Patent Publication Number: US-7217321-B2

Title: Microfluidic protein crystallography techniques

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The instant patent application is a continuation-in-part of U.S. nonprovisional patent application Ser. No. 10/637,847, filed Aug. 7, 2003, which is a continuation-in-part of U.S. nonprovisional patent application Ser. No. 10/117,978, filed Apr. 5, 2002, which claims priority as a nonprovisional application from U.S. provisional patent application No. 60/323,524 filed Sep. 17, 2001, and which is also a continuation-in-part of U.S. nonprovisional application Ser. No. 09/887,997 filed Jun. 22, 2001, now U.S. Pat. No. 7,052,545, which is in turn a continuation-in-part of U.S. nonprovisional patent application Ser. No. 09/826,583 filed Apr. 6, 2001, now U.S. Pat. No. 6,899,137. The instant patent application is also a continuation-in-part of U.S. nonprovisional patent application Ser. No. 10/265,473, filed Oct. 4, 2002. The instant patent application further claims priority from U.S. provisional patent application No. 60/527,168 filed Dec. 5, 2003, and from U.S. provisional patent application No. 60/527,625 filed Dec. 5, 2003. These prior patent applications are hereby incorporated by reference for all purposes. 

   STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
   Work described herein has been supported, in part, by NSF (xyz in a chip program); National Institute of Health grant CA 77373; NSERC (Julie Payette Fellowship); David H. &amp; Lucille M. Packard Foundation; and G. Harold and Leila Y. Mathers Charitable Foundation. Work described herein has also been supported in part by National Science Foundation Grant No. CTS 0088649, National Institutes of Health Grant Nos. CA 77373 and HG 1642-02, and Army Research Office Grant No. DAAD19-00-1-0392 awarded by DARPA. The United States Government may therefore have certain rights in the invention. 

   BACKGROUND OF THE INVENTION 
   Crystallization is an important technique to the biological and chemical arts. Specifically, a high-quality crystal of a target compound can be analyzed by x-ray diffraction techniques to produce an accurate three-dimensional structure of the target. This three-dimensional structure information can then be utilized to predict functionality and behavior of the target. 
   In theory, the crystallization process is simple. A target compound in pure form is dissolved in solvent. The chemical environment of the dissolved target material is then altered such that the target is less soluble and reverts to the solid phase in crystalline form. This change in chemical environment typically accomplished by introducing a crystallizing agent that makes the target material is less soluble, although changes in temperature and pressure can also influence solubility of the target material. 
   In practice however, forming a high quality crystal is generally difficult and sometimes impossible, requiring much trial and error and patience on the part of the researcher. Specifically, the highly complex structure of even simple biological compounds means that they are not amenable to forming a highly ordered crystalline structure. Therefore, a researcher must be patient and methodical, experimenting with a large number of conditions for crystallization, altering parameters such as sample concentration, solvent type, countersolvent type, temperature, and duration in order to obtain a high quality crystal, if in fact a crystal can be obtained at all. 
   Accordingly, there is a need in the art for methods and structures for performing high throughput screening of crystallization of target materials. 
   SUMMARY OF THE INVENTION 
   The present invention relates to microfluidic devices and methods facilitating the growth and analysis of crystallized materials such as proteins. In accordance with one embodiment, a crystal growth architecture is separated by a permeable membrane from an adjacent well having a much larger volume. The well may be configured to contain a fluid having an identity and concentration similar to the solvent and crystallizing agent employed in crystal growth, with diffusion across the membrane stabilizing that process. Alternatively, the well may be configured to contain a fluid having an identity calculated to affect the crystallization process. In accordance with the still other embodiment, the well may be configured to contain a material such as a cryo-protectant, which is useful in protecting the crystalline material once formed. 
   An embodiment of a method in accordance with the present invention for growing crystals, comprises, disposing a solution including a solvent and containing a crystal source material in a first chamber defined by an elastomer structure. A crystallizing agent is disposed in a second chamber defined by the elastomer structure. A material is disposed in a well defined by the elastomer structure adjacent to and separated from the first and second chambers by an elastomer membrane. The first and second chambers are placed in fluid communication to alter a solubility of the crystal source material, such that a presence of the material affects formation of a crystal from the crystal source material. 
   An embodiment of apparatus in accordance with the present invention for forming crystals, comprises, an elastomer structure defining a first chamber in selective fluid communication with a second chamber. The first chamber is configured to contain a solution containing a crystal material dissolved in a solvent, and the second chamber is configured to contain a crystallizing agent, the first and second chambers separated from an adjacent well by a thin elastomer membrane. 
   An embodiment of an apparatus in accordance with the present invention for extracting a crystal from an elastomer microfluidic device, comprises, an enclosure and a piston slidable within the enclosure and comprising an ejector portion having pins moveable relative to a blade portion having blades. A loose spring is positioned between the blade portion and the enclosure and configured to bias the piston. A tight spring is positioned between the blade portion and the ejector portion and configured to bias the ejector portion relative to the blade portion. 
   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. 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. 
       FIG. 7H  is a front sectional view showing the valve of  FIG. 7B  in an actuated state. 
       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 front sectional view of the valve of  FIG. 7B  showing actuation of the membrane. 
       FIG. 11  is a front sectional view of an alternative embodiment of a valve having a flow channel with a curved upper surface. 
       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. 17  shows a CAD drawing of a large-capacity embodiment of a crystal growth chip in accordance with the present invention. 
       FIG. 18  shows a micrograph of a protein crystal grown utilizing the device of  FIG. 17 . 
       FIG. 19A  is a simplified cross-sectional schematic view of an embodiment of a crystal growth/analysis device in accordance with the present invention. 
       FIG. 19B  is an enlarged micrograph showing a perspective view of the embodiment of a crystal growth/analysis device shown in  FIG. 19A . 
       FIG. 19C  is an enlarged micrograph showing a plan view of the embodiment of a crystal growth/analysis device shown in  FIGS. 19A–B . 
       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 . 
       FIG. 21  is an enlarged cross-sectional view of the inverted membrane of the device shown in  FIG. 19 . 
       FIG. 22  is a micrograph showing a plan view of a crystal disk in accordance with an embodiment of the present invention. 
       FIG. 23  is a micrograph showing a crystal disk in accordance with an embodiment of the present invention held in a mounting clip. 
       FIG. 24  shows protein crystals grown utilizing the design of  FIG. 21 . 
       FIG. 25  shows a high resolution diffraction from a protein crystals grown, frozen and harvested utilizing the design of  FIG. 21 . 
       FIGS. 26A–26D  show plan views illustrating operation of one embodiment of a cell pen structure in accordance with the present invention. 
       FIGS. 27A–27B  show plan and cross-sectional views illustrating operation of one embodiment of a cell cage structure in accordance with the present invention. 
       FIGS. 28A–28D  show plan views of operation of a structure utilizing cross-channel injection in accordance with the embodiment of the present invention. 
       FIGS. 29A–D  are simplified schematic diagrams plotting concentration versus distance for two fluids in diffusing across a microfluidic free interface in accordance with an embodiment of the present invention. 
       FIGS. 30A–B  show simplified cross-sectional views of the attempted formation of a macroscopic free-interface in a capillary tube. 
       FIGS. 31A–B  show simplified cross-sectional views of convective mixing between a first solution and a second solution in a capillary tube resulting from a parabolic velocity distribution of pressure driven Poiseuille flow 
       FIGS. 32A–C  show simplified cross-sectional views of interaction in a capillary tube between a first solution having a density greater than the density of second solution. 
       FIG. 33A  shows a simplified cross-sectional view of a microfluidic free interface in accordance with an embodiment of the present invention. 
       FIG. 33B  shows a simplified cross-sectional view of a conventional non-microfluidic interface. 
       FIGS. 34A–D  show plan views of the priming of a flow channel and formation of a microfluidic free interface in accordance with an embodiment of the present invention. 
       FIGS. 35A–E  show simplified schematic views of the use of “break-through” valves to create a microfluidic free interface. 
       FIG. 36A  shows a simplified schematic view of a protein crystal being formed utilizing a conventional macroscopic free interface diffusion technique. 
       FIG. 36B  shows a simplified schematic view of a protein crystal being formed utilizing diffusion across a microfluidic free interface in accordance with an embodiment of the present invention. 
       FIG. 37A  shows a simplified plan view of a flow channel overlapped at intervals by a forked control channel to define a plurality of chambers (A–G) positioned on either side of a separately-actuated interface valve. 
       FIGS. 37B–D  plot solvent concentration at different times for the flow channel shown in  FIG. 37A . 
       FIG. 38A  shows three sets of pairs of chambers connected by microchannels of a different length. 
       FIG. 38B  plots equilibration time versus channel length. 
       FIG. 39  shows four pairs of chambers, each having different arrangements of connecting microchannel(s). 
       FIG. 40A  shows a plan view of a simple embodiment of a microfluidic structure in accordance with the present invention. 
       FIG. 40B  is a simplified plot of concentration versus distance for the structure of  FIG. 40A . 
       FIG. 41  plots the time required for the concentration in one of the reservoirs of  FIG. 40  to reach 0.6 of the final equilibration concentration, versus channel length. 
       FIG. 42  plots the inverse of the time required for the concentration in one of the reservoirs to reach 0.6 of the final equilibration concentration (T 0.6 ), versus the area of the fluidic interface of  FIG. 40 . 
       FIG. 43  presents a phase diagram depicting the phase space between fluids A and B, and the path in phase space traversed in the reservoirs as the fluids diffuse across the microfluidic free interface of  FIG. 40   
       FIG. 44  shows an enlarged view of one embodiment of a chip holder in accordance with the present invention. 
       FIG. 45A  shows a simplified plan view of the alternative embodiment of the chip utilized to obtain experimental results. 
       FIG. 45B  shows as simplified enlarged plan view of a set of three compound wells of the chip shown in  FIG. 45A . 
       FIG. 45C  shows a simplified cross-sectional view of the compound wells of  FIGS. 45A–B . 
       FIG. 46  shows a plan view of the creation of a microfluidic free interface between a flowing fluid and a dead-ended branch channel. 
       FIG. 47  shows a simplified plan view of one embodiment of a microfluidic structure in accordance with the present invention for creating diffusion gradients of two different species in different dimensions. 
       FIG. 48  shows a simplified plan view of an alternative embodiment of a microfluidic structure in accordance with the present invention for creating diffusion gradients of two different species in different dimensions. 
       FIG. 49  shows a simplified plan view of a sorting device in accordance with an embodiment of the present invention. 
       FIG. 50  plots Log(R/B) vs. number of slugs injected for one embodiment of a cross-flow injection system in accordance with the present invention. 
       FIG. 51  plots injected volume versus injection cycles during operation of one embodiment of a cross-flow injection structure in accordance with the present invention. 
       FIG. 52  plots crystallization hits utilizing a microfluidic chip in accordance with the present invention. 
       FIGS. 53A–B  show plan and cross-sectional views of one embodiment of a crystal growing/harvesting chip in accordance with of one embodiment of the present invention. 
       FIG. 54A  shows a plan view of one embodiment of a combinatoric mixing/storage structure in accordance with the present invention. 
       FIG. 54B  shows an enlarged view of the mixing portion of the structure of  FIG. 54A . 
       FIG. 54C  shows an enlarged view of the storage array of the structure of  FIG. 54A . 
       FIG. 54D  shows an enlarged view of one cell of the storage array of the structure of  FIG. 54C . 
       FIG. 55  shows a simplified cross-sectional view of one embodiment of a tool which allows the removal of crystal-containing portions of a chip for analysis. 
       FIG. 56  shows a simplified enlarged view of the tool of  FIG. 55 . 
       FIG. 57  plots trajectory through the phase space achieved by a number of crystallization approaches. 
       FIG. 58A–D  illustrates a schematic view of storage technique utilizing a gated serpentine storage line. 
       FIG. 59  illustrates a schematic view of an embodiment wherein a multiplexer could be used to direct each experimental condition into parallel storage channels. 
       FIG. 60  illustrates a schematic view of an embodiment wherein each reaction condition is dead-end filled to the end of a storage line. 
   

   DETAILED DESCRIPTION OF THE PRESENT INVENTION 
   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. Micromachined 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. Micromachined 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 heterogeneous 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 heterogeneous 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:1 B. 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 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 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. 
   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. 
   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 diamines (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 elasticity&#39;s, 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. 
   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, τclose is expected to be smaller than τopen. 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: topen=3.63 ms, τopen=1.88 ms, tclose=2.15 ms, τclose=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. 10 , 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. 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 2n) control lines. 
   8. 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  FIGS. 20A–D  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 . 
   9. Cell Pen/Cell Cage 
   In yet a further application of the present invention, an elastomeric structure can be utilized to manipulate organisms or other biological material.  FIGS. 26A–26D  show plan views of one embodiment of a cell pen structure in accordance with the present invention. 
   Cell pen array  4400  features an array of orthogonally-oriented flow channels  4402 , with an enlarged “pen” structure  4404  at the intersection of alternating flow channels. Valve  4406  is positioned at the entrance and exit of each pen structure  4404 . Peristaltic pump structures  4408  are positioned on each horizontal flow channel and on the vertical flow channels lacking a cell pen structure. 
   Cell pen array  4400  of  FIG. 26A  has been loaded with cells A–H that have been previously sorted.  FIGS. 26B–26C  show the accessing and removal of individually stored cell C by 1) opening valves  4406  on either side of adjacent pens  4404   a  and  4404   b,  2) pumping horizontal flow channel  4402   a  to displace cells C and G, and then 3) pumping vertical flow channel  4402   b  to remove cell C.  FIG. 26D  shows that second cell G is moved back into its prior position in cell pen array  4400  by reversing the direction of liquid flow through horizontal flow channel  4402   a.    
   The cell pen array  4404  described above is capable of storing materials within a selected, addressable position for ready access. However, living organisms such as cells may require a continuous intake of foods and expulsion of wastes in order to remain viable. Accordingly,  FIGS. 27A and 27B  show plan and cross-sectional views (along line  45 B– 45 B′) respectively, of one embodiment of a cell cage structure in accordance with the present invention. 
   Cell cage  4500  is formed as an enlarged portion  4500   a  of a flow channel  4501  in an elastomeric block  4503  in contact with substrate  4505 . Cell cage  4500  is similar to an individual cell pen as described above in  FIGS. 26A–26D , except that ends  4500   b  and  4500   c  of cell cage  4500  do not completely enclose interior region  4500   a . Rather, ends  4500   a  and  4500   b  of cage  4500  are formed by a plurality of retractable pillars  4502 . 
   Specifically, control channel  4504  overlies pillars  4502 . When the pressure in control channel  4504  is reduced, elastomeric pillars  4502  are drawn upward into control channel  4504 , thereby opening end  4500   b  of cell cage  4500  and permitting a cell to enter. Upon elevation of pressure in control channel  4504 , pillars  4502  relax downward against substrate  4505  and prevent a cell from exiting cage  4500 . 
   Elastomeric pillars  4502  are of a sufficient size and number to prevent movement of a cell out of cage  4500 , but also include gaps  4508  which allow the flow of nutrients into cage interior  4500   a  in order to sustain cell(s) stored therein. Pillars  4502  on opposite end  4500   c  are similarly configured beneath second control channel  4506  to permit opening of the cage and removal of the cell as desired. 
   The cross-flow channel architecture illustrated shown in  FIGS. 26A–26D  can be used to perform functions other than the cell pen just described. For example, the cross-flow channel architecture can be utilized in mixing applications. 
   This is shown in  FIGS. 28A–E , which illustrate a plan view of mixing steps performed by a microfabricated structures in accordance another embodiment of the present invention. Specifically, portion  7400  of a microfabricated mixing structure comprises first flow channel  7402  orthogonal to and intersecting with second flow channel  7404 . Control channels  7406  overlie flow channels  7402  and  7404  and form valve pairs  7408   a - b  and  7408   c–d  that surround each intersection  7412 . 
   As shown in  FIG. 28A , valve pair  7408   c–d  is initially opened while valve pair  7408   a–b  is closed, and fluid sample  7410  is flowed to intersection  7412  through flow channel  7404 . Valve pair  7408   a–b  is then actuated, trapping fluid sample  7410  at intersection  7412 . 
   Next, as shown in  FIG. 28B , valve pairs  7408   c–d  are closed and  7408   a–b  are opened, such that fluid sample  7410  is injected from intersection  7412  into flow channel  7402  bearing a cross-flow of fluid. The process shown in  FIGS. 28A–B  can be repeated to accurately dispense any number of fluid samples down cross-flow channel  7402 . 
   While the embodiment of a process-channel flow injector structure shown in  FIGS. 28A–B  feature channels intersecting at a single junction, this is not required by the present invention. Thus  FIG. 28C  shows a simplified plan view of another embodiment of an injection structure in accordance with the present invention, wherein junction  7450  between intersecting flow channels  7452  is extended to provide additional volume capacity.  FIG. 28D  shows a simplified plan view of yet another embodiment of an injection structure in accordance with the present invention, wherein elongated junction  7460  between intersecting flow channels  7462  includes branches  7464  to provide still more injection volume capacity. 
   And while the embodiment shown and described above in connection with  FIGS. 28A–28D  utilizes linked valve pairs on opposite sides of the flow channel intersections, this is not required by the present invention. Other configurations, including linking of adjacent valves of an intersection, or independent actuation of each valve surrounding an intersection, are possible to provide the desired flow characteristics. With the independent valve actuation approach however, it should be recognized that separate control structures would be utilized for each valve, complicating device layout. 
     FIG. 50  plots Log(R/B) vs. number of slugs injected for one embodiment of a cross-flow injection system in accordance with the present invention. The reproducibility and relative independence of metering by cross-flow injection from process parameters such as flow resistance is further evidenced by  FIG. 51 , which plots injected volume versus number of injection cycles for cross-channel flow injection under a variety of flow conditions.  FIG. 51  shows that volumes metered by cross-flow injection techniques increase on a linear basis over a succession of injection cycles. This linear relationship between volume and number of injection cycles is relatively independent of flow resistance parameters such as elevated fluid viscosity (imparted by adding 25% glycerol) and the length of the flow channel (1.0–2.5 cm). 
   II. Crystallization Structures and Methods 
   High throughput screening of crystallization of a target material, or purification of small samples of target material by recrystallization, may be accomplished by simultaneously introducing a solution of the target material at known concentrations into a plurality of chambers of a microfabricated fluidic device. The microfabricated fluidic device is then manipulated to vary solution conditions in the chambers, thereby simultaneously providing a large number of crystallization environments. Control over changed solvent conditions may result from a variety of techniques, including but not limited to metering of volumes of a crystallizing agent into the chamber by volume exclusion, by entrapment of liquid volumes determined by the dimensions of the microfabricated structure, or by cross-channel injection into a matrix of junctions defined by intersecting orthogonal flow channels. 
   Crystals resulting from crystallization in accordance with embodiments of the present invention can be utilized for x-ray crystallography to determine three-dimensional molecular structure. Alternatively, where high throughput screening in accordance with embodiments of the present invention does not produce crystals of sufficient size for direct x-ray crystallography, the crystals can be utilized as seed crystals for further crystallization experiments. Promising screening results can also be utilized as a basis for further screening focusing on a narrower spectrum of crystallization conditions, in a manner analogous to the use of standardized sparse matrix techniques. 
   Systems and methods in accordance with embodiments of the present invention are particularly suited to crystallizing larger biological macromolecules or aggregates thereof, such as proteins, nucleic acids, viruses, and protein/ligand complexes. However, crystallization in accordance with the present invention is not limited to any particular type of target material. 
   As employed in the following discussion, the term “crystallizing agent” describes a substance that is introduced to a solution of target material to lessen solubility of the target material and thereby induce crystal formation. Crystallizing agents typically include countersolvents in which the target exhibits reduced solubility, but may also describe materials affecting solution pH or materials such as polyethylene glycol that effectively reduce the volume of solvent available to the target material. The term “countersolvent” is used interchangeably with “crystallizing agent”. 
   1. Crystallization by Volume Entrapment 
     FIG. 45A  shows a simplified plan view of an embodiment of a crystallization system wherein metering of different volumes of countersolvent is determined by photolithography during formation of the flow channels.  FIG. 45B  shows a simplified enlarged plan view of a set of three compound wells of the device of  FIG. 45A .  FIG. 45C  shows a simplified cross-sectional view of the wells of  FIG. 45B  along line C–C′. This chip design employed metering of target solution and crystallizing agent utilizing the volume entrapment technique. 
   Specifically, each chip  9100  contains three compound wells  9102  for each of the 48 different screen conditions, for a total of 144 assays per chip. A compound well  9102  consists of two adjacent wells  9102   a  and  9102   b  etched in a glass substrate  9104 , and in fluidic contact via a microchannel  9106  In each of the compound wells  9102 , the protein solution is combined with the screen solution at a ratio that is defined by the relative size of the adjacent wells  9102   a–b . In the particular embodiment shown in  FIGS. 45A–C , the three ratios were (protein:solution) 4:1, 1:1, and 1:4. The total volume of each assay, including screen solution, is approximately 25 nL. However, the present invention is not limited to any particular volume or range of volumes. Alternative embodiments in accordance with the present invention may utilize total assay volumes of less than 10 nL, less than 5 nL, less than 2.5 nL, less than 1.0 nL, and less than 0.5 nL. 
   The chip control layer  9106  includes an interface control line  9108 , a containment control line  9110  and two safety control lines  9112 . Control lines  9108 ,  9110 , and  9112  are filled with water rather than air in order to maintain a humid environment within the chip and to prevent dehydration of the flow channels and chambers in which crystallization is to be performed. 
   The interface valves  9114  bisect the compound wells  9102 , separating the protein from the screen until completion of loading. Containment valves  9116  block the ports of each compound well  9102 , isolating each condition for the duration of the experiment. The two safety valves  9118  are actuated during protein loading, and prevent spillage of protein solution in the event of a failed interface valve. 
   Fabrication of the microfluidic devices utilized in the experiments were prepared by standard multilayer soft lithography techniques and sealed to an etched glass slide by baking at 80° C. for 5 hours or greater. The glass substrate is masked with a 16 um layer of  5740  photoresist, and is patterned using standard photolithography. The glass substrate is then etched in a solution of 1:1:1 (BOE:H 2 O:2N HCl) for 60 minutes, creating micro-wells with a maximum depth of approximately 80 μm. 
   The chip fabrication protocol just described is only one example of a possible embodiment of the present invention. In accordance with alternative embodiments, the crystallization chambers and flow channels could be defined between a planar substrate and a pattern of recesses formed entirely in the lower surface of the elastomer portion. Still further alternatively, the crystallization chambers and flow channels could be defined between a planar, featureless lower surface of the elastomer portion and a pattern of recesses formed entirely in the substrate. 
   Crystallization on chip is set up as follows. All control lines in chip control layer  9106  are loaded with water at a pressure of 15–17 psi. Once the control lines are filled and valves  9114  and  9116  are completely actuated, the containment valve  9116  is released, and protein is loaded through the center via  9120  using about 5–7 psi. The protein solution completely fills the protein side of each compound well  9102 . Failed valves, if present, are then identified, and vacuum grease is placed over the corresponding screen via to prevent subsequent pressurization, and possible contamination of the remaining conditions. 2.5 to 4 μL of a sparse matrix screen (typically Hampton Crystal Screen I, 1–48) are then pipetted into the screen vias  9122 . The safety valves  9118  are released, and a specially designed chip holder (described below) is used to create a pressurized (5–7 psi) seal over all 48 screen vias  9122 . The screen solutions are dead end loaded, filling the screen side of each compound well. Protein and crystal screen reagents are kept separate with the interface valve until all wells are loaded, at which point the containment valve is closed and the interface valve opened to allow diffusion between liquid volumes present in the two halves of the compound wells  9102 . 
   For these experiments, the average time spent setting up an experiment, including filling control lines, was approximately 35 min, with the fastest experiment taking only 20 minutes to set up. This set up time could potentially be reduced even further through the use of robotic pipetting of solutions to the chip, or through the use of pressures to load and prime delivered solutions, or through use of a microfluidic metering device, for example the combinatorial mixing structure previously described. 
   As previously illustrated, embodiments of microfluidic devices in accordance with the present invention may utilize on-chip reservoirs or wells. However, in a microfluidic device requiring the loading of a large number of solutions, the use of a corresponding large number of input tubes with separate pins for interfacing each well may be impractical given the relatively small dimensions of the fluidic device. In addition, the automated use of pipettes for dispensing small volumes of liquid is known, and thus it therefore may prove easiest to utilize such techniques to pipette solutions directly on to wells present on the face of a chip. 
   Capillary action may not be sufficient to draw solutions from on-chip wells into active regions of the chip, particularly where dead-ended chambers are to be primed with material. In such embodiments, one way of loading materials into the chip is through the use of external pressurization. Again however, the small dimensions of the device coupled with a large number of possible material sources may render impractical the application of pressure to individual wells through pins or tubing. 
   Accordingly,  FIG. 44  shows an exploded view of a chip holder  11000  in accordance with one embodiment of the present invention. Bottom portion  11002  of chip holder  11000  includes raised peripheral portion  11004  surrounding recessed area  11006  corresponding in size to the dimensions of chip  11008 , allowing microfluidic chip  11008  to be positioned therein. Peripheral region  11004  further defines screw holes  11010 . 
   Microfluidic device  11008  is positioned within recessed area  11006  of bottom portion  11002  of chip holder  11000 . Microfluidic device  11008  comprises an active region  11011  that is in fluidic communication with peripheral wells  11012  configured in first and second rows  11012   a  and  11012   b , respectively. Wells  11012  hold sufficient volumes of material to allow device  11008  to function. Wells  11012  may contain, for example, solutions of crystallizing agents, solutions of target materials, or other chemical reagents such as stains. Bottom portion  11002  contains a window  11003  that enables active region  11011  of chip  11008  to be observed. 
   Top portion  11014  of chip holder  11000  fits over bottom chip holder portion  11002  and microfluidic chip  11008  positioned therein. For ease of illustration, in  FIG. 80  top chip holder portion  11014  is shown inverted relative to its actual position in the assembly. Top chip holder portion  11014  includes screw holes  11010  aligned with screw holes  11010  of lower holder portion  11002 , such that screws  11016  may be inserted through holes  11010  secure chip between portions  11002  and  11014  of holder  11000 . Chip holder upper portion  11014  contains a window  11005  that enables active region  11011  of chip  11008  to be observed. 
   Lower surface  11014   a  of top holder portion  11014  includes raised annular rings  11020  and  11022  surrounding recesses  11024  and  11026 , respectively. When top portion  11014  of chip holder  11000  is pressed into contact with chip  11008  utilizing screws  11016 , rings  11020  and  11022  press into the soft elastomeric material on the upper surface of chip  11008 , such that recess  11024  defines a first chamber over top row  11012   a  of wells  11012 , and recess  11026  defines a second chamber over bottom row  11012   b  of wells  11012 . Holes  11030  and  11032  in the side of top holder portion  11014  are in communication with recesses  11024  and  11026  respectively, to enable a positive pressure to be applied to the chambers through pins  11034  inserted into holes  11030  and  11032 , respectively. A positive pressure can thus simultaneously be applied to all wells within a row, obviating the need to utilize separate connecting devices to each well. 
   In operation, solutions are pipetted into the wells  11012 , and then chip  11008  is placed into bottom portion  11002  of holder  11000 . The top holder portion  11014  is placed over chip  11008 , and is pressed down by screws. Raised annular rings  11020  and  11022  on the lower surface of top holder portion  11014  make a seal with the upper surface of the chip where the wells are located. Solutions within the wells are exposed to positive pressures within the chamber, and are thereby pushed into the active area of microfluidic device. 
   The downward pressure exerted by the chip holder may also pose the advantage of preventing delamination of the chip from the substrate during loading. This prevention of delamination may enable the use of higher priming pressures. 
   The chip holder shown in  FIG. 44  represents only one possible embodiment of a structure in accordance with the present invention. 
   2. Control Over Other Factors Influencing Crystallization 
   While the above crystallization structures describe altering the environment of the target material through introduction of volumes of an appropriate crystallization agent, many other factors are relevant to crystallization. Such additional factors include, but are not limited to, temperature, pressure, concentration of target material in solution, equilibration dynamics, and the presence of seed materials. 
   In specific embodiments of the present invention, control over temperature during crystallization may be accomplished utilizing a composite elastomer/silicon structure previously described. Specifically, a Peltier temperature control structure may be fabricated in an underlying silicon substrate, with the elastomer aligned to the silicon such that a crystallization chamber is proximate to the Peltier device. Application of voltage of an appropriate polarity and magnitude to the Peltier device may control the temperature of solvent and countersolvent within the chamber. 
   Alternatively, as described by Wu et al. in “MEMS Flow Sensors for Nano-fluidic Applications”,  Sensors and Actuators  A 89 152–158 (2001), crystallization chambers could be heated and cooled through the selective application of current to a micromachined resistor structure resulting in ohmic heating. Moreover, the temperature of crystallization could be detected by monitoring the resistance of the heater over time. The Wu et al. paper is hereby incorporated by reference for all purposes. 
   It may also be useful to establish a temperature gradient across a microfabricated elastomeric crystallization structure in accordance with the present invention. Such a temperature gradient would subject target materials to a broad spectrum of temperatures during crystallization, allowing for extremely precise determination of optimum temperatures for crystallization. 
   With regard to controlling pressure during crystallization, embodiments of the present invention employing metering of countersolvent by volume exclusion are particularly advantageous. Specifically, once the chamber has been charged with appropriate volumes of solvent and countersolvent, a chamber inlet valve may be maintained shut while the membrane overlying the chamber is actuated, thereby causing pressure to increase in the chamber. Structures in accordance with the present invention employing techniques other than volume exclusion could exert pressure control by including flow channels and associated membranes adjacent to the crystallization chamber and specifically relegated to controlling pressure within the channel. 
   Another factor influencing crystallization is the amount of target material available in the solution. As a crystal forms, it acts as a sink to target material available in solution, to the point where the amount of target material remaining in solution may be inadequate to sustain continued crystal growth. Therefore, in order to grow sufficiently large crystals it may be necessary to provide additional target material during the crystallization process. 
   Accordingly, the cell pen structure previously described in connection with  FIGS. 27A–27B  may be advantageously employed in crystallization structures in accordance with embodiments of the present invention to confine growing crystals within a chamber. This obviates the danger of washing growing crystals down a flow channel that is providing additional target material, causing the growing crystals to be lost in the waste. 
   Moreover, the cell cage structure of  FIGS. 27A–27B  may also be useful during the process of crystal identification. Specifically, salts are often present in the sample or countersolvent, and these salts may form crystals during crystallization attempts. One popular method of distinguishing the growth of salt crystals from the target crystals of interest is through exposure to a staining dye such as IZIT™, manufactured by Hampton Research of Laguna Niguel, Calif. This IZIT™ dye stains protein crystals blue, but does not stain salt crystals. 
   However, in the process of flowing the IZIT™ dye to the crystallization chamber holding the crystals, the crystals may be dislodged, swept away, and lost. Therefore, the cell pen structure can further be employed in crystallization structures and methods in accordance with the present invention to secure crystals in place during the staining process. 
     FIG. 49  shows an embodiment of a sorting device for crystals based upon the cell cage concept. Specifically, crystals  8501  of varying sizes may be formed in flow channel  8502  upstream of sorting device  8500 . Sorting device  8500  comprises successive rows  8504  of pillars  8506  spaced at different distances. Inlets  8508  of branch channels  8510  are positioned in front of rows  8504 . As crystals  8501  flow down channel  8502 , they encounter rows  8504  of pillars  8506 . The largest crystals are unable to pass between gap Y between pillars  8506  of first row  8504   a , and accumulate in front of row  8504   a . Smaller sized crystals are gathered in front of successive rows having successively smaller spacings between pillars. Once sorted in the manner described above, the crystals of various sizes can be collected in chambers  8512  by pumping fluid through branch channels  8510  utilizing peristaltic pumps  8514  as previously described. Larger crystals collected by the sorting structure may be subjected to x-ray crystallographic analysis. Smaller crystals collected by the sorting structure may be utilized as seed crystals in further crystallization attempts. 
   Another factor influencing crystal growth is seeding. Introduction of a seed crystal to the target solution can greatly enhance crystal formation by providing a template to which molecules in solution can align. Where no seed crystal is available, embodiments of microfluidic crystallization methods and systems in accordance with the present invention may utilize other structures to perform a similar function. 
   For example, as discussed above, flow channels and chambers of structures in accordance with the present invention are typically defined by placing an elastomeric layer containing microfabricated features into contact with an underlying substrate such as glass. This substrate need not be planar, but rather may include projections or recesses of a size and/or shape calculated to induce crystal formation. In accordance with one embodiment of the present invention, the underlying substrate could be a mineral matrix exhibiting a regular desired morphology. Alternatively, the underlying substrate could be patterned (i.e. by conventional semiconductor lithography techniques) to exhibit a desired morphology or a spectrum of morphologies calculated to induce crystal formation. The optimal form of such a substrate surface morphology could be determined by prior knowledge of the target crystals. 
   Embodiments of crystallization structures and methods in accordance with the present invention offer a number of advantages over conventional approaches. One advantage is that the extremely small volumes (nanoliter/sub-nanoliter) of sample and crystallizing agent permit a wide variety of recrystallization conditions to be employed utilizing a relatively small amount of sample. 
   Another advantage of crystallization structures and methods in accordance with embodiments of the present invention is that the small size of the crystallization chambers allows crystallization attempts under hundreds or even thousands of different sets of conditions to be performed simultaneously. The small volumes of sample and crystallizing agent employed in recrystallization also result in a minimum waste of valuable purified target material. 
   A further advantage of crystallization in accordance with embodiments of the present invention is relative simplicity of operation. Specifically, control over flow utilizing parallel actuation requires the presence of only a few control lines, with the introduction of sample and crystallizing agent automatically performed by operation of the microfabricated device permits very rapid preparation times for a large number of samples with the added advantages of parsimonious use of sample solutions, ease of set-up, creation of well defined fluidic interfaces, control over equilibration dynamics, and the ability to conduct high-throughput parallel experimentation. These advantages are made possible by a number of features of the instant invention. 
   Microfluidics enables the handling of fluids on the sub-nanoliter scale. Consequently, there is no need to use large containment chambers, and hence, assays may be performed on the nanoliter, or subnanoliter scale. The utilization of extremely small volumes allows for thousands of assays to be performed to consume the same sample volume required for one macroscopic free-interface diffusion experiment. This reduces costly and time-consuming amplification and purification steps, and makes possible the screening of proteins that are not easily expressed, and hence must be purified from a bulk sample. 
   Microfluidics further offers savings in preparation times, as hundreds, or even thousands of assays may be performed simultaneously. The use of scaleable metering techniques as previously described, allow for parallel experimentation to be conducted without increased complexity in control mechanisms. 
   Still another advantage of crystallization systems in accordance with embodiments of the present invention is the ability to control solution equilibration rates. Crystal growth is often very slow, and no crystals will be formed if the solution rapidly passes through an optimal concentration on the way to equilibrium. It may therefore be advantageous to control the rate of equilibration and thereby promote crystal growth at intermediate concentrations. In conventional approaches to crystallization, slow-paced equilibrium is achieved using such techniques as vapor diffusion, slow dialysis, and very small physical interfaces. 
   However, crystallization in accordance with embodiments of the present invention allows for control over the rate of solution equilibrium. In systems metering crystallizing agent by volume exclusion, the overlying membrane can be repeatedly deformed, with each deformation giving rise to the introduction of additional crystallizing agent. In systems that meter crystallizing agent by volume entrapment, the valves separating sample from crystallizing agent may be opened for a short time to allow for partial diffusive mixing, and then closed to allow chamber equilibration at an intermediate concentration. The process is repeated until the final concentration is reached. Either the volume exclusion or entrapment approaches enables a whole range of intermediate concentrations to be screened in one experiment utilizing a single reaction chamber. As discussed in detail below, control over kinetics of the crystallization process may be controlled by varying the length or cross-sectional area of a capillary connection between reservoirs containing the sample and crystallizing agent, respectively. 
   The manipulation of solution equilibrium over time also exploits differential rates of diffusion of macromolecules such as proteins versus much smaller crystallizing agents such as salts. As large protein molecules diffuse much more slowly than the salts, rapidly opening and closing interface valves allows the concentration of crystallizing agent to be significantly changed, while at the same time very little sample is lost by diffusion into the larger volume of crystallizing agent. Moreover, as described above, many crystallization structures described readily allow for introduction of different crystallizing agents at different times to the same reaction chamber. This allows for crystallization protocols prescribing changed solvent conditions over time. Temperature control over equilibration is discussed in detail below. 
   3. Analysis of Crystal Structure from Protein on Chip 
   The utility of the chip is ultimately dependent on its&#39; ability to quickly generate high quality diffraction patterns at a reduced cost. A clear path from the chip-to-protein structure is therefore invaluable. Several paths from in-chip crystals to diffraction data are discussed below. 
   One possible application for a chip is determination of favorable crystallization conditions that can subsequently be reproduced using conventional techniques. Correspondence between the chip and two conventional techniques (micro batch and hanging drop) has been shown to be variable (between 45% and 80%). However, this variation is not a feature unique to the chip. These widely used crystallization techniques show only marginal correspondence (e.g. 14 of 16 hanging drop hits for lysozyme do not occur in microbatch), and often show variation within themselves. As a tool for screening initial crystallization conditions, the chip may be able to identify as many promising conditions. 
     FIG. 52  shows a comparison of the number of hits generated on six different protein samples (lysozyme, glucose isomerase, proteinase K, B subunit of topoisomerase VI, xylanase, and bovine pancrease trypsin) using the three different technologies. In  FIG. 52  only crystals, microcrystals, rods, and needles are counted as hits, while spherulites and precipitation is not counted. The data on Proteinase K is a sum of the experiments with and without PMSF, and data for the B subunit of topoisomerase VI has not been included for lack of hanging drop data (although the chip far outperformed micro-batch in this case). Inspection of  FIG. 52  shows that in four of the six cases, the chip produced more hits than either conventional method. 
   In order to understand differences between crystallization methods to identify possible reasons for productivity of the chip, we must appreciate that the three methods produce different thermodynamic conditions on both short and long time scales. In order to induce protein crystallization, one must make the crystallization energetically favorable (supersaturation condition), and maintain these conditions long enough for crystal growth to occur. 
   There are also different degrees of supersaturation. In low supersaturation, crystal growth tends to be supported, while nucleation of new crystals is relatively unlikely. In high supersaturation, nucleation is rapid, and many small low quality crystals may often be formed. In the three methods considered here, the condition of supersaturation is achieved through the manipulation of the relative, and absolute, concentration of protein and counter-solvent. 
   A comparison of the phase space evolution/equilibration of the three methods is shown in  FIG. 57 . For the micro batch technique, mixing of the two solutions is quick, and when kept under impermeable oil layers, little significant concentration occurs over time. Micro batch therefore tends to sample only a single point in phase space, maintaining approximately the same condition over time. 
   Hanging drop starts out like micro batch, with rapid mixing of the two solutions, but then undergoes a concentration on a longer timescale (hours to days) due to vapor equilibration with the more concentrated salt/precipitant reservoir. During the evaporative dehydration of the drop, the ratio of protein to precipitant remains constant. 
   As described in detail below in the description of Microfluidic Free Interface Diffusion, on the short time scale the chip dynamics most closely resemble a free interface diffusion experiment. Mixing is slow, and the rate of species equilibration (protein/precipitant/proton/salt) depends on species&#39; diffusion constants. Small molecules such as salts have large diffusion constants, and hence equilibrate quickly. Large molecules (e.g. proteins) have small diffusion constants, and equilibrate more slowly. 
   The crystallization technique of free interface diffusion in capillaries may more closely emulate the chip results. Traditionally this method is not often used due to the difficulty of reliably setting up a well-defined interface. However, in microfluidic environments it is relatively easy to establish reliable free-interface diffusion experiments. Additional discussion of the formation of microfluidic free-interfaces is presented below. In another application of the crystallization chip, crystals may be grown for harvesting using conventional methods. 
   If high quality crystals can be grown in, and extracted from the chip, crystallization conditions need not be exported. Since the chip can be removed from the glass substrate, it is also possible to extract protein crystals. 
   4. Temporal Control Over Equilibration 
   The growth and quality of crystals is determined not only by thermodynamic conditions explored during the equilibration, but also by the rate at which equilibration takes place. It is therefore potentially valuable to control the dynamics of equilibration. 
   In conventional crystallization methods, course control only over the dynamics of equilibration may be available through manipulation of initial conditions. For macroscopic free interface diffusion, once diffusion begins, the experimenter has no control over the subsequent equilibration rate. For hanging drop experiments, the equilibration rate may be changed by modifying the size of the initial drop, the total size of the reservoir, or the temperature of incubation. In microbatch experiments, the rate at which the sample is concentrated may be varied by manipulating the size of the drop, and the identity and amount of the surrounding oil. Since the equilibration rates depend in a complicated manner on these parameters, the dynamics of equilibration may only be changed in a coarsely manner. Moreover, once the experiment has begun, no further control over the equilibration dynamics is available. 
   By contrast, the fluidic interface in a gated μ-Fib experiment may be controlled by manipulation of the dimensions of the reaction chambers and of the connecting channels. To good approximation, the time required for equilibration varies as the required diffusion length. The equilibration rate also depends on the cross-sectional area of the connecting channels. The required time for equilibration may therefore be controlled by changing both the length, and the cross-sectional area of the connecting channels. 
     FIG. 38A  shows three sets of pairs of compound chambers  9800 ,  9802 , and  9804 , each pair connected by microchannels  9806  of a different length Δx.  FIG. 38B  plots equilibration time versus equilibration distance.  FIG. 38B  shows that the required time for equilibration of the chambers of  FIG. 38A  varies as the length of the connecting channels. 
     FIG. 39  shows four compound chambers  9900 ,  9902 ,  9904 , and  9906 , each having different arrangements of connecting microchannel(s)  9908 . Microchannels  9908  have the same length, but differ in cross-sectional area and/or number of connecting channels. The rate of equilibration may thus be increased/decreased by decreasing/increasing the cross-sectional area, for example by decreasing/increasing the number of connecting channels or the dimensions of those channels. 
   Varying the equilibration rate by changing the geometry of connecting channels may be used on a single device to explore the effect of equilibration dynamics on crystal growth.  FIGS. 37A–D  show an embodiment in which a gradient of concentrations, initially established by the partial diffusive equilibration of two solutions from a micro-free interface, can be maintained by the actuation of containment valves. 
     FIG. 37A  shows flow channel  10000  that is overlapped at intervals by a forked control channel  10002  to define a plurality of chambers (A–G) positioned on either side of a separately-actuated interface valve  10004 .  FIG. 37B  plots solvent concentration at an initial time, when interface valve  10004  is actuated and a first half  10000   a  of the flow channel has been mixed with a first solution, and a second half  10000   b  of the flow channel has been primed with a second solution.  FIG. 37C  plots solvent concentration at a subsequent time T 1 , when control channel  10002  is actuated to define the seven chambers (A–G), which capture the concentration gradient at that particular point in time.  FIG. 37D  plots relative concentration of the chambers (A–G) at time T 1 . 
   In the embodiment shown in  FIG. 37A , actuation of the forked control channel simultaneously creates the plurality of chambers A–G. However, this is not required, and in alternative embodiments of the present invention multiple control channels could be utilized to allow independent creation of chambers A–G at different time intervals, thereby allow additional diffusion to occur after an initial set of chambers are created immediately adjacent to the free interface. 
   An embodiment of a method of capturing a concentration gradient between two fluids comprises providing a first fluid on a first side of an elastomer membrane present within a microfluidic flow channel, and providing a second fluid on a second side of the elastomer membrane. The elastomer membrane is displaced from the microfluidic flow channel to define a microfluidic free interface between the first fluid and the second fluid. The first fluid and the second fluid are allowed to diffuse across the microfluidic free interface. A group of elastomer valves positioned along the flow channel at increasing distances from the microfluidic free interface are actuated to define a succession of chambers whose relative concentration of the first fluid and the second fluid reflects a time of diffusion across the microfluidic free interface. 
   5. Target Materials 
   Typical targets for crystallization are diverse. A target for crystallization may include but is not limited to: 1) biological macromolecules (cytosolic proteins, extracellular proteins, membrane proteins, DNA, RNA, and complex combinations thereof), 2) pre- and post-translationally modified biological molecules (including but not limited to, phosphorylated, sulfolated, glycosylated, ubiquitinated, etc. proteins, as well as halogenated, abasic, alkylated, etc. nucleic acids); 3) deliberately derivatized macromolecules, such as heavy-atom labeled DNAs, RNAs, and proteins (and complexes thereof), selenomethionine-labeled proteins and nucleic acids (and complexes thereof), halogenated DNAs, RNAs, and proteins (and complexes thereof), 4) whole viruses or large cellular particles (such as the ribosome, replisome, spliceosome, tubulin filaments, actin filaments, chromosomes, etc.), 5) small-molecule compounds such as drugs, lead compounds, ligands, salts, and organic or metallo-organic compounds, and 6) small-molecule/biological macromolecule complexes (e.g., drug/protein complexes, enzyme/substrate complexes, enzyme/product complexes, enzyme/regulator complexes, enzyme/inhibitor complexes, and combinations thereof). Such targets are the focus of study for a wide range of scientific disciplines encompassing biology, biochemistry, material sciences, pharmaceutics, chemistry, and physics. 
   A nonexclusive listing of possible protein modifications is as follows: 5′ dephospho; Desmosine (from Lysine); decomposed carboxymethylated Methionine; Ornithine (from Arginine); Lysinoalanine (from Cysteine); Lanthionine (from Cysteine); Dehydroalanine (from Cysteine); Homoserine formed from Met by CNBr treatment; Dehydration (—H2O); S-gamma-Glutamyl (crosslinked to Cysteine); O-gamma-Glutamyl-(Crosslink to Serine); Serine to Dehydroalanine; Alaninohistidine (Serine crosslinked to theta or pi carbon of Histidine); Pyroglutamic Acid formed from Gln; N-pyrrolidone carboxyl (N terminus); N alpha-(gamma-Glutamyl)-lysine; N-(beta-Aspartyl)-Lysine (Crosslink); 3,3′,5,5′-TerTyr (Crosslink); Disulphide bond formation (Cystine); S-(2-Histidyl)-(Crosslinked to Cysteine); S-(3-Tyr) (Crosslinked to Cysteine); 3,3′-BiTyr (Crosslink); IsodiTyr (Crosslink); Allysine (from Lysine); Amide formation (C terminus); Deamidation of Asparagine and Glutamine to Aspartate and Glutamate; Citruline (from Arginine); Syndesine (from Lysine); Methylation (N terminus, N epsilon of Lysine, O of Serine, Threonine or C terminus, N of Asparagine); delta-Hydroxy-allysine (from Lysine); Hydroxylation (of delta C of Lysine, beta C of Tryptophan, C3 or C4 of Proline, beta C of Aspartate); Oxidation of Methionine (to Sulphoxide); Sulfenic Acid (from Cysteine); Pyruvoyl-(Serine); 3,4-Dihydroxy-Phenylalanine (from Tyrosine) (DOPA); Sodium; Ethyl; N,N dimethylation (of Arginine or Lysine); 2,4-BisTrp-6,7-dione (from Tryptophan); Formylation (CHO); 6,7 Dione (from Tryptophan); 3,4,6-Trihydroxy-Phenylalanine (from Tyrosine) (TOPA); 3,4-Dihydroxylation (of Proline); Oxidation of Methionine (to Sulphone); 3-Chlorination (of Tyrosine with 35Cl); 3-Chlorination (of Tyrosine with 37Cl); Potassium; Carbamylation; Acetylation (N terminus, N epsilon of Lysine, O of Serine) (Ac); N-Trimethylation (of Lysine); gamma Carboxylation of Glutamate or beta Carboxylation of Aspartate; disodium; Nitro (NO2); t-butyl ester (OtBu) and t-butyl (tBu); Glycyl (-G-, -Gly-); Carboxymethyl (on Cystine); sodium+potassium; Selenocysteine (from Serine); 3,5-Dichlorination (of Tyrosine with 35Cl); Dehydroalanine (Dha); 3,5-Dichlorination (of Tyrosine with mixture of 35Cl and 37Cl)); Pyruvate; Acrylamidyl or Acrylamide adduct; Sarcosyl; Alanyl (-A-, -Ala-); Acetamidomethyl (Acm); 3,5-Dichlorination (of Tyrosine with 37Cl); S-(sn-1-Glyceryl) (on Cysteine); Glycerol Ester (on Glutamic acid side chain); Glycine (G, Gly); Beta mercaptoethanol adduct; Phenyl ester (OPh) (on acidic); 3-Bromination (of Tyrosine with 79Br); Phosphorylation (O of Serine, Threonine, Tyrosine and Aspartate, N epsilon of Lysine); 3-Bromination (of Tyrosine with 81Br); Sulphonation (SO3H) (of PMC group); Sulphation (of O of Tyrosine); Cyclohexyl ester (OcHex); Homoseryl lactone; Dehydroamino butyric acid (Dhb); Gamma Aminobutyryl; 2-Aminobutyric acid (Abu); 2-Aminoisobutyric acid (Aib); Diaminopropionyl; t-butyloxymethyl (Bum); N-(4-NH2-2-OH-butyl)-(of Lysine) (Hypusine); Seryl (-S-, -Ser-); t-butylsulfenyl (StBu); Alanine (A, Ala); Sarcosine (Sar); Anisyl; Benzyl (Bzl) and benzyl ester (OBzl); 1,2-ethanedithiol (EDT); Dehydroprolyl; Triflouroacetyl (TFA); N-hydroxysuccinimide (ONSu, OSu); Prolyl (-P-, -Pro-); Valyl (-V-, -Val-); Isovalyl (-I-, -Iva-); t-Butyloxycarbonyl (tBoc); Threoyl (-T-, -Thr-); Homoseryl (-Hse-); Cystyl (-C-, -Cys-); Benzoyl (Bz); 4-Methylbenzyl (Meb); Serine (S, Ser); HMP (hydroxymethylphenyl) linker; Thioanisyl; Thiocresyl; Diphthamide (from Histidine); Pyroglutamyl; 2-Piperidinecarboxylic acid (Pip); Hydroxyprolyl (-Hyp-); Norleucyl (-Nle-); Isoleucyl (-I-, -Ile-); Leucyl (-L-, -Leu-); Ornithyl (-Orn-); Asparagyl (-N-, -Asn-); t-amyloxycarbonyl (Aoc); Proline (P, Pro); Aspartyl (-D-, -Asp-); Succinyl; Valine (V, Val); Hydroxybenzotriazole ester (HOBt); Dimethylbenzyl (diMeBzl); Threonine (T, Thr); Cysteinylation; Benzyloxymethyl (Bom); p-methoxybenzyl (Mob, Mbzl); 4-Nitrophenyl, p-Nitrophenyl (ONp); Cysteine (C, Cys); Chlorobenzyl (ClBzl); Iodination (of Histidine[C4] or Tyrosine[C3]); Glutamyl (-Q-, -Gln-); N-methyl Lysyl; Lysyl (-K-, -Lys-); O-Methyl Aspartamyl; Glutamyl (-E-, -Glu-); N alpha-(gamma-Glutamyl)-Glu; Norleucine (Nle); Hydroxy Aspartamyl; Hydroxyproline (Hyp); bb-dimethyl Cystenyl; Isoleucine (I, Ile); Leucine (L, Leu); Methionyl (-M-, -Met-); Asparagine (N, Asn); Pentoses (Ara, Rib, Xyl); Aspartic Acid (D, Asp); Dmob (Dimethoxybenzyl); Benzyloxycarbonyl (Z); Adamantyl (Ada); p-Nitrobenzyl ester (ONb); Histidyl (-H-, -His-); N-methyl Glutamyl; O-methyl Glutamyl; Hydroxy Lysyl (-Hyl-); Methyl Methionyl; Glutamine (Q, Gln); Aminoethyl Cystenyl; Pentosyl; Deoxyhexoses (Fuc, Rha); Lysine (K, Lys); Aminoethyl cystenyl (-AECys-); 4-Glycosyloxy-(pentosyl, C5) (of Proline); Methionyl Sulfoxide; Glutamic Acid (E, Glu); Phenylalanyl- (-F-, -Phe-); Pyridyl Alanyl; Flourophenylalanyl; 2-Nitrobenzoyl (NBz); Methionine (M, Met); 3-methyl Histidyl; 2-Nitrophenylsulphenyl (Nps); 4-Toluenesulphonyl (Tosyl, Tos); 3-nitro-2-pyridinesulfenyl (Npys); Histidine (H, His); 3,5-Dibromination (of Tyrosine with 79Br); Arginyl (-R-, -Arg-); Citrulline; 3,5-Dibromination (of Tyrosine with mixture of 79Br and 81Br); Dichlorobenzyl (Dcb); 3,5-Dibromination (of Tyrosine with 81Br); Carboxyamidomethyl Cystenyl; Carboxymethyl Cystenyl; Methylphenylalanyl; Hexosamines (GalN, GlcN); Carboxymethyl cysteine (Cmc); N-Glucosyl (N terminus or N epsilon of Lysine) (Aminoketose); O-Glycosyl-(to Serine or Threonine); Hexoses (Fru, Gal, Glc, Man); Inositol; MethionylSulphone; Tyrosinyl (-Y-, -Tyr-); Phenylalanine (F, Phe); 2,4-dinitrophenyl (Dnp); Pentaflourophenyl (Pfp); Diphenylmethyl (Dpm); Phospho Seryl; 2-Chlorobenzyloxycarbonyl (ClZ); Napthyl acetyl; Isopropyl Lysyl; N-methyl Arginyl; Ethaneditohiol/TFA cyclic adduct; Carboxy Glutamyl (Gla); Acetamidomethyl Cystenyl; Acrylamidyl Cystenyl; Arginine (R, Arg); N-Glucuronyl (N terminus); delta-Glycosyloxy-(of Lysine) or beta-Glycosyloxy-(of Phenylalanine or Tyrosine); 4-Glycosyloxy-(hexosyl, C6) (of Proline); Benzyl Seryl; N-methyl Tyrosinyl; p-Nitrobenzyloxycarbonyl (4Nz); 2,4,5-Trichlorophenyl; 2,4,6-trimethyloxybenzyl (Tmob); Xanthyl (Xan); Phospho Threonyl; Tyrosine (Y, Tyr); Chlorophenylalanyl; Mesitylene-2-sulfonyl (Mts); Carboxymethyl Lysyl; Tryptophanyl (-W-, -Trp-); N-Lipoyl-(on Lysine); Matrix alpha cyano MH+; Benzyl Threonyl; Benzyl Cystenyl; Napthyl Alanyl; Succinyl Aspartamyl; Succinimidophenyl carb.; HMP (hydroxymethylphenyl)/TFA adduct; N-acetylhexosamines (GalNAc, GlcNAc); Tryptophan (W, Trp); Cystine ((Cys)2); Farnesylation; S-Farnesyl-; Myristoleylation (myristoyl with one double bond); Pyridylethyl Cystenyl; Myristoylation; 4-Methoxy-2,3,6-trimethylbenzenesulfonyl (Mtr); 2-Bromobenzyloxycarbonyl (BrZ); Formyl Tryptophanyl; Benzyl Glutamyl; Anisole Adducted Glutamyl; S-cystenyl Cystenyl; 9-Flourenylmethyloxycarbonyl (Fmoc); Lipoic acid (amide bond to lysine); Biotinylation (amide bond to lysine); Dimethoxybenzhydryl (Mbh); N-Pyridoxyl (on Lysine); Pyridoxal phosphate (Schiff Base formed to lysine); Nicotinyl Lysyl; Dansyl (Dns); 2-(p-biphenyl)isopropyl-oxycarbonyl (Bpoc); Pahnitoylation; “Triphenylmethyl (Trityl, Trt)”; Tyrosinyl Sulphate; Phospho Tyrosinyl; Pbf (pentamethyldihydrobenzofuransulfonyl); 3,5-Diiodination (of Tyrosine); 3,5-di-I″; N alpha-(gamma-Glutamyl)-Glu2; O-GlcNAc-1-phosphorylation (of Serine); “2,2,5,7,8-Pentamethylchroman-6-sulphonyl (Pmc)”; Stearoylation; Geranylgeranylation; S-Geranylgeranyl; 5′phos dCytidinyl; iodo Tyrosinyl; Aldohexosyl Lysyl; Sialyl; N-acetylneuraminic acid (Sialic acid, NeuAc, NANA, SA); 5′phos dThymidinyl; 5′phos Cytidinyl; Glutathionation; O-Uridinylylation (of Tyrosine); 5′phos Uridinyl; S-farnesyl Cystenyl; N-glycolneuraminic acid (NeuGc); 5′phos dAdenosyl; O-pantetheinephosphorylation (of Serine); SucPhencarb Lysyl; 5′phos dGuanosyl; 5′phos Adenosinyl; O-5′-Adenosylation (of Tyrosine); 4′-Phosphopantetheine; GL2; S-palmityl Cystenyl; 5′phos Guanosyl; Biotinyl Lysyl; Hex-HexNAc; N alpha-(gamma-Glutamyl)-Glu3; Dioctyl Phthalate; PMC Lysyl; Aedans Cystenyl; Dioctyl Phthalate Sodium Adduct; di-iodo Tyrosinyl; PMC Arginyl; S-Coenzyme A; AMP Lysyl; 3,5,3′-Triiodothyronine (from Tyrosine); S-(sn-1-Dipalmitoyl-glyceryl)-(on Cysteine); S-(ADP-ribosyl)-(on Cysteine); N-(ADP-ribosyl)-(on Arginine); O-ADP-ribosylation (on Glutamate or C terminus); ADP-rybosylation (from NAD); S-Phycocyanobilin (on Cysteine); S-Heme (on Cysteine); N theta-(ADP-ribosyl)diphthamide (of Histidine); NeuAc-Hex-HexNAc; MGDG; O-8 alpha-Flavin [FAD])-(of Tyrosine); S-(6-Flavin [FAD])-(on Cysteine); N theta and N pi-(8alpha-Flavin) (on Histidine); (Hex)3-HexNAc-HexNAc; (Hex)3-HexNAc-(dHex)HexNAc. 
   A nonexclusive listing of possible nucleic acid modifications, such as base-specific, sugar-specific, or phospho-specific is as follows: halogenation (F, Cl, Br, I); Abasic sites; Alkylation; Crosslinkable adducts such as thiols or azides; Thiolation; Deamidation; Fluorescent-group labeling, and glycosylation. 
   A nonexclusive listing of possible heavy atom derivatizing agents is as follows: potassium hexachloroiridate (III); Potassium hexachloroiridate (IV); Sodium hexachloroiridate (IV); Sodium hexachloroiridate (III); Potassium hexanitritoiridate (III); Ammnoium hexachloroiridate (III); Iridium (III) chloride; Potassium hexanitratoiridate (III); Iridium (III) bromide; Barium (II) chloride; Barium (II) acetate; Cadmium (II) nitrate; Cadmium (II) iodide; Lead (II) nitrate; Lead (II) acetate; Trimethyl lead (IV) chloride; Trimethyl lead (IV) acetate; Ammonium hexachloro plumbate (IV); Lead (II) chloride; Sodium hexachlororhodiate (III); Strontium (II) acetate; Disodium thiomalonato aurate (I); Potassium dicyano aurate (I); Sodium dicyano aurate (I); Sodium thiosulphato aurate (III); Potassium tetracyano aurate (III); Potassium tetrachloro aurate (III); Hydrogen tetrachloro aurate (III); Sodium tetrachloro aurate (III); Potassium tetraiodo aurate (III); Potassium tetrabromo aurate (III); (acetato-o)methylmercury; Methyl (nitrato-o) mercury; Chloromethylmercury; Iodomethylmercury; Chloroethylmercury; Methyl mercury cation; Triethyl(m3-phosphato(3-)-0,0′,0″)tri mercury eth; [3-[(aminocarbonyl)amino]-2-methoxypropyl]chlorome; 1,4 diacetoxymercury 2-3 dimethoxy butane; Meroxyl mercuhydrin; Tetrakis(acetoxy mercuri)-methane; 1,4-bis(chloromercuri)-2,3-butanediol; Ethyl diacetoxymercurichloro acetate (dame); Mercuric (II) oxide; Methyl mercuri-2-mercaptoethanol; 3,6 bis(mercurimethyl dioxane acetate); Ethyl mercury cation; Billman&#39;s dimercurial; Para chloromercury phenyl acetate (pcma); Mercury phenyl glyoxal (mpg); Thiomersal, ethyl mercury thiosalicylate [emts]; 4-chloromercuribenesulphonic acid; 2,6 dichloromercuri-4-nitrophenol (dcmnp); [3-[[2(carboxymethoxy)benzoyl]mino-2 methoxy prop; Parachloromercury benzoate (pcmb), 4-chloromercury; (acetato-o)phenyl mercury; Phenyl mercuri benzoate (pmb); Para hydroxy mercuri benzoate (phmb); Mercuric imidosuccinate/mercury succinimide; 3-hydroxymercurybenzaldehyde; 2-acetoxy mercuri sulhpanilamide; 3-acetoxymercuri-4-aminobenzenesulphonamide; Methyl mercuri thioclycolic acid (mmtga); 2-hydroxymercuri-tolulen-4-sulphonic acid (hmts); Acetamino phenyl mercury acetate (apma); [3-[(aminocarbonyl)amino]-3-methoxypropyl 2-chloro; Para-hydroxymercuri benzene sulphonate (phmbs); Ortho-chloromercuri phenol (ocmp); Diacetoxymercury dipopylene dioxide (dmdx); Para-acetoxymercuri aniline (pama); (4-aminophenyl) chloromercury; Aniline mercury cation; 3-hydroxy-mercuri-s sulphosalicylic acid (msss); 3 or 5 hydroxymercuri salicylic acid (hmsa); Diphenyl mercury; 2,6 diacetoxymercurimethyl 1–4 thioxane (dmmt); 2,5-b1s (chloromercury) furan; Ortho-chloromercuri nitrophenol (ocmnp); 5-mercurydeoxyuridine monosulphate; Mercury salicylate; [3-[[2-(carboxymethoxy)benzoyl]amino-2-methoxypro; 3,3 bis(hydroximercuri)-3-nitratomercuri pyruvic; 3-chloro mercuri pyridine; 3,5 bis acetoxymercuri methyl morpholine; Ortho-mercury phenol cation; Para-carboxymethyl mercaptomercuri benzensulphonyl; Para-mercuribenzoyl glucosamine; 3-acetetoxymercuri-5-nitrosalicyladehyde (msa); Ammonium tetrachloro mercurate (II); Potassium tetrathiocyanato mercurate (II); Sodium tetrathiocyanato mercurate (II); Potassium tetraisothiocyanto mercurate (II); Potassium tetraido mercurate (II); Ammonium tetrathiocyananato mercurate (II); Potassium tetrabromo mercurate (II); Potassium tetracyano mercurate (II); Mercury (II) bromide; Mercury (II) thiocyanate; Mercury (II) cyanide; Mercury (II) iodide; Mercuric (II) chloride; Mercury (II) acetate; Mercury (I) acetate; Dichlorodiamino mercurate (II); Beta mercury-mercapto-ethylamine hydrochloride; Mercury (II) sulphate; Mercury (II) chloroanilate; Dimercuriacetate; Chloro(2-oxoethyl) mercury; Phenol mercury nitrate; Mercury mercaptoethanol; Mercury mercaptoethylamine chloride; Mercury thioglycollic acid (sodium salt); 0-hydroxymercuri-p-nitrophenol/2-hydroxymercuri-4-; Para chloromercuri phenol (pcmp); Acetylmercurithiosalicylate (amts); Iodine; Potassium iodide (iodine); 4-iodopyrazole; O-iodobenzoylglucasamine; P-iodobenzoylglucasamine; Potassium iodide/chloramine t; Ammonium iodide; 3-isothio-cyanato-4-iodobenzene sulphonate; Potassium iodide; 3′-iodo phenyltrazine; 4′-iodo phenyltrazine; Sodium iodide/iodine; Silver nitrate; Silver ( ) trinitridosulphoxylate; Tobenamed; Samarium (III) chloride; Thulium (III) chloride; Lutetium (III) chloride; Europium (III) chloride; Terbium (III) chloride; Gadolinium (III) chloride; Erbium (III) chloride; Lanthanum (III) chloride; Samarium (III) nitrate; Samarium (III) acetate; Samarium (III) cation; Praseodymium (III) chloride; Neodymium (III) chloride; Ytterbium (III) chloride; Thulium (III) sulphate; Ytterbium (III) sulphate; Gadolinium (III) sulphate; Gadolinium (III) acetate; Dysprosium (III) chloride; Erbium (III) nitrate; Holmium (III) chloride; Penta amino ruthenium (III) chloride; Cesium nitridotiroxo osmium (viii); Potassium tetraoxo osmiate; Hexa amino osmium (III) iodide; Ammonium hexachloroosmiate (IV); Osmium (III) chloride; Potassium hexachloro osmiate (IV); Cesium trichloro triscarbonyl osmiate (?); Dinitritodiamine platinum (II); Cis dichlorodimethylammido platinum (II); Dichlorodiammine platinum (II); Dibromodiammine platinum (II); Dichloroethylene diamine platinum (II); Potassium dicholodinitrito platinate (II); Diethylenediamene platinum (II); Potassium dioxylato platinate (II); Dichlorobis (pyridine) platinum (II); Potassium (thimethyl dibenzyloamine) platinum; Potassium tetrabromoplatinate (II); Potassium tetrachloro platinate (II); Potassium tetranitrito platinum (II); Potassium tetracyano platinate (II); Sodium tetracyano platinate (II); Potassium tetrathiocyanato platinate (II); Ammonium tetranitrito platinate (II); Potassium tetraisocyanato platinate (II); Ammonium tetracyano platinum (II); Ammonium tetrachloro platinate (II); Potassium dinitritodioxalato platinate (IV); Dichlorotetraammino platinium (IV); Dibromodinitrito diammine platinium (IV); Potassium hexanitrito platinate (IV); Potassium hexachloro platinate (IV); Potassium hexabromo platinate (IV); Sodium hexachloroplatinate (IV); Potassium hexaiodo platinate (IV); Potassium hexathiocyanato platinate (IV); Tetrachloro bis(pyridine) platinum (IV); Ammonium hexachloro platinate (IV); Di-mu-iodo bis(ethylenediamine) di platinum (II) n; Potassium hexaisothiocyanato platinate (IV); Potassium tetraiodo platinate (II); 2,2′,2″terpyridyl platinium (II); 2 hydroxyethanethiolate (2,2′,2″ terpyeidine) pla; Potassium tetranitro platinate (II); Trimethyl platinum (II) nitrate; Sodium tetraoxo rhenate (VII); Potassium tetraoxo rhenate (VI); Potassium tetraoxo rhenate (VII); Potassium hexachloro rhenium (IV); Rhenium (III) chloride; Ammonium hexachloro rhenate (IV); Dimethyltin (II) dichloride; Thorium (IV) nitrate; Uranium (VI) oxychloride; Uranium (VI) oxynitrate; Uranium (VI) oxyacetate; Uranium (VI) oxypyrophosphate; Potassium pentafluoro oxyuranate (VI); Sodium pentafluoro oxyuranate (VI); Potassium nanofluoro dioxyuranate (VI); Sodium triacetate oxyuranate (VI); Uranium (VI) oxyoxalate; Selenocyanate anion; Sodium tungstate; Sodium 12-tungstophosphate; Thallium (I) acetate; Thallium (I) fluoride; Thallium (I) nitrate; Potassium tetrachloro palladate (II); Potassium tetrabromo palladate (II); Potassium tetracyano palladate (II); Potassium tetraiodo palladate (II); Cobalt (II) chloride. 
   The PDMS material from which the chip can be formed is well suited for many of these targets, particularly biological samples. PDMS is a non-reactive and biologically inert compound that allows such molecules to maintain their appropriate shape, fold, and activity in a solublized state. The matrix and system can accommodate a range of target sizes and molecular weights, from a few hundred Daltons to the mega-Dalton regime. Biological targets, from small proteins and peptides to viruses and macromolecular complexes, fall within this range, and are generally anywhere from 3–10 kDa to &gt;1–2 MDa in size. 
   6. Solute/Reagent Types 
   During crystallization screening, a large number of chemical compounds may be employed. These compounds include salts, small and large molecular weight organic compounds, buffers, ligands, small-molecule agents, detergents, peptides, crosslinking agents, and derivatizing agents. Together, these chemicals can be used to vary the ionic strength, pH, solute concentration, and target concentration in the drop, and can even be used to modify the target. The desired concentration of these chemicals to achieve crystallization is variable, and can range from nanomolar to molar concentrations. A typical crystallization mix contains set of fixed, but empirically-determined, types and concentrations of ‘precipitants’, buffers, salts, and other chemical additives (e.g., metal ions, salts, small molecular chemical additives, cryo protectants, etc.). Water is a key solvent in many crystallization trials of biological targets, as many of these molecules may require hydration to stay active and folded. 
   As described above in connection with the pressurized out-gas priming (POP) technique, the permeability of PDMS to gases, and the compatibility of solvents with PDMS may be a significant factor in deciding upon precipitating agents to be used. 
   ‘Precipitating’ agents act to push targets from a soluble to insoluble state, and may work by volume exclusion, changing the dielectric constant of the solvent, charge shielding, and molecular crowding. Precipitating agents compatible with the PDMS material of certain embodiments of the chip include, but are not limited to, non-volatile salts, high molecular weight polymers, polar solvents, aqueous solutions, high molecular weight alcohols, divalent metals. 
   Precipitating compounds, which include large and small molecular weight organics, as well as certain salts are used from under 1% to upwards of 40% concentration, or from &lt;0.5M to greater than 4M concentration. Water itself can act in a precipitating manner for samples that require a certain level of ionic strength to stay soluble. Many precipitants may also be mixed with one another to increase the chemical diversity of the crystallization screen. The microfluidics devices described in this document are readily compatible with a broad range of such compounds. Moreover, many precipitating agents (such as long- and short-chain organics) are quite viscous at high concentrations, presenting a problem for most fluid handling devices, such as pipettes or robotic systems. The pump and valve action of microfluidics devices in accordance with embodiments of the present invention enable handling of viscous agents. 
   An investigation of solvent/precipitating agent compatibility with particular elastomer materials may be conducted to identify optimum crystallizing agents, which may be employed develop crystallization screening reactions tailored for the chip that are more effective than standard screens. 
   A nonexclusive list of salts which may be used as precipitants is as follows: Tartrate (Li, Na, K, Na/K, NH4); Phosphate (Li, Na, K, Na/K, NH4); Acetate (Li, Na, K, Na/K, Mg, Ca, Zn, NH4); Formate (Li, Na, K, Na/K, Mg, NH4); Citrate (Li, Na, K, Na/K, NH4); Chloride (Li, Na, K, Na/K, Mg, Ca, Zn, Mn, Cs, Rb, NH4); Sulfate (Li, Na, K, Na/K, NH4); Malate (Li, Na, K, Na/K, NH4); Glutamate (Li, Na, K, Na/K, NH4. 
   A nonexclusive list of organic materials which may be used as precipitants is as follows: PEG 400; PEG 1000; PEG 1500; PEG 2k; PEG 3350; PEG 4k; PEG 6k; PEG 8k; PEG 10k; PEG 20k; PEG-MME 550; PEG-MME 750; PEG-MME 2k; PEG-MME 5k; PEG-DME 2k; Dioxane; Methanol; Ethanol; 2-Butanol; n-Butanol; t-Butanol; Jeffamine M-600; Isopropanol; 2-methyl-2,4-pentanediol; 1,6 hexanediol. 
   Solution pH can be varied by the inclusion of buffering agents; typical pH ranges for biological materials lie anywhere between values of 3.5–10.5 and the concentration of buffer, generally lies between 0.01 and 0.25 M. The microfluidics devices described in this document are readily compatible with a broad range of pH values, particularly those suited to biological targets. 
   A nonexclusive list of possible buffers is as follows: Na-Acetate; HEPES; Na-Cacodylate; Na-Citrate; Na-Succinate; Na-K-Phosphate; TRIS; TRIS-Maleate; Imidazole-Maleate; BisTris Propane; CAPSO, CHAPS, MES, and imidizole. 
   Additives are small molecules that affect the solubility and/or activity behavior of the target. Such compounds can speed crystallization screening or produce alternate crystal forms of the target. Additives can take nearly any conceivable form of chemical, but are typically mono and polyvalent salts (inorganic or organic), enzyme ligands (substrates, products, allosteric effectors), chemical crosslinking agents, detergents and/or lipids, heavy metals, organo-metallic compounds, trace amounts of precipitating agents, and small molecular weight organics. 
   The following is a nonexclusive list of possible additives: 2-Butanol; DMSO; Hexanediol; Ethanol; Methanol; Isopropanol; sodium fluoride; potassium fluoride; ammonium fluoride; lithium chloride anhydrous; magnesium chloride hexahydrate; sodium chloride; Calcium chloride dihydrate; potassium chloride; ammonium chloride; sodium iodide; potassium iodide; ammonium iodide; sodium thiocyanate; potassium thiocyanate; lithium nitrate; magnesium nitrate hexahydrate; sodium nitrate; potassium nitrate; ammonium nitrate; magnesium formate; sodium formate; potassium formate; ammonium formate; lithium acetate dihydrate; magnesium acetate tetrahydrate; zinc acetate dihydrate; sodium acetate trihydrate; calcium acetate hydrate; potassium acetate; ammonium acetate; lithium sulfate monohydrate; magnesium sulfate heptahydrate; sodium sulfate decahydrate; potassium sulfate; ammonium sulfate; di-sodium tartate dihydrate; potassium sodium tartrate tetrahydrate; di-ammonium tartrate; sodium dihydrogen phosphate monohydrate; di-sodium hydrogen phosphate dihydrate; potassium dihydrogen phosphate; di-potassium hydrogen phosphate; ammonium dihydrogen phosphate; di-ammonium hydrogen phosphate; tri-lithium citrate tetrahydrate; tri-sodium citrate dihydrate; tri-potassium citrate monohydrate; di-ammonium hydrogen citrate; barium chloride; cadmium chloride dihydrate; cobaltous chloride dihydrate; cupric chloride dihydrate; strontium chloride hexahydrate; yttrium chloride hexahydrate; ethylene glycol; Glycerol anhydrous; 1,6 hexanediol; MPD; polyethylene glycol 400; trimethylamine HCl; guanidine HCl; urea; 1,2,3-heptanetriol; benzamidine HCl; dioxane; ethanol; iso-propanol; methanol; sodium iodide; L-cysteine; EDTA sodium salt; NAD; ATP disodium salt; D(+)-glucose monohydrate; D(+)-sucrose; xylitol; spermidine; spermine tetra-HCl; 6-aminocaproic acid; 1,5-diaminopentane di-HCl; 1,6-diaminohexane; 1,8-diaminooctane; glycine; glycyl-glycyl-glycine; hexaminecobalt trichloride; taurine; betaine monohydrate; polyvinylpyrrolidone K15; non-detergent sulfo-betaine 195; non-detergent sulfo-betaine 201; phenol; DMSO; dextran sulfate sodium salt; jeffamine M-600; 2,5 Hexanediol; (+/−)-1,3 butanediol; polypropylene glycol P400; 1,4 butanediol; tert-butanol; 1,3 propanediol; acetonitrile; gamma butyrolactone; propanol; ethyl acetate; acetone; dichloromethane; n-butanol; 2,2,2 trifluoroethanol; DTT; TCEP; nonaethylene glycol monododecyl ether, nonaethylene glycol monolauryl ether; polyoxyethylene (9) ether; octaethylene glycol monododecyl ether, octaethylene glycol monolauryl ether; polyoxyethylene (8) lauryl ether; Dodecyl-β-D-maltopyranoside; Lauric acid sucrose ester; Cyclohexyl-pentyl-β-D-maltoside; Nonaethylene glycol octylphenol ether; Cetyltrimethylammonium bromide; N,N-bis(3-D-gluconamidopropyl)-deoxycholamine; Decyl-β-D-maltopyranoside; Lauryldimethylamine oxide; Cyclohexyl-pentyl-β-D-maltoside; n-Dodecylsulfobetaine, 3-(Dodecyldimethylammonio)propane-1-sulfonate; Nonyl-β-D-glucopyranoside; Octyl-β-D-thioglucopyranoside, OSG; N,N-Dimethyldecylamine-β-oxide; Methyl-6-O-(N-heptylcarbamoyl)-a-D-glucopyranoside; Sucrose monocaproylate; n-Octanoyl-β-D-fructofuranosyl-a-D-glucopyranoside; Heptyl-β-D-thioglucopyranoside; Octyl-β-D-glucopyranoside, OG; Cyclohexyl-propyl-β-D-maltoside; Cyclohexylbutanoyl-N-hydroxyethylglucamide; n-decylsulfobetaine, 3-(Decyldimethylammonio)propane-1-sulfonate; Octanoyl-N-methylglucamide, OMEGA; Hexyl-β-D-glucopyranoside; Brij 35; Brij 58; Triton X-114; Triton X-305; Triton X-405; Tween 20; Tween 80; polyoxyethylene(6)decyl ether; polyoxyethylene(9)decyl ether; polyoxyethylene(10)dodecyl ether; polyoxyethylene(8)tridecyl ether; Isopropyl-β-D-thiogalactoside; Decanoyl-N-hydroxyethylglucamide; Pentaethylene glycol monooctyl ether; 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate; 3-[(3-Cholamidopropyl)-dimethylammonio]-2-hydroxy-1-propane sulfonate; Cyclohexylpentanoyl-N-hydroxyethylglucamide; Nonanoyl-N-hydroxyethyglucamide; Cyclohexylpropanol-N-hydroxyethylglucamide; Octanoyl-N-hydroxyethylglucamide; Cyclohexylethanoyl-N-hydroxyethylglucamide; Benzyldimethyldodecyl ammonium bromide; n-Hexadecyl-β-D-maltopyranoside; n-Tetradecyl-β-D-maltopyranoside; n-Tridecyl-β-D-maltopyranoside; Dodecylpoly(ethyleneglycoether)n; n-Tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate; n-Undecyl-β-D-maltopyranoside; n-Decyl-β-D-thiomaltopyranoside; n-dodecylphosphocholine; a-D-glucopyranoside, β-D-fructofuranosyl monodecanoate, sucrose mono-caprate; 1-s-Nonyl-β-D-thioglucopyranoside; n-Nonyl-β-D-thiomaltoyranoside; N-Dodecyl-N,N-(dimethlammonio)butyrate; n-Nonyl-β-D-maltopyranoside; Cyclohexyl-butyl-β-D-maltoside; n-Octyl-β-D-thiomaltopyranoside; n-Decylphosphocholine; n-Nonylphosphocholine; Nonanoyl-N-methylglucamide; 1-s-Heptyl-β-D-thioglucopyranoside; n-Octylphosphocholine; Cyclohexyl-ethyl-β-D-maltoside; n-Octyl-N,N-dimethyl-3-ammonio-1-propanesulfonate; Cyclohexyl-methyl-β-D-malto side. 
   Cryosolvents are agents that stabilize a target crystal to flash-cooling in a cryogen such as liquid nitrogen, liquid propane, liquid ethane, or gaseous nitrogen or helium (all at approximately 100–120° K.) such that crystal becomes embedded in a vitreous glass rather than ice. Any number of salts or small molecular weight organic compounds can be used as a cryoprotectant, and typical ones include but are not limited to: MPD, PEG-400 (as well as both PEG derivatives and higher molecular-weight PEG compounds), glycerol, sugars (xylitol, sorbitol, erythritol, sucrose, glucose, etc.), ethylene glycol, alcohols (both short- and long chain, both volatile and nonvolatile), LiOAc, LiCl, LiCHO 2 , LiNO 3 , Li2SO 4 , Mg(OAc) 2 , NaCl, NaCHO 2 , NaNO 3 , etc. Again, materials from which microfluidics devices in accordance with the present invention are fabricated may be compatible with a range of such compounds. 
   Many of these chemicals can be obtained in predefined screening kits from a variety of vendors, including but not limited to Hampton Research of Laguna Niguel, Calif., Emerald Biostructures of Bainbridge Island, Wash., and Jena BioScience of Jena, Germany, that allow the researcher to perform both ‘sparse matrix’ and ‘grid’ screening experiments. Sparse matrix screens attempt to randomly sample as much of precipitant, buffer, and additive chemical space as possible with as few conditions as possible. Grid screens typically consist of systematic variations of two or three parameters against one another (e.g., precipitant concentration vs. pH). Both types of screens have been employed with success in crystallization trials, and the majority of chemicals and chemical combinations used in these screens are compatible with the chip design and matrices in accordance with embodiments of the present invention. 
   Moreover, current and future designs of microfluidic devices may enable flexibly combinatorial screening of an array of different chemicals against a particular target or set of targets, a process that is difficult with either robotic or hand screening. This latter aspect is particularly important for optimizing initial successes generated by first-pass screens. 
   7. Additional Screening Variables for Crystallization 
   In addition to chemical variability, a host of other parameters can be varied during crystallization screening. Such parameters include but are not limited to: 1) volume of crystallization trial, 2) ratio of target solution to crystallization solution, 3) target concentration, 4) co-crystallization of the target with a secondary small or macromolecule, 5) hydration, 6) incubation time, 7) temperature, 8) pressure, 9) contact surfaces, 10) modifications to target molecules, and 11) gravity. 
   Volumes of crystallization trials can be of any conceivable value, from the picoliter to milliliter range. Typical values may include but are not limited to: 0.1, 0.2, 0.25, 0.4, 0.5, 0.75, 1, 2, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 75, 80, 90, 100, 125, 150, 175, 200, 225, 250, 275, 300, 350, 400, 450, 500, 550, 600, 700, 750, 800, 900, 1000, 1100, 1200, 1250, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2000, 2250, 2500, 3000, 4000, 5000, 6000, 7000, 7500, 8000, 9000, and 10000 nL. The microfluidics devices previously described can access these values. 
   In particular, access to the low volume range for crystallization trials (&lt;100 nL) is a distinct advantage of embodiments of the microfluidics chips in accordance with embodiments of the present invention, as such small-volume crystallization chambers can be readily designed and fabricated, minimizing the need the need for large quantities of precious target molecules. The low consumption of target material of embodiments in accordance with the present invention is particularly useful in attempting to crystallize scarce biological samples such as membrane proteins, protein/protein and protein/nucleic acid complexes, and small-molecule drug screening of lead libraries for binding to targets of interest. 
   The ratios of a target solution to crystallization mix can also constitute an important variable in crystallization screening and optimization. These rations can be of any conceivable value, but are typically in the range of 1:100 to 100:1 target: crystallization-solution. Typical target: crystallization-solution or crystallization-solution: target ratios may include but are not limited to: 1:100, 1:90, 1:80, 1:70, 1:60, 1:50, 1:40, 1:30, 1:25, 1:20, 1:15, 1:10, 1:9, 1:8, 1:7.5, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2.5, 1:2, 1:1, 2:3, 3:4, 3:5, 4:5, 5:6, 5:7, 5:9, 6:7, 7:8, 8:9, and 9:10. As previously described, microfluidics devices in accordance with embodiments of the present invention can be designed to access multiple ratios simultaneously on a single chip. 
   Target concentration, like crystallization chemical concentration, can lie in a range of values and is an important variable in crystallization screening. Typical ranges of concentrations can be anywhere from &lt;0.5 mg/ml to &gt;100 mg/ml, with most commonly used values between 5–30 mg/ml. The microfluidics devices in accordance with embodiments of the present invention are readily compatible with this range of values. 
   Co-crystallization generally describes the crystallization of a target with a secondary factor that is a natural or non-natural binding partner. Such secondary factors can be small, on the order of about 10–1000 Da, or may be large macromolecules. Co-crystallization molecules can include but are not limited to small-molecule enzyme ligands (substrates, products, allosteric effectors, etc.), small-molecule drug leads, single-stranded or double-stranded DNAs or RNAs, complement proteins (such as a partner or target protein or subunit), monoclonal antibodies, and fusion-proteins (e.g., maltose binding proteins, glutathione S-transferase, protein-G, or other tags that can aid expression, solubility, and target behavior). As many of these compounds are either biological or of a reasonable molecular weight, co-crystallization molecules can be routinely included with screens in the microfluidics chips. Indeed, because many of these reagents are expensive and/or of limited quantity, the small-volumes afforded by the microfluidics chips in accordance with embodiment of the present invention make them ideally suited for co-crystallization screening. 
   Hydration of targets can be an important consideration. In particular, water is by far the dominant solvent for biological targets and samples. The microfluidics devices described in this document are relatively hydrophobic, and are compatible with water-based solutions. 
   The length of time for crystallization experiments can range from minutes or hours to weeks or months. Most experiments on biological systems typically show results within 24 hours to 2 weeks. This regime of incubation time can be accommodated by the microfluidics devices in accordance with embodiments of the present invention. 
   The temperature of a crystallization experiment can have a great impact on success or failure rates. This is particularly true for biological samples, where temperatures of crystallization experiments can range from 0–42° C. Some of the most common crystallization temperatures are: 0, 1, 2, 4, 5, 8, 10, 12, 15, 18, 20, 22, 25, 30, 35, 37, and 42. Microfluidics devices in accordance with embodiments of the present invention can be stored at the temperatures listed, or alternatively may be placed into thermal contact with small temperature control structures such as resistive heaters or Peltier cooling structures. 
   In addition, the small footprint and rapid setup time of embodiments in accordance with the present invention allow faster equilibration to desired target temperatures and storage in smaller incubators at a range of temperatures. Moreover, as the microfluidics systems in accordance with embodiments of the present invention do not place the crystallization experiment in contact with the vapor phase, condensation of water from the vapor phase into the drop as temperatures change, a problem associated with conventional macroscopic vapor-diffusion techniques, is avoided. This feature represents an advance over many conventional manual or robotic systems, where either the system must be maintained at the desired temperature, or the experiment must remain at room temperature for a period before being transferred to a new temperature. 
   Variation in pressure is an as yet understudied crystallization parameter, in part because conventional vapor-diffusion and microbatch protocols do not readily allow for screening at anything typically other than atmospheric pressure. The rigidity of the PDMS matrix enables experiments to probe the effects of pressure on target crystallization on-chip. 
   The surface on which the crystallization ‘drop’ sits can affect experimental success and crystal quality. Examples of solid support contact surfaces used in vapor diffusion and microbatch protocols include either polystyrene or silanized glass. Both types of supports can show different propensities to promote or inhibit crystal growth, depending on the target. In addition, the crystallization ‘drop’ is in contact with either air or some type of poly-carbon oil, depending on whether the experiment is a vapor-diffusion or microbatch setup, respectively. Air contact has the disadvantage in that free oxygen reacts readily with biological targets, which can lead to protein denaturation and inhibit or degrade crystallization success. Oil allows trace hydrocarbons to leach into the crystallization experiment, and can similarly inhibit or degrade crystallization success. 
   Microfluidics device designs in accordance with embodiments of the present invention may overcome these limitations by providing a nonreactive, biocompatible environment that completely surrounds the crystallization reaction. Moreover, the composition of the crystallization chambers in the microfluidics chips can conceivably be varied to provide new surfaces for contacting the crystallization reaction; this would allow for routine screening of different surfaces and surface properties to promote crystallization. 
   Crystallization targets, particularly those of biological origin, may often be modified to enable crystallization. Such modifications include but are not limited to truncations, limited proteolytic digests, site-directed mutants, inhibited or activated states, chemical modification or derivatization, etc. Target modifications can be time consuming and costly; modified targets require the same thorough screening as do unmodified targets. Microfluidics devices of the present invention work with such modified targets as readily as with the original target, and provide the same benefits. 
   The effect of gravity as a parameter for crystallization is yet another understudied crystallization parameter, because of the difficulty of varying such a physical property. Nonetheless, crystallization experiments of biological samples in zero gravity environments have resulted in the growth of crystals of superior quality than those obtained on Earth under the influence of gravity. 
   The absence of gravity presents problems for traditional vapor-diffusion and microbatch setups, because all fluids must be held in place by surface tension. The need to often set up such experiments by hand also poses difficulties because of the expense of maintaining personnel in space. Microfluidics devices in accordance with embodiments of the present invention, however, would enable further exploration of microgravity as a crystallization condition. A compact, automated metering and crystal growth system would allow for: 1) launching of satellite factory containing target molecules in a cooled, but liquid state, 2) distribution of targets and growth of crystals, 3) harvesting and cryofreezing of resultant crystals, and 4) return of cryo-stored crystals to land-based stations for analysis. 
   8. In Situ Crystallization Screening 
   The ability to observe the growth of crystals with a microscope is a step in deciding upon success or failure of crystallization trials. Conventional crystallization protocols may use transparent materials such as polystyrene or silanized glass to allow for visualization. The transparency of the PDMS matrix of embodiments in accordance with the present invention is particularly suited to the two primary methods by which crystallization trials are traditionally scored: 1) direct observation in the visible light regime by optical microscopes and 2) birefringence of polarized light. 
   Birefringence may be difficult to judge in conventional experiments as many plastics are themselves birefringent, interfering with sample assessment. However, the microfluidics devices described herein can be made without such optical interference properties, allowing for the design of an automated scanning system that routinely allows direct visualization with both polarizing and non-polarizing features. 
   In addition, robotic and, in particular, manually-set crystallization experiments can vary the placement of a crystallization drop on a surface by tens to hundreds of microns. This variability presents a problem for automated scanning systems, as it is difficult to program in the need for such flexible positioning without stable fiducials. However, the fixed placement of crystallization chambers in the microfluidics chips of embodiments of the present invention overcomes such problems, as every well can be positioned in a particular location with submicron accuracy. Moreover, such a system is readily scalable for the design of differently sized and positioned crystallization chambers, as masks and other templates used to design microfluidics devices in accordance with embodiments of the present invention can be simply digitized and ported into scanning software for visualization. 
   Once crystals are obtained by visual inspection, it may be possible to screen for diffraction directly through the chip itself. For example, a crystallization chamber within a chip could be outfitted with transparent ‘windows’ comprising glass, quartz, or thinned portions of the elastomer material itself, on opposite walls of the chamber. Crystals could then be exposed directly to x-rays through the chip to assay for diffraction capabilities, eliminating the need to remove, and thereby possibly damage, the crystalline sample. Such an approach could be used to screen successes from initial crystallization trials to determine the best starting candidate conditions for follow-up study. Similarly, crystals grown under a particular set of conditions could be ‘re-equilibrated’ with new solutions (e.g., cryo-stabilizing agents, small-molecule drug leads or ligands, etc.), and the stability of the crystals to such environment changes monitored directly by x-ray diffraction. 
   9. Utilizing Microfluidic Devices for Purification/Crystallization 
   Crystallization of target biological samples such as proteins is actually the culmination of a large number of prior complex and difficult steps, including but not limited to protein expression, purification, derivatization, and labeling. Such steps prior to crystallization comprise shuttling liquids from a chamber with one set of solution properties to another area with a different set of properties. Mircofluidics technology is suited to perform such tasks, allowing for the combination of all necessary steps within the confines of a single chip. 
   Examples of microfluidic handling structures enabling performance of pre-crystallization steps have been described above. For example, a microfluidics chip could act as a regulated bioreactor, allowing nutrients to flow into growing cells contained in cell pen structure while removing wastes and inducing recombinantly-modified organisms to produce target molecules (e.g., proteins) at a desired stage of cell growth. Following induction, these cells could be shunted from the cell pen to a different region of the chip for lysis by enzymatic or mechanical means. Solubilized target molecules could then be separated from cellular debris by molecular filtration units incorporated directly on-chip. 
   The crude mixture of target molecules and contaminating cellular proteins and nucleic acids could then be funneled through porous matrices of differing chemical properties (e.g., cation-exchange, anion-exchange, affinity, size-exclusion) to achieve separation. If a target molecule were tagged with a fusion protein of a particular type to promote solubility, it could be affinity purified, briefly treated with a similarly-tagged, site-specific protease to separate the fusion product, and then repassaged though the affinity matrix as a clean-up step. 
   Once pure, the target could be mixed with different stabilizing agents, assayed for activity, and then transported to crystallization staging areas. Localized heating (such as an electrode) and refrigeration (such as a Peltier cooler) units stationed at various points on a chip or a chip holder would allow for differential temperature regulation at all stages throughout the processing and crystallization. Thus, the production, purification, and crystallization of proteins may be accomplished on an embodiment of a single microfluidics device in accordance with the present invention. 
   Furthermore, since many thousands of unique solutions may be mixed directly on chip, the present invention may be used to do exhaustive screening of protein crystallization conditions. This screening may be done in a random or systematic way. Once mixed, crystallization reactions may be routed to a locations device for storage and inspection. 
   III. Micro-Free Interface Diffusion 
   A conventional approach to crystallization has been to effect a gradual change in target solution conditions by introducing a crystallizing agent through slow diffusion. One method that is particularly effective at sampling a wide range of conditions is macroscopic free-interface diffusion. This technique requires the creation of a well-defined fluidic interface between two or more solutions, typically the protein stock, and the precipitating agent, and the subsequent equilibration of the two solutions via a diffusive process. As the solutions diffuse into one another, a gradient is established along the diffusion path, and a continuum of conditions is simultaneously sampled. Since there is a variation in the conditions, both in space, and in time, information regarding the location and time of crystal formation may be used in further optimization.  FIGS. 29A–29D  are simplified schematic diagrams plotting concentration versus distance for a solution A and a solution B in contact along a free interface.  FIGS. 29A–D  show that over time, a continuous and broad range of concentration profiles of the two solutions is ultimately created. 
   Despite the efficiency of macroscopic free-interface diffusion techniques, technical difficulties have rendered it unsuitable for high throughput screening applications, and it is not widely used in the crystallographic community for several reasons. First, the fluidic interfaces are typically established by dispensing the solutions into a narrow container; such as a capillary tube or a deep well in a culture plate.  FIGS. 30A–B  show simplified cross-sectional views of the attempted formation of a macroscopic free-interface in a capillary tube  9300 . The act of dispensing a second solution  9302  into a first solution  9304  creates convective mixing and results in a poorly defined fluidic interface  9306 . 
   Moreover, the solutions may not be sucked into a capillary serially to eliminate this problem.  FIGS. 31A–B  show the mixing, between a first solution  9400  and a second solution  9402  in a capillary tube  9404  that would result due to the parabolic velocity distribution of pressure driven Poiseuille flow, resulting in a poorly defined fluidic interface  9406 . Furthermore, the container for a macro free-interface crystallization regime must have dimensions making them accessible to a pipette tip or dispensing tool, and necessitating the use of large (10–100 μl) volumes of protein and precipitant solutions. 
   In order to avoid unwanted convective mixing, care must be taken both during dispensing and during crystal incubation. For this reason cumbersome protocols are often used to define a macro free-interface. For example, freezing one solution prior to the addition of the second. Moreover, two solutions of differing density will mix by gravity induced convection if they are not stored at the proper orientation, additionally complicating the storage of reactions. This is shown in  FIGS. 32A–C , wherein over time first solution  9500  having a density greater than the density of second solution  9502  merely sinks to form a static bottom layer  9504  that is not conducive to formation of a diffusion gradient along the length of a capillary tube. 
   In accordance with embodiments of the present invention, a crystallization technique analogous to traditional macro-free interface diffusion, called gated micro free interface diffusion (Gated μ-FID), has been developed. Gated μ-FID retains the efficient sampling of phase space achieved by macroscopic free interface diffusion techniques. 
   A microfluidic free interface (μFI) in accordance with embodiments of the present invention is a localized interface between at least one static fluid and another fluid wherein mixing between them is dominated by diffusion rather than by convective flow. For the purposes of this application, the term “fluid” refers to a material having a viscosity below a particular maximum. Examples of such maximum viscosities include but are not limited to 1000 CPoise, 900 CPoise, 800 CPoise, 700 CPoise, 600 CPoise, 500 CPoise, 400 CPoise, 300 CPoise, 250 CPoise, and 100 CPoise, and therefore exclude gels or polymers containing materials trapped therein. 
   In a microfluidic free interface in accordance with an embodiment of the present invention, at least one dimension of the interface is restricted in magnitude such that viscous forces dominate other forces. For example, in a microfluidic free interface in accordance with an embodiment of the present invention, the dominant forces acting upon the fluids are viscous rather than buoyant, and hence the microfluidic free interface may be characterized by an extremely low Grashof number (see discussion below). The microfluidic free interface may also be characterized by its localized nature relative to the total volumes of the fluids, such that the volumes of fluid exposed to the steep transient concentration gradients present initially after formation of the interface between the pure fluids is limited. 
   The properties of a microfluidic free interface created in accordance with embodiments of the present invention may be contrasted with a non-free microfluidic interface, as illustrated in  FIGS. 33A and 33B . Specifically,  FIG. 33A  shows a simplified cross-sectional view of a microfluidic free interface in accordance with an embodiment of the present invention. Microfluidic free interface  7500  of  FIG. 33A  is formed between first fluid A and second fluid B present within channel  7502 . The free microfluidic interface  7500  is substantially linear, with the result that the steep concentration gradient arising between fluids A and B is highly localized within the channel. 
   As described above, the dimensions of channel  7502  are extremely small, with the result that non-slip layers immediately adjacent to the walls of the channel in fact occupy most of the volume of the channel. As a result, viscosity forces are much greater than buoyant forces, and mixing between fluids A and B along interface  7500  occurs almost entirely as a result of diffusion, with little or no convective mixing. 
   Conditions associated with the microfluidic free interface of embodiments of the present invention can be expressed in terms of the Grashof number (Gr) per Equation (1) below, an expression of the relative magnitude of buoyant and viscous forces: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         Gr 
                         = 
                         
                           
                             B 
                             / 
                             V 
                           
                           = 
                           
                             
                               α 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               cg 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 L 
                                 3 
                               
                             
                             
                               v 
                               2 
                             
                           
                         
                       
                       , 
                     
                   
                 
                 
                   
                     
                       where 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     
                       
                         Gr 
                         = 
                         
                           Grashof 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           number 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         B 
                         = 
                         
                           buoyancy 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           force 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         = 
                         
                           viscous 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           force 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         α 
                         = 
                         
                           solutal 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           expansivity 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         c 
                         = 
                         concentration 
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         g 
                         = 
                         
                           acceleration 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           gravity 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         L 
                         = 
                         
                           chamber 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           critical 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           dimension 
                         
                       
                       ; 
                       and 
                     
                   
                 
                 
                   
                     
                       v 
                       = 
                       
                         kinematic 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           viscosity 
                           . 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   According to Equation (1), a number of approaches may be taken to reduce the Grashof number and hence the presence of unwanted corrective flow. One such approach is to reduce g, and this is the tactic adopted by microgravity crystallization experiments conducted in space. Another approach is to increase v, and this is the tactic adopted by investigators working with gel acupuncture techniques, as described generally by Garcia-Ruiz et al., “Agarose as Crystallization Media for Proteins I: Transport Processes”,  J. Crystal Growth  232, 165–172 (2001), hereby incorporated by reference for all purposes. 
   Embodiments in accordance with the present invention seek to reduce L and through the use of microfluid flow channels and vessels having extremely small dimensions. The effect of this approach is amplified by the cubed power of the variable (L) in Equation (1). 
   Microfluidic free interfaces in accordance with embodiments of the present invention would be expected to exhibit a Grashof number of 1 or less. The Grashof number expected with two fluids having the same density is zero, and thus Grashof, numbers very close to zero would be expected to be attained. 
   The embodiment of a microfluidic free interface illustrated above in  FIG. 33A  may be contrasted with the conventional non-microfluidic free interface shown in  FIG. 33B . Specifically, first and second fluids A and B are separated by an interface  7504  that is not uniform or limited by the cross-sectional width of channel  7502 . The steep concentration gradient occurring at the interface is not localized, but is instead present at various points along the length of the channel, exposing correspondingly large volumes of the fluids to the steep gradients. In addition, viscosity forces do not necessarily dominate over buoyancy forces, with the result that mixing of fluids A and B across interface  7504  can occur both as the result of diffusion and of convective flow. The Grashof number exhibited by a conventional non-microfluidic interface would be expected to exceed one. 
   1. Creation of Microfluidic Free Interface 
   A microfluidic free interface in accordance with embodiments of the present invention may be created in a variety of ways. One approach is to utilize the microfabricated elastomer structures previously described. Specifically, in certain embodiments the elastomeric material from which microfluidic structures are formed is relatively permeable to certain gases. This gas permeability property may be exploited utilizing the technique of pressurized out-gas priming (POP) to form well-defined, reproducible fluidic interfaces. 
     FIG. 34A  shows a plan view of a flow channel  9600  of a microfluidic device in accordance with an embodiment of the present invention. Flow channel  9600  is separated into two halves by actuated valve  9602 . Prior to the introduction of material, flow channel  9600  contains a gas  9604 . 
     FIG. 34B  shows the introduction of a first solution  9606  to first flow channel portion  9600   a  under pressure, and the introduction of a second solution  9608  to second flow channel portion  9600   b  under pressure. Because of the gas permeability of the surrounding elastomer material  9607 , gas  9604  is displaced by the incoming solutions  9608  and  9610  and out-gasses through elastomer  9607 . 
   As shown in  FIG. 34C , the pressurized out-gas priming of flow channel portions  9600   a  and  9600   b  allows uniform filling of these dead-ended flow channel portions without air bubbles. Upon deactuation of valve  9602  as shown in  FIG. 34D , microfluidic free interface  9612  is defined, allowing for formation of a diffusion gradient between the fluids. 
   The formation of protein crystals utilizing gated μ-FID retains the efficient sampling of phase space achieved by macroscopic free interface diffusion techniques, with a number of added advantages, including the parsimonious use of sample solutions, ease of set-up, creation of well defined fluidic interfaces, control over equilibration dynamics, and the ability to conduct high-throughput parallel experimentation. 
   Another possible advantage of the formation of protein crystals utilizing gated μ-FID is the formation of high quality crystals, as illustrated in connection with  FIGS. 36A  and  36 B.  FIG. 36A  shows a simplified schematic view of a protein crystal being formed utilizing a conventional macroscopic free interface diffusion technique. Specifically, nascent protein crystal  9200  is exposed to sample from solution  9202  that is experiencing a net conductive flow of sample as a result of the action of buoyant forces. As a result of the directionality of this conductive flow, the growth of protein crystal  9200  is also directional. However, as described in Nerad et al., “Ground-Based Experiments on the Minimization of Convection During the Growth of Crystals from Solution”, Journal of Crystal Growth 75, 591–608 (1986), assymetrical growth of a protein crystal can give rise to unwanted strain in the lattice of the growing crystal, promoting dislocations and/or the incorporation of impurities within the lattice, and otherwise adversely affecting the crystal quality. 
   By contrast,  FIG. 36B  shows a simplified schematic view of a protein crystal being formed utilizing diffusion across a microfluidic free interface in accordance with an embodiment of the present invention. Nascent protein crystal  9204  is exposed to sample solution  9206  that is diffusing within the crystallizing agent. This diffusion is nondirectional, and the growth of protein crystal  9204  is also correspondingly nondirectional. Accordingly, the growing crystal avoids strain on the lattice and the attendant incorporation of impurities and dislocations experienced by the growing crystal shown in  FIG. 36A . Accordingly, the quality of the crystal in  FIG. 36B  is of high quality. 
   While the specific embodiment just described exploits the permeability of the bulk material to dead end fill two or more chambers or channels separated by a closed valve, and creates a microfluidic free interface between the static fluids by the subsequent opening of this valve, other mechanisms for realizing a microfluidic free interface are possible. 
   For example,  FIG. 46  shows one potential alternative method for establishing a microfluidic free interface diffusion in accordance with the present invention. Microfluidic channel  8100  carries fluid A experiencing a convective flow in the direction indicated by the arrow, such that static non-slip layers  8102  are created along the walls of flow channel  8100 . Branch channel  8104  and dead-ended chamber  8106  contain static fluid B. Because material surrounding the dead-ended channel and chamber provide a back pressure, fluid B remains static and microfluidic free interface  8110  is created at mouth  8112  of branch channel  8108  between flowing fluid A and static fluid B. As described below, diffusion of fluid A or components thereof across the microfluidic free interface can be exploited to obtain useful results. While the embodiment shown in  FIG. 46  includes a dead-ended branch channel and chamber, this is not required by the present invention, and the branch channel could connect with another portion of the device, as long as a sufficient counter pressure was maintained to prevent any net flow of fluid through the channel. 
   Another potential alternative method for establishing a microfluidic free interface diffusion assay is the use of break-through valves and chambers. A break-through valve is not a true closing valve, but rather a structure that uses the surface tension of the working fluid to stop the advance of the fluid. Since these valves depend on the surface tension of the fluid they can only work while a free surface exists at the valve; not when the fluid continuously fills both sides and the interior of the valve structure. 
   A non-exclusive list of ways to achieve such a valve include but are not limited to patches of hydrophobic material, hydrophobic treatment of certain areas, geometric constrictions (both in height and width) of a channel, geometric expansions (both in height and in width of a channel), changes in surface roughness on walls of a channel, and applied electric potentials on the walls. 
   These “break-through” valves may be designed to withstand a fixed and well defined pressure before they “break through” and allow fluid to pass nearly unimpeded. The pressure in the channel can be controlled and hence the fluid can be caused to advance when desired. Different methods of controlling this pressure include but are not limited to externally applied pressure at an input or output port, pressure derived from centrifugal force (i.e. by spinning the device), pressure derived from linear acceleration (i.e. applying an acceleration to the device with a component parallel to the channel), elecrokinetic pressure, internally generated pressure from bubble formation (by chemical reaction or by hydrolysis), pressure derived from mechanical pumping, or osmotic pressure. 
   “Break-through” valves may be used to create a microfluidic free interface as shown and described in connection with  FIGS. 35A–E .  FIG. 35A  shows a simplified plan view of a device for creating a microfluidic free interface utilizing break through valves. First chamber  9100  is in fluid communication with second chamber  9102  through branches  9104   a  and  9104   b  respectively, of T-shaped channel  9104 . 
   First break through valve  9106  is located at outlet  9108  of first chamber  9100 . Second break through valve  9110  is located in branch  9104   b  upstream of inlet  9105  of second chamber  9102 . Third break through valve  9112  is located at outlet  9114  of second chamber  9102 . Breakthrough valves  9106 ,  9110 , and  9112  may be formed from hydrophobic patches, a constriction in the width of the flow channel, or some other way as described generally above. In  FIGS. 35A–E , an open break through valve is depicted as an unshaded circle, and a closed break through valve is depicted as a shaded circle. 
   In the initial stage shown in  FIG. 35B , first chamber  9100  is charged with first fluid  9116  introduced through stem  9104   c  and branch  9104   a  of T-shaped channel  9104  and chamber inlet  9107  at a pressure below the break through pressure of any of the valves  9106 ,  9110 , and  9112 . In the second stage shown in  FIG. 35C , second chamber  9102  is charged with a buffer or other intermediate fluid  9118  introduced through stem  9104   c  of T-shaped channel  9104  and inlet  9105  at a pressure below the break through pressure of valve  9106  but greater than the break through pressures of valves  9110  and  9112 . 
   In the third stage shown in  FIG. 35D , intermediate fluid  9118  is replaced in second chamber  9102  with second fluid  9120  introduced through stem  9104   c  of T-shaped channel  9104  and inlet  9105  at a pressure below the break through pressure of valve  9106  but greater than the break through pressures of valves  9110  and  9112 . In the final stage depicted in  FIG. 35E , second fluid  9120  has replaced the intermediate fluid, leaving the first and second fluids  9116  and  9120  in separate chambers but fluidically connected through T-junction  9104 , creating a microfluidic free interface  9122 . 
   The use of break through valves to create a microfluidic free interface in accordance with embodiments of the present invention is not limited to the specific example given above. For example, in alternative embodiments the step of flushing with a buffer or intermediate solution is not required, and the first solution could be removed by flushing directly with the second solution, with potential unwanted by-products of mixing removed by the initial flow of the second solution through the channels and chambers. 
   While the embodiments just described create the microfluidic free interface in a closed microfluidic device, this is not required by embodiments in accordance with the present invention. For example, an alternative embodiment in accordance with the present invention may utilize capillary forces to connect two reservoirs of fluid. In one approach, the open wells of a micro-titer plate could be connected by a segment of a glass capillary. The first solution would be dispensed into one well such that it fills the well and is in contact with the glass capillary. Capillary forces cause the first solution to enter and flow to the end of the capillary. Once at the end, the fluid motion ceases. Next, the second solution is added to the second well. This solution is in contact with the first solution at the capillary inlet and creates a microfluidic interface between the two wells at the end of the capillary. 
   The connecting path between the two wells need not be a glass capillary, and in alternative embodiments could instead comprise a strip of hydrophilic material, for example a strip of glass or a line of silica deposited by conventional CVD or PVD techniques. Alternatively, the connecting paths could be established by paths of less hydrophobic material between patterned regions of highly hydrophobic material. Moreover, there could be a plurality of such connections between the wells, or a plurality of interconnected chambers in various configurations. Such interconnections could be established by the user prior to use of the device, allowing for rapid and efficient variation in fluidic conditions. 
   Where as in the previous example the two reservoirs are not enclosed by a microfluidic device but are connected instead through a microfluidic path, an alternative embodiment could have reservoirs both enclosed and not enclosed. For example, sample could be loaded into a microfluidic device and pushed to the end of an exit capillary or orifice (by any of the pressure methods described above). Once at the end of the exit capillary, the capillary could be immersed in a reservoir of reagent. In this way, the microfluidic free interface is created between the external reservoir and the reservoir of reagent in the chip. This method could be used in parallel with many different output capillaries or orifices to screen a single sample against a plurality of different reagents using microfluidic free interface diffusion. 
   In the example just described, the reagent is delivered from one or many inlets to one or many different outlets “through” a microfluidic device. Alternatively, this reagent can be introduced through the same orifice that is to be used to create the microfluidic interface. The sample-containing solution could be aspirated into a capillary (either by applying suction, or by capillary forces, or by applying pressure to the solution) and then the capillary may be immersed in a reservoir of counter-reagent, creating a microfluidic interface between the end of the capillary and the reservoir. This could be done in a large array of capillaries for the parallel screening of many different reagents. Very small volumes of sample could be used since the capillaries can have a fixed length beyond which the sample will not advance. For crystallization applications (see below), the capillaries could be removed and mounted in an x-ray beam for diffraction studies, without requiring handling of the crystals. 
   2. Reproducible Control Over Equilibration Parameters 
   One advantage of the use of microfluidic free interface diffusion in accordance with embodiments of the present invention is the ability to create uniform and continuous concentration gradients that reproducibly sample a wide range of conditions. As the fluids on either side of the interface diffuse into one another, a gradient is established along the diffusion path, and a continuum of conditions is simultaneously sampled. Since there is a variation in the conditions, both in space and time, information regarding the location and time of positive results (i.e. crystal formation) may be used in further optimization. 
   In many applications it is desirable to create a gradient of a condition such as pH, concentration, or temperature. Such gradients may be used for screening applications, optimization of reaction conditions, kinetics studies, determination of binding affinities, dissociation constants, enzyme-rate profiling, separation of macromolecules, and many other applications. Due principally to the suppression of convective flow, diffusion across a microfluidic free interface in accordance with an embodiment of the present invention may be used to establish reliable and well-defined gradient. 
   The dimensional Einstein equation (2) may be employed to obtain a rough estimate of diffusion times across a microfluidic free interface. 
                         t   =       x   2       4   ⁢   D         ;               where   ⁢     :                   t   =     diffusion   ⁢           ⁢   time       ;                 x   =     longest   ⁢           ⁢   diffusion   ⁢           ⁢   length       ;   and               D   =     diffusion   ⁢           ⁢   coefficient                   (   2   )               
Generally, as shown in Equation (3) below, the diffusion coefficient varies inversely with the radius of gyration, and therefore as one over the cube root of molecular weight.
 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         D 
                         ∝ 
                         
                           1 
                           r 
                         
                         ∝ 
                         
                           1 
                           
                             m 
                             
                               1 
                               / 
                               3 
                             
                           
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       where 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     
                       
                         D 
                         = 
                         
                           diffusion 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           coefficient 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         r 
                         = 
                         
                           radius 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           gyration 
                         
                       
                       ; 
                       and 
                     
                   
                 
                 
                   
                     
                       m 
                       = 
                       
                         molecular 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         weight 
                       
                     
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   In reviewing equation (3), it is important to recognize that correlation between the radius of gyration (r) and the molecular weight (m) is only an approximation. Because of the dominance of viscous forces over inertial forces, the diffusion coefficient is in fact independent of molecular weight and is instead dependent upon the size and hence drag experienced by the diffusing particle. 
   As compared with the rough 1.5 hr equilibration time for a dye, an approximate equilibration time for a protein of 20 KDa over the same distance is estimated to be approximately 45 hours. The equilibration time for a small salt of a molecular weight of 100 Da over the same distance is about 45 minutes. 
   The relative concentrations resulting from diffusion across a fluidic interface is determined not only by thermodynamic conditions explored during the equilibration, but also by the rate at which equilibration takes place. It is therefore potentially valuable to control the dynamics of equilibration. 
   In conventional macroscopic diffusion methods, only coarse control over the dynamics of equilibration may be available through manipulation of initial conditions. For macroscopic free interface diffusion, once diffusion begins, the experimenter has no control over the subsequent equilibration rate. For hanging drop experiments, the equilibration rate may be changed by modifying the size of the initial drop, the total size of the reservoir, or the temperature of incubation. In microbatch experiments, the rate at which the sample is concentrated may be varied by manipulating the size of the drop, and the identity and amount of the surrounding oil. Since the equilibration rates depend in a complicated manner on these parameters, the dynamics of equilibration may only be changed in a coarse manner. Moreover, once the experiment has begun, no further control over the equilibration dynamics is available. 
   By contrast, in a fluidic free interface experiment in accordance with an embodiment of the present invention, the parameters of diffusive equilibration rate may also be controlled by manipulating dimensions of chambers and connecting channels of a microfluidic structure. For example, in a microfluidic structure comprising reservoirs in fluid communication through a constricted channel, where no appreciable gradient exists in the reservoirs due to high concentrations or replenishment of material, to good approximation the time required for equilibration varies linearly with the required diffusion length. The equilibration rate also depends on the cross-sectional area of the connecting channels. The required time for equilibration may therefore be controlled by changing both the length, and the cross-sectional area of the connecting channels. 
   For example,  FIG. 40A  shows a plan view of a simple embodiment of a microfluidic structure in accordance with the present invention. Microfluidic structure  9701  comprises reservoirs  9700  and  9702  containing first fluid A and second fluid B, respectively. Reservoirs  9700  and  9702  are connected by channel  9704 . Valve  9706  is positioned on the connecting channel between reservoirs  9700  and  9702 . 
   Connecting channel  9704  has a much smaller cross-sectional area than either of the reservoirs. For example, in particular embodiments of microfluidic structures in accordance with the present invention, the ratio of reservoir/channel cross-sectional area and thus the ratio of maximum ratio of cross-sectional area separating the two fluids, may fall between 500 and 25,000. The minimum of this range describes a 50×50×50 μm chamber connected to a 50×10 μm channel, and the maximum of this range describes a 500×500×500 μm chamber connected to a 10×1 μm channel. 
   Initially, reservoirs  9700  and  9702  are filled with respective fluids, and valve  9706  is closed. Upon opening valve  9706 , a microfluidic free interface in accordance with an embodiment of the present invention is created, and fluids A and B diffuse across this interface through the channel into the respective reservoirs. Moreover, where the amount of diffusing material present in one reservoir is large and the capacity of the other reservoir to receive material without undergoing a significant concentration change is also large, the concentrations of material in the reservoirs will not change appreciably over time, and a steady state of diffusion will be established. 
   Diffusion of fluids in the simple microfluidic structure shown in  FIG. 40  may be described by relatively simple equations. For example, the net flux of a chemical species from one chamber to the other may be simply described by equation (4): 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         J 
                         = 
                         
                           D 
                           * 
                           A 
                           * 
                           
                             
                               Δ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               C 
                             
                             L 
                           
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       where 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     
                       J 
                       = 
                       
                         net 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         flux 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         chemical 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         species 
                       
                     
                   
                 
                 
                   
                     
                       
                         D 
                         = 
                         
                           diffusion 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           constant 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chemical 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           species 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         A 
                         = 
                         
                           cross 
                           ⁢ 
                           
                             - 
                           
                           ⁢ 
                           sectional 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           area 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           connecting 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           channel 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         
                           Δ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           C 
                         
                         = 
                         
                           concentration 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           difference 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           between 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           two 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           channels 
                         
                       
                       ; 
                       and 
                     
                   
                 
                 
                   
                     
                       L 
                       = 
                       
                         length 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         the 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         connection 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           channel 
                           . 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Following integration and extensive manipulation of the terms of equation (4), the characteristic time τ for the equilibration of the two chambers, where one volume V 1  is originally at concentration C and the other volume V 2  is originally at concentration 0, can therefore be taken to be as shown in Equation (5) below: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         τ 
                         = 
                         
                           
                             1 
                             
                               
                                 
                                   V 
                                   1 
                                 
                                 / 
                                 
                                   V 
                                   2 
                                 
                               
                               + 
                               1 
                             
                           
                           * 
                           
                             1 
                             D 
                           
                           * 
                           
                             L 
                             
                               A 
                               / 
                               
                                 V 
                                 1 
                               
                             
                           
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       where 
                       ⁢ 
                       
                         : 
                       
                     
                   
                 
                 
                   
                     
                       
                         τ 
                         = 
                         
                           equilibration 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           time 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         V 
                         1 
                       
                       = 
                       
                         volume 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         chamber 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         initially 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         containing 
                       
                     
                   
                 
                 
                   
                     
                       
                           
                       
                       ⁢ 
                       
                         
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chemical 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           species 
                         
                         ; 
                       
                     
                   
                 
                 
                   
                     
                       
                         V 
                         2 
                       
                       = 
                       
                         volume 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         chamber 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         into 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         which 
                       
                     
                   
                 
                 
                   
                     
                       
                           
                       
                       ⁢ 
                       
                         
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chemical 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           species 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           is 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           diffusing 
                         
                         ; 
                       
                     
                   
                 
                 
                   
                     
                       
                         D 
                         = 
                         
                           diffusion 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           constant 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           chemical 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           species 
                         
                       
                       ; 
                     
                   
                 
                 
                   
                     
                       
                         A 
                         = 
                         
                           cross 
                           ⁢ 
                           
                             - 
                           
                           ⁢ 
                           sectional 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           area 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           of 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           the 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           connecting 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           channel 
                         
                       
                       ; 
                       and 
                     
                   
                 
                 
                   
                     
                       L 
                       = 
                       
                         length 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         of 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         the 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         connection 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           channel 
                           . 
                         
                       
                     
                   
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   Therefore, for a given initial concentration of a chemical species in a chamber of a defined volume, the characteristic equilibration time depends in a linear manner from the diffusive length L and the ratio of the cross-sectional area to the volume (hereafter referred to simply as the “area”), with the understanding that the term “area” refers to the area normalized by the volume of the relevant chamber. Where two chambers are connected by a constricted channel, as in the structure of  FIG. 40A , the concentration drop from one channel to the other occurs primarily along the connecting channel and there is no appreciable gradient present in the chamber. This is shown in  FIG. 40B , which is a simplified plot of concentration versus distance for the structure of  FIG. 40A . 
   The behavior of diffusion between the chambers of the microfluidic structure of  FIG. 40A  can be modeled, for example, utilizing the PDE toolbox of the MATLAB® software program sold by The MathWorks Inc. of Natick, Mass.  FIGS. 41 and 42  accordingly show the results of simulating diffusion of sodium chloride from a 300 um×300 um×100 um chamber to another chamber of equal dimensions, through a 300 um long channel with a cross-sectional area of 1000 um. The initial concentrations of the chambers are 1 M and 0 M, respectively. 
     FIG. 41  plots the time required for the concentration in one of the reservoirs to reach 0.6 of the final equilibration concentration, versus channel length.  FIG. 41  shows the linear relationship between diffusion time and channel length for this simple microfluidic system. 
     FIG. 42  plots the inverse of the time required for the concentration in one of the reservoirs to reach 0.6 of the final equilibration concentration (T 0.6 ), versus the area of the fluidic interface created upon opening of the valve.  FIG. 42  shows the linear relationship between these parameters. The simple relationship between the equilibration time constant and the parameters of channel length and 1/channel area allows for a reliable and intuitive method for controlling the rate of diffusive mixing across a microfluidic free interface in accordance with an embodiment of the present invention. 
   This relationship further allows for one reagent to be diffusively mixed with a plurality of others at different rates that may be controlled by the connecting channel geometry. For example,  FIG. 38A  shows three sets of pairs of compound chambers  9800 ,  9802 , and  9804 , each pair connected by microchannels  9806  of a different length Ax.  FIG. 38B  plots equilibration time versus equilibration distance.  FIG. 38B  shows that the required time for equilibration of the chambers of  FIG. 38A  varies as the length of the connecting channels. 
     FIG. 39  shows four compound chambers  9900 ,  9902 ,  9904 , and  9906 , each having different arrangements of connecting microchannel(s)  9908 . Microchannels  9908  have the same length, but differ in cross-sectional area and/or number of connecting channels. The rate of equilibration may thus be increased/decreased by decreasing/increasing the cross-sectional area, for example by decreasing/increasing the number of connecting channels or the dimensions of those channels. 
   Another desirable aspect of microfluidic free interface diffusion studies in accordance with embodiments of the present invention is the ability to reproducibly explore a wide range of phase space. For example, it may be difficult to determine, a priori, which thermodynamic conditions will be favorable for a particular application (i.e. nucleation/growth of protein crystals), and therefore it is desirable that a screening method sample as much of phase-space (as many conditions) as possible. This can be accomplished by conducting a plurality of assays, and also through the phase space sampled during the evolution of each assay in time. 
     FIG. 43  shows the results of simulating the counter-diffusion of lysozyme and sodium chloride utilizing the microfluidic structure shown in  FIG. 40 , with different relative volumes of the two reservoirs, and with initial concentrations normalized to 1.  FIG. 43  presents a phase diagram depicting the phase space between fluids A and B, and the path in phase space traversed in the reservoirs as the fluids diffuse across the microfluidic free interface created by the opening of the valve in  FIG. 40 .  FIG. 43  shows that the phase space sampled depends upon the initial relative volumes of the fluids contained in the two reservoirs. By utilizing arrays of chamber with different sample volumes, and then identifying instances where diffusion across the channel yielded desirable results (i.e. crystal formation), promising starting points for additional experimentation can be determined. 
   As described above, varying the length or cross-sectional area of a channel connecting two reservoirs changes the rate at which the species are mixed. However, so long as the channel volume remains small compared as compared with the total reaction volume, there is little or no effect on the evolution of concentration in the chambers through phase space. The kinetics of the mixing are therefore decoupled from the phase-space evolution of the reaction, allowing the exercise independent control over the kinetic and thermodynamic behavior of the diffusion. 
   For example, it is often desirable in crystallography to slow down the equilibration so as to allow for the growth of fewer and higher quality crystals. In conventional techniques this is often attempted by adding new chemical constituents such as glycerol, or by using microbatch methods. However, this addition of constituents is not well characterized, is not always effective, and may inhibit the formation of crystals. Microbatch methods also may pose the disadvantage of lacking a driving force to promote continued crystal growth as protein in the solution surrounding the crystal is depleted. Through the use of diffusion across a microfluidic free interface in accordance with an embodiment of the present invention, crystal formation may be slowed by a well-defined amount without altering the phase-space evolution, simply by varying the width or cross-sectional area of the connecting channel. 
   The ability to control the rate at which equilibration proceeds has further consequences in cases were one wishes to increase the total volume of a reaction while conserving both the thermodynamics and the microfluidic free interface diffusion mixing. One such case arises again in the context of protein crystallography, in which an initial, small volume crystallization assay results in crystals of insufficient size for diffraction studies. In such a case, it is desirable to increase the reaction volume and thereby provide more protein available for crystal growth, while at the same time maintaining the same diffusive mixing and path through phase space. By increasing the chamber volumes proportionally and decreasing the area of the channel, the area of the interface relative to the total assay volume is reduced, and a larger volume would pass through the same phase space as in the original small volume conditions. 
   While the above description has focused upon diffusion of a single species, gradients of two or more of species which do not interact with each other may be created simultaneously and superimposed to create an array of concentration conditions.  FIG. 47  shows a plan view of one example of a microfluidic structure for creating such superimposed gradients. Flat, shallow chamber  8600  constricted in the vertical direction is connected at its periphery to reservoirs  8602  and  8604  having fixed concentrations of chemical species A and B, respectively. Sink  8606  in the form of a reservoir is maintained at a substantially lower concentration of species A and B. After the initial transient equilibration, stable and well-defined gradients  8608  and  8610  of species A and B respectively, are established in two dimensions. 
   As evident from inspection of  FIG. 47 , the precise shape and profile of the concentration gradient will vary according to a host of factors, including but not limited to the relative location and number of inlets to the chamber, which can also act as concentration sinks for the chemical species not contained therein (i.e. reservoir  8604  may act as a sink for chemical species A). However, the spatial concentration profiles of each chemical species within the chamber may readily be modeled using the MATLAB program previously described to describe a two-dimensional, well-defined, and continuous spatial gradient. 
   The specific embodiment illustrated in  FIG. 47  offers the disadvantage of continuous diffusion of materials. Hence, where diffusion of products of reaction between the diffusing species is sought to be discerned, these products will themselves diffuse in the continuous gradient, thereby complicating analysis. 
   Accordingly,  FIG. 48  shows a simplified plan view of an alternative embodiment of a microfluidic structure for accomplishing diffusion in two dimensions. Grid  8700  of intersecting orthogonal channels  8702  establishes a spatial concentration gradient. Reservoirs  8704  and  8708  of fixed concentrations of chemical species A and B are positioned on adjacent edges of grid  8700 . Opposite to these reservoirs on the grid are two sinks  8703  of lower concentration of the chemical present in the opposing reservoirs. 
   Surrounding each channel junction  8710  are two pairs of valves  8712  and  8714  which control diffusion through the grid in the vertical and horizontal directions, respectively. Initially, only valve pairs  8714  are opened to create a well-defined diffusion gradient of the first chemical in the horizontal direction. Next, valve pairs  8714  are closed and valve pairs  8712  opened to create a well-defined diffusion gradient of the second chemical in the vertical direction. Isolated by adjacent horizontal valves, the gradient of the first chemical species remains present in regions between the junctions. 
   Once the second (vertical) gradient is established, the two gradients can be combined and by opening all the valve pairs for a short time to allow partial diffusive equilibration. After the period of diffusion has passed, all the valve pairs are closed to contain the superimposed gradient. Alternatively, valve pairs  8712  and  8714  can be closed to halt diffusion in the vertical direction, with every second horizontal valve opened to create separate isolated chambers. 
   To summarize, conventional macro free-interface techniques employ capillary tubes or other containers having dimensions on the order of mm. By contrast, the fluidic interface in accordance with embodiments of the present invention is created in a microchannel having dimensions on the order of μm. At such small dimensions, unwanted convection is suppressed due to viscosity effects, and mixing is dominated by diffusion. A well-defined fluidic interface may thus be established without significant undesirable convective mixing. 
   IV. Crystal Storage and Analysis 
   Combining the basic metering and mixing functionality with a fluidic storage structure, allows for a complete protein crystallization workstation to be implemented on chip. In this way a researcher may explore the solubility of a protein in various chemistries, decide which are the most promising crystallization conditions, and then set and incubate reactions for crystal growth. In this way, screening, phase space exploration, optimization, and incubation may be achieved on a single microfluidic workstation. A non-exclusive list of possible methods of storage is provided below. 
   In accordance with one embodiment of the present invention, reactions may be stored by pumping pre-mixed reagent (crystallizing agents, additives, cryo-protectants . . . , sample) into a storage channel and separating the experiments by an immiscible fluid (eg. Paraffin oil).  FIGS. 58A–C  are schematic views of one implementation of this storage technique. 
   In  FIG. 58A , a sample is flowed into circular mixing device  5800  through the peristaltic pumping action of serial valves  5802   a–c . Mixture  5803  is then created and flowed into cross-junction  5804 . In  FIG. 58B , the sample within cross-junction  5804  is routed to serpentine, storage channel  5806  having end  5806   a  controlled by valve  5810 . In  FIG. 58C , the valve configuration is changed, and an inert separating fluid  5808  such as oil is flowed into cross-junction  5804 . In  FIG. 58D , the separating fluid  5808  is flowed into storage channel  5806 . The cycle illustrated in  FIGS. 58A–D  may then be repeated to provide a new sample volume within storage line  5806 . Samples stored within channel  5806  may be recovered through end  5806   a  through gate valve  5810 . 
   Assuming that the storage channel has dimensions 100 μm wide*100 μm tall, a 1 nL sample would fill a length of channel equal to 100 μm. Assuming that the channel is serpentine and that adjacent legs are separated by 100 μm, the total length of channel that would fit on a 1 cm square storage area is approximately 1 cm*100/2=0.5 m. This would allow the storage of 0.5 m/100 μm=5000 reactions. 
   Since the entire length of fluid must be advanced for every addition, it may prove difficult to pump this long length of fluid. To avoid this problem,  FIG. 59  shows a schematic view of embodiment  5900  of a reaction/storage scheme wherein multiplexer  5902  could be used to direct an experimental sample and inert separating fluid into one of a plurality of parallel storage channels  5904 . 
   In accordance with still another alternative embodiment, each reaction could be dead-end filled to the end of the storage line so that the entire column of fluid need never be moved together.  FIG. 60  shows a simplified schematic view of such an approach, wherein storage channel  6000  is dead-ended. 
   A flow of air could be utilized to bias the samples and inert separating liquid into the storage channel, with the air ultimately diffusing out of the channel through the elastomer material. In such an embodiment, the relatively high pressures required to accomplish dead-ended filling could be achieved using an external pressure source, thereby eliminating the need for a separate pump on the oil line. This dead-end filling technique could be used to fill a single storage line as in the embodiment shown in  FIGS. 58A–D , or many parallel storage lines through a multiplexer as in the embodiment shown in  FIG. 59 . 
   While  FIGS. 58A–60  show the storage line as being integrated in a planar fashion as a channel on the chip, this is not required by the present invention. In accordance with alternative embodiments of the present invention the reagents may be off-loaded from the chip in the vertical direction, for example into a glass capillary. 
   Still another approach for storing chemicals is to utilize diffusion assays.  FIGS. 54A–D  show a layout of a device that combines combinatoric mixing structure  5400  with an addressable storage array  5402 . Array  5402  allows for incubation on chip of 256 individual batch experiments or 128 free interface diffusion experiments. 
     FIG. 54B  shows a blow-up of the entrance to storage array  5402 . Array  5402  works on the dead-end loading principal discussed above. Once the reagents are mixed in ring  5406 , they are pumped into serpentine channel  5408  for temporary storage. Multiplexer  5410  at the storage array inlet is then actuated to open one of the sixteen possible inlets, thereby selecting the array row to be addressed. 
     FIG. 54C  shows an enlarged view of the storage array of  FIG. 54A .  FIG. 54D  shows an enlarged view of a single cell in the array of  FIG. 54D . 
   Each row of the storage array has a control line  5452  that actuates valves  5454  separating storage chambers  5456  from the channel inlets, and a control line  5458  that separates the columns of the storage array. A single control line  5460  is further routed to every pair of fluidically coupled chambers  5462   a–b  to separate them until it is desired to create a fluidic interface. 
   Once the row is selected by the multiplexer, the array column  5470  is selected by actuating a corresponding column valve  5472 . In this way a single chamber of the array is selected for filling. 
   Valves near the outlet of the ring are actuated to connect the serpentine storage line to the multiplexer inlet, and the stored solution is pushed back out of the serpentine storage line and into the multiplexer area by pressurized air. This pressurization drives the fluid into the appropriate row of the storage array, pressurizing the air ahead of it and causing it to diffuse into the polymer. 
   While the chamber inlet valves remain closed, the fluid does not enter the chamber, but rather remains in the dead volume between the multiplexer and the storage array (or partly in this volume and partly in the storage array channels). A new line of the multiplexer is then selected and the column valve is temporarily opened to allow the new row to be flushed with buffer as a precaution to avoid cross contamination. Since only one line of the multiplexer is open the other rows of the storage array are held fixed. 
   This new row is then emptied by blowing air through it, preparing it for the next solution. These steps are repeated until all rows are filled with a unique solution. 
   The column valve is then actuated, and the inlet valves opened, and all rows are simultaneously pressurized. This drives the solutions into their respective chambers. This entire process can be repeated for every column until the array is filled with solutions (potentially a different solution in every chamber). 
   If the interface valves are held closed, the array of  FIGS. 54A–D  will accommodate 256 (8 columns×16 rows) batch reactions. In applications such as protein crystallography where diffusion across a microfluidic free interface is desired, the interface valves may be opened to commence the reactions. Since all solutions have previously been separately mixed in the ring, the experimenter has control over the solutions. 
   For example, free-interface diffusion experiments for crystallization may be conducted in which one or more of the following is varied: identity and/or initial concentration of the precipitating agent; identity and/or initial concentration of the crystallized species; identity and/or initial concentration of additives; and identity and/or initial concentration of cryo-protectants. The ability to mix a host of different agents into small volumes of protein solution prior to free interface diffusion experiments offers an important advantage over conventional crystallization approaches, where typically a standard protein stock is used against different crystallization agents. The microfluidic network described above thus offers a flexible platform for crystallization. 
   The array of  FIGS. 54A–D  also allows for sample recovery, addressable well flushing, and the reusing of reaction chambers. Specifically, to recover sample or flush a well the appropriate inlet valves are opened on the column that has the well to be emptied/flushed. One of the rows of the multiplexer that connects one of the chambers of the pair to be flushed is selected. 
   The array row that connects the pair that was not opened at the multiplexer is then opened at the end of the array, and the row that was opened at the multiplexer is closed at the end of the array. 
   This manipulation causes and open fluidic path through the selected row of the multiplexer, through the chamber pair to be emptied/flushed, and out the row selected at the outlet. In this way a single chamber pair can be addressed and flushed. 
   As previously described, once a protein crystal has been formed, information about its three dimensional structure can be obtained from diffraction of x-rays by the crystal. However, application of highly energetic radiation to the protein tends to generate creates heat. X-rays are also ionizing, and can result in the production of free radicals and broken covalent bonds. Either heat or ionization may destroy or degrade the ability of a crystal to diffract incident x-rays. 
   Accordingly, upon formation of a crystal a cryogenic material is typically added to preserve the crystalline material in its altered state. However, the sudden addition of cryogen can also damage a crystal. Therefore, it would be advantageous for an embodiment of a crystallization chip in accordance with the present invention to enable the direct addition of cryogen to the crystallization chamber once a crystalline material is formed therein. 
   In addition, protein crystals are extremely delicate, and can quickly crumble or collapse in response to physical trauma. Accordingly, harvesting a crystal unharmed from the small chambers of a chip poses a potential obstacle to obtaining valuable information about the crystalline material. 
   Therefore, it would also be advantageous for an alternative embodiment of a crystallization chip in accordance with the present invention to allow direct interrogation by x-ray radiation of crystalline materials formed in a chip, thereby obviating entirely the need for separate crystal harvesting procedures. 
   Accordingly,  FIG. 53A  shows a plan view of a simplified embodiment of a crystal growing chip in accordance with the present invention.  FIG. 53B  shows a simplified cross-sectional view of the embodiment of the crystal growing chip shown in  FIG. 53A  along line B–B′. 
   Harvesting/growing chip  9200  comprises elastomer portion  9202  overlying glass substrate  9204 . Glass substrate  9204  computes three etched wells  9206   a ,  9206   b  and  9206   c . Placement of elastomer portion  9202  over glass substrate  9204  thus defines three corresponding chambers in fluid communication with each other through flow channels  9208 . The flow of materials through flow channels  9208  is controlled by valves  9210  defined by the overlap of control lines  9212  over control channels  9208 . 
   During operation of growth/harvesting chip  9200 , valves  9210  are initially activated to prevent contact between the contents of chambers  9206   a ,  9206   b  and  9206   c . Chambers  9206   a ,  9206   b  and  9206   c  are then separately charged through wells  9214  with different materials for effecting crystallization. For example, chamber  9206   a  may be charged with a protein solution, chamber  9206   b  may be charged with a crystallizing agent, and chamber  9206   c  may be charged with a cryogen. 
   The first control line  9212  may then be deactivated to open valve  9210   a , and thereby allowing diffusion of protein solution and crystallizing agent. Upon formation of a crystal  9216 , the remaining control lines  9212  may be deactivated to allow the diffusion of cryogen from chamber  9206   c  to preserve the crystal  9216 . 
   Next, the entire chip  9200  may be mounted in an x-ray diffraction apparatus, with x-ray beam  9218  applied from source  9220  against crystal  9216  with diffraction sensed by detector  9222 . As shown in  FIG. 53B , the general location of wells  9206  corresponds to regions of reduced thickness of both elastomer portion  9202  and underlying glass portion  9204 . In this manner, radiation beam  9218  is required to traverse a minimum amount of elastomer and glass material prior to and subsequent to encountering crystal  9216 , thereby reducing the deleterious effect of noise on the diffracted signal received. 
   While one example of a protein growth/harvesting chip has been described above in connection with  FIGS. 53A–B , embodiments in accordance with the present invention are not limited to this particular structure. For example, while the embodiment of the current embodiment that is described utilizes a glass substrate in which microchambers have been etched, fabrication of microfluidic structures in accordance with the present invention is not limited to the use of glass substrates. Possible alternatives for fabricating features in a substrate include injection molding of plastics, hot embossing of plastics such as PMMA, or fabricating wells utilizing a photocurable polymer such as SU8 photoresist. In addition, features could be formed on a substrate such as glass utilizing laser ablation, or features could be formed by isotropic or aniosotropic etching of a substrate other than glass, such as silicon. 
   Potential advantages conferred by alternative fabrication methods include but are not limited to, more accurate definition of features allowing for more dense integration, and ease of production (e.g. hot embossing). Moreover, certain materials such as carbon based plastics impose less scattering of X-rays, thereby facilitating collection of diffraction data directly from a chip. 
   An additional possibility for the harvesting of crystals is to have a method of offloading from chip. Off-loading could be performed once crystals have formed, or alternatively, prior to incubation. These off-loaded crystals could then be used to seed macroscopic reactions, or be extracted and mounted in a cryo-loop. If a method for the addition of cryogen was also developed, the crystals could be flash frozen and mounted directly into the x-ray beam. 
   Protein crystals grown in relatively small volumes (approximately 10 nL) may be extracted from chip and used in diffraction studies. The small volumes of protein sample used in each reaction on these devices, however, may limits the size of the protein crystals grown, making them unsuitable for high resolution diffraction studies. Furthermore, since protein crystals are often very fragile, it is technically difficult to harvest crystals without damaging them. Finally, since the unique mixing obtained in the microfluidic environment enables the higher success rate of crystallization, it is not necessarily a straight forward process to reproduce crystallization conditions in a traditional large (microliter) volume format. 
   Accordingly, an alternative embodiment of the present invention provides a large volume crystal growth format that has a high correspondence with the previously reported microfluidic screening chip. This device addresses the need to grow larger crystals and facilitates that harvesting of crystals from the chip. 
   The device of this embodiment is a made from an elastomer material and consists of a series of microfluidic chambers connected in pairs via microfluidic channels. Active valves are used to separated the wells of each pair during loading and to isolate the pairs from the rest of the chip during incubation and crystal growth. An autoCAD drawing of a group of five chamber pairs is shown in  FIG. 17 . 
   Operation of the device of  FIG. 17  is identical to that of screening chip described previously. This embodiment is distinct in that the volume of the chambers has been greatly increased, and in that the wells may be easily made accessible by cutting and removing a thin piece of elastomer on the bottom of the device. 
   The dimensions of the wells (in microns) for this embodiment are shown in  FIG. 17 . The depth of the wells is approximately 100 microns. The total reaction volume for this design is approximately 500 nL, allowing for the growth of protein crystals suitable for diffraction studies. Since the mixing of the reagents is still accomplished via diffusion across a constricted channel, the kinetics that gave crystallization in the original device are conserved, and a high degree of correspondence with the screening chip is achieved. 
     FIG. 18  shows a micrograph of a crystal grown and harvested utilizing the device of  FIG. 17 . The crystal shown in  FIG. 18  is a large (450 kDa) complex comprising the DnaB replicative helicase protein and the DnaC helicase loader protein. 
   Protein crystal growth devices in accordance with embodiments of the present invention can be fabricated using at least two methods. In the first method, a multilayer elastomer chip having buried control channels and open flow channels (approximately 20 microns high) is sealed to a thin (approx 1 mm) layer of elastomer with molded cavities (approximately 200 microns high). The control channels may be actuated to pinch off the underlying flow layer. The seal between the flow and well layer is not permanent, so that when crystals are identified, the thin well layer may be cut with a blade and a section pealed away from the device to expose the crystals. Once the crystals are exposed, a solution containing a cryo-protectant may be added. The crystals are then looped out and frozen using conventional techniques. 
   According to a second fabrication method, the wells are molded on the same layer as the flow channels from a negative master having both flow features (approximately 20 microns high) and well features (approximately 200 microns high). This layer is bonded on top of a control structure, so that the pressurization of the control channels causes a membrane to deflect up into the flow channel, closing it. The control layer is sealed to a blank membrane that can be cut to expose the crystals for harvesting. 
   Embodiments in accordance with the present invention can be made with a variety of chamber sizes, aspect ratios, channel geometries, mixing ratios, number of inlets, and thicknesses. By making the device sufficiently thin, it is possible to collect diffraction data through the chip without significant x-ray scattering or attenuation. By adding more fluidics it is possible to do more advanced functions, including but not limited to control over the addition of cryo-protectant or ligands, seeding experiments, or protein replenishment. 
   Once a crystallization condition is identified in the screening chip, a method must be found to determine if the crystals diffract, optimize the growth conditions, grow crystals of sufficient size, add cryo-protectants or heavy metals, harvest them, get them into an x-ray beam, and collect data over a significant angle of rotation. Various problems and requirements which may be associated with this process are described below. 
   First, crystallization conditions discovered in the microfluidic device are not necessarily transportable to conventional large-scale formats. This is because the unique mixing kinetics achieved on chip can not be reproduced in conventional formats. Therefore a technique must preserve the kinetics of microfluidic mixing while providing larger reaction volumes. Moreover, the chip typically undergoes dehydration during the course of incubation that is not well controlled and thus difficult to reproduce in larger formats. Therefore there must be a technique to screen for various levels of dehydration in the growth device. 
   Second, protein crystals are often very fragile and the manipulations required during conventional harvesting, or harvesting from the “Microfluidic Protein Crystal Growth Chip” can damage them. 
   Third, although it is advantageous to freeze a crystal during data collection, the freezing process may damage the crystals. Therefore a general technique is required to facilitate mounting of crystals at room temperature and at cryo temperatures. 
   Fourth, the freezing of a protein crystal requires the presence of a cryo-protectant so that the solvent around the crystal does not crystallize (form ice) but rather freezes in an amorphous glass. Therefore a technique is required to add cryo-protectant to the crystal. 
   Fifth, many cryo-protectants will damage the crystal. Therefore there must be an easy way to screen a variety of cryo-protectants. 
   Sixth, the sudden addition of cryo protectant to a crystal can cause damage to the crystal (e.g. due to osmotic shock). Therefore there must be a mechanism for the slow and controlled addition of cryo protectant. 
   Seventh, when a cryo protectant is added, care must be taken to match the relevant chemical variables, including but not limited to salt concentration, pH, and additive concentrations. If these variables change appreciably, the crystal contacts may be disturbed, compromising the perfection and hence diffracting power of the crystal. In order to match these variables precise control over the chemical concentrations, and hence the dehydration, is desired. Therefore a growth device must allow for exact control over the final state of the reaction. 
   Eighth, once crystals grow to sufficient size they should be harvested and frozen as soon as possible to avoid deterioration. Since crystals in different wells of a device may appear and grow at different times it is necessary to be able to harvest single reaction sites from a chip without disturbing the others. 
   Ninth, any background scatter or attenuation of x-ray reflections during data collection compromises the quality of the collected data. Therefore, a mounting technique must be arranged that minimizes the interaction of the x-rays with material that does not contribute to the diffraction signal. 
   Tenth, to maximize throughput during crystal screening (i.e. to determine which crystals diffract the best), it should be easy to mount and align crystals in the diffraction beam. Therefore, the crystals must be easily visible in the beam with minimal optical aberrations, and mounting more then one crystal at a time is advantageous. 
   Twelfth, the dead-end filling technique used to fill the previously described microfluidic devices is slow for large volumes. The harvesting device should allow for faster filling. 
   Embodiments in accordance with the present invention address these and other issues related to obtaining high quality diffraction data using microfluidic devices. In particular, these embodiments make it straightforward to proceed from initial crystallization conditions (identified using the previously described crystallization chip) to high resolution diffraction data. 
     FIG. 19A  shows a simplified cross-sectional view of an embodiment of a “Microfluidic Protein Crystal Growth and Diffraction Chip” in accordance with the present invention. Crystal Growth and Diffraction Chip  1700  comprises a thick (approximately 1 cm) elastomer layer  1702  with large through-holes  1704  (approximately ⅜″ diameter) bonded to a thin (approximately 250 micron) multilayer elastomer  1706  containing flow channels  1718 , microwells  1716 , and control channels (not shown in  FIG. 19A ). Thick layer  1702  is aligned so that every reaction center comprised of 5 pairs of microwells  1716   a–b  connected via a microfluidic channel  1718 , is located at the bottom of one through-hole  1704 . The liquid  1705  disposed in through-holes  1720  may be referred to as “osmotic baths” or “vapor diffusion barrier”. 
     FIGS. 19B–C  are micrographs showing perspective and plan views, respectively, of a reaction center at the bottom of an osmotic bath. The current design of the device has twenty such reaction centers (through-holes). The wells in the reaction centers can have a variety of relative volumes, total volumes, and connecting channel geometries. 
   A cross-section of the membrane separating the osmotic bath from the reaction center, comprising three bonded layers, is shown in  FIG. 21 . Note that the photograph is inverted, with the bottom of the device at the top of the picture. The bottom layer is a thin (approximately 20 micron) featureless membrane that seals the control channels. The center layer is the control layer that contains the control channels. The top layer is the thickest layer in the membrane (approximately 200 microns) and contains the flow channels (approximately 20 microns high) and the reaction chambers (approximately 150 microns high). The entire membrane is approximately 265 microns thick. Depending upon the particular embodiment, PDMS elastomer material separating crystal forming regions from the adjacent osmotic bath or vapor diffusion barrier, could have a thickness as large as 500 μm. 
   The device of  FIGS. 19A–C and 21  is operated as follows. All control lines are first dead-end filled with a fluid (e.g. water or fluorinated oil). The interface control line that separates the pairs of micro-wells is actuated and the solutions (protein and precipitant) are dead-end filled up to the valve. The osmotic baths are filled with different fluids and are sealed with clear tape. The containment valves are actuated to contain the reactions, and the interface valves are opened to begin the mixing. 
   When crystals form, they may be harvested by punching out the membrane at the bottom of the osmotic bath. This “crystal-disk” can then be held in a clip and mounted in the x-ray beam for data collection. 
     FIG. 55  shows a simplified cross-sectional view of one embodiment of a tool which allows the removal of crystal-containing portions of a chip for analysis. Specifically, tool  2200  comprises piston  2204  sildable within enclosure  2206 . Piston  2204  comprises handle  2208 , ejector portion  2210  including ejector pin  2214 , and blade portion  2211  including blade  2212 . Tight spring  2216  and loose spring  2218  govern the relative motion of blade portion  2211  and punch portion  2210  relative to the housing and to one another. 
     FIG. 56  shows a simplified enlarged view of the tool of  FIG. 55  in operation. Specifically, tool  2200  is positioned over the region of chip  2230  containing the crystal  2232  of interest. Next, handle  2208  is biased in the direction of the chip, compressing loose spring  2218  and allowing blade portion  2211  and ejector portion  2210  of piston  2204  to move in concert unit within housing  2206 , such that blade  2212  engages and penetrates the elastomer material to separate the crystal-disk from the surrounding chip. 
   Next, the tool is withdrawn from the elastomer, such that the crystal disk remains held between the blades. Then, in order to free to the crystal disk from the tool for X-ray diffractometry, handle  2208  is continued to be biased forward, such that such that tight spring  2216  is compressed, and ejector pins  2214  of ejector portion  2211  extend forward relative to the now-stationary blade  2212 , displacing the crystal disk for analysis. 
     FIG. 22  is a micrograph showing a plan view of an extracted crystal disk in accordance with an embodiment of the present invention.  FIG. 23  is a micrograph showing a crystal disk in accordance with an embodiment of the present invention held in a mounting clip. 
   An embodiment of a device in accordance with FIGS.  19  and  21 – 22  satisfies the previously listed requirements at least as follows. First, since the mixing is accomplished by diffusion across a microfluidic channel, the kinetics that were successful in the growth chip are preserved. These kinetics may be further manipulated by changing the geometry of the connecting channel as described above. Protein crystals grown using the present invention are shown in  FIG. 24 . 
   Moreover, since the elastomer is gas permeable, water vapor can easily pass through the thin membrane separating the wells from the osmotic baths. The wells will therefore come into thermodynamic equilibrium with the osmotic baths. Since the volume of the osmotic baths is much larger than that of the wells the final concentration of the solutions in the wells is determined by the osmotic baths. By changing the composition of the solutions in each osmotic bath the level of dehydration necessary for crystallization may be determined. 
   Furthermore, since the crystals do not need to be removed from the “crystal disks” there is no need for damaging manipulations. 
   Once the disk is removed from the chip, it may be directly placed into the x-ray beam for room temperature data collection. Since the fluid is encased in elastomer, dehydration is slow and data may be collected for several hours. If cryo mounting is desired, the crystal disk may be submerged in liquid nitrogen or some other cryogen. As the disk is very thin, it has low thermal mass and allows for quick freezing of the crystal. 
   The addition of cryo-protectant may be accomplished in several ways. One method is to replace the osmotic bath with the cryo-protectant solution and then slice the membrane over the well connected to the well containing the crystal. This creates a fluidic connection to the crystal well, allowing the cryo-protectant to diffuse in over time. Alternatively, the membrane over the well containing the crystal could be sliced to allow for more rapid addition of cryo-protectant. 
   Another method of adding cryo-protectant is to remove the disk from the device and submerge it in cryo-protectant. This will allow diffusion of the cryo-protectant into the wells through the channels open at the edge of the disk. Alternatively, a separate channel for the addition of the cryo-protectant could be added to the fluidic layer. 
   Alternatively, in some cases the crystals may grow in the presence of a cryo-protectant. Specifically, the elastomer material may be permeable to a cryo-protectant, for example certain types of alcohols. The osmotic bath may comprise such a cryo-protectant. 
   Where the concentration of this cryo-protectant is sufficient to ensure freezing of the solution in an amorphous glass, the disks may be extracted and frozen directly. Where the concentration is below that required for formation of an amorphous glass, the cryo-protectant may be concentrated through vapor diffusion with a concentrate solution in the osmotic bath, thereby obviating any need to perforate the membrane. 
   Embodiments in accordance with the present invention allow different cryo-protectant to be screened in each osmotic bath. Moreover, the cryo-protectant may be added very slowly by diffusion as described above. In addition, the final concentration of the well solutions is precisely controlled by the osmotic baths as described above. 
   While embodiments in accordance with the present invention have described a structure wherein an osmotic bath comprises water, solute, crystallizing agent, or cryo-protectant, the well adjacent to the reaction is need limited to containing one or more of these materials. In accordance with another alternative embodiment of the present invention, the elastomer may be permeable to an additive such as a cross-linking reagent, a ligand, or a small drug molecule. 
   In accordance with one such alternative embodiment, diffusion of a cross-linking reagent across the membrane may help to stabilize an crystal that has formed, for example by forming covalent cross-links between individual proteins within a crystal. The formation of such covalent cross-links can result from interaction between functional groups including but not limited to amines, thiols, and carboxylic acids. One specific reagent useful in forming such cross-links is glutaraldehyde. A detailed discussion of some approaches for protein cross-linking may be found at http://www.probes.com/handbook/sections/0502.html, incorporated by reference herein for all purposes. 
   Embodiments in accordance with the present invention offer the advantage that the control lines isolating the reaction sites do not intersect with the edge of the osmotic baths. Therefore, punching out a crystal disk does not severe the control lines, and therefore does not affect the other reaction sites. 
   Moreover, the membrane is very thin to minimize background scatter and attenuation. The X-rays pass through 50 microns of elastomer, followed by 150 microns of solvent and crystal, followed by 50 microns of elastomer. Thus the scatter and absorption is superior or comparable to standard techniques. 
   Furthermore, data may be collected over greater then 120 degrees of rotation. High resolution diffraction from a protein crystals grown, frozen and harvested using the present invention is shown in  FIG. 25 . 
   Since an entire disk is harvested, frozen, and mounted in one step, this technique allows for the parallel processing of many crystals, which is not possible by conventional approaches. And since the wells are only 150 microns thick, the chance of two crystals lying on top of one another is small. An x-ray beam focused to 100 microns can be used to sequentially interrogate single protein crystals. In this way 100 crystals can be screened from a single disk. 
   Since the elastomer is clear, it is easy to view the crystals for beam alignment. Furthermore, since the disk is very thin, there is negligible parallax in the case of slight deviation from normal optical incidence, which can pose a problem with thicker devices. Finally, since the gas vents through a very thin membrane during loading, the loading time may be accelerated, and harvesting may be easily automated. 
   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.