Fiber‐Format Dielectric Elastomer Actuators by the Meter

Dielectric elastomers in the shape of thin films are heavily investigated, but they do not produce beneficial strains and forces comparable to that of skeletal muscles without entailing extremely complicated fabrication processes, rendering their practical use limited. Here, a silicone elastomer fiber is reported that can be produced by the meter and turned into an actuator by a simple process entailing the injection of an ionic liquid as the inner electrode and dipping in an ionogel to form the outer electrode. The fibers range from 174 to 439 µm in outer diameter with wall thicknesses from 62 to 108 µm and can be produced by the meter with regular diameter and thickness. The mechanical properties of the fibers are unaffected by the electrodes and actuation strains of 10% can be achieved, in both dry and wet environments. The fiber actuator can be bundled to mimic human skeletal muscle bundles or used in folded configurations, rendering it ideal as a building block for macroscopic actuators that can be used for many products such as body‐compliant actuators and wearables.


Introduction
Soft actuators are gaining popularity, such as fishing lines and sewing thread actuators, [1] twisted and coiled artificial muscles, [2] aerial and marine soft robots, [3] and silicone-based fiber pumps, [4] just to mention a few.Dielectric elastomer actuators (DEAs) have for years been promising candidates for soft DOI: 10.1002/adfm.202314056actuators and have commonly been referred to as artificial muscles since their emergence in the 1990s, primarily due to their elastic nature, activation by electricity, high actuation speeds, and high work densities, all resembling the properties of human skeletal muscles. [5]Ideally, the DEAs should be fiber-shaped to mimic the skeletal muscles truly and to enable novel structures, but fibers with the required dimensions and properties have not been easily achieved.
In principle, a DEA is very simple, but to achieve useful forces one needs multilayer structures and very thin elastomer films. [6]hese two aspects have hindered the wide implementation of DEAs due to complicated production processes, resulting in low realized efficiencies compared to their promised energy densities and competition from other soft actuator technologies, such as hydraulically amplified selfhealing electrostatic (HASEL) actuators. [7] DEA is a flexible capacitor, consisting of an elastomer sandwiched by two compliant electrodes.When a voltage is applied over the electrodes, the combination of the attraction of unlike charges on the opposing electrodes and the repulsion of like charges on each electrode generates an electrostatic pressure leading to decreased thickness and area expansion.[8] However, the resulting planar motion is unsuitable for most applications where axial motion, like that of human muscles, is desired.Thus, various configurations of DEAs have been developed by converting planar DEA films into rolls and stacks, but these approaches cause a significant loss of simplicity, achievable strains, and efficiency due to the assembly, [9] and the actuator technology loses its significant advantages.Thus, a new approach is needed to exploit the potential of the high energy density dielectric elastomers fully, and fiber geometry is believed to be key.
Hollow-fiber dielectric elastomer actuators hold promise as a direct path, requiring no film assembly, to generate the more useful linear strains, but fiber actuators providing large strains have until now only been reported on the scale of millimeters in length.Cameron et al. [10] prepared polyurethane fibers by the meter via coextrusion but achieved actuation strains of 1.8% only.Chortos et al. [11] developed a fiber dielectric elastomer actuator by 3D printing combined with a co-extrusion of a silicone elastomer and conductive inner and outer electrodes.The 7 cm long fiber actuator presented a maximum axial strain of up to 10% and was assembled into bundles providing powerful actuation energy densities (134 W kg −1 at 4 Hz) but limited to <1 cm overall length.3D printing is a relatively complicated process for the required dimensions of the hollow fiber elastomers, and, on the relevant size scale of commercial applications for dielectric elastomers, it is currently only suitable for producing solid core fibers.The solid core imposes mechanical constraints on the inner surface of the fiber, and actuation is hindered significantly with a reduced actu- , where Y and D are the elastic modulus and outer diameter, respectively, of the fiber (f) and electrode (e).As an example, if they have same Young's modulus and D f = 2D e , then the achievable strain is reduced by at least 33% compared to the achievable strain with a liquid electrode via the simple composite calculation above, ignoring the effect of the free surface in the hollow fiber geometry that will further ease the actuation of the hollow core fiber compared to the fiber with solid-core electrode.Shimizu et al. [12] prepared a 7.5 cm silicone dielectric fiber with outer and inner diameters of 6 and 5 mm, respectively, with liquid electrodes but did not get beyond 1.3% axial strain due to the large fiber wall thickness (1 mm).Arora et al. [13] used a relatively thick silicone hollow fiber (wall thickness 130 μm) that was highly pre-stretched via a constant supply of pressurized air.It was shown to result in 7% axial strain in the first actuation cycle, but actuation strains decreased steadily over the investigated 25 cycles.The fibers could not actuate without the pressurized air supply.To summarize, fiber dielectric elastomers have been shown to hold promise but are yet to be fully explored and exploited in the shape of a facile-produced fiber actuator with thin walls and liquid or liquid-like electrodes allowing for significant and reproducible strains without the need for pre-stretch.The fibers enable a new type of fiber actuator based on electro-static forces as opposed to the classical fiber actuators that are usually based on large deformations or thermal transitions. [14]e present a dielectric elastomer fiber that is fabricated by a continuous wet spinning process of a mixture of two commercially available poly-dimethyl siloxane (PDMS) polymers, namely mercaptopropyl and vinyl-terminated PDMS that cure under UV light in the presence of a photo initiator (Figure 1A,B).Very long fibers (>10 m per batch) (Figure 1C, Figure S1, and Video S1, Supporting Information) can be prepared, and this enables the fabrication of long single-fiber actuators by the injection of an ionic liquid as the inner electrode and dip-coating an ionogel as the outer electrode (Figure 1D).The electrodes are chosen to give minimum mechanical resistance.An important feature of the fiber is that the two surfaces of the PDMS will have different charge densities when a voltage is supplied due to the different curvatures, as illustrated in Figure 1E, and this is shown to improve the electro-mechanical properties significantly over the flat-film configuration (single-layer DEA) prepared with the same material.The fiber actuators enable strain of up to 10% in both wet and dry conditions.With this strain of individual fibers, higher strains will be possible in the future by structural design in the same manner that, e.g.skeletal muscles are constructed.The length, at which these actuators can be produced, and the stability of the resulting actuators enable the production of larger arrays of fiber actuators or very long single fiber actuators for a plethora of novel applications.

Dielectric Elastomer Optimization and Fiber Actuator Fabrication
Thin PDMS fibers are prepared via a wet spinning method with a coaxial spinneret needle using a photoinitiated thiol-ene reaction between the sulfhydryl (-SH) groups and alkene (C═C) groups of functionalized polydimethylsiloxane (PDMS) polymers to form an alkyl sulfide linkage between the silicone polymers. [15]The detailed analysis of the starting materials can be seen in Figure S2 (Supporting Information).The properties of the resulting dielectric elastomer are adjusted by changing the stoichiometry (r) between the -SH and C═C groups via changing the mixing ratio of the two polymers (Table S1, Supporting Information), and by varying the exposure time (t) of ultraviolet light allowing for the UV curing of the elastomer. [16]Details of the optimization with respect to mechanical and electromechanical properties, amongst others, can be found in Figures S3-S5 and Table S2 (Supporting Information).An optimum, namely excess of -SH of 3 (r = 3) and UV exposure time of t = 60 s, is identified for elastomers with a high crosslinking density, which is needed for actuator robustness.The deviation from the ideal stoichiometry can be explained by the side reaction of two -S-H. [17]More details can be found in the supplementary text.The given parameters are used for all prepared actuators.The effect of the flow rate ratios of PDMS and internal removable solvent (2-ethyl-1-hexanol) on the fiber geometry and properties was investigated.By simple variations of this ratio, uniform fibers ranging from 174 to 439 μm in outer diameter with wall thicknesses from 62 to 108 μm resulted (Figure 2A; Table S3, Supporting Information).The fibers are uniform along the fiber length, as illustrated in Figure 2B where 0.5 m of fiber (i) is investigated by means of microscopy at 4 different locations along the fiber (ii-v).The fibers possess excellent mechanical properties with extensions beyond 600% (Figure 2C,D; Table S4, and Video S2, Supporting Information) and unchanged mechanical properties after 1000 cycles of 50% deformation, both for pure fibers and fiber actuators (Figure 2E).Mechanical properties and physical dimensions are not changed significantly by the assembly of the fiber into an actuator with outer and inner electrodes (Figure 2E,F; Figure S6, Supporting Information) as well as the transparency is almost unaltered, also after repeated cyclic strains (Figure 2G) indicating that silicone does not interact with the liquid electrodes over time.Migration of the ionic liquid to the silicone phase would cause an initial improved actuation due to the increased permittivity due to the presence of the high-permittivity IL in the silicone phase, as shown in previous work, [18] followed by significantly decreased performance due to the conductive nature of the ionic liquid.The fiber geometry with the thinnest walls (fiber d) self-sticks internally upon collapse (Figure S7, Supporting Information) which renders its practical use limited, and thus actuators are made from fibers b and c only.
The fiber DEA (as illustrated in Figure 3A) is assembled by injecting ionic liquid into the core, sealing the inner core with the same formulation as for the dielectric elastomer itself, and dip-coating an ionogel as the outer electrode (Figure S8, Supporting Information).Various electrode systems have been tested, and a suitable combination is identified as the ionic liquid (1ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide) (also denoted [Emim][TFSI]) for the inner electrode and for the outer electrode an ionogel system of poly(ethyl acrylate)based elastomer and ionic liquid ([Emim][TFSI]), [19] and this pair of electrodes is used if not elsewise stated.The as-used [Emim][TFSI] is characterized as a hydrophobic and non-volatile ionic liquid with high electrochemical stability and conductivity (0.2 S cm −1 at 1 MHz) whereas the ionogel has been optimized for the use (details in Supplementary text and Figure S9, Supporting Information).

Dry Fiber Actuator Performance
The actuation properties of fiber actuators have been characterized extensively (Figure 3B-D; Figure S12 and Video S3, Supporting Information).Fiber c was proven the best fiber geometry, due to the dielectric layer being sufficiently thin while the fiber structure remains structurally stable, enabling an actuation strain of 9% at 2.7 kV, both right after preparation and after one year of storage (Figure S13 and Table S5, Supporting Information).Response and recovery times for a 2.5 cm actuator to 4% strain were determined to be of the order of 2 s (Figure 3D), being significantly slower than classical dielectric elastomer actuators, but on the other hand significantly faster than many other polymer actuator technologies (see Table S6, Supporting Information for details).The slow response is due to the two electrode systems not being ideal conductors so a charging time is needed.
Due to the nonideal conductivity of the two electrodes, the fiber length can not be increased infinitely without losing actuator performance, unless repeatedly bent, as will be discussed later.The absolute displacement of actuators of varied length is reported in Figure 3E where it can be seen that up to ≈60 cm of fiber the relative displacement (i.e., the slope of the curve) only decreases slightly whereas >60 cm fiber length both the absolute and relative actuation decrease.This is due to a disconnection between the connecting electrodes caused by an increase in the inner electrode volume.This phenomenon will be discussed in the following.

Actuation Deformation of Hollow Fiber Dielectric Elastomer
The fiber geometry introduces a benefit to the fiber actuators compared to their linear analogs, namely decreased sensitivity toward thickness inhomogeneities, compared to our experience with the flat film configuration where small variations in film thickness cause premature breakdown via immediate breakdown, [21] or via electromechanical instability. [22]Despite the small variations in wall thicknesses around the core, we do not observe significantly premature breakdowns for the fiber geometry.The fibers have slightly lower breakdown strengths than the corresponding planar single-layer DEA (42-54 vs 76 V μm −1 for planar DEA with highly controlled thickness).We believe that this favorable insensitivity of thickness variations is due to the geometrical constraints imposed by the electrical field that cause the inner surface to move toward the outer surface that remains stationary throughout the actuation, as illustrated in Figure 3A and with data in (Figure S14 and Video S3, Supporting Information).Both ionogel and dielectric elastomer are virtually incompressible (Poisson's ratio of ≈0.49) so the behavior can not be ascribed to compressibility.This atypical deformation in actuation can be explained by the asymmetry in surface charges of the inner and outer surfaces (Figure 1E) with the inner surface having a charge density 30% higher than the outer surface (see Figure S15 Supporting Information for calculations).For the classical flat film configuration, the surface charge densities of both electrodes are identical and increase equally upon the actuation of the film.Furthermore, for films, the like-like repulsion and the opposite charge attraction across the dielectric film contribute equally to the actuation. [23]For the fiber geometry, the high like-like repulsion on the inner surface combined with the energy of the opposite charge attraction across the dielectric elastomer drives the inner surface toward the outer surface, which, importantly, remains stationary to minimize the involved energies.The actuators are extremely soft, yet they can still deliver work densities of 2.1 J kg −1 of dry fiber and operated at a frequency of 0.3 Hz, they can deliver 0.63 W kg −1 of dry fiber or ≈0.42 W kg −1 of fiber actuator without any need for pre-stretch.
This stationarity of the outer surface poses a design challenge since the volume of the cavity inside the fiber increases upon actuation because it increases in both length and cross-sectional area.For small fiber segments and small actuations, we do not observe a disappearance of colored ionic liquid electrodes at the top of the fiber actuator.For fiber lengths >0.6 m, we observe the depletion of ionic liquid in the top, and, at some point, a disconnection from the connecting electrode upon actuation takes place.The volume change of the core is calculated by means

cm). (i) A
bundle with 12 segments lifting 8 g plastic bricks (actuation strain of 3% at 2700 V). (ii) A bundle with 22 segments lifting a 14 g monkey (actuation strain of 3% at 2700 V).D) Actuation strain and loading weight as a function of number of actuation segments (segment length: 2.5 cm).E) Voltage along the fiber as function of applied voltage for two different electrodes.F) Experimental setup for a bundle with 2 m length fiber winding into 8 actuation segments (segment: 25 cm).G) Actuation strain and loading weight as a function of length of fiber 1 to 8 segments (segment: 25 cm).J) Actuation strain and loading weight as a function of length of fiber (1 -8 segments with fiber length 0.25 m -2 m).H) A lifting system constructed of plastic bricks and a long fiber with nine separated actuation segments at 2700 V. Data presented as mean ± SD (k = 1) with sample size n = 3.
of setting the outer diameter constant and calculating the wall thickness assuming the elastomer is incompressible.The theoretically calculated volume change at 10% strain is ≈ 10% (Figure S16, Supporting Information).While the volume increase constitutes a small design challenge when designing long fibers, the confined movement of the fiber imparts extremely favorable electro-mechanical properties to the fiber geometry which by far outweigh the electrode disadvantage.The design issue could be resolved by the inclusion of a liquid reservoir or by using solid, stretchable inner electrodes.While fiber c has identical Young's modulus to the planar film configuration, it can be stretched >5 times longer and has a 5 times higher tensile strength in pure mechanical deformation (Figure 2D).Furthermore, the fibers possess no hysteresis (Figure 2E).Even more important, the actuation performance is improved significantly for the fiber geometries over the single layer DEA planar geometry, and at a field of 54 V μm −1 fiber actuator c actuates 9% compared to 2% for the film (Figure S12C, Supporting Information).

Actuator of Fibers Connected in Parallel
The use of a series of individually operated short fiber actuators in parallel, as illustrated in Figure 3F,G and Figure S17 (Supporting Information), has been explored.Four fiber actuators with an active length of 2.5 cm with individual control of voltage ) (actuation at 3600 V: 9%, response time: 0.1 s).B) Actuation strain as a function of voltage (direct current).C) Actuation of fiber at wet conditions at 3000 V and 0.1 Hz (sinusoidal current).D) Actuation of fiber at wet conditions at 3000 V and 1 Hz (sinusoidal current).E) Four parallel fiber pumps (pump length: 5 cm) constructed by fiber actuators.F) Flow rates at different frequencies and voltage (the voltage is 2100 V where the frequency is varied, and the frequency is 0.1 Hz where the voltage is varied).Data presented as mean ± SD (k = 1) with sample size n = 3. applied were assembled.Figure 3H and Video S5 (Supporting Information) illustrate the actuation strain when actuating a given number of fiber actuators where the strain increases with number of actuators activated, as expected.Remarkedly, 75% relative actuation efficiency could be achieved at 2 kV with 3% actuation by 4 parallel actuators, compared to 4% strain of a single fiber actuator.The non-activated actuators contribute elastically against the actuation, thus the reduced actuation at a given voltage.On the other hand, the parallel actuator can continue working despite one actuator malfunctioning which for practical uses is very convenient.

Initial Exploitation of Long Fibers
A simpler way than the parallel coupling the fibers is to utilize the length of the fiber and bundle one fiber up into an actuator consisting of multiple actuator segments (in principle a serial connection of actuators), as illustrated in Figure 4A and with a cross-section of a prepared bundle shown in Figure 4B.Two bundled fibers with different numbers of actuator segments are shown in Figure 4C and Video S6 (Supporting Information).The achieved strains and loads are shown in Figure 4D.Even for the short length of segments, at 12 and 22 segments (corresponding to 30 and 55 cm of active fiber length) the charging dynamics can be observed (Video S6, Supporting Information) by the actuation initially resulting in a twisting motion due to the full activation of the first part of the actuator whereas the last part is not activated, leading to a rotation due to the asymmetry caused around the lower load-bearing ring in the setup.Also, the voltage drop along the fiber is significant with the ionogel electrode (Figure 4E) and reaches 60 V per 10 cm at 2.5 kV.Therefore, the ionogel was replaced with carbon grease with a voltage drop of <3 V per 10 cm at 2.5 kV, yet still with comparable charge/discharge times (≈10-15% faster than the ionogel-based actuators) (Figure S18, Video S6, Supporting Information), for the following experiments with focus on the utilization of long fibers.The choice of electrode does not affect the actuation significantly up to ≈20 cm (Figure S19, Supporting Information).A long fiber actuator with 25 cm segments (Figure 4F) is prepared and actuated (Figure 4G; Figure S20, and Video S7, Supporting Information).The carbon grease outer electrode increases the relative strain of short fibers slightly over the transparent electrode system and enables actuation of a 2 m fiber configured into 8 serial actuator segments (Figure 4F) that are shown to lift 4.8 g with a strain of 1%.This actuator length is possible due to the bending of the fiber resulting in the top part of each individual segment not being in contact with the liquid electrode, yet the inner electrode remains conductive throughout.Furthermore, to show the versatility and appliance of the fiber actuator, it is shown how the actuator can be winded up in different configurations, also asymmetrical ones where each fiber segment has a different length, yet still works as one actuator, as shown in Figure 4H; Figure S21 and Video S6 (Supporting Information).It is obvious that the separated segment construct is better than the bundle segments construct, as a result of the overlap of the fibers at the two rings employed.The passivation of these areas does however have the favorable property that we can use significantly longer fibers than the 60 cm otherwise posing the limitation for useful actuation.

Wet and Semi-Wet Fiber Actuator Performance
Commonly actuators either work in wet or in dry environments only and are specifically designed to work in the given environment.Here we provide an actuator that can work across different environments and still deliver relatively fast actuation.Fiber actuator c immersed directly in conducting liquid without the ionogel outer electrode shows a 9% actuation (Figure 5A,B; Video S9, Supporting Information).In the wet environment, the actuator can be actuated to higher voltage without failure (3.6 kV vs 2.7 kV in dry environment) but with a slightly lower actuation due to the folded geometry.The achieved actuation is stable for both slow (0.1 Hz) and relatively fast actuation (1 Hz) (Figure 5C,D; Video S9, Supporting Information).
Another option for our fiber is to explore semi-dry conditions where the pumped fluid is used as the inner electrode (Figure 5E) and the ionogel is used on the outer surface of the fibers.The relatively slow charging and discharging process can be used in a favorable way via the resulting field gradients that will cause local expansion and contractions of the inner surface.This is explored in a fiber actuator pump configuration where the frequency and amplitude of the applied voltage can be changed.Under these conditions, it is shown that the flow rate scales approximately inversely with the frequency and linearly with the voltage in good agreement with theory.Flow rates of 2.5 mg min −1 are realized at 2.1 kV and 0.1 Hz, while a pressure of 1.5 cm ionic liquid can be generated (Figure S24-S26, Video S10, Supporting Information).

Conclusion
We have demonstrated a dielectric elastomer fiber actuator that can be produced by the meter and that allows for lifting 200 times its own weight with actuation of 10% in both dry and wet environments causing, and with excellent electro-mechanical properties that remain stable over an extended time.The actuators are extremely soft, yet they can still deliver work densities of 2.1 J kg −1 and operated with a frequency of 0.3 Hz, they can deliver 0.63 W kg −1 of dry fiber.These properties allow for constructing bundles of fibers closely mimicking human skeletal muscle fibers in physical properties and in actuation performance.The scalability and simplicity of the process, the multifaceted applications across wet and dry environments, and the stability of the resulting fiber actuators allow for unprecedented soft actuator technologies that are currently not possible with conventional dielectric elastomer films.The fibers can be used as modular building blocks for advanced actuators in the future.

Figure 1 .
Figure 1.Fabrication of fiber actuators.A) UV-based curing with DMPA (2, 2-Dimethoxy-2-phenylacetophenone) as initiator.B) Fiber fabrication process with the coaxial needle causing hollow fibers after 2-ethyl-1-hexanol removal.C) Produced fiber (9 m).D) Fabrication of fiber actuators.First, the ionic liquid is injected into the fiber and then the ends are sealed by a silicone elastomer reaction, and copper wires are attached.Finally, the fiber is dipped into an ionogel.E) Actuation mechanism of the fiber actuator, repulsion of like charges, and attraction of opposite charges.

Figure 2 .
Figure 2. Characterization of fibers and fiber actuators.A) Different fiber geometries.(i) Fiber a with an outer diameter (D out ) of 347 ± 2 μm, (ii) Fiber b with D out of 389 ± 6 μm and wall thickness (t wa ): of 108 ± 4 μm, (iii) Fiber c with D out of 463 ± 4 μm and t wa of 78 ± 3 μm, and (iv) Fiber d with D out of 563 ± 7 μm and t wa of 62 ± 5 μm.B) Homogeneity of the resulting fiber illustrated at different lengths of the fiber.(i) Image of fiber c with a length of 50 cm.(ii-v) Optical images taken at 5, 15, 25, 35, and 45 cm.C) Illustration of extensibility of the fiber (≈6 times its original length).D) Tensile properties of fibers and planar single-layer EAP for comparison.E) Tensile properties of fiber c and fiber actuator c in repeated deformation.F) SEM pictures of the fiber actuator (cross-section and length) with ionogel electrode (ionogel thickness: 6.0 ± 0.3 μm).G) Optical transmittance of original silicone film (diameter: 5 cm, thickness: 80 ± 4 μm) and silicone film (diameter: 5 cm, thickness: 80 ± 4 μm) coated with 6 μm ionogel electrode before and after repeated deformation, and silicone film coated with 6 μm ionogel in front of a red DTU logo.Data presented as mean ± SD (k = 1) with sample size n = 3.

Figure 3 .
Figure 3. Dry fiber actuators act in parallel.A) Experimental setup and working principle for dry actuators.B) Actuation displacement of a single fiber with a length of 65 cm under applied voltage of 2700 V. C) Axial actuation strain of two different fiber actuators.D) Cyclic response of single fiber actuator (2.5 cm length).Response time: 2 s and recovery time: 2 s.E) Actuation displacement for fibers with different lengths at 2700 V. F) Experimental setup for four individually actuated fibers.G) Bundle actuator with four individually actuated fibers.(i) Image of bundle with four individually actuated fibers.(ii) Four parallel fibers with a length of 2.5 cm loaded with 2.2 g weight.H) Actuation of four-individual-fibers actuator with one to four fibers actuated.Data presented as mean ± SD (k = 1) with sample size n = 3.

Figure 4 .
Figure 4. Exploiting the length of the dry fiber actuators.A) Bundle actuator assembled by winding a long fiber into 10 actuation segments.B) Optical image of a bundle with 10 fibers.C) Bundle lifting systems with a long fiber winding into different actuation segments (segment length: 2.5 cm).(i) A bundle with 12 segments lifting 8 g plastic bricks (actuation strain of 3% at 2700 V). (ii) A bundle with 22 segments lifting a 14 g monkey (actuation strain of 3% at 2700 V).D) Actuation strain and loading weight as a function of number of actuation segments (segment length: 2.5 cm).E) Voltage along the fiber as function of applied voltage for two different electrodes.F) Experimental setup for a bundle with 2 m length fiber winding into 8 actuation segments (segment: 25 cm).G) Actuation strain and loading weight as a function of length of fiber 1 to 8 segments (segment: 25 cm).J) Actuation strain and loading weight as a function of length of fiber (1 -8 segments with fiber length 0.25 m -2 m).H) A lifting system constructed of plastic bricks and a long fiber with nine separated actuation segments at 2700 V. Data presented as mean ± SD (k = 1) with sample size n = 3.

Figure 5 .
Figure 5. Fiber actuators operated in wet or semi-wet states.A) Actuation in wet conditions.Fiber actuator c immersed in conducting liquid ([Emim][TFSI]) (actuation at 3600 V: 9%, response time: 0.1 s).B) Actuation strain as a function of voltage (direct current).C) Actuation of fiber at wet conditions at 3000 V and 0.1 Hz (sinusoidal current).D) Actuation of fiber at wet conditions at 3000 V and 1 Hz (sinusoidal current).E) Four parallel fiber pumps (pump length: 5 cm) constructed by fiber actuators.F) Flow rates at different frequencies and voltage (the voltage is 2100 V where the frequency is varied, and the frequency is 0.1 Hz where the voltage is varied).Data presented as mean ± SD (k = 1) with sample size n = 3.