Abstract:
A structure for use with a microfluidic channel to reduce the effects of surface tension and capillary forces. A macroscale reservoir is connected to a microscale channel by a microscale section extending from the reservoir, which fills with fluid and flows smoothly into the microscale channel.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    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. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to microfluidic devices for performing analytic testing, and, in particular, to a device for reducing the effect of surface tension on fluids flowing in microfluidic channels.  
           [0004]    2. Description of the Related Art  
           [0005]    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.  
           [0006]    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 fluid 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.  
           [0007]    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.  
           [0008]    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.  
           [0009]    Surface effects describe the character of a surface on a micro scale. Materials often have unbound electrons, exposed polar molecules, or other molecular level features that generate a surface charge or reactivity characteristic. Due to scaling, these surface effects or surface forces are substantially more pronounced in microstructures than they are in traditionally sized devices. This is particularly true in microscale fluid handling systems where the dynamics of fluid movement are governed by external pressures and by attractions between liquids and the materials they are flowing through.  
           [0010]    This invention deals with the passive control of fluids within a microfluidic circuit. The passive control is generated by using the natural forces that exist on a microscale, specifically capillarity, which is caused by the attraction or repulsion of a fluid toward certain materials.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the present invention to provide a device for reducing the effect of surface tension on fluids flowing within a microfluidic channel.  
           [0012]    It is a further object of the present invention to provide a microfluidic structure in which fluids flow from a macrochannel into a microchannel to insure smooth flow within the microfluidic structure.  
           [0013]    These and other objects of the present invention will be more readily apparent from the description and drawings that follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a plan view of a microfluidic structure including an H-Filter using the principles of the present invention; and  
         [0015]    [0015]FIG. 2 is a fragmentary, cross-sectional side view of the microfluidic structure shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0016]    [0016]FIG. 1 shows a microfluidic analysis card  10  which contains an H-Filter  12 , which structure is described in detail in U.S. Pat. No. 5,932,100, incorporating the present invention. H-Filter  12  includes a first reservoir  14  and a second reservoir  16 . An outlet channel  18  of reservoir  14  and an outlet channel  20  of reservoir  16  are both connected to a flow channel  24  at a first end  26 . A second end  28  of flow channel  24  is coupled to an exit channel  30 , which is connected to a reservoir  32  and also to an exit channel  34 , which is coupled to a reservoir  36 . Reservoir  36  is also coupled to a bellows  38  via a channel  40 . It should be understood that H-Filter  12  will also operate using gravity as a driving force.  
         [0017]    Reservoir  14  contains a vent hole  42  and an inlet port  44 , while reservoir  16  contains an inlet port  46 . Reservoir  14  also contains a narrowed lower section  50 , which extends across the lower length of reservoir  14 , while reservoir  16  also contains a similarly narrowed lower section  52  across the lower length of reservoir  16 .  
         [0018]    Operation of H-Filter  12  is as follows: a sample fluid is placed into inlet port  46  of reservoir  16  while an extractor solution is placed into port  44  of reservoir  14 . The fluids form a stream and flow through channels  20 ,  18  respectively to end  26  of channel  24 . The fluids form a stream and flow laminarly within channel  24  while particles from the sample fluid diffuse across the laminar junction into the extractor fluid. As the stream reaches end  28  of channel  24 , the extractor fluid containing particles flow through channel  30  into reservoir  32 , while the sample fluid flows through channel  34  into reservoir  36 .  
         [0019]    Narrowed section  50  of reservoir  14  fills with sample fluid when the sample is loaded into inlet port  44 . Since the structure of reservoir  14  is not microscale, and outlet channel  18  is of a microscale dimension, the effect of surface tension would generally prevent the fluid from flowing smoothly from reservoir  14  to channel  18 . However, as can be clearly seen in FIGS. 1 and 2, the narrow lower section  50 , which runs the entire length of reservoir  14 , is of essentially the same microdimensions of channel  18 ; thus, fluid moves smoothly and consistently from reservoir  14  into channel  18  and through the rest of the H-Filter structure. This is also true for fluids flowing from reservoir  16  into channel  20 , as the narrow lower section  52  of reservoir  16  fills with fluid and flows smoothly into channel  20  with little or no surface tension effect.  
         [0020]    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.