention relates to methods of focusing particles in microchannels, e g , to improve assay throughput, to sort particles, to count particles, or the like In the methods of the invention, cells and other particles are focused in the center of, to one side of, or in other selected regions of microscale channels, thereby avoiding, e g , the above noted difficulties inherent in pressure-based flow of particles Furthermore, the device structures of the present invention are optionally integrated with other microfluidic systems Other reactions or manipulations involving cells, other particles, or fluids upstream of the detection zone are also optionally performed, e g , monitoπng drug interactions with cells or other particles In one aspect, the invention provides methods of providing substantially uniform flow velocity to particles flowing in a first microchannel In the methods, the particles are optionally flowed in the microchannel, e g , using pressure - based flow, in which the particles flow with a substantially non-uniform flow velocity Pπor to performing the flowing step, the particles are optionally sampled with at least one capillary element, e g , by dipping the capillary element into a well containing the particles on a microwell plate and drawing the particles into, e g , reservoirs, microchannels, or other chambers of the device The particles (e g , a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a set of functionalized microbeads, a molecule, a set of molecules, etc ) are optionally focused hoπzontally and or vertically in the first microchannel to provide substantially uniform flow velocity to the particles in the first microchannel Particles are optionally focused using one or more fluid direction components (e g . a fluid pressure force modulator, an electrokinetic force modulator, a capillary force modulator, a fluid wick ng element, or the like) Additional options include sorting, detecting or otherwise manipulating the focused particles The particles are hoπzontally focused in the microchannel, e g , by introducing a low density fluid and a high density fluid into the microchannel, causing the particles to be focused in an intermediate density fluid present between the high density fluid and the low density fluid The particles are also optionally focused in a top or a bottom portion of the microchannel by introducing a high or a low density fluid into the microchannel with the flowing particles The particles are vertically or hoπzontally focused in the microchannel, e g , by simultaneously introducing fluid flow from two opposing microchannels into the first microchannel duπng flow of the particles in the first channel Vertical focusing is also optional achieved to one side of a microchannel by simultaneously introducing fluid flow from, e g , a second microchannel into the first microchannel duπng flow of the particles in the first microchannel In another aspect, the invention also provides particle washing or exchange techniques For example, focused cells or other particles are optionally washed free of diffusible mateπal by introducing a diluent into the first microchannel from at least a second channel and removing the resulting diluted diffused product compπsing diluent mixed with the diffusible mateπal through at least a third microchannel Alternating arrangements of diluent input and diffused product output channels are also optionally used to further wash the particles For example, in one aspect the methods of the invention include simultaneously introducing the diluent into the first microchannel from the second microchannel and a fourth microchannel, where the second and fourth microchannel intersect the first microchannel at a common intersection region Optionally, the methods include sequentially introducing the diluent into the first microchannel from the second microchannel and a fourth microchannel, wherein the second and fourth microchannels intersect the first microchannel at an offset intersection region The diffused product is typically removed through the third microchannel and a fifth microchannel, which third and fifth microchannels intersect the first microchannel at a common intersection region In further washing steps, the diluent is introduced through sixth and seventh microchannels which intersect the first microchannel at a common intersection The resulting further diluted diffused product is removed through eighth and ninth microchannels, which intersect the first microchannel at a common intersection Diluent is optionally introduced into the first microchannel by pressure or electrokinetic flow In one preferred assay of the invention, the particles are cells and the method includes performing a TUNEL assay or an Annexin-V on the cells in the channel to measure apoptosis Integrated systems for performing the above methods, including the particle sorting embodiments, are also provided An integrated system for providing substantially uniform flow velocity to flowing members of at least one particle population in a microfluidic device optionally includes a body structure that includes at least a first microchannel disposed therein A first fluid direction component (e g , a fluid pressure force modulator) is typically coupled to the first microchannel for inducing flow of a fluidic mateπal that includes the members of the at least one particle population in the first microchannel The first fluid direction component generally induces non-uniform flow A source of at least one fluidic mateπal is optionally fluidly coupled to the first microchannel The system also optionally includes at least a second microchannel that intersects the first microchannel for introducing at least one fluid into the first microchannel to hoπzontally or vertically focus the members of the at least one particle population in the first microchannel The at least one fluid is optionally introduced using a second fluid direction component that includes one or more of a fluid pressure force modulator, an electrokinetic force modulator, a capillary force modulator, a fluid wicking element, or the like At least one flow control regulator for regulating flow of the fluidic mateπal or the fluid in the first or second microchannel is also optionally provided A computer including an instruction set directing simultaneous flow of the members of the at least one particle population in the first microchannel and simultaneous introduction of the at least one fluid from the second microchannel into the first microchannel is optionally also operably coupled to a fluid movement system for directing flow of mateπals in the microchannels As a further option this integrated system additionally includes at least a third microchannel which intersects the first microchannel in an intersection region common to the second microchannel The flow control regulator of this system optionally further regulates flow of the at least one fluid in the second and the third microchannels In this embodiment, the computer typically also includes an instruction set for simultaneously flowing fluids from the second and third microchannels into the first microchannel In particle washing systems, typically, at least fourth and fifth channels which intersect the first microchannel at a common intersection downstream of the second and third microchannels are provided The computer further includes an instruction set for simultaneously flowing mateπal from the first microchannel into the fourth and fifth microchannels Sixth and seventh microchannels which intersect the first microchannel at a common intersection downstream of the fourth and fifth microchannels, with the computer further compπsing an instruction set for simultaneously flowing mateπal from the sixth and seventh microchannels into the first microchannel are optionally provided Similarly, eighth and ninth microchannels which intersect the first microchannel at a common intersection downstream of the sixth and seventh microchannels, the computer further including an instruction set for simultaneously flowing mateπal from the first microchannel into the eighth and ninth microchannels are optionally provided The integrated system optionally includes sources for any reagent or particle used in the methods noted above, such as one or more sources of terminal deoxynucleotide transferase, one or more sources of one or more fluorescein labeled nucleotides or other labeled polynucleotides, one or more sources of Annexm V, one or more sources of an AnnexinV-biotin conjugate, one or more sources of a DNA dye, one or more sources of Campthotecin, one or more sources of Calcein-AM, one or more sources of a control cell, one or more sources of a test cell, etc Signal detector(s) mounted proximal to the first microchannel for detecting a detectable signal produced by one or more of the members of the at least one particle population in the microchannel are typically provided in the integrated systems of the invention The detector also optionally includes, e g , a fluorescent excitation source and a fluorescent emission detection element Optionally, the computer is operably linked to the signal detector and has an instruction set for converting detected signal information into digital data The integrated system of the present invention is also optionally used to sort the members of a particle population (e.g., a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a set of functionalized microbeads, a molecule, a set of molecules, or the like). In this embodiment, the integrated system typically additionally includes a third and a fourth microchannel which intersect the first microchannel downstream from the intersection of the second microchannel with the first microchannel The fourth microchannel also generally intersects the first microchannel downstream from the intersection of the third microchannel with the first microchannel. The flow control regulator of this system optionally further regulates flow of the at least one fluid in the third or the fourth microchannels. Furthermore, the signal detector typically detects a detectable signal produced by a selected member of the particle population between the intersections of the second and the third microchannels with the first microchannel. In this particle sorting embodiment, the computer is optionally operably linked to the first or other fluid direction component(s), the flow control regulator, and the signal detector. Additionally, the instruction set typically directs simultaneous introduction of the at least one fluid from the third microchannel into the first microchannel to hoπzontally or vertically focus the selected member of the particle population such that the selected member is directed into the fourth microchannel in response to the detectable signal produced by the selected member. Optionally, the instruction set further directs simultaneous introduction of the at least one fluid from the third microchannel by activating a heating element (e.g., a Joule heating electrode, a conductively coated microchannel portion, etc.) disposed within the third microchannel or a well that fluidly communicates with the third microchannel. In another embodiment, at least a portion of the first microchannel optionally includes a separation element disposed therein. The separation element optionally includes, e.g , two sides and at least a portion of the separation element is typically disposed upstream of the fourth microchannel In this embodiment, a selected member of the particle population is generally directed to one of the two sides of the separation element and into the fourth microchannel that intersects the first microchannel in response to the detectable signal produced by the selected member The integrated system for use in particle sorting also optionally includes a fifth microchannel which intersects the first microchannel in an intersection region common to the second microchannel In this case, the flow control regulator also typically regulates flow of the at least one fluid in the second and the fifth microchannels, and the computer optionally includes an instruction set for simultaneously flowing fluids from the second and the fifth microchannels into the first microchannel Similarly, the system also optionally includes a sixth microchannel which intersects the first microchannel in an intersection region common to the third microchannel. In this embodiment, the flow control regulator optionally additionally regulates flow of the at least one fluid in the third and the sixth microchannels. Furthermore, the computer also typically includes an instruction set for flowing fluids from the third and the sixth microchannels into the first microchannel. Optionally, the instruction set directs individual or simultaneous fluid flow from the third and sixth microchannels by individually or simultaneously activating at least one heating element (e.g., a Joule heating electrode, a conductively coated microchannel portion, or the like) disposed within each of the third and sixth microchannels or within at least one well that fluidly communicates with each of the third and sixth microchannels. Many additional aspects of the invention will be apparent upon review, including uses of the devices and systems of the invention, methods of manufacture of the devices and systems of the invention, kits for practicing the methods of the invention and the like. For example, kits compπsing any of the devices or systems set forth above, or elements thereof, in conjunction with packaging mateπals (e.g., containers, sealable plastic bags, etc.) and instructions for using the devices, e.g., to practice the methods herein, are also contemplated BRIEF DESCRIPTION OF THE FIGURES Figure 1A is a schematic showing focusing of cells in a microscale system by simultaneous flow from side channels into a main channel through which the cells are being flowed Figure IB is a photomicrograph of focused labeled cells flowing in a microchannel Figure 2A is a schematic of a microfluidic system with a pressure- source (in this case a vacuum source) for achieving fluid movement Figure 2B shows a cross-sectional view down a channel having high, medium and low fluid density. Figure 3 is a schematic drawing of a microfluidic system adapted to washing reagents from microparticles. Figure 4 is a data graph illustrating a control analysis, e.g., U937 cells not treated to induce apoptosis The bottom line corresponds to the SYTO®-62, which indicates the mere presence of cells, whereas the top line corresponds to the fluorescein end labeled nucleic acids. Figure 5 is a data graph illustrating U937 cells treated with Campthotecin to induce apoptosis. As can be seen, corresponding peaks are seen on both the lower and upper lines, indicating the presence of apoptotic cells. Figure 6 is a data graph illustrating an analysis of U937 cells that were not treated to induce apoptosis. In this case, the bottom line indicates live cell count (Calcem), while the top line indicates the presence of apoptotic cells (Annexm-V- Cy5). As can be seen, a few apoptotic cells are present within the control expeπment. Figure 7 is a data graph showing an analysis of U937 cells treated with Campthotecin to induce apoptosis. The top trace includes a much greater number of peaks representing apoptotic cells, and particularly as a percentage of total cells in the analysis, e.g., as compared to the lower line. Figure 8 is a data graph showing results from a TUNEL assay with HL-60 cells treated with Campthotecin for 6 hours to induce apoptosis. The bottom line traces the total cell count The top line traces apoptotic cells labeled at DNA damage points. In this case, almost all cells had sustained apoptotic DNA damage Figure 9 is a data graph showing results from a TUNEL assay. HL-60 cells (not treated for apoptosis) were used as control cells The bottom trace is for SYTO®-62 indicating total cell count The top trace is for fluorescein labeled nucleotide which signifies the presence of apoptosis related DNA damage Figure 10 is a data graph showing results from TUNEL assays from Figures 8 and 9 analyzed using appropπate software Data is presented as scatter plot format in Figure 10. Figure 11 is a data graph showing results from TUNEL assays from data represented in bar graph format. Additional data for different time points and two other cell lines is included. Figure 12 is a data graph showing an Annexin-V assay. HL60 cells were treated with Campothotecin for four hours to induce apoptosis The bottom trace indicates live cell count (Calcein). The top trace indicates Annexιn-V-Cy5 labeling. Cells labeled only for Calcein are live cells and not apoptotic. In this case, almost all cells are apoptotic Figure 13 is a data graph showing an Annexin-V assay. HL-60 cells (not treated for apoptosis) are control cells. The bottom trace indicates live cell count The top trace indicates Annexm-V-Cy5 labeling. Cells labeled with both dyes are apoptotic. Cells labeled only for Annexιn-Cy5 are dead cells. In this case, only a few cells were apoptotic. Figure 14 is a data graph showing data analysis of Figures 12 and 13. Data is presented in scatter plot format in Figure 14. Figure 15 is a data graph showing data represented in bar graph format. Figure 16 shows a "T-junction" microchannel configuration for hydrodynamically focusing cells for use in, e.g., flow cytometry. Figure 17 depicts a chip design used to conduct fluorescence population focusing expeπments. Figure 18 is a histogram showing the distπbution of results obtained in a fluorescence intensity population expeπment conducted utilizing hydrodynamic focusing. Figures 19 and 20 are histograms showing the distπbution of results obtained in fluorescence intensity population expeπments conducted in the absence of hydrodynamic focusing. Figure 21 is a microchannel network that is optionally utilized to hydrodynamically focus particles. Figure 22 is a schematic illustration of a particle sorting configuration utilizing sets of opposing microchannels to focus and/or otherwise direct the flow of the members of, e.g., a cell population to achieve cell sorting. Figure 23 is a schematic representation of a microchannel configuration that includes one separation element embodiment. Figure 24 is a schematic depiction of a microchannel configuration that includes Joule heating electrodes for use in particle sorting. Figures 25A, B, and C are histograms showing the distribution of results obtained in resolution experiments using various microbead concentrations. Figure 26 is a histogram showing the distribution of results obtained in a resolution experiment using viable cells stained with an intercalating dye. DETAILED DISCUSSION OF THE INVENTION The invention relates to particle focusing to improve assay throughput by regularizing flow velocity. As used herein, the term "particle" refers to a discretely describable material element, such as a cell, microbead (e.g., functionalized or non-functionalized), molecule (e.g., a nucleic acid, a polypeptide, etc.), or the like. For example, a particle typically includes, e.g., a soluble or non-soluble component of a fluid mixture, such as a solubilized or suspended molecule, liposome, cell, organelle, bead, or the like. Ordinarily, the particle is a detectable unit; for example, in the case of molecular particles, the particle is detectable, e.g., via a label. Similarly, microbeads and cells are detected, e.g., optically or through an associated label (this detection is also optionally, e.g., optical or via fluorescence). The following provides details regarding various aspects of the methods of focusing particles, e.g., vertically and/or horizontally within the microscale systems of the invention. It also provides details pertaining to methods of washing cells and to particular assays, such as cell viability screening, which are optionally performed using the methods and devices of the present invention. Devices and integrated systems are also discussed extensively. LAMINAR FLOW VERTICAL FOCUSING OF CELLS AND OTHER PARTICLES IN MICROSCALE SYSTEMS Microfluidic hydrodynamic focusing is a highly effective technique when used, e g., in flow cytometry applications, such as the assessment of live/dead cell ratios, the analysis of transfection efficiencies, the sizing of vaπous molecular components (e g., polynucleotides, polypeptides, etc ), the study of apoptosis, or the like. Fluid flow streams are optionally "pinched" or "focused" into a narrow region of a microchannel to facilitate single particle or narrow streamline detection using a vaπety of optical detection schemes (discussed further, below) In one embodiment, particle focusing is achieved electrodynamically (see also, Ramsey et al., "Apparatus and Method for Performing Electrodynamic Focusing on a Microchip," U.S. Pat. No 5,858,187, issued January 12, 1999) In other preferred embodiments, pressure, Joule heating, and or other fluid movement methods are used to focus cells or other particles. As used herein, a "vertically" focused particle stream refers to a particle stream that is substantially focused, pinched, narrowed, or otherwise confined along or proximal to a plane that extends, in one dimension, from the highest to the lowest internal surface of a microchannel (e.g., along the length of a cross-sectional line or axis of the microchannel), and along a selected length of the microchannel, in another dimension, when the microfluidic device is oπented for typical operational usage. A vertically focused particle stream corresponds to a plane that is oπented approximately 90° from a plane in which a hoπzontally focused particle stream (discussed below) would be oπented. Vertical focusing is optionally achieved by flowing fluids into a microchannel (e.g., from one or more sides of the microchannel) in which a particle stream is flowing to focus the particles along or proximal to the vertical plane having a dimension that corresponds to a selected vertical line or axis of the microchannel. The level of focusing along or proximal to the vertical plane is optionally vaπed. Furthermore, as indicated, a particle stream is also optionally vertically focused away from the vertical cross-sectional axis, e.g., to one side of a microchannel. An additional option includes simultaneously hoπzontally and vertically focusing a particle stream to position the stream within a desired region of a microchannel. For example, the measurement of fluorescent signals associated with single cells (or other particles) in microfluidic channels is difficult if the cells or particles are randomly disbursed in the channels The channels are usually about 100 μm or larger in width to prevent clogging by particle aggregates; however, particles such as cells are typically on the order of 10 μm in diameter An interrogating light beam is focused on a spot usually much less than the 100 μm width of the channel As a result, it is useful, e g , to center particles in the channel, to position particles to one side of a channel, or to otherwise focus flowing particles in a portion of a channel pπor to detection to obtain accurate fluorescent readings on all the particles passing through the channel Figure 1 A shows an example cell (or other particle) analysis chip design for on-chip focusing The microchannel network geometry includes a cross formed by four microchannels that intersect in a common intersection region Figure IB shows a photomicrograph of this microchannel configuration with focused labeled cells flowing in a main or analysis microchannel Cells or other particles are optionally flowed in the main microchannel and/or focused using vaπous methods, which include the use of electrokinetic forces, pressure gradients, surface tension gradients, gravitational forces, or the like Furthermore, combinations of these fluid flow/focusing methods are also optionally used. A preferred method of flowing particles in the mam microchannel utilizes pressure-based flow. In the embodiment depicted in Figure 1, cells 100 (or other particles) are typically flowed from one microchannel into the cross-junction and focused by introducing hydrodynamic flows 102 from the two orthogonal microchannels Non- orthogonal (e.g., opposing or non-opposmg) microchannels are also optionally used For example, as discussed herein, particles are optionally focused with a single focusing microchannel, or alternately, by using a seπes of offset focusing microchannels to achieve focusing by seπal introduction of fluids from the offset channels As depicted in both panels of Figure 1, cells 100 are optionally constrained to the center of a detection microchannel downstream from the two orthogonal microchannels by hydrodynamic flows 102 introduced from both sides as cells 100 pass through detector 104 Using these methods, an expeπment is optionally conducted which uses, e.g , a small optical detection system on the order of the size of the particular cell or other particle being analyzed to deπve accurate and precise measurements Figure 21 depicts a microchannel network that is optionally used to hydrodynamically focus particles, e g., cells, microbeads, molecules, or the like in parallel through common detection zone 2100 For example, five cell suspensions are typically loaded into wells 2102 and flowed towards common detection zone 2100. Cell buffer solutions are also typically loaded into particular wells (e.g., well 2104) in the device and flowed towards common detection zone 2100 As shown, in the region just pπor to common detection zone 2100, each microchannel in which cells are flowed intersects with two buffer flowing microchannels in a cross-configuration to focus the cells as they pass through common detection zone 2100 In another embodiment of these methods, a "T-junction" microchannel geometry is optionally used to focus cells or other particles 1600 to one side of detection microchannel 1604. (FIG. 16). As shown in Figure 16, a "T-junction" typically includes an orthogonal intersection of two microchannels However, microchannel geometπes that include non-orthogonal intersections are also optionally used (e.g., a "Y-junction" or the like). In this embodiment, cells or other particles 1600 are typically flowed from one well 1602 and pinched to one side of detection microchannel 1604 by only one other introduced fluid stream 1606 and the wall of detection microchannel 1604 opposing the "T-junction" as cells or other particles 1600 pass through detector 1608. This embodiment generally achieves the same effect as the cross-microchannel formats, but with the added advantage of using less space on the microfluidic device due to the use of one focusing channel, rather than two which, in turn, provides for greater parallelism or complexity in chip design, and for more control of the hydrodynamic focusing mechanism. The focusing of flow to one side of a microchannel also provides a solution to vaπous problems associated with controlling fluid flow under lower pressures (e.g., difficulties regulating fluid direction pumps, etc.). For example, the resistance produced by pinching fluid mateπals against microchannel walls functions to reduce the velocity of the pinched fluid stream As a result, pinched fluids are optionally flowed under higher pressures, e.g., for greater control, while achieving comparable low pressure flow velocities Resistance is also altered, e.g., by varying the viscosity of pinched fluid mateπals. which provides an added level of control over flow rates and the extent of fluid pinching FOCUSING CELLS HORIZONTALLY IN A MICROCHANNEL In high throughput screening applications using cells or microparticles, the throughput is increased if all of the cells or microparticles move at a common velocity The parabolic nature of pressure-based flow causes cells at the center of a flow stream to move faster than those near the walls of a channel This phenomenon causes dispersion in the system since cells can move from one sample into another To focus cells in the center of a channel, high and low density fluid streams are incorporated into fluid flow of the cells or particles The high density fluid keeps the cells or other microparticles off of the bottom of the channel, while the low density fluid keeps cells or microparticles off of the top of the microchannel, thereby focusing the cells in the center of the fluid stream Even after diffusion bπngs the high, low, and middle density fluids into equi bπum, the cells generally remain focused in the center of a microchannel Optionally, the point in the microchannel in which the high and low density fluids are flowed into the mam channel is deeper than other channel regions to improve distπbution of the different density fluids A "hoπzontally" focused particle stream, as used herein, refers to a particle stream that is substantially focused, pinched, narrowed, or otherwise confined along or proximal to a plane that extends, in one dimension, from one substantially vertical internal microchannel surface to another (e g., substantially parallel to the dimension of the lowest internal microchannel surface that extends from one vertical internal surface to another), and along a selected length of the microchannel, m another dimension, when the microfluidic device is oπented for typical operational usage. A hoπzontally focused particle stream corresponds to a plane that is oπented approximately 90° from a plane in which a vertically focused particle stream (discussed above) would be onented. As mentioned, hoπzontal particle focusing typically involves using focusing fluids having densities that differ relative to the density of a particle stream For example, higher density fluids are optionally used to substantially prevent the particle stream from contacting the lowest internal surface of the microchannel over a particular length. In contrast, a less dense focusing fluid is optionally used to substantially prevent the particle stream from contacting the highest internal surface of the microchannel over a selected length Additionally, both lower and higher density focusing fluids are optionally flowed simultaneously to substantially prevent the particle stream from contacting either the highest or lowest internal microchannel surfaces. The extent of hoπzontal focusing achieved is optionally regulated, e.g., by varying the density and/or the volume of focusing fluid flowed into a microchannel A further option includes simultaneously hoπzontally and vertically focusing a particle stream to position the stream within a desired region of a microchannel. Figure 2A is a schematic representation of a microfluidic system that uses vacuum source 210 to achieve fluid movement In this embodiment, test compounds are typically flowed through capillary channel 200 (capillary channels or elements are discussed in greater detail, below) into a detection microchannel of the device. High density buffer and low density buffer are optionally flowed into the detection microchannel through high density buffer microchannel 202 and low density buffer microchannel 204, respectively, both of which intersect the detection microchannel upstream from detector 208. Cell microchannel 206 typically intersects the detection microchannel downstream from the intersections of high density buffer microchannel 202 and low density buffer microchannel 204, but upstream from detector 208. Cells in medium density buffer are typically flowed into the detection channel from cell microchannel 206, becoming focused between the high and low density buffers, and exposed to the test compounds pπor to flowing through detector 208. Figure 2B shows a cross-sectional view down the detection microchannel which includes low density buffer 214, cells 212, and high density buffer 216. Fluid density-based focusing also optionally entails focusing cells or other particles either at the top or the bottom of a microchannel. For example, a high density fluid is optionally flowed into a microchannel to keep cells or other particles off of the bottom of the microchannel, i.e., focused at the top of the microchannel. Alternatively, a low density fluid is flowed into a microchannel to keep cells or other particles off of the top of the microchannel, i.e , focused on the bottom of the microchannel It is also worth noting that hoπzontal and vertical focusing are optionally both performed to center cells vertically and hoπzontally in the center of microscale channels Furthermore, the extent of hydrodynamic focusing, using either or both methods, is typically controlled by a vaπety of vaπables including viscosity in pressure dπven flows, electrokinetic mobility in electrokinetic flow, fluid density in gravity dπven flow, or the like As will be recognized by those of skill, the enhanced control of focusing, provided by the present invention, allows for a large gamut of flow-based applications, including any particle and/or bulk fluid application that involves small, uniform detection regions. USE OF FOCUSING TO SORT PARTICLES The separation of fluorochrome-labeled cells with fluorescence- activated cell sorters is well-known in the art, as mentioned above. In bπef, the technique generally involves incubating a mixed population cells with a fluorescently- labeled antibody against a specific antigenic determinant displayed on the surfaces of target cells. The suspension of cells is then typically expelled, one cell at a time, from a sample chamber of the device through a small vibrating nozzle that generates microdroplets. Each rmcrodroplet contains a single cell that the FACS assays for the presence or absence of the fluorochrome label using a laser source and a detector. Droplets that fluoresce are electπcally charged in proportion to their fluorescence and in turn separated as they pass between charged plates. General references descπbmg FACS include, e.g., Kuby, Immunology (3 rd Ed.) W.H. Freeman and Company, New York (1997), Watson, et al., Recombinant DNA (2 nd Ed.) W.H. Freeman and Company, New York (1992), and Alberts, et al. Molecular Biology of the Cell (3 rd Ed.) Garland Publishing, Inc., New York (1994). Other references relating to flow cytometry include, e.g., Radbruch (Ed.) Flow Cvtometrv and Cell Sorting, Spπnger- Verlag, New York (1992), Owens and Loken, Flow Cytometry Pnnciples for Clinical Laboratory Practice, Wiley-Liss, New York (1995), and Jarosqeski and Heller (Eds.) Flow Cvtometrv Protocols' Methods in Molecular Biology, Vol. 91, Humana Press (1997). The present invention provides microfluidic devices and methods for sorting particles (e.g., fluorescently-labeled particles) that use hydrodynamic flow to focus and/or sort the particles The methods for sorting members of a particle population (e g , a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a set of functionalized microbeads, a molecule, a set of molecules, or the like) typically include flowing the members of a particle population in a first microchannel The members of the particle population are focused hoπzontally and/or vertically in the first microchannel such that selected individual members are directed into at least a second microchannel that intersects with the first microchannel In one embodiment, a portion of the first microchannel optionally includes at least one separation element disposed therein The separation element generally includes at least two sides in which at least a portion of the separation element is disposed upstream of the second microchannel In this embodiment, the members of the particle population are optionally focused hoπzontally and/or vertically in the first microchannel such that selected individual members are directed to at least one of the at least two sides of the separation element and into, e.g , at least a second microchannel that intersects the first microchannel In other embodiments, this separation element is omitted The focusing steps of the particle sorting methods, descπbed above, optionally include hoπzontally and or vertically focusing the members of the particle population in the first microchannel by introducing at least one fluid flow (e g., a buffer, a high density fluid, a low density fluid, or the like) from at least a third microchannel that intersects with the first microchannel upstream from, e.g., the second microchannel. This is optionally performed by any method for flowing fluid in a microfluidic device, including electrokinetic flow, pressure-based flow, and the like. For example, methods employing electrostatic forces to transport and focus samples generally involve inducing sample flow in a transport channel by applying a first electπcal potential to the channel Thereafter, a second electπcal potential is typically applied to focusing channels that intersect with the transport channel in a cross-configuration to vertically focus the transported sample in the center of the transport channel This electrokinetic-based system is also optionally utilized to direct or "nudge" flowing samples to effect a sorting function, e g., by applying an electπcal potential to at least one of the intersecting focusing channels to direct sample flow to ards or away from that focusing channel and into, e g., an additional intersecting channel or into a channel region That is, electrokinetic force can be applied to "nudge" mateπals across the width of a first channel at an intersection of the first channel and a second channel, resulting in downstream focusing of mateπals in the channel This application of electrokinetic force is optionally electrokinetic, electroosmotic, or both See also, e g , Ramsey et al , "Apparatus and Method for Performing Electrodynamic Focusing on a Microchip," U S Pat No 5,858,187, issued January 12, 1999 As mentioned, other alternative techniques for inducing the flow of focusing fluids to sort particles according to the methods of the present invention include pressure, hydrostatic, wicking, capillary, and other forces Fluid direction components based upon these forces are discussed, below In another embodiment, the fluid flow is induced by heating (e.g., Joule heating, etc ) a fluid in the third microchannel Alternatively, the focusing steps include hoπzontally and/or vertically focusing the members of the particle population in the first microchannel by introducing at least one fluid flow from at least one of at least a third and at least a fourth microchannel that oppose one another and that intersect with the first microchannel upstream from the second microchannel. Similarly, the fluid flow is optionally introduced by heating (e.g , Joule heating or the like) a fluid in at least one of the third or fourth microchannels Embodiments that involve heat induced fluid flow are discussed further below Figure 22 schematically illustrates one particle sorting configuration utilizing these methods. As shown, cells 2200 are generally flowed in a main microchannel that includes at least two sets of opposing microchannels for focusing and/or otherwise directing the flow of cells 2200 using hydrodynamic flow 2202 (e.g , cell buffer flow). One set of opposing microchannels is typically located, e.g , upstream from detector 2204 for simultaneously introducing hydrodynamic flow 2202 from both microchannels to focus cells 2200, as descπbed above A second set of opposing microchannels is typically located downstream from detector 2204 for introducing at least one hydrodynamic flow 2202 so as to direct selected cells 2208 (e.g , fluorescently-labeled cells) and non-selected cells 2206 into, in this case, one of two microchannels, each terminating in particular collection wells 2210 Figure 23 schematically depicts a microchannel configuration that includes one embodiment of separation element 2310 In this example, cells 2300 are typically flowed in a microchannel that intersects with another microchannel located upstream from detector 2304 The upstream intersecting microchannel is optionally used to focus cells 2300 to one side of the microchannel as cells 2300 pass through detector 2304 using hydrodynamic flow 2302 This illustration also includes a set of opposing microchannels located downstream from the detector for introducing at least one hydrodynamic flow 2302 to direct selected cells 2308 (e.g , fluorescently-labeled cells) and non-selected cells 2306 to either side of separation element 2310 and into, in this case, one of two microchannels, each terminating in particular collection wells 2312. Separation element 2310 is optionally omitted, with cells 2300 or other particles being directed into destination regions as desired The inverse relationship between temperature and viscosity for vaπous buffers, gels, and other mateπals is also optionally exploited in the present invention to effect particle sorting For example, upon detection of a desired particle, e.g., a buffer or gel disposed in a downstream side-channel is typically heated to decrease fluid viscosity (i.e., to induce fluid flow) to thus direct particles within the microfluidic device. Although any heating source, functional m the devices of the present invention, is optionally used in these embodiments, Joule heating is a preferred method. Joule heating is typically produced by flowing current through an electrode or other conductive component positioned within a well, microscale channel, or other cavity within the device. The resulting flow of current into fluid within the well, channel, or cavity results in resistive heating of the fluid. By substantially increasing the current across the channel, rapid temperature changes are induced that are optionally monitored by conductivity. Because nano ter volumes of fluid have tiny thermal mass, transitions between temperatures are typically extremel) short. For example, oscillations between any two temperatures above 0°C and below 100°C in 100 milliseconds have been performed Thus, the present invention optionally uses power sources that pass electπcal current through, e.g., a focusing channel region for heating purposes In exemplary embodiments, fluid passes through a channel of a desired cross-section (e g , diameter) to enhance thermal transfer of energy from the current to the fluid The channels are optionally formed on almost any type of substrate mateπal such as, amorphous mateπals (e g , glass, plastic, silicon), composites, multi-layered mateπals, combinations thereof, or the like In general, electπc current passing through fluid in a channel produces heat by dissipating energy through the electπcal resistance of the fluid. Power dissipates as the current passes through the fluid and goes into the fluid as energy as a function of time to heat the fluid The following well-known mathematical expression generally relates the power dissipated in a fluid ("P") to the electπc current passing through the fluid ("I") and the electπcal resistance of fluid ("R"): P=I 2 R In these embodiments, a portion of the power goes into kinetic energy for moving the fluid through the channel. However, it is also possible to use a selected portion of the power to controllably heat fluid in a channel or selected channel regions, e.g., to further induce fluid movement by reducing fluid viscosity. A channel region suitable for heating is optionally narrower or smaller in cross-section than other channel regions in the channel structure, as a smaller cross-section provides higher resistance in the fluid, which increases the temperature of the fluid as electπc current passes through. Alternatively, the electπc current is increased across the length of the channel by increased voltage, which also increases the amount of power dissipated into the fluid to correspondingly increase fluid temperature. To selectively control the temperature of fluid in a region of a channel, a power supply applies voltage and/or current in vaπous ways. For instance, a power supply optionally applies direct current (i.e., DC) or alternating current (AC), which passes through the channel and into a channel region which is, e.g., smaller m cross- section to heat fluid in the region. Alternatively, a power supply applies a pulse or impulse of current and or voltage, which passes through the channel and into a channel region to heat fluid in the region Pulse width, shape, and/or intensity are optionally adjusted, e.g., to heat the fluid substantially while moving the fluid. Still further, the power supply optionally applies any combination of DC, AC, and pulse, depending upon the application In practice, direct application of electπc current to fluids in the microchannels of the invention results in extremely rapid and easily controlled changes in temperature. A controller or computer such as a personal computer is generally used to monitor the temperature of the fluid in the region of the channel where the fluid is heated. The controller or computer typically receives current and voltage information from, e.g., the power supply and identifies or detects fluid temperature in the channel region. The controller or computer also typically receives current information from an operably connected detector, e.g., when a selected particle is detected, which triggers the flow of current through, e.g., one or more Joule heating electrodes. Depending upon the desired temperature of fluid in the region, the controller or computer adjusts voltage and/or current to meet the desired fluid temperature. In certain embodiments, the region which is heated optionally includes a "coil" which is typically in a planar arrangement. Transfer of heat from the coil to a channel through a substrate material is used to heat the focusing fluids, gels or other materials disposed in the channel. Alternatively, the coil itself is optionally the focusing channel. A voltage is applied between regions of the coil to direct current through the fluid for heating purposes. In particular, a power supply provides a voltage differential between regions of the coil. Current flows between the regions and traverses a plurality of coils or coil loops, which are defined by a substrate. The shape and size of the coils typically influences an ability of current to heat the fluid in the coil. As current traverses through the fluid, energy is transferred to the fluid for heating purposes. Cooling coils are also optionally included. One embodiment of Joule heating-based particle sorting is schematically illustrated in Figure 24. As shown, particles are typically flowed from particle well 2400 into main microchannel 2402. In the configuration depicted, focusing microchannel 2404 intersects with main microchannel 2402 in a "T- junction" upstream from detector 2408. Similar to the device represented in Figure 23 (discussed above), focusing flow stream 2406 is optionally used to focus or pinch the particles against the wall of main microchannel 2402 opposing the "T-junction" as the particles flow through detector 2408. When selected particle 2410 (e.g., a fluorescently-labeled particle) is detected, an electrical signal from detector 2408 typically triggers the flow of current to Joule heating electrodes 2416, which are disposed within wells of branched particle sorting microchannel 2418. Optionally, branched particle sorting microchannel 2418 includes more than two branches or is a single, unbranched microchannel. Additionally, greater or less than two Joule heating electrodes 2416 are optionally included. Although not shown in Figure 24, conductive coatings are also optionally included in lieu of or in addition to the heating electrodes for effecting the flow of a focusing fluid. The heat produced by current flowing from Joule heating electrodes 2416 raises the temperature of, e.g., a buffer or gel disposed within branched particle sorting microchannel 2418, thus reducing hydrodynamic resistance in the buffer or gel. The reduced viscosity, in conjunction with vacuum source(s) that are typically operably connected to the device at non-selected particle collection well 2420 and/or selected particle collection well 2422, induce or increase flow of the buffer or gel from branched particle sorting microchannel 2418, which intersects main microchannel 2402 downstream from detector 2408. The induced flow of buffer or gel into main microchannel 2402 thus deflects or redirects the flow of selected particle 2410 into selected particle collection well 2422. In this embodiment, non- selected particle 2412 does not trigger Joule heating/redirecting flow and as such, flows unimpeded into non-selected particle collection well 2420. Many alternative embodiments that incorporate Joule heating to control fluid viscosity and particle flow are possible. For example, although not shown in Figure 22, one or both microchannels of the second set of opposing microchannels located downstream from detector 2204 optionally include one or more Joule heating electrodes (e.g., 1, 2, 3, 4, 5, or more electrodes) and/or conductive coating portions. One or both of the opposing microchannels also optionally includes a branched channel structure similar to the branched structure depicted in Figure 24, above (i.e., branched particle sorting microchannel 2418). Optionally, one or both of the opposing microchannels includes greater than two branches, each branch of which optionally includes one or more Joule heating electrode(s). In other embodiments, separation elements, such as those discussed above are optionally included. Preferred buffers, gels, and other materials used the in these heating embodiments have relatively sharp temperature/viscosity transitions in a narrow temperature range and are generally known in the art. Joule heating is described further in, e.g., in 08/977,528, entitled "Electrical Cuπent for Controlling Fluid Temperatures in Microchannels," filed November 25, 1997 by Calvin Chow, Anne R. Kopf-Sill and J. Wallace Parce; and in PCT/US98/06723, entitled "Closed-Loop Biochemical Analyzers," filed April 3, 1998, by Knapp. There are various other techniques that are also optionally used to heat buffers, gels, and other materials to effect particle focusing. For example, resistive heating typically results from current applied to conductively coated well or microchannel portions, from one or more electrodes directly (e.g., a thermocouple, etc.), or the like. Other heating methods optionally include directing, e.g., light from a laser source through one or more fiber optic cables to, e.g., wells that fluidly communicate with focusing channels. Heat from other external sources is also optionally utilized in the methods described herein. WASHING CELLS IN MICROFLUIDIC CHANNEL SYSTEMS In one aspect, the invention provides in-system cell washing capability. This concept is also optionally extended to washing any other particulate sample. For an extensive description of microfluidic systems comprising particles, see, U.S. Provisional Patent Application No. 60/128,643, filed April 4, 1999, entitled "Manipulation of microparticles in microfluidic systems," by Mehta et al. In this embodiment, diffusible substances such as antibodies, proteins, salts, drugs, etc. are removed by washing the materials away from the particles in a microscale system. For this embodiment of the invention, lateral channels are placed downstream of the point at which diffusible substances are present in a central main channel. For example, as illustrated in Figure 3, cells flowed from sample well 300 are optionally incubated with labeled antibody flowed from test compound well 302 in incubation channel region 304, with lateral channels being placed downstream of incubation channel region 304. Wash buffer is typically injected simultaneously from both sides of the main channel from lateral wash channels 306. Although not shown, wash buffer is optionally sequentially introduced into the main channel from lateral wash channels that intersect the main channel in an offset intersection region (i.e., the lateral wash channels are at least somewhat staggered, instead of intersecting directly across from one another). This buffer is then generally removed by flowing the diffusible material into another set of lateral wash channels 308. Further washing is optionally achieved by placing additional lateral wash channels 310 farther downstream from incubation channel region 304 for flowing wash buffer into the main channel and additional wash channels 312 are also optionally included for flowing wash buffer out of the main channel. By repeating wash in-wash out steps, diffusible materials are removed from the main channel. Thereafter, labeled cells are optionally detected in detection zone 314 and subsequently flowed into waste well 316. Assuming laminar flow, the cells or other particles remain centered in the main channel and do not flow into the lateral wash channels, preventing clogging of the wash channels. Flow in the main channel, and in the wash channels is optionally performed by electrokinetic or pressure-based flow of materials. CELL VIABILITY SCREENING The methods and systems of the present invention are particularly applicable in performing cell viability assays as well as for screening test compounds for their effects on cell viability. Such assays are generally utilized in performing toxicity studies, antibiotic screening methods, or the like, and are particularly suitable for the methods and systems of the present invention. Accordingly, in these aspects, the cellular function specific indicator is an indicator of cell viability. In operation, the suspension of cells typically includes a label. The cells are optionally treated with a second function labeling group that indicates the viability of the cells in the suspension. Specifically, the function label preferentially stains or labels either viable or non-viable cells. A variety of viability indicative dyes are generally commercially available. For example, fluorogenic esterase substrates, such as Calcein AM, BCECF AM and fluorescein diacetate, are optionally loaded into adherent or nonadherent cells, and are suitable indicators of cell viability. Specifically, these esterase substrates measure both esterase activity, which is required to activate the fluorescence of the dye, as well as cell-membrane integrity, which retains the fluorescent materials intracellular] y. Other suitable viability indicators include polyfluorinated fluorescein derivatives (i.e., DFFDA, TFFDA, HFFDA and Br TFFDA), polar nucleic acid based dyes (i.e., SYTOX Green™), dimeric and monomeric cyanine dyes (i.e., TOTO™ and TO-PRO™ series dyes from Molecular Probes), ethidium and propidium dyes (i.e., ethidium bromide, ethidium homodimer and propidium iodide). Depending upon the viability indicator used, the level of function label is indicative of the number of either viable or non-viable cells, while the level of reference label is indicative of the number of total cells, e.g., viable and non-viable. Comparison of the levels of the two labels then provides an indication of the relative viability of the cells in the suspension, regardless of the number of cells being detected, e.g., multiple cells, aggregates, or individual cells). In particular, where two cell populations show a similar level of reference label, but one population shows a lower level of viability indicator, it will be understood that the second population is less viable, e.g., has more nonviable cells. It will be appreciated that many dyes or labels described for use as reference labels are also often used as viability labels. Accordingly, it will generally be desirable to select a reference label that labels both viable and nonviable cellular material, and which is distinguishable from the function label. Examples of such reference labels include, e.g., lipophilic membrane labels, or the like. In performing screening assays, cell suspensions that are exposed to different test compounds or agents are flowed past the detection point and the relative viability of the cells is determined, as compared to a control. Increases or decreases in cellular viability indicate that the compound or agent improves or decreases cellular viability. Such assays are readily employed in identifying antimicrobial, antibiotic or other viability affecting agents. Similarly, such assays are optionally employed in screening for effectors of pathways involved in apoptosis or programmed cell death, e.g., ras mediated pathways. The methods and devices of the present invention are optionally used to perform the cell viability assays described herein. In one embodiment, e.g., cells exposed to selected test compounds are washed free of excess staining dyes or the like before being focused horizontally and/or vertically as they flow past a detection point to obtain accurate fluorescent readings (e.g., derived from function and/or reference labels). Thereafter, cells (e.g., viable and non-viable cells) are optionally sorted similarly using hydrodynamic flow. These embodiments are depicted, e.g., in Figures 22 and 23, which are discussed further above. ELIMINATING ADSORPTION OF MATERIALS DURING FLOW Adhesion of materials during pressure-based flow using prior art methods can be problematic, because the flow velocity at the channel wall is low. This low flow velocity increases the time that a material remains in position proximal to a given region of the microscale channel. This increased proximity to a single region leads to formation of strong interactions between the channel region and the material. Thus, one particular advantage of the present method is that focusing of materials in the center of microchannels inhibits adsorption of the materials to the walls of microscale channels and other microscale elements during flow of the materials. ASSAY SYSTEMS As noted above, the methods and systems of the present invention are useful in assaying for virtually any cellular function, provided that either the function or a result of the function is independently detectable. In biological applications, and particularly pharmaceutical research, a number of specific types of assays are generally used as screening models for the identification of potential drug candidates, or "lead compounds." The assay types most frequently used in these screening operations generally include transport assays, binding assays, viability assays, and expression assays. Transport In a first aspect, the methods and systems of the present invention are used in assaying cellular transport functions, i.e., ion flux, and intracellular pH regulation. In particular, cellular transport channels have been generally shown to be responsive to important cellular events, e.g., receptor mediated cell activation, and the like. For example, G-protein coupled receptors have been shown to directly or indirectly activate or inactivate ion channels in the plasma membrane or endosomal membranes of cells, thereby altering their ion permeability and thus effecting the excitability of the membrane and intracellular ion concentrations. See, Hille, Ionic Channels of Excitable Membranes, Sinauer Assoc. (1984). In accordance with this aspect of the present invention, therefore, the function specific label comprises an indicator of the level of a particular intracellular species. In particularly preferred aspects, the intracellular species is an ionic species, such as Ca 2+ , Na + , K + , Cl " , or H * (e.g., for pH measurements). A variety of intracellular indicator compounds are commercially available for these ionic species (e.g., from Molecular Probes, Eugene OR). For example, commonly used calcium indicators include analogs of BAPTA (l,2-bis(2-aminophenoxy)ethane-N,N,N',N'- tetraacetic acid), such as Fura-2, Fluo-2 and Indo-1, which produce shifts in the fluorescent excitation or emission maxima upon binding calcium, and Fluo-3 and Calcium Green-2, which produce increases in fluorescence intensity upon binding calcium. See also, U.S. Patent No. 5,516,911. Sodium and potassium sensitive dyes include SBFI and PBFI, respectively (also commercially available from Molecular Probes). Examples of commercially available chloride sensitive indicators include 6- methoxy-N-(sulfopropyl)quinolinium (SPQ), N-(sulfopropyl)acridinium (SPA), N-(6- methoxyquinolyl)acetic acid, and N-(6-methoxyquinolyl)acetoethyl ester (Molecular Probes, Inc.), all of which are generally quenched in the presence of chloride ions. In a related aspect, the function specific indicator is an intracellular pH indicator compound. Specifically, intracellular pH changes have been found to be associated with biologically and pharmaceutically important cellular events, including cell proliferation, apoptosis, fertilization, malignancy, ion transport, drug resistance, lysosomal storage disorders, and Alzheimer's disease. A variety of indicator compounds are commercially available to indicate the intracellular pH of cells, and are readily applicable to the present invention as indicators of cellular function. Examples of these pH indicators include, e.g., SNARFL, SNARF, BCECF, and HPTS, available from Molecular Probes, Inc. In operation, a suspension of cells that is to be assayed is flowed along a channel. The cells include a reference label as described above, e.g., SYTO® dyes available from Molecular Probes. The cells are also treated with an intracellular indicator of the level of the species for which relative transport levels are to be determined, and which indicator is distinguishable from the reference label. As a specific example, the cells are optionally stained with, e.g., S YTO®-62 as a reference label. SYTO®-62 is a red nucleic acid dye that is generally excited by light at approximately 655 nm, and which emits light at approximately 675 nm. The cells are also optionally treated with an intracellular calcium indicator, e.g., Fluo-3, also available from Molecular Probes, which is excited at 488 nm and emits at approximately 530 nm. The two labels are easily distinguishable based upon their differing fluorescent emission maxima. At a point in the channel, the cells are illuminated with a broad spectrum of light, e.g., light that encompasses the excitation maxima of both the SYTO®-62 and Fluo-3 labels. Emitted fluorescence is then passed through optical filtering systems that separate and separately detect the SYTO®-62 fluorescence and the Fluo-3 fluorescence. The levels of fluorescence from each dye are then compared. For example, the comparison optionally includes plotting the level of reference label versus the level of function label. Over the course of the assay, a number of separate data points are gathered that represent different cells or groups of cells that are detected. These are plotted and the slope of the resulting line is calculated. Changes in this slope are indicative of changes in the level of the function that is being assayed. Binding I. Generally In an alternate aspect, the methods and systems of the present invention are used in assaying cellular binding functions, such as ligand-receptor binding, nucleic acid hybridization, antigen/antibody binding, cell-cell interactions, and the like. As with transport functions, cellular binding functions are often necessary precursors to a variety of cellular functions. Specifically, many biological responses are often triggered and or controlled by the binding of a receptor to its ligand. For example, interaction of growth factors, i.e., EGF, FGF, PDGF, etc., with their receptors stimulates a wide variety of biological responses including, e.g., cell proliferation and differentiation, activation of mediating enzymes, stimulation of messenger turnover, alterations in ion fluxes, activation of enzymes, changes in cell shape and the alteration in genetic expression levels. Accordingly, control of the interaction of the receptor and its ligand may offer control of the biological responses caused by that interaction. As noted, included within these biological functions controlled by binding reactions are many transport functions, e.g., G-protein linked receptor activation, as set forth above Accordingly, these binding functions may be detected by detecting the downstream event for which binding is a precursor, e.g., enhanced or decreased transport function, expression of receptor linked reporter label, protein translocation, or by detecting actual binding of cells with a binding agent, e.g., a ligand, nucleic acid or the like, through the inclusion in the ligand of a binding indicator, e.g , fluorescent resonance energy transfer dyes (FRET), molecular beacons, etc. For example, in the case of cell-cell interactions, detection of binding may be accomplished by labeling the cells' surfaces with both elements of appropπate FRET dyes, e.g., energy donor and energy acceptor Upon cell-cell binding, these elements are placed in sufficient proximity for energy transfer, allowing their detection. Alternatively, fluorescence polaπzation detection methods are used to detect binding of relatively small molecules, e g , ligands, antibodies, etc., to relatively large structures, e.g., cells. Fluorescence polaπzation assays for use in microfluidic systems are generally descπbed in Provisional U.S Application No. 60/088,650, filed June 8, 1998, incorporated herein by reference. A vaπety of other detection/labeling mechanisms are also available for detecting binding of one molecule, e.g., a ligand or antibody, to another molecule, e.g., a cell surface receptor. For example, a number of labeling mateπals change their fluorescent properties upon binding to hydrophobic sites on proteins, e.g., cell surface proteins. Such labels include, e.g., 8-ammo-l -naphthalene sulfonate (ANS), 2-p- toluιdιnylnaphthalene-6-sulfonate (TNS) and the like Alternatively, detectable enzyme labels are utilized that cause precipitation of fluorescent products on solid phases, i.e., cell surfaces are optionally used as function indicators of binding. For example, alkaline phosphatase substrates that yield fluorescent precipitates are optionally employed in conjunction with alkaline phosphatase conjugates of cell binding components Such substrates are generally available from Molecular Probes, Inc., and are descπbed in, e.g., U.S. 5,316,906 and U.S. 5,443,986. II. Cell Rolling Assays In a related but alternative aspect, the present invention provides methods, devices and systems for use in performing in vitro cell rolling assays. In particular, it has been reported that several classes of cell adhesion molecules participate in a wide range of important physiological functions, including wound healing, inflammation and cancer metastasis. Some examples of these molecules include selectins and integrins which mediate the rolling and subsequent immobilization of white blood cells along the endothelial lining of blood vessel, thus allowing them to migrate out of the blood vessel and toward the target tissue. Cell rolling assays are designed to mimic in vitro the rolling phenomenon in vivo, to create a more effective model for use in screening potential effectors of that phenomenon. See, e.g., Lawrence et al., J. Immunol., (1993) 151:6338-6346 and Brunk et al., Biophys. J. (1997) 72:2820-2833. Generally, the assay is performed by flowing a suspension of cells over a surface upon which ligands are immobilized, and observing the numbers of firmly attached and or rolling cells on that surface as well as the velocity of the rolling cells. The present invention employs the microfluidic systems described herein, in the performance of these assay types. In particular, as described in greater detail below, the cell suspension bearing an appropriate reference label is introduced into a channel in which an appropriate ligand of interest is immobilized on the inner surface. Immobilization of ligands on the interior surface of channels is optionally accomplished by covalently attaching the ligands to the surface or by adsorbing the ligands on the surface. Covalent attachment of ligands to surfaces of solid substrates has been described in the art. See, e.g., Sundberg, et al., J. Am. Chem. Soc. (1995) 117:12050-57. In accordance with the present invention, the cell suspension is flowed through the channel, i.e., using pressure flow, as described in greater detail below, and the number of cells that are rolling over or firmly attached to the interior surface of the channel is monitored using an appropriate detection system. Alternatively, cells are pulsed through the channel to facilitate their monitoring. Typically, such systems employ a video imaging system that images and identifies the cells as they enter the imaged area, and tracks the cells path through the field, determining their relative velocity. Alternatively, point detection systems, e.g., as described herein, are used which detect cells at two separate points in the channel, and determine their relative velocity. In the latter case, it is generally desirable to provide the cells in suspension that is sufficiently dilute so as to permit correlation between the two detectors. Alternatively, cells may be coded with mixtures of different, distinguishable labels to permit the correlation among cells between points. Such coded cells optionally include wide varieties of different labels, or alternatively, include a set of two, three, four, five, six, seven or eight different labels at varying relative levels, where the profile of the relative levels of labels identifies the different cells. In screening assays, the test compounds are introduced into a channel, e.g., via an external sample accessing capillary (e.g., an electrokinetic injector or other capillary element) where they contact the suspension of cells. The cell suspension is then assayed for rolling or firmly attached cells, and the effect of the test compound, if any, on the cell rolling or binding is determined as compared to the control, e.g., in the absence of the test compound. III. Expression In a further aspect, the methods and systems of the present invention are used to assay cellular expression functions, and particularly, for the effect of test compounds on such cellular expression. Such assays are generally utilized in screening for effectors of given biological processes, which effectors target those functions at the gene expression level. In accordance with the present invention, therefore, the function label is indicative of the level of gene expression, for a particular gene of interest. Gene expression levels are typically assayed by quantifying the level of gene product from the gene of interest, e.g., the amount of protein produced by the cells. Alternate methods of gene expression analysis examine the amount of RNA transcribed from the gene of interest. Typically, such assays involve the use of a nucleic acid hybridization assay to identify a pattern of RNA transcription following an activating event. The methods and systems of the present invention are readily applied to such expression analyses. In particular, in accordance with the present invention, the function label is typically provided that is expressed by the cells during the expression function. For example, chimeric reporter systems are optionally employed as function labels or indicators of gene expression. Chimeric reporter systems typically incorporate a heterogeneous reporter system integrated into the coding sequence of the gene of interest. The expression of the gene of interest is then accompanied by the expression of the reporter, which is then detected. For example, a receptor may be a fusion between the product of the gene of interest and heterologous protein, e.g., an enzyme whose activity is readily assayable, or an otherwise detectable protein, e.g., luciferase, aequoπn, green fluorescent protein (GFP), β-galactosidase, alkaline phosphatase, or the like. The expressed reporter is then detected and compared with the level of reference label, to provide a quantitative determination of expression levels on a per cell basis. Expression of gene products to a detectable level can require varying amounts of time, e.g., several minutes to hours Accordingly, the assay time is vaπed to allow such expression. As noted herein, such vaπation is generally accomplished by slowing the flow rates of the cell suspension through a channel and/or lengthening the channel. Alternatively, the function label is provided as an element of a binding molecule that specifically associates with the downstream indicator of gene expression, e.g., an expressed protein, wherein the binding of the binding molecule (beaπng the function label) to the gene product of interest produces a detectable property within the cell, e.g., as descπbed with reference to the binding assays, above. The assay methods are then earned out in the same manner as descπbed with reference to the binding functions, descπbed above. In the case of expressed proteins, the binding molecule optionally includes an antibody specific for the gene product, or a specific binding partner, where the expressed protein is a member of a binding pair, e.g., a receptor or ligand. Because gene expression assays typically require much longer incubation times than other assay types descπbed herein, modified methods are optionally employed. For example, in one aspect, cells are flowed through the channel of the system, and preferably, through multiple parallel channels of the system, and contacted with surfaces of the channel(s) that cause them to adhere. Test compounds are then introduced into the channel(s) and allowed to flow over the adhered cells, e.g., for from 5 to 60 minutes. Any effects of these test compounds on the level of function label, and therefore, gene expression, is determined in the channel(s), and compared to the level of reference label. The compaπson then allows quantification of the level of expression on a per cell basis Optionally, the reaction channel is provided such that the travel time of the cells from the point of test compound addition to detection is sufficient to permit appropπate expression analysis In certain aspects, tortuous channels, e g serpentine channels, and the like, are used to extend channel lengths to expand the travel time Alternatively or additionally, flow rates are substantially reduced to increase this travel time SCREENING ASSAYS As noted repeatedly above, the assays descπbed herein are particularly useful in performing high-throughput screening assays As used herein, the term "test compound" refers to the collection of compounds that are to be screened for their ability to affect a particular biochemical system Test compounds optionally include a wide vaπety of different compounds, including chemical compounds, mixtures of chemical compounds, e g , polysacchaπdes small organic or inorganic molecules, biological macromolecules, e g , peptides, proteins, nucleic acids, or an extract made from biological mateπals such as bacteπa, plants, fungi, or animal cells or tissues, naturally occurπng or synthetic compositions Depending upon the particular embodiment being practiced, the test compounds may be provided, e.g., injected, free in solution, or may be attached to a earner, or a solid support, e g , beads. A number of suitable solid supports may be employed for immobilization of the test compounds Examples of suitable solid supports include agarose, cellulose, dextran (commercially available as, e.g , Sephadex®, Sepharose®, etc ), carboxymethyl cellulose, polystyrene, polyethylene glycol (PEG), filter paper, nitrocellulose, ion exchange resins, plastic films, glass beads, polyaminemethylvinylether maleic acid copolymer, amino acid copolymer, ethylene-maleic acid copolymer, nylon, silk, etc. Additionally, for the methods and apparatuses descπbed herein, test compounds may be screened individually, or in groups Group screening is particularly useful where hit rates for effective test compounds are expected to be low such that one would not expect more than one positive result for a given group Alternatively, such group screening is optionally used where the effects of different test compounds are differentially detected in a single system, e.g , through electrophoretic separation of the effects, or differential labeling which enables separate detection Typically, vast hbraπes of test compounds are separately tested for potential effects on different cellular functions In preferred aspects, large braπes of chemical compounds prepared using combinatoπal synthesis techniques are typically employed as test compounds in high-throughput screening applications, to identify any such compounds that may have pharmacologically beneficial activities. In optional preferred aspects, test compounds include large braπes of naturally occurnng mateπals or compounds, hbraπes of genetic mateπal, protein fragments, In general, the test compounds are separately introduced into the assay systems descπbed herein The relative level of a particular cellular function is then assessed in the presence of the test compound, and this relative level of function is then compared to a control system, which lacks an introduced test compound Increases or decreases in relative cellular function are indicative that the test compound is an enhancer or an inhibitor of the particular cellular function, respectively OVERALL SYSTEMS As noted above, the present invention also provides systems and devices used to practice the above-descπbed methods. The system includes a channel along which is flowed a suspension of cells or other particles, and cell focusing means as noted above (laminar flow focusing channels, density gradient layers, or both). A source of different test compounds is optionally linked to the channel, for introducing the different test compounds into the channel whereupon they are contacted with, e.g., the cells One or more detectors are also provided in sensory communication with the channel, e.g., for detecting and quantifying both the level of reference label and the level of function label present on the cells. As used herein, the phrase "sensory communication" refers to onentation of the detector such that it is capable of obtaining an appropπate signal from the point of interest In the case of optical detectors, sensory communication provides a detector oπented such that it is capable of receiving an optical signal from a channel of a microfluidic device. Such detection is optionally direct, or includes an intervening optical pathway, e.g., lenses, fiber optics, etc In the case of chemical detectors, such sensory communication typically requires a sensor component disposed in contact with the fluid mateπal within the channel The detector(s) is/are operably linked to a processor, e.g., a computer, for recording the detected signals, and providing a report of relative activity of the cells that are being assayed. The computer also typically includes appropriate programming for determining whether one assay, e.g., a first screening assay, shows greater or lesser cellular function than another assay, e.g., a control. Detector systems optionally includes one or more different detector elements, and are selected to detect relevant labels present in the cells. For example, in the case of cells that include reference and function labels that are fluorescent, the detector typically includes a dual wavelength fluorescent detector. This detector typically includes a light source. Appropriate light sources typically vary depending upon the type of detection being employed. For example, in some cases broad spectrum illumination is desirable while in other cases, a narrower spectrum illumination is desired. Typically, the light source is a coherent light source, such as a laser, or laser diode, although other light sources, such as LEDs, lamps or other available light sources are also optionally employed. In the case of a fluorescent detector, excitation light, e.g., light of appropriate wavelength to excite labels, from the light source is directed at an analysis channel, e.g., disposed in a microfluidic device, via an optical train that includes optional lens, beam splitters, and objective lenses In some instances, electrokinetic material transport systems are used to direct one or more of the flow of cell suspensions, the injection of test compounds, and other material movement parameters. In such cases, the overall system used in performing the assay will typically include an appropriate controller and interface for controlling such electrokinetic material transport. Typically, such transport systems include one or more electrical power supplies that are operably coupled to the termini of the channels in a microfluidic device, e.g., as described in greater detail below. The connection of the power supply(ies) with the channels is typically accomplished via electrodes placed into reservoirs at the termini of the channels, which electrodes are coupled to the power supply(ies). The power supply(ies) then deliver(s) appropriate voltage levels to the various electrodes to yield a desired flow pattern within the channels of the device, e.g., flowing the cell suspension and periodically injecting a test compound. The power supply is typically linked to an appropriately programmed computer which directs the application of voltages in accordance with a user selected flow profile ASSAY DEVICES Microfluidic devices and assay components which are optionally adapted to the present invention are descπbed in vaπous PCT applications and issued U.S. Patents, such as, U.S. Patent Nos. 5,699,157 (J Wallace Parce) issued 12/16/97, 5,779,868 (J Wallace Parce et al.) issued 07/14/98, 5,800,690 (Calvin Y.H. Chow et al.) issued 09/01/98, 5,842,787 (Anne R. Kopf-Sill et al.) issued 12/01/98, 5,852,495 (J. Wallace Parce) issued 12/22/98, 5,869,004 (J Wallace Parce et al ) issued 02/09/99, 5,876,675 (Colin B. Kennedy) issued 03/02/99, 5,880,071 (J Wallace Parce et al.) issued 03/09/99, 5,882,465 (Richard J. McReynolds) issued 03/16/99, 5,885,470 ( J. Wallace Parce et al.) issued 03/23/99, 5,942,443 (J Wallace Parce et al.) issued 08/24/99, 5,948,227 (Robert S. Dubrow) issued 09/07/99, 5,955,028 (Calvin Y.H. Chow) issued 09/21/99, 5,957,579 (Anne R. Kopf-Sill et al.) issued 09/28/99, 5,958,203 (J. Wallace Parce et al.) issued 09/28/99, 5,958,694 (Theo T. Nikiforov) issued 09/28/99, and 5,959,291 (Morten J. Jensen) issued 09/28/99; and published PCT applications, such as, WO 98/00231, WO 98/00705, WO 98/00707, WO 98/02728, WO 98/05424, WO 98/22811, WO 98/45481, WO 98/45929, WO 98/46438, and WO 98/49548, WO 98/55852, WO 98/56505, WO 98/56956, WO 99/00649, WO 99/10735, WO 99/12016, WO 99/16162, WO 99/19056, WO 99/19516, WO 99/29497, WO 99/31495, WO 99/34205, WO 99/43432, and WO 99/44217, which are all incorporated herein by reference. As noted above, the assays of the present invention are earned out within fluidic channels, along which the cell suspensions and or other particles are flowed. In some cases, the channels may simply be present in a capillary tube, e.g., a glass, fused silica, quartz or plastic capillary. The capillary channel is fluidly coupled to a source of the suspension of cells or other particles, which are then flowed along the capillary channel. In particularly preferred aspects, the channel is integrated into the body structure of a microfluidic device. As used herein, the term "microfluidic" generally refers to one or more fluid passages, chambers or conduits which have at least one internal cross-sectional dimension, e.g., depth, width, length, diameter, etc., that is less than 500 μm, and typically between about 0.1 μm and about 500 μm In the devices of the present invention, the microscale channels or chambers preferably have at least one cross-sectional dimension between about 0 1 μm and 200 μm, more preferably between about 0 1 μm and 100 μm, and often between about 0 1 μm and 50 μm Accordingly, the microfluidic devices or systems prepared in accordance with the present invention typically include at least one microscale channel, usually at least two intersecting microscale channels, and often, three or more intersecting channels disposed within a single body structure Channel intersections may exist in a number of formats, including cross intersections, "Y" or "T" intersections, or any number of other structures, whereby two channels are in fluid communication The body structure of the microfluidic devices, descπbed herein, typically compπses an aggregation of two or more separate layers which when appropnately mated or joined together, form the microfluidic device of the invention, e.g., containing the channels and/or chambers descπbed herein Typically, the microfluidic devices descπbed herein will compπse a top portion, a bottom portion, and an inteπor portion, wherein the inteπor portion substantially defines the channels and chambers of the device In preferred aspects, the bottom portion of the device compnses a solid substrate that is substantially planar in structure, and which has at least one substantially flat upper surface. A vaπety of substrate mateπals may be employed as the bottom portion Typically, because the devices are microfabπcated, substrate matenals will be selected based upon their compatibility with known microfabπcation techniques, e.g., photolithography, wet chemical etching, laser ablation, air abrasion techniques, injection molding, embossing, and other techniques The substrate mateπals are also generally selected for their compatibility with the full range of conditions to which the microfluidic devices are typically exposed, including extremes of pH, temperature, salt concentration, and application of electπc fields Accordingly, in some preferred aspects, the substrate mateπal optionally includes matenals normally associated with the semiconductor industry in which such microfabπcation techniques are regularly employed, including, e.g , silica based substrates, such as glass, quartz, silicon or polysi con, as well as other substrate matenals, such as gallium arsenide and the like In the case of semiconductive mateπals, it will often be desirable to provide an insulating coating or layer, e.g , silicon oxide, over the substrate matenal, and particularly in those applications where electπc fields are to be applied to the device or its contents In additional preferred aspects, the substrate mateπals will compπse polymeπc mateπals, e.g., plastics, such as polymethylmethacrylate (PMMA), polycarbonate, polytetrafluoroethylene (TEFLON™), polyv ylchloπde (PVC), polydimethylsiloxane (PDMS), polysulfone, polystyrene, polymethylpentene, polypropylene, polyethylene, polyviny dine fluonde, ABS (acrylomtnle-butadiene- styrene copolymer), and the like Such polymeπc substrates are readily manufactured using available microfabπcation techniques, as descπbed above, or from microfabπcated masters, using well known molding techniques, such as injection molding, embossing or stamping, or by polymeπzing the polymenc precursor matenal within the mold See, e.g., U.S. Pat. No 5,512,131 Such polymeπc substrate mateπals are preferred for their ease of manufacture, low cost and disposability, as well as their general inertness to most extreme reaction conditions. Again, these polymeπc mateπals optionally include treated surfaces, e.g., deπvatized or coated surfaces, to enhance their utility in the microfluidic system, e.g., provide enhanced fluid direction, e.g., as descπbed in U.S. Patent Application Seπal No. 08/843,212, filed Apnl 14, 1997, and which is incorporated herein by reference in its entirety for all purposes. The channels and/or chambers of the microfluidic devices are typically fabπcated into the upper surface of the bottom substrate or portion of the device, as microscale grooves or indentations, using the above descπbed microfabncation techniques. The top portion or substrate also compπses a first planar surface, and a second surface opposite the first planar surface. In the microfluidic devices prepared in accordance with certain aspects of the methods descπbed herein, the top portion also includes a plurality of apertures, holes or ports disposed therethrough, e.g., from the first planar surface to the second surface opposite the first planar surface. The first planar surface of the top substrate is then mated, e.g., placed into contact with, and bonded to the planar surface of the bottom substrate, coveπng and sealing the grooves and/or indentations in the surface of the bottom substrate, to form the channels and/or chambers (I e., the intenor portion) of the device at the interface of these two components Bonding of substrates is typically earned out by any of a number of different methods, e.g., thermal bonding, solvent bonding, ultrasonic welding, and the like. The holes in the top portion of the device are oπented such that they are in communication with at least one of the channels and/or chambers formed in the inteπor portion of the device from the grooves or indentations in the bottom substrate In the completed device, these holes function as reservoirs for facilitating fluid or mateπal introduction into the channels or chambers of the inteπor portion of the device, as well as providing ports at which electrodes may be placed into contact with fluids within the device, allowing application of electπc fields along the channels of the device to control and direct fluid transport within the device In many embodiments, extensions are provided over these reservoirs to allow for increased fluid volumes, permitting longer running assays, and better controlling fluid flow parameters, e.g., hydrostatic pressures. Examples of methods and apparatuses for providing such extensions are descnbed in, e.g., Dubrow et al , "Microfluidic Devices and Systems Incorporating Cover Layers," WO 99/43432, published September 2, 1999, which are incorporated herein by reference in their entirety for all purposes These devices are optionally coupled to other sample introduction ports, e.g., one or more pipettor or capillary elements (e.g., 1, 2, 3, 4, 6, 8, 10, 12, or more elements) which senally introduce multiple samples, e.g., from the wells of a microwell plate. Thus, in some embodiments, both reservoirs in the upper surface and external capillary elements are present in a single device. Alternatively, the devices include only one or more capillary elements for sample or other mateπal introduction The sources of reagents, samples (e.g., cells, microbeads, DNA or other molecules, etc.), buffers, and other mateπals are optionally fluidly coupled to the microchannels in any of a vaπety of ways. In particular, those systems compπsing sources of matenals set forth m Knapp et al., "Closed Loop Biochemical Analyzers" (WO 98/45481 ; PCT/US98/06723) and U.S. Pat. No. 5,942,443, issued August 24, 1999, entitled "High Throughput Screening Assay Systems in Microscale Fluidic Devices" to J. Wallace Parce et al. and, e.g., in PCT/USOO/04522, filed February 22, 2000, entitled "Manipulation of Microparticles In Microfluidic Systems," by Mehta et al. are applicable. In these systems, one or more capillary or pipettor elements (i.e., an element that includes, e.g., a channel in which components are optionally moved from a source to a microscale element such as a second channel or reservoir) is temporaπly or permanently coupled to a source of mateπal The source is optionally internal or external to a microfluidic device that includes the pipettor or capillary element Example sources include microwell plates, membranes or other solid substrates compnsing lyophilized components, wells or reservoirs in the body of the microscale device itself and others In one preferred embodiment, one or more capillary elements are used to draw cell or other samples from microwell plates into the body structure of the device The channel disposed in the capillary element typically includes a diameter in the range of about 1 μm to about 100 μm, more preferably in the range of about 25 μm to about 75 μm, e g , about 50 μm m diameter These devices are typically included as part of an automated or integrated system which further enhances device throughput Integrated systems are discussed further above and in the references cited herein In many embodiments, the microfluidic devices will include an optical detection window disposed across one or more channels and/or chambers of the device Optical detection windows are typically transparent such that they are capable of transmitting an optical signal from the channel/chamber over which they are disposed Optical detection windows are optionally merely a region of a transparent cover layer, e.g., where the cover layer is glass or quartz, or a transparent polymer mateπal, e.g , PMMA, polycarbonate, etc Alternatively, where opaque substrates are used in manufactunng the devices, transparent detection windows fabπcated from the above mateπals are optionally separately manufactured into the device In many aspects, it is desirable to provide the inteπor surfaces of the channels with an appropπate treatment to prevent the adhesion of cells or other particles to that surface For example, in the case of glass or other highly charged channel surfaces, some cell types often have a tendency to stick to the channel surfaces, interfenng with the flowing of cells through the channels For example, in the case of mammalian cell-based assays, many mammalian cell types are particularly adherent to certain types of surfaces, e g , glass and some plastics Accordingly, in some embodiments, it is desirable to treat or coat the mtenor surfaces of the channels to prevent cell adhesion A vaπety of surface treatments are optionally employed to accomplish this goal For example, charge masking coatings such as polyols (e g., polyvmylalcohol (PVA)), polyethyleneimme (PEI), polyethylene glycol (PEG), polyacrylamides (e.g., polyacrylamide, polymethylacryalamide, polydimethacrylamide, and the like), carbohydrates such as polysucrose (Ficoll™), polyglucose (dextran and cellulose), and polytetrafluoroethylene (Teflon™), or the like. Alternatively, covalent surface treatments are also optionally used to prevent surface adhesion of cells, such as silanization (e.g., using dimethyl or dichlorosilane) of glass or plastic surfaces. Other surface treatments are generally described, with reference to device fabrication techniques, above. The flowing of the suspension of cells or other particles along one or more channels of the devices described herein is optionally carried out by a number of mechanisms, including pressure based-flow, electrokinetic flow, or other mechanisms or combinations of mechanisms. In a first preferred aspect, a pressure differential is used to flow the suspension of cells along a channel. Application of a pressure differential along a channel is carried out by a number of means. For example, in a simple passive aspect, the cell suspension is deposited in a reservoir at one end of a channel and at a sufficient volume or depth, that the cell suspension creates a hydrostatic pressure differential along the length of the channel, e.g., by virtue of its having greater depth than a reservoir at an opposite terminus of the channel. The hydrostatic pressure then causes the cell suspension to flow along the length of the channel. Typically, the reservoir volume is quite large in comparison to the volume or flow through rate of the channel, e.g., 10 μl reservoirs, vs. 1000 μm 2 channel cross- section. As such, over the time course of the assay, the flow rate of the cell suspension will remain substantially constant, as the volume of the reservoir, and thus, the hydrostatic pressure changes very slowly. Applied pressure is then readily varied to yield different cell suspension flow rates through the channel. In screening applications, varying the flow rate of the cell suspension is optionally used to vary the incubation time of the cells with the test compound. In particular, by slowing the cells flow rate along the channel, one can effectively lengthen the amount of time between introduction of test compounds and detection of their effects. Alternatively, channel lengths, detection points, or test compound introduction points are varied in fabrication of the devices, to vary incubation times. In many applications, it may be desirable to provide relatively precise control of the flow rate of the cell suspension and/or other particles, e.g., to precisely control incubation times, or the like. As such, in many preferred aspects, flow systems that are more active than hydrostatic pressure dπven systems are employed For example, the cell suspension is optionally flowed by applying a pressure differential across the length of a channel For example, a pressure source (positive or negative) is applied at the cell suspension reservoir at one end of a channel, and the applied pressure forces the suspension through the channel The pressure source is optionally pneumatic, e g , a pressuπzed gas, or alternatively is a positive displacement mechanism, i.e., a plunger fitted into a cell suspension reservoir, for forcing the cell suspension through the analysis channel Alternatively, a vacuum source is applied to a reservoir at the opposite end of the channel to draw the suspension through the channel Pressure or vacuum sources are optionally supplied external to the device or system, e.g., external vacuum or pressure pumps sealably fitted to the inlet or outlet of a channel, or they are internal to the device, e g , microfabncated pumps integrated into the device and operably linked to a channel Examples of microfabπcated pumps have been widely descπbed in the art. See, e.g., published International Application No. WO 97/02357. In alternate aspects, other flow systems are employed in transporting or focusing the cellular suspension and/or other particles in a channel. In one embodiment, cells are initially flowed under pressure, but focused using alternative fluid direction components, such as an electrokinetic force modulator. While electrokinetic forces typically produce substantially uniform fluid flow in microchannels, they tend to disrupt cellular membranes Thus, the use of electrokinetic motive forces is typically limited to focusing pressure -based cellular flows, which as discussed have non-uniform flow velocities Electrokinetic transport systems typically utilize electnc fields applied along the length of channels that have a surface potential or charge associated therewith When fluid is introduced into the channel, the charged groups on the inner surface of the channel ionize, creating locally concentrated levels of ions near the fluid surface interface. Under an electπc field, this charged sheath migrates toward the cathode or anode (depending upon whether the sheath compnses positive or negative ions) and pulls the encompassed fluid along with it, resulting in bulk fluid flow This flow of fluid is generally termed electroosmotic flow Where the fluid includes a cell or other particle suspension, those components are also pulled along A more detailed descnption of controlled electrokinetic matenal transport systems in microfluidic systems is descπbed in published International Patent Application No WO 96/04547, which is incorporated herein by reference Hydrostatic, wicking and capillary forces are also optionally used to provide for fluid flow See, e g , "Method and Apparatus for Continuous Liquid Flow in Microscale Channels Using Pressure Injection, Wick-ing and Electrokinetic Injection," by Alajoki et al , Attorney Docket Number 017646-007100, filed February 5, 1999 In alternative aspects, flow of the cell suspension and/or other particles is dπven by mertial forces In particular, channels are optionally disposed in a substrate that has the conformation of a rotor, with channels extending radially outward from the center of the rotor The cell suspension is deposited in a reservoir that is located at the inteπor portion of the rotor and is fluidly connected to the channels Dunng rotation of the rotor, the centπpetal force on the cell suspension forces the cell suspension through the channels, outward toward the edge of the rotor Multiple channels are typically provided in the rotor to perform multiple different analyses. Detection of the function and reference labels is then earned out by placing a detector under the spinning rotor and detecting the signal as a channel passes over the detector. Examples of rotor systems have been previously descnbed for performing a number of different assay types See, e.g , Published International Application No. WO 95/02189 Test compound reservoirs are optionally provided in the rotor, in fluid communication with one or more channels, such that the rotation of the rotor also forces the test compounds into the one or more channels Although illustrated in the figures herein as a single channel and accessing capillary, it will be readily appreciated that these aspects may be provided as multiple parallel channels and accessing capillaπes (discussed above), m order to substantially increase the throughput of the system Specifically, single body structures are optionally provided with multiple parallel channels coupled to multiple sample accessing capillaπes that are positioned to sample multiple samples at a time from sample branes, e.g , multiwell plates As such, these capillanes are generally spaced at regular distances that correspond with the spacing of wells in multiwell plates, e.g , 9 mm centers for 96 well plates, 4 5 mm for 384 well plates, and 2.25 mm for 1536 well plates The present invention is further illustrated with reference to the following non-limiting examples. EXAMPLES TUNEL ASSAY AND ANNEXIN-V ASSAY FOR APOPTOSIS DETECTION A TUNEL assay (TdT (Terminal deoxynucleotidyl transferase)- mediated dUTP Nick End Labeling) was used to measure cellular apoptosis in accordance with the above-described methods and systems. In a TUNEL Assay, damaged DNA is labeled with fluorescinated nucleotides. Terminal deoxynucleotidyl transferase (TdT) binds to exposed 3' ends of DNA fragments generated in response to apoptotic signals and catalyzes the addition of fluorescein-labeled deoxynucleotides. U937 cells were treated with Campthotecin to induce apoptosis. The cells were then harvested and fixed with 4% formaldehyde and stored in 80% ethanol at +4°C until labeling was performed. The cells were rehydrated in TBS buffer. They were then treated with 20 μg/ml of proteinase K for 5 minutes and resuspended in equilibration buffer. The cells were incubated in a working TdT labeling reaction mixture from Oncogene Research Products containing TdtT enzyme and FragEL TdT labeling reaction mix. The cells were resuspended in TBS and then counterstained with a DNA dye (SYTO®-62 DNA dye at 1 μM for 10 minutes RT) to obtain a total cell count. After washing the cells in TBS, they were resuspended in Hanks' Balanced Salt Solution with 10% OPTIPREP at 5 x 10 6 /ml and loaded in a microfluidic device having the channel geometry shown in Figure 1A, for analysis. Fluorescence was detected using 488nm excitation, while emission was read at 525 nm and 680 nm. Figure 4 illustrates a control analysis, e.g., U937 cells not treated to induce apoptosis. The bottom line corresponds to the SYTO®-62, which indicates the mere presence of cells, whereas the top line corresponds to the fluorescein end labeled nucleic acids. Apoptotic cells would yield a peak on both the upper and lower lines, indicating the presence of an apoptotic cell. As can be seen, there are substantially no discernible peaks in the upper line, indicating a lack of apoptosis in the control. Figure 5 represents U937 cells treated with Campthotecin to induce apoptosis. As can be seen, corresponding peaks are seen on both the lower and upper lines, indicating the presence of apoptotic cells. In an Annexin-V assay, change in the outer membrane of apoptotic cells is detected A membrane component, phosphatidylseπne (PS) is translocated to the outer layer Annexin-V binds to PS and is conjugated to biotin to allow secondary reactions to fluorescently labeled Streptavidin for detection. U937 cells were treated with Campthotecin to induce apoptosis The cells were harvested and washed m phosphate buffered saline (PBS). Cells were incubated with Annexin-V-Biotin (Boehπnger Mannheim) in HEPES buffer, and then incubated with 5 μg streptavιdιn-Cy5 (Amersham) and 1 mM of Calcem-AM dye in HEPES buffer The HEPES buffer solution contained lOmM HEPES, pH 7.4; 140 mM NaCl, and a 5 mM CaC The cells were washed and resuspended m HEPES buffer with 8.5% sucrose and loaded in a microfluidic device for analysis Fluorescence was detected using excitation at 488 nm and 635 nm, and emission was read at 525 nm and 682 nm. Figure 6 indicates analysis of U937 cells that were not treated to induce apoptosis. In this case, the bottom line indicates live cell count (Calcem), while the top line indicates the presence of apoptotic cells (Annexιn-V-Cy5). As can be seen, a few apoptotic cells are present within the control expenment. Figure 7, on the other hand, shows an analysis of the same cells treated with Campthotecin to induce apoptosis. As can be seen, the top trace includes a much greater number of peaks representing apoptotic cells, and particularly as a percentage of total cells in the analysis, e.g., as compared to the lower line Figure 8 shows results from a TUNEL assay with HL-60 cells treated with Campthotecin for 6 hours to induce apoptosis. The bottom line traces the total cell count. The top line traces apoptotic cells labeled at DNA damage points. In this case, almost all cells had sustained apoptotic DNA damage. Figure 9 shows results from a TUNEL assay HL-60 cells (not treated for apoptosis) were used as control cells. The bottom trace is for SYTO®-62 indicating total cell count. The top trace is for Fluorescein labeled nucleotide which signifies the presence of apoptosis related DNA damage. Figures 10 and 11 shows results from TUNEL assays from Figures 8 and 9 analyzed using appropnate software. Data is presented as scatter plot format in Figure 10. In Figure 11, data is represented in bar graph format. Additional data for different time points and two other cell lines is included. Figure 12 shows and Annexin-V assay. HL60 cells were treated with Campothotecin for four hours to induce apoptosis. The bottom trace indicates live cell count (Calcein). The top trace indicates Annexin-V-Cy5 labeling. Cells labeled only for Calcein are live cells and not apoptotic. In this case, almost all cells are apoptotic. Figure 13 shows an Annexin-V assay. HL-60 cells (not treated for apoptosis) are control cells. The bottom trace indicates live cell count. The top trace indicates Annexin-V-Cy5 labeling. Cells labeled with both dyes are apoptotic. Cells labeled only for Annexin-CY5 are dead cells. In this case, only a few cells were apoptotic. Figures 14 and 15 show data analysis of figures 12 and 13. Data is presented in scatter plot format in Figure 14. In Figure 15, data is represented in bar graph format. Additional data for different time points and two other cell lines is included. FLUORESCENCE POPULATION AND CONCENTRATION EXPERIMENTS Figure 17 depicts a chip employing a channel geometry that was used to conduct fluorescence population experiments to confirm the feasibility of the device as a microfluidic flow cytometer. The experiment entailed comparing microfluidic hydrodynamic focusing data obtained by flowing fluorescently-labeled calibration beads (Quantum 26 Fluorescein Microbeads Standards B-111698 from Flow Cytometry Standards Corp.) through the microfluidic flow cytometer (both focused and unfocused) with data provided in a certificate of calibration that accompanied the beads. Flow Cytometry Standards Corp. obtained the calibration data by flowing the same type of bead through a traditional flow cytometer. Figure 18 is a histogram showing the distribution of results obtained in the fluorescence intensity population experiment conducted utilizing hydrodynamic focusing in the microfluidic flow cytometer. Figures 19 and 20 are histograms showing the distribution of results obtained in the fluorescence intensity population experiment conducted, in the same microfluidic flow cytometer, in the absence of hydrodynamic focusing. As shown, the focused beads resolve into a more uniform fluorescence intensity population than the unfocused beads which are more widely distπbuted Furthermore, the focused beads are comparable to results achievable using traditional flow cytometers, which typically include a coefficient of vaπation (%CV) of about 6%, according to the calibration data provided by Flow Cytometry Standards Corp The calculated %CV for the focused beads in the microfluidic system was about 6 8%, whereas that calculated for the unfocused beads was only about 28 8% In general, an acceptable %CV is less than 15%, which further demonstrates the feasibility of the device, e g , as a flow cytometer Figures 25A, B, and C are histograms showing the distπbution of results obtained in resolution expeπments using vaπous microbead to buffer concentrations, namely, 1 1, 1 3, and 1 7, respectively The expeπments were conducted using an Agilent bioanalyzer microfluidic system to determine whether microbead doublet populations could be resolved As shown, even at the highest microbead concentration, where the largest population of doublets was expected, significant resolution of the two populations was observed with a %CV of about 6 7 % (FIG. 25A) Similar %CVs were obtained at the two other concentrations To further confirm the capabilities of the devices and methods of the present invention an additional resolution expenment was conducted using viable THP-1 cells stained with SYTO®-62 intercalating dye As shown in Figure 26, significant resolution was achieved with a %CV comparable to those obtained using traditional flow cytometnc devices The discussion above is generally applicable to the aspects and embodiments of the invention descnbed herein Moreover, modifications are optionally made to the methods and devices descnbed herein without departing from the spmt and scope of the invention as claimed, and the invention is optionally put to a number of different uses including the following: The use of a microfluidic system containing at least a first substrate and having a first channel and a second channel intersecting the first channel, at least one of the channels having at least one cross-sectional dimension in a range from 0 1 to 500 μm, in order to test the effect of each of a plurality of test compounds on a biochemical system compnsing one or more focused cells or particles The use of a microfluidic system as descπbed herein, wherein a biochemical system flows through one of said channels substantially continuously, providing for, e g , sequential testing of a plurality of test compounds The use of a microfluidic device as descπbed herein to modulate reactions within microchannels or microchambers The use of electrokinetic injection in a microfluidic device as descπbed herein to modulate or achieve flow in the channels The use of a combination of wicks, electrokinetic injection and pressure based flow elements in a microfluidic device as descπbed herein to modulate, focus, or achieve flow of mateπals, e g , in the channels of the device An assay utilizing a use of any one of the microfluidic systems or substrates descnbed herein While the foregoing invention has been descπbed in some detail for purposes of claπty and understanding, it will be clear to one skilled in the art from a reading of this disclosure that vaπous changes in form and detail can be made without departing from the true scope of the invention For example, all the techniques and apparatus descπbed above may be used in vaπous combinations All publications, patent applications and other documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were individually so denoted

Claims: WHAT IS CLAIMED IS 1. A method of providing substantially uniform flow velocity to particles flowing in a first microchannel, the method compnsing flowing the particles in the first microchannel, wherein the particles compπse a non-uniform flow velocity, and, focusing the particles honzontally or vertically in the first microchannel, thereby providing substantially uniform flow velocity to the particles flowing in the first microchannel 2. The method of claim 1, further compπsing sampling the particles with at least one capillary element pπor to performing the flowing step 3. The method of claim 1, compπsing flowing the particles using pressure-based flow. 4. The method of claim 1, compnsing focusing the particles using one or more fluid direction component compπsing one or more of a fluid pressure force modulator, an electrokinetic force modulator, a capillary force modulator, or a fluid wicking element. 5. The method of claim 1 , wherein the focusing step further compnses focusing the particles honzontally and vertically in the first microchannel. 6. The method of claim 1, compnsing focusing the particles vertically or honzontally in the first microchannel by simultaneously introducing fluid flow from two opposing microchannels into the first microchannel 7. The method of claim 1, compnsing focusing the particles honzontally in the microchannel by introducing a low density fluid and a high density fluid into the microchannel, wherein the particles are focused in an intermediate density fluid present between the high density fluid and the low density fluid. 8. The method of claim 1, compπsing focusing the particles honzontally or vertically in the first microchannel by simultaneously introducing at least one fluid flow from at least a second microchannel that compπses at least one intersection with the first microchannel 9. The method of claim 1, further compπsing sorting the particles 10. The method of claim 9, wherein the particles compnse one or more of a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a set of functionalized microbeads, a molecule, or a set of molecules 11. The method of claim 1, further compπsing washing a diffusible mateπal from the particles by introducing a diluent into the first microchannel from at least a second microchannel and removing the resulting diluted diffused product compπsing diluent mixed with the diffusible mateπal through at least a third microchannel. 12. The method of claim 11, the method compπsing sequentially introducing the diluent into the first microchannel from the second microchannel and a fourth microchannel, wherein the second and fourth microchannels intersect the first microchannel at an offset intersection region. 13. The method of claim 11, the method compπsing simultaneously introducing the diluent into the first microchannel from the second microchannel and a fourth microchannel, wherein the second and fourth microchannel intersect the first microchannel at a common intersection region. 14. The method of claim 13, the method compπsing removing the diffused product through the third microchannel and a fifth microchannel, which third and fifth microchannels intersect the first microchannel at a common intersection region. 15. The method of claim 14, further compπsing introducing the diluent through a sixth and seventh microchannel which sixth and seventh intersect the first microchannel at a common intersection and removing the resulting further diluted diffused product through an eighth and ninth microchannel, which eighth and ninth microchannel intersect the first microchannel at a common intersection. 16. The method of claim 15, wherein the diluent is introduced into the first microchannel by pressure-based flow. 17. The method of claim 15, wherein the diluent is introduced into the first microchannel by electrokinetic flow. 18. The method of claim 1, wherein the particles comprise cells and the method further comprises performing a TUNEL assay or an Annexin-V assay on the cells in the channel to measure apoptosis. 19. A method of sorting members of at least one particle population, the method comprising: flowing the members of the at least one particle population in a first microchannel; and, focusing the members of the at least one particle population horizontally or vertically in the first microchannel such that selected individual members are directed into at least a second microchannel that intersects with the first microchannel, thereby sorting the members of the at least one particle population. 20. The method of claim 19, wherein at least a portion of the first microchannel comprises at least one separation element disposed therein, the at least one separation element comprising at least two sides, wherein at least a portion of the at least one separation element is disposed upstream of the at least the second microchannel. 21. The method of claim 20, wherein the focusing step further comprises focusing the members of the at least one particle population horizontally or vertically in the first microchannel such that the selected individual members are directed to at least one of the at least two sides of the at least one separation element and into the second microchannel that intersects the first microchannel. 22. The method of claims 19 or 21, wherein the members of the at least one particle population comprise one or more of: a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a set of functionalized microbeads, a molecule, or a set of molecules. 23. The method of claims 19 or 21, wherein the focusing step further comprises horizontally or vertically focusing the members of the at least one particle population in the first microchannel by introducing at least one fluid flow from a third microchannel that intersects with the first microchannel upstream from the second microchannel. 24. The method of claim 23, comprising introducing the at least one fluid flow by heating at least one fluid in the third microchannel. 25. The method of claim 24, wherein the heating step comprises Joule heating the at least one fluid. 26. The method of claims 19 or 21, wherein the focusing step further comprises horizontally or vertically focusing the members of the at least one particle population in the first microchannel by introducing at least one fluid flow from at least one of at least a third and at least a fourth microchannel that oppose one another and that intersect with the first microchannel upstream from the second microchannel. 27. The method of claim 26, comprising introducing the at least one fluid flow by heating at least one fluid in at least one of the third or fourth microchannels. 28. The method of claim 27, wherein the heating step comprises Joule heating the at least one fluid. 29. An integrated system for providing substantially uniform flow velocity to flowing members of at least one particle population in a microfluidic device, the system comprising: a body structure comprising at least a first microchannel disposed therein; at least a first fluid direction component coupled to the first microchannel for inducing flow of at least one fluidic material comprising the members of the at least one particle population in the first microchannel; at least one source of the at least one fluidic material fluidly coupled to the first microchannel; at least a second microchannel which intersects the first microchannel for introducing at least one fluid into the first microchannel to horizontally or vertically focus the members of the at least one particle population in the first microchannel; at least one flow control regulator for regulating flow of the at least one fluidic material or the at least one fluid in the first or the second microchannels; and, a computer comprising an instruction set directing simultaneous flow of the members of the at least one particle population in the first microchannel and simultaneous introduction of the at least one fluid from the second microchannel into the first microchannel. 30. The integrated system of claim 29, wherein the first fluid direction component induces non-uniform flow. 31. The integrated system of claim 29, wherein the first fluid direction component comprises a fluid pressure force modulator. 32. The integrated system of claim 29, wherein the at least one fluid is introduced using at least a second fluid direction component comprising one or more of: a fluid pressure force modulator, an electrokinetic force modulator, a capillary force modulator, or a fluid wicking element. 33. The integrated system of claim 29, further comprising at least a third microchannel which intersects the first microchannel in an intersection region common to the second microchannel, wherein the at least one flow control regulator further regulates flow of the at least one fluid in the second and the third microchannels, and wherein the computer further comprises an instruction set for simultaneously flowing fluids from the second and third microchannels into the first microchannel. 34. The integrated system of claims 29 or 33, further comprising at least a fourth and fifth channel which intersect the first microchannel at a common intersection downstream of the second and third microchannels, the computer further comprising an instruction set for simultaneously flowing material from the first microchannel into the fourth and fifth microchannels. 35. The integrated system of claims 34, further comprising sixth and seventh microchannels which intersect the first microchannel at a common intersection downstream of the fourth and fifth microchannels, the computer further comprising an instruction set for simultaneously flowing material from the sixth and seventh microchannels into the first microchannel. 36. The integrated system of claim 35, further comprising eighth and ninth microchannels which intersect the first microchannel at a common intersection downstream of the sixth and seventh microchannels, the computer further comprising an instruction set for simultaneously flowing material from the first microchannel into the eighth and ninth microchannels. 37. The integrated system of claim 29, comprising one or more source of reagent selected from the group of reagents consisting of: terminal deoxynucleotide transferase, one or more fluoresceins labeled nucleotide, Annexin V, an AnnexinV-biotin conjugate, a DNA dye, Campthotecin, Calcein-AM, a control cell, and a test cell. 38. The integrated system of claim 29, further comprising at least one signal detector mounted proximal to the first microchannel for detecting a detectable signal produced by one or more of the members of the at least one particle population in the first microchannel. 39. The integrated system of claim 38, further comprising at least a third and at least a fourth microchannel which intersect the first microchannel downstream from the intersection of the second microchannel with the first microchannel, wherein the fourth microchannel intersects the first microchannel downstream from the intersection of the third microchannel with the first microchannel 40. The integrated system of claim 39, wherein the at least one flow control regulator further regulates flow of the at least one fluid in the third or the fourth microchannels 41. The integrated system of claim 40, wherein the at least one signal detector detects a detectable signal produced by at least one selected member of the at least one particle population between the intersections of the second and the third microchannels with the first microchannel 42. The integrated system of claim 41, wherein the computer is operably linked to the first fluid direction component, the at least one flow control regulator, and the at least one signal detector, wherein the instruction set further directs simultaneous introduction of the at least one fluid from the third microchannel into the first microchannel to honzontally or vertically focus the at least one selected member of the at least one particle population such that the at least one selected member is directed into the fourth microchannel at least in response to the detectable signal produced by the at least one selected member 43. The integrated system of claim 42, wherein the instruction set further directs simultaneous introduction of the at least one fluid from the third microchannel by activating at least one heating element disposed within the third microchannel or at least one well that fluidly communicates with the third microchannel. 44. The integrated system of claim 43, wherein the at least one heating element compnses at least one Joule heating electrode or at least one a conductively coated microchannel portion 45. The integrated system of claim 42, wherein at least a portion of the first microchannel compπses at least one separation element disposed therein, the at least one separation element compπsing at least two sides, wherein at least a portion of the at least one separation element is disposed upstream of the fourth microchannel 46. The integrated system of claim 45, wherein the at least one selected member of the at least one particle population is directed to at least one of the at least two sides of the at least one separation element and into the fourth microchannel that intersects the first microchannel at least in response to the detectable signal produced by the at least one selected member 47. The integrated system of claims 38, 42, or 46, wherein the detector compnses a fluorescent excitation source and a fluorescent emission detection element 48. The integrated system of claim 47, wherein the computer is operably linked to the signal detector and compnses an instruction set for converting detected signal information into digital data. 49. The integrated system of claims 42 or 46, wherein the at least one selected member of the at least one particle population compπses one or more of: a cell, a set of cells, a microbead, a set of microbeads, a functionalized microbead, a molecule, or a set of molecules. 50. The integrated system of claims 42 or 46, further compnsing at least a fifth microchannel which intersects the first microchannel in an intersection region common to the second microchannel, wherein the at least one flow control regulator further regulates flow of the at least one fluid in the second and the fifth microchannels, and wherein the computer further compπses an instruction set for simultaneously flowing fluids from the second and the fifth microchannels into the first microchannel. 51. The integrated system of claim 50, further compπsing at least a sixth microchannel which intersects the first microchannel in an intersection region common to the third microchannel, wherein the at least one flow control regulator further regulates flow of the at least one fluid in the third and the sixth microchannels, and wherein the computer further compnses an instruction set for flowing fluids from the third and the sixth microchannels into the first microchannel 52. The integrated system of claim 51 , wherein the instruction set directs individual or simultaneous fluid flow from the third and sixth microchannels by individually or simultaneously activating at least one heating element disposed within each of the third and sixth microchannels or within at least one well that fluidly communicates with each of the third and sixth microchannels. 53. The integrated system of claim 52, wherein the at least one heating element compnses at least one Joule heating electrode or at least one conductively coated microchannel portion.

Inventors: ALAJOKI MARJA LIISA, CHOW ANDREA W, DUBROW ROBERT S, KOPF-SILL ANNE R, PARCE J WALLACE, WADA H GARRETT, WANG BENJAMIN N, ALAJOKI, MARJA, LIISA, CHOW, ANDREA, W., DUBROW, ROBERT, S., KOPF-SILL, ANNE, R., PARCE, J., WALLACE, WADA, H., GARRETT, WANG, BENJAMIN, N.
Applicants: ALAJOKI MARJA LIISA, CALIPER TECHN CORP, CHOW ANDREA W, DUBROW ROBERT S, KOPF SILL ANNE R, PARCE J WALLACE, WADA H GARRETT, WANG BENJAMIN N, ALAJOKI, MARJA, LIISA, CALIPER TECHNOLOGIES CORP., CHOW, ANDREA, W., DUBROW, ROBERT, S., KOPF-SILL, ANNE, R., PARCE, J., WALLACE, WADA, H., GARRETT, WANG, BENJAMIN, N.
Application year: 2000
Main classification: C12Q1
IPC codes: B01F13, B01J19, B01L3, G01N15, G01N27