Abstract:
A semipermeable ultrathin polymer membrane is a microfluidic device that comprises a substantially optically transparent polymer film having a surface area to thickness ratio of at least 1,000,000:1, and an array of precisely spatially ordered pores of a user-selected diameter defined therethrough. Such membranes can be fabricated by providing a mold having a patterned array of nanoholes femtosecond laser ablated in a surface thereof; applying a first polymer solution onto the mold surface so that the first polymer solution infiltrates the nanoholes; allowing the first polymer solution to dry and form a replica of the mold having a plurality of freestanding nanoneedles extending from a surface of the replica; removing the replica from the mold; coating the replica surface with a second polymer solution; drying the second polymer solution to form a porous polymer film; and dissolving the replica in a solvent to release the film from the replica as a semipermeable ultrathin polymer membrane. Also disclosed are multi-chambered microfluidic devices for studying cell biology in vitro that incorporate one or more such semipermeable ultrathin polymer membranes.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/331,077, entitled “Preparation of an ultrathin perfusion membrane with micron diameter pores from femtosecond laser machined template transfer process,” filed May 3, 2016, the entire disclosure of which is hereby incorporated herein by this reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OF DEVELOPMENT 
       [0002]    Subject matter described herein was supported in part by the Tennessee Higher Education Commission through a grant to the Center for Laser Applications, University of Tennessee Space Institute, and in part by the National Center for Advancing Translational Sciences of the NIH under Award No. UH2TR000491. The government may have certain rights in the subject matter disclosed herein. 
     
    
     REFERENCE TO SEQUENCE LISTING OR COMPUTER PROGRAM LISTING APPENDIX 
       [0003]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0004]    The present disclosure relates generally to semipermeable membranes, in particular very thin membranes with nonrandom pores, and more particularly to tunable polymer nanofilm perfusion membranes, which are believed to be advantageous for use in microfluidic devices for cell biology studies. 
         [0005]    The basal lamina or basement membrane is a key physiological system that supports diverse epithelial cell types and participates in physicochemical signaling and transport between tissue types [4]. Its formation and function are essential in tissue maintenance, growth, angiogenesis, disease progression, and immunology. 
         [0006]    In vitro models of the basement membrane, such as Boyden and transwell chambers, are essential to cell biology studies and are common in lab-on-a-chip devices where cells require apical and basolateral polarization. Extravasation, intravasation, membrane transport of chemokines, cytokines, chemotaxis of cells, and other key functions are routinely studied using these models. For example, transwells have been widely adopted for perfused, polarized cells, and migration assays since Boyden&#39;s initial chemotaxis experiment [1]. 
         [0007]    The promise of organs-on-a-chip technology (i.e., 3-D microfluidic cell culture chips that simulate the activities, mechanics, and physiological response of entire organs and/or organ systems) is essentially to create a more robust in vitro model of the complex electrophysicochemical systems that control cell function and fate in multicellular organisms [2]. However, a key element of any such model, including both static transwells and complex microfluidic organ models, is the semipermeable membrane used to simulate the basement membrane [3]. 
         [0008]    Critical properties of a semipermeable membrane suitable for use in a microfluidic device include controlled porosity, high species flux, mechanical strength, surface biocompatibility, and optical transparency. Although semi-permeable membranes formed from polycarbonate (PC) and polyester (PE) have traditionally been used for transwell cell culture and diffusion experiments, such membranes are far thicker than the basement membrane. Transwell membranes are typically more than 100 microns thick and the basement membrane is much less than one micron thick, typically tens of nanometers, three to four orders of magnitude thinner than currently available polycarbonate and polyester membranes, the transparency of which is not optimal in bright field light and confounds observation by differential interference contrast. 
         [0009]    By contrast, ultrathin polymer films having a thickness of less than one micron (referred to herein as “nanofilms”) are promising candidates for use as an in vitro model of the basement membrane because they are closer in thickness to native basement membrane and are compatible with cell culture. Nanofilms with thicknesses ranging from tens to hundreds of nanometers belong to a class of polymeric nanomaterials having huge surface to thickness ratios (&gt;10 6 ), unique thickness-dependent interfacial and mechanical properties, and optical transparency. Previous studies have revealed that such films can have non-covalent high adhesiveness to surfaces, tunable flexibility and molecular permeability [5], defined structural color [6] and mechanical strength [7], and possibly conductive [8] and magnetic [9] properties, all of which enable such nanofilms to closely mimic the  lamina basalis  of the extracellular matrix in human tissues, making such films an ideal structure to direct cellular organization and regulate organ regeneration and function in vitro. 
         [0010]    Nanofilm fabrication techniques are diverse, and include layer-by-layer assembly [10] methods, and the Langmuir-Blodgett method [11], among others. However, the simplest route to fabricate freestanding and easy-to-handle nanofilms is spin coating from a liquid polymer-solvent solution [12]. In previous works, full characterization of plain nanofilm structures was performed. The adhesion and proliferation of different cell types on poly (lactic acid) (“PLLA”) nanofilms [13] and polyelectrolytic films [14] was confirmed. 
         [0011]    Existing examples of organs-on-a-chip have used perforated polydimethylsiloxane (“PDMS”) membranes [15, 16], polycarbonate [17] or photoresist membranes [18]. While the PDMS membranes have the advantages of optical transparency, tunable elastic modulus, and can be easily integrated in a monolithic PDMS device, it is still difficult to reduce the thickness of the membrane below 5 μm. This represents a major drawback of PDMS membranes because submicron membrane thickness is essential for physical contact and paracrine communication between cells growing on both sides. 
         [0012]    Numerous approaches have been used to fabricate organic semipermeable membranes [19], such as polymer synthesis [20] and ion track etching of polymers [21]. However, these methods have numerous drawbacks mainly related to random arrangement of pores with wide size distribution or nanometric pore size. Other methods are based on direct micro- and nanofabrication on the plain polymer sheet, by e-beam lithography and by focused ion beam milling [22, 23]. These approaches have the advantage of control of the design and arrangement of the pores [24], but they also present drawbacks, such as long working time and seriality, and thus high cost. 
         [0013]    Moreover, while direct etching of any polymeric film should be theoretically possible for example by tuning current and voltage of an ion or electron beam [24], it is practically impossible to drill holes with micrometric diameter through an approximately 100 nm thin film because the Gaussian distribution of the ion beam and the control of current and milling depth are not sufficient to avoid local polymer melting. Debris from the milling process limits the depth to 10× the diameter of the hole. Furthermore, direct milling of the film would implicate local changes in the film&#39;s mechanical and structural characteristics and thus increase the risk of breakage and tears in the polymer film. 
         [0014]    Accordingly, what is needed are improvements in semipermeable membranes for use in microfluidic devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    The presently disclosed subject matter overcomes some or all of the above-identified deficiencies of the prior art, as will become evident to those of ordinary skill in the art after a study of the information provided in this document. 
         [0016]    Disclosed herein are engineered semipermeable ultrathin polymer membranes, methods for the fabrication of same, and microfluidic devices comprising such membranes. The semipermeable membranes disclosed herein are advantageously transparent and include precisely patterned micrometric pores, enabling them to function as support for growth of cell layers as well as a separation between distinct microfluidic chambers. As such, they are expected to be of significant utility in cellular assays, chemotaxis studies, and organs-on-a-chip applications. 
         [0017]    The semipermeable membranes disclosed herein can be formed by spin coating-assisted deposition of a polymer-solvent solution such as PLLA atop a sacrificial array of spatially ordered polyvinyl-alcohol (“PVA”) nanoneedles, followed by removal of the sacrificial PVA nanoneedle array. The PVA nanoneedles are formed by PVA replication of an array of surface nanometric holes formed on a fused silica wafer using femtosecond laser machining. 
         [0018]    Accordingly, in one aspect, the disclosure provides a method of fabricating a semipermeable ultrathin polymer membrane, comprising providing a mold having a patterned array of nanoholes femtosecond laser ablated in a surface thereof; applying a first polymer solution onto the mold surface so that the first polymer solution infiltrates the nanoholes; allowing the first polymer solution to dry and form a negative replica of the mold, the replica having a first surface, a second surface opposite the first surface, and a plurality of freestanding nanoneedles extending a nanoneedle height from the first surface; removing the replica from the mold; coating the first replica surface with a second polymer solution whereby the first replica surface is covered with a layer of the second polymer solution having a thickness that is less than the nanoneedle height such that the nanoneedles extend through said layer above a surface of the layer of second polymer solution; drying the second polymer solution to form a porous polymer film; and dissolving the replica in a solvent to release the film from the replica as a semipermeable ultrathin polymer membrane. 
         [0019]    In another aspect, the disclosure provides a semipermeable ultrathin polymer membrane comprising an optically transparent polymer film having a surface area to thickness ratio of at least 1,000,000:1, and an array of spatially ordered pores having a preselected micrometric diameter defined therethrough. 
         [0020]    In yet another aspect, the disclosure provides a microfluidic device for studying cell biology in vitro, comprising a first microfluidic layer having an upper surface, a lower surface, an apical chamber defined in the lower surface, and an aperture in fluid communication with the apical chamber extending through the upper surface; a second microfluidic layer having an upper surface, a lower surface, and at least one basal chamber defined in the upper surface; and an optically transparent, biocompatible polymer nanofilm having an upper surface, a lower surface, a surface area to thickness ratio of at least 1,000,000:1, and an ordered array of micrometric pores defined therethrough, the film contacting the lower surface of the first microfluidic layer over the apical chamber and the upper surface of the second microfluidic chamber over the basal chamber such that the apical chamber is in fluid communication with the basal chamber through the pores of the film. 
         [0021]    Numerous other objects, advantages and features of the present disclosure will be readily apparent to those of skill in the art upon a review of the following drawings and description of a preferred embodiment. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0022]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color draying(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0023]      FIG. 1  depicts an embodiment of a method of fabricating a semipermeable ultrathin membrane from PLLA. 
           [0024]      FIG. 2A  is an SEM image of a representative femtosecond laser machined nanohole on the surface of a fused silica wafer having a plurality of such nanohole. The nanohole was opened using a single 3.2 μJ, 790 ηm, 160 femtosecond laser pulse. 
           [0025]      FIG. 2B  is an SEM image of a PVA replica of the surface of the fused silica wafer of  FIG. 2A  (Pt coated, 45° stage tilt). Replicas of individual nanoholes are referred to herein as “nanoneedles.” 
           [0026]      FIG. 2C  is an SEM image of a single PVA nanoneedle of  FIG. 2B  having a final length of 10 μm. 
           [0027]      FIG. 2D  is an SEM image of the PVA nanoneedle of  FIG. 2C  at 30° stage tilt. 
           [0028]      FIG. 3A  is an optical photograph of a PLLA film floating on the surface of water after the PVA template has been dissolved using the method of  FIG. 1 . Film edges are indicated by white arrows. 
           [0029]      FIG. 3B  is an optical microscope image of the PLLA film of  FIG. 3A . Pores in the film are shown as white dots. 
           [0030]      FIG. 3C  is an SEM image of the array of pores in the PLLA film of  FIG. 3A . A gold coating of 2 nm was used to visualize the pores. 
           [0031]      FIG. 3D  is an SEM image showing a detail view of a single pore in the film of  FIG. 3A . 
           [0032]      FIG. 4A  is a perspective view of the PLLA film of  FIG. 3A  adhered to the channeled surface of a PDMS layer. Microfluidic channels in the surface of the PDMS layer are 200 μm-wide. 
           [0033]      FIG. 4B  is a top plan view of the PLLA film and channeled PDMS layer of  FIG. 4A . The lateral sample identification label is indicated by a white arrow. 
           [0034]      FIG. 4C  is a magnified view of the PLLA film and PDMS layer of  FIG. 4B  showing a group of four micrometric pores (indicated by white arrows) in the PLLA film positioned above the 200 μm-wide microfluidic channels of the PDMS layer. 
           [0035]      FIG. 4D  is another top plan view of the PLLA film and channeled PDMS layer of  FIG. 4C  with alignment features indicated by white arrows. 
           [0036]      FIG. 5A  depicts an embodiment of an assembled multi-chamber microfluidic device typical of perfused organ-on-a-chip devices incorporating a semipermeable ultrathin polymer membrane constructed in accordance with the present disclosure. The upper chamber volume is indicated by blue dye. The lower channeled chamber volume is indicated by red dye. 
           [0037]      FIG. 5B  is an optical differential interference contrast image of HUVECs loaded into the microfluidic device of  FIG. 5A  at the time of loading (DO, 4x, ph). 
           [0038]      FIG. 5C  is an optical fluorescence image of the HUVECs of  FIG. 5B  stained to differentiate live and dead cells. After seven days the live cells were stained blue and dead cells stained green. Scale bar is 400 μm. 
           [0039]      FIG. 5D  is an optical fluorescence image of the HUVECs of  FIG. 5B  subject to actin staining. Actin of HUVECs inside the microfluidic device is stained green. Scale bar is 400 μm. The inset is at 40× magnification. 
           [0040]      FIG. 6  is a bar graph showing the relative permeability to FITC-dextran of a perforated semipermeable ultrathin PLLA membrane constructed in accordance with the present disclosure (left bar) and a non-perforated PLLA control membrane (right bar). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The details of one or more embodiments of the presently disclosed subject matter are set forth in this document. Modifications to embodiments described in this document, and other embodiments, will be evident to those of ordinary skill in the art after a study of the information provided herein. The information provided in this document, and particularly the specific details of the described exemplary embodiments, is provided primarily for clearness of  9  understanding and no unnecessary limitations are to be understood therefrom. In case of conflict, the specification of this document, including definitions, will control. 
         [0042]    While the terms used herein are believed to be well understood by one of ordinary skill in the art, definitions are set forth herein to facilitate explanation of the subject matter disclosed herein. 
         [0043]    Unless define otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter disclosed herein belongs. Although any methods, devices, and materials similar or equivalent to those described herein can be used in the practice or testing of the presently disclosed subject matter, representative methods, devices and materials are now described. 
         [0044]    The terms “a”, “an”, and “the” refer to “one or more” when used in this application, including the claims. Thus, for example, reference to “a contaminant” includes a plurality of particles of the contaminant, and so forth. The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims and/or the specification may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one,” and “one or more than one.” 
         [0045]    All references to singular characteristics or limitations of the present disclosure shall include the corresponding plural characteristic(s) or limitation(s) and vice versa, unless otherwise specified or clearly implied to the contrary by the context in which the reference is made. 
         [0046]    All combinations of method or process steps as used herein can be performed in any order, unless otherwise specified or clearly implied to the contrary by the context in which the referenced combination is made. 
         [0047]    The methods and devices of the present disclosure, including components thereof, can comprise, consist of, or consist essentially of the essential elements and limitations of the embodiments described herein, as well as any additional or optional components or limitations described herein or otherwise useful. 
         [0048]    Unless otherwise indicated, all numbers expressing physical dimensions, quantities of ingredients, properties such as reaction conditions, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently disclosed subject matter. 
         [0049]    As used herein, the term “about,” when referring to a value or to an amount of mass, weight, time, volume, concentration, percentage or a physical dimension such as length, width, or diameter, is meant to encompass variations of in some embodiments ±20%, in some embodiments ±10%, in some embodiments ±5%, in some embodiments ±1%, in some embodiments ±0.5%, and in some embodiments ±0.1% from the specified value or amount, as such variations are appropriate to perform the disclosed methods. 
         [0050]    As used herein, ranges can be expressed as from “about” one particular value, and/or to “about” another particular value. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed. 
         [0051]    For purposes of clarity, the term “nanohole” is used herein to refer to a minute opening or aperture (commonly referred to as a hole or pore) that has been laser ablated into the surface of a substrate. In some embodiment, a nanohole has a diameter less than 1 μm. By contrast, the term “pore” is used herein to refer to a minute opening or aperture extending through a semipermeable membrane formed in accordance with the present disclosure. 
         [0052]    As used herein, the term “semipermeable membrane” refers to a nanofilm with an array of ordered pores of a specific user-defined size, distribution, and shape; precisely patterned and extending completely through the nanofilm to allow passage of chemical species and cells of a specific preselected size range, while blocking migration of other chemical species and cells outside the specific preselected size range. The term “array” as used herein refers to a plurality of structures (such as nanoholes, pores, and nanoneedles) having user-defined physical dimensions and spatial parameters. 
         [0053]    User-tunable physical characteristics of the semipermeable membranes disclosed herein include pore spacing, pore diameter, number of pores per unit of surface area, membrane thickness, and membrane constituent material. For example, semipermeable membranes disclosed herein can comprise millions of pores per square centimeter of surface area, with pore diameter and spacing, composition configured to allow passage of chemical species and cells of a specific preselected size range, while blocking migration of other chemical species and cells outside the specific preselected size range. In some embodiments, one or more of pore surface area density, diameter, spacing, and shape are selected to perform a perfusion function. In certain embodiments, the semipermeable membranes can be optimized or “tuned” to perform a specific perfusion function or target a preselected chemical species or cells or protein size. 
         [0054]    The semipermeable membranes disclosed herein can include pores spaced along an X-axis and a Y-axis at the same or different intervals along either axis. In some embodiments, the pores can be spaced from about 1 μm to about 100 μm apart on one or both of the X-axis and the Y-axis. In some embodiments, the pores can be spaced from about 5 μm to about 10 μm apart on one or both of the X-axis and the Y-axis. In still some embodiments, the pores can be spaced about 2 μm apart on one or both of the X-axis and the Y-axis. In some embodiments advantageous to the study of cell migration or chemotaxis, the pores may be spaced as far as about 100 μm apart. However, using the methods disclosed herein, it is possible to fabricate semipermeable membranes having pores spaced as closely as about 1 μm apart. 
         [0055]    In some embodiments, a semipermeable membrane disclosed herein can include pores having an average diameter of from about 100 nm to about 3 μm. In certain embodiments, a semipermeable membrane disclosed herein can include pores having an average diameter of from about 300 nm to about 2 μm. 
         [0056]    Replicas of individual nanoholes are referred to herein as “nanoneedles.” As used herein, the term “nanoneedle” refers to a nanostructure having a diameter of less than 1000 nanometers for more than half the length of the structure. In some embodiments, the nanoneedles disclosed herein can comprise a tapered base portion and a relatively longer fiber portion which extends from the base portion to a terminal tip. In such embodiments, the fiber portion has a diameter of less than 1000 nm and a length greater than that of the base portion, and the base portion can have a diameter of from about 4.0 μm to less than 1.0 μm. Additionally, in some embodiments, the base portion can also have a length of from about 1.0 μm to about 10 μm, and the fiber portion can have a length of from about 10 to 100 times greater than the length of the base portion. 
         [0057]    Semipermeable membranes disclosed herein can be composed of virtually any biocompatible polymer that is not water soluble. Non-limiting examples of suitable polymers include poly(ε-caprolactone) (PCL), polyethylene oxide (PEO), polyvinyl chloride (PVC), polyvinyl formal (PVF), polyisoprene, trans (PI), polypropylene (PP), low-density polyethylene (LDPE), high-density polyethylene (HDPE), polyvinylidene fluoride (PVDF), poly-lactic acid (PLA), and poly-L-lactic acid (PLLA). It should be understood that a blend of two or more such polymers can also be used. 
         [0058]    The presently disclosed subject matter is further illustrated by the following specific but non-limiting example. The following example may include compilations of data that are representative of data gathered at various times during the course of development and experimentation related to the present invention. 
       EXAMPLE 1 
       [0059]    The goal of the present study was to integrate a semipermeable ultrathin polymer membrane with precisely positioned pores of 2 μm diameter in a microfluidic device with apical and basolateral chambers. We selected poly (L-lactic acid) (“PLLA”), a transparent biocompatible polymer, to prepare the semipermeable ultrathin membranes. The pores were generated by pattern transfer using a three-step method coupling femtosecond laser machining, polymer replication, and spin coating. Each step of the fabrication process was characterized by scanning electron microscopy to investigate reliability of the process and fidelity of pattern transfer. 
         [0060]    In order to evaluate the compatibility of the fabrication method with organs-on-a-chip technology, porous PLLA membranes were embedded in polydimethylsiloxane (“PDMS”) microfluidic devices and used to grow human umbilical vein endothelial cells (“HUVECS”) on top of the membrane with perfusion through the basolateral chamber. Viability of cells, optical transparency of membranes, and strong adhesion of PLLA to PDMS were observed, thus confirming the suitability of the prepared membranes for use in organs-on-a-chip. 
       Materials and Methods 
     Fabrication of Fused Silica Molds 
       [0061]    A 10 mm×10 mm regular array of 201×201 nanoholes was patterned on the surface of a 500 μm thick UV grade fused silica wafer (Mark Optics, Inc.) using the single-pulse femtosecond laser machining technique first described by White, et al. [25]. In their 2008 paper, White and co-workers demonstrated that high numerical aperture single-pulse femtosecond laser machining can create uniquely-shaped nanoholes at the surface of fused silica, exhibiting high aspect ratios, with depths that exceed 10 μm and diameters below 200 nm. In the present work, femtosecond laser machining was carried out using the system described by Rajput [26]. A dry microscope objective, namely a Nikon CF Plan Achromat 79173, was used to focus the femtosecond laser beam on the surface of the fused silica wafer. Energy per pulse of 3.2 μJ was used to open each nanohole. The laser-patterned wafer was soaked first in a 5 M KOH solution at 80° C. for 2 hours, and then in de-ionized water also at 80° C. for another 2 hours, in order to remove any machining debris. The washed wafer was dried under a stream of nitrogen, and used as a mold to prepare PLLA membranes for characterization purposes. 
         [0062]    A second fused silica mold, designed to prepare PLLA membranes for use in organs-on-a-chip trials was femtosecond laser machined in a similar way. This mold was patterned with a 16 row×100 column rectangular array of surface nanoholes. The distance between rows was 400 μm, while the distance between columns was 100 μm. 
       Preparation of PVA Nanoneedle Arrays 
       [0063]    Recently, Rajput and co-workers reported on a novel method of preparing arrays of freestanding polymer nanoneedles from femtosecond laser patterned fused silica molds using the solution casting mold-replication technique [26]. In the present work, arrays of freestanding polyvinyl alcohol (PVA) nanoneedles (or replicas) were prepared using the water/alcohol-based PVA mold release agent Partall® Film #10 (by Rexco). In particular, each fused silica mold containing an array of surface nanoholes was coated with a thin layer of Partall® Film #10 using a foam paintbrush. The layer was exposed to a flow of nitrogen and allowed to dry. The resulting ˜25 μm thick PVA film was peeled-off the fused silica mold and mounted on a 22×22 mm 2  glass or polyvinyl chloride (PVC) coverslip using double-sided adhesive tape. 
       Preparation of PLLA Membranes 
       [0064]    As shown in  FIG. 1 , spin coating-assisted deposition of PLLA (molecular weight 100 kDa; 10 mg/mL solution in dichloromethane; Polysciences, Inc.) was performed (at 3000 rpm for 30 s) on PVA nanoneedle arrays, inside a Class 100 clean room. The resulting PLLA films were allowed to dry for 1 minute at 80° C. and stored in polystyrene Petri dishes. 
       Assembly of Microfluidic Devices 
       [0065]    A simple two-compartment device was assembled to test the semipermeable PLLA membranes in an organs-on-a-chip assembly. The device consists of two microfabricated layers in polydimethylsiloxane (PDMS, Sylgard® 184 silicone elastomer kit from Dow Corning, Mich., USA), obtained by SU8 softlithography [27]. Briefly, the bottom layer consists of 16 parallel channels (200 μm width, 100 mm height, 10 mm length), obtained by casting and curing liquid PDMS (10:1) on SU8-2100 mold (from Microchem, Mass., USA) fabricated in a Class 100 clean room. The upper chamber was prepared by punching a thick 3 mm layer of PDMS with a sterile disposable 6 mm diameter punch. Ports to access the microfluidic channels were opened by punching 1.59 mm ( 1/16 inch) diameter holes. 
         [0066]    In order to bond a PLLA membrane between the two PDMS layers, the channeled PDMS layer was gently pressed against the PLLA layer formed atop a PVA replica as shown in  FIG. 1 . These two pieces were then immersed in de-ionized water for 6 to 12 hours to allow water to gradually dissolve and remove the sacrificial PVA template, leaving behind a semipermeable ultrathin PLLA membrane with precisely patterned micrometer scale pores. The resulting PLLA membrane remained adherent to the channeled PDMS layer. The upper PDMS layer chamber and the PLLA membrane were then oxygen-plasma treated (600 mTorr, 100 W, for 45 sec) and finally bonded together. Oxygen-plasma treatment renders the exposed surfaces hydrophilic. Once bonded the devices were immediately filled with deionized sterile water and stored at 4° C. until used. 
       Loading of Microfluidic Devices 
       [0067]    The devices were sterilized by UV irradiation for  30  minutes prior to loading with cell medium and cells. Mimicking seeding protocol used for cell culture in traditional transwell inserts, the water was replaced with full medium, first loading the lower microfluidic channels and then the upper chamber. The nanofilm was not coated with any adhesive protein. The wet device was thus closed in a Petri dish and equilibrated at 37° C. for 12 hours inside the incubator. Cell growth on the PLLA nanofilm was monitored for 7 days as proof of principle. 
       Cell Culture, Staining and Assays 
       [0068]    Human umbilical vein endothelial cells (“HUVECS”) [28] were isolated from umbilical cord obtained from a de-identified placenta collected from patients who underwent elective cesarean sections between 37 and 39 weeks of gestation. All procedures related to the consent and collection of tissues were approved by the Vanderbilt University Institutional Review Board. Cells were cultured in EBM-2 media supplemented with 10% of fetal bovine serum (Lonza, USA) and with 1% antibiotics/antimycotics. Purity of the isolation was validated by morphological and immunofluorescent staining for CD31 (DAKO, USA) before loading in the devices. Cells were maintained at 37° C. in a saturated humidity atmosphere containing 95% air/5% CO 2 , and they were sub-cultured before reaching 70% confluence (approximately every 2 days). 
         [0069]    After tripsinization and centrifugation, cells were suspended in full medium (2000 cells/μL) and 50 μL were injected in the device. Medium was refreshed in the device every 2 days. 
         [0070]    Vitality was investigated after 7 days in culture by using NucBlue® Live ReadyProbes® Reagent and NucGreen® Dead 488 ReadyProbes® Reagent (Molecular Probes, R37605 and R37109). 20 μL of staining solutions were added directly to cells in full media and incubated for 20 minutes. NucBlue® Live Cell Stain emits at 460 nm when bound to DNA while NucGreen® Dead 488 reagent is a membrane-impermeable stain DNA of dead cells (excitation/emission at 504/523 nm). Cellular cytoskeleton was visualized by F-actin staining using ActinGreen™ 488 ReadyProbes® Reagent from Molecular Probes. Cells were prepared for staining by 4% paraformaldehyde fixation. 
         [0071]    Cells were finally observed with an inverted fluorescent microscope (EVOS, FL Cell Imaging System) and with Image J software (Rasband, W. S., ImageJ, U. S. National Institutes of Health, Bethesda, Md., USA). 
       Results 
     SEM Characterization of Fused Silica Molds and PVA Replicas 
       [0072]    The surfaces of both the femtosecond laser patterned fused silica mold and the corresponding PVA replica were imaged using a JEOL 6320 field emission scanning electron microscope. Both samples were sputter-coated with a few nanometers of Platinum for successful imaging. An image of a single-pulse femtosecond laser machined nanohole is shown in  FIG. 2A . Images of the PVA replica, at various magnifications, are shown in  FIGS. 2B through 2D . 
       SEM Characterization of PLLA Membrane 
       [0073]    Spin coating assisted deposition of PLLA on PVA replicas was performed using the same processing parameter values used to prepare plain ˜100 nm thin PLLA films reported in previous work [29]. As shown in  FIG. 3D , the PLLA solution spread and coated the full surface of the replica. Due to the high aspect ratio of the PVA nanoneedles and lack of wettability, needles were not completely covered during the spin coating. 
         [0074]    The sacrificial layer PVA was dissolved by immersion in de-ionized water, and once removed, the PVA needle sites become open pores in the PLLA nanofilm membrane. The hydrophobic PLLA film floats freestanding on the water surface ( FIG. 3A ). The patterned pores can be visualized in bright field once the film is collected and dried on a flat surface, such as a silicon wafer or a glass coverslip ( FIGS. 3B and 3C ). The PLLA nanofilm replicates perfectly the surface morphology and any irregularity of the PVA replica, including the features imposed by the laser in the original silica mold ( FIG. 3D ). 
         [0075]    Different views of a PLLA semipermeable membrane bonded to the bottom channeled PDMS layer are shown in  FIGS. 4A through 4D . 
         [0076]    The PLLA membrane bonded wrinkle-free to the channeled PDMS layer (see  FIGS. 4A-4D ). The sixteen rows of micrometric pores of the PLLA membrane aligned precisely above the sixteen microfluidic channels. The sacrificial PVA layer provides a means of handling the nanofilm and mounting the film on the channeled PDMS layer preventing damage to the film; a key concern in manipulation of the ultrathin membrane. 
         [0077]    Since both PDMS and PLLA are optically transparent, alignment of various components was done under a stereomicroscope. Together with the pattern of surface pores, the original silica mold has femtosecond laser machined surface alignment marks and a lateral identification label ( FIGS. 4B-4D ); these microscopic features transfer to the PVA and PLLA films and serve as alignment guides during device assembly. 
         [0078]    We observed that the PLLA membrane tightly adhered to the channeled PDMS layer and did not collapse inside the channels ( FIG. 5A ). Cells were then seeded directly on the PLLA membrane in the upper chamber without previous coating of the membrane surface. Approximately 100,000 cells were seeded at Day 0 in the device, filling the upper chamber without leakage between the layers. It was possible to observe the cell behavior and monitor growth by optical microscopy due to the transparency of the PLLA membrane in bright field. 
         [0079]    Oxygen plasma treatment did not adversely affect the biological properties of the PLLA film surfaces; the grow rate and cell morphology of the cells on PLLA inside the device were comparable to the control flask. As shown in  FIGS. 5C and 5D , cells proliferate rapidly in the device and with very low mortality (below 1%). The medium of the two chambers was replaced every 48 hours and after 7 days the adhesion of the cells on the semipermeable PLLA membrane was confirmed by F-actin staining ( FIG. 5D ). The media from the two chambers were collected and the amount of cells in the two chambers was measured by using a cell counter. Cells were found only in medium from the upper chamber, thus proving that the size of the pores did not exceed the nominal value of 1 μm in diameter (data not shown). At the same time, permeability of the membrane was confirmed by 150 kDa FITC-dextran diffusion measurements. 
       Permeability of the PLLA Nanofilms 
       [0080]    Diffusion of FITC dextran (70 kDa molecular weight, Sigma Aldrich, USA) through the perforated PLLA membranes was measured by using a UV-Vis Spectrophotometer (Varian Cary 50, Agilent Technologies, Santa Clara, Calif.). 
         [0081]    The experiment was performed in a 96-well plate. Five devices were assembled, following the protocols described above in the Materials and Methods section, each one integrating one PLLA perforated membrane. FITC dextran solution (2 mg/mL in MilliQ water) was used to fill the upper chamber of the device, while the lower channels were filled with MilliQ water. Liquid from the lower channels was collected after 6 hours. One additional device was assembled with a PLLA membrane prepared with the same process parameters on a flat silicon wafer, which thus did not have holes and was not permeable. The relative permeability to FITC-dextran of perforated PLLA membranes and a non-perforated PLLA control membranes are shown in  FIG. 6 . 
       DISCUSSION AND CONCLUSIONS 
       [0082]    Fabrication of biocompatible, transparent, semipermeable ultrathin polymer membranes with precisely patterned micropores is difficult by perforation using e-beam lithography, focused ion beam milling, or laser micromachining, all of which processes lack production scalability. Forming membranes using a molding approach is also difficult to implement. First of all, the polymer melt, polymer precursor, or the polymer-solvent solution being used must have sufficiently low viscosity to spread and fill the surface of the mold completely and homogeneously, without leaving a residual layer on top of the mold nanoneedles or micropillars. Mechanical peeling of the resulting membrane is also problematic because the thinness of the membrane makes it very delicate and susceptible to tearing during removal from the mold. Once released from the mold, the membrane must also be transferred and precisely attached to the microfluidic device and without any wrinkles or folds. 
         [0083]    The method reported herein provides a quick, repeatable and convenient route to prepare multiple copies of the semipermeable membranes having an engineered porosity. First, the direct one-step femtosecond laser machining of the fused silica molds can be carried out at kilohertz rates. With the current system, this translates to patterning a regular array of four million surface nanoholes inside a one square centimeter area in less than one hour. Molds can be prepared with a distance between nanoholes as small as 2 μm. Chemically inert and mechanically hard, the fused silica mold can be used to prepare thousands of PVA replicas. The PVA formulation used does not stick to fused silica. Each PVA replica precisely and repeatedly duplicates all the features of the mold, yielding straight, standing, and aligned high aspect-ratio nanoneedles. PVA replicas can be produced relatively quickly (15-30 minutes) and the mold can be easily cleaned from residual traces of PVA by hot water. The water-solubility and organic solvent-resistance of PVA makes it a suitable sacrificial material to use as template in preparing semipermeable membranes for many organic solvent-soluble polymers of interest. Finally, the PVA template provides a convenient vehicle to transfer, align and attach the ultrathin membrane to the microfluidic device without introducing undesirable wrinkles or folds in the final assembly. 
         [0084]    While the selection of biocompatible polymers is wide and the fabrication of films with micrometric thickness can be achieved with several methods, handling of perforated thin membrane represents a technological challenge. In this work we selected PLLA since its biocompatibility is well demonstrated. Furthermore, the sacrificial layer is fundamental for handling and positioning of the ultrathin membrane: while a plain sacrificial layer is traditionally used to release positioning polymeric films from their support, for example in stretchable and epidermal electronics [30, 31], in this case the dissolvable PVA includes not only the needles but also temporary labels and frames to align the pattern of needles with the microfabricated channels in the PDMS. 
         [0085]    The semipermeable property of the PLLA membranes disclosed herein guarantees the possibility to separate two fluidic compartments and to select the size of particulates (i.e. cells) able to pass through membrane pores. This is a key characteristic in lab-on-a-chip technology, where cells extravasation and passage of chemical species between communicating compartments need to be controlled. Organs-on-a-chip seeks to understand complex processes; e.g. organ development, embryogenesis, tumor metastasis, and leukocyte infiltration that are regulated by cellular responses to multiple (competing) chemokines as well as autocrine feedback loops, cell-cell interactions, and mechanical stress. The result outlined herein of a scalable, tunable semipermeable nanofilm membrane will contribute to the success of organs-on-a-chip models and enable more faithful recapitulation of in vivo conditions. 
         [0086]    The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements within the spirit and scope of the invention as set forth in the appended claims. 
         [0087]    Throughout this document, various references are mentioned. All such references are incorporated herein by references, including the references set forth in the following list: 
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