Abstract:
This invention relates to methods and apparatus for performing microanalytic and microsynthetic analyses and procedures. The invention provides a microsystem platform and a micromanipulation device for manipulating the platform that utilizes the centripetal force resulting from rotation of the platform to motivate fluid movement through microchannels. 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. Methods specific for the apparatus of the invention for performing any of a wide variety of microanalytical or microsynthetic processes are provided.

Description:
This application claims priority to U.S. Provisional Applications Serial Nos. 60/436,335 and 60/436,384, each filed Dec. 24, 2002, the disclosure of each of which is explicitly incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to methods and apparatus for performing microanalytic and microsynthetic analyses and procedures. In particular, the invention relates to microminiaturization of genetic, biochemical and bioanalytic processes. Specifically, the present invention provides devices and methods for the performance of miniaturized biochemical 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. Methods for performing any of a wide variety of such microanalytical or microsynthetic processes using the microsystems apparatus of the invention are also provided. 
     2. Background of the Related Art 
     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, for example, 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. 
     More traditional approaches to compound library development are also yielding growing numbers of candidates, including the use of naturally-derived compounds extracted from plants, fungi, and bacteria. In part, this is due to an increased understanding of the function of these compounds, including how they affect the metabolic pathways of the organisms which synthesize and use them; the increasing refinement in identifying and understanding compounds based on small structural and compositional differences; and improved methods for extracting and purifying these compounds. 
     Increased numbers of potential targets for these drug candidates are also being identified. Recent advances in biology, most notably the Human Genome Project, have discovered many molecules whose biochemical activity is implicated in various disease states. Although these novel targets can provide exquisitely precise and specific indicia of how biological processes underlying disease can be effectively controlled and manipulated, drugs must be identified, usually by screening processes, to find compounds that can enhance, diminish, or otherwise alter these targets&#39; ability to affect the metabolic pathways associated with disease. 
     The function of drug candidates, targets, and the effect of the candidates on targets is assessed in the early stages of pharmaceutical development through a process of screening that typically includes: binding of a drug candidate to a portion or domain of the target molecule; immunoassays that bind to drug candidate target domains correlated with drug efficacy; enzymatic assays, in which the inhibition of an enzymatic activity of the target by the drug candidate can be used as a sign of efficacy; protein/protein binding; and protein/DNA(RNA) binding. Additional assays involve the use of living cells and include gene expression, in which levels of transcription in response to a drug candidate are 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. (Wallace &amp; Goldman, 1997, “Bioassay Design and Implementation”, in  High - Throughput Screening: The Discovery of Bioactive Substances , J. P. Devlin, ed., Marcel Dekker, Inc.: New York, pp. 279-305). 
     In initial screening of compounds against targets, the number of possible screens is roughly the number of candidates multiplied by the number of targets. As a result of the growth in both the number of candidates and the number of targets, the number of assays that must be performed is growing rapidly. In addition to the increasing number of assays to be performed, it is desirable to reduce the time required to perform the assays in order to obtain results of such screenings in a timely and useful fashion. Finally, “multiplexing” technology that allows the performance of multiple assays on one sample within a single reaction well—for example, by using readily-distinguishable signals, such as fluorescent moieties with different characteristic wavelengths—can be used to increase throughput. 
     In addition to drug screening assays, biological research has uncovered a vast reservoir of genetic information and diversity having little if any correlation with the function of the gene products encoded by the deciphered DNA. On the one hand, the identification of the nucleotide sequence of the human genome, coupled with bioinformatics analysis of these sequences, has identified a larger number of protein coding sequences (termed “open reading frames”) that can and probably do encode functional proteins. However, since these sequences have been uncovered by simply “reading” a sequence without any information (such as the correlation of a genetic locus with a mutation associated with a disease), the function of the gene products of such a locus must be determined in order to fully understand and identify what protein target is encoded thereby and what utility drug candidates directed to such a target might have. On the other hand, human genome sequencing efforts have also identified genetic mutations (such as single nucleotide polymorphisms, or “SNPs”) that may or may not be associated with human disease. In either instance, the products of this human genetic information must be assayed to determine the activity of the genes, both “wild-type” and mutant, encoded at each new genetic locus. Progress in life sciences research requires researchers to perform large numbers of assays as they investigate the structure and function of proteins coded by the growing number of identified genes in the human genome. Many of the same assays and assay formats used in drug screening may be used in other life sciences research. 
     Large numbers of assays must also be performed in the field of molecular diagnostics, in which individuals can now be assayed for genetic mutation associated with a disease state or the propensity to develop a disease state. For example, any particular disease or propensity for disease may be associated with several different mutations in more than one gene that can determine disease susceptibility or severity. In the monitoring of a disease state, a disease may have a “fingerprint” consisting of certain genes the expression level of which can be used diagnostically to predict the severity of the disease. Monitoring expression levels of these genes can provide an indication of the response (or lack of response) to different treatment modalities. 
     For these and other applications in drug discovery, life sciences research, and molecular and clinical diagnostics there exists a need for systems and assay methods that can perform very many assays in a highly-parallel fashion at low cost. A central strategy has been the miniaturization of existing assays or development of new assays that work with very small volumes of drug compound and reagents. Miniaturization has been accompanied by the development of more sensitive detection schemes, including both better detectors for conventional signals (e.g., calorimetric absorption, fluorescence, and chemiluminescence) as well as new chemistries or assay formats (e.g., imaging, optical scanning, and confocal microscopy). 
     Miniaturization can also confer performance advantages. At short length scales, diffisionally-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. Microfabricated systems also enable assays that rely on a large surface area to volume ratio such as those that require binding to a surface and a variety of chromatographic approaches. 
     The development of fluid-handling and processing for miniaturized assays has primarily involved the scaling down of conventional methods. The vast majority of initial drug screens have been performed conventionally in 96-well microtiter plates with operating volumes of less than 0.1-0.5 mL. The wells of these plates serve as “test tubes” for reactions as well as optical cuvettes for detection. Fluids are typically delivered to these plates using automated pipetting stations or external tubing and pumps; automation is also required for handling of plates and delivery to sub-systems such as plate washers (used in solid phase assays, for example). 
     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. 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. Another drawback of open plates is the existence of a fluid meniscus 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. 
     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 a few microliters) 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. 
     Integration of microdevices with existent laboratory infrastructure is also desirable and has been poorly addressed in the art. This integration is one of both scale and format. Regarding scale, fluids must be transferred to devices from the external world, where the volumes in which they are handled are typically one or more orders of magnitude greater than the volumes required by the microdevice. It is desirable that this transition be done in a way that does not introduce excessively complex processes or machinery and which does not create excessive errors, such as in the volume of fluid transferred. Regarding format, it is desirable that microdevices have a similar physical aspect to macroscale devices already used in laboratories, especially in regard to the manner in which fluids are added to or removed from the devices. Microdevices that can be loaded with fluids using standard methods, such as pipettors, will be more easily and widely used in a variety of settings. 
     Fluid processing in microtiter plates is also difficult. The small dimensions of the wells, while enhancing diffusional mixing, suppress turbulence and make difficult mixing on length scales between a few tens of microns and a few millimeters. For similar reasons, washing, an important step in many assays can be problematic. Methods that reduce both the number of manipulations of fluids on the plate as well as manipulations of the plate itself (such as passing the plate to and from washing stations) can reduce cost while improving assay quality through suppression of contamination, carry-over, and fluid loss. 
     Thus, there is a need in the art for improved micromanipulation apparatus and methods for performing bioanalytic assays more rapidly and economically using less biological sample material and which may be easily interfaced with existing laboratory instrumentation. 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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     SUMMARY OF THE INVENTION 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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, collection chambers, detection chambers and sample outlet ports, generically termed “microfluidic structures.” In particular, the invention provides such microfluidic structures adapted to and arranged to accomplish efficient dilution of a sample with a diluent. Especially provided are such structures that enable serial dilution, for example, over several ten-fold increments (logs) of dilution, providing for efficient dilution of solutes from a sample, most preferably a biological sample. The microsystems platforms of the invention so provided permit microminiaturization of assays, most preferably biological assays, such as, for example, immunoassays, enzymatic assays including enzyme inhibition assays, and specific binding assays of members of a specific binding pair. 
     In the practice of the methods of the invention, the disk is rotated at speeds from about 1 to about 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. Specific sites on the disk also preferably comprise elements that allow fluids to be analyzed, as well as detectors for each of these effectors. 
     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. Secondly, the small dimensions conferred by microfabrication enable the use of “passive” valving, dependent upon capillary forces, over much wider ranges of rotational velocities and with greater reliability than in more macroscopic systems. To this are added the already described advantages of miniaturization. 
     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 as 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. The micromanipulation devices of this invention are more particularly described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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 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 more preferably a multiplicity of collection, and more preferably detection, chambers, and the microfluidics components arranged so that a specific volume of the reagent solution is delivered to each collection chamber. More preferably, said reagent reservoirs are fluidly connected to mixing structures, most preferably a mixing microchannel that is also fluidly connected to a sample reservoir, so that one or a plurality of reagents are mixed with sample and the resulting mixture delivered into the detection chamber. In preferred embodiments, the platform comprises a multiplicity of sample reservoirs and mixing structures fluidly connected with a multiplicity of detection chambers. In additional preferred embodiments, the platform comprises microfluidic structures, including sample reservoirs, diluent reservoirs, dilution reservoirs, and overflow chambers, for the effective and efficient dilution of solutes or particulates from a sample solution or suspension, most preferably a biological solution or suspension. 
     In the use of the platforms of the invention, fluids (including samples 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 the assay, whereby the collection chambers comprise optical cuvettes, preferably at the outer edge of the platform, most preferably wherein the platform is scanned past a fixed detector through the action of the rotary motor. 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. 
     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 a small volume, 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. 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. 
     The disclosed invention also provides microfluidic disc comprising metering structures and a microfluidic network that is 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 one or more common reagents. The fluidic network, defined as the overall pattern of channels, reservoirs, microvalves, and air vents, may be planar or three-dimensional, depending upon the application under consideration. The use of such metering and distribution 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. 
     The microfluidic discs of the invention also comprise microfluidics structures particularly adapted and arranged to provide dilution, most preferably serial dilution, of a solute or particulate from a sample, most preferably a biological sample. Such structures are arrayed in the platform, preferably in conjunction and in fluidic contact with the metering and other microfluidics structures comprising the disc. As provided in further detail herein, said dilution-specific microfluidics structures and arrangements thereof preferably comprise sample reservoirs, overflow reservoirs, one or a plurality of diluent reservoirs, one or a plurality of dilution reservoirs, a metering manifold, wherein the metering manifold is fluidly connected to each of the plurality of dilution reservoirs and the manifold is further fluidly connected to the overflow reservoir, and a plurality of microchannels embedded in the microsystems platform and in fluidic contact with the reagent reservoir, manifold, diluent reservoir, and plurality of dilution reservoirs. As used in the practice of the invention, rotation of the platform motivates flow of a sample and a diluent through the metering manifold and into one or a plurality of dilution reservoirs, and wherein the sample is uniformly diluted from a first concentration in the sample reservoir to a lesser, more dilute sample concentration in the one or plurality of dilution reservoirs. 
     The assembly of a multiplicity of collection chambers on the platforms of the invention also permits simplified detectors to be used, whereby each individual collection/detection 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). 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, 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. 
     Second, the total number of fluid “delivery” events on the microfluidic platform is reduced relative to 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; and are disclosed below. 
     Third, the requirement that samples, particularly biological samples be dilute, especially serially diluted in accordance with the practice of a variety of assays such as immunological detection and specificity assays, is automated using the dilution structures on the disc, that permit effective and automatic (i.e., without user intervention) dilution, particularly serial dilution, from a multiplicity of biological samples. 
     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 
         FIG. 1  depicts an exploded, oblique view of a microsystems platform of the invention. 
         FIG. 2   a  depicts a plan view of the upper face of one component of the microsystems platform shown in exploded, oblique view in  FIG. 1 , the reservoir layer. 
         FIG. 2   b  is a detail of one component of the upper face of the reservoir layer illustrated in  FIG. 2   a.    
         FIG. 3  is a plan view of another component of the microsystems platform of  FIG. 1 , the upper sealing film. 
         FIG. 4   a  depicts a plan view of the lower face of the reservoir layer. 
         FIG. 4   b  is a detail of the lower face of the reservoir layer showing the second reagent addition reservoir and first reagent overflow reservoir. 
         FIG. 4   c  is a detail of the lower face of the reservoir layer showing the first reagent addition reservoir and second reagent overflow reservoir. 
         FIG. 4   d  is a detail of the lower face of the reservoir layer showing one series of sample and reagent reservoirs, channels, and a detection cuvette for a single microdevice that performs a single measurement. 
         FIG. 5   a  depicts a plan view of another component of the microsystems platform of  FIG. 1 , the microfluidic layer. 
         FIG. 5   b  is a detail of one segment of the microfluidic layer of  FIG. 5  comprising the microfluidic channels of a single microfluidic assay structure. 
         FIG. 5   c  is a detail of the microfluidic layer of  FIG. 5  illustrating the overflow valve and channels for the first reagent. 
         FIG. 5   d  is a detail of the microfluidic layer of  FIG. 5  illustrating the overflow valve and channels for the second reagent. 
         FIG. 6  is a segment of the assembled reservoir and microfluidic layers comprising the microsystems platform of  FIG. 1 . 
         FIGS. 7   a  through  7   g  illustrate the sequence of fluid motions as sample, reagent  1 , and reagent  2  are distributed to the reservoirs of the device of  FIG. 1 . 
         FIGS. 8   a  through  8   l  illustrate the sequence of fluid motions into a single microfluidic assay structure of the device of  FIG. 1 . 
         FIG. 9  illustrates a first alternative construction of the microsystems platform in which the sample volume is metered by the construction of the device. 
         FIG. 10  illustrates a second alternative construction of the microsystems platform in which two common reagents are isolated through the introduction of a bubble. 
         FIG. 11  illustrates a third alternative construction of the microsystems platform in which a series of mixtures of two reagents are delivered to the assay structures of the device. 
         FIG. 12  shows a device developed for the performance of 24 parallel assays. 
         FIG. 13  shows a dose-response curve illustrating enzymatic activity as a function of inhibitor concentration for enzymatic inhibition assays performed with the devices of the invention, as disclosed in Example 1 and illustrated in  FIG. 12 . 
         FIGS. 14A through 14I  shows an arrangement of microfluidics structures on a disc of the invention for performing dilution of a biological sample (e.g., a drug). 
         FIGS. 15A and 15B  shows an arrangement of microfluidics structures on a disc of the invention for performing parallel dilution of a biological sample (e.g., a drug). 
         FIGS. 16A through 16C  shows an arrangement of microfluidics structures on a disc of the invention for performing serial dilution of a biological sample (e.g., a drug). 
         FIGS. 17 ,  17 A and  17 B show an arrangement of microfluidics structures on a disc of the invention for performing multiple dilution of a biological sample (e.g., a drug). 
         FIG. 18  is an enhanced view of a structure for a mixing element that can be included in the dilution configuration of the microfluidics structures of the platforms of this invention. 
         FIGS. 19A and 19B  illustrates longitudinal mixing accompanying fluid flow of two fluids mixing in a microchannel ( FIG. 19A ) and mixing of a first fluid with a second fluid located more distal from the center of rotation in a mixing microchannel. 
         FIG. 20  shows a mixing microchannel comprising a center channel and two side channels. 
         FIG. 21  is a stop-action photograph of fluid mixing using the mixing microchannel illustrated in  FIG. 20 . 
         FIG. 22  shows the arrangement of microfluidic structures in a single-level, two-split mixing microchannel. 
         FIG. 23  shows the arrangement of microfluidic structures in a two-level, two-split mixing microchannel. 
         FIG. 24  shows the arrangement of microfluidic structures in a two-level, two-split mixing microchannel. 
         FIG. 25  shows the arrangement of microfluidic structures in a two-level, six-split mixing microchannel. 
         FIG. 26  shows the arrangement of microfluidic structures in a two-level, two-split design fork mixing microchannel. 
         FIG. 27  shows the arrangement of microfluidic structures in an alternative embodiment of a two-level, two-split design fork mixing microchannel. 
         FIG. 28  shows the arrangement of microfluidic structures in a two-level, six-split design fork mixing microchannel. 
         FIG. 29  shows the arrangement of microfluidic structures in a two-level, two-split shuffle mixer design of a mixing microchannel. 
         FIG. 30  shows the arrangement of microfluidic structures in a two-level, six-split shuffle mixer design of a mixing microchannel. 
         FIG. 31  shows the arrangement of microfluidic structures in a single-channel, comb mixer. 
         FIG. 32  shows an alternative embodiment of an arrangement of microfluidic structures in a single-channel, comb mixer. 
         FIG. 33  shows an alternative embodiment of an arrangement of microfluidic structures in a single-channel, comb mixer. 
         FIG. 34  shows an alternative embodiment of an arrangement of microfluidic structures in a single-channel, comb mixer. 
         FIGS. 35A and 35B  are graphs showing the results of the dilution assays performed according to the disclosure in Example 2 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     This invention provides a microsystem platform 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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein, adapted for performing microanalytical and microsynthetic assays of biological samples. 
     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. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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. 
     For the purposes of this invention, the term “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. 
     For the purposes of this invention, the terms “entry port,” “sample input,” 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. 
     For the purposes of this invention, the terms “reagent 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. 
     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. 
     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; Ser. No. 08/768,990, filed Dec. 18, 1996; and Ser. No. 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). 
     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. 
     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. 
     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. 
     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. 
     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. 
     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. 
     The microplatforms of the invention (preferably and hereinafter collectively referred to as “disks” or “discs”; for the purposes of this invention, the terms “microplatform”, “microsystems platform”, “disc” 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. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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 interned 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; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, 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 500μL. 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. 
     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. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996 and Ser. No. 08/768,990, filed Dec. 18, 1996, incorporated by reference). 
     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. 
     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. 
     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. 
     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. 
     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. 
     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 applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser. No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. 
     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 . 
     This invention specifically provides microfluidic structures adapted for and arranged for sample dilution as part of a platform using centripetal acceleration to actuate fluid flow. Structures are described herein that can be used to perform serial dilution, parallel dilution, or a combination of both dilution types. In certain embodiments, the invention provides apparatus that can dilute a single volume of sample fluid into many less concentrated sample volumes by combining various volumes of sample with various volumes of diluent. For accomplishing serial dilution, a volume that was previously-diluted on the platform is further diluted by one or more successive stages of dilution with the same or a different diluent. 
     The apparatus of the invention advantageously permits dilution of a sample, preferably a chemical sample and most preferably a biological sample, requiring a decreased number of liquid handling steps, particularly in instances using distribution of a common reagent. Such a common reagent needs to be manually or automatically pipetted only once or twice onto the platform rather than for each separate assay, as in the case, for example, of microtiter plates. Liquid handling steps are further decreased by allowing the other component of the assay (for example, a drug, compound, or sample) to be diluted directly on the platform, instead of by a liquid handler prior to loading. Because many assays are run with a dilution series of a particular drug, compound, or sample, such an automatic dilution structure is widely applicable to many types of assays. 
     As provided herein, the dilution-specific microfluidic structures are directly incorporated into the platform. Because very small volume features (≦1 nl) can be fabricated in the platform, it is possible to accurate and reproducibly meter such volumes in the platform. This is well beyond the capability of most liquid handling machines, many of which cannot accurately dispense less than 1 microliter. Moreover, although some liquid handlers can dispense less than 1 microliter, they are unable to do so without handling at least 1 microliter of sample, meaning that there is still substantial waste. In contrast, the apparatus provided by the invention can reproducibly and accurately dispense small volumes while utilizing a minimal amount of liquid. Moreover, the apparatus and practice of the methods of the invention permits dilution to much lower concentrations and over a large range of orders of magnitude than most other methods. 
     As disclosed herein, dilution according to the invention is accomplished by loading a sample, for example a drug, directly onto the platform. Subsequently, after motivation of fluid flow by centripetal acceleration the initial drug solution is subsequently diluted to one or more lower concentrations. Each dilution is performed by combining a metered volume of drug with an amount of a diluent until a final metered volume is obtained. Additional dilutions can be performed in two ways. Parallel dilutions can be performed at the same time by changing the amount of drug or diluent that is metered. Ser. dilutions can be performed by using all or a portion of the first dilution in a second dilution stage. In the preferred embodiment, a plurality of dilution chambers are used in a first stage (parallel dilution) and one or more of the volumes of the first-stage dilution chambers are used as sample for a second dilution stage (serial dilution). 
     For high-throughput multiplexed assays, it is convenient to start with the same metered sample volume for each assay. Therefore, in preferred embodiments volumes of drug and diluent are added to provide a final mixed (diluted) volume that is the same in the case of each dilution. When a plurality of dilutions are performed in parallel, the volume of drug to be used in each dilutions varies inversely with the volume of diluent to be used. For example, if a total dilution volume of 2 μl is desired for each dilution, then the sum of volumes of drug and diluent must equal 2 μl. Therefore, as the metered drug volume increases, the metered diluent volume decreases. 
     Platform Manufacture and Assembly 
     Referring now to the Figures for a more thorough description of the invention,  FIG. 1  shows an exploded view of an example of a disc appropriate for large numbers of similar or identical microfluidics structures for performing, inter alia, liquid-phase assays. The disc shown here enables the performance of 96 parallel assays of the same form. The assays have the general form: mix first fluid A with second fluid B, and then mix the combined fluids (A+B) with third fluid C. In practice, the fluid A may actually be a number of fluids, A 1 , A 2 , . . . A i , . . . A n , where n is the total number of assays to be performed on independent fluids. Furthermore, the disc is designed for loading fluids thereupon through ports constructed in the platform. For example, one fluid, A, herein termed “sample”, is loaded through 96 independent entry ports. The second fluid, B, is loaded in a volume somewhat greater than 96 times the volume required for each assay into a single entry port. The third fluid, C, is similarly loaded into a single entry port. The volumes of each of the reservoirs containing fluids A, B and C, or the amount of fluid A, B or C loaded onto the disk, can be different, depending on the requirements of the assay. The disc is configured so that rotation of the disc after loading, under a prescribed rotational profile, effects the following fluid motions: Delivery of individual samples A i  to assay structures within the disc; delivery of metered aliquots of fluid B to individual assay structures within the disc; delivery of metered aliquots of fluid C to individual assay structures within the disc; “isolation” of individual assay structure volumes of fluids B and C through the use of overflow reservoirs which take excess fluid and introduce air bubbles between individual assay structures; and the performance of the assay as described above, wherein the discs effect two fluid mixing steps. The number of fluids and sequence of mixing steps can be arbitrary, e.g., (A i +B)+(B+C). The total number of fluids can also be arbitrary, and the three-dimensional nature of the microfluidic network allows the “crossing over” of numerous channels. Features of this device include: a) “samples” that must be loaded in large numbers can be loaded into a standard format accessible to laboratory robotics or standard automated pipetting systems; b) common reagents need to be loaded only once each, and without high precision. These features integrate the device into existing laboratory infrastructure while providing the advantages of reduced operating time for fluid transfers, enclosed assays for reduction of evaporation and contamination, and the removal of liquid-air meniscus for detection. 
     This disc illustrates that identical assays may be made by repeating assay structures around the disc at a given radius as well as modifying the structures for placement at different radial positions. 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 cay 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. 
     In  FIG. 1 , platform  100  is composed of at least 3 component layers. A reservoir layer  201  having features on the lower face, or both the upper and lower faces, is used. In cases where there are fluidic channels on the upper surface, a sealing film  301  is used to enclose those channels. The “lower” surface of the reservoir layer  201  is bonded to a microfluidics layer  500 . The upper surface of the reservoir layer contains the fluid entry ports; it may also contain one or more distribution manifolds as described herein for distributing one or more common fluids. The bottom face of the reservoir layer, when mated with the microfluidic layer described below, forms a complete network of enclosed channels and reservoirs through which fluids flows under the impetus of centripetal force created by rotation of the platform about a central axis. Fluid flow permits mixing of various component fluids in the assay and movement of the fluids from sample and reagent chambers through mixing structures and into assay reaction chambers. In addition, fluid flow can be effectuated to include incubation and wash steps, using structures disclosed in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000 and incorporated by reference herein. Fluid flow rates range from about 1 nL/s to about 1000 μL/s at rotational speeds of from about 4-30,000 rpm. “Passive” or capillary valves are preferably used to control fluid flow in the platform as described in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, and co-owned and co-pending patent applications U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser, No. 08/768,990, filed Dec. 18, 1996; Ser. No. 08/910,726, filed Aug. 12, 1997; Ser, No. 08/995,056, filed Dec. 19, 1997; Ser. No. 09/315,114, filed May 19, 1999, and Ser. No. 09/858,318, filed May 15, 2001, the disclosures of each of which are explicitly incorporated by reference herein. In the operation of the platforms of the invention, competition between rotationally-induced hydrostatic pressure and the capillary pressure exerted in small channels and orifices are exploited to provide a rotation-depending gating or valving system. After fluids are deposited in detection chambers positioned towards the outer edge of the platform, an optical signal is detected. 
     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. 
     Reservoir Layer 
     The structure of top surface of the reservoir layer  201  is shown in  FIG. 2   a.    
     Reservoir layer  201  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. The layer preferably comprises a center hole  202  for mounting on a spindle, having a diameter of from about 1 mm to about 20 mm. Center hole  202  can be replaced by an extruded fitting for connection to a spindle, or may be absent entirely, in which case registry and connection to the spindle is accomplished using the attached microfluidic layer or another portion of the surface of the platform. 
       FIG. 2   a  illustrates a variety of structures necessary for device function. These include “sample” or fluid entry ports that are comprised of a through hole  204  communicating between the two faces of the disc and in some embodiments a conical or cup-like depression  203 . The depression aids in the placement of pipette tips when the device is used manually. These entry holes are typically arrayed in a rectangular pattern with a spacing to permit an automated pipetting device such as an 8-tip linear or 96-tip rectangular pipetting head to be used. For such devices the spacing of entry ports is typically 9 mm or 4.5 mm, and the arrays are typically 8×12 or 16×24 elements in size. These ports may also be placed in the pattern of a 1536-well plate, which has a spacing of 2.25 mm and consists of 32×48 elements. They may also be placed in an arbitrary pattern for manual use or use with custom devices. The upper surface also contains entry ports for the addition of the two common reagents, comprised respectively of  214 + 215  and  216 + 217 . These ports have dimensions adapted to automated loading devices such as micropipettors, for example, a standard 200 μL plastic pipette tip of tip diameter 1.5 mm; micropipette tips of diameter 1 mm; piezoelectric or ceramic drop delivery systems; and inkjet-based fluid delivery systems. For non-contact delivery systems such as piezoelectric or inkjet delivery, the dimensions of the ports must be a few times greater than the size of the droplets, e.g., 0.2 mm for a 1 nL drop. Corresponding to these latter entry ports are air ports  205  and  206  that allow air to escape from the common reagent reservoirs  401  and  402  (discussed below). The disc is able to distribute three fluids to an arbitrary port only by having a three-dimensional construction: fluid paths must “cross over” one another. As a result, some of the fluid motion occurs on the upper face of  201 . Reagent aliquotting manifold  210  is such a distribution channel; its connection to bulk reagent reservoir  402  is via through hole  207  and exit channel  208 . Along the manifold  210  are vias  211  which penetrate from one side of the disc to the other, allowing fluids to be distributed from the top to the bottom of the disc. The manifold  210  terminates in a via  218  which communicates with an overflow reservoir  403  discussed below. Also visible on the upper surface are air-ports  212  and  213  whose function will be discussed further below. 
       FIG. 2   b  is a detail of the fluid entry to the manifold. The through-hole  207  connects the upper surface of the disc within the channel  208  to the reservoir  402  on the lower surface of the disc. 
       FIG. 3  shows the sealing film  301 . The sealing film is typically of a thin, flexible material that can be sealed to the upper surface of the disc using an adhesive or heat-bonded into place, such that it seals all fluid channels. It is also shaped such that fluid entry ports and air vents are not blocked. 
       FIG. 4   a  illustrates the bottom surface of the reservoir layer  201 . Shown here are a number of through features from  FIG. 2   b , including the entry vias  204 . Also shown are the common reagent reservoirs  401  and  402 . Reservoir  401  distributes fluids along the lower face of the reservoir layer, while  402  distributes fluids through manifold  210  on the upper surface. Also shown are the overflow reservoirs  403  and  404  corresponding to reagent reservoirs  402  and  401 , respectively. 
       FIG. 4   b  is a detail of the region around reagent reservoir  401 . The reservoir is accessed through hole  215  and entry passageway  406 ; this may be shaped as shown to prevent flow of fluid toward the air vent  206 . In addition, the depth of the reservoir  407  can be contoured to prevent fluid from reaching the hole  206  before the remainder of the reservoir has been filled. The reservoir  401  is connected via a channel  408  to a distribution manifold  409  through which the reagent is distributed. The fluid samples enter via ports  204  and channels  411 . 
       FIG. 4   c  is a detail of the region around reagent reservoir  402 . The reservoir is accessed through hole  217  and entry passageway  412 ; this may be shaped as shown to prevent flow of fluid toward the air vent  205 . In addition, a restriction in depth of the reservoir  407  can successfully prevent fluid from reaching the hole  206  before the remainder of the reservoir has been filled. The reservoir  402  also contains a via  207  which communicates with the manifold  210  on the upper surface of the disc. 
       FIG. 4   d  illustrates an expanded view of a section of the reservoir layer showing the reservoirs involved in a single assay. As shown in the Figure, this embodiment of the platforms of the invention contains three reservoirs plus one detection chamber for each assay. Each reservoir has dimensions of from about 0.05 mm to about 5 mm wide, from about 0.05 mm to about 20 mm long, and from about 0.05 mm to about 5 mm thick, and has a volumetric capacity of from about 0.1 nL to about 500 μL. Reservoirs  417 ,  418 , and  416  are designed to contain fluids A i (in some embodiments, this will the a sample), B, and C. For the purposes of this invention, reservoir such as reservoir  418  that is fluidly connected to the reagent manifold is terms an “aliquotted reagent reservoir”. The detection cuvette for this assay is detection chamber  420  with air-port  213  leading to air displacement hole  214 . Air displacement holes  214  that allow air displaced by the motion of fluids to escape, have a cross-sectional dimension of from about 100 to about 500 μm. These holes may optionally be replaced by a manifold or series of channels connecting the receiving reservoirs to one or more air holes. The detecting reservoirs are designed to be accessible to optical interrogation, for example, by being composed of optically-transparent plastics or other materials. Also shown is the distribution manifold  409  that communicates with the filling channel  415 . Filling channel  415  terminates at aliquotted reagent reservoir  418 . At the proximal portion of  415  a narrow passageway  414  is connected to filling channel  415 . Passageway  414  passes through one or more capillary junctions  413  to air port  212 . Reservoir  416  is connected via passageway  419  and via  211  to the distribution manifold  210  on the upper surface of the disc. Finally, reservoir  417  is connected via sample input port  204  to the interface 
     This collection of reservoirs and structures— 416 ,  417 ,  418 ,  420 ,  415 ,  414 ,  413 ,  212 ,  416 ,  419 ,  211 ,  417 , and  411 —is repeated on the illustrated platform of the invention a total of 96 times azimuthally around the disc with an angular spacing of about 3.5°. The 96 arrays are sub-divided into two groups of 48; which are placed with azimuthal symmetry around the disc. Platforms having a smaller or greater number of arrays of such reservoirs are within the scope of the invention being most preferably evenly spaced around the surface area of the platform in configurations that match the pattern of microfluidics components on the microfluidics layer. 
     Microfluidics Layer 
     The microfluidics layer of the embodiment of the platform of the invention is shown in  FIGS. 5   a  through  5   d.    
     Microfluidics layer  500  is optimally of the same lateral dimensions as the reservoir layer. There is also an optional center hole for mounting on a spindle, although this is not required in all configurations. 
     The microfluidics layer contains an array  501  of microfluidic structures  502 , the number of structures in the array being equal to the number of parallel assays to be run on the platform. In the embodiment illustrated in the Figures, there are 96 such structures evenly repeated with angular spacing of about 3.5°. Microfluidics structures  502  preferably comprise microchannels having cross-sectional dimensions of from about 5 μm to about 500 μm and a depth in the microfluidics layer of from about 10 μm to about 3 mm. 
       FIG. 5   b  is an expanded view of a single unit of microfluidic structures. Each microfluidics structure comprises one microfluidics assay. The microfluidic structure consists of depressions in the surface of the microfluidic disc of a single or multiple depths ranging between 2 microns and 1000 microns, while the widths of the depressions varies from about 2 μm to about 500 μm, as further described below. 
     The structure of the microfluidics components of the assay structure are as follows. Microchannels  505  and  506  are aligned by assembly between the reservoir layer and microfluidics layer so that the microchannels protrude into reservoirs  416  and  417 , respectively. Microchannels  505  and  506  in some embodiments narrow to form capillary junctions  508  before joining mixing microchannel  509 . Mixing microchannels are configured to provide mixing of different solutions as the mixture traverses the longitudinal extent of the microchannel. The degree of mixing is dependent on the flow rate of the fluids and the longitudinal extent of the mixing microchannel, which is proportional to the amount of time the two fluids are in contact and are mixed together. The degree of mixing is also dependent on the lateral extent of the mixing microchannel, and is further dependent on the diffusion constants of the fluids to be mixed. In order to accommodate mixing microchannels having sufficient lengths for mixing fluids having a useful range of viscosities, the mixing microchannels are provided as shown in  FIG. 5   b , wherein mixing is promoted as illustrated in  FIG. 5   b  by configuring the microchannel to bend several times as it traverses a path on the platform surface that is perpendicular to the direction of rotation, but extends radially on the surface of the platform from a position more proximal to a position more distal to the axis of rotation. Mixing microchannel  509  has a length of from about 1 mm to about 100 mm, its length in some cases achieved through the use of bends. Mixing microchannel  509  is provided with a capillary junction of a restriction in the lateral dimension at  510  wherein the interior diameter of the microchannel is reduced by about 0 to 95%, and then joins capillary junction  511 . Capillary junction  511  is larger in the lateral or vertical direction or both than the restriction  510 . 
     Mixing in the device is promoted through diffusion. If two small volumes A and B are added to a single container, diffusion of A into B and/or B into A will effect mixing. The amount of time required for this mixing will depend upon the diffusion constants of the molecules within the solutions whose mixing is desired and the distances over which the molecules must diffuse. For example, 0.5 microliter of solution A comprising a molecule with diffusion constant D is added to a reservoir 1 mm on a side. Solution B comprising a molecule whose diffusion constant is also D is added. The solutions will initially occupy the volume with an interface partitioning them. Even if the fluids are highly miscible, the diffusion times to create a completely homogeneous solution will be approximately t=2x 2 /D. For x=0.05 cm (0.5 mm) and D=10 −5  cm 2 /s, the mixing time is 500 seconds, an unacceptably long time for most reactions. This mixing time may be reduced by mechanical stirring, for example, but stirring is difficult to obtain in fluids confined in small structures because the flow of the fluid is laminar and does not contain turbulent eddies that are known to promote mixing. If, instead of placing fluids A and then B in a 1 mm 3  container, fluids A and B were placed side-by-side in a long, thin capillary of lateral dimension d, the relevant time for mixing is much shorter. If, for example, d is 100 microns, mixing time t is 20 seconds. The mixing channels of the device simulate the placement of fluid in a long capillary by co-injecting fluid streams A and B into a capillary microchannel. These fluids flow side-by-side down the channel initially. As the fluid is pushed through the microchannel due to centrifugal force produced by rotation of the platform, diffusion occurs between the fluids. By choosing a capillary of sufficiently narrow diameter, sufficient length, and a pumping rate that is sufficiently low, the portion of A and B of the total volumes of A and B present in the channel during pumping can be caused to mix. 
     These choices may be determined by setting the required time for mixing equal to the amount of time necessary for the fluid to traverse the channel. The required time for diffusion is 
               t   m     ≈       2   ⁢     w   2       D           
where w is the lateral size of the channel. The amount of time necessary to traverse the channel is simply the length of the channel divided by the fluid velocity, the velocity being calculated as described in co-owned and co-pending U.S. Ser. No. 08/910,726, filed Aug. 12, 1997, and Duffy et al. (1999 , Anal. Chem . 71: 4669-4678):
 
               t   t     =       l   U     =       l     (       ρ   ⁢           ⁢     ω   2     ⁢   Δ   ⁢           ⁢   R   ⁢           ⁢     〈   R   〉     ⁢       (     d   H     )     2         32   ⁢           ⁢   η   ⁢           ⁢   l       )       =       32   ⁢           ⁢   η   ⁢           ⁢     l   2         ρ   ⁢           ⁢     ω   2     ⁢   Δ   ⁢           ⁢   R   ⁢           ⁢     〈   R   〉     ⁢       (     d   H     )     2                   
where the fluid properties are the density ρ and viscosity η, ΔR and &lt;R&gt; are the extent along the radius and average radial position of the fluid subject to centripetal acceleration, and l and d H  are the length and hydraulic diameter of the channel. By choosing variables such that t t  is at least equal to or greater than t m , mixing in the microchannels is achieved.
 
     Entry  512  to microchannel  514  protrudes into aliquotted reagent reservoir  418  and preferably forms capillary junction  513 , having dimensions substantially the same as capillary junction  511 . Microchannel  514  passes through a restriction in the lateral dimension at  515  wherein the interior diameter of the microchannel is reduced by about 1-99%, and then joins capillary junction  511 . The capillary junction leads to a further mixing microchannel  516  that terminates at end  517  and that protrudes into detection chamber  420 . Mixing microchannel  516  has a length of from about 1 mm to about 100 mm. 
     Additional structures on the microfluidics disc include overflow reservoirs  503  and  504  as shown in  FIG. 5   c  and  5   d . Each overflow structure abuts at  518  with the terminus of a distribution manifold. Entry  518  passes through passageway  519  to enlargement  520 , forming a capillary junction. This is then connected via channel  521  that ends at  522 , internal to an overflow chamber. 
     Structure of the Assembled Microsystems Platform 
       FIG. 6  illustrates three assay sectors of the assembled platform, in which the reservoirs of the reservoir layer are mated to microchannels from the microfluidics layer. The platform layers are mated as described in more detail below. 
     Because the principles by which the fluidic elements of the platforms are combined are understood, these platforms can be used for a variety of bioanalytical methods. Passive or capillary valving of two fluids to bring them into a channel and the use of that channel to facilitate fluid mixing by diffusion may be used to include any number of fluids, and is not limited to the mixture of two fluids followed by further combination of the first mixture with a third fluid, as illustrated herein. In addition, since the mixing ratios depend on the geometric shapes of the reservoirs containing the solutions to be mixed (as described more fully in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000 and incorporated by reference herein), alternative arrangements of these geometries result in mixing ratios over a large range. 
     Similarly, capillary valving is understood to depend on geometry, fluid properties and rotational rate, as disclosed more fully in U.S. Ser. Nos. 08/761,063, filed Dec. 5, 1996; Ser. No. 08/768,990, filed Dec. 18, 1996; and Ser. No. 08/910,726, filed Aug. 12, 1997, incorporated by reference herein. Alternative arrangements of the microfluidic layers of the platforms of the invention can be provided to contain any number of concentric rings of assays consistent with the amount of surface area available on the platform surface and the extent of the surface taken up by any one embodiment of microfluidics required to perform an assay. 
     The fluid channels described here are preferably of a size 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 application U.S. Ser. No. 09/595,239, filed Jun. 16, 2000, incorporated by reference. 
     An example of alternative platforms for performing assays of the general form disclosed herein, having a number of reservoirs whose volume is equal to the total fluid volume of an assay, VA, connected by microchannels to a collection/detection chamber having the same volume VA. For reservoirs having a common depth, t, and microchannels and inter-assay areas occupying approximately the same area as the reagent reservoirs and collection/detection chambers, the total number of assays possible on a disc of radius R is approximately
 
Number of assays=π R   2   t /(4VA)
 
For a platform having a radius of R=6 cm, a reservoir depth of 0.1 cm and a total fluid volume of 4 microliters, the total number of assays that can be fit onto the disc is 706.
 
     Other considerations include the placement of the components on the platform relative to the axis of rotation. Generally, the collection/detection chamber should be closer to the edge of the platform than the reagent reservoirs, so that there will be sufficient hydrodynamic pressure produced by convenient rotational speeds to motivate the fluid through the microchannels and mixing elements and into the collection/detection chambers. Placement of the collection/detection chambers at the outer edge of the platform also facilitates detection using a fixed optical detector. However, for extremely high-density platforms this may not be the most efficient way to arrange the assay components. For example, if the desired number of assays can be achieved only by placing the collection/detection chambers nearer the reservoirs containing the unreacted samples and reagents, it may be necessary to use a detector that can access cuvettes at a variety of radial and azimuthal positions. An example of a scanning optical system would be one in which the optical signal is scanned radially, while the disc could be indexed beneath the optics azimuthally. In this way the optics can address any point on the disc surface. Scanning methods include a detector on a linear drive that moves radially; alternately, the optical signal may be scanned radially through the use of a galvanometrically-controlled mirror system. 
     The platform of the invention is used for the performance of assays in the following fashion, referring to  FIGS. 7 and 8 , which illustrate fluid motion in the vicinity of the reservoirs  401 ,  402 ,  403 ,  404  and an assay structure, respectively. The assays were run as follows. Aliquots of fluid A are added ports  203 + 204 ; the volume of the individual aliquots are identical, but may range depending upon the design of the device from 1 nL to 50 microliters. The chosen volume for the aliquot is such that, when fluid fills reservoir  417 , it protrudes into  411  for a short distance, typically less than ⅓ of the length of  417 . A total volume of fluid B equal to the number assays multiplied by the volume of fluid B required for each assay, plus an additional volume greater than or equal to the volume enclosed in channels  409  and in all channels  415 , is added reservoir  401  through entry port  215 .  FIGS. 7 and 8  illustrate the sequence of fluid flows in the vicinity of the reservoirs of microfluidic assay elements.  FIG. 7   a  illustrates the flow of fluids entering reservoirs  401  and  402 , as well as of samples of fluid A enter ports  217  and channels  414 . The platform is then placed on a device that is capable of performing rotations. At a first rotational speed ranging from 100-1000 rpm, fluids A are driven through channels  411  into the reservoirs  417 , where they are retained by capillary junctions  508 . This is illustrated in  FIGS. 7   a - 7   d  and  8   a  through  8   d . Also at this rotation rate, fluid B is driven through hole  207  into manifold  210 . As fluid B travels  210 , it flows through the vias  211  into passageways  419  and finally reservoirs  416 , where it was retained by capillary junctions  508 . Fluid B also passes through via  218  and is retained at capillary junction  520  of overflow structure  503 . Also at this rotation rate, fluid C is driven through channel  408  and manifold  409  to the fill channels  414 , and then to aliquotted reagent reservoirs  418 . Fluid in aliquotted reagent reservoir  418  is stopped at capillary junction  513 . Fluid C also enters capillary  414  and is stopped at capillary junction  413 . Fluid C completely fills  409  and is also stopped at capillary junction  520  over overflow structure  503 . The rotational rate is then increased to a second speed. At this speed, the overflow capillary valves formed  520  on the overflow structures  503  and  504  release. Fluid A in manifold  210  flows into the overflow reservoir  403 , leaving behind solution in the reservoirs  416 . Similarly, fluid C flows into reservoir  402 ; as it flows, the fluid in each assay structure at capillary junctions  413  is “pulled”. However, the fact that aliquotted reagent reservoir  418  is radially-outward from  413  means that there is rotationally-induced resistance to drawing the fluid from aliquotted reagent reservoir  418 . The tension in the fluid at  413  is relieved through the introduction of air via the channel and port  212 ; by introduction of this air bubble, solution C within the manifold  409 + 415  is effectively separated from fluid remaining in aliquotted reagent reservoir  418 . The speed is then increased to a third value, at which point the capillary junctions at  508  and  513  allows fluid to flow. The fluids from reservoirs  505  and  506  flow through channel  509  and are halted at the junction  510 ; similarly, the fluid flowing through  514  was halted at the junction  515 . At a fourth rotational rate, the capillary junctions at  511  allow the fluids to flow. The mixing fluids are then pumped via centrifugation into the detection reservoir  420 . In the case of the junctions  507  and  511 , whichever fluid flows first is forced to wet the exit capillary of the other fluid in the capillary junction, thereby inducing it to flow as well. 
     In some alternative constructions, the various rotational rates need not be monotonically increasing. The velocity may be “spiked” momentarily from a low value to high value when a capillary valving event is desired; if it is then reduced quickly to a lower value, the next capillary valving event may be designed to operate at the same rotational rate as the first or even a lower rate. By using the delay time required for fluid to transit from capillary valve to capillary valve in a sequence, a large number of events may be designed to function serially. 
     Three alternative constructions of the fluidic design are now discussed.  FIG. 9  illustrates a detail of the lower face of the fluidics layer  201  of the platform depicted in  FIGS. 1 through 5 . Most of the features illustrated correspond to those of  FIGS. 2 through 4 . It will be understood that the structures in microfluidic layer  500  are sized and spaced so that they mate with those of layer  201  in the preceding example. The additional elements in this embodiment are capillary  901 , a combined capillary junction and overflow chamber  902 , and air-vent  903 . This embodiment is designed to meter an imprecise volume of “sample” fluid for each assay structure. The function of this alternative is the same as in the previous embodiment, with these additional features: At the first rotational speed, sample fluid is delivered via channel  411  to reservoir  417 . As it flows to the reservoir, some fluid enters capillary  901  but is retained at the expansion of  901  into capillary junction  902 . The fluid fills the reservoir and is halted by capillary junction  508  in the microfluidics layer (not shown in this figure). The dimensions of capillary  901  are chosen so that fluid is able to pass opening  902  at a rotational speed intermediate between the first and second rotational speeds discussed above. Displaced air is vented through  903 , and fluid that extends radially-inward of the intersection of channel  411  and capillary junction  902  flows into capillary junction  902 . Air vent  903  may be chosen to be small enough that fluids cannot escape it at any rotational speed normally used for the device. The selection of the diameter of capillary  901  depends on the expected amount of excess volume. For example, for an assay volume of 0.5 μL and a conventional pipetting device used to apply fluids to the disc having a precision of ±0.2 μL. The device may be designed so that the user is required to add 0.75 μL, as long as the volume of channel  901  is 0.05 μL or less. For example, if the radial position of  902  is the same as that of capillary junction  508  on microfluidic layer  500 , the diameter of capillary  901  at capillary junction  902  need only be somewhat larger than that of the channels that meet at capillary junction  507  in order to function. A 100 μm wide and 100 μm deep channel  901  meets requirements. 
     A second alternative construct is shown in  FIG. 10 . In this embodiment, the manifold  210  is positioned radially-outward of reservoirs  416 ,  417 , and  418 . Via  211  connects with channel  1003  that leads radially-inward to a microchannel configuration identical to that associated with aliquotted reagent reservoir  418  described above, consisting of capillary  1004 , capillary junctions  1005 , and air vent  1006 . It will be understood that this structure behaves like that associated with aliquotted reagent reservoir  418  in the previous description, by isolating fluids in the multiple reservoirs  416  from one another. Modifications in which channel  1003  resides on the “upper” face of the layer  201  are within the scope of the invention, as long as a via connecting the fluid path to the lower face is made, for example, at the radially-proximal end of  416 ; this may be advantageous in order to “pack” structures most effectively in the azimuthal direction. 
     This alternate construction may provide advantages for liquids that have unpredictable properties such as viscosity and surface tension. With such fluids, it is possible that bubbles may accidentally be introduced in manifold  210  in the previously described embodiment. These bubbles make draining of the manifold, and isolation of the reservoirs  416 , difficult. By introducing bubbles at each reservoir, the need for maintaining a single plug of fluid in manifold  210  is relaxed 
     A third alternative construction with additional functionality is shown  FIG. 11 . In this Figure, a microfluidic network for the creation of a dilution series is illustrated and is part of a larger network of structures used for various purposes. The fluid distribution scheme illustrated here is advantageous not only for the three-fluid homogeneous assays disclosed herein but can be applied to any centrifugally-based process that requires the creation of such dilution series. 
     In this embodiment, the platform has only three ports and reservoirs for fluid addition: Reservoirs  401  and  402  for common reagents, as previously discussed, and reservoirs  601  and  602  placed on a surface of the platform  201  and accessed by entry ports  618  and  619  respectively. Reservoirs  401  and  402  lead to distribution manifolds, overflows, and assay reservoirs as previously discussed. Fluid channel  603  exits reservoir  601  and is split into two components at what will be called a T-junction  604 , a portion of which continues to further T-junctions and a portion of which,  607 , terminates at a 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 that leads to reservoir  417  of the left-most assay structure  650 . The structure  650 , composed of reservoirs and channels as previously disclosed, embodies the same functionality as that given in  FIG. 6 . Channel  419 , for example, is the passage for the entry of first reagent into reservoir  416 . The other portion of channel  603  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 which continues to assay structure  651 . 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 assay structures  653  and  654 . Channel  614  is further split at a 4-armed junction  617  into 3 channels: A continuation of  614 , which leads assay structure  652 , and side channels  618  and  619  which terminate at capillary junctions  615  and  616 , respectively. 
     The disc may be used to distribute liquids for an assay in the following fashion. Common reagents are loaded into reservoirs  401  and  402 . “Sample” (fluid A) is loaded into reservoirs  601  and a diluent buffer (fluid B) into reservoir  602 . Under the influence of rotation, the common reagents are distributed to reservoirs  416  and  418  as previously described. Sample and diluent 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 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 application U.S. Ser. No. 09/595,239, filed Jun. 16, 2000, incorporated by reference. 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  is denoted fluid C 1 . 
     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, denoted C 2 , is in channel  640  and 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. 
     The functioning of the remainder of the network of channels is clear from this demonstration. As shown, the fluidic network delivers 5 concentrations of A—1.0, 0.75, 0.5, 0.25, 0.0—to the structures  650 ,  651 ,  652 ,  653 , and  654 , 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. 
     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, 0.0625, 0.0. 
     It will further be recognized that this mixing scheme need not be restricted to use with the three-fluid homogeneous assays previously discussed, but may be used to deliver a fluid of arbitrary composition to a point on the platform. 
     A number of variations in fluidic design are possible, either dictated by assay requirements, fluidic requirements, ease-of-use or reduction in automation or all of these factors. For example, capillary valves have been shown to retain fluids in an intermediate chamber at elevated temperatures, used for incubation (as disclosed more extensively in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000, incorporated by reference). Assays that require intermediate incubations, for example, because of slow chemical kinetics, may be performed in such structures. 
     Alternatively, assays for which diffusional mixing is insufficient may require agitation of the fluid to effect mixing. In such a case, active valves can be used, which retain the fluids against the sudden pressure changes induced by agitation, as described in more fully in co-owned and co-pending U.S. Ser. No. 09/315,114, filed May 19, 1999. 
     It may also be desirable to treat the platform surfaces to change the liquid contact angle for controlling capillary valving properties, as disclosed in co-owned and co-pending U.S. Ser. No. 08/910,726, filed Aug. 12, 1997, incorporated by reference. 
     Sample Dilution 
     One embodiment of an arrangement of microfluidics structure adapted for and arranged to accomplish dilution of a sample on the platforms of the invention is shown in  FIGS. 14A through 14I . In this embodiment, sample reservoir  1201  is fluidly connected to input port  1202  and to air displacement channel  1203  and vent  1204 . Sample reservoir  1201  has a volume of 1 nanoliter to 1 ml and dimensions in the platform of 50 microns to 5 mm. Sample reservoir  1201  is directly connected (i.e., without valving) to metering manifold  1205 , having a volume of 1 nl to 1 ml. Metering manifold  1205  is connected to metering channel  1207  through reverse capillary junction  1206 , which provides little to no resistance to flow in a direction away from the center of rotation, but prevents backfilling of the metering manifold. Metering channel  1207  has at its end distal to the axis of rotation a capillary valve  1208 , which prevents fluid flow except when motivated by sufficient centrifugal force from rotation of the platform. Capillary valve  1208  is fluidly connected distally to the axis of rotation to mixing channel  1217 , which is in turn fluidly connected to dilution reservoir  1218 , having a volume of 1 nl to 1 ml. Metering manifold  1205  is also fluidly connected to overflow capillary junction  1212  and diluent capillary junction  1211 , both of which prevent fluid flow except when motivated by sufficient centrifugal force from rotation of the platform. Overflow capillary  1212  is connected to overflow reservoir  1209 , which is vented through vent  1210 . In this arrangement, flow into the overflow reservoir  1209  is restricted by reverse capillary junction  1206 , wherein fluid occupying metering channel  1207  does not drain into or toward the overflow reservoir and an air-liquid meniscus is formed at reverse capillary junction  1206 . In some embodiments, there may exist a spacing  1283  that exists between the diluent capillary junction  1211  and the closest reverse capillary junction  1206 . During the sample filling process, this volume may remain filled with air. Such an air bubble may interfere with the operation of the device from that point forward. Therefore, the introduction of a trap  1282  provides a place for the air bubble to reside to prevent it from interfering with device operation. To be effective, this trap need only appear at any location in the system where the air bubble will pass by. 
     Also arranged on the platform, proximal to the axis of rotation with respect to metering manifold  1205 , is diluent reservoir  1213  having a volume of 1 microliter to 1 ml. Diluent reservoir  1213  is also fluidly connected to input port  1214 , air displacement channel  1215  and vent  1216 , and to diluent capillary junction  1211 . In certain arrangements, diluent capillary junction  1211  permits diluent to flow into metering channel  1207  with adequate pressure to push sample fluid in metering channel  1207  through the meter capillary junction  1208 , through mixing channel  1217  and into dilution reservoir  1218 . Dilution reservoir  1218  is also connected to air displacement channel  1219 , vent  1220  and diluent reservoir capillary junction  1221 . In certain embodiments, dilution reservoir  1218  further comprises capillary junction  1222  that is fluidly connected to assay channel  1223 . In these embodiments, capillary junction  1222  and assay channel  1223  are located on the disk distal to the axis of rotation from diluent reservoir capillary junction  1221 . 
     The practice of the methods of the invention are illustrated for drug dilution in  FIGS. 14A through 14I . Drug is loaded into the sample reservoir  1201  through input port  1202 . Air that previously occupied the sample reservoir will vent through air displacement channel  1203  and vent  1204 . The sample then fills metering manifold  1205 . The sample preferentially fills metering manifold  1205  in a specific order. It first fills metering channel  1207  after passing through reverse capillary junction  1206 , which has minimal to no resistance to flow in the direction from the metering manifold  1205  into the metering channel. The sample fills metering channel  1207  until reaching sample metering capillary junction  1208 . The remainder of the metering manifold  1205  fills up to the diluent capillary junction  1211  and the overflow capillary junction  1212 . Then, the remainder of the sample fluid exits the metering manifold through the overflow capillary junction  1212  into the overflow reservoir  1209 , which is vented through vent  1210 , flow being restricted by reverse capillary junction  1206  to prevent fluid occupying metering channel  1207  from draining into or toward the overflow reservoir and an air-liquid meniscus is formed at reverse capillary junction  1206 . 
     After sample has been distributed on the disk, diluent is loaded into the diluent reservoir  1213  through input port  1214 , and air vented through air displacement channel  1215  and vent  1216 . In alternative and equivalent embodiments, diluent is loaded onto the disc into the diluent reservoir  1213  through input port  1214 , and air vented through air displacement channel  1215  and vent  1216  prior to or concomitantly with sample loading. In these embodiments, capillary valve  1211  is configured to prevent diluent fluid flow through the valve at the disc rotation speeds used to motivate sample fluid flow into metering manifold  1205  and overflow reservoir  1209 . Diluent enters metering manifold  1205  through diluent capillary junction  1211 . Preferably, it first fills metering channel  1207  with adequate pressure to push the sample fluid in metering channel  1207  through the meter capillary junction  1208 , through mixing channel  1217  and into dilution reservoir  1218 . Air that previously occupied dilution reservoir  1218  is vented through air displacement channel  1219  and vent  1220 . Diluent continues to flow into metering channel  1207 , mixing channel  1217 , and dilution reservoir  1218  until flow stops when liquid reaches diluent reservoir capillary junction  1221 . At this point, the remainder of the diluent exits the metering manifold through overflow capillary junction  1212  into overflow reservoir  1209 . As when the sample fluid exits into the overflow reservoir  1209 , the air-water meniscus is located at reverse capillary junction  1206 . The final diluted sample is now located in the volume occupied by dilution reservoir  1218  (which contains the majority of the volume) as well as the volume of mixing channel  1217  and metering channel  1207 . In additional embodiments, the diluted sample may later exit these volumes through capillary junction  1222  and into assay channel  1223 . 
     Alternative embodiments are arranged to provide parallel dilution of a multiplicity of samples on a single disc. One such arrangement is shown in  FIGS. 15A and 15B . In these embodiments, a plurality of metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  are arranged and fluidly connected to sample reservoir  1201 . Each of the metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  may have a different volume, depending on the desired dilution factor. Each metering channel  1207 ,  1224 ,  1225 ,  1226 , and  1227  is preceded by a reverse capillary junction  1206 ,  1228 ,  1229 ,  1230 , and  1231  and leads to a dilution reservoir  1218 ,  1238 ,  1239 ,  1240 , and  1241 . By varying the volume of the metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  but keeping the volume of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  the same, the dilution factor of the resulting fluid volume that fills each dilutions reservoir  1218 ,  1238 ,  1239 ,  1240 , and  1241  will be different. For example, the volume of each dilution reservoir  1218 ,  1238 ,  1239 ,  1240 , and  1241  may be 2 μl. The volumes of the metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  may be set to 0.25, 0.5, 0.75, 1 and 1.5 μl respectively. These will be the total volume of sample that will fill the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241 , respectively. Therefore, the amount of diluent that will fill each of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  is 1.75, 1.5, 1.25, 1.0 and 0.5 μl, respectively. Thus, the ratio of original sample concentration to final diluted concentration in each of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241 , respectively, will be 0.125, 0.25, 0.375, 0.5, and 0.75. The operation of the apparatus and practice of the methods of the invention using these embodiments of the apparatus are as described above, wherein a multiplicity of different dilutions are performed on the same sample. 
     In another embodiment, varying dilution factors are achieved by keeping the volume of each of the metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  the same but varying the volume of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241 . For example, all metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  could have a volume of 1 μl. The dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  could have volumes of 2, 3, 4, 5 and 6 μl, respectively. The residual volume of each dilution reservoir  1218 ,  1238 ,  1239 ,  1240 , and  1241  will fill with diluent. Therefore, the final dilution ratios (as defined herein) of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  will be 0.5, 0.333, 0.25, 0.2, and 0.167. 
     Additionally, the volumes of both the metering channels  1207 ,  1224 ,  1225 ,  1226 , and  1227  and the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  can be varied to achieve the desired array of dilution ratios. One particularly useful function of this embodiment is to provide adequate diluted volume when some of the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  lead to a larger number of fluidic structures than others. For example, a scenario may require 2 μl of each dilution volume, therefore requiring that the dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  have a volume of at least 2 μl. However, if one of the diluted volumes is to be split between providing sample for an assay and some additional function (such as supplying initial sample to a second dilution array), a larger dilution volume is required. It may therefore be desirable in such a scenario to use a range of volumes of dilution reservoirs  1218 ,  1238 ,  1239 ,  1240 , and  1241  of 2, 2, 2, 2, and 4 μl, respectively. As will be evident to those skilled in the art, many other final volumes may be desired for each dilution reservoir  1218 ,  1238 ,  1239 ,  1240 , and  1241 . 
     In some embodiments, it may be desirable to have one dilution reservoir that contains pure, undiluted sample. This is particularly desirable when performing an assay in which is desired to use sample at its highest possible (undiluted) concentration. In such a case, reverse capillary junction  1232  leads directly to metering channel  1233 , where the absence of a mixing channel and dilution capillary junction results in pure diluent flowing through mixing channel  1233 , thereby completely filling dilution reservoir  1234  during the initial sample distribution phase of filling. As with all volumes to be filled, there is associated air displacement channel  1235 , vent  1236 , and capillary valve  1237 , as shown in  FIG. 18B . 
     Conversely, in some embodiments it may be desirable to have one dilution reservoir that contains pure diluent without any sample present. In this case, the reverse capillary junction and dilution capillary junction are combined a single element, capillary junction  1242 . The absence of a metering channel means that no sample will enter the channel and rather than the entire dilution reservoir  1234  will fill with diluent through mixing channel  1243 . 
     In yet further alternative embodiments, microfluidic structures are arranged to provide serial dilution of samples on a single disc. These structure arrangements are shown in  FIGS. 16A through 16C . In these embodiments, dilution reservoir  1218  is fluidly connected to two different microfluidic structures. Dilution reservoir  1218  is connected to feed channel  1244 , passing through capillary junction  1245 , and connected to dilution reservoir  1246 . Dilution reservoir  1246  is connected to air displacement channel  1247 , capillary junction  1248 , and vent  1249 . Dilution reservoir  1246  is further equipped with an assay channel  1250  and capillary junction  1251  for using this dilution volume in an assay or other operation. Dilution reservoir  1218  is also fluidly connected to connecting channel  1223 , capillary junction  1222 , and metering manifold  1252 . Metering manifold  1252  is fluidly connected to metering channel  1257  through reverse capillary junction  1258 . Metering manifold  1252  is also fluidly connected to overflow channel  1254  and overflow reservoir  1255  through overflow capillary junction  1253 . Overflow reservoir  1255  is further connected to vent  1256 . The platform in these embodiments further comprises diluent reservoir  1267 , fluidly connected to input port  1273 , air displacement channel  1271  and vent  1272 . Diluent reservoir  1267  is fluidly connected to channel  1268 , capillary junction  1269 , and channel  1270 , and then to metering manifold  1252 . Metering channel  1257  is fluidly connected to dilution reservoir  1261  through capillary junction  1259  and mixing channel  1260 . Dilution reservoir  1261  is further connected to air displacement channel  1264 , capillary junction  1265 , and vent  1266 . 
     In certain embodiments, dilution reservoir  1261  further comprises capillary junction  1263  that is fluidly connected to assay channel  1262 . In these embodiments, capillary junction  1263  and assay channel  1262  are located on the disk distal to the axis of rotation from diluent reservoir capillary junction  1261 . 
     Alternatively (as shown in  FIG. 16B ), diluent may be provided to the second stage of dilution through secondary diluent channel  1274  and capillary valve  1275 , connected directly to metering manifold  1252 . In other alternative embodiments (shown in  FIG. 16C ), diluent can be provided to the second stage of dilution through the first diluent reservoir  1213 , with diluent traveling through all of the intermediate microfluidic structures to reach dilution reservoir  1261 . 
     In the practice of the serial dilution methods of the invention, after dilution reservoir  1218  is filled, diluent first enters feed channel  1244 , passing through capillary junction  1245 , and fills dilution reservoir  1246 . Air in any or all of these microfluidics structures is vented through air displacement channel  1247 , capillary junction  1248 , and vent  1249 . The remainder of the volume contained in dilution reservoir  1218  flows into connecting channel  1223 , passed through a capillary junction  1222 , and enters a metering manifold  1252 . The filling process is identical to that described above. Sample fills the metering manifold  1252  and enters a metering channel  1257  through a reverse capillary junction  1258 . The remainder of sample then passes through overflow capillary junction  1253 , into overflow channel  1254 , and overflow reservoir  1255 . Air escapes the overflow reservoir through vent  1256 . Next, diluent is provided from diluent reservoir  1267 , which was loaded through input port  1273 , utilizing air displacement channel  1271  and vent  1272 . Diluent flows through channel  1268 , capillary junction  1269 , and channel  1270 , and then enters metering manifold  1252 . Dilution reservoir  1261  is then filled by a mixture of sample and diluent through dilution capillary junction  1259  and mixing channel  1260 , and air is displaced through air displacement channel  1264 , capillary junction  1265 , and vent  1266 . Assay channel  1262  and capillary junction  1263  are used for removing fluid from dilution reservoir  1261  for an assay or other purpose. Alternatively, diluent can be provided for the second stage of dilution through secondary diluent channel  1274  and capillary valve  1275 , which connects directly to metering manifold  1252 . Alternatively, diluent can be provided to the second stage of dilution through the first diluent reservoir  1213 , with diluent traveling through all of the intermediate microfluidic structures to reach dilution reservoir  1261 . 
     The invention also provides embodiments of such serial dilution arrangements comprising a plurality of such serial dilution stages, all connected in a serial fashion. In these embodiments, volumes taken from each serial dilution stage (such as in the dilution reservoir  1246 ) can be used for several subsequence operations, including further dilution. In addition, parallel dilution can be further performed using any or all of the serial dilution stages such that multiple dilutions are formed during any or all of the dilution stages. Such an arrangement microfluidic structures illustrating these embodiments is shown in  FIG. 17 , having a first dilution stage with 6 parallel dilutions. The sixth such parallel dilution provides the input for the sample reservoir of the second dilution stage, which itself performs 5 dilutions. Note that this Figure also illustrates other reagent reservoirs  1301 - 1310  that contain a second assay component to be mixed with each dilution. 
     An example of such embodiments as shown in  FIG. 17  are microfluidic platforms that integrate aqueous drug dilution into a disc design comprising a 96-well cuvette format. This design advantageously permits dilution to be performed in the disc, rather than, for example, using off-disc microtitre plates or other dilution fluidics. One advantage of such integration of sample dilution on the disc is an increase in experimental throughput due to the elimination of off-disc serial dilution processed and also the reduction in significant disc loading times. Further advantages are the ability to meter and distribute extremely small volumes (≦15 nl) of sample without the need for complex liquid handling systems, therefore increasing the reliability of the data at high dilution. 
     The initial drug solution is loaded onto the disc and subsequently fills a distribution manifold, which then leads to the filling of various defined ‘metering’ volumes, as described above. It is the volume of the metering chamber (along with the volume of the subsequent intermediate cuvette) that defines the dilution ratio for that step. Since the metering volumes are independent of each other, these structures in this arrangement avoids propagation of error that can result from microtitre plate-based serial dilution methods. Any dilution errors that arise using the microsystem platforms in this embodiments of the invention will be due to production errors in defining the metering chamber volume and any errors in filling or emptying the chamber, all of which can be minimized by quality control measures. 
     Dilution microfluidics structures are arrayed to perform dilution in two stages, schematically shown in  FIG. 17A . This Figure illustrates structures useful in the practice of microsystems platforms equivalent to a 96-well microtitre plate format. This format requires 2 μl final drug sample volumes, and for this volume it is near impossible to meter volumes small enough to obtain anything close to 5 logs (100,000-fold) of dilution in a single stage. Therefore, the first stage performs 5 dilutions in half log increments for a total of 2.5 logs of dilution plus one volume that is undiluted. The 2.5 log dilution point is then the inlet reservoir for the 2 nd  stage, which is a repeat of the first stage in that an additional 4 half log increments of dilution are performed plus one volume that is undiluted (the 2.5 log dilution concentration from the 1 st  stage). This results in a 4.5 log (˜32,000) dilution, plus one volume for the originally loaded drug sample. 
       FIG. 18  shows a close-up view of a portion of the dilution structure shown in  FIG. 17 . Specifically,  FIG. 18  shows an additional component a mixing element  1281  that can be located within mixing channel  1279 . This mixing element may help ensure that the sample and diluent components are more fully mixed as the fluids are processed through the dilution apparatus. Specifically, when both fluids reach the dilution reservoir  1238 , it is preferable that they be fully mixed before any further fluid flow occurs. 
     Dilution of the original drug sample occurs over the steps depicted in  FIG. 17B . Drug and diluent are loaded into the sample reservoir and diluent reservoir(s), respectively. Next, upon initiation of the spin profile, the drug sample fills the distribution manifold, the channel to the overflow reservoir, the metering chambers, and the intermediate cuvette. As the drug fills the manifold, it will trap an air bubble in the diluent valve. Once the metered volumes are filled with the drug sample, the valve holding the diluent reservoir breaks. Then the bubble trapped in the diluent valve pushes down the manifold while the trailing diluent washes out the drug sample from the manifold and into the overflow reservoir (the drug sample remains in the metering chambers). The bubble, when at the end of the manifold gets trapped in the bump-out in the manifold near the drug inlet. This prevents the bubble from blocking the channel leading to the overflow chamber. Next, the rotation rate of the disc is increased and the exit valve to the metered chambers breaks and the corresponding drug samples are diluted by the diluent as the fluids fill individual intermediate cuvettes. The exits to the intermediate cuvettes are valved. Finally, the remaining diluent and drug flow out of the manifold and into the overflow chamber, thus breaking the fluid trail above each dilution volume. This process is repeated for the second stage, while the diluted drug samples remain in the intermediate cuvettes of the first stage. 
     The degree of dilution, or dilution ratio, DR, can be theoretically predicted based on the volume of the metered chamber, V meter , and the volume of the intermediate cuvette, V int , that the fluid enters according to the following formula 
     
       
         
           
             DR 
             = 
             
               
                 V 
                 meter 
               
               
                 
                   V 
                   meter 
                 
                 + 
                 
                   V 
                   int 
                 
               
             
           
         
       
     
     As provided herein, the mixing element is any means that performs mixing on the fluids. In some cases, mixing may be thorough mixing while in other cases it may be only partial mixing. There are numerous microfluidic structures that can act as a means for performing mixing. Some mixers are transverse, in the sense that fluid elements are mixed in a transverse fashion relative to the flow direction. In other cases, mixing is axial, in the sense that fluid elements are mixed along the direction of flow. These different mixing alternatives are illustrated schematically in  FIGS. 19A and 19B . 
     In certain embodiments, the mixer is a mixer described in co-owned U.S. provisional patent application Ser. No. 60/436335 (filed Dec. 24, 2002, incorporated by reference). The structure of this mixer is shown in  FIG. 20 . This mixer is primarily an axial mixer, although it has a transverse component as well. The mixer can be placed in line with any microfluidic channel and provides three paths for fluid to flow, some with different residence times. As will be evident to those skilled in the art, such a mixing element could be comprised of as few as 2 channels or a substantially larger number of channels, from about 4 to 100. The differing flow residence time of the legs of this mixing element is shown in  FIG. 21 . 
     Another in-line mixer element is one that splits and then recombines fluid flow. In some instances, flow is spilt into smaller channels so that diffusion distances to achieve mixing are shorter, thereby decreasing the time or flow distance for mixing (see Bessoth et al., 1999 , Anal. Comm . 36: 213-215). In other cases, bends or turns are introduced into the microchannel to induce rotation and/or helical flow in the fluids to increase mixing (see Liu et al., 2000 , J. Microelectromechanical Systems  9: 190.). In other cases, grooves or other non-uniformities are introduced into the microchannel to induce ebbing, rotation, and/or helical flow (see Stroock et al., 2002 , Science  295: 647). 
     All of the above mixers can be used separately or in combination to provide some degree of in-line mixing. Numerous examples of these types of mixers are illustrated in  FIG. 22-34 . 
       FIG. 22  shows the structure of a single-level split-mixer, wherein each of two fluids enter through their own respective ports, and combine together in co-parallel laminar flow. The main channel, which has cross-sectional dimensions of 250 μm by 250 μm, is periodically interrupted by bifurcations, where half of the flow in the main channel is directed to each branch of the bifurcation. The flow then recombines into a merged portion of the main channel. 
       FIG. 23  shows the structure of a double-level split-mixer, wherein each of two fluids enter through their own respective ports, and combine together in co-parallel laminar flow. The main channel, which has cross-sectional dimensions of 250 μm by 250 μm, is periodically interrupted by bifurcations, where half of the flow in the main channel is directed to each branch of the bifurcation. In this case, the thickness of each branch of the bifurcation is halved at approximately the halfway point of the length of each bifurcation branch. Because the narrower half of each bifurcation branch is located in a different level, one branch has its narrow channel in the upper level, while the other branch has its narrow channel in the lower level. The flow then recombines into a merged portion of the main channel. At the next bifurcation, the arrangement of narrowed channels is reversed such that the channel narrowings are located in the opposite layer than in the previous bifurcation. 
       FIG. 24  shows an alternate design of the double-level split-mixer that is identical to that of  FIG. 23  except that the channel narrowing are not alternated, such that the narrow channels on either side of the main channel are always located in either the top or bottom layer, respectively. 
       FIG. 25  shows a double-level split-mixer design of  FIG. 23  extended to show 6 consecutive subunits of the mixer. 
       FIG. 26  shows a fork split-mixer, wherein each of two fluids enter through their own respective ports, and combine together in co-parallel laminar flow. The main channel, which has cross-sectional dimensions of 125 μm by 250 μm, is interrupted by a single bifurcation, where half of the flow in the main channel is directed to each branch of the bifurcation. Each branch is fluidly connected to a second main channel, where the fluid from the two branches recombines in a serial format. This differs from the examples shown in  FIGS. 22-25  in that those examples show recombinations that occur in a parallel format. The second main channel represents a second subunit, where flow is later bifurcated again. 
       FIG. 27  shows the structure of a split-mixer is identical to the example in  FIG. 26  except that the main channel cross-sectional dimensions are 50 μm by 250 μm, and  FIG. 28  shows the structure of 6 subunits of the mixer shown in  FIG. 26 . 
       FIG. 29  shows the structure of a shuffle-mixer, wherein each of two fluids enter through their own respective ports, and combine together in co-parallel laminar flow. The main channel, which has cross-sectional dimensions of 125 μm by 250 μm, connects fluidicly to an S-shaped channel located in the other layer. The end of this S-shaped channel connects to a second S-shaped channel located in the original layer. The overall affect is a spiral-like pathway that caused the fluid to take sequential turns in different directions. 
       FIG. 30  shows the structure of 6 subunits of the mixer shown in  FIG. 29 . 
       FIG. 31  shows the structure of a single-channel comb mixer, wherein each of two fluids enter through their own respective ports, and combine together in co-parallel laminar flow. The main channel, which has cross-sectional dimensions of 50 μm by 50 μm, has associated chambers located on either side of the main channel in alternating fashion. Such features increase the average residence time inside the device, while also causing a spatial shift in the fluids that promotes mixing. 
       FIG. 32  shows the structure of a modification of the mixer in  FIG. 31 , where the main channel thickness has been increased to 100 μm. 
       FIG. 33  shows the structure of a top-bottom inlet design of the comb mixer, wherein each fluid enters through multiple, spaced inlet ports. Each of the ports for at least one fluid are on the top side of the device, while each of the ports for another fluid are located on the bottom side of the device. Each inlet port has cross-sectional dimensions of 50 μm by 50 μm. Ultimately, all of the fluid segments merge into a laminar flow field that enters the main channel. The main channel converges in one dimension in step-wise fashion, resulting in a triangular-shaped planar channel.  FIG. 34  shows the structure of a flat comb mixer, wherein all of the input fluids enter in a single plane, rather than from the op or the bottom. In this case, multiple input ports are arranged in a linear array, with each fluid entering through alternating input ports. All of the input fluids merge in a laminar flow field that enters the main channel. As in the example in  FIG. 33 , the main channel converges in one dimension in step-wise fashion, resulting in a triangular-shaped planar channel. 
     It will be recognized by those with skill in the art that a plurality of such arrangements of microfluidic structures can be fabricated on a platform disk according to the invention, permitting parallel dilution series of a plurality of different samples to be performed simultaneously or on the same platform. 
     The following Examples are intended to further illustrate certain preferred embodiments of the invention and are not limiting in nature. 
     EXAMPLE 1 
     Simultaneous Enzyme Inhibition Assays 
     A platform depicted in the  FIG. 11  was used in this example. The platform of  FIG. 11  is functionally identical to that of  FIGS. 1-8 , the only significant difference being that that of  FIG. 11  is designed for the performance of 24 assays, while that of  FIGS. 1-8  is designed for the performance of 96 assays. 
     The platform was prepared as follows. The microfluidic reservoir layer  201  was 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.). 
     The sealing film of as shown in  FIG. 3  was made by applying a double-sided tape to a thin sheet of heat-stabilized polyester (mylar). A section of mylar bonded to tape was cut from the combined sheet to the correct shape, leaving one adhesive face of the tape for application to the macrofluidic layer. 
     The microfluidics layer was manufactured as follows. A microfluidics structure such as the structure shown in  FIGS. 5   a  through  5   d  was designed using in a computer aided design package such as AutoCAD (Autodesk, San Rafael Calif.) and Freehand (Macromedia Inc., San Francisco, Calif.). This design was converted into a photomask by printing at high resolution (3386 dpi) on a transparent plastic sheet. A 125-mm diameter silicon wafer was coated with a layer of negative photoresist (SU-8(50)) and spun on a spin-coater (Chemat Technology, Northridge, Calif.) at a speed sufficient (200 to 8000 rpm) to give a desired thickness between 5 μm and 500 μm. The silicon wafer was baked to have a smooth surface and then the photoresist partially cured. The silicon wafer was exposed to ultraviolet (UV) light using a conventional UV source and mask aligner. The photoresist was then developed in propylene glycol methyl ether acetate and non-crosslinked photoresist removed through washing in dichloromethane. The resulting relief was then passivated by exposure to a vapor of tridecafluoro- 1,1,2,2-tetrhydrooctyl-1-trichlorosilane and used as a mold for microfabrication (Duffy et al., 1998 , Anal. Chem . 70: 4974-4984). 
     A 10:1 mixture of polydimethylsiloxane (PDMS) oligomer and crosslinking agent (Sylgard 184, Dow Corning) was poured onto the mold after degassing under vacuum. PDMS is a clear material; by adding 1 wt % liquid pigment (containing TiO 2 ), the disc was made white for reflectance optical measurements and black for fluorescence measurements. The poured elastomer was then cured at 65° C. for 1 hour. The resulting microfabricated PDMS part was peeled from the mold. The mold could then be re-used to fabricate additional copies of the microfluidics layer. 
     The microplatform was assembled in the following way. Sealing film with adhesive exposed was first applied to the upper surface of the macrofluidic layer so that it completely covered the holes and channels  207 ,  208 ,  211 ,  218 . Final assembly was completed by forming a reversible, conforming seal between the PDMS microfluidics layer and the bottom surface of the microfluidic reservoir layer  201  made through simple physical contact of the two components. This seal is based on physical adhesion forces alone—van der Waals attraction forces and potentially static electrical charge present on the surfaces—and was sufficient to seal the disc against leakage due to the centripetally-induced pressures used. 
     The platform shown in  FIG. 11  and prepared as described herein was used to perform simultaneously and in parallel 24 enzyme inhibition assays. Fluids were deposited in the reservoirs formed in reservoir layer  201  when reservoir layer is mated or bonded with microfluidics layer  500 . Platform  100  is then rotated using a rotational profile designed to drive fluids through the channels within the macrofluidic disc  201  and the microfluidic disc  500 . 
     The platform shown in  FIG. 11  possesses the same features as that shown in  FIGS. 1 through 6 , and reference to features will be made those on the latter figures. It was used to perform 24 simultaneous enzyme inhibition assays as model homogeneous assays. In an enzyme inhibition assay, the effect of a compound present in a first fluid (“A”) upon the capacity of an enzyme present in a second fluid (fluid “B”) to catalyze a reaction, typically of a substrate in a third fluid (“C”) is determined. The reaction was chosen to give a change in a readily-measured parameter of the fluid, such as its optical density, or to produce a fluorescent moiety. When no inhibitor was present in fluid A, mixing solution A with enzyme solutions had no effect: The enzyme activity detected in this assay was the maximum detected and provided the largest change in the measured parameter. However, if an inhibitor was present in fluid A, mixing fluid A with fluid B will, after a sufficient time resulted in a chemical reaction or other change induced by the inhibitor in most or all of the enzyme molecules, rendering them incapable of catalyzing the desired reaction. If this solution was mixed with the substrate solution, little or no change in the measured parameter was seen. 
     The system chosen to model homogeneous assays consisted of theophylline as inhibitor, alkaline phosphatase as the enzyme, and p-nitrophenol phosphate (PNPP) as the substrate. In the presence of alkaline phosphatase, PNPP, which is colorless, is converted to p-nitrophenol (PNP), which absorbs in the blue and therefore appears yellow. Theophylline was used in concentrations of 0.01 mM to 100 mM to provide a standard dose-response curve in the inhibitor. Alkaline phosphatase was used in a 1 mg/mL solution. PNPP was used as a 0.5 mM solution. All solutions were made in a buffer of 0.1 M glycine and 0.5 mM MgCl 2  in deionized water. 
     The dimensions of the platform used for these assays were as follows. The overall platform diameter was 12 cm. The macrofluidic disc was about 1.6 mm thick. The diameter of the sample entry ports through-holes  204  was 0.5 mm; the depression  203  was conical, with an outer diameter of 2 mm and a depth of 1 mm, allowing the cones to “guide” the placement of pipette tips when loaded manually. The common reagent entry ports consisted here of only through holes  215  and  217 , also of diameter 0.5 mm. The air ports  205  and  206  were also 0.5 mm in diameter. The reagent manifold through-hole  207  was also 0.5 mm in diameter, and the exit channel  208  was of larger diameter and depth of 0.43 mm. The exit channel narrowed to join the channel  209  of width 0.45 mm. The manifold  210  was also 0.43 mm deep and 0.45 mm wide. The vias  211  were 0.45 mm in diameter and penetrated through the macrofluidic disc from the top surface to the bottom surface. The terminal via  218  had a diameter of 0.53 mm and penetrated to the bottom surface of the layer. 
     The dimensions of the features on the lower face of the macrofluidic disc were as follows. The common reagent reservoirs  401  and  402  had a depth of 1.14 mm. The radial positions of the ends of these reservoirs most proximal to the center of the disc were 1.59 cm and that of the ends most distal from the center of the disc were 3.4 cm. The volume allowable in reagent reservoir  402  was 1 mL, and that of reservoir  401  was 1 mL. Entry passageway  406  was 0.25 mm deep and 0.25 mm wide. Channel  408  was 0.25 mm wide×0.25 mm deep, as was manifold  409 . Overflow reservoir  403 , corresponding to reagent reservoir  402 , had an outer radius of about 5.6 cm and an inner radius of about 4.3 cm and a depth of 1.5 mm. The angle subtended by the reservoir was chosen to accommodate a volume of 30 microliters. Similarly, overflow reservoir  404 , corresponding to reagent reservoir  401 , had an outer radius of about 5.6 cm and an inner radius of about 4.3 cm and a depth of 1.5 mm. The angle subtended by the reservoir was chosen such that it could accommodate a volume of 30 microliters. Air holes  410  had a diameter of 0.5 mm. 
     Referring to  FIG. 4   d , the features of the individual assay structures were as follows. Entry channels  411  were 0.25 mm wide×0.25 mm deep; their lengths varied according to the relative positions of the reservoir  417  and entry port  204  that each channel connected, from about 12 mm to about 27 mm. Channel  411  preferably opens at its connection to reservoir  417  such that acute angles are not presented within the plane of the disc that might impede fluid flow. The inner radii of reservoirs  416 ,  417 , and  418  were about 4.1 cm and outer of reservoirs  416 ,  417 , and  418  were about 4.5 cm. Both inner and outer ends of the reservoirs were rounded with radii of 0.5 m to prevent fluid from both “stopping” at the inner end during loading due to capillary forces and being retained as the reservoirs emptied. Reservoirs  416 ,  417 , and  418  were about 0.57 mm, 0.57 mm, and 1.14 mm deep, respectively. Each reservoir was 0.6 mm in width. The lengths, depths, and widths of the reservoirs were chosen such that the volumes contained within  416  and  417  were 0.91 microliters while that contained with aliquotted reagent reservoir  418  was twice that volume, 1.82 microliters. Detection reservoirs  420  were constructed of optically-transparent material and had an outer radius of about 5.7 cm and an inner radius of about 5.2 cm, were 0.7 mm deep, and had a subtended angle of 3.2° and were thus designed to accommodate the combined volumes of reservoirs  416 ,  417 , and  418  (approximately 3.6 microliters). Distribution manifold  409  is connected to aliquotted reagent reservoir  418  via a filling channel  415  which as 0.25 mm wide×0.25 mm deep; the end of  415  was also enlarged at its connection to aliquotted reagent reservoir  418 . Capillary channel  414  was 0.13 mm wide×0.13 mm deep. The capillary junctions  413  were 0.25 mm deep×0.5 mm wide; they were formed so that the opening of  414  into  414  formed a backward angle of 45°, thus increasing the capillary stopping power of the junction. The connection to reservoir  416  is via passsageway  419 , which was about 0.25 mm deep and widened by 0.25 mm wide to 0.6 mm wide at its joining with  416 . 
     The microfluidics layer  500  was also 12 cm in diameter and had a thickness between 1 and 2 mm (although the thickness is not important) and was composed of white PDMS. The depth of all microfluidic structures (that was determined by the height of the SU-8 relief) was 100 μm. The width of the entrance to the mixing channels,  505 ,  506 , and  512 , was 200 μm. The channels narrowed to 100 μm before reaching the capillary junction at  508 . The width of the junction  508  was 120 μm. Similarly, the entrance  512  narrowed to 100 μm prior to the junction  513  that was 200 μm. The widths of channels  509  and  514  was 100 μm. The lengths of the mixing channels was chosen to provide sufficient time for mixing via diffusion with liquids of moderate diffusion constant (5×10 −6  cm 2 /s) as fluids are pumped through them under the influence of centripetal acceleration. The lengths of channels  509  and  514  was about 17 mm. These channels narrowed to 50 μm at  510  and  515  before joining the junction  511 , which was about 0.4 mm square. Channel  516  was 100 μm wide and about 38 mm long. These dimensions resulted in the fluids taking ≧2 sec to transit the mixing microchannels. 
     Also on the microfluidic layer are the overflow structures  503  and  504 . The entry  518  was 0.4 mm×1.8 mm, while the capillary passageway  519  was 120 μm wide. Enlargement  520  was about 200 μm wide by 200 μm long, and  521  was also 100 μm long. 
     The assays were run as follows. 1 μL aliquots of theophylline solutions having the concentrations set forth above were loaded into entry ports  203 + 204  using a pipette. Alkaline phosphatase was loaded into reservoir  402  through entry port  217 , and PNPP solution was loaded into reservoir  401  through entry port  215 . The platform was placed on the spindle of an instrument containing a diffuse reflectance optical head capable of three-color measurements.  FIGS. 7 and 8  illustrate the sequence of fluid flows in the vicinity of the reservoirs of microfluidic assay elements. The platform was first rotated at 300 rpm for 30 seconds. At this rotation rate, the theophylline solutions were driven completely through channels  411  into the reservoirs  417 , where it was retained by capillary junctions  508 . Also at this rotation rate, the alkaline phosphatase solutions were driven through hole  207  into manifold  210 . As the alkaline phosphatase solution traveled  210 , it flowed through the vias  211  into passageways  419  and finally reservoirs  416 , where it was retained by capillary junctions  508 . Also at this rotation rate, the PNPP solution was driven through channel  408  and manifold  409  to the fill channels  415 . The PNPP solution was driven through  414  to the aliquotted reagent reservoirs  418 ; PNPP solution also entered capillary  414  and was stopped at capillary junction  413  and at capillary junction  513 . The rotational rate was then increased to 500 rpm. At this speed, the overflow capillary valves formed by  519  and  520  on the overflow structures released. The alkaline phosphatase solution in manifold  210  then flowed into the overflow reservoir  403 , leaving behind solution in the reservoirs  416 . At the same time, the draining of excess PNPP solution into the overflow reservoir  402  exerted an inward “pull” on the solution at the capillary junctions  413 . The tension in the fluid in channel  415  so created was relieved through the introduction of air via channel  413 ; this effectively separated the draining PNPP solution in  415 + 409  from the solution in the aliquotted reagent reservoirs  418 . The speed was increased to 600 rpm, at which point the capillary junctions at  508  and  513  allowed fluid to flow. The fluids from reservoirs  505  and  506  flow through channel  509  and were halted at the junction  510 ; similarly, the fluid flowing through  514  was halted at the junction  515 . At 700 RPM, the capillary junctions at  511  allowed the fluids to flow. The mixed fluids were then pumped into the detection reservoir  420 . In the case of the junctions  507  and  511 , whichever fluid flows first is forced to wet the exit capillary of the other fluid in the capillary junction, thereby inducing it to flow as well. 
     An important feature of mixing in the device is made possible through the narrowness of microchannels  509 ,  514 , and  516 . The resistance to flow due to rotationally-induced pressure of a channel that is denoted by R H  is given by 
             Q   =     P     R   H                     R   H     =     C   ⁢           ⁢     l       (     d   H     )     4               
where Q is a flow-rate, P is the induced pressure, C is a constant, l is the length of the channel through which the fluid flows and d H  is the hydraulic diameter. Because the diameter of microchannels  509 ,  514 , and  516  are much narrower than that of the reservoirs  416 ,  417 , and  418 , resistance to flow is dominated by the microchannels, and hence the pressure drop across the flowing fluid is sustained almost exclusively over the length of the mixing microchannels. This insures that the fluids flowing from reagent reservoirs into mixing channels do so in a strict, known ratio. In particular, assume that fluid begins to flow from one reservoir into the mixing channel at a rate faster than the fluid flows from the other reservoir. The resulting pressure drop from the meniscus of the fluid at the inner edge of the reservoir to the point where the fluids mix for the fluid that flowed faster will be less than that of the other fluid, because the rotationally-induced pressure is proportional to the radial extent of the fluid (ΔR discussed earlier). Because a higher pressure now exists across the fluid that moved more slowly, it is induced to flow more rapidly. This process of feedback provides a pressure-equalization phenomenon that results in the inner meniscuses of fluids in reservoirs  416 ,  417 , and  418  progressing outward at the same radial velocity (same distance in the radial direction per unit time). As a result, the ratio of the alkaline phosphatase and theophylline flow-rates as a function of time in mixing microchannel  509  is given exactly by
 
where A A  and B B  are the cross-sectional area of the reservoirs  416  and  417  as a function of time, or alternately, radial position of the meniscus as fluid is removed from the reservoirs. If it was desired that the ratio of flows is constant (as was the case here), it was sufficient to maintain a constant ratio of cross-sectional areas as a function of radial position. Note that this does not imply that the cross sections are constant, just that their ratio is. The ratio expressed in the equation can be manipulated by altering the ratio of cross-sectional areas of the reservoirs, as disclosed more fully in co-owned U.S. Pat. No. 6,063,589, issued May 16, 2000 and incorporated by reference.
 
     This equation and analysis also accurately describes the significance of the ratio of the three fluids in mixing microchannel  516 . Because microchannel  509  and  516  are long and the flow rates can be controlled by rotational rate, the co-flowing streams are present in those channels for a time sufficient enough for diffusion across the interface between these streams to effect complete mixing of the solutions. 
     After fluid was delivered to the detection reservoirs  420 , reflectance optics was used to measure the reflected radiation at an off-specular (diffuse) angle at two wavelengths, 430 nm (absorbing for the expected reaction product, PNP) and 630 nm. As there is no absorbance from reaction product PNP at 630 nm, this wavelength can be used to correct for optical imperfections in the platform, stray scattering, or unintended air bubbles in the optically-transparent chamber. The optical system also advantageously contained a beam-splitter that sent a fraction of the incident light to a reference photodiode. Two detectors used in this optics system were the assay detector, which measured diffusely-reflected light; and the reference detector, which measured a fraction of the incident light. Measurements at each detector were made when both the 430 nm and 630 nm light sources were active as well as when they were “dark” or off. The measured voltages were thus: 
     V D   D  dark measurement in assay detector 
     V R   D  dark measurement in reference detector 
     V D   1  measurement at absorbing wavelength λ 1  (430 nm) in assay detector 
     V D   2  measurement at non-absorbing wavelength in λ 2  (660 nm) assay detector 
     V R   1  measurement in reference detector at absorbing wavelength λ 1  (430 nm) 
     V R   2  measurement in reference detector at non-absorbing wavelength λ 2  (660 nm) 
     The absorbance at 430 nm is calculated from 
     
       
         
           
             K 
             = 
             
               
                 ( 
                 
                   
                     
                       V 
                       D 
                       1 
                     
                     - 
                     
                       V 
                       D 
                       D 
                     
                   
                   
                     
                       V 
                       R 
                       1 
                     
                     - 
                     
                       V 
                       R 
                       D 
                     
                   
                 
                 ) 
               
               
                 ( 
                 
                   
                     
                       V 
                       D 
                       2 
                     
                     - 
                     
                       V 
                       D 
                       D 
                     
                   
                   
                     
                       V 
                       R 
                       2 
                     
                     - 
                     
                       V 
                       R 
                       D 
                     
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             A 
             = 
             
               
                 
                   - 
                   log 
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   ( 
                   K 
                   ) 
                 
               
               ∝ 
               
                 c 
                 PNP 
               
             
           
         
       
     
     Here, C PNP  is the concentration of yellow product, p-nitrophenol; this concentration is inversely related to the concentration of theophylline in the initial solution. 
     Data was collected continuously as the platform was rotated at 60 or 100 rpm. Because data can be taken continuously, the kinetics of the chemical reactions could be observed.  FIG. 12  shows data for 24 assays run simultaneously on the platform, representing four-fold replicates for each of six theophylline concentrations ranging from 0 to 10 mM. The data are consistent with those generated on the laboratory bench using absorbance and that contained in co-owned and co-pending U.S. application Ser. No. 09/595,239, filed Jun. 16, 2000, incorporated by reference herein. 
     These results demonstrated that microplatform systems according to the invention can be used as a substitute for conventional 96-well microtiter plates for performing enzyme assays to determine enzymatic activity thereof. 
     EXAMPLE 2 
     Dilution Methods 
     A microsystem platform as shown in  FIG. 17  was used to dilute a fluorescein-containing sample. This test was performed in two stages. First 1 mM fluorescein in DMSO was used to assess the performance of the first stage. Next 20 mM fluorescein in DMSO was used to assess the performance from the 4 th  dilution to the 9 th . The 20 mM fluorescein experiments were necessary to resolve the concentrations at the higher dilution range, which were not detectable using 1 mM fluorescein. 
     Meter chamber and intermediate cuvette volumes as well as theoretical dilution ratios for 1 st  stage dilution structures are shown in Table 1. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 1 st  Stage characteristics 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                 5 th   
               
               
                   
                 Non- 
                 1 st   
                 2 nd   
                 3 rd   
                 4 th   
                 Dilution/ 
               
               
                   
                 metered 
                 Dilution 
                 Dilution 
                 Dilution 
                 Dilution 
                 Reservoir 
               
               
                   
               
               
                 Meter (μl) 
                 0.0 
                 0.6357 
                 0.2147 
                 0.0671 
                 0.0210 
                 0.0153 
               
               
                 Meter Dimensions 
                   
                 10888 × 200 × 288 
                 3926 × 200 × 261 
                 1582 × 200 × 190 
                 1847 × 127 × 85 
                 1652 × 127 × 71.5 
               
               
                 L × W × D (μm) 
               
               
                 Intermediate (μl)* 
                 2.0 
                 1.3687 
                 1.7911 
                 1.9364 
                 1.9844 
                 5.1439 
               
               
                 Theoretical 
                 0.0 
                 3.17 · 10 −1   
                 1.07 · 10 −1   
                 3.35 · 10 −2   
                 1.05 · 10 −2   
                 2.97 · 10 −3   
               
               
                 Dilution Ratio 
               
               
                   
               
             
          
         
       
     
     The intermediate cuvette volumes varied so that the combined V meter +V int =2.0 μl. This ensured a 1:1 mixture. 
     Since the second stage structure is the same as the 1 st  stage, and assuming that the solution in the 5 th  dilution/reservoir is fully mixed prior to feeding the 2 nd  stage (i.e. no error propagation), the dilution ratios for the 2 nd  stage are shown in Table II. 
     
       
         
               
             
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 2 nd  Stage characteristics 
               
             
          
           
               
                   
                 Non-metered/ 
                 6 th   
                 7 th   
                 8 th   
                 9 th   
               
               
                   
                 5 th  Dilution 
                 Dilution 
                 Dilution 
                 Dilution 
                 Dilution 
               
               
                   
               
               
                 Theoretical 
                 2.97 · 10 −3   
                 9.40 · 10 −4   
                 3.18 · 10 −4   
                 9.93 · 10 −5   
                 3.11 · 10 −5   
               
               
                 Dilution Ratio 
               
               
                   
               
             
          
         
       
     
     The dimensions of the valving structures used in this disc were as follows: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Valve Dimensions 
               
             
          
           
               
                   
                   
                 Valve Dimensions 
               
               
                   
                 Valve Location 
                 (Depth × Width, μm) 
               
               
                   
                   
               
               
                   
                 Dilution 1-3 meter valve 
                  75 × 200 
               
               
                   
                 Dilution 4 meter valve 
                  85 × 200 
               
               
                   
                 Dilution 5 meter valve 
                 71.5 × 154  
               
               
                   
                 Dilution 6-8 meter valve 
                 150 × 150 
               
               
                   
                 Dilution 9 meter valve 
                 150 × 200 
               
               
                   
                 Intermediate Cuvette bottom exit 
                  35 × ~80 
               
               
                   
                 Intermediate Cuvette top exit 
                  45 × ~80 
               
               
                   
                 Dilution 5 Intermediate bottom exit 
                  45 × ~80 
               
               
                   
                 Dilution 5 Intermediate top exit 
                 250 × 400 
               
               
                   
                 Diluent Valve - 1 st  stage 
                 250 × 200 
               
               
                   
                 Diluent Valve - 2 nd  stage 
                  35 × ~80 
               
               
                   
                   
               
             
          
         
       
     
     The following pattern of disc acceleration (the “spin profile”) was used to perform dilution of fluorescein using microfluidics structures as shown in  FIG. 17  and the sizes and volumes set forth in Tables I and II: The platform was loaded at platform rest (rpm=0) and then the disc accelerated to 300 rpm. At this speed the fluid comprising the fluorescein solution was motivated from the sample reservoir and into the manifold. Thereafter, disc speed was increased to 535 rpm, at which speed the non-metered intermediate cuvette and the metered chambers were filled. As the speed was increased to 560 rpm the capillary valve at the overflow chamber was overcome and excess fluorescein solution passed into the overflow chamber. Concomitantly, the diluent chamber valve was overcome and the bubble created in the microfluidics motivated to the transverse manifold and entered the bubble capture region. The speed was increased a final time to 900 rpm, and capillary valves controlling all metered chambers were overcome and all intermediate cuvettes filled. Disc speed was then reduced to 600 rpm to prevent intermediate exit valve from breaking prematurely and to permit time for the manifold to drain of excess diluent and fluorescein solution. 
     2 nd  Stage 
     The spin profile for the second stage was commenced manually once the manifold from the first stage completely drained (˜15 sec). Disc speed was increased to 1125 rpm to overcome the valve retaining the diluted fluorescein solution from the fifth dilution reservoir; disc speed was controlled to be no greater than 1125 rpm, since higher speeds could result in the capillary valves controlling the first stage intermediate exit valves being overcome. Once the fluid exits the 5 th  dilution reservoir, disc speed was reduced to 500 rpm, at which speed the non-metered volume and the metered chambers were filled. Thereafter, disc speed was again increased to 650 rpm, to overcome the capillary valve at the overflow chamber and to overcome the capillary valve controlling the diluent reservoir for the second stage. The disc speed was then sequentially increased to 775 rpm to overcome the capillary valve controlling the 9th dilution meter, then to 850 rpm to overcome the capillary valve controlling the 7 th  and 8 th  meter valves, then to 900 rpm to overcome the 6 th  dilution meter valve, and finally to 1000 rpm to overcome the capillary valve controlling the capillary valve controlling the overflow chamber. Rotation of the disc at 1000 rpm was continued to drain the remaining fluid in the manifold into the overflow chamber. 
     Filling the Detection Cuvettes 
     To empty the intermediate cuvettes and fill the detection cuvettes the disc speed (rpm) were increased from 1000 rpm to 2500 rpm at 500 rpm/s 
     The results of these experiments are shown in Table III. These results are a combination of 3 discs for each of first, the 1 st  stage results and second the results for the 4 th  dilution to the 9 th . The results are presented in terms of normalized fluorescein concentration, therefore the first non-metered detection cuvette will have a concentration, C 1 , of unity, while the additional detection cuvettes should theoretically follow C i =DR i * C 1 , where, i refers to the i th  dilution. 
     As seen in Table III, the experimental results are very similar to the theoretical predictions, therefore signifying that the disc is working according to the design. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 Summary of fluorescein dilution experiments 
               
               
                 on 5-log dilution disc 
               
             
          
           
               
                 Detection 
                 Theoretical 
                 Experimental 
               
               
                 Cuvette 
                 Concentration 
                 Concentration 
               
               
                   
               
             
          
           
               
                 1 
                 1.00 · 10 −0   
                 1.00 · 10 −0   
               
               
                 2 
                 3.17 · 10 −1   
                 2.72 · 10 −1   
               
               
                 3 
                 1.07 · 10 −1   
                 0.88 · 10 −1   
               
               
                 4 
                 3.35 · 10 −2   
                 2.71 · 10 −2   
               
               
                 5 
                 1.05 · 10 −2   
                 0.95 · 10 −2   
               
               
                 6 
                 2.97 · 10 −3   
                 2.68 · 10 −3   
               
               
                 7 
                 9.40 · 10 −4   
                 N.D.* 
               
               
                 8 
                 3.18 · 10 −4   
                 3.09 · 10 −4   
               
               
                 9 
                 9.93 · 10 −5   
                 10.8 · 10 −5   
               
               
                 10 
                 3.11 · 10 −5   
                 4.11 · 10 −5   
               
               
                   
               
               
                 *N.D. = Not determined 
               
             
          
         
       
     
     These results are also depicted graphically in  FIGS. 35A and 35B . 
     The experimental fluorescein concentration from the dilution disc was determined using results from an experimentally determined calibration curve of concentration vs. fluorescence signal. There is a slight curvature to the calibration curve for fluorescein concentrations above ˜5 μM, and the curve is essentially linear below this concentration. Therefore the concentrations in the 2 nd  stage were determined using a linear relationship. No background subtraction was performed. 
     Based on the experimental dilution factors shown in Table I, a well-characterized CYP assay (CYP3A4/DBF/Ketoconazole, as described in Kobayashi et al., 2003 , Drug Metab Dispos . 31:833-6; Margolis &amp; Obach, 2003 , Drug Metab Dispos . 31:606-11; Shou et al;., 1994 , Biochemistry  33: 6450-5; Remmel &amp; Burchell, 1993 , Biochem Pharmacol  46:559-66) was next performed to assess the performance of the dilution disc. The results, from a combination of data obtained from three independently produced and tested microsystems platforms demonstrated the expected degree of dilution of the assay components on the disc. 
     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.