Open microfluidic system and various functional arrangements therefore

An open microfluidic system is provided. The open microfluidic system including the extreme wettability of exclusive liquid repellency (ELR), open microchannels with high lateral resolution and low profile, various valve arrangements, capable of a broad range flow rates, and capable of spatially and temporally trapping particles in open fluid.

FIELD OF THE INVENTION

This invention related generally to microfluidics, and in particular, to an open microfluidic system including the extreme wettability of exclusive liquid repellency (ELR), open microchannels with high lateral resolution and low profile and with various valve arrangements, capable of a broad range flow rates, and capable of spatially and temporally trapping particles.

BACKGROUND AND SUMMARY OF THE INVENTION

Open microfluidics has been defined as a microfluidic system with at least one solid boundary confining the fluid removed, exposing the fluid either to air (i.e., single-liquid-phase) or a second fluid (i.e., multi-liquid-phase). One disadvantage of single-liquid-phase open systems is their sensitivity to evaporation. To overcome this limitation many open microfluidic systems employ an oil overlay (similar to the oil-overlaid microdroplets used for decades for the in vitro study of early embryo development) to prevent detrimental fluid loss via evaporation and airborne contamination through the liquid-air interface. Important advantages of open microfluidics include accessibility, bubble elimination and ease of use. The liquid/air or liquid/liquid interface above and surrounding the channel provides direct physical access to the fluid of interest, e.g., enabling localized interrogation of cellular samples with their biophysics or biochemistry. Also, without the need to bond to another surface, open microfluidic devices are generally easy-to-make and easy-to-use (e.g., elimination of bubble trapping and associated device failures), reducing the adoption barrier to end users.

Under oil open microfluidics has been limited in its functional/operational range due to the lack of lateral flow. Recently, lateral flow was introduced to under oil open microfluidics to expand its functionality. Fundamental to most microfluidic systems is their ability to control mass transport (e.g., maintaining steady flow, varying flow rate, and turning flow on and off). However, reported under oil open-channel systems exhibit a limited flow range (from both the upper and lower limits) thus limiting potential applications. For example, the maximum flow rate is several orders of magnitude lower than the typical range for closed channels. The reported techniques form channels via a two-step process whereby the channels are initially filled in air and then subsequently overlaid with oil. This two-step process presents a number of technical challenges that limit the scope of their geometries and application. From a practical operational perspective, the initial in-air step (prior to the introduction of an oil overlay) restricts the channel width to the millimeter scale due to volume loss via evaporation. Millimeter scale channels are limited in their ability to spatially and temporally organize cellular samples (e.g., mammalian and bacterial cells are less than 10 μm), and flexibility in control of mass transport. In addition to extending the range of flow rates, open microchannels should also be able to provide the ability to turn flow on and off.

Therefore, it is a primary object and feature of the present invention to provide an open microfluidic system that allows for increased accessibility, minimized evaporation and airborne contamination, and ease of use over prior systems.

It is a further object and feature of the present invention to provide an open microfluidic system that allows for a high lateral resolution of fluidic channels (e.g., a few microns in channel width, spacing, and height).

It is a still further object and feature of the present invention to provide an open microfluidic system that allows for confinement of various cellular samples (e.g., mammalian cells, bacteria, and fungi) in open fluid.

It is a still further object and feature of the present invention to provide an open microfluidic system that enables flow control covering several orders of magnitude.

It is a still further object and feature of the present invention to provide an open microfluidic system that includes various valve arrangements with the ability to reversibly turn fluid flow through the system on and off.

It is a still further object and feature of the present invention to provide an open microfluidic system including a single-use valve for use in a sample loading for streamlined multi-step assays.

It is a still further object and feature of the present invention to provide an open microfluidic system that is simple to utilize and inexpensive to manufacture.

In accordance with the present invention, an open microfluidic system is provided. The open microfluidic system includes a microfluidic device having a reservoir adapted for receiving an oil therein. The reservoir is defined by a surface configured to repel an aqueous solution. A hydrophilic input and a hydrophilic output are patterned on the surface. The output is spaced from the input. A hydrophilic strip interconnects the input and the output.

The strip includes a first channel having a first end connected to the input, a second channel having a first end connected to the output, and a valve configured to selectively fluidically connect the second ends of the first and second channel. The valve includes a second end of the first channel and a second end of the second channel. The valve may have one of plurality of configurations. By way of example, the valve may include a dried reagent fluidically interconnecting the second end of the first channel and the second end of the second channel. Fluid flowing over the dried reagent picks-up and re-dissolves the dried reagent therein, thereby exposing a portion of the surface between the first and second hydrophilic channels and fluidically isolates the first channel from the second channel.

Alternatively, the second end of the second channel may have a horseshoe configuration. In addition, the second end of the first channel has a horseshoe configuration. A droplet having a first dimension may be deposited on the surface. When the droplet communicates with the second end of the first channel and the second end of the second channel, the valve is closed. When the droplet has a second dimension, the droplet fluidically isolates the second end of the first channel form the second end of the second channel, thereby opening the valve.

In accordance with a further aspect of the present invention, a method of fabricating an open microfluidic system is provided. The method includes the step of providing a microfluidic device including a reservoir defined by a surface configured to repel an aqueous solution. A hydrophilic input, a hydrophilic output and a hydrophilic strip interconnecting the input and output are patterned on the surface. The reservoir is filled with an oil. An input droplet of the aqueous solution is positioned on the input and an output droplet of the aqueous solution is positioned on the output. The input droplet and the output droplet are fluidically connected along the strip with the aqueous solution.

The step of fluidically connecting the input droplet and the output droplet along the strip includes the step of generating an external perturbation on the oil in the reservoir. The external perturbation on the oil may be generated by an anti-static gun Which repetitively pumps ionized air at the oil. The strip may include a valve, the valve having a first open configuration fluidically isolating the input from the output and a second closed configuration wherein the input and the output are in fluidic communication. The strip includes a first channel having a first end connected to the input and a second channel having a first end connected to the output. The valve is configured to selectively fluidically connect the first and second channels. The valve may include a second end of the first channel and a second end of the second channel. A dried reagent may fluidically interconnecting the second end of the first channel and the second end of the second channel. A fluid flowing over the dried reagent picks-up and re-dissolves the dried reagent therein so as to expose a portion of the surface between the first and second hydrophilic channels, thereby opening the valve.

Alternatively, the second end of the second channel may have a horseshoe configuration and the second end of the first channel may have a horseshoe configuration. The valve includes a droplet having a first dimension wherein the droplet communicates with the second end of the first channel and the second end of the second channel thereby closing the valve and a second dimension wherein the droplet fluidically isolates the second end of the first channel form the second end of the second channel thereby opening the valve.

In accordance with a still further aspect of the present invention, a single-use valve is provided. The valve includes a microfluidic device having a reservoir defined by a surface configured to repel an aqueous solution. The reservoir is configured for receiving oil therein. First and second hydrophilic channels are patterned on surface. The first and second hydrophilic channels spaced from each other. A dried reagent interconnects the first and second hydrophilic channels. Fluid flowing over the dried reagent picks-up and re-dissolves the dried reagent therein, thereby exposing a portion of the surface between the first and second hydrophilic channels and fluidically isolating the first hydrophilic channel from the second hydrophilic channel.

DETAILED DESCRIPTION OF THE DRAWINGS

ELR is a phenomenon observed in solid-liquid-liquid three phase systems, where a solid surface shows complete repellency to a liquid (with a contact angle (CA)=180°) when exposed to a second liquid. This phenomenon is observed when a particular thermodynamic boundary condition is satisfied, for example, by the equation:
γS/Lcp+γLdp/Lcp≤γS/LdpEquation (1)
wherein: γ is the interfacial tension; S is solid; Lcp is a liquid of continuous phase; and Ldp is a liquid of dispersed phase. ELR enables additional fluidic control, robust on-chip cell culture, and improved processing of rare cell samples in open aqueous fluid under oil.

In systems employing double-ELR, there is selective and complete repellency of two immiscible liquids from adjacent surfaces through surface chemistry patterning. Fluids are naturally contained on “preferred” surfaces of having a lower contact angle, which lowers free energy of the system. The boundary between surface patterns thus creates a virtual barrier (i.e., an energetic impediment) to fluid expansion from its footprint (i.e., the contact area between the fluid and its preferred surface). Double-ELR offers the theoretical maximum virtual barrier to both aqueous fluid and oil with a CA of 180° on their “non-preferred” surfaces (i.e., oil on glass, media on PDMS), and thus robustly confining the fluids to their preferred surfaces (i.e., oil on PDMS, media on glass). This virtual barrier is important to stabilize the three phase contact line. If fluid spreads from its original footprint, it is completely repelled by the non-preferred surface and recedes to the original pattern when the system is allowed to equilibrate. Double-ELR allows for spontaneous, uncompromised oil/media separation (without the need for surfactant), and thus, under oil sweep in open microfluidic designs. Under oil sweep is accomplished by simply dragging media across the patterned surface under oil, resulting in a specific volume of media (with or without cells) being spontaneously dispensed onto the patterned areas (i.e., microchannels or spots) and leaving the background clean with minimized sample loss and device fouling. As hereinafter described, by exploiting double-ELR and the technique of under oil sweep, open microchannels under oil are achieved with improved lateral resolution (for example, ˜30 μm in both width and spacing), low profiles (for example, ˜1 μm in height) capable of cell trapping, convective bulk flow covering eight orders of magnitude (for example, from 6 mL/min to 13 pL/min), and fully reversible fluidic valves.

Referring toFIGS.1-3, a schematic drawing depicting a system for carrying out a first aspect of the methodology of the present invention is generally designated by the reference numeral10. System10includes microfluidic device12defined by first and second generally parallel, spaced side walls14and16respectively, interconnected by first and second generally parallel, spaced end walls18and20, respectively. First and second side walls14and16respectively, and first and second end walls18and20respectively, define reservoir22for receiving a fluid, such as oil24or like, for reasons hereinafter described. It can be understood that oil24may be any liquid showing limited miscibility with water or any aqueous media. Upper edges14aand16aof first and second side walls14and16respectively, and upper edges18aand20aof end walls18and20, respectively, define an opening26for allowing access to reservoir22. Lower edges14band16bof first and second side walls14and16respectively, and lower edges18band20bof end walls18and20, respectively, are interconnected by an exclusive liquid repellency (ELR) surface28which communicates with reservoir22. ELR surface28is a hydrophobic solid surface having specific surface chemical and physical conditions intended to repel aqueous solutions, as hereinafter described. It can appreciated that while microfluidic device10has a generally rectangular, box-like configuration, other configurations are possible without deviating from the scope of the present invention.

In the depicted embodiment, surface patterning is provided on ELR surface28of open microfluidic system10of the present invention. For example, surface patterning of ELR surface28may be done on a PDMS-grafted glass substrate using a reusable PDMS stamp and O2 plasma diffusion treatment. More specifically, a hydrophilic input spot30is pre-patterned on ELR surface28of microfluidic device12at a selected first location and a hydrophilic output spot32is pre-patterned on ELR surface28of microfluidic device12at a selected second location, axially spaced from the first location. Input spot30has an outer periphery intersecting ELR surface28at boundary35. Similarly, output spot32has an outer periphery intersecting ELR surface28at boundary38. It is contemplated for output spot32to have a greater cross-sectional area than input spot30, for reasons hereinafter described. However, input spot30and output spot32may be other configurations without deviating from the scope of the present invention.

Input spot30and output spot32are interconnected by a hydrophilic strip40pre-patterned on and extending axially along ELR surface28. Strip40is defined by first and second, generally parallel edges42and44, respectively, which intersect ELR surface28. First and second edges42and44, respectively, have corresponding first ends42aand44a, respectively, which intersect boundary35of input spot32and corresponding second ends42band44b, respectively, which intersect boundary38of output spot32. The functionality of microfluidics leverages channel dimensions of tens to hundreds of microns. By way of example, strip40has width ranging from 10 to 200 micrometers (μm). It is further contemplated for the surface patterning on ELR surface28to have other configurations. For example, strip40may be wider at second ends42band44band narrower at first ends42aand44ato promote passive pumping of an aqueous media in system10, hereinafter described. In addition, it is contemplated to provide input and output spots30and32, respectively, at different locations along strip40, without deviating from the scope of the present invention.

In operation, reservoir22of microfluidic device12is filled with a selected fluid, such as oil24. After flooding the reservoir22with oil24, one of more injectors34are configured to deliver input droplet46of a desired aqueous media on input spot30and outlet droplet48of a desired aqueous media on output spot32. In order to generate flow between input spot30and output spot32along strip40, it can be understood that the aqueous media must displace a thin layer of oil24at the interface of oil24and strip40come into contact with strip40. To overcome the energy barrier for the displacement of oil24and facilitate the flow of the aqueous media along strip40, it is contemplated to provide an external perturbation on the interface of oil24and strip40, e.g., utilizing anti-static gun50. Other mechanisms such as an on-chip micro transducer, a surface acoustic wave generator, and/or paramagnetic beads moved with a magnet may be used to provide the external perturbation on the interface of oil24and strip40, without deviating scope of the present invention.

To generate the external perturbation on the interface of oil24and strip40, anti-static gun50is positioned a selected distance, e.g., 5 centimeters, above the upper surface of oil24. Anti-static gun50repetitively pumps streams of ionized air at oil24so as to provide alternate positive and negative charges on oil24. Oil24vibrates in response to the repetitive pumping of the ionized air by anti-static gun50, thus causing a momentum to be applied to the interface of oil24and strip40. This momentum helps the aqueous media overcome the energy barrier for the displacement of oil24and allow for the aqueous media to flow along strip40to interconnect input spot30and output spot32. The aqueous media along strip40defines a microchannel41having a height h, a width w and a length,FIG.3a.

In order to generate fluid flow from input spot30to output spot32, injector34, operatively connected to an aqueous media source37, may be used to deliver aqueous media to input droplet46such that the fluidic pressure in input droplet46urges the aqueous media along strip40toward output droplet48. Alternatively, because input droplet46has a smaller radius of curvature than output droplet48, a larger pressure exists on input spot30. The resulting pressure gradient causes aqueous media to flow from input droplet46, along strip40, towards output droplet48. It can be understood that by sequentially depositing additional drops of aqueous media to input droplet46on input spot30with injector34, the resulting pressure gradient will cause the aqueous media to flow along strip40towards output droplet48on output spot32. As a result, fluid flows along strip40from input droplet46to output droplet48. It can be understood that in addition to generating the flow of aqueous media in system10utilizing passive pumping, as heretofore described, other mechanisms (e.g., a syringe pump) may be used without deviating from the scope of the present invention.

It has been determined that microchannel41has a similar height/width (h/w) ratio (e.g., approximately 1:13), which is independent of the width of strip40. Hence, it can be appreciate that cellular samples flowing in microchannel41may be confined within a designated area simply by adjusting a dimension (i.e., either the width or the height) of portion43of microchannel41to be comparable or smaller than the objects being confined (e.g., a single cell). The small height/width ratio of the open microchannels means confinement of cellular samples can be achieved with microchannels having relatively large widths. In contrast to closed systems, the geometry and effect of surface tension in open systems result in a gradual rather than abrupt channel entrance. This produces a saddle-shaped geometry at the connection of microchannel41to input and output spots30and32, respectively,FIG.13. Due to the unique entrance geometry, cells entering microchannel41follow a contour line whose height is comparable to the size of a single cell, named an entrance plume. The entrance plume can affect the constraint of cellular samples in adjacent spots. If the length of a channel is less than two times the length of the entrance plume, then the height at the center of the channel is enough for cells to flow from input spot30to output spot32.

Critical to the function of microfluidic systems is the ability to have flow across multiple scales. To extend the function of open microfluidic systems, it is necessary that the flow rates at both the upper and lower limits (i.e., for maximum and minimum flow respectively) be expanded. Flow is important for many applications in biomedical research where control of mass transport (e.g., a constant drug delivery rate) and/or mechanical cues (e.g., stable shear force on cell-surface adhesion) is necessary. The volumetric flow rate (Q) of fluid in a channel may be calculated according to the expression:
ΔP=RhQ  Equation (2)
wherein: ΔP is the pressure drop and Rhis the hydrodynamic resistance.

Given the lack of physical walls in open microfluidic system10, robust confinement of fluid in open channels can be challenging, especially when high ΔP is applied. To increase the upper limit of flow rate in open-channel designs, a maximum virtual barrier to the fluid, such as that enabled by double-ELR, is desired. On the other end, to establish a low flow rate, a sufficiently high Rhis necessary to limit convective bulk flow. It can be understood that Rhcan be controlled by varying the dimensions of microchannel, either by making microchannel41longer or by reducing the cross sectional area thereof. More specifically, for a given resistance, the length of microchannel41must be increased exponentially to offset the change in the cross sectional area of microchannel41. Hence, to establish a low flow rate across a short distance (e.g., a few hundred microns, a typical distance in which capillary exchange occurs in vivo), a reduced cross sectional area is necessary. To maintain Δcritical, and thus a steady flow through microchannel41, a syringe pump may be utilized to add volume of aqueous media to inlet droplet46, while simultaneously removing the same volume of aqueous media from outlet droplet48.

Referring toFIG.4, it is contemplated for system10to include a dissolvable single-use smart “valve.” By way of example, strip40may be defined by first and second hydrophilic channels66aand66bpatterned on ELR surface28of microfluidic device12. With reservoir22dry and free of fluids, reagent60of interest in solution is deposited onto ELR surface28at a location64interconnecting first and second hydrophilic channels66aand66b, respectively. Reagent60is allowed to dry and physically adsorb onto surface28. Once reagent60is dried on surface28, reservoir22of microfluidic device12is filled with a selected fluid, such as oil24.

With reservoir22filled with oil, an aqueous solution of interest may be flowed from first hydrophilic channel66a, over reagent60at location64, to second hydrophilic channel66b, as heretofore described. It can be understood that as the aqueous solution of interest flows from first hydrophilic channel66a, over reagent60at location64, to second hydrophilic channel66b, the aqueous solution of interest picks-up and re-dissolves the desiccated reagent60therein so as to carry reagent60to second hydrophilic channel66b. Once all the available reagent60is dissolved, ELR surface28is returned back to a liquid repellent state, thus disconnecting first hydrophilic channel66afrom second hydrophilic channel66b. This arrangement allows for reagent60at location64to act as a valve, serving as both a reagent delivery device and an autonomous self-regulating timer that shuts off liquid flow once all reagent60is delivered to second hydrophilic channel66b.

Alternatively, it is contemplated to incorporate a reversible valve into system10. It can be appreciated that open channels present a unique challenge in the design of reversible valves due to the lack of physical walls whereby a mechanism capable of connecting, disconnecting and reconnecting fluid flow can be easily deployed.

Referring toFIGS.5-8, an alternate embodiment of a valve is generally designated by the reference numeral80. By way of example strip40may be defined by first and second hydrophilic channels82and84patterned on ELR surface28of microfluidic device12. First channel82extends along an axis and is defined by first and second, generally parallel edges86and88, respectively, which intersect ELR surface28. First and second edges86and88, respectively, have first ends which intersect boundary35of input spot30and corresponding second ends which intersect each other and define hydrophilic spot89at terminal end90of first channel82. Second channel84extends along an axis and is defined by first and second edges92and94, respectively, and concave edge95which intersect ELR surface28. More specifically, first and second edges92and94respectively, include first ends92aand94a, respectively, which define output end97of second channel84and intersect boundary38of output spot32. Parallel portions96and98of first and second edges92and94, respectively, extend from first ends92aand94aand intersect second ends92band94bof first and second edges92and94, respectively. Second ends92band94bof first and second edges92and94, respectively, diverge from each other and extend on opposite sides of terminal end90of first channel82. Second ends92band94bof first and second edges92and94respectively, are interconnected by concave edge95which extends about terminal end90of first channel82. Second ends92band94bof first and second edges92and94, respectively, and concave edge95define a generally horseshoe-shaped input end99of second channel84.

In operation, reservoir22of microfluidic device12is filled with a selected fluid, such as oil24. One of more injectors34are configured to deliver input droplet100of a desired aqueous media on input spot30, outlet droplet102of a desired aqueous media on output spot32, and bridge droplet104on spot89at terminal end90of first channel82. It is intended for bridge droplet104to be of sufficient dimension to overlap and communicate with input end99of second channel84so as to fluidically connected first and second channels82and84, respectively,FIGS.5-6. It is intended for bridge droplet104to fluidically connect first and second channels82and84, respectively, thereby closing valve80and allowing for fluid flow therebetween.

In order to generate flow between input spot30and output spot32along strip40, it is contemplated to provide an external perturbation on the interface of oil24and strip40utilizing anti-static gun50. As heretofore described, oil24vibrates in response to the repetitive pumping of the ionized air by anti-static gun50, thus causing a momentum to be applied to the interface of oil24and strip40. This momentum helps the aqueous media overcome the energy barrier for the displacement of oil24and allow for the aqueous media to flow along strip40to fluidically connect input droplet100on input spot30to bridge droplet104and to fluidically connect bridge droplet104and output droplet102on output spot32. Because input droplet100has a smaller radius of curvature than bridge droplet104, a larger pressure exists on input spot30. The resulting pressure gradient causes aqueous media to flow from input droplet100, along first channel82, towards bridge droplet104. Similarly, because bridge droplet104has a smaller radius of curvature than output droplet102, a larger pressure exists on spot89. The resulting pressure gradient causes aqueous media to flow from bridge droplet104along second channel84, towards output droplet102. It can be understood that by sequentially depositing additional drops101of aqueous media to input droplet100on input spot30with injector34, the resulting pressure gradient will cause the aqueous media to flow along first channel82, through bridge droplet104, along second channel84towards output droplet102on output spot32. As a result, fluid flows along strip40from input droplet100to output droplet102,FIG.6.

By terminating the depositing of drops101of aqueous media, it can be understood that the volume of aqueous media flowing in bridge droplet104decreases, while the aqueous media continues to output droplet102, thereby reducing the dimension of bridge droplet104,FIGS.7-8. More specifically, as the dimension abridge droplet104is reduced, the volume in bridge droplet104reaches a critical point (i.e., the minimum amount of liquid required to maintain the fluidic connection between first and second channels82and84, respectively (hereinafter referred to as “Vcritical”). Once bridge droplet104reaches Vcritical, termination end90of first channel82becomes isolated from input end99of second channel84, thereby opening or disconnecting valve80and terminating the fluid flow between input spot30and output spot32along strip40. Reinstitution of fluid flow between input spot30and output spot32along strip40may be achieved by simply adding aqueous media to bridge droplet104to reestablish a fluidic connection between first and second channels82and84, respectively, and sequentially depositing additional drops101of aqueous media on input droplet100.

It can be understood that Vcriticalis dependent upon the geometry and the size of the portion of ELR surface28between termination end90of first channel82and input end99of second channel84(hereinafter referred to as the ELR gap). A larger ELR gap requires a larger Vcriticalto maintain the connection between first and second channels82and84, respectively. The connection time (Δtconnection) of valve80is defined as the time to reduce the initial volume of bridge droplet104(Vinitial) to Vcritical. It can be appreciated that a larger Vcritical(e.g., a valve with a larger ELR gap) results in a shortened Δtconnectionfor a given Q.

It is noted that fluid flow through valve80may be reversed by providing outlet droplet102with a smaller radius of curvature than bridge droplet104and by providing bridge droplet104with a smaller radius of curvature than input droplet100. The resulting pressure gradient causes aqueous media to flow from output droplet104, along second channel84, towards bridge droplet104. Similarly, because bridge droplet104has a smaller radius of curvature than input droplet100, the resulting pressure gradient causes aqueous media to flow from bridge droplet104, along first channel82, towards input droplet100. It can be understood that by sequentially depositing additional drops101of aqueous media to outlet droplet102, the resulting pressure gradient will cause the aqueous media to flow along second channel84, through bridge droplet104, along first channel82towards input droplet100. As a result, fluid flows along strip40from output droplet102to input droplet100.

Referring toFIGS.9-12, a still further embodiment of a valve is generally designated by the reference numeral110. By way of example, strip40may be defined by first and second hydrophilic channels1112and114patterned on ELR surface28of microfluidic device12. First channel112extends along an axis and is defined by first and second edges116and118, respectively, and concave edge120which intersect ELR surface28. More specifically, first and second edges116and118, respectively, include first ends116aand118a, respectively, which define input end122of first channel112and intersect boundary35of input spot30. Parallel portions124and126of first and second edges116and118, respectively, extend from first ends116aand118aand intersect second ends116band118bthereof. Second ends116band118bof first and second edges116and118, respectively, diverge from each other. Second ends116band118bof first and second edges116and118, respectively, are interconnected by concave edge120. Second ends116band118bof first and second edges116and118, respectively, and concave edge120define a generally horseshoe-shaped output end128of first channel112.

Second channel114extends along an axis and is defined by first and second edges132and134, respectively, and concave edge136which intersect ELR surface28. More specifically, first and second edges132and134, respectively, include first ends132aand134a, respectively, which define output end138of second channel114and intersect boundary38of output spot32. Parallel portions140and142of first and second edges132and134, respectively, extend from first ends132aand134aand intersect second ends1132band134bof first and second edges1132and134, respectively. Second ends132band134bof first and second edges132and134, respectively, diverge from each other. Second ends132band134bof first and second edges132and134, respectively, are interconnected by concave edge136. Second ends132band134bof first and second edges132and134, respectively, and concave edge136define a generally horseshoe-shaped input end144of second channel114. As hereinafter described, it can be appreciated that output end128of first channel112and input end144of second channel114may be utilized as valve110to terminate fluid flow from input spot30and output spot32along strip40.

In operation, reservoir22of microfluidic device12is filled with a selected fluid, such as oil24. One of more injectors34are configured to deliver input droplet150of a desired aqueous media on input spot30, outlet droplet152of a desired aqueous media on output spot32, and bridge droplet154so as to overlap output end128of first channel112and input end144of second channel114. It is intended for bridge droplet154to fluidically connect first and second channels112and114, respectively, thereby closing valve110and allowing for fluid flow therebetween.

In order to generate flow between input spot30and output spot32along strip40, it is contemplated to provide an external perturbation on the interface of oil24and strip40utilizing anti-static gun50. As heretofore described, oil24vibrates in response to the repetitive pumping of the ionized air by anti-static gun50, thus causing a momentum to be applied to the interface of oil24and strip40. This momentum helps the aqueous media overcome the energy barrier for the displacement of oil24and allows for the aqueous media to flow along strip40to fluidically connect input droplet150on input spot30to bridge droplet154and to fluidically connect bridge droplet154and output droplet152on output spot32. Because input droplet150has a smaller radius of curvature than bridge droplet154, a larger pressure exists on input spot30. The resulting pressure gradient causes aqueous media to flow from input droplet150, along first channel112, towards bridge droplet154. Similarly, bridge droplet154has a smaller radius of curvature than output droplet152, a larger pressure exists on input end144of second channel114. The resulting pressure gradient causes aqueous media to flow from bridge droplet104, along second channel114, towards output droplet152. It can be understood that by sequentially depositing additional drops155of aqueous media to input droplet150on input spot30with injector34, the resulting pressure gradient will cause the aqueous media to flow along first channel112, through bridge droplet154, along second channel114towards output droplet152on output spot32. As a result, fluid flows along strip40from input droplet150to output droplet152,FIG.10.

As heretofore described with respect to valve80, by terminating the depositing of drops155of aqueous media, it can be understood that the volume of aqueous media in bridge droplet154decreases thereby reducing the dimension thereof,FIGS.11-12. As the dimension of bridge droplet154is reduced to Vcritical, output end128of first channel112becomes isolated from input end144of second channel114, thereby opening valve110and terminating the fluid flow between input spot30and output spot32along strip40. Reinstitution of fluid flow between first and second channels112and114, respectively, along strip40may be achieved by simply adding aqueous media to bridge droplet154to reestablish a fluidic connection between first and second channels112and114, respectively, and sequentially depositing additional drops155of aqueous media on input droplet150. Hence, by simply adding aqueous media to or removing aqueous media from bridge droplet154, connection or disconnection of valve110can be achieved reversibly.

Once again, it is noted that Vcriticalis dependent upon the geometry and the size of the portion of ELR surface28between output end128of first channel112and input end144of second channel114(hereinafter referred to as the ELR gap). A larger ELR gap requires a larger Vcriticalto maintain the connection between first and second channels112and114, respectively. The connection time (Δtconnection) of valve1110is defined as the time to reduce the initial volume of bridge droplet154(Vinitial) to Vcritical. It can be appreciated that a larger Vcritical(e.g., a valve with a larger ELR gap) results in a shortened for a given Q.

It is noted that fluid flow through valve110may be reversed by providing outlet droplet152with a smaller radius of curvature than bridge droplet154and by providing bridge droplet1104with a smaller radius of curvature than input droplet150. The resulting pressure gradient causes aqueous media to flow from output droplet154, along second channel114, towards bridge droplet154. Similarly, because bridge droplet154has a smaller radius of curvature than input droplet150, the resulting pressure gradient causes aqueous media to flow from bridge droplet154, along first channel112, towards input droplet150. It can be understood that by sequentially depositing additional drops155of aqueous media to outlet droplet152, the resulting pressure gradient will cause the aqueous media to flow along second channel114, through bridge droplet154, along first channel112towards input droplet150. As a result, fluid flows along strip40from output droplet152to input droplet150.

Referring toFIGS.14-15, it is contemplated for system10to include surface patterning provided on ELR surface28to allow for the generation of a gradient of particles between a first source region230to a second sink region232over a predetermined time period (the “gradient development period”). More specifically, in the depicted embodiment, hydrophilic source region230is pre-patterned on ELR surface28of microfluidic device12at a selected first location and a hydrophilic sink region232is pre-patterned on ELR surface28of microfluidic device12at a selected second location, axially spaced from the first location. Source region230has an outer periphery intersecting ELR surface28at boundary235. Similarly, sink region232has an outer periphery intersecting ELR surface28at boundary238. It is contemplated for sink region232to have a cross-sectional area generally equal to a cross-sectional area of source region230. However, source region230and sink region232may have other configurations without deviating from the scope of the present invention.

Source region230and sink region232are interconnected by a hydrophilic strip240pre-patterned on and extending axially along ELR surface28. Strip240is defined by first and second, generally parallel edges242and244, respectively, which intersect ELR surface28. First and second edges242and244, respectively, have corresponding first ends242aand244a, respectively, which intersect boundary235of source region230and corresponding second ends242band244b, respectively, which intersect boundary238of sink region32. It is further contemplated for the surface patterning on ELR surface28to have other configurations. For example, strip240may be wider at second ends242band244band narrower at first ends242aand244ato promote passive pumping of an aqueous media in system10, as heretofore described. In addition, it is contemplated to provide source region230and sink region232, respectively, at different locations along strip240, without deviating from the scope of the present invention.

In operation, reservoir22of microfluidic device12is filled with a selected fluid, such as oil24. After flooding the reservoir22with oil24, one of more injectors34,FIG.1, are configured to deliver first input droplet (no shown) of a desired aqueous media on source region230and outlet droplet248of a desired aqueous media on sink region232. Aqueous media is flowed along strip240, as heretofore described, to interconnect source region230and sink region232, thereby defining microchannel241along strip240having a height h, a width w and a length.

In order to generate a gradient of particles across microchannel241between source region230and sink region232, the convective flow of fluid in microchannel241must be minimized. It can be appreciated that a strong convective flow through microchannel241will transport the particles through, which reduces the ability to establish a gradient in microchannel241over an extended period of time. As such, to generate a gradient, it is necessary to minimize the pressure differential between the inlet of microchannel241, microchannel241, and the outlet of microchannel241. Fox example, to minimize the pressure differential between the inlet of microchannel241, microchannel241, and the outlet of microchannel241, the radii of the curvature of the droplets at the inlet of microchannel24and the outlet of microchannel must be maintained as close as possible. The combination of the size of source region230and sink region232patterned on ELR surface28and the volume of the droplets provided on source region230and sink region232, as hereinafter described, determine the local radius of curvature of the droplets. Consequently, it can be understood that size of source region230and the size of sink region232do not have to be the same, if the volumes of the droplet provided on source region230and sink region232are different.

In the depicted embodiment, one of more injectors34are configured to deliver second input droplet252of a desired aqueous media having a known concentration of particles, such as cells, molecules, chemical species, organisms or the like, on source region230. The particles in second input droplet252on source region230diffuse into microchannel241such that after a predetermined time period, a concentration gradient is created along the length of microchannel241. It can be understood that an ideal source/sink setup may be achieved by providing source and sink regions230and232, respectively, with volumes that are significantly larger that the volume of microchannel241. The large volume sink region232at the output end of microchannel241can help maintain the concentration gradient by not allowing the particles to accumulate in microchannel241. As such, the source/sink concept heretofore described may be used to create a pseudo-steady state in microchannel241wherein the concentration at a point within microchannel241does not vary dramatically with time.

Alternatively, it can be appreciated an ideal source/sink setup may be constructed by maintaining by a constant concentration of particles in microchannel241by providing an infinite source of particles at source region230and by providing a sink region232of infinite size. This may be accomplished by providing a steady flow through microchannel241utilizing a syringe pump to add volume to second inlet droplet46, while simultaneously removing the same volume of outlet droplet248.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter that is regarded as the invention.