Letter pubs.acs.org/NanoLett
Knitted Fabrics Made from Highly Conductive Stretchable Fibers Rujun Ma,†,‡ Jiyong Lee,†,‡ Dongmin Choi,§ Hyungpil Moon,§ and Seunghyun Baik*,†,§ †
IBS Center for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Daejeon, 305-701, Korea Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Korea § School of Mechanical Engineering, Sungkyunkwan University, Suwon, 440-746, Korea ‡
S Supporting Information *
ABSTRACT: We report knitted fabrics made from highly conductive stretchable fibers. The maximum initial conductivity of fibers synthesized by wet spinning was 17460 S cm−1 with a rupture tensile strain of 50%. The maximum strain could be increased to 490% by decreasing the conductivity to 236 S cm−1. The knitted fabric was mechanically and electrically reversible up to 100% tensile strain when coated by poly(dimethylsiloxane). The normalized resistance of the poly(dimethylsiloxane)-coated fabric decreased to 0.65 at 100% strain. KEYWORDS: Silver particles, carbon nanotubes, conductive stretchable fibers, knitted fabrics, reversibility
S
by nanotubes with high aspect ratios.7,20 Moreover, the contact interface was significantly improved by agglomeration between microscale Ag particles and nanoscale Ag particles preadsorbed on the side wall of carbon nanotubes during the curing process.7,20,21 There were two drawbacks in the previous stretchable conductive film.7 There was a large increase in resistance when the film was stretched more than 30−40% tensile strain. Besides, the film could not return to the initial state due to the plastic deformation at high strain region.7 Here we significantly improved the resistance change and mechanical elasticity at high strain region by fabrics knitted using highly conductive stretchable fibers. The fibers composed of 100−150 nm Ag particles, nAg-MWNTs, and PVDF-HFP matrix were synthesized by wet spinning. Figure 1a shows a schematic of the wet spinning process (see the Experimental Section for details). Briefly, 100−150 nm Ag particles were dispersed in 4-methyl-2pentanone by ultrasonication. The mixture gel of nAg-MWNTs and ionic liquid (1-butyl-4-methylpyridinium tetrafluoroborate) was then added and ultrasonicated. The ionic liquid improved dispersion of nAg-MWNTs and increased conductivity of composites during stretching.5−7 The relative concentration of self-assembled Ag nanoparticles over MWNTs was previously optimized at ∼35 wt %.7 Finally, a PVDF-HFP solution was added and stirred for 2 h in order to form a spinnable dope. The dope (∼2 mL) was extruded through a nozzle with a diameter of 200 μm into a coagulant by aid of a syringe pump (3 mL/h) to remove the solvent.18 The average fiber diameter
tretchable electronic devices have received considerable attention recently.1−11 To realize such devices, stretchable metals as well as semiconductors need to be developed. Wavy structural configurations where thin metals or semiconductors are applied on prestretched polymer substrates have been intensively investigated.1,2,12,13 On the other hand, bulk composites possessing stretchability and conductivity have been developed by imbedding functional nanoparticles in stretchable polymers to address metallic parts.5−7,14,15 However, the composites demonstrated an inevitable decrease in conductivity in the high strain region due to the volumetric expansion of the polymer matrix or disconnection of conductive fillers.5,7,14 Besides, it is very challenging to achieve reversible stretching of composites. On the other hand, stretchability can be obtained by fabrics even they are knitted using rigid fibers. Multifunctional fibers made of carbon nanotubes (CNTs) have been explored with potential applications in intelligent clothings, structural textiles, woven electrodes, and artificial muscles.16−18 These include fibers synthesized by wet spinning and yarns drawn from multiwalled carbon nanotube (MWNT) forests providing lightweight, high specific strength, high electrical and thermal conductivity, and torsional/tensile actuation.16−18 Wet spinning is a scalable technology and widely employed in industry to manufacture ballistic fibers such as Kevlar.18,19 We previously demonstrated conductive flexible adhesives and stretchable composite films using microscale silver (Ag) particles and multiwalled carbon nanotubes decorated with 3−5 nm silver particles (nAg-MWNTs).7,20 The conductivity of bulk composite films significantly increased by a small addition of nAg-MWNTs. Microscale Ag particles embedded in flexible nitrile butadiene rubber or stretchable poly(vinylidene fluorideco-hexafluoropropylene) (PVDF-HFP) matrix were connected © 2014 American Chemical Society
Received: December 27, 2013 Revised: March 13, 2014 Published: March 24, 2014 1944
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Figure 1. Synthesis of conductive stretchable fibers. (a) A schematic of the wet spinning apparatus. An SEM image of 100−150 nm Ag particles and a HRTEM image of nAg-MWNTs are shown. (b) The synthesized fiber was continuously collected on a winding drum. The fiber length was longer than 10 m. (c) SEM image of the spun fiber with a diameter of ∼100 μm. (d) SEM images of the hot-rolled fiber. The width and thickness were ∼250 and ∼17 μm.
was ∼100 μm. The diameter of fibers could be controlled by the nozzle size of wet spinning apparatus. A number of different coagulants were tested, and hexane provided highest stretchability of the fiber (see Supporting Figure S1). The 4-methyl-2pentanone was extracted while the ionic liquid was retained inside the fiber due to the immiscibility with hexane.22 The filament longer than 10 m was collected on a winding drum (Figure 1b) and further cured in an oven at 135 °C. As shown in Figure 1c, the diameter of the fiber was uniform (∼100 μm). The density of the wet-spun fiber was 2.17 g cm−3 measured by the Archimedes method (electronic densimeter MD-300S). The density increased to 2.58 g cm−3 after the hot rolling process (Figure 1d). The fillers were more uniformly distributed, and the fiber was densified.7 The corresponding relative densities were 0.752 and 0.894 before and after the hot rolling process (see Supporting Information for details). The typical width and thickness of the fiber were ∼250 and ∼17 μm after the hot rolling process. The hot-rolled thin fiber could be easily deformed in a helical shape by hand as shown in the inset of Figure 1d. Figure 2a shows the conductivity and maximum tensile strain of spun fibers as a function of the 100−150 nm Ag particle volume fraction in the fiber. The corresponding Ag particle mass fraction in the initial mixture is shown with the x-axis displayed on top. The mass and volume fraction of Ag particles in the fiber became higher due to the extraction of solvent during the wet spinning and curing process. The mass of other components in the initial mixture was identical for four fibers. For simplicity, the 100−150 nm Ag particle mass fraction in the initial mixture was used to designate each fiber synthesized in this study. The mass fraction of nAg-MWNTs in the initial mixture was 1.06 wt % for the specimen with 8.5 wt % of 100− 150 nm Ag particles. The conductivity increased but
stretchability decreased as the mass fraction of Ag particles increased. The addition of Ag particles increased brittleness of the fiber upon stretching. The conductivity and maximum strain at rupture were 236 S cm−1 and 490% at 7.4 wt % of Ag particles. As the silver concentration increased to 10.4 wt %, the conductivity increased to 8344 S cm−1, but stretchability decreased to 55%. Nevertheless, these fibers exhibited substantially greater stretchability compared with other CNTbased fibers.16−18 The electrical conductivity of composites was theoretically calculated using the power law relationship7,23 σ = σ0(Vf − Vc)s
(1)
where σ is the electrical conductivity of the composite, σ0 is the conductivity of the conductive filler, Vf is the volumetric fraction of the filler, Vc is the volumetric fraction at percolation threshold, and s is the fitting exponent. Ag particles were modeled as uniformly distributed spheres, and the percolation threshold was calculated using the average interparticle distance model (see the Supporting Information for details).7,23 There was a good agreement between the theory and data (Figure 2a). An optimized silver concentration should be determined based on the requirements of target application. Figure 2b shows the conductivity change of fibers at two different Ag particle concentrations (8.5 and 10.4 wt %) as a function of tensile strain. The initial conductivity was 8344 S cm−1 for the fiber with 10.4 wt % of Ag particles. The initial conductivity decreased to 2681 S cm−1 when the Ag particle concentration decreased to 8.5 wt %. However, the maximum strain at rupture increased from 55% to 350%. Both fibers demonstrated a decrease in conductivity with increasing tensile strain. The theoretically calculated conductivity change of a fiber (Ag: 10.4 wt %) with increasing strain is also shown.7,23 A 1945
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Figure 2. Electrical and mechanical properties of conductive stretchable fibers. (a) The effect of 100−150 nm Ag particle concentration on conductivity and stretchability. Three specimens were characterized for each condition, and the standard deviation is shown. (b) The conductivities of fibers are shown as a function of tensile strain. The conductivity calculated using a three-dimensional percolation theory is also shown (Ag: 10.4 wt %, without the hot-rolling process). (c) Optical images of the fiber (Ag: 8.5 wt %) before and after stretching. (d) The initial conductivities and maxium tensile strains at rupture are compared with those of control materials. Filled symbols: fibers synthesized in this study. Open symbols: black open square, CNT fiber;18 dark cyan open down triangle, red open circle, navy open diamond, wine open pentagon, olive open star, polymer/CNT composites;5,6,15,25,27 blue open up triangle, dark yellow open right triangle, polymer/metal composites;14,26 magenta open left triangle, pink open hexagon, polymer/Ag/CNT composites.7,24
Fibers with four different diameters (70, 100, 190, and 380 μm) were synthesized by changing the diameter of nozzles (100, 200, 310, and 500 μm) as shown in Supporting Figure S2. The conductivity−tensile strain characteristics were similar regardless of the diameter of fibers in the investigated range. Supporting Figure S3 compares the conductivities of three fibers synthesized under identical conditions (Ag: 8.5 wt %) as a function of tensile strain. The conductivity−tensile strain characteristics were similar for all three fibers demonstrating good reproducibility. Figure 2c shows optical images of the fiber (Ag: 8.5 wt %) before and after stretching demonstrating excellent stretchability. SEM images of fibers with and without the hot-rolling process are provided in Supporting Figure S4. Ag particles were more uniformly embedded in the polymer matrix after the hotrolling process.7 The increased longitudinal distance between Ag particles could be observed after stretching decreasing conductivity as discussed above. Figure 2d compares the initial conductivity and maximum strain of spun fibers (filled
detailed derivation is provided in the Supporting Information. Briefly, the volume change of the matrix polymer as a function of strain modulates Vf of eq 1.7 The diameter change of the fiber was experimentally measured, resulting in an isotropic Poisson’s ratio of 0.243. As the strain increased, the volume of the composite fiber (Vmatrix) increased, whereas the volume of Ag particles (Vsilver) was invariant. This led to a decrease in Vf = Vsilver/Vmatrix and σ. There was a reasonable agreement between the data and theory validating the proposed mechanism.7 The theoretical prediction slightly overestimated the data. This could be due to the disconnection of conductive fillers (Ag particles and nAg-MWNTs) upon stretching which was not considered in the model. The conductivity of fibers increased after the hot-rolling process (Figure 2b). The film was densified after the hot-rolling process increasing Vf. This resulted in an increase in conductivity. The maximum initial conductivity was 17460 S cm−1 (Ag: 10.4 wt %). However, the decreasing trend in conductivity with increasing strain was similar. 1946
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Figure 3. Ropes and weft knitted fabrics made from the conductive stretchable fibers. The 100−150 nm Ag particle concentration in the initial mixture was 8.5 wt % for all fibers. (a) Optical images of the 2-ply rope, 4-ply rope, and weft knitted fabric (purl stitch). The images of the weft knitted fabric before and after stretching are also shown. (b) The normalized resistance is shown as a function of tensile strain. (c) The fabric was stretched between 0 and 100% tensile strain up to 3000 cycles. (d−e) The fabric was attached on a robot finger to transmit touch sensing data to the main controller by a CAN bus system. The force (F, mN), moment (M, mN·m), and position (Pos, mm) information was successfully analyzed as the robot finger touched a piano key. The length of an arrow in the inset image is proportional to force.
decreased to 236 S cm−1. Only one control specimen (high density carbon nanotube fibers synthesized by wet spinning) provided a greater conductivity than our fibers.18 However, the stretchability was very small (∼1.4%).18 Polymer−carbon nanotube composites provided generally lower conductivity but higher stretchability. These include vinylidenefluoride− hexafluoropropylene SWNT film,5 vinylidenefluoride−tetrafluoroethylene−hexafluoropropylene SWNT film,6 textile
symbols) with those of control materials in literatures (open symbols).5−7,14,15,18,24−27 Generally, stretchability could be enhanced by sacrificing conductivity. This could be controlled by Ag particle concentration in this study. The conductivity of the hot-rolled fiber was as high as 17460 S cm−1 with a reasonably high strain of 50% (Ag: 10.4 wt %). The maximum strain at rupture increased to 490% by decreasing the concentration of silver (7.4 wt %), but the initial conductivity 1947
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Figure 4. Reversible performance of the single fiber and fabric (Ag: 8.5 wt %) coated by PDMS. (a) The PDMS-coated single fiber was stretched to 100% strain and released to 0% strain. (b) Optical images of the tightly knitted fabric. (c) The normalized resistances of the PDMS-coated single fiber and PDMS-coated fabric are shown during the stretching and releasing cycle. (d, e). The PDMS-coated single fiber and PDMS-coated fabric were repeatedly stretched and released between 0 and 100% tensile strain. Three specimens were characterized for each condition, and the standard deviation is shown. The inset images show specimens after 300 stretching cycles. (f) The current−voltage characteristics of LEDs attached to PDMScoated two fibers are shown. The applied bias was 3.5 V. Visual images of LEDs before (top) and after (bottom) stretching are provided in the inset.
SWNT film,15 poly(dimethylsiloxane) SWNT film,25 and polyurethane MWNT film.27 A higher conductivity could be obtained by polymer−metal or polymer−metal−carbon nanotube composites sacrificing stretchability. The polyurethane− gold film,14 poly(styrene-block-butadiene-block-styrene)−Ag mat,26 PVDF−Ag MWNT film,7 and polystyrene−polyisoprene−polystyrene−Ag MWNT film24 belong to this category. It is challenging to achieve both high conductivity and stretchability by composite films or fibers. On the other hand, the stretchability of clothes has been obtained by fabrics even they are knitted using rigid fibers. Figure 3a shows optical images of 2-ply rope, 4-ply rope, and weft knitted fabric (purl stitch) made from the conductive stretchable fibers. The 2-ply rope was fabricated by twisting and relaxing two fibers to reach a torsional balance.28 The 4-ply rope was obtained by twisting and relaxing two 2-ply ropes.28 The fabric was knitted by hand using a long spun fiber. The purl stitch is widely employed in baby and children clothes due to the excellent stretchability.29 The stretchability depended on the pattern of fabrics. A plain weave fabric was also made for comparison, but stretchability was not enhanced (see Supporting Figure S5). The optical images of the weft knitted fabric as a function of strain are shown in Figure 3a. The fabric structure was first deformed followed by stretching of the fiber. A schematic of the fabric before and after stretching is provided in Supporting Figure S6.
Figure 3b compares the normalized resistance change of the single spun fiber, 2-ply rope, 4-ply rope, and weft knitted fabric as a function of tensile strain. The corresponding initial resistances were 4.27, 3.31, 2.43, and 7.8 Ω, respectively. The conductivity change could not be calculated due to the deformation of nonuniform cross-section. The stretchability of ropes was greater than that of the single fiber. This increase in stretchability of ropes was also previously observed by CNT yarns.17,28,30 The increase in normalized resistance with increasing strain was retarded by making 2-ply and 4-ply ropes compared with the single fiber. The resistance was measured by clipping each end of all fibers. However, the resistance of the 4-ply rope still increased ∼333 times at 200% strain. In contrast, the resistance increase of the weft knitted fabric was negligible up to ∼200% strain. The resistance was measured by clipping each end of a fiber used for the fabric. The resistance even slightly decreased up to ∼150% strain. This could be due the tighter contact of the fiber at the stitch crosssection. This remarkable stability in resistance up to about ∼200% strain is good enough to employ this conductive stretchable fabric for practical applications in human clothes and industrial robots. The resistance increased ∼22 times even at 450% strain where the fabric was ruptured. Figure 3c shows stretching cycleability of the fabric. The fabric was stretched and released between 0 and 100% tensile strain. The resistance 1948
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The effective Poisson’s ratio of the PDMS-coated fiber increased to 0.333 (see Supporting Figure S8 for optical measurement of the effective Poisson’s ratio). This increase in the effective Poisson’s ratio reduced the volumetric increase of the PDMS-coated fiber with increasing strain. This resulted in the less strain-dependent resistance change.7 The normalized resistance of the PDMS-coated fabric even slightly decreased (R/Ro = 0.65) when stretched to 100% strain with a nearly complete mechanical and electrical reversibility. This is a significant breakthrough compared with the previous conductive stretchable film where R/Ro = 67.8 at 100% strain.7 The cycleability was also excellent as shown in Figure 4d and e. There was no change in electrical resistance and mechanical structure when the PDMS-coated single fiber and PDMScoated fabric were stretched between 0 and 100% tensile strain up to 300 cycles. The electrical performance of PDMS-coated fiber was visually demonstrated using light-emitting diode (LED) chips as shown in Figure 4f. Visual images of LEDs before (top) and after (bottom) stretching are also shown in the inset. The current slightly decreased as the fiber was stretched to 100% strain due to the small increase in resistance (Figure 4c). However, the currents at 0 and 100% strain after 100 stretching cycles were identical to the initial currents at 0 and 100% strain, respectively, confirming excellent cycleability. In summary, highly conductive stretchable fibers composed of 100−150 nm Ag particles, nAg-MWNTs, and PVDF-HFP matrix were synthesized by the wet-spinning method. The initial conductivity increased, but stretchability decreased as the Ag particle concentration increased. The maximum initial conductivity of fibers was 17460 S cm−1 with a rupture tensile strain of 50%. The maximum strain could be increased to 490% by decreasing the initial conductivity to 236 S cm−1. There was an inevitable decrease in conductivity of the fiber as the strain increased due to the decrease in volumetric filler fraction and disconnection of conductive fillers. This was significantly improved by the weft knitted fabric where the resistance increase was negligible up to ∼200% strain. Excellent mechanical elasticity and electrical reversibility could be achieved when the fiber and fabric were coated with PDMS. The cycleability between 0 and 100% strain was very good. The successful implementation of the fabric for the CAN data bus system of a robot finger demonstrates the feasibility for practical application in electronics and robot industries. Experimental Section. Synthesis of Conductive Stretchable Fibers. Multiwalled carbon nanotubes decorated with 3−5 nm silver nanoparticles (nAg-MWNTs) were synthesized using a previously published protocol.7,20,21 Briefly, benzyl mercaptan in ethanol (0.1 mol L−1, 2.4 mL, Sigma Aldrich) was mixed with AgNO3 solution in ethanol (0.02 mol L−1, 300 mL, Junsei) by stirring for 48 h to synthesize Ag nanoparticles. In the next step, MWNTs (100 mg, Hanwha Nanotech) dispersed in ethanol (200 mL) were added and additionally sonicated (bath sonicator, 200 W, 8 h). Finally, nAg-MWNTs were collected by filtration (PTFE membrane, 0.2 μm), rinsing with ethanol, and drying in a vacuum chamber at room temperature. The conductive stretchable fiber was synthesized by the wet spinning technology. First, 100−150 nm Ag particles were dispersed in 4-methyl-2-pentanone (10 mL) by ultrasonication (560 W, 20 min). A gel mixture of ionic liquid (1-butyl-4methylpyridinium tetrafluoroborate, 600 mg) and nAgMWNTs (200 mg) was also prepared by mixing and grinding for 20 min using a pestle and mortar.5−7 In the next step, the gel mixture (800 mg) was added to the Ag particle suspension
measured at 100% strain did not change up to 3000 stretching cycles. The conventional rigid electrical wires impede stretchinginvolved joint motion of robots. The bending of robot fingers or arms needs stretching of accompanying wires for sensors and power supply. Typically, multiple rigid wires are installed on outside of robots, and extra length is provided to address this hurdle since the conventional wires are not stretchable. These long wires are detached from the surface of robots and occupy additional space when the finger or arm is in straight position. This makes the compact design difficult. The stretchable wires without extra length can be attached to the surface of robots or installed inside robots advancing robotics design. Figure 3d and e show the performance demonstration of the conductive stretchable fabric employed for the controller area network (CAN) data bus system of a robot finger. A detailed fabrication process of the robot finger was provided elsewhere,31 and a schematic of the dissembled robot finger is shown in Supporting Figure S7. Briefly, the 6-axis force and moment information was calculated by an embedded microprocessor, when the robot finger-tip was touched by an object, using the data from six strain gauges installed in a finger-tip sensor frame.31,32 Two electrical wires were required to transmit processed data over the CAN interface, and one wire was replaced by the fabric. The fabric was attached on the exterior of the robot finger for visual demonstration. The force and moment information was transmitted to the main controller through the fabric, at a sampling rate of 100 Hz, as the robot finger touched a piano key. The position information was also calculated using a customized software in the main controller.31 The touch position and force was visualized using an arrow in the controller image. The signal was successfully transmitted although the fabric went through more than 100% stretching during the finger touching maneuver. This demonstrates excellent compatibility of the fabric for the CAN data bus system which is commonly used in robotic and automotive industries. It is challenging to obtain reversible behavior of conductive stretchable films when stretched more than 30−40% tensile strain.7 Here we achieved excellent mechanical elasticity and electrical reversibility by coating the fiber and fabric using poly(dimethylsiloxane) (PDMS). Figure 4a shows the single fiber (Ag: 8.5 wt %) coated by PDMS (40 × 3 × 0.4 mm3). The PDMS-coated single fiber was mechanically elastic when stretched and released between 0 and 100% tensile strain. Figure 4b shows a tightly knitted fabric (Ag: 8.5 wt %) with smaller pores compared with the fabric shown in Figure 3a. The knitting pattern density can be defined as the average number of pores per unit area (mm2). The corresponding knitting pattern densities were 1.65/mm2 (Figure 3a) and 5.03/mm2 (Figure 4b). The stretchability generally decreased as the knitting pattern density increased. The normalized resistance change of the PDMS-coated single fiber and PDMS-coated fabric are shown in Figure 4c during the stretching and releasing cycle. The normalized resistance of the PDMS-coated single fiber was 7.0 at 100% tensile strain which is much smaller than that of the bare single fiber shown in Figure 3b. Besides, there was almost no hysteresis demonstrating excellent mechanical and electrical reversibility. The pure PDMS film is mechanically elastic with a Poisson’s ratio of 0.5.33 The elasticity of the PDMS-coated fiber was enhanced due to the elastic property of the surrounding PDMS. The Poisson’s ratio of the wet spun fiber was 0.243. 1949
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(7) Chun, K. Y.; Oh, Y.; Rho, J.; Ahn, J. H.; Kim, Y. J.; Choi, H. R.; Baik, S. Nat. Nanotechnol. 2010, 5 (12), 853−857. (8) Chae, S. H.; Yu, W. J.; Bae, J. J.; Duong, D. L.; Perello, D.; Jeong, H. Y.; Ta, Q. H.; Ly, T. H.; Vu, Q. A.; Yun, M.; Duan, X. F.; Lee, Y. H. Nat. Mater. 2013, 12 (5), 403−409. (9) Benight, S. J.; Wang, C.; Tok, J. B. H.; Bao, Z. A. Prog. Polym. Sci. 2013, 38 (12), 1961−1977. (10) Wagner, S.; Bauer, S. MRS Bull. 2012, 37 (3), 207−213. (11) Kaltenbrunner, M.; Kettlgruber, G.; Siket, C.; Schwodiauer, R.; Bauer, S. Adv. Mater. 2010, 22 (18), 2065−2067. (12) Kim, D. H.; Song, J. Z.; Choi, W. M.; Kim, H. S.; Kim, R. H.; Liu, Z. J.; Huang, Y. Y.; Hwang, K. C.; Zhang, Y. W.; Rogers, J. A. Proc. Natl. Acad. Sci. U.S.A. 2008, 105 (48), 18675−18680. (13) Wang, X. L.; Hu, H.; Shen, Y. D.; Zhou, X. C.; Zheng, Z. J. Adv. Mater. 2011, 23 (27), 3090−3094. (14) Kim, Y.; Zhu, J.; Yeom, B.; Di Prima, M.; Su, X.; Kim, J. G.; Yoo, S. J.; Uher, C.; Kotov, N. A. Nature 2013, 500 (7460), 59−63. (15) Hu, L. B.; Pasta, M.; La Mantia, F.; Cui, L. F.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Nano Lett. 2010, 10 (2), 708−714. (16) Lima, M. D.; Fang, S. L.; Lepro, X.; Lewis, C.; Ovalle-Robles, R.; Carretero-Gonzalez, J.; Castillo-Martinez, E.; Kozlov, M. E.; Oh, J. Y.; Rawat, N.; Haines, C. S.; Haque, M. H.; Aare, V.; Stoughton, S.; Zakhidov, A. A.; Baughman, R. H. Science 2011, 331 (6013), 51−55. (17) Lima, M. D.; Li, N.; de Andrade, M. J.; Fang, S. L.; Oh, J.; Spinks, G. M.; Kozlov, M. E.; Haines, C. S.; Suh, D.; Foroughi, J.; Kim, S. J.; Chen, Y. S.; Ware, T.; Shin, M. K.; Machado, L. D.; Fonseca, A. F.; Madden, J. D. W.; Voit, W. E.; Galvao, D. S.; Baughman, R. H. Science 2012, 338 (6109), 928−932. (18) Behabtu, N.; Young, C. C.; Tsentalovich, D. E.; Kleinerman, O.; Wang, X.; Ma, A. W. K.; Bengio, E. A.; ter Waarbeek, R. F.; de Jong, J. J.; Hoogerwerf, R. E.; Fairchild, S. B.; Ferguson, J. B.; Maruyama, B.; Kono, J.; Talmon, Y.; Cohen, Y.; Otto, M. J.; Pasquali, M. Science 2013, 339 (6116), 182−186. (19) Yang, H. H. Aromatic high-strength fibers; Wiley: New York, 1989. (20) Ma, R.; Kwon, S.; Zheng, Q.; Kwon, H. Y.; Kim, J. I.; Choi, H. R.; Baik, S. Adv. Mater. 2012, 24 (25), 3344−3349. (21) Oh, Y.; Chun, K. Y.; Lee, E.; Kim, Y. J.; Baik, S. J. Mater. Chem. 2010, 20 (18), 3579−3582. (22) Moniruzzaman, M.; Nakashima, K.; Kamiya, N.; Goto, M. Biochem. Eng. J. 2010, 48 (3), 295−314. (23) Li, J.; Kim, J. K. Compos. Sci. Technol. 2007, 67 (10), 2114− 2120. (24) Chun, K. Y.; Kim, S. H.; Shin, M. K.; Kim, Y. T.; Spinks, G. M.; Aliev, A. E.; Baughman, R. H.; Kim, S. J. Nanotechnology 2013, 24 (16), 165401. (25) Lipomi, D. J.; Vosgueritchian, M.; Tee, B. C. K.; Hellstrom, S. L.; Lee, J. A.; Fox, C. H.; Bao, Z. N. Nat. Nanotechnol. 2011, 6 (12), 788−792. (26) Park, M.; Im, J.; Shin, M.; Min, Y.; Park, J.; Cho, H.; Park, S.; Shim, M. B.; Jeon, S.; Chung, D. Y.; Bae, J.; Park, J.; Jeong, U.; Kim, K. Nat. Nanotechnol. 2012, 7 (12), 803−809. (27) Shin, M. K.; Oh, J.; Lima, M.; Kozlov, M. E.; Kim, S. J.; Baughman, R. H. Adv. Mater. 2010, 22 (24), 2663−2667. (28) Zhang, M.; Atkinson, K. R.; Baughman, R. H. Science 2004, 306 (5700), 1358−1361. (29) Yip, J.; Ng, S. P. J. Mater. Process Tech. 2008, 206 (1−3), 359− 364. (30) Zhang, X. F.; Li, Q. W.; Tu, Y.; Li, Y. A.; Coulter, J. Y.; Zheng, L. X.; Zhao, Y. H.; Jia, Q. X.; Peterson, D. E.; Zhu, Y. T. Small 2007, 3 (2), 244−248. (31) Jung, K.; Shin, S.; Lee, K.; Park, S.; Koo, J. C.; Choi, H. R.; Moon, H. 2011 8th International Conference on Ubiquitous Robots and Ambient Intelligence (URAI), Nov 23−26, 2011; pp 442−445. (32) Butterfass, J.; Grebenstein, M.; Liu, H.; Hirzinger, G. Proceedings of the 2001 IEEE International Conference on Robotics and Automation; May 21−26, 2001; pp 109−114.
and additionally ultrasonicated (560 W, 20 min). Finally, the PVDF−HFP solution (5.875 wt % in 4-methyl-2-pentanone, 10 mL) was added and mixed by stirring for 2 h to form a spinnable dope.7 The dope (2 mL) was extruded through a spinneret with a diameter of 200 μm into a coagulant (hexane, Sigma Aldrich) by aid of a syringe pump (3 mL/h) to remove the solvent.18 The spun fiber was continuously collected on a winding drum. The fiber was further cured in an oven at 135 °C for 45 min after air-drying for 12 h. Characterization. The Ag particles and nAg-MWNTs were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL, 300 kV) and field-emission scanning electron microscopy (SEM, JEOL, JSM 890). The stretching experiments were carried out using an in-house-built device.7 The resistance of the fiber was measured as a function of tensile strain by the two-probe method (Fluke, 177 TRUE RMS multimeter).
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ASSOCIATED CONTENT
S Supporting Information *
Detailed synthesis procedures and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
R.M. and S.B. conceived and designed the experiments, which were carried out by R.M. and J.L. The robot finger experiment was carried out by D.C. and H.M. R.M. and S.B. wrote the paper. All authors contributed to data analysis and scientific discussion. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBS) and a grant (Code No. 2011-0031635) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. H.M. acknowledges Industrial Strategic Technology development program (project number: 10033888, Development of Anthropomorphic Robot Hand Technology for Delicate Dynamic Grasp) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).
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REFERENCES
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(33) Yu, Y. S.; Zhao, Y. P. J. Colloid Interface Sci. 2009, 332 (2), 467− 476.
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Knitted Fabrics Made from Highly Conductive Stretchable Fibers Rujun Ma,†,‡ Jiyong Lee,†,‡ Dongmin Choi,§ Hyungpil Moon,§ and Seunghyun Baik*,†,§ †
IBS Center for Integrated Nanostructure Physics (CINAP), Institute of Basic Science (IBS), Daejeon, 305-701, Korea Department of Energy Science, Sungkyunkwan University, Suwon, 440-746, Korea § School of Mechanical Engineering, Sungkyunkwan University, Suwon, 440-746, Korea ‡
S Supporting Information *
ABSTRACT: We report knitted fabrics made from highly conductive stretchable fibers. The maximum initial conductivity of fibers synthesized by wet spinning was 17460 S cm−1 with a rupture tensile strain of 50%. The maximum strain could be increased to 490% by decreasing the conductivity to 236 S cm−1. The knitted fabric was mechanically and electrically reversible up to 100% tensile strain when coated by poly(dimethylsiloxane). The normalized resistance of the poly(dimethylsiloxane)-coated fabric decreased to 0.65 at 100% strain. KEYWORDS: Silver particles, carbon nanotubes, conductive stretchable fibers, knitted fabrics, reversibility
S
by nanotubes with high aspect ratios.7,20 Moreover, the contact interface was significantly improved by agglomeration between microscale Ag particles and nanoscale Ag particles preadsorbed on the side wall of carbon nanotubes during the curing process.7,20,21 There were two drawbacks in the previous stretchable conductive film.7 There was a large increase in resistance when the film was stretched more than 30−40% tensile strain. Besides, the film could not return to the initial state due to the plastic deformation at high strain region.7 Here we significantly improved the resistance change and mechanical elasticity at high strain region by fabrics knitted using highly conductive stretchable fibers. The fibers composed of 100−150 nm Ag particles, nAg-MWNTs, and PVDF-HFP matrix were synthesized by wet spinning. Figure 1a shows a schematic of the wet spinning process (see the Experimental Section for details). Briefly, 100−150 nm Ag particles were dispersed in 4-methyl-2pentanone by ultrasonication. The mixture gel of nAg-MWNTs and ionic liquid (1-butyl-4-methylpyridinium tetrafluoroborate) was then added and ultrasonicated. The ionic liquid improved dispersion of nAg-MWNTs and increased conductivity of composites during stretching.5−7 The relative concentration of self-assembled Ag nanoparticles over MWNTs was previously optimized at ∼35 wt %.7 Finally, a PVDF-HFP solution was added and stirred for 2 h in order to form a spinnable dope. The dope (∼2 mL) was extruded through a nozzle with a diameter of 200 μm into a coagulant by aid of a syringe pump (3 mL/h) to remove the solvent.18 The average fiber diameter
tretchable electronic devices have received considerable attention recently.1−11 To realize such devices, stretchable metals as well as semiconductors need to be developed. Wavy structural configurations where thin metals or semiconductors are applied on prestretched polymer substrates have been intensively investigated.1,2,12,13 On the other hand, bulk composites possessing stretchability and conductivity have been developed by imbedding functional nanoparticles in stretchable polymers to address metallic parts.5−7,14,15 However, the composites demonstrated an inevitable decrease in conductivity in the high strain region due to the volumetric expansion of the polymer matrix or disconnection of conductive fillers.5,7,14 Besides, it is very challenging to achieve reversible stretching of composites. On the other hand, stretchability can be obtained by fabrics even they are knitted using rigid fibers. Multifunctional fibers made of carbon nanotubes (CNTs) have been explored with potential applications in intelligent clothings, structural textiles, woven electrodes, and artificial muscles.16−18 These include fibers synthesized by wet spinning and yarns drawn from multiwalled carbon nanotube (MWNT) forests providing lightweight, high specific strength, high electrical and thermal conductivity, and torsional/tensile actuation.16−18 Wet spinning is a scalable technology and widely employed in industry to manufacture ballistic fibers such as Kevlar.18,19 We previously demonstrated conductive flexible adhesives and stretchable composite films using microscale silver (Ag) particles and multiwalled carbon nanotubes decorated with 3−5 nm silver particles (nAg-MWNTs).7,20 The conductivity of bulk composite films significantly increased by a small addition of nAg-MWNTs. Microscale Ag particles embedded in flexible nitrile butadiene rubber or stretchable poly(vinylidene fluorideco-hexafluoropropylene) (PVDF-HFP) matrix were connected © 2014 American Chemical Society
Received: December 27, 2013 Revised: March 13, 2014 Published: March 24, 2014 1944
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Figure 1. Synthesis of conductive stretchable fibers. (a) A schematic of the wet spinning apparatus. An SEM image of 100−150 nm Ag particles and a HRTEM image of nAg-MWNTs are shown. (b) The synthesized fiber was continuously collected on a winding drum. The fiber length was longer than 10 m. (c) SEM image of the spun fiber with a diameter of ∼100 μm. (d) SEM images of the hot-rolled fiber. The width and thickness were ∼250 and ∼17 μm.
was ∼100 μm. The diameter of fibers could be controlled by the nozzle size of wet spinning apparatus. A number of different coagulants were tested, and hexane provided highest stretchability of the fiber (see Supporting Figure S1). The 4-methyl-2pentanone was extracted while the ionic liquid was retained inside the fiber due to the immiscibility with hexane.22 The filament longer than 10 m was collected on a winding drum (Figure 1b) and further cured in an oven at 135 °C. As shown in Figure 1c, the diameter of the fiber was uniform (∼100 μm). The density of the wet-spun fiber was 2.17 g cm−3 measured by the Archimedes method (electronic densimeter MD-300S). The density increased to 2.58 g cm−3 after the hot rolling process (Figure 1d). The fillers were more uniformly distributed, and the fiber was densified.7 The corresponding relative densities were 0.752 and 0.894 before and after the hot rolling process (see Supporting Information for details). The typical width and thickness of the fiber were ∼250 and ∼17 μm after the hot rolling process. The hot-rolled thin fiber could be easily deformed in a helical shape by hand as shown in the inset of Figure 1d. Figure 2a shows the conductivity and maximum tensile strain of spun fibers as a function of the 100−150 nm Ag particle volume fraction in the fiber. The corresponding Ag particle mass fraction in the initial mixture is shown with the x-axis displayed on top. The mass and volume fraction of Ag particles in the fiber became higher due to the extraction of solvent during the wet spinning and curing process. The mass of other components in the initial mixture was identical for four fibers. For simplicity, the 100−150 nm Ag particle mass fraction in the initial mixture was used to designate each fiber synthesized in this study. The mass fraction of nAg-MWNTs in the initial mixture was 1.06 wt % for the specimen with 8.5 wt % of 100− 150 nm Ag particles. The conductivity increased but
stretchability decreased as the mass fraction of Ag particles increased. The addition of Ag particles increased brittleness of the fiber upon stretching. The conductivity and maximum strain at rupture were 236 S cm−1 and 490% at 7.4 wt % of Ag particles. As the silver concentration increased to 10.4 wt %, the conductivity increased to 8344 S cm−1, but stretchability decreased to 55%. Nevertheless, these fibers exhibited substantially greater stretchability compared with other CNTbased fibers.16−18 The electrical conductivity of composites was theoretically calculated using the power law relationship7,23 σ = σ0(Vf − Vc)s
(1)
where σ is the electrical conductivity of the composite, σ0 is the conductivity of the conductive filler, Vf is the volumetric fraction of the filler, Vc is the volumetric fraction at percolation threshold, and s is the fitting exponent. Ag particles were modeled as uniformly distributed spheres, and the percolation threshold was calculated using the average interparticle distance model (see the Supporting Information for details).7,23 There was a good agreement between the theory and data (Figure 2a). An optimized silver concentration should be determined based on the requirements of target application. Figure 2b shows the conductivity change of fibers at two different Ag particle concentrations (8.5 and 10.4 wt %) as a function of tensile strain. The initial conductivity was 8344 S cm−1 for the fiber with 10.4 wt % of Ag particles. The initial conductivity decreased to 2681 S cm−1 when the Ag particle concentration decreased to 8.5 wt %. However, the maximum strain at rupture increased from 55% to 350%. Both fibers demonstrated a decrease in conductivity with increasing tensile strain. The theoretically calculated conductivity change of a fiber (Ag: 10.4 wt %) with increasing strain is also shown.7,23 A 1945
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Figure 2. Electrical and mechanical properties of conductive stretchable fibers. (a) The effect of 100−150 nm Ag particle concentration on conductivity and stretchability. Three specimens were characterized for each condition, and the standard deviation is shown. (b) The conductivities of fibers are shown as a function of tensile strain. The conductivity calculated using a three-dimensional percolation theory is also shown (Ag: 10.4 wt %, without the hot-rolling process). (c) Optical images of the fiber (Ag: 8.5 wt %) before and after stretching. (d) The initial conductivities and maxium tensile strains at rupture are compared with those of control materials. Filled symbols: fibers synthesized in this study. Open symbols: black open square, CNT fiber;18 dark cyan open down triangle, red open circle, navy open diamond, wine open pentagon, olive open star, polymer/CNT composites;5,6,15,25,27 blue open up triangle, dark yellow open right triangle, polymer/metal composites;14,26 magenta open left triangle, pink open hexagon, polymer/Ag/CNT composites.7,24
Fibers with four different diameters (70, 100, 190, and 380 μm) were synthesized by changing the diameter of nozzles (100, 200, 310, and 500 μm) as shown in Supporting Figure S2. The conductivity−tensile strain characteristics were similar regardless of the diameter of fibers in the investigated range. Supporting Figure S3 compares the conductivities of three fibers synthesized under identical conditions (Ag: 8.5 wt %) as a function of tensile strain. The conductivity−tensile strain characteristics were similar for all three fibers demonstrating good reproducibility. Figure 2c shows optical images of the fiber (Ag: 8.5 wt %) before and after stretching demonstrating excellent stretchability. SEM images of fibers with and without the hot-rolling process are provided in Supporting Figure S4. Ag particles were more uniformly embedded in the polymer matrix after the hotrolling process.7 The increased longitudinal distance between Ag particles could be observed after stretching decreasing conductivity as discussed above. Figure 2d compares the initial conductivity and maximum strain of spun fibers (filled
detailed derivation is provided in the Supporting Information. Briefly, the volume change of the matrix polymer as a function of strain modulates Vf of eq 1.7 The diameter change of the fiber was experimentally measured, resulting in an isotropic Poisson’s ratio of 0.243. As the strain increased, the volume of the composite fiber (Vmatrix) increased, whereas the volume of Ag particles (Vsilver) was invariant. This led to a decrease in Vf = Vsilver/Vmatrix and σ. There was a reasonable agreement between the data and theory validating the proposed mechanism.7 The theoretical prediction slightly overestimated the data. This could be due to the disconnection of conductive fillers (Ag particles and nAg-MWNTs) upon stretching which was not considered in the model. The conductivity of fibers increased after the hot-rolling process (Figure 2b). The film was densified after the hot-rolling process increasing Vf. This resulted in an increase in conductivity. The maximum initial conductivity was 17460 S cm−1 (Ag: 10.4 wt %). However, the decreasing trend in conductivity with increasing strain was similar. 1946
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Figure 3. Ropes and weft knitted fabrics made from the conductive stretchable fibers. The 100−150 nm Ag particle concentration in the initial mixture was 8.5 wt % for all fibers. (a) Optical images of the 2-ply rope, 4-ply rope, and weft knitted fabric (purl stitch). The images of the weft knitted fabric before and after stretching are also shown. (b) The normalized resistance is shown as a function of tensile strain. (c) The fabric was stretched between 0 and 100% tensile strain up to 3000 cycles. (d−e) The fabric was attached on a robot finger to transmit touch sensing data to the main controller by a CAN bus system. The force (F, mN), moment (M, mN·m), and position (Pos, mm) information was successfully analyzed as the robot finger touched a piano key. The length of an arrow in the inset image is proportional to force.
decreased to 236 S cm−1. Only one control specimen (high density carbon nanotube fibers synthesized by wet spinning) provided a greater conductivity than our fibers.18 However, the stretchability was very small (∼1.4%).18 Polymer−carbon nanotube composites provided generally lower conductivity but higher stretchability. These include vinylidenefluoride− hexafluoropropylene SWNT film,5 vinylidenefluoride−tetrafluoroethylene−hexafluoropropylene SWNT film,6 textile
symbols) with those of control materials in literatures (open symbols).5−7,14,15,18,24−27 Generally, stretchability could be enhanced by sacrificing conductivity. This could be controlled by Ag particle concentration in this study. The conductivity of the hot-rolled fiber was as high as 17460 S cm−1 with a reasonably high strain of 50% (Ag: 10.4 wt %). The maximum strain at rupture increased to 490% by decreasing the concentration of silver (7.4 wt %), but the initial conductivity 1947
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Figure 4. Reversible performance of the single fiber and fabric (Ag: 8.5 wt %) coated by PDMS. (a) The PDMS-coated single fiber was stretched to 100% strain and released to 0% strain. (b) Optical images of the tightly knitted fabric. (c) The normalized resistances of the PDMS-coated single fiber and PDMS-coated fabric are shown during the stretching and releasing cycle. (d, e). The PDMS-coated single fiber and PDMS-coated fabric were repeatedly stretched and released between 0 and 100% tensile strain. Three specimens were characterized for each condition, and the standard deviation is shown. The inset images show specimens after 300 stretching cycles. (f) The current−voltage characteristics of LEDs attached to PDMScoated two fibers are shown. The applied bias was 3.5 V. Visual images of LEDs before (top) and after (bottom) stretching are provided in the inset.
SWNT film,15 poly(dimethylsiloxane) SWNT film,25 and polyurethane MWNT film.27 A higher conductivity could be obtained by polymer−metal or polymer−metal−carbon nanotube composites sacrificing stretchability. The polyurethane− gold film,14 poly(styrene-block-butadiene-block-styrene)−Ag mat,26 PVDF−Ag MWNT film,7 and polystyrene−polyisoprene−polystyrene−Ag MWNT film24 belong to this category. It is challenging to achieve both high conductivity and stretchability by composite films or fibers. On the other hand, the stretchability of clothes has been obtained by fabrics even they are knitted using rigid fibers. Figure 3a shows optical images of 2-ply rope, 4-ply rope, and weft knitted fabric (purl stitch) made from the conductive stretchable fibers. The 2-ply rope was fabricated by twisting and relaxing two fibers to reach a torsional balance.28 The 4-ply rope was obtained by twisting and relaxing two 2-ply ropes.28 The fabric was knitted by hand using a long spun fiber. The purl stitch is widely employed in baby and children clothes due to the excellent stretchability.29 The stretchability depended on the pattern of fabrics. A plain weave fabric was also made for comparison, but stretchability was not enhanced (see Supporting Figure S5). The optical images of the weft knitted fabric as a function of strain are shown in Figure 3a. The fabric structure was first deformed followed by stretching of the fiber. A schematic of the fabric before and after stretching is provided in Supporting Figure S6.
Figure 3b compares the normalized resistance change of the single spun fiber, 2-ply rope, 4-ply rope, and weft knitted fabric as a function of tensile strain. The corresponding initial resistances were 4.27, 3.31, 2.43, and 7.8 Ω, respectively. The conductivity change could not be calculated due to the deformation of nonuniform cross-section. The stretchability of ropes was greater than that of the single fiber. This increase in stretchability of ropes was also previously observed by CNT yarns.17,28,30 The increase in normalized resistance with increasing strain was retarded by making 2-ply and 4-ply ropes compared with the single fiber. The resistance was measured by clipping each end of all fibers. However, the resistance of the 4-ply rope still increased ∼333 times at 200% strain. In contrast, the resistance increase of the weft knitted fabric was negligible up to ∼200% strain. The resistance was measured by clipping each end of a fiber used for the fabric. The resistance even slightly decreased up to ∼150% strain. This could be due the tighter contact of the fiber at the stitch crosssection. This remarkable stability in resistance up to about ∼200% strain is good enough to employ this conductive stretchable fabric for practical applications in human clothes and industrial robots. The resistance increased ∼22 times even at 450% strain where the fabric was ruptured. Figure 3c shows stretching cycleability of the fabric. The fabric was stretched and released between 0 and 100% tensile strain. The resistance 1948
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The effective Poisson’s ratio of the PDMS-coated fiber increased to 0.333 (see Supporting Figure S8 for optical measurement of the effective Poisson’s ratio). This increase in the effective Poisson’s ratio reduced the volumetric increase of the PDMS-coated fiber with increasing strain. This resulted in the less strain-dependent resistance change.7 The normalized resistance of the PDMS-coated fabric even slightly decreased (R/Ro = 0.65) when stretched to 100% strain with a nearly complete mechanical and electrical reversibility. This is a significant breakthrough compared with the previous conductive stretchable film where R/Ro = 67.8 at 100% strain.7 The cycleability was also excellent as shown in Figure 4d and e. There was no change in electrical resistance and mechanical structure when the PDMS-coated single fiber and PDMScoated fabric were stretched between 0 and 100% tensile strain up to 300 cycles. The electrical performance of PDMS-coated fiber was visually demonstrated using light-emitting diode (LED) chips as shown in Figure 4f. Visual images of LEDs before (top) and after (bottom) stretching are also shown in the inset. The current slightly decreased as the fiber was stretched to 100% strain due to the small increase in resistance (Figure 4c). However, the currents at 0 and 100% strain after 100 stretching cycles were identical to the initial currents at 0 and 100% strain, respectively, confirming excellent cycleability. In summary, highly conductive stretchable fibers composed of 100−150 nm Ag particles, nAg-MWNTs, and PVDF-HFP matrix were synthesized by the wet-spinning method. The initial conductivity increased, but stretchability decreased as the Ag particle concentration increased. The maximum initial conductivity of fibers was 17460 S cm−1 with a rupture tensile strain of 50%. The maximum strain could be increased to 490% by decreasing the initial conductivity to 236 S cm−1. There was an inevitable decrease in conductivity of the fiber as the strain increased due to the decrease in volumetric filler fraction and disconnection of conductive fillers. This was significantly improved by the weft knitted fabric where the resistance increase was negligible up to ∼200% strain. Excellent mechanical elasticity and electrical reversibility could be achieved when the fiber and fabric were coated with PDMS. The cycleability between 0 and 100% strain was very good. The successful implementation of the fabric for the CAN data bus system of a robot finger demonstrates the feasibility for practical application in electronics and robot industries. Experimental Section. Synthesis of Conductive Stretchable Fibers. Multiwalled carbon nanotubes decorated with 3−5 nm silver nanoparticles (nAg-MWNTs) were synthesized using a previously published protocol.7,20,21 Briefly, benzyl mercaptan in ethanol (0.1 mol L−1, 2.4 mL, Sigma Aldrich) was mixed with AgNO3 solution in ethanol (0.02 mol L−1, 300 mL, Junsei) by stirring for 48 h to synthesize Ag nanoparticles. In the next step, MWNTs (100 mg, Hanwha Nanotech) dispersed in ethanol (200 mL) were added and additionally sonicated (bath sonicator, 200 W, 8 h). Finally, nAg-MWNTs were collected by filtration (PTFE membrane, 0.2 μm), rinsing with ethanol, and drying in a vacuum chamber at room temperature. The conductive stretchable fiber was synthesized by the wet spinning technology. First, 100−150 nm Ag particles were dispersed in 4-methyl-2-pentanone (10 mL) by ultrasonication (560 W, 20 min). A gel mixture of ionic liquid (1-butyl-4methylpyridinium tetrafluoroborate, 600 mg) and nAgMWNTs (200 mg) was also prepared by mixing and grinding for 20 min using a pestle and mortar.5−7 In the next step, the gel mixture (800 mg) was added to the Ag particle suspension
measured at 100% strain did not change up to 3000 stretching cycles. The conventional rigid electrical wires impede stretchinginvolved joint motion of robots. The bending of robot fingers or arms needs stretching of accompanying wires for sensors and power supply. Typically, multiple rigid wires are installed on outside of robots, and extra length is provided to address this hurdle since the conventional wires are not stretchable. These long wires are detached from the surface of robots and occupy additional space when the finger or arm is in straight position. This makes the compact design difficult. The stretchable wires without extra length can be attached to the surface of robots or installed inside robots advancing robotics design. Figure 3d and e show the performance demonstration of the conductive stretchable fabric employed for the controller area network (CAN) data bus system of a robot finger. A detailed fabrication process of the robot finger was provided elsewhere,31 and a schematic of the dissembled robot finger is shown in Supporting Figure S7. Briefly, the 6-axis force and moment information was calculated by an embedded microprocessor, when the robot finger-tip was touched by an object, using the data from six strain gauges installed in a finger-tip sensor frame.31,32 Two electrical wires were required to transmit processed data over the CAN interface, and one wire was replaced by the fabric. The fabric was attached on the exterior of the robot finger for visual demonstration. The force and moment information was transmitted to the main controller through the fabric, at a sampling rate of 100 Hz, as the robot finger touched a piano key. The position information was also calculated using a customized software in the main controller.31 The touch position and force was visualized using an arrow in the controller image. The signal was successfully transmitted although the fabric went through more than 100% stretching during the finger touching maneuver. This demonstrates excellent compatibility of the fabric for the CAN data bus system which is commonly used in robotic and automotive industries. It is challenging to obtain reversible behavior of conductive stretchable films when stretched more than 30−40% tensile strain.7 Here we achieved excellent mechanical elasticity and electrical reversibility by coating the fiber and fabric using poly(dimethylsiloxane) (PDMS). Figure 4a shows the single fiber (Ag: 8.5 wt %) coated by PDMS (40 × 3 × 0.4 mm3). The PDMS-coated single fiber was mechanically elastic when stretched and released between 0 and 100% tensile strain. Figure 4b shows a tightly knitted fabric (Ag: 8.5 wt %) with smaller pores compared with the fabric shown in Figure 3a. The knitting pattern density can be defined as the average number of pores per unit area (mm2). The corresponding knitting pattern densities were 1.65/mm2 (Figure 3a) and 5.03/mm2 (Figure 4b). The stretchability generally decreased as the knitting pattern density increased. The normalized resistance change of the PDMS-coated single fiber and PDMS-coated fabric are shown in Figure 4c during the stretching and releasing cycle. The normalized resistance of the PDMS-coated single fiber was 7.0 at 100% tensile strain which is much smaller than that of the bare single fiber shown in Figure 3b. Besides, there was almost no hysteresis demonstrating excellent mechanical and electrical reversibility. The pure PDMS film is mechanically elastic with a Poisson’s ratio of 0.5.33 The elasticity of the PDMS-coated fiber was enhanced due to the elastic property of the surrounding PDMS. The Poisson’s ratio of the wet spun fiber was 0.243. 1949
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and additionally ultrasonicated (560 W, 20 min). Finally, the PVDF−HFP solution (5.875 wt % in 4-methyl-2-pentanone, 10 mL) was added and mixed by stirring for 2 h to form a spinnable dope.7 The dope (2 mL) was extruded through a spinneret with a diameter of 200 μm into a coagulant (hexane, Sigma Aldrich) by aid of a syringe pump (3 mL/h) to remove the solvent.18 The spun fiber was continuously collected on a winding drum. The fiber was further cured in an oven at 135 °C for 45 min after air-drying for 12 h. Characterization. The Ag particles and nAg-MWNTs were characterized by high-resolution transmission electron microscopy (HRTEM, JEOL, 300 kV) and field-emission scanning electron microscopy (SEM, JEOL, JSM 890). The stretching experiments were carried out using an in-house-built device.7 The resistance of the fiber was measured as a function of tensile strain by the two-probe method (Fluke, 177 TRUE RMS multimeter).
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ASSOCIATED CONTENT
S Supporting Information *
Detailed synthesis procedures and additional data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions
R.M. and S.B. conceived and designed the experiments, which were carried out by R.M. and J.L. The robot finger experiment was carried out by D.C. and H.M. R.M. and S.B. wrote the paper. All authors contributed to data analysis and scientific discussion. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBS) and a grant (Code No. 2011-0031635) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Education, Science and Technology, Korea. H.M. acknowledges Industrial Strategic Technology development program (project number: 10033888, Development of Anthropomorphic Robot Hand Technology for Delicate Dynamic Grasp) funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea).
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REFERENCES
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