US 20020160518 A1
A device for promoting sedimentation within microfluidic channels which uses gravity to separate particles from fluid. Particles such as blood cells or beads are separated from a carrier fluid using gravity combined with various devices such as membranes and sonic energy in different embodiments.
1. A microfluidic device comprising:
a microfluidic structure having an inlet and a sedimentation region;
a fluid containing particles having a density different from that of said fluid;
and means for moving said fluid through said inlet into said sedimentation region such that a concentration gradient of said particle is established across the vertical dimension of said sedimentation region.
2. A microfluidic device, comprising:
a microfluidic channel having a depth and a width, and an inlet and a first and a second outlet,
with first outlet placed higher than said second outlet with respect to the vertical axis of said microfluidic channel;
and a fluid containing particles that have a different density from the fluid flowing through said channel such that said particles exit said channel preferentially through said first or said second outlet.
3. A microfluidic device, comprising:
a first channel and a second channel;
means for preventing solid material from moving from said first channel to said second channel, said means being located between said first channel and said second channel to form a collection of solid particles located in said first channel;
and means for moving a fluid through said first and said second channel such that said fluid interacts with said particles located in said first channel.
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 This application claims benefit from U.S. Provisional Patent Application Serial No. 60/281,114, filed Apr. 3, 2001, which application is incorporated herein by reference.
 1. Field of the Invention
 This invention relates generally to microfluidic devices for performing analytic testing, and, in particular, to devices for rapidly increasing sedimentation within microfluidic channels.
 2. Description of the Related Art
 Microfluidic devices have recently become popular for performing analytic testing. Using tools developed by the semiconductor industry to miniaturize electronics, it has become possible to fabricate intricate fluid systems which can be inexpensively means produced. Systems have been developed to perform a variety of analytical techniques for the acquisition of information for the medical field.
 Microfluidic devices may be constructed in a multi-layer laminated structure where each layer has channels and structures fabricated from a laminate material to form microscale voids or channels where fluids flow. A microscale channel is generally defined as a fluid passage which has at least one internal cross-sectional dimension that is less than 500 μm and typically between about 0.1 μm and about 500 μm. The control and pumping of fluids through these channels is affected by either external pressurized fluid forced into the laminate, or by structures located within the laminate.
 U.S. Pat. No. 5,716,852 teaches a method for analyzing the presence and concentration of small particles in a flow cell using diffusion principles. This patent, the disclosure of which is incorporated herein by reference, discloses a channel cell system for detecting the presence of analyte particles in a sample stream using a laminar flow channel having at least two inlet means which provide an indicator stream and a sample stream, where the laminar flow channel has a depth sufficiently small to allow laminar flow of the streams and length sufficient to allow diffusion of particles of the analyte into the indicator stream to form a detection area, and having an outlet out of the channel to form a single mixed stream. This device, which is known at a T-Sensor, may contain an external detecting means for detecting changes in the indicator stream. This detecting means may be provided by any means known in the art, including optical means such as optical spectroscopy, or absorption spectroscopy of fluorescence.
 U.S. Pat. No. 5,932,100, which patent is also incorporated herein by reference, teaches another method for analyzing particles within microfluidic channels using diffusion principles. A mixture of particles suspended in a sample stream enters an extraction channel from one upper arm of a structure, which comprises microchannels in the shape of an “H”. An extraction stream (a dilution stream) enters from the lower arm on the same side of the extraction channel and due to the size of the microfluidic extraction channel, the flow is laminar and the streams do not mix. The sample stream exits as a by-product stream at the upper arm at the end of the extraction channel, while the extraction stream exits as a product stream at the lower arm. While the streams are in parallel laminar flow is in the extraction channel, particles having a greater diffusion coefficient (smaller particles such as albumin, sugars, and small ions) have time to diffuse into the extraction stream, while the larger particles (blood cells) remain in the sample stream. Particles in the exiting extraction stream (now called the product stream) may be analyzed without interference from the larger particles. This microfluidic structure, commonly known as an “H-Filter,” can be used for extracting desired particles from a sample stream containing those particles.
 It is often desirable to remove particles from a liquid for analysis purposes. One method of performing this procedure is to use a centrifuge. Centrifugation is a process by which particles in suspension in a fluid are separated by spinning the fluid, usually in a test tube, such that centrifugal force throws the particles to the periphery of the rotated vessel. Sedimentation is also an important method to separate particles by density. In many cases, the difference in the rate of sedimentation of particles to be separated is very small, as is the rate of separation itself. Frequently, small particles such as blood cells sediment at a rate of only a few micrometers per second. This problem is usually solved by increasing the apparent gravitational force that drives the sedimentation by using a centrifuge.
 In microfluidic structures, sedimentation structures can be used to achieve sedimentation without the use of centrifuges. The rate of sedimentation of a few micrometers/second provides a sufficient speed in channels that have dimensions in the order of hundreds of micrometers. For example, blood cells will settle in a channel of 100 micrometer depth in about 100 seconds at standard gravity.
 It is therefore an object of the present invention to provide a device which will allow sedimentation in microfluidic channels.
 It is a further object of the present invention to provide a device using microfluidic channels to separate blood cells from plasma.
 It is still a further object of the present invention to provide a microfluidic sedimentation device which is simple and easy to use.
 These and other objects of the present invention will be more readily apparent in the description and drawings which follow.
FIG. 1 is a side view of a sedimentation device according to the present invention;
FIG. 2 is a side view of an alternative embodiment of the device of FIG. 1;
FIG. 3 is another embodiment of a sedimentation device according to the present invention;
FIG. 4 is a side view of a microfluidic channel having a second channel passing beneath for use in the present invention;
FIG. 5 is a bottom view of the device of FIG. 4;
FIG. 6 is another embodiment for carrying out the present invention;
 FIGS. 7A-I show several different embodiments of a device for capturing beads for use in the present invention; and
FIG. 8 is a plan view of an analysis card according to the principles of the present invention.
FIG. 1 shows a microfluidic device used for promoting sedimentation. Referring now to FIG. 1, a microfluidic channel 10 is filled with whole blood. An audio speaker 12 is positioned below channel 10. Speaker 10 is then activated, subjecting channel 10 to sonic energy, vibrating blood cells 14 within the whole blood sample. This vibration is sufficient to exceed the minimum shear stress in the fluid surrounding cells 14, allowing motion of cells 14 in response to gravity. After sufficient time for sedimentation, a pusher fluid is used to flush the plasma from above and around settled cells 14 by passing it through channel 10 in the direction of arrows A. This technique is insensitive to channel geometry except for a requirement that the height of channel 10 be small enough that sedimentation occurs rapidly. Although this device is shown as a sedimentation device for blood cells, it could also be used to isolate beads within a channel to be used for analysis purposes.
 An alternative structure for the device of FIG. 1 is shown in FIG. 2. In this embodiment, channel 10 is saturated at an angle above the horizontal plane. Whole blood is loaded into channel 10 and speaker 12 activated to subject the sample to sonic energy. Blood cells 14 settle along the bottom of channel 10, and as channel 10 is angled, cells 14 tend to move along the bottom surface of channel 10 in the direction of arrows B. As a pusher fluid is injected into channel 10, plasma from the blood sample travels in the direction of arrows C, which is in the opposite direction of the movement of cells 14. The speed of sedimentation can be varied by varying the angle of inclination of channel 10.
 Another embodiment which can be used for promoting sedimentation is shown in FIG. 3. A sample fluid containing particles 28 which are denser than the sample fluid is inputted into a microfluidic channel 30. Channel 30 contains a recessed well section 32 on the bottom surface of channel 30. As particles 28 flow along within channel 30, they drop down into section 32 of channel 30, as they are denser than the fluid. As a result, a particle-free sample passes out of channel 30 as shown at arrow D.
 Another embodiment of the principles of this invention is shown in FIGS. 4 and 5. Referring now to FIG. 4, a main microfluidic channel 40 is shown having a circuitous or S-shaped channel 42 coupled to the bottom surface and is open to channel 40 in periodic locations along channel 40 as can be clearly seen in FIG. 5. In addition, a filter or membrane 44 is situated on the bottom surface of channel 40. A diluted fluid containing particles 46 flows into channel 40 at 48. Particles 46 tend to move slightly away from the walls of channel 40 to avoid the shear gradient that is present in that area. Membrane 44, which is fluid permeable, excludes particles 46 from entering into channel 42; however, when channel 42 is held at a lower absolute pressure than main channel 40, a small portion of the fluid will flow through membrane 44 into channel 42 at each intersection. This clear fluid flowing within channel 42 may be collected at the end of channel 42, while the particle 46 suspension within channel 40 becomes more concentrated as it moves through main channel 40. This structure may be used for the extraction of undiluted plasma from whole blood.
 Another structure which may be used to separate plasma from whole blood is shown in FIG. 6. Referring now to FIG. 6, a microfluidic channel 60 is shown. The inner walls 62 of channel 60 contain a chemical that initiates aggregation of blood cells into dense formations called rouleaux. A sample of whole blood flows into channel 60 at 64, and blood cells 66 react with the chemical on walls 62 and begin to aggregate. After a sufficient amount of time has passed, a pusher fluid enters channel 60 at 64, and flows through aggregated cells 66 to flush the plasma from between the rouleaux and out of channel 60 at 68.
 FIGS. 7A-I represent different embodiments in which beads may be trapped within a microfluidic channel to assist in analyzing a particular fluid. Referring now to FIG. 7A, there is shown a microfluidic channel 80 through which a plurality of beads 82 are transmitted. Beads 82 are preferably functionalized with antibodies such that the beads will fluoresce upon contact with a specific substance. A membrane or filter 84 is located within channel 80 such that beads will not pass through channel 80, but a fluid can flow across beads 82 for analysis purposes and flow out through opening 84. Other means for capturing beads 82 are also shown in the figures; channel 80 may have a narrow section 90 which will restrict passage of beads 82 (FIG. 7B); beads 82 may be denser that the fluid flowing in channel such that they will settle on the bottom surface 92 of channel 80 due to gravity (FIG. 7C); beads 80 may have magnetic properties such that their travel within channel 80 is stopped using a magnet 94 located outside channel 80 (FIG. 7D); channel 80 may have an inlet 96 in which beads 82 are inserted into a wide section 98 of channel 80 whereas beads 82 cannot pass into channel 80 from section 98 (FIG. 7E); beads 82 may be less dense than the fluid flowing in channel 80 such that they would settle into a section 100 on the upper surface of channel 80 and remain in section 100 (FIG. 7F); channel 80 may have a section 102 which is above the level of channel 80 wherein beads 82 which are less dense than the fluid in channel 80 such that they will be trapped in section 102 (FIG. 7G); channel 80 may have a recessed section 104 wherein beads 82 which are more dense than the fluid will settle in section 104 (FIG. 7H); and channel 80 may have a downwardly depending section 106 such that beads 82 which are more dense than the fluid remain in section 106 (FIG. 7I). In all of these embodiments, beads 82 will react of a specific substance within the fluid such that they will fluoresce to indicate a particular concentration of that substance.
FIG. 8 shows a laminate analysis card 120 which also embodies the principles of the present invention. Card 120 has a first input 122 into which a solution of beads that are functionalized with antibodies is injected, a second input 124 into which a sample such as whole blood is injected, and a third input 126 into which a wash solution is injected. Input 122 is coupled through a channel 128 to a junction 130, input 124 is coupled to junction 130 through a channel 132, and input 126 is coupled to junction 130 through a channel 134.
 Junction 130 is connected to a channel 140 having a series of recessed well-like structures 141 similar to well 32 shown in FIG. 3. The output of channel 140 is coupled to a reservoir 142 through a channel 144.
 The operation of analysis card 120 is as follows: a bead solution is injected into input 122, a whole blood sample into inlet 124, and a wash solution into inlet 126. Bead solution is first pumped into channel 140 through a valve 150, and the beads in the solution settle into well structures 141. Then the blood sample is pumped into channel 140 through a valve 152, where the blood analytes interact with the antibodies on the beads in wells 141. Finally, the wash solution is pumped through a valve 154 through channel 140 to wash the blood away. The beads in wells 141 will change color or fluoresce to indicate the presence or concentration of the desired substance in the blood.
 While the present invention has been shown and described in terms of a preferred embodiment thereof, it will be understood that this invention is not limited to this particular embodiment and that changes and medications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.