Patent Publication Number: US-11027278-B2

Title: Methods for controlling fluid flow in a microfluidic system

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/941,793, entitled “Microfluidic Device” and filed Mar. 30, 2018, which is a continuation of U.S. patent application Ser. No. 14/689,508, filed Apr. 17, 2015, and now U.S. Pat. No. 9,943,847, which is a continuation of U.S. patent application Ser. No. 13/527,331, entitled “Microfluidic System Including a Bubble Valve for Regulating Fluid Flow Through A Microchannel”, filed on Jun. 19, 2012 and now U.S. Pat. No. 9,011,797, which is a continuation of U.S. patent application Ser. No. 11/433,781, entitled “Microfluidic System Including a Bubble Valve for Regulating Fluid Flow Through A Microchannel”, filed on May 12, 2006 and now U.S. Pat. No. 8,210,209, which is a continuation of U.S. patent application Ser. No. 11/021,251 filed Dec. 21, 2004 and now U.S. Pat. No. 7,069,943, which is a continuation of U.S. patent application Ser. No. 10/179,586, filed Jun. 24, 2002 and now U.S. Pat. No. 6,877,528, which claims priority to U.S. Provisional Patent Application No. 60/373,256 filed Apr. 17, 2002; and is related to U.S. patent application Ser. No. 10/179,488, entitled “Method and Apparatus for Sorting Particles”, filed Jun. 24, 2002 and now U.S. Pat. No. 6,808,075. The contents of all of the above-referenced applications are herein incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microscale fluid handling devices and systems. More particularly, the present invention relates to a method and system for controlling liquid flow in a microchannel by the introduction of a gas bubble to a microfluidic system. 
     BACKGROUND OF THE INVENTION 
     In the chemical, biomedical, bioscience and pharmaceutical industries, it has become increasingly desirable to perform large numbers of chemical operations, such as reactions, separations and subsequent detection steps, in a highly parallel fashion. The high throughput synthesis, screening and analysis of (bio)chemical compounds, enables the economic discovery of new drugs and drug candidates, and the implementation of sophisticated medical diagnostic equipment. Of key importance for the improvement of the chemical operations required in these applications are an increased speed, enhanced reproducibility, decreased consumption of expensive samples and reagents, and the reduction of waste materials. 
     Microfluidic devices and systems provide improved methods of performing chemical, biochemical and biological analysis and synthesis. Microfluidic devices and systems allow for the performance of multi-step, multi-species chemical operations in chip-based micro chemical analysis systems. Chip-based microfluidic systems generally comprise conventional ‘microfluidic’ elements, particularly capable of handling and analyzing chemical and biological specimens. Typically, the term microfluidic in the art refers to systems or devices having a network of processing nodes, chambers and reservoirs connected by channels, in which the channels have typical cross-sectional dimensions in the range between about 1.0 μm and about 500 μm. In the art, channels having these cross-sectional dimensions are referred to as ‘microchannels’. 
     By performing the chemical operations in a microfluidic system, potentially a number of the above-mentioned desirable improvements can be realized. Downscaling dimensions allows for diffusional processes, such as heating, cooling and passive transport of species (diffusional mass-transport), to proceed faster. One example is the thermal processing of liquids, which is typically a required step in chemical synthesis and analysis. In comparison with the heating and cooling of liquids in beakers as performed in a conventional laboratory setting, the thermal processing of liquids is accelerated in a microchannel due to reduced diffusional distances. Another example of the efficiency of microfluidic systems is the mixing of dissolved species in a liquid, a process that is also diffusion limited. Downscaling the typical dimensions of the mixing chamber thereby reduces the typical distance to be overcome by diffusional mass-transport, and consequently results in a reduction of mixing times. Like thermal processing, the mixing of dissolved chemical species, such as reagents, with a sample or precursors for a synthesis step, is an operation that is required in virtually all chemical synthesis and analysis processes. Therefore, the ability to reduce the time involved in mixing provides significant advantages to most chemical synthesis and analysis processes. 
     Another aspect of the reduction of dimensions is the reduction of required volumes of sample, reagents, precursors and other often very expensive chemical substances. Milliliter-sized systems typically require milliliter volumes of these substances, while microliter sized microfluidic systems only require microliters volumes. The ability to perform these processes using smaller volumes results in significant cost savings, allowing the economic operation of chemical synthesis and analysis operations. As a consequence of the reduced volume requirement, the amount of chemical waste produced during the chemical operations is correspondingly reduced. 
     In microfluidic systems, regulation of minute fluid flows through a microchannel is of prime importance, as the processes performed in these systems highly depend on the delivery and movement of various liquids such as sample and reagents. A flow control device may be used to regulate the flow of liquid through a microchannel. Regulation includes control of flow rate, impeding of flow, switching of flows between various input channels and output channels as well as volumetric dosing. 
     U.S. Pat. No. 6,062,681 describes a bubble valve for a liquid flow channel in which the flow of a liquid is controlled by the generation of a gas bubble in the channel using a heater placed in the liquid. As the heater is activated, a bubble is formed which can be enlarged or reduced in size by increasing or decreasing, respectively, the temperature of the heater. The described system presents a number of disadvantages, namely, the required power to operate the valve and the inherent requirement that liquid in the channel be heated upon passing the valve. Even small increases in liquid temperature, by only a couple of degrees, can have disastrous effects on the highly heat sensitive biochemical substances present in the liquids to be controlled in many microfluidic systems. In addition, the required on-chip electric circuitry for the heater increases the complexity of the described valve and consequently results in unacceptably high costs, particularly if the fluidic system employing the bubble valve only used for a single application. 
     Other valves in the prior art use electrochemical means to produce a bubble in a liquid. 
     SUMMARY OF THE INVENTION 
     The present invention provides a bubble valve for controlling, regulating or varying fluid flow through a microfluidic system. The bubble valve regulates fluid flow through a channel using an externally operated mechanical or pneumatic actuator. The actuator causes a deflection of a fluid meniscus into the interior of the channel to regulate liquid flow. The actuator may mechanically force a gas bubble into a fluid carrying microchannel to inhibit liquid flow or to cause liquid flow by applying a sufficiently high pressure to the meniscus. The bubble valve effectively controls the flow of liquids in microfluidic systems, without heating the fluid and without complex on-chip circuitry. 
     The microfluidic system includes a microchannel and a sealed, gas-filled reservoir positioned adjacent to and connected to the microchannel. The gas filled reservoir has a movable wall and a meniscus formed by a liquid in the microchannel that forms an interface between the reservoir and the microchannel interior. The meniscus may form a portion of the side wall of the microchannel. An external mechanical actuator may be used to deflect the movable wall of the reservoir. As the movable wall is deflected, the volume of the reservoir decreases and the gas pressure inside the reservoir increases, causing the meniscus to deflect into the microchannel, thereby modifying the cross-sectional area of the microchannel and consequently varying the flow of liquid through the channel. The increased pressure in the reservoir pushes gas from the reservoir into the microchannel. The gas may result in a local gas bubble being forced into the microchannel from the gas-filled reservoir. The resulting gas bubble occupies a portion of the cross-section of the channel, allowing liquid flow through the channel to be effectively controlled by controlling the size of the gas bubble via the external actuator. 
     The meniscus may comprise a virtual wall formed in a side wall of the microchannel. The virtual wall is a meniscus formed by a liquid in the microchannel that fills an aperture formed in the side wall of the microchannel and essentially replaces the removed portion of the side wall without affecting the properties of liquid flow through the channel. A gas bubble can be forced into the channel by applying a gas pressure at the opening using an external pneumatic actuator. The gas pressure forces the meniscus inside the channel, which varies the flow of liquid through the channel interior. 
     According to one embodiment, the microchannel includes a hydrophobic patch spanning the width of the microchannel at the location where the gas bubble is introduced to enhanced on-off switching of the bubble valve. The hydrophobic patch anchors the bubble in a particular location in the microchannel. If the introduced gas bubble covers the whole area of the patch, the bubble is effectively retained by capillary forces and blocks any liquid flow up to a certain pressure difference, depending on the level of hydrophobicity of the patch. 
     Alternatively or in combination with a hydrophobic patch, the microchannel can be locally shaped into a cavity for receiving and anchoring the gas bubble. By providing an appropriate cavity, the bubble can be kept in place during operation, reducing the risk that the gas bubble is carried away with the liquid. 
     According to one aspect of the invention, a microfluidic device is provided. The microfluidic device comprises a microchannel having an interior bounded by a side wall and a valve for regulating the flow of fluid through the microchannel. The valve comprises a gas-filled reservoir, a fluid meniscus interfacing the reservoir and the interior and an actuator for varying the volume of the reservoir to increase an internal pressure of the reservoir to vary the flow of liquid through the channel. 
     According to another aspect, a microfluidic device is provided, comprising a first plate having a groove formed therein defining a microchannel, a second plate for enclosing the microchannel and a flexible membrane. The second plate is bonded to the first plate and has an aperture adjacent to the groove sized and dimensioned to form a meniscus when the microchannel is filled with a liquid. The aperture defines a reservoir adjacent to the microchannel, wherein the meniscus forms an interface between the microchannel and the reservoir. The flexible membrane is bonded to the second plate to seal the reservoir. 
     According to another aspect, a method of making a bubble valve is provided, the method comprises providing a microchannel having an interior bounded by a side wall, an aperture formed in the side wall and a valve chamber adjacent to the aperture in communication with the interior, filling the microchannel with a liquid to form a meniscus of the liquid in the aperture, whereby the step of filling traps a gas in the valve chamber and providing an actuator for increasing the pressure in the valve chamber to deflect the meniscus into the interior. 
     According to yet another aspect, a method of making a bubble valve is provided. The method comprises providing a microchannel having an interior bounded by a side wall, an aperture formed in the side wall and a valve chamber adjacent to the aperture in communication with the interior, filling the microchannel with a liquid to form a meniscus of the liquid in the aperture and applying and sealing an actuator comprising a chamber to a top surface of the microchannel to form a gas-filled chamber adjacent to the meniscus. The actuator varies the pressure in the gas-filled chamber to deflect the meniscus into the interior, thereby regulating fluid flow. 
     According to still another aspect, a microfluidic device is provided comprising a microchannel having an interior bounded by a side wall, a bubble valve for creating and injecting a bubble into the microchannel interior to regulate fluid flow through the microchannel and a hydrophobic patch for retaining the bubble in a predetermined position in the microchannel interior. 
     According to yet another aspect a bubble valve in a particle sorting device for separating particles having a predetermined characteristic from particles not having a predetermined characteristic is provided. The bubble valve comprises a gas-filled reservoir, a side channel in communication with a channel through which a stream of particles in a carrier fluid passes, wherein the carrier fluid forms a meniscus in the side channel adjacent to the gas-filled reservoir and an actuator for deflecting the meniscus to create a pressure pulse to selectively deflect a particle having the predetermined characteristic from the stream of particles. 
     According to still another aspect, a method of varying an electrical resistance in a microchannel is provided. The method comprises generating a bubble and injecting the bubble into a liquid in the microchannel, whereby the bubble varies the electrical resistance of the microchannel. 
     According to yet another aspect, an electrophoretic system is provided, comprising an electrokinetically operated microchannel, a sample well for providing an sample to the microchannel, a voltage source and a bubble valve for injecting a bubble into the microchannel to vary the electrical resistance of the microchannel. 
     According to a final aspect of the invention, an electrokinetic column to column switch is provided, comprising a first electrokinetically operated microchannel, a second electrokinetically operated microchannel in communication with the first electrokinetically operated microchannel and a bubble valve for selectively blocking flow from the first electrokinetically operated microchannel to the second electrokinetically operated microchannel by selectively injecting a bubble into a microchannel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a microfluidic system suitable for implementing the illustrative embodiment of the invention. 
         FIG. 2  shows an exploded view of a bubble valve according to an illustrative embodiment of the present invention. 
         FIG. 3  shows an isometric view of the bubble valve of  FIG. 2 . 
         FIG. 4  shows a top view of the bubble valve of  FIG. 2 . 
         FIGS. 5 a - c    are cross-sectional views of the bubble valve of  FIG. 2  in operation. 
         FIG. 6  shows an exploded view of an alternative embodiment of a bubble valve according to the present invention. 
         FIG. 7  shows a top view of the bubble valve of  FIG. 6 . 
         FIGS. 8 a - c    are cross-sectional view of the bubble valve of  FIG. 6  in operation. 
         FIG. 9  shows an application of the bubble valve of an illustrative embodiment of the present invention in a microchannel. 
         FIG. 10  shows a Y-intersection in a microfluidic system of an embodiment of the invention that implements a bubble valve to control liquid flow according to the teachings of the present invention. 
         FIG. 11  shows a Y-intersection in a microfluidic system of another embodiment of the invention that implements a bubble valve to control liquid flow according to the teachings of the present invention. 
         FIGS. 12 a - c    shows an electrophoresis system that implements a bubble valve to control electrical current during electrokinetic injection according to the teachings of the present invention. 
         FIG. 13 a    shows an electrokinetic column-column switch implementing a bubble valve according to the teachings of the present invention. 
         FIG. 13 b    shows an alternate electrokinetic column-column switch implementing a bubble valve according to another embodiment of the present invention. 
         FIG. 14  shows a selective resistance circuit that employs a bubble valve to control electrical current according to the teachings of the present invention. 
         FIG. 15  shows an alternative selective resistance circuit that employs a bubble valve to control electrical current according to the teachings of the present invention. 
         FIG. 16  shows a particle sorting system that implements a bubble valve of the present invention to produce fluid impulses to sort particles. 
         FIGS. 17 a , 17 b  and 17 c    illustrate the operation of the particle sorting system of  FIG. 16 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides an improved bubble valve for controlling fluid flow through a microchannel in a microfluidic system. The invention further provides a method of forming the bubble valve. The bubble valve of the present invention can be applied in numerous microfluidic systems for controlling and switching fluid flows. Examples of suitable applications include, but are not limited to: flow cytometry, column switching, 2-D separations, cell or particle sorting applications on a chip, regulating pressurized fluid flows including on-off switching, regulating electrokinetic fluid flows and electrokinetically induced processes including on-off switching and electrokinetic sample injection and channel to channel switching. 
       FIG. 1  illustrates a microfluidic system suitable for implementing the illustrative embodiment of the present invention. The illustrative microfluidic system  100  comprises a substrate  101  having one or more microchannels  21  disposed therein. The microchannels transport fluid through the microfluidic system  100  for processing, handling, and/or performing any suitable operation on a liquid sample. As used herein, the term “microfluidic” refers to a system or device for handling, processing, ejecting and/or analyzing a fluid sample including at least one channel having microscale dimensions. The term “channel” as used herein refers to a pathway formed in or through a medium that allows for movement of fluids, such as liquids and gases. The term “microchannel” refers to a channel preferably formed in a microfluidic system or device having cross-sectional dimensions in the range between about 1.0 μm and about 500 μm, preferably between about 25 μm and about 250 μm and most preferably between about 50 μm and about 100 μm. One of ordinary skill in the art will be able to determine an appropriate volume and length of the microchannel. The ranges are intended to include the above-recited values as upper or lower limits. The microchannel can have any selected shape or arrangement, examples of which include a linear or non-linear configuration and a U-shaped configuration. The microfluidic system  100  may comprise any suitable number of microchannels  21  for transporting fluids through the microfluidic system  100 . 
     The microfluidic system  100  includes a bubble valve  10 ,  10 ′ shown in  FIGS. 2-8   c  for controlling liquid flow through a microchannel of the system. According to the illustrative embodiment, the microchannel is defined by a side wall having any suitable shape enclosing at least a portion of the interior of the channel. The bubble valve may be formed by a gas-filled reservoir positioned adjacent to the microchannel including a meniscus that forms the interface between the reservoir and the microchannel interior. The meniscus may form a portion of the side wall of the microchannel. The bubble valve includes an actuator for modifying the pressure in the reservoir to deflect the meniscus into the channel interior, thereby modifying the cross-sectional area of the microchannel and consequently varying the flow of liquid through the channel. 
     According to an alternate embodiment, the bubble valve is formed by a meniscus in a separate side channel that communicates with and intersects a microchannel through which a liquid to be controlled flows. One skilled in the art will recognize that the meniscus can be located at any location relative to the microchannel through which liquid flows. 
     The gas-filled reservoir may be formed when filling the microchannel having an aperture in a side wall and a reservoir formed adjacent to the aperture. An empty microchannel may be filled with liquid, forming the meniscus in the aperture, which traps the gas that forms the gas bubble and forms a gas pocket in the reservoir adjacent to the meniscus. The creation of the gas pocket on filling provides a sterile gas bubble and reduces contaminants in the system. Alternatively, the air pocket may be created by introducing a gas to the reservoir after filling of the microchannel. 
       FIG. 2  shows an exploded view of an embodiment of an illustrative bubble valve  10  of the present invention. The microfluidic system may be formed by a plurality of stacked layers. As shown in  FIG. 2 , the illustrative microfluidic system  100  includes a first plate  20  in which a groove defining the microchannel  21  is provided. A hydrophobic patch  22  may be applied to an inner wall of the microchannel  21 . A second plate  30  for enclosing the microchannel is bonded to the first plate  20  and includes an aperture  31 . A third plate  40  is bonded on top of second plate  30  to close and seal the stacked structure. The aperture  31  of the intermediate second plate  30  defines a void in the system adjacent to the microchannel  21 . 
       FIGS. 3 and 4  illustrates the assembled bubble valve  10 . As shown, the stacked first plate  20 , second plate  30  and third plate  40  define a closed, gas filled gas reservoir  70 , which can be actuated with a displacement actuator  50 . According to the illustrative embodiment, the aperture  31  defining the reservoir  70  comprises a main body  31   a  and a slot  31   b  extending from the main body  31   a . When the microfluidic system  10  is assembled, the slot  31   b  of the aperture  31  defines a gap in the side wall of the microchannel  21  that provides access to and communicates with the interior of the microchannel  21 . 
     The bubble valve  10  operates to control the flow of liquid through the microchannel  21 . A meniscus is formed in the aperture  31 , which interfaces with and separates the microchannel interior from the reservoir  70 . According to the embodiment shown in  FIGS. 2-8   c , the meniscus is formed by a liquid filling the microchannel in the slot  31   b . One skilled in the art will recognize that other suitable devices may be used to form the meniscus. The liquid in the slot  31   b  is retained in the microchannel by capillary forces. The actuator  50  deflects the upper wall of the reservoir, defined by plate  40 , which decreases the volume of the reservoir  70 . The actuator  50  may comprise any suitable device for deflecting the wall, such as an electromagnetic actuator or a piezoelectric element. The plate  40  may comprise a flexible membrane. The decreased volume consequently increases the pressure of the reservoir  70  and causes the meniscus  80  to deflect into the channel interior to create a constriction in the channel, thereby impeding fluid flow or pushing fluid away from the meniscus. If a sufficient pressure is applied to the meniscus, the actuator generates and enlarges a bubble in the liquid of the microchannel, which blocks fluid flow. The hydrophobic patch  22  provides an anchor for the bubble and retains the bubble at a selected location in the microchannel. 
     After the microchannel is filled with a liquid  60 , the bubble valve  10  is ready for operation.  FIG. 5 a - c    illustrate the operation of the bubble valve according to the teachings of the invention.  FIG. 5 a    shows the bubble valve in an ‘open’ state. According to the embodiment of  FIGS. 5 a -5 c   , the meniscus between the reservoir and the interior of the microchannel is defined by the meniscus  80 , formed by the liquid  60  in the slot  31   b  in the second plate  30 . In the ‘open’ state, liquid flows freely through the microchannel  21  and the valve does not impose any additional flow resistance in the channel. 
     The slot  31   b  may be sized and dimensioned to form a “virtual wall” in the microchannel. As used herein, “virtual wall” refers to the meniscus  80  formed by the first liquid  60  in the aperture formed in the side wall of the microchannel  20 , which essentially replaces the removed portion of the side wall without affecting the properties of the microchannel. The meniscus surface can be, although not required, substantially co-planar with the wall of the microchannel in which the meniscus is formed. The word “virtual” is chosen to express the effect that the overall liquid flow through the microchannel  21  of the microfluidic system  100  is not influenced by the virtual wall, i.e. the flow of liquid in the microfluidic system having a virtual wall is substantially identical to the flow of liquid through an identical microfluidic system in which no virtual wall is present. One of ordinary skill will recognize that the meniscus may be convex or concave, depending on the appropriate system pressure. 
     When the actuator  50  is actuated, the bubble valve  10  switches to a “pinched” state, as shown in  FIG. 5 b   , to inhibit fluid flow through the channel interior. In the ‘pinched’ state, the actuator  50  deflects the top of the gas reservoir  70  for a certain fraction, increasing the pressure in the reservoir  70  and forcing the meniscus  80  down into the channel  21 . The deflection of the meniscus locally reduces the cross-section of the channel  21  and introduces an additional flow resistance to the liquid flow. The degree of reduction in the liquid flow through the microchannel corresponds to the amplitude, frequency and duration of the displacement of the meniscus  80 , which are controllable by the actuator  50 . One skilled in the art will recognize that any suitable means for varying the pressure within the reservoir  70  may be used to deflect the meniscus  80 , thereby regulating fluid flow. 
     When the actuator is fully actuated, the bubble valve  10  is switched to a closed state, as illustrated in  FIG. 5 c   . As shown, in the closed state, the meniscus  80  deflects fully to form and introduce a gas bubble  81  into the microchannel  21 . The gas bubble  81  is retained by the hydrophobic patch  22  formed in the channel wall opposite the slot  31   b . As a result, the liquid flow in the channel is substantially blocked. By reducing the pressure on the meniscus  80 , the bubble valve  10  can be brought from the ‘closed’ state of  FIG. 5 c    via the ‘pinched’ state of  FIG. 5 b    back to the ‘open’ state of  FIG. 5   a.    
     According to one embodiment, the bubble valve  10  may be used as a check valve for regulating pressure in the microchannel. When the pressure in the microchannel exceeds a maximum breaking pressure, the bubble collapses, opening the valve and allowing fluid to flow through the channel, thereby reducing the pressure in the microchannel. The breaking pressure depends on the hydrophobicity of the hydrophobic patch  22 , as well as the geometry of the microchannel. 
     Alternatively or in combination with the hydrophobic patch  22 , the microchannel  21  can be locally shaped into a cavity for receiving and anchoring the gas bubble  81 . By providing an appropriate cavity, the bubble can be kept in place during operation, reducing the risk that the gas bubble is carried away with the liquid. 
     According to the embodiments shown in  FIGS. 2-5   c , the actuator  50  is integrated in the microfluidic chip  100 . However, one skilled in the art will recognize that an external, reusable actuator may also be used to control formation of a gas bubble in the microchannel. 
       FIG. 6  shows an alternative embodiment of a microfluidic system  100 ′ including a bubble valve  10 ′ having an external actuator  90  according to the teachings of the invention. In the embodiment shown in  FIG. 6 , the microfluidic system  100 ′ includes a first plate  200  including a groove defining the microchannel  210  and a second plate  300  bonded to the first plate  200  for enclosing the microchannel  210  and in which a virtual wall opening  32  is formed. The virtual wall opening  32  is sized and dimensioned to form a “virtual wall” when the microchannel  210  is filled with a liquid. 
     Upon filling of the microchannel  21  with a liquid, a virtual wall  32   a  is formed in virtual wall opening  32 . As shown in  FIGS. 7, 8   a - 8   c , the microfluidic system further includes an external actuator, illustrated as pressurizer  90 , pressed and sealed onto the top of the second plate  300  to form a tight seal. The external pressurizer  90  defines a sealed pressurizing chamber  92  adjacent to the virtual wall  32   a . The pressurizer varies the pressure within the pressurizing chamber  92  to control liquid flow through the microchannel  210  by modifying the position of the virtual wall. The pressurizer  90  may include a source of pressurized gas (not shown) and a gas inlet  91  to allow a gas pressure to be applied to the virtual wall  32   a  in order to move the virtual wall. The pressurizer may alternatively include a flexible wall that deflects to vary the volume of the chamber  92  upon activation of an actuator, such as a piezoelectric element or electromagnetic actuator. 
       FIG. 8 a    shows a cross-section of the bubble valve  10 ′ of  FIGS. 6 and 7  in the ‘open’ state. As shown, when the microchannel  210  is filled with liquid  600 , a virtual wall  32   a , defined by a meniscus  800 , is formed within the virtual wall opening  32 . The meniscus essentially replaces the absent portion of the side wall of the microchannel and allows liquid to flow through the channel interior unimpeded and uninfluenced by the virtual wall. 
       FIG. 8 b    depicts the ‘pinched’ state of the bubble valve, when the pressurizer  90  is activated. As shown, activation of the pressurizer  90  increases the internal pressure within the pressurizing chamber  92 . The increased pressure moves the meniscus  800  down the channel height and into the microchannel interior, thereby regulating liquid flow. The pressurized controls the level of the internal pressure in order to control the amount of deflection of the meniscus and therefore the rate of fluid flow. 
     To switch the valve to a ‘closed’ state, as shown in  FIG. 8 c   , the pressurizer  90  applies a large pressure that is sufficient to form and introduce a gas bubble  810  into the channel  210 . The hydrophobic patch  220  retains the gas bubble  810  in place. As a result, the liquid flow in the channel is blocked up to a ‘breaking pressure’, which depends on the hydrophobicity of hydrophobic patch  220 . A higher hydrophobicity results in a larger breaking pressure. By reducing the pressure on the meniscus  800  the bubble valve  10 ′ can be brought from the ‘closed’ state of  FIG. 8 c    via the ‘pinched’ state of  FIG. 8 b    back to the ‘open’ state of  FIG. 8   a.    
       FIG. 9  shows an application of a bubble valve  10  for flow regulation in a microchannel  121  according to one embodiment of the invention. During typical operation of a microfluidic system, a pressure difference is applied over the length of a microchannel  121 . A bubble valve  10  can be employed to regulate the flow through the microchannel between zero and a maximum flow rate, depending on the applied pressure difference. 
       FIG. 10  shows a portion of a microfluidic system according to an embodiment of the invention forming a Y-intersection comprising two inlet microchannels  121   a  and  121   b  and an outlet channel  121   c  that combines the fluids flowing through the two inlet microchannels. The first microchannel  121   a  carries a first liquid and the second microchannel  121   b  carries a second liquid. The microchannels  121   a  and  121   b  are each controlled by a corresponding bubble valve,  10   a  and  10   b , respectively, for regulating the combined composition and flow rate through the outlet microchannel  121   c . One skilled in the art will recognize that that the number of inlet channels is not limited to two, but is presented here merely as an example. 
       FIG. 11  shows a Y-intersection of a microfluidic system according to another embodiment of the invention. As shown the Y-intersection comprises an inlet microchannel  221   c  and two outlet microchannels  221   a  and  221   b  for splitting the incoming liquid flow from the inlet microchannel  221   c . The flow of each outlet channel is regulated by a corresponding bubble valve  200   a  and  200   b , respectively. The incoming liquid flow from the inlet microchannel  221   c  can be split between microchannel  221   a  and microchannel  221   b  in any required ratio. One skilled in the art will recognize that that the number of inlet channels is not limited to two, but is presented here merely as an example. 
       FIG. 12 a -12 c    shows the implementation of an electrophoresis system  110  comprising five bubble valves,  10   a - e  of the present invention, arranged with a crossed microchannel configuration. Regulation of the bubble valve  10   a  regulates the electric current through the associated electrokinetically operated microchannel  115   a . The pinching of the liquid in the electrokinetically operated microchannel  115   a  by a bubble valve will result in an increased electrical resistance in the microchannel  115   a . As a result, the migration of charged species and electro-osmotic flow in the electrokinetically operated microchannel  115   a  can be regulated. A voltages difference for the injection of sample from a well storing a supply of a sample  125  between the bubble valve  10   a  and the crossing point  111  of the microchannels and consecutive separation are provided via wells V+ and V0. Electrodes are placed in the V+ and V0 wells and energized with a constant voltage difference during the operation of the electrophoresis system. 
     In the injection phase, valves  10   a ,  10   b ,  10   c  and  10   d  are substantially in the open position, allowing passage of electrical current up to a required level for injection ( FIG. 12 b    shows direction of current/sample), while bubble valve  10   e  is closed. The valves are kept in this position long enough for the sample to move from the sample well  125  towards and past the central injection crossing  111 . Varying the opening ratio of valves  10   a ,  10   b  and  10   d  can be adjusted to confine the sample to a narrow flow through the injection cross  111 , as shown in  FIG. 12 c    (i.e. ‘pinched injection’). 
     Immediately after the injection phase, a plug of sample is injected and separated in the separation column  115   a  (microchannel which runs horizontally in figure) by closing bubble valves  10   a ,  10   d  and  10   c  and opening valves  10   b  and  10   e . Now the total voltage difference is applied longitudinally over the separation channel, resulting in the separation of the constituents in the sample ( FIG. 12 c   ). 
       FIG. 13 a    illustrates another application of the bubble valve of the present invention implemented in a column-column switch  130  for electrokinetically transferring a substance from a first electrokinetically operated microchannel  215   a  to a second electrokinetically operated microchannel  215   b . The column-column switch  130  comprises a bubble valve  10   b  of the present invention that connects the first electrokinetically operated microchannel  215   a  to the second electrokinetically operated microchannel  215   b . Both electrokinetically operated microchannel  215   a  and electrokinetically operated microchannel  215   b  are connected to corresponding wells well  220   a - d . In the following example, it is assumed that the substances electrokinetically move from a positive electric potential to ground and that well  220   c  and well  220   b  are provided with a positive potential whilst well  220   a  and well  220   d  are grounded. 
     At first, the two electrokinetically operated microchannel  215   a  and electrokinetically operated microchannel  215   b  are operated independently and the connecting bubble valve  10   b  is in the ‘closed’ state whilst bubble valve  10   a  and bubble valve  10   c  are in the ‘open’ state. To electrokinetically transfer substance from electrokinetically operated microchannel  215   a  to the electrokinetically operated microchannel  215   b , bubble valve  10   a  and bubble valve  10   c  are switched to the closed state, and the connecting bubble valve  10   b  is opened momentarily to allow passage of an amount of substance from the electrokinetically operated microchannel  215   a  to the electrokinetically operated microchannel  215   b . The amount transferred depends directly upon the time bubble valve  10   b  is opened. After the required amount of substance is transferred, the connecting bubble valve  10   b  is closed and the bubble valve  10   a  and bubble valve  10   b  are opened again. 
       FIG. 13 b    illustrates the implementation of another column to column switch  130 ′ to exchange liquid from a first column  215   a ′ selectively into a second column  215   b ′. The first column and the second column are each crossed by a transfer column  250 , operated by a first bubble valve  10   a  arranged on one end of the transfer column  250  and a second bubble valve  10   b  arranged on the opposite end of the transfer column  250 . The first one of the bubble valves is attached to the first column  215   a ′ and is actuated upon transiently by an external actuator for increasing the pressure within the bubble valve reservoir. Increasing the pressure on the bubble valve will deflect the meniscus  80 , inducing a transient flow in the transfer column  250  from the pressurized bubble valve  10   a  towards the second bubble valve  10   b , which comprises a compressible volume of gas. This transient flow effectively transfers a liquid volume from the first column  215   a ′ to the second column  215   b ′. Upon deactivation of the external actuator, the flow in the transfer column  250  reverses as the compressed gas volume in the second bubble valve  10   b  exerts a pressure on the liquid in the transfer column  250 . 
       FIG. 14  shows a selective resistance circuit employing a bubble valve of the present invention for selectively including a predefined electrical resistance in an electrokinetic circuit. The circuit  140  comprises an inlet microchannel  321 , which splits into two paths. The first path  322  includes a fluidic resistor  240   a  and a bubble valve  10   a , the second parallel path  323  includes a fluidic resistor  240   b  and a bubble valve  10   b . The fluidic resistors  240   a - b  comprise a channel of appropriate length to results in a certain electrical resistance. The bubble valve  10   a  and the bubble valve  10   b  can be switched each to either on to allow fluid flow through the associated microchannel or off to block fluid flow through the associated microchannel. As a result, the overall electrical resistance of the electrokinetic circuit can be switched between four values: infinite (both bubble valves  10   a - b  are off), the resistance of fluidic resistor  240   a  (bubble valve  10   a  on, bubble valve  10   b  off), the resistance of fluidic resistor  240   b  (bubble valve  10   a  off, bubble valve  10   b  on) and the parallel resistance of fluidic resistor  240   a - b  (both bubble valves  10   a  and  10   b  on). 
       FIG. 15  shows an alternative resistance circuit  150  according to another application of the invention, now for the selective application of a voltage. By opening either the bubble valve  10   a  or bubble valve  10   b , the voltage imposed on an outgoing channel  523  can be selected. Fluidic resistor  245   a  and  245   b  function to limit the electric current in either of the two states to a predetermined value. 
       FIG. 16  illustrates another application of the bubble valve  10  of the present invention in a particle sorting application, wherein the bubble valve is positioned in a side channel that communicates with a channel through which particles in suspension flow. According to one application of the present invention, a particle sorter  160  comprises a closed channel system of capillary size for sorting particles, such as cells. The channel system comprises a first supply duct  162  for introducing a stream of particles and a second supply duct  164  for supplying a carrier liquid. The first supply duct  162  ends in a nozzle, and a stream of particles is introduced into the flow of carrier liquid. The first supply duct  162  and the second supply duct  164  enter a measurement duct  166 , which branches into a first branch  172   a  and a second branch  172   b  at a branch point  171 . A measurement region  182   a  is defined in the measurement duct  166  and is associated with a detector  182   b  to sense a predetermined characteristic of particles in the measurement region  182   a . Two opposed bubble valves  10   a  and  10   b  are positioned in communication with the measurement duct  166  and are spaced opposite each other. The bubble valves  10   a ,  10   b  communicate with the measurement duct  166  through a pair of opposed side passages  174   a  and  174   b , respectively. Liquid is allowed to partly fill these side passages  174   a  and  174   b  to form a meniscus  175  therein. An external actuator  176  is also provided for actuating the bubble valves  10   a ,  10   b , which momentarily causes a flow disturbance in the duct to deflect the flow therein when activated by the actuator  176 . 
     In a suspension introduced by the first supply duct  162 , two types of particles can be distinguished, normal particles  180   a  and particles of interest  180   b . The flow rates in both branches  172   a  and  172   b  are adjusted so that the stream of particles normally flows through the second branch  172   b . Upon sensing the predetermined characteristic in the particles in the measurement region  182   a , the detector  182   b  raises a signal. The external actuator  176  activates the bubble valves  10   a ,  10   b  when signaled by the detector  182   b  in response to sensing the predetermined characteristic, to create a flow disturbance in the measurement duct  166  between the sideway passages  174   a ,  174   b , to deflect the particle having the predetermined characteristic so that it flows down the first branch duct  172   a  rather than the second branch duct  172   b . The detector communicates with the actuator  176 , so that when the detector  182   b  senses a predetermined characteristic in a particle, the actuator activates the first bubble valve  10   a  to cause pressure variations in the reservoir  70  of the first bubble valve. The activation of the first bubble valves causes a transient pressure variation in the first side passage  174   a . The second side passage  174   b  and the second bubble valve  10   b  absorb the transient pressure variations in the measurement duct  166  induced via the actuator  176 . Basically, the reservoir  70   b  of the second bubble valve  10   b  is a chamber having a resilient wall or contains a compressible fluid such as a gas. The resilient properties allow the flow of liquid from the measurement duct into the second side passage  174   b.    
       FIGS. 17 a -17 c    illustrate the operation of the particle sorting system  160  of  FIG. 16 . In  FIG. 17 a   , the detector raises a signal to activate the actuator. Upon activation of the actuator, the pressure within the reservoir of the first bubble valve  10   a  is increased, causing a transient discharge of liquid from the first side passage  174   a  as indicated by the arrow. The sudden pressure increase caused at this point in the duct causes liquid to flow into the second side passage  174   b  because of the resilient properties of the reservoir of the second bubble valve  10   b . This movement of liquid into the second side passage  174   b  is indicated with an arrow. Resultingly, as can be seen in the figure, the flow through the duct is deflected causing the selected particle of interest  178   b  located between the first side passage  174   a  and the second side passage  174   b  to be shifted perpendicular to its flow direction in the normal state. The flow resistances to the measurement duct  166 , the first branch  172   a  and the second branch  172   b  is chosen so that the preferred direction of the flow to and from the first side passage  174   a  and the second side passage  174   b  has an appreciable component perpendicular to the normal flow through the measurement duct  166 . This goal can for instance be reached by the first branch  172   a  and the second branch  172   b  so that their resistances to flow is large in comparison with the flow resistances of the first side passage  174   a  and the second side passage  174   b.    
       FIG. 17 b    shows the particle sorting system  160  during the relief of the first bubble valve reservoir when the particle of interest  178   b  has left the volume between the first side passage  174   a  and the second side passage  174   b . The actuator  176  is deactivated, causing the pressure inside the reservoir to return to the normal pressure. During this relief phase there is a negative pressure difference between the two reservoirs of the bubble valves, causing a liquid flow through the first side passage  174   a  and the second side passage  174   b  opposite to the liquid flow shown in the previous figure and is indicated by the arrows. 
       FIG. 17 c    shows the particle sorting system  160  after completion of the switching sequence. The pressures inside the reservoirs of the bubble valves has been equalized so the flow through the measurement duct  166  is normalized. As the particle of interest  178   b  has been displaced radially, it will flow into the first branch  172   a  as was the objective of the switching operation. 
     According to yet another embodiment, the cross-sectional dimensions of a microchannel including a bubble valve according to the teachings of the present invention may be varied locally to affect the pressure within the microchannel interior. For example, the microchannel may be narrowed or widened at certain locations to increase or decrease the capillary forces acting on a fluid in the microchannel interior. One of ordinary skill in the art will be able to determine a suitable cross-sectional dimension to achieve a desired pressure within the microchannel interior. 
     The bubble valve of the present invention may be implemented in a variety of microfluidic devices used for many applications. In a particular application, the bubble valve is implemented in a flow-cytometer based instrument for sorting or physically separating particles of interest from a sample or for measuring selected physical and chemical characteristics of cells or particles in suspension as they travel past a particular site. The bubble valve may also be employed in devices for sequencing or manipulating DNA, medical diagnostic instruments, devices for drug discovery, chemical analysis and so on. 
     The present invention provides an improved system and method for regulating fluid flow in a microchannel for a variety of applications. The bubble valve of the present invention is easy to operate and control, simple to manufacture and economical. In addition, the bubble valve does not adversely affect the liquid in the microchannel. The bubble valve effectively controls the flow of liquids in microfluidic systems, without heating the fluid and without complex on-chip circuitry. 
     The present invention has been described relative to an illustrative embodiment. Since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are to cover all generic and specific features of the invention described herein, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.