Patent Publication Number: US-2016220995-A1

Title: Microfluidic systems with microchannels and a method of making the same

Description:
RELATED APPLICATION 
     This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/876,820, filed Sep. 12, 2013, entitled “MICROFLUIDIC SYSTEMS WITH MICROCHANNELS AND A METHOD OF MAKING THE SAME,” which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present concept relates generally to a microfluidic system having microchannels and electrodes, and to a method of manufacturing the same. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present disclosure includes a flexible microfluidic device, having a first substrate with micro-rough microchannels formed in a first surface of the first substrate and a second substrate having conductive electrodes disposed on a second surface of the second substrate. A bonding layer of curable polymeric material secures the second substrate to the first substrate. 
     In another aspect, the present disclosure includes a flexible microfluidic device, having a first substrate with micro-rough channels formed on a first surface thereof. A second substrate has conductive electrodes disposed on a second surface thereof. At least one of the first bonding surface and the second bonding surface is treated to form a treated surface. The treated surface has an increased bonding activity as compared to the treated surface before it was treated. 
     In another aspect, the present disclosure includes a method of manufacturing a master mold for a microfluidic device. The method includes the steps of forming a microchannel mold with raised lines extending generally orthogonally from a top surface of the microchannel mold, wherein the raised lines are formed using at least one of PCB manufacturing methods and additive printing methods. The microchannel mold is positioned in a mold cavity to form the master mold. 
     In yet another aspect, the present disclosure includes a method of manufacturing a microfluidic device, the method including the steps of forming a microchannel mold having a bottom surface and a top surface, and having raised lines extending generally orthogonally from the top surface. The microchannel mold is positioned within a mold cavity of a block, with the bottom surface of the microchannel mold supported by the block to create a master mold. A first substrate material is added to the master mold and cured to form a first substrate having a first surface with microchannels formed therein. Electrodes are printed on a second surface of a second substrate. A bonding layer is applied to at least one of the first surface of the first substrate and the second surface of the second substrate. The first substrate and the second substrate are positioned to align the electrodes with the microchannels with the bonding layer between the first substrate and the second substrate. The bonding layer is cured. 
     In yet another aspect, the present disclosure includes a method of manufacturing a microfluidic device including the steps of forming a microchannel mold having a bottom surface and a top surface and raised lines extending generally orthogonally from the top surface. The microchannel mold is positioned within a mold cavity of a block, with the bottom surface of the microchannel mold supported by the block to create a master mold. A first substrate material is added to the master mold and cured to form a first substrate with microchannels formed in a first surface thereof. Electrodes are printed on a second surface of a second substrate. At least one of the first surface of the first substrate and the second surface of the second substrate is treated to increase bonding activity. The microchannels of the first substrate and the electrodes of the second substrate are aligned and the first surface is allowed to bond with the second surface. 
     These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross sectional view of a microfluidic device having microchannels formed therein and a bonding layer; 
         FIG. 1B  is a cross sectional view of another embodiment of a microfluidic device having microchannels formed therein; 
         FIG. 2  is a top view of a first substrate for a microfluidic device having microchannels formed therein; 
         FIG. 3  is a top view of a second substrate for a microfluidic device, with electrodes provided thereon; 
         FIG. 3A  is an enlargement of the electrode shown in  FIG. 3 ; 
         FIG. 4  is a top view of a master mold for forming microchannels in the substrate shown in  FIGS. 1A and 1B ; 
         FIG. 5  is a schematic of a microchannel mold for a master mold as shown in  FIG. 4  designed using PCB software; 
         FIG. 6  is a top perspective view of a block for use in a master mold as shown in  FIG. 4 ; 
         FIG. 7  is a side view of the master mold shown in  FIG. 4 ; 
         FIG. 8  is a top perspective cutaway view of a microchannel; 
         FIG. 9  is a schematic of an experimental setup using a microfluidic device having a substrate with microchannels and electrodes; and 
         FIG. 10  is a graph illustrating the electrochemical response of the microfluidic device experimental setup shown in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of description herein, the terms “upper,” “lower,” “right,” “left,” “rear,” “front,” “vertical,” “horizontal,” and derivatives thereof shall relate to the concept as oriented in  FIGS. 1A and 1B  (and  FIG. 4 , as applicable). However, it is to be understood that the concept may assume various alternative orientations, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     As shown in the embodiment depicted in  FIG. 1A , the present concept generally includes a flexible microfluidic device  10  which includes a first substrate  12  having at least one microchannel  14  formed therein, and a second substrate  16  having electrodes  18  provided thereon. The second substrate  16  is bonded to the first substrate  12 , with the electrodes  18  facing the first substrate  12 , by applying a bonding layer  20  to the first substrate  12 , the second substrate  16 , or both the first and second substrates  12 ,  16 , and positioning the first and second substrates  12 ,  16  with respect to each other prior to the bonding layer  20  being cured. The microfluidic device  10  described herein can be used, for example, as a sensor to detect analytes  22  in a fluid  24 , including dissolved or suspended analytes  22 . 
     As shown in the embodiment depicted in  FIG. 1B , the present concept generally includes the flexible microfluidic device  10  which includes the first substrate  12  having at least one microchannel  14  formed therein. The microchannels  14  are formed in a first surface  26  of the first substrate  12 , and an opposing surface  28  of the first substrate  12  preferably includes inlet and/or outlet ports  30  to permit the fluid  24  to be supplied to the microchannels  14 . The second substrate  16  has electrodes  18  disposed on a second surface  32  of the second substrate  16 . The second substrate  16  is bonded to the first substrate  12 , with the electrodes  18  facing the first substrate  12 , by treating the first surface  26  of the first substrate  12 , the second surface  32  of the second substrate  16 , or both, to modify and activate the surface(s)  26 ,  32  for bonding, and then aligning the surfaces  26 ,  32  and allowing them to bond to form the microfluidic device  10 . The microfluidic device  10  described herein can be used, for example, as a sensor to detect analytes  22  in the fluid  24 , including dissolved or suspended analytes  22 . 
     As shown in the embodiments depicted in  FIGS. 1A, 1B, and 2 , the first substrate  12  is generally planar, with the first surface  26  and the opposing second surface  28 . Microchannels  14  are sized to permit the passage of very small amounts of the fluid  24  to be analyzed. “Microchannels” as used herein includes all fluid passageways on the first substrate  12 , including without limitation reservoirs, mixing channels and chambers, separation junctions, addition junctions, reaction chambers and channels. The inlet and/or outlet ports  30  are also formed in the first substrate  12 , to permit the fluid  24  to be supplied to the microchannels  14  from a fluid source (not shown) through the opposing surface  28  of the first substrate  12 . The first substrate  12  is generally made from a curable polymeric material, which has a liquid or flowable consistency prior to curing, and a flexible, though solid consistency after curing. 
     The second substrate  16 , as shown in the embodiments depicted in  FIGS. 1A-1B and 3-3A , is a thin film with a generally planar shape, and has electrodes  18  on a second surface  32  thereof. The second substrate  16  is bonded to the first substrate  12 , with the second surface  32  of the second substrate  16  (having the electrodes  18  thereon) facing the first surface  26  of the first substrate  12  (having the microchannels  14  formed therein). The electrodes  18  align with and interact with the microchannels  14  to allow the application of electrical signals to the fluid  24  in the microchannels  14 . 
     In the embodiment depicted in  FIG. 1A , the second substrate  16  is bonded to the first substrate  12  using a coated adhesive bonding layer  20 , such as a curable polymeric material, which is optionally the same material that is used to make the first substrate  12 . Acrylates, polyester resins, and laminate films are additional non-limiting examples of curable materials that can act as the bonding layer  20 . After coating the adhesive bonding layer  20 , the first substrate  12  and second substrate  16  are aligned and the bonding layer  20  is permitted to cure. In the embodiment depicted in  FIG. 1B , the second substrate  16  is bonded to the first substrate  12  by treating one or both surfaces  26 ,  32  to modify and activate the surfaces  26 ,  32  for bonding. Exemplary treatments include, without limitation, treating with a silane coating, including 3-aminopropyl triethoxysilane; treating with solvents, including alcohols, acetone, DMSO, and acetonitrile; treating with acids; treating with heat; treating with plasma energy; treating with UV/ozone; and treating with a corona discharge. Such treatments promote the bonding of the surfaces  26 ,  32  to each other. Optionally, one or both surfaces  26 ,  32  can act as an adhesive surface by partial curing or cross-linker variation of the first or second substrates  12 ,  16 . After treatment, the first surface  26  of the first substrate  12  and the second surface  32  of the second substrate are aligned, and then pressed together and allowed to bond to form a microfluidic device  10 . In another embodiment, after treatment to activate one or both of the surfaces  26 ,  32 , one or both of the surfaces  26 ,  32  can be coated with an adhesive bonding layer  20 . The first substrate  12  and second substrate  16  are then aligned and the bonding layer is permitted to cure. 
     To design and fabricate the microfluidic system  10 , a master mold  40 , as shown in the embodiments depicted in  FIGS. 4 and 7 , is used to form the first substrate  12 . As shown in  FIG. 4 , the master mold  40  preferably includes two parts, a microchannel mold  42  and a block  44 . As best shown in the embodiment depicted in  FIG. 7 , the microchannel mold  42  has a top surface  46  and an opposing bottom surface  48 , with raised copper lines  50  extending generally orthogonally upward from the top surface  46 . Although the lines  50  are referred to herein as “raised copper lines” it is understood that the lines can comprise any material which can be etched using PCB manufacturing technology or deposited using additive printing methods, like gravure, screen or inkjet printing. 
     The block  44 , one embodiment of which is shown in  FIG. 6 , has a top surface  52  with a mold cavity  54  formed therein. The mold cavity  54  has a generally flat bottom surface  56  and side walls  58  extending upwardly from the flat bottom surface  56  to define a perimeter of the mold cavity  54 . To assemble the master mold  40 , the microchannel mold  42  is placed along the flat bottom surface  56  of the mold cavity  54 , with the raised copper lines  50  extending upwardly into the mold cavity  54 . 
     As shown in the embodiment depicted in  FIG. 5 , traditional printed circuit board (“PCB”) design and manufacturing methods can be used to design and implement the pattern of raised copper lines  50  on the top surface  46  of the microchannel mold  42 , and therefore the corresponding microchannels  14  on the first surface  26  of the first substrate  12 . For example, software such as ExpressPCB™ software can be used to design the desired layout of raised copper lines  50  on the microchannel mold  42 . The raised copper lines  50  are then created using known PCB manufacturing methods, whereby a copper sheet is deposited on the top surface  46  of the microchannel mold  42 , and is then masked and etched to create the raised copper lines  50 . The raised copper lines  50  created in this way have micro-rough areas at both sides of the copper lines  50  that become smooth as the edges of the copper lines  50  taper to the top surface  46  of the microchannel mold  42 . The raised copper lines  50  are used in the master mold  40  to form the micro-rough microchannels  14  in the first substrate  12  as further described below. 
     Additive printing methods, including gravure, screen, or inkjet printing, could also be used in place of PCB manufacturing methods to create micro-rough, raised copper lines  50  on the microchannel mold  42  to form micro-rough microchannels  14  in the first substrate  12  as further described herein. 
     As best shown in the embodiment depicted in  FIGS. 6-7 , the microchannel mold  42  is placed within the mold cavity  54  of the block  44  to create the master mold  40 . The mold cavity  54  is of a size and shape to receive the microchannel mold  42 , with the bottom surface  48  of the microchannel mold  42  supported by the flat bottom surface  56 . The block  44  provides rigidity and structure to the master mold  40 , and provides support for the microchannel mold  42 , as well as defining side walls  58  for the master mold  40  to contain material used to form the first substrate  12  of the microfluidic device  10 . The material used for the block  44  can include any material which provides sufficient structure and rigidity to the master mold  40  over the temperature range that the master mold  40  is intended to be used. Preferable materials also permit the release of the material used to form the first substrate  12  after formation. Non-limiting examples of suitable materials include plastic resins, wood, or metal, with any of the foregoing having an optional coating to provide desired characteristics, such as release of the first substrate  12  material. 
     To form the first substrate  12 , a curable polymeric material is added to the master mold  40  in its liquid or flowable state and is then cured, to form the flexible first substrate  12 . The raised copper lines  50  form indentations on the first side of the first substrate  12 , which are the microchannels  14  on the first substrate  12 . Following curing, the first substrate  12  is removed from the master mold  40 , and inlet and/or outlet ports  30  for the fluid  24  are cored out of the first substrate  12 . Suitable tools for forming the inlet and/or outlet ports  30  for the microchannels  14  include biopsy punch tools, or other tools capable of making small-scale holes in the flexible solidified material of the first substrate  12 . 
     Suitable materials for making the first substrate  12  generally include polymeric materials, such as PDMS, polymethylmethacrylate (PMMS), polycarbonate, polyepoxide, cyclic olefin polymers (COP), cyclic olefin copolymers (COC), or other materials suitable for making a flexible microfluidic device  10 , so long as the materials used for the first substrate  12  can be formed using the mold  40  described herein (e.g., the material is curable and is able to conform to the master mold  40  at a temperature that does not melt the master mold  40  material). 
     As shown in the embodiment depicted in  FIG. 8 , the microchannels  14  formed in the first side  26  of the first substrate  12  have micro-roughened edges as a result of the raised copper lines  50 . The 3-dimensional topography of the microchannels  14 , as shown in  FIG. 8 , was visualized and measured by vertical scanning interferometry, using a Bruker Contour GTL EN 61010 laser profilometer (Bruker Biosciences Corporation, USA), with Bruker Vision software operating in hybrid mode. In this embodiment, the depth of the microchannel  14  was found to be 55 μm. 
     The second substrate  16  is a thin film, including without limitation a polymeric film or a PET film, or polymeric materials such as PDMS, polymethyl-methacrylate (PMMS), polycarbonate, polyepoxide, cyclic olefin polymers (COP), cyclic olefin copolymers (COC), with electrodes  18  formed thereon, as shown in  FIG. 3 . To form the electrodes  18  a conductive ink is preferably printed onto the first surface  32  of the thin film second substrate  16  to form interdigitated electrodes  18 , as shown in greater detail in  FIG. 3A . Suitable printing methods for printing the electrodes  18  include inkjet printing, screen printing, gravure printing, or other methods capable of printing conductive inks. 
     To complete manufacture of the microfluidic device  10 , the first substrate  12  having microchannels  14  formed therein and the second substrate  16  having electrodes  18  thereon are assembled to form the microfluidic device  10 . In one embodiment, as shown in  FIG. 1A , assembly of the first and second substrates  12 ,  16  includes masking the electrodes  18  on the second substrate  16  and coating a thin layer of curable liquid polymeric material on the first surface  32  of the second substrate  16  to form a bonding layer  20 . The bonding layer  20  functions as an adhesive. Alternatively, assembly of the first and second substrates  12 ,  16  includes filling the microchannels  14  with a removable material, and coating a thin layer of curable liquid polymeric material on the first surface  26  of the first substrate  12  to form a bonding layer  20 . Suitable removable materials include, without limitation, wax or ice, which are used to fill the microchannels  14 . Coating methods such as bar coating, which provides a uniform coating, are preferred for applying the bonding layer  20  to the first substrate  12  or the second substrate  16 , to ensure even and complete bonding between the first substrate  12  and the second substrate  16 . 
     In another embodiment, as shown in  FIG. 1B , the assembly of the first and second substrates  12 ,  16  includes treating the first and second substrates  12 ,  16  to promote bonding. Preferably, the first and second substrates  12 ,  16  are cleaned by placing the first and second substrates  12 ,  16  on a non-conducting surface with the first surface  26  of the first substrate  12  and second surface  32  of the second substrate  16  exposed. One non-limiting, exemplary cleaning agent is an isopropyl alcohol solution. One or both of the first surface  26  of the first substrate and the second surface  32  of the second substrate  16  are treated to promote bonding. For example, a corona discharge treatment can be performed on the surfaces  26 ,  32  by passing a corona discharge device over each of the surfaces  26 ,  32  in order to promote bonding. The treated surfaces  26 ,  32  are then pressed together and permitted to bond to form the microfluidic device  10  as shown in  FIG. 1B . Alternate treatment methods, as described above, include without limitation: treating with a silane coating, including 3-aminopropyl triethoxysilane; treating with solvents, including alcohols, acetone, DMSO, and acetonitrile; treating with acids; treating with heat; treating with plasma energy; and treating with UV/ozone. 
     In yet another embodiment, to improve the bonding of the first and second substrates  12 ,  16 , one or both surfaces  26 ,  32  can be treated to activate the surface  26 ,  32  for bonding before applying a bonding layer  20 . The resulting microfluidic device  10  would generally have the structure as shown in  FIG. 1A . 
     In one embodiment of the manufacture of a microfluidic device  10 , the layout of the desired microchannels  14  is designed using ExpressPCB™ software. The PCB microchannel mold  42  is designed to have overall dimensions that correspond to the desired height and width of the first substrate  12 . For example, in this embodiment, the microchannel mold  42  has overall dimensions of about 96.5 mm (height) by about 63.5 mm (width) by about 1.57 mm (thickness). The raised copper line thickness  50  of the microchannel mold  42  is set to about 55 μm. The PCB microchannel mold  42  is manufactured from traditional PCB materials, using traditional PCB manufacturing methods. PCB manufacturing methods create raised copper lines having micro-rough edges, by etching copper sheets on the non-conductive top surface  46  of the microchannel mold  42 . 
     The PCB microchannel mold  42  is then placed into the mold cavity  54  in the block  44 . One material that is suitable for use in manufacturing the block  44  is a Delrin® Acetal block. Such blocks can be purchased from McMaster-Carr® with dimensions of about 101.6 mm (height) by about 76.2 mm (width) by about 12.7 mm (thickness). The mold cavity  54  is formed by machining a cavity of the desired size and shape out of a top surface  52  of the block  44 , in this example, a machined area of about 96.5 mm (height) by about 63.5 mm(width) by about 5 mm(depth) accommodates the microchannel mold  42  described above. In this particular embodiment, the side walls  58  of the block  44  extend upwards approximately 3.5 mm from the top surface  46  of the microchannel mold  42 , defining the mold cavity  54  where the polymeric material can be poured. 
     The first substrate  12  is formed by filling the mold cavity  54  with a curable polymeric material, where the material is constrained by the side walls  58  of the mold cavity  54 , and covers the top surface  46  of the microchannel mold  42  at a thickness sufficient to cover the raised copper lines  50 . One material that can be used to form the first substrate  12  is polydimethylsiloxane (PDMS), which is sold as a two-part heat curable silicone elastomer kit (Sylgard® 184 from Dow Corning) including a pre-polymer and a curing agent. To use PDMS, the Sylgard® 184 pre-polymer and curing agent are combined in a 10:1 (w/w) ratio, and stirred vigorously until well mixed. Bubbles introduced by the mixing are removed by allowing the mixture to rest at room temperature for a sufficient length of time, such as 30 minutes. Alternative methods for removing air from the solution could also be employed. The PDMS is then poured into the master mold  40  described herein and cured at 90° C. for thirty (30) minutes in a VWR oven. Following curing, the PDMS can be peeled from the master mold  40 , forming the first substrate  12 . In the embodiment described herein, having raised copper lines  50  with a height of 55 μm, the average width and thickness of microchannels  14  formed in the first substrate  12  were measured to be about 500 μm and about 45 μm. Microchannels  14  having varying width or thickness can be created by using different patterns for formation of raised copper wires  50  on the PCB microchannel mold  42 , and by use of an alternative method, like an additive printing method, for creating the microchannel mold  42 . The printing technique can be chosen based on the desired height or depth of the microchannel, with different printing methods resulting in different thicknesses of the raised lines  50 . 
     In an alternative embodiment, a microchannel mold  42  is created by producing a design layout of microchannels  14  with CoventorWare software. A stainless steel mesh pattern of the microchannels  14  was produced following the design layout and used for screen printing the microchannel mold  42  using a silver-based ink to print a microchannel mold  42  with overall dimensions of about 96.5 mm by 63.5 mm by 1.58 mm, with a raised line  50  thickness of about 10 μm. The microchannel mold  42  is placed in the corresponding mold cavity  54  in the block  44  to form a master mold  40 . The microchannel mold  42  is used to form the first substrate  12 , by adding a curable polymeric material to the master mold  40  in its liquid or flowable state to a depth sufficient to cover the raised lines  50 , and then curing the polymeric material. The screen-printed microchannel mold  42  used in a master mold  40  produced a microfluidic device  10  that had micro-rough microchannels  14  having a depth of 9 μm. 
     Inlet and/or outlet ports  30  for the microchannels  14  are then formed in the first substrate  12 , preferably using tools that can remove cores  30  having a diameter of about 1 mm. One example of such a tool is biopsy puncher model 33-31AA from Miltex®. Alternative tools can also be used to create inlet and/or outlet ports  30  communicating with the microchannels  14  in the first substrate  12 . 
     Further, in this embodiment the second substrate  16  is formed on a flexible thin film, such as a polyethylene terephthalate (PET) film. 
     In one embodiment, conductive silver-based ink is printed onto the first surface of the thin film to form electrodes  18  using a Dimatix 2831 inkjet printer. In the embodiment shown in  FIGS. 3 and 3A , two pairs of electrodes  18  are provided for each of a plurality of biosensors present on the microfluidic device  10 . Each of the pairs of electrodes is 5.4 mm long, with a width of 200 μm and a spacing of 600 μm. 
     In one embodiment, the assembly of the first and second substrates  12 ,  16  includes the steps of masking the electrodes  18  on the second substrate  16 , and bar-coating liquid PDMS onto the PET second substrate  16  to form a bonding layer  20  with a thickness of about 12.7 μm on the first surface  32  of the second substrate  16  as shown in  FIG. 1A . The second substrate  16  is then positioned as desired with respect to the first substrate  12 , and the bonding layer  20  is cured and solidified by heating the assembly in a VWR oven for 30 minutes at 90° C. to complete production of the microfluidic device  10  as shown in  FIG. 1A . 
     In another embodiment, the assembly of the first and second substrates  12 ,  16  includes the steps of cleaning the first and second substrates  12 ,  16  and placing the first and second substrates  12 ,  16  on a non-conducting surface with the first surface  26  of the first substrate  12  and second surface  32  of the second substrate exposed. One non-limiting, exemplary cleaning agent is an isopropyl alcohol solution. A corona discharge treatment is performed on the surfaces  26 ,  32  by passing a corona discharge device over each of the surfaces  26 ,  32  at a height of about 6.4 mm above each of the surfaces  26 ,  32  for about 15 seconds, activating the surfaces  26 ,  32  for bonding. A suitable corona discharge device for providing the corona discharge treatment at the parameters described herein includes, without limitation, a laboratory corona treater (model BD-20AC, sold by Electro-Technic Products Inc.). The treated surfaces  26 ,  32  are then pressed together and permitted to bond by leaving undisturbed overnight to form the microfluidic device  10  as shown in  FIG. 1B . Alternative corona discharge treatment protocols may be used to execute the corona discharge treatment step. 
     As illustrated in  FIG. 9 , to use one embodiment of a microfluidic device  10  as described herein, a programmable syringe pump (not shown) was connected to the inlet port  30  of the microchannel  14  for loading a test sample of fluid  24 , such as a KDS210P syringe pump from KD Scientific. An LCR meter  60  was connected to the printed electrodes  18  via a test clip (not shown) to measure impedance. One example of a suitable LCR meter  60  is an Agilent model E4980A Precision LCR meter, and an example of a suitable test clip is a 5251 SOIC test clip from Pomona Electronics. Deionized water is loaded into the microfluidic device  10  to set a reference signal for the fluid  24 , and then sample solutions with different concentrations (1 pM and 1 nM) of an analyte  22  such as potassium chloride were loaded into the microfluidic device  10 . The impedance of the microfluidic device  10  was measured at a frequency of 1 kHz with a 1 mV voltage excitation. The response of the potentiostat was observed and analyzed on a PC  62  using a custom built LabView program. 
     As shown in  FIG. 10 , using the microfluidic device  10  described herein, the reference signal for the deionized water fluid  24  was established around 520 kΩ. Impedance measurements of around 700 kΩ and 1.1 MΩ were measured for the 1 pM and 1 nM concentration of KCl solution fluids  24 , respectively. The microfluidic device  10  was shown to be reversible by introducing deionized water after each concentration of KCl solution was tested, as the impedance of the microfluidic device  10  returned to the base value of 520 kΩ. This response of the microfluidic device  10  demonstrated the capability of the microfluidic device  10  to distinguish among various concentrations of potassium chloride in a test sample of fluid  24 . 
     Microfluidic devices  10  as described herein are capable of handling very low volumes of fluid  24  at a low cost per assay. The microfluidic devices  10  can be designed to carry out desired functions, such as cell separation, DNA sequencing, enzyme/substrate reaction systems, biosensors, and implanted drug delivery or metabolite analysis systems. These devices  10  are a promising way to realize an efficient, rapid response, portable, and cost effective approach to microfluidic applications. The microfluidic devices  10  and methods for manufacturing the devices  10  disclosed herein are also intended to be more cost effective and to have fewer barriers for preparation and manufacture than more traditional and expensive silicon mold based systems and conventional lithography techniques. This permits creation of inexpensive or disposable microfluidic devices  10  for mass market use, such as in multiple cancer marker analyses and on-site portable analytic systems, as non-limiting examples. It also allows further development and testing of the microfluidic devices  10 , particularly by time-bound and/or budget-constricted non-experts. The microfluidic devices  10  and methods described herein also reduce the amount of material and energy wasted during fabrication of the devices  10 . 
     It is also important to note that the construction and arrangement of the elements of the concept as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures and/or members or connector or other elements of the system may be varied, the nature or number of adjustment positions provided between the elements may be varied. It should be noted that the elements and/or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability, in any of a wide variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations. 
     It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present concept. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting. 
     It is also to be understood that variations and modifications can be made on the aforementioned structures and methods without departing from the concepts of the present concept, and further it is to be understood that such concepts are intended to be covered by the following claims unless these claims by their language expressly state otherwise.