Abstract:
A microfluidic system and method for its fabrication is disclosed comprising an interposed intermediate layer covering the channel-containing layers, said intermediate layers comprising an integral valve made from the intermediate layer material. A microfluidic system and method is also disclosed for actively pumping a fluid through an integrated layered device incorporating the above-mentioned channels, valves in communication with a chamber, the volume of which can be predictably controlled by interaction between a magnetizable assembly placed on pre-selected sides of the chamber.

Description:
BACKGROUND  
       [0001]     The present invention is directed generally to the field of microfluidic devices. More particularly the present invention is directed to novel valving components for microfluidic devices where such valve components are fabricated integrally on the device substrate.  
         [0002]     In microfluidic systems, development of on-chip propulsion and valving components is important, for example, to reduce or eliminate the sample dead volumes and, thus, to improve the analytical performance of a microfluidic system. Use of microfluidic chip valves is known. See U.S. Pat. Nos. 6,581,899; 6,575,188; 6,561,224; 6,527,003; 6,523,559; 6,448,090; 6,431,212; 6,406,605; 6,395,232; 6,382,254; 6,318,970; 6,068,751; 5,932,799, all of which are incorporated by reference herein as if made part of the present specification. However, the need exists in improving parameters of existing reported valves for microfluidic systems. In particular, existing valves suffer from being too large, too expensive, having poor respond time, or not being sufficiently robust. In addition, there is a need to integrate such valves with a diaphram chamber, to achieve the positive flow or pumping of the fluid in a microfluidic device Use of magnet for movement in a laminated structure is known. See U.S. Pat. No. 5,472,539, which is incorporated by reference herein. However, the integration of magnet activation for a pump chamber incorporating valves in a laminated microfluidic device is not known.  
       SUMMARY  
       [0003]     It is highly desirable to develop a valve that is intrinsically located on a microfluidic chip and meets the requirements of small size and low fabrication cost. Embodiments of the invention address the limitations of known valves for microfluidic systems and are directed to a new type of valve for incorporation in microfluidic systems.  
         [0004]     Embodiments of the invention are further directed to a valve, preferably a microfluidic valve, fabricated on the same substrate as the microfluidic channels in a microfluidic device.  
         [0005]     In addition, embodiments of the invention are directed to a microfluidic device having a first layer made from a first material having a channel, and a second layer made from a second layer material. The second layer is in intimate contact with the first layer, and the second layer comprises an integral valve made from the same material as the second layer material.  
         [0006]     Still further, embodiments of the invention are directed to a microfluidic device having a multilayered structure with a first layer made from a first layer material and having at least one channel, and a second layer made from a second layer material, with second layer in intimate contact with the first layer. The second layer comprises an integral valve made from the second layer material, with the valve aligned and dimensioned to cover a channel.  
         [0007]     Yet, still further, embodiments of the invention are directed to a method for analyzing an analyte by providing a microfluidic device comprising a multilayered structure. The structure includes a first layer made from a first layer material and having at least one channel, and a second layer made from a second layer material, with the second layer in intimate contact with the first layer. The second layer comprises an integral valve made from the second layer material, and the valve aligned and dimensioned to cover a channel. An amount of analyte is then provided and introduced to the microfluidic device, and is then analyzed.  
         [0008]     Embodiments of the invention are also directed to a method for analyzing an analyte including the steps of providing a microfluidic device having a first channel-containing layer and a second channel-containing layer with an intermediate layer interposed between, and in intimate contact with the first and second channel-containing layers. The intermediate layer comprises an integral valve aligned and dimensioned to cover at least one channel. An amount of analyte is provided and introduced to the microfluidic device, and is then analyzed.  
         [0009]     According to another embodiments, there is provided a structure for actively pumping a fluid through an integrated, layered device including above-mentioned channels and valves. In such a preferred structure, the device has a chamber acting as a diaphram, with the volume of the chamber controlled by the interaction between a magnet placed on one side of the chamber and an electrical coil place on another side of the chamber. Activation of the coil to attract the magnet compresses the chamber, pushing fluid out through one check valve, while coil activation to repel the magnet expands the chamber, bringing fluid into it through another check valve. Such a structure can be used to control the amount and type of analyte provided to other areas of the microfluidic device.  
         [0010]     These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIGS. 1A and 1B  are schematic representations of one embodiment constructed in accordance with the invention showing an integrated valve of the device in operation as pressure differentials occur within channels.  
         [0012]      FIGS. 2A and 2B  are schematic representations of another embodiment constructed in accordance with the invention showing an integrated valve of the device in operation.  
         [0013]      FIG. 3  is a cross-sectional side view of another embodiment of the microfluidic valve device of the present invention.  
         [0014]      FIGS. 4A and 4B  are overhead and cross-sectional side views, respectively, of another embodiment of the microfluidic valve device of the invention showing the glass substrate and first channel layer.  
         [0015]      FIGS. 5A and 5B  are overhead and cross-sectional side views, respectively, of the microfluidic valve device of  FIGS. 4A and 4B  showing the addition of connecting via layer on the first channel layer.  
         [0016]      FIGS. 6A and 6B  are overhead and cross-sectional side views, respectively, of the microfluidic valve device of  FIGS. 5A and 5B  showing the addition of gold release and valve layers on the connecting via layer.  
         [0017]      FIGS. 7A and 7B  are overhead and cross-sectional side views, respectively, of the microfluidic valve device of  FIGS. 6A and 6B  showing the second channel layer on the via layer.  
         [0018]      FIG. 8A  is an overhead view of a device constructed in accordance with embodiments of the invention, the device having multiple check valves in place to form a microfluidic circuit.  
         [0019]      FIG. 8B  is a schematic representation of the device shown in  FIG. 6A .  
         [0020]      FIG. 9  is a cross-sectional schematic diagram of a microfluidic device constructed in accordance with embodiments of the invention and incorporating a diaphragm chamber activated by interaction between a magnet placed on top of the uppermost layer, and a coil patterned on the underlying substrate. Arrows on the schematic show the direction of fluidic flow during operation of the diaphragm chamber. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0021]     Embodiments of the invention are directed to a valve incorporated in microfluidic systems with one or more of the following features. The microfluidic valve is located integrally within the microfluidic system and therefore is desirably dimensioned to selectively and predictably seal channels or otherwise direct flow within a channel of a microfluidic device. The valve is preferably made from the same material as the microfluidic device substrate and therefore has a desirably low cost and low fabrication processing cost.  
         [0022]     Operation of the valve is depicted in  FIGS. 1A-1B  and  2 A- 2 B. Upon applying pressure P 0  to a valve where P 0  is less than the critical opening pressure P C  created by the main flow (with arrow indicating flow direction), the valve remains closed and only the main flow is passing through the microfluidic channel (see  FIG. 1A ). Upon applying pressure P 1  to the valve where P 1 &gt;P C , the valve is open and the second flow is passing into the microfluidic channel (see  FIG. 1B ).  
         [0023]     Alternatively, the valve can be used as a passive one-way flow controller. For example, as shown in  FIGS. 2A and 2B , when the pressure in a Channel  1  is greater than the pressure in the second Channel  2 , the pressure difference will force the valve to open and allow flow from the Channel  1  toward the Channel  2 . However, when the pressure in the Channel  2  is greater than the pressure in the Channel  1 , the position of the valve will be dictated by the valve seat, resulting in a closure of the valve and no further liquid flow, in this case, from Channel  1  into Channel  2  or from Channel  2  into Channel  1 .  
         [0024]      FIG. 3  shows a cross-sectional side view of a microfluidic device having a check valve  9 . A first layer  10  made from a first layer material, is in contact with, and preferably laminated to, a second layer  12  made from a second layer material. A substrate  24 , such as a silicon—(Si), glass-, or plastic-based substrate, having a channel  20 , is provided and brought into contact with a third layer  22  made from a third layer material. Preferably the substrate  24  is adhered to the third layer  22 . The third layer  22  also has a channel  18 . At least a portion of the channel  20  of the glass substrate  24  overlaps at least a portion of the channel  18  of the third layer  22 . A metal, preferably gold, or other non-stick layer  16  of a predetermined size is positioned to cover the channel  18  and preferably extends beyond the channel  18 . The layers are preferably adhered together. A laser cut  14  made in the second layer  12 , thereby forming a check valve, or flap-like structure  9 . According to the present invention, the laser cut is typically made in a “U” shape to allow formation of a flap thus making the check valve  9 .  
         [0025]     As shown in  FIG. 3 , the solid arrow indicate the fluid flow direction. The check valve  9  opens in response to vacuum. The small area opening  21  on the input side of the device may be designed to inhibit pressure to push the check valve open. The check valve  9  should, however, preferably allow a slight vacuum on the output side  23  of the device to open the check valve  9 . Thus, according to this embodiment, the presence of the check valve  9  insures unidirectional movement of the fluid flowing through the channel  20 .  
         [0026]      FIGS. 4A and 4B  respectively show a top view and a cross-sectional side view of a partial construction of a device made according to one embodiment of the invention. As shown, a glass substrate  26  is in contact with a layer  28  having a first channel  29 . Preferably, the first channel  29  is laser micro-machined into layer  28 . Fluid can access channel  29  through substrate  26  from channel  17 .  
         [0027]      FIGS. 5A and 5B  respectively show a top view and cross-sectional side view of the construction of  FIGS. 4A and 4B . An additional layer  22  is in contact with layer  28  with opening via  30  machined through layer  22 .  
         [0028]      FIGS. 6A and 6B  show the progressive construction shown in  FIGS. 5A and 5B  with an additional metal release layer  16  placed over via  30  and thus, over a portion of layer  22 . A valve layer  12  is then placed over the metal release layer  16 . A “U”-shaped cut  14  is made through valve layer  12  to the metal release layer  16  as shown in a top view in  FIG. 6A  forming check valve  9 . Finally, as shown in  FIGS. 7A and 7B , a second channel layer  42  having a channel  37  with fluid input  38   a  and fluid output  38   b  is aligned over valve  9  and applied to form the microfluidic device. It is understood that adhesive layers  25  are applied between substrate. Representative thicknesses are exaggerated for illustrative purposes and not for the purpose of depicting actual or relative layer thicknesses.  
         [0029]      FIGS. 7A-7B  and  8 A- 8 B show various embodiments of the microfluidic devices having multiple channels. A vacuum can be applied to one or more channels for such devices by pulling fluid through the microfluidic device. According to the present invention, the placement of the integral check valve(s) allows the predictable and desired regulation of fluid flow through the channels.  
         [0030]      FIG. 9  shows an integrated microfluidic device incorporating both check valves,  9   a  and  9   b  at two different levels of the layered device having a chamber  54  in layer  55 . The substrate  26  on which the device is fabricated has an electrical coil structure  52  patterned thereon, over which subsequent layers are applied. Fabrication of the channels and valve structures are carried out as previously described. After completion of the microfluidic portion of the device, which includes substrate  26  with patterned electrical coil structure  52 , fluidic channel  20 , chamber  54 , valves  9   a  and  9   b , output channel  58 , and magnet  56  are applied to the top of the device and positioned over chamber  54  and coils  52 . Preferably, the magnet may be a molded magnet structure that is subsequently magnetized in an electric field, or consists of a permanent magnet that is positioned and preferably held in place by an adhesive. As will be appreciated by one skilled in the field, depending upon the polarity applied to the coils  52 , a magnetic field is produced which either attracts or repels the magnet  56 , vertically moving the layers of the device  55 ,  57 , and  59  either toward or away from the layers  12 ,  19 , and  22 , consequently predictably changing the volume of chamber  54 . The volume change will cause fluid movement through check valves  9   a  and  9   b , resulting in a pumping action through the microfluidic device. Applying polarity to the coils in a ramp function or microstep function versus a large step function will cause the magnet to move more slowly and consequently cause the chamber to expand or contract slowly, thus minimizing any damage to cells or other fragile structures that may be present in the fluid being pumped through the device.  
         [0031]     Magnet  56  is a micromolded permanent magnet adhered to a substrate. As will be appreciated by one skilled in the field, substrate  24  is representative of a variety of substrates that may comprise movable elements of micromechanical structures. Magnet  56  preferably is a rare earth NdFeB magnet comprising powdered NdFeB metal suspended in a thermosetting plastic, cured, and magnetized employing, for example, a magnetic field strength in the order of about 20 kOe, produced by a suitable electromagnet.  
         [0032]     The fluids presented to the channels and chambers in the devices of the present invention may comprise an analyte, which is understood to be a substance or chemical constituent that is undergoing analysis. Typically, the analyte can be of chemical, biological or physical nature. Examples of analytes include molecules, living cells, bacteria, other organisms and fractions of organisms and tissue, clusters of molecules and atoms, nanocrystals, etc. In one embodiment, the preferred diaphragm/magnet assembly is analogous to a heart chamber with the channels/valves/fluid taking on the role of a circulatory system, possibly containing cells (e.g. blood). A further embodiment is contemplated to be useful in modeling a biological system for use in bio-research, potentially reducing the need for animal testing.  
       EXAMPLE  
       [0033]     A flexible structure was made from Kapton® (polyimide) as a microfluidic valve component. The Kapton® structure, combined with a gold release layer, and an opening to direct fluid flow, created the reliable integral microfluidic check valve of the present invention.  
         [0034]      FIG. 3  shows a cross section of a device fabrication where a Kapton® layer  22  was laminated onto a Si, glass or plastic substrate  24 . As shown in  FIGS. 6A-6B , a patterned gold release layer  16  was deposited onto the Kapton® layer surface  22 , followed by the deposition of the layer  12  in which the flap valve was to be cut. A laser cut “U”shape  14  was made through the flap layer  12  to the gold release layer  16  forming a flexible structure with an effective hinge at one end (the base of the “U”).  
         [0035]     According to one embodiment, these aforementioned structures are preferably fabricated out of thermally laminated Kapton® structures with laser micro-machining to produce channels and valve structures, but could be made from any suitable microfluidic system substrate material as would be understood by one skilled in the field of microfluidics. For example, if light transmission through the laminated structure is desired down to 350 nm or below, more (near UV) transparent films, such as Bayer Apec Polycarbonate, Solvay Udel, or Radel Polysulfone, or Dyneon THV-220 Fluorothermoplastic can be used in place of the Kapton® film. According to one embodiment, each layer is preferably hot press laminated to the previous laser-machined layer. In this way, registration of all except the top most layer, is not necessary during the lamination process. All alignment preferably is done at the laser operation, such that each laser-machining step is in registration relative to the previous layers. In this way, the structure is built up much like an integrated circuit chip rather than a multi-level circuit board where pre-patterned layers are pinned together and only laminated as a final step. The top-most layer, in which a channel has been pre-micro-machined, must be aligned over the check valve to provide it to a cavity to operate while also providing a channel for fluid to flow.  
         [0036]     The preferred adhesives used for laminating the multiple layers used in the microfluidic devices preferably must adhere well to the underlying substrate on which the fluidic device is fabricated, and to the layers of material forming the device. They must be thermally stable during multiple lamination processes. They must be resistant to the fluids used in the channels during device operation that might include water of different pH and/or chemical solvents. Further, the preferred adhesives must be laser-processable to allow formation of the channels and valves. Adhesives which can be used for this application preferably include thermoplastic polymers such as polyimide, polysulfone, polycarbonate and acrylic materials and blends of such polymers with cycloaliphatic epoxy with a thermal epoxy curing catalyst present such that a thermoset layer is formed during lamination. One preferred adhesive to be used for lamination is a GE developed material, composed of a siloxane containing polyimide, SPI-135, available from MicroSi Corp, Phoenix, Ariz., blended with ERL-4221 epoxy, available from Dow Chemical, Midland, Mich. and UV9380C catalyst, available from General Electric Specialty Materials, Waterford, N.Y. This adhesive blend has excellent adhesion to Kapton®, is resistant to attack from water and most solvents, but releases cleanly from a metal surface, especially a gold surface.  
         [0037]     The adhesives used in connection with embodiments of the invention preferably facilitate the use of Kapton® structures where selected flaps can move to create micro-fluidic check valves, when photolithography is used to define small gold areas that act as release layers.  
         [0038]     The “U”-shaped cuts made in the films of embodiments of the invention are preferably made with a tripled (355 nm) or quadrupled (266 nm) YAG laser, or an excimer laser at 308 nm or 248 nm. The thickness of the layers to be laser-machined may be from about 12 to about 25 μm thick, with the precise thickness dependent upon the material characteristic, such as, for example, flexibility.  
         [0039]     The layers must have similar properties relative to the selected adhesive, such as resistance to water and solvents, thermal stability relative to multiple lamination cycles (to retain channel integrity), and laser processability. Such preferred materials include polyimides such as Kapton®, Upilex® and Ultem®, high temperature polycarbonates such as Bayer Apec (especially if clear, transparent and colorless fluidic devices are desired for possible optical analysis), polysulfone films, PEEK (polyether ether ketone) and possibly PVDF film made from Kynar® plastic, also available from Westlake Plastics.  
         [0040]     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of he described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.