Functional Polymer Sheet Patterning Using Microfluidics - American

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Functional Polymer Sheet Patterning Using Microfluidics Minggan Li, Mouhita Humayun, Janusz A. Kozinski, and Dae Kun Hwang Langmuir, Just Accepted Manuscript • DOI: 10.1021/la501723n • Publication Date (Web): 26 Jun 2014 Downloaded from http://pubs.acs.org on June 30, 2014

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Functional Polymer Sheet Patterning Using Microfluidics

Minggan Li†, Mouhita Humayun†, Janusz A. Kozinski‡, and Dae Kun Hwang*† †

Department of Chemical Engineering, Ryerson University 350 Victoria Street, Toronto, Ontario, M5B 2K3, Canada ‡

Lassonde School of Engineering, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada Correspondence to *E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Poly(dimethylsiloxane) (PDMS)-based microfluidics provide a novel approach for advanced material synthesis. While PDMS has been successfully used in a wide range of industrial applications, due to the weak mechanical property, channels generally possess low aspect ratios (AR), and thus produce micro-particles with similarly low AR’s. By increasing the channel width to nearly one centimeter, AR to 267 and implementing flow lithography, we were able to establish slit-channel lithography. Not only does this allow us to synthesize sheet materials bearing multi-scale features and tunable chemical anisotropy, but it also allows us to fabricate functional layered sheet structures in a onestep, high throughput fashion. We showcased the technique’s potential role in various applications, such as synthesis of planar material with micro- and nano-scale features, surface morphologies, construction of tubular and 3D layered hydrogel tissue scaffolds and one-step formation of radio frequency identification (RFID) tags. The method introduced offers a novel route for functional sheet material synthesis and sheet system fabrication.

Keywords: slit-channel lithography, sheet material, layered structure, RFID bonding


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Introduction Given the low cost of micro-fabrication using soft lithography and the optical transparency, gas permeability, and biocompatibility properties of polydimethylsiloxane (PDMS), PDMS-based microfluidic systems have been successfully used throughout a broad range of applications, from chemical and biological assay to material synthesis. For material synthesis, PDMS-based microfluidic systems have demonstrated an alternative pathway to generate polymeric micro-particles with highly mono-dispersed, non-spherical, and multi-functional features not seen in conventional methods. In particular, flow-lithography (FL) techniques1-4 enhance the control over particle shape and functionality, and have been successfully applied to generate 2D,5,

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multiplexed,7-11 cell-laden ,12 magnetic Janus13 and 3D particles14, 15 as well as complex microstructures.16 The success of PDMS, used as a microfluidic platform for FL-based material synthesis, is primarily due to material properties such as optical transparency (for ultraviolet (UV) polymerization and monitoring) and oxygen diffusion capacity (for lubrication layers).17 However, because of the weak mechanical strength of PDMS, the channels for FL generally exhibit low aspect ratios (AR: the ratio of channel width to height). With a typical AR of less than 20,18 production is limited to micro-scale objects, with similarly low ARs. In practice, PDMS channels are generally designed to be at most, O(100) µm in width.18 Channels with larger widths typically experience sagging, preventing the synthesis of materials with high ARs through FL techniques and thus limiting their range of application.


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By simply increasing the PDMS channel AR (>100), FL-based synthesis methods can enable fabrication of high AR sheet materials with desirable features for various applications. For instance, sheet materials patterned with micro- and nano-features are key functional components in a number of lab-on-a-chip technologies, in which they can be utilized for multi-fraction separation,19, 20 gas sensing,21 cell trapping and analysis,22, 23 bioassay,24 and bioreactors.11,

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Furthermore, layered sheet materials with functional

components can be employed in the construction of 3D channeled microstructures from patterned hydrogel sheets to vascularize artificial tissues,

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the bonding of radio-

frequency-identification (RFID) microchips with organic planar circuits to form RFID tags,28, 29 and the assembly of electronically-patterned sheets to form flexible displays30 among others. In PDMS-based microfluidics, the addition of post or segmented arrays allows for high AR channels; however, they occupy the space used for sheet formation, thus preventing the synthesis of sheet materials using FL techniques. Alternatively, embedding a glass support into the top of a PDMS channel adds strength to the arch and prevents it from sagging.18 This approach has been successfully used for DNA analysis in nano-sized channels.31 For FL, the introduction of a glass support not only frees the sheet formation space while maintaining the mechanical strength, but it also ensures the presence of the oxygen-lubrication layers (top and bottom surfaces of the PDMS channel). This feature is key in ensuring a continuous collection of the synthesized materials without them adhering to the channel wall. Therefore, glass-supported slit channels, in conjunction with FL-techniques (slit-channel lithography), can offer a novel route for


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fabricating various patterned sheet materials and structures with features not possible or difficult to achieve using conventional methods, in a high throughput manner. For our demonstration, using slit-channel lithography, we first synthesized singlelayer sheets with features including micro- and nano-pores, controlled pore shape profiles and desired surface morphologies potentially for membrane technology. We then produced three dimensional (3D) hydrogel structures by layering sheets, with and without cells, which could have a potential use in tissue engineering. Finally, we generated an electrically conductive sheet that serves as a functional unit in RFID tags.

Materials and Methods Slit microfluidic channel fabrication. Polydimethyl-siloxane (PDMS, Sylgard 184, Dow Corning, Midland, MI, USA) precursor with a mixing ratio of 10:1 (PDMS: curing agent) was used for the fabrication of the slit channel. After partial curing of PDMS on a SU-8 photoresist (Microchem, Newton, MA, USA) positive relief pattern, a piece of glass, slightly wider than the channel, was placed on the PDMS surface. Then fresh PDMS precursor was poured onto the surface and subsequently baked at 65 °C for another 2 h, making the slit PDMS channel with glass support inside. The channels were attached to glass slides with partially cured PDMS coating and then baked at 65 °C for another 2 h for bonding. Photopolymerization setup. Among FL techniques, we used stop-flow lithography for sheet synthesis.6 In our system configuration, a metal arc lamp was used as the UV source (Lumen 200, Prior Scientific, Rockland, MA, USA) and a UV shutter (Lambda SC, Sutter Instruments, Novato, CA, USA) was installed in the UV light path to control


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the UV exposure time. The intensity of the UV light was set to 100% for all experiments. The pneumatic solution feeding system consisted of a pressure regulator (Type 100LR, ControlAir, Amherst, NH USA), serially connected to a three-way solenoid valve (Model 6014, Burkert, Germany) and PDMS channel. The UV shutter and the solenoid valve were both controlled by a program in Labview (National Instruments, Austin, TX, USA) through a digital controller (NI 9472, National Instruments, Austin, TX, USA) to coordinate the synthesis process of flow-stop, UV exposure (synthesis) and flow-resume, in a repeating pace. An inverted microscope Axio Observer (Carl Zeiss, Jena, Germany) equipped with objectives of 5X/0.13, 10X/0.3 and 20X/0.4 (N-Achroplan, Ec planNeofluar and korr LD Plan-Neofluar, Carl Zeiss, Jena, Germany) was used for this study. A UV filter set (11000v3, Chroma, VT, USA) was used to filter the UV light source to obtain desired UV excitation for polymerization. The red and green filters (XF101-2, XF100-2 Omegafilters, VT, USA) were used for the fluorescence imaging. The transparency photomasks were designed with AUTOCAD 2011 and printed at a resolution of 25,000 dpi (CAD/Art Services, OR, USA).

Single layer sheet synthesis. In single layer sheet synthesis, channels of 30 µm high and 8 mm wide were used as shown in Figure 1. Sheets with micro-pores in Figure 2a-c were synthesized using poly(ethylene glycol)diacrylate (PEG-DA 700, Sigma-Aldrich) with 5% photoinitiator 2-hydroxy-2-methylpropiophenon (Darocur 1173, Sigma-Aldrich) and 0.1% rhodamine B (Sigma-Aldrich) by 5 s UV exposure through a 5X objective. The cylindrical and conical pores were made by using 10X objective with a numerical aperture of 0.3 and 1 s UV exposure. The cylindrical pore profile in Figure 2e was obtained by placing the focal plane in the middle of the channel height, and the conical


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pore profile in Figure 2f was produced by lowering the focal plane by ~30 µm. The surface textured sheets in Figure 2g-i were made by varying the components of the prepolymer solution and their concentrations. They were all polymerized in the slit channel using 5X objective and 5 s UV exposure. Smooth (Figure 2g) and wrinkled surfaces (Figure 2i) were created using 0% and 60% Ethanol (Sigma-Aldrich) respectively in Poly(ethylene glycol) (575) diacrylate (PEG-DA 575, Sigma-Aldrich) both with 5% Darocur 1173. The porous surface was synthesized using 60% Ethanol and 1% Darocur 1173 in poly(ethylene glycol) (200) diacrylate (PEG-DA 200, SigmaAldrich). The fluorescent images were taken by a Nikon D300s camera (Nikon, Canada) and SEM images were obtained through a scanning electron microscopy (FE-SEM S4500, Hitachi). Cell-laden sheets and hydrogel sheets assembly. 3T3 fibroblasts (ATCC, Manassas, VA, USA) were grown in Dulbecco’s Modified Eagle’s medium (DMEM) (SigmaAldrich, St. Louis, MO, USA) supplemented with 10% fetal bovine serum (FBS) and 1% antibiotic. The cells were then incubated at 37°C in a humidified atmosphere of 95% air and 5% CO2. 2-Hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959, SigmaAldrich) was used as photoinitiator and dissolved in the mixture of PEG-DA 700 and phosphate buffered saline (PBS). The prepolymer solution without cells was a mixture of 20% (v/v) PEG-DA 700, 3% (w/v) Irgacure 2959, and 80% (v/v) PBS. 3T3 fibroblasts (ATCC, Manassas, VA, USA) containing prepolymer solution was a mixture of 20% (v/v) PEG-DA 700, 3% (w/v) Irgacure 2959, and 80% (v/v) PBS with cell density of


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5×106 cells mL-1. Before mixing with cells, the prepolymer solution was filter-sterilized through a 0.22 µm filter. A slit channel with

8mm wide and 60 µm height was used. The prepolymer

solutions, with and without cells, were allowed to flow into the channel to form the middle and side streams, respectively. The co-flow streams were polymerized by 6 s UV light exposure through a rectangular photomask (Figure 3a). The sheets were washed by PBS 3 times followed by 30 min. incubation. They were then stained by live/dead cell viability assay agent (L-3224, Invitrogen, Canada) for 10 min. Both bright field and fluorescent images (Figure 3b, c) were taken by a Nikon D300s camera. An elongated hydrogel sheet (Figure 3d) was produced by polymerizing the sheet with overlapped edges. Rhodamine B was added into the prepolymer solution for fluorescent imaging. The elongated hydrogel sheet was pulled out and rolled onto a 1.5 mm diameter glass rod (Figure 3e). A similar process was used to produce the doublestream elongated sheet, in which Rhodamine B was added into one stream (red), while Fluoresbrite YG carboxylate microspheres solution (1 µm beads, Polysciences Inc., Warrington, PA, USA) was added to another stream (green). The obtained sheet was then rolled around a 2.1 mm diameter metal tube at a certain angle to form a long hydrogel tube. The fluorescent images were taken by a Nikon D300s camera. To make the magnetic hydrogel (Figure 3g-i ), a prepolymer solution containing 10% (v/v) magnetic beads solution (1µm, Sera-Mag, Thermo Scientific, Canada), 40% (v/v) PEG-DA 700, 50% (v/v) DI water and 6% (w/v) Irgacure 2959 was used as the side streams and non-magnetic prepolymer solution containing 40% (v/v) PEG-DA 700, 60%


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(v/v) DI water and 5% (w/v) Irgacure 2959 was used as the middle stream (Figure 3g). 5X objective was used to project the UV light through a pore-arrayed photomask. The fabricated magnetic hydrogel sheets were rinsed and placed in a rectangular PDMS reservoir filled with water (Figure 3h inset). As the magnetic field was applied and the water was gently agitated by a pipette, the sheets were drawn to the corner and stacked up (Figure 3h). Images were taken by a charge coupled device (CCD) camera (QImaing, Canada). RFID tag fabrication. RFID dies were obtained from Alien Technology (Morgan Hill, CA, USA). 2mg/mL CNT prepolymer solution was obtained by dispersing CNT (Cheaptubes, Brattleboro, VT, USA) in PEG-DA 700 (40%) and DI water (60%) solution. A slit channel with 8 mm wide and 150 µm height was used and three inlets was designed to form flow focusing streams (Figure 4a). The die was placed in the middle inlet reservoir. CNT prepolymer solution was passed through the two inlets, forming three streams. The flow rate of the two side streams were set at 3 µL s-1 and the middle one at 1 µL s-1. The dies were focused into the middle of the channel and were stopped at a downstream location by pausing the flows (Figure 4a). An antenna-designed photomask was adjusted to align the antenna and the bumps. Then a UV light was projected to the channel for 6 s through the photomask to polymerize the CNT antenna with the die attached (Figure 4b), to form an RFID tag. After washing, a stereomicroscope (AccuScope, Commack, NY, USA) was used to observe the tag and the images were captured using a Nikon D300s camera (Figure 4c). The tag was then encapsulated in PEG-DA using prepolymer solution containing 95% PEG-DA 700 and 5% Darcure 1173 and subsequent UV polymerization, making the tag inlay (Figure 4d).


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Results and Discussions Single-layer sheet patterning

Fig 1a. is a schematic diagram of slit-channel lithography for single-layer sheet synthesis. Because of the high AR (267) of the PDMS channel, without support, the channel deformed. This sagging deformation (Figure S1) at the middle was up to 33% of the original height of the channel in Figure 1a . The deformed channel will directly affect the thickness uniformity of synthesized membranes, degrading their quality. We introduced a glass support above the channel within the PDMS, as shown in Figure 1a and 1b, to ensure the prevention of the deformation. The thickness of the resulting synthesized membranes (Figure 1c and S2) were uniform (Figure S2). In the glass supported PDMS slit channel, we flowed a photocurable prepolymer solution. As the flow was stopped, a UV light was projected to the channel through a photomask. The applied UV light polymerized the solution with the photomask-defined pattern. By taking advantage of the lubrication layer at the channel walls caused by oxygen inhibition (Figure 1b), we were able to flush out the patterned sheet immediately by resuming the flow. This process was repeated and patterned sheets were fabricated continuously, without adherence to the channel walls. Figure 1c shows an example of patterned sheets with micro-scale holes using a slit channel with an AR of 267. In a typical FL technique, the channel height and UV exposure time defined the sheet thickness, while the size and shape of the sheet and pores were determined by photomasks and objective used. We experimentally determined the thickness of sheets and lubrication layers with respect to UV exposure times for different objectives (Table


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S1). Decreasing the UV exposure time increases the thickness of the oxygen inhabitation layers and thus produces thinner membranes (Figure S3). The ratio of photomask feature sizes to that of synthesized membranes using different objectives were also experimentally measured (Table S1). Thus, by simply changing photomasks, we were able to generate complex geometric patterns with controllable pore size and sheet thickness in one single-step. Figure 2 shows the resulting single-layer sheets embedded with complex micro-pore patterns and demonstrates precise patterning capabilities. The gradient pore-patterns (starting from 5 µm in diameter) have displayed beneficial effects in multi-fraction separation and filtration.20 With our current setup, we have fabricated a sheet size of up to 6.2 mm in diameter with an AR of 620 and a regular micro-pore down to 5 µm. The size of the sheet can be readily increased by using a wider channel and a larger size UV beam. In addition to pore patterning, we were able to tune pore shape-profiles by adjusting the focal plane of UV along the height of the channel. The UV projection-profile, within the sheet formation zone in the channel, defined the shape of the pores as demonstrated in Figure 2d-f. We can approximate the geometry of UV light to be linear in depth at the focal plane (depth of focus is 135 µm for 10X objective with numerical aperture NA=0.315, Figure 2d). Placing the focal plane in the middle of the channel resulted in cylindrical pores (Figure 2e) whereas lowering the focal plane inside of the channel produced conical pores (Figure 2f). The cone angle is mainly determined by the NA of the objective and can thus be changed by selecting objectives with different NAs. Although cylindrical pores are widely used for cell trapping and analysis,23, 32 conical


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pores are also beneficial since they enhance oxygen and medium delivery for cell culture33, 34 and improve assay sensitivity for biochemical analysis.35 By adjusting the prepolymer compositions of ethanol, PEG-DA molecular weight, and photoinitator, we further manipulated surface textures onto the sheets, including smooth, porous and wrinkled surface morphologies (Figure 2g-i). Without ethanol, polymerizing a prepolymer solution produced sheets with smooth surfaces. The addition of ethanol into the PEG-DA 200 prepolymer solution induced phase separation during the polymerization process, resulting in porous sheets with pore sizes down to O(100) nm (Figure 2h). In combination with the micro-scale pore gradients, these porous sheets can be used for multi-scale and multi-fraction filtration and separation.19 For a wrinkled surface, we added ethanol into a solution of PEG-DA 575 (Figure 2i). The wrinkles were generated by the swelling of the partially polymerized surface layer caused by the oxygen inhabitation and the shrinkage of the sheet body due to the evaporation of the ethanol. The width of the wrinkles are down to 1 µm (Figure 2i) and can be used in smart adhesives,36 protein analysis,37 thin film metrology,38 and responsive microfluidic channels39. These surface textured sheets have different light transparencies due to their different surface morphologies (Figure S4). This one-step sheet synthesis technique may greatly improve current sheet material patterning processes. The existing sheet patterning techniques, such as scanning beam lithography,40, 41 photolithography,42, 43 and soft and hard mold lithography,18, 44 are either of low efficiency because of their essential batch production nature, or not cost-effective because of the requirements of mold fabrication. Our slit channel lithography will


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simplify these processes into a simple microfluidic channel and improve their efficiency by its versatile sheet synthesis ability and high throughput capacity. 3D hydrogel microstructures By combining the ability of the single-layer sheet patterning with the laminar co-flow property of microfluidics, this technique allows us to fabricate functional multilayered sheet-structures that have potential use as cell-laden hydrogel sheets with designed micro-patterns (for formation of interconnected micro-channel networks) and multiple cell types and tailored growth factor distributions (necessary for functional tissue regeneration). Figure 3 shows different sheet patterning strategies for multilayer sheetstructures and assembled 3D tissue scaffold using our technique.

Figure 3a-c demonstrates the single layer cell-laden sheet patterning. We used a three-stream slit channel to selectively incorporate cells into a hydrogel sheet (Figure 3a): 3T3 fibroblasts mixed with PEG-DA 700 water solution for the middle stream of the channel and PEG-DA 700 water solution without cells for the two side streams. Downstream of the channel, these three streams were stopped and exposed to UV light through a rectangular photomask and simultaneously polymerized into a cell-laden hydrogel sheet with a thickness of about 50 µm (Figure 3b). This ability of selective incorporation using our fabrication technique may allow for the design of cell-laden hydrogel sheets with multiple cell types and tailored growth factor distributions. By optimizing the process parameters such as PEG-DA, photoinitiator concentrations and UV exposure time,12 the maximum cell viability was retained. Figure 3c shows the fluorescent image of the cell-laden hydrogel sheet, in which cells are stained by live/dead


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cell viability assay. Cell viability examination revealed that about 75% of cells remain alive after polymerization. To build multilayered structures, we used an elongated hydrogel-sheet obtained by polymerizing overlapped edges of consecutive sheets (Figure 3d). The elongated sheet was further formed into a tubular structure by rolling the sheet around a rod (Figure 3e). The tubular hydrogel structure shown in Figure 3e has four layers. This tubular structure has potential use in vascular or nerve conduit tissue engineering. Moreover, by using laminar co-flow, the incorporation of different chemical components into one sheet is feasible with our fabrication method. This feature is useful in controlling cell and growth factor distributions in the scaffold. The sharp interface shown in Figure 3f implies that multiple chemicals and cell types can be incorporated into one hydrogel sheet with controlled lateral distribution. By rolling a sheet at a certain angle along the rod, a long tubular structure with chemical anisotropy can be obtained (Figure 3f). The continuous product collection feature of our technique may greatly simplify heterogeneous sheet patterning. Current techniques need multi-step alignment and protection procedures or complex devices to create chemically heterogeneous sheets,45, 46 making it difficult to perform in a simple and high-throughput fashion. An alternative approach to create a multilayered structure is to use an external magnetic field. Magnetic assembly of multiple layers of micro-patterned hydrogel sheets created a 3D scaffold, as seen in Figure 3g-i. We generated the magnetic hydrogel sheets by encapsulating magnetic micro-particles at two edges of the hydrogel sheets. We collected these sheets (5 mm side-length dimensions) in a rectangular water-filled PDMS reservoir for assembly (Figure 3h). By applying a magnetic field in the presence of slight


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water agitation, the sheets were dragged to the corner and aligned in the vertical direction of the reservoir (Figure 3h). A magnified image in Figure 3i shows the vertical alignment of the 60 µm square pores and formation of a thoroughfare 3D hydrogel structure, which is essential for nutrition supply and waste removal in large tissue construction. Creating individually tuned hydrogel sheets and magnetically assembling them into 3D structures offers a unique method for fabricating 3D tissue scaffold with precisely controlled distribution of cells and growth factors. One-step formation of RFID tag In addition to the aforementioned sheet patterning and structure formation, we can extend our fabrication technique to generate patterned sheets. Such sheets can be used as functional elements in devices and simplify and miniaturize many multi-step sheet assembly processes. As proof of concept, we demonstrated the feasibility of our technique with electronic packaging, particularly for RFID tag fabrication, shown in Figure 4. Assembly of RFID tags involves the attachment of the bumps on a micro-scale RFID die (Figure 4a) to a conductive antenna so that the energy collected by the antenna through the electromagnetic waves can power the die that permits sending and receiving of information to and from a remote reader. By integrating the unique particle-focusing feature of microfluidics and geometrical patterning ability of our synthesis technique, we were able to position micro-dies, fabricate the antenna and bond the die to the antenna insitu, all in a simple slit channel.


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Figure 4a shows the entire RFID tag fabrication compacted into a slit microfluidic channel. We used carbon nanotubes (CNT) as conductors in the PEG-DA solution to make the polymerized antenna electrically conductive. The RFID die, with the bumps facing down, was hydrodynamically focused in the middle of the channel by the two side streams and positioned downstream by stopping the flow. As the photomask was aligned with the die bumps, UV light was projected through the photomask (aligned with the die bumps) to polymerize the prepolymer solution for a designed antenna shape while simultaneously connecting the bumps of the die to the two ends of the antenna (Figure 4b, c). Various antenna patterns can be readily generated and bonded onto dies by changing photomasks (Supplementary Information Figure S5). Although the conductivity of the antenna (~0.5 S/m) was not high enough to electrically power the die, we can significantly improve it through several methods to reach the commercial requirements, such as efficient CNT loading and processing47,

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and using silver nanowires as

conductive filler.49 The photograph of the bottom surface of the connected area confirms the bonding between the bumps and antenna (Figure 4d). The tag inlay was made by encapsulating the tag into PEG-DA polymer by a second polymerization (Figure 4e). In current RFID tags manufacturing, antennas are first fabricated through etching or screen printing and then fed into an assembly line for packaging through multi-step procedures, generally including antenna aligning, adhesive dispensing, pick-and-place die bonding, and adhesive curing. Due to the complex and high precision antenna fabrication and die handling processes, the tag packaging contribute the most significant portion to the RFID tag manufacturing cost.50 Instead of prefabrication of the antenna and subsequent


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multistep positioning and bonding processes involved in current practice, this technique uses in-situ polymerization to condense antenna fabrication and die bonding into a single step after the facile flow focused alignment. Furthermore, by exploiting the focusing ability of microfluidics, our slit-channel lithography can easily manage much smaller RFID dies than typically used, while the current industry practice might require additional and more precision processing steps, resulting in a higher packaging cost.50 By simplifying and miniaturizing the tag fabrication process, this technique would greatly improve current RFID packaging efficiency while significantly reducing the cost of the tags, promoting the massive deployment of RFID tags.

Conclusions We have demonstrated that slit-channel lithography can pattern high AR functional sheet materials bearing geometrical and chemical anisotropy with controllable surface textures in a simple, one-step process. In addition, with designed patterns on each sheet, we were able to construct layered 3D structures for potential use in tissue engineering. Moreover, slit-channel lithography shows the ability to fabricate a functional unit of a device and simplify its assembly process as demonstrated in the dramatically miniaturized process for RFID tag fabrication by polymerizing an electrical conductive planar antenna and simultaneously bonding a microchip. With its versatile sheet material synthesis and planar structure fabrication capability, we believe slit-channel lithography will encourage more innovations with a broad range of potential uses such as with membrane-based sensors, advanced membrane synthesis, organic electronics and complex layered


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structures. Additionally, its flow production strategy and agile manufacturing ability will significantly simplify sheet material patterning processes, highly demanded in many fastevolving industries such as biotechnology and material chemistry. Supporting Information Available This information is available free of charge via the Internet at http://pubs.acs.org/. Acknowledgment The authors acknowledge discussion with Prof. Milica Radisic and Dr. Nicole Feric and for providing cells. They also thank Prof. Costin N. Antonescu for access to cell culture facilities. The authors acknowledge the Alien Technology Ltd. for providing the RFID dies. This work is supported by Natural Sciences and Engineering Research Council of Canada (Discovery grant no. RGPIN/386092). References: (1) Dendukuri, D.; Pregibon, D. C.; Collins, J.; Hatton, T. A.; Doyle, P. S. Continuousflow lithography for high-throughput microparticle synthesis. Nat. Mater. 2006, 5 (5), 365-369. (2) Dendukuri, D.; Gu, S. S.; Pregibon, D. C.; Hatton, T. A.; Doyle, P. S. Stop-flow lithography in a microfluidic device. Lab Chip 2007, 7 (7), 818-828. (3) Bong, K. W.; Pregibon, D. C.; Doyle, P. S. Lock release lithography for 3D and composite microparticles. Lab Chip 2009, 9 (7), 863-866. (4) Chung, S. E.; Park, W.; Park, H.; Yu, K.; Park, N.; Kwon, S. Optofluidic maskless lithography system for real-time synthesis of photopolymerized microstructures in microfluidic channels. Appl. Phys. Lett. 2007, 91 (4). (5) Shepherd, R. F.; Panda, P.; Bao, Z.; Sandhage, K. H.; Hatton, T. A.; Lewis, J. A.; Doyle, P. S. Stop-Flow Lithography of Colloidal, Glass, and Silicon Microcomponents. Adv. Mater. 2008, 20 (24), 4734-4739. (6) Hwang, D. K.; Oakey, J.; Toner, M.; Arthur, J. A.; Anseth, K. S.; Lee, S.; Zeiger, A.; Van Vliet, K. J.; Doyle, P. S. Stop-Flow Lithography for the Production of ShapeEvolving Degradable Microgel Particles. J. Am. Chem. Soc. 2009, 131 (12), 4499-4504. (7) Pregibon, D. C.; Toner, M.; Doyle, P. S. Multifunctional encoded particles for high-throughput biomolecule analysis. Science 2007, 315 (5817), 1393-1396. (8) Appleyard, D. C.; Chapin, S. C.; Srinivas, R. L.; Doyle, P. S. Bar-coded hydrogel microparticles for protein detection: synthesis, assay and scanning. Nat. Protoc. 2011, 6 (11), 1761-1774. (9) Chapin, S. C.; Pregibon, D. C.; Doyle, P. S. High-throughput flow alignment of barcoded hydrogel microparticles. Lab Chip 2009, 9 (21), 3100-3109.


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(10) Bong, K. W.; Chapin, S. C.; Doyle, P. S. Magnetic Barcoded Hydrogel Microparticles for Multiplexed Detection. Langmuir 2010, 26 (11), 8008-8014. (11) Choi, Y. Y.; Chung, B. G.; Lee, D. H.; Khademhosseini, A.; Kim, J. H.; Lee, S. H. Controlled-size embryoid body formation in concave microwell arrays. Biomaterials 2010, 31 (15), 4296-4303. (12) Panda, P.; Ali, S.; Lo, E.; Chung, B. G.; Hatton, T. A.; Khademhosseini, A.; Doyle, P. S. Stop-flow lithography to generate cell-laden microgel particles. Lab Chip 2008, 8 (7), 1056-1061. (13) Suh, S. K.; Yuet, K.; Hwang, D. K.; Bong, K. W.; Doyle, P. S.; Hatton, T. A. Synthesis of Nonspherical Superparamagnetic Particles: In Situ Coprecipitation of Magnetic Nanoparticles in Microgels Prepared by Stop-Flow Lithography. J. Am. Chem. Soc. 2012, 134 (17), 7337-7343. (14) Lee, S. A.; Chung, S. E.; Park, W.; Lee, S. H.; Kwon, S. Three-dimensional fabrication of heterogeneous microstructures using soft membrane deformation and optofluidic maskless lithography. Lab Chip 2009, 9 (12), 1670-1675. (15) Hakimi, N.; Tsai, S. S. H.; Cheng, C.-H.; Hwang, D. K. One-Step Two-Dimensional Microfluidics-Based Synthesis of Three-Dimensional Particles. Adv. Mater. 2014, 26 (9), 1393-1398. (16) Chung, S. E.; Park, W.; Shin, S.; Lee, S. A.; Kwon, S. Guided and fluidic selfassembly of microstructures using railed microfluidic channels. Nat. Mater. 2008, 7 (7), 581-587. (17) Dendukuri, D.; Panda, P.; Haghgooie, R.; Kim, J. M.; Hatton, T. A.; Doyle, P. S. Modeling of Oxygen-Inhibited Free Radical Photopolymerization in a PDMS Microfluidic Device. Macromolecules 2008, 41 (22), 8547-8556. (18) Xia, Y. N.; Whitesides, G. M. Soft lithography. Annual Review of Materials Science 1998, 28, 153-184. (19) Moorthy, J.; Beebe, D. J. In situ fabricated porous filters for microsystems. Lab Chip 2003, 3 (2), 62-66. (20) Wei, H. B.; Chueh, B. H.; Wu, H. L.; Hall, E. W.; Li, C. W.; Schirhagl, R.; Lin, J. M.; Zare, R. N. Particle sorting using a porous membrane in a microfluidic device. Lab Chip 2011, 11 (2), 238-245. (21) Mitrovski, S. M.; Elliott, L. C. C.; Nuzzo, R. G. Microfluidic devices for energy conversion: Planar integration and performance of a passive, fully immersed H2-O2 fuel cell. Langmuir 2004, 20 (17), 6974-6976. (22) Rettig, J. R.; Folch, A. Large-scale single-cell trapping and imaging using microwell arrays. Anal. Chem. 2005, 77 (17), 5628-5634. (23) Yamamura, S.; Kishi, H.; Tokimitsu, Y.; Kondo, S.; Honda, R.; Rao, S. R.; Omori, M.; Tamiya, E.; Muraguchi, A. Single-cell microarray for analyzing cellular response. Anal. Chem. 2005, 77 (24), 8050-8056. (24) Kim, P.; Jeong, H. E.; Khademhosseini, A.; Suh, K. Y. Fabrication of nonbiofouling polyethylene glycol micro- and nanochannels by ultraviolet-assisted irreversible sealing. Lab Chip 2006, 6 (11), 1432-1437. (25) Karp, J. M.; Yeh, J.; Eng, G.; Fukuda, J.; Blumling, J.; Suh, K. Y.; Cheng, J.; Mahdavi, A.; Borenstein, J.; Langer, R.; Khademhosseini, A. Controlling size, shape and homogeneity of embryoid bodies using poly(ethylene glycol) microwells. Lab Chip 2007, 7 (6), 786-794.


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(26) Choi, N. W.; Cabodi, M.; Held, B.; Gleghorn, J. P.; Bonassar, L. J.; Stroock, A. D. Microfluidic scaffolds for tissue engineering. Nat. Mater. 2007, 6 (11), 908-915. (27) Cuchiara, M. P.; Allen, A. C. B.; Chen, T. M.; Miller, J. S.; West, J. L. Multilayer microfluidic PEGDA hydrogels. Biomaterials 2010, 31 (21), 5491-5497. (28) Cichos, S.; Haberland, J.; Reichl, H. Performance analysis of polymer based antenna-coils for RFID. Polytronic 2002: 2Nd International Ieee Conference on Polymers and Adhesives in Microelectronics and Photonics, Conference Proceedings 2002, 120-124. (29) Kirsch, N. J.; Vacirca, N. A.; Plowman, E. E.; Kurzweg, T. P.; Fontecchio, A. K.; Dandekar, K. R. Optically Transparent Conductive Polymer RFID Meandering Dipole Antenna. Ieee Rfid: 2009 Ieee International Conference on RFID 2009, 270-274. (30) Chen, Y.; Au, J.; Kazlas, P.; Ritenour, A.; Gates, H.; McCreary, M. Electronic paper: Flexible active-matrix electronic ink display. Nature 2003, 423 (6936), 136. (31) Lee, J.; Yun, Y. K.; Kim, Y.; Jo, K. PDMS Nanoslits without Roof Collapse. Bull. Korean Chem. Soc. 2009, 30 (8), 1793-1797. (32) Hwang, Y. S.; Chung, B. G.; Ortmann, D.; Hattori, N.; Moeller, H. C.; Khademhosseini, A. Microwell-mediated control of embryoid body size regulates embryonic stem cell fate via differential expression of WNT5a and WNT11. Proc. Natl. Acad. Sci. U. S. A. 2009, 106 (40), 16978-16983. (33) Selimovic, S.; Piraino, F.; Bae, H.; Rasponi, M.; Redaelli, A.; Khademhosseini, A. Microfabricated polyester conical microwells for cell culture applications. Lab Chip 2011, 11 (14), 2325-2332. (34) Schiffenbauer, Y. S.; Kalma, Y.; Trubniykov, E.; Gal-Garber, O.; Weisz, L.; Halamish, A.; Sister, M.; Berke, G. A cell chip for sequential imaging of individual non-adherent live cells reveals transients and oscillations. Lab Chip 2009, 9 (20), 2965-2972. (35) Bostwick, J. R.; Le, W. D. A Tyrosine-Hydroxylase Assay in Microwells Using Coupled Nonenzymatic Decarboxylation of Dopa. Anal. Biochem. 1991, 192 (1), 125130. (36) Chan, E. P.; Smith, E. J.; Hayward, R. C.; Crosby, A. J. Surface wrinkles for smart adhesion. Adv. Mater. 2008, 20 (4), 711-716. (37) Chung, S.; Lee, J. H.; Moon, M. W.; Han, J.; Kamm, R. D. Non-lithographic wrinkle nanochannels for protein preconcentration. Adv. Mater. 2008, 20 (16), 3011-3016. (38) Chan, E. P.; Page, K. A.; Im, S. H.; Patton, D. L.; Huang, R.; Stafford, C. M. Viscoelastic properties of confined polymer films measured via thermal wrinkling. Soft Matter 2009, 5 (23), 4638-4641. (39) Kim, H. S.; Crosby, A. J. Solvent-Responsive Surface via Wrinkling Instability. Adv. Mater. 2011, 23 (36), 4188-4192. (40) Rensch, C.; Hell, S.; Vonschickfus, M.; Hunklinger, S. Laser Scanner for Direct Writing Lithography. Appl. Opt. 1989, 28 (17), 3754-3758. (41) Ghislain, L. P.; Elings, V. B.; Crozier, K. B.; Manalis, S. R.; Minne, S. C.; Wilder, K.; Kino, G. S.; Quate, C. F. Near-field photolithography with a solid immersion lens. Appl. Phys. Lett. 1999, 74 (4), 501-503.


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(42) Revzin, A.; Russell, R. J.; Yadavalli, V. K.; Koh, W. G.; Deister, C.; Hile, D. D.; Mellott, M. B.; Pishko, M. V. Fabrication of poly(ethylene glycol) hydrogel microstructures using photolithography. Langmuir 2001, 17 (18), 5440-5447. (43) Qin, D.; Xia, Y. N.; Whitesides, G. M. Rapid prototyping of complex structures with feature sizes larger than 20 mu m. Adv. Mater. 1996, 8 (11), 917-&. (44) Gates, B. D.; Xu, Q. B.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. New approaches to nanofabrication: Molding, printing, and other techniques. Chem. Rev. 2005, 105 (4), 1171-1196. (45) Bracher, P. J.; Gupta, M.; Mack, E. T.; Whitesides, G. M. Heterogeneous Films of Ionotropic Hydrogels Fabricated from Delivery Templates of Patterned Paper. ACS Appl. Mater. Interfaces 2009, 1 (8), 1807-1812. (46) Burdick, J. A.; Khademhosseini, A.; Langer, R. Fabrication of gradient hydrogels using a microfluidics/photopolymerization process. Langmuir 2004, 20 (13), 5153-5156. (47) Mehdipour, A.; Rosca, I. D.; Sebak, A. R.; Trueman, C.; Hoa, S. V. Carbon Nanotube Composites for Wideband Millimeter-Wave Antenna Applications. IEEE Trans. Antennas. Propag. 2011, 59 (10), 3572-3578. (48) Wu, Z. C.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Transparent, conductive carbon nanotube films. Science 2004, 305 (5688), 1273-1276. (49) Zeng, X. Y.; Zhang, Q. K.; Yu, R. M.; Lu, C. Z. A New Transparent Conductor: Silver Nanowire Film Buried at the Surface of a Transparent Polymer. Adv. Mater. 2010, 22 (40), 4484-4488. (50) Preradovic, S.; Karmakar, N. C. Low Cost Chipless RFID Systems. In Multiresonator-Based Chipless RFID; Springer US: New York, USA, 2012.


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Figure 1. Sheet synthesis using slit channel lithography. a) Schematic illustration of sheet patterning. The PDMS channel is strengthened by a glass plate to avoid sagging of the channel. A prepolymer solution in the channel is selectively polymerized by UV light through a photomask. Sheets with desirable patterns can be readily produced by changing the photomask. b) Oxygen inhabitation layers (5 µm for each layer, with 1 s UV exposure using 10X objective) at channel walls caused by oxygen diffusion through PDMS serve as boundary lubrication and ensure the smooth collection of synthesized sheets. c) SEM image of a synthesized sheet. Scale bars 400 µm (c), 200 µm (c inset).


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Figure 2. Versatile functional sheet synthesis using a channel with height 30 µm and width 8mm (AR=267). a) Bright field image of a Pacman-arena styled sheet, b) Fluorescent image of a fingerprint patterned sheet, and c) DIC image of a sheet with a pore size gradient ranging from 5 to 300 µm. d) Illustration of cylindrical and cone shape micro-pores fabrication process. Cylindrical pores are obtained by focusing a photomask in the middle of the channel while cone shaped pores are formed by lowering the focal plane. e) and f) are the SEM images of the cylindrical and cone shaped pores in the sheets respectively. g-i) are the SEM image of sheets with different surface morphologies. These


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sheets are made by changing the concentrations of ethanol in PEG-DA. g) 100% PEGDA 575 and i) 60% ethanol, 35% PEG-DA 575. Both are with 5% Darcure 1173. h) 60% ethanol, 39% PEG-DA 200 and 1% Darcure 1173. Scale bars 1 mm (a-c), 40 µm (e, f), 2 µm (g, h) and 5 µm (i).


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Figure 3. Cell encapsulated tissue sheet and the assembly of 3D tissue scaffolds using a channel with height 60 µm and width 8mm. a) Illustration of synthesis of tissue sheet with patterned cell layers. Two side streams are PEG-DA water solution and the middle stream is PEG-DA solution in PBS with NIH 3T3 cells. b) Bright field image of a cellladen sheet. Two sides of the sheet are PEG-DA hydrogel layers and the middle layer is the encapsulated cell. c) Fluorescent image of the tissue sheet in b) for the cell viability expressed by calcein AM (live cells, green) and ethidium homodimer (dead cells, orange). d) Schematic of tubular hydrogel scaffold formation by rolling an elongated hydrogel sheet on a glass rod. After one piece of hydrogel is formed by UV exposure, the


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projection area is moved upstream so that the next piece of hydrogel can be formed with their edges connected. e) A fluorescent image of the tubular hydrogel scaffold. f) A fluorescent image of double-stream elongated sheets rolled around a metal tube at a certain angle for forming a long hydrogel tube. g) Illustration of hydrogel sheet formation with magnetic particles in the outer layers. Two side streams are PEG-DA water solution with 10% magnetic suspension and the middle streams are PEG-DA water solution. After making three of these magnetic hydrogel sheets, they are assembled in a PDMS reservoir at a right-angled corner by using a magnetic field (h). Inset is the three sheets before assembly. i) The pores are aligned in the vertical direction. Scale bars 500 µm (b, c), 1 mm (e, h), 400 µm (f) and 100 µm (i).


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Figure 4. One-step fabrication of RFID tags using a channel with height 150 µm and width 8mm. a) Schematic illustration of the flow lithography fabrication process. After focusing RFID microchips in the middle of the channel by two sheath flows, the flow is stopped. Then UV is exposed to the channel through an antenna-shaped photomask to polymerize the CNT prepolymer solution, simultaneously bonding the two bumps to the antenna. b) Microchip focusing and bonding process. Left is a bright field image of a microchip entering the sheath flow. Right is a DIC image of a polymerized antenna connected to the RFID chip. c) Bright field image of the fabricated RFID tag. d) Photographs of top and bottom views of the microchip bonding area. e) Photograph of the RFID tag inlay. After cleaning the bonded RFID tag, it is immersed in PEG-DA prepolymer solution and encapsulated inside by polymerizing prepolymer solution. Scale bars 1mm (b, c) and 500 µm (d).


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Table of Contents graphic

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Figure 1. Sheet synthesis using slit channel lithography. a) Schematic illustration of sheet patterning. The PDMS channel is strengthened by a glass plate to avoid sagging of the channel. A prepolymer solution in the channel is selectively polymerized by UV light through a photomask. Sheets with desirable patterns can be readily produced by changing the photomask. b) Oxygen inhabitation layers (5 µm for each layer, with 1 s UV exposure using 10X objective) at channel walls caused by oxygen diffusion through PDMS serve as boundary lubrication and ensure the smooth collection of synthesized sheets. c) SEM image of a synthesized sheet. Scale bars 400 µm (c), 200 µm (c inset). 1183x731mm (72 x 72 DPI)


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Figure 2. Versatile functional sheet synthesis using a channel with height 30 µm and width 8mm (AR=267). a) Bright field image of a Pacman-arena styled sheet, b) Fluorescent image of a fingerprint patterned sheet, and c) DIC image of a sheet with a pore size gradient ranging from 5 to 300 µm. d) Illustration of cylindrical and cone shape micro-pores fabrication process. Cylindrical pores are obtained by focusing a photomask in the middle of the channel while cone shaped pores are formed by lowering the focal plane. e) and f) are the SEM images of the cylindrical and cone shaped pores in the sheets respectively. g-i) are the SEM image of sheets with different surface morphologies. These sheets are made by changing the concentrations of ethanol in PEG-DA. g) 100% PEG-DA 575 and i) 60% ethanol, 35% PEG-DA 575. Both are with 5% Darcure 1173. h) 60% ethanol, 39% PEG-DA 200 and 1% Darcure 1173. Scale bars 1 mm (a-c), 40 µm (e, f), 2 µm (g, h) and 5 µm (i). 183x182mm (300 x 300 DPI)


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Figure 3. Cell encapsulated tissue sheet and the assembly of 3D tissue scaffolds using a channel with height 60 µm and width 8mm. a) Illustration of synthesis of tissue sheet with patterned cell layers. Two side streams are PEG-DA water solution and the middle stream is PEG-DA solution in PBS with NIH 3T3 cells. b) Bright field image of a cell-laden sheet. Two sides of the sheet are PEG-DA hydrogel layers and the middle layer is the encapsulated cell. c) Fluorescent image of the tissue sheet in b) for the cell viability expressed by calcein AM (live cells, green) and ethidium homodimer (dead cells, orange). d) Schematic of tubular hydrogel scaffold formation by rolling an elongated hydrogel sheet on a glass rod. After one piece of hydrogel is formed by UV exposure, the projection area is moved upstream so that the next piece of hydrogel can be formed with their edges connected. e) A fluorescent image of the tubular hydrogel scaffold. f) A fluorescent image of double-stream elongated sheets rolled around a metal tube at a certain angle for forming a long hydrogel tube. g) Illustration of hydrogel sheet formation with magnetic particles in the outer layers. Two side streams are PEG-DA water solution with 10% magnetic suspension and the middle streams are PEG-DA water solution. After making three of these magnetic hydrogel sheets, they are assembled in a PDMS reservoir at a right-angled corner by using a magnetic field (h). Inset is the three sheets before assembly. i) The pores are aligned in the vertical direction. Scale bars 500 µm (b, c), 1 mm (e, h), 400 µm (f) and 100 µm (i). 184x137mm (300 x 300 DPI)


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Figure 4. One-step fabrication of RFID tags using a channel with height 150 µm and width 8mm. a) Schematic illustration of the flow lithography fabrication process. After focusing RFID microchips in the middle of the channel by two sheath flows, the flow is stopped. Then UV is exposed to the channel through an antenna-shaped photomask to polymerize the CNT prepolymer solution, simultaneously bonding the two bumps to the antenna. b) Microchip focusing and bonding process. Left is a bright field image of a microchip entering the sheath flow. Right is a DIC image of a polymerized antenna connected to the RFID chip. c) Bright field image of the fabricated RFID tag. d) Photographs of top and bottom views of the microchip bonding area. e) Photograph of the RFID tag inlay. After cleaning the bonded RFID tag, it is immersed in PEG-DA prepolymer solution and encapsulated inside by polymerizing prepolymer solution. Scale bars 1mm (b, c) and 500 µm (d). 176x105mm (300 x 300 DPI)


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619x506mm (72 x 72 DPI)


Functional Polymer Sheet Patterning Using Microfluidics - American

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