THERMOPLASTIC NANOFLUIDIC DEVICES

Related Applications

This application claims the benefit of United States Provisional Patent Application Serial No. 62/050,237, filed September 15, 2014, the disclosure of which is incorporated by reference herein in its entirety.

Background of the Invention

Fluidic devices that employ structures less than 100 m in one or two dimensions (i.e., nanofluidics) are generating great interest due to the unique properties afforded by this size domain when compared to their micro-scale counterparts. For example, new DNA sequencing machines are being conceived, which employ nanofluidic devices.

Over the past decade, thermoplastics have been extensively investigated as alternative substrates to glass and Silicon for the fabrication of microfluidic devices because of the availability of diverse and robust fabrication protocols that can be used to produce the desired structures in a high production mode and at low cost, the extensive array of physiochemical properties they possess, and the simple modification strategies that can be employed to tune their surface chemistry.

While the advantages of polymer microfluidics are currently being realized, the evolution of functional thermoplastic nanofluidic devices is fraught with challenges. One major challenge is assembly of the device, which consists of sealing a cover plate to the fluidic substrate. Channel collapse or substrate dissolution can result when sealing the nanofluidic substrate with the cover plate, making the device inoperable. Accordingly, there is a need for new ways to manufacture nanofluidic devices.

Summary of the Invention

In this invention, we describe a hybrid assembly approach for the generation of functional nanofluidic devices. The invention involves using a high glass transition temperature (Tg) substrate containing the nanofluidic structures thermally sealed to a cover plate possessing a lower Tg compared to the substrate. As a proof-of-concept, nanofluidic devices with dimensions ranging between about 40 and 200 nanometers were fabricated in a higher Tg substrate and sealed with a different cover plate having a lower Tg compared to the substrate. The results obtained from sealing tests revealed continuity along all nanochamiels with the integrity of the nanochaimels intact following assembly.

The present invention provides a method of making a nanofluidic device, comprising the steps of: providing a thermoplastic substrate having a top surface portion, the top surface portion having at least one nanofluidic feature formed therein; providing a thermoplastic cover having a bottom surface portion, the thermoplastic cover having a glass transition temperature less than that of the substrate; optionally activating one or both of the thermoplastic substrate top surface portion and the thermoplastic cover bottom surface portion; and then thermally bonding the thermoplastic cover top surface portion to the thermoplastic cover bottom surface portion to produce the nanofluidic device.

Thermal fusion bonding has been suggested for the manufacture of microfluidic devices (see, e.g., C.-W. Tsao and D. DeVoe, Bonding of thermoplastic polymer microfluidics, Microfluid Nanofluid 6, 1-16 (2009)), but has neither been suggested nor described for the production of nanofluidic devices.

The present invention is explained in greater detail in the drawings herein and the

specification set forth below.

Brief Description of the Drawings

Figure 1. Process scheme for the fabrication and assembly of the thermoplastic-based nanofluidic devices. (A) Fabrication of the Si master, which consisted of micron-scale access channels and the nanochannels; (B) Fabrication of the protrusive polymer stamp in a UV-curable resin from the Si master; (C) Generation of the fluid ic structures in the thermoplastic substrate from the UV-curable resin stamp by thermal imprinting and bonding of the substrate with the low T _g cover plate to build the enclosed mixed-scale thermoplastic fluidic device.

Figure 2. Scanning electron micrographs (SEM) of the Si master, resin stamp and PMMA substrate for the nanoslits (a, b, c) and nanochannel (d, e, f), respectively. - Inset shows the off-axis (52°) cross section SEM images of the Si masters. The dimensions (/ ^χ w ^χ h) were 21 urn x 1 μηι x 50 nm for each of the 4 nansoslits and 46 um ^χ 120 nm ^χ 120 nm for each of the 7 nanochannels. SEM images of the 40 ^χ 40 nm nanochannels for the Si Master (g) with Al layer and (h) without Al layer (i) on axis view of the Si Master (j) UV-resin stamp and (k) PMMA substrate. (Note that the observed roughness on the SEM image of the stamp and substrate are artifacts from coating with 3 nm AuPd for imaging).

Figure 3. Fluorescence images of the sealed (a) 1 μτη x 50 nm nanoslit, (b) 120 x 120 nm nanochannel (c) 2D nanochannels with sizes between 200 and 35 nm fabricated following the steps shown in Figure 2g - 2k (left to right - 200 nm, lOOnm, 50 nm, 35 nm, 80nm and 45 nm). All sealed devices were seeded with 10 mM FITC in 0.5X TBE buffer and acquired at exposure times of 200 ms (a and b) and 1 s (c). (d) Photograph of the thermally assembled hybrid nanofluidic devices fabricated in PMMA and sealed with COC.

Figure 4. (a) Depiction of multi-structured device showing an entropic trap and nanopiilar separated by a single nanochannel (120 nm x 120 nm). (b) SEM image showing an array of the device with 1.3 μν diameter nanopiilar (see insert), entropic traps ranging from 0 nm to 400 nm diameter with a spacing of 9.18 μηι. (c) and (d) shows the results obtained from the sealing test for the hybrid (PMMA - COC) device and PMMA - PMMA devices, respectively, acquired at 500 ms exposure time. The obscurity of the nanochannel/nanopillar region in the PMMA-PMMA devices is due to possible collapse of the device during assembly, (e) Microscope image of Lambda DNA translocation events through the hybrid device.

Detailed Description of Illustrative Embodiments

The present invention is now described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

In the figures, the thickness of certain lines, layers, components, elements or features may be exaggerated for clarity. Where used, broken lines illustrate optional features or operations unless specified otherwise.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a," "an" and "the" are intended to include plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements components and/or groups or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups or combinations thereof.

As used herein, the term "and/or" includes any and all possible combinations or one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative ("or").

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and claims and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well- known functions or constructions may not be described in detail for brevity and/or clarity. It will be understood that when an element is referred to as being "on," "attached" to,

"connected" to, "coupled" with, "contacting," etc., another element, it can be directly on, attached to, connected to, coupled with and/or contacting the other element or intervening elements can also be present. In contrast, when an element is referred to as being, for example, "directly on," "directly attached" to, "directly connected" to, "directly coupled" with or "directly contacting" another element, there are no intervening elements present. It will also be appreciated by those of skill in the art that references to a structure or feature that is disposed "adjacent" another feature can have portions that overlap or underlie the adjacent feature.

Spatially relative terms, such as "under," "below," "lower," "over," "upper" and the like, may be used herein for ease of description to describe an element's or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is inverted, elements described as "under" or "beneath" other elements or features would then be oriented "over" the other elements or features. Thus the exemplary term "under" can encompass both an orientation of over and under. The device may otherwise be oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms "upwardly," "downwardly," "vertical. ^" "horizontal" and the like are used herein for the purpose of explanation only, unless specifically indicated otherwise.

It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. Rather, these terms are only used to distinguish one element, component, region, layer and/or section, from another element, component, region, layer and/or section. Thus, a first element, component, region, layer or section discussed herein could be termed a second element, component, region, layer or section without departing from the teachings of the present invention. The sequence of operations (or steps) is not limited to the order presented in the claims or figures unless specifically indicated otherwise.

"Metal" as used herein to describe thermoplastic materials from which a substrate or cover may be formed includes any suitable metal, such as sodium, magnesium, calcium, lead, lanthanum, boron, thorium, cerium, antimony, gold, aluminum, cobalt, indium, molybdnenum, nickel, palladium, platinum, tin, titanium, tungsten, zinc, silver, copper and combinations thereof.

"Polymer" as used herein to describe thermoplastic materials from which a substrate or cover may be formed includes any suitable polymer (including copolymers), including but not limited to acrylate polymers such as poly(methyl methacrylate) (PMMA), polyamides such as nylon, polybenzimidazoles, polyethylene, polypropylene, polystyrene, polyvinyl chloride (PVC), polyethylene terephthalate (PET), polytetrafluoroethylene, etc., including blends or combinations thereof such as interpenetrating polymer networks formed therefrom.

"Amorphous inorganic solid" to describe thermoplastic materials from which a substrate or cover may be formed includes any suitable material, including but not limited to fused silica glass, soda-lime-silica glass, sodium borosilicate glass, lead-oxide glass, aluminosilicate glass, oxide glass, silicon oxide, silicone nitride, colloidal glasses, glass- ceramics, etc., including combinations thereof. Such materials may contain dopants or other ingredients. Many such materials are generically referred to as "glass."

"Nanofluidic feature" as used herein include wells, channels (or slits or trenches), pillars, and the like, including combinations and networks thereof, where the average width and/or depth dimensions thereof are typically about 1 or 2 to 100 or 200 nanometers in size. In the present invention the features preferably include channels, formed on the top surface of a substrate in the horizontal plane thereof, which are then enclosed as described herein.

As noted above, the present invention provides a method of making a nanofluidic device, comprising the steps of: providing a thermoplastic substrate having a top surface portion, the top surface portion having at least one nanofluidic feature formed therein; providing a thermoplastic cover having a bottom surface portion, the thermoplastic cover having a glass transition temperature less than that of the substrate; optionally activating one or both of the thermoplastic substrate top surface portion and the thermoplastic cover bottom surface portion; and then thermally bonding the thermoplastic cover top surface portion to the thermoplastic cover bottom surface portion to produce the nanofluidic device.

In some embodiments, the thermoplastic cover has a glass transition temperature at least 1, 2, 5, 10 or 20 degrees less than that of the thermoplastic substrate.

In some embodiments, the thermoplastic cover has a glass transition temperature not more than 30, 40, 60 or 80 degrees less than that of the thermoplastic substrate.

In some embodiments, the thermoplastic substrate comprises an organic polymer, an amorphous inorganic solid, or a metal. In some embodiments, the thermoplastic substrate has a Young's modulus not greater than 200, 100, 50, or 20.

In some embodiments, the thermoplastic cover comprises an organic polymer, an amorphous inorganic solid, or a metal.

In some embodiments, the thermoplastic cover has a Young's modulus not greater than 200, 100, 50, or 20.

In some embodiments, the substrate comprises a first poly(methyl methacrylate) and the cover comprises a second poly(methyl methacrylate) different from the first poly (methyl methacrylate).

In some embodiments, the thermally bonding step is carried out at an elevated temperature and elevated pressure, with the elevated temperature not greater than the glass transition temperature of the thermoplastic cover.

In some embodiments, the activating step is carried out by plasma etching.

The present invention is explained in greater detail in the following non-limiting Examples.

EXPERIMENTAL

ID and 2D nanofluidic networks with dimensions ranging between about 40 and 200 nanometers were fabricated in PMMA substrate and sealed with a lower Tg COC cover plate. The substrate and cover plate were pretreated under controlled oxygen plasma conditions and assembled at a temperature ~5°C lower than that the Tg of the cover plate. Bond strengths approaching those of the native polymers assembled at temperatures above their glass transition temperatures are demonstrated with the integrity of the fluidic channels being retained. Sealed devices were seeded with 10 mM FITC and visualized under a fluorescence microscope to check for uniformity and continuity along the length of the nanochannels. Functionality of our bonding scheme was tested by monitoring the translocation of lambda DNA through an assembled device containing multiple nanostructures. Overall, this simple, low cost and high throughput device assembly scheme was found to enable strong bonds between thermoplastic nanofluidic substrates and their cover plates and useful in the designing of functional thermoplastic based sub-50 nm 2D nanofluidic devices. METHODS

Materials and Reagents. PMMA sheets (Tg ~ 104 °C) and cover plates used for the device fabrication were purchased from Good Fellow (Berwyn, PA). Cycloolefin copolymer sheets, COC 6017 (Tg ~ 170°C) used as the backbone for fabricating the nanoimprinting stamp and COC 8007 (Tg ~ 75°C) used as the cover plate, were purchased from TOPAS Advanced Polymers (Florence KY). Si <100> wafers were purchased from University wafers (Boston, MA). Anti-adhesion monolayer of (Tridecafluoro - 1,1,2,2 - Tetrahydrooctyl) Tricholorosilane (T-Silane) was purchased from Gelest, Inc. Fluorescein Isothiocyanate (FITC) salt and 10X Tris Borate EDTA (TBE) buffer were purchased from Sigma- Aldrich (Saint Louis, MO). All required dilutions were performed using 18 ΜΩ/cm milliQ water (Millipore technologies) and all measurements were performed at 25 °C unless specified otherwise.

Fabrication of Nanofluidic Devices. Previously, we have reported the development of nanoslits and nanochannels in polymer substrates following a single step fabrication scheme that is based on nanoimprint lithography (NIL). ¹ ' ² The schematic showing the steps involved in the development of the nanofluidic device is shown in Figure 1. Device fabrication involves four key steps; (i) Fabrication of Silicon master with nanochannels using FIB lithography, (ii) Fabrication of the UV -resin stamp with COC backbone, (iii) Thermal imprinting into thermoplastic substrate, and (iv) Sealing of the fluidic channels with low Tg cover plate.

The silicon master was developed by initially patterning two V-shaped access microfluidic channels, 55 urn wide, 12 um deep 1.5 cm long in Si < 1 ()()> wafer using standard photolithography followed by anisotropic etching with 50% KOH solution. Next, nanofluidic channels were patterned across the microchannels by FIB milling using a Helios NanoLab 600 Dual Beam instrument (FEI Company). For the fabrication of the smaller 2D nanochannels (<50 nm), 100 nm Al layer was deposited on the Si wafer prior to FIB milling following the method described by Menard et al. ³ After fabrication, the Al was stripped-off using Al etching solution. In all cases, the spot size (beam current), sputtering rate and dwell time were carefully controlled to ensure that the desired channel dimensions were designed.

Following this, an anti-adhesion monolayer of T-Silane was coated on the Si master from gas phase in a desiccator under vacuum for 2 h to facilitate the demolding process. The structures on the Si master were then carefully transferred into a UV-curable resin polymeric blend, containing 68 wt% TPGA as the base, 28 wt% TMPA as the crosslinking agent and 4 wt% Irgacure 651 as photo- initiator) coated onto a cyloolefm copolymer (COC) base plate, via UV-NIL to produce polymer stamps with protrusive structures. To achieve this, the Si master (mold) was initially coated with the UV resin by dispensing with a pipette, followed by gentle pressing of the COC base plate on the resin- coated master to ensure complete filling of the resin into mold cavities. This was followed by exposure to a 365nm UV light (10 J/m ² ) through the COC backbone for 5 min in a CL-100 Ultraviolet Crosslinker. After curing, the UV-curable resin was gently demolded from the Si mold to get the negative copy on UV-curable resin.

Next, the patterned UV-curable resin was used as the stamp to hot emboss into a 3 mm-thick PMMA sheet (Lucite CP) (2 cm x 2 cm) with access holes for reservoirs, drilled prior to embossing. The imprinting was performed at a pressure of 1910 kN/m ² for 120 s with the top and bottom plates maintained at a temperature of 125°C using the Hex03 hot-embosser (JenOptik). Pressure was applied after 30 s preheating of the stamp and the substrate at the desired molding temperature, and was maintained during the imprinting process until the system was cooled down to 45°C. Upon cooling, PMMA copy was easily demolded from the UV-resin stamp. A 120 μιη thick COC 8007 sheet was used as the cover plate. Both the patterned PMMA sheet and cover plate were pre-activated with oxygen plasma at 50 W for 35 s and 7 seem gas flow rate. Device assembly was performed immediately at 70°C for 900 s under a 680 kN/m ² pressure. This approach not only aids in achieving a low temperature device assembly with high bond strength but also contributes to the effective functionalization of the nanochannel surface with carboxyl (hydrophilic) functional groups, which may be necessary in subsequent experiments.

RESULTS AND DISCUSSIONS

The Silicon (Si) master, which consisted of micron-scale access channels (fabricated using photolithography and wet chemical etching) and an array of connecting nanoslits or nanochannels (fabricated using Focused Ion Beam (FIB) milling), was used to fabricate the protrusive polymer stamp, which was made from a UV-curable resin. Thermal imprinting was used to transfer the nanofluidic structures into the PMMA substrate from the UV-curable resin stamp and the device was sealed with a COC 8007 cover plate using low-temperature plasma assisted bonding to build the enclosed mixed-scale polymer device. UV-NIL conditions were carefully controlled to ensure that patterns from the Si master were transferred with high fidelity and minimum deformations into the resin stamp.

We fabricated and assembled nanofluidic devices with nanoslits (1 μπι wide and 50 nm deep) and 2D nanochannels (500 nm to 40 nm) in a PMMA substrate, the 40 nm 2D nanochannels being unprecedented in the literature. Figure 2 shows scanning electron microscope (SEM) images of the original Si master, cured UV-resin stamp and thermally imprinted PMMA substrate with an array of four nanoslits (Figures 2a - 2c) and seven nanochannels (Figures 2d - 2f). The original channel fabricated by FIB milling into the Si master was purposely designed with dimensions (width ^χ depth) of 995 nm ^χ 50 nm and 1 18 nm ^χ 122 nm for the nanoslit and nanochannel, respectively, to account for slight deformation of the UV resin stamp during the high pressure and temperature thermal imprinting. ²

The final PMMA devices had the dimensions of 1 um ^χ 50 nm and 120 nm ^χ 120 nm, with the same polarity as the structures in the Si master. Figures 2g and h shows the off-axis (52°) SEM images of the Si master with the 40 ^χ 40 nm nanochannel before and after the removal of 80 nm Al layer. On axis view of the produced Si master, UV-resin stamp and PMMA substrate are shown in Figures i - k. Compared to Si as stamp material, the UV resin possessed a low Young's modulus value of 600-800 MPa4 and a thermal expansion coefficient similar to that of the PMMA substrate (6 ^χ 10-5/°C), which leads to reduced adhesion and thermal stress during thermal NIL production of the nanofluidic device.5,6 Moreover, the UV resin stamp can be easily and repeatedly fabricated through replication from the Si master using UV-NIL. A single UV resin stamp was used for thermal imprinting for up to 10 times without any noticeable damage to the structures.

The PMMA substrates were sealed to the low Tg COC cover plate using the low temperature thermal fusion bonding scheme described above. The formation of leak-free fluidic devices or discontinuities due to channel collapse during assembly was evaluated by introducing 10 mM fluorescein isothiocyanate (FITC) solution in 0.5X TBE buffer into the fluidic network and allowing the channels to be filled by capillarity. As shown in Figures 3b - 3c, the fluidic channels did not show any leakage between the substrate and the cover plate.

To assess the functionality of the devices assembled with our bonding scheme, we fabricated a multi-nanostructured device (Figure 4a) in PMMA and bonded it to a lower Tg COC cover plate. The sealing test, shown in Figure 4b, revealed uniformity in the structures and the absence of collapse or deformation in the structures. For comparison, we assembled a device with a PMMA cover plate possessing the same Tg as the substrate. From the sealing test (Figure 4d) there were regions of possible collapse around the nanochannel and the entropic trap as observed from the non- uniformity in the filling. DNA translocation through the hybrid devices (Figure 4e) confirmed that thermoplastic devices assembled following our scheme will be useful in several bioanalytical applications. CONCLUSION

In this work, we developed a low temperature hybrid bonding scheme useful for the assembly of thermoplastic-based nanoslits and nanochannels. With this scheme, we have addressed the most notable challenge associated with the use of low Tg substrate materials for the fabrication of nanofluidic devices - the relatively small Young's modulus associated with these materials, which makes cover plate assembly to the patterned substrate difficult due to cover plate collapse and/or nanostructure deformation using either thermal or chemical bonding to enclose the fluidic network. The imprinting with polymer stamps showed good replication fidelity for multiple replication processes, preventing the damage of the nanopatterned master and reduced undesirable deformation in the molded polymer substrate.

We have generated current-voltage plots acquired after filling the nanofluidic devices with 1 mM KCl. From the data obtained, the measured currents for voltages of opposite polarity had similar absolute values and showed good linearity (non-rectification). The absence of voltage gating and rectification indicated homogeneity in surface charge along the walls of the nanoslits and nanochannels when using symmetrical electrolyte conditions. Using low thermal bonding temperatures (~70°C) also significantly minimized the amount of surface reorganization of the polar - - functional groups for the functionalized devices. We are currently investigating the utility of these devices for the studies of translocation and elongation behavior of double-stranded DNAs and assessing the magnitude of heterogeneity in the EOF profile especially for applications related to nano-chromatography where solutes are transported and separated based on their solute/wall interactions and differences in their electrophoretic mobility. We are also currently evaluating the stability of the devices sealed after plasma treatment and how this may affect the operation of the bonded nanofluidic devices. In summary, this assembly scheme will not only aid in building cheap and disposable polymer nanofluidic devices for single molecule analysis, but also help in assessing the performance metrics of sub-50 nm channels for comparison with their glass-based counterparts in bioanalytical applications.

REFERENCES

1. Chantiwas, R.; Hupert, M. L.; Pullagurla, S. R.: Balamurugan. S.; Tamarit-Lopez, J.; Park, S.; Datta, P.; Goettert, J.; Cho, Y.-K.; Soper, S. A. Simple replication methods for producing nanoslits in thermoplastics and the transport dynamics of double-stranded DNA through these slits. Lab on a Chip 2010, 10, 3255-3264.

2. Wu, J.; Chantiwas, R.; Amirsadeghi, A.; Soper, S. A.; Park, S. Complete plastic nanofluidic devices for DNA analysis via direct imprinting with polymer stamps. Lab on a Chip 2011, 11, 2984-2989.

3. Menard, L. D.; Ramsey, J. M. Fabrication of Sub-5 nm Nanochannels in Insulating Substrates Using Focused Ion Beam Milling. Nano Letters 201 1 , 11, 512-517.

4. Amirsadeghi, A.; Lee, J. J.; Park, S. Surface adhesion and demolding force dependence on resist composition in ultraviolet nanoimprint lithography. Applied Surface Science 2011, 258, 1272- 1278.

5. Chan-Park, M. B.; Yan. Y.: Neo, W. K.; Zhou. W.; Zhang, J.; Yue, C. Y. Fabrication of High Aspect Ratio Poly( ethylene glycol)-Containing icrostructures by UV Embossing. Langmuir

2003, 19, 4371-4380.

6. Becker, H.; Gartner, C. Polymer microfabrication technologies for microfluidic systems. Anal Bioanal Chem 2008, 390, 89-1 1 1. The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.