WITH REDUCED CHANNEL HEIGHT

[0001] This application claims the priority benefit of U.S. Provisional Patent Application Serial No. 61/684,796, filed August 19, 2012, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION [0002] The present invention relates to methods for preparing low volume microfluidic devices, the resulting micro fluidic devices, and the use thereof.

BACKGROUND OF THE INVENTION [0003] Prior art microfluidic devices are characterized by the presence of a relief patterned substrate that forms a plurality of microchannels, as well as any inlet/outlet ports, mixing chambers, etc., and an elastomeric polymer film bonded to the upper surface of the substrate. One example of this type of microfluidic device 10 is shown in Figure 1 A to includes a silicon substrate 12 with one or more etched microchannels 14 (one shown) and an essentially planar polydimethoxysilane ("PDMS") cap 16. The only control over channel height of the microchannels in these types of devices is the use of thinned substrate (e.g., a thinned silicon wafer). One example of this type of microfluidic device 20 is shown in Figure IB to includes a thinned silicon substrate 22 with one or more etched microchannels 24 (one shown) and an essentially planar PDMS cap 26. This is problematic insofar as the thinned substrate has increased cost and is more susceptible to damage.

[0004] It would be desirable to identify materials and methods for producing microfluidic devices that achieve reduced channel height and, therefore, volume while also using materials in a way that does not substantially increase the cost of the device.

[0005] The present invention is intended to overcome these and other deficiencies in the art. SUMMARY OF THE INVENTION

[0006] A first aspect of the invention relates to a method of making a microfluidic device that includes the steps of: forming a mold substrate having a relief pattern formed in a surface thereof and a device substrate having a relief pattern formed in a surface thereof, wherein the relief patterns formed into the surface of the mold and device substrates are identical or nearly identical except the relief pattern of the mold substrate has a depth that is smaller than a depth of the relief pattern of the device substrate;

forming a polymer layer over the patterned surface of the mold substrate, whereby the polymer layer has a conforming surface that conforms to the relief pattern thereof;

removing the polymer layer from the mold substrate; and bonding the conforming surface of the polymer layer to the patterned surface of the device substrate, thereby forming a microfluidic device having a microfluidic channel with a height essentially equal to the difference between the depths of the mold substrate and device substrate relief patterns.

[0007] A second aspect of the invention relates to a method of making a microfluidic device that includes the steps of: forming a polymer layer over a relief patterned surface of a mold substrate, whereby the polymer layer has a conforming surface that conforms to the relief pattern thereof; removing the polymer layer from the mold substrate; and bonding the conforming surface of the polymer layer to a relief patterned surface of a device substrate, where the relief pattern on the device substrate and mold substrate differ in that the relief pattern of the mold substrate has a depth that is smaller than a depth of the relief pattern of the device substrate, thereby forming a microfluidic device having a microfluidic channel with a height essentially equal to the difference between the depths of the mold substrate and device substrate relief patterns.

[0008] A third aspect of the invention relates to a microfluidic device prepared according to a process of the first or second aspects of the invention.

[0009] A fourth aspect of the invention relates to a microfluidic device that includes a device substrate comprising a pattern of one or more microchannels formed in a surface thereof, the microchannels being defined by a floor membrane and sidewalls; a polymer layer having a conforming surface that conforms to the pattern of the one or more microchannels, whereby the conforming surface is bonded to the substrate surface and the sidewalls of the one or more microchannels; and a port in fluid communication with the one or more microchannels. [0010] A fifth aspect of the present invention relates to a kit that includes a mold substrate having a relief pattern formed in a surface thereof; and a device substrate having a relief pattern formed in a surface thereof, wherein the relief patterns formed into the surface of the mold and device substrates are identical or nearly identical except the relief pattern of the mold substrate has a depth that is smaller than a depth of the relief pattern of the device substrate.

[0011] A sixth aspect of the present invention relates to a kit that includes a mold substrate having a relief patterned surface; and a device substrate having a relief patterned surface thereof, where the relief pattern on the device substrate and mold substrate differ in that the relief pattern of the mold substrate has a depth that is smaller than a depth of the relief pattern of the device substrate.

[0012] The present invention affords a process for the manufacture of

microfluidic devices that have improved control over microchannel height and, hence, microchannel volume. This method also provides the flexibility to change channel height dimensions without altering the design of the device chip itself. By using different mold substrates, it is possible to form different polymer covers with differently dimensioned relief patterns so as to allow for the generation of microfluidic devices having different channel heights and, hence, channel volumes. It is also advantageous to use the same mask to create both the device channels and the mold for the elastomeric polymer layer. The resulting microfluidic device delivers advantages over traditional systems by eliminating the need for more expensive and fragile thinned wafers to reduce channel height.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A is a cross section of a prior art microfluidic device formed using a substrate (e.g., silicon wafer) that is etched to form one or more microchannels, and an elastomeric polymer (e.g., PDMS) cap over the substrate. The materials identified are exemplary.

[0014] Figure IB is a cross section of a prior art microfluidic device formed using a reduced thickness substrate (e.g., silicon wafer) that is etched to form one or more reduced height microchannels, and an elastomeric polymer (e.g., PDMS) cap over the substrate. The thinned substrate, while sufficient to reduce the microchannel height, is problematic insofar as the substrate is much more fragile and prone to damage. The materials identified are exemplary. [0015] Figures 2A-B illustrate a method of preparing the mold and device substrates, fabricating the elastomeric polymer layer, and combining the device substrate and elastomeric polymer layer to form the microfluidic device of the present invention. The materials identified are exemplary.

[0016] Figure 3 is a cross section of a microfluidic device formed using the present invention, which includes one or more reduced height microchannels formed between the patterned substrate (e.g., silicon) and the conforming surface of the elastomeric polymer (e.g., PDMS) cap. The materials identified are exemplary.

[0017] Figure 4 illustrates a microfluidic device prepared in accordance with the present invention, which includes a plurality of parallel microfluid channels that are fed by a common passage from the inlet port and relieved by a common passage to the outlet port. The bottoms of the microfluid channels are porous and communicate via the pores with a common channel that passes a separate fluid via inlet and outlet.

[0018] Figure 5 is a cross sectional view of PDMS sheet capping micromachined channels shows the height reduction of the channels, which have a trapezoidal cross section. The PDMS was bonded to a multichannel membrane device and reduced the channel height from 300 μιη to 200 μιη. The cross section is created with a razor blade which creates grooves in the PDMS and leaves debris on the rough exposed edge of the Si.

DETAILED DESCRIPTION OF THE INVENTION

[0019] The present invention relates to methods for making microfluidic devices, the resulting microfluidic devices, and kits that can be used to prepare the same.

[0020] The microfluidic device is formed of a substrate and an elastomeric polymer layer, which together define the features of the microfluidic device. These features may include, without limitation, one or more microfluidic channels, one or more inlet and outlet ports, one or more sample chambers, one or more mixing chambers, one or more heating chambers, one or more valve structures, and one or more nanoporous or microporous membrane structures. These features can be connected together so as to achieve a desired function (e.g., mix, pump, redirect, allow reactions to occur, filter, etc.).

[0021] The materials and structures formed in accordance with the present invention are particularly, though not exclusively, suitable for photolithography and electrochemical etching processes for the formation thereof. [0022] Although silicon is by far the most common substrate used in forming microfluidic devices of this type, persons of skill in the art should appreciate the other materials can be used including, without limitation, undoped germanium, p-doped silicon or germanium, n-doped silicon or germanium, a silicon-germanium alloy, and Group III element nitrides. Dopants are well known in the art and may include, without limitation, (CH ₃ ) ₂ Zn, (C ₂ H ₅ ) ₂ Zn, (C ₂ H ₅ ) ₂ Be, (CH ₃ ) ₂ Cd, (C ₂ H ₅ ) ₂ Mg, B, Al, Ga, In, H ₂ Se, H ₂ S, CH ₃ Sn, (C ₂ H ₅ ) ₃ S, SiH ₄ , Si ₂ ¾, P, As, and Sb. The dopants can be present in any suitable amount.

[0023] The elastomeric polymer material is preferably a silicone elastomeric material such as polydimethylsiloxane ("PDMS", e.g., Dow Corning Sylgard ^® 184) (McDonald et al, "Fabrication of Microfluidic Systems in Poly(dimethylsiloxane)," Electrophoresis 21 :27-40 (2000), which is hereby incorporated by reference in its entirety). PDMS is a particularly well studied material for the construction of microfluidic systems. It is optically transparent, and has a refractive index that is much lower than that of silicon. PDMS has a hydrophobic surface after polymerization, but the surface of PDMS can be treated with a surfactant, oxygen and plasma, or atmospheric RF to become hydrophilic (Hong et al., "Hydrophilic Surface Modification of PDMS Using Atmospheric RF Plasma," Journal of Physics: Conference Series 34:656-661 (2006), which is hereby incorporated by reference in its entirety). This hydrophilicity assists not only in bonding the polymer layer to the substrate, but also decreases surface tension and bio fouling within the microchannels to allow fluids to move easily along those channels. Chemical treatment methods are also available for improving the performance of PDMS (Lee and Voros, "An Aqueous-based Surface Modification of poly(dimethylsiloxane) with poly(ethylene glycol) to Prevent Biofouling," Langmuir 21 : 11957-11962 (2004), which is hereby incorporated by reference in its entirety).

[0024] Photolithography and etching are commonly used to produce

microstructures in a variety of substrates, particularly those described above. In a typical process, a masking agent is applied to the surface of a material using lithography to form an array of elements that will dictate the manner in which the unprotected substrate will be etched. The portions of the surface that are not protected by the masking agent are then chemically etched using an etchant. Any of a variety of suitable anisotropic or isotropic (i.e., wet) etchants can be used. Exemplary anisotropic etchants include, without limitation, buffered oxide etchant solutions containing about 5 to about 25 wt %, more preferably about 5 to about 15 wt % HF; KOH (potassium hydroxide) etchants containing about 20 wt% to about 60 wt% KOH, TMAH (tetramethylammonium hydroxide) etchants containing about 2 wt% to about 10 wt% TMAH, and EDP etchants (containing ethylenediamine, pyrocatechol, pyrazine, and water). Exemplary isotropic etchants include, without limitation, various combinations and mixtures of HNO ₃ , HF, CH ₃ COOH, HCIO ₄ , or KMn0 ₄ . Regardless of the etchant, after etching the substrate can be removed from the etch cell, rinsed with ethanol, then water, and dried under a stream of N ₂ gas. The mask can be removed with an appropriate solvent such as acetone, methyl ethyl ketone (MEK), or methyl isobutyl ketone (MIBK). Used in this manner, lithography and etching can produce highly detailed microfluidic structures in the substrate.

[0025] Depending on the extent of the etching process and subsequent post-etch processing, the resulting device substrate can have the bottom of its microfluidic channels formed into nanoporous membranes that are less than 500 nm thick, more preferably less than 100 nm thick, and have properties that include a porosity of at least about 1% percent, a pore size cutoff below 100 nm, and combinations thereof. Nanoporous membranes of this type can be formed according to the techniques described in U.S. Patent No. 8,182,590 to Striemer et al, Fang et al, "Methods for Controlling the

Morphology of Ultra-thin Porous Nanocrystalline Silicon Membranes," J. Phys: Condens Matter 22(45) :4134 (2010); Fang et al, "Pore Size Control of Ultra-thin Silicon

Membranes By Rapid Thermal Carbonization," Nano Letters 10(10):3904-8 (2010); Striemer, CC, "Application of Silicon Nanostructures Compatible with Existing

Manufacturing Technology," Ph.D. Thesis, University of Rochester, Rochester, NY (2004), each of which is hereby incorporated by reference in its entirety. Subsequent to membrane formation, silicon nitride support structures can be applied to the backside of the membrane if desired. Alternatively, microporous membranes can be formed using deep reactive ion etching (DRIE) for pore drilling through the silicon (Zazpe et al., "Ion- transfer voltammetry at silicon membrane-based arrays of micro -liquid-liquid interfaces," Lab Chip 7: 1732-1737 (2007), which is hereby incorporated by reference in its entirety. These membrane structures can be used to allow filtration between adjacent

microchannels or microchannel/chamber formations within the microfluidic device.

[0026] In addition to etching the substrates, structures can also be formed on the surface of the mold substrate. This can be achieved either before masking, described above, or after etching the microstructures into the substrate. Application before etching is preferred. As discussed in greater detail hereinafter, the structures formed on the surface of the mold substrate are negatives insofar as they allow for formation of any one of the above described cavities or channels or chambers, etc., when the elastomeric polymer layer cast on the mold is removed, transferred, and bonded to the device substrate.

[0027] To form structures on the surface of the mold substrate, a photoresist material such as SU-8 can be applied to the surface in any desired configuration or dimension, and then cured via baking. SU-8 is known to bond permanently to silicon substrate via epoxy cross-linking so as not to interfere with removal of the elastomeric polymer layer as discussed below. SU-8 is mechanically very strong, whereas PDMS is not. This makes SU-8 use on the mold surface highly compatible with the use of PDMS as the elastomeric polymer layer, because the SU-8 will allow for release of the PDMS from the mold substrate.

[0028] To inhibit elastomeric polymer material from adhering to the patterned surface of the mold substrate, the mold substrate can be surface treated to render it substantially inert to cross-linking with the polymer during the cure process, discussed below. Briefly, the mold substrate can be silanized using any of a variety of techniques. According to one approach, the mold substrate can be exposed to

perfluorooctyltrichlorosilane under vacuum conditions to form a self-assembled monolayer on the relief patterned surface of the mold substrate.

[0029] In accordance with the present invention, and referring now to Figures 2A-

B, these techniques can used to form a relief pattern in each of a microfluidic device substrate and a mold substrate.

[0030] In preferred embodiments, the surfaces of both the device substrate 32 and mold substrate 52 are masked using an identical mask pattern (34, 54). This ensures that the eventual relief pattern formed into the respective substrates is identical or nearly identical. As used herein, the term "nearly identical" is intended to mean that the formed relief patterns (and mask patterns used to form them) are structurally similar, within tolerances afforded by the materials, so as to allow formation of a fluid-tight microfluidic device. Because the mold substrate is used to form structural features of the polymer material, persons of skill in the art will readily appreciate that the elastomeric nature of the polymer material will dictate the degree of tolerance to variations between the mold and device substrates.

[0031] To the extent that surface structures 56 have also been applied to the surface of the mold substrate 52, this will not affect the relief pattern formed into the mold substrate by etching. During the etching process, it is desirable to etch the relief patterns into the mold and device substrates using the same reaction conditions, including etchant, electrochemical conditions (current/voltage), and temperature. In most embodiments, the only different variable during formation of the mold and device substrates is the time of the etching steps, which affords a deeper etch pattern in the device substrate 32 as compared to the mold substrate 52. The difference between the depth of the relief patterns formed in the mold and device substrates (52, 32) will translate to the final height and, thus, volume of the microchannel formations of the resulting microfluidic device.

[0032] After etching and removal of the mask (34, 54), the mold and device substrates (60, 50) can be cleaned, rinsed, and dried as described above. The mold substrate 60 should be surface treated, after which it is ready for its use, whereas the device substrate 50 can be retained for later use.

[0033] The mold substrate 60 is used to form the elastomeric polymer layer 62 that will be used with the device substrate 40 to form the microfluidic device. The elastomeric polymer layer 62 is formed by introducing a liquid composition that includes the polymer precursors onto the surface of the mold substrate 60, and then allowing the liquid composition to cure under conditions suitable to form the elastomeric polymer layer. Briefly, for PDMS, the base and curing agents are thoroughly mixed together in a roughly 10: 1 weight ratio, although variations in this ratio can also be used. After mixing, the liquid mixture should be placed in a desiccator under vacuum (e.g., 22 in. Hg) until the liquid mixture is free of bubbles, which should take about 10-20 minutes. The mold substrate 60 can then be placed into a holding device and the degassed liquid mixture can be poured slowly over the mold substrate so as to avoid trapping air. The mold and liquid are then placed in a vacuum oven (e.g., 80°C, 5 in. Hg) and cured. These conditions should avoid formation of gas bubbles or voids at the mold surface. After curing, the edges of the elastomeric polymer layer 62 can be trimmed, as needed, and the layer removed from the mold substrate 60. The elastic polymer layer 62 thus formed has a conforming surface 64. [0034] At this time the elastomeric polymer layer 62 has a hydrophobic surface, which will inhibit bonding to the mating relief-patterned surface of the device substrate 40. The conforming surface 64 of the elastomeric polymer layer 62 can be treated under suitable conditions with an activating agent that will render the surface hydrophilic. Examples of suitable activating agents are described above (0 ₂ plasma, surfactants, atmospheric RF treatment). By way of example, the elastomeric polymer layer can loaded into a reactive ion etcher using 25% 0 ₂ at 0.200 torr, 33.3% RF for 30 sec (or equivalent conditions).

[0035] Once treated, the elastomeric polymer layer 62 is then bonded to the device substrate 40. This is carried out by contacting the conforming surface 64 of the elastomeric polymer layer 62 to the relief patterned surface of the device substrate 40. To facilitate handling and placement of the elastomeric polymer layer, it may be wetted in water or ethanol prior to initiating contact with the device substrate. (Contacting with the device substrate should be made carried out without significant delay after surface activation of PDMS, because the PDMS surface will return to its hydrophobic state after time.) Once dry, bonding is complete. To facilitate a thorough fluid-tight seal between the elastomeric polymer layer 62 and the device substrate 40, slight pressure can be applied while contacting, i.e., during the bonding process.

[0036] As shown in Figure 3, the micro fluidic device 70 thus formed possesses a reduced height microchannel 72 defined between the polymer layer 62 and the device substrate 40. The base of the microchannel 72 can be defined by a micro or nanoporous membrane 74 that is formed using, e.g., the processes described above. The microfluidic device 70 also includes a microchannel 76 formed on the upper surface of the device substrate 40.

[0037] As is well known in the art, fluid ports can be designed for either introduction through the device substrate or through the elastomeric polymer layer. For example, ports can be formed by creating vertical channels in the elastomeric polymer layer such that short glass tubing may be inserted into channels formed in the PDMS (defined by the SU-8 on the mold substrate). Flexible tubing can be attached to these short glass tubes. Alternatively, a common or shared channel in the elastomeric polymer layer can be carried to the edge of the device substrate and accessed from a single port. Side entrance to the device will allow multiple devices to be stacked upon each other in parallel. [0038] Once the fluid ports are installed, the microfluidic device can be pressure- tested by applying a volume of a flowable fluid to the fluid inlet port, and flowing the flowable fluid through the one or more microfluidic channels toward the fluid outlet port. Thereafter the device is ready for use.

[0039] Using the present invention, it is possible to produce microchannels that are less than about 500 μιη in height. In certain embodiments, the microchannels are less than about 400 μιη in height, less than about 300 μιη in height, less than about 200 μιη in height, or less than about 100 μιη in height. In other embodiments, the microchannels are less than about 90 or about 80 μιη in height, less than about 70 or about 60 μιη in height, less than about 50 or about 40 μιη in height, less than about 30 or about 20 μιη in height, or less than about 10 μιη in height. In addition, it is possible to form microchannels that are about 100 μιη to about 1 mm wide at the membrane surface.

[0040] The ability to control the microchannel height to width ratio allows the resulting microfluidic devices to possess a microchannel volume to membrane surface area ratio that in some embodiments is lower than that found in other microfluidic devices. For example, a microfluidic device with 300 μιη deep, 1mm wide channel (at membrane), has a V/A ratio of 0.345, whereas a channel reduced to 100 μιη but having the same width has a V/A ratio of 0.105 and a channel reduced to 10 μιη but having the same width has a V/A ratio of 0.01005.

[0041] According to one embodiment, illustrated in Figure 4, the microfluidic device 80 includes a device substrate 90 formed of silicon and having a plurality of parallel, reduced-height microchannels with a porous nanocrystalline membrane (less than 500 nm thick) formed at the base of each microchannel. Above substrate 90 is a polymer layer 82 that has a conforming lower surface that partially defines the plurality of microchannels. Formed in the polymer layer 82 is a pair of lateral microchannels 84, 86, which allow for coupling of the plurality of parallel, reduced-height microchannels to a common inlet 88 that communicates with the microchannel 84 and a common outlet 90 that communicates with the microchannel 86. The polymer layer 82 is capped by a layer 92, which is formed by PDMS, glass, or a thermoplastic material. Below device substrate 90 is a polymer layer 94 that defines a common chamber communicating with each of the plurality of parallel, reduced-height microchannels via the porous nanocrystalline membranes. The common chamber is also capped by a layer 96, which is formed by PDMS, glass, or a thermoplastic material. The common chamber includes an inlet 98 and an outlet 100.

[0042] This embodiment can be used as a parallel flow filtration system for, e.g., dialysis. A fluid to be filtered is delivered via inlet 88 through each of the microchannels, where the fluid flows above the porous membrane, and exits via outlet 90 as a filtered fluid. A counter-flow fluid is delivered via inlet 98 to the common chamber, where it collects the filtered materials before exiting via outlet 100.

EXAMPLE

[0043] The following Example is intended to illustrate the practice of the present invention, but it is in no way intended to limit the scope of the claimed invention.

[0044] A silicon wafer was used to form both a mold substrate and device substrate. The lower side of the device substrate was first prepared to generate a porous nanocrystalline nanolayer in accordance with the techniques described in U.S. Patent No. 8,182,590 to Striemer et al., Fang et al., "Methods for Controlling the Morphology of Ultra-thin Porous Nanocrystalline Silicon Membranes," J. Phys: Condens Matter

22(45):4134 (2010); Fang et al, "Pore Size Control of Ultra-thin Silicon Membranes By Rapid Thermal Carbonization," Nano Letters 10(10):3904-8 (2010), each of which is hereby incorporated by reference in its entirety. This nanolayer will eventually form the membrane at the base of the device microchannel.

[0045] These mold substrate surface was printed with SU-8 and both substrates were masked with a nitride film. After masking, the substrates were etched with EDP etchant. The mold substrate was etched for a shorter duration to form a 100 μιη deep channel, whereas the device substrate was etched to a depth of 300 μιη. This allowed for a reduction in channel height from 300 μιη to 200 μιη. After removal of the nitride mask, the mold substrate was cleaned, dried, and then surface treated with Cole-Parmer® Micro-90® as a release agent. Thereafter, PDMS was prepared, applied to the mold substrate, and cured. After curing, the PDMS layer was removed from the mold substrate.

[0046] Both the PDMS layer and the Si device substrate were surface activated

(0 ₂ , plasma) and then soaked in ethanol. Thereafter, the PDMS later and Si device substrate were mated, with pressure, until dry. A cross sectional image of the finished device is illustrated in Figure 5. The cross section was created by cutting the device with a razor blade, which created grooves in the PDMS and left debris on the rough exposed edge of the Si. In Figure 5, the location of the bulk silicon (forming the trapezoidal channel sidewalls), the porous nanoscale nc-Si membrane, and PDMS are identified. The reduction of the channel height from 300 μιη to 200 μιη is also illustrated.

[0047] Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations,

improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Accordingly, the invention is limited only by the following claims and equivalents thereto.