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Materials and Interfaces

Roll-to-Roll Fabrication of Residual Layer Free Micro/Nanoscale Membranes with Precise Pore Architectures and Tuneable Surface Textures Him Cheng Wong, Gianluca Grenci, Jumiati Wu, Virgile Viasnoff, and Hong Yee Low Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03867 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 29, 2018

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Industrial & Engineering Chemistry Research

Roll-to-Roll Fabrication of Residual Layer Free Micro/Nanoscale Membranes with Precise Pore Architectures and Tuneable Surface Textures Him Cheng Wong,a* Gianluca Grenci, bc Jumiati Wu,a Virgile Viasnoff,bd Hong Yee Lowa* a

Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, Singapore 487372. b MechanoBiology Institute, National University of Singapore, 5A Engineering Drive 1, Singapore 117411, Singapore c Biomedical Engineering Department, Faculty of Engineering, National University of Singapore, 4 Engineering Drive 3, 117583 Singapore d Bio Mechanics of Cellular Contacts, Centre National de la Recherche Scientifique, UMI 3639 Singapore 117411, Singapore E-mail: [email protected]; [email protected]

Abstract Many technologies are increasingly dependent on the development of membranes with highly precise pore size and spatial order. While mold-based lithographic techniques are adopted for the fabrication of such ordered porous membranes, their transition toward large-scale fabrication remains challenging due to manual and laborious process requirements. To facilitate the transition from batch to continuous membrane production, a novel roll-to-roll (R2R) process was designed based on an established capillary-driven molding technique. Establishing conformal contact between patterned mold and web support and the capillary filling of UV-curable resin form the basis of the molding process, which is automated by the R2R platform with precise positioning and reproducible contact force. This results to in-situ formation of residual layer free membranes without additional complex, expensive etching steps and more importantly, with high throughput up to 3000 mm2/min. The versatile process allows the combination of mold and web’s pattern geometry and lengthscale, yielding multilevel and multiscale membranes that are difficult to achieve with existing methods. Membranes with double-side surface textures and hierarchical pore architectures spanning lengthscale from microscale to nanoscale (down to ~200nm) can be fabricated by overcoming the intrinsically slow resin capillary filling at sub-micrometer lengthscale. The R2R platform’s scalability and customisability open avenues toward manufacturing porous membranes with complex yet precise architectures for a broad range of applications. Introduction Membranes play important roles in many processes from cellular level up to industrial scales but majority of today’s membranes have random pore morphology and tortuous paths which may not meet the requirements of increasing number of applications. In particular, recent demands in purification, cell biology, and stencilling applications are driving the development of new classes of membranes with very uniform pore architectures (size, shape, depth, order, density) and straight pore channels,1,2 hereafter referred to as ordered porous membranes. The ordered pore structures can emulate in vivo cellular microenvironments3,4 by incorporating precise biochemical, topographic and rheological cues, allowing in vitro studies of (mechano)biological processes by which cells sense, interact and respond to their surroundings. By virtue of their excellent pore size and shape uniformity, ordered porous membranes also allow highly size-selective separation/capturing processes2,5–7 and can also serve as stencils for resistless patterning of wet (protein) or dry (metal) structures.2 A plethora of fabrication methods which enable excellent control over membrane architectures have been reported.8 Amongst the traditional top-down methods, mold-based

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imprinting9 and replica molding techniques are widely adopted for replicating highly ordered pattern from a prefabricated mold onto a target material at elevated temperature and pressure or under ultraviolet radiation. These approaches however, typically leave behind thin residual layer due to incomplete polymer resin displacement by the mold protrusions. The residual layer removal typically requires additional etching procedures5,9–11 which are prohibitively complex and laborious. Other fabrication methods include photolithography/laser interference lithography;10,12 or sequential writing/machining methods such as electron-beam lithography and focus ion beam milling13,14 but their adoption are also limited by process repetition/complexity and low throughput which result to high cost. The burgeoning applications for ordered porous membranes in the fields of biomedical,1 biological15,16 and material science17 result to increasingly stringent membrane morphological requirements . For example, extending membrane pore architecture down to the nanoscale proves to be very challenging in terms of process complexity and throughput. Also, membrane thickness in the nanoscale will inevitably reduce the membrane’s mechanical durability for handling and implementation. In order to circumvent this, increasing efforts are made toward fabricating membranes with hierarchical architectures.2,6,18 where multiple membranes are arranged in tandem over adjacent levels. Distinct pore morphology at each hierarchy can therefore synergistically maximise the benefits associated with nanoscale membranes such as enhanced flux and optical transparency without compromising mechanical stability. Conventional methods such as photolithography however are poorly suited for pattern structures with multiple levels and curvature due to depth of focus limitations.

Figure 1. Schematics depicting the capillary-driven molding process. (i) Elastomeric mold carrying a protrusion structure is placed in good contact with a plastic web substrate, forming the enclosed network channels resembling an ordered porous membrane. (ii) Low viscosity UV-curable resin is dispensed at the mold edge for filling into the channel network by capillary action. (iii) The molded resin is cured with UV light before (iv) the mold separation process reveals the pattern feature.

The last two decades have witnessed significant progress in soft lithography19 which is well known for its remarkable simplicity, cost efficiency and good patterning fidelity. Micromolding in Capillaries17,20 is a variant of soft lithography capable of generating numerous residual layer free polymeric structures with ordered dimensions. Figure 1 illustrates the capillary-driven molding process schematically. Briefly, an elastomeric mold containing the inverse of the desired pattern is placed on a substrate (Figure 1(i)), forming a network of enclosed and interconnected channels/capillaries for spontaneous filling of UV-curable resin by capillary action (Figure 1(ii)). The molded resin is cured with a UV light source (Figure 1(iii)) before the mold separation step reveals the patterned structures (Figure 1(iv)). It is a thermodynamically driven process, with the resin filling rate and distance an interplay between interfacial thermodynamics and viscous drag of the resin in the capillary. Therefore, the three main components of the process, namely the mold, support substrate and resin must meet certain criteria: (i) the mold must be elastomeric to allow conformal


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contact with the substrate and be inert to the resin used; (ii) the inner walls of the formed network capillaries (substrate and mold) must be completely or at least partially wet by the resin. To promote capillary filling, the contact angle of the resin on the mold and support surface must be less than 90°. Resins with low viscosity, low surface tension are therefore most suitable, with typical capillary filling occurs quickly over centimeter distance. With the utilisation of the now ubiquitous polydimethylsiloxane (PDMS) elastomer as patterning mold and the exclusion of expensive instrumentations and hard-to-implement procedures such as those of residual layer etching and photolithography, capillary-driven molding process has been increasingly explored2,6,7,21–25 to produce ordered porous membranes which are used in wide range of applications. From generating flow patterns in 3D microfluidic devices;25 geometry-selective microparticle arrays,23 multiprotein pattern landscapes24 to being used for high selectivity particle filtration and enrichment,2,7 in vitro cell isolation,6 3D cell culture21 and super resolution imaging.22 The increasing demands for ordered porous membranes in disposable format, particularly in biological applications also open the case for continuous and scalable production for better economics, higher throughput and standardised production quality. Recent reports6,7 have demonstrated the large-scale fabrication of through-hole membranes with wafer scale mold. However, the fabrication steps which include sealing of mold onto support substrate, resin injection, resin curing and mold separation are still carried out manually, one step at a time. The transition from batch to continuous production requires precise process parameter control and automation which is still a subject of intense research with tremendous commercial potential. Among many up-scaling strategies, roll-to-roll (R2R) fabrication26–32 is rapidly emerging as a manufacturing route that facilitates continuous operation of various batch-based micronanofabrication processes involving lithographically patterned molds. In this paper, by leveraging the simplicity of capillary-driven molding and the efficacy of R2R fabrication, we introduce and describe a novel R2R platform which was custom designed for the continuous fabrication of ordered porous membranes. As the resin capillary filling rate and distance scale correspondingly with the dimensions of the filling capillaries,33 the resin filling will be slower for smaller capillaries over a large filling distance. Therefore, we also adapted the R2R process for the fabrication of membranes with multilevel, multiscale and precise architectures down to the nanoscale, a resolution previously not possible with capillary-driven molding process due to long resin filling times. Results and discussion R2R platform design principles and working sequence The core idea of the R2R platform is to adopt roller comprises a series of flexible and reusable patterned molds, and to allow all fabrication steps to be automated concurrently with precise positioning and reproducible contact force. Despite some basic similarities with conventional R2R UV nanoimprinting unit, a R2R platform tailored for ordered porous membrane fabrication based on capillary-driven molding process requires many different design considerations: (i) the methodologies and force required to bring the mold and support substrate into conformal contact to create the capillaries for resin filling; (ii) the engineering of surface properties of the enclosed network capillaries (substrate and mold) that promotes liquid resin capillary filling and subsequent mold separation and membrane transfer; (iii) the optimum mold pattern design to achieve good balance between resin capillary filling time, fabrication throughput and the resulting membrane pore size. The aforementioned design considerations will be discussed in detail in subsequent sections.



Figure 2. (a) Schematic and (b) photograph illustrating various components of the custom designed R2R platform. (c) Schematic and (e) photograph showing the mounted PDMS patterned molds onto the central roller’s mold slots followed by the installation of web substrate at a pre-set web tension. (d) photograph of a custom fabricated PDMS mold with precise dimensions. Scanning electron microscopy image in inset of (d) shows the micropillar structures replicated from silicon master template. The location at the R2R platform where each step (see Figure 1 (i)-(iv)) takes place is highlighted in (a).

First, the schematic and photograph detailing the anatomy of the R2R platform are shown in Figure 2a and b. The custom designed platform comprises the following main components: an unwinding unit which feeds 30 mm wide and 50 µm thick flexible web substrate; a central roller where PDMS patterned molds are attached to; and a gripper unit that forwards the web substrate. The mold and web components adopt a vertical orientation which ensures a downward resin filling direction. The location at the R2R platform where each process step (see Figure 1 (i)-(iv)) takes place is highlighted in Figure 2a and will be referenced throughout the manuscript. The unwinding unit feeds the flexible web substrate along the route indicated by the red arrows in Figure 2a, leading to the central roller where it first makes contact with the patterned side of the PDMS molds before being fixed to the gripper unit at the other side of the central roller. Web cleanliness is of great importance, an ionising air knife is installed to eliminate dust particles and statics on the web before making contact with the PDMS mold. There is also a web alignment system comprises an ultrasonic sensor which detects the web edge position, ensuring the web is aligned vertically by a steering roller within ±0.1 mm accuracy. The R2R platform has two tension zones (Tzone 1 just after the unwinding unit and T-zone 2 before the central roller) where load cell at each zone measures and feedbacks the web tension to the control system. The system maintains the web tension at setpoint value by controlling the central roller’s rotation speed and the gripper unit’s pull force which are in turn synchronised with the unwind speed of the web roll. The central roller has a circumference of 1000 mm divided into ten individual stainless steel mold slots as depicted schematically and photographically in Figure 2c and e, respectively. The external diameter of the central roller is 320 mm, which is sufficiently large not to cause any


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curvature-induced lengthscale deviation to the mold pattern features. The choice of PDMS as mold material is also important that: it faithfully replicates the features of the master template; its elastomeric and compliant character enables good contact with the web substrate during resin capillary filling; its relatively low surface energy allows easy separation from cured resin yet does not retard resin capillary flow. Step features are added at the sides of the PDMS mold to allow mounting by mechanical fixation to the central roller’s mold slots. Each PDMS mold also has a built-in resin dispense area, see Figure 2c and 2d. In order to precisely shape the aforementioned designed features and to replicate the pattern features from a Silicon master template, a metal casting jig was custom fabricated. The jig surface was polished and coated with diamond like carbon in order to ensure PDMS molds possess highly planar and uniform surface finishing. The detailed PDMS mold fabrication process is detailed in Figure S1. Each custom fabricated PDMS mold is 90-mm-long, 30-mm-wide and 6-mmthick (see Figure 2d). The thickness (6 mm) of the PDMS mold is important such that it helps to cushion any long-range surface waviness present at the surface of the stainless steel mold slot and prevent it from being transmitted into the pattern surface.34 Up to ten PDMS molds can be fixed onto the mold slots, with the micropillar structures facing outward (see inset in Figure 2d). Additional fixation force is provided via vacuum suction through the grooves engraved on the mold slots (Figure 2c). Finally, the continuous web feed is installed at a pre-set web tension around the central roller, making conformal contact with the PDMS molds during the process, see Figure 2c and 2e.



Figure 3. (a-c) Photographs and (d-f) schematics illustrating the R2R fabrication of ordered porous membranes. (a, d) automated and continuous resin dispensing onto PDMS molds at the built-in resin dispense area and the capillary force driven filling of the formed capillaries between the mold and web in conformal contact; (b, e) UV curing of the molded resin; (c, f) separation of cured membrane from PDMS mold, and the latter returns to the dispensing station. Photograph, planar SEM, and perspective SEM images of the ordered porous membrane are shown in (g), (h) and (i) respectively.

At central roller position (i) (refer to schematic Figure 2a), UV-curable resin such as (but not limited to) polyurethane NOA73 is continuously dispensed by a software controlled volumetric dispenser (PreciFluid System) onto the designated area along a 90 mm-mold which is in contact with the continuous web feed (Figure 3a, d). The dispenser is mounted on an X-Y-Z translation stage which allows fine adjustments to the dispense location. As an integral part of the R2R process, the dispenser operation is synchronised to the central roller position and rotation speed; allowing ondemand resin dispensing to selected mold position and with precise dispense rate down to 0.63 µl/s. The dispensing system also has disposable syringes and pistons which allows resin change with minimal preparation time. In order to accommodate the capillary filling time (few minutes), the filled resin is UV cured through the web further downstream at roller position (iii) shown in Figure 2a, using a UV LED system (FireFly FF200, Phoseon Technology, λ = 365 nm) having a 20 mm × 25 mm irradiation window and a peak irradiance of 1.5 W/cm2, see Figure 3b, e. Upon curing, the web supported membrane finally detaches from the PDMS mold and exits the central roller at position (iv), see Figure 3c, f. The separation angle of the cured membrane against the mold is kept

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