High-Performance Multiresponsive Paper Actuators - ACS Nano (ACS

High-Performance Multiresponsive Paper Actuators Morteza Amjadi†,‡ and Metin Sitti*,†,‡ †

Physical Intelligence Department and ‡Max Planck−ETH Center for Learning Systems, Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, 70569, Stuttgart, Germany

ACS Nano 2016.10:10202-10210. Downloaded from pubs.acs.org by RUTGERS UNIV on 01/07/19. For personal use only.

S Supporting Information *

ABSTRACT: There is an increasing demand for soft actuators because of their importance in soft robotics, artificial muscles, biomimetic devices, and beyond. However, the development of soft actuators capable of low-voltage operation, powerful actuation, and programmable shape-changing is still challenging. In this work, we propose programmable bilayer actuators that operate based on the large hygroscopic contraction of the copy paper and simultaneously large thermal expansion of the polypropylene film upon increasing the temperature. The electrothermally activated bending actuators can function with low voltages (≤ 8 V), low input electric power per area (P ≤ 0.14 W cm−2), and low temperature changes (≤ 35 °C). They exhibit reversible shape-changing behavior with curvature radii up to 1.07 cm−1 and bending angle of 360°, accompanied by powerful actuation. Besides the electrical activation, they can be powered by humidity or light irradiation. We finally demonstrate the use of our paper actuators as a soft gripper robot and a lightweight paper wing for aerial robotics. KEYWORDS: soft actuators, electrothermal actuators, copy paper, hybrid films, silver nanowires bending motions.1,4,7 A bilayer system typically consists of an active layer that contracts or expands by an external stimulation and a passive layer that remains intact. To date, thermo-, hygro-, and solvent-responsive materials have successfully been adopted as active layer materials.1−3,5,9,12,15,16,19,20 Indeed, several biological systems can also convert physical and chemical signals into macroscopic bending. Well-known examples include bending movements of a wheat awn or scales in pinecones due to the daily change of the relative humidity (RH).1,16 Herein, we design bilayer actuators that operate based on the large hygroscopic contraction of the copy paper and simultaneously large thermal expansion of the polypropylene (PP) film upon increasing the temperature. Different from the traditional bilayer designs that consist of an active layer bonded to a passive layer, our bilayer actuators are composed of two active layer materials. We show that foldability and high anisotropic behavior of the copy paper allow a series of programmable mechanical motions such as bending, angular, linear, and torsional movements. Such electrothermally activated bending actuators can function with low voltages (≤ 8 V), low input electric power per area (≤ 0.14 W cm−2),

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oft actuator materials are being actively pursued owing to their importance in soft robotics, artificial muscles, biomimetic devices, and beyond.1−6 Electrically, chemically, and light-activated actuators are the mostly explored soft actuators. Recently, significant efforts have been made to reduce the driving voltage and temperature of thermoresponsive actuators, develop chemical actuators that can function in air, and enhance the energy efficiency of light-responsive actuators.1,3,7−12 Although some performance improvements have been reported for such actuators, there are still several challenges that need to be further improved. For example, very low output power generation of soft actuators is an obstacle to their practical use.1,5,13−15 Instead of low-modulus materials, high-strength yet flexible actuator materials can improve the force generation of actuators. Large deformation and various shape outputs are other important challenges in the soft actuator design. Programmable site-specific actuation would enable soft actuators to create diverse shapes and complex structures.16,17 Shape-changing capability of many actuators, however, is limited to only simple and monotonous contraction−expansion or bending−straightening motions.1,3,9,12−14,18 Thus, the development of soft actuators capable of low-voltage operation, powerful actuation, and programmable shape-changing still remains a grand challenge. Among various actuating structures, bilayer structures have widely been used to convert isotropic dimensional changes into © 2016 American Chemical Society

Received: August 17, 2016 Accepted: October 15, 2016 Published: October 16, 2016 10202

DOI: 10.1021/acsnano.6b05545 ACS Nano 2016, 10, 10202−10210

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Figure 1. (a) Schematic illustration of a bilayer structure composed of two active layers. (b) Fabrication process of the paper-based actuators. (c) Optical images of a patterned hybrid film on the paper substrate before and after folding. (d) SEM image of the surface of the AgNW network coated paper. (e) Magnified SEM image of the AgNW-coated paper, showing overlapped AgNWs within the percolation network. (f) SEM image on the top surface of the AgNW-coated paper after PEDOT:PSS coating, showing that AgNWs are buried under the PEDOT:PSS film.

and low temperature changes (≤ 35 °C). They exhibit reversible shape-changing behavior with curvature radii up to 1.07 cm−1 and bending angles up to 360°. The paper actuators can produce high output forces and lift objects 53.7 times heavier than their own weight. Besides electrical activation, they can be powered by humidity or light irradiation. We finally demonstrate the use of our paper actuators as a soft gripper robot and a lightweight paper wing for aerial robotics.

thermal- and moisture-induced dimensional changes of layer materials. Therefore, the overall response of the bilayer actuator due to the thermal energy is determined by the thermal and hygroscopic expansion of each layer (Figure 1a). To achieve the maximum bending actuation, we need a material with a very high CHE and a very low CTE for layer 1 and contrarily another material with a very low CHE and a very high CTE for layer 2. We found that normal copy paper can potentially function as a layer 1 material due to several of its distinguishing features that can influence the actuation performance. First, it possesses a very high CHE of ∼ 0.1 C−1 coupled with a very low CTE value of (4−16) × 10−6 K−1 (see Table S1 in the Supporting Information).22,23 In fact, the copy paper shrinks by increasing the temperature due to its moisture loss. Second, paper is a porous hydrophilic network of intertwined cellulose fibers. The porosity would give a large actuation and fast responsiveness of the actuators by accelerating the sorption and desorption of moisture. Third, the CHE of the paper in the cross-direction of the fibers is 2−3 times higher than the CHE in the fiber direction.23 This anisotropic hygroscopic expansion can lead to a more efficient shape-changing capability for the bending movements.16 Additionally, a wide variety of structurally programmable actuations could become possible by the anisotropic design of the actuators.15,16 Next, unlike polymer materials, the paper substrate is foldable, biodegradable,

RESULTS AND DISCUSSION To realize the thermal actuation mechanics of a bilayer actuator composed of thermoexpansive and hygroexpansive materials, we first modeled the curvature of the actuator using the linear elastic theory of beams (see Actuator Modeling section and Figure S1 in the Supporting Information).21 The temperatureinduced bending curvature of a bilayer actuator consisting of layer 1 and 2 materials can be expressed as (see Figure 1a): k ∝ (α2 − α1)ΔT + β2ΔC2 − β1ΔC1

where k is the bending curvature of the actuator, α1 and α2 are the respective coefficient of thermal expansion (CTE) for layers 1 and 2, ΔT is the temperature change, β1 and β2 are the respective coefficient of hygroscopic expansion (CHE) of layer 1 and 2 materials, and ΔC1 and ΔC2 are change of moisture concentration in layers 1 and 2 due to the temperature gradient, respectively. Moreover, CTE and CHE are responsible for the 10203


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Figure 2. Electromechanical and electrothermal characterization of hybrid thin films. (a) Effect of the positive and negative folding on the sheet resistance of hybrid thin films on the paper substrate; inset: a schematic illustration of the hybrid thin film on the paper under different folding angles (here N = 5, where N is the number of tested samples). (b) Durability test of a hybrid thin film under 1000 cyclic folding− unfolding cycles; inset: the loading profile of the hybrid thin film on the paper for the durability test. (c) Surface temperature of hybrid thin films as a function of the input electric power; inset: the temperature morphology of a hybrid thin film in a flat state and under negative and positive folding conditions (N = 3). (d) Temporal response of a hybrid thin film (response time < 20 s and recovery time < 24 s).

material) or expansion of the PP film (layer 1 as a passive material) (see Figure S2b in the Supporting Information). After selection of actuating materials, integrated electrically driven resistive heaters are required as local thermal energy sources. Low sheet resistivity is necessary for the low-voltage and energy-efficient operation of resistive heaters.38,39 In addition, heating elements must have stable electromechanical and electrothermal performance when the paper actuators are bent, twisted, and even folded. Graphene and silver nanowire (AgNW) thin films on the paper substrate offer high electrical conductivity.25,40 However, they delaminate from the paper substrate upon mechanical loading due to their poor adhesion to the paper.24,33 Alternatively, poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PPS) coated on the paper substrate has been shown to have a stable electromechanical performance under different mechanical deformations.22,24 Furthermore, the PEDOT:PSS solution can penetrate into the porous network of the paper and form a strong nanocomposite structure.24 The PEDOT:PSS−paper nanocomposites, however, exhibited less electrical conductivity than metal thin films coated on the paper substrate.22,24 As a feasible approach, we combined high conductivity of the AgNW thin film and robustness of the PEDOT:PSS film in the form of a hybrid thin film on the paper substrate as a high-performance resistive heater. Figure 1b illustrates the fabrication process of our bilayer actuators (see Experimental Section and Figures S3−S5 in the Supporting Information for details). Briefly, the heater geometry was first printed on a normal A4 copy paper by a laser printer (Figure S3 in the Supporting Information). The

lightweight, and abundant. A paper-based foldable actuator can create various mechanical motions through its different structural folding.17,22 Finally, paper offers high flexibility and high mechanical strength due to its fibrous nature (Young’s modulus, EPaper = 2−6 GPa).9 The strength of the paper enables actuators to produce large specific output forces. Although paper has extensively been studied for electronic, microfluidic, energy storage and harvesting, and sensor applications,24−33 there have been only a few studies on the paper-based actuators.22,34,35 For the layer 2 material, we selected the PP self-adhesive film because of its high CTE of ∼ 137.5 × 10−6 K−1 and almost moisture impermeability (see Table S1 in the Supporting Information).18,36,37 Consequently, α1 and β2 are almost negligible for the paper layer and PP film, respectively. To evaluate the bending curvature of the actuators, we measured the moisture content of the copy paper against the temperature change (see Experimental Section). The moisture concentration linearly declined as the temperature was increased (Figure S2a in the Supporting Information). On the basis of this linear relationship, the k value was predicted as a function of the temperature change (Figure S2b in the Supporting Information). For comparison, the curvature of actuators work only with the contraction of the paper layer (layer 2 as a passive layer, α2 = 0 and β2 = 0) or expansion of the PP layer (layer 1 as a passive layer, α1 = 0 and β1 = 0) was also calculated. Our results indicate that the bending curvature of the actuator with the contraction−expansion mechanism is 2.64 and 1.61 times larger than the curvature of actuators that operate by the contraction of the paper (layer 2 as a passive 10204


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Figure 3. Various mechanical motions of the paper-based foldable actuators. (a) Bending motion of a straight actuator. (b) Unbending shapechanging of a precurved actuator. (c) Left-handed twisting−bending motion of an actuator. (d) Linear back and forth motion of a folded actuator. (e) Programmable shape-changing capability of paper-based actuators, creating a snake-like undulatory body deformation.

influence of long-term folding−unfolding cycles on the conductivity of a hybrid film. More than 1000 loading cycles were applied to the sample while its electrical resistance was continuously recorded (see inset of Figure 2b). The resistance of the sample increased from 1.03 to 2.76 ± 0.68 Ω sq−1 after the durability test (Figure 2b). We also evaluated the electromechanical behavior of the pristine PEDOT:PSS and PEDOT:PSS−EG thin films coated on the paper substrate (Figure S5 in the Supporting Information). The hybrid thin films exhibited better performance considering their low sheet resistivity, stability, and durability. Figure 2c illustrates the surface temperature of the Jouleheated hybrid thin films. They exhibited a linear temperature increase as the input power increased. By increasing the input power to 0.14 W cm−2, their surface temperature reached 70 °C. The temperature morphology of hybrid films was uniform and stable in the flat state and under both negative and positive folding conditions (see inset of Figure 2c). There was no degradation of the temperature level at the back side of the paper, which is important for the effective thermal transport to the PP film (Figure S6 in the Supporting Information). Figure 2d depicts the transient electrothermal response of a hybrid film. The temporal response time of the hybrid thin film was less than 20 s, while its recovery time was influenced by the temperature level. For example, the temperature recovery time of the hybrid film was around 8 and 24 s when the temperature decreased from 33 and 65.3 °C to room temperature. The electrothermal performance of our paper-based heaters is comparable with recently reported resistive heaters (see Table S2 in the Supporting Information).39,42−47 Figure 3 demonstrates various programmable shape-changing capabilities of our electrically activated paper actuators. Figure 3a shows a straight actuator with two free ends. When a voltage was applied, the actuator bent toward the paper side due to the

AgNW conductive network was formed on the patterned paper by drop-casting the AgNW solution and subsequent solution evaporation under light heating. A mixture of PEDOT:PSS and ethylene glycol (EG) was then coated over the AgNW network and dried upon light heating, yielding a robust hybrid thin film of AgNWs and PEDOT:PSS on the paper substrate (Figure 1c). It is noted that EG was added into the PEDOT:PSS solution to enhance the conductivity of the pristine PEDOT:PSS by the depletion of PSS components (see Conductivity Optimization section in the Supporting Information).41 Finally, the PP self-adhesive film was adhered to the back side of the patterned paper, which resulted in a bilayer electrothermal actuator. The entire fabrication process is simple, scalable, mask-free, and cost-effective. Figure 1d and e show scanning electron microscopy (SEM) images of the top surface of the AgNW-coated paper. Uniformly distributed AgNWs provided high electrical conductivity through mutual connections between randomly oriented AgNWs. Figure 1f illustrates the surface morphology of the AgNW network on the paper substrate after PEDOT:PSS coating. PEDOT:PSS evenly filled the gaps among the paper fibers, and all AgNWs were buried below the PEDOT:PSS layer. Compared with the complete detachment of the AgNW thin film on the paper substrate by a mild touch, the resultant hybrid thin film on the paper was highly durable under bending, twisting, and folding (Figure 1c). The results of electromechanical and electrothermal analyses of the hybrid thin films on the paper substrate are depicted in Figure 2. Figure 2a illustrates the effect of the folding angle on the conductivity of the hybrid films. The electrical resistance decreased upon negative folding, while it increased under positive folding conditions. The relative change of the resistance remained in the range of 1.00 ± 0.05, revealing high robustness of the hybrid thin films. Figure 2b depicts the 10205


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Figure 4. Actuation performance of 4.5 cm long actuators. (a) Shape-changing behavior of an actuator under different voltages and electric powers. (b) Curvature of an actuator as a function of the input electric power and temperature change. (c) Axial and vertical blocking force measurements of actuators as the input electric power was increased; inset: axial and vertical configurations of the actuators (N = 3). (d) Stability test of a 4.5 cm long actuator under 500 cycles; insets: optical images of the actuator at different loading cycles. (e) Effect of the relative thickness and modulus of layers on the maximum bending curvature of the actuators.

hygroscopic contraction of the paper and thermal expansion of the PP film. Figure 3b illustrates an initially bent actuator that can perform the exact opposite motion of the straight actuator. The originally bent actuator was fabricated by sticking the PP self-adhesive film to the precurved paper substrate. These obvious bending actuations were observed only when the actuation was in the width direction of the copy paper (see Figure S7 in the Supporting Information). On the contrary, there was no bending movement once the actuators were fabricated along the length direction of the paper. This high anisotropic shape-changing behavior is attributed to the anisotropic fiber orientation of the copy paper. The surface topology revealed that most cellulose fibers are aligned in the length direction of the copy paper (see Figure S8 in the Supporting Information). Both left-handed and right-handed twisting were achieved by simply changing the angle between the actuation direction and fiber orientation (see Figure 3c and Figure S7 in the Supporting Information). Furthermore, the actuators showed reversible and powerful actuation under folding conditions (see Movie S1 in the Supporting Information). Figure 3d illustrates a folded actuator that can

perform linear back and forth motions. In this design, the PP film was attached to the back side of the paper, except in the folded regions. Conversely, another folded actuator exhibited angular actuation when the PP self-adhesive film was specifically adhered to the folded area (Figure S9 in the Supporting Information). Additionally, multiple mechanical motions could be programmed by connecting several actuators along each other. Figure 3e illustrates a sinusoidal-shaped structure programmed by two initially bent actuators. When 8 V was applied, its shape became straight in 15 s and then sinusoidal but with a 180° phase shift in 30 s (see Movie S2 in the Supporting Information). Notably, the structure could move itself with a snake-like undulatory motion under repeated on−off actuation cycles. The programmable and site-specific shape-changing of paper actuators is advantageous over most polymer ones, which can generate only single-mode actuation.3,7,18 To quantitatively analyze the actuation of paper-based actuators, we further investigated the shape-changing behavior, energy efficiency, output power generation, actuation speed, and bending stability of 4.5 cm long actuators. Figure 4a 10206


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allows shorter actuators to generate larger forces.22 For example, a 3 cm long actuator could lift a 1.878 g PDMS cube, already 53.7 times heavier than its own weight (1.878 g/ 0.035 g). Overall, we found that the input electric power of around 0.03 W cm−2 and temperature variation of 10 °C were sufficient for obvious shape-changing of our paper-based actuators. This input electric power can easily be provided by an onboard power supply, which is very important for practical applications. As a trial, our actuators showed high-performance actuation with a small 9 V battery (see Movie S4 in the Supporting Information). The attained actuation performance of our paper actuators outperforms many recently reported electrothermal actuators (see Table S3 in the Supporting Information).1,3,14,15,18,22 The experimentally obtained bending curvatures were well matched to the predicted values considering a scaling factor of 2 in our model (see Figure S11 in the Supporting Information). This scaling factor indicates that the CHE of the PEDOT:PSScoated paper is higher than the CHE of the pristine copy paper that was considered in the model. To further confirm this hypothesis, the shape-changing behavior of the PEDOT:PSScoated paper−PP bilayer was compared with the pristine paper−PP double layer (see Figure S12 in the Supporting Information). The bending actuation of the former bilayer structure was considerably larger than the latter one. Moreover, the high hygroscopic behavior of PEDOT:PSS greatly promoted the CHE of the PEDOT:PSS-coated paper.1,48 We then calculated the CHE of the PEDOT:PSS-coated paper by fitting our experimental data (Figure 4b) into the model, considering PEDOT:PSS-coated paper as a nanocomposite layer. The predicted CHE of the PEDOT:PSS-coated paper was around 0.37 C−1, which is 3.7 times higher than the CHE of the pristine paper. With the given thermoexpansive properties of the PEDOT:PSS-coated paper and PP film, we computationally exploited the influence of the relative thickness and modulus of layers on the resultant curvature of the actuators (see Figure 4e and Figure S13 in the Supporting Information). As observed in the figures, the bending actuation is strongly affected by the relative thickness and Young’s modulus of the layers. For the paper−PP film bilayer actuator (EPaper = 2 GPa and EPP film = 1.5 GPa), the relative thickness of 0.57 gives a maximum curvature of 0.68 cm−1. The curvature can be further increased to 0.78 cm−1 by choosing a softer paper layer (EPaper = 1 GPa) and optimal relative thickness of 0.42. Therefore, the bending actuation of the paper actuators depends not only on the thermal behavior of each layer but also on the relative thickness and modulus of the layers. We believe that the large shapechanging and powerful actuation of our actuators are attributed to the high CHE of the PEDOT:PSS-coated paper combined with the high CTE of the PP film and proper stiffness of both layers. For instance, a bilayer actuator made of the PEDOT:PSS-coated paper and polyimide film (CTEPolyimide = 17 × 10−6 K−1, 8 times less than the CTE of the PP film with the same thickness) exhibited much smaller and slower shapechanging, revealing the critical role of the complementary expansion layer on the total performance of the paper actuators (see Figure S14 in the Supporting Information). The responsiveness of actuators to the other stimuli such as light and humidity was also examined. An actuator was placed in front of a light source (a 100 W lamp) at a distance of around 30 cm. When the lamp was switched on, it quickly

demonstrates shape-changing of a straight actuator. One end of the actuator was fixed while the other end was free to move. Sequential dc voltages were applied to the actuator, and the resulting bending curvature was recorded by a side-view digital camera. Figure 4b shows that the curvature of the actuator appeared linearly proportional to the input electric power. When 4 V was applied to the actuator, its curvature increased to 0.45 cm−1 with a large displacement of 35 mm at the actuator tip (Figure 4a). By increasing the voltage to 7 V, the maximum curvature of the actuator reached 1.07 cm−1 with a bending angle of around 360°. As shown in the figure, the curvature and temperature change of the actuator are fairly overlapped, indicating a predictable response of the actuator. Moreover, the maximum temperature change of the actuator was around 35 °C. The low-temperature actuation of our actuators is comparable with the high-temperature (T ≥ 100 °C) operation of polymer thermoresponsive actuators.1,12,14,15 The energy efficiency of a 4.5 cm long actuator was 0.028%, 9.34 and 3.5 times higher than the efficiency of the aligned carbon nanotube (CNT) thin-film polydimethylsiloxane (PDMS) and singlelayer superaligned CNT sheet-polymer electrothermal actuators, respectively (see Energy Efficiency section in the Supporting Information).15,18 Figure 4c depicts the output power generation of 4.5 cm long actuators. Blocking force, the equivalent force needed to maintain the actuator tip in a fixed position, was measured in both axial and vertical configurations (see inset of Figure 4c). Both axial and vertical forces were linearly increased as the input electric power was increased (Figure 4c). The required time for actuators to produce the maximum stable force was 16 s. The maximum axial and vertical blocking forces were around 11 and 5 mN, respectively, when the actuators were powered by 0.14 W cm−2. Blocking force divided by the weight of an actuator (i.e., the force density), σ, is a measure of the effective force generation of an actuator. A bilayer system requires σ > 1 for an effective bending motion. σ was over 18 for our actuators, showing powerful actuation of the paper-based bilayer structures. Further tests were carried out to characterize the actuation speed and bending stability of 4.5 cm long actuators. The actuation speed was proportional to the input electric power (see Movies S3 in the Supporting Information). When the input power of 0.1 W cm−2 was applied to the actuator, it rapidly responded to the input power. The actuator then gradually reverted to its original shape once the power was switched off. The average tip actuation and recovery speed of the actuator were around 3.00 and 0.67 mm/s, respectively. The slower recovery speed of the actuator is understandable given the slower moisture sorption of the copy paper. The actuation stability of an actuator was evaluated upon 500 repetitive cycles. The actuator was first powered with 4 V for 1 min and then subjected to a square-waveform voltage (0−4 V) with a frequency of 0.025 Hz. The maximum curvature of the actuator was almost constant under frequent cyclic loading (Figure 4d). The slight increase of the curvature is possibly due to the induced thermal stress at the interface of the layers.12 The actuation performance of shorter (3 cm) and longer (6.5 cm) actuators was additionally studied (Figure S10 in the Supporting Information). The longer actuators were more energy efficient for the large shape-changing, while shorter actuators were more powerful. The powerful actuation of shorter actuators could be explained by the increase in their stiffness (spring constant) by decreasing their length, which 10207


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Figure 5. Robotic application concept demonstrations of the prototype paper-based actuators. (a) Paper wing design toward aerial robot wing steering actuation application. (b) Cooperative actuation of three 4.5 cm long actuator legs triggered by a change in humidity.

The main advantage of stimuli-triggered actuators is that they could be wirelessly driven and remotely controlled, without any external energy supply systems (i.e., harvesting the stimuli such as temperature, humidity, or light change in the environment) or coupled instruments. This gives actuators other beneficial functionalities such as cooperative actuation, i.e., a group of individual actuators that can work cooperatively to program complicated structures or accomplish different tasks. Figure 5b describes an example of the corporative work of three paperbased actuators. The actuators were flat at 40% RH. When the RH was increased to 60%, the actuators completely lifted their own weight. At 80% RH, the actuators cooperatively could hold objects 20 times heavier than their own weight.

responded to the light exposure. The actuator tip exhibited a 19 mm displacement in 30 s (see Figure S15a in the Supporting Information). The infrared thermal image on the surface of the actuator revealed that PEDOT:PSS can absorb the light irradiation. Moreover, the temperature change of the actuator was around 4.5 °C upon illumination (see Figure S15b in the Supporting Information). This temperature change can bend the actuator toward the paper side due to the moisture loss of the paper and thermal expansion of the PP layer. The sensitivity of the actuator to the humidity was further explored. The actuator was almost flat in 40% RH. When the RH was gradually increased to 80%, the actuator ended up with a curvature of 0.75 cm−1 and bending angle of 230° (Figure S15c in the Supporting Information). The moisture-induced shapechanging of our actuators is comparable with the recently reported stimuli-responsive actuators, taking the benefits of solvent-free actuation and a facile fabrication process.2,16,19,20 It is noteworthy to mention that the RH can change the initial bending state of the actuators. However, the effect of the RH on the final curvature state of the Joule-heated actuators was insignificant. As an example of the robotic application demonstration, a soft two-finger gripper was constructed by two bending actuators oppositely attached to a holder (see Figure S16 and Movie S5 in the Supporting Information). The gripper could grasp, lift, and manipulate miniature objects (e.g., a 0.19 g polymer foam). Because our paper actuators are lightweight and can function with an onboard power supply, they could possibly be used in miniature mobile robots also. As a proof of concept demonstration for a flying miniature robot’s wing steering actuation, an actuator was attached to a 25 cm2 wingshaped paper with a total weight of 0.35 g (Figure 5a). By powering the actuator with a small 9 V battery, the incidence angle of the wing was changed with a speed of 1 deg/s. We believe that the actuation speed could be further increased by reducing the size of the wing and actuator.

CONCLUSION In summary, we engineered the basic physical properties of active materials to develop high-performance bilayer actuators through a simple, scalable, mask-free, and cost-effective approach. The high CHE of the PEDOT:PSS-coated copy paper suitably complemented the high CTE of the PP film to construct actuators capable of low-voltage (≤ 8 V), low-power (≤ 0.14 W cm−2), and low-temperature (< 35 °C) operation, accompanied by powerful actuation (σ > 18). The foldability and anisotropic hygroscopic expansion of the copy paper allowed various programmable site-specific shape-changing. In addition, environmental multiresponsiveness of these actuators enabled electricity-free and cooperative actuation. The actuation of paper-based actuators can be further improved and tailored by changing the porosity and fiber orientation of the paper, relative thickness of layers, and even coupling of different stimuli. We believe that our material approach can also be implemented for the development of high-performance foldable electronic circuits and wearable systems. 10208


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EXPERIMENTAL SECTION

ASSOCIATED CONTENT S Supporting Information *

Preparation of Materials. AgNWs (length 20−50 μm and diameter 115 nm) with 0.5 wt % dispersion in isopropyl alcohol (IPA) were purchased from Sigma-Aldrich. The solution was sonicated for 30 min and subsequently centrifuged at 5000 rpm for 5 min. The IPA was then replaced by EG, and the AgNWs were redispersed in EG by stirring the solution for 5 min. The well-dispersed AgNW solution was then used for further experiments. The mass concentration of AgNWs was controlled by the amount of added EG. PEDOT:PSS (1.3 wt % dispersion in water) was purchased from Sigma-Aldrich. PP selfadhesive films with a thickness of 40 μm were purchased from TESA (Tesafilm transparent). Normal A4 copy paper (80 g m−2, Inapa Tecno) with a thickness of 110 μm was used for the fabrication of actuators. Fabrication Process of Paper-Based Actuators. First, Ushaped heating circuits with lateral dimensions of 4.5 cm × 0.9 cm (length × width) were printed on the copy paper by a laser printer (Canon C5250i). The printing ink was used to prevent spreading of conductive solutions on the whole paper substrate (Figure S4 in the Supporting Information). Then, 60 μL cm−2 of the 0.1 wt % AgNW solution was drop-cast on the patterned areas and dried under light heating. Light heating provided a uniform and gradual increase of the temperature in the entire substrate, preventing the agglomeration of AgNWs.49 A 60 μL cm−2 mixture of PEDOT:PSS and EG was then drop-cast on the AgNW patterns and dried under light heating, yielding hybrid thin films of AgNWs and PEDOT:PSS on the copy paper. The excess paper was then trimmed away, and the PP film was attached to the back side of the patterned paper to construct bilayer actuators. The electrical connection of actuators in the fixed ends was easily established using small flat alligator clips. Thin copper wires were connected to the free ends of the actuators by a silver paste (Sigma-Aldrich). The curvature and displacement of the actuators were analyzed by ImageJ software. All experiments were conducted in a laboratory with an average temperature and RH of around 24 °C and 50%, respectively. Microscopic Imaging. The SEM images were taken by a scanning electron microscope (SmartSEM, Supra55VP, ZEISS). The 3D surface profile of the copy paper was measured by a 3D laser scanning confocal microscope (KEYENCE, VK-X200 Series). Moisture Loss of Paper. To measure the moisture loss of the copy paper, the weight of the paper was measured at room temperature with a precision balance (Sartorius ENTRIS 623-1S). The copy paper was then put into an oven under different temperatures (40−100 °C). At each temperature level, the paper was kept inside the oven for 10 min and immediately weighed afterward. Assuming that the paper is completely dry at 100 °C, the moisture concentration was calculated as C = (MT − MDry)/MDry, where MT is the weight of the paper at different temperatures and MDry is the weight of paper at 100 °C. Electromechanical Experiments. Hybrid thin films with rectangular shapes (40 mm × 4 mm) were formed on the paper substrate, and copper films were connected to the two ends of hybrid films by a silver paste. The samples were then subjected to repeated folding−unfolding cycles by a motorized moving stage (M-605 HighAccuracy Translation Stage, Physik Instrumente (PI)), and their electrical resistance was simultaneously measured by a DAQ system (USB X Series Multifunctional DAQ, National Instruments). Electrothermal Analysis. Hybrid thin film heaters and actuators were subjected to different input electric powers, and their surface temperature was measured by an infrared thermal imaging camera (FLIR ONE thermal imaging camera). Blocking Force Measurement. The tips of 4.5 cm long actuators were connected to a PDMS cube (17.6 mN) as a preload. They were then placed on a precision balance with axial and vertical configurations. The generated forces of the actuators were measured as a function of the input electric power. Shape Change of Paper-Based Actuators under Humidity. The actuators were put into a closed glass chamber. The RH was controlled by regulating the flow of the water vapor and argon gas.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05545. Actuator modeling, conductivity optimization, anisotropic behavior of the copy paper, energy efficiency, light and humid responsiveness, and soft gripper demonstration (PDF) Movie 1 (MPG) Movie 2 (MPG) Movie 3 (MPG) Movie 4 (MPG) Movie 5 (MPG)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +49-711 689 3401. Present Address

Physical Intelligence Department, Max Planck Institute for Intelligent Systems, Heisenbergstraße 3, 70569, Stuttgart, Germany. Notes

The authors declare no competing financial interest.

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