pubs.acs.org/Langmuir © 2009 American Chemical Society
Microcontact Printing and Lithographic Patterning of Electrospun Nanofibers J. Shi,† L. Wang,† and Y. Chen*,†,‡ †
Ecole Normale Sup erieure, CNRS-ENS-UPMC UMR 8640, 24 rue Lhomond, 75005 Paris, France, and ‡ Institute for Integrated Cell-Material Science (iCeMS), Kyoto University, Kyoto 606-8507, Japan Received March 6, 2009. Revised Manuscript Received April 10, 2009
We describe a method for printing electrospun nanofibers from a collector onto another substrate. The electrospinning collector that we used is made of a thin layer of polydimethylsiloxane (PDMS) on glass with or without patterned electrodes, allowing the fabrication of either aligned or randomly deposited nanofibers. Afterwards, the electrospun fibers are printed onto a glass substrate, and such a process can be repeatedly used to obtain multilayer fibers with a good reproducibility. We also show a postprocessing method for the pattern formation of electrospun nanofibers by using standard photolithography and reactive ion etching techniques, thereby providing a way of fabricating microarrays of single fibers or distributed fiber matrices.
Introduction Electrospinning was proposed 70 years ago, and it has attracted much attention during the past few years because of the rapid progress in and increasing demand for nanomaterials.1-3 Electrospun fibers can have different mechanical, wetting, and permeability properties.4,5 By using functional polymers, for example, the fiber assemblies can also provide unique electrical, magnetic, or optical functionalities.6-8 Therefore, a large number of applications can be expected, including filter membranes, cutedge texturing, chemical and biochemical sensors, and tissue engineering.9-12 Electrospinning can be done in a very simple manner. A viscous polymer solution is loaded by syringe pump and driven to a metallic needle tip where a droplet is formed. A high electrical voltage is applied between the needle tip and a grounded collector. At a critical voltage, the electrostatic repulsion within the charged solution overcomes the surface tension, resulting in stretching of the polymer droplet into a Taylor cone at the tip of the needle and ejection of a charged fluid jet. The unstable jet undergoes elongation, traveling through the air with continuous solvent evaporation, and finally deposition on the collector. In general, a *Corresponding author. Tel: +33 1 44 32 24 21. Fax: +33 1 44 32 24 02. E-mail:
[email protected]. (1) Ramakirshma, S.; Fujihara, K; Teo, W. E.; Lim, T. C.; Ma, Z. In An Introduction to Electrospinning and Nanofibers; World Scientific: Singapore, 2005. (2) Huang, Z. M.; Zhang, Y. Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223. (3) Li, D.; Xia, Y. N. Adv. Mater. 2004, 16, 1151. (4) Gu, S. Y.; Wu, Q. L.; Ren, J.; Vancso, G. J. Macromol. Rapid Commun. 2005, 26, 716. (5) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210. (6) Norris, I. D.; Shaker, M. M.; Ko, F. K.; MacDiarmid, A. G. Synth. Met. 2000, 114, 109. (7) Li, D.; Herricks, T.; Xia, Y. N. Appl. Phys. Lett. 2003, 83, 4586. (8) Wang, X. Y.; Drew, C.; Lee, S. H.; Senecal, K. J.; Kumar, J.; Sarnuelson, L. A. Nano Lett. 2002, 2, 1273. (9) Gibson, P.; Schreuder-Gibson, H.; Rivin, D. Colloids Surf., A 2000, 187188, 469. (10) Pawlowski, K. J.; Belvin, H. L.; Raney, D. L.; Su, J.; Harrison, J. S.; Siochi, E. J. Polymer 2003, 44, 1309. (11) Liu, H. Q.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Nano Lett. 2004, 4, 671. (12) Li, W. J.; Laurencin, C. T.; Caterson, E. J.; Tuan, R. S.; Ko, F. K. J. Biomed. Mater. Res. 2002, 60, 613.
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conductive substrate is used as a collector to accumulate the nanofibers. With insulating substrates such as glass, quartz, and organic materials, fibers can also be collected if they are placed in front of or between collecting electrodes. It has been shown that the fiber matrices producing a nonconductive substrate have a lower fiber packing density and hence a higher porosity compared to those collected on a conductive surface.13 For some applications, high porosity may not be necessary or desirable. For example, a relative low porosity should be more favorable for cell colonization inside of the fiber matrix and the formation of 3D tissues. Alternative, nanofibers can be fabricated with collectors of different shapes and arrangements such as a rotating drum and frame,14 a pair of split electrodes,15 and an apparatus producing focused electric or magnetic fields.16-18 These methods lack versatility because they generally require a particular electrode design or complicated setup and cannot be applied to pattern nanofibers features on the micrometer scales that are often required for integrated devices. In this letter, we propose a general method of nanofiber manipulation that can be applied to transfer electrospun fibers from one substrate to another type of substrate and to produce lithographically defined nanofiber patterns. This method is based on commonly used microcontact printing, lithography, and pattern-transfer techniques. Microcontact printing has been widely used to produce surface chemical patterns.19 A stamp is first inked with a molecular solution and then placed on a substrate for pattern transfer. To adapt this technique for our purposes, nanofibers are “inked” by electrospinning on the surface of the stamp (collector) and then printed in a conventional way but with a heated substrate. Randomly deposited and aligned nanofibers of different diameters (200 nm-1.5 μm) could be printed by single- or double-layer printing. We also show the (13) Liu, H. Q.; Hsieh, Y. L. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2119. (14) Kim, J. S.; Reneker, D. H. Polym. Eng. Sci. 1999, 39, 849. (15) Singh, G.; Bittner, A. M.; Loscher, S.; Malinowski, N.; Kern, K. Adv. Mater. 2008, 20, 2332. (16) Fennessey, S. F.; Farris, R. J. Polymer 2004, 45, 4217. (17) Yang, D. Y.; Lu, B.; Zhao, Y.; Jiang, X. Y. Adv. Mater. 2007, 19, 3702. (18) Deitzel, J. M.; Kleinmeyer, J. D.; Hirvonen, J. K.; Tan, N. C. B. Polymer 2001, 42, 8163. (19) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 28, 153.
Published on Web 4/28/2009
DOI: 10.1021/la900811k
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Figure 1. Schematic process flow of (A) PDMS stamp preparation and aligned fibers electrospinning, (B) microcontact printing of fibers deposited on a PDMS stamp, and (C) lithographic patterning of fibers on a PDMS stamp by using photolithography and reactive ion etching techniques.
fabrication of nanofiber micropatterns by using standard photolithography and reactive ion etching techniques, and we believe that both proposed techniques can be used for a wide range of applications.
Experimental Section Electrospinning. Solutions for electrospinning were prepared by dissolving polymethylglutarimide (PMGI) in tetraethylammonium hydroxide at concentrations ranging from 6 to 19% (w/v) with or without adding red (rhodamine 6-G) and green (FITC) dye molecules. The custom electrospinning apparatus was built to obtain random and aligned PMGI nanofibers of different diameters. The polymer solution was loaded into a syringe with a stainless steel needle (18 gauge). The needle was connected to a high-voltage supply (Stanford Research Systems). The voltage used for electrospinning was 5 kV, and the distance between the needle and the collector was 10 cm. The solution was continually supplied using a syringe pump at a rate of 10 μL/min. Stamp Preparation. A thin layer of PDMS was used as a collector for electrospinning and as a stamp for microcontact printing. It was obtained by casting a mixture of two liquid components (kit RTV615, from General Electric Co.) in ratio of 1:10 onto a flat Petri dish, and after being cured at 80 °C for several hours, the solidified PDMS was separated from the Petri dish and cleaned with ethanol before use. For random fiber collection, 30 nm of gold was deposited on the surface of PDMS with an electron beam evaporator (BOC Edward Auto 500) before electrospinning. For aligned fiber collection, two parallel gold electrodes were first fabricated by contact mode photolithography and liftoff on a glass substrate (Figure 1A). The gap distance between the two electrodes was about 1 cm. Then, a PDMS mixture in a ratio of 1:20 was spin coated onto the whole substrate to a thickness of about 20 μm. After being cured on a hot plate at 120 °C for 5 min, the PDMS film above the two electrodes was peeled off, and electrospinning was performed for 30 s. As a result, we obtained a single layer of aligned nanofibers on PDMS between the two parallel electrodes. Finally, the PDMS thin film was peeled off and bonded to a thicker PDMS block at 80 °C for 1 h. Before being bonded, the surfaces of both PDMS layers were
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Figure 2. Variation of electrospun fiber diameter as a function of polymer (PMGI) concentration. treated with oxygen plasma for 1 min using a plasma cleaner (Harrick Scientific). Microcontact Printing. The electrospun fibers were transferred onto a glass substrate by microcontact printing (Figure 1B). First, the PDMS stamp coated with fibers was brought into contact with a clean glass substrate. Then, the stamp-substrate assembly was placed on a hot plate of 200 °C for less than 30 s. After cooling to room temperature and undergoing separation, nanofibers were irreversibly transferred onto the glass substrate that can then be used for applications or further processing (see below). The substrate for electrospinning with the two parallel electrodes could be reused many times. Similarly, the pattern transfer described here can also be performed for layer-by-layer microcontact printing to achieve a multilayer fabrication of nanofibers. In this work, only double-layer printing has been tested under the same working condition. Lithographic Patterning. Once the electrospun nanofibers are deposited or fixed on a substrate, photolithography and different pattern-transfer techniques can be applied. For demonstration, a thin layer of photoresist (AZ5214E) was directly deposited by spin coating at 5000 rpm on the PDMS stamp with electrospun nanofibers (Figure 1C). Contact mode photolithography was then performed. After development, the sample was Langmuir 2009, 25(11), 6015–6018
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Figure 3. Scanning electron micrographs of randomly distributed (A and B) and aligned (C) PMGI fibers electrospun on a PDMS layer. The mean diameters of the fibers are 200 nm (A, C) and 500 nm (B). etched by oxygen plasma in a reactive ion etching chamber (Nextral N100) with a gas flow rate of 10 sccm, a pressure of 30 mTorr, and a power of 50 mW. Afterwards, the resist was removed in an acetone bath, and the patterned nanofibers were finally transferred onto a glass substrate. Characterization. All optical images were recorded using a fluorescence microscope (Zeiss, Axiovert 200) equipped with a high-sensitivity B&W CCD camera (PhotoMax512B, Princeton Instruments). Multicolor images of dye-doped PMGI fibers were obtained with the help of Image Pro Plus software. High-resolution observations were performed with a scanning electron microscope (Hitachi S-800) operated at 8 kV. Before observation, a 5-nm-thick gold layer was deposited on the samples by sputtering.
Results and Discussion In electrospinning, the diameter and the morphology of the produced fibers depend mainly on the amplitude of the applied voltage, the working distance, the flow rate, and the polymer concentration. We fixed the first three parameters (5 kV, 10 cm, and 10 μL/min) in order to focus on the problems of printing and lithographic patterning performance of electrospun fibers. The diameter of the fibers could be controlled by varying the polymer concentration. As expected, we observed a linear dependence between the fiber diameter and the polymer concentration for the range from 6 to 19% (Figure 2). The morphology of the fiber scaffold was characterized by scanning electronic microscopy (SEM). Figure 3A,B shows SEM images of electrospun fibers of 200 and 500 nm diameters, obtained by random deposition using 6 and 11% PMGI solutions, respectively. The largediameter nodes that appeared in the fiber matrix produced with the 6% PMGI solution can be attributed to the low surface tension of the solution jet due to the relatively low viscosity. Electrospinning of aligned fibers could be done with a collector having two parallel electrodes, similar to the method of Singh et al.15 Such a configuration results in suitable electrostatic forces during the electrospinning, allowing the charged fibers to be aligned in the gap perpendicularly to the edge of the two electrodes. To perform microcontact printing of deposited fibers, a thin layer of PDMS has been inserted. One obvious advantage of such a modified configuration is that the substrate with patterned electrodes can be reused many times so that multilayer deposition becomes possible with the same substrate or electrospun collector. Figure 3C shows an SEM image of aligned fibers with a diameter of about 200 nm,which were deposited on the PDMS film for 5 min with 6% PMGI solution. As can be seen, they were densely packed with regard to alignment but had some apparent largediameters nodes. Microcontact printing of deposited fibers was possible with PDMS as part of the electrospinning collector. In fact, the relative low Young’s modulus of PDMS (360-870 kPa) allows Langmuir 2009, 25(11), 6015–6018
for the conformable, micrometer-scale contact of PDMS with a substrate. In addition, PDMS has a low surface free energy (ca. 21.6 10-3 J m-2) 19, which is suitable for material transfer with an easy release. When PDMS is used as a stamp for microcontact printing, it allows the production of homogeneous patterns over a large surface area. It has also been demonstrated that PDMS could be used to pattern protein line arrays with a feature size down to 50 nm.20 To print nanofibers, we used a flat PDMS layer as a collector, and the fibers were arranged directly on the PDMS surface. Because of the absence of a self-bonding or assembling mechanism, substrate heating is needed during printing in order to fix the transferred materials. We found that when the PDMS stamp was brought into contact with a heated glass substrate, the fibers adhered more strongly onto the glass than onto the PDMS stamp surface because of the low surface energy of PDMS. To fix the nanofibers efficiently on the glass substrate without significant deformation, the printing temperature has to be optimized. In our experiment, the glass substrate was heated to 200 °C, which is slightly higher than the glass-transition temperature (Tg) of PMGI (190 °C). We noticed that if the printing temperature is not high enough, the fibers cannot be totally transferred onto the substrate, and they can be easily broken under large printing forces. This can be explained by the fact that below Tg, the polymer stays in a glassy state. If the printing temperature is too high, then the polymer will be in a viscous state so that the printed fiber matrix can be completely destroyed. Thus, the printing temperature should be slightly above the glasstransition temperature of the polymer for irreversible bonding without a significant change in the fiber morphology. The above-described process also works for other types of thermoplastic polymers such as PMMA, PS, and PC. Here, we choose PMGI because of its good thermal stability at relative high temperature and reasonable chemical resistance to commonly used resin solvents such as acetone and ethanol, being mostly compatible with both microcontact printing and lithography processes. To demonstrate the feasibility of nanofiber printing, dye-doped polymer solution was used. Figure 4A,B shows fluorescent images of FTIC-doped fibers (with a diameter of about 1 μm) with (B) or without (A) alignment, both printed on a glass substrate. Doublelayer printing has been tested with nanofibers with different doping molecules but the same processing parameters. Figure 4C,D shows the resulting fluorescent images of parallel and cross-bar arrays of polymer fibers doped with FITC (green) (20) Li, H. W.; Muir, B. V. O.; Fichet, G.; Huck, W. T. S. Langmuir 2003, 19, 1963.
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Figure 4. Fluorescence images of dye-doped PMGI nanofibers on glass substrates, obtained by electrospinning (random distribution for A and aligned distribution for B-D) on PDMS layers and single-layer (A, B) or double-layer (C, D) microcontact printing.
Figure 6. (A, B) Microphotographs of an electrospun PMGI nanofiber before and after lithographic patterning on a PDMS stamp. (C, D) Microphotographs of patterned dot and line arrays of electrospun nanofibers after lithography, reactive ion etching, and transfer from a PDMS stamp (collector) onto a glass substrate.
well-defined geometry. For example, contact mode photolithography could be used to pattern the fiber matrix before or after pattern transfer. After photolithography, reactive ion etching can be applied to remove the fiber areas without photoresist protection. Figure 6 shows microphotographs of a 1-μm-diameter PMGI fiber on a PDMS stamp before (A) and after (B) lithographic patterning and reactive ion etching in oxygen plasma for 10 min. Figure 6C,D shows microphotographs of dot and line arrays of electrospun PMGI nanofibers after pattern transfer on a glass substrate. Depending on the electrospinning parameters, the size and density of patterned nanofibers in each feature can be varied, and both randomly distributed and aligned fibers can be processed in a similar manner. Figure 5. Scanning electron micrographs of 200-nm-diameter PMGI nanofibers on glass substrates, obtained by single-layer printing (A) and double-layer cross printing (B) with the PDMS stamp.
and rhodamine (red) dye molecules, respectively. The pattern stability during the transfer has also been checked by observing the configuration of a nanofibers matrix before and after microcontact printing, and we did not observed noticeable pattern deformation. Figure 5A,B shows SEM images of printed 200 nm fibers, aligned in parallel or perpendicularly on a glass substrate. In general, fibers produced by electrospinning are collected as nonwoven or randomly arranged because of the bending instabilities associated with the electrical charged jets. Hence, the fabrication of fiber matrices with complex and ordered structures is difficult. Though some of the patterns can be produced on the substrate by using patterned electrodes, their usefulness should be limited. Therefore, microcontact printing of electrospun nanofibers provides a new method for the exploration of their application fields. Another possibility is to pattern the nanofiber matrix directly by using standard lithography and etching techniques with a
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Conclusions Microcontact printing has been used to transfer electrospun nanofibers with PDMS stamps. This method is simple and versatile and is applicable to both randomly distributed and aligned fibers. Dye-doped fibers of two different colors have also been used to show the feasibility of printing double-layer fibers. Furthermore, standard photolithography and reactive ion etching techniques have been applied to create nanofibers arrays with well-defined pattern size and shape. We believe that this method is promising for a number of applications, including nanofiberintegrated nanodevices and extracellular matrixes for cell biology studies. Acknowledgment. This work was supported by the European Commission through project contract CP-FP 214566-2 (Nanoscale) and the French National Research Agency through project contract ANR-06-NANO-028-03 (Active Nanopore). J.S. is grateful to the region of Ile-de-France for the research grant (program DIM cells and stem cells). The content of this work is the sole responsibility of the authors.
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