Capillary junction

Abstract

A microfluidic device with reduced risk of bubble formation at a capillary junction between two conduits is provided. In some embodiments, the microfluidic device comprises a supply reservoir, a first conduit and a second conduit. The first conduit is configured such that liquid flows by capillary effect from the supply reservoir into the first conduit. The second conduit is connected to the first conduit through an opening in a wall of the first conduit and is configured such that liquid flows from the first conduit to the second conduit by capillary effect. A width of the second conduit, along a direction of liquid flow in the first conduit, is greater than a depth of the second conduit and a depth of the second conduit is less than a depth of the first conduit.

Claims

1. A microfluidic device comprising: a supply reservoir; a first vented conduit in fluidic communication with the supply reservoir such that liquid flows by capillary effect from the supply reservoir into the first conduit; and a second unvented conduit connected to the first vented conduit through an opening in a wall of the first vented conduit and configured such that liquid flows from the first vented conduit to the second unvented conduit by capillary effect; wherein a width of the second unvented conduit, along a direction of liquid flow in the first vented conduit, is greater than a depth of the second unvented conduit; and wherein the depth of the second unvented conduit is less than a depth of the first vented conduit.

2. A device as claimed in claim 1, further comprising: a first vent structure comprising an air channel connecting the first vented conduit to the supply reservoir, the first vent structure enabling gas in the first vented conduit displaced by flow of liquid from the supply reservoir into the first vented conduit to escape from the first vented conduit and into the supply reservoir.

3. A device as claimed in claim 1, wherein a ratio of a depth of the first vented conduit upstream of a junction between the first vented conduit and second unvented conduit to the depth of the second unvented conduit is greater than or equal to 3.

4. A device as claimed in claim 1, wherein the first vented conduit comprises a surface tension barrier downstream of a junction between the first vented conduit and second unvented conduit.

5. A device as claimed in claim 1, wherein a portion of the first vented conduit downstream of a junction between the first vented conduit and second unvented conduit has a transverse dimension greater than a depth of the first vented conduit upstream of the junction between the first vented conduit and second unvented conduit.

6. A device as claimed in claim 5, wherein a ratio of the depth of the first vented conduit upstream of the junction between the first vented conduit and second unvented conduit to a depth of the aforesaid portion of the first vented conduit downstream of the junction between the first vented conduit and second unvented conduit is less than or equal to 0.75.

7. A device as claimed in claim 1, wherein a portion of the first vented conduit downstream of a junction between the first vented conduit and second unvented conduit has a depth greater than a depth of the first vented conduit upstream of the junction between the first vented conduit and second unvented conduit.

8. A device as claimed in claim 1, wherein a depth decrease from the first vented conduit to the second unvented conduit is configured as a step.

9. A device as claimed in claim 1, wherein a ratio of the width of the second unvented conduit to the depth of the second unvented conduit is greater than or equal to 42.5.

10. A device as claimed in claim 1, wherein at least one of the first vented conduit and second unvented conduit contains a dry reagent.

11. A device as claimed in claim 1, wherein the second unvented conduit is configured as a detection conduit to allow liquid to be imaged as it flows through the second unvented conduit.

12. A device as claimed in claim 11, wherein a wall of the second unvented conduit is substantially transparent.

13. A device as claimed in claim 1, wherein the second unvented conduit is substantially perpendicular to the first vented conduit.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The following description of specific embodiments is made by way of example and illustration and not limitation, with reference to the drawings in which:

(2) FIG. 1A illustrates schematically a microfluidic device in a first cross-section; and

(3) FIG. 1B illustrates schematically a second cross-section of the microfluidic device, perpendicular to that in FIG. 1A; and

(4) FIGS. 2A-C illustrate schematically the operation of the microfluidic device.

DETAILED DESCRIPTION

(5) With reference to FIGS. 1A and 1B, a microfluidic device 2, in some embodiments a disc-shaped device arranged to drive liquid flow by rotating the disc, in addition to the capillary flows described below, comprises a supply reservoir 4 with an inlet 6, the location of which is illustrated with a dashed line. The supply reservoir 4 is in fluidic communication with a first conduit 8 with a depth of 200 m and a width of 1 mm defined by a first wall 10 and a second wall 12. A first air flow structure illustrated schematically by an arrow 14 connects the first conduit 8 to the supply reservoir 4 such that gas in the first conduit which is displaced by flow of liquid from the supply reservoir 4 into the first conduit 8 can escape from the first conduit 8 and into the supply reservoir.

(6) For the avoidance of doubt, the depths of the various microfluidic structures including the first and second conduits, as referred to herein, are measured perpendicular to a plane containing the first conduit 8 and a second conduit 16 (described below). The widths of the first and second conduits 8, 16, are measured in the plane, perpendicular to the direction of flow of liquid in the respective conduit.

(7) The first conduit 8 is also in fluidic communication with a second conduit 16 which has a depth of 20 m and a width of 850 m defined by a first wall 18 and a second wall 20. A second air flow structure is illustrated schematically by an arrow 22 and connects the second conduit 16 to the supply reservoir 4 such that gas in the second conduit which is displaced by flow of liquid from the first conduit 8 into the second conduit 16 can escape from the second conduit 16 and into the supply reservoir 4.

(8) The second conduit 16 connects to the first conduit 8 through an opening 24 in the wall 10 and extends perpendicularly from the wall 10. The wall 10 in the region of the opening 24 forms a step change in flow depth between the first and second conduits 8, 16. In some embodiments, the change may be more gradual and may comprise a series of steps or a sloping portion.

(9) The second conduit 16 is thus connected to the first conduit 8 through the wall 10 of the first conduit 8, extending from the side of the first conduit. Due to the difference in depth, liquid in the first conduit flows into the second conduit by capillary effect.

(10) The first conduit 8 comprises a first portion 26 and a second portion 28. The second portion 28 of the first conduit 8 is downstream of the opening 24 and has a depth of 300 m. At the boundary between the first portion 26 and the second portion 28 of the first conduit, the depth of the conduit changes in one step, forming a surface tension barrier to flow. Liquid is not able to flow past the surface tension barrier, under the action of capillary forces, but air is able to move past the barrier and is vented out of the first conduit 8 via the first vent structure, as indicated with arrow 14. The surface tension barrier therefore also helps to reduce the risk of the air flow structure 14 becoming clogged with liquid.

(11) In some embodiments, instead of being located downstream of the opening 24, the step 16 is located at the downstream end of the opening 24.

(12) Liquid flows within the microfluidic device will now be described with reference to FIGS. 2A-C.

(13) Initially, a liquid 30 is introduced into the device via the inlet 6. This may be done by a user with a pipette or capillary tube, for example. The liquid may be a blood sample, but in other embodiments may be serum or aqueous solutions. Liquid enters the supply reservoir 4 via the inlet 6 and subsequently flows into the first conduit 8.

(14) Once liquid has entered the first conduit 8, it flows along the first conduit 8 until it reaches the opening 24, as illustrated in FIG. 2A. Since the second conduit 16 extends off from a side of the first conduit 8, a liquid front 32 of the liquid 30 travelling along the first conduit 8 first encounters the second conduit 16 in a well-defined location at the upstream corner of the opening 24 between the walls 10, 18. As illustrated in FIG. 2B, as the liquid front 32 advances in the first conduit 8, the surface tension of the liquid encourages the integrity of the liquid front 32 and the advance of the liquid front 32 in the first conduit 8 causes the second conduit 16 to be filled from the wall 18 to the wall 20 in a controlled manner as the liquid front 32 is drawn across the opening 24 by its advance in the first conduit 8. While the liquid front 32 is drawn across the opening 24, a volume of air 34 in the second conduit 16 between the liquid 30 and the opening 24 remains connected to the air flow structure 14 through the first conduit 8, thereby venting any bubble that may otherwise form in the second conduit 16 through the first conduit 8.

(15) Referring now to FIG. 2C, liquid proceeds to fill the entire width of the second conduit 16 and flows along it. Liquid 30 also continues to flow in the first conduit 8 after the integrity of the liquid front 32 (due to surface tension) has forced all, or at least some, of any air present in the second conduit 16 and the opening 24 to vent via the air flow structure 14. As the liquid continues to flow along the first conduit 8 it reaches the step 16, acting as a barrier to capillary flow to stop liquid flowing into the portion 28 of the first conduit. In this way, the majority of the volume of liquid that is transferred from the supply reservoir 4 to the first conduit 8 is directed into the second conduit 16, rather than continuing down the first conduit 8.

(16) In some embodiments, the second conduit 16 is configured as a detection conduit. An upper surface of the second conduit 16 is transparent so as to allow an imaging device to capture one or more images of the liquid as it flows through the second conduit 16.

(17) In some embodiments, one or both of the first conduit and the second conduit may contain one or more dry reagents. Such reagents may include a haemolysing agent for selective lysis of erythrocytes in a blood sample flowing through the conduit and/or a staining agent for selectively staining leukocytes in such a blood sample.

(18) One or both of the first conduit and the second conduit may, in some embodiments, contain a surfactant in dry form and/or a stabiliser agent in dry form.

(19) While specific embodiments have been described above for illustration, many variations, alterations and juxtapositions of the specific features in addition to those specifically mentioned above, will occur to the skilled reader upon reading the description and are within the scope of the present disclosure and of the appended claims. For example, capillary flow of a liquid through a conduit may be enhanced or facilitated by lining the conduit with hydrophilic material or by constructing the conduit out of hydrophilic material. Likewise, the surface tension barrier can be implemented by hydrophobic surface modification rather than by virtue of the shape of the conduit.

(20) Other configurations with regards to the venting of the first and second conduits 8, 16 are also possible. In the embodiment described above, with reference to FIGS. 1A and 1B and also 2A-C, the device 2 comprises first air flow structure illustrated schematically by an arrow 14, which connects the first conduit 8 to the supply reservoir 4 such that gas in the first conduit which is displaced by flow of liquid from the supply reservoir 4 into the first conduit 8 can escape from the first conduit 8 and into the supply reservoir.

(21) Venting the first conduit may be advantageous in that it enables gas displaced by flow of liquid from the supply reservoir 4 into the first conduit 8 to escape the first conduit 8, instead of becoming trapped in the first conduit, in particular in the region of the junction between the first 8 and second 16 conduits. By avoiding, or at least minimising the risk of, trapped gas at the junction between the two conduits 8, 16, there is a reduced risk of liquid entering the second conduit 16 without filling the second conduit across its entire width at the opening 24, thus reducing the effective cross-section of the second conduit.

(22) In some embodiments, the first conduit 8 is not vented. In this case, as liquid flows from the supply reservoir 4 into the first conduit 8, gas present in the first conduit 8 is pushed along it and as the liquid flows, the gas pressure at the end of the first conduit increases. It will be appreciated that there will come a point at which the capillary forces acting on the liquid, drawing it along the first conduit, will be balanced by the gas pressure at the end of the first conduit. Accordingly, the liquid front in the first conduit will reach an equilibrium position. Provided that this equilibrium position is downstream of the opening, the second conduit will fill with liquid across its full width and liquid will flow from the first conduit 8 and into the second conduit 16 freely. In the absence of a vent for the first conduit, the first conduit is, in some embodiments, configured such that its volume downstream of the opening 24 is sufficiently large so as to contain the entire volume of trapped air at the point when capillary and pressure forces balance. Equally, a chamber or other microfluidic structure may be provided at the end of the first conduit in order to contain the required volume of air.

(23) Similarly, the second conduit, in some embodiments, is not vented. In this case, liquid will flow along the second conduit and as it does so, the pressure of the volume of air at the end of the second conduit will increase. Eventually, capillary forces acting on the liquid in the second conduit will balanced by the pressure of this trapped gas and flow of liquid along the second conduit will stop. As will be appreciated, this is not necessarily problematic if liquid is able to flow as far down the second conduit as is required for the application in question.