Abstract:
A microfluidic-based flow assay and methods of manufacturing the same are provided. Specifically, the microfluidic flow assay includes a micropatterned surface that induces clot formation and an array of microfluidic channels though which blood flows. The micropatterned surface contains two clotting stimuli, one for inducing platelet adhesion and another for inducing the coagulation cascade.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/313,257, filed Mar. 12, 2010, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The invention relates to a microfluidic-based flow assay for use in analyzing bleeding and thrombotic disorders, dosing anticoagulant and antiplatelet drugs, tracking the effects of pharmacological interventions on thrombosis, and methods of making the same. 
     BACKGROUND OF INVENTION 
     Maintaining the balance between bleeding and thrombosis remains one of the greatest challenges facing the biomedical community. Excessive bleeding is an important medical issue. For example, post partum bleeding represents a leading cause of maternal mortality and causes serious morbidity in developing countries. Individuals with genetic bleeding disorders, such as hemophilia, have a decreased ability to clot blood because of deficiencies in certain coagulation factors. 
     On the other end of the spectrum, excessive clotting, or thrombosis, is a major complication of surgery and is integrally involved in atherosclerosis, obesity, infection, diabetes, cancer, and autoimmune disorders. Over the last decade, significant advances have been made in understanding the molecular basis of bleeding and thrombotic disorders; however, a large portion of the observed variability remains unknown. 
     Parallel with these discoveries, there has been a rapid development of new drugs like recombinant proteins for replacement and interventional therapies. Interestingly, what remains strikingly deficient in clinical hematology are techniques to diagnose a very broad range of disorders of both deficient and excessive clotting as well as to monitor the effects of therapeutic interventions. 
     Diagnosing the severity of bleeding disorder is impossible with current bleeding assays, particularly because most current bleeding assays test for either platelet function or coagulation, but not both. Thus, most existing solutions do not properly create an environment which properly simulates a natural human wound or point of bleeding. In addition, most of these conventional assays occur under static, or no flow, conditions. Since blood is a moving fluid, however, there are several advantages to studying it under flow in bleeding diagnostics. 
     SUMMARY OF INVENTION 
     It is, therefore, one aspect of the present invention to provide a device which contains two clotting stimuli, one for inducing platelet adhesion and another for inducing the coagulation cascade. 
     It is another aspect of the present invention to provide a device which allows blood to flow over a micro-patterned surface which induces clot formation. In some embodiments, a microfluidic channel is provided with one or more clot inducing areas. Each of the one or more clot inducing areas may include a micro-patterned surface that induces clot formation via two different stimuli (e.g., inducing platelet adhesion and inducing coagulation). 
     It is another aspect of the present invention to combine the physics of blood flow and the biology of the clotting system into a single device. 
     In accordance with at least some embodiments of the present invention, a microfluidic flow assay is provided which accounts for the three main factors which contribute to the formation of a blood clot: platelets, coagulation, and blood flow. 
     Platelets are the first responders to a vascular injury. A vascular injury can be due to trauma or the rupture of an atherosclerotic plaque. Platelets adhere to proteins, especially collagens, found underneath the cells that line blood vessels and von Willebrand factor, which is secreted by endothelial cells and platelets. Following platelet adhesion, a series of enzymatic reactions occur that are collectively known as the coagulation cascade. The main catalyst for the coagulation cascade is a transmembrane protein called tissue factor. 
     Embodiments of the present invention provide a microfluidic device having a clot inducing area in a microfluidic channel though which blood is allowed to flow, where the clot inducing area includes a mixture of collagen, von Willebrand factor, and tissue factor. In some embodiments, the area(s) of tissue factor which are exposed to blood flowing thereby are interspersed in the collagen in a predetermined pattern. 
     Because there is significant variability in clotting factors and blood cell counts in the healthy population, it is useful to provide a microfluidic device in which the microfluidic channel(s) and clot inducing areas, which are also referred to as prothrombotic surfaces, are homogeneous and repeatable. Otherwise, it may become difficult to determine whether differences in platelet and fibrin accumulation are variations in blood constituent or variability in the prothrombotic surface. It is, therefore, another aspect of the present invention to standardize the methods for patterning molecules that stimulate these two mechanisms and evaluate the microfluidic flow assay in a clinical setting. More specifically, embodiments of the present invention provide a homogeneous, repeatable collagen patterning method for measuring platelet adhesion. Embodiments of the present invention also provide a repeatable method for co-patterning collage and tissue factor for measuring coagulation defects. 
     It is another aspect of the present invention to provide a flow assay which allows the in vitro study of platelet response to defined surfaces at controlled wall shear stresses (e.g., via use of a microfluidic channel). 
     In accordance with at least some embodiments of the present invention, a microfluidic device is provided which generally comprises: 
     at least one microfluidic channel; and 
     at least one prothrombotic surface provided in the at least one microfluidic channel, wherein the at least one prothrombotic surface is capable of inducing both platelet adhesion and coagulation cascade. 
     In accordance with at least some embodiments of the present invention, a method of manufacturing a microfluidic device is provided which generally comprises: 
     providing a substrate; 
     creating at least one prothrombotic surface on the substrate, wherein the at least one prothrombotic surface is capable of inducing both platelet adhesion and coagulation cascade; and 
     establishing at least one microfluidic channel which intersects at least a portion of the at least one prothrombotic surface. 
     In accordance with at least some embodiments of the present invention, a microfluidic device made by the above-described method is also provided. 
     In accordance with at least some embodiments of the present invention, a microfluidic channel through which blood is capable of flowing is provided that generally comprises: 
     at least one prothrombotic surface provided as a part of at least a portion of one surface in the channel, wherein the at least one prothrombotic surface is capable of inducing both platelet adhesion and coagulation cascade in the blood. 
     In accordance with at least some embodiments of the present invention, a kit for measuring clot formation is provided which generally comprises: 
     a vacuum or hemetically sealed microfluidic device, the microfluidic device comprising at least one microfluidic channel and at least one prothrombotic surface provided in the at least one microfluidic channel, wherein the at least one prothrombotic surface is capable of inducing both platelet adhesion and the coagulation cascade. 
     In accordance with at least some embodiments of the present invention, a method of measuring clot formation or clotting characteristics is provided which generally comprises: 
     causing blood to flow through a microfluidic channel under laminar flow conditions, wherein the blood flows through the microfluidic channel and across at least one prothrombotic surface provided in the microfluidic channel, wherein the at least one prothrombotic surface is capable of inducing both platelet adhesion and coagulation cascade; and 
     analyzing, around the at least one prothrombotic surface, a number of blood cells which have substantially stopped flowing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  depicts a top view of an exemplary microfluidic device in accordance with at least some embodiments of the present invention; 
         FIG. 2   a  depicts an exploded top view of a portion of an exemplary microfluidic channel in accordance with at least some embodiments of the present invention; 
         FIG. 2   b  depicts an exploded partial cross-sectional view across line  2 - 2  of an exemplary microfluidic channel in accordance with at least some embodiments of the present invention; 
         FIG. 3  depicts an exemplary method of manufacturing a microfluidic device in accordance with at least some embodiments of the present invention; 
         FIG. 4  depicts a partial cross-sectional view of a microfluidic channel at a first step of manufacturing in accordance with at least some embodiments of the present invention; 
         FIG. 5  depicts a partial cross-sectional view of a microfluidic channel at a second step of manufacturing in accordance with at least some embodiments of the present invention; 
         FIG. 6  depicts a partial cross-sectional view of a microfluidic channel at a third step of manufacturing in accordance with at least some embodiments of the present invention; 
         FIG. 7  depicts a partial cross-sectional view of a microfluidic channel at a fourth step of manufacturing in accordance with at least some embodiments of the present invention; 
         FIG. 8  depicts a partial cross-sectional view of a microfluidic channel at a fifth step of manufacturing in accordance with at least some embodiments of the present invention; and 
         FIG. 9  depicts a partial cross-sectional view of a microfluidic channel at a sixth step of manufacturing in accordance with at least some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in connection with methods, devices, and systems used for testing blood clotting or determining whether an individual is prone to blood-clotting issues. However, those skilled in the art will appreciate that embodiments of the present invention are not limited to the field of blood flow and can be utilized in other fields without departing from the scope of the present invention. 
     Referring initially to  FIG. 1 , an exemplary microfluidic device  100  will be described in accordance with at least some embodiments of the present invention. More specifically, the microfluidic device  100  may include a plurality of fluid-receiving passages  104   a ,  104   b ,  104   c , which are capable of receiving fluid at a receiving end  108  and allowing said fluid to flow through a microfluidic channel  116  to a terminal end  112 . 
     In accordance with at least some embodiments of the present invention, the fluid flowing through the microfluidic channel  116  may be blood, such as human blood. Additional details of microfluidic channels which facilitate laminar flow conditions are described, for example, in U.S. Pat. No. 7,318,902 to Oakey et al., the entire contents of which are hereby incorporated herein by reference. 
     Although three receiving ends  108  are depicted, one skilled in the art will appreciate that one or more of the microfluidic channels  116  may split into multiple channels, thereby resulting in a number of terminal ends  112  which exceeds the number of receiving ends  108 . As can be appreciated by one skilled in the art, however, the number of receiving ends  108  may equal the number of terminal ends  112 . The configuration and design of the microfluidic channels  116  can vary without departing from the scope of the present invention. 
     In addition to comprising microfluidic channels  116 , the microfluidic device  100  may also comprise one or more prothrombotic structures  120   a ,  120   b  which intersect one or more of the microfluidic channels  116 . In particular, a prothrombotic structure  120   a ,  120   b  include a first end  124 , a second end  128 , and a prothrombotic surface  132  therebetween. The prothrombotic surface  132  may intersect the microfluidic channel  116  at an area of intersection generally referred to as a clot forming area  136 . This clot forming area  136  is an area within the microfluidic channel  116  in which both platelet adhesion and coagulation cascade is induced in the blood flowing through the microfluidic channel  116 . 
     The specific properties of the prothrombotic surface  132  which induce both platelet adhesion and coagulation cascade will now be described in connection with  FIGS. 2   a  and  2   b . In particular, the clot forming area  136  is depicted in further detail in  FIGS. 2   a  and  2   b . In accordance with at least some embodiments of the present invention, the prothrombotic surface  132  includes collagen  212  or a similar material known to induce platelet adhesion. The prothrombotic surface  132  may also include structures of tissue factor  216 , which are designed to induce the coagulation cascade. By providing the prothrombotic surface  132  with both collagen  212  and tissue factor  216 , the prothrombotic surface  132  is capable of inducing both platelet adhesion and coagulation cascade in blood flowing through the microfluidic channel  116  when the blood traverses the clot forming area  136 . 
     Surrounding the clot forming area  136  in the microfluidic channel  116  are a first  204  and second  208  passive surface, which may or may not include endothelial cells, and which is generally neutral with respect to inducing blood clotting. Accordingly, the amount of blood clotting induced within the microfluidic channel  116  can be tightly controlled by precisely controlling the size of the prothrombotic surface  132  and the amount of tissue factor  216  provided therein. 
     In accordance with at least some embodiments of the present invention, the first  204  and second  208  surface may be considered a neutral or passivated surface. In some embodiments, a lipid is used for the first  204  and second  208  surface. In particular, a bovine serum albumin (BSA) may be utilized as a passivity protein. This particular protein is known not to induce any type of blood clotting, such as platelet adhesion or the coagulation cascade. 
     The width of the prothrombotic surface  132  may vary depending upon desired clotting or the size of the microfluidic channel  116  (e.g., cross-sectional area of the microfluidic channel  116 ). In some embodiments, the width of the prothrombotic surface  132  may be about 100 microns. This may be a particularly useful size of prothrombotic surface  132  if the microfluidic channel  116  comprises a cross sectional area of about 50 microns×250 microns. This particular geometry is useful because it provides an area of constant shear stress across the middle of the channel  116 . As can be appreciated by one skilled in the art, however, the actual width of the prothrombotic surface  132  can have a greater or lesser size without departing from the scope of the present invention. 
     The structures of tissue factor  216  may comprise any type of shape. For example, although the structures of tissue factor  216  are depicted as having a generally circular cross-section, the structures of tissue factor  216  may comprise a square cross-section, oval cross-section, rectangular cross-section, or unshaped cross-section. In some embodiments, a circular cross-section tissue factor  216  may comprise a diameter of between 10-100 microns. Smaller or larger structures of tissue factor  216  may be used. It should be noted that a 10 micron diameter island of tissue factor  216  substantially represents a single cell in a human. Thus, utilization of a structure of tissue factor  216  having a diameter of about 10 microns may be preferable for modeling a typical human bleeding environment. Additionally, the number of structures of tissue factor  216  provided in the clot forming area  136  can be any number larger than one and the distribution of the structures of tissue factor  216  within the collagen can either be symmetrical, asymmetrical, or random. As one exemplary distribution, lanes of tissue factor  216  may be provided in the clot forming area  136  that traverse substantially the length of the clot forming area  136  but do not traverse the width of the clot forming area  136 . The lanes of tissue factor  216  may be separated by non-tissue factor lanes. 
     Moreover, the ratio of collagen  212  surface area to tissue factor  216  surface area in a given prothrombotic surface  132  is less than 1:1. In more preferred embodiments, there is at least a 2:1 ratio of collage  212  surface area to tissue factor  216  surface area, meaning that for every square nm of tissue factor  216  exposed there is at least two square nm of collagen  212  exposed. This ratio can also vary according to conditions and the size of the microfluidic channel  216  without departing from the scope of the present invention. 
     In accordance with at least some embodiments of the present invention, the collagen  212  is constructed of a fibrillar type  1  collagen. In some embodiments, an equine or rat tail-based collagen can be used. In certain embodiments where a tighter control on variables within the microfluidic device  100  is required, an acid soluble collagen having a more homogeneous surface than a non-acid soluble collagen can be utilized. For example, rat tail digested in a pH 3 environment can be utilized as a purer form of collagen than a non-treated collagen. 
     In accordance with at least some embodiments of the present invention, the structures of tissue factor  216  can be constructed of a lipid or lipid membrane that is used as an expressed surface of “activated” cells. This creates a significant amount of a molecule known as thrombin when blood plasma interacts with the lipid. Thrombin is a known serine protease that creates a biopolymer of fibrin by cleaving fibrinopeptide from the plasma protein fibrinogen. Fibrin forms a highly entangled hydrogel that provides the scaffold onto which a blood clot grows. Generally speaking, high concentrations of thrombin are created during via the extrinsic or tissue factor pathway of the coagulation cascade; this is why tissue factor is known as a coagulation cascade inducing agent. 
     As can be seen in  FIG. 2   b , the first  204  and second  208  areas of neutral material as well as the prothrombotic surface  132  (comprising the layer of collagen  212  and structures of tissue factor  216 ) may be provided on a substrate  218 . Additionally, the microfluidic channel  116  may be enclosed with a lid or top layer  220 . In some embodiments the substrate  218  and/or lid or top layer  220  is constructed of polydimethylsiloxane (PDMS) or a similar type of silicone. As an alternative, or in addition, the substrate  218  and/or lid or top layer  220  is constructed of glass, plastic, gold, combinations thereof, or any other type of known substrate material used in surface chemistry. 
     Referring now to  FIGS. 3-9 , an exemplary method of constructing a microfluidic device  100  will be described in accordance with at least some embodiments of the present invention. The method begins by providing a substrate  218  ( FIG. 4 ; step  304 ). Thereafter, the substrate  218  is functionalized (step  308 ). In this step, the substrate  218  may be treated with octadecyltrichlorosilane (OTS), thereby creating a monolayer of OTS on the upper surface of the substrate  218 . Methods of rendering substrates, such as glass substrates, hydrophobic are well known in the art. Methods of functionalizing the substrate  218  include, without limitation, rendering the substrate hydrophobic, hydrophilic, reactive (via amine or carboxylic acid groups), or some other chemistry. In particular, different surface chemistries may allow different molecules to be patterned in a specific configuration. In one embodiment, silane chemistries may be used on glass substrates. 
     After the substrate  218  has been functionalized, the method continues by forming the prothrombotic surface  132 , which will ultimately include the clot forming area  136  ( FIG. 5 ; step  312 ). In particular, the BSA may be provided as the first  204  and second  208  surface. The void between the first  204  and second  208  surfaces generally corresponds to the channel in which the prothrombotic structure  120   a  or  120   b  will be created. This void may be created by providing a masking layer on the substrate  218  prior to applying the BSA to the substrate  218 . After the BSA has been provided, the masking layer may be removed from the substrate  218 , thereby exposing the void between the first  204  and second  208  areas. 
     After the BSA has been laid down on the substrate  218 , the method continues by positioning a pillar-creating structure  604  on the substrate  218 , particularly in the clot forming area  136  ( FIG. 6 ; step  316 ). This pillar-creating structure  604  may comprise a PDMS structure that is formed to have pillars or structures of a size substantially the same as a desired size of the structures of tissue factor  216 . In some embodiments, the pillar-creating structure  604  comprises a single structure having a plurality of posts extending therefrom which touch the surface of the substrate  218  in the clot forming area  136 . In other embodiments, the pillar-creating structure  604  comprises a plurality of distinct posts placed on the surface of the substrate  218  in the clot forming area  136 . The posts of the pillar-creating structure  604  are generally used as masks to prevent collagen from adhering to the substrate  218  during subsequent manufacturing steps. 
     Once the pillar-creating structure  604  is in place, the method continues by adding a collagen material  212  to the substrate  218  in the clot forming area  136  ( FIG. 7 ; step  320 ). The collagen material  212  interacts with the exposed surface of the substrate  218  and eventually adheres thereto. Also, the collagen material  212  does not adhere to the substrate  218  in areas where the pillar-creating structure  604  is present. In some embodiments, the collagen material  212  may be added to the substrate  218  by submerging the substrate  218  in a collagen bath for a sufficient time to ensure that an even layer of collagen material  212  has been created in the clot forming area  136 . 
     Thereafter, the method continues by removing the pillar-creating structure  604  from the substrate  218  to reveal collagen voids  804  ( FIG. 8 ; step  324 ). The lipid  216  is then added to the substrate  218  to fill the collagen voids  804 , thereby creating the structures of tissue factor  216  ( FIG. 9 ; step  328 ). Again, the lipid  216  can be added to the substrate  218  by submerging the substrate  218  into a lipid bath until the collagen voids  804  have been sufficiently filled and a substantially smooth lower surface of the microfluidic channel  116  has been created (e.g., substantially smooth surface is created between the first area  204  of BSA, the collagen material  212 , the structures of tissue factor  216 , and the second area  208  of BSA). 
     At this point, the creation of the prothrombotic structure  120   a  or  120   b  is complete. As can be appreciated by one skilled in the art, although the method of creating only one prothrombotic structure  120   a  or  120   b  was depicted and described, multiple prothrombotic structures  120   a  and  120   b  (or more) can be created at substantially the same time on a single substrate  218  by following the steps described above. Accordingly, multiple prothrombotic structures  120   a  and  120   b  can be created at substantially the same time, thereby reducing the amount of time required to construct a microfluidic device  100 . 
     As an alternative to creating pillar structures  604 , embodiments of the present invention are capable of utilizing laminar flow patterning in which both protein and lipid solutions are introduced simultaneously under flow to the substrate  218  to form lanes (i.e., alternating lanes of protein-lipid-protein-lipid-protein-etc. across the width of the clot forming area  136 ) Accordingly, rather than creating pillar structures in the clot forming area  136 , lane structures that have a long axis substantially parallel to the fluid flow direction are created. In the event that laminar flow patterning is employed to create lane structures, the laminar flow patterning step may replace or augment one or more of steps  316 ,  320 ,  324 , and  328 . 
     Once the prothrombotic structure(s)  120   a  and/or  120   b  are in place, the method continues by positioning a microfluidic channel structure or lid  220  across the clot forming area(s)  136  ( FIG. 9 ; step  332 ). As discussed in connection with  FIG. 1 , the microfluidic channels  116  may be created within a PDMS material and may be positioned to intersect the prothrombotic structure(s)  120   a  and/or  120   b  at only selected locations, thereby controlling the amount of clot forming that is induced when blood is flowed through the microfluidic channels  116 . 
     At this point, a microfluidic device  100  has been created. In accordance with at least some embodiments of the present invention, the microfluidic device  100  may be hygienically sealed in a sterile environment (e.g., hermetic plastic package) (step  336 ) such that the microfluidic device  100  can be distributed as a clot testing kit to medical personnel and other interested parties (step  340 ). One possible complication with a kit that may need to be addressed is the fact that lipids must be stored in an aqueous environment, in other words, they can&#39;t be dried out. Accordingly, prior to hermetically sealing the microfluidic device  100  in a sterile environment, an aqueous solution may be injected into the hermetic packaging prior to final sealing. This will enhance the shelf life of the kit as well as enhance its ability to be distributed great distances away from its source of manufacture. 
     In other embodiments, a vacuum source can be utilized to vacuum seal the substrate  218  to the lid  220 . Such a device  100  can be used in various testing facilities. Moreover, the vacuum assisted sealing of the lid  220  and substrate  218  is a reversible bonding technique which may allow testing personnel to reposition the lid  220  relative to the substrate  218  without damaging either component. 
     In accordance with at least some embodiments of the present invention, once the microfluidic device  100  has been prepared, one or more blood samples can be passed through the microfluidic channels  116  of the device  100  to analyze bleeding and thrombotic disorders, dosing anticoagulant and antiplatelet drugs, tracking the effects of pharmacological interventions on thrombosis, and the like. In particular, platelet flow can be analyzed and platelet adhesion can be quantified in any number ways. As one example, platelet labeling and detection schemes can be employed whereby blood platelets are fluorescently labeled with a small molecule or platelet specific antibody that is fluorescently visible. As the blood with the labeled platelets flows through the microfluidic channels  116  of the device  100 , and more specifically across the clot forming areas  136  in the channels  116 , the number (specific or relative) of platelets that have adhered to and/or around the clot forming area  136  may be observed. Such observations can be made in real-time with fluorescent imaging devices, cameras, recording devices, and the like, or after an experiment has been performed. Real-time and/or post-testing analysis can help yield quantifiable results as to the number of platelets that have adhered to the clot forming area  136 , which can then be correlated to standard test results or other variables to determine whether the patient is prone to excessive bleeding or the like. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain the best mode known for practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with various modifications required by the particular applications or uses of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.