Abstract:
This invention provides methods and apparatus for performing microanalytic analyses and procedures, particularly miniaturized cell based assays. These methods are useful for performing a variety of cell-based assays, including drug candidate screening, life sciences research, and clinical and molecular diagnostics.

Description:
[0001]    This application claims priority to U.S. Provisional Application Serial No. 60/204,264, filed May 15, 2000, the disclosure of which is explicitly incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates to methods and apparatus for performing microanalytic analyses and procedures. In particular, the present invention provides devices and methods for the performance of miniaturized cell based assays. These assays may be performed for a variety of purposes, including but not limited to screening of drug candidate compounds, life sciences research, and clinical and molecular diagnostics.  
           [0004]    2. Background of the Related Art  
           [0005]    Recent developments in a variety of investigational and research fields have created a need for improved methods and apparatus for performing analytical, particularly bioanalytical assays at microscale (i.e., in volumes of less than 100 μL). In the field of pharmaceuticals, an increasing number of potential drug candidates require assessment of their biological function. As an example, the field of combinatorial chemistry combines various structural sub-units with differing chemical affinities or configurations into molecules; in theory, a new molecule having potentially unique biochemical properties can be created for each permutation of the sub-units. In this way, large libraries of compounds may be synthesized from relatively small numbers of constituents, each such compound being a potential drug lead compound of usually unknown biological activity and potency. Similarly, increasingly large numbers of targets for these putative therapeutic compounds are being discovered, many as a result of the growing information derived from such large-scale biological research as the sequencing of the human genome.  
           [0006]    As the first phase of drug discovery, compounds that represent potential drugs are screened against targets in a process known as High Throughput Screening (HTS) or ultra-High Throughput Screening (uHTS). An advantage of these screening methods is that they usually consist of simple solution phase biochemical assays that can be performed quickly and with small amounts of expensive compounds and reagents. However, a significant drawback to HTS is that the targets do not provide a functional assessment of compounds&#39; effects on the complex biochemical pathways inherent in the normal and abnormal (mutant or disease-state) functioning of cells, tissues, organs, and organisms. As a result, compounds that have shown biochemical activity of interest in initial screens are usually put through cell-based screens, in which the affect of the compounds on cellular function is independently assayed.  
           [0007]    There are a wide range of assays that may be performed using living cells. Assays that involve the use of living cells include gene expression, in which levels of transcription in response to a drug candidate are monitored; cell permeability assays, in which the ability of drugs to traverse membranes of cells is monitored; and functional assays designed to investigate both macroscopic effects, such as cell viability, as well as biochemical effects and products produced in and by the cells as a result of treatment with the drug lead compound.  
           [0008]    These assays include cytotoxicity and cell proliferation to measure the viability of a population of cells, often in the presence of a putative therapeutic compound (drug candidate). A variety of methods have been developed for this purpose. These include the use of tetrazolium salts, in which mitochondria in living cells use dehydrogenases to reduce tetrazolium salts to colored formazan salts. Soluble or insoluble precipitates may be formed, depending on the nature of the tetrazolium salt used. A typical assay procedure is to culture the cells, add a solution of tetrazolium salt, phenazine methosulfate and DPBS, incubate, and determine absorbance at 490 nm. The absorbance measured is larger for viable cell populations that have metabolized the salt. Another such assay uses alamarBlue, which uses a fluorometric/colorimetric growth indicator that is reduced to a membrane-soluble, red, fluorescent form by the products of metabolic activity. A variety of other indicators are either taken up by living cells, dead cells, or both; for example, neutral red is taken up only by live cells, while trypan blue is excluded by live cells. Dyes that bind to or intercalate with DNA can be used to visualize or quantitate the number of live or dead cells, since DNA synthesis only occurs in living cells.  
           [0009]    Another important class of cell based assays in reporter gene assays. These assays are used to study the control of gene transcription. They can also be used as a secondary detection method for a number of other molecules present in or acting on a cell. Pharmaceutical companies and others involved in drug development commonly utilize reporter gene assays to determine the effects of their compounds on transcription of specific genes whose promoter sequences are known. For example, the production of proteins associated with a condition of interest can be quantified by using a reporter gene operatively linked to the promoter of the gene encoding the protein. The method employed in reporter gene assays varies with the type of reporter gene used and the application. Initially, the promoter from the gene of interest in operable combination with the reporter gene is inserted into a commercially available plasmid comprising an antibiotic resistance gene, which is then transfected into the cells. Cells that have been successfully transfected can be selected by addition of the antibiotic, thereby eliminating the cells that have not been successfully transduced with the plasmid. When studying gene transcription, the cells are subsequently plated, compound(s) to be tested are introduced, and the assay for the reporter protein is conducted. These assays range from extremely simple to complex, with reporter proteins ranging from enzymes to hormones and photoproteins. Typically, enzymes are assayed using rate assays, hormones are detected using immunoassays, and photoproteins (e.g., green fluorescent protein, aequorin) are imaged optically.  
           [0010]    Cell permeability assays measure the transport of compounds across cells. The commonly-used example is the CaCo-2 cell line derived from human intestinal endothelial cells. When grown to confluency over a porous membrane, these cells form a “biologically active” filter: Transport of compound through the cell layer is accepted in the art to be correlated with absorbsion by the digestive system.  
           [0011]    The compounds available for such cell-based testing have increased dramatically in recent years. In the decade from 1985 to 1995, drug library development through methods such as combinatorial chemistry and the discovery of new targets have created an explosive growth in both the number of compounds with promising biochemical properties. In order to effectively assay these “hits” using cell-based assays, an equivalent system of high throughput screening for such cell-based assays is needed.  
           [0012]    To achieve the primary need of high throughput for cell based assays, a number of secondary features are desirable. First, it is advantageous to have a high degree of process automation, such as fluid transfer, cell plating and washing, and detection. It is also advantageous for the processes to be integrated so as to require a minimum of human intervention. Compound consumption (non-specific adsorption onto the materials comprising the assay apparatus) must be minimized, in order to prevent depletion of rare and/or expensive components of the compound libraries. This is most readily addressed through miniaturization of assays from their current scale of hundreds of microliters to ten microliters or less. A goal in the art is to provide automated, integrated and miniaturized apparatus for performing assays that are reliable and produce results consistent with the results produced by current, more laborious, expensive and time-consuming assays.  
           [0013]    In addition to these advantages, miniaturization itself can confer performance advantages. At short length scales, diffusionally-limited mixing is rapid and can be exploited to create sensitive assays (Brody et al., 1996,  Biophysical J.  71: 3430-3431). Because fluid flow in miniaturized pressure-driven systems is laminar, rather than turbulent, processes such as washing and fluid replacement are well-controlled. Miniaturized, most advantageously microfabricated systems also enable assays that rely on a large ratio of surface area to volume, such chromatographic assays generally and assays that require binding to a surface.  
           [0014]    Miniaturization has led to the creation of 384-well and 1536-well microtiter plates for total reaction volumes of between 0.015 and 0.1 mL. However, a number of problems arise when miniaturizing standard plate technology, especially for use in conjunction with cells. First, because the total volumes are smaller and the plates are open to the environment, evaporation of fluid during the course of an assay can compromise results; this is especially problematic for cell based assays that may require incubation at elevated temperatures for up to several days. Another drawback of open plates is the existence of the meniscus of fluid in the well. Meniscuses of varying configurations (due, for example to imperfections in the plate or differences in contact angle and surface tension) can distort the optical signals used to interrogate the samples. As the strength of the optical signals decreases with decreasing assay volume, correction for background distortions becomes more difficult. Finally, optical scanning systems for high-density plates are often complex and expensive. Methods that minimize evaporation, provide a more uniform optical pathway, and provide simpler detection schemes are desirable.  
           [0015]    Highly accurate pipetting technologies have been developed to deliver fluids in precisely metered quantities to these plates. Most of these fluid-delivery methods for low volumes (below approximately 0.5 μL) rely on expensive piezoelectric pipetting heads that are complex and difficult to combine or “gang” into large numbers of independent pipettors so that many wells may be addressed independently. As a result, fluid delivery is either completely or partially serial (i.e., a single micropipettor, or a small number of parallel delivery systems used repeatedly to address the entire plate). Serial pipetting defeats the aim of parallelism by increasing the amount of time required to address the plate. Methods that reduce the number and precision of fluid transfer steps are therefore needed.  
           [0016]    Attempts to produce microfabricated devices for performing cell-based assays have been reported in the art. For example, International Patent Application WO98/028623, published Jul. 2, 1998 by several of the instant inventors, discloses a microfluidics platform for detecting particulates in a fluid, specifically including cells.  
           [0017]    A microfabricated device explicitly for the performance of cell based assays in a centrifugal format has been disclosed in International Patent Application WO 99/55827, published November 1999. The operative principles of this device include the use of hydrophobic coatings along a radial channel punctuated by cell culturing chambers and optical cuvettes. However, this device cannot perform distinct assays on subpopulations of the cells cultured on the device. By providing only a single entry to a multiplicity of cell culturing chambers, all chambers are exposed to the same solutions, such as cell suspension, cell culture medium, test compounds and any reagents used for detection of the effects of these compounds. Furthermore, the format disclosed in WO 99/55827 relies on the manufactured surface of the microplatform to provide the support for cell attachment and proliferation, or the use of carrier beads. This may not be adequate for all cell types of interest. Finally, no provision is made for selectively trapping and incubating certain cells or cell types rather than others. In applications such as diagnostics, in which a variety of cells may be present in a biological sample such as blood, means for separating cells based on type or other features may be required.  
           [0018]    Thus, there is a need in the art for improved micromanipulation apparatus and methods for performing cell based assays more rapidly and economically using less biological sample material. Relevant to this need in the art, some of the present inventors have developed a microsystem platform and a micromanipulation device to manipulate said platform by rotation, thereby utilizing the centripetal forces resulting from rotation of the platform to motivate fluid movement through microchannels embedded in the microplatform, as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         SUMMARY OF THE INVENTION  
         [0019]    The invention disclosed herein relates to microfluidic devices for performing cell based assays for a variety of applications such as life sciences, diagnostics and drug screening. In particular, these devices have been developed to carry out various steps common to many cell-based assays. The devices comprise an entry port or other means for adding cellular suspensions, most preferably in vitro cell cultures, into the devices of the invention. Surfaces and supports comprising the devices have been adapted or treated to permit cell attachment and growth to occur on appropriate surfaces and supports in the devices, while alternatively other surfaces or components of the devices have been fabricated or treated to inhibit cell attachment and growth. The components of the devices are arranged to permit cell growth on the surface of the device, including such attendant process as exchange of growth media, exchange of gases like carbon dioxide naturally respired during growth, and incubation at temperatures appropriate for cell culture. The devices of the invention are produced to facilitate distribution of test solutions to the cells cultured within the device, said solutions preferably carrying test compounds or other reagents. Finally, the components of the devices of the invention are provided so that metabolites, secretions, or excretions from cells on the device can be detected, either directly or through reaction with appropriate reagents. Another preferred form of detection provided is the direct visualization and imaging of cells.  
           [0020]    This invention provides microsystems platforms as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/61,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein, adapted to permit cell attachment and growth on the surface thereof, most preferably in specific components such as cell growth chambers as described herein. Additional microfluidics components that facilitate the performance of cell based assays are also provided, as described in more detail herein.  
           [0021]    The invention provides apparatus and methods for performing microscale processes on a microplatform, whereby fluid is moved on the platform in defined channels motivated by centripetal force arising from rotation of the platform. The first element of the apparatus of the invention is a microplatform that is a rotatable structure, most preferably a disk, the disk comprising fluid (sample) inlet ports, fluidic microchannels, reagent reservoirs, cell growth or aggregation chambers, detection chambers and sample outlet ports, generically termed “microfluidic structures,” and also comprise heating elements that make up a portion of the surface area of the platform for heating fluids contained therein to temperatures greater than ambient temperature. In preferred embodiments, said heating elements are positioned on the disk in sufficient proximity to the cell growth or aggregation chambers to allow cell growth in said chambers on the disk surface without inhibiting cell growth or otherwise compromising the viability of growing cells. The disk is rotated at speeds from about 1-30,000 rpm for generating centripetal acceleration that enables fluid movement through the microfluidic structures of the platform. The disks of the invention also preferably comprise air outlet ports and air displacement channels. The air outlet ports and in particular the air displacement ports provide a means for fluids to displace air, thus ensuring uninhibited movement of fluids on the disk. These air outlet ports also permit gas exchange between cell growth or aggregation chambers and the atmosphere, to allow the culture media to be oxygenated and to eliminate gaseous waste such as carbon monoxide. Specific sites on the disk also preferably comprise elements that allow fluids to be analyzed, as well as detectors for each of these effectors. Alternatively, some or all of these elements can be contained on a second disk that is placed in optical or direct physical contact, most preferably thermal contact, with the first platform disk.  
           [0022]    The discs of this invention have several advantages over those that exist in the centrifugal analyzer art. Foremost is the fact that flow is laminar due to the small dimensions of the fluid channels; this allows for better control of processes such as mixing and washing. To this are added the already described advantages of miniaturization, as described in more detail above.  
           [0023]    The second element of the invention is a micromanipulation device that is a disk player/reader device that controls the function of the disk. This device comprises mechanisms and motors that enable the disk to be loaded and rotated. In addition, the device provides means for a user to operate the Microsystems in the disk and access and analyze data, preferably using a keypad and computer display. The micromanipulation device also advantageous provides means for actuation of on-disc elements, such active valves; the application and control of heat to the disc for purposes of chemical or biological incubation; and means for adding fluids to and removing fluids from the discs. In preferred embodiments, the apparatus also comprises means for insulating the platforms of the invention from the environment, so that cells growing on the disc can be maintained at the proper temperature, oxygen tension, acidity, humidity levels, and other parameters understood by those with skill in the cell culture arts.  
           [0024]    The invention specifically provides Microsystems platforms comprising microfluidics components contained in one or a multiplicity of platform layers that are fluidly connected to permit transfer, mixing and assay performance on the sealed surface of the platform. The platforms preferably comprise one or more entry ports through which cell suspensions may be added in volumes ranging from about 1 nL to about 1 mL. The platforms preferably comprise one or more reagent reservoirs containing a sufficient volume, preferably from about 1 nL to about 1 mL, of a reagent solution for a multiplicity of individual assays. The reagent reservoirs are fluidly connected by microchannels to one or preferably a multiplicity of cell incubation chambers. These cell incubation chambers are preferably equipped with a surface that has been constructed or specifically adapted for attachment and growth of cells, and may also be sealed with selectively-permeable membranes for which allow passage of gases in and out of the chambers from the exterior environment. Cell incubation chambers may be equipped with devices for capturing cells passed through the chambers. Additionally, the cell incubation chambers may be fluidly connected to detection chambers and waste chambers. In some preferred embodiments, the platform comprises a multiplicity mixing channels and reservoirs for the mixing of reagents in various ratios and for the creation of dilution series for performing cell-based assays of drugs and other compounds.  
           [0025]    In the use of the platforms of the invention, fluids (including cell suspensions and reagents) are added to the platform when the platform is at rest. Thereafter, rotation of the platform on a simple motor motivates fluid movement through microchannels for various processing steps. In preferred embodiments, the platforms of the invention permit the use of a detector, most preferably an optical detector, for detecting the products of an assay, most preferably a biochemical assay, whereby the assay reaction chambers comprise optical cuvettes, preferably positioned at the outer edge of the platform, and most preferably wherein the platform is scanned past a fixed detector through the action of the rotary motor. In other embodiments, the platforms permit the use of optical imaging systems for the direct visualization of cells that have attached to support surfaces within the platform Because the platforms of the invention are most preferably constructed using microfabrication techniques as described more fully below, the volumes of fluids used may be made arbitrarily small as long as the detectors used have sufficient sensitivity.  
           [0026]    The present invention solves problems in the current art through the use of a microfluidic disc in which centripetal acceleration is used to move fluids. It is an advantage of the microfluidics platforms of the present invention that the fluid-containing components are constructed to contain small volumes, thus reducing reagent costs, reaction times and the amount of biological material required to perform an assay. It is also an advantage that the fluid-containing components are sealed, thus eliminating experimental error due to differential evaporation of different fluids and the resulting changes in reagent concentration, as well as reducing the risk of contamination, either of the cell culture or the operator. Because the microfluidic devices of the invention are completely enclosed, both evaporation and optical distortion are reduced to negligible levels. The platforms of the invention also advantageously permit “passive” mixing and valving, i.e., mixing and valving are performed as a consequence of the structural arrangements of the components on the platforms (such as shape, length, position on the platform surface relative to the axis of rotation, and surface properties of the interior surfaces of the components, such as wettability as discussed below), and the dynamics of platform rotation (speed, acceleration, direction and change-of-direction), and permit control of assay timing and reagent delivery.  
           [0027]    In alternative embodiments of the platforms of the invention, metering structures as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000 and incorporated by reference herein, are used to distribute aliquots of reagent to each of a multiplicity of mixing structures, each mixing structure being fluidly connected to one of a multiplicity of sample reservoirs, thereby permitting parallel processing and mixing of the samples with a common reagent. This reduces the need for automated reagent distribution mechanisms, reduces the amount of time required for reagent dispensing (that can be performed in parallel with distribution of reagent to a multiplicity of reaction chambers), and permits delivery of small (nL-to-μL) volumes without using externally-applied electromotive means. It also enables the performance of multiplexed assays, in which cell populations may be divided and the microfluidics of the device used to perform a variety of assays on different sub-populations in parallel, on one population serially, or on a single population simultaneously.  
           [0028]    A further advantage of the platforms of the invention is the use of elements that can serve to selectively capture cells based on properties such as size (though the use of porous filters) or type (through the use of immunochemical methods, as disclosed in co-owned U.S. Pat. No. 6,143,247, issued Nov. 7, 2000 and International Application Publication No. WO98/28623, published Jul. 2, 1999, the teachings of each of which are explicitly incorporated by reference herein).  
           [0029]    The assembly of a multiplicity of cell incubation chambers on the platforms of the invention also permits simplified detectors to be used, whereby each individual reaction chamber can be scanned using mechanisms well-developed in the art for use with, for example, CD-ROM technology. Finally, the platforms of the invention are advantageously provided with sample and reagent entry ports for filling with samples and reagents, respectively, that can be adapted to liquid delivery means known in the art (such as micropipettors).  
           [0030]    The platforms of the invention reduce the demands on automation in at least three ways. First, the need for precise metering of delivered fluids is relaxed through the use of on-disc metering structures, as described more fully in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein. By loading imprecise volumes, slightly in excess of those needed for the assay, and allowing the rotation of the disc and use of appropriate microfluidic structures to meter the fluids, much simpler (and less expensive) fluid delivery technology may be employed than is the conventionally required for high-density microtitre plate assays.  
           [0031]    Second, the total number of fluid “delivery” events on the microfluidic platform is reduced relative to conventional assay devices such as microtiter plates. By using microfluidic structures that sub-divide and aliquot common reagents (such as reagent solutions, buffers, and enzyme substrates) used in all assays performed on the platform, the number of manual or automated pipetting steps are reduced by at least half (depending on the complexity of the assay). Examples of these structures have been disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and incorporated by reference herein. In some examples shown explicitly herein, the microfluidic structures of the platform may be used to create, for example, multiple mixtures of reagents with different mixing ratios for application to cultured cells. These structures provide automation, for example, for serial dilution assays, a laborious process when performed conventionally. This process is replaced by “parallel dilution” on the platforms of the invention.  
           [0032]    Finally, the invention also provides on-platform means for adding incubation media, washing cell incubation chambers, and media replacement. These features also reduce manipulation of the assay device by, for example, robotic washing stations, as well as providing controlled and integrated fluid processing.  
           [0033]    Certain preferred embodiments of the apparatus of the invention are described in greater detail in the following sections of this application and in the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 depicts an oblique, exploded view of a disc developed with a flow-through membrane for capturing cells  
         [0035]    [0035]FIG. 2 depicts a detail of a structure for growing and assaying 4 populations of cells on the disc of FIG. 1.  
         [0036]    [0036]FIG. 3 is a cross-sectional view of the flow-through cell culturing chamber of the disc of FIG. 1.  
         [0037]    [0037]FIG. 4 illustrates an alternate construction of the disc of FIG. 1 for the performance of 48 cell-based assays.  
         [0038]    [0038]FIG. 5 is a detail of a single assay microstructure of the disc of FIG. 4.  
         [0039]    [0039]FIG. 6 is an alternate construction of the microstructure of FIG. 5.  
         [0040]    [0040]FIG. 7 is a version of the disc of FIG. 1 designed for the delivery of a dilution series to the chambers.  
         [0041]    [0041]FIG. 8 is a version of the microstructure of FIG. 2 used for a viability assay.  
         [0042]    [0042]FIG. 9 is an image of stained cells showing viability and proliferation in the disc of the invention (FIG. 8).  
         [0043]    [0043]FIG. 10 is a time-course of fluorescence data from cells cultured within cell incubation chambers of the disc using alamarBlue reagents. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]    This invention provides a microplatform and a micromanipulation device as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein, adapted for performing microanalytical and microsynthetic assays of biological samples.  
         [0045]    For the purposes of this invention, the term “sample” will be understood to encompass any fluid, solution or mixture, either isolated or detected as a constituent of a more complex mixture, or synthesized from precursor species. In particular, the term “sample” will be understood to encompass any biological species of interest. The term “biological sample” or “biological fluid sample” will be understood to mean any biologically-derived sample, including but not limited to blood, plasma, serum, lymph, saliva, tears, cerebrospinal fluid, urine, sweat, plant and vegetable extracts, semen, and ascites fluid.  
         [0046]    For the purposes of this invention, the term “a centripetally motivated fluid micromanipulation apparatus” is intended to include analytical centrifuges and rotors, microscale centrifugal separation apparatuses, and most particularly the Microsystems platforms and disk handling apparatuses as described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0047]    For the purposes of this invention, the term “Microsystems platform” is intended to include centripetally-motivated microfluidics arrays as described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0048]    In one aspect of the platforms of the invention is provided a surface or cell growth or accumulation chamber treated to comprise a specific binding reagent. For the purposes of this invention, the term “specific binding reagent” is intended to encompass biomolecules having a specific binding affinity between pairs thereof providing a specific molecular binding interaction with a binding affinity constant of between about 10 −4  and 10 −15  M. Examples of such pairs of specific binding reagents include but are not limited to antigen and antibody, including antisera, polyclonal antibodies and most preferably monoclonal antibodies; receptor and ligands, including cell-surface receptors; integrins and adhesion proteins, including ICAM-I and ICAM-II; and carbohydrates and lectins, including phytohemagglutinin. As provided by the invention, specific binding reagents comprising a first member of a specific binding pair is provided coating a surface or cell culture reservoir of a platform designed or intended to permit cell growth or accumulation thereupon, most preferably a cell expressing a cognate antigen, receptor or adhesion protein or having a carbohydrate moiety at the cell surface specific for a particular lectin. Said specific binding reagent is applied to the surface or cell growth or accumulation chamber of the platform by depositing the reagent on the surface using any appropriate means, including inkjet printing, computer-positioned syringes, microetching and microlithographic methods, including photolithography, screen and airbrush printing methods, solution coating, dipping, and conventional microtitre-well techniques.  
         [0049]    For the purposes of this invention, the terms “capillary”, “microcapillary” and “microchannel” will be understood to be interchangeable and to be constructed of either wetting or non-wetting materials where appropriate.  
         [0050]    For the purposes of this invention, the term “reagent reservoir,” “assay chamber,” “fluid holding chamber,” “collection chamber” and “detection chamber” will be understood to mean a defined volume on a microsystems platform of the invention comprising a fluid.  
         [0051]    For the purposes of this invention, the terms “entry port” and “fluid input port” will be understood to mean an opening on a microsystems platform of the invention comprising a means for applying a fluid to the platform.  
         [0052]    For the purposes of this invention, the terms “exit port” and “fluid outlet port” will be understood to mean a defined volume on a microsystems platform of the invention comprising a means for removing a fluid from the platform.  
         [0053]    For the purposes of this invention, the term “capillary junction” will be understood to mean a region in a capillary or other flow path where surface or capillary forces are exploited to retard or promote fluid flow. A capillary junction is provided as a pocket, depression or chamber in a hydrophilic substrate that has a greater depth (vertically within the platform layer) and/or a greater width (horizontally within the platform layer) that the fluidics component (such as a microchannel) to which it is fluidly connected. For liquids having a contact angle less than 90° (such as aqueous solutions on platforms made with most plastics, glass and silica), flow is impeded as the channel cross-section increases at the interface of the capillary junction. The force hindering flow is produced by capillary pressure, that is inversely proportional to the cross sectional dimensions of the channel and directly proportional to the surface tension of the liquid, multiplied by the cosine of the contact angle of the fluid in contact with the material comprising the channel. The factors relating to capillarity in microchannels according to this invention have been discussed in co-owned U.S. Pat. No. 6,063,589, issued May 12, 2000 and in co-owned and co-pending U.S. patent application Ser. No. 08/910,726, filed Aug. 12, 1997, incorporated by reference in its entirety herein.  
         [0054]    Capillary junctions can be constructed in at least three ways. In one embodiment, a capillary junction is formed at the junction of two components wherein one or both of the lateral dimensions of one component is larger than the lateral dimension(s) of the other component. As an example, in microfluidics components made from “wetting” or “wettable” materials, such a junction occurs at an enlargement of a capillary as described in co-owned and co-pending U.S. Ser. Nos. U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; and 08/910,726, filed Aug. 12, 1997. Fluid flow through capillaries is inhibited at such junctions. At junctions of components made from non-wetting or non-wettable materials, on the other hand, a constriction in the fluid path, such as the exit from a chamber or reservoir into a capillary, produces a capillary junction that inhibits flow. In general, it will be understood that capillary junctions are formed when the dimensions of the components change from a small diameter (such as a capillary) to a larger diameter (such as a chamber) in wetting systems, in contrast to non-wettable systems, where capillary junctions form when the dimensions of the components change from a larger diameter (such as a chamber) to a small diameter (such as a capillary).  
         [0055]    A second embodiment of a capillary junction is formed using a component having differential surface treatment of a capillary or flow-path. For example, a channel that is hydrophilic (that is, wettable) may be treated to have discrete regions of hydrophobicity (that is, non-wettable). A fluid flowing through such a channel will do so through the hydrophilic areas, while flow will be impeded as the fluid-vapor meniscus impinges upon the hydrophobic zone.  
         [0056]    The third embodiment of a capillary junction according to the invention is provided for components having changes in both lateral dimension and surface properties. An example of such a junction is a microchannel opening into a hydrophobic component (microchannel or reservoir) having a larger lateral dimension. Those of ordinary skill will appreciate how capillary junctions according to the invention can be created at the juncture of components having different sizes in their lateral dimensions, different hydrophilic properties, or both.  
         [0057]    For the purposes of this invention, the term “capillary action” will be understood to mean fluid flow in the absence of rotational motion or centripetal force applied to a fluid on a rotor or platform of the invention and is due to a partially or completely wettable surface.  
         [0058]    For the purposes of this invention, the term “capillary microvalve” will be understood to mean a capillary microchannel comprising a capillary junction whereby fluid flow is impeded and can be motivated by the application of pressure on a fluid, typically by centripetal force created by rotation of the rotor or platform of the invention. Capillary microvalves will be understood to comprise capillary junctions that can be overcome by increasing the hydrodynamic pressure on the fluid at the junction, most preferably by increasing the rotational speed of the platform.  
         [0059]    For the purposes of this invention, the term “in fluid communication” or “fluidly connected” is intended to define components that are operably interconnected to allow fluid flow between components.  
         [0060]    For the purposes of this invention, the term “air displacement channels” will be understood to include ports in the surface of the platform that are contiguous with the components (such as microchannels, chambers and reservoirs) on the platform, and that comprise vents and microchannels that permit displacement of air from components of the platforms and rotors by fluid movement.  
         [0061]    The microplatforms of the invention (preferably and hereinafter collectively referred to as “disks”; for the purposes of this invention, the terms “microplatform”, “Microsystems platform” and “disk” are considered to be interchangeable) are provided to comprise one or a multiplicity of microsynthetic or microanalytic systems (termed “microfluidics structures” herein). Such microfluidics structures in turn comprise combinations of related components as described in further detail herein that are operably interconnected to allow fluid flow between components upon rotation of the disk. These components can be microfabricated as described below either integral to the disk or as modules attached to, placed upon, in contact with or embedded in the disk. For the purposes of this invention, the term “microfabricated” refers to processes that allow production of these structures on the sub-millimeter scale. These processes include but are not restricted to molding, photolithography, etching, stamping and other means that are familiar to those skilled in the art.  
         [0062]    The invention also comprises a micromanipulation device for manipulating the disks of the invention, wherein the disk is rotated within the device to provide centripetal force to effect fluid flow on the disk. Accordingly, the device provides means for rotating the disk at a controlled rotational velocity, for stopping and starting disk rotation, and advantageously for changing the direction of rotation of the disk. Both electromechanical means and control means, as further described herein, are provided as components of the devices of the invention. User interface means (such as a keypad and a display) are also provided, as further described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0063]    The invention provides a combination of specifically-adapted microplatforms that are rotatable, analytic/synthetic microvolume assay platforms, and a micromanipulation device for manipulating the platform to achieve fluid movement on the platform arising from centripetal force on the platform as result of rotation. The platform of the invention is preferably and advantageously a circular disk; however, any platform capable of being rotated to impart centripetal for a fluid on the platform is intended to fall within the scope of the invention. The micromanipulation devices of the invention are more fully described in co-owned and co-pending U.S. Ser. Nos. U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0064]    Fluid (including reagents, samples and other liquid components) movement is controlled by centripetal acceleration due to rotation of the platform. The magnitude of centripetal acceleration required for fluid to flow at a rate and under a pressure appropriate for a particular microfluidics structure on the microsystems platform is determined by factors including but not limited to the effective radius of the platform, the interior diameter of microchannels, the position angle of the microchannels on the platform with respect to the direction of rotation, and the speed of rotation of the platform. In certain embodiments of the methods of the invention an unmetered amount of a fluid (either a sample or reagent solution) is applied to the platform and a metered amount is transferred from a fluid reservoir to a microchannel, as described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein, In preferred embodiments, the metered amount of the fluid sample provided on an inventive platform is from about 1 nL to about 500 μL. In these embodiments, metering manifolds comprising one or a multiplicity of metering capillaries are provided to distribute the fluid to a plurality of components of the microfluidics structure.  
         [0065]    The components of the platforms of the invention are in fluidic contract with one another. In preferred embodiments, fluidic contact is provided by microchannels comprising the surface of the platforms of the invention. Microchannel sizes are optimally determined by specific applications and by the amount of and delivery rates of fluids required for each particular embodiment of the platforms and methods of the invention. Microchannel sizes can range from 0.1 μm to a value close to the thickness of the disk (e.g., about 1 mm); in preferred embodiments, the interior dimension of the microchannel is from 0.5 μm to about 500 μm. Microchannel and reservoir shapes can be trapezoid, circular or other geometric shapes as required. Microchannels preferably are embedded in a microsystem platform having a thickness of about 0.1 to 25 mm, wherein the cross-sectional dimension of the microchannels across the thickness dimension of the platform is less than 1 mm, and can be from 1 to 90 percent of said cross-sectional dimension of the platform. Sample reservoirs, reagent reservoirs, reaction chambers, collection chambers, detections chambers and sample inlet and outlet ports preferably are embedded in a microsystem platform having a thickness of about 0.1 to 25 mm, wherein the cross-sectional dimension of the microchannels across the thickness dimension of the platform is from 1 to 75 percent of said cross-sectional dimension of the platform. In preferred embodiments, delivery of fluids through such channels is achieved by the coincident rotation of the platform for a time and at a rotational velocity sufficient to motivate fluid movement between the desired components.  
         [0066]    The flow rate through a microchannel of the invention is inversely proportional to the length of the longitudinal extent or path of the microchannel and the viscosity of the fluid and directly proportional to the product of the square of the hydraulic diameter of the microchannel, the square of the rotational speed of the platform, the average distance of the fluid in the channels from the center of the disk and the radial extent of the fluid subject to the centripetal force. Since the hydraulic diameter of a channel is proportional to the ratio of the cross-sectional area to cross-sectional perimeter of a channel, one can judiciously vary the depth and width of a channel to affect fluid flow (see Duffy et al., 1998,  Anal. Chem.  71: 4669-4678 and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996 and 08/768,990, filed Dec. 18, 1996, incorporated by reference).  
         [0067]    For example, fluids of higher densities flow more rapidly than those of lower densities given the same geometric and rotational parameters. Similarly, fluids of lower viscosity flow more rapidly than fluids of higher viscosity given the same geometric and rotational parameters. If a microfluidics structure is displaced along the radial direction, thereby changing the average distance of the fluid from the center of the disc but maintaining all other parameters, the flow rate is affected: greater distances from the center result in greater flow rates. An increase or a decrease in the radial extent of the fluid also leads to an increase or decrease in the flow rate. These dependencies are all linear. Variation in the hydraulic diameter results in a quartic dependence of flow rate on hydraulic diameter (or quadratic dependence of fluid flow velocity on hydraulic diameter), with larger flow rates corresponding to larger diameters. Finally, an increase in the rotational rate results in a quadratic increase in the flow rate or fluid flow velocity.  
         [0068]    Input and output (entry and exit) ports are components of the microplatforms of the invention that are used for the introduction or removal of fluid components. Entry ports are provided to allow samples and reagents to be placed on or injected onto the disk; these types of ports are generally located towards the center of the disk. Exit ports are also provided to allow products to be removed from the disk. Port shape and design vary according specific applications. For example, sample input ports are designed, inter alia, to allow capillary action to efficiently draw the sample into the disk. In addition, ports can be configured to enable automated sample/reagent loading or product removal. Entry and exit ports are most advantageously provided in arrays, whereby multiple samples are applied to the disk or to effect product removal from the microplatform.  
         [0069]    In some embodiments of the platforms of the invention, the inlet and outlet ports are adapted to the use of manual pipettors and other means of delivering fluids to the reservoirs of the platform. In alternative, advantageous embodiments, the platform is adapted to the use of automated fluid loading devices. One example of such an automated device is a single pipette head located on a robotic arm that moves in a direction radially along the surface of the platform. In this embodiment, the platform could be indexed upon the spindle of the rotary motor in the azimuthal direction beneath the pipette head, which would travel in the radial direction to address the appropriate reservoir.  
         [0070]    Another embodiment is a pipettor head adapted to address multiple reservoirs, either a subset of or all of the reservoirs on the platform surface. For embodiments where the pipettor head addresses a subset of the reservoirs, a single head may for example be composed of a linear array of pipette heads. For example, the entry ports of FIG. 1 might be addressed by indexing such a linear head in the direction transverse to the pipette tips. In other embodiments, pipette heads may be used which can simultaneously address all entry ports (for example, a 96-tip head). In these embodiments, there may be a distinction between sample entry ports—needed for the delivery of many samples—and reagent entry ports, through which larger volumes or reagent are delivered for use in reactions with all samples. A pipetting device that can simultaneously address all sample entry ports as well as reagent ports might consist of a standard multipipettor with a few added, large-volume delivery tips.  
         [0071]    Also included in air handling systems on the disk are air displacement channels, whereby the movement of fluids displaces air through channels that connect to the fluid-containing microchannels retrograde to the direction of movement of the fluid, thereby providing a positive pressure to further motivate movement of the fluid.  
         [0072]    Platforms of the invention such as disks and the microfluidics components comprising such platforms are advantageously provided having a variety of composition and surface coatings appropriate for particular applications. Platform composition will be a function of structural requirements, manufacturing processes, and reagent compatibility/chemical resistance properties. Specifically, platforms are provided that are made from inorganic crystalline or amorphous materials, e.g. silicon, silica, quartz, inert metals, or from organic materials such as plastics, for example, poly(methyl methacrylate) (PMMA), acetonitrile-butadiene-styrene (ABS), polycarbonate, polyethylene, polystyrene, polyolefins, polypropylene and metallocene. These may be used with unmodified or modified surfaces as described below. The platforms may also be made from thermoset materials such as polyurethane and poly(dimethyl siloxane) (PDMS). Also provided by the invention are platforms made of composites or combinations of these materials; for example, platforms manufactures of a plastic material having embedded therein an optically transparent glass surface comprising the detection chamber of the platform. Alternately, platforms composed of layers made from different materials may be made. The surface properties of these materials may be modified for specific applications, as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0073]    Preferably, the disk incorporates microfabricated mechanical, optical, and fluidic control components on platforms made from, for example, plastic, silica, quartz, metal or ceramic. These structures are constructed on a sub-millimeter scale by molding, photolithography, etching, stamping or other appropriate means, as described in more detail below. It will also be recognized that platforms comprising a multiplicity of the microfluidic structures are also encompassed by the invention, wherein individual combinations of microfluidics and reservoirs, or such reservoirs shared in common, are provided fluidly connected thereto. An example of such a platform is shown in FIG. 1.  
         [0074]    Platform Manufacture and Assembly  
         [0075]    Referring now to the Figures for a more thorough description of the invention, FIG. 1 shows an exploded oblique view of an example of a disc appropriate for the performance of 4 parallel cell based assays in quadruplicate. In this embodiment, platform  100  is composed of two layers, an upper reservoir layer  101  and a lower reservoir layer  199 . In this example, the two layers are mirrored versions of one another except for two features described in detail below.  
         [0076]    Assays are performed in the following general manner: A cell suspension is added to the disc and driven through channels and reservoirs under the influence of centripetal force produce by rotation of the platform. The suspended cells are distributed by flow across a porous support or filter, while the medium that carries the cells can traverse the pores of the support. The platform can also be sealed with a gas permeable membrane in the vicinity of the cell-retaining support, and the entire disc can be placed inside an incubator so that the trapped cells may attach and proliferate. Additionally media or reagents may be added through the same fluidic channels, for example exchanging nutrients or administering test compounds, dyes, and other compounds. All excess fluids are washed into waste reservoirs contained on the disc.  
         [0077]    The disc depicted in FIG. 1 illustrates how a multiplicity of identical assays can be performed on a platform having repeating assay structures around the disc at a particular radius positioned at equivalent distances from the axis of rotation, as well as modifying the structures for placement at different radial positions. In FIG. 1, structure  103  (shown in more detail in FIG. 2) is repeated azimuthally around the platform layers  101  and  199 . In this way, it is possible to fully cover the surface of the disc with microfluidics structures for performing assays. The maximum number of assays that may be performed will depend upon the volume of fluid that may be manipulated reproducibly, i.e., the minimum reproducible dimensions with which the disc may be fabricated, and the amount of hydrodynamic pressure required to drive small volumes of fluid through microchannels at convenient rotational rates. Taking these considerations into account, it is estimated that greater than 10,000 assays having volumes of 1-5 nL can be created in a circular platform having a 6 cm radius.  
         [0078]    In FIG. 1, platform  100  is composed of at least 2 component layers. A fluidics layer  101  having features on one or both faces is used. In cases where both sides contain features, some features may be through-holes or vias connecting the fluidic networks, allowing for fluidic “cross-overs” and three-dimensional flow from one side of the disc to the other. In such a case, a sealing film  198  (not shown) is preferably used to seal the channels on the “upper” side of the layer  101 . The fluidic channels on the lower face of layer  101  must be sealed partially or completely through the use of one or more sealing layers or a second fluidics layer  199 . The sealing layer may be a featureless material. It can also be composed of selectively-permeable material for the transmission of gases necessary for cell metabolism and elimination of gaseous waste products produced by that metabolism. Alternatively, the sealing layer may be replaced by a second fluidics layer with channels and reservoirs constructed to that channels and reservoirs on the two layers overlap. The embodiment shown in FIG. 1 is composed of two layers  101  and  199  that are mirrored versions that permit at all features to overlap on the two layers.  
         [0079]    Platform  100  is preferably provided in the shape of a disc, a circular planar platform having a diameter of from about 10 mm to about 50 mm and a thickness of from about 0.1 mm to about 25 mm. Each layer comprising the platform preferably has a diameter that is substantially the same as the other layers, although in some embodiments the diameters of the different layers are not required to completely match. Each layer has a thickness ranging from about 0.1 mm to about 25 mm, said thickness depending in part on the volumetric capacity of the microfluidics components contained therein.  
         [0080]    Referring to FIG. 2, a single cell based assay structure is illustrated and will be described in the case of a single fluidics layer with sealing layer. The Figure illustrates structures necessary for the device&#39;s function. These include a fluid entry port  218  through which cell culture media, nutrient media, test compounds, dyes, and other components of cell based assays can be loaded. A distribution manifold  201  additionally has sub-volumes  202  that are used to divide the fluids into equal aliquots for delivery to the multiple incubation chambers. Also provided is an overflow channel  212  leading to an overflow reservoir  214 ; this may optionally pass through a capillary or physical valve  213 . Air displaced as the fluid moves to the overflow chamber is vented via channel  215  and air-vent  217 . Air displaced as fluid moves into the volumes  202  is vented through channels  203 , chambers  205 , cell support or filtering element  206  channels  211  and vents  216  in cases where no physical valve blocks channels  203 . In some instances a physical valve may be placed such that it blocks  203  at position  204 . In these instances, displaced air will either be expelled through the inward surface of fluid in subvolumes  202  or may be expelled via channels and vents not shown. The cell culturing chambers  205  may have channels  219  and air vents  220  and is optionally shaped to permit insertion of a cell support or filtering element  206 , forming a leak-tight seal  207  with the surface of the platform through the use of solvent bonding, adhesive, or snap-in washers, screw-in elements, or a pressure seal between the layer  101  and sealing layer  199 . Note that the cell support or filtering element  206  need not be placed in both layers  101  and  199 .  
         [0081]    The cell culturing chamber itself is shown in cross-section in FIG. 3. As shown, the upper layer  101  contains the inlet channel  203  and the cell support or filtering element  206 , while layer  199  contains outlet channel  209  that communicates to overflow reservoir  210 , air channel  211  and port  216 . Channels  209  and  208  may also be coated with a hydrophobic coating material (e.g. PerFluorCoat, obtained from PFC MH-Series, Cytonix Corporation, Beltsville, Md. 20705). These coatings have the beneficial effect of preventing capillary action (“wicking”) of fluids from one chamber to another, e.g., between overflow reservoir  210  and cell culturing chamber  205 , when the disc is stationary, thus preventing waste or wash fluid from backflowing into the cell culturing chamber. They may also function as a temporary valve, inhibiting radially-outward flow much as a capillary valve does until disc rotation speed is sufficiently high to overcome surface forces.  
         [0082]    To summarize the differences between  101  and  199 , layer  101  contains the entry port  218 , channel  203  and support  206  as well as sealings means  207 , while layer  199  contains channel  209 . Fluid passage between layers occurs at overlaps in reservoirs such as distributor manifold  201  and overflow reservoir  210  and at the culturing chamber  205 . In alternative constructions, layer  101  may contain all of the features described above on its lower face which are radially inward, and inclusive, of cell culturing chamber  205 . Layer  199  may then carry all structures that are radially outward of cell culturing chamber  205 . Similarly, since the sequence of flow is to pass unidirectionally through cell culturing chamber  205 , one layer may contain all components necessary for flow prior to reaching cell culturing chamber  205 , while the other layer contains the remaining features for flow subsequent to cell culturing chamber  205 .  
         [0083]    In use, the disc functions as follows. A liquid sample containing cells, herein termed a cell suspension, is loaded through port  218 . The disc is rotated at a first rate, and under centrifugation is distributed among sub-volumes  202 . Fluid also passes into channel  212 , but flow at this rotational speed is blocked, either by a capillary valve or physical valve  213 . At a second rotational speed, the capillary forces at valve  213  are overcome and excess fluid flows into overflow chamber  214 . Alternatively, a physical valve at  213  is opened, either at the first rotational speed or a second speed. Excess suspension is decanted into overflow chamber  214 . At a third rotational speed, capillary valves  204  are overcome and suspension flows into cell culture chambers  205 . Through use of a filter element  206  with sufficiently small pores, the hydraulic resistance of the filter is far greater than that of the channels; as result, the cell incubation chambers fills behind the filters, and then excess fluid can be spun through the filter into overflow chamber  210 , if necessary.  
         [0084]    The device may then be halted to allow the trapped cells to attach to cell support or filter element  206 ; incubation at 37° C. may follow, in order for the cells to multiply. Subsequent additions of nutrient media may be made using the same steps of addition, rotation, and valve actuation detailed above.  
         [0085]    A reagent, for example a compound whose toxicity is being evaluated with the cells, is then added to port  218 . Through repetition of the above steps, this second reagent enters cell culture chamber  205  and displaces the medium. The device may then be incubated once again to permit the adhered cells to absorb the reagent.  
         [0086]    Further reagent additions may occur, such as indicator compounds; fixing stains which preferentially or differentially stain living or dead cells; color-generating or fluorescence-generating compounds that indicate the presence of specific metabolites generated by cultured cells; spectrophotometrically detect metabolites or altered forms of co-factors, and other detection methods known to those with skill in the art. The cells may be imaged in situ on the cell supports  206  and their morphology, number, or color determined. The eluent washed into chambers  210  may be spectroscopically or fluorometrically interrogated.  
         [0087]    In some alternative embodiments of the device of FIG. 2, cell capturing elements  206  are not used. In such embodiments, hydrophobic coating of channel  209  may be used to retain fluids in the cell incubation chamber. Alternatively, channel  209  may be constructed so that it bends by 180 degrees, traces an inward radial path, and then bends 180 degrees again to trace an outward radial path toward chamber  210 . If the radial position of the second bending is interior to the radially-inward edge of cell culture chamber  205 , cell culture chamber  205  will remain filled with fluids as long as the connection of channel  209  to chamber  210  is designed to prevent siphoning of fluids.  
         [0088]    It will be understood that the disc may be extended to multiple samples of cells and multiple kinds of assays through the combination of the individual cell-culturing chambers disclosed herein and the liquid-handling capabilities known in the art, for example as disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent application U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; 08/768,990, filed Dec. 18, 1996; 08/910,726, filed Aug. 12, 1997; 08/995,056, filed Dec. 19, 1997; and 09/315,114, filed May 19, 1999, the disclosures of each of which are explicitly incorporated by reference herein.  
         [0089]    Another alternative embodiment is shown in FIG. 4. The disc of FIG. 4 is designed for the performance of 48 identical assays on identical cell populations, and hence is appropriate for large numbers of “repeats”. In FIG. 4, platform  300  is composed of at least 2 component layers, a fluidics layer  101  and sealing layer  199  as discussed above. Fluidics layer  101  contains a central hole or mounting features  301  for attachment to the rotary spindle of the manipulation device. It further comprises a reagent reservoir  303  accessed by a port  302 . This reservoir is connected by channel  304  to a distribution manifold  305 . The distribution manifold is further connected to the individual assay structures  306 . Fluids that pass through structures  306  are collected in waste reservoir  307 .  
         [0090]    An individual cell-based assay structure comprising the disc shown in FIG. 4 is set forth in FIG. 5 and will be described in the case of a single fluidics layer with sealing layer. Referring to FIG. 5, a variety of features necessary for device function are shown. These include the fluid distribution manifold  305  that is fluidly connected to cell assay structure  306 , which is comprised of cell culture chamber  205 , channels  310  and  311  and passage  314 . Manifold  305  is fluidly connected to channel  310 ; the cell culture chamber  205  is shown here with a cell-retaining filter  206 . Cell culture chamber  205  is connected to waste reservoir  307  through a “U-channel”  311  as described above. Channel  311  passes through a passage  314 , which is at least twice as deep and twice as wide as the channel itself A single air-vent (not shown) in the waste reservoir  307  is sufficient for the removal of displaced air.  
         [0091]    An alternative construction of the cell incubation structure  306  is shown in FIG. 6. This structure additionally contains an optical detection cuvette  313  connected via channel  312  to cell culture chamber  205 . The channel then leads from detection cuvette  313  to waste reservoir  307 , via passage  314 .  
         [0092]    In use, these embodiments of the disc function as follows. First, a liquid sample containing cells, herein termed a cell suspension, is loaded through port  303 . The disc is rotated at a first rotational speed, and the cell suspension is displaced into manifold  305 . By using channels  310  with very small diameters (10-250 microns), fluid resistance leading to the incubation chambers is large, such that fluid is distributed throughout the annular manifold  305 . As rotation continues, the cell suspension flows into the plurality of cell culture chambers  205  and, in some embodiments, detection cuvette  313 . In some embodiments, a cell-trapping filter retains the cells in chamber  205 , leaving the detection cuvette free of cells. In other embodiments that do not use filtering elements, the surface of  313  may be treated in manufacture to minimize or eliminate cell attachment. As a result, cells allowed to settle by gravity onto the surface of  313  will not proliferate. In alternative embodiments, the channel  312  may be treated to be hydrophobic, preventing the first cell suspension solution from passing into chamber  313  at a first rotational speed; the cell suspension medium will pass through  312  to  313  and thence  307  at a second, higher rotational speed under conditions whereby the cells do not settle in detection cuvette  313 . In cell culture chamber  205 , however, cells are allowed to attach and grow. Incubation at 37° C. follows, in order for the cells to attach and multiply. Subsequent additions of nutrient media may be made using the same steps of addition, rotation, and valve actuation detailed above.  
         [0093]    The shape of channel  311  and presence of passage  314  insure that cell culture chamber  205  and detection chamber  313  (where present) always remain filled with fluid. When fluid first passes through cell culture chamber  205  and thence into detection chamber  311 , the fluid is driven by the hydrostatic pressure of the fluid in cell culture chamber  205  and remaining in manifold  305  to travel radially inward in channel  311  and thence into channel  314 . Because channel  314  is sized significantly larger than channel  311 , the emergent fluid will “drip”, forming droplets, which are driven under centrifugation into overflow reservoir  307  more rapidly than fluid can flow through the channels and chambers of the structure. In particular, channel  311  presents the largest “resistance” to the flow of liquid due to its significant length and small diameter, while  314  provides less resistance and is sized large enough for the formation of droplets, rather than a continuous flow. As a result, when all fluid in manifold  305  has been pumped outward into channel  310 , flow will cease when the meniscus of fluid entering the structure  306  is at the radial position of the opening of channel  311  into channel  314 . If channel  311  were used without channel  314 , that is, channel  311  continued out to be fluidly connect with overflow reservoir  307 , fluids would be “siphoned” through the cell culturing chamber, eventually emptying it and detection chamber  313  (if present) of all fluid.  
         [0094]    A second reagent, for example a compound whose toxicity is being evaluated with the cells, is now added to port  303 . Through repetition of the above steps, this second reagent enters cell culture chamber  205  and displaces the cell culture medium therein. The device may then be incubated once again such that the adherent cells may absorb or otherwise interact with the compounds of the reagent mix.  
         [0095]    Further reagent additions may occur, for example, of indicator compounds, such as fixing stains that preferentially or differentially stain living or dead cells; color-generating or fluorescence-generating compounds that indicator the presence of specific metabolites generated by cultured cells; and other detection means known to those with skill in the art. The cells may be imaged on the cell supports  206  and their morphology, number, or color determined. The eluent washed into detection chamber  313  may be spectroscopically or fluorometrically interrogated.  
         [0096]    Alternate constructions of the discs of FIGS.  4 - 6  may include, for example, overflow channels and reservoirs as discussed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, the disclosure of which are explicitly incorporated by reference herein., that permits the use of imprecisely metered volumes of fluid. It will also be understood by those with skill in the art that the platform shown in FIGS. 4 through 6 can be constructed to permit multiple assays on multiple cell populations to be performed. This may be accomplished, for example, by providing multiple reservoirs  303  and multiple manifolds  305 . For example, a disc that would be used to run 4 independent assays on 4 populations of 12 cell incubations would comprise 4 reservoirs  303  connected to four manifolds  305 . Each manifold  305  would in turn be fluidly connected to 12 cell assay structures  306 .  
         [0097]    [0097]FIG. 7 is a third alternative embodiment, shown schematically. In this Figure, a microfluidic network (collectively termed “a branching dilution microchannel”) for the creation of a dilution series is illustrated and is part of a larger network of structures used for cell based assays. Reservoirs  601  and  602  are accessed by entry ports  618  and  619  respectively. Fluid channel  603  exits reservoir  601  and is split into two components at T-junction  604 , a portion of which continues to further T-junctions and a portion of which,  607 , terminates at capillary junction  609 . Similarly, reservoir  602  leads to channel  605 , which is split at T-junction  606 ; one arm of the split channel continues to further T-junctions, while the other arm,  608 , terminates at the capillary junction  609 . Following channel  603  past T-junction  604 , it is again split at T-junction  610  into a portion which leads to cell culturing chamber  620 ; the other portion,  611 , terminates at capillary junction  615 . Similarly, channel  605  leads to T-junction  612 , where it is split into channel  613 , which terminates at capillary junction  618  and a portion that continues to cell culturing chamber  631 . The capillary junctions  609 ,  615 , and  616  all are fluidly connected to channels  614 ,  640 , and  641 , respectively. Channels  640  and  641  lead respectively to cell culturing chambers  625  and  629 . Channel  614  is further split at a 4-armed junction  617  into 3 channels: A continuation of  614 , which leads to cell culturing chamber  627 , and side channels  618  and  619  which terminate at capillary junctions  615  and  616 , respectively.  
         [0098]    The cell culture chamber  620  is further connected to channel  621  that emerges radially-inward of the chamber and then leads to waste reservoir  623 ; an air vent  624  is provided for the removal of displaced air. Cell culture chambers  625 ,  627 ,  629 , and  631  are likewise connected via channels  626 ,  628 ,  630 , and  632  to the waste reservoir  623 .  
         [0099]    The fluid channels described here are preferably sized such that the residence time within the channel of a fluid element under centrifugal flow is sufficient to allow diffusional mixing across the diameter of the channel. The design of such mixing elements is defined in co-owned and co-pending U.S. application Ser. No. 09/595,239, filed Jun. 16, 2000, incorporated by reference herein.  
         [0100]    The disc is used as follows. Cell suspensions are loaded into both reservoirs  601  and  602 . Under the influence of rotation, the suspensions are driven into channels  603  and  604 . As they are pumped via centrifugation, the fluids divide and recombine. Since they are identical fluids, identical cell suspension is delivered to all five incubation chambers. In an alternative construction, separate delivery means to the cell culture chambers may be provided, for example, in the form of channels and entry ports leading to the chambers, so that they are individually loaded; or a separate distribution reservoir, potentially on the other face of the microplatform or on another microplatform connected to that shown here. The manner in which multiple liquids are distributed to an arbitrary position on the microdevice by using a three-dimensional network of channels is more fully disclosed in U.S. Serial No. 60/204,272, filed May 15, 2000, U.S. Ser. No. 09/______, filed May 15, 2001 (Attorney Docket No. 95,1408-GGG, and International Application No. PCT/US01/______, filed May 15, 2001, Attorney Docket No. 95,1408-HHH, incorporated by reference herein.  
         [0101]    Cells are allowed to attach to the surfaces of the culturing chambers, and the entire platform can be placed inside an incubator at an appropriate temperature. Exchange of nutrient medium may be accomplished as described for the initial distribution of cellular suspension.  
         [0102]    The disc is then used to create a series of mixtures of two solutions in which the ratio of solutions is varied. If one solution contains a compound suspected of some biological effect on the cells and the other is a buffer or cell culture medium, the series of solutions may be called a dilution series. Fluid A is pipetted through  618  into reservoir  601 , while fluid B is pipetted through port  619  into reservoir  602 . The disc is rotated at a first rotational rate. The fluids enter channels  603  and  605 . Fluid A reaches the T-junction  604 , at which point a portion of the fluid continues down channel  603  and a portion flows into channel  607 . Similarly, fluid B splits at  606  into channels  605  and  608 . The portion of Fluid A present in  607  reaches capillary junction  609 , as does the fluid B present in  608 . As the disc is spun to overcome capillary force at  609 , the fluids are brought together and flow into meandering mixing channel  610 . Mixing in this channel is described in co-owned and co-pending U.S. application Ser. No. 09/595,239, filed Jun. 16, 2000, incorporated by reference herein. The fluid in channel  614 , after sufficient time for diffusional mixing in the channel, arrives at junction  617  with a volume fraction of A equal to 0.5 and B equal to 0.5, i.e., the fluids A and B are “mixed”. The mixed fluid arriving at  617  may be denoted as fluid C 1 .  
         [0103]    Fluid C 1  is split into 3 streams at junction  617 . A portion of that mixed liquid C 1  now mixes with the original A solution which has been directed by channel  603  to junction  610  and channel  611 , by passing through capillary junction  615 . This fluid C 2  in channel  640  has volume fraction of A of 0.75 and B of 0.25. Similarly, the fluid in  641  has volume fraction of A of 0.25 and B of 0.75.  
         [0104]    As shown, the fluidic network delivers 5 concentrations of A—1.0,0.75,0.5,0.25,0.0—to the cuvettes  620 ,  625 ,  627 ,  629 , and  631 , respectively. In order to achieve these ratios, the flow rates of the two fluids entering any mixing channel  614 ,  640 , and  641  must be equal. This is assured by the diameter of the channels, as fluid flow is controlled by the fluidic impedances of the various mixing channels.  
         [0105]    It will be understood that the process of dividing and recombining channels illustrated may be continued indefinitely. One further splitting and recombination in the manner shown would lead to a total of 9 concentrations of A: 1.0, 0.875, 0.75, 0.625, 0.5, 0.375, 0.125, 0.125, and 0.0.  
         [0106]    The following Examples are intended to further illustrate certain preferred embodiments of the invention and are not limiting in nature.  
       EXAMPLE 1  
       [0107]    The disc disclosed in FIGS.  1 - 3  was used in order to illustrate filtration of small particles using a two-level disc construction.  
         [0108]    The Microsystems platform was prepared as follows. The fluidic layers were manufactured through machining of acrylic using computer/numerical code machining using a Light Machines VMC5000 milling machine running Light Machines “Benchman” software (Light Machines Corporation, Manchester, N.H.).  
         [0109]    The particle-capturing cell growth substrate  206  comprised a Whatman Microfibre glass filter with a 2.7 μm diameter pore size. In some experiments these were substituted with Whatman Cyclopore polycarbonate membranes having 3.0 μm diameter pores. This filtering element was sealed into the cell culturing chamber  205  along its circumference  207  using solvent bonding using dimethyl chloride solvent, epoxy, and nail polish for different experiments.  
         [0110]    In some experiments, channel  209  was coated with a hydrophobic coating, PerFluorCoat (PFC MH-Series, Cytonix Corporation, Beltsville, Md. 20705).  
         [0111]    The two discs were affixed to one another using double-sided tape after alignment.  
         [0112]    The dimensions of the platform used for these assays were as follows. The overall platform diameter was 12 cm. Fluidics layer  101  was 12 cm in diameter and 1.5 mm thick, as was layer  199 . The distribution manifold  201  ranged from about 1.5 cm radius to 2.5 cm and subtended an angle of 67 degrees. The manifold was about 1 mm deep, and the sub-volumes  202  were designed to hold 40 μL. The overflow channel  212  had a width and depth of 0.5 mm and there was a capillary junction at  213  that was wider and deeper than channel  212 . The cell culturing chamber  205  had a diameter of 7 mm and a volumetric capacity of 30 μL. Channels  203  and  209  were 0.5 mm wide and 0.5 mm deep. Of these channels, channel  203  was present on layer  101 , while channel  209  was present on layer  199 . In this way, fluid was forced to pass through the filtering element  206 .  
         [0113]    Experiments were performed using fluorescent latex beads of a variety of sizes, ranging from about 4.5 to about 6 μm. Bead solutions containing approximately 0.25% beads (by volume) were added to entry port  218  and the device was spun to 300 rpm. At this rotational speed, capillary valve  213  allowed excess fluid to flow into overflow reservoir  214 . Fluid in the subvolumes  202  were retained by capillary valves  204  until the rotational rate was increased to 500 rpm, at which point they flowed into the chambers  205 . Spinning at 1000 rpm drove fluid through support  206 . The device disc was then placed under a Nikon fluorescence microscope. Comparison of the number of beads detected in cell culture chamber  205  and overflow reservoir  210  showed that more than 90% of the beads were retained by filter  206 .  
       EXAMPLE 2  
       [0114]    The disc shown in FIG. 8 was used for a cell viability and staining assay. This disc is a simplified version that illustrates that cells may be loaded onto, cultured, and maintained within the structures of a microfluidic disc.  
         [0115]    The assay structure consists of a fluid addition port  801  and channel  802  with allows fluids to be added to cell culturing chamber  205 . There is also an air displacement channel  803  and vent  804  for the removal of displaced air. Cell culturing chamber  205  is connected via channel  805  to a waste reservoir  806 . There is also an air displacement channel  807  and vent  808  for the removal of displaced air. Cell incubation chamber was designed to hold approximately 40 μL of liquid.  
         [0116]    The disc was fabricated using CNC machining, as described above, and assembled using double-sided tape. A further passivation step, in which parylene was conformally deposited through chemical vapor deposition, was explored, though found unnecessary.  
         [0117]    The experiments were performed as follows. A mammalian cell culture of 3T3 Swiss mouse fibroblast cells was grown using conventional methods. The cell culture was harvested and diluted to 70 cells/μL. The cells were then loaded into multiple structures comprising cell culturing chamber  205  via port  801 , chamber  802  while the air port  808  was covered, to prevent liquid from flowing into  806 . The disc was then placed in an incubator at 37° C. and incubated for 3 hours. It was removed from the incubator and the vent  808  uncovered. The cell culture medium was spun out at a rotational speed of 1000 rpm into the waste chamber  806 . Live/Dead reagents (Molecular Probes) were then added to cell culture chamber  205 . These reagents are fluorogenic esterase substrates, which, upon diffusion into the cell, are cleaved by nonspecific intracellular esterases, producing fluorescent compounds. As a result, living cells may be fluorescently imaged. The fluorscent compounds rapidly diffuse through the broken cell walls of dead cells, making it possible to distinguish between live and dead cells. The disc was incubated at 37° C. for 30 minutes and examined under a microscope.  
         [0118]    Observing the surface of cell culture chamber  205 , cell counts showed that the number of cells had increased during the incubation, and that the dye had been fixed; an exemplary image is shown in FIG. 9. This result shows that the cells were alive when the dye was added to the cell culture chamber  205 . Furthermore, the experiment shows that assays that had attached to the surface of cell culture chamber  205  remained attached even under centrifugation at 1000 rpm.  
       EXAMPLE 3  
       [0119]    In this example, a simplified device consisting solely of a cell culturing chamber with an entry port and air displacement channel and vent was used to demonstrate the ability to culture cells within a chamber on the device and to detect metabolic processes through the use of an indicating chemistry.  
         [0120]    In this assay, the layer  101  was fabricated and assembled as in the first two Examples. A cell culture was assayed for cell number using a hemacytometer and diluted with nutrient medium/alamarBlue solution to concentrations of 2.5×10 3  cells/100 μL, 5×10 3  cells/100 μL, 7.5×10 3  cells/100 μL, and 10×10 3  cells/100 μL. The alamarBlue/nutrient solution was 10% by weight alamarBlue reagent (Accumed). AlamarBlue reagent is blue in its unreduced, native state. It can function as an oxidizing agent in metabolic pathways, and thereby be reduced to a red form which is also fluorescent.  
         [0121]    [0121]FIG. 10 illustrates a the fluorescence reading for the various cell populations pipetted into optical cuvettes of the disc. The difference in cell numbers is readily detected at the time of cell plating (t 0 ). Fluorescence increases with time, indicating a build-up of reduced product; and the exponential nature of the initial increase shows that the number of cells is increasing in each trace, that is, the cells are living and multiplying. The details of the shapes of the curves at long time are unimportant for this assessment of viability, and are due to exhaustion of reagent and breakdown of the fluorescent product.  
         [0122]    This example demonstrates not only the viability of cells within chambers of the invention, but the ability to monitor their state in real time.  
         [0123]    It should be understood that the foregoing disclosure emphasizes certain specific embodiments of the invention and that all modifications or alternatives equivalent thereto are within the spirit and scope of the invention.