Patent Publication Number: US-8992858-B2

Title: Microfluidic devices and methods of use

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 09/970,122, entitled “MICROFLUIDIC DEVICES AND METHODS OF USE,” filed Oct. 2, 2001 (“the parent application”). The parent patent application claims priority from U.S. Provisional Patent Application No. 60/237,938, entitled MICROFLUIDIC DEVICES AND METHODS OF USE, filed Oct. 3, 2000. The parent application also claims priority from the U.S. Provisional Patent Application No. 60/237,937, entitled VELOCITY INDEPENDENT MICROFLUIDIC FLOW CYTOMETRY, filed Oct. 3, 2000. All of the foregoing patent applications are incorporated by reference herein for all purposes. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under Grant No. HG-01642-02 awarded by the National Institute of Health. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Sorting of objects based upon size is extremely useful in many technical fields. For example, many assays in biology require determination of the size of molecular-sized entities. Of particular importance is the measurement of length distribution of DNA molecules in a heterogeneous solution. This is commonly done using gel electrophoresis, in which the molecules are separated by their differing mobility in a gel matrix in an applied electric field, and their positions detected by absorption or emission of radiation. The lengths of the DNA molecules are then inferred from their mobility. 
     While powerful, electrophoretic methods pose disadvantages. For medium to large DNA molecules, resolution, i.e. the minimum length difference at which different molecular lengths may be distinguished, is limited to approximately 10% of the total length. For extremely large DNA molecules, the conventional sorting procedure is not workable. Moreover, gel electrophoresis is a relatively lengthy procedure, and may require on the order of hours or days to perform. 
     The sorting of cellular-sized entities is also an important task. Conventional flow cell sorters are designed to have a flow chamber with a nozzle and are based on the principle of hydrodynamic focusing with sheath flow. Most conventional cell sorters combine the technology of piezo-electric drop generation and electrostatic deflection to achieve droplet generation and high sorting rates. However, this approach offers some important disadvantages. One disadvantage is that the complexity, size, and expense of the sorting device requires that it be reusable in order to be cost-effective. Reuse can in turn lead to problems with residual materials causing contamination of samples and turbulent fluid flow. 
     Another disadvantage of conventional sorting technologies is an inability to readily integrate sorting with other activities. For example, due to the mechanical complexity of conventional sorting apparatuses, pre-sorting activities such as labeling and post-sorting activities such as crystallization are typically performed on different devices, requiring physical transfer of the pre- and post-sorted sample to the sampling apparatus. 
     This transfer requires precise and careful handling in order to prevent any loss of the frequently small volumes of sample involved. Moreover, sample handling for conventional sorter structures is time-consuming. The resulting delay may hinder analysis of materials having limited lifetimes, or prevent sorting that is based upon time-dependent criteria. 
     Therefore, there is a need in the art for a simple, inexpensive, and easily fabricated integratable sorting device which relies upon the mechanical control of fluid flow rather than upon electrical interaction between the particle and the solute. 
     SUMMARY OF THE INVENTION 
     An embodiment of a microfluidic device in accordance with the present invention comprises pumps, valves, and fluid oscillation dampers. In a device employed for sorting, an entity is flowed by the pump along a flow channel through a detection region to a junction. Based upon an identity of the entity determined in the detection region, a waste or collection valve located on opposite branches of the flow channel is actuated, thereby routing the entity to either a waste pool or a collection pool. A damper structure may be located between the pump and the junction. The damper reduces the amplitude of oscillation pressure in the flow channel due to operation of the pump, thereby lessening oscillation in velocity of the entity during sorting process. The microfluidic device may be formed in a block of elastomer material, with thin membranes of the elastomer material deflectable into the flow channel to provide pump or valve functionality. 
     An embodiment of a microfluidic device in accordance with the present invention comprises a flow channel, a pump operatively interconnected to said flow channel for moving a fluid in said flow channel, and a damper operatively interconnected to said flow channel for reducing the fluid oscillation in said flow channel. 
     An embodiment of a microfluidic sorting device comprises a first flow channel formed in a first layer of elastomer material, a first end of the first flow channel in fluid communication with a collection pool and a second end of the first flow channel in fluid communication with a waste pool. A second flow channel is formed in the first elastomer layer, a first end of the second flow channel in fluid communication with an injection pool and a second end of the second flow channel in fluid communication with the first flow channel at a junction. A collection valve is adjacent to a first side of the junction proximate to the collection pool, the collection valve comprising a first recess formed in a second elastomer layer overlying the first elastomer layer, the first recess separated from the first flow channel by a first membrane portion of the second elastomer layer deflectable into the first flow channel. A waste valve is adjacent to a second side of the junction proximate to the waste pool, the waste valve comprising a second recess formed in the second elastomer layer separated from the second flow channel by a second membrane portion of the second elastomer layer deflectable into the first flow channel. A pump adjacent to a third side of the junction proximate to the injection pool, the pump comprising at least three pressure channels formed in the second elastomer layer and separated from second flow channel by third membrane portions of the second elastomer layer deflectable into the second flow channel. A detection region is positioned between the injection pool and the junction, one of an open and closed state of the collection valve and the waste valve determined by an identity of a sortable entity detected in the detection region. 
     An embodiment of a damper in accordance with the present invention for a microfluidic device comprises a flow channel formed in an elastomer material, and an energy absorber adjacent to the flow channel and configured to absorb an energy of oscillation of a fluid positioned within the flow channel. 
     An embodiment of a sorting method in accordance with the present invention comprises deflecting a first elastomer membrane of an elastomer block into a flow channel to cause a sortable entity to flow into a detection region positioned upstream of a junction in the flow channel. The detection region is interrogated to identify the sortable entity within the detection region. Based upon an identity of the sortable entity, one of a second membrane and a third membrane of the elastomer block are deflected into one of a first branch flow channel portion and a second branch flow channel portion respectively, located downstream of the junction. This causes the sortable entity to flow to one of a collection pool or a waste pool. 
     An embodiment of a method for dampening pressure oscillations in a flow channel comprises providing an energy absorber adjacent to the flow channel, such that the energy absorber experiences a change in response the pressure oscillations. 
     These and other embodiments of the present invention, as well as its advantages and features, are described in more detail in conjunction with the text below and attached figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a first elastomeric layer formed on top of a micromachined mold. 
         FIG. 2  is an illustration of a second elastomeric layer formed on top of a micromachined mold. 
         FIG. 3  is an illustration of the elastomeric layer of  FIG. 2  removed from the micromachined mold and positioned over the top of the elastomeric layer of  FIG. 1   
         FIG. 4  is an illustration corresponding to  FIG. 3 , but showing the second elastomeric layer positioned on top of the first elastomeric layer. 
         FIG. 5  is an illustration corresponding to  FIG. 4 , but showing the first and second elastomeric layers bonded together. 
         FIG. 6  is an illustration corresponding to  FIG. 5 , but showing the first micromachined mold removed and a planar substrate positioned in its place. 
         FIG. 7A  is an illustration corresponding to  FIG. 6 , but showing the elastomeric structure sealed onto the planar substrate.  FIG. 7H  is a corresponding view showing first flow channel  30  closed by pressurization of the second flow channel  32 . 
         FIG. 7B  is a front sectional view corresponding to  FIG. 7A , showing an open flow channel. 
         FIGS. 7C-7G  are illustrations showing steps of a method for forming an elastomeric structure having a membrane formed from a separate elastomeric layer. 
         FIGS. 8A and 8B  illustrates valve opening vs. applied pressure for various flow channels. 
         FIG. 9  illustrates time response of a 100 μm×100 μm×10 μm RTV microvalve. 
         FIG. 10  is a cross sectional view of a microfluidic device similar to that of  FIG. 7B , with flow channel  30  being rectangular in cross sectional shape. When flow channel  32  is pressurized, the membrane portion  25  of elastomeric block  24  separating flow channels  30  and  32  will move downwardly to the successive positions shown by the dotted lines  25 A,  25 B,  25 C,  25 D, and  25 E. 
         FIG. 11  is a cross sectional view of a microfluidic device in which channel  30 A has a curved upper wall  25 A. When flow channel  32  is pressurized, membrane portion  25 A will move downwardly to the successive positions shown by dotted lines  25 A 2 ,  25 A 3 ,  25 A 4  and  25 A 5 . 
         FIG. 12A  is a top schematic view of an on/off valve. 
         FIG. 12B  is a sectional elevation view along line  23 B- 23 B in  FIG. 12A   
         FIG. 13A  is a top schematic view of a peristaltic pumping system. 
         FIG. 13B  is a sectional elevation view along line  24 B- 24 B in  FIG. 13A   
         FIG. 14  is a graph showing experimentally achieved pumping rates vs. frequency for an embodiment of the peristaltic pumping system of  FIG. 13 . 
         FIG. 15A  is a top schematic view of one control line actuating multiple flow lines simultaneously. 
         FIG. 15B  is a sectional elevation view along line  26 B- 26 B in  FIG. 15A   
         FIG. 16  is a schematic illustration of a multiplexed system adapted to permit flow through various channels. 
         FIG. 17A  is a plan view of a flow layer of an addressable reaction chamber structure. 
         FIG. 17B  is a bottom plan view of a control channel layer of an addressable reaction chamber structure. 
         FIG. 17C  is an exploded perspective view of the addressable reaction chamber structure formed by bonding the control channel layer of  FIG. 17B  to the top of the flow layer of  FIG. 17A . 
         FIG. 17D  is a sectional elevation view corresponding to  FIG. 17C , taken along line  28 D- 28 D in  FIG. 17C . 
         FIG. 18  is a schematic of a system adapted to selectively direct fluid flow into any of an array of reaction wells. 
         FIG. 19  is a schematic of a system adapted for selectable lateral flow between parallel flow channels. 
         FIG. 20A  is a bottom plan view of first layer (i.e.: the flow channel layer) of elastomer of a switchable flow array. 
         FIG. 20B  is a bottom plan view of a control channel layer of a switchable flow array. 
         FIG. 20C  shows the alignment of the first layer of elastomer of  FIG. 20A  with one set of control channels in the second layer of elastomer of  FIG. 20B . 
         FIG. 20D  also shows the alignment of the first layer of elastomer of  FIG. 20A  with the other set of control channels in the second layer of elastomer of  FIG. 20B . 
         FIGS. 21A-21J  show views of one embodiment of a normally-closed valve structure in accordance with the present invention. 
         FIGS. 22A and 22B  show plan views illustrating operation of one embodiment of a side-actuated valve structure in accordance with the present invention. 
         FIG. 23  shows a cross-sectional view of one embodiment of a composite structure in accordance with the present invention. 
         FIG. 24  shows a cross-sectional view of another embodiment of a composite structure in accordance with the present invention. 
         FIG. 25  shows a cross-sectional view of another embodiment of a composite structure in accordance with the present invention. 
         FIG. 26  is a cross-sectional view of one embodiment of a damper structure in accordance with the present invention. 
         FIG. 27  is a cross-sectional view of an alternative embodiment of a damper structure in accordance with the present invention. 
         FIG. 28  is a plan view of another alternative embodiment of a damper structure in accordance with the present invention. 
         FIGS. 29A-B  are cross-sectional views of operation of one embodiment of a circulation apparatus including a damper structure in accordance with the present invention. 
         FIG. 30A  is a schematic plan view of a cell sorter structure in accordance with one embodiment of the present invention. 
         FIG. 30B  is a photograph of a plan view of the cell sorter shown in  FIG. 30A . 
         FIGS. 31A and 31B  are schematic views of a T-junction of a sorter structure in accordance with an embodiment of the present invention engaged in reversible sorting. 
         FIG. 32A  plots cell velocity versus pump frequency for one embodiment of a cell sorter in accordance with the present invention. 
         FIG. 32B  plots mean reversible time versus pump frequency for the embodiment of a cell sorter of  FIG. 32A . 
         FIG. 33A  plots optical intensity over time for the cell sorter structure of  FIG. 33C  operated at first frequency. 
         FIG. 33B  plots optical intensity over time for the cell sorter structure of  FIG. 33C  operated at first frequency. 
         FIG. 33C  is a schematic view of a cell sorter structure in accordance with an alternative embodiment in accordance with the present invention. 
         FIG. 34  plots flow velocity versus pump frequency for cell sorters fabricated from different elastomeric materials. 
     
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     I. Microfabrication Overview 
     The following discussion relates to formation of microfabricated fluidic devices utilizing elastomer materials, as described generally in U.S. patent application Ser. No. 09/826,585 filed Apr. 6, 2001, Ser. No. 09/724,784 filed Nov. 28, 2000, and Ser. No. 09/605,520, filed Jun. 27, 2000. These patent applications are hereby incorporated by reference. 
     1. Methods of Fabricating 
     Exemplary methods of fabricating the present invention are provided herein. It is to be understood that the present invention is not limited to fabrication by one or the other of these methods. Rather, other suitable methods of fabricating the present microstructures, including modifying the present methods, are also contemplated. 
       FIGS. 1 to 7B  illustrate sequential steps of a first preferred method of fabricating the present microstructure, (which may be used as a pump or valve).  FIGS. 8 to 18  illustrate sequential steps of a second preferred method of fabricating the present microstructure, (which also may be used as a pump or valve). 
     As will be explained, the preferred method of  FIGS. 1 to 7B  involves using pre-cured elastomer layers which are assembled and bonded. In an alternative method, each layer of elastomer may be cured “in place”. In the following description “channel” refers to a recess in the elastomeric structure which can contain a flow of fluid or gas. 
     Referring to  FIG. 1 , a first micro-machined mold  10  is provided. Micro-machined mold  10  may be fabricated by a number of conventional silicon processing methods, including but not limited to photolithography, ion-milling, and electron beam lithography. 
     As can be seen, micro-machined mold  10  has a raised line or protrusion  11  extending therealong. A first elastomeric layer  20  is cast on top of mold  10  such that a first recess  21  will be formed in the bottom surface of elastomeric layer  20 , (recess  21  corresponding in dimension to protrusion  11 ), as shown. 
     As can be seen in  FIG. 2 , a second micro-machined mold  12  having a raised protrusion  13  extending therealong is also provided. A second elastomeric layer  22  is cast on top of mold  12 , as shown, such that a recess  23  will be formed in its bottom surface corresponding to the dimensions of protrusion  13 . 
     As can be seen in the sequential steps illustrated in  FIGS. 3 and 4 , second elastomeric layer  22  is then removed from mold  12  and placed on top of first elastomeric layer  20 . As can be seen, recess  23  extending along the bottom surface of second elastomeric layer  22  will form a flow channel  32 . 
     Referring to  FIG. 5 , the separate first and second elastomeric layers  20  and  22  ( FIG. 4 ) are then bonded together to form an integrated (i.e.: monolithic) elastomeric structure  24 . 
     As can been seen in the sequential step of  FIGS. 6 and 7A , elastomeric structure  24  is then removed from mold  10  and positioned on top of a planar substrate  14 . As can be seen in  FIGS. 7A and 7B , when elastomeric structure  24  has been sealed at its bottom surface to planar substrate  14 , recess  21  will form a flow channel  30 . 
     The present elastomeric structures form a reversible hermetic seal with nearly any smooth planar substrate. An advantage to forming a seal this way is that the elastomeric structures may be peeled up, washed, and re-used. In preferred aspects, planar substrate  14  is glass. A further advantage of using glass is that glass is transparent, allowing optical interrogation of elastomer channels and reservoirs. Alternatively, the elastomeric structure may be bonded onto a flat elastomer layer by the same method as described above, forming a permanent and high-strength bond. This may prove advantageous when higher back pressures are used. 
     As can be seen in  FIGS. 7A and 7B , flow channels  30  and  32  are preferably disposed at an angle to one another with a small membrane  25  of substrate  24  separating the top of flow channel  30  from the bottom of flow channel  32 . 
     In preferred aspects, planar substrate  14  is glass. An advantage of using glass is that the present elastomeric structures may be peeled up, washed and reused. A further advantage of using glass is that optical sensing may be employed. Alternatively, planar substrate  14  may be an elastomer itself, which may prove advantageous when higher back pressures are used. 
     The method of fabrication just described may be varied to form a structure having a membrane composed of an elastomeric material different than that forming the walls of the channels of the device. This variant fabrication method is illustrated in  FIGS. 7C-7G . 
     Referring to  FIG. 7C , a first micro-machined mold  10  is provided. Micro-machined mold  10  has a raised line or protrusion  11  extending therealong. In  FIG. 7D , first elastomeric layer  20  is cast on top of first micro-machined mold  10  such that the top of the first elastomeric layer  20  is flush with the top of raised line or protrusion  11 . This may be accomplished by carefully controlling the volume of elastomeric material spun onto mold  10  relative to the known height of raised line  11 . Alternatively, the desired shape could be formed by injection molding. 
     In  FIG. 7E , second micro-machined mold  12  having a raised protrusion  13  extending therealong is also provided. Second elastomeric layer  22  is cast on top of second mold  12  as shown, such that recess  23  is formed in its bottom surface corresponding to the dimensions of protrusion  13 . 
     In  FIG. 7F , second elastomeric layer  22  is removed from mold  12  and placed on top of third elastomeric layer  222 . Second elastomeric layer  22  is bonded to third elastomeric layer  20  to form integral elastomeric block  224  using techniques described in detail below. At this point in the process, recess  23  formerly occupied by raised line  13  will form flow channel  23 . 
     In  FIG. 7G , elastomeric block  224  is placed on top of first micro-machined mold  10  and first elastomeric layer  20 . Elastomeric block and first elastomeric layer  20  are then bonded together to form an integrated (i.e.: monolithic) elastomeric structure  24  having a membrane composed of a separate elastomeric layer  222 . 
     When elastomeric structure  24  has been sealed at its bottom surface to a planar substrate in the manner described above in connection with  FIG. 7A , the recess formerly occupied by raised line  11  will form flow channel  30 . 
     The variant fabrication method illustrated above in conjunction with  FIGS. 7C-7G  offers the advantage of permitting the membrane portion to be composed of a separate material than the elastomeric material of the remainder of the structure. This is important because the thickness and elastic properties of the membrane play a key role in operation of the device. Moreover, this method allows the separate elastomer layer to readily be subjected to conditioning prior to incorporation into the elastomer structure. As discussed in detail below, examples of potentially desirable condition include the introduction of magnetic or electrically conducting species to permit actuation of the membrane, and/or the introduction of dopant into the membrane in order to alter its elasticity. 
     While the above method is illustrated in connection with forming various shaped elastomeric layers formed by replication molding on top of a micromachined mold, the present invention is not limited to this technique. Other techniques could be employed to form the individual layers of shaped elastomeric material that are to be bonded together. For example, a shaped layer of elastomeric material could be formed by laser cutting or injection molding, or by methods utilizing chemical etching and/or sacrificial materials as discussed below in conjunction with the second exemplary method. 
     An alternative method fabricates a patterned elastomer structure utilizing development of photoresist encapsulated within elastomer material. However, the methods in accordance with the present invention are not limited to utilizing photoresist. Other materials such as metals could also serve as sacrificial materials to be removed selective to the surrounding elastomer material, and the method would remain within the scope of the present invention. For example, gold metal may be etched selective to RTV 615 elastomer utilizing the appropriate chemical mixture. 
     2. Layer and Channel Dimensions 
     Microfabricated refers to the size of features of an elastomeric structure fabricated in accordance with an embodiment of the present invention. In general, variation in at least one dimension of microfabricated structures is controlled to the micron level, with at least one dimension being microscopic (i.e. below 1000 μm). Microfabrication typically involves semiconductor or MEMS fabrication techniques such as photolithography and spincoating that are designed for to produce feature dimensions on the microscopic level, with at least some of the dimension of the microfabricated structure requiring a microscope to reasonably resolve/image the structure. 
     In preferred aspects, flow channels  30 ,  32 ,  60  and  62  preferably have width-to-depth ratios of about 10:1. A non-exclusive list of other ranges of width-to-depth ratios in accordance with embodiments of the present invention is 0.1:1 to 100:1, more preferably 1:1 to 50:1, more preferably 2:1 to 20:1, and most preferably 3:1 to 15:1. In an exemplary aspect, flow channels  30 ,  32 ,  60  and  62  have widths of about 1 to 1000 microns. A non-exclusive list of other ranges of widths of flow channels in accordance with embodiments of the present invention is 0.01 to 1000 microns, more preferably 0.05 to 1000 microns, more preferably 0.2 to 500 microns, more preferably 1 to 250 microns, and most preferably 10 to 200 microns. Exemplary channel widths include 0.1 μm, 1 μm, 2 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, 200 μm, 210 μm, 220 μm, 230 μm, 240 μm, and 250 μm. 
     Flow channels  30 ,  32 ,  60 , and  62  have depths of about 1 to 100 microns. A non-exclusive list of other ranges of depths of flow channels in accordance with embodiments of the present invention is 0.01 to 1000 microns, more preferably 0.05 to 500 microns, more preferably 0.2 to 250 microns, and more preferably 1 to 100 microns, more preferably 2 to 20 microns, and most preferably 5 to 10 microns. Exemplary channel depths include including 0.01 μm, 0.02 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 17.5 μm, 20 μm, 22.5 μm, 25 μm, 30 μm, 40 μm, 50 μm, 75 μm, 100 μm, 150 μm, 200 μm, and 250 μm. 
     The flow channels are not limited to these specific dimension ranges and examples given above, and may vary in width in order to affect the magnitude of force required to deflect the membrane as discussed at length below in conjunction with  FIG. 27 . For example, extremely narrow flow channels having a width on the order of 0.01 μm may be useful in optical and other applications, as discussed in detail below. Elastomeric structures which include portions having channels of even greater width than described above are also contemplated by the present invention, and examples of applications of utilizing such wider flow channels include fluid reservoir and mixing channel structures. 
     The Elastomeric layers may be cast thick for mechanical stability. In an exemplary embodiment, elastomeric layer  22  of  FIG. 1  is 50 microns to several centimeters thick, and more preferably approximately 4 mm thick. A non-exclusive list of ranges of thickness of the elastomer layer in accordance with other embodiments of the present invention is between about 0.1 micron to 10 cm, 1 micron to 5 cm, 10 microns to 2 cm, 100 microns to 10 mm. 
     Accordingly, membrane  25  of  FIG. 7B  separating flow channels  30  and  32  has a typical thickness of between about 0.01 and 1000 microns, more preferably 0.05 to 500 microns, more preferably 0.2 to 250, more preferably 1 to 100 microns, more preferably 2 to 50 microns, and most preferably 5 to 40 microns. As such, the thickness of elastomeric layer  22  is about 100 times the thickness of elastomeric layer  20 . Exemplary membrane thicknesses include 0.01 μm, 0.02 μm, 0.03 μm, 0.05 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 5 μm, 7.5 μm, 10 μm, 12.5 μm, 15 μm, 17.5 μm, 20 μm, 22.5 μm, 25 μm, 30 μm, 40 μm, 50 μm, 75 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm, 400 μm, 500 μm, 750 μm, and 1000 μm. 
     3. Soft Lithographic Bonding 
     Preferably, elastomeric layers are bonded together chemically, using chemistry that is intrinsic to the polymers comprising the patterned elastomer layers. Most preferably, the bonding comprises two component “addition cure” bonding. 
     In a preferred aspect, the various layers of elastomer are bound together in a heterogenous bonding in which the layers have a different chemistry. Alternatively, a homogenous bonding may be used in which all layers would be of the same chemistry. Thirdly, the respective elastomer layers may optionally be glued together by an adhesive instead. In a fourth aspect, the elastomeric layers may be thermoset elastomers bonded together by heating. 
     In one aspect of homogeneous bonding, the elastomeric layers are composed of the same elastomer material, with the same chemical entity in one layer reacting with the same chemical entity in the other layer to bond the layers together. In one embodiment, bonding between polymer chains of like elastomer layers may result from activation of a crosslinking agent due to light, heat, or chemical reaction with a separate chemical species. 
     Alternatively in a heterogeneous aspect, the elastomeric layers are composed of different elastomeric materials, with a first chemical entity in one layer reacting with a second chemical entity in another layer. In one exemplary heterogenous aspect, the bonding process used to bind respective elastomeric layers together may comprise bonding together two layers of RTV 615 silicone. RTV 615 silicone is a two-part addition-cure silicone rubber. Part A contains vinyl groups and catalyst; part B contains silicon hydride (Si—H) groups. The conventional ratio for RTV 615 is 10A:1B. For bonding, one layer may be made with 30A:1B (i.e. excess vinyl groups) and the other with 3A:1B (i.e. excess Si—H groups). Each layer is cured separately. When the two layers are brought into contact and heated at elevated temperature, they bond irreversibly forming a monolithic elastomeric substrate. 
     In an exemplary aspect of the present invention, elastomeric structures are formed utilizing Dow Corning Sylgard 182, 184 or 186, or aliphatic urethane diacrylates such as (but not limited to) Ebecryl 270 or Irr 245 from UCB Chemical. 
     In one embodiment in accordance with the present invention, two-layer elastomeric structures were fabricated from Dow Corning Sylgard 184. The layer containing the flow channels had an A:B ratio of 20:1 was spin coated at 5000 rpm. The layer containing the control channels had an A:B ratio of 5:1 
     In another embodiment in accordance with the present invention, two-layer elastomeric structures were fabricated from pure acrylated Urethane Ebe 270. A thin bottom layer was spin coated at 8000 rpm for 15 seconds at 170° C. The top and bottom layers were initially cured under ultraviolet light for 10 minutes under nitrogen utilizing a Model ELC 500 device manufactured by Electrolite corporation. The assembled layers were then cured for an additional 30 minutes. Reaction was catalyzed by a 0.5% vol/vol mixture of Irgacure 500 manufactured by Ciba-Geigy Chemicals. The resulting elastomeric material exhibited moderate elasticity and adhesion to glass. 
     In another embodiment in accordance with the present invention, two-layer elastomeric structures were fabricated from a combination of 25% Ebe 270/50% Irr245/25% isopropyl alcohol for a thin bottom layer, and pure acrylated Urethane Ebe 270 as a top layer. The thin bottom layer was initially cured for 5 min, and the top layer initially cured for 10 minutes, under ultraviolet light under nitrogen utilizing a Model ELC 500 device manufactured by Electrolite corporation. The assembled layers were then cured for an additional 30 minutes. Reaction was catalyzed by a 0.5% vol/vol mixture of Irgacure 500 manufactured by Ciba-Geigy Chemicals. The resulting elastomeric material exhibited moderate elasticity and adhered to glass. 
     Alternatively, other bonding methods may be used, including activating the elastomer surface, for example by plasma exposure, so that the elastomer layers/substrate will bond when placed in contact. For example, one possible approach to bonding together elastomer layers composed of the same material is set forth by Duffy et al, “Rapid Prototyping of Microfluidic Systems in Poly(dimethylsiloxane)”,  Analytical Chemistry  (1998), 70, 4974-4984, incorporated herein by reference. This paper discusses that exposing polydimethylsiloxane (PDMS) layers to oxygen plasma causes oxidation of the surface, with irreversible bonding occurring when the two oxidized layers are placed into contact. 
     Yet another approach to bonding together successive layers of elastomer is to utilize the adhesive properties of uncured elastomer. Specifically, a thin layer of uncured elastomer such as RTV 615 is applied on top of a first cured elastomeric layer. Next, a second cured elastomeric layer is placed on top of the uncured elastomeric layer. The thin middle layer of uncured elastomer is then cured to produce a monolithic elastomeric structure. Alternatively, uncured elastomer can be applied to the bottom of a first cured elastomer layer, with the first cured elastomer layer placed on top of a second cured elastomer layer. Curing the middle thin elastomer layer again results in formation of a monolithic elastomeric structure. 
     Bonding together of successive elastomer layers in accordance with embodiments of the present invention need not result in a monolithic elastomeric structure. Successive layers of different elastomer materials can be bonded together to form an embodiment of a structure in accordance with the present invention. For example General Electric RTV and Dow Corning Sylgard 184 can be bonded together to form a multilayer structure. 
     Moreover, bonding together of successive elastomer layers in accordance with the present invention need not be permanent. In certain embodiments, the strength of the bond between elastomer layers need only maintain contact and resist separation under the forces encountered during membrane actuation. Application of greater force will cause the elastomer layers to separate, for example allowing flushing and reuse of flow or control channels present in the separated layers. 
     Where encapsulation of sacrificial layers is employed to fabricate the elastomer structure, bonding of successive elastomeric layers may be accomplished by pouring uncured elastomer over a previously cured elastomeric layer and any sacrificial material patterned thereupon. Bonding between elastomer layers occurs due to interpenetration and reaction of the polymer chains of an uncured elastomer layer with the polymer chains of a cured elastomer layer. Subsequent curing of the elastomeric layer will create a bond between the elastomeric layers and create a monolithic elastomeric structure. 
     Referring to the first method of  FIGS. 1 to 7B , first elastomeric layer  20  may be created by spin-coating an RTV mixture on microfabricated mold  12  at 2000 rpm&#39;s for 30 seconds yielding a thickness of approximately 40 microns. Second elastomeric layer  22  may be created by spin-coating an RTV mixture on microfabricated mold  11 . Both layers  20  and  22  may be separately baked or cured at about 80° C. for 1.5 hours. The second elastomeric layer  22  may be bonded onto first elastomeric layer  20  at about 80° C. for about 1.5 hours. 
     Micromachined molds  10  and  12  may be patterned photoresist on silicon wafers. In an exemplary aspect, a Shipley SJR 5740 photoresist was spun at 2000 rpm patterned with a high resolution transparency film as a mask and then developed yielding an inverse channel of approximately 10 microns in height. When baked at approximately 200° C. for about 30 minutes, the photoresist reflows and the inverse channels become rounded. In preferred aspects, the molds may be treated with trimethylchlorosilane (TMCS) vapor for about a minute before each use in order to prevent adhesion of silicone rubber. 
     4. Suitable Elastomeric Materials 
     Allcock et al, Contemporary  Polymer Chemistry,  2 nd  Ed. describes elastomers in general as polymers existing at a temperature between their glass transition temperature and liquefaction temperature. Elastomeric materials exhibit elastic properties because the polymer chains readily undergo torsional motion to permit uncoiling of the backbone chains in response to a force, with the backbone chains recoiling to assume the prior shape in the absence of the force. In general, elastomers deform when force is applied, but then return to their original shape when the force is removed. The elasticity exhibited by elastomeric materials may be characterized by a Young&#39;s modulus. Elastomeric materials having a Young&#39;s modulus of between about 1 Pa-1 TPa, more preferably between about 10 Pa-100 GPa, more preferably between about 20 Pa-1 GPa, more preferably between about 50 Pa-10 MPa, and more preferably between about 100 Pa-1 MPa are useful in accordance with the present invention, although elastomeric materials having a Young&#39;s modulus outside of these ranges could also be utilized depending upon the needs of a particular application. 
     The systems of the present invention may be fabricated from a wide variety of elastomers. In an exemplary aspect, the elastomeric layers may preferably be fabricated from silicone rubber. However, other suitable elastomers may also be used. 
     In an exemplary aspect of the present invention, the present systems are fabricated from an elastomeric polymer such as GE RTV 615 (formulation), a vinyl-silane crosslinked (type) silicone elastomer (family). However, the present systems are not limited to this one formulation, type or even this family of polymer; rather, nearly any elastomeric polymer is suitable. An important requirement for the preferred method of fabrication of the present microvalves is the ability to bond multiple layers of elastomers together. In the case of multilayer soft lithography, layers of elastomer are cured separately and then bonded together. This scheme requires that cured layers possess sufficient reactivity to bond together. Either the layers may be of the same type, and are capable of bonding to themselves, or they may be of two different types, and are capable of bonding to each other. Other possibilities include the use an adhesive between layers and the use of thermoset elastomers. 
     Given the tremendous diversity of polymer chemistries, precursors, synthetic methods, reaction conditions, and potential additives, there are a huge number of possible elastomer systems that could be used to make monolithic elastomeric microvalves and pumps. Variations in the materials used will most likely be driven by the need for particular material properties, i.e. solvent resistance, stiffness, gas permeability, or temperature stability. 
     For example, a diluent may be included during formation of the elastomer material to alter its properties. In one embodiment, a diluent is added to the elastomer comprising the membrane layer to lessen the stiffness of the membrane and thereby reduce the actuation force required. Two examples of diluent for elastomer materials are General Electric SF96, and DMV-V21 manufactured by Gelest, Inc. of Tullytown, Pa. In general, the diluent is mixed with the elastomer at a ratio of between about 15% and 30%. 
     There are many, many types of elastomeric polymers. A brief description of the most common classes of elastomers is presented here, with the intent of showing that even with relatively “standard” polymers, many possibilities for bonding exist. Common elastomeric polymers include polyisoprene, polybutadiene, polychloroprene, polyisobutylene, poly(styrene-butadiene-styrene), the polyurethanes, and silicones. 
     Polyisoprene, polybutadiene, polychloroprene: 
     
         
         
           
             Polyisoprene, polybutadiene, and polychloroprene are all polymerized from diene monomers, and therefore have one double bond per monomer when polymerized. This double bond allows the polymers to be converted to elastomers by vulcanization (essentially, sulfur is used to form crosslinks between the double bonds by heating). This would easily allow homogeneous multilayer soft lithography by incomplete vulcanization of the layers to be bonded; photoresist encapsulation would be possible by a similar mechanism.
 
Polyisobutylene:
 
             Pure polyisobutylene has no double bonds, but is crosslinked to use as an elastomer by including a small amount (˜1%) of isoprene in the polymerization. The isoprene monomers give pendant double bonds on the polyisobutylene backbone, which may then be vulcanized as above.
 
Poly(styrene-butadiene-styrene):
 
             Poly(styrene-butadiene-styrene) is produced by living anionic polymerization (that is, there is no natural chain-terminating step in the reaction), so “live” polymer ends can exist in the cured polymer. This makes it a natural candidate for the present photoresist encapsulation system (where there will be plenty of unreacted monomer in the liquid layer poured on top of the cured layer). Incomplete curing would allow homogeneous multilayer soft lithography (A to A bonding). The chemistry also facilitates making one layer with extra butadiene (“A”) and coupling agent and the other layer (“B”) with a butadiene deficit (for heterogeneous multilayer soft lithography). SBS is a “thermoset elastomer”, meaning that above a certain temperature it melts and becomes plastic (as opposed to elastic); reducing the temperature yields the elastomer again. Thus, layers can be bonded together by heating.
 
Polyurethanes:
 
             Polyurethanes are produced from di-isocyanates (A-A) and di-alcohols or di-amines (B-B); since there are a large variety of di-isocyanates and di-alcohols/amines, the number of different types of polyurethanes is huge. The A vs. B nature of the polymers, however, would make them useful for heterogeneous multilayer soft lithography just as RTV 615 is: by using excess A-A in one layer and excess B-B in the other layer.
 
Silicones:
 
             Silicone polymers probably have the greatest structural variety, and almost certainly have the greatest number of commercially available formulations. The vinyl-to-(Si—H) crosslinking of RTV 615 (which allows both heterogeneous multilayer soft lithography and photoresist encapsulation) has already been discussed, but this is only one of several crosslinking methods used in silicone polymer chemistry. 
           
         
       
    
     5. Operation of Device 
       FIGS. 7B and 7H  together show the closing of a first flow channel by pressurizing a second flow channel, with  FIG. 7B  (a front sectional view cutting through flow channel  32  in corresponding  FIG. 7A ), showing an open first flow channel  30 ; with  FIG. 7H  showing first flow channel  30  closed by pressurization of the second flow channel  32 . 
     Referring to  FIG. 7B , first flow channel  30  and second flow channel  32  are shown. Membrane  25  separates the flow channels, forming the top of first flow channel  30  and the bottom of second flow channel  32 . As can be seen, flow channel  30  is “open”. 
     As can be seen in  FIG. 7H , pressurization of flow channel  32  (either by gas or liquid introduced therein) causes membrane  25  to deflect downward, thereby pinching off flow F passing through flow channel  30 . Accordingly, by varying the pressure in channel  32 , a linearly actuable valving system is provided such that flow channel  30  can be opened or closed by moving membrane  25  as desired. (For illustration purposes only, channel  30  in  FIG. 7G  is shown in a “mostly closed” position, rather than a “fully closed” position). 
     Since such valves are actuated by moving the roof of the channels themselves (i.e.: moving membrane  25 ) valves and pumps produced by this technique have a truly zero dead volume, and switching valves made by this technique have a dead volume approximately equal to the active volume of the valve, for example about 100×100×10 μm=100 pL. Such dead volumes and areas consumed by the moving membrane are approximately two orders of magnitude smaller than known conventional microvalves. Smaller and larger valves and switching valves are contemplated in the present invention, and a non-exclusive list of ranges of dead volume includes 1 aL to 1 uL, 100 aL to 100 nL, 1 fL to 10 nL, 100 fL to 1 nL, and 1 pL to 100 pL 
     The extremely small volumes capable of being delivered by pumps and valves in accordance with the present invention represent a substantial advantage. Specifically, the smallest known volumes of fluid capable of being manually metered is around 0.1 μl. The smallest known volumes capable of being metered by automated systems is about ten-times larger (1 μl). Utilizing pumps and valves in accordance with the present invention, volumes of liquid of 10 nl or smaller can routinely be metered and dispensed. The accurate metering of extremely small volumes of fluid enabled by the present invention would be extremely valuable in a large number of biological applications, including diagnostic tests and assays. 
     Equation 1 represents a highly simplified mathematical model of deflection of a rectangular, linear, elastic, isotropic plate of uniform thickness by an applied pressure:
 
 w =( BPb   4 )/( Eh   3 ), where:  (1)
         w=deflection of plate;   B=shape coefficient (dependent upon length vs. width and support of edges of plate);   P=applied pressure;   b=plate width   E=Young&#39;s modulus; and   h=plate thickness.
 
Thus even in this extremely simplified expression, deflection of an elastomeric membrane in response to a pressure will be a function of: the length, width, and thickness of the membrane, the flexibility of the membrane (Young&#39;s modulus), and the applied actuation force. Because each of these parameters will vary widely depending upon the actual dimensions and physical composition of a particular elastomeric device in accordance with the present invention, a wide range of membrane thicknesses and elasticities, channel widths, and actuation forces are contemplated by the present invention.
       

     It should be understood that the formula just presented is only an approximation, since in general the membrane does not have uniform thickness, the membrane thickness is not necessarily small compared to the length and width, and the deflection is not necessarily small compared to length, width, or thickness of the membrane. Nevertheless, the equation serves as a useful guide for adjusting variable parameters to achieve a desired response of deflection versus applied force. 
       FIGS. 8A and 8B  illustrate valve opening vs. applied pressure for a 100 μm wide first flow channel  30  and a 50 μm wide second flow channel  32 . The membrane of this device was formed by a layer of General Electric Silicones RTV 615 having a thickness of approximately 30 μm and a Young&#39;s modulus of approximately 750 kPa.  FIGS. 21   a  and  21   b  show the extent of opening of the valve to be substantially linear over most of the range of applied pressures. 
     Air pressure was applied to actuate the membrane of the device through a 10 cm long piece of plastic tubing having an outer diameter of 0.025″ connected to a 25 mm piece of stainless steel hypodermic tubing with an outer diameter of 0.025″ and an inner diameter of 0.013″. This tubing was placed into contact with the control channel by insertion into the elastomeric block in a direction normal to the control channel. Air pressure was applied to the hypodermic tubing from an external LHDA miniature solenoid valve manufactured by Lee Co. 
     In certain embodiments, it may be useful to apply both positive and negative pressures to actuate the membrane. For example, where the elastomer material is relatively inflexible, an extremely rapid response time is desired, or the flow channel dimensions are small, it may be useful to apply a positive pressure to a control channel actuate the membrane into the flow channel, followed by a negative pressure to cause the membrane to be displaced out of the flow channel. 
     Moreover, it may also be useful to cause movement of fluid through the microfluidic device by the direct application of pressure to the flow channel, such as the application of positive pressure directly to the flow channel inlet, or application of negative pressure directly to the flow channel outlet. Direct application of pressure alone can drive the flow of fluid within the microfluidic device, or direct pressure may be utilized in conjunction with actuation of membranes overlying the flow channel. Pressure applied directly to the flow channel can also serve to alter the speed of movement of materials thought the flow channel as desired. 
     While control of the flow of material through the device has so far been described utilizing applied gas pressure, other fluids could be used. For example, air is compressible, and thus experiences some finite delay between the time of application of pressure by the external solenoid valve and the time that this pressure is experienced by the membrane. In an alternative embodiment of the present invention, pressure could be applied from an external source to a noncompressible fluid such as water or hydraulic oils, resulting in a near-instantaneous transfer of applied pressure to the membrane. However, if the displaced volume of the valve is large or the control channel is narrow, higher viscosity of a control fluid may contribute to delay in actuation. The optimal medium for transferring pressure will therefore depend upon the particular application and device configuration, and both gaseous and liquid media are contemplated by the invention. 
     While external applied pressure as described above has been applied by a pump/tank system through a pressure regulator and external miniature valve, other methods of applying external pressure are also contemplated in the present invention, including gas tanks, compressors, piston systems, and columns of liquid. Also contemplated is the use of naturally occurring pressure sources such as may be found inside living organisms, such as blood pressure, gastric pressure, the pressure present in the cerebro-spinal fluid, pressure present in the intra-ocular space, and the pressure exerted by muscles during normal flexure. Other methods of regulating external pressure are also contemplated, such as miniature valves, pumps, macroscopic peristaltic pumps, pinch valves, and other types of fluid regulating equipment such as is known in the art. 
     As can be seen, the response of valves in accordance with embodiments of the present invention have been experimentally shown to be almost perfectly linear over a large portion of its range of travel, with minimal hysteresis. Accordingly, the present valves are ideally suited for microfluidic metering and fluid control. The linearity of the valve response demonstrates that the individual valves are well modeled as Hooke&#39;s Law springs. Furthermore, high pressures in the flow channel (i.e.: back pressure) can be countered simply by increasing the actuation pressure. Experimentally, the present inventors have achieved valve closure at back pressures of 70 kPa, but higher pressures are also contemplated. The following is a nonexclusive list of pressure ranges encompassed by the present invention: 10 Pa-25 MPa; 100 Pa-10 Mpa, 1 kPa-1 MPa, 1 kPa-300 kPa, 5 kPa-200 kPa, and 15 kPa-100 kPa. 
     While valves and pumps do not require linear actuation to open and close, linear response does allow valves to more easily be used as metering devices. In one embodiment of the invention, the opening of the valve is used to control flow rate by being partially actuated to a known degree of closure. Linear valve actuation makes it easier to determine the amount of actuation force required to close the valve to a desired degree of closure. Another benefit of linear actuation is that the force required for valve actuation may be easily determined from the pressure in the flow channel. If actuation is linear, increased pressure in the flow channel may be countered by adding the same pressure (force per unit area) to the actuated portion of the valve. 
     Linearity of a valve depends on the structure, composition, and method of actuation of the valve structure. Furthermore, whether linearity is a desirable characteristic in a valve depends on the application. Therefore, both linearly and non-linearly actuable valves are contemplated in the present invention, and the pressure ranges over which a valve is linearly actuable will vary with the specific embodiment. 
       FIG. 9  illustrates time response (i.e.: closure of valve as a function of time in response to a change in applied pressure) of a 100 μm×100 μm×10 μm RTV microvalve with 10-cm-long air tubing connected from the chip to a pneumatic valve as described above. 
     Two periods of digital control signal, actual air pressure at the end of the tubing and valve opening are shown in  FIG. 9 . The pressure applied on the control line is 100 kPa, which is substantially higher than the ˜40 kPa required to close the valve. Thus, when closing, the valve is pushed closed with a pressure 60 kPa greater than required. When opening, however, the valve is driven back to its rest position only by its own spring force (≦40 kPa). Thus, τ close  is expected to be smaller than τ open . There is also a lag between the control signal and control pressure response, due to the limitations of the miniature valve used to control the pressure. Calling such lags t and the 1/e time constants τ, the values are: t open =3.63 ms, τ open =1.88 ms, t close =2.15 ms, τ close =0.51 ms. If 3τ each are allowed for opening and closing, the valve runs comfortably at 75 Hz when filled with aqueous solution. 
     If one used another actuation method which did not suffer from opening and closing lag, this valve would run at ˜375 Hz. Note also that the spring constant can be adjusted by changing the membrane thickness; this allows optimization for either fast opening or fast closing. The spring constant could also be adjusted by changing the elasticity (Young&#39;s modulus) of the membrane, as is possible by introducing dopant into the membrane or by utilizing a different elastomeric material to serve as the membrane (described above in conjunction with  FIGS. 7C-7H .) 
     When experimentally measuring the valve properties as illustrated in  FIG. 9  the valve opening was measured by fluorescence. In these experiments, the flow channel was filled with a solution of fluorescein isothiocyanate (FITC) in buffer (pH≧8) and the fluorescence of a square area occupying the center ˜⅓rd of the channel is monitored on an epi-fluorescence microscope with a photomultiplier tube with a 10 kHz bandwidth. The pressure was monitored with a Wheatstone-bridge pressure sensor (SenSym SCC15GD2) pressurized simultaneously with the control line through nearly identical pneumatic connections. 
     6. Flow Channel Cross Sections 
     The flow channels of the present invention may optionally be designed with different cross sectional sizes and shapes, offering different advantages, depending upon their desired application. For example, the cross sectional shape of the lower flow channel may have a curved upper surface, either along its entire length or in the region disposed under an upper cross channel). Such a curved upper surface facilitates valve sealing, as follows. 
     Referring to  FIG. 10 , a cross sectional view (similar to that of  FIG. 7B ) through flow channels  30  and  32  is shown. As can be seen, flow channel  30  is rectangular in cross sectional shape. In an alternate preferred aspect of the invention, as shown in  FIG. 20 , the cross-section of a flow channel  30  instead has an upper curved surface. 
     Referring first to  FIG. 10 , when flow channel  32  is pressurized, the membrane portion  25  of elastomeric block  24  separating flow channels  30  and  32  will move downwardly to the successive positions shown by the dotted lines  25 A,  25 B,  25 C,  25 D, and  25 E. As can be seen, incomplete sealing may possibly result at the edges of flow channel  30  adjacent planar substrate  14 . 
     In the alternate preferred embodiment of  FIG. 11 , flow channel  30   a  has a curved upper wall  25 A. When flow channel  32  is pressurized, membrane portion  25  will move downwardly to the successive positions shown by dotted lines  25 A 2 ,  25 A 3 ,  25 A 4  and  25 A 5 , with edge portions of the membrane moving first into the flow channel, followed by top membrane portions. An advantage of having such a curved upper surface at membrane  25 A is that a more complete seal will be provided when flow channel  32  is pressurized. Specifically, the upper wall of the flow channel  30  will provide a continuous contacting edge against planar substrate  14 , thereby avoiding the “island” of contact seen between wall  25  and the bottom of flow channel  30  in  FIG. 10 . 
     Another advantage of having a curved upper flow channel surface at membrane  25 A is that the membrane can more readily conform to the shape and volume of the flow channel in response to actuation. Specifically, where a rectangular flow channel is employed, the entire perimeter (2× flow channel height, plus the flow channel width) must be forced into the flow channel. However where an arched flow channel is used, a smaller perimeter of material (only the semi-circular arched portion) must be forced into the channel. In this manner, the membrane requires less change in perimeter for actuation and is therefore more responsive to an applied actuation force to block the flow channel 
     In an alternate aspect, (not illustrated), the bottom of flow channel  30  is rounded such that its curved surface mates with the curved upper wall  25 A as seen in  FIG. 20  described above. 
     In summary, the actual conformational change experienced by the membrane upon actuation will depend upon the configuration of the particular elastomeric structure. Specifically, the conformational change will depend upon the length, width, and thickness profile of the membrane, its attachment to the remainder of the structure, and the height, width, and shape of the flow and control channels and the material properties of the elastomer used. The conformational change may also depend upon the method of actuation, as actuation of the membrane in response to an applied pressure will vary somewhat from actuation in response to a magnetic or electrostatic force. 
     Moreover, the desired conformational change in the membrane will also vary depending upon the particular application for the elastomeric structure. In the simplest embodiments described above, the valve may either be open or closed, with metering to control the degree of closure of the valve. In other embodiments however, it may be desirable to alter the shape of the membrane and/or the flow channel in order to achieve more complex flow regulation. For instance, the flow channel could be provided with raised protrusions beneath the membrane portion, such that upon actuation the membrane shuts off only a percentage of the flow through the flow channel, with the percentage of flow blocked insensitive to the applied actuation force. 
     Many membrane thickness profiles and flow channel cross-sections are contemplated by the present invention, including rectangular, trapezoidal, circular, ellipsoidal, parabolic, hyperbolic, and polygonal, as well as sections of the above shapes. More complex cross-sectional shapes, such as the embodiment with protrusions discussed immediately above or an embodiment having concavities in the flow channel, are also contemplated by the present invention. 
     In addition, while the invention is described primarily above in conjunction with an embodiment wherein the walls and ceiling of the flow channel are formed from elastomer, and the floor of the channel is formed from an underlying substrate, the present invention is not limited to this particular orientation. Walls and floors of channels could also be formed in the underlying substrate, with only the ceiling of the flow channel constructed from elastomer. This elastomer flow channel ceiling would project downward into the channel in response to an applied actuation force, thereby controlling the flow of material through the flow channel. In general, monolithic elastomer structures as described elsewhere in the instant application are preferred for microfluidic applications. However, it may be useful to employ channels formed in the substrate where such an arrangement provides advantages. For instance, a substrate including optical waveguides could be constructed so that the optical waveguides direct light specifically to the side of a microfluidic channel. 
     7. Alternate Valve Actuation Techniques 
     In addition to pressure based actuation systems described above, optional electrostatic and magnetic actuation systems are also contemplated, as follows. 
     Electrostatic actuation can be accomplished by forming oppositely charged electrodes (which will tend to attract one another when a voltage differential is applied to them) directly into the monolithic elastomeric structure. For example, referring to  FIG. 7B , an optional first electrode  70  (shown in phantom) can be positioned on (or in) membrane  25  and an optional second electrode  72  (also shown in phantom) can be positioned on (or in) planar substrate  14 . When electrodes  70  and  72  are charged with opposite polarities, an attractive force between the two electrodes will cause membrane  25  to deflect downwardly, thereby closing the “valve” (i.e.: closing flow channel  30 ). 
     For the membrane electrode to be sufficiently conductive to support electrostatic actuation, but not so mechanically stiff so as to impede the valve&#39;s motion, a sufficiently flexible electrode must be provided in or over membrane  25 . Such an electrode may be provided by a thin metallization layer, doping the polymer with conductive material, or making the surface layer out of a conductive material. 
     In an exemplary aspect, the electrode present at the deflecting membrane can be provided by a thin metallization layer which can be provided, for example, by sputtering a thin layer of metal such as 20 nm of gold. In addition to the formation of a metallized membrane by sputtering, other metallization approaches such as chemical epitaxy, evaporation, electroplating, and electroless plating are also available. Physical transfer of a metal layer to the surface of the elastomer is also available, for example by evaporating a metal onto a flat substrate to which it adheres poorly, and then placing the elastomer onto the metal and peeling the metal off of the substrate. 
     A conductive electrode  70  may also be formed by depositing carbon black (i.e. Cabot Vulcan XC72R) on the elastomer surface, either by wiping on the dry powder or by exposing the elastomer to a suspension of carbon black in a solvent which causes swelling of the elastomer, (such as a chlorinated solvent in the case of PDMS). Alternatively, the electrode  70  may be formed by constructing the entire layer  20  out of elastomer doped with conductive material (i.e. carbon black or finely divided metal particles). Yet further alternatively, the electrode may be formed by electrostatic deposition, or by a chemical reaction that produces carbon. In experiments conducted by the present inventors, conductivity was shown to increase with carbon black concentration from 5.6×10 −16  to about 5×10 −3  (Ω-cm) −1 . The lower electrode  72 , which is not required to move, may be either a compliant electrode as described above, or a conventional electrode such as evaporated gold, a metal plate, or a doped semiconductor electrode. 
     Magnetic actuation of the flow channels can be achieved by fabricating the membrane separating the flow channels with a magnetically polarizable material such as iron, or a permanently magnetized material such as polarized NdFeB. In experiments conducted by the present inventors, magnetic silicone was created by the addition of iron powder (about 1 um particle size), up to 20% iron by weight. 
     Where the membrane is fabricated with a magnetically polarizable material, the membrane can be actuated by attraction in response to an applied magnetic field Where the membrane is fabricated with a material capable of maintaining permanent magnetization, the material can first be magnetized by exposure to a sufficiently high magnetic field, and then actuated either by attraction or repulsion in response to the polarity of an applied inhomogenous magnetic field. 
     The magnetic field causing actuation of the membrane can be generated in a variety of ways. In one embodiment, the magnetic field is generated by an extremely small inductive coil formed in or proximate to the elastomer membrane. The actuation effect of such a magnetic coil would be localized, allowing actuation of individual pump and/or valve structures. Alternatively, the magnetic field could be generated by a larger, more powerful source, in which case actuation would be global and would actuate multiple pump and/or valve structures at one time. 
     It is also possible to actuate the device by causing a fluid flow in the control channel based upon the application of thermal energy, either by thermal expansion or by production of gas from liquid. For example, in one alternative embodiment in accordance with the present invention, a pocket of fluid (e.g. in a fluid-filled control channel) is positioned over the flow channel. Fluid in the pocket can be in communication with a temperature variation system, for example a heater. Thermal expansion of the fluid, or conversion of material from the liquid to the gas phase, could result in an increase in pressure, closing the adjacent flow channel. Subsequent cooling of the fluid would relieve pressure and permit the flow channel to open. 
     8. Networked Systems 
       FIGS. 12A and 12B  show a views of a single on/off valve, identical to the systems set forth above, (for example in  FIG. 7A ).  FIGS. 13A and 13B  shows a peristaltic pumping system comprised of a plurality of the single addressable on/off valves as seen in  FIG. 12 , but networked together.  FIG. 14  is a graph showing experimentally achieved pumping rates vs. frequency for the peristaltic pumping system of  FIG. 13 .  FIGS. 15A and 15B  show a schematic view of a plurality of flow channels which are controllable by a single control line. This system is also comprised of a plurality of the single addressable on/off valves of  FIG. 12 , multiplexed together, but in a different arrangement than that of  FIG. 12 .  FIG. 16  is a schematic illustration of a multiplexing system adapted to permit fluid flow through selected channels, comprised of a plurality of the single on/off valves of  FIG. 12 , joined or networked together. 
     Referring first to  FIGS. 12A and 12B , a schematic of flow channels  30  and  32  is shown. Flow channel  30  preferably has a fluid (or gas) flow F passing therethrough. Flow channel  32 , (which crosses over flow channel  30 , as was already explained herein), is pressurized such that membrane  25  separating the flow channels may be depressed into the path of flow channel  30 , shutting off the passage of flow F therethrough, as has been explained. As such, “flow channel”  32  can also be referred to as a “control line” which actuates a single valve in flow channel  30 . In  FIGS. 12 to 15 , a plurality of such addressable valves are joined or networked together in various arrangements to produce pumps, capable of peristaltic pumping, and other fluidic logic applications. 
     Referring to  FIGS. 13A and 13B , a system for peristaltic pumping is provided, as follows. A flow channel  30  has a plurality of generally parallel flow channels (i.e.: control lines)  32 A,  32 B and  32 C passing thereover. By pressurizing control line  32 A, flow F through flow channel  30  is shut off under membrane  25 A at the intersection of control line  32 A and flow channel  30 . Similarly, (but not shown), by pressurizing control line  32 B, flow F through flow channel  30  is shut off under membrane  25 B at the intersection of control line  32 B and flow channel  30 , etc. 
     Each of control lines  32 A,  32 B, and  32 C is separately addressable. Therefore, peristalsis may be actuated by the pattern of actuating  32 A and  32 C together, followed by  32 A, followed by  32 A and  32 B together, followed by  32 B, followed by  32 B and C together, etc. This corresponds to a successive “101, 100, 110, 010, 011, 001” pattern, where “0” indicates “valve open” and “1” indicates “valve closed.” This peristaltic pattern is also known as a 120° pattern (referring to the phase angle of actuation between three valves). Other peristaltic patterns are equally possible, including 60° and 90° patterns. 
     In experiments performed by the inventors, a pumping rate of 2.35 nL/s was measured by measuring the distance traveled by a column of water in thin (0.5 mm i.d.) tubing; with 100×100×10 μm valves under an actuation pressure of 40 kPa. The pumping rate increased with actuation frequency until approximately 75 Hz, and then was nearly constant until above 200 Hz. The valves and pumps are also quite durable and the elastomer membrane, control channels, or bond have never been observed to fail. In experiments performed by the inventors, none of the valves in the peristaltic pump described herein show any sign of wear or fatigue after more than 4 million actuations. In addition to their durability, they are also gentle. A solution of  E. Coli  pumped through a channel and tested for viability showed a 94% survival rate. 
       FIG. 14  is a graph showing experimentally achieved pumping rates vs. frequency for the peristaltic pumping system of  FIG. 13 . 
       FIGS. 15A and 15B  illustrates another way of assembling a plurality of the addressable valves of  FIG. 12 . Specifically, a plurality of parallel flow channels  30 A,  30 B, and  30 C are provided. Flow channel (i.e.: control line)  32  passes thereover across flow channels  30 A,  30 B, and  30 C. Pressurization of control line  32  simultaneously shuts off flows F 1 , F 2  and F 3  by depressing membranes  25 A,  25 B, and  25 C located at the intersections of control line  32  and flow channels  30 A,  30 B, and  30 C. 
       FIG. 16  is a schematic illustration of a multiplexing system adapted to selectively permit fluid to flow through selected channels, as follows. The downward deflection of membranes separating the respective flow channels from a control line passing thereabove (for example, membranes  25 A,  25 B, and  25 C in  FIGS. 15A and 15B ) depends strongly upon the membrane dimensions. Accordingly, by varying the widths of flow channel control line  32  in  FIGS. 15A and 15B , it is possible to have a control line pass over multiple flow channels, yet only actuate (i.e.: seal) desired flow channels.  FIG. 16  illustrates a schematic of such a system, as follows. 
     A plurality of parallel flow channels  30 A,  30 B,  30 C,  30 D,  30 E and  30 F are positioned under a plurality of parallel control lines  32 A,  32 B,  32 C,  32 D,  32 E and  32 F. Control channels  32 A,  32 B,  32 C,  32 D,  32 E and  32 F are adapted to shut off fluid flows F 1 , F 2 , F 3 , F 4 , F 5  and F 6  passing through parallel flow channels  30 A,  30 B,  30 C,  30 D,  30 E and  30 F using any of the valving systems described above, with the following modification. 
     Each of control lines  32 A,  32 B,  32 C,  32 D,  32 E and  32 F have both wide and narrow portions. For example, control line  32 A is wide in locations disposed over flow channels  30 A,  30 C and  30 E. Similarly, control line  32 B is wide in locations disposed over flow channels  30 B,  30 D and  30 F, and control line  32 C is wide in locations disposed over flow channels  30 A,  30 B,  30 E and  30 F. 
     At the locations where the respective control line is wide, its pressurization will cause the membrane ( 25 ) separating the flow channel and the control line to depress significantly into the flow channel, thereby blocking the flow passage therethrough. Conversely, in the locations where the respective control line is narrow, membrane ( 25 ) will also be narrow. Accordingly, the same degree of pressurization will not result in membrane ( 25 ) becoming depressed into the flow channel ( 30 ). Therefore, fluid passage thereunder will not be blocked. 
     For example, when control line  32 A is pressurized, it will block flows F 1 , F 3  and F 5  in flow channels  30 A,  30 C and  30 E. Similarly, when control line  32 C is pressurized, it will block flows F 1 , F 2 , F 5  and F 6  in flow channels  30 A,  30 B,  30 E and  30 F. As can be appreciated, more than one control line can be actuated at the same time. For example, control lines  32 A and  32 C can be pressurized simultaneously to block all fluid flow except F 4  (with  32 A blocking F 1 , F 3  and F 5 ; and  32 C blocking F 1 , F 2 , F 5  and F 6 ). 
     By selectively pressurizing different control lines ( 32 ) both together and in various sequences, a great degree of fluid flow control can be achieved. Moreover, by extending the present system to more than six parallel flow channels ( 30 ) and more than four parallel control lines ( 32 ), and by varying the positioning of the wide and narrow regions of the control lines, very complex fluid flow control systems may be fabricated. A property of such systems is that it is possible to turn on any one flow channel out of n flow channels with only 2(log 2 n) control lines. 
     9. Selectively Addressable Reaction Chambers Along Flow Lines 
     In a further embodiment of the invention, illustrated in  FIGS. 17A ,  17 B,  17 C and  17 D, a system for selectively directing fluid flow into one more of a plurality of reaction chambers disposed along a flow line is provided. 
       FIG. 17A  shows a top view of a flow channel  30  having a plurality of reaction chambers  80 A and  80 B disposed therealong. Preferably flow channel  30  and reaction chambers  80 A and  80 B are formed together as recesses into the bottom surface of a first layer  100  of elastomer. 
       FIG. 17B  shows a bottom plan view of another elastomeric layer  110  with two control lines  32 A and  32 B each being generally narrow, but having wide extending portions  33 A and  33 B formed as recesses therein. 
     As seen in the exploded view of  FIG. 17C , and assembled view of  FIG. 17D , elastomeric layer  110  is placed over elastomeric layer  100 . Layers  100  and  110  are then bonded together, and the integrated system operates to selectively direct fluid flow F (through flow channel  30 ) into either or both of reaction chambers  80 A and  80 B, as follows. Pressurization of control line  32 A will cause the membrane  25  (i.e.: the thin portion of elastomer layer  100  located below extending portion  33 A and over regions  82 A of reaction chamber  80 A) to become depressed, thereby shutting off fluid flow passage in regions  82 A, effectively sealing reaction chamber  80  from flow channel  30 . As can also be seen, extending portion  33 A is wider than the remainder of control line  32 A. As such, pressurization of control line  32 A will not result in control line  32 A sealing flow channel  30 . 
     As can be appreciated, either or both of control lines  32 A and  32 B can be actuated at once. When both control lines  32 A and  32 B are pressurized together, sample flow in flow channel  30  will enter neither of reaction chambers  80 A or  80 B. 
     The concept of selectably controlling fluid introduction into various addressable reaction chambers disposed along a flow line ( FIGS. 17A-D ) can be combined with concept of selectably controlling fluid flow through one or more of a plurality of parallel flow lines ( FIG. 16 ) to yield a system in which a fluid sample or samples can be can be sent to any particular reaction chamber in an array of reaction chambers. An example of such a system is provided in  FIG. 18 , in which parallel control channels  32 A,  32 B and  32 C with extending portions  34  (all shown in phantom) selectively direct fluid flows F 1  and F 2  into any of the array of reaction wells  80 A,  80 B,  80 C or  80 D as explained above; while pressurization of control lines  32 C and  32 D selectively shuts off flows F 2  and F 1 , respectively. 
     In yet another novel embodiment, fluid passage between parallel flow channels is possible. Referring to  FIG. 19 , either or both of control lines  32 A or  32 D can be depressurized such that fluid flow through lateral passageways  35  (between parallel flow channels  30 A and  30 B) is permitted. In this aspect of the invention, pressurization of control lines  32 C and  32 D would shut flow channel  30 A between  35 A and  35 B, and would also shut lateral passageways  35 B. As such, flow entering as flow F 1  would sequentially travel through  30 A,  35 A and leave  30 B as flow F 4 . 
     10. Switchable Flow Arrays 
     In yet another novel embodiment, fluid passage can be selectively directed to flow in either of two perpendicular directions. An example of such a “switchable flow array” system is provided in  FIGS. 20A to 20D .  FIG. 20A  shows a bottom view of a first layer of elastomer  90 , (or any other suitable substrate), having a bottom surface with a pattern of recesses forming a flow channel grid defined by an array of solid posts  92 , each having flow channels passing therearound. 
     In preferred aspects, an additional layer of elastomer is bound to the top surface of layer  90  such that fluid flow can be selectively directed to move either in direction F 1 , or perpendicular direction F 2 .  FIG. 20  is a bottom view of the bottom surface of the second layer of elastomer  95  showing recesses formed in the shape of alternating “vertical” control lines  96  and “horizontal” control lines  94 . “Vertical” control lines  96  have the same width therealong, whereas “horizontal” control lines  94  have alternating wide and narrow portions, as shown. 
     Elastomeric layer  95  is positioned over top of elastomeric layer  90  such that “vertical” control lines  96  are positioned over posts  92  as shown in  FIG. 20C  and “horizontal” control lines  94  are positioned with their wide portions between posts  92 , as shown in  FIG. 20D . 
     As can be seen in  FIG. 20C , when “vertical” control lines  96  are pressurized, the membrane of the integrated structure formed by the elastomeric layer initially positioned between layers  90  and  95  in regions  98  will be deflected downwardly over the array of flow channels such that flow in only able to pass in flow direction F 2  (i.e.: vertically), as shown. 
     As can be seen in  FIG. 20D , when “horizontal” control lines  94  are pressurized, the membrane of the integrated structure formed by the elastomeric layer initially positioned between layers  90  and  95  in regions  99  will be deflected downwardly over the array of flow channels, (but only in the regions where they are widest), such that flow in only able to pass in flow direction F 1  (i.e.: horizontally), as shown. 
     The design illustrated in  FIG. 20  allows a switchable flow array to be constructed from only two elastomeric layers, with no vertical vias passing between control lines in different elastomeric layers required. If all vertical flow control lines  94  are connected, they may be pressurized from one input. The same is true for all horizontal flow control lines  96 . 
     11. Normally-Closed Valve Structure 
       FIGS. 7B and 7H  above depict a valve structure in which the elastomeric membrane is moveable from a first relaxed position to a second actuated position in which the flow channel is blocked. However, the present invention is not limited to this particular valve configuration. 
       FIGS. 21A-21J  show a variety of views of a normally-closed valve structure in which the elastomeric membrane is moveable from a first relaxed position blocking a flow channel, to a second actuated position in which the flow channel is open, utilizing a negative control pressure. 
       FIG. 21A  shows a plan view, and  FIG. 21B  shows a cross sectional view along line  42 B- 42 B′, of normally-closed valve  4200  in an unactuated state. Flow channel  4202  and control channel  4204  are formed in elastomeric block  4206  overlying substrate  4205 . Flow channel  4202  includes a first portion  4202   a  and a second portion  4202   b  separated by separating portion  4208 . Control channel  4204  overlies separating portion  4208 . As shown in  FIG. 42B , in its relaxed, unactuated position, separating portion  4008  remains positioned between flow channel portions  4202   a  and  4202   b , interrupting flow channel  4202 . 
       FIG. 21C  shows a cross-sectional view of valve  4200  wherein separating portion  4208  is in an actuated position. When the pressure within control channel  4204  is reduced to below the pressure in the flow channel (for example by vacuum pump), separating portion  4208  experiences an actuating force drawing it into control channel  4204 . As a result of this actuation force membrane  4208  projects into control channel  4204 , thereby removing the obstacle to a flow of material through flow channel  4202  and creating a passageway  4203 . Upon elevation of pressure within control channel  4204 , separating portion  4208  will assume its natural position, relaxing back into and obstructing flow channel  4202 . 
     The behavior of the membrane in response to an actuation force may be changed by varying the width of the overlying control channel. Accordingly,  FIGS. 21D-42H  show plan and cross-sectional views of an alternative embodiment of a normally-closed valve  4201  in which control channel  4207  is substantially wider than separating portion  4208 . As shown in cross-sectional views  FIG. 21E-F  along line  42 E- 42 E′ of  FIG. 21D , because a larger area of elastomeric material is required to be moved during actuation, the actuation force necessary to be applied is reduced. 
       FIGS. 21G  and H show a cross-sectional views along line  40 G- 40 G′ of  FIG. 21D . In comparison with the unactuated valve configuration shown in  FIG. 21G ,  FIG. 21H  shows that reduced pressure within wider control channel  4207  may under certain circumstances have the unwanted effect of pulling underlying elastomer  4206  away from substrate  4205 , thereby creating undesirable void  4212 . 
     Accordingly,  FIG. 21I  shows a plan view, and  FIG. 21J  shows a cross-sectional view along line  21 J- 21 J′ of  FIG. 21I , of valve structure  4220  which avoids this problem by featuring control line  4204  with a minimum width except in segment  4204   a  overlapping separating portion  4208 . As shown in  FIG. 21J , even under actuated conditions the narrower cross-section of control channel  4204  reduces the attractive force on the underlying elastomer material  4206 , thereby preventing this elastomer material from being drawn away from substrate  4205  and creating an undesirable void. 
     While a normally-closed valve structure actuated in response to pressure is shown in  FIGS. 21A-21J , a normally-closed valve in accordance with the present invention is not limited to this configuration. For example, the separating portion obstructing the flow channel could alternatively be manipulated by electric or magnetic fields, as described extensively above. 
     12. Side-Actuated Valve 
     While the above description has focused upon microfabricated elastomeric valve structures in which a control channel is positioned above and separated by an intervening elastomeric membrane from an underlying flow channel, the present invention is not limited to this configuration.  FIGS. 22A and 22B  show plan views of one embodiment of a side-actuated valve structure in accordance with one embodiment of the present invention. 
       FIG. 22A  shows side-actuated valve structure  4800  in an unactuated position. Flow channel  4802  is formed in elastomeric layer  4804 . Control channel  4806  abutting flow channel  4802  is also formed in elastomeric layer  4804 . Control channel  4806  is separated from flow channel  4802  by elastomeric membrane portion  4808 . A second elastomeric layer (not shown) is bonded over bottom elastomeric layer  4804  to enclose flow channel  4802  and control channel  4806 . 
       FIG. 22B  shows side-actuated valve structure  4800  in an actuated position. In response to a build up of pressure within control channel  4806 , membrane  4808  deforms into flow channel  4802 , blocking flow channel  4802 . Upon release of pressure within control channel  4806 , membrane  4808  would relax back into control channel  4806  and open flow channel  4802 . 
     While a side-actuated valve structure actuated in response to pressure is shown in  FIGS. 22A and 22B , a side-actuated valve in accordance with the present invention is not limited to this configuration. For example, the elastomeric membrane portion located between the abutting flow and control channels could alternatively be manipulated by electric or magnetic fields, as described extensively above. 
     13. Composite Structures 
     Microfabricated elastomeric structures of the present invention may be combined with non-elastomeric materials to create composite structures.  FIG. 23  shows a cross-sectional view of one embodiment of a composite structure in accordance with the present invention.  FIG. 23  shows composite valve structure  5700  including first, thin elastomer layer  5702  overlying semiconductor-type substrate  5704  having channel  5706  formed therein. Second, thicker elastomer layer  5708  overlies first elastomer layer  5702 . Actuation of first elastomer layer  5702  to drive it into channel  5706 , will cause composite structure  5700  to operate as a valve. 
       FIG. 24  shows a cross-sectional view of a variation on this theme, wherein thin elastomer layer  5802  is sandwiched between two hard, semiconductor substrates  5804  and  5806 , with lower substrate  5804  featuring channel  5808 . Again, actuation of thin elastomer layer  5802  to drive it into channel  5808  will cause composite structure  5810  to operate as a valve. 
     The structures shown in  FIG. 23  or  24  may be fabricated utilizing either the multilayer soft lithography or encapsulation techniques described above. In the multilayer soft lithography method, the elastomer layer(s) would be formed and then placed over the semiconductor substrate bearing the channel. In the encapsulation method, the channel would be first formed in the semiconductor substrate, and then the channel would be filled with a sacrificial material such as photoresist. The elastomer would then be formed in place over the substrate, with removal of the sacrificial material producing the channel overlaid by the elastomer membrane. As is discussed in detail below in connection with bonding of elastomer to other types of materials, the encapsulation approach may result in a stronger seal between the elastomer membrane component and the underlying nonelastomer substrate component. 
     As shown in  FIGS. 23 and 24 , a composite structure in accordance with embodiments of the present invention may include a hard substrate that bears a passive feature such as a channels. However, the present invention is not limited to this approach, and the underlying hard substrate may bear active features that interact with an elastomer component bearing a recess. This is shown in  FIG. 25 , wherein composite structure  5900  includes elastomer component  5902  containing recess  5904  having walls  5906  and ceiling  5908 . Ceiling  5908  forms flexible membrane portion  5909 . Elastomer component  5902  is sealed against substantially planar nonelastomeric component  5910  that includes active device  5912 . Active device  5912  may interact with material present in recess  5904  and/or flexible membrane portion  5909 . 
     Many types of active structures may be present in the nonelastomer substrate. Active structures that could be present in an underlying hard substrate include, but are not limited to, resistors, capacitors, photodiodes, transistors, chemical field effect transistors (chem FET&#39;s), amperometric/coulometric electrochemical sensors, fiber optics, fiber optic interconnects, light emitting diodes, laser diodes, vertical cavity surface emitting lasers (VCSEL&#39;s), micromirrors, accelerometers, pressure sensors, flow sensors, CMOS imaging arrays, CCD cameras, electronic logic, microprocessors, thermistors, Peltier coolers, waveguides, resistive heaters, chemical sensors, strain gauges, inductors, actuators (including electrostatic, magnetic, electromagnetic, bimetallic, piezoelectric, shape-memory-alloy based, and others), coils, magnets, electromagnets, magnetic sensors (such as those used in hard drives, superconducting quantum interference devices (SQUIDS) and other types), radio frequency sources and receivers, microwave frequency sources and receivers, sources and receivers for other regions of the electromagnetic spectrum, radioactive particle counters, and electrometers. 
     As is well known in the art, a vast variety of technologies can be utilized to fabricate active features in semiconductor and other types of hard substrates, including but not limited printed circuit board (PCB) technology, CMOS, surface micromachining, bulk micromachining, printable polymer electronics, and TFT and other amorphous/polycrystalline techniques as are employed to fabricate laptop and flat screen displays. 
     A variety of approaches can be employed to seal the elastomeric structure against the nonelastomeric substrate, ranging from the creation of a Van der Waals bond between the elastomeric and nonelastomeric components, to creation of covalent or ionic bonds between the elastomeric and nonelastomeric components of the composite structure. Example approaches to sealing the components together are discussed below, approximately in order of increasing strength. 
     A first approach is to rely upon the simple hermetic seal resulting from Van der Waals bonds formed when a substantially planar elastomer layer is placed into contact with a substantially planar layer of a harder, non-elastomer material. In one embodiment, bonding of RTV elastomer to a glass substrate created a composite structure capable of withstanding up to about 3-4 psi of pressure. This may be sufficient for many potential applications. 
     A second approach is to utilize a liquid layer to assist in bonding. One example of this involves bonding elastomer to a hard glass substrate, wherein a weakly acidic solution (5 μl HCl in H 2 O, pH 2) was applied to a glass substrate. The elastomer component was then placed into contact with the glass substrate, and the composite structure baked at 37° C. to remove the water. This resulted in a bond between elastomer and non-elastomer able to withstand a pressure of about 20 psi. In this case, the acid may neutralize silanol groups present on the glass surface, permitting the elastomer and non-elastomer to enter into good Van der Waals contact with each other. 
     Exposure to ethanol can also cause device components to adhere together. In one embodiment, an RTV elastomer material and a glass substrate were washed with ethanol and then dried under Nitrogen. The RTV elastomer was then placed into contact with the glass and the combination baked for 3 hours at 80° C. Optionally, the RTV may also be exposed to a vacuum to remove any air bubbles trapped between the slice and the RTV. The strength of the adhesion between elastomer and glass using this method has withstood pressures in excess of 35 psi. The adhesion created using this method is not permanent, and the elastomer may be peeled off of the glass, washed, and resealed against the glass. This ethanol washing approach can also be employed used to cause successive layers of elastomer to bond together with sufficient strength to resist a pressure of 30 psi. In alternative embodiments, chemicals such as other alcohols or diols could be used to promote adhesion between layers. 
     An embodiment of a method of promoting adhesion between layers of a microfabricated structure in accordance with the present invention comprises exposing a surface of a first component layer to a chemical, exposing a surface of a second component layer to the chemical, and placing the surface of the first component layer into contact with the surface of the second elastomer layer. 
     A third approach is to create a covalent chemical bond between the elastomer component and functional groups introduced onto the surface of a nonelastomer component. Examples of derivitization of a nonelastomer substrate surface to produce such functional groups include exposing a glass substrate to agents such as vinyl silane or aminopropyltriethoxy silane (APTES), which may be useful to allow bonding of the glass to silicone elastomer and polyurethane elastomer materials, respectively. 
     A fourth approach is to create a covalent chemical bond between the elastomer component and a functional group native to the surface of the nonelastomer component. For example, RTV elastomer can be created with an excess of vinyl groups on its surface. These vinyl groups can be caused to react with corresponding functional groups present on the exterior of a hard substrate material, for example the Si—H bonds prevalent on the surface of a single crystal silicon substrate after removal of native oxide by etching. In this example, the strength of the bond created between the elastomer component and the nonelastomer component has been observed to exceed the materials strength of the elastomer components. 
     II. Damper Structure 
     Embodiments of apparatuses and methods in accordance with the present invention are directed to microfluidic devices comprising pumps, valves, and fluid oscillation dampers. In this respect, the microfluidic devices of the present invention is similar to that described in Unger et al.  Science,  2000, 288, 113-116, which is incorporated herein by reference in its entirety. However, microfluidic devices of the present invention may further comprise a damper. 
     The advantages of microfluidic devices of the present invention include reduced fluid oscillation within a flow channel which reduces potential variability in detection means. As the fluid is pushed through the flow channel by the pumps, there is a tendency for the fluid to oscillate, i.e., the fluid is pushed through the flow channel in a sinusoidal wave-like fashion. As this oscillating fluid passes through a detector region, different fluid depth passes through the detector region. And depending on a particular detection means used, this difference in fluid depth can cause a higher “background noise” or an inaccurate reading by the detector. By reducing or eliminating this fluid oscillation, the “background noise” is reduced and a more accurate reading by the detector can be achieved. 
     Preferred devices are constructed by single and multilayer soft lithography (MLSL) as detailed in commonly assigned U.S. patent application Ser. No. 09/605,520, filed Jun. 27, 2000, which is incorporated herein by reference in its entirety. 
     Microfluidic devices of the present invention comprise an integrated pump which can be electronic, magnetic, mechanical, or preferably pneumatic pumps. By using a pneumatic pump, microfluidic devices of the present invention allow more precise control of the fluid flow within the fluid channel. In addition, unlike electro-osmotic driven fluid flow, pneumatic pump allows the flow of fluids in both directions, thereby allowing reversible sorting of materials, as discussed in greater detail below. Furthermore, a pneumatic pump provides at least 10 times, preferably at least about 20 times, and more preferably at least about 30 times the fluid flow rate capacity compared to the capacity of electro-osmotic fluid flow. 
     In addition, microfluidic devices of the present invention may comprise a damper which reduces or eliminates the fluid oscillation within the fluid channel. The damper can any device which attenuates the fluid oscillation. For example, the damper can simply be a channel which is open to the ambient atmosphere and has a thin elastic membrane between the channel and the fluid flow channel. Preferably, the damper is an encapsulated pocket of fluid medium with a thin elastic membrane above the fluid flow channel. The fluid medium can be a liquid or, preferably, a gas. The damper is generally located above the fluid flow channels with a thin membrane, preferably an elastic membrane, between the fluid flow channel and the damper. Typically, there is at least 1 damper posterior to the pump in the direction of the fluid flow. Preferably, there is at least 2 dampers, more preferably at least 3 dampers and most preferably at least about 5 dampers posterior to the pump. 
     The width of damper is at least as wide as the width of flow channel that is located below the damper. In this manner, the entire cross-section of the flow channel is covered by the damper to ensure attenuation of fluid oscillation across the entire width of the flow channel. Preferably, the width of damper is at least about 1.1 times the width of flow channel, more preferably at least about 1.3 times the width of the flow channel, and most preferably at least about 1.5 times the width of the flow channel. In this manner, the need for a precise alignment of the damper on top of the flow channel is eliminated. 
     The damper is separated from the fluid flow channel by a thin membrane. Preferably this thin membrane has sufficient elasticity to deflect “upward” when a fluid having a peak of sinusoidal wave-like passes underneath. In this manner, some of the fluid oscillation energy is absorbed by the damper, thereby reducing the height (i.e., peak) of fluid oscillation. Typically, the thickness of the membrane between the damper and the fluid flow channel depends on a variety of factors including the depth and width of the flow channel, the amount of fluid oscillation produced by the pump and the elasticity (i.e., the material) of the membrane. One of ordinary skill in the art can readily determine the proper membrane thickness to achieve a desired attenuation of fluid oscillation depending on a desired application and materials used. 
     In general, damper structures in accordance with embodiments of the present invention feature an energy absorber adjacent to the flow channel, the energy absorber configured to experience a physical change in response to an oscillation within the flow channel. As described below, the energy absorber can take several forms, including but not limited to a flexible membrane, a pocket filled with a compressible fluid, and/or flexible walls of the flow channel itself. 
       FIG. 26  shows a cross-sectional view of a first embodiment of a damper structure in accordance with the present invention. Damper structure  2600  comprises cavity  2602  separated from underlying flow channel  2604  by membrane  2606  of elastomer layer  2608  in which cavity  2602  is formed. Oscillation in pressure  2610  in the fluid flowing through underlying flow channel  2604  causes membrane  2606  to flex up and down, thereby absorbing some of the energy of oscillation and providing for a more uniform flow of material through channel  2604 . The degree to which damper structure  2600  is capable of absorbing oscillation energy is dictated by a number of factors, including but not limited to, the length of the flow channel covered by the membrane, the elasticity of the membrane, and the compressability of any liquid material present in the cavity. The longer the membrane and the greater its flexion, the larger amount of oscillation energy that can be absorbed. Moreover, as stated above, the damping effect can be amplified by positioning a series of damper structures along the flow channel. 
       FIG. 27  shows a cross-sectional view of yet another embodiment of a damper structure in accordance with the present invention, wherein damper structure  2700  comprises portion  2702   a  of flow channel  2702  having a larger cross-section, an upper region of portion  2702  being filled with air or some other type of fluid  2704 . As material  2706  flowing through channel  2702  experiences pressure oscillations  2708  and eventually encounters damper  2700 , energy is expended as material  2706  pushes against fluid  2704 . This expenditure of energy serves to reduce the amplitude of energy of oscillation of material in the flow channel. 
       FIG. 28  shows a plan view of yet another embodiment of a damper structure in accordance with the present invention. Damper structure  2800  comprises the combination of cavity  2802  overlying and separated from underlying flow channel  2804  by membrane  2806 , and constriction  2808  in the width of flow channel  2804  positioned downstream of cavity/membrane combination  2802 / 2806 . In a manner analogous to operation of a resistance-capacitance (RC) element of an electronic circuit, cavity/membrane combination  2802 / 2806  serves as a flow capacitor while constriction  2808  serves as a flow resistor, resulting in a reduction in amplitude of pressure oscillations  2810  downstream of damper structure  2800 . 
     While the above embodiments have described dampers that are specifically implemented as separate structures in the architecture of a microfluidic device, embodiments of the present invention are not limited to such structures. For example, the elastomer material in which flow channels are formed may itself serve to absorb pressure oscillations within the flow, independent of the presence of separate membrane/cavity structures or fluid filled portions of enlarged flow channels. The damping effect of the elastomer material upon pressure oscillations would depend upon the elasticity of the particular elastomer, and hence its ability to change shape during absorption of energy from the oscillating flow. 
     Damper structures in accordance with embodiments of the present invention function reduce the amplitude of oscillations by absorbing energy through displacement of a moveable portion of the damper structure, for example a flexible membrane or a fluid pocket. The damper structures will generally operate most efficiently to reduce the amplitude of oscillations within a given frequency range. This frequency range is related to the speed of displacement and recovery of the moveable portion relative to the oscillation frequency. The speed of displacement and recovery of the moveable element is in turn dictated by such factors as material composition, and the design and dimensions of a specific damper structure. 
     Damper structures in accordance with embodiments of the present invention may also be utilized to create fluid circulation substructures. This is illustrated and described in conjunction with  FIGS. 29A-29B , which show cross-sectional views of embodiments of circulation structures utilizing damper structures in accordance with the present invention. 
     Fluid circulation system  2900  comprises flow channel  2902  formed in elastomer material  2904  and featuring valves  2904  and  2906  positioned at either end. End valves  2904  and  2906  are actuated, such that elastomer valve membranes  2904   a  and  2906   a  project into and block flow channel  2902 , forming sealed flow channel segment  2902   a.    
     Pump  2908  and damper  2910  are positioned adjacent to sealed flow segment  2902   a . Pump  2908  comprises recess  2908   a  separated from underlying flow channel  2902  by pump membrane  2908   b . Damper  2910  comprises cavity  2910   a  separated from underlying flow channel  2902  by damper membrane  2910   b.    
     As shown in  FIGS. 29A-B , pump  2908  and damper  2910  cooperate to permit a continuous circulation of fluid within sealed segment  2902   a . Specifically, pump  2908  is first actuated such that pump membrane  2908   b  deflects into flow channel  2902 , increasing the pressure within sealed segment  2902   a . In response to this increased pressure, damper membrane  2910   b  is displaced into cavity  2910   a  and fluid within segment  2902   a  circulates to occupy the additional space. 
     Subsequently, pump  2908  is deactuated such that pump membrane  2908   b  relaxes to its original position, out of flow channel  2902 . Because of the reduced pressure experienced by segment  2902   a  as a result of the deactuation of pump  2908 , damper membrane  2910   b  also relaxes back into its original position, displacing material back into flow channel  2902 . As a result of this action, the material within sealed segment  2902   a  experiences a back flow, and circulation is accomplished. 
     The circulation of material as just described may prove useful in a number of applications. For example, where a mixture comprising several components is being manipulated by a microfluidic apparatus, the circulation may serve to ensure homogeneity of the mixture. Similarly, where a suspension is being manipulated, the circulating action may serve to maintain particles in suspension. 
     Moreover, certain components of a fluid being manipulated by a microfluidic device, such as cellular material, may stick to flow channel sidewalls. Maintenance of a continuous circulation within the flow channels may help prevent loss of material to the channel walls. 
     III. Sorting Applications 
     In one particular embodiment of the present invention, the microfluidic device comprises a T-channel for sorting materials (e.g., cells or large molecules such as peptides, DNA&#39;s and other polymers) with fluid flow channel dimensions of about 50 μm×35 μm (width×depth). The width of pressure channels (i.e., pneumatic pump) and the damper is 100 μm and 80 μm, respectively. The gap between the flow channel and the damper (or the pressure channel) is about 5 to 6 μm. In order to produce such a thin first layer, the MLSL process requires providing a layer of an elastomer (e.g., by spreading) which is typically thinner than most other previously disclosed microfluidic devices. For example, when using GE RTV 615 PDMS silicon rubber, previous microfluidic devices typically used 30:1 ratio of 615A:615B at 2000 rpm spin-coating to fabricate the first (i.e., bottom) layer of the elastomer and 3:1 ratio of A:B for the second elastomer layer. However, it has been found by the present inventors that the silicon rubber does not cure when the ratio of 30:1 is used in fabricating the above described dimensions of fluid flow channels in the first elastomer layer. 
     Moreover, in order to produce a thin first elastomer layer, a higher spin-coating rate was required. For example, without using any diluent, GE RTV 615 PDMS silicon rubber A and B components in the ratio of about 20:1 was required at 8000 rpm to produce the first elastomer layer having about 3.5 μm flow channel depth and about 5-6 μm thickness between the flow channel and the damper (or the pressure channels). When SF-96 diluent was used, spin-coating at about 3000 rpm can be used to achieve a similar dimension first elastomer layer. 
     During fabrication of a mold, the photoresist is typically etched using a mask, developed and heated. Heating of the developed photoresist reshapes trapezoid-shaped “ridges”, which ultimately form the channels, to a smooth rounded ridges and reduces the height of ridges from about 20 μm to about 5 μm. This method, however, does not provide channels having depth of about 3.5 μm. The present inventors have found that this limitation can be overcome by treating the developed photoresist with oxygen plasma (e.g., using SPI Plasma Prep II from SPI Supplies a Division of Structure Probe, Inc., West Chester, Pa.) and heating the photoresist at a lower heat setting. Unlike previous methods, where a higher heat setting appear to chemically modify the photoresist, the lower heat setting used in the present invention does not chemically alter the photoresist. 
     Microfluidic devices of the present invention can be used in a variety of applications such as sorting cells as disclosed in commonly assigned U.S. patent application Ser. No. 09/325,667 and the corresponding published PCT Application No. US99/13050, and sorting DNA&#39;s as disclosed in commonly assigned U.S. patent application Ser. No. 09/499,943, all of which are incorporated herein by reference in their entirety. 
     The actual dimensions of a particular microfluidic device depend on its application. For example, for sorting bacteria which typically have cell size of about 1 μm, the width of fluid flow channel is generally in the range of from about 5 μm to about 50 μm and the depth of at least about 5 μm. For sorting mammalian cells which have typically have cell size of about 30 μm, the width of fluid flow channel is generally in the range of from about 40 μm to about 60 μm and the depth of at least about 40 μm. For DNA sorting, the dimensions of fluid flow channels can be significantly less. 
     TABLE A below provides a nonexclusive, nonlimiting list of candidate sortable entities, their approximate size range, and the approximate range of flow channel widths of a microfluidic apparatus at the point of detection of the entity. 
     
       
         
           
               
               
               
             
               
                 TABLE A 
               
               
                   
               
               
                   
                 APPROXIMATE SIZE 
                 APPROXIMATE RANGE 
               
               
                   
                 RANGE OF 
                 OF FLOW CHANNEL 
               
               
                 SORTABLE 
                 SORTABLE 
                 WIDTH AT DETECTION 
               
               
                 ENTITY 
                 ENTITY (μm) 
                 POINT (μm) 
               
               
                   
               
             
            
               
                 bacterial cell 
                 1-10 
                 5-50 
               
               
                 mammalian cell 
                  5-100 
                 10-500 
               
               
                 egg cell 
                  10-1000 
                  10-1000 
               
               
                 sperm cell 
                 1-10 
                 10-100 
               
               
                 DNA strands 
                 0.003-1    
                 0.1-10   
               
               
                 proteins 
                 0.01-1    
                 1-10 
               
               
                 micelles 
                 0.1-100  
                  1-500 
               
               
                 viruses 
                 0.05-1    
                 1-10 
               
               
                 larvae 
                 600-6500 
                 VARIABLE 
               
               
                 beads 
                 0.01-100   
                 VARIABLE 
               
               
                   
               
            
           
         
       
     
     The information presented in TABLE A is exemplary in nature, and is intended only as a nonexclusive listing of candidates for sorting utilizing embodiments in accordance with the present invention. The sorting of other entities, and variation in approximate channel widths utilized to sort the entities listed above, are possible and would vary according to the particular application. 
     One particular embodiment of the present invention is shown in  FIGS. 30A and 30B , where  FIG. 30A  is a schematic drawing of the microfluidic device shown in  FIG. 30B . In this embodiment, the microfluidic device comprises an injection pool  3022 , where a fluid containing a material can be introduced. The fluid is then pumped through the fluid flow channel  3034  via a pneumatic pump  3010  which comprises three pressure channels. By alternately pressurizing the three pressure channels, one can pump the fluid through the fluid flow channel  3034  in a similar fashion to a peristaltic pump. 
     The fluid exiting the pump oscillates due to actions of the pump. The fluid oscillation amplitude is attenuated by dampers  3014  which is located above the flow channel  3034 , behind the pump  3010  and before a detector (not shown). Initially, the collection valve  3018 A is closed and the waste valve  3018 B is open to allow the fluid to flow from the injection pool  3022  through the T-junction  3038  and into the waste pool  3030 . When a desired material is detected by the detector (not shown), the waste valve  3018 B is closed and the collection valve  3018 A is opened to allow the material to be collected in the collection pool  3026 . The valve  3018 A and  3018 B are interconnected to the detector through a computer or other automated system to allow opening and closing of appropriate valves depending on whether a desired material is detected or not. 
     Dimensions of the embodiment shown above in  FIGS. 30A and 30B  are as follows. The width of the peristaltic pumps is 100 μm. The width of the dampers is 80 μm. The width of the switch valves is 30 and 50 μm. The dimensions of the T-channel is 50×3.5 μm. The dimensions at the T-junction are 5×3.5 μm. The thickness of the interlayer is 6 μm. 
     One such application for the microfluidic device described above is in a reverse sorting of a material (e.g., beads, DNA&#39;s, peptides or other polymers, or cells) as shown in  FIGS. 31A and 31B . In the reverse sorting process, the material is allowed to flow towards the waste pool  3030  as shown in  FIG. 31A . When a desired material is detected by a detector (not shown) the pump (not shown) is reversed until the material is again detected by the detector. At this point, the waste valve  3018 B is closed and the collection valve  3018 A is opened, as shown in  FIG. 31B , and the flow of material is again reversed to allow the material to flow into the collection pool  3026 . After which the collection valve  3018 A is closed and the waste valve  3018 B is opened. This entire process is repeated until a desired amount of materials in the injection (or input) pool  3022  is “sorted”. The reverse movement of materials in the flow channel as just described can be assisted by changing pressures applied directly to flow channel inlets and outlets. 
     In one embodiment of the present invention,  E. Coli  expressing GFP is sorted using the reversible sorting process described above. As shown in  FIGS. 32A and 32B , the cell velocity depends on the frequency of the pump. Thus, the cell velocity reaches a maximum of about 16 mm/sec at about 100 Hz of pump rate. Moreover, as expected, the mean reverse time in  FIG. 31B , which represents the time interval between detection of  E. Coli  expressing GFP, reversing the pump, and detection of the same  E. Coli , decreases as the pump frequency is increased. 
     In another embodiment of the present invention provides sorting materials according to ratio of wavelengths (e.g., from laser induced fluorescence). For example, by measuring two different fluorescence wavelengths (e.g., λ1 and λ2) and calculating the ratio of λ1 and λ2, one can determine a variety of information regarding the material, such as the life cycle stage of cells, the stage of evolution of cells, the strength of enzyme-substrate binding, the strength of drug interactions with cells, receptors or enzymes, and other useful biological and non-biological interactions. 
     Another embodiment of the present invention provides multiple interrogation (i.e., observation or detection) of the same material at different time intervals. For example, by closing of the valves  3018 A or  3018 B in  FIG. 33C  and alternately pumping the fluid to and from the input well  3022  at a particular intervals, the material can be made to flow to and from the input well  3022  through the detector (not shown). By oscillating this material through the detection window  3040 , one can observe the material at different time intervals. For example, a sample can be interrogated at 10 Hz pump frequency as shown in  FIG. 33A  or at 75 Hz pump frequency as shown in  FIG. 33B . As expected, at a higher pump frequency, the material can be observed at shorter intervals. Such observation of materials at different time has variety of applications including monitoring cell developments, enzyme-substrate interactions, affect of drugs on a given cell or enzyme; measuring half-life of a given material including drugs, compounds, polymers and the like; as well as other biological applications. 
     The performance of a sorter structure in accordance with embodiments of the present invention may be dependent upon the elastomer material utilized to fabricate the device.  FIG. 34  plots flow velocity versus pump frequency for cell sorters fabricated from different elastomeric materials, namely General Electric RTV 615 and Dow Corning Sylgard 184. 
       FIG. 34  shows that the flow velocity of cells through the cell sorters reached a maximum at a pumping frequency of about 50 Hz. The decline in flow velocity above this frequency may be attributable to incomplete opening and closing of the valves with each pumping cycle. 
     Moreover, different values for maximum pumping rates of the two sorting structures are different. The RTV 615 cell sorter exhibits a maximum pumping rate of about 10,000 μm/sec, while the Sylgard 184 cell sorter exhibits a maximum pumping rate of about 14,000 μm/sec. Maximum flow rates of other elastomeric microfluidic devices in accordance with the present invention have ranged from about 6000 μm/sec to about 17,000 μm/sec, but should be understood as merely exemplary and not limiting to the scope of the present invention. 
       FIG. 34  indicates that the pumping rate of a cell sorter device may be controlled by the identity, and hence flexibility, of the particular elastomer used. In the instant case, based upon the relative flexibility of RTV 615 and Sylgard 184, the greater the elasticity of the elastomer results in a faster rate of pumping. 
     Other changes, for example the addition of diluents to the elastomer or the mixing of different ratios of A and B components of fluidic layer, may allow even further fine tuning of the pumping rate. Changing the dimensions of the fluidic channel may also allow tuning of the pumping rate, as different volumes of fluid in the channels will be moved with each actuation of the membrane. 
     While the sorting device described above utilizes a T-shaped junction between flow channels, this is not required by the present invention. Other types of junctions, including but not limited to Y-shaped, or even junctions formed by the intersection of four or more flow channels, could be utilized for sorting and the device would remain within the scope of the present invention. 
     Moreover, while only a single sorting structure is illustrated above, the invention is not limited to this particular configuration. A sorter in accordance with embodiments of the present invention is readily integratable with other structures on the same microfabricated device. For example, embodiments in accordance with the present invention may include a series of consecutively-arranged sorting structures useful for segregating different components of a mixture through successive sorting operations. 
     In addition, a microfluidic device in accordance with embodiments of the present invention could also include structures for pre-sorting and post-sorting activities that are in direct fluid communication with the sorter. Examples of pre- or post-sorting activities that can be integrated directly into a microfluidic device in accordance with the present invention include, but are not limited to, crystallization, cell lysis, labeling, staining, filtering, separation, dialysis, chromatography, mixing, reaction, polymerase chain reaction, and incubation. Chambers and other structures for performing these activities can be integrated directly onto the microfabricated elastomeric structure. 
     While the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosure, and it will be appreciated that in some instances some features of the invention will be employed without a corresponding use of other features without departing from the scope of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments and equivalents falling within the scope of the claims.