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Oxygen-Inhibition Lithography for the Fabrication of Multipolymeric Structures Alessandra Vitale,* Marzia Quaglio,* Angelica Chiodoni, Katarzyna Bejtka, Matteo Cocuzza, Candido F. Pirri, and Roberta Bongiovanni The use of polymeric patterned structures and devices is nowadays essential for a number of different applications, including microfluidics,[1–3] biotechnology,[4–7] surface science,[8,9] and energy storage and production.[10–12] Although current microfabrication technologies, such as standard lithographic and printing techniques, have proved to be very successful in many cases, they still present several limitations. For instance, they are generally costly and require either a complicated series of subsequent steps or the fabrication of high-resolution molds, being therefore unsuitable for rapid prototyping. Moreover they may limit the selection of the feature of the final pattern, and in most cases they allow to process only one polymer at a time, producing exclusively monomaterial structures. The fabrication of multifunctional and multimaterial patterns or devices is proving to be important for various applications,[13, 14] in order to control and localize wettability,[15] bioadhesion,[16] mechanical[17] and swelling[18] properties. Eventually, when the devices are made of different materials, the issue of a good adhesion among the layers has still to be solved. The development of new high-throughput manufacturing technologies with low cost is therefore required. Different critical issues such as reliability, speed, and overlay accuracy need to be considered in developing new fabrication techniques. Other crucial requisites are versatility (e.g., the capability to produce structures whose properties can be tailored for specific applications) and simplicity (e.g., no need of sophisticated equipment, and possibility to carry out the process at room temperature and without employing solvents). As the final pattern dimensions should Dr. A. Vitale,[+] Dr. M. Cocuzza, Prof. C. F. Pirri, Prof. R. Bongiovanni Department of Applied Science and Technology Politecnico di Torino 10129 Torino, Italy E-mail: [email protected] Dr. M. Quaglio, Dr. A. Chiodoni, Dr. K. Bejtka, Dr. M. Cocuzza, Prof. C. F. Pirri Center for Space Human Robotics@PoliTo Istituto Italiano di Tecnologia 10129 Torino, Italy E-mail: [email protected] Dr. M. Cocuzza CNR-IMEM 43124 Parma, Italy Prof. R. Bongiovanni INSTM RU Politecnico di Torino 10129 Torino, Italy [+]Present address: Department of Chemical Engineering, Imperial College London, SW7 2AZ London, UK
DOI: 10.1002/adma.201501737
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be carefully engineered with regard to the specific application, the resolution capability of the manufacturing process has to be designed accordingly. An ideal technology should be capable of producing multiscale features (i.e., both large and small, as well as thin and thick) in various combinations and distributions, as in real applications microdevices usually present a mixture of arbitrary patterns with various sizes. Moreover, creating multipolymeric patterned surfaces where different chemistries are involved is a very challenging task. Herein, we propose a new highly innovative fabrication technology meeting the above described needs for an efficient fabrication of multipolymeric patterned structures. This advanced manufacturing technology, named oxygen-inhibition lithography (OIL), is a photolithographic technique that exploits the inhibitory effects of oxygen during UV curing in air to fabricate multiscale and multimaterial patterns and devices. Although oxygen inhibition is generally treated as a problem in photopolymerization and photocuring[19,20] and for fabrication techniques that imply the photoinduced free radical polymerization of resins in air, the formation of an inhibition layer (IL) can be exploited to control the polymer surface and can thus turn to be advantageous.[21,22] For instance, Jeong et al.[23] obtained hierarchical patterns on micro- and nanolayers using a two-step capillary molding process, and Chandra and Crosby[24] used this inhibition phenomenon to make wrinkles on the surface of a UV-cured film. Moreover, very recently Tumbleston et al.[25] proposed a new continuous stereolithography technology based on oxygen inhibition. However, none of these techniques offers a simultaneous control over pattern shape, dimensions, and composition. When a photocurable resin is exposed to UV light in atmospheric conditions (i.e., in the presence of oxygen), only the bulk is cured becoming a solid crosslinked network, while a layer of uncured liquid monomer (i.e., IL) remains on the top of the sample surface, as shown in Figure 1a (see Figure S1 in the Supporting Information for more details). In the OIL technology, the formation of the IL is coupled with a photolithographic approach to generate polymeric patterns, simply selecting the areas to be UV irradiated and consequently converted into solid polymer (Figure 1b). As long as the UV exposure is performed in air, an IL showing the same reactive groups of the starting liquid resin (Figure S2 in the Supporting Information) is present, and can be further processed by sequential photolithographic steps. This layer is the main building block for the OIL process due to the reactive functionalities still active on its free surface. If the UV irradiation is instead performed in an inert atmosphere (e.g., nitrogen) also the surface layer is cured, thus becoming no more processable. The OIL technology is based on subsequent UV exposure steps in air with different
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photomasks (accurately selected depending on the geometry to be obtained), and a last UV exposure under nitrogen to completely and definitely cure selected regions. Then, with a final development step, the uncured resin still remaining on or within the structure is selectively removed. The OIL technology is extremely versatile and simple; in fact, neither expensive equipment nor cleanroom conditions are needed. It enables to obtain multiscale (from micro- to millimeter sizes) open-faced-patterned structures as well as closed devices with high reproducibility, low cost and very rapidly, exploiting the speed of photopolymerization reaction. OIL can be applied to all materials polymerizing through a radical mechanism, namely to the family of (meth)acrylates, which includes a number of different commercially available polymers used in several application fields,[26,27] such as coatings and adhesives,[28] dental and biomedical applications,[29, 30] patterning and 3D prototyping.[31,32] Among the (meth) acrylates, there are resins that show different polymerization kinetics (bearing acrylic or methacrylic functional end groups) and various final bulk and surface properties (due to the chemistry of the chain backbone). In this paper we use poly(ethylene glycol) diacrylate (PEG-DA), poly(ethylene glycol) dimethacrylate (PEG-DMA), poly(dimethylsiloxane) diacrylate (PDMSDA), and perfluoropolyether dimethacrylate (PFPE-DMA) as model resins. The former two polymers are hydrophilic and
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Figure 1. Mechanism of formation and processing of the oxygen-inhibited layer by OIL. a) Schematic representation of the UV curing reaction of multifunctional (meth)acrylic monomers, which are made of a backbone chain and two (meth)acrylic functionalities, in the presence of oxygen. When the resins are UV irradiated, the bulk of the sample is transformed into a crosslinked network, while the surface remains liquid keeping unmodified its chemical and physical behavior. b) Coupling of the formation of the IL with a photolithographic approach to generate polymeric patterns. The IL is patterned by a UV irradiation step in air in the presence of a photomask, thus creating polymeric patterns on top of a polymeric substrate. As the exposure is performed in an ambient atmosphere, reactive functional groups are still active on the sample surface even after irradiation. A simplified representation of the basic process flow is reported in the Supporting Information.
biocompatible,[33,34] PDMS-based materials are often used in micro- and nanoreplication processes due to their low Young’s modulus,[35,36] while PFPEs are characterized by hydro- and oleophobicity and very high chemical resistance.[37, 38] At first the experiments were addressed to characterize the oxygen-inhibited layer: its understanding is fundamental for the OIL fabrication process as it corresponds to the final pattern height. The IL thickness depends on the partial pressure of oxygen surrounding the sample, the rate of diffusion of oxygen into the resin, the photoinitiator concentration, the sample thickness, as well as the UV curing conditions (i.e., light intensity and time of irradiation).[39] Figure 2a shows the decrease of IL thickness with exposure time for PEG-DA, with the IL becoming thinner as the light intensity increases. In the domain of conditions explored, the height of PEG-DA structures ranged from 2 µm up to 75 µm. Very thin IL thicknesses were obtained due to hydroperoxide decomposition[40] during photocuring (Figure S3 in the Supporting Information). Data collapse into a single curve (Figure 2b) by expressing the IL thickness values as a function of UV dose d, which is defined as the product of exposure time t and incident light intensity I0 (i.e., d ≡ t I0). Other factors remaining constant, the scaling of the oxygen-inhibited layer thickness with the dose d is given by Equation (1): (1)
where −γ corresponds to the slope of the power law and is equal to −1.77. This relationship is similar to the one proposed by Dendukuri et al.[41] and revisited by Tumbleston et al.[25] (see Figure S3 in the Supporting Information). Figure 2a also demonstrates that structures about 10–20 µm wide could be easily transferred even when arranged in complex patterns. All features are precise and well reproducible, showing actual dimensions almost identical to the nominal ones on the photomask (the maximum variation is close to 1%). The thickness of the patterns is always consistent, demonstrating the independence of the IL thickness on the photomask used. The monolithic structures obtained by OIL do not show any interface between the bulk and the patterned layer, which are chemically bonded, ensuring an excellent cohesion and granting the best mechanical and chemical performance (see the Supporting Information). Figure 2c shows the evolution of the IL thickness with irradiation time for each resin under investigation, for constant light intensity. The IL thickness diminishes while increasing UV irradiation time; however, different values were obtained depending on the chemistry of the materials. The results presented in Figure 2c demonstrate that by selecting the resin
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Figure 2. OIL technology for the fabrication of all-polymeric patterned structures. a) IL thickness as a function of irradiation time at three different light intensities (1.37, 2.74, and 6.86 mW cm−2) for PEG-DA. The insets represent two examples of pattern transfer onto the IL, showing 20 µm wide pillars and 25 µm wide parallel lines. Images were acquired by a field-emission scanning electron microscope (FESEM). b) The results in (a) are collapsed into one curve by plotting in log–log scale the IL thickness versus the dose d. The inset details the power dependence of IL thickness by the light intensity, for different exposure times t. c) Comparison of oxygen inhibition for different (meth)acrylic resins: IL thickness as a function of irradiation time for UV light intensity I = 2.74 mW cm−2 for PEG-DA (䊉), PEG-DMA (䊏), PDMS-DA (䉱), and PFPE-DMA (䉬). d) Master curve representing the IL thickness for different polymers. The inset details the dependence of the IL thickness by the reactivity of the resin k and the oxygen solubility S and diffusivity D in it. The error bars represent the error of at least five measurements of the same conditions; when the error bar is not visible on data points of graphs in (a) and (c), it means that it is within the symbol size. The maximum error found among all studied resins and UV curing conditions explored is lower than 4%. Additional schematics of the process flow describing the OIL technology are reported in Figure S4 in the Supporting Information.
functionality or its main chain it is possible to tune the IL thickness, generating patterns whose thickness ranges from few up to hundreds of micrometers. These control parameters are related to the oxygen-inhibited layer thickness according to a simple power law (Figure 2d): IL thickness = Cd −γ
(2)
where C is a proportionality constant for a fixed material system, which is dependent on the monomer reactivity k and the oxygen solubility S and diffusivity D into the liquid resin. Increasing the reactivity of the resin or decreasing its capability to dissolve oxygen, as well as its permeability to this gas, makes the IL thinner. In fact, methacrylic resins resulted in higher IL thicknesses since they have a slower polymerization kinetics compared to acrylics; siloxanes and fluorinated resins provided higher IL thicknesses since they dissolve more oxygen[42] compared to hydrogenated monomers (i.e., PEG-DA and PEG-DMA). With these model resins we demonstrate that OIL is suitable for processing a broad range of materials. The physicochemical nature of the polymers and the properties of the patterned structures (e.g., wettability, mechanical strength and elasticity, protein adsorption) can be selected on demand, depending on the final application.
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Being able to control the IL, the OIL technology opens a novel route to the fabrication and patterning of multiscale systems and devices with tunable functionalities and thicknesses. This latter aspect is fundamental as the pattern dimensions have to be carefully designed with regard to the specific device application. A common way to create structures with a high thickness is to resort to multilayers. As the interlayer adhesion is a critical issue, frequently surface treatments[43] and functionalization reactions[44] are required to obtain well-bonded layers. With the here presented technology there is not such a problem, since the IL itself acts as an adhesion promoter. In fact, as already shown in Figure 2, for a given resin it is possible to dramatically reduce the IL thickness by increasing the light intensity. Then, fresh monomer can be added on top of the thin undercured layer to be patterned. This approach is a layer-bylayer process, evidently close to additive manufacturing concept. Figure 3a schematically illustrates the OIL technology for the fabrication of multilayer-based structures taking advantage of oxygen inhibition. In this process the height of the pattern is not limited by the UV exposure conditions, but depends only on the amount of liquid resin that is added for the construction of each layer. During the irradiation step in the presence of a photomask, strong chemical bonds are generated between two layers and continuous thick patterned films are formed. As shown in Figure 3a, monolithic structures with thickness up to
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to generate multimaterial structures, where each layer contains a different chemistry. This opens the possibility to combine in the same device different sets of properties belonging to different polymers, optimizing the behavior of the system. Even in this case, during each UV exposure, chemical bonds are formed between the layers although they are not made of the same polymer, ensuring an excellent adhesion (Figure S6 and S7 and Movie S1 in the Supporting Information). As an example of a multimaterial and multifunctional structure, Figure 3b shows a nice lotus flower floating on water. The water and the lotus petals are made of hydrophilic PEG-DMA and PEG-DA, respectively, while the leaf is made of hydrophobic PFPE-DMA. Each layer of the structure is characterized by a different wettability, however a good quality of the interface between the different materials is ensured. Since highly fluorinated polymers such as PFPEs usually exhibit low adhesion due to their very low surface energy,[45] it is worth noting the good adhesion between PFPE-DMA and both PEG-DA and PEG-DMA without application of a pretreatment. With the OIL technology each layer of the patterned structure can be finely tuned, choosing its chemical composition and as a consequence its physical and chemical properties. Multifunctional samples can hence be created without using complicate surface chemistry-based treatments (e.g., molecular grafting, polymer brushes, self-assembly monolayers), highly defining topographical and chemical features in the micrometer range. In addition, the OIL technology enables the fabrication of closed and embedded structures by a precise control of the photopolymFigure 3. OIL technology for the fabrication of mono- and multimaterial thick polymeric struc- erization reaction. After obtaining a complex tures. a) Schematic view of the process flow of OIL to produce patterned layers with high thick- micropatterned structure, the last exposure nesses. Examples of PEG-DA and PEG-DMA thick structures obtained through OIL are also step can be performed in nitrogen atmosphere shown. The PEG-DA circular pillars have a diameter of 200 µm and a height of 1.5 mm (left and used to close the device (Figure 4a). For structure) and 1.2 mm (right structure). The PEG-DMA squared pillars are 200 µm wide and 1.4 mm high. b) Schematic view of the process flow of the OIL process to produce multimate- this step the exposure time should be finely rial patterned structures. The image shows a lotus flower floating on water. The flower is made controlled in order to cure only a desired surof PEG-DA, the leaf of PFPE-DMA, and the water of PEG-DMA. Colorants were added to the face thickness, without completely closing the reactive UV curable mixtures prior to photopolymerization. The image shows the hydrophilic pattern structures. Due to absorption of the character of PEG-DA (contact angle with water = 51.5°) and the hydrophobic behavior of PFPE- incident light by the external layer, the photoDMA (contact angle with water = 114°). Additional schematics of the process flow describing sensitive resin within the structures does not the OIL technology are reported in Figure S4 in the Supporting Information. receive a sufficient dose to cure. Examples of this kind of systems are quite interesting for application fields as microfluidics. Figure 4 shows two conmillimeters can be easily fabricated with high accuracy (see also veniently produced all-polymeric microfluidic devices with 950 Figure S5 in the Supporting Information). Selecting the proper and 50 µm wide (Figure 4b,c, respectively) squared microchanmaterial and the desired pattern thickness, one can simply fabnels, fabricated by OIL. It can be noticed that the last UV curing ricate thick structures showing the appropriate functional propstep (responsible for obtaining close-faced structures) does not erties for the chosen application. modify the pattern geometry, which is maintained regular and Even more interestingly, OIL can easily work also if difprecise. Both entire devices were fabricated in less than 5 min. ferent resins are added one over the other (Figure 3b) in order
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In conclusion, we presented a novel photolithographic technology that takes advantage of the oxygen-inhibition phenomenon and exploits it to fabricate multiscale and multimaterial structures, such as polymeric openfaced patterns and close-faced devices. The uncured liquid layer, which is spontaneously generated on the surface of a crosslinked polymer when UV curing is performed in air, can be further processed by sequential photolithographic steps. We demonstrate that OIL is a general patterning route for (meth-) acrylic resins that can be used to obtain wellcontrolled patterns by tuning the curing kinetics and parameters. Exposure time, UV light intensity, and material chemistry were selected as critical variables to finely control the thickness of the IL. The OIL technology is a simple approach that controls with high fidelity both surface topography and functionality over a variety of feature sizes and Figure 4. Fabrication of embedded structures by the OIL technology. a) Schematic view of the process flow of the OIL technology for the fabrication of all-polymeric-closed patterned struc- heights. We demonstrate the generation of tures. b) Example of a microfluidic device made of PFPE-DMA, filled with colored water-based devices with sizes in the range of centimsolution. Squared channels are 950 µm wide and high. c) Cross-section of a microfluidic device eters, showing a minimal features resolumade of PFPE-DMA, with microchannels 50 µm wide and high, observed by optical microtion of 10 µm. OIL enables also the fabricascopy. d) Multimaterial microfluidic device for emulsion separation. First, the emulsion, which is formed by droplets of an aqueous solution (Aq) in a continuous oil flow (Oil), is created tion of multimaterial and multifunctional (enlargement zone 1) by a T-junction in a PDMS-DA channel. Then, in the separation area, the structures of varied and complex shapes, containing different compositions and thus channel is formed by a PDMS-DA side and a PEG-DA side and this allows the streams of oil (red) and water (blue) to flow separately (enlargement zone 2). Dyes were added to water- and showing a synergy of properties. With the oil-based solutions to increase the contrast. The insets are optical images of two representative OIL technology polymer-based devices with areas of the device. Details on the fabrication process of the microfluidic devices are reported prescribed dimensions can be structured in the Supporting Information. simply, without any further use of specialized microfabrication equipment, and very rapidly (i.e., in few minutes). Excellent interface and adhesion between Embedded structures can be successfully created by OIL the patterned layers are guaranteed, even in the fabrication of also into multimaterial systems: one can thereby select and multimaterial structures. This technology offers immediate tune the physicochemical properties of each layer of the device. advantages over other conventional microfabrication techAs an exemplar, we fabricated a multipolymeric microfluidic niques. Thanks to its generality, robustness, reproducibility, device for efficient separation of water-in-oil emulsions into and cost-efficiency, it undoubtedly has potential applications in their individual components (Figure 4d), which can selectively areas such as biotechnology, microfluidics, flexible electronics, interact with the different materials forming the microchansensors, and energy devices. nels. The microfluidic device shown in Figure 4d includes a first part where the emulsion is formed, and a second part where separation occurs due to surface tension differences (further details on the separation device fabrication and operaExperimental Section tion are provided in the Supporting Information, Figure S8). In fact the rectangular microchannel is defined by a hydrophilic Materials: PEG-DA with a molecular weight of 700 g mol−1 and PEG-DMA with a molecular weight of 750 g mol−1 were purchased from PEG-DA boundary and a hydrophobic PDMS-DA boundary. Sigma–Aldrich and used as received. Fluorolink MD700 (PFPE-DMA) The different wettability properties of these two resins are used with a molecular weight of 1800 g mol−1 was kindly provided by Solvay– to stabilize the adjacent concurrent laminar flows of the two Solexis. Ebecryl 350 (PDMS-DA) with a molecular weight of 1500 g mol−1 immiscible phases: the aqueous stream (Aq) is stabilized along was obtained from Allnex (formerly Cytec Ltd). 2-hydroxy-2-methyl-1the PEG-DA channel side due to hydrophilic interactions, while phenyl-propan-1-one (Darocur 1173, Sigma–Aldrich) was chosen as the organic stream (Oil) is stabilized along the PDMS-DA side photoinitiator and used at 2 wt%. Sylgard 184 PDMS elastomer kit was purchased from Dow Corning. All other chemicals were obtained from due to weak nonpolar interactions. Thus, the device fabricated Sigma–Aldrich. by the OIL technology is a passive system that allows emulPatterning of the Oxygen-Inhibited Layer: The photocurable mixture sion separation without resorting to filtration, complex channel (containing no solvent, only the monomer and the photoinitiator) geometries (to control the hydrodynamic forces), application of was poured into 1.5 cm × 2 cm molds with heights ranging from external fields, or chemical surface modification. This exemplar 0.5 to 3 mm; the thickness of the resin layer was set by the depth of demonstrates that multimaterial and multifunctional 3D structhe mold employed and confirmed by a volumetric control. The system was irradiated in air by means of a fiber optic UV lamp (Hamamatsu tures can be easily built up by OIL in a layer-by-layer fashion.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The work of A.V. was partially funded by Consorzio INSTM (Firenze, Italy). Received: April 13, 2015 Revised: May 14, 2015 Published online: July 14, 2015
[1] S. Xu, Y. Zhang, L. Jia, K. E. Mathewson, K.-I. Jang, J. Kim, H. Fu, X. Huang, P. Chava, R. Wang, S. Bhole, L. Wang, Y. J. Na, Y. Guan, M. Flavin, Z. Han, Y. Huang, J. A. Rogers, Science 2014, 344, 70. [2] L. Gervais, N. de Rooij, E. Delamarche, Adv. Mater. 2011, 23, H151. [3] P. Neuzil, S. Giselbrecht, K. Lange, T. J. Huang, A. Manz, Nat. Rev. Drug Discov. 2012, 11, 620. [4] P. Zorlutuna, N. Annabi, G. Camci-Unal, M. Nikkhah, J. M. Cha, J. W. Nichol, A. Manbachi, H. Bae, S. Chen, A. Khademhosseini, Adv. Mater. 2012, 24, 1782. [5] Y. Shao, J. Fu, Adv. Mater. 2014, 26, 1494. [6] B. Derby, Science 2012, 338, 921. [7] K.-Y. Suh, M. C. Park, P. Kim, Adv. Funct. Mater. 2009, 19, 2699. [8] E. Ueda, P. A. Levkin, Adv. Mater. 2013, 25, 1234. [9] A. M. Brzozowska, F. J. Parra-Velandia, R. Quintana, Z. Xiaoying, S. S. C. Lee, L. Chin-Sing, D. Jan´czewski, S. L. M. Teo, J. G. Vancso, Langmuir 2014, 30, 9165. [10] Q. Zhang, Y. Sun, W. Xu, D. Zhu, Adv. Mater. 2014, 26, 6829. [11] J. Kong, S. Song, M. Yoo, G. Y. Lee, O. Kwon, J. K. Park, H. Back, G. Kim, S. H. Lee, H. Suh, K. Lee, Nat. Commun. 2014, 5, 5688.
Adv. Mater. 2015, 27, 4560–4565
[12] H. Kang, S. Jung, S. Jeong, G. Kim, K. Lee, Nat. Commun. 2015, 6, 6503. [13] K.-H. Roh, D. C. Martin, J. Lahann, Nat. Mater. 2005, 4, 759. [14] C. Liao, M. Zhang, M. Y. Yao, T. Hua, L. Li, F. Yan, Adv. Mater. 2014, DOI: 10.1002/adma201402625. [15] Q. Cheng, M. Li, Y. Zheng, B. Su, S. Wang, L. Jiang, Soft Matter 2011, 7, 5948. [16] R. Ogaki, M. Alexander, P. Kingshott, Mater. Today 2010, 13, 22. [17] S. P. R. Kobaku, G. Kwon, A. K. Kota, R. G. Karunakaran, P. Wong, D. H. Lee, A. Tuteja, ACS Appl. Mater. Interfaces 2015, 7, 4075. [18] J. Yom, S. M. Lane, R. A. Vaia, Soft Matter 2012, 8, 12009. [19] Y. Yagci, S. Jockusch, N. J. Turro, Macromolecules 2010, 43, 6245. [20] S. C. Ligon, B. Husár, H. Wutzel, R. Holman, R. Liska, Chem. Rev. 2014, 114, 557. [21] D. Dendukuri, D. C. Pregibon, J. Collins, T. A. Hatton, P. S. Doyle, Nat. Mater. 2006, 5, 365. [22] J. Shi, M. B. Chan-Park, C. Gong, H. Yang, Y. Gan, C. M. Li, Chem. Mater. 2010, 22, 2341. [23] H. E. Jeong, R. Kwak, J. K. Kim, K. Y. Suh, Small 2008, 4, 1913. [24] D. Chandra, A. J. Crosby, Adv. Mater. 2011, 23, 3441. [25] J. R. Tumbleston, D. Shirvanyants, N. Ermoshkin, R. Janusziewicz, A. R. Johnson, D. Kelly, K. Chen, R. Pinschmidt, J. P. Rolland, A. Ermoshkin, E. T. Samulski, J. M. DeSimone, Science 2015, 347, 1349. [26] C. N. Bowman, C. J. Kloxin, AlChE J. 2008, 54, 2775. [27] J.-P. Fouassier, X. Allonas, Basics and Applications of Photopolymerization Reactions, Research Signpost, Trivandrum, India 2010. [28] R. S. Davidson, Exploring the Science, Technology and Applications of UV and EB Curing, Sita Tecnology Limited, London, UK 1999. [29] M. H. Bland, N. A. Peppas, Biomaterials 1996, 17, 1109. [30] N. Moszner, T. Hirt, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 4369. [31] S. S. Williams, S. Retterer, R. Lopez, R. Ruiz, E. T. Samulski, J. M. DeSimone, Nano Lett. 2010, 10, 1421. [32] A. Vitale, M. Quaglio, S. L. Marasso, A. Chiodoni, M. Cocuzza, R. Bongiovanni, Langmuir 2013, 29, 15711. [33] M. S. Hahn, J. S. Miller, J. L. West, Adv. Mater. 2006, 18, 2679. [34] G. K. Such, Y. Yan, A. P. R. Johnston, S. T. Gunawan, F. Caruso, Adv. Mater. 2015, 27, 2278. [35] Y. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550. [36] A. Carlson, A. M. Bowen, Y. Huang, R. G. Nuzzo, J. A. Rogers, Adv. Mater. 2012, 24, 5284. [37] J. P. Rolland, R. M. Van Dam, D. A. Schorzman, S. R. Quake, J. M. DeSimone, J. Am. Chem. Soc. 2004, 126, 2322. [38] A. Vitale, M. Quaglio, M. Cocuzza, C. F. Pirri, R. Bongiovanni, Eur. Polym. J. 2012, 48, 1118. [39] V. V. Krongauz, C. P. Chawla, J. Dupre, in Photoinitiated Polymerization, Vol. 847 (Eds: K. D. Belfield, J. V. Crivello), American Chemical Society, Washington DC, USA 2003, p.165. [40] R. Pynaert, J. Buguet, C. Croutxe-Barghorn, P. Moireau, X. Allonas, Polym. Chem. 2013, 4, 2475. [41] D. Dendukuri, P. Panda, R. Haghgooie, J. M. Kim, T. A. Hatton, P. S. Doyle, Macromolecules 2008, 41, 8547. [42] M. F. Costa Gomes, J. Deschamps, D. H. Menz, J. Fluorine Chem. 2004, 125, 1325. [43] J. Zhou, A. V. Ellis, N. H. Voelcker, Electrophoresis 2010, 31, 2. [44] T. Rohr, D. F. Ogletree, F. Svec, J. M. J. Fréchet, Adv. Funct. Mater. 2003, 13, 264. [45] A. Vitale, A. Priola, C. Tonelli, R. Bongiovanni, Int. J. Adhes. Adhes. 2014, 48, 303.
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LC8). The following exposure steps were performed in the presence of a photomask. UV light intensity (1–10 mW cm−2) and exposure time were varied. Oxygen partial pressure of the system was 20.7 kPa. The photolithographic process was optimized using both Mylar and quartz masks (further details are given in the Supporting Information): different photomasks with features ranging from 10 to 500 µm were used. The final exposure step was carried out in nitrogen atmosphere. After multiple UV light exposures, the samples were developed by soaking them in a solvent for few seconds (no more than 30 s) to remove the uncured monomer. Development was operated in acetone and ethanol for PFPE-DMA and in isopropyl alcohol for PEG-DA, PEG-DMA, and PDMS-DA. For the preparation of colored patterns, colorants were added to the monomer before the first UV curing step. Additional details on the photopatterning process are reported in the Supporting Information. PDMS molds were fabricated by casting (pouring 10:1 wt/wt monomer/crosslinker mixture, followed by baking at 75 °C for 3 h). PMMA molds were fabricated by milling. Pattern Analysis: Patterns were analyzed by a Nikon Eclipse ME600 optical microscope and a ZEISS Dual Beam Auriga FESEM. Prior to FESEM examination samples were coated with an 8 nm thick Au film via sputtering. The thickness of the oxygen-inhibited layer was determined by imaging the cross-section of the patterns.
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Oxygen-Inhibition Lithography for the Fabrication of Multipolymeric Structures Alessandra Vitale,* Marzia Quaglio,* Angelica Chiodoni, Katarzyna Bejtka, Matteo Cocuzza, Candido F. Pirri, and Roberta Bongiovanni The use of polymeric patterned structures and devices is nowadays essential for a number of different applications, including microfluidics,[1–3] biotechnology,[4–7] surface science,[8,9] and energy storage and production.[10–12] Although current microfabrication technologies, such as standard lithographic and printing techniques, have proved to be very successful in many cases, they still present several limitations. For instance, they are generally costly and require either a complicated series of subsequent steps or the fabrication of high-resolution molds, being therefore unsuitable for rapid prototyping. Moreover they may limit the selection of the feature of the final pattern, and in most cases they allow to process only one polymer at a time, producing exclusively monomaterial structures. The fabrication of multifunctional and multimaterial patterns or devices is proving to be important for various applications,[13, 14] in order to control and localize wettability,[15] bioadhesion,[16] mechanical[17] and swelling[18] properties. Eventually, when the devices are made of different materials, the issue of a good adhesion among the layers has still to be solved. The development of new high-throughput manufacturing technologies with low cost is therefore required. Different critical issues such as reliability, speed, and overlay accuracy need to be considered in developing new fabrication techniques. Other crucial requisites are versatility (e.g., the capability to produce structures whose properties can be tailored for specific applications) and simplicity (e.g., no need of sophisticated equipment, and possibility to carry out the process at room temperature and without employing solvents). As the final pattern dimensions should Dr. A. Vitale,[+] Dr. M. Cocuzza, Prof. C. F. Pirri, Prof. R. Bongiovanni Department of Applied Science and Technology Politecnico di Torino 10129 Torino, Italy E-mail: [email protected] Dr. M. Quaglio, Dr. A. Chiodoni, Dr. K. Bejtka, Dr. M. Cocuzza, Prof. C. F. Pirri Center for Space Human Robotics@PoliTo Istituto Italiano di Tecnologia 10129 Torino, Italy E-mail: [email protected] Dr. M. Cocuzza CNR-IMEM 43124 Parma, Italy Prof. R. Bongiovanni INSTM RU Politecnico di Torino 10129 Torino, Italy [+]Present address: Department of Chemical Engineering, Imperial College London, SW7 2AZ London, UK
DOI: 10.1002/adma.201501737
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be carefully engineered with regard to the specific application, the resolution capability of the manufacturing process has to be designed accordingly. An ideal technology should be capable of producing multiscale features (i.e., both large and small, as well as thin and thick) in various combinations and distributions, as in real applications microdevices usually present a mixture of arbitrary patterns with various sizes. Moreover, creating multipolymeric patterned surfaces where different chemistries are involved is a very challenging task. Herein, we propose a new highly innovative fabrication technology meeting the above described needs for an efficient fabrication of multipolymeric patterned structures. This advanced manufacturing technology, named oxygen-inhibition lithography (OIL), is a photolithographic technique that exploits the inhibitory effects of oxygen during UV curing in air to fabricate multiscale and multimaterial patterns and devices. Although oxygen inhibition is generally treated as a problem in photopolymerization and photocuring[19,20] and for fabrication techniques that imply the photoinduced free radical polymerization of resins in air, the formation of an inhibition layer (IL) can be exploited to control the polymer surface and can thus turn to be advantageous.[21,22] For instance, Jeong et al.[23] obtained hierarchical patterns on micro- and nanolayers using a two-step capillary molding process, and Chandra and Crosby[24] used this inhibition phenomenon to make wrinkles on the surface of a UV-cured film. Moreover, very recently Tumbleston et al.[25] proposed a new continuous stereolithography technology based on oxygen inhibition. However, none of these techniques offers a simultaneous control over pattern shape, dimensions, and composition. When a photocurable resin is exposed to UV light in atmospheric conditions (i.e., in the presence of oxygen), only the bulk is cured becoming a solid crosslinked network, while a layer of uncured liquid monomer (i.e., IL) remains on the top of the sample surface, as shown in Figure 1a (see Figure S1 in the Supporting Information for more details). In the OIL technology, the formation of the IL is coupled with a photolithographic approach to generate polymeric patterns, simply selecting the areas to be UV irradiated and consequently converted into solid polymer (Figure 1b). As long as the UV exposure is performed in air, an IL showing the same reactive groups of the starting liquid resin (Figure S2 in the Supporting Information) is present, and can be further processed by sequential photolithographic steps. This layer is the main building block for the OIL process due to the reactive functionalities still active on its free surface. If the UV irradiation is instead performed in an inert atmosphere (e.g., nitrogen) also the surface layer is cured, thus becoming no more processable. The OIL technology is based on subsequent UV exposure steps in air with different
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photomasks (accurately selected depending on the geometry to be obtained), and a last UV exposure under nitrogen to completely and definitely cure selected regions. Then, with a final development step, the uncured resin still remaining on or within the structure is selectively removed. The OIL technology is extremely versatile and simple; in fact, neither expensive equipment nor cleanroom conditions are needed. It enables to obtain multiscale (from micro- to millimeter sizes) open-faced-patterned structures as well as closed devices with high reproducibility, low cost and very rapidly, exploiting the speed of photopolymerization reaction. OIL can be applied to all materials polymerizing through a radical mechanism, namely to the family of (meth)acrylates, which includes a number of different commercially available polymers used in several application fields,[26,27] such as coatings and adhesives,[28] dental and biomedical applications,[29, 30] patterning and 3D prototyping.[31,32] Among the (meth) acrylates, there are resins that show different polymerization kinetics (bearing acrylic or methacrylic functional end groups) and various final bulk and surface properties (due to the chemistry of the chain backbone). In this paper we use poly(ethylene glycol) diacrylate (PEG-DA), poly(ethylene glycol) dimethacrylate (PEG-DMA), poly(dimethylsiloxane) diacrylate (PDMSDA), and perfluoropolyether dimethacrylate (PFPE-DMA) as model resins. The former two polymers are hydrophilic and
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Figure 1. Mechanism of formation and processing of the oxygen-inhibited layer by OIL. a) Schematic representation of the UV curing reaction of multifunctional (meth)acrylic monomers, which are made of a backbone chain and two (meth)acrylic functionalities, in the presence of oxygen. When the resins are UV irradiated, the bulk of the sample is transformed into a crosslinked network, while the surface remains liquid keeping unmodified its chemical and physical behavior. b) Coupling of the formation of the IL with a photolithographic approach to generate polymeric patterns. The IL is patterned by a UV irradiation step in air in the presence of a photomask, thus creating polymeric patterns on top of a polymeric substrate. As the exposure is performed in an ambient atmosphere, reactive functional groups are still active on the sample surface even after irradiation. A simplified representation of the basic process flow is reported in the Supporting Information.
biocompatible,[33,34] PDMS-based materials are often used in micro- and nanoreplication processes due to their low Young’s modulus,[35,36] while PFPEs are characterized by hydro- and oleophobicity and very high chemical resistance.[37, 38] At first the experiments were addressed to characterize the oxygen-inhibited layer: its understanding is fundamental for the OIL fabrication process as it corresponds to the final pattern height. The IL thickness depends on the partial pressure of oxygen surrounding the sample, the rate of diffusion of oxygen into the resin, the photoinitiator concentration, the sample thickness, as well as the UV curing conditions (i.e., light intensity and time of irradiation).[39] Figure 2a shows the decrease of IL thickness with exposure time for PEG-DA, with the IL becoming thinner as the light intensity increases. In the domain of conditions explored, the height of PEG-DA structures ranged from 2 µm up to 75 µm. Very thin IL thicknesses were obtained due to hydroperoxide decomposition[40] during photocuring (Figure S3 in the Supporting Information). Data collapse into a single curve (Figure 2b) by expressing the IL thickness values as a function of UV dose d, which is defined as the product of exposure time t and incident light intensity I0 (i.e., d ≡ t I0). Other factors remaining constant, the scaling of the oxygen-inhibited layer thickness with the dose d is given by Equation (1): (1)
where −γ corresponds to the slope of the power law and is equal to −1.77. This relationship is similar to the one proposed by Dendukuri et al.[41] and revisited by Tumbleston et al.[25] (see Figure S3 in the Supporting Information). Figure 2a also demonstrates that structures about 10–20 µm wide could be easily transferred even when arranged in complex patterns. All features are precise and well reproducible, showing actual dimensions almost identical to the nominal ones on the photomask (the maximum variation is close to 1%). The thickness of the patterns is always consistent, demonstrating the independence of the IL thickness on the photomask used. The monolithic structures obtained by OIL do not show any interface between the bulk and the patterned layer, which are chemically bonded, ensuring an excellent cohesion and granting the best mechanical and chemical performance (see the Supporting Information). Figure 2c shows the evolution of the IL thickness with irradiation time for each resin under investigation, for constant light intensity. The IL thickness diminishes while increasing UV irradiation time; however, different values were obtained depending on the chemistry of the materials. The results presented in Figure 2c demonstrate that by selecting the resin
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Figure 2. OIL technology for the fabrication of all-polymeric patterned structures. a) IL thickness as a function of irradiation time at three different light intensities (1.37, 2.74, and 6.86 mW cm−2) for PEG-DA. The insets represent two examples of pattern transfer onto the IL, showing 20 µm wide pillars and 25 µm wide parallel lines. Images were acquired by a field-emission scanning electron microscope (FESEM). b) The results in (a) are collapsed into one curve by plotting in log–log scale the IL thickness versus the dose d. The inset details the power dependence of IL thickness by the light intensity, for different exposure times t. c) Comparison of oxygen inhibition for different (meth)acrylic resins: IL thickness as a function of irradiation time for UV light intensity I = 2.74 mW cm−2 for PEG-DA (䊉), PEG-DMA (䊏), PDMS-DA (䉱), and PFPE-DMA (䉬). d) Master curve representing the IL thickness for different polymers. The inset details the dependence of the IL thickness by the reactivity of the resin k and the oxygen solubility S and diffusivity D in it. The error bars represent the error of at least five measurements of the same conditions; when the error bar is not visible on data points of graphs in (a) and (c), it means that it is within the symbol size. The maximum error found among all studied resins and UV curing conditions explored is lower than 4%. Additional schematics of the process flow describing the OIL technology are reported in Figure S4 in the Supporting Information.
functionality or its main chain it is possible to tune the IL thickness, generating patterns whose thickness ranges from few up to hundreds of micrometers. These control parameters are related to the oxygen-inhibited layer thickness according to a simple power law (Figure 2d): IL thickness = Cd −γ
(2)
where C is a proportionality constant for a fixed material system, which is dependent on the monomer reactivity k and the oxygen solubility S and diffusivity D into the liquid resin. Increasing the reactivity of the resin or decreasing its capability to dissolve oxygen, as well as its permeability to this gas, makes the IL thinner. In fact, methacrylic resins resulted in higher IL thicknesses since they have a slower polymerization kinetics compared to acrylics; siloxanes and fluorinated resins provided higher IL thicknesses since they dissolve more oxygen[42] compared to hydrogenated monomers (i.e., PEG-DA and PEG-DMA). With these model resins we demonstrate that OIL is suitable for processing a broad range of materials. The physicochemical nature of the polymers and the properties of the patterned structures (e.g., wettability, mechanical strength and elasticity, protein adsorption) can be selected on demand, depending on the final application.
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Being able to control the IL, the OIL technology opens a novel route to the fabrication and patterning of multiscale systems and devices with tunable functionalities and thicknesses. This latter aspect is fundamental as the pattern dimensions have to be carefully designed with regard to the specific device application. A common way to create structures with a high thickness is to resort to multilayers. As the interlayer adhesion is a critical issue, frequently surface treatments[43] and functionalization reactions[44] are required to obtain well-bonded layers. With the here presented technology there is not such a problem, since the IL itself acts as an adhesion promoter. In fact, as already shown in Figure 2, for a given resin it is possible to dramatically reduce the IL thickness by increasing the light intensity. Then, fresh monomer can be added on top of the thin undercured layer to be patterned. This approach is a layer-bylayer process, evidently close to additive manufacturing concept. Figure 3a schematically illustrates the OIL technology for the fabrication of multilayer-based structures taking advantage of oxygen inhibition. In this process the height of the pattern is not limited by the UV exposure conditions, but depends only on the amount of liquid resin that is added for the construction of each layer. During the irradiation step in the presence of a photomask, strong chemical bonds are generated between two layers and continuous thick patterned films are formed. As shown in Figure 3a, monolithic structures with thickness up to
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to generate multimaterial structures, where each layer contains a different chemistry. This opens the possibility to combine in the same device different sets of properties belonging to different polymers, optimizing the behavior of the system. Even in this case, during each UV exposure, chemical bonds are formed between the layers although they are not made of the same polymer, ensuring an excellent adhesion (Figure S6 and S7 and Movie S1 in the Supporting Information). As an example of a multimaterial and multifunctional structure, Figure 3b shows a nice lotus flower floating on water. The water and the lotus petals are made of hydrophilic PEG-DMA and PEG-DA, respectively, while the leaf is made of hydrophobic PFPE-DMA. Each layer of the structure is characterized by a different wettability, however a good quality of the interface between the different materials is ensured. Since highly fluorinated polymers such as PFPEs usually exhibit low adhesion due to their very low surface energy,[45] it is worth noting the good adhesion between PFPE-DMA and both PEG-DA and PEG-DMA without application of a pretreatment. With the OIL technology each layer of the patterned structure can be finely tuned, choosing its chemical composition and as a consequence its physical and chemical properties. Multifunctional samples can hence be created without using complicate surface chemistry-based treatments (e.g., molecular grafting, polymer brushes, self-assembly monolayers), highly defining topographical and chemical features in the micrometer range. In addition, the OIL technology enables the fabrication of closed and embedded structures by a precise control of the photopolymFigure 3. OIL technology for the fabrication of mono- and multimaterial thick polymeric struc- erization reaction. After obtaining a complex tures. a) Schematic view of the process flow of OIL to produce patterned layers with high thick- micropatterned structure, the last exposure nesses. Examples of PEG-DA and PEG-DMA thick structures obtained through OIL are also step can be performed in nitrogen atmosphere shown. The PEG-DA circular pillars have a diameter of 200 µm and a height of 1.5 mm (left and used to close the device (Figure 4a). For structure) and 1.2 mm (right structure). The PEG-DMA squared pillars are 200 µm wide and 1.4 mm high. b) Schematic view of the process flow of the OIL process to produce multimate- this step the exposure time should be finely rial patterned structures. The image shows a lotus flower floating on water. The flower is made controlled in order to cure only a desired surof PEG-DA, the leaf of PFPE-DMA, and the water of PEG-DMA. Colorants were added to the face thickness, without completely closing the reactive UV curable mixtures prior to photopolymerization. The image shows the hydrophilic pattern structures. Due to absorption of the character of PEG-DA (contact angle with water = 51.5°) and the hydrophobic behavior of PFPE- incident light by the external layer, the photoDMA (contact angle with water = 114°). Additional schematics of the process flow describing sensitive resin within the structures does not the OIL technology are reported in Figure S4 in the Supporting Information. receive a sufficient dose to cure. Examples of this kind of systems are quite interesting for application fields as microfluidics. Figure 4 shows two conmillimeters can be easily fabricated with high accuracy (see also veniently produced all-polymeric microfluidic devices with 950 Figure S5 in the Supporting Information). Selecting the proper and 50 µm wide (Figure 4b,c, respectively) squared microchanmaterial and the desired pattern thickness, one can simply fabnels, fabricated by OIL. It can be noticed that the last UV curing ricate thick structures showing the appropriate functional propstep (responsible for obtaining close-faced structures) does not erties for the chosen application. modify the pattern geometry, which is maintained regular and Even more interestingly, OIL can easily work also if difprecise. Both entire devices were fabricated in less than 5 min. ferent resins are added one over the other (Figure 3b) in order
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In conclusion, we presented a novel photolithographic technology that takes advantage of the oxygen-inhibition phenomenon and exploits it to fabricate multiscale and multimaterial structures, such as polymeric openfaced patterns and close-faced devices. The uncured liquid layer, which is spontaneously generated on the surface of a crosslinked polymer when UV curing is performed in air, can be further processed by sequential photolithographic steps. We demonstrate that OIL is a general patterning route for (meth-) acrylic resins that can be used to obtain wellcontrolled patterns by tuning the curing kinetics and parameters. Exposure time, UV light intensity, and material chemistry were selected as critical variables to finely control the thickness of the IL. The OIL technology is a simple approach that controls with high fidelity both surface topography and functionality over a variety of feature sizes and Figure 4. Fabrication of embedded structures by the OIL technology. a) Schematic view of the process flow of the OIL technology for the fabrication of all-polymeric-closed patterned struc- heights. We demonstrate the generation of tures. b) Example of a microfluidic device made of PFPE-DMA, filled with colored water-based devices with sizes in the range of centimsolution. Squared channels are 950 µm wide and high. c) Cross-section of a microfluidic device eters, showing a minimal features resolumade of PFPE-DMA, with microchannels 50 µm wide and high, observed by optical microtion of 10 µm. OIL enables also the fabricascopy. d) Multimaterial microfluidic device for emulsion separation. First, the emulsion, which is formed by droplets of an aqueous solution (Aq) in a continuous oil flow (Oil), is created tion of multimaterial and multifunctional (enlargement zone 1) by a T-junction in a PDMS-DA channel. Then, in the separation area, the structures of varied and complex shapes, containing different compositions and thus channel is formed by a PDMS-DA side and a PEG-DA side and this allows the streams of oil (red) and water (blue) to flow separately (enlargement zone 2). Dyes were added to water- and showing a synergy of properties. With the oil-based solutions to increase the contrast. The insets are optical images of two representative OIL technology polymer-based devices with areas of the device. Details on the fabrication process of the microfluidic devices are reported prescribed dimensions can be structured in the Supporting Information. simply, without any further use of specialized microfabrication equipment, and very rapidly (i.e., in few minutes). Excellent interface and adhesion between Embedded structures can be successfully created by OIL the patterned layers are guaranteed, even in the fabrication of also into multimaterial systems: one can thereby select and multimaterial structures. This technology offers immediate tune the physicochemical properties of each layer of the device. advantages over other conventional microfabrication techAs an exemplar, we fabricated a multipolymeric microfluidic niques. Thanks to its generality, robustness, reproducibility, device for efficient separation of water-in-oil emulsions into and cost-efficiency, it undoubtedly has potential applications in their individual components (Figure 4d), which can selectively areas such as biotechnology, microfluidics, flexible electronics, interact with the different materials forming the microchansensors, and energy devices. nels. The microfluidic device shown in Figure 4d includes a first part where the emulsion is formed, and a second part where separation occurs due to surface tension differences (further details on the separation device fabrication and operaExperimental Section tion are provided in the Supporting Information, Figure S8). In fact the rectangular microchannel is defined by a hydrophilic Materials: PEG-DA with a molecular weight of 700 g mol−1 and PEG-DMA with a molecular weight of 750 g mol−1 were purchased from PEG-DA boundary and a hydrophobic PDMS-DA boundary. Sigma–Aldrich and used as received. Fluorolink MD700 (PFPE-DMA) The different wettability properties of these two resins are used with a molecular weight of 1800 g mol−1 was kindly provided by Solvay– to stabilize the adjacent concurrent laminar flows of the two Solexis. Ebecryl 350 (PDMS-DA) with a molecular weight of 1500 g mol−1 immiscible phases: the aqueous stream (Aq) is stabilized along was obtained from Allnex (formerly Cytec Ltd). 2-hydroxy-2-methyl-1the PEG-DA channel side due to hydrophilic interactions, while phenyl-propan-1-one (Darocur 1173, Sigma–Aldrich) was chosen as the organic stream (Oil) is stabilized along the PDMS-DA side photoinitiator and used at 2 wt%. Sylgard 184 PDMS elastomer kit was purchased from Dow Corning. All other chemicals were obtained from due to weak nonpolar interactions. Thus, the device fabricated Sigma–Aldrich. by the OIL technology is a passive system that allows emulPatterning of the Oxygen-Inhibited Layer: The photocurable mixture sion separation without resorting to filtration, complex channel (containing no solvent, only the monomer and the photoinitiator) geometries (to control the hydrodynamic forces), application of was poured into 1.5 cm × 2 cm molds with heights ranging from external fields, or chemical surface modification. This exemplar 0.5 to 3 mm; the thickness of the resin layer was set by the depth of demonstrates that multimaterial and multifunctional 3D structhe mold employed and confirmed by a volumetric control. The system was irradiated in air by means of a fiber optic UV lamp (Hamamatsu tures can be easily built up by OIL in a layer-by-layer fashion.
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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements The work of A.V. was partially funded by Consorzio INSTM (Firenze, Italy). Received: April 13, 2015 Revised: May 14, 2015 Published online: July 14, 2015
[1] S. Xu, Y. Zhang, L. Jia, K. E. Mathewson, K.-I. Jang, J. Kim, H. Fu, X. Huang, P. Chava, R. Wang, S. Bhole, L. Wang, Y. J. Na, Y. Guan, M. Flavin, Z. Han, Y. Huang, J. A. Rogers, Science 2014, 344, 70. [2] L. Gervais, N. de Rooij, E. Delamarche, Adv. Mater. 2011, 23, H151. [3] P. Neuzil, S. Giselbrecht, K. Lange, T. J. Huang, A. Manz, Nat. Rev. Drug Discov. 2012, 11, 620. [4] P. Zorlutuna, N. Annabi, G. Camci-Unal, M. Nikkhah, J. M. Cha, J. W. Nichol, A. Manbachi, H. Bae, S. Chen, A. Khademhosseini, Adv. Mater. 2012, 24, 1782. [5] Y. Shao, J. Fu, Adv. Mater. 2014, 26, 1494. [6] B. Derby, Science 2012, 338, 921. [7] K.-Y. Suh, M. C. Park, P. Kim, Adv. Funct. Mater. 2009, 19, 2699. [8] E. Ueda, P. A. Levkin, Adv. Mater. 2013, 25, 1234. [9] A. M. Brzozowska, F. J. Parra-Velandia, R. Quintana, Z. Xiaoying, S. S. C. Lee, L. Chin-Sing, D. Jan´czewski, S. L. M. Teo, J. G. Vancso, Langmuir 2014, 30, 9165. [10] Q. Zhang, Y. Sun, W. Xu, D. Zhu, Adv. Mater. 2014, 26, 6829. [11] J. Kong, S. Song, M. Yoo, G. Y. Lee, O. Kwon, J. K. Park, H. Back, G. Kim, S. H. Lee, H. Suh, K. Lee, Nat. Commun. 2014, 5, 5688.
Adv. Mater. 2015, 27, 4560–4565
[12] H. Kang, S. Jung, S. Jeong, G. Kim, K. Lee, Nat. Commun. 2015, 6, 6503. [13] K.-H. Roh, D. C. Martin, J. Lahann, Nat. Mater. 2005, 4, 759. [14] C. Liao, M. Zhang, M. Y. Yao, T. Hua, L. Li, F. Yan, Adv. Mater. 2014, DOI: 10.1002/adma201402625. [15] Q. Cheng, M. Li, Y. Zheng, B. Su, S. Wang, L. Jiang, Soft Matter 2011, 7, 5948. [16] R. Ogaki, M. Alexander, P. Kingshott, Mater. Today 2010, 13, 22. [17] S. P. R. Kobaku, G. Kwon, A. K. Kota, R. G. Karunakaran, P. Wong, D. H. Lee, A. Tuteja, ACS Appl. Mater. Interfaces 2015, 7, 4075. [18] J. Yom, S. M. Lane, R. A. Vaia, Soft Matter 2012, 8, 12009. [19] Y. Yagci, S. Jockusch, N. J. Turro, Macromolecules 2010, 43, 6245. [20] S. C. Ligon, B. Husár, H. Wutzel, R. Holman, R. Liska, Chem. Rev. 2014, 114, 557. [21] D. Dendukuri, D. C. Pregibon, J. Collins, T. A. Hatton, P. S. Doyle, Nat. Mater. 2006, 5, 365. [22] J. Shi, M. B. Chan-Park, C. Gong, H. Yang, Y. Gan, C. M. Li, Chem. Mater. 2010, 22, 2341. [23] H. E. Jeong, R. Kwak, J. K. Kim, K. Y. Suh, Small 2008, 4, 1913. [24] D. Chandra, A. J. Crosby, Adv. Mater. 2011, 23, 3441. [25] J. R. Tumbleston, D. Shirvanyants, N. Ermoshkin, R. Janusziewicz, A. R. Johnson, D. Kelly, K. Chen, R. Pinschmidt, J. P. Rolland, A. Ermoshkin, E. T. Samulski, J. M. DeSimone, Science 2015, 347, 1349. [26] C. N. Bowman, C. J. Kloxin, AlChE J. 2008, 54, 2775. [27] J.-P. Fouassier, X. Allonas, Basics and Applications of Photopolymerization Reactions, Research Signpost, Trivandrum, India 2010. [28] R. S. Davidson, Exploring the Science, Technology and Applications of UV and EB Curing, Sita Tecnology Limited, London, UK 1999. [29] M. H. Bland, N. A. Peppas, Biomaterials 1996, 17, 1109. [30] N. Moszner, T. Hirt, J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 4369. [31] S. S. Williams, S. Retterer, R. Lopez, R. Ruiz, E. T. Samulski, J. M. DeSimone, Nano Lett. 2010, 10, 1421. [32] A. Vitale, M. Quaglio, S. L. Marasso, A. Chiodoni, M. Cocuzza, R. Bongiovanni, Langmuir 2013, 29, 15711. [33] M. S. Hahn, J. S. Miller, J. L. West, Adv. Mater. 2006, 18, 2679. [34] G. K. Such, Y. Yan, A. P. R. Johnston, S. T. Gunawan, F. Caruso, Adv. Mater. 2015, 27, 2278. [35] Y. Xia, G. M. Whitesides, Angew. Chem. Int. Ed. 1998, 37, 550. [36] A. Carlson, A. M. Bowen, Y. Huang, R. G. Nuzzo, J. A. Rogers, Adv. Mater. 2012, 24, 5284. [37] J. P. Rolland, R. M. Van Dam, D. A. Schorzman, S. R. Quake, J. M. DeSimone, J. Am. Chem. Soc. 2004, 126, 2322. [38] A. Vitale, M. Quaglio, M. Cocuzza, C. F. Pirri, R. Bongiovanni, Eur. Polym. J. 2012, 48, 1118. [39] V. V. Krongauz, C. P. Chawla, J. Dupre, in Photoinitiated Polymerization, Vol. 847 (Eds: K. D. Belfield, J. V. Crivello), American Chemical Society, Washington DC, USA 2003, p.165. [40] R. Pynaert, J. Buguet, C. Croutxe-Barghorn, P. Moireau, X. Allonas, Polym. Chem. 2013, 4, 2475. [41] D. Dendukuri, P. Panda, R. Haghgooie, J. M. Kim, T. A. Hatton, P. S. Doyle, Macromolecules 2008, 41, 8547. [42] M. F. Costa Gomes, J. Deschamps, D. H. Menz, J. Fluorine Chem. 2004, 125, 1325. [43] J. Zhou, A. V. Ellis, N. H. Voelcker, Electrophoresis 2010, 31, 2. [44] T. Rohr, D. F. Ogletree, F. Svec, J. M. J. Fréchet, Adv. Funct. Mater. 2003, 13, 264. [45] A. Vitale, A. Priola, C. Tonelli, R. Bongiovanni, Int. J. Adhes. Adhes. 2014, 48, 303.
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LC8). The following exposure steps were performed in the presence of a photomask. UV light intensity (1–10 mW cm−2) and exposure time were varied. Oxygen partial pressure of the system was 20.7 kPa. The photolithographic process was optimized using both Mylar and quartz masks (further details are given in the Supporting Information): different photomasks with features ranging from 10 to 500 µm were used. The final exposure step was carried out in nitrogen atmosphere. After multiple UV light exposures, the samples were developed by soaking them in a solvent for few seconds (no more than 30 s) to remove the uncured monomer. Development was operated in acetone and ethanol for PFPE-DMA and in isopropyl alcohol for PEG-DA, PEG-DMA, and PDMS-DA. For the preparation of colored patterns, colorants were added to the monomer before the first UV curing step. Additional details on the photopatterning process are reported in the Supporting Information. PDMS molds were fabricated by casting (pouring 10:1 wt/wt monomer/crosslinker mixture, followed by baking at 75 °C for 3 h). PMMA molds were fabricated by milling. Pattern Analysis: Patterns were analyzed by a Nikon Eclipse ME600 optical microscope and a ZEISS Dual Beam Auriga FESEM. Prior to FESEM examination samples were coated with an 8 nm thick Au film via sputtering. The thickness of the oxygen-inhibited layer was determined by imaging the cross-section of the patterns.
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