Highly stretchable electric circuits from a composite material of silver

LETTERS PUBLISHED ONLINE: 25 NOVEMBER 2012 | DOI: 10.1038/NNANO.2012.206

Highly stretchable electric circuits from a composite material of silver nanoparticles and elastomeric fibres Minwoo Park1†, Jungkyun Im2†, Minkwan Shin1, Yuho Min1, Jaeyoon Park1, Heesook Cho3, Soojin Park3, Mun-Bo Shim2, Sanghun Jeon2, Dae-Young Chung2, Jihyun Bae2, Jongjin Park2 *, Unyong Jeong1 * and Kinam Kim2 Conductive electrodes and electric circuits that can remain active and electrically stable under large mechanical deformations are highly desirable for applications such as flexible displays1–3, field-effect transistors4,5, energy-related devices6,7, smart clothing8 and actuators9–11. However, high conductivity and stretchability seem to be mutually exclusive parameters. The most promising solution to this problem has been to use one-dimensional nanostructures such as carbon nanotubes and metal nanowires coated on a stretchable fabric12,13, metal stripes with a wavy geometry14,15, composite elastomers embedding conductive fillers16,17 and interpenetrating networks of a liquid metal and rubber18. At present, the conductivity values at large strains remain too low to satisfy requirements for practical applications. Moreover, the ability to make arbitrary patterns over large areas is also desirable. Here, we introduce a conductive composite mat of silver nanoparticles and rubber fibres that allows the formation of highly stretchable circuits through a fabrication process that is compatible with any substrate and scalable for large-area applications. A silver nanoparticle precursor is absorbed in electrospun poly (styrene-block-butadiene-block-styrene) (SBS) rubber fibres and then converted into silver nanoparticles directly in the fibre mat. Percolation of the silver nanoparticles inside the fibres leads to a high bulk conductivity, which is preserved at large deformations (s ≈ 2,200 S cm–1 at 100% strain for a 150-mm-thick mat). We design electric circuits directly on the electrospun fibre mat by nozzle printing, inkjet printing and spray printing of the precursor solution and fabricate a highly stretchable antenna, a strain sensor and a highly stretchable light-emitting diode as examples of applications. Figure 1a presents a schematic illustration of the overall process, in which a non-woven mat of electrospun SBS fibres collected on a surface-treated silicon wafer (Supplementary Fig. S1) is peeled off, then dipped in a silver precursor solution (AgCF3COO in ethanol). The precursor and the solvent are absorbed by the fibres, such that the fibre mat becomes swollen. After drying, the precursor is reduced by a solution of hydrazine hydrate (N2H4.4H2O), generating silver nanoparticles inside the fibres and silver shells at the surfaces of the fibres. The elasticity of the composite fibre mat is similar to that of the as-spun mat, as demonstrated in the photograph in Fig. 1a. At large strains, the silver shell breaks into small pieces of debris. However, electrical conductance is maintained by percolation of the silver nanoparticles inside the

fibres, as well as by inter-fibre bridges formed from the pieces of silver shell. We used AgCF3COO in time-of-flight (TOF) mass spectroscopy to tag positive charges in the molecules, because donation of C¼Cp electrons to the free 5s and 5p orbitals of silver allows interaction between Agþ and unsaturated or aromatic hydrocarbons19–21. The trifluoroacetate anions (CF3COO2) can form an ion-dipole interaction with the hydroxyl groups (–OH) of alcohols, enabling rapid absorption of both precursor and alcohols into the fibres. Figure 1b shows the degree of swelling of the fibre mat measured by the change in lateral dimension as the precursor concentration varies. The degree of swelling increases abruptly by 50% at a concentration of 2.0 wt%, and gradually approaches a saturated value (73%) at 15 wt% concentration. The saturated mat contained 62 wt% silver content after chemical reduction (Supplementary Fig. S2). A similar tendency was observed with other alcohols such as methanol, 2-propanol and 1-butanol. Absorption of the precursor inside the fibres was verified by Fourier-transform infrared spectroscopy (FTIR) after washing the precursor coated on the fibre surfaces with water (Fig. 1c). The absorbance of C–F stretching at 1,128 cm21 and 1,182 cm21 clearly indicates the presence of the precursor inside the fibres. After drying, the embedded precursors prevented the fibre mat from shrinking back to the original size of the as-spun mat (Supplementary Fig. S3). Figure 2 presents scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the swollen fibres (Fig. 2a,b) and their chemically reduced state (Fig. 2c–f ). Figure 2a,b reveals that the precursor formed a conformal coating on the 2-mm-thick SBS fibres. After chemical reduction, the surfaces of the fibres are covered by silver nanoparticles (diameter, 20–40 nm), which merged with one another to form a continuous shell (Fig. 2c,d). A cross-sectional TEM image of an SBS/Ag fibre shows that the silver nanoparticles are densely embedded in the interior of the fibres (Fig. 2e). Higher magnification of the TEM image shows that the silver nanoparticles (average diameter, 20 nm) inside the fibres are connected with each other, forming networked percolation (Fig. 2f ) and that they have a face-centred cubic crystal structure (Supplementary Fig. S4). There are no discernible microdomains in the SBS block copolymer, because the rapid evaporation of the solvent solidifies the polymer chains before they self-assemble22. When the composite fibre mat was annealed at 150 8C, the polymer chains began to self-assemble, so that the silver nanoparticles in the core moved to the outer shell

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Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seoul, Korea, 2 Samsung Advanced Institute of Technology, Mt.14-1, Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do 446–712, Korea, 3 Interdisciplinary School of Green Energy, UNIST, Ulsan 689–798, Korea, † These authors contributed equally to this work. * e-mail: [email protected]; [email protected] NATURE NANOTECHNOLOGY | ADVANCE ONLINE PUBLICATION | www.nature.com/naturenanotechnology

© 2012 Macmillan Publishers Limited. All rights reserved.

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Figure 1 | Fabrication and quantitative analysis of the composite non-woven mat of elastomer fibres and silver nanoparticles. a, Schematic of the process with images corresponding to the steps. Silver precursor (AgCF3COO) and alcohol are absorbed on dipping the electrospun fibre mat into a precursor solution, swelling the fibre mats. After drying, the precursor is reduced into silver nanoparticles inside and on the surfaces of the fibres. The silver nanoparticles percolate, making the fibre mat electrically conductive. The composite fibre mat is highly stretchable, without losing its conductivity at large strains. b, Swelling ratio of the fibre mat after dipping in a precursor solution with varying concentration. Precursor absorption saturates at a concentration of 15 wt%. c, FTIR analysis of the swollen fibre mat after selectively dissolving the precursor deposited at the fibre surface. Because water is a poor solvent for the polymer, the precursor inside the fibres did not dissolve. Black and red lines indicate results for the pure SBS fibre mat and the composite fibre mat, respectively. Asymmetric stretching of C–F bonds in AgCF3COO is reflected in the two strong peaks at 1,128 cm21 and 1,182 cm21.

(Supplementary Fig. S5). Once phase separation took place, the fibre mats lost electrical conductance at a low strain (1 ¼ 0.3). Figure 3a presents cyclic stress–strain curves for the composite fibre mat. A dog-bone-shaped specimen (3 cm × 1 cm × 150 mm) was used for the measurements. The elastic modulus of the composite mat (E ¼ 1.05 MPa) at low strains was higher than that of the as-spun fibre mat (E ¼ 0.47 MPa). It is known that hard fillers increase the elastic modulus of rubber composites when they have conformal interfaces in an elastomeric matrix23,24. The composite fibre mat shows a typical elastomeric behaviour at 1 ¼ 0.4. Residual strain appears at higher strains (1 ≥ 0.6); this is also found in pure SBS fibre mats (Supplementary Fig. S6). Note that the stress–strain curve measures the overall elastic property of the mat, rather than of each fibre. Curvy fibres are straightened at low strains, with negligible local stress accumulation in each fibre. The conformational elasticity of the fibres restores the initial configuration after the strains are released. At large strains, however, the straightened fibres undergo irreversible yielding. Based on the stress–strain curve, the distribution of axial strain in the fibre mat was simulated using the finite-element method ABAQUS. Figure 3b,c presents the simulation at two tensile strains (1 ¼ 0.2, Fig. 3b; 1 ¼ 1.0, Fig. 3c). At 1 ¼ 0.2, the strain near the grips is 2

smaller than the applied strain, which is concentrated in the centre of the specimen. At 1 ¼ 1.0, the strain is distributed uniformly throughout the composite mat. The uneven distribution of the strain at low tensile strain implies that straightening of the fibres and cracking of the silver shells are the main contributions to stretching of the fibre mat. The solid silver shells of the fibres crack at low strain (1 ¼ 0.05, Fig. 3d), as observed in thin metal films on a rubber substrate with weak adhesion25. At increased strains, including 1 ¼ 0.2 in Fig. 3e, the silver layer is completely ruptured. At higher strains (1 ¼ 1.0), the silver shells peel off the fibre surface to form bark-like debris (Fig. 3f ). The enhancement in the modulus is therefore due to the silver nanoparticles embedded inside each fibre, not the silver shells on the outer surface of the fibres26. The SBS/Ag composite mat was torn apart at 1 ¼ 4.9, a value lower than the upper limit of the as-spun fibre mat (Supplementary Fig. S7). The electrical conductivity, s, of the fibre mat reflects the stress in each fibre rather than the overall stress of the mat. Because the silver shell in the surface was completely ruptured at 1 ≤ 0.2, the conductivity of a single fibre is mainly governed by percolation of the silver nanoparticles inside each fibre. Figure 4a shows the change in conductivity under uniaxial elongation of a single fibre,

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Figure 2 | SEM and TEM studies of the SBS/Ag composite fibres. a,b, SEM images of a SBS/AgCF3COO fibre mat before chemical reduction. c,d, SEM images after chemical reduction of AgCF3COO. Silver nanoparticles are generated on each SBS fibre and the SBS surface is uniformly coated with the silver nanoparticles. e,f, Cross-sectional TEM image of an SBS/Ag composite fibre. The silver nanoparticles within each SBS fibre are interconnected, providing the main contribution to electrical conductivity.

comparing the experimental data (solid dots) and the theoretical calculation (solid line) obtained from three-dimensional percolation theory27 and the interparticle distance model28. The fibre was straightened by a marginal strain for setting 1 ¼ 0, and then elongated (Supplementary Fig. S8). The single fibre broke when 1 . 0.6. A theoretical analysis is provided in the Supplementary section, ‘Conductivity calculation of single fibre based on 3D percolation theory’. There was excellent agreement between the theory and experimental data, confirming that percolation of the silver nanoparticles inside the fibre is the main contribution to the conductivity of the mat. A large reduction in the conductivity of the single fibre took place around 1 ¼ 0.2. The electrical conductivity of the composite fibre mat was proportional to the thickness t of the fibre mat, consistent with the theoretical prediction. Details of the calculation are provided in the Supplementary section, ‘Measurement of the conductivity of a composite fibre mat’. Figure 4b shows changes in the conductivity with thickness of the fibre mats under uniaxial stretching. The conductivity of the fibre mats saturates at all strains when t ≥ 150 mm. The high electrical conductivity (s ≈ 5,400 S cm21) at 1 ¼ 0 is attributed to percolation of the silver nanoparticles inside the fibres, together with the electrical connection of the fibres via the silver shells. To investigate the role of the silver shells, these

were scraped off by repeatedly pressing 3M tape onto the fibre mat after the shells had been broken into debris at 1 ¼ 1.0. The pieces of debris were well separated from one another, with no evidence of cold-sintering (Supplementary Fig. S9). After scraping, the conductivity of the composite fibre mat (t ¼ 150 mm) at 1 ¼ 0 decreased to 1,100 S cm21. We assume that the silver shell pieces of debris act as conductive bridges connecting the fibres and lowering the contact resistance between them. The conductivity of the composite fibre mat at 1 ¼ 0 without scraping fell to 4,100 S cm21 when the thickness was 30 mm due to incomplete interconnection. A thickness of 50 mm gives rise to adequate connections in the absence of external stretching. The relation between the conductivity and the strain also depends on the thickness of the fibre mats. For t ¼ 150 mm, the conductivity decreased to 2,200 S cm21 at 1 ¼ 1.0, and to 610 S cm21 at 1 ¼ 1.4, which is comparable to the best data in zero-strain films of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulphonate) (PEDOT:PSS)29. The conductivities of the SBS/Ag fibre mat at 1 ¼ 1.0 were 1,600, 877, 125 and 71 S cm21 for t ¼ 100, 70, 50 and 30 mm, respectively. To compare the conductivity of a single fibre with that of the composite fibre mat, it is necessary to consider the curvy contour path of the fibres. The large drop in conductivity of the composite



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mat around 1 ¼ 0.6 (Fig. 4b) coincides with the emergence of the strain offset in Fig. 2a. The strain for a straightened single fibre (1 ¼ 0.2) seems to correspond to 1 ¼ 0.6 for the fibre mat. The strain offset is responsible for a permanent reduction in the conductivity of the fibre mat after releasing the applied strain. The conductivity of the mat at a fixed strain during repeated stretching is stable, because the yielding of each fibre at a fixed strain is the same. The curvy architecture of the fibres allows stable recovery of the conductivity after the strain is released (Fig. 4c). Up to 1 ¼ 0.4, the conductivity of the mat (t ¼ 150 mm) returns accurately to its initial value (s ¼ 5,450 S cm21). The elastic dependence of the conductivity at low strains is due to the curved shape of the electrospun fibres involved in physical contact with one another. Most of the mechanical stress applied to the mats is absorbed by straightening of the curved fibres, with negligible stress accumulation inside those fibres. Once the composite mats have been elongated by larger strains (1 . 0.4), the stress accumulated in the fibres causes yielding, so that some of the strain remains even after releasing the applied strain. The conductivity at 1 ¼ 0 fell to 5,215 S cm21 after stretching up to 1 ¼ 1.4. Once a large strain had been applied to the mats, the conductivity after releasing became the permanent value for the zero-strain state. After one-time stretching at 1 ¼ 1.4, for instance, the conductivity at zero strain state was the same (s ¼ 5,215 S cm21) for any subsequent strains smaller than 1.4. The composite fibre mats fabricated using this simple solution process offer diverse applications as a bulk stretchable material. As an example, Fig. 5a–c demonstrates a stretchable radiofrequency antenna made of a composite fibre mat on top of an elastomer platform (Ecoflex, type 0010, Smooth-On; for structure see Fig. 5a and inset schematic). The electrospun SBS fibres were 4

collected on the rubber substrate. Printing of the silver precursor solution by nozzle printing, followed by chemical reduction, then created a linear conductor (45 mm × 2.5 mm) with a rectangular shape (5 mm × 5.5 mm) overlaid in the centre. An SMA (SubMiniature version A) male connector with coaxial cable was attached to the centre rectangle via a small amount of liquid metal EGaIn (eutectic Ga–In), so as to ensure electrical contact between the elastomer composite and the connector. To determine the antenna performance under strain, the prepared half-wave dipole antenna was mounted in a jig. The resonance frequency was measured as 2.2 GHz at 1 ¼ 0 (Fig. 5b). When the antenna was stretched by up to 1 ¼ 0.8, it maintained a similar level of high-quality radiation efficiency. The stretchability of the antenna allowed tunability over a wide range of frequencies. Figure 5c shows the resonance frequency as a function of tensile strain. The resonance frequency ( f, in MHz) of a dipole antenna can be estimated by the simple equation30,31 f ¼ 143/(l.1 1/2 eff ), where l is the length of the antenna (in metres) and 1eff is the effective dielectric constant of the environment surrounding the conductor (SBS and air in this analysis). The resonance frequency of the antenna decreased linearly from 2.2 to 1.0 GHz as it was stretched to 1 ¼ 0.8, demonstrating the functionality of a tunable antenna. The value of 1eff was found to be 2.08, in the range of polymers and air. The antenna is also highly reliable, showing no noticeable change in the resonance frequency during repeated stretching cycles at 1 ¼ 0.8 (Supplementary Fig. S10). Additional applications of the bulk material are set out in Supplementary Figs S11–S13. These include a conductive cloth lighting material with an LED chip, a polymer/fibre composite employing the porous character of the fibre mat, a flexible bar coated with



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Figure 4 | Electrical conductivity of a single composite fibre and conductivity change with mat thickness t and strain 1 applied to the substrate. a, Measured conductivity (symbols) of a single composite fibre straightened between two electrodes. The calculated conductivity (line) of the fibre is based on the silver nanoparticle distribution and threedimensional percolation theory. The measured and calculated results are in a good agreement, indicating that percolation of the silver nanoparticles in the fibres is the main contribution to conductivity. b, Changes in conductivity of SBS/Ag fibre mats of different thicknesses with increasing strain. Conductivity increases with mat thickness and saturates at t ¼ 150 mm. c, Changes in conductivity of the composite fibre mat (t ¼ 150 mm) after the strain is released. The conductivity is large and stable at high strains.

conductive fibre mat, a conductive stretchable yarn and a transparent highly conductive polymer substrate with a thin layer of fibre mat on top. For practical applications in electronic devices, the ability to pattern electric circuits over a large area is vital. The upper

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surface of our electrospun fibre mat collected on a silicon wafer is porous, with pores of a few micrometres (Fig. 2a), so the composite fibre mats have some roughness. The roughness is greatly reduced by sandwiching the as-spun fibre mat between two silicon wafers, followed by thermal annealing at 90 8C for a short time (10 min). The thickness of the polymer fibre mat reduces from 150 mm to 120 mm, producing a rubbery substrate with an average roughness of 134 nm and a surface coverage of 85% (Supplementary Fig. S14). The annealed composite fibre mat has the same conductivity as its counterpart without thermal annealing. Direct printing of the precursor solution can be achieved by conventional techniques such as nozzle printing and inkjet printing of the precursor solution followed by direct chemical reduction. An example of a pattern array (linewidth, 200 mm) on a thin SBS fibre layer (thickness, 30 mm), and its related application in stretchable LED lighting, are demonstrated in Supplementary Fig. S15. A small amount of liquid metal, EGaIn, was used for the contact between the stretchable electrode and the terminals of the LED chips. Patterning with clear definition was achieved by spray printing the precursor solution through masks (Fig. 5d). Figure 5e shows a 50 mm line-and-space pattern obtained by spray printing. The conductivity of the pattern could be controlled by regulating the spraying time of the precursor solution in ethanol (15 wt% precursor solution, flow rate ¼ 10 ml h21). When the silver content in the mat is ≥15 wt% (spraying for 120 s), the resistance of the pattern is invariant in the small strain region (Fig. 5f ). The sensitivity to strain, relative resistance change versus resistance at zero strain (DR/Ro), is almost zero. In contrast, the resistance change of the pattern is sensitive when the silver content of the mat is less than 10 wt% (spraying for 90 s); indeed, the sensitivity is monotonically proportional to the applied strain value. This result facilitates an approach towards the direct patterning of strain sensors composed of a metallic interconnection and a strain-sensitive unit. Our process uses electrospun fibre mats, which are cheap and already scalable for industry. The direct printing of the conductive circuits over large areas takes advantage of simple solution processes routinely used in current microfabrication. Although this study demonstrates a conductive non-woven mat made of electrospun rubber fibres and silver nanoparticles, the concept can be extended to diverse material combinations for the preparation of functionalized composite materials.

Methods Materials. SBS (Mn ¼ 142,000 g mol21, weight fraction of styrene ¼ 28.4%) was purchased from Kraton. Octadecyltrichlorosilane (OTS, ≥90%), AgCF3COO (98%), hydrazine hydrate (N2H4.4H2O, 50–60%) and EGaIn (99%) were purchased from Sigma Aldrich. Tetrahydrofuran (THF, 99.5%), N,N-dimethylformamide (DMF, 99.5%), toluene (99.5%) and ethanol (95.0%) were purchased from JT Baker. Poly(dimethylsiloxane) (PDMS) was purchased from Dow Corning, and Ecoflex (type-0010) from Smooth-On. OTS treatment of silicon wafers. A silicon wafer was treated with oxygen plasma to generate hydroxyl groups on its surface. It was then dipped in an OTS/toluene solution (0.5 wt%) for 3 min. Because the trichlorosilane group of OTS has high reactivity to the hydroxyl group, a self-assembled monolayer quickly formed on the surface of the wafer. To remove the residual OTS, the wafer was gently washed with toluene and dried in a N2 atmosphere. Preparation of the composite mat of SBS fibre and silver nanoparticles. SBS was dissolved in a solvent mixture of THF/DMF (wt/wt ¼ 3:1). The polymer solution (10 wt% concentration) was electrospun onto the OTS-treated silicon wafer at a fixed feed rate (20 ml min21) and voltage (18.0 kV). The nozzle-to-collector distance was 15 cm. The resulting electrospun fibre mat (SBS fibre mat) was readily peeled away from the wafer. The electrospun fibre mat was dipped in an ethanol solution of AgCF3COO (15 wt%). The fibre mat was removed from the solution after 30 min and dried at room temperature. Hydrazine hydrate (50%) in ethanol/water mixture as solvent (vol/vol ¼ 1:1) was dropped onto the fibre mat to reduce the absorbed precursor. After 5 min, the residual reducing agent was rinsed out several times using deionized water. Patterning circuits on the SBS fibre mat on an elastomer substrate. The SBS fibre mats (thickness, 30–150 mm) were transferred to PDMS (monomer:initiator ¼



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Figure 5 | Applications of the conductive composite mat as a stretchable antenna and as a strain sensor enabled by a micropattern of conductive lines. a, Photograph of a half-wave dipole antenna created by nozzle-printing on top of an Ecoflex platform. Inset: schematic of the structure. b, Frequency response of the reflected power from the antenna at the initial length. c, Resonance frequency of the antenna as a function of tensile strain. d,e, Photograph and SEM image of a micropattern of conductive lines drawn by spray-printing of the precursor solution through a mask. f, Strain sensitivity (DR/Ro) of the pattern according to the silver content in the mat. Patterns with a large silver content (≥15 wt%) had no resistance change at small strains, but the sensitivity of patterns with low silver content (≤10 wt%) was proportional to the applied strain. 25:1, wt/wt) or Ecoflex pads from the OTS-treated silicon wafer. AgCF3COO in ethanol solution (15 wt%) was injected from a nozzle printer or inkjet printer to the SBS/PDMS substrate through a soft needle with a diameter of 100 mm. The injection parameters were controlled by an air-pressure controller and a dispensing machine (SUPER x and SHOTMASTER 300, Musashi Engineering). The precursor solution was applied by air pressure (2.2 kPa) and the nozzle was moved at 40 mm s21 so as to draw discrete patterns on the substrate. AgCF3COO was initially reduced by N2H4 gas for 2 min, and further reduced by N2H4 solution in an ethanol/water mixture solvent. After 5 min, the patterned circuit was dried in vacuum. Characterization. SEM and energy-dispersive X-ray (EDX) mapping images were taken by field-emission SEM (FESEM, JSM-7001F, JEOL). A 100-nm-thick cross-section of the SBS/Ag composite fibre was obtained using a cryomicrotome (PowerTome PC, SIMS Co.). High-resolution TEM (HRTEM) analysis was undertaken with a JEOL 2100F. Thermogravimetric analysis (TGA) was performed to determine the weight fraction of silver in the fibre 6

mat (Q500, TA Instruments). Atomic force microscopy (AFM) images were taken in tapping mode (Dimension 3100, Digital Instrument Co.). FTIR analysis (FTIR NICOLET 6700, Thermo Scientific Co.) was performed in the attenuated total reflection (ATR) mode. A stress–strain curve was obtained using a tensile stress tester (Linkam Scientific Instruments). Sheet resistances were measured using a four-point probe (CMT-Series, Chang Min Co.). I–V characteristics and the conductivity of the SBS/Ag fibre mats were measured using an Agilent 4156A.

Received 18 April 2012; accepted 22 October 2012; published online 25 November 2012

References 1. Park, S-I. et al. Printed assemblies of inorganic light-emitting diodes for deformable and semitransparent displays. Science 325, 977–981 (2009).




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Acknowledgements This research was supported in part by a National Research Foundation (NRF) grant funded by the Korean Government (MEST) through the Active Polymer Center Pattern Integration (no. R11-2007-050-01004-0), by the Advanced Soft Electronics under the Global Frontier Research Program (2011-0031659) and by the World Class University Program (R32-20031).

Author contributions U.J. and M.P. designed the experiments. M.P. and J.I. performed the experiments. M.S. and J.Y.P. contributed materials. Y.M., H.C. and S.P. performed the cryo-microtoming and TEM analysis. M-B.S. and D-Y.C. analysed the mechanical properties with the finite element method. S.J. and J.B. characterized the performance of the stretchable antenna. M.P. and J.I. co-wrote the paper. U.J., J.P. and K.K. conceived and guided the project. All authors discussed the results and commented on the manuscript at all stages.

Additional information Supplementary information is available in the online version of the paper. Reprints and permission information is available online at http://www.nature.com/reprints. Correspondence and requests for materials should be addressed to J.P. and U.J.

Competing financial interests The authors declare no competing financial interests.



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