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Newton Output Blocking Force under Low-Voltage Stimulation for Carbon Nanotube-Electroactive Polymer Composite Artificial Muscles I-Wen Peter Chen, Ming-Chia Yang, Chia-Hui Yang, Yiwen Chen, Dai-Xuan Zhong, and Ming-Chun Hsu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13759 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 23, 2017

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ACS Applied Materials & Interfaces

Newton Output Blocking Force under Low-Voltage Stimulation for Carbon Nanotube-Electroactive Polymer Composite Artificial Muscles I-Wen Peter Chen,1,* Ming-Chia Yang,4 Chia-Hui Yang,2,3 YiWen Chen,2,3,* Dai-Xuan Zhong,1and Ming-Chun Hsu1 1

Department of Applied Science, National Taitung University, 369, Sec. 2, University Road,

Taitung City 95092,Taiwan 2

3D Printing Medical Research Center, China Medical University Hospital, 2 Tuh-Der Road,

Taichung City, 40447, Taiwan 3

Graduate Institute of Clinical Medical Science, China Medical University, No. 91, Hsueh-Shih

Road, Taichung City, 40402, Taiwan 4

High-Performance Materials Institute, Florida State University, 2005 Levy, Tallahassee, FL

32310, USA KEYWORDS (Carbon nanotube buckypaper; Electroactive polymers; Ionic liquids; Composite actuator; Blocking force)

ACS Paragon Plus Environment

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Abstract

This is a study on the development of carbon nanotube-based composite actuators using a new ionic liquid-doped electroactive ionic polymer. For scalable production purposes, a simple hotpressing method was used. Carbon nanotube/ionic liquid-Nafion/carbon nanotube composite films were fabricated that exhibited a large output blocking force and a stable cycling life with low alternating voltage stimuli in air. Of particular interest and importance, a blocking force of 1.5 N was achieved at an applied voltage of 6 V. Operational durability was confirmed by testing in air for over 30,000 cycles (or 43 h). The superior actuation performance of the carbon nanotube/ionic liquid-Nafion/carbon nanotube composite, coupled with easy manufacturability, low driving voltage, and reliable operation, promises great potential for artificial muscle and biomimetic applications.

1. Introduction Bio-inspired artificial muscles that can be precisely controlled by applying electrical stimuli have received extensive attention from scientists and engineers in the fields of actuators and smart materials.1,2 Among several types of artificial muscles, electroactive ionic polymer (EIP) actuators, which have a core of ionic electroactive polymers sandwiched between electrodes, have been investigated as promising candidates owing to their lightweight, low driving voltage, quick response, relatively high degree of deformation and facile processability at low costs.3,4 Some well-known EIPs, including poly(vinylidenefluoride)-based polymers,5,6 block copolymers,4,7 conjugated polymers8 and perfluorinated ionomers with acidic moieties (i.e. Nafion)2,9,10, have been widely investigated as ionic transportation materials for polymer actuators. Currently state-of-the-art EIP actuators require improvements in several aspects,


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especially with regard to enhancing the low blocking force and drastically deteriorated displacements under a low operating voltage.11,12 These obstacles and difficulties have inspired researchers to use ionic liquids due to certain characteristics they possess, such as a rigid chemical structure, a wide electrochemical potential window, high ionic conductivity and low vapor pressure13 when used in EIP-based actuators.6,10,14,15 Ionic liquid-doped EIP actuators have led to significant progress for dry EIP actuators due to the use of π-π interactions or crosslinking reactions as promising routes to enhance performance.4,16,17 Research and development of ionicdoped composite actuators have led them to meet the criterion of a high degree of deformation, but their blocking force is still far inferior to that of muscle. One of the most important families of EIPs are ionic polymer-metal composite (IPMC) actuators, which are composed of ion-exchangeable polymer materials and metal electrodes. Their electromechanical motion occurs via redistributing mobile ions against oppositely charged metal electrodes under an applied driving voltage, which causes a significant degree of deformation, thus generating displacements. However, IPMC actuators have several limitations, such as a low generative blocking force, a limited working period under ambient conditions, the formation of cracks in the metal layer of the electrodes that lead to poor durability, and the timeconsuming process of electroless metal plating. To overcome the problem of crack formation, the development of new high-performance ionic polymer actuators with carbon-based electrodes possessing superior mechanical, electrical conductivity and electromechanical properties is highly desirable. With rapid advances in the production of high-quality single-walled carbon nanotubes (SWCNTs) and experimental techniques, SWCNTs are now a reasonable choice for actuator electrodes due to their excellent degree of flexibility, superior mechanical properties and exceptional electrical conductivity. Moreover, SWCNTs can be used to make buckypapers (BP)


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with a porous structure. A porous electrode makes more effective use of ions or molecules to enhance the actuation durability without sacrificing performance. A simple sandwich structure for the proposed composite actuator, composed of alternating layers of ionic polymers and conductive electrodes, is one of the most widely used configurations.5-7,18-20 This structure has several benefits, including easy manufacturability and the potential for large-scale production.1 However, fabricating a sandwich structure actuator involves many parameters such as material compatibility between electrodes and polymers as well as the hot-pressing temperature, pressure and time. Therefore, the authors have created a Design of Experiment (DOE) methodology to systematically study and evaluate the intercorrelation between these parameters in sandwiched tri-layer structured actuators. The DOE fractional factor design allows researchers to significantly reduce the number of experimental runs while still providing acceptable information at a minimal cost. This study employed a tri-layer structured actuator comprising of a highly conductive SWCNT BP and anionic liquid/Nafion membrane. The four ionic liquids of 1-butyl-3methylimidazolium tetrafluoroborate

tetrafluoroborate (EMI+BF4−),

(BMI+BF4−),

1-ethyl-3-methylimidazolium

1-ethyl-3-methylimidazolium trifluoromethanesulfonate

(EMI+Otf−) and 1-ethyl-3-methylimidazolium thiocyanate (EMI+SCN−) were used in studying the effect of molecular structures on actuation performance. In addition to determining the intercorrelation of parameters and finding the optimal conditions, the DOE methodology helped gain an understanding of the optimal fabrication process of the tri-layer structured actuator. The fabricated actuator composed of SWCNTBP/EMI+SCN−-Nafion/SWCNTBP showed an output force of nearly 1.5 Newtons (N) driven by a low voltage (6 volts). Moreover, it could be operated for over 43 hours (or 30,000 cycles) without any decline in performance quality.


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2. Experimental Section 2.1 Materials. The ionic liquids used in this study were 1-butyl-3-methylimidazolium tetrafluoroborate (BMI+BF4−; Sigma-Aldrich), 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI+BF4−;

Sigma-Aldrich),

1-ethyl-3-methylimidazolium

trifluoromethanesulfonate

(EMI+Otf−; Sigma-Aldrich) and 1-ethyl-3-methylimidazolium thiocyanate (EMI+SCN−; SigmaAldrich).Nafion film (N117) and Nafion solution (DE2020) were purchased from Du PontCo., single-walled carbon nanotubes (SWCNTs) were purchased from Thomas Swan Inc. and multiwalled carbon nanotubes (MWCNT) were purchased from Golden Innovation Business Co. Ltd. Triton X-100 (Golden Innovation Business Co. Ltd.) was used as a dispersant for carbon nanotube dispersion. All materials were used as received. 2.2 Preparation of BP electrode. The SWCNT and MWCNT BP were made through a dispersion and filtration process. The carbon nanotubes were dispersed in water (40 mg/L) with the Triton X-100 (0.4 wt.%) and sonicated for 3 h. After filtering

the

suspension,

randomly

dispersed BP was formed.10,21 2.3 Actuator fabrication. Figure S1 shows the fabricated process of the actuator strip with a BP/ionic liquid-Nafion film/BP configuration. Nafion and ionic liquids were baked at 120oC in a vacuum oven for 3 h to remove water molecules. Then, the dried Nafion was immersed in ionic liquid overnight to allow for total saturation. The ionic liquid-doped Nafion (35 mm long ×7 mm wide) was sandwiched by two 35 mm long × 7 mm wide BPs, which were pressed in a hotpressing set at a controlled pressure (60 or 120 psi) and temperature (80 or 120oC) for 10 min to integrate the three layers, as shown in Figure S2. 2.4 Characterization. The electrical conductivity of the doped Nafions was measured using the four-point probe technique with a probe diameter of 80 µm and a probe-to-probe spacing of 1.6


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mm. Measurements were accomplished using a KeithLink source meter in order to provide a current in the range of-10 mA to 10 mA. The morphology of the BP/ionic liquid-Nafion/BP membrane was analyzed by a field emission scanning electron microscope (FE-SEM, JEOL JSM-7600F). A hot-pressing machine (GOTECH, GT-7014-30C) was used to apply pressure and control temperature to fabricate the composite actuator. The sample dimensions were 35 mm long × 7 mm wide. Tensile strength testing was carried out using an EZ-test machine (Shimadzu) at a constant loading rate of 0.5 mm/min. For each parameter sample, three technical replicates were performed. 2.5 Displacement and blocking force measurement. The displacement and blocking force had to be measured separately. When the bias voltage was applied, an MTI Microtrak II laser displacement meter (LTC-200-100; MTI Instruments, Inc.) measured the displacement of the BP/ionic liquid-Nafion/BP film. While measuring the output force of the composite actuator, the blocking force was measured by an ultra-small-capacity load cell (LVS-100GA; KYOWA, Japan).Voltage application was executed with an Agilent 33220A function generator. All measurements were performed and recorded at a controlled temperature (25°C) and humidity (40%). 3. Results and Discussion 3.1 Mechanical properties of ionic liquid-Nafion film The chemical structures of the Nafion and ionic liquids are shown in Figures 1a and 1b, respectively. After each Nafion N117 was treated with different ionic liquids, the mechanical properties of each ionic liquid-doped-Nafion film were significantly different. To understand the reason for this unexpected phenomenon, the chemical structures of the ionic liquids were compared. While the ionic liquids BMI+BF4− and EMI+BF4− have the same anion, the cation of


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the BMI+ ionic liquid made the doped-Nafion film more supple than the EMI+ ionic liquid (Figure 1c). One of the possible reasons is that the diffused cations can expand the porous structure of the Nafion. While the ionic liquids EMI+BF4−, EMI+Otf− and EMI+SCN− have the same cation, the flexibility of the doped-Nafion films was affected by the anion, as shown in Figure 1c. The EMI+SCN− treated Nafion film possessed the highest degree of stiffness, one of the plausible reasons for this phenomenon is the hydrogen bond formation between SCN− and sulfonate groups of the Nafion film.22,23 This observation indicates that the flexibility of the doped-Nafion films was affected by both the cations and anions of the ionic liquids. The mechanical properties of the ionic liquid-Nafion film samples are shown in Figure 1d. The EMI+SCN−-Nafion film samples (35 mm long × 7 mm wide) exhibited better mechanical properties than those of the Nafion films doped with BMI+BF4−, EMI+BF4− and EMI+Otf−. Table S1 shows that the EMI+SCN−-Nafion film exhibited the average Young’s modulus of 104.9 ±3.0 MPa.

Figure 1. a) Chemical structure of Nafion film. b) Chemical structure of investigated ionic liquids. c) Side view photographs of ionic liquid-Nafion films. The red dotted line is parallel to the surface (the ionic liquid-doped Nafion films underwent variable deformations due to gravity).


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d) Stress-strain curves of BMI+BF4−-Nafion, EMI+BF4−-Nafion, EMI+Otf−-Nafion and EMI+SCN−-Nafion films. 3.2 Actuation performance of tri-layer bimorph configuration Although hot-pressing is a quick, simple method, the compatibility of the materials as well as hot-pressing temperature and pressure are critical parameters that determine the performance of the composite actuator. After applying a hot-pressing set with a temperature of 80oC and a pressure of 60 psi, the interface exhibited crevices between the SWCNT BP and the ionic liquid-Nafion film, as shown in Figure 2a. These crevices indicate that the ionic liquidNafion could not effectively penetrate into the porous structure of the SWCNT BP to strengthen interface adhesion. Therefore, the actuation performance of this SWCNT BP/EMI+SCN−-Nafion film/SWCNT BP (Figures 2a and 2b) actuator was inferior to that of the actuator without crevices (Figures 2c and 2d) at an applied voltage of ±2 V. Figure S3 shows that the mechanical properties of the tri-layer configuration was significantly enhanced. The Young’s modulus of the SWCNT BP/EMI+SCN−-Nafion/SWCNT BP actuator was 473 ±150 MPa. a)

b)

c)

d)

Figure 2. a) Cross-sectional FE-SEM image of SWCNT BP/EMI+SCN−-Nafion/SWCNT BP actuator prepared at a hot-pressing temperature of 80oC and a pressure of 60 psi. b) Bending


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movement of a) at an applied voltage of ±2 V with a frequency of 0.1 Hz. c) Cross-sectional FESEM image of SWCNT BP/EMI+SCN−-Nafion/SWCNT BP actuator prepared at a hot-pressing temperature of 120oC and a pressure of 120 psi. d) Bending movement of c) at an applied voltage of ±2 V with a frequency of 0.1 Hz. 3.3 Effect of fabrication parameters on performance of composite actuator As described above, blocking force and actuation behavior are affected by many variables such as the additive of ionic liquid as well as hot-pressing pressure and temperature. To gain a further understanding of actuation performance of the tri-layer bimorph configuration, two more parameters, the type of carbon nanotube BP and Nafion, were assessed. Therefore, five parameters of the tri-layer configuration actuator were used to assess the intercorrelation between parameters. The DOE methodology was used to analyze the relationships between parameters and provided several advantages, for instance, determining the important controllable variables and their significance, eliminating unimportant variables, reducing the number of experimental runs, and systematic optimization. Based on the authors’ previously reported results10 and the discussion in Table S1, two types of ionic liquids, BMI+BF4− and EMI+SCN−, were selected as the DOE ionic liquid factor options because the BMI+BF4−-Nafion BP composite actuator showed a high degree of deformation10 and the EMI+SCN− showed the highest degree of electrical conductivity. For the DOE analysis, multi-walled carbon nanotubes (MWCNTs) were used as the actuator electrode in addition to SWCNTs since MWCNTs cost less. The remaining parameters, i.e. Nafion film vs. Nafion solution, 60 psi vs. 120 psi, and 80°C vs. 120°C, are shown in Table S2. Each sample was prepared with the parameter settings which were suggested by the DOE two-fractional 25−1 factor (16 samples; Table S3) design. Eight samples of Nafion solution-adapted actuators were fabricated using a different process than that of the Nafion film-


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adapted actuators. The Nafion solution mixed with the ionic liquid and cured fully after standing for five hours. The subsequent steps were the same as those of the fabrication procedures for the other BP actuators.10 Figure S4 demonstrates the displacement of each sample against its blocking force under continuous operation in response to ±2 V square-wave input signals at a frequency of 0.15 Hz. The actuators can be roughly grouped into four categories. Sample 1 showed the greatest displacement and blocking force among all 16 samples. It is noteworthy that these parameters (Table S3) greatly dominated the as-prepared actuator's performance. The impacts of each significant factor on displacement and blocking force, based on DOE methodology analysis, are shown in Figure S4. A greater absolute value of the mean effect slope indicates that the parameter had a greater impact. Therefore, BP and ionic liquid type were identified as the most significant factors regarding displacement and blocking force, as shown in Figures S5a and S5b, respectively. The most effective parameter settings (SWCNT BP, EMI+SCN− ionic liquid, Nafion film, a hot-pressing temperature of 120oC and a hot-pressing pressure of 60 psi) were suggested by the DOE results. The thickness of the sample was 233.5 ± 5.0 µm. Then, we analyzed actuation performance and durability behavior based on the optimal DOE results. 3.4 Newton level blocking force of composite actuator The SWCNT BP/EMI+SCN−-Nafion/SWCNT BP composite actuator was fabricated by using SWCNT BP, EMI+SCN− ionic liquid, Nafion film, a hot-pressing temperature of 120oC and a hot-pressing pressure of 60 psi as the optimized parameters. Figure 3a shows that a blocking force exceeding 1 N was achieved at an applied voltage of 6 V. With an applied low voltage of only 2 V, the blocking force value of the composite actuator exceeded 0.5 N (in the range of 2.3 to14 MPa, ~50 gf or ~2000 gf/g). The measured result was nearly three orders of


10

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magnitude superior to the reported results of EIP-based actuators, i.e. a carbon nanotube/Nafionbased IPMC (0.3 mN),2 a graphitic carbon nitride nanosheet electrode (0.93 mN),20 blended IPMC (3 mN),24 multi-walled carbon nanotubes/Nafion (0.236 gf/mm2),25 reduced graphene oxide/PEDOT:PSS

(0.119

oxide/PEDOT:PSS

(0.128

gf),26 gf),26

sulfur a

and

nitrogen

co-doped

graphene/Nafion-based

reduced

IPMC

(0.4

graphene gf),9

a

polypyrrole/alumina/Nafion-based IPMC (0.82 gf),27 and a graphene oxide/Nafion-based IPMC (30 gf/g).28 Figure 3b shows that the blocking forces were nearly saturated at 5 V at frequencies of 50 and 100 mHz. With an applied voltage of 10 V, the blocking force of the composite actuator was 1.5 N (in the range of 6.9 to 42 MPa). Therefore, the measured blocking force for the fabricated composite actuator was comparable to the peak capacity of human muscle.29 a)

b) 1200 1000

1000

Force (mN)

800

Force (mN)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


600 400

100 50mHz 100mHz

200 0

10 0

10

20

Time (s)

30

40

0

2

4

6

8

10

Applied voltage (V)

Figure 3. Performance of SWCNT BP/EMI+SCN−-Nafion/SWCNT BP composite actuator. a) Blocking force of composite actuator at applied voltage of ±3 V with a frequency of 50 mHz. b) Voltage-dependent blocking force at frequencies of 50 and 100 mHz. Furthermore, the operational stability of the optimal SWCNT BP/EMI+SCN−Nafion/SWCNT BP actuator was examined by continually alternating the square-wave voltage (movie 1). The SWCNT BP/EMI+SCN−-Nafion/SWCNT BP composite actuator possessed long-


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term stability and had a high degree of displacement upon applying a low voltage stimulus in air. Figure 4a shows the displacement in response to ±2.5 V square-wave input signals at a frequency of 0.2 Hz. The actuation continuously operated for more than 30,000 cycles (or 43 h) with negligible displacement deterioration in the actuator stroke, as shown in Figure 4b. The deformation stability of the composite actuators at ±2.5 V inspired the authors to assess their bending performance at higher applied voltage conditions geared towards bio-mimetic technologies for artificial muscles. Figure 4c shows that the bending displacement was unmeasurable by the laser displacement sensor with ±5 V square-wave input signals. Moreover, to demonstrate that there was nearly no sample-to-sample variation, multiple tri-layer actuators were tested in a synchronized manner (movie 2). To the best of the authors’ knowledge, such a Newton-level blocking force and high degree of displacement in response to low-driving voltage with excellent stability in air has never been demonstrated.

Figure 4. Durability and displacement of SWCNT BP/EMI+SCN−-Nafion/SWCNT BP actuator. a) Initial 25 cycles of actuator at an applied voltage of ±2.5 V with a frequency of 0.2 Hz. b)


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Applied voltage of ±2.5 V with a frequency of 0.2 Hz for 43 h. c) Bending motion of actuator at ±5 V with a frequency of 0.15 Hz. To get an understanding of the function of the ionic liquid of EMI+SCN−in the composite actuator, we assessed the actuation mechanism of the composite sample with different ionic liquids that were subjected to the same fabrication parameters for electromechanical characterization. As depicted in Figure S4, ionic liquids with SCN− as the anion performed best. One of the plausible reasons is that hydrogen bonds were formed by the sulfonate groups of Nafion and thiocyanate. The EMI+ cation exhibited fewer chemical bonding interactions than the thiocyanate anion. Table S1 shows that the electrical conductivity of EMI+SCN− was the highest, and thus its charge transportation properties were the best among these four ionic liquids. When an electric field was applied between two electrodes, the fast electromechanical actuation of the actuator was forced by the rapid establishment of dimensional gradients within the SWCNT BP/EMI+SCN−-Nafion/SWCNT BP actuator. Using the best-performing actuator, i.e. SWCNT BP/EMI+SCN−-Nafion/SWCNT BP, the time-dependent displacement was examined by applying a voltage of 2V. This demonstration ensured a durable response of the composite actuator by providing sufficient time for ion transportation. When an electric field was generated between two electrodes, the fast electromechanical actuation of the actuator was forced by the rapid

establishment

of

dimensional

gradients

within

the

SWCNT

BP/EMI+SCN−-

Nafion/SWCNT BP actuator. As shown in Figure S6, the actuation displacement was as short as one second, whereas the bending movement was gradually saturated at around 500s. Interestingly, no back relaxation was detected for an elongated interval of up to 1,000s, overcoming one of the universal problems of conventional actuators, that is, their limited accuracy of motion control. A proposed model is shown in Figure S7. Note that the swelling and


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shrinking of electrodes caused by ions redistribution has been omitted from the schematic illustration in Figure S7 for clarity. 4. Conclusions This study used sulfonate groups of Nafion integrated with EMI+SCN− ionic liquid to fabricate a composite actuator that significantly improved a BP composite actuator, not only in terms of the Newton scale blocking force, but also regarding deformation performance without signs of back relaxation. Moreover, the SWCNT BP/EMI+SCN−-Nafion/SWCNT BP composite actuator had a large Young’s modulus (473 ±150 MPa) and high actuation stability (43 h) with an operational applied voltage of ±2.5V under ambient conditions. To the best of the authors’ knowledge, this is the first time a Newton-level blocking force has ever been exhibited with low applied voltage for an EIP-based actuator. Considering the lightweight, superior mechanical properties and large degree of electromechanical motion of this design, this new composite actuator will have great potential in real-world applications for advanced biomimetic technologies. ASSOCIATED CONTENT Supporting Information. Physical properties of ionic liquids. Young's modulus of the composite actuators. DOE parameter settings of the 25-1 fractional factorial design. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] ORCID


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I-Wen Peter Chen: 0000-0003-2532-1380 YiWen Chen: 0000-0001-7537-5251 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the Ministry of Science and Technology (MOST), Taiwan (MOST 103-2113-M-143-003-MY2 (I-W.P.C., M.C.Y., D.X.Z.), MOST 105-2119-M-143-001MY2 (I-W.P.C., M.C.Y., M.C.H.) and MOST 105-2622-E-039-003-CC2(Y.W.C., M.C.Y., C.H.Y.) for financial and research support. REFERENCES (1)Kong, L.; Chen, W. Carbon Nanotube and Graphene-based Bioinspired Electrochemical Actuators. Adv. Mater. 2014, 26, 1025-1043. (2)Liu, S.; Liu, Y.; Cebeci, H.; Villoria, R. G. d.; Lin, J.-H.; Wardle, B. L.; Zhang, Q. M. High Electromechanical Response of Ionic Polymer Actuators with Controlled-Morphology Aligned Carbon Nanotube/Nafion Nanocomposite Electrodes. Adv. Funct. Mater. 2010, 20, 3266-3271. (3)Shahinpoor, M.; Bar-Cohen, Y.; Simpson, J. O.; Smith, J. Ionic Polymer-Metal Composites (IPMCs) as Biomimetic Sensors, Actuators and Artificial Muscles-A Review. Smart Mater. Struct. 1998, 7, R15-R30. (4)Cheedarala, R. K.; Jeon, J.-H.; Kee, C.-D.; Oh, I.-K. Bio-Inspired All-Organic Soft Actuator Based on a π-π Stacked 3D Ionic Network Membrane and Ultra-Fast Solution Processing. Adv. Funct. Mater. 2014, 24, 6005-6015.


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