Abstract:
A passive valve for use within microfluidic structures. Surface tension forces developed within microscale channels are used to control flow within the channels. Flow can be halted within a channel until fluid force reaches a predetermined pressure to allow the channel to open.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application claims benefit from U.S. Provisional Patent Application Ser. No. 60/206,878, filed May 24, 2000, which application is incorporated herein in its entirety by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to microscale devices for performing analytical testing and, in particular, to surface tension valves for controlling flow within microfluidic channels.  
           [0004]    2. Description of the Prior Art  
           [0005]    Microfluidic devices have recently become popular for performing analytic testing. Using tools developed by the semiconductor industry to miniaturize electronics, it has become possible to fabricate intricate fluid systems which can be inexpensively mass produced. These techniques may be used to enable the development of miniaturized fluidic circuits as building blocks for an advancement in the fields of medical diagnostics and chemical analysis.  
           [0006]    One aspect of microfluidics technology is based on the very special behavior of fluids when flowing in channels approximately the size of a human hair. This phenomenon, known as laminar flow, exhibits very different properties within a microscale channel than fluids flowing within the macro world of everyday experience. Due to the extremely small inertial forces in microscale structures, practically all flow in microfluidic channels is laminar. This allows the movement of different layers of fluid and particles next to each other in a channel without any mixing, except for diffusion.  
           [0007]    Microfluidic technology can be used to deliver a variety of in vitro diagnostic applications at the point of care, including blood cell counting and characterization, and calibration-free assays directly in whole blood. There are also other applications for this technology, including food safety, industrial process control, and environmental monitoring. The reduction in size and ease of use of these systems allows the devices to be deployed closer to the patient, where quick results facilitate better patient care management, thus lowering healthcare costs and minimizing inconvenience. In addition, this technology has potential applications in drug discovery, synthetic chemistry, and genetic research.  
           [0008]    Control of fluid movement within microfluidic channels is usually accomplished by the use of mechanical valves. An example of such a valve is taught in U.S. patent application Ser. No. 09/677,250, entitled “Valve for Use In Microfluidic Structures”, filed Oct. 2, 2000, and is assigned to the assignee of the present invention. This application describes a valve manufactured from a flexible material which allows one-way flow through microfluidic channels for directing fluids through a microfabricated analysis cartridge. This type of valve, however, is often difficult to fabricate due to its extremely small dimensions.  
           [0009]    It has also been proposed to use passive or nonmechanical means to control fluid movement in microfluidic channels. U.S. Pat. No. 6,193,471 is directed to a process and system for introducing menisci, arresting the movement of menisci at defined locations within the system, and for removing menisci from capillary volumes of a liquid sample, as well as delivering precise small volumes of liquid samples to a point of use.  
           [0010]    U.S. Pat. No. 6,130,098, which issued on Oct. 10, 2000, is directed to microscale devices using flow-directing means including a surface tension gradient mechanism in which discrete droplets are differentially heated and propelled through etched channels. Electronic components are fabricated on the same substrate material, allowing sensors and controlling circuitry to be incorporated in the same device.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the present invention to provide a passive valve within a microfluidic system which uses surface tension forces to control flow within the microfluidic channels.  
           [0012]    It is also an object of the present invention to provide a valve within a microfluidic channel such that the channel will open at a predetermined fluid pressure.  
           [0013]    These and other objects and advantages of the present invention will be readily apparent in the description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is an illustration of a microfluidic channel having sharp edges;  
         [0015]    [0015]FIG. 2 is an illustration of the channel of FIG. 1 containing a fluid having a meniscus extending beyond its edge;  
         [0016]    [0016]FIG. 3 is an illustration of a microfluidic channel having a plurality of branched channels;  
         [0017]    [0017]FIG. 4 is an illustration of a microfluidic channel having a central barrier within the channel;  
         [0018]    [0018]FIG. 5 is an illustration of a microfluidic channel having stepped branches;  
         [0019]    [0019]FIG. 6 is an illustration of an embodiment of a valve according to the present invention at intersecting microfluidic channels depicting a fluid in one channel;  
         [0020]    [0020]FIG. 7 shows the channels of FIG. 6 depicting fluids within both channels; and  
         [0021]    [0021]FIG. 8 is an illustration of a microfluidic channel having a soluble material deposited on its walls. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring now to FIG. 1, there is shown a microfluidic channel  10  having an end  11  and containing a fluid  12  within its walls  14 ,  16 . A concave meniscus  18  is formed at the leading edge of flowing fluid  12  within channel  10 . Edges  14   a,    16   a  of channel walls  14 ,  16  are formed at approximately 90° which constitute “sharp edges”, thus causing surface tension forces within flowing fluid  12 . As can be clearly seen in FIG. 2, fluid  12  moves within channel  10  due to a positive pressure upstream or a positive displacement. Its flow velocity is determined by several factors, including the magnitude of the pressure and the fluidic resistance of channel  10 . When fluid  12  reaches end  11  of channel  10  which contains sharp edges  14   a  and  16   a,  the fluidic resistance increases, and if the driving pressure is less than the force needed to overcome the surface tension resistance at edges  14   a,    16   a,  the flow of fluid  12  will stop, and meniscus  18  will distend into the open space beyond edges  14   a,    16   a.    
         [0023]    The shape of the meniscus depends on several factors, such as properties of the material that composes the channel along with properties of the flowing fluid. For example, meniscus  18  may adopt a convex shape if the properties of the fluid and channel walls are conducive to the formation of that shape. Another factor which is related to this phenomenon is the angle of contact. If a liquid is in contact with a solid and with air along a line, the angle θ between the solid-liquid interface and the liquid-air interface is called the angle of contact. If θ=0, the liquid is said to wet the channel thoroughly. If θ is less than 90°, the liquid moves within the channel and forms a concave meniscus; and if more than 90°, the liquid does not wet the solid and is depressed within the channel, forming a convex meniscus.  
         [0024]    This phenomenon can also be used as a stream splitter when desirable. Referring now to FIG. 3, a main channel  30  contains a fluid  32  which flows toward a series of channel branches  34 ,  36 ,  38  at the distal end  40  of channel  30 . As fluid  32  flows toward end  40 , it will partition and flow at different velocities in each of channels  34 ,  36 ,  38  due to variation in the resistance within each channel. When fluid within fastest flowing channel  34  reaches a sharp edge boundary  34   a,  flow will stop. Fluid in the second fastest flowing channel  36  will then reach a sharp edge boundary  36  and stop, while fluid within the slowest flowing channel  38  will finally reach a sharp edge boundary  38   a.  The sizes and characteristics of channels  34 ,  36 ,  38  can be varied to control the speed of the flow in each channel.  
         [0025]    [0025]FIG. 4 shows another embodiment which uses branched fluidic channels to control fluid flow. A channel  41  divides into two arcuate paths  41   a,    41   b  which converge at a channel  42  at a distance from channel  41 . A fluid traveling within channel  41  will divide and flow into channels  41   a,    41   b  at different velocities until surface tension forces stop the flow and form menisci  43   a,    43   b  at the junction of channels  41   a,    41   b  and  42 . These junctions act as passive valves to control flow into channel  42 . The type of channel, materials, sizes, and fluid pressure all contribute to the forces necessary to overcome the surface tension which forms menisci  43   a,    43   b.    
         [0026]    [0026]FIG. 5 shows a further embodiment using branched fluidic channels for fluid control. A main channel  44  divides into two separate branch channels  44   a,    44   b.  Channel  44   a  is connected to a wider channel  45 , while channel  44   b  is also connected to a wider channel  46 . Edges  45   a,    45   b  of the junction of channels  44   a  and  45  constitute “sharp edges” as discussed earlier while edges  46   a,    46   b  of the junction of channels  44   b  and  46  also contain sharp edges.  
         [0027]    As fluid flows within channel  44  and divides into channels  44   a  and  44   b,  the fluid will stop as it reaches edges  45   a,    45   b  and  46   a,    46   b  respectively, and if the driving pressure of the fluid is less than the force needed to overcome the surface tension at these edges, menisci  47 ,  48  will form at the junction of the respective channels. Each channel can be constructed of the appropriate materials, or treated with hydrophobic or hydrophilic materials, to provide the proper surface tension resistance to the flow through channel  44  to achieve the desired flow timing from channels  44   a  and  44   b.    
         [0028]    [0028]FIG. 6 shows an embodiment of microfluidic channels containing a passive valve using the principles of the present invention. Referring now to FIG. 6, a first microfluidic channel  50  is intersected by a second microfluidic channel  52 . The intersection of channels  50 ,  52  is formed by a pair of sharp edges  54 ,  56  which are offset such that channel  52  is separated into two channels  52   a  and  52   b  having different widths.  
         [0029]    A fluid stream  58  enters channel  50  via a port  60  and flows until it contacts sharp edges  54 ,  56  at the intersection of channels  50  and  52 , where the flow stops due to surface tension. Stopped stream  58  forms a meniscus  62  which distends into channel  52 . To restart fluid flow within channel  50 , a fluid stream  64  is initiated in channel section  52   a  in the direction indicated by arrow A, as can be seen in FIG. 7. As fluid stream  64  contacts meniscus  62 , the surface tension holding fluid stream  58  within channel  50  is overcome, thus reinitiating fluid flow from port  60  through channel  50  and into channel section  52   b.  Although meniscus  62  is convex, this valve will operate if the meniscus is concave, as fluid stream  64  would contact the meniscus in channel  50  and reinitiate the flow.  
         [0030]    Surface tension valves may also be created in microfluidic channels by the use of hydrophobic or hydrophilic materials. For example, if a hydrophobic material is deposited in one or several spots within a channel, it would act like a valve in a microfluidic circuit for aqueous fluids. Referring now to FIG. 8, there is shown a microfluidic channel  80  having a pair of parallel walls  82 ,  84 . A track  86  of material is deposited across the width of channel  80 . This material may be hydrophobic, such that an aqueous fluid flowing within channel  80  would stop when it reached material  86  if the fluid pressure within channel  80  was below the pressure level needed to overcome the surface tension at that point. Once the pressure exceeds the surface tension, the fluid will flow past material  86 , and once channel  80  is witted, the fluid would continue to flow. Material  86  can be added at several positions within channel  80 .  
         [0031]    It is also possible to deposit a soluble material in the microfluidic channel such that it will act as a valve until the flowing fluid is able to dissolve the material, thus permanently opening the passageway. This material can also be hydrophobic or hydrophilic and can present a certain definable initial resistance due to surface tension.  
         [0032]    While this invention has been shown and described in terms of a preferred embodiment, it will be understood that this invention is not limited to any particular embodiment and that changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.