| Title of Invention | MICROFLUIDIC SYSTEM AND METHOD |
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| Abstract | ABSTRACT (IN/PCT/2002/01345/CHE) "MULTI-PORT PRESSURE CONTROL SYTEMS" Improved microfluidic devices, systems, and methods allow selective transportation of fluids within microfluidic channels of a microfluidic network by applying, controlling and varying pressures at a plurality of reservoirs. Modeling the microfluidic network as a series of nodes connected together by channel segments and determining the flow resistance characteristics of the channel segments may allow calculation of fluid flows through the channel segments resulting from a given pressure configuration at the reservoirs. To affect a desired flow within a particular channel or series of channels, reservoir pressures may be identified using the network model. Viscometers or other flow sensors may measure flow characteristics within the channels and the measured flow characteristics can be used to calculate pressures to generate a desired flow. Multi-reservoir pressure modulator and pressure controller systems can optionally be used in conjunction with electrokinetic or other fluid transport mechanisms. |
| Full Text | BACKGROUND OF THE INVENTION The present invention relates to microfluidic systems, and methods for selectively transporting fluids within microfluidic channels of a microfluidic network often using a plurality of selectively variable pressures. Microfluidic systems are now in use for the acquisition of chemical and biological information. These microfluidic systems are often fabricated using techniques commonly associated with the semiconductor electronics industry, such as photolithography, wet chemical etching, and the like. As used herein, "microfluidic" means a system or device having channels and chambers which are at the micron or submicron scale, e.g., having at least one cross-sectional dimension in a range from about 0.1 I’m to about 500 Applications for microfluidic systems are myriad. Microfluidic systems have been proposed for capillary electrophoresis, liquid chromatography, flow injection analysis, and chemical reaction and synthesis. Microfluidic systems also have wide ranging applications in rapidly assaying compounds for their effects on various chemical, and preferably, biochemical systems. These interactions include the fib range of catabolic and anabolic reactions which occur in living systems, including enzymatic, binding, signaling, and other reactions. A variety of methods have been described to affect the transport of fluids between a pair of reservoirs within a microfluidic system or device. Incorporation of mechanical micro pumps and valves within a microfluidic device has been described to move the fluids within a microfluidic channel. The use of acoustic energy to move fluid samples within a device by the effects of acoustic streaming has been proposed, along with the use of external pumps to directly force hquids through microfluidic channels. The cavities and use of microfluidic systems advanced significantly with the advent of electrokinetics: the use of electrical fields (and the resulting electrokinetic forces) to move fluid materials through the channels of a microfluidic system. Electrokinetic forces have the advantages of direct control, fast response, and simplicity, and allow fluid materials to be selectively moved through a complex network of channels so as to provide a wide variety of chemical and biochemical analyses. An exemplary electrokinetic system providing variable control of electro-osmotic and/or electrophoretic forces within a fluid-containing structure is described in U.S. Patent No. 5,965,001, the fuU disclosure of which is incorporated herein by reference. Despite the above-described advancements m the field of microfluidic, as with ash success’, still further improvements are desirable. For example, while electrokinetic material transport systems provide many benefits in the mic3x>-scale movement, mixing, and aliquoting of fluids, the apphcation of electrical fields can have detrimental effects in some instances. In the case of charged rsagaits, electrical fields can cause electrophoretic biasing of material volumes, e.g., highly charged materials moving to the front or back of a fluid volume. Where transporting cellular material is desired, elevated electrical fields can, in some cases, result in. a perforation or electroporation of the cells, which may effect their climate use in the system. To mitigate the difficulties of electrokinetic systems, sinmlified transport systems for time domain multiplexing of reagents has been descrihed in WO 00/45172 (assigned to the assignee of the present invention), the fiill disclosure of which is incorporated herein by reference. In this exemplary time domain multiplexing system, structural characteristics of chamiels carrying reagents can, at least in part, regulate the timing and amount of reagent additions to reactions (rather than relying solely on the specific times at which pumps are turned on and'or valves are actuated to regulate when and how much of a particular reagait is added to a reaction). While other solutions to the disadvantageous aspects of electrokinstic matCTial transport within a microfluidic system have be«i d’cribed, still finHKr aftjKnative fluid transport mechanisms and control meShodoiogies would be advaatagec«2s to enhance the flexibility and capabilities of R’ardless of fee TTn-rhaTTigm vised to effect movement of fluid and other materials \’ifein a miduQuidic diannd network, accuracy and repeatainhty of specific flows can be problematic. There may be variations in, for example, electroosmotic flow between two cMps having similar designs, and even between different operations run on a single chip at different times. Quahty control can be more challenging hi hght of this variabiHty, as accurate control over microfluidic flows in applications such as high throughput screening would benefit significantly from stable and reliable assays. In hght of the above, it would be advantageous to pro\dde improved microfluidic devices, systems, and methods for selectively transporting fluids within one or more microfluidic channels of a microfluidic network. It w’ould be desirable if these improved transport techniques provided selective fluid movement c’abilities similar to those of electrokinetic microfluidic systems, while mitigating ihe disadvantageous aspects of the application of electrical fields to chemical and biochemical fluids in at least some of the microfluidic channels of the network. It would also be beneficial to provide improved devices, s\3tsms, metiiods and kits ibr enhancing the accuracy, rehability, and stability of miCTofluidic flows wimin a microfluidic network. It would be beneficial if these enhanced flow control techniques provided real-time and/or quality control feedback on the actual flows, ideally withoct relying on significantiy increased system coniplexity or cost SUMMARY OF THE INVENTION The present invention generally provides improved microfluidic devices, systems, and me&ods. The devices and systems of th!e invention gensaliy allow flexible and selective transportation of fluids within microfluidic channels of a microfluidic network by applying, controlling, and varying pressures at a plinrality of reservoirs or ports. By modeling the microfluidic network as a series of nodes (including the reservoirs, channel intersections, and the hke) connected together by channel segments, and by determining the flow resistance characteristics of the channel segments, the fluid flows through the channel segments resulting fi-om a given pressmre configuration at the reservoirs can be determined. Reservoir pressures to effect a desired flow profile may also be calculated Msing, the network model A sitnple multi-reservoir pressure modulator and pressure controller system can optionally be used in conjunction with electrokinetic CT o&xec fluid nangxirt mechanisms. The invention also provides techniques to avoid fluid muUn-e degradatioai witim a microflmdic channel by maintaining sufficient osdBmcsi to avoid s’Kcaticxi of tiie ftnyd mixmre wbsn. no gros movem’eat of the Smd isdearod- >d5cxoflmdic sysiasB and mefiK)ds having ‘iscometss or other flow sensors are particularly useful for determining pressures so as to hydrodynamically induce a desire to flow in response to a measured flow within a microfluidic channel. Regardless of the mechanism used to effect movement of fluids within a microfluidic network, the techniques of the present invention may be used to provide feedback on the actual flow and/or network system characteristics, allowing (for example) more accurate, stable md reliable assays. In a first aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plui-aiity of reservoirs in fluid communication with the network. The network includes a channel. A plurality of pressure modulators are also included, each pressure modulator providing a selectably variable pressure. A plurality of pressure transmission lumens transmit me pressures fi-om the pressure modulators to the reservoirs so as to induce a desired flow within the channel. Generally, the lumens will transmit tiie pressures to the ports Tsiih significantly less resistance to the lumen flow than the resistance of the channel to the associated micnofluific flow. Each pressure modolaror will typically be in fluid communication with an associated port via an associated lumen. In many embodiments, a netwoik flow controller wQl be coiq)led to the pressure modulators and will send signals to the pressure modulators so that the modulators vary the pressures. The network controller will generally include chaimel network dat’'which correlate the channel flows with the pressures fi?om the pressure modulators. In some embodiments, the network will comprise a pluraiity of microfluidic channels in fluid communication at channel intCTsections. The intersections and reservoirs will define nodes coupled by chaimel segments. The network data can indicate correlations between the flows in the channel segments and the plurality of pressures. In other embodiments, a network data generator may be coupled to the network controller. The netwoik data generator may comprise a netwoik flow model, a viscomete coupled to the channel, aad'or a astwoii tester ad’ted to measure at least one parametCT indicating the pressure-flow correlation. The pressure controller or controllers ■win oSea m’ce use of signals from piesaire sensors so as to provide a pressure feedback ■ps&L Opdocany, the pressure oosiadlkxs mzy include c’ilsnaJioD. flata corrdating drive signals wife the resulting reservoir prcssmcs. Preferably, ihe pressure modulators will ccm’prise pncaroatic di’lacanezit pon’js. Typically, at least one sample test liquid will be disposed in the channel networL A pressure-transmission fluid can be disposed in the lumens, with a fluid/fluid-pressure-transmission interface disposed therebetween. Typically, the pressure-transmission fluid will comprise a compressible gas, which qaa coropliantly couple the pressure modulators with the channel flow. Typically, the system will include at least four independently variable pressure modulators. Preferably, the system will make use of at least eight independently variable pressure modulators. A pressure intaiace manifold can be used to releasably engage the microfluidic body, the manifold providing sealed fluid commum'cation between the lumens and the associated reservoirs. Ideally, a plurality of electrodes will also be coupled to the microfluidic network with an electrokinetic controller coiq>led to the electrodes so as to induce electrokinetic movement of fluids within the network. In general, whsi a hydrodynamic pressure differential is used to move fluid within the microflnidic network, the pressure differential will be sigrnficanfly greater flian a c’illary pressure of flmds within the reservoirs. In another aspect, the invention provides a body defining a microfluidic chaimel network with a plurality of reservoirs in fluid communication with the network. The network includes a first channel. A plurality of pressure modulators is also provided, with each pressure modulator in fluid communication with a reservoir for varying a pressure applied thereto. A network flow controller ii coupled to the pressire modulators. The network controller comprises channel network data correlatiug a flow within the first channel and the pressures fix>m the pressure modulators. TTie network controller independently varies the pressures fixjm the pressure modulators in response to a desired flow within the first channel in the network data. Optionally, the system may further include means for generating the network data coupled to the network controller. The network data generating means may comprise a model of the network, a viscometer, an electrical resistance sensor for saising electrical resistance witinn tiie network, OT the libs. In another aspect, the inveotioai jHXDvides a microfluidic system comprising a body controller with caUbration data couples the pressure modulators with the network controllers. The pressure controllers transmit drive signals to the pressure modulators in response to desired pressure signals fi"om the network flow controller and the calibration data. in a first method aspect, the invention provides a microfiuidic method comprising transmitting a first plurality of pressures to an associated pluraliiy of reservoirs using a plurality of pressure transmission systems. A first flow is induced within a first microfiuidic channel of a microfluidic network in re’onse to the first pressures. A second plurahty of pressures in determined so as to effect a desired second flow within the first microfluidic channel. The determined second pluraliw of pressures are applied with the pressure transmission systems and the second flow is induced within the first microfluidic chaimel -with the second pressures. The methods of the present invention are particularly well suited for precisely combining selected fluids within a microfluidic network, such as for muitiport dilution in which concentrations of first and second fluids fi"om first and second reservoirs can be combined at different concentrations. hi another method aspect, the invention provides a microfluidic method comprising detCTmining pressure-induced flow characteristics of a microfluidic channel within a microfiuidic network. A first plurality of pressures are derived fiiom the characteristics of the microfluidic network so as to proWde a first desired flow in a first microfluidic charmeL The first desired flow is induced by applying the first pressure to a plurahty of ports in communication with the microfluidic network. In yet another method aspect, the invention provides a method for use with a fluid mixture which can degrade when held stationary. The method comprises introducing the fluid mixture into a microfluidic channel of a microfluidic network. The mixture is maintained by oscillating the fluid mixture within the channel. The maintained fluid mixture is then transported along the diannel. While analysis of the mlcrofimdic network based on the known chaimel geometry can significantiy facilitate cdculation of pressures to be apphed for generation of a desired hydrodynamic flow, work in coraaection with the present invention has shown tiiat the comjdex nature of fiie flows within a cdCTofluidic channel can make calculation Qic’ecCyc flmd viscosity witian a naaofluidic netwOTk highly problematic. SpecificaBy, tbe flows wiftiin a sjngje dianod of a iniat>ihn and the like. To over come this complication, the invention often makes use of viscometers and other flow sensing systems to determine actual flow characteristics from a known microfluidic driving force. Based on these measurements, a desired flow may then be generated hydrod>7iamicalIy by adjusting the appropriate reservoir pressures. In a related method aspect, the invention provides, a microfluidic method comprising inducing flow within a microfluidic channel of a microfluidic network. Ths flow is measured and a pressing is calculated from the measured flow so as to generate a desired flow. The desired flow is generated within the channel by applying the calculated pressure to the microfluidic network. The flow is optionally measured by generating a detectable sign’ within the flow at a first location, and by measuring a time for the signal to reach a second location. The signal may comprise a change in a fluid of the flow, particularly where the first location comprises an intersection between a pluralit\' of microfluidic channels. Such a change in the flow may be initiated hydrodynamically by applying a pressore pulse to a nssar/oir in cananunication with the intersection, and'cH- elsctrc&inetically by varying an electrical field across the first intersection. Optionally, a plurality of detectable signals from a plurality of channel intersections maybe sensed as each of tbese signals reaches the second location, hi many embodiments, a signal will comprise a change in an optical quality of fluid in the flow. For exan’le, the signal may comprise a change in a concentration of a dye from a channel intersection, as desaibed above. Alternatively, where the fluid con:q)rises a photobleachable dye, the dye may be photobleached by a laser at the first location with the photobleaching sensed at the second location. Many of these methods will allow a speed of the flow to be determined, particularly when a distance between the first and second locations is known. In some embodiments, the speed of the flow maybe determined by, for example. Dopier velocimetry, tracer particle videography, or the like. Ideally, a viscosity of the flow can be calculated using a first pressure (which induces the measured flow) and the speed of the flow. This viscosity can thsi be used ki detennination of the calculated pressure so as to geierated tiie desired flow. In a related system a.’>ect, the invention provides a microfluidic system comprising a body defining a miciofiuidic channel network and a plurality of reservoirs in flsid ccKEnnnication wife tae nctwcs±. The netwodc icdudes a microflnidic dianneL A viscometer is cotroied to fee dannd fe deteraiimng a viscoaty of a flow tiiaein. In yet another system aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid communication with the network. The network includes a microfluidic channel. A plurality of pressure modulators are in fluid commimication with the reservoirs. A sensor is coupled to the channel for transmission of flow signals in response to flow within the channel. The controller couples the sensor to the pressure modulators. The controller transmits pressure commands in response to the flow signals to provide a desired flow. In yet another aspect, the invention provides a microfluidic system comprising a body defining a microfluidic channel network and a plurality of reservoirs in fluid commimication with the network. The system also includes means fer selectively and independently varying presstires within the reservoirs. The pressure varying means is in fluid communication with the reservoirs. In yet another aspect, the invention provides a microfluidic method comprising inducing a potnrbation in a flow through a microfluidic channel of a microfluidic network by gjplyicg a pressure transient to the roicrofiuidic network. A characteristic of the flow or microfluidic network is determined by monitoring progress of the perturbation. The pressure transient may convenienfly be applied by spontaneous injection of an introduced fluid into an injection channbl of the microfluidic network Such spontaneous injection may draw the introduced fluid into the injection channel using capillary forces between the injection channel and the introduced fluid. Typically, the perturbation wiU comprise a change in a material of the flow downstream of an intersection. This change wfll often comprise a change in quantity of a fluid fix>m a first channel, with the pressure transient being ‘pUed at the first channel. The use of pressure induced flow perturbations may be used to determine flow or network characteristics in systems having flow that is pressure induced, electrically induced, or any mixture of flow inducing mechanisms. Typically, flow characteristics such as effective flow viscosiw, flow speed, and the like may be deiermined. In some embodimems, metwaas. characteristics such as flow resistance of one or ttwre chaimds may be detenniaed- Tbe progres of the pertabaticKi cay be mroritared at least in part with a secsor chsposed downstream of a pcrtntbatkai source ktcstian (sodi an intexsection of channds). A speed of the flow may be dstennined &om, for acamplc, a time interval extending from the pressure transient to detection of the perturbation at the sensor location, and from a distance along the channel or channels extending from the source location to the sensor location. More complex analyses are also possible, such as determining a second speed of a second flow. This second speed may be generated in response to a time interval defined in part by detection of a second flow perturbation, and a second distance defined in part by a second perturbation source location (such as a second channel intersection). As the different speeds along intersecting channels may be determined, the amount of materials combined from different channeb at an intersection may be calculated. In a related system aspect, the invention provides a microfliddic system comprising a body having channel walls defining a microfluidic netwoii. A pressure transient generator is in communication with a channel intersection of the microfluidic network for initiation of a flow perturbation. A sensor is coupled to the flow within the network at a sensor location. A processor coupled to the pressure generaxor and frie sensor detemilnes a characteristic of the flow or the netwoik in response to detection of the perturbation at the saisor location. BRIEF DESCRIPTION OF THE DRAWBsGS Fig. 1 schematically illustrates a microfluidic system having a multi-reservoir pressure modulation system according to the principles of the present invention. Fig. 2 is a plan view of a representative microfluidic device having microfluidic channels with enhanced fluid flow resistance for use in the miorofluidic system of Fig. 1. Figs. 3xA. and 3B are perspective views of a pressure manifcld for releasably sealing reservoirs of the microfluidic de’’ice of channel 2 in fluid communication with the pressure modulators of the system of Fig. 1. Fig. 4 schematically illustrates a control system for indq)endentiy varying reservoir pressures in the microfluidic sj’tan of Fig. 1. Figs. 5 A-C schematically iHostrate a method and computer program for dets'-n'riiing p-essures to provi Figs. 7A and 7B illustrate well-pair dilution in which concentration variations are produced by selectively varying the relative flow rates from two reservoirs connected at an intersection. Figs. 7C-E graphically illustrate measured dilution verses set or intended dilution for a multi-reservoir pressure controlled well-pair dilution. Figs. 8 and 8A-8D graphically illustrate an enzyme assay using a multi-reservoir pressure controlled microfluidic system, and more speci£cally: Fig. 8 illustrates the reaction. Fig. 8A is a titration curve for different substrate concentrations. Fig. SB is a plot of the corrected signal verses substrate concentration. Fig. 8C is a plot for determination of the MichaeHs constant, and Fig. 8D is a substrate titration plot Figs. 9A-C illustrate a microfluidic Protein Kinase A (PKA) reaction assay with variations in concentration achieved using hydrodynamic pressure modulation. Figs. lOA and lOB illustrate a mobility shift assay microfhiidic network and assay test results at different concaatrations. Figs. 11A and UB are a perspective and plane view, respectively, of an exemplary hydrodynamic and electrokinetic interface stracture fyr coaling to a microfluidic body. Fig. 12 schematically illustrates an exemplar’' microfhndic viscometer. Figs. 13A and 13B schematically illustrate a microfhndic networii and method for imposing detectable signals on a microfluidic flow for measurement of flow characteristics whidi can be used to calculate pressures to affect a desired flow. Figs. 14A and 14B graphically illustrate flow characteristic signals which may be used to determine effective viscosity. Fig. 15 is a perspective view of a microfluidic chip having a plurality of capillaries for spontaneous injection of flmds into the microfluidic network. Fig. 16 is a top view of a simple microfluidic chip having a single capillary for spontaneous injection. Figs. 16A-16C gr’Mcally lUostrate methods for monitoring progress of permrbations induced by spontaneous iigection of fluids, for use in determining characteristics of a flow and/or rmcioOuidk iKstwoik. Figs. 17A and 17B are perspective aad plan view of fluorogenic multi- Figs. ISA and 1 SB arc perspective and plan view of a naobility-shift capillary ch’j. Fig. 19 graphically illustrates the detection of a perturbation generated at an intersection of microfluidic channels by spontaneous injection. DESCRIPTION OF THE SPECIFIC EMBODIMENTS The present invention generally makes use of a multi-reseivoir pressure controller coupled to a pluralit>' of independently variable pressure modulators to effect movement of fluids within microfluidic networks. By selectively controUing and changing the pressure applied to the reservoirs of a w' i.cQuidic device, hydn ayiiamic flow at very low flow rates may be accurately controlled within intersecting microfluidic channels. Such pressure-induced flows can help to decrease (or entirely avoid) any detrimental-effects of the electrical fields associated with electrokinetic trHnsportation methods, such as sample bias, ceU perforation, electroporation, and the like. Additionalh', such pressure-induced microfluidic flows may, throng proper chip design, reduce fiow’ variabilities as compared to electrokinetic techniques throu’ tie use of pressure diSerentials (and/or channel resistances that are significanfly greater than flow’ variations nadnced by secondary effects, such as inflow/outflow capiUary fijrce differentials within the reservoirs). Advantageously, the pressure-induced flows of the present invention may also be combined with electrokinetic and/or other fluid transportation mechanisms thereby providiag composite pressure/electroldnetic microfluidic systems. The techniques of the present invention wiU often make use of data regarding the network of channels within a microfluidic device. This netwoik data may be calculated using a model of the microfluidic network, measured by testing a microfluidic de\'ice, sensed using a sensor, and/or the Hke. The netwoik data will often be in the form of hydrostatic resistances along microfluidic channel segments connecting nodes, with the nodes often being intersections beti’'een channels, ports or reservoirs, connections between channel segments having differing cross-sectional dimensions and/br flow characteristics, and the like. As used herein, the term "reservoir" eacorrroass’ ports for interfecing with a microfliddic network within a microfluidic body, inckding ports which do not have cross-Kcti which might be difficult and/or impossible to control using alternative fluid transportation mechanisms. Advantageously, the present invention may provide flow rates of less than 0.1 nanoUters per second, the flow rates often being less ttian 1 nanoliters per second, and the pressure induced flow rates typically being less than 10 nanoliters per second within the xnicrofluidic channel. To accurately ‘ply the pressures within the microfluidic network, the invention generally makes use of a pressure transmission system having relatively large lumens coupling the pressure modulators to the reservoirs of the micfofluidic device, with the pressure transmission lumens ideally containing a compressible gas. Pressure is often transmitted through this relatively low resistance pressure transmission s>'srsm to flmds disposed within the resa’oirs of the microfluidic system via a gas/fluid intsrface within the reservoir. The resistance of the microfluidic channels to the fliud flows iheran is typically much greater than the resistance of the pressure transmission lumens to the associated flow of compressible gas. Generally, the channel resistance is at least 10 muss the transmission system resistance, preferably being at least 100 times, and ideally being at least 1000 times the transmission system resistance of the compressible gas used to induce the channel flows. In other words, a response time constant of the pressure transmission system will generally be lower than the time constant of the channel network, preferably being much lower, and ideally being at least one, two, or three orders of magnitude lower. The head space of a fluid (for example, in the pressaire modulator pump and/or in the port or reservoir) times the resistance of the fluid flow (for exairqjle, in the channels or lumens) may generally define the response time constant Siuprisingly, it is often advantageous to enhance the resistance of the microfluidic channels to provide the desired relative resistance factors. The channels may have reduced cross-sectional dimensions, pressure drop maabers (such as a small cross-section pressure orifice, a flow restricting substance or coating, or the like), and/or lengths of some, most, or even all of the microfluidic channel segments may be increased by including serpentine segment paths. As me resistance of the pressure transmission system can be several ordars of magnitude less than the resistance of the channels, pressure dinaTcntials can be accurately transmitted from the pr’sure modidalors to the reservoirs of the miciuiluidic device. Additionaliy, reduced, transmission system resistances can bsfcio eohzace me r’)OPse of tig ‘essuiK system, providing a fa’sar response time coctstaoL Referring now to Fig. 1, a microfluidic system 10 includes a microfluidic device 12 coupled to a bank of pressure modulators 14 by a pressure transmission system 16. Pressure modulator bank 14 includes a plurality of pressure modulators 14a, i4b,... Modulator bank 14 will generally include at least three independent’, selectively variable pressure modulators, typically having at least four modulators, and ideally having eight or more modulators. Each modulator is in fluid communication with a reservoir 18 of microfluidic device 12 via an associated tube 20, the tube having a pressure transmission lumen with a compressible gas therein. Modulator bank 14 generally provides independently selectable pressures to the lumens of tubing 20 under the direction of a controller(s) 22. Feedback may be provided to controllef 22 from pressure sensors 24, as wiU be described hersnbelow. Processor 22 will often conrorise a machine-readable code embodied by a tangfole media 26, with the machine-readable code comprising program instructions and/'or data for effecting the methods of the present invention. Processor 22 may conq)rise a personal computer having at least an Intel Pentium® or Pentium II® processor having a speed of at least 200 MHz, 300 MHz, or more. Tangible media 26 may cx)ngirise one or more floppy disks, conq)act disks, or "CDs," magnetic recording tape, a read-only memory, a random access memory, or the like. In some embodiments, the programmiag instructions may be ii’ut into controller 22 via a disk drive or other ir’jut/oulput system such as an internet, intranet, modem reservoir, or the like. Suitable programs may be written in a variety of programming languages, including the LabView'‘‘ language, as available from National Instruments of Austin, Texas. Controller 22 transmits drive signals to modulator bank 14, ideally via an RS232/R5485 serial connection. In addition to tubing 20, pressure transmission system 16 includes a manifold 28. Manifold 28 releasably seals the lumen of each tube 20 with an associated reservoir 18 of microfluidic device 12. Tubing 20 may comprise a relatively high-strength polymer such as polyetheretherketone (PEEK), or apolytetrafluoroethylene (sudi as a Teflon""* material), or the like. The tnhjng typically has an inner diameter in a range from about 0.01" to about 0.05’, wim a lengih from about Im to about 3m. A'T" connector couples ttie pressure output fitan each pi’sure modulator to an associated p’essuie sensor 24. EadimodnlaEtDr 14a, 14b ... geosrally coo’irises a pomp or other pressure scxETce ‘ffiadi pressanzes the caaxpns’biie gas wifinn ‘x hniKai of asscxaated tzdang 20. TTK mo’iktors preferably canqrise positive diq)Iacaneait pnn’K, wifli the exeniplaiy modulators comprising a piston which is selectively positioned within a siuroundirig cylinder by an actuator. Preferably, the actuators are adapted to allow accurate positioning of the piston in response to drive signals from controller 22, the exemplary actuators comprising stepper motors. The exemplary piston/cylinder arrangement is similar to a syringe. Exemplary modulator banks may be provided by (or modified from components available through) a variety of commercial sources, including Kloehn of Las Vegas, Nevada, Cavaxo of Sunny\'aie, California, and the like. Microfluidic device 12 is seen more clearly in Fig. 2. Microfiuidic device 12 includes an array of reservoirs 18a, 18b,... coupled together by microscale channels defining a microfluidic network 30. As used herein, the term "microscale" or "microfabricated" generally refers to stractural elements or features of a device winch have at least one febricated dimension in the range of from about 0.1 jim to about 500 fmi. Thus, a de\ice referred to as being microfabricated or microscale will include at least one structural element or feature having such a dimensiorL When usee to descriDe a fluidic element, such as a passage, chamber or conduit, the terms "microscale", '*microfebricated" or "microfiuidic" genCTaDy refer to one or more fiuid passages, chambers or conduits which have at least one internal cross-sectional dimension, e.g., depth, width, length, diameter, etc., that is less than 500 ‘m, and typically between about 0.1 (im and about 500 um. In the devices of the present invention, the microscale channels or chambers preferably have at least one cross-sectional dimension between about 0.1 Jim and 200 um, more preferably between about 0.1 [im and 100 •rm, and often between about 0.1 um and 50 fun. The microfluidic devices or systems of the present invention typically include at least one microscale channel, usxially at least two intersecting microscale channel segments, and often, three or more intersecting channel segments disposed within a single body structure. Channel intersections may exist in a number of formats, including cross intersections, "T" intersections, or any number of other structures wherefoy two channels are in fluid commnnication. The body structures of tiie devices which integrate various microfluidic channels, diambCTS or other elements may be M)ricated from a number of individual parts, wMcii wben competed form the integrated microfluidic devices described horeiiL FcH- exarapfc, tfe body fracture can be Msicsctcd from a number of separate cajrillary elemez’ wacroBczle ciiambers, and flie HVf all of which are connected togedaer to define an integrated body sUuctme. Altsrtgtfively and in piefeued aspects, the rotegrated body structure is fabricated from two or more substrate layers which are mated together to define a body structure having the channel and chamber networks of the devices within. In particular, a desired channel network is laid out upon a typically planar surface of at least one of the two substrate layers as a series of grooves or indentations in tiaat surface. A second substrate layer is overlaid and bonded to the first substrate layer, covering and sealing the grooves, to define the channels within the interior of the device. In order to provide fluid and/or control access to the channels of the device, a series of reservoirs or reservoirs is typically provided in at least one of the substrate layers, which reservoirs or reservoirs are in fluid communication with the various channels of the device. A variety of different substrate materials may be used to fabricate the devices of the invention, including sihca-based substrates, i.e., glass, quartz, fused silica, silicon and the Hke, polymeric substrates, i.e., acrylics (e.g., polymethylmethacryiate) polycarbonate, polypropylene, polystyrene, and the hke. Examples of preferred polymeric substrates are desaibed in commonly owned pubhabed international patent appKcation no. WO 98/'46438 which is incorporated herein by reference for all purposes. Silica-based substrates are generally amoiable to microfabrication techniques feat are well-known in the art including, e.g., photolithographic techniques, wet chemical etching, reactive ion etching (RJE) and the like. Fabrication of polymeric substrates is generally carried out using known polymer fabrication methods, e.g., injection moldiag, embossing, or the like. In particular, master molds or stamps are 'optionally created from solid substrates, such as g’ass, sflicon, nickel electro forms, and the like, using well-iaown micro fabrication techniques. These techniques include photolithography followed by wet chemical etching, LIGA methods, laser ablation, thin film deposition technologies, chemical vapor deposition, and the hke. These masters are then used to injection mold, cast or emboss the channel structures in the planar surface of the first substrate surface, in particularly preferred aspects, the channel or chamber structures are embossed in the planar surface of the fiorst substrate. Methods of fabricating and bonding polymeric substrates are described in commonly owned U.S. Patent AppHcation No, 09/073,710, filed May 6,1998, and incorporated herein by reference in its entirety for all purposes. Further preferred aspects of fee nricrofluidic devices of the present •mveTTTion are more fuHy (kscribed in co-peadrng U.S. Patent Application No. 09/238,467, as fifed cm January 2S, 1999 (coismcmiy assignod wiSi SK present s’jpHcation), tbs full disckjsnrr of wirici is incofporated honeic by referajce. These jsrefesred adjects include, for exarq>le, a reactkai Txms disposed within the overall body structure of the device, a (generally reterrmg to a chemical interaction that involves molecules of the type generally found within living organisms), sensing systems for detecting and’or quantifying the results of a particular reaction (often by sensing an optical or other detectable signal of the reaction), and the like. Referring once again to Fig. 2, reservoirs 18 wiU often be defined by openings in an overlaying substrate layer. Reservoirs 18 are coupled together by channels 32 of microfluidic network 30, with the channels generally being defined by indentations in an underlying layer of the substrate, as was also described above. Microfluidic channels 32 are in fluid communication with each other at channel intersections 34a, 34b,... (generally referred to as intersections 34). To simplifj' analysis of microfluidic net\’'oik 30, channels 32 may be analyzed as channel segments extending between nodes defined at reservoirs 18 and/or channel intersections 34. To provide enhanced control over movement of fluids within xmarofluidic network 30 by reducing the effects of secondary hydrostatic forces (sadi as camRary forces within r’ervoirs 18), the resistance of channels 32 to flow through the nricroQuidic netwOTk may be rahanced. These aahanced channel rssistaiKes may be provided by having a channel length greater than the normal separation between the nodes defining the channel segment, such as by having serpentine areas 36 along the channel segments. Alternatively, a cross-sectional dimension of the channd may be decreased along at least a portion of the channel, or flgrw may be blocked by a flow restrictor such as a local orifice, a coating or material disposed in the channel, 03: the like, hi general, to take advantage of the fiill range of flow control provided by the p’ssure modulators, microfluidic device 12 should be optimized for hydrodynamic flow. Flow control is generally enhanced by providing sufficient flow resistance between each reservoir 18 and the adjacent nodes so as to allow a sufficient variation in flow rate to be achieved wilhin the various channel segments given Ihe dynamic operating pressure range of the pressure modulators. Pressure manifold 28 can be serai njore clearly in Figs. 3A and 3B. Marffbld 28 has at least one device engaging sm&ce 40 for engaging microfluidic device 12, wifli the Qagagememt surfece hzving an array of pressure lumens 42 coCTesporaiii’ to irseivoirs 18 of the device. Each of pressure Imnsis 42 is in fluid conEnuakaikai with z Siting 44 fiar rfm’Hrnv each reservoir with an associated pressure modnlator via an associated tofoe. Sealing body 46 hdps maintain a seal between the associated presKirs modulator and resavcgr, and mandfoid 28 is reieas’ly secured to '7 device 12 by a securing mechanism 48, which here includes openings for threaded fasteners, or the like. Manifold 28 may comprise a polymer, a metal such as 6061-T6 aluminum, or a wide variety of altemative materials. Lumens 42 may have a dimension in a range from about 2 mm to about 3 mm. Fittings 44 optionally comprise standard YA-IS fittings. Sealing body 46 Avill often comprise an elastomer such as a natural or synthetic robber. The pressure transmission system (including manifold 28) will preferably maintain a seal when transmitting pressures greater than atmospheric pressure (positive gauge pressures) and less than atmospheric pressure (negative gauge pressures or vacuum). The pressure transmission system and modulator bank 14 will generally be :;apable of applying pressure differentials which are significantly higher than hydrostatic and capillary pressures exerted by, for example, a buSa: or other fluid in reservoirs 18, so as to avoid variabliitj* or noise hi the pressure differential and resulting flow rates. As capillary pressure within reservoirs 18 are typically less than 1/10 of a psi, oirai bdng less than l/l(K)th of a psi, the system will preferably be capable of varying pressure at reservoirs 18 ihrou’out a range of at least Vz psi, more often having a pressure range of at least 1 psi, and most often having a pressure range of at least +/-1 psig (so as to provide a 2 psi pressure differentiaL) Many systems will be capable of ‘jpl’ing at least about a 5 psi pressure differential, optionally having pressure transmission capabilities so as to apply pressure anywhere throughout a range of dt least about +/- 5 psig. A controi system for selecting the pressures apphed to reser/oirs 18 is schematically illustrated in Fig. 4. Controller 22 generally includes circuity and/or programming which allows the controller to determine reservoir pressures which will provide a desired flow within a channel of microfluidic network 30 (here schematically iQustxated. as microfluidic network controller 52) and also includes circuitry and/or programming to direct the modulators of modulator bank 14 to provide the desired individual reservoir pressures (here schematically illustrated as a plurality of pressure controllKS 54.) It should be understood that netwoik controller 52 and pressure controller 54 may be integrated within a single hardware and/or software system, for example, running on a angle processor board, or that a wide variety of distributing process tetitnkp’s might be employed. Srmilariy, while pressure controEers 54 are schematically ‘rfnsgaKd here as separate picssuic cosfirolkrs &r each modulaior, a single pressure comn’ler might be used with diez. sampKng zod/otmjitdpladDS tedmiqnes. In general, pressure controller 54 transmits drive signals to an actuator 56, and the actuator moves a piston of displacement pump or syringe 58 in response to the drive signals. Movement of the piston within pump 58 changes a pressure in pressure transmission system 20, and the change in pressure is sensed by pressure sensor 24. Pressm-e sensor 24 provides a feedback signal to the pressure controller 54, and the pressure controller will optionally make use of the feedback signal so as to tailor the drive signals and accurately position the piston. To enhance the time response of the pressure control systcm, pressure controller 54 may include pressure calibration data 60. The caUbration data vtiU generally indicate a correlation between drive signals transmitted to actuator 56 and the pressure provided from the pressure modulator. Pressure calibration data 60 will pref’’bly be determined by initially cahbrating the pressure change system, ideally before initiation of testing using the microfluidic network. Generation of calibration data 60 may be effected by transmitiing a calibration drive signal to actuator 56 and soising the pressure response using pressure sensor 24. The change of pressure from this calibration test msy be stored in the program as caHbration data 60. The calibration signal will typically cause a known displacement of the piston within pump 58. Usrag this known displacement and tiie measured change in pressure, the overall pressure system response may be calculated for future drive signals using the ideal gas law, PV = TJRT (in which B'is pressure, V is the total compressible air volume, n is the number of moles of gas in the volume, R is the gas constant, and T is the temperature). Calibration may be preformed for each modulator/pressure transmission systems/reservoir (so as to accommodate varying reagent quantities within the reservoirs, and the like), or may be preformed on a single reservoir pressurization system as an estimate for calibration for all of the modulators of the system. Once cahbration data 60 has been generated, pressure controller 54 can geneirale drive signals for actuator 56 quite qvdcksY in response to a desired pressure sigi’ transmitted from network controHs" 52. Ii diould be noted that these estimate will prefeg’bly acctanmodate the changing overall voiame of the compressible gas within the system, ro T’TTT the calcniated change in ptessme SDT a given displacement of the piston •sifinn pcnap 58 aEkrwpresanresina’-bemficretgfeaitiiesamcdi’lacaiieatoftiie pistcE at higli prcssarts (Lc., the aspiacaneatqsvssarc onrdatioo plot is not linear, but curves.) In the exemplary embodiment, actuator 56 comprises a stepper motor coupled to a linear output mechanism. Pump 58 comprises a syringe having a length of about 100 mm, and a diameter of about 20 mm. Overall response time for the system may depend on a variety of parameters, including dead volume, syringe size, and the iike. Preferably, the response time will be less than about 1 sec/psi of pressure change, ideally being less than about 500 msecs/psi for a pressure change from zero to 1 psi. Network controller 52 generally calculates the desired pressure from each pressure modulator in respon’w lo a desired flow in one or more of the channels of microfluidic network 30. Given a desired channel flow, network confroller 52 derives these pressures using network data 62, with the network data typically being sqjplied by eithCT a mathematical model of the microfluidic network 64 and/or a tester 66. Network data 62 will generally indicate a correlation between pressure differentials rophed to reservoirs 18 and flows wi&in the microfluidic channels. Network model 64 preferably comprises programming to help translate desired hydrodynamic flow rates into pressures to be applied at reservoirs 18. An exemplar}' network model 64 gaierates a hydrodynamic multi-level r’istaKje nstn'o±: correlating to each microfluidic networic 30, as can be understood with reference lo Figs. 5A-5C. Referring now to Figs. 5 A and 2, nodes can be defined at each well 18 and at each intersection 34. Hydrodynamic resistances of channel segments coupling the nodes can be calculated fiiom the chip design. More specifically, calculation of hydrodynamic resistances may be preformed using hydrostatic pressure loss calculations based on the cross sectional dimensions of channels 32, the length of channel segments connecting the nodes, the channel surface properties, the fluid properties of the fluids included m the flows, and the hke. Analysis of the multi-level flow resistance network may be performed using techniques oftai used for analysis of current in electrical circuits, as can be understood with reference to Figs. 5B-5C. Hydrodynamic resistances of the channel segments connecting reservoirs 18 to ad’iscssz iKxfcs may be analyzed as the lowest level of a multi4ev-d ndwcnk. The channel segments ai’orning these lowest level segments form ftje seccaad icvd of hydrodynamic rcsisaoccs of the network. This level-by-level anahrsas caatsaDcs TSSH aSi channels of miuulhudk: netwo’ 30 are iiK’luded in the nawcakmodd. Therdatrveflowratsofan’channdinfhemicrofhiidicnetwoikcan then be obtained once the flow rates from each of the reservoirs 18 in the lowest level have been calculated. As described above, flow resistances maybe calculated based upon hydrodynamic chip design alone. It is also possible to measure these resistances using, for example, electrical sensors, pressure drop sensors, or the like. In other words, resistances to hydrodynamic flow of the channels and channel segments may be measured by, for example, measuring electrical resistance between reservoirs 18 while a conductive fluid is disposed within the network. Regardless, once the channel resistances are known, the pressure drop in each channel segment in the network can be obtained by simply multiplying the flow rate of that channel with its associated channel resistance. The pressure of each reservoir 18 can then be calculated by summing up all the pressure drops along the network 30 starting at the top level of the network. Referring now to the exemplary program for calculating pressures illustrated in Figs. 5B and 5C, hydrodynamic flow rate Q is related to flow resistance B’ and pressure differential AP by the equation: This relationship is quite similar to that used in electrokinetic calculations, in which current I and electrical resistance R are related to voltage V by the equaiioii: V = I • R. This sioaplifies the application of circuit analysis techniques to the hydrodynamic analysis. Determination of reservoir pressures so as to provide a desired flow rate will preferably be performed using a pressure calculation program 70, as illustrated ia Fig. 5C. Desired flow rates are input in step 72 from each reservoir 18. These flow rates may be input by flie user, by an automated test matrix gsieration program, or the like. Flow r’istances are obtained 74 as described above, and the input flow rate propagates throng the network to obtain flow rates for f’’lf. Irsndi 76. The pressure drop of each brandi is fliendetenninediising the netwcHiressstaiice circuit 78. These pressure branches are feen aDowed to propagate tlir«'Hioh ftte n’woric to obtain reservoir pressures 80 so as to efectfijc desired flow. Rgfii.'Mig to Fig. 6, an alaznative eaBbodunent of a microfluidic sj’stem m?kc> Tise ofbofii eiectrokinetic transoOTt and hydnxhmannc tran’XHt mechanisms to move fluids within microfluidic channels of the system. Electrokinetic transfer of fluids has significant advantages when electro osmosis and/or electrophoresis are desired. Electrokinetic fluid transport is also both fast and conveniait, and modifications of the channel surfaces are possible to avoid and/'or deviate elecfrokinetic tran’x)rt ■ disadvantages. The plug profiles of fluid plugs moved within a electrokinetic transport system can also be well-controlled and defined. Electrokinetic/hydrodynamic system 90 also provides the advantages of hydrodynamic transport described above. This hydrodynamic transport is quite reliable, and is independent of charges and electrical surface properties of the channels. Hydrodynamic transport is particularly well-suited for biocompounds which are sensitive to electrical fields. Electrokinetic/hydrodynamic microfluidic system 90 includes many of the pressurization, microfluidic network and control components described above. In this embodiment, manifold 92 includes fittings 44 opening laterally firom the manifold to provide sealed fhdd coimnunication fiom each pressure transmission tube 20 to an associated reservoir 18 of tiie microfluidic device 12. Additionally, electrodes 94 are coupled to each reservoir 18 via manifold 92. In the exemplary embodiment the electrodes comxjrise platinum surfeces which extend down from manifoid 92 into electrical contact with fluids disposed within reservoirs 18 when the manifold provides a sealing engagement between fittings 44 and the reservoirs. Coupling of the electrodes with the fittings 44 may be provided by using'T" connectors within the manifeld for each well, and inserting a platinum electrode across and through the "T". The appropriate (upper, iQ this example) connector branch of the T-connector can be sealed and the electrode affixed in place with a sealing material such as epoxy. By coupling electrodes 94 to computer 22, and by including within computer 22 an electrokinetic fluid transport controller capable of inducing electro-osmosis and electrophoresis, the system of Fig. 6 is capable of emulating pumps, valves, Qi’>easers, reactors, separation systems, and other laboratory fluid handling mechanisms, often witiKJOt having to resort to moving parts on microfluidic device 12. Electrokinetic tian’xjrtation and control are desaibed in, for exanq)le, U.S. Patent No. 5,965,001, pggvion’ incoiporated herein by lefiaeuce. Ooe particular advant’eocs use of me ptessme modulated flow ctaitrol can be undasfcood wi& reference to Figs. 7A and 7B. Jnmzny rhcmicsO analysis, n is desirable to varv tbe rdative flow rates from two reservoirs connected to a common node ‘X so as to vary a concentration of a test solution, reagent, or the hke, particularly for defining standard curves of chemical reactions. As illustrated in Fig. 7A, it is possible to vary the flows firom two reservoirs electroJdnetically, with the relative fluid concentrations being indicated by the changes in fluorescence intensity over time. Unfortunately, control over the relative flow rates (and hence, &e concentration) may be less than ideal due to variation in capillary forces within the reservoirs and the like. An alternative well-pair dilution plot in Fig. 7B can be generated by varying concentrations using multi-pressure control. This plot illustrates the reduced noise and enhanced flow control provided by the pressure control systems of the present invention. As generally described above, hydrodynamic control can be enhanced by increasing resistance of the channel segments. In the exemplary microfluidic device 12 illustrated in Fig. 2, channels 32 coupling weUs 18b, 18c, 18d, ISe, ISf’ and 18g to Ihe adjacent nodes have a resistance of 1.3 x 10’’ g/cm'* s. Channel 32 coupling ressvoir 18a to the adjacent intersection 34 has a resistance of 4.8 x 10’° g/cm'* s. Such a chip is well-suited for use with flows having a pressure drop between reservoirs of about 2 psi, so as to provide a mixing time of about 6 seconds, and a reaction time of about 20 seconds. Fig. 7C is a plot of measured dilution vs. set dilution for a dilution well-pair with a hydrodynamic flow system, showing the accuracy and controllability of feese dilution methods. Figs. 7D and 7E are plots of the measured dilution near the iipper and lower extremes, respectively, showing that a small amount of mixing at a channel intersection may occur whsn. flow fi-om a channel is at least substantially halted. As can be understood with reference to these figures, some modification of the overall flow fijom one or more channels at an intersection may be used to effect a desired dilution jercentage adjacent a maximimi and/or a minimmn of the dilution range. For example, relative flow adjustments within 5% of a maximum or miniTnum desired dilution, and often within 2.5% of a desired maximum and/or mtnimum may be employed. More specifically, to achieve a near 0% actual dilution fix)m a given channel at an intersection. Quid may flow’ into the channel at the intersecdoa. Sicrdlarly, to achieve 100% measured diiutkm from the channel, more than 100% of the desired flow may be provided firom the sutH>ly channel into fee iniCTsection. CStaracterization of an sEvme ofeai involves determination of maximum i’actksi -rdkxity and aKfichadis con>acir fer eatx. sifesUaie. The enzymatic reaction of ABcaiinc Pboqaaflase oc dFMUP (as iBnstratoi in Fig. 8) was studied on a micxofkddic device 12 q>timized fer pressure (iiven flow. Fig. 8A is a titiadcm carve for difTeail concentrations with and without substrate. A plot of background corrected signal vs. substrate concCTitration is shown in Fig. 8B, while a Lineweaver-Burk plot for the Michaelis constant (Km) is provided in Fig. 8C. Results of a substrate titration assay for the reaction are shown in Fig. 8D. 5 Additional exemplary assay reactions, assay results, and microfluidic networks to provide those results are illustrated in Figs. 9A through lOB. More specificaUy, Figs. 9A-C illustrate the reaction and assay results for a Protein Kinase A (PKA) assay performed at different ATP concentrations. Fig. 1OA illustrates a chip design having a microfluidic network 130 of microfluidic channels 32 connecting 10 reservoirs 18, in which the network is adapted for a mobility shift assay. Fig. 1 OB are exemplary results of a mobility shift assay at different concentrations of ATP as may be measured using the chip design of Fig. lOA. Referring now to Figs. 1 lA and 1 IB, an exemplary manifold or chip interface structure 92' is illustrated in more detail. Exemplary manifold 92' is adapted to 15 provide both hydrpdynamjc coimling and electroldnetic coupling betweai a micajoflmdic body and an associated controller, as described above. Electrical conduit passages 140 for coupling electrodes 94 to a system controller 22 (see Fig. 6) are illustrated in Fig. 11 A. Fig. 1 IB illustrates manifold pressure transmission lumais 142 which provide fluid communication betweeai fittings 44 and a microfluidic body interfece surface 144 within 20 manifold 92'. Manifold lumeais 142 are illustrated in phantom. Accurate control of the flow of fluids within a network of microfluidic channels can be quite challenging within even a relatively simple network of channels. More specifically, in many microfluidic apphcations, a variety of different fluids (with different characteristics) may be present in a single channel segment. As described 25 above, where tihe hydro-resistance of each channel segment can be obtained, it may be possible to simulate and calculate the flow of fluids throughout fee networic for a given pressure configuration. Unfortunately, it can be quite difficult to accurately calculate \iscc«ilies (and, haice, resistances and flow rates) when several different buffers are used within a diamieL often together with one or more different test fluid samples. 30 FoitPsufply, a relatively sirnple flow sensor can be provided to measure an actaalSowwifhinadttaimdof amcrofimcficQrtwosk. "Whse the measured flow results from a known driving ftsrce (scffih as a fcoown ptetaaia differential) can be dgtermrned, prcssires to be ‘fcoed aSL fee fluid reseryoks so as to affea a desired flow conditkai may thai be calculated. Referring now to Fig. 12, a relatively simple viscometer 150 makes use of a channel intersection 152 at a first location and a detector 154 at a second location to measure fluid flow characteristics. In general, a steady-state flow within a microfluidic channel 32 between intersection 152 and sensor 154 may be produced using a pressure 5 differential between reservoirs 18, as described above. Intersection 152 may impose a signal on the steady-state flow by applying a pressure pulse to one or more of the reservoirs 18, by applying an electrokinetic pulse across intersection 152, or the like. The signal imposed at int«-sectioii 152 win often be in the form of a small flow pertmbation, typically for a short duration. For example, where reservoir 18d includes a detectable 10 dye, the flow perturbation or signal may comprise an increase or decrease in the dye concentration in the flow of microfluidic channel 32 from intersection 152 toward detector 154. Detector 154 is downstream from intersection 152, and can be used to detect die arrival time of Ae signal, for example, as a peak or dip in &e iatensit\' of a 15 fluorescent signal fix>m the dye. Thus, the time difference between iniposition of the signal at intersection 152 and sensing of ftie signal flow at d’ector 154 mzy be readily measured. CaUing this time differential At, and knowing the distance along channel 32 between intersection 152 and detector 154, Ad, from the microfluidic netwoik geometry, the flow rate Q can be calculated from the equation: 20 Q = A{Ad/At) in which A is the cross-sectional area of the channel. This measured flow rate of a steady-state flow for a given initial driving force greatly facilitates calculation of an appropriate pressure configuration to achieve a desired flow. Where viscosity is to be determined by the system of Fig. 12, reservoirs 25 18d and 18e coupled to channel 32 by intersection 152 may individually or in combination introduce fluid of known or unknown viscosity into the microfluidic channel at the intersection to provide a flow within the Cdiannd ba\Tng an unknown total flow resistance. Wife channel 32 c’tionall}' ccHitaitcng only a trace amount of fluorescent dye (to inhfoit any sSect of the dye on the unknown orsali viscosity), a substantially 30 caostaai pressure cxmSgars&m at ports 18 may drive Sow from intCTsecticm 152 toward detecErl54. TljissteKiy’-statefkmrcaockknmaybeeffijctedby acoaistant reservoir 18a a’’aced detector 154, positrve piegguies ‘?plied at leseryoirs 18d, 18e ac’aceat intersectiaE 152, or a combin’iOTi of botih. Regardless, the steafy-state flow ■bc:’ with a constant pressiire differential will result in a volumetric flow rate Q in channel 32 which is linearly proportional to the pressure differential AP and inversely proportional to the fluid viscosity rjas follows: Q = KAP/7j 5 K is a proportionality constant which depends on the geometry of the channel netw’ork. II can be calculated from the channel geometry, or can be determined through a calibration standard test, or the like. A variety of alternative structures may be used to sense flow characteristics so as to apply a proper pressure configuration to generate a desired flow. 10 For example, a signal may be imposed on a flow within a microflmdic channel by photobleaching of a fluorescent dye, rather than imposiag a flow perturbation at z intersection. Alternative flow velocimetry ‘proaches such as laser Dopier velocimetrv-, tracer particle videxsgrapby, and the like are also possible. Usiag such tedmiques, a simple straight channel coimecting a fluid supply reservoir and a waste fluid reservoir 15 may suffice, with the fluid supply resacvoir containing a fluid comprising a photobleachable fluorescent tracer dye or appropriate tracer particles. As can be understood with reference to the calculations of flow rate O above, sensors may also be used to determine alternative flow characteristics within a microfluidic channel, including flow rate, viscosity; the proportionahty constant for a 20 segment or network (by use of fluids having known and/or uniform viscosities) and/'or other flow characteristics, hi fact, in addition to providing a tool to study effective viscosity of two or more mixed fluids (of optionally unequal viscosity) stiU further measurements are possible. Mixing of DMSO and an acquiesce buffer can yield a nonmonotonic viscosity-composition relationship. By applying different levels of pressure 25 differential AP and measuring the flow rate Q, viscometer 150 could be used to estabhsh a relationshigj of the effective viscosity during mixing as a function of mixing length This nifoTmation may be pertinCTit to chip design for tests ■which involve geometric dilution. "Where temperature depawksicy of viscc’ty is of interest, systems such as vTscOTBEtsr 150 can be coi’led to a ten5)erature control Systran con:q)risiQg an external 30 heater blodc in coEtac’ with febo are a function of the sheer rate experienced by the fluid. One example of a non-Newtonian fluid is a polymer solution containing high molecular weight molecules. A microfluidic viscometo- similar to viscometer 150 of Fig. 12 might have a channel geometry and/or channel network intersection structure and/or flow arranged so that the application of a pressure differential creates a range of sheer stresses so as to accurately measure such non-Newtonian viscosity. Real-time flow and viscosity measurements for microfluidic systems based on transient pressure pulse techniques can be further understood with reference to Figs. 13 A and 13B. A microfluidic network structure 30 with a single branch channel coupling each node to a main channel 32' is used. Each branch can be connected to a single reservoir 18 for a different buffer, sample, enzyme, or hke. In the simplest embodiment, reservoir 18e at the end of the microfluidic channel network contains a dye solution to provide a detectable signal. A steady flow can be directed toward reservoir 18a by applying initial pressures on wells 18. A short pressure pulse may be applied to well 18e and/or some or all of the other reservoirs of fee microfluidic system. This prrasure pulse will uiopagaie substantially instantly to alter flow at some or all of the intersections 34 of network 30. This disturbance of the flow at the node poitits can change the dilution ratio ffom one or more of the side branches. After the pressure pulse, steady state flow is resumed. As can be understood with reference to Fig. 13B, a time series of signals 160a, 160b, and 160c occur at times Ti, T2, and T3, respectively. The flow rate from some or all of the side branches may then be obtained from the difference of flow rates between successive node points. Once the flow rates of the branches have been obtained, as the pressures at reservoirs 18 are known, the resistances of the branch channels may then be calculated. From the known channel geometry, the viscosity of the solution in the side branches may also be detemuned. This information can thai be fed back to the netwoii: model to derive the pressures for a desired flow rate from each reservoir. Refexing now to Figs. i4A and l-Q, ex€n3|jlary time signature data indicates that prs’ure pulse signals can effectrvdy be inxpoxd on the flow within a miCToSuidic system, and can accurately and r’)eats2h' be sensed by a detector (such as an optical detector, or tiK fifce) for meaOTreoaciit of few characteristics. Hjpdrodyjsanic’ dectrtdcHictic, sod odser fluid transport mechanisms may be used in a variety of ways to provide specialized functions within a mit’oflnidic system. For exanqjle, finid mixtores sach as biological fltoid san5)les having particulates and/or cells in suspension within a hquid are often introduced into microfluidic systems. A particularly advantageous system and method for introducing a large number of samples into a microfluidic system is described in U.S. Patent Nos. 5,779,868 sad 5,942,443, the full disclosure of which is incorporated herein by reference. In tiiat 5 system, a vacuum may be used to draw a sequential series of fluid samples firom the wells of a multi-well plate into a capillaiy tube in fluid communication with the microfluidic system. In the above-described system, it may be desirable to maintain fluids at a substantially stationary location within the microfluidic channel, for example, during tiie 10 time delay while a sample in a last well of a first multi-well plate is moved away fi-om the capillary tube and before a sample in a first well of a second multi-well plate is in fhnd communication with the capiEary tube. Maintaining the fluids within the microfluidic channel at a substantiaUy fixed location can avoid introducing significant amounts of air iato the microfluific system, which mi’t interfere with its operation. In general, it may 15 be desirable to maintaia fluid mixtures at a given location within a microfluidic network for a wide variety of reasons. Unfortunately, work in connection with the present iavention has found that halting movement of some fluid mixtures within a microfluidic network may have significant disadvantages. Specifically, ceU-based assays performed using a fluid mixture 20 including cells suspended in a liquid are susceptible to sticking of the cells to the channel walls if flow is completely halted. Similarly, other fluids may deteriorate if flow within the channel is sxifficientiy low for a sufficient amount of time. To avoid deterioration of fluid mixtures, the present invention can provide a small amplitude oscillatory movement.of a fluid mixture so as to maintaia the fluid 25 mixture within a microfluidic channel. Modulator bank 14 is capable of providing a small amplitude oscillatory pressure such that there is no significant inflow or outflow of materials Scorn the channel. This small ampHtude oscillatory pressure will preferably be sufficiem to continuously move the fluid mixmre (and. for example, the ceUs within the liquid) contimiously back and forth. The osdiiation frequency should be high enough 30 such that the instantar’ous fluid mixture velocity is sumciaitiy high to avoid detedaration of tie mixmre, while ampHirinp: sho«M be small enou’ such that there is Ettie cs" no ‘‘nrr’’’"‘'‘ let trai’xaiaiicai inso car oct of the dranrgl fiom afiacait rese5rvx>iis, reserroiis saad intesecting rjvi’’fpt4v. Once tiie desired deday in fluid mixiiire movancnt has been provided it wiH often be desirable to flow an intervening Hquid sach 5’ channel ends have been flushed. It should be noted that this small amplitude oscillatory motion may optionally be provided using electroldnetic forces, such as providing an alternating 5 current, particularly ifthe alternating current is not harmful to ceUs or other components of the fluid mixture. It may also be beneficial to insure that cells in the channel do not lyse when subjected to the alternating ciurent if electroldnetic forces are to be used to induce the oscillatory motion. Referring now to Fig. 15, the systems and methods described above may 10 optionally take advantage of a wide variety of pressure transient generators so as to ioitiate a flow perturbation. A multiple c’iliaiy assembly 170 includes a microfhadic body or chip 172 mounted a polymer interface housing 174. A plurality of capillaries 176 contain fluid introduction channels. As explained in detail in U.S. Patent No. 6,149,787, the fiiU disclosure of winch is incorporated herein by reference, the capiUsry channels can 15 be used to spontaneonsly inject fluids into the microfluidic networic of chip 172 using capillary forces betweai fee iigected fluid and liie capillary channels. Sudi spontaneous injection is sufficient to induce a pressure transient for measurement of hydrodynamic and/or electrokinetic flow. Such flow measurements allow the derivation of infcnmation regarding the properties of the chip, microfluidic network, and/or fluids. 20 The use of multiple capillary assembly 170 is beneficial for parallel assays using a plurality of test samples, and the like. Referring now to Fig. 16, a sinq)le ckip 178 having a relatively straightforward micro flxiidic network may be used to understand tiis derivation of flow and/or chip properties from spontaneous icy ection. In many embodiments, the open end of capillary 176 wiU be placed in a fluid, typically by 25 introducing the end of the capillary into a microtiter plate (or any other structure stq>porting one or more fluid test samples). This may be efiected by moving the capillary 176 and diip 178 relative to the microtiter plate, by moving the microtiter plate relative to the capuhry or by moving both structures rslativs to each other. Regardless, placing capillary 176 into a fluid results in spontanecHB introdiKiion of the fluid into the capillary 30 channel. 3y applying a constant vacuum on st least one well of the microfluidic system, a steady flow may tiiec be jHOvided along a channel caapiing the c’illary to the well. If; for sxxnpis’ a steady-staie flow is induced &am csfoUiBy 176, a substrate rsservoir 180a, and/cjr an enzynw reservoir 180b toward a vacunm reservoir or waste wdl 180c akmg a dormd 182, a flow pcrtarb’on can be initiated at intersecticin '1 186 between the capillary channel and the microfluidic network at the time the capillary is withdrawn out from the well containing the introduced flxiid. This flow perturbation may, for example, comprise a change in composition of the flow progressing along . channel 182 toward vacuum reservoir 180c. This change in composition may be sensed 5 at a detection location 184 as, for exairq)le, a change in fluorescent intensit}'. Similar flow perturbations might be induced by applying other pressure transients at intersection 186, for example, when capillaiy 176 is introduced into the spontaneously injected fluid, or by applying a change in pressure using a pressure modulation pump as described above, again changing the composition of the flow within channel 182. By monitoring 10 the propaly of the composition at detection point 184, progress of the perturbations 11133/ be detected. A time delay between initiation of the perturbation and their respective detections at the detection point when combined with a known length of channel 1S2, can be used to determine a speed of the flow within the channel. From this actual, real-time speed, a variety of information regarding the fluid and/or network system may be 15 determined. Referring now to Figs. 16A and 16B, each time c’illary 176 is dipped into and are removed flxim a fluid well, a perturbation will be gaierated at a cspOlBiy intersection 186 coupHng tbe ci’riliaiy channel with the microfluidic network. Additionally, as the pressure perturbation will propag’e throughout the microfluidic 20 network, another flow perturbation may be simultaneously initiated at a second intersection 186a downstream of tiie sipper intersection 186. If we assume that fluid is flowing from reservoirs afBxed to the nMcrofluidic network toward a vacuum reservoir 180c, the pressure transient appUed by spontaneous injection at capillary 176 will alter tiie mixtures occurring at each intersection. 25 Where the channel lengths may be designated, and AJi, Adj a time delay may be measured at detector 184 between initiation of the pressure transient (at t = 0) and sensing of a 5rst flow pealurbation as a signal 1 SSa sr detector 184. The first signal 188a may be sffid to have occurred after a time dday of Ati, witii this time being the time required fix- flow to prop’ate from fee intssection immediately upstream of detector 30 184. A simSartiri’deisyAfi will tfaec be required for flie flow to propagate from the second vipscrssm iriecseaxxi (186 in tbe sJn’jie ikctwcai of Fig. 16A). Where the channel leagdis b’ween inteisection are known, fee various time ddays can be used to deierraxDe A'‘' the various fluid speeds between intersections. Where the channel cross-sections are known, this information can be used to determined contributions fi-om branch chaimels to the flow volume, and the Mke, regardless of whether the flows throughout the microfluidic system are induced hydrod}'namically, electrokinetically, electroosmoticalJy, or the like. 5 Referring now to Fig. 16C, capillary 176 may be dipped into and removed firom a variety of fluids in a sequential series. P indicates pressure. Si is a signal indicating a flow perturbation caused at a first intersection by spontaneous injection into the capniaiy, and signals S2 indicates a flow perturbation signal generated at a second intersection by the same spontaneous injection at the capillary. A series of pressure 10 transients 190 will be generated by capillary 176 when the capillary is, for example, dipped into and removed fi-om a dye, followed by dipping of the axillary iato abxiffsr solution, followed dipping of the c’illary into a first test substance well, and the Hke. This sequOTce of spontaneous injection events at capillary 176 may result in generation of a series of Si signals due to a series of flow perturbations at, for example, intersection 15 186a. Simultaneously, a sari’ of seccmd flow perturbation signals S2 will also be generated at intersection 186, with detection of the second series following the first series by a time delay At2 which is depaident on the speed of fluid within the netwoik: channels. The total signal St measured at detector 184 will be a combination oftins offset series of signals with the more immediate Si signals. Furthermore, the composition of the overall 20 flow arriving at the detector may vary significantly with the different materials introduced by capillary 176. Regardless, by properly identifying the time delays between signals, flows between the nodes of the microfluidic system may be calculated. Referring now to Fig. 16A, placing a detector 184a downstream of an electrode vi may facilitate measurements of electrically induced flow, such as 25 electroosmotic flows induced by a differential voltage between Vi and V2. As described above, pressure perturbations will be initiated at the channel intersections, so that an initial signal may be generated at the detector firim the downstream electrode Vi, followed by another signal generated at the upsuream eiectnxie V2. Setting A/i, as the time delay between these electrode intersections and A*2.- 2s the time delay for a 30 siibsequani; agnal generasd by a reacticni rhannP'; at intesecticsi 186, and knowing the lengths of the chanoels AJi, A’ we can calculate the electroosmotic EO flow as follows: 3l With voltage between the electrodes off, using only pressure to drive fluids within the network, we can determine velocities along the channels between nodes caused by pressure vip, V2p from: AL M Ad, -' ‘‘ Ad, ' 5 While leaving the same pressure differentia] on the voltage differential may then bs turned on, allowing us to calculate the electroosmotic flow velocity as follows: At' At' =- = V-, — = Vi + V A J ‘P and KJ ‘P ‘ ; which gives us >'e. Ail-Ad, This electroosmotic velocity may then be used to calculate electroosmotic Z/„ = ‘"' 10 mobihty using the equation: ' ‘° /T > ‘ which Ej is the electric field strengdi between the first and second voltages Vi, V2. Fig. 1’ gr;apbically illustrates data fix)m a detector or sensor firam which the time delays discussed above may be taken. The multiple capillary assembly and simplified capillary networks ofFigs. 15,16 and 16A are examples of microfluidic devices which might benefit from 15 monitoring of pressure induced flow perturbations for analysis and/or confrol of flows, quality control, and the like. Additional examples of microfluidic structures which may benefit from these techniques are illustrated in Figs. 17A, 17B, ISA and 18B. Referring now to Figs. 17A and 17B, more complex microfluidic networks may include a plurality of capillary joints or intetsectkHS 192 and substrate weUs or 20 resavoirs 194, enzyme weDs 196, wastewells 198, znd the like. One or more detection or sensor windows or locations 200 may be provkiBd fer mcsdtoring of propagation of the fiow patmb’kns. The irdCToflmdic assembly and Dctwadc ofFigs. 17A and 17B may be useful fix gmiti-capifiaryfiixKogecicassa’ A nmtth-ca|Kllzry base mobility-’uft nncrofioidkasscnAJyaodiKtwodchavingsinrilarstrtKtaresisfl ISA >oi.. and 18B. This structure also includes a plurality of electrode wets 202 for applying voltages to the microfluidic network, as described above. While the exemplary embodiments have been described in some detail, by way of example and for clarity of example, a variety of modifications, changes, and adaptation will be obvious to those of small in the art. Hence, the scope of the present- WE CLAIM: 1. A microfluidic system (10) comprising: a body forming a microfluidic channel network (30) and a plurality of reservoirs (18) in fluid communication with the network, the network comprising a first microfluidic channel (32) and means (14) for independently varying pressures within the reservoirs (18), the pressure varying means (14) in fluid communication with the reservoirs. 2. The microfluidic system as claimed in claim 1, wherein the means (14) for independently varying pressures within the reservoirs comprises a plurality of pressure modulators each pressure modulator providing an independently variable pressure. 3. The microfluidic system as claimed in claim 2, comprising a plurality of pressure transmission lumens, the lumens transmitting the pressures from the pressure modulators to the reservoirs so as to induce a desired flow within the channel. 4. The microfluidic system as claimed in claim 3, wherein the first microfluidic channel has a resistance to the channel flow, and wherein the lumens transmit the pressures to the reservoirs with a lumen flow, resistance of the lumens to the lumen flow being less than the channel resistance. 5. The microfluidic system as claimed in claim 3, wherein each pressure modulator is in fluid communication with an associated reservoir via an associated 34 lumen, and comprising a network flow controller coupled to the pressure modulators, the network flow controller transmitting signals to the pressure modulators, the pressure modulators independently varying the pressures in response to the signals so as to induce a channel flow within the channel. 6. The microfluidic system as claimed in claim 5, wherein the network flow controller comprises channel network data correlating the channel flows and the pressures from the pressure modulators. 7. The microfluidic system as claimed in claim 6, wherein the network flow controller calculates desired pressures from the pressure modulators in response to the network data and a desired flow in the first microfluidic channel. 8. The microfluidic system as claimed in claim 6, wherein the network comprises a plurality of microfluidic channels in fluid communication at channel intersections, the intersections and reservoirs forming nodes coupled by channel segments, and wherein the network data indicates correlations between flows in the channel segments and the plurality of pressure. 9. The microfluidic system as claimed in claim 8, comprising a network data generator coupled to the network flow controller, the data generator comprising at least one member selected from the group consisting of a network flow model, a viscometer coupled to the first microfluidic channel, and a network tester adapted to measure at least one parameter indicating the pressure-flow correlation. 35 10. The microfluidic system as claimed in claim 2, comprising at least one pressure controller, the pressure modulators varying the pressures in response to drive signals from the at least one pressure controller. 11. The microfluidic system as claimed in claim 10 comprising a plurality of pressure sensors each pressure sensor transmitting pressure signals to at least one pressure controller along a pressure feedback path in response to the pressures, wherein the pressure controllers transmit the drive signals to the pressure modulators in response to the pressure signals. 12. The microfluidic system as claimed in claim 10, wherein the pressure controllers comprise calibration data correlating the drive signals and the pressures. 13. The microfluidic system as claimed in claim 10, wherein the pressure modulators comprise pneumatic displacement pumps. 14. The microfluidic system as claimed in claim 3, wherein at least one sample test liquid is disposed in the channel network and pressure-transmission fluid is disposed in the lumens with a fluid/fluid pressure-transmission interface disposed therebetween. 15. The microfluidic system as claimed in claim 14, wherein the pressure-transmission fluid comprises a compressible gas. 16. The microfluidic system as claimed in claim 3, wherein the lumens compliantly couple the pressure modulators with the channel flow. 17. The microfluidic system as claimed in claim 2, wherein the plurality of pressure modulator comprise at least four independently variable pressure modulators. 18. The microfluidic system as claimed in claim 2, wherein the plurality of pressure modulator comprise at least eight independently variable pressure modulators. 19. The microfluidic system as claimed in claim 3, comprising a pressure interface manifold releasably engaging the body, the manifold providing sealed fluid communication between the lumens and the reservoirs. 20. The microfluidic system as claimed in claim 1, comprising a plurality of electrodes coupled to the network and an electrokinetic controller coupled to the electrodes so as to induce electrokinetic movement of fluids within the network. 21. The microfluidic system as claimed in claim 1, wherein a difference between the pressures greater than a capillary pressure of fluids within the reservoirs. 22. The microfluidic system as claimed in claim 2 comprising a network flow controller coupled to the pressure modulators, the network flow controller comprising channel network data correlating a flow within the first microfluidic channel and the pressures from the pressure modulators, the network flow controller independently varying the pressures from the pressure modulators in response to a desired flow within the first microfluidic charnel and the network data. 37 23. The microfluidic system as claimed in claim 22 comprising means for generating the network data (62) coupled to the network controller. 24. The microfluidic system as claimed in claim 23, wherein the network comprises a plurality of channels having a plurality of intersections, each reservoir and each intersection forming a node, wherein the means for generating the network data comprises a model of the network nodes and channel segments connecting the nodes, wherein the model determines resistances of the channel segments. 25. The microfluidic system as claimed in claim 23, wherein the means for generating network data comprises an electrical resistance sensor for sensing electrical resistance through the network. 26. The microfluidic system as claimed in claim 2 comprising: a network flow controller, the network flow controller generating independent desired pressure signals in response to a desired flow within the first microfluidic channel, wherein the plurality of pressure modulators is coupled to the network flow controller, each pressure modulator in fluid communication with an associated reservoir; and a pressure controller with calibration data coupling the pressure modulators with the network flow controller, the pressure controllers transmitting drive signals to the pressure modulators in response to desired pressure signals from the network flow controller and the calibration data. 27. The microfluidic system as claimed in claim 2 comprising: a sensor coupled to the first microfluidic channel for transmission of signals in 38 response to flow within the channel; and a controller coupling the sensor to the pressure modulators, the controller transmitting pressure commands in response to the signals to provide a desired flow. 28. A microfluidic method comprising: determining pressure-induced flow characteristics of a first microfluidic channel in a microfluidic network; deriving a first plurality of pressures from the characteristics of the microfluidic network so as to provide a first flow in the first microfluidic channel; and inducing the first fiow by applying the first plurality of pressures to a plurality of reservoirs in communication with the microfluidic network. 29. The microfluidic method as claimed in claim 28, wherein the first plurality of pressures is applied to the plurality of reservoirs using a plurality of pressure transmission systems. 30. The microfluidic method as claimed in claim 29 comprising: deriving a second plurality of pressures fi-om the characteristics of the microfluidic network so as to provide a second flow in the first microfluidic channel; and inducing the second flow by applying the determined second plurality of pressures with the pressure transmission systems. 31. The microfluidic method as claimed in claim 30, wherein the pressure transmission systems have resistances to pressure-transmission flows which are 39 significantly less than a resistance of the microfluidic network to the pressure-induced flow during the flow inducing steps. 32. The microfluidic method as claimed in claim 30, wherein a first reservoir has a first fluid and a second reservoir has a second fluid, wherein the first and second reservoirs are couple to the first microfluidic channel, the first flow comprising a first solution with concentrations of the first and second fluids and the second flow comprising a second solution with concentrations of the first and second fluids different than the first solution, and wherein the determining step is performed so as to provide the second flow with the second solution. 33. The microfluidic method as claimed in claim 28, wherein the network defines a plurality of nodes at the reservoirs and at intersections of microfluidic chaimels, wherein the determining step comprises determining flow resistances of the charmels between the nodes. 34. The microfluidic method as claimed in claim 28 for use with a fluid mixture which can degrade when held stationary, wherein the step of inducing the first desired flow comprises: introducing the fluid mixture into a microfluidic channel of a microfluidic network; maintaining the fluid mixture by oscillating the fluid mixture within the channel; and transporting the maintained fluid mixture along the channel. 40 35. The microfluidic method as claimed in claim 30, comprising sensing the first flow thin the first channel, wherein the second pressures are determined in response to the sensed flow. 36. The microfluidic method as claimed in claim 35, wherein the first flow comprises a substantially steady-state flow, and comprising initiating a change in the first flow at a first channel intersection by applying a pressure pulse, determining a flow time for the change in the first flow to propagate to the sensor, the second pressures being determined using the flow time. 37. The microfluidic method as claimed in claim 28, wherein determining pressure-induced flow characteristics of a first microfiuidic channel within a microfluidic network comprises: inducing flow within the first microfluidic channel; determining a first measured flow; and calculating a pressure from the first measured flow. 38. The microfluidic method as claimed in claim 37, wherein determining a first measured flow comprises: generating a detectable signal within the flow at a first location; and measuring a time for the signal to reach a second location. 39. The microfluidic method as claimed in claim 38, wherein the signal comprises a change in fluid of the flow, the first location comprising a first intersection of a plurality of microfluidic channels. 41 40. The microfluidic method as claimed in claim 39, comprising initiating the change in the fluid at the first location hydrodynamically by applying a pressure pulse to a reservoir in fluid communication with a first intersecting channel. 41. The microfluidic method as claimed in claim 39, comprising initiating the change in the fluid at the first location electrokinetically by varying an electrical field across the first intersection. 42. The microfiuidic method as claimed in claim 38, comprising measuring a plurality of detectable signals from a plurality of channel intersections by sensing a time the signals reach the second location. 43. The microfluidic method as claimed in claim 38, wherein the signal comprises a change in optical quality of the fluid at the first location. 44. The microfluidic method as claimed in claim 43, wherein the fluid comprises a dye, the method comprising: photobleaching the dye at the first location; and sensing the photobleached dye at the second location. 45. The microfluidic method as claimed in claim 37, comprising determining a speed of the flow. 46. The microfluidic method as claimed in claim 45, wherein the speed of the flow is determined by laser Doppler velocimetry or tracer particle videography. 42 47. The microfluidic method as claimed in claim 45, comprising calculating a viscosity of the flow using a first pressure used to induce the flow and the speed of the flow, wherein the calculated pressure is calculated using the viscosity. |
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in-pct-2002-1345-che abstract-duplicate.pdf
in-pct-2002-1345-che abstract.pdf
in-pct-2002-1345-che claims-duplicate.pdf
in-pct-2002-1345-che claims.pdf
in-pct-2002-1345-che correspondence-others.pdf
in-pct-2002-1345-che correspondence-po.pdf
in-pct-2002-1345-che description(complete)-duplicate.pdf
in-pct-2002-1345-che description(complete).pdf
in-pct-2002-1345-che drawings-duplicate.pdf
in-pct-2002-1345-che drawings.pdf
in-pct-2002-1345-che form-1.pdf
in-pct-2002-1345-che form-13.pdf
in-pct-2002-1345-che form-18.pdf
in-pct-2002-1345-che form-26.pdf
in-pct-2002-1345-che form-3.pdf
in-pct-2002-1345-che form-5.pdf
in-pct-2002-1345-che others.pdf
in-pct-2002-1345-che petition.pdf
| Patent Number | 218884 | ||||||||||||||||||
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| Indian Patent Application Number | IN/PCT/2002/1345/CHE | ||||||||||||||||||
| PG Journal Number | 23/2008 | ||||||||||||||||||
| Publication Date | 06-Jun-2008 | ||||||||||||||||||
| Grant Date | 16-Apr-2008 | ||||||||||||||||||
| Date of Filing | 27-Aug-2002 | ||||||||||||||||||
| Name of Patentee | CALIPER LIFE SCIENCES, INC | ||||||||||||||||||
| Applicant Address | |||||||||||||||||||
Inventors:
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| PCT International Classification Number | G01N 27/26 | ||||||||||||||||||
| PCT International Application Number | PCT/US2001/005960 | ||||||||||||||||||
| PCT International Filing date | 2001-02-23 | ||||||||||||||||||
PCT Conventions:
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