Abstract:
A microfluidic device which operates without the need for an external power source. The device includes a body structure, at least one microscale channel within the structure, a port for introducing fluid into the channel, and a power source internal to the structure for propelling the fluid through the channel. Various structures are described which embody the invention.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation-in-part application of U.S. patent application Ser. No. 09/415,404, filed Oct. 8, 1999, and also of U.S. Provisional Patent Application Serial No. 60/213,865, filed Jun. 23, 2000 which applications are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     This invention relates generally to microfluidic systems, and, in particular, to a microfluidic device in which the operation is conducted entirely without the benefit of an external fluidic driver. 
     2. Description of the Prior Art 
     Microfluidic devices have become very popular in recent years for performing analytical testing. Using tools developed by the semiconductor industry to miniaturize electronics, it has become possible to fabricate intricate fluid systems which can be inexpensively mass produced. Systems have been developed to perform a variety of analytical techniques for the acquisition and processing of information. 
     U.S. Pat. No. 5,716,852 is an example of such a device. This patent teaches a microfluidic system for detecting the presence of analyte particles in a sample stream using a laminar flow channel having at least two input channels 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 as a T-sensor, allows the movement of different fluidic layers next to each other within a channel without mixing other than by diffusion. 
     Microfluidic systems of this type require some type of external fluidic driver, such as piezoelectric pumps, microsyringe pumps, electroosmotic pumps and the like, to operate. It would be desirable, in many situations, to produce a device which is entirely driven by a readily available force, such as gravity, capillary action, absorption in porous materials, chemically induced pressures or vacuums (e.g., by a reaction of water with a drying agent), or by vacuum and pressure generated by simple manual action, rather than by an external fluidic driver requiring a separate power source having moving parts. Such a device could be extremely simple to operate, could be manufactured very inexpensively, and could be used to perform many diagnostic assays using a variety of microfluidic methods. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a microfluidic device which can be operating without a fluid driver that requires a power source. 
     It is a further object of the present invention to provide a low cost disposable qualitative assay which can be adapted to medical or environmental uses, among others. 
     It is still a further object of the present invention to provide a simple microfluidic system which can perform analytical functions without the necessity of an external electrical or mechanical fluid driver system. 
     These and other objects are accomplished in the present invention by a simple cartridge device containing microfluidic channels which perform a variety of analytical techniques for the acquisition of information without the use of any electrical or mechanical driving forces applied to the cartridge. The cartridge may be constructed from a single material, such as plastic, by conventional manufacturing methods, such as injection molding, to create a low cost device which can be discarded after a single use. Inherently available forces such as gravity, hydrostatic pressure, capillary force, absorptive force manually generated pressure, or vacuum, move the fluids through the microfluidic channels to accomplish the desired analytical analyses. Other applications for this technology include toys and advertising devices. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a microfluidic device manufactured according to the present invention; 
     FIG. 2 is a plan view depicting an alternative embodiment of the cartridge of FIG. 1; 
     FIG. 3 is another alternative embodiment of the cartridge of the present invention which is driven by absorption; 
     FIG. 4 is a sectional view taken along lines  4 — 4  of FIG. 3; 
     FIG. 5 is another alternative embodiment of the cartridge of the present invention which is driven by capillary action; 
     FIG. 6 is another alternative embodiment of the cartridge of the present invention which is driven by hydrostatic pressure; 
     FIG. 7 is another embodiment of the cartridge of the present invention which is driven by aspiration within the cartridge itself; 
     FIG. 8 is another embodiment of the cartridge of the present invention which is driven by a chemical process or a pump from within the cartridge; 
     FIG. 9 is another embodiment of the cartridge shown in FIG. 6 having an H-Filter within the cartridge; 
     FIG. 10 is a graph showing the relationship between hydrostatic pressure and Reynolds number for a typical microfluidic mixing structure; 
     FIG. 11 is another embodiment of the cartridge of FIG. 3 which is driven by absorption; 
     FIG. 12 is an enlarged view of an alternative embodiment of an absorption material for use in the present invention; 
     FIG. 13 is another embodiment of the cartridge of the present invention exhibiting improved fluid flow within the cartridge; 
     FIG. 14 is an enlarged view of a portion of the cartridge of FIG. 13; 
     FIG. 15 is a graphic representation of a T-Sensor for use in the present invention; 
     FIG. 16 is a graphic representation similar to FIG. 15 showing a T-Sensor using different analytes; 
     FIG. 17 is a graphic representation of a T-Sensor using a readout device according to the present invention; and 
     FIG. 18 is a fragmentary view of a microfluidic channel joining two reservoirs in a Y-junction constructed according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, there is shown a cartridge generally indicated at  10  containing the elements of the present invention. Note that like parts are given like reference numerals in the embodiments contained in the present application. Cartridge  10  is preferably constructed from a single material, such as plastic, using a method such as injection molding, and is approximately the size and thickness of a typical credit card. Located within cartridge  10  are several flow channel systems  12 , preferably comprising T-Sensors which are described in detail in U.S. Pat. No. 5,716,852, which disclosure incorporated by reference herein. Each T-Sensor  12  consists of a first sample channel  14 , a second sample channel  16 , a common channel  18 , and a terminating storage chamber or reservoir  20 . At the end of each channel  14 ,  16  opposite common channel  18  is located a circular input port  14   a ,  16   a  respectively, where droplets of reagents and samples can be placed for analysis. 
     In operation, T-Sensor  12  allows the movement of different fluidic layers next to each other within channel  18  without mixing other than diffusion, as fluids generally show laminar behavior within microfluidic channels. A sample solution placed in port  14   a  passes through channel  14  and an indicator solution placed in port  16   a  passes through channel  16 , and the streams from channels  14 ,  16  merge in common channel  18  and flow next to each other until they exit into reservoir  20 . Smaller particles such as ions or small proteins diffuse rapidly across the fluid boundaries within channel  18 , whereas larger molecules diffuse more slowly. Large particles, such as blood cells, show no significant diffusion within the time the two flow streams are in contact. An interface zone is formed between the fluid layers. The signal strength of a particular optical or electrochemical property, such as fluorescence intensity of the interface zone is a function of the concentration of the analyte. In addition, it is sometimes desirable to add a third stream to T-Sensor  12 , with one channel containing a reference stream for analysis purposes. This is described in detail in U.S. Pat. No. 5,948,684, which issued Sep. 7, 1999. 
     Manually operated microfluidic devices such as T-Sensor  12  can be used to qualitatively or semi-quantitatively determine analyte concentrations. A practical use may be the determination of several parameters directly in whole blood. A color change in the diffusion zone of a T-Sensor detection channel can provide qualitative information about the presence of an analyte. This method can be made semi-quantitative by providing a comparator color chart with which to compare the color of the diffusion zone. This method would work somewhat similar to a paper test strip, but with much better control and reproducibility. In addition, long term monitoring functions can be accomplished by placing such a device in line with a sample feed. With a T-Sensor, assays can be performed directly with whole blood, whereas paperstrip readings can be affected by the color and consistency of whole blood. 
     The accuracy of this method can be enhanced by combining the device with a readout system which may consist of an absorbance, fluorescence, chemiluminescence, light scatter, or turbidity detector placed so that the detector can observe an optically detectable change which is caused by the presence or absence of a sample analyte or particle in the detection channel. Alternatively, electrodes can be placed within the device to observe electrochemically observable changes caused by the presence or absence of a sample analyte or particle in the in the detection channel. 
     One embodiment of this device is a disposable cartridge combined with a mass market digital camera-like detector system: a flash would illuminate the sensor area, and any type of optically detectable signal would be interpreted by image processing software and yield a chemical concentration or count output. 
     FIG. 2 shows an alternative embodiment for cartridge  10  of FIG.  1 . Note that in the various embodiments, similar elements have been given similar numerals. Cartridge  10   a  contains four separate T-Sensor circuits  12   a ,  12   b ,  12   c ,  12   d . These T-Sensors include common channels  16   a ,  16   b ,  16   c ,  16   d  having different configurations. 
     The driving force for operating cartridges such as those shown in FIGS. 1 and 2 is generally supplied from an external source. U.S. patent application Ser. No. 09/080,691, which was filed on May 18, 1998, the disclosure of which is hereby incorporated by reference in its entirety, is an example of a device which requires external motive forces to operate properly. Various syringe pumps located external to the cartridge are coupled to the device for operation. FIG. 3, however, shows a device which uses an internal force to drive the circuitry. 
     Turning now to FIG. 3, there is shown a T-Sensor  12  within cartridge  10   b , substantially similar to that taught in FIG.  1 . Channels  14 ,  16  intersect to form common channel  18  which terminates in reservoir  20 . Input ports  14   a ,  16   a  are connected to channels  14 ,  16  respectively at the ends opposite channel  18 . A porous absorbent material  30  is positioned within channels  14 ,  16 ,  18  and reservoir  20  of cartridge  10   b , as can be clearly seen in FIG.  4 . To operate the T-Sensor of FIG. 3, a liquid sample of reagent is applied to port  14   a  while a liquid sample of a specimen is applied to port  16   a . Material  30  acts to propel the liquids through channels  14 ,  16 ,  18  into reservoir  20 . Material  30  is preferably an absorbent hydrophilic substance such as cellulose, calcium chloride, calcium sulfate, or silica gel. 
     FIG. 11 shows another embodiment of a cartridge using the absorption principles of the present invention. Referring now to FIG. 11, cartridge  10   b  uses a T-Sensor like device  12  having three input ports  14   a ,  15   a ,  16   a  coupled to channels  14 ,  15 ,  16  which converge to form common channel  18 . Channel  18  terminates at a reservoir  32  which contains a circular absorbent material  33 . Channel  18  is connected to reservoir  32 , and thus material  33 , by a series of small apertures  34 , which, in the present embodiment, allow a fluid traveling within channel  18  to initially reach the center of circular absorbent material  32 . As fluid from channel  18  reaches material  33 , material  33  acts to propel fluid through channels  14 ,  15 ,  16  and common channel into reservoir  32 . In this embodiment, fluid exits from apertures  34  onto the center of material  33  and travels radially at a constant speed toward the outer circumference of circular material  33 , thus providing a constant flow speed within channel  18  as the fluid is absorbed by material  33 . The absorption begins at the center of material  33 , and expands exponentially the amount of absorption front as the absorption spot expands. By varying the shape, geometry, and directional preference of absorbent pad  33 , the flow speed of the fluid absorbed by material  33  can be controlled as desired. 
     FIG. 12 shows an enlarged section of a cartridge using the absorption principles shown in FIG.  11 . In this embodiment, common channel  18  terminates in a triangular section  36  of absorbent material. As fluid travels through channel  18  in the direction shown by arrow A, it reaches material  36 , and since it is triangularly shaped, the length of the diffusion front is increased as diffusion progresses, thereby compensating for an increase in fluid resistance in material  36  as more fluid must pass through already wetted absorbent material. Other geometries which can be used in the present invention include hyperbolically expanding triangular shapes. 
     Different shapes may not only be used for both keeping the flow speed constant, but also for increasing or decreasing the flow speed, or for cyclically varying the flow speed. For example, a sinusoidal expanding “triangular” shape can be used to keep the average flow speed constant, but let it vary periodically. 
     The movement of fluids through cartridge  10   b  can be enhanced by means of a surface treatment on the walls of channels  14 ,  15 ,  16  and  18  formed by contacting the surface with low-pressure plasma. In addition, other features which enhance fluid flow (which will be discussed in greater detail hereinafter), such as capillary forces and gravity feed, can be employed to assist this method. 
     Another method available for use in cartridge  10   b  of FIG. 11 is driven by evaporation, where absorbent material  33  is not used, and reservoir  32  is exposed to the atmosphere outside cartridge  10   b , where a portion of the fluid from channel  18  evaporates. If the area of fluid that is exposed to atmosphere at the outlet is larger than that at the inlet, then fluid movement will occur in principle. Alternatively, one or both of the outlet or inlet can be cooled or heated, thereby controlling the evaporation rate and, thus, the fluid flow rate and direction, to maintain a constant flow rate or to vary the rate. 
     According to this method, the device in FIG. 11 can be operated beyond the time at which the absorptive capacity of the absorbent pad is exhausted. Reservoir  32  has a relatively large surface area, causing the evaporation of fluid at a higher rate than the combined areas of inlet reservoirs  14   a ,  15   a , and  16   a . This creates a motive force, driving the fluid contained in cartridge  10   b  towards the apertures  34 . In this embodiment, material  33  is not required for the function of this device. 
     The movement of fluid through a microfluidic cartridge by differential evaporation at the inlet and outlet reservoirs is, provided that all evaporation areas are kept at the same temperature, only a function of the surface area of these evaporation areas. If the surfaces are liquid-filled, then the actual area differences determine the force driving the fluid movement. If the surfaces are a wetted porous material such as a filter paper, then the surface roughness also increases the effective surface area. The rougher a surface is, the more “evaporative force” it creates. 
     The differential “evaporative force” can be enhanced by keeping the different evaporative areas at different temperatures, as evaporation is enhanced by increased temperature. For example, an inlet area at a low temperature, and an outlet area of the same size as the inlet area, but at a higher temperature, would create fluid movement towards the outlet. 
     Alternatively, a magnetic material can be placed within reservoir  20 , and the liquids applied to ports  14   a ,  16   a  comprise ferroflulds which would be propelled through channels  14 ,  16 ,  18  by magnetic forces into reservoir  20 . These ferrofluids could also be used in combination with reagents and specimen samples to either push or pull these fluids through T-Sensor  12  by magnetic forces. 
     FIG. 5 shows another alternative design for operating the T-Sensor of FIG.  1 . In this embodiment, a liquid sample of reagent is applied to port  14   a  of cartridge  10   c  while a liquid sample of specimen is applied to port  16   a . Channels  14 ,  16 , and  18  are sized such that capillary action draws the reagent and specimen through channels  14 ,  16  and  18  of T-Sensor  12  into terminating reservoir  20 . Channels  14 ,  16 , and  18  preferably have dimensions between 10 nanometers and 500 micrometers in width, which act to create capillary action to power T-Sensor  12 . 
     FIG. 6 is still another alternative embodiment for T-Sensor operation. In this cartridge  10   d , T-Sensor  12  includes a first pressure head  40  for receiving a reagent sample and a second pressure head  42  for receiving a specimen sample. To operate T-Sensor  12 , the cartridge is placed vertically and a sample of reagent is introduced into channel  14  via head  40  and a sample of specimen introduced into channel  16  via head  42 . The hydrostatic pressure present by virtue of the orientation of cartridge  10   d  drives the liquids through channel  18  of T-Sensor  12  and into reservoir  20 . The velocity at which the liquids travel through cartridge  10   d  can be varied by tilting the cartridge from the vertical position. 
     The flow of fluids within cartridge  10   d  of FIG. 6 can be significantly enhanced using a structural feature that uses the reservoir edges to improve the “wet-out” of the cartridge. Referring now to FIG. 13, cartridge  10   d  includes pressure heads  40  and  42  which are coupled to channels  14  and  16  respectively, which flow into common channel  18 . Channel  18  eventually connects to a reservoir  20 , which is preferably vented to the atmosphere external to cartridge  10   d  via a venting aperture  21 . Connection  22  between channel  18  and reservoir  20  is a right angle bend which is located at some distance from bottom wall section  23  and reservoir  20 . 
     FIG. 14 is an enlarged view of the connection between channel  18  and reservoir  20 . While the horizontal dimension of the channel mouth at connection  22  may be narrow or wide, vertical dimension  24  must be large enough that the surface tension of a liquid  25  flowing with channel  18  is inadequate to support a liquid drop, if this requirement is fulfilled within the structure of cartridge  10   d , the liquid  25  entering reservoir forms a thin stream as shown at  26  from the bottom of connection  22  and flows down the wall  27  of reservoir  20 . By using this structure, pressure oscillations in channel  18  that would occur as drops form and fall within reservoir  20  is avoided, thus ensuring a smooth liquid flow. 
     Hydrostatically driven microfluidic circuits use gravity as their driving force. The pressure generated is a function of the difference in inlet and outlet liquid height level. This pressure is a constant for a given height difference, and generates a fluid flow in the microfluidic circuit. Flow speed and flow ratios for a given pressure are largely a function of the fluid resistance in these microchannels, and of the viscosity of the solutions. For example, in a given T-Sensor, the fluid flow ratio between the two fluids will depend on the viscosity ratio of the two fluids. Blood has a viscosity about 3-4.5 times greater than that of aqueous solutions. Therefore, blood will flow more slowly than an aqueous solution as the two streams flow along each other in a T-sensor channel. However, for a given hydrostatic pressure, the dividing line between the two streams will remain independent of a change of viscosity (i.e., for the same hydrostatic pressure, the dividing line will be at the center of the channel). The volumes that flow through the channel per time however, are a function of their viscosity, and higher viscosity solutions will flow at a lower volume flow rate. This will eventually cause the water levels of the two inlet streams to become different, since the low-viscosity fluid will drain more quickly. This change in water levels between the two inlet streams then would have an effect on the hydrostatic pressure, and the resulting flow patterns. This can easily be remedied by using comparably large volume pressure heads which change very little during the measurement time. 
     This phenomenon is different from flow patterns in a volume-controlled flow situation (as is usual for syringe-pump positive displacement pumps). In this case, a constant volume flow rate will move the same amount of liquid per time through the channel. In case of different viscosities of sample and reagent, the center line will shift to the side of the lower-viscosity stream, as the higher viscosity stream flows more slowly. 
     The advantages of pressure driven flow in T-Sensors is therefore that the position of the center line is independent of viscosity. This phenomenon can be used to simplify detection in these devices. 
     One method to compensate for a change in flow speed due to a change in viscosity is to include a small “metering bubble” in the inlet channels. The time it takes to fill this bubble is proportional to the flow speed of each solution, which again is proportional to the viscosity of each solution. 
     While hydrostatic pressure is a very constant source of power for microfluidic structures, it may be necessary at times for some types of structures to overcome some initial surface effects while filling a cartridge prior to allowing gravity or capillary forces to keep the flow constant and well controlled. A variety of methods for “jump-starting” the flow can be used: tapping on the inlets, using a syringe to either aspirate or press fluid into the disposable, using a “bubble pump”, or a variety of other processes. 
     Microfluidic cartridges can also be surface-treated (for example, by a so-called Oxygen Plasma process) to create surfaces that can be more easily wetted by aqueous solutions. Such cartridges generally do not need to be “jump-started”, as the capillary forces draw fluids into these cartridges autonomously. By selective surface treatment, some areas of the cartridge can be filled, while others can create so-called surface tension valves, which require a certain amount of pressure to overcome before they wet. 
     An alternate embodiment of the cartridge of FIG. 6 is shown in FIG.  7 . In this cartridge  10   e , a vent passage  50  is connected to reservoir  20 . Vent passage  50  includes a compressible aspiration bubble pump  52  and a closure means  54 . To operate T-Sensor  12  in cartridge  10   e , aspiration bubble  52  is depressed while a reagent sample and a specimen sample are introduced into pressure heads  40 , 42 . After pressure heads  40 , 42  are filled, closure means  54  is activated to seal vent passage  50  from atmosphere. Bubble  52  is then released causing the reagent and specimen to enter T-Sensor  12  via channels  14 ,  16  and  18  and into reservoir  20 . 
     FIG. 8 shows yet another version of the cartridge of the present invention. Cartridge  10   f  contains a compressible pressure bubble  60  which is connected to ports  14   a  and  16   a  via passages  62 ,  64  respectively, while vent passage  50  connects reservoir  20  to atmosphere. T-Sensor  12  in cartridge  10   f  operates as follows: bubble  60  is depressed, forcing air through passages  62 ,  64 , and consequently causing reagent in port  14   a  and specimen in port  16   a  to enter T-Sensor  12  via channels  14 ,  16  and  18  and into reservoir  20 . 
     Bubble  60  can alternatively contain a liquid in equilibrium with its gas phase; some liquids, such as alcohol, acetone, methanol, have a vapor pressure high enough at room temperature to provide a constant pressure source to drive liquids through T-Sensor  12 . The pressure source will only cease once all the liquid in bubble  60  has been converted to the gas phase; until then, pressure will be constant, and only a function of temperature. 
     Another type of microfluidic device which can be readily adapted to manual operation is the H-Filter. The H-Filter structure is described in detail in U.S. Pat. No. 5,932,100, the disclosure of which is hereby incorporated by reference. Manually operated H-Filters can be used to separate components from particulateladen samples such as whole blood, or to manufacture small quantities of chemicals. 
     FIG. 9 shows a cartridge similar to FIG. 6 embodying the present invention in the form of an H-Filter. Cartridge log contains a first pressure head  40  for receiving a reagent sample and a second pressure head  42  for receiving a specimen sample. An H-Filter  70  is coupled to pressure heads  40 ,  42  via channels  14 ,  16  respectively. H-Filter  70  consists of a channel  72  which connects channels  14 ,  16  to a pair of separate reservoirs  74 ,  76 , which are connected only through a common vent  80 . In the embodiment shown in FIG. 9, channel  72  of H-Filter  70  follows a serpentine path which assists in creating turbulence to enhance mixing; however, channel  72  can be straight, such as channel  16   c  shown in T-Sensor  12   c  in FIG.  2 . 
     Referring now to FIG. 9, the operation of cartridge  10   g  will now be described. A sample, such as whole blood, is inserted into pressure head  40 , while an acceptor reagent, such as water or saline is inserted into pressure head  42 . Two parallel laminar streams will flow through channel  72  as the liquids travel from channels  14 ,  16 . Smaller components of the sample stream will diffuse into the acceptor stream. The two parallel flows are then split up into separate reservoirs  74 ,  76  at the end of H-Filter  70 . Reservoir  74  will then contain a sample solution with a reduced concentration of the extracted component, while reservoir  76  contains the acceptor reagent containing the extracted reagent at a level of some fraction of its original concentration in the sample. The contents of both reservoirs  74 ,  76  can then be harvested from cartridge  10   g  for future use, or be processed through further integrated microfluidic structures. 
     It may be advantageous to provide the disposables pre-filled with reagents on the cartridge. Such reagents can be placed in inlet reservoirs, and sealed with a membrane, tape, or the like. Prior to the measurement, the user fills the sample inlet reservoir, and then removes the reagent seal. This process allows the reagents to start flowing down the channel. Alternative sealing methods may be surface-tension controlled orifices which require a certain initial pressure to overcome the surface effects, and to start flowing, or other types of manual or mechanical valves. 
     By combining a flexible manual “bubble pump” (as shown in FIGS. 7 and 8) with a one-way valve, fluids can be pumped in a closed circuit for a period of time. This may be advantageous when extracting or washing samples or reagents, and multiple passes through a microfluidic structure would increase the yield. 
     Mixing solutions in manually driven microfluidic circuits can be either diffusion-based in a laminar channel (as in H-Filters and T-Sensors), or quasi-turbulent. Several structures can be used to mix solutions in a quasi-turbulent fashion. Generally, such structures consist of a succession of corners, edges, zig-zag channels, or other features which increase the likelihood that laminar recirculation or turbulence can be generated in microchannels at the flow speeds available with a manually driven microfluidic circuit FIG. 10 shows the relationship between hydrostatic pressure and Reynolds number for a typical microfluidic mixing structure. A Reynolds number of 10 is usually sufficient to induce mixing. Turbulent or quasi-turbulent mixing is required for relatively large particles (Molecular weight of 1000 KD or more). Alternatively, bubble pumps can generate enough pressure to turbulently mix solutions in most microfluidic structures. 
     Other assay methods that lend themselves to manually driven microfluidic circuits are assays that use agglutination or sedimentation. For example, antibody-antigen complexes may either sediment into a microfluidic reservoir for detection or further processing, or they may form particles that intentionally get stuck in small microfluidic features such as orifices. The presence or absence of these complexes in particular parts of the microstructures can serve as an indicator for the presence, absence, or quantity of an analyte. This could be used to develop a method for blood typing. 
     A simple detection method for analyzing the results of an assay performed in a microfluidic format according to the present invention is shown in FIGS. 15-17. Referring now to FIG. 15, a basic T-Sensor device  12  is shown having an indicator port  14   a  and an analyte port  16   a . Port  14   a  is connected to main channel  18  by a sample channel  14 , while port  16   a  is connected to channel  18  by an analyte channel  16 . In FIG. 15, a high concentration analyte is loaded into port  16   a , and when T-Sensor  12  is operated with an indicator solution within port  14   a , a diffusion pattern forms as shown at  90 . In FIG. 16, a low concentration analyte is loaded into port  16   a , and a different diffusion pattern  92  is generated. 
     At some point along channel  18 , the reaction between the analyte and indicator will become sufficiently intense to be seen visually. This point in channel  18 , which is located some distance from channels  14  and  16 , will correlate with a particular concentration of analyte within channel  18 . Optical aids, such as a magnifying lens, colored filter layer, or slit may aid in the manual visual interpretation of the concentration. 
     An example of a device for simple quantitation of a microfluidic device which requires no external instruments is shown in FIG.  17 . Referring now to FIG. 17, a convoluted T-Sensor  12   e  having ports  14   a ,  16   a  and channels  14 ,  16  contains a main channel  18   a  upon which a viewing window  100  has been inserted. In addition, a chart  102  is placed near T-Sensor  12   e  which contains indicia representative of different concentrations of the desired analyte. During operation of T-Sensor  12   e , quantitation is achieved by interpreting the point at which visible reaction has occurred at the interface between the sample and indicator. The only portion of channel  18   a  visible is seen through viewing window  100 . In this embodiment the analyte may be at a 4+ to 5+ amount (1+ being low, 10+ being high) because in the 6+ view area of window  100 , there is barely an reaction visible 
     This embodiment is aimed at providing a quantitative assessment of analyte concentration in a format that is compatible with manual performance and visual readout. This format is particularly appropriate for use with kinetic assays—in which product builds up with time—since time in a microfluidic channel corresponds to geographic distance. Examples of kinetic assays are enzyme assays, and some slow finding reactions, such as some antibody-antigen reaction. However, this technique may also work with “fast” reactions, because visual interpretation incorporates both intensity of color and “thickness” of color at the interface. 
     FIG. 18 shows a microfluidic device according to the present invention which joins two fluid streams in a manner which prevents the formation of air bubbles within the channels that may hamper the performance of the device. Referring now to FIG. 18, a pair of reservoirs  14   a ,  16   a  are coupled to a main microfluidic outlet channel  18  by a pair of inlet channels  14 ,  16  respectively. In this embodiment, channel  18  is tapered such that it is wider at section  18   a  where it connects to channels  14 ,  16 . Channel  14  connects to reservoir  14   a  at  110 , while channel  16  connects to reservoir  16   a  at  112 . 
     When reservoirs  14   a ,  16   a  are initially filled, a geometry of channels  14 ,  16  and  18  make it difficult to avoid trapping an air bubble within channel  18 , because typically the liquid in one channel flows in section  18   a  of channel  18  before the liquid in the other channel breaks through the surface tension barrier where the channel meets the bottom of the reservoir. As shown in FIG. 18, fluid from reservoir  14   a  has already reached channel  18 , as shown at  114 , whereas the fluid in reservoir  16   a  is still trapped by the surface tension of the liquid-gas interface where reservoir  16   a  meets channel  16 , as shown at  112 . 
     To avoid trapping an air bubble in this microfluidic device, two constraints must be addressed in the design of the microfluidic circuitry. First, the outlet channel  18  must be large enough to contain the volume of inlet channel  14  from reservoir  14   a  without trapping the air in channel  16 . Second, the pressure differential between the pressure in outlet channel  18  and the pressure at  110 , which is due to the fluid resistance to flow within channel  14 , must be greater than the pressure differential between the pressure in outlet channel  18  and the pressure at  112 , which is due to the surface tension of the gas-liquid interface at  112 . This is accomplished by narrowing the width of channel  16  as needed for the surface tension characteristic of the liquid and gas pair. 
     Surface tension valves are based on the difference in resistance contained in unwetted orifice as compared to a wetted orifice. These valves are considered “single use” valves, as once they are wetted, they behave essentially like a low in resistance flow channel (dynamic resistance); however, these valves present enough resistance (static resistance) to block the fluid flow, even if some force is applied to them such as pressure, capillary forces, or hydrostatic pressure. Once this static force is overcome by a “breaking force,” fluid will flow through the valve. 
     Resistance of the inlet channels  14 ,  16 , however, is only dynamic resistance. The more the inlet channels are filled, the higher the resistance (provided that there are no capillary forces to drive fluids through these inlet channels). 
     In operation, as the cartridge is being filled, only the resistance of one of the inlet channels  14 ,  16  becomes higher than the static resistance of the surface tension valve of the other inlet channel, the surface tension valve will break, and fluids from both reservoirs  14   a ,  16   a  will flow into outlet channel  18 . Surface tension valves typically consist of an orifice, a channel terminating in sharp edges, or other structures which create a high surface tension. Alternatively, the surface of a channel can be coated with a specific material in the valve area, thereby increasing the surface tension resistance at that point. However, the actual resistance exhibited by these valves depends to a large extent on the exact dimensions and surface properties; thus, manufacturing tolerances can have a significant impact on the accuracy of these valves. 
     One method of improving the reliability of surface tension valves would be to provide multiple redundant surface tension valves in parallel rather than a single valve. Although the multiple valves will vary statistically in resistance, but the average of the group will be close to that of a single valve. For example, by placing multiple small orifices between two channels, the same approximate surface tension can be achieved each time. FIG. 11 shows the use of multiple orifices within a channel to exhibit this principle. 
     Finally, an alternative design for a surface tension valve would provide a maximum breaking resistance, along with a minimum breaking resistance, for the valve. 
     The principles of the present invention can be applied to many other types of products. For example, a cartridge containing a microfluidic device as described can be used as science kits, such as a miniature chemical laboratory, for educational purposes. Another use could be as a novelty device that uses fluid flow to visualize specific patterns, such as company logos, names, signatures, and the like on a small plastic card roughly the size of a standard credit card. 
     While the invention has been shown and described in terms of several preferred embodiments, it will be understood that this invention is not limited to these particular embodiments and that many changes and modifications may be made without departing from the true spirit and scope of the invention as defined in the appended claims.