CROSS REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
This application claims benefit from U.S. Provisional Patent Application Ser. 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 a device in which the concentration of a particle in a solvent is increased by flowing it in contact with a solution that extracts solvent.
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.
- SUMMARY OF THE INVENTION
There are occasions in which a sample to be analyzed within a microfluidic channel is of such a low concentration that it is difficult, if not impossible, to get useful or reliable information from the analyte. Thus, it is necessary to increase the concentration of the sample to make it possible to get meaningful results.
It is therefore an object of the present invention to provide a device for increasing the concentration of a sample flowing within a microfluidic channel.
It is a further object of the present invention to provide a device which can reverse some of the dilution affects of an H-Filter or similar device.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects of the present invention will be more readily apparent from the descriptions and drawings that follow.
FIG. 1 is a top view of a T-Sensor which operates according to the principles of the present invention; and
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 2 is a top view of a diffusion channel of an H-Filter which operates according to the principles of the present invention.
FIG. 1 shows a standard T-Sensor device, designated at 10, the operation of which is described in detail in U.S. Pat. No. 5,716,852. T-Sensor 10 consists of a first channel 12 having an input port 18. Channels 12 and 16 meet at a diffusion channel 20 having an output 21, as shown in FIG. 1. The characteristics of T-Sensor 10 are such that fluids from channels 12 and 16 will flow laminarly within diffusion channel 20.
To accomplish the desired concentration using T-Sensor 10, a sample 22 to be concentrated, which contains constituents which diffuse more slowly than the sample solvent molecules, is injected into input port 14, while a concentrating solution 24 is injected into port 18. The fluids flow through channels 12 and 16 respectively and finally into diffusion channel 20. Flow within channel 20 is laminar such that a diffusion interface region 26 is formed. Concentrating solution 24 is formulated such that is extracts fluid from sample 22, and may contain large ionic compounds, such as surfactant molecules, which do not diffusion significantly into the sample stream, whereas sample fluid 22 molecules, typically small solvent molecules such as water, diffuse into concentration solution 24 very quickly, as indicated by arrows A, thus concentrating all molecules contained in sample 22 that have a smaller diffusion coefficient (i.e., a larger size) than the solvent molecules.
As an example, a sample solution of urine containing bacteria is injected into port 14, while a concentrating solution such as icodextrin is injected into port 18. Molecules from the sample diffuse quickly into the icodextrin solution, and at output 21 of T-Sensor 10, the bacteria would be concentrated in a small volume of fluid.
This process can be accelerated by providing a large diffusion interface area, and a small diffusion distance. This is shown in a patent application entitled “Microfluidic Device for Rotational Manipulation of the Fluidic Interface between Multiple Flow Streams,” Ser. No. 09/956,497, filed Sep. 18, 2001; the disclosure of which is incorporated by reference herein.
An alternative embodiment for carrying out the present invention is shown in FIG. 2. Referring now to FIG. 2, the diffusion channel 50 of an H-Filter structure is shown. The velocity distribution of fluid flow in microchannels usually follows a combination of a parabolic flow profile and a plug flow profile, depending on viscosity, flow speed, channel dimensions, etc. For a circular or square cross-sectional channel, the flow profile is more or less uniformly parabolic, whereas for a rectangular cross section, the flow profile is parabolic only in the narrow dimension, and a combination of parabolic (close to the walls) and plug flow (closer to the center of the channel), as shown at 52 in FIG. 2.
If two fluids of similar viscosity flow parallel next to each other in a T-Sensor or an H-Filter, such that one of the two flows takes up only a narrow slice of the complete channel next to a wall as seen at 54 in FIG. 2, then the average flow speed of this flow will be lower than that of the other flow that takes up space in the channel both in the center and on the other side of the channel.
Separation by size in H-Filters and T-Sensors occurs because the particles of different sizes initially contained in one of the two flows diffuse across the fluid interface into the other flow at different rates determined by the size of the particles. The driving force for the diffusion is a concentration gradient present between the two flows, which is initially very high, but, as diffusion progresses, is reduced. This process is applicable to both miscible and immiscible fluids.
If the average flow speed of the two flows is different, i.e., if the bulk of the sample flows closer to the wall and relatively slowly, while the bulk of the receiver solution flows more in the center of the channel and relatively fast, then the concentration of extracted molecules in the receiver solution is increased more slowly, therefore increasing the effective diffusion across the diffusion interface, and hence speeding up the separation compared to an H-Filter in which both fluids flow at the same rate.
This effect is frequently enhanced by having a sample with a higher velocity than the receiver solution, thus further slowing down the sample and increasing the separation speed. The separation process can be further increased by providing a large diffusion interface area and a small diffusion distance. In addition, separation of fluids having different flow speeds by a permeable membrane within a microchannel will also enhance diffusion across the membrane.
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 modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.