Three-dimensional microfluidics incorporating passive fluid control structures

• US 20040109793A1
• Filed: 08/07/2003
• Published: 06/10/2004
• Est. Priority Date: 02/07/2002
• Status: Abandoned Application

• Current Assignee: BioMicro Systems, Inc. (Danaher Corporation)
• Original Assignee: BioMicro Systems, Inc. (Danaher Corporation)
• Inventors: McNeely, Michael R, Adey, Nils, Spute, Mark
• Application Number:
  US10/467,585
• Publication Number:
  US 20040109793A1
• Time in Patent Office: Days
• US Class Current: 422/100
• CPC Class Codes: B01F 25/43211 using a simple by-pass for separating and recombining the flow, e.g. by using branches of different length
• CPC Class Codes: B01F 25/4323 using elements provided with a plurality of channels or using a plurality of tubes which can either be placed between common spaces or collectors
• CPC Class Codes: B01F 25/433 Mixing tubes wherein the shape of the tube influences the mixing, e.g. mixing tubes with varying cross-section or provided with inwardly extending profiles
• CPC Class Codes: B01F 25/4331 Mixers with bended, curved, coiled, wounded mixing tubes or comprising elements for bending the flow
• CPC Class Codes: B01F 31/31 using receptacles with deformable parts, e.g. membranes, to which a motion is imparted
• CPC Class Codes: B01F 33/30 Micromixers
• CPC Class Codes: B01J 19/0093 Microreactors, e.g. miniaturised or microfabricated reactors laboratory containers with capillary fluid transport in microfabricated channels or chambers B01L3/5027
• CPC Class Codes: B01J 2219/00783 Laminate assemblies, i.e. the reactor comprising a stack of plates
• CPC Class Codes: B01J 2219/00804 Plurality of plates
• CPC Class Codes: B01J 2219/00837 comprising coatings other than catalytically active coatings
• CPC Class Codes: B01J 2219/00873 Heat exchange
• CPC Class Codes: B01J 2219/00889 Mixing
• CPC Class Codes: B01L 2200/025 Align devices or objects to ensure defined positions relative to each other
• CPC Class Codes: B01L 2200/0621 Control of the sequence of chambers filled or emptied
• CPC Class Codes: B01L 2200/10 Integrating sample preparation and analysis in single entity, e.g. lab-on-a-chip concept
• CPC Class Codes: B01L 2300/0861 Configuration of multiple channels and/or chambers in a single devices
• CPC Class Codes: B01L 2300/0874 Three dimensional network
• CPC Class Codes: B01L 2300/0887 Laminated structure
• CPC Class Codes: B01L 2400/0406 capillary forces
• CPC Class Codes: B01L 2400/0655 pinch valves
• CPC Class Codes: B01L 2400/0688 : surface tension valves, capillary stop, capillary break
• CPC Class Codes: B01L 3/5027 : by integrated microfluidic structures, i.e. dimensions of channels and chambers are such that surface tension forces are important, e.g. lab-on-a-chip B01L3/5023 takes precedence; micromixers B01F33/30; microreactors for synthesis B01J19/0093; microcapillary devices in general B81B1/00
• CPC Class Codes: B01L 3/502715 : characterised by interfacing components, e.g. fluidic, electrical, optical or mechanical interfaces
• CPC Class Codes: B01L 7/52 : with provision for submitting samples to a predetermined sequence of different temperatures, e.g. for treating nucleic acid samples amplification or hybridisation processes per se C12Q1/68; controlling sequential reactions for synthesis B01J19/0046
• CPC Class Codes: B81B 2201/058 : Microfluidics not provided for in B81B2201/051 - B81B2201/054
• CPC Class Codes: B81C 1/00119 : Arrangement of basic structures like cavities or channels, e.g. suitable for microfluidic systems
• CPC Class Codes: B81C 2201/019 : Bonding or gluing multiple substrate layers
• CPC Class Codes: F28F 2260/02 : having microchannels

Abstract

A three-dimensional microfluidic device (100) formed from a plurality of substantially planar layers (101, 102, 103) sealed together is disclosed

Claims

1. A multi-layered microfluidic device comprising:

a. a plurality of substantially planar layers assembled together in sealing relationship;

b. microfluidic structures lying in at least two planes corresponding to at least two said planar layers of said microfluidic device; and

c. at least one microfluidic structure passing through one or more adjacent planar layers and providing fluid communication between microfluidic structures in different planes;

wherein said microfluidic structures comprise one or more channels, wells, dividers, mixers, valves, air ducts, or air vents; and

wherein at least one of said plurality of planar layers has a hydrophobic surface.

2. The microfluidic device of claim 1, further comprising at least one active element selected from the group consisting of heating elements, electrodes, sensors, mixing elements, and active valves.

3. The microfluidic device of claim 2, wherein said active element comprises a mixing element selected from the group consisting of piezoelectric transducers, pneumatically actuated, operated bladders, and hydraulically actuated bladders.

4. The microfluidic device of claim 2, wherein said active element comprises a sensor selected from the group consisting of optical sensors, pressure transducers, flow transducers, and temperature sensors.

5. The microfluidic device of claim 1, wherein said at least one layer is formed of a hydrophobic material.

6. The microfluidic device of claim 1, wherein said at least one layer is formed of a non-hydrophobic base material, and said hydrophobic surface is formed by a hydrophobic coating on said non-hydrophobic base material.

7. The microfluidic device of claim 1, wherein said layers are aligned in an alignment frame prior to being assembled together.

8. The microfluidic device of claim 1, each said layer comprises at least two alignment holes formed there through, and wherein said layers are aligned by rods passing through said alignment holes.

9. The microfluidic device of claim 1, wherein at least two of said layers are assembled together in sealing relationship by being clamped together.

10. The microfluidic device of claim 9, wherein a fluid-tight seal between said at least two layers is obtained by providing a compressible gasket layer between non-compressible layers.

11. The microfluidic device of claim 9, wherein a fluid-tight seal between said at least two layers is obtained by providing hydrophobic surfaces at the interface between said two layers.

12. The microfluidic device of claim 1, wherein at least two of said layers are assembled together in sealing relationship with an adhesive.

13. The microfluidic device of claim 12, wherein said adhesive is releasable from at least one of said at least two layers.

14. The microfluidic device of claim 1, wherein the seal between at least two of said layers can be released to allow said at least two layers to be separated.

15. The microfluidic device of claim 14, wherein said device can be separated between said at least two layers into a disposable portion and a reusable portion.

16. The microfluidic device of claim 15, wherein one of said layers is a glass slide.

17. The microfluidic device of claim 15, wherein one of said layers is a microtiter plate.

18. The microfluidic device of claim 15, wherein at least one of said layers comprises at least one region having biomolecules immobilized thereon.

19. The microfluidic device of claim 1, wherein said planar layers are formed of hydrophobic base material.

20. The microfluidic device of claim 1, wherein at least one said valves is a passive valve.

21. The microfluidic device of claim 1, wherein at least one said valves is a remote valve.

22. The microfluidic device of claim 1, wherein microfluidic structures in one said plane are formed through the entire thickness of at least one layer, so that the boundaries of the microfluidic structures are formed by said at least one layer, and upper and lower surface are formed by adjacent layers.

23. The microfluidic device of claim 1, wherein at least a portion of the microfluidic structures in one said plane are formed in a surface of at least one said layer but do not pass through the entire thickness of the layer.

24. A multi-layered microfluidic device comprising:

a. a plurality of substantially planar layers assembled together in sealing relationship;

b. microfluidic structures lying in at least two planes corresponding to at least two said planar layers of said microfluidic device; and

c. at least one microfluidic structure passing through one or more adjacent planar layers and providing fluid communication between microfluidic structures in different planes;

wherein at least a portion of said microfluidic structures lying in said at least two planes are formed in a surface of at least one said layer but do not pass through the entire thickness of said layer, and wherein said microfluidic structures in said at least two planes and passing through one or more planar layers comprise at least one passive valve and at least one additional microfluidic structure selected from the group consisting of channels, wells, dividers, mixers, valves, air ducts, and air vents.

25. The microfluidic device of claim 24, further comprising at least one active element selected from the group consisting of heating elements, electrodes, sensors, mixing elements, and active valves.

26. The microfluidic device of claim 25, wherein said active element comprises a mixing element selected from the group consisting of piezoelectric transducers, pneumatically actuated, operated bladders, and hydraulically actuated bladders.

27. The microfluidic device of claim 25, wherein said active element comprises a sensor selected from the group consisting of optical sensors, pressure transducers, flow transducers, and temperature sensors.

28. The microfluidic device of claim 24, wherein at least one layer has a hydrophobic surface.

29. The microfluidic device of claim 28, wherein said at least one layer is formed of a hydrophobic material.

30. The microfluidic device of claim 28, wherein said at least one layer is formed of a non-hydrophobic base material, and said hydrophobic surface is formed by a hydrophobic coating on said non-hydrophobic base material.

31. The microfluidic device of claim 24, wherein at least two of said layers are assembled together in sealing relationship by being clamped together.

32. The microfluidic device of claim 31, wherein a fluid-tight seal between said at least two layers is obtained by providing a compressible gasket layer between non-compressible layers.

33. The microfluidic device of claim 31, wherein a fluid-tight seal between said at least two layers is obtained by providing hydrophobic surfaces at the interface between said two layers.

34. The microfluidic device of claim 24, wherein at least two of said layers are assembled together in sealing relationship with an adhesive.

35. The microfluidic device of claim 34, wherein said adhesive is releasable from at least one of said at least two layers.

36. The microfluidic device of claim 24, wherein the seal between at least two of said layers can be released to allow said at least two layers to be separated.

37. The microfluidic device of claim 36, wherein said device can be separated between said at least two layers into a disposable portion and a reusable portion.

38. The microfluidic device of claim 36, wherein one of said layers is a glass slide.

39. The microfluidic device of claim 36, wherein one of said layers is a microtiter plate.

40. The microfluidic device of claim 36, wherein at least one of said layers comprises at least one region having biomolecules immobilized thereon.

41. The microfluidic device of claim 24, wherein at least one of said valves is a remote valve.

42. The microfluidic device of claim 24, wherein microfluidic structures in at least one said plane are formed through the entire thickness of at least one layer, so that the boundaries of the microfluidic structures are formed by said at least one layer, and upper and lower surface are formed by adjacent layers.

43. A multi-layer microfluidic device for performing a biochemical reaction including a heating step, comprising:

a. a plurality of substantially planar layers assembled together;

b. at least one sample inlet formed in at least one said layer;

c. at least one thermal reaction well in fluid communication with said sample inlet;

d. at least one read well in fluid communication with said thermal reaction well;

and

e. at least one active valve located between said thermal reaction well and said read well to control flow of fluid between said thermal reaction well and said read well.

44. The multi-layer microfluidic device of claim 43 further comprising a heating element, wherein said heating element is formed in a different layer than said at least one thermal reaction well and is configured to provide heating to said thermal reaction well.

45. The multi-layer microfluidic device of claim 43, wherein said layers are clamped together.

46. The multi-layer microfluidic device of claim 45, wherein fluid-tight connections between layers are obtained by providing a compressible gasket layer between non-compressible layers.

47. The multi-layer microfluidic device of claim 45, wherein fluid-tight connections between layers are obtained by providing hydrophobic surfaces at said junctions.

48. The multi-layer microfluidic device of claim 47, wherein said hydrophobic surfaces are surfaces of hydrophobic base material.

49. The multi-layer microfluidic device of claim 47, wherein said hydrophobic surfaces are formed by hydrophobic coatings on non-hydrophobic base material.

50. A method of performing DNA processing in a multi-layer microfluidic device, comprising the steps of:

a. loading a solution containing a DNA sample of interest into said multi-layer microfluidic device;

b. distributing said solution into at least one thermal reaction well in said microfluidic device, said at least one thermal reaction well being provided with additional materials required for amplifying a specific DNA sequence of interest;

c. closing a valve downstream of said least one thermal reaction well to block the downstream movement of gas or liquid from said at least one thermal reaction well,

d. heating solution and additional materials in said at least one thermal reaction well in a manner sufficient to produce amplification of said specific DNA sequence of interest if it is present in the DNA sample of interest in said thermal reaction well;

e. opening said valve downstream of at least one said thermal reaction well;

f. washing contents of said at least one thermal reaction well out of said thermal reaction well, through said channel downstream of said thermal reaction well and into a corresponding read well; and

g. detecting the presence or absence of DNA in said read well.

51. The method of claim 50 adapted for performing PCR analysis, wherein said DNA solution comprises PCR cocktail without primers, wherein said additional materials comprise primer pairs specific for said specific DNA sequence of interest, and wherein said step of heating solution and additional materials in said at least one thermal reaction well comprises performing thermal cycling.

52. The method of claim 50 adapted for performing LCR analysis, wherein said step of heating solution and additional materials in said at least one thermal reaction well comprises an isothermal heating step.

53. The method of claim 50 adapted for performing RCA analysis, wherein said step of heating solution and additional materials in said at least one thermal reaction well comprises an isothermal heating step.

54. A method of performing a biochemical reaction in a multi-layer microfluidic device, comprising the steps of:

a. loading a solution into said multi-layer microfluidic device;

b. distributing said solution to at least one thermal reaction well in said microfluidic device;

c. closing a valve downstream of said at least one thermal reaction well to block the downstream movement of gas or liquid from said thermal reaction well;

d. heating said at least one thermal reaction well in the manner required for performing the biochemical reaction of interest;

e. opening said valve downstream of said at least one thermal reaction well;

f. washing contents of each said thermal reaction well out of each said thermal reaction well and into a corresponding downstream read well; and

g. detecting the presence or absence of a product of said biochemical reaction in said read well.

55. A three-dimensional microfluidic device for performing a binding reaction to detect an analyte of interest in a sample, comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. at least one inlet for receiving a sample solution in which the analyte of interest may be present;

c. a read well downstream of said inlet and containing a binding moiety adapted to bind said analyte of interest;

d. at least one waste well downstream of said read well for receiving fluid washed from said read well;

e. at least one passive valve for temporarily stopping the flow of fluid to retain fluid within said read well; and

f. at least one passive valve for at least temporarily stopping the flow of fluid to retain fluid within said waste well.

56. The three-dimensional microfluidic device of claim 55, wherein said inlet, read well, waste well, and passive valves are located in at least two different planes corresponding to two different planar layers of said microfluidic device.

57. A three-dimensional microfluidic device for performing ELISA to detect an analyte of interest in a sample, comprising:

a. a plurality of substantially planar layers;

b. at least one ELISA circuit having components formed in at least two of said layers, comprising;

i. a main channel adapted to receive a sample solution in which an analyte of interest may be present;

ii. a read well in fluid communication with said main channel and containing immobilized capture antibody specific for said analyte of interest;

iii. a conjugate well in fluid communication with said main channel and said read well, and containing a quantity of conjugate comprising antibody specific for said analyte of interest conjugated to an enzyme;

iv. a substrate well in fluid communication with said main channel and said read well, and containing a quantity of enzyme substrate capable of reacting with said enzyme to produce a detectable reaction product; and

v. a plurality of passive valves for directing the flow of fluid through said main channel, said read well, said conjugate well, and said read well in sequence to deliver, in order, sample solution, conjugate, and substrate to said read well.

58. The three-dimensional microfluidic device of claim 57, further comprising:

a. at least one additional ELISA circuit having components formed in at least two of said layers, comprising;

i. a main channel adapted to receive a sample solution in which an analyte of interest may be present;

ii. a read well in fluid communication with said main channel and containing immobilized capture antibody specific for said analyte of interest;

iii. a conjugate well in fluid communication with said main channel and said read well, and containing a quantity of conjugate comprising antibody specific for said analyte of interest conjugated to an enzyme;

iv. a substrate well in fluid communication with said main channel and said read well, and containing a quantity of enzyme substrate capable of reacting with said enzyme to produce a detectable reaction product; and

v. a plurality of passive valves for directing the flow of fluid through said main channel, said read well, said conjugate well, and said read well in sequence to deliver, in order, sample solution, conjugate, and substrate to said read well;

b. a first sample well located upstream of all additional sample wells and adapted to receive undiluted sample injected into said microfluidic device;

c. at least one additional sample well adapted to receive diluted sample from an upstream sample well; and

d. at least one mixing circuit positioned between each said additional sample well and an upstream sample well, said mixing circuit configured to mix sample from said upstream sample well with a diluent to form a diluted sample solution that is collected in said at least one additional sample well;

wherein sample solution from said first sample well is delivered to said at least one ELISA circuit and diluted sample solution from said at least one additional sample well is delivered to said at least one additional ELISA circuit, wherein each said ELISA circuit is used to detect an analyte of interest in said sample solution or a dilution of said sample solution

59. A three-dimensional microfluidic device for processing hybridization solution and to delivering it to the surface of a microarray slide, comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. an inlet channel through which at least a first portion of said hybridization solution may be loaded into the device;

c. microfluidic processing circuitry downstream of said inlet channel, comprising at least one component selected from the group consisting of;

a well containing a reagent or other component of said hybridization solution to be combined with said first portion of said hybridization solution, a separation column for performing a separation step on at least a portion of said hybridization solution, a mixing circuit for mixing at least a portion of said hybridization solution with a diluent, and a branch circuit for dividing at least a portion of said hybridization solution among two or more channels;

d. at least one passive valve for regulating the flow of said hybridization solution through said microfluidic processing circuitry; and

e. a via channel for delivering at least a portion of said hybridization solution to the surface of the microarray slide;

wherein in use said microarray slide is assembled to said three-dimensional microfluidic device in sealing relationship so that at least one hybridization chamber is formed at the interface between said microarray slide and said three-dimensional microfluidic device, and wherein said via channel is in fluid communication with said hybridization chamber.

60. A three-dimensional microfluidic structure comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. microfluidic circuitry formed in at least two planes defined by said planar layers;

c. at least one microscale channel formed in a plane defined by at least one said layer; and

d. a passive valve comprising a short, abrupt narrowing within said at least one microscale channel;

wherein the interior surfaces of said channel and said passive valve are hydrophobic.

61. The three-dimensional microfluidic structure of claim 60, wherein said layers are formed of hydrophobic material.

62. The three-dimensional microfluidic structure of claim 60, wherein said layers are formed of non-hydrophobic base material with a hydrophobic coating.

63. A three-dimensional microfluidic structure comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. microfluidic circuitry formed in at least two planes defined by said planar layers;

c. at least one microscale channel formed through at least one said layer and providing fluid communication between microfluidic circuitry in at least two different planes defined by said planar layers; and

d. a passive valve comprising a short, abrupt narrowing within said at least one microscale channel.

64. The three-dimensional microfluidic structure of claim 63, wherein the interior surfaces of said channel and said passive valve are hydrophobic.

65. The three-dimensional microfluidic structure of claim 63, wherein said channel comprises aligned openings in at least three layers of said microfluidic structure, and wherein said passive valve is formed by at least one layer of said at least three layers in which said opening has a smaller cross-sectional area than said openings in others of said at least three layers.

66. The three-dimensional microfluidic structure of claim 63, wherein said channel comprises aligned openings in at least first and second layers of said microfluidic structure, wherein said first layer has an opening with a narrow section and a wide section, wherein said narrow section is narrower than the opening in said second layer, and wherein said first and second layers are assembled together such that said narrow section is position adjacent said second layer, and wherein said passive valve comprises said narrow section.

67. A three-dimensional microfluidic structure comprising:

a. a plurality of substantially planar hydrophobic layers assembled in sealing relationship; and

b. a well formed within said microfluidic structure, comprising a plurality of aligned holes in a plurality of adjacent layers.

68. A three-dimensional microfluidic structure comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

a mixing circuit;

b. a mixing circuit formed within said three-dimensional structure comprising;

i. a first channel;

ii. a branch point downstream of said first channel at which said first channel branches into a main channel and a side channel;

iii. a first passive valve located downstream of said branch point on said main channel;

iv. a junction downstream of said branch point where said side channel rejoins said main channel;

v. a second passive valve located on said side channel just upstream of said junction, wherein said second passive valve is stronger than said first passive valve; and

vi. an outlet channel downstream of said junction.

69. The three-dimensional microfluidic structure of claim 68, wherein said mixing circuit comprises components formed in at least two different planes corresponding to at least two said planar layers.

71. A method of mixing two fluids in a microfluidic structure, comprising the steps of:

a. injecting a quantity of a first fluid into the first channel of the mixing circuit of claim 69;

b. injecting a quantity of a second fluid into said first channel behind said second fluid, wherein said first fluid is diverted into said side channel by said first passive valve as it is pushed into said mixing circuit by said second fluid, and wherein said quantity of said first fluid is just sufficient to fill said side channel up to said second passive valve;

c. injecting additional second fluid into said first channel at a pressure sufficient to overcome said first passive valve to move first fluid into said main channel until it reaches said junction; and

d. injecting additional second fluid into said first channel to move said first fluid out of said second channel, past said junction, whereupon said first fluid combines with said second fluid in said outlet channel downstream of said junction.

70. A three-dimensional microfluid structure adapted for performing serial dilution of a sample, comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

a mixing circuit;

b. a first mixing circuit formed within said three-dimensional structure comprising;

i. a first inlet channel;

ii. a branch point downstream of said first channel at which said first inlet channel branches into a first main channel and a first side channel;

iii. a first passive valve located downstream of said branch point on said main channel;

iv. a first junction downstream of said branch point where said first side channel rejoins said first main channel;

v. a second passive valve located on said first side channel just upstream of said first junction, wherein said second passive valve is stronger than said first passive valve; and

vi. a first outlet channel downstream of said junction;

c. at least one additional mixing circuit formed within said three-dimensional structure downstream of said first mixing circuit, comprising;

i. a second inlet channel downstream of said first outlet channel;

ii. a second branch point downstream of said second inlet channel at which said second inlet channel branches into a second main channel and a second side channel;

iii. a third passive valve located downstream of said second branch point on said second main channel;

iv. a second junction downstream of said branch second point where said second side channel rejoins said second main channel;

v. a fourth passive valve located on said second side channel just upstream of said second junction, wherein said fourth passive valve is stronger than said third passive valve; and

vi. a second outlet channel downstream of said second junction.

72. A three-dimensional microfluidic branching circuit comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. an inlet channel passing through at least a first layer;

c. a primary branch channel formed in a second layer adjacent said first layer, wherein said inlet channel intersects said primary branch channel at its central region;

d. two primary via channels passing through a third layer adjacent said second layer, wherein one of said primary via channels intersects said primary branch channel at each of its ends;

e. two secondary branch channels formed in a fourth layer adjacent said third layer, wherein each of said via channels intersects one of said secondary branch channels at its central region;

f. four secondary via channels passing through a fifth layer adjacent said fourth layer, wherein one of said secondary via channels intersects each said secondary branch channel at each of its ends;

wherein said two primary via channels have smaller cross sectional areas than said primary branch channel, and wherein said four secondary via channels have smaller cross-sectional areas than said primary via channels.

73. A three-dimensional microfluidic branching circuit comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. an inlet channel passing through at least a first layer;

c. a branched channel formed in a second layer adjacent said first layer, wherein said inlet channel communicates with a central region of said branched channel, and wherein said branched channel has a plurality of arms extending outward from said central region;

d. a plurality of outlet channels formed in a third layer adjacent said second layer, each said outlet channel communicating with the end of one of said plurality of arms of said branched channel;

wherein each of said outlet channels provides a greater resistance to fluid flow than do said arms of said branched channel, thereby causing fluid entering said branched channel to fill all of said arms before entering any of said outlet channels.

74. A three-dimensional microfluidic structure comprising:

a. a plurality of substantially planar layers assembled in sealing relationship;

b. a microfluidic circuit including microfluidic structures lying in at least two planes corresponding to at least two said planar layers of said microfluidic device, said microfluidic circuit comprising;

i. a main channel;

ii. a side channel branching off of said main channel at a branch point;

iii. a first passive valve located in said side channel just downstream of said branch point;

iv. at least one microfluidic structure located downstream of said branch point in fluid communication with said main channel, said microfluidic structure comprising a well or a channel;

v. a second passive valve located downstream of said microfluidic structure;

wherein said first passive valve has a strength sufficient to cause fluid first entering said main channel under pressure to flow preferentially into said main channel rather than said side channel at said branch point; and

wherein said second passive valve has a strength sufficient to divert fluid flow into said side channel after said main channel has been filled to said second passive valve.

75. The three-dimensional microfluidic structure of claim 74, wherein said side channel rejoins said main channel downstream of said branch point but upstream of said second passive valve;

wherein said side channel comprises an air duct adjacent the point where said side channel rejoins said main channel; and

wherein said side channel has a diameter sufficiently greater than that of said main channel that when said main channel and said side channel are filled with fluid, additional fluid injected into said main channel flows preferentially through said side channel at said branch point.

76. The three-dimensional microfluidic structure of claim 74, wherein said main channel, said side channel, said first passive valve, said microfluidic structure, and said second passive valve lie in one of said a least two planes, and wherein said microfluidic circuit comprises at least one additional microfluidic structure lying in another of said at least two planes.