STATEMENT OF GOVERNMENT SUPPORT

[0001] This research was supported by the Department of Energy (DOE) under award no. ER45852, and the Defense Threat Reduction Agency (DTRA) under award no. HDTRA1-14- C-0037. The U. S. Government has certain rights in this invention.

BACKGROUND

[0002] Cellulose-based paper has emerged as a useful material for the fabrication of microfluidic devices, especially in the area of diagnostics. As an example, paper has the following characteristics that differentiate it from glass, elastomers (e.g., PDMS), and synthetic polymers (e.g., polycarbonate) as a material for microfluidics. Paper is porous and hydrophilic. Porosity allows for immobilization of biomolecules inside the pores of paper, and hydrophilicity allows spontaneous wicking of water which is useful for microfluidic applications. Paper is also compatible with large-scale methods of fabrication, (e.g., screen-, or reel-to-reel printing, embossing, and folding). Printing of hydrophobic regions into paper (e.g., using wax printing), and stacking of paper sheets, enables the fabrication of three- dimensional paper microfluidic devices. Further, printing of electronic inks, on the surface of, or inside, paper allows for the fabrication of electroanalytical paper devices. Paper is also safe during handling and disposal, (e.g., paper does not fracture and produce "sharps," and it can be incinerated).

[0003] Paper microfluidic systems are based on spontaneous wicking of liquids

(especially water and aqueous buffers) through the porous matrix of cellulose-based paper, often through a fluid path defined by hydrophobic wax barriers. Widely used methodologies such as electrochemical monitoring of glucose, and lateral flow immunoassays were the first (and still largest volume) paper-based devices to enter large-scale fabrication, not counting simple sensing paper strips. Paper microfluidics systems provide an alternative technology for applications in medical diagnostics for resource-limited settings, environmental monitoring, food safety testing, and molecular diagnostics. [0004] Despite its useful characteristics, some physical properties of paper fabricated from cellulose fibers limit its use in applications in which wicking flow is not desirable. These limitations include transport of complex fluids that contain suspended particles, and the development of multiphase systems involving drops or bubbles. Paper also scatters light and is not transparent. Optical opacity limits its use for microfluidic applications (e.g., fluorescence, electrochemiluminescence ECL, or absorption spectroscopy) that require transmission and/or emission of light.

[0005] Paper-based microfluidic systems utilize an open-channel with pressure driven flows. These microfluidic devices are fabricated using embossing or cutting (using a machine-driven knife) to shape channels in paper, silanization to make the paper

hydrophobic; and plastic tape to seal the top of the channels. Hollow channels may be created in the spacing between layers of paper, where wax-printed barriers confine the liquid in the channel. These hybrid systems successfully demonstrate the classical, diffusion- limited co-flows that are characteristic of open-channel microfluidic devices, but they still suffer from drawbacks, such as the use of synthetic polymers, and the lack of optical transparency across the device. Index-matching has been used to increase the transmission of light through paper in paper microfluidics systems for quantitative colorimetric POC diagnostics, by wetting the paper with a liquid with an index of refraction similar to that of cellulose. However, index matching does not render paper fully transparent, and the liquids used for index matching fill the pores of paper, rendering capillary flows ineffective.

SUMMARY

[0006] Under one aspect, a microplate includes a plate; and an array of wells formed in the plate, wherein the plate comprises cellophane. One of more embodiments include one or more of the following features. The cellophane is nitrocellulose and/or polyvinyl chloride coated. The microplate is a 96-well plate.

[0007] Under one aspect, a method of fabricating a microplate includes providing a cellophane sheet; and embossing wells into the sheet. One of more embodiments include one or more of the following features. The cellophane is nitrocellulose coated. The cellophane is polyvinyl chloride coated. The uncoated cellophane sheet is stored in water prior to the embossing. The embossing comprises compressing the sheet between molds to emboss the wells. [0008] Under one aspect, a microfluidic device includes at least one sheet of cellophane; a second sheet of material sealed to the sheet of cellophane; a channel formed between the at least one sheet of cellophane and the second sheet; and at least a first liquid inlet in communication with the channel. One of more embodiments include one or more of the following features. The second sheet of material is cellophane and the sheets are coated with polyvinyl chloride and/or and nitrocellulose. A second liquid inlet is provided, wherein the first liquid inlet and the second liquid inlet communicate with separate portions that merge into the channel. The channel has a width of approximately 500 μπι or approximately 1 mm. The sheets are transparent and the channel has a serpentine shape. The channel includes a porous mixer, which may be paper. The microfluidic device comprises a window for optical measurement. The microfluidic device comprises integrated electrodes that are patterned on at least one of the sheets. The microfluidic device comprises an integrated heater. The heater is a resistive heating device provided by printing a structure on a backside of the microfluidic device. The microfluidic device comprises multiple channels, wherein the channels face each other and are separated by an intermediate layer. The intermediate layer is uncoated cellophane and is bound by the sheets, the sheets being coated cellophane.

[0009] Under one aspect, a method of fabricating a microfluidic device comprises compressing a first cellophane sheet between two stamps to emboss a microfluidic feature; placing a second sheet against the embossed first cellophane sheet; subjecting the first and second sheets to heat to seal the sheets together; and forming at least one hole in the sealed sheets for providing a fluidic connection. One of more embodiments include one or more of the following features. The second sheet is cellophane and both sheets are polyvinyl chloride and/or nitrocellulose coated. Electrodes are patterned on at least one of the sheets. A heater is printed on at least one of the sheets. The sheets are stored in a vacuum desiccator chamber for at least one day prior to use.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The following Figures are presented for the purpose of illustration only, and are not intended to be limiting:

[0011] FIG. 1 A depicts contact angles and surface tensions of materials, according to some embodiments.

[0012] FIG. IB depicts light transmission of materials, according to some embodiments. [0013] FIG. 2A illustrates an exemplary fabrication flow diagram, according to some embodiments.

[0014] FIG. 2B depicts UV absorption of different analytes, according to some embodiments.

[0015] FIG. 2C is a cross section of a well plate, according to some embodiments.

[0016] FIG. 2D is a top view of a well plate, according to some embodiments.

[0017] FIG. 2E is a perspective view of the well plate in FIG. 2D.

[0018] FIGS. 3A-3B depict exemplary molds used for fabrication of microplates, according to some embodiments.

[0019] FIG. 4 is a flow diagram for the construction of an exemplary microfluidic device, according to some embodiments.

[0020] FIGS. 5A-5C depict exemplary molds used for fabrication of microfluidic devices, according to some embodiments.

[0021] FIGS. 6A-6B depict exemplary molds used for fabrication of microfluidic devices, according to some embodiments.

[0022] FIG. 7A depicts top views of exemplary microfluidic devices, according to some embodiments.

[0023] FIGS. 7B-7D depicts top views of exemplary PVC-coated cellophane microfluidic devices with upward facing embossed features, according to some embodiments.

[0024] FIGS. 7E and 7F show cross sections of channels fabricated according to some embodiments.

[0025] FIG. 7G is a perspective view of an exemplary embodiment showing

transparency, according to some embodiments.

[0026] FIG. 8A illustrates an exemplary Y-junction microfluidic device displaying a laminar flow, according to some embodiments.

[0027] FIG. 8B illustrates inputs and an output of the device in FIGS. 8 A and 8B.

[0028] FIG. 8C illustrates the implementation of the device in FIGS. 8 A and 8B.

[0029] FIG. 9 depicts solution injected into channels of an exemplary microfluidic device, according to some embodiments.

[0030] FIG. 10A illustrates an exemplary fabrication flow diagram of a microfluidic device with a porous static mixer, according to some embodiments.

[0031] FIG. 10B illustrates an exemplary microfluidic device with a porous static mixer, according to some embodiments. [0032] FIG. IOC illustrates the flow of dyes through an exemplary microfluidic device, according to some embodiments.

[0033] FIG. 1 1 A illustrates an exemplary fabrication flow diagram of a microfluidic device with electrodes, according to some embodiments

[0034] FIGS. 1 IB and 1 ID are graphs showing normalized current verses concentration, according to some embodiments.

[0035] FIGS. l lC and 1 IE depict exemplary microfluidic devices with integrated electrodes, according to some embodiments.

[0036] FIG. 12 shows a time-current response of a continuous flow experiment using an exemplary electroanalytical device, according to some embodiments.

[0037] FIG. 13 shows cyclic voltammograms produced using an exemplary microfluidic device, according to some embodiments.

[0038] FIGS. 14A and 14C depict electrodes printed on backsides of exemplary microfluidic devices, according to some embodiments.

[0039] FIGS. 14B and 14D depict infrared images of exemplary microfluidic devices during operation, according to some embodiments.

[0040] FIG. 15 shows a crossectional view of a multi-layer microfluidic device, according to some embodiments.

DETAILED DESCRIPTION

[0041] Cellophane is the trademark name for cellulose films that are manufactured by regenerating cellulose from cellulose xanthate (viscose). Today cellulose films are mainly used as an environmentally friendly packaging material. As shown further below in Table 1, Cellophane has properties that are complementary to those of paper as a substrate for microfluidic devices. For example, it is readily available and inexpensive (less than 1 USD/m ² , or 0.05 cent/5 cm ² — the typical size of a paper microfluidic chip) in large quantities. Cellophane is easily disposed of by incineration without producing sharps.

Further, cellophane is, in its unmodified form, biocompatible and biodegradable. It does not dissolve in most organic solvents, or aqueous solutions (such as solutions commonly used in biochemistry and molecular biology applications, or human blood). It is semi-permeable, and its permeability can be tuned to allow different molecular/weight cutoffs for use as dialysis membranes. Cellophane has a printable surface (like paper) but with less surface roughness than paper. It also has excellent optical transparency— comparable to glass and most plastics— over the entire visible range, as well as in the UV range.

[0042] Table 1. Properties of regenerated cellulose films (uncoated cellophane)

[0043] The compatibility of uncoated cellophane (C ) was tested with the following solvents: hexadecane, glycerol, ethylene glycol, ethanol, DMSO, NMP, THF, NMP, and DMF, by immersing the uncoated cellophane in each solvent for a period of 2 days. The cellophane sheets were removed from the solvent and dried in an oven at 110 °C, for 30 min. No apparent qualitative change in the mechanical or optical structures of the dry solvent- exposed cellophane from sheets that had not been exposed to any solvent was observed. [0044] Surface Wetting Properties of Uncoated, Coated and Silanized Cellophane

[0045] Using measurements of apparent static contact angles, 0app, the wettability of cellophane with different modifications using a series of organic liquids spanning a wide range of surface tensions were examined. The uncoated cellophane C ^0H used in these experiments was silanized with either a fluorinated or a non-fluorinated alkylsilane in the vapor phase, or was used as received (the pure, untreated cellophane and the cellophane coated with nitrocellulose). FIG. 1 A shows that silanization rendered the uncoated cellophane hydrophobic. The wettability of the alkyl-silanized cellophane and cellophane C ^NS were similar under our experimental conditions, while cellophane C ^PVC showed lower contact angles than C ^NS . In particular, FIG. 1 A shows a correlation of static contact angles on surface tension for 10 [iL droplets of liquid on uncoated cellophane C ^0H , cellophane with polyvinyl chloride (PVC) and nitrocellulose coatings (C ^PVC , C ^NS ), and cellophane C ^0H silanized with either a fluorinated (CIO F) or a non-fluorinated (CIO H) alkyl silane (standard deviation are based on n=7). The liquids used in these experiments and their respective surface tensions at 20 °C in mN/m are: anhydrous ethanol (22.3), hexadecane (27.4), ethylene glycol (46.3), glycerol (63.7), water (72.8). The contact angle measurements (n=7 for each contact angle) may be performed by a contact angle goniometer (e.g., Rame-Hart model 100, Rame-Hart Instrument Co.) at room temperature (20 - 25 °C) with -30% relative humidity.

[0046] Silanization with a fluoroalkyl silane afforded only a modest increase in contact angle for a given liquid. Since nitrocellulose and PVC-coated cellophane are commercially available, they were used in most investigations. Cellophane silanized with fluorinated alkylsilanes may, however, be useful in applications that require omniphobic surfaces.

[0047] A silanization reaction may be conducted in a chamber with a volume of 0.01 m3 at a temperature of 95 °C. The organosilane is transferred into a glass vial under an inert gas atmosphere, and placed inside the chamber together with the samples. Each experiment required approximately 100 mg of silane in 3 mL of anhydrous toluene. The organosilane was vaporized at 95 °C under reduced pressure (-30 mbar, 0.03 atm) and was allowed to react for 20 minutes.

[0048] UV/Vis Transmission and Absorption

[0049] Cellophane is transparent across a remarkably wide wavelength window, compared to most plastics, and typical glasses. FIG. IB shows normalized UV/Vis transmission spectra of cellophane sheets, in comparison with cellulose acetate, a commercial plastic cuvette (PMMA), and a quartz cuvette. The cellophane sheets are highly transparent in the visible regime, and also in the UV regime, with over 65% transmittance for uncoated cellophane C ^0H , and 40% transmittance for coated cellophane C ^PVC , at 250 nm. The different sheets of cellophane in FIG. IB include an uncoated 34 μιη thick sheet, a nitrocellulose- coated 42 μπι thick sheet, a PVC-coated 42 μιη thick sheet, PMMA cuvette, and quartz cuvette.

[0050] Microplates

[0051] The high UV transparency of cellophane in the UV regime makes it a possible alternative to quartz cuvettes or quartz microplates for certain applications. The unmodified surface properties of uncoated cellophane, which is chemically similar to paper, may also provide a platform for cell-culture, or for analytical adaptations requiring immobilization of DNA or proteins on its surface.

[0052] FIG. 2A shows a schematic flow diagram for the fabrication and use of uncoated cellophane (C ^0H ) 96-well microplates. According to an aspect, weld plates are made using molds with patterns analogous to those of a 96-well plate to emboss wells into uncoated cellophane C ^0H . It was found that wells embossed in dry cellophane C ^0H buckled and deformed when placed in contact with water, since water swells uncoated cellophane. To help prevent buckling of the wells the cellophane sheets were pre-soaked in deionized water prior to embossing. When the cellophane C ^0H is pre-soaked, the aqueous solutions in the wells cannot be absorbed by the cellophane, and they only transport inside the cellophane and spread with diffusion, which is slow compared to the measurement times used for the absorption measurement.

[0053] FIG. 2B shows UV Absorption of different analytes measured in cellophane C ^0H 96-well plates using 10 μΙ_, of solution in each well (n=7). Cellophane 96-well plates were used to measure the UV absorption of four analytes: bovine serum albumin (BSA), adenine, acetaminophen, and thiamine (vitamin Bl). These analytes were chosen because of their range of absorption maxima within the UV region ma 280, 260, 242 and 238 nm

respectively). This range allowed us to examine the suitability of cellophane C ^0H as an alternative material to quartz for UV absorption measurements across these wavelengths.

[0054] The linear response range was measured for all four analytes with a cellophane 96-well plate, in which each well contained 10 μΙ_, solutions of the analytes in deionized water. Table 2, reproduced below, summarizes analytical figures of merit for absorption measurement of different compounds in uncoated cellophane 96-well plates and provides limits of detection (LODs). The confidence interval was 95% and the number of samples was n=7.

[0055] Table 2

Analyte max Measured linear R ² LOD ^a

(nm) dynamic range ^g/mL)

^g/mL)

Thiamine 238 20 - 500 0.995 8

Acetaminophen 242 20 - 500 0.995 8

Adenine 260 2.6■ - 675 0.998 6

BSA 280 70 - 2000 0.993 377

LOD =—

s

[0056] Table 3, reproduced below, shows analytical figures of merit for absorption of different analytes, in PVC coated cellophane well plates (95% CI, n=7). The absorption measurements were done by placing 10 μL of solution in each well and sealing the wells with an adhesive tape (adhesive silicone film for PCR plates)

[0057] Table 3

a) LOD = ^

[0058] The concentration range and LOD for BSA and adenine, were comparable to the concentrations that can be measured using instruments frequently used for determining concentrations of DNA/protein such as the NanoDrop™, which measures absorption through liquids without using a container that holds the liquid. The linear range for both acetaminophen and thiamine in the cellophane microplates is appropriate for analytical measurements in routine pharmaceutical and food-science settings, but not for high- sensitivity measurements.

[0059] FIG. 2C is a macro image of the cross section of a cellophane C ^0H (pre-wetted) 96-well plate. FIG. 2D is a top view image of a cellophane (non-coated) 96 well plate, in which each well is filled with 10 μΙ_, of aqueous solutions showing red: acid red, blue:

reactive blue, yellow: food color). FIG. 2E is a bottom view macro image of the microplate shown in 2D.

[0060] Embossed cellophane C ^0H well plates may need to be kept wet for the duration of the analysis because wells can distort and buckle upon drying, which may be a limiting factor in storage and shipping. In accordance with a further embodiment, microplates may be embossed using dry sheets of coated cellophane (C ^PVC , and C ^NS ). Embossed wells in coated cellophane do not swell since water in the wells cannot penetrate through the coating and into the cellophane.

[0061] Due to the high liquid contact angles of aqueous solutions for coated cellophane (C ^PVC , and C ^NS ), it is possible that certain samples may not evenly coat the surface of the wells, which could potentially result in inconsistencies in the path-length of the samples. According to an embodiment, the wells may be sealed with, for example, tape, such as polyolefin films with silicone adhesive from VWR®, used for PCR in well-plates, after having placed the liquid in the well, to enclose the liquids and ensure a more consistent path length. The standard error from the coated cellophane C ^PVC , and C ^NS microplates was notably higher for all four analytes than with cellophane C ^0H well plates, as noted above. This may be a result of higher UV absorption by the coating and the tape. In some applications, microplates fabricated using coated cellophane (C ^PVC , and C ^NS ) may be more appropriate than those from C ^0H for applications that do not require UV transparency. Furthermore, they can be prepared and stored prior to use.

[0062] FIGS. 3A and 3B show molds, fabricated in polyurethane using 3D printing, used for the fabrication of microplates, according to an aspect. The molds for 96-well plates were designed using Autodesk 3dsMax. The molds (one with a positive and one with a negative relief) were fabricated in polyurethane using a Connex500 3D Printer from Stratasys Ltd. Uncoated cellophane (C ^0H ) sheets were stored in deionized water for at least two hours prior to embossing. This step helped prevent the cellophane C ^0H from further swelling when a liquid was placed into the wells. Next, the cellophane C ^0H sheet was compressed between the molds to form the wells, suspended the cellophane sheet on a standard frame for 96-well plates. The wells were filled with 10 μΙ_, of solution using a micropipette, and carried out the optical measurement using a standard 96-well plate reader (SpectraMax M2e Multi-Mode Microplate reader). UV- Visible Spectra were collected for analytes (0.01 μg/mL in deionized water) in 1-cm path-length quartz cells to determine the absorption maxima.

[0063] Cellophane C ^NS /C ^PVC Microfluidic Devices

[0064] According to one aspect, open-channel microfluidic devices are appropriate for applications where optical transparency (especially in the visible regime), resistance to damage by water, biocompatibility and biodegradability are important. Cellophane microfluidic systems may complement existing cellulose-based paper microfluidic systems, and provide an alternative to other materials used in microfluidics, such as synthetic polymers or glass. Cellulose films are plausible materials for uses in integrated microfluidic systems for diagnostics, analyses, cell-culture, and MEMS.

[0065] FIG. 4 illustrates an exemplary flow diagram for the construction of coated cellophane (C ^PVC , or C ^NS ) microfluidic devices. Since pure cellophane swells in aqueous solutions and is not heat-sealable, coated cellophane (C ^PVC , and C ^NS ) may be used. The PVC and NS coatings allow different sheets of cellophane to be heat-sealed against one another, and also provide a barrier to prevent water from entering and swelling the cellophane.

[0066] In accordance with an exemplary embodiment, an initial step of the process shown in FIG. 4 includes compressing a dry, coated cellophane sheet between two metallic stamps kept at -120 °C, for 10s, to emboss the desired microfluidic features. Second, an additional coated cellophane sheet is placed against the embossed film. A heated metallic mold (-120 °C) is then placed on top to heat-seal the two cellophane films, and to enclose the embossed channels. Third, small holes are punched into the cellophane, either before or after sealing, for fluidic connections. As shown in FIG. 4, the tubing may be placed in communication with the holes using a connector and tape. The inlet and outlet holes may be connected to standard flangeless ferrules using double-sided tape (VHB ^® from 3M ^® ). Silicone tubing may be used to drive fluids into the channels. Metallic stamps may be used to form the microfluidic structures, and to seal microfluidic channels. Metallic stamps can be readily manufactured with standard industrial techniques, such as, CNC machining, laser cutting and printing, or water-jet cutting, for example.

[0067] The microfluidic structures may be formed in the first step through purely mechanical deformation of the cellophane since cellophane is not thermoplastic. As a result, when the cellophane is heated in the second step, the surface coating melts to provide heat- sealing, but the microfluidic structure is not altered by the heat. In one embodiment, both the embossing and heat-sealing steps can be completed in less than two minutes. The use of only dry materials may simplify the procedure and reduces cost. The process for forming the microfluidic devices, therefore, has potential to be scaled to large volumes using high- throughput printing techniques such as roll-to-roll manufacturing, for example.

[0068] In an embodiment, the sheets are stored in a vacuum desiccator chamber for at least one day prior to use. This step is helpful to remove water that is pre-absorbed from air in the cellophane so that the sheets do not shrink during the molding and sealing processes, due to drying of the pre-absorbed water. In the fabrication process, the metallic molds are first heated to -120 °C in an oven. A dry cellophane (C ^PVC , or C ^NS ) sheet is compressed between the two molds, using a pressure of 1-5 psi for the embossing. A second dry cellophane (C ^PVC , or C ^NS ) sheet is then compressed against the embossed sheet at a higher pressure, around 10 psi, with a second set of molds. The final step seals the microfluidic channel by heat-sealing of the coatings. In an exemplary embodiment, the embossing step takes 10-30 seconds while the heat sealing requires 30-60 seconds. The microfluidic channels are connected to silicone tubing (e.g., 1/16"), using flangeless ferrules (e.g., Tefzel, Item # EW-02007-54) affixed to the device with a thick transparent double-sided tape (e.g., VHB from 3M). A two-channel syringe pump (e.g., Harvard PHD 2000) may be used to drive fluids from the inlets to the outlets of the open micro channels at different flow rates.

[0069] Molds for microfluidic devices may be fabricated in aluminum, using

computerized numerical control machines (CNC), and sandblasted for a smooth finish.

FIGS. 5A-5C show exemplary molds made in brushed aluminum with CNC machining, used for the fabrication of microfluidic devices (channel width 500 μπι). FIG. 5A shows a negative mold (indented structures), and FIGS. 5B and 5C show positive molds (protruded structures).

FIGS. 6A and 6B show exemplary molds made in brushed aluminum with CNC machining, and used for the fabrication of microfluidic devices (channel width 1 mm). FIG. 6A shows a negative mold (indented structures) and FIG. 6B shows a positive mold (protruded structures).

[0070] FIG. 7A illustrates three exemplary cellophane microfluidic devices including a "Y-junction device," a "cross junction device," and a "T-junction device." The microfluidic devices may include serpentine channels. Repeating C-shaped implementations can make the fluid turn 180° to induce chaotic advection and passively enhance mixing of the streams. The devices may include a circular window intended for optical measurement.

[0071] FIGS. 7B-7D are images of PVC-coated cellophane (C ^PVC ) microfluidic devices with upwardly facing embossed features. FIGS. 7B and 7C show an exemplary channel width of 500 μιη. FIG. 7D shows an exemplary channel width of 1 mm. The channels in each of the devices may be 250 μιη deep, for example. It will be appreciated that other channel widths and depths may be implemented.

[0072] FIG. 7E is a microscope image showing a cross section of a channel from the embodiments of FIGS. 7B and 7C. FIG. 7F is a microscope image showing the cross section of a channel from the embodiment of FIG. 7D. FIG. 7G is an image of a C ^PVC microfluidic device illustrating its transparency by allowing background clouds to be seen through the device.

[0073] Basic Microfluidic Operation

[0074] FIG. 8A shows an image of the Y-junction microfluidic device, fabricated with nitrocellulose-coated cellophane (C ^NS ), displaying a laminar flow created by pumping red (input 2) and blue (input 1) aqueous dyes at a speed of 50 μΙ τήη. Since the microfluidic devices may comprise, for example, only two sheets of cellophane, including the thin coating on each side of the sheets, they are relatively narrow with a thickness of around 75 μιτι, and they are flexible. The right side of FIG. 8 A shows a microfluidic device that is bent during a continuous flow of the two liquids. The devices may be bendable to a radius of curvature of 1-2 cm, for example, without any noticeable change in the flow behavior or damage of the microfluidic structure. FIG. 8B shows an enlarged image of 8A.

[0075] FIG. 8C shows images of multiphase flow using the T-junction cellophane microfluidic device, showing the generation of slugs in a hexadecane / water system (both solutions had a flow rate of 30 μΙ τιίη). In particular, hexadecane (transparent) and an aqueous solution of a blue dye (reactive blue) were used to show the compatibility of the microfluidic channels to other solvents and the formation of different liquids. Hexadecane and the aqueous solution were injected into the T-junction device to form slugs.

[0076] With continued reference to FIG. 8A, two inlets are provided for connecting the Y-junction to a serpentine channel and one outlet. In an experiment using pressure-driven flows, a series of dyes were put into transmission mode. In particular, two aqueous solutions of dyes (Acid Red, and Reactive Blue) were injected at flow rates ranging from 10 to 200 and laminar flow was observed. It will be appreciated that either pressure- or vacuum-driven flows could be implemented. The Reynolds numbers for this example are below 10, which is an approximate value, and based on water flowing in a channel with a circular cross-section. The edges of the microfluidic channels were observed as being sharp, which indicates tight and well-defined sealing. No leakage or breakage from the inlets or at the edges of the channels occurred, which otherwise potentially could occur either as a result of delamination or film puncture during molding. Thus, experiments have shown that cellophane microfluidic devices according to exemplary embodiments can support flow rates up to 200 μΐ,/πήη, under pressure-driven flow, without leakage.

[0077] FIG. 9 shows a fluorescence image of a solution (fluorescein 0.001 wt/v % in water) injected into the channels of a nitrocellulose-coated cellophane (C ^NS ) microfluidic device. A Typhoon FLA 9000 instrument was used to image the device, with a 494 nm emission laser, a filter for 521 nm emission, and an image resolution 40 μιη/pixel. The dark spots correspond to bubbles or buckled channels.

[0078] Static Mixers

[0079] FIG. 10A shows a schematic diagram of an exemplary fabrication process for a cellophane C ^PVC microfluidic device with a porous static mixer having a Y-shaped input. As will be appreciated, mixing is important for many microfluidic applications. FIG. 10B shows an image of a cellophane C ^PVC microfluidic device with a porous static mixer. The porous mixer may be a piece of paper placed within the channel, as shown in the cross-section image of FIG. 10B(1).

[0080] Filter paper (Whatman grade 1) having a width of 1 mm is placed in a section of the channel before sealing the device, as shown in the fabrication process of FIG. 10A. In use, two aqueous solutions of a blue and a green dye (food color) flowed at a speed of 20 μΙ7πήη before and after entering into the mixer. FIG. 10B(2) shows the flow before the mixer and FIG. 10B(3) shows the flow after the mixer. The porosity and tortuosity of the filter paper accelerated the mixing of the two fluids. An advantage of this mixer is that it is more rapid, and requires shorter channels, than serpentine mixers. The length of the paper zone may be, for example, 10 mm. Paper has been extensively used as a substrate for storage of biomolecules, so the ability to incorporate paper into the cellophane-based device, may also take advantage of its large surface area and surface chemistry for the storage and release of reagents.

[0081] FIG. IOC shows an image of a T-junction microfluidic device with two aqueous dyes (blue color at input 1, and yellow color at input 2) being pumped at a flow speed of 2 μΙ7ηήη for each solution. The dyes have a laminar flow behavior at the beginning of the channel and subsequently mix with each other with diffusion mixing, as shown in FIGS. 10C(1)-10C(4). At this flow rate, the dyes co-flow without convective mixing. After passing through the serpentine channels, the flows mix and form a single green-colored stream. Mixing occured through diffusion, and could also be enhanced by chaotic advection using the turns in serpentine channels.

[0082] Electroanalytical Devices

[0083] The integration of electrodes in microfluidic systems enables the development of electroanalytical devices, and is often achieved by patterning electrodes (usually metals, patterned using vapor deposition) in one of the plates of the device (e.g., on polymer or glass substrates) and then sealing a second plate that includes microfluidic channels. The use of carbon electrodes in electroanalysis is attractive because they have a wide potential window in aqueous solutions, and low cost. Off-device configurations, using screen-printed carbon electrodes on substrates placed perpendicularly to closed microfluidic devices, or channels that incorporate embedded carbon paste, may be used.

[0084] Cellophane has the ability (like paper) to be used as a substrate for printing, with the added advantage of very low surface roughness. In accordance with exemplary embodiments, electrodes are integrated into cellophane microfluidic devices. Electrodes may be printed, for example, with graphite ink, using stencil printing, on coated cellophane (C ^PVC ). The cellophane sheets are sealed against the embossed features, as shown in FIG. 1 1 A. In particular, FIG. 1 1 A shows the fabrication of a coated cellophane microfluidic device with carbon, and Ag/AgCl electrodes printed inside the channels. The printed electrodes adhere strongly to the cellophane sheets and could also be sealed tightly against the embossed sheet, so that the device does not delaminate. The sealing is conformal between the regions with the electrodes and the bare substrate so that no water escapes from the channel into the regions around the electrodes. As a result, printed electrodes are integrated into the microfluidic channels.

[0085] To evaluate the performance of the integrated electrochemical cell electrodes, a flow injection analysis was performed by pumping a supporting electrolyte over a three- electrode electrochemical cell where all the electrodes were printed in a single step using carbon ink. In an embodiment, the working electrode (e.g., 1-mm width) is positioned before the counter electrode (e.g., 2.5 mm width), so that any substances that could be produced by electrolysis at the counter electrode would not reach the working electrode and cause interference. A carbon electrode (e.g., 1 mm width) acts as a quasi -reference electrode, as shown in FIG. 1 1C. FIG. 1 1C is an image of a cellophane C ^PVC device with carbon electrodes integrated into the channel, which is filled with a highly concentrated solution of ferrocyanide flowing at 50 μΐ,/πήη. The electrodes may be connected to a potentiostat using small copper clips.

[0086] A common electrochemical technique used in microfluidic devices is

potentiostatic amperometry. In this technique, a fixed potential— at which electron transfer occurs— is applied to the working electrode and the current is continuously recorded vs. time. After the baseline (continuous flow of the buffer) is allowed to stabilize, the sample is injected, and the signal it generates (the plateau current) is recorded vs. time.

[0087] FIG. 1 IB shows results obtained, using the cellophane C ^PVC device shown in FIG. 1 1C, by potentiostatic amperometry (+0.4 V vs. carbon quasi-reference) for potassium ferrocyanide. The ferrocyanide (in phosphate-buffered saline lxPBS) was injected from the sample inlet at different concentration (0.1 to 10 mM) at a constant flow rate of 50 μΐ,/πήη. A buffer solution (phosphate-buffered saline lxPBS) was pumped at 50 μΙ7πήη between each injected sample solution, to acquire a baseline current for normalization (n=7).

Solutions were pumped at 50 μΐ. per minute using two syringe pumps in such a way that pumping was carried out using one syringe pump at a time (either the carrier buffer or the ferrocyanide).

[0088] FIG. 1 ID shows results obtained for EC, using a cellophane C ^PVC device shown in FIG. 1 IE, by potentiostatic amperometry (1.25 V vs. Ag/AgCl) for the reaction of

Ru(bpy) ₃ ²⁺ (2 mM) with L-proline (0.1 to 1 μΜ). Both Ru(bpy) ₃ ²⁺ and L-proline were injected at a constant flow rate of 10 μΐ. per minute (n=9). FIG. 1 IE is an image of an ECL cellophane C ^PVC device with a carbon working- and counter electrode, and an Ag/AgCl reference electrode integrated into the channel. The channel is filled with a solution of Ru(bpy) ₃ ²⁺ flowing at 10 μΕ/πήη.

[0089] In the continuous flow ECL experiments, the analyte and the luminophore were injected into a common channel where they co-flow and pass the electrodes at a constant flow rate. With reference to FIG. 1 IE, for the ECL measurements in cellophane CPVC devices, both the analyte and the luminophore were injected at a constant flow rate of 10 μΕ/ιηίη, applied a potential of 1.25 V (vs. Ag/AgCl), and the resulting ECL light response was measured for a period of 0.5 seconds. A linear relationship was observed between the integrated area of the ECL response and the concentration of proline, in the concentration range between 0.1 μΜ to 10 μΜ (n=9), as shown in FIG. 1 ID. This proof-of-concept demonstration promises useful future applications of cellophane in ECL diagnostic applications.

[0090] FIG. 12 shows a time-current response of a continuous flow experiment with an electroanalytical device, such as those represented by FIGS. 11 A-l IE. The data were obtained by potentiostatic amperometry (+0.4 V) for potassium ferrocyanide. The ferrocyanide (in phosphate-buffered saline lxPBS, pH 7.0) was injected from the sample inlet at different concentrations (0.1 mM, 1 mM and 10 mM) at a constant flow rate of 50 μΐνηήη. A buffer solution (lxPBS) was pumped at 50 μΐνηήη between each injected sample solution (the sample was not pumped when the buffer was running), to acquire a baseline current for normalization.

[0091] The cellophane-based system was used for amperometric detection of the redox probe, potassium ferrocyanide (concentrations between 10 μΜ to 10 mM), at a potential of 0.4 V (vs. a carbon quasi-reference electrode) applied to the working electrode. A linear relationship was observed between the ferrocyanide concentration and the ratio of the plateau current to the baseline current (FIG. 1 IB), over this concentration range (n=7). This linearity indicates that this system can be used for quantitative determination of an electroactive analyte. To further exploit the inherent transparency of cellophane in combination with electroanalysis, electrochemical cellophane microfluidic devices were fabricated for use in continuous flow ECL detection. These devices were similar to those used for continuous flow electroanalysis of ferrocyanide, with the exception that the reference electrode was printed with an Ag/AgCl ink. ECL is among the most sensitive analytical techniques, and it is commonly employed for immunoassays at many routine hospital laboratories using clinical autoanalyzers.

[0092] For the cellophane ECL system, a reaction between Ru(bpy) ₃ ²⁺ (luminophore) and L-proline (analyte) was chosen because this reaction has been extensively studied. The ECL reaction mechanism, in short, starts with the electrochemical oxidation of Ru(bpy) ₃ ²⁺ to Ru(bpy) ₃ ³⁺ at the electrode surface. Proline is then oxidized by Ru(bpy) ₃ ³⁺ to produce a free radical proline ^* , which undergoes spontaneous deprotonation to produce an intermediate radical ion that reacts with Ru(bpy) ₃ ³⁺ to produce the excited state [Ru(bpy) ₃ ²⁺ ] ^* . The excited [Ru(bpy) ₃ ²⁺ ] ^* finally decays to its ground state, Ru(bpy) ₃ ²⁺ , and emits orange light at 620 nm. (See SI for the balanced chemical equations for these reactions). If the concentration of the luminophore is kept constant, the intensity of the emitted light is directly related to the concentration of the analyte. Mechanisms of proline/Ru(bpy)3 ²⁺ based ECL are shown below.

Ru(bpy) ₃ ²⁺ → Ru(bpy) ₃ ³⁺ + e ^" (1)

Proline + Ru(bpy)3 ³⁺ → proline ⁺ * + Ru(bpy)3 ²⁺ (2) proline ⁺ * ^→ proline* + H ⁺ (3)

Ru(bpy)3 ³⁺ + proline*→ [Ru(bpy)3 ²⁺ ] * + products (4)

[Ru(bpy) ₃ ²⁺ ]*→ Ru(bpy) ₃ ²⁺ + hv (light) (5)

[0093] Next, 0.2 mM Ru(bpy) ₃ ²⁺ and L-standards proline (0.1 to 1 μΜ) in 0.1 M phosphate buffer (pH 7.5) are provided. The working electrode of the flow cell is positioned directly above a 4 mm2 Si photodiode (e.g., DET36A, Thorlabs), interfaced to the ADC164 module of the potentiostat using a BNC cable. The Ru(bpy)3 ²⁺ (2 mM) and L-proline (at varying concentrations) was injected into the two inlets at a constant flow rate of 10 μΐ. per minute. The potentiostat is configured to apply 0.5-second potential pulses at a potential of 1.25 V vs. Ag/AgCl, with a 5-second delay time between measurements, and recorded the light intensity. The ECL signal was calculated by integrating the area under the measured light intensity, using NOVA software. The sample may be varied by manually changing the sample inlet between syringe pumps with different concentrations of L-proline. According to embodiment, for ECL measurements, an Ag/AgCl quasi-reference electrode may stencil printed, instead of a carbon reference electrode.

[0094] A cyclic voltammogram of Ru(bpy)32+ showed that the electrodes printed inside cellophane microfluidic channels were stable in the range of potentials used for the experiments, and that the channels did not leak or change dimension significantly during operation, as shown in FIG. 13. FIG. 13 represents cyclic voltammograms of Ru(bpy)32+ (lmM in lxPBS) measured at 100 mV/s, using a microfluidic device similar to that seen in FIG. HE.

[0095] Integrated Heaters

[0096] The ability to print conductors on cellophane enables the integration of electrical function intended for applications other than electroanalysis. One useful example is electrical heating. In an embodiment, a resistive heating device is integrated into the cellophane C ^PVC microfluidic devices by printing a carbon structure on the backside of a device, as shown in FIGS. 14A-14D. Since the cellophane is thin, heating may be restricted to the regions immediately adjacent to the heater, and heating of the liquid in the channel is rapid.

[0097] In particular, FIGS. 14A and 14C show carbon electrodes printed on the backside of PVC-coated cellophane (C ^PVC ) microfluidic devices to act as resistive heaters. FIG. 14A shows the heater printed along a single channel. FIG. 14C shows the heater printed on the entire area of serpentine channels. FIGS. 14B and 14D are infrared images from the heaters in FIGS. 14A and 14C, respectively, during operation. Both heaters operated using a power input of around 1W, for example. The temperature shown in FIGS. 14B and 14D is given in Celsius.

[0098] Fabrication of the Electrochemical Devices and Heaters

[0099] According to an exemplary embodiment, a laser cutter (e.g., VL-300, 50 Watt, Versa Laser) may be used to cut shapes in an adhesive tape (e.g., polypropylene low-tack adhesive-back film, 0.002" thick). The adhesive tape can serve as a stencil mask by gluing it onto a single sheet of cellophane. A carbon ink (Ercon Inc.) is applied using a clean-room swab through the stencil mask, to print the electrodes. The mask is then removed, and the ink is allowed to dry at room temperature for 1 hour and then the cellophane sheets are stored in a desiccator for 1 day, for example, to remove any excess solvent from the ink, and to dry the cellophane sheet. The cellophane sheets with the carbon print are then used for making microfluidic devices. By placing the printed electrodes facing the microfluidic channel, electrodes are fabricated inside of the channel, as shown in FIG. 1 1 A. By placing the printed electrodes facing the outer part of the device, the resistive heating elements are fabricated, as shown in FIG. 14.

[0100] FIG. 15 shows a crossectional view of an exemplary microfluidic device made of cellophane that has two channels separated by a cellophane layer. The middle cellophane layer (layer 2) can act as a membrane. The birefringence properties of cellophane can be exploited as optical filters. According to aspect, multilayered cellophane based devices, which have integrated microfluidic channels for the delivery of analytes and filters to allow for both excitation and detection at specified wavelengths. Such devices can be used for UV- Visible spectrophotometric detection or fluorescence detection of various analytes of interest. Multiplexed detection methods can also be provided by positioning removable cellophane filters throughout the device. According to embodiment, the multi-channel microfluidic devices can be adapted to produce a low-cost, portable, fully integrated detection system for various analytes of interest. [0101] Cellophane may be used as a membrane for several applications. The pore size of cellophane is modifiable. With further reference to the three-layer cellophane microfluidic device shown in FIG. 15, the inner layer (layer 2) can be uncoated cellophane while the two outer layers (layers 1 and 3) can be a coated cellophane, for example. This device can be used for liquid-liquid and gas-liquid applications where selective membrane properties are needed, for example.

[0102] Materials and Methods

[0103] According to exemplary embodiments, commercially available uncoated cellophane (C ^0H ), and cellophane with thin coatings (several microns-thick) of nitrocellulose (C ^NS ) or polyvinylidene chloride (C ^PVC ) are used. The coatings allow different sheets of cellophane to be heat-sealed against each other and provide a barrier to prevent water from entering and swelling the cellophane. According to embodiments, sheets having the following properties can be used: 28 μπι and 35 μιη-thick uncoated cellophane (cellophane P), 30 μιη-thick cellophane with a double-sided nitrocellulose coating (cellophane LMS), and 42 μιη-thick cellophane with a double-sided polyvinylidene chloride coating (cellophane XS). According to further embodiments, silanes, 3,3,4,4,5,5,6,6,7,7,8,8,9,9,10, 10,10 heptadecafluorodecyl trichlorosilane, CF ₃ (CF2)7CH2CH2SiCl ₃ (Cio ^F ), and decyltrichlorosilane (Cio) can be used. The following were also used with respect to the above noted

experiments: hexadecane, glycerol, ethylene glycol, ethanol, dimethyl sulfoxide (DMSO), N-Methyl-2-pyrrolidone ( MP), tetrahydrofuran (TFIF) , methyl-2-pyrrolidone,

dimethylformamide (DMF), L-proline, tris(2,2'bipyridyl)ruthenium(II) chloride (Ru(bpy) ₃ ²⁺ ), sodium 4-(2-hydroxy-l-naphthalenylazo)-naphthalenesulfonate), fluorescein and Reactive Blue 2, adenine, bovine serum albumin BSA, acetaminophen, and thiamine.

[0104] Conclusions

[0105] In accordance with exemplary embodiments discussed herein, microfluidic systems are provided based on films from regenerated cellulose (cellophane), which is either uncoated, (C ^0H ), or has thin surface coatings of nitrocellulose (C ^NS ), or polyvinyl chloride (C ^PVC ). Cellophane C ^0H can be used to fabricate microplates and cellophane C ^NS , or C ^PVC can be used to fabricate open-channel microfluidic devices that incorporate channels with small dimensions and complex geometries. Electrodes can be printed onto the microfluidic devices to integrate resistive heating elements, and embedded in the microfluidic channels, to enable the fabrication of electroanalytical devices such as continuous-flow ECL devices. [0106] Cellophane C and C are attractive materials for microfluidic systems that require high throughput manufacturing at low cost, and optical transparency as they heat seal under conveniently attainable conditions. Cellophane microfluidic devices combine some of the advantages of cellulose-based paper microfluidics with new capabilities. They have at least the following useful characteristics: i) the manufacturing processes are potentially scalable, and low in cost (through, for example, roll-to-roll manufacturing); and ii) they enable open-channel systems with useful resolution (~ 500μιη in this work, but resolution down to tens of μιη is possible). They also enable transparent electroanalytical microfluidic devices that can be fabricated using high-throughput printing techniques. For example, cellophane materials have a high optical transparency in both the UV and visible regime. Cellophane C ^0H has the highest transparency in the UV region of the cellophane varieties that were analyzed. Cellophane materials are also resistant to destruction by many solvents and are thin (e.g., around 70 μιη) and flexible. Further, they are easily disposable, through biodegradation or incineration. In some cases, a potential disadvantage of cellophane C ^0H as a microfluidic material is it absorbs water and could therefore change dimensions.

Furthermore, bonding of cellophane C ^0H may need an adhesive, since cellulose does not dissolve easily in solvent, nor is it thermoplastic. Cellophane C ^0H materials are useful for microplate applications that require optical transparency (especially in the UV region), solvent resistance, and biocompatibility.

[0107] Cellophane is both a complement to cellulose-based paper for microfluidic devices, and also an alternative to other materials currently used in microfluidics, such as PDMS, cyclic olefin copolymers (COC), PMMA, PS, and glass or quartz. Regenerated cellulose films or other forms of cellulose films, such as nanocellulose paper, may be used in disposable integrated microfluidic systems for diagnostics, cell culture, and MEMS.

[0108] As will be apparent to one of ordinary skill in the art from a reading of this disclosure, the disclosed subject matter can be embodied in forms other than those specifically disclosed above. The particular embodiments described above are, therefore, to be considered as illustrative and not restrictive. Those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific embodiments described herein.