Highly stretchable electric circuits from a composite material of silver nanoparticles and elastomeric fibres Minwoo Park 1† , Jungkyun Im 2† , Minkwan Shin 1 , Yuho Min 1 , Jaeyoon Park 1 , Heesook Cho 3 , Soojin Park 3 , Mun-Bo Shim 2 , Sanghun Jeon 2 , Dae-Young Chung 2 , Jihyun Bae 2 , Jongjin Park 2 * , Unyong Jeong 1 * and Kinam Kim 2 Conductive electrodes and electric circuits that can remain active and electrically stable under large mechanical defor- mations are highly desirable for applications such as flexible dis- plays 1–3 , field-effect transistors 4,5 , energy-related devices 6,7 , smart clothing 8 and actuators 9–11 . 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 fabric 12,13 , metal stripes with a wavy geometry 14,15 , composite elastomers embedding conductive fillers 16,17 and interpenetrating networks of a liquid metal and rubber 18 . At present, the conductivity values at large strains remain too low to satisfy requirements for practical applications. Moreover, the ability to make arbi- trary patterns over large areas is also desirable. Here, we intro- duce a conductive composite mat of silver nanoparticles and rubber fibres that allows the formation of highly stretchable cir- cuits 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 (AgCF 3 COO 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 (N 2 H 4 . 4H 2 O), 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 demon- strated 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 AgCF 3 COO 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 Ag þ and unsaturated or aromatic hydrocarbons 19–21 . The trifluoroacetate anions (CF 3 COO 2 ) can form an ion-dipole inter- action 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 con- centration 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 pre- cursor 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 cm 21 and 1,182 cm 21 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 trans- mission 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 dis- cernible microdomains in the SBS block copolymer, because the rapid evaporation of the solvent solidifies the polymer chains before they self-assemble 22 . 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 1 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: ujeong@yonsei.ac.kr; jongjin00.park@samsung.com LETTERS PUBLISHED ONLINE: 25 NOVEMBER 2012 | DOI: 10.1038/NNANO.2012.206 NATURE NANOTECHNOLOGY | VOL 7 | DECEMBER 2012 | www.nature.com/naturenanotechnology 803 © 201 2 M acmillan Publishers Limited. All rights reserved.