Humidity-Driven Soft Actuator Built up Layer-by-Layer and

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Humidity-Driven Soft Actuator Built up Layer-by-Layer and Theoretical Insight into Its Mechanism of Energy Conversion Huiyan Tan, Xiunan Yu, Yaqing Tu, and Lidong Zhang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02249 • Publication Date (Web): 02 Sep 2019 Downloaded from pubs.acs.org on September 2, 2019

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The Journal of Physical Chemistry Letters

Humidity-Driven Soft Actuator Built up Layer-byLayer and Theoretical Insight into Its Mechanism of Energy Conversion Huiyan Tan, Xiunan Yu, Yaqing Tu, Lidong Zhang* School of Chemistry and Molecular Engineering, East China Normal University, Shanghai, 200241, People’s Republic of China. *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT. An improved protocol is proposed for preparation of a humidity-sensitive soft actuator through the layer-by-layer assembling of weight-ratio-variable composites of sodium alginate (SA) and polyvinyl alcohol (PVA) into laminated structures. The design induces nonuniform hygroscopicity in thickness direction and gives rise to strong interfacial interaction between layers, making the actuator with directional motility. A mathematic model reveals that the directional motion is driven by chemical potential of humidity, and its energy conversion efficiency from humidity to mechanical work reaches 81.2% at 25 °C. By coating with CoCl2, the composite film of SA@PVA/CoCl2 can act as a warning sign that provides a reminding information to prevent people from slipping or falling by conspicuous red sign during the high-humidity environment. When the film is involved in a bidirectional switch, it is capable of turning on/off light emitting diodes by humidity, showing a promising potential in control over humidity-dependent devices.

TOC GRAPHICS


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KEYWORDS. layer-by-layer assembling, mathematic models, soft actuators, humidity, energy conversion


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Biomimetics is an amazing concept that has attracted more and more attentions and given rise to various effective technologies in the research fields of soft actuators and robotics at macro and nanoscales.1-5 There are lots of magical plants in nature which can change shapes and states to adapt to the living environments when subjected to external stimuli such as pressure or humidity changes. Upon contacting by a fly, flytrap is capable of sensing the pressure with generating a grabbing action to capture the fly.1,4,6 This feature has enabled pressure-sensitive soft robotics and switchers.4,7 The dynamic habit of pine cones is associated with their moisture content; cones are open when dry and closed by rehydration.811

The reversible interaction with water molecules steered energy conversion from humidity

to mechanical work, which can be used in diverse artificial actuators that were prepared from soft polymer materials.2,12-15 Additionally, Selaginella lepidophylla, a "resurrection plant", has an ability to survive almost complete desiccation, and can protect itself from excess dehydration/hydration by curling its stem into a tight ball during dry weather in its native habitat and opening upon exposure to moisture.16 Its outer stems curl into circular rings at a higher rate relative to the inner stems due to the hydro-actuated strain gradient along their length.17-18 These features above provide reference models for researchers to design soft smart actuators that can be driven by external stimuli such as humidity,2,12-15 light,3,19-21 and heating.18,22 Typical examples include humidity-driven motion models of soft polymer bilayers,23-26 three-layers27 or multilayers.28-32 These models applied active layers in


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combination with inert layers to promote directional motion in response to constant supply of humidity gradients,4,5,33-36 or periodical humidity gradients37-40 for the controlled deformation in an appointed direction. However, the interaction between the layers was difficult to be well coordinated during the response to humidity owing to the differential properties of the materials of all layers. The structural integrity was also less-than-perfect in the period of repeated deformations. Therefore, it remains challenging to achieve a layered actuator that deforms through the synergistic effect of layered structures and can maintain its structural integrity in response to long-period shape deformations. These points differ from the stem of Selaginella lepidophylla, which is an organic entirety; the curling and uncurling of the stems comes from the synergistic action of an organism.17 Inspired by the structure of the stem of Selaginella lepidophylla and its functions, we proposed an improved protocol for the preparation of soft polymer films. This protocol produced film that was similar to multilayer polymer films in structures and functions, but resembled single-layered films with the capability to keep the structural integrity during the repeated shape deformations. To meet this goal, we hybridized and assembled sodium alginate (SA) and polyvinyl alcohol (PVA) into a laminated structure. Both the SA and PVA are hydrophilic materials, and contain plenty of hydroxyl groups in the structures. Therefore, their composites could be integrated by inter/intramolecular hydrogen bonding interactions. The laminated structure was constructed through layer-by-layer assembling of the composites of SA@PVA. By keeping the total amount of the composite constant in each


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layer, the weight ratio of SA to PVA was decreased in sequence from 5.6 to 1.1 from the first layer to the final one. This design delivered two critical strong points: non-uniform hygroscopicity in thickness and strong interfacial interaction between layers. The resulting film demonstrated a synergistic action in response to the gradients of humidity, similar to the movements of Selaginella lepidophylla.16-17,41 The non-uniform hygroscopicity in the thickness direction endowed the film capability to respond to humidity in a directional manner, while the strong interfacial interaction combined all layers into an inseparable unit that enabled the film to keep its structural integrity during the continuous and long-term shape deformation. It is well known that energy is a quantity that provides soft polymer actuators with the capability to perform work such as jumping, lifting, and walking.3,12,21,33,38,43 According to the conservation law, energy can be transferred between different locations or objects, but it cannot be created or destroyed.43-46 To get insights into the mechanism of energy conversion from humidity to the directional shape deformation, we showed a universal mathematic model that quantitatively demonstrated energy changes of absorbed humidity from one form to another. The energy conversion is important; nevertheless, only a few examples described the specific energy changes during the actuating processes of soft actuators in response to external stimuli.38,47-48 For the humidity-driven energy conversion, there is an ever-present suspicion on what a kind of energy has been converted and how does it proceed. To address them, we designed another film actuator based on polystyrene (PS) and a common tough


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hydrophilic polymer agarose (AG). The PS@AG actuator was capable of providing higher stiffness that enabled more controllable motility in response to humidity, and made the calculation more accurate though the mathematic model. This experimental model not only verifies the theory of chemical potential change of humidity, but also demonstrates the powerful potential of humidity to be converted into mechanical energies. By coating with CoCl2 elements, the composite film of SA@PVA/CoCl2 can be designed into a warning sign that provides a reminding information to prevent people from slipping or falling by conspicuous red sign during the high-humidity environment. We further reveal the potential of SA@PVA film by involving it in a humidity-driven bidirectional switch to control the light emitting diodes (LEDs), which shows a promising potential in humiditydependent devices.


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Figure 1. Preparation of humidity-responsive laminated films. (a) Materials for assembling the composite films. (b) The weight ratio of SA to PVA in each coating layer. (c) Schematic diagram showing the procedures of layer-by-layer assembling of humidity-responsive laminated films. The weight ratio of SA to PVA was gradually decreased from AP1 to AP5 layer, whereby there were gradational properties in the thickness direction of the laminated films.


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Figure 2. Laminated structure effects on humidity-responsive kinematics. (a) Sketch demonstrating that the laminated structure could be assembled into an integrity owing to the hydrogen bonding interaction between SA and PVA molecules. (b) The laminated film displaying gradational water swellability and humidity sensitivity along its thickness


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direction. (c) Time-dependent bending direction of the laminated film driven by gradient of humidity. (d) Overlaid snapshots of laminated Film-5 revealing the time-dependent bending. The film first bent to AP1 side and then bent back into a uniform curling strip. (e) Bending motions of various laminated films. Film-1 bent irregularly, and other films bent as an order of Film-5 > Film-4 > Film-3 > Film-2 in response to the same humidity gradients. The SA and PVA presented differential hygroscopicity by their hydrophilic hydroxyl and carboxylate groups.49-53 Therefore, the hybridization of these two elements allowed adjusting the hygroscopicity of their composite film by variation of the ratios of two elements (Figure 1a). We first prepared five sets of hybrid aqueous solutions of SA@PVA, where the weight ratio of SA to PVA was 5.6 (AP1 solution), 2.8 (AP2 solution), 1.9 (AP3 solution), 1.4 (AP4 solution) and 1.1 (AP5 solution) respectively (Figure 1b). The solutions were then assembled into laminated films through layer-by-layer coating over a pre-cleaned glass template. Typically, the AP1 solution was coated on the glass template to give the first layer (named Film-1). After drying at 40 °C in a natural convection oven, the film-1 was treated by oxygen plasma for 10 s, and coated with the AP2 solution to form the AP1/AP2 bilayer (named Film-2). These processes were repeated until the formation of laminated Film-5 in Figure 1c. Total five layers were assembled for optimal motility and energy conversion by harnessing humidity gradients; too many layers, for example > 6 layers would increase the total thickness and decrease the responsive speed of the film. Even though the films were constructed through the layer-by-layer assembling, the interfacial interaction was


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very strong owing to the similar physical and chemical features of SA and PVA (Figure 2a). All layers combined tightly into an inseparable unit, and could not be peeled off from each other. We examined its interfacial interaction by a standard 90° peeling test. A laminated Film-5 was fixed between a sticky substrate and a strip of non-elastic adhesive tape. Upon peeling at a rate of 10 mm/min, the Film-5 was separated from the substrate with still keeping its integrity. The separation between laminated layers was not caused (Figure S1). Thus, this method has its unique advantages over the traditional preparation of bilayer or multilayer systems, in which the interfacial failure normally happened during the deformation.12,42 Generally, humidity-driven bilayer polymer actuators was prepared by combining an active layer with an inert layer to realize directional movements.32-36 However, owing to the different physical and chemical properties of the elements, the bilayer structures might be easily destroyed during the responding processes. Additionally, the stress of active layer caused by sorption/desorption of humidity could not be well transmitted to the inert layer to trigger synergistic movements of the actuators.42 Differently, we hybridized the two polymer materials of SA and PVA that could generate molecular hydrogen bonding interactions for the structural integrity. By assembling them into a laminated structure, there was no so called active and inert layers; however, it generated a non-uniform hygroscopicity in the thickness direction that was achieved by variation of the weight ratio of SA to PVA in each layer. As a result, the laminated film such as Film-5 demonstrated a gradually-increased


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humidity sensitivity, but a gradually-decreased water swellability along the thickness from AP5 to AP1 layers (Figure 2b). Upon exposure to humidity, the AP1 side adsorbed water molecules more rapidly than the AP5 side. For example, an 80 m thick Film-5 bent toward the AP5 side in initial 40 s. By extending the humidity exposure period, the AP1 side was saturated with water molecules uptake and reached equilibrium in swelling/deswelling; yet the AP5 side was still active in humidity sorption which triggered the Film-5 bent back and further curled toward AP1 side until reaching a maximum curvature (Figure 2c). Overlapped images indicated that the 80 m thick Film-5 first bent to AP5 side and then curled back into a uniform circle in ~200 s with continual sorption of humidity (Figure 2d and Movie S1, SI). The responsive speed was dependent on the thickness of the film. As the thickness was decreased to < 5 m, the film was able to curl into a maximum curvature in Film4 > Film-3 > Film-2 in response to the same humidity gradients (Figure 2e, Movie S2). This was caused by the gradually-increased PVA contents from Film-2 to Film-5. Noted that the Film-1 deformed without regularity and directionality, since there was no hydroscopic difference along its thickness (Figure 2e, Movie S2 ). To get more insights into the order regarding the maximum curling curvatures by humidity, we measured the mechanical properties of SA and PVA single films and all laminated films. As shown in Figure S2a, the maximum bending stress of SA single film was ~1.5 MPa that was three times higher than that of PVA single film (~0.5 MPa). Upon


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stretching by a tensile tester at a rate of 200 mm/min, the rupture stress of SA single film reached 38 MPa, while it was 28 MPa for the PVA film (Figure S2b). The both tests indicated that the SA film was more rigid than the PVA film. Therefore, for the composite films of SA@PVA containing the more content of PVA, the films would be easier to be bent by external force. The results were in agreement with the curvatures variation as an order of Film-5 > Film-4 > Film-3 > Film-2 in response to the same humidity gradients (Figure 2e). This curvature variation was also confirmed by three-point bending tests, where the Film-1 that contained the lowest content of PVA (SA:PVA=5.6 in weights), showed a maximum bending stress of ~0.6 MPa. By increasing the content ratio of PVA (e.g. in Film-5), the maximum bending stress was reduced to ~0.2 MPa, endowing the laminated film a better flexibility (Figure S2c). Additionally, increasing the PVA ratio improved the elongation of the composite films (Figure S2d). Therefore, the Film-5 that contained more PVA content with the higher flexibility, could bend with the ultimate curvature higher than the other films by exposure to the same humid environment.


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Figure 3. Characterization on non-uniform swellability and sensitivity. (a) Swellability of neat SA and PVA elements. (b) The SA to PVA weight ratio effects on the dehydration of various laminated films that were heated from 25 to 100 °C at a heating rate of 15 °C/min. (c) Simple models for examining the differential responsiveness of SA and PVA to humidity. (d)


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Quantitative analysis on time-dependent tip deflection of the bilayers of SA/PVDF and PVA/PVDF. (e,f) Humidity-driven bending of laminated Film-5 with (e) uniform thickness, and (f) non-uniform thickness. To verify the non-uniform swellability along the film thickness, the water swellability of SA and PVA elements was measured respectively by hanging their neat strips in humid environment (RH: 90%) for 1 h. The initial weight was recorded as m0, and the watersaturated weight was defined as mt. The water swellability was calculated by the equation of (mt – m0) ×100%/m0. The result indicated that the water swellability of neat SA film strip reached (92 ±10) % while it was (217±15) % for neat PVA film (Figure 3a). Therefore, as the content of PVA in laminated film was increased, the water swellability increased as well from AP1 to AP5 layers. The higher water swellability resulted in the larger expansion, whereby the Film-5 was capable of curling with the maximum curvature higher than other films in response to the same humid condition. The heating-driven desorption test revealed that the higher content of PVA also caused the slower release of water molecules from the laminated film (Figure 3b). As a result, the Film-5 after being deformed by humidity sorption required longer time to recover itself than other films by dehydration. To support the conclusion on the non-uniform sensitivity to humidity along the thickness of Film-5 and other laminated films (Figure 2b), a simple actuating model was designed in Figure 3c, where two kinds of bilayer films were prepared. The first one was composed of humidity-responsive SA layer over an inert PVDF (polyvinylidene fluoride)


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matrix, while the second one consisted of humidity-responsive PVA layer over another inert PVDF matrix. The equal-size two films were exposed to the same humid condition, and their motions were recorded. Upon sorption of humidity, the SA/PVDF bilayer bent towards PVDF side with the tip deflection up to ~280 pixels in initial 40 s; the average bending speed was ~7 pixels/s. After that, its bending motion increased slowly and the tip deflection kept constant at ~330 pixels. The PVA/PVDF bent at an average rate of ~2 pixels/s in the initial 40 s; however, it was capable of bending until reaching a constant tip deflection of ~550 pixels (Figure 3c and d, Movie S3). These two simple models provided a visualized evidence; the higher the film contaning SA content, the higher the humidity sensitivity. This conclusion was also supported by the bending tests, where 80 m thick strips of Film-1 to Film-5 were exposed to the same environment, and their bending angles were recorded and calculated. The relative humidity (RH) for inducing the initial bending was 56% for Film-1, 62% for Film-2, 64% for Film-3, 72% for Film-4, and 75% for Film-5 (Figure S3a and Table S1). Evidently, the Film-1 that conatined the highest content of SA, was most sensitive to humidity and capable of bending with the lowest RH. By exposure to RH of 90%, all films could bend immediately, and their bending processes in initial 18 s were recorded and analyzed by calculating the curvatures. Their curvatures at 18 s changed as an order of Film1 > Film-2 > Film-3 ≥ Film-4 > Film-5 (Figure S3b). Th bending tests further proved that the humidity sensitivity of all films was related to the content of SA. Therefore, when exposed to humidity, the Film-5 first bent to AP5 side in initial 40 s because AP1 side was more sensitive to humidity owing to the higher SA content (Figure 2d). As the AP1 was saturated


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with humidity, the AP5 was still active in humidity sorption by the higher PVA content, which resulted in the Film-5 bending back into a maximum curling curvature towards the AP1 side. For a Film-5 strip with a uniform thickness of 72 m, it first bent to AP5 side within 40 s and then curled back into a uniform circle along its long axis in response to the sorption of humidity (Figure 3e), which was caused by the equal expansion from AP5 side. The tip deflection-time profiles demonstrated a point of inflection during the curling that appeared at ~40 s. This point of inflection was still existed when the thickness was reduced to 36 m (Figure S4). Such a tip deflection-time variation further reflected the non-uniform sensitivity and swellability along the thickness of the laminated film of SA@PVA composites. Driven by the sorption/desorption of gradient of humidity, all films were able to bend reversibly. The Film-5 strip (2 cm × 2 mm × 80 m) was taken to examine its bending cycling capability by fixing it on a support by glue. Its bending was recorded and analyzed by a common software (HSSC Link1.3.4.34) by tracking its tip deflection, which indicated that a bending cycle required ~50 s, and this process was capable of proceeding for 50 cycles with still keeping its responsiveness to humidity (Figure S5). This reversible bending generated the reversible variation of contractile stress that was tested on a tensile tester. The Film-5 strip (2 cm × 2 mm × 80 m) was clapped between two sample holders and kept straight by a pre-load of 0.02 N. The gauge length was constant during the whole testing process. Upon being approached by water-immersed cotton, the film adsorbed humidity and expanded to


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generate a negative contractile stress. As the cotton was removed away, the film dehydrated with giving a positive contractile stress. The contractile stress increased with an increase in the PVA content ratio in the composite from Film-2 to Film-5 (Figure S6). Not only would this test reflect the contractile stress in the film, but also further demonstrated the reversible actuating capability in response external humidity. As the laminated Film-5 was assembled with a non-uniform thickness along its long axis, it also first bent to AP5 side and then returned with further curling into the maximum curvature toward AP1 side. However, its curling process was not uniform; the thinner segment curled more rapidly with the maximum curling curvature higher than the thicker segment (Figure 3f, Movie S4), highly similar to that of Selaginella lepidophylla in response to gradient of humidity. 16-18 To further demonstrate such non-uniform curling motion, a laminated strip of Film-5 with non-uniform thickness (the thickness varied from 36 to 72 m along the long axis) was fixed by tweezers and exposed to the gradient of humidity. Its curling motion was recorded and compared with that of uniform-thickness strip of Film-5. As shown in Figure S7a and b, we tracked the time-dependent deflection of the positions A and B for the non-/uniform-thickness strips of Film-5, where position A reflected the motion of the tip of strip and position B was 5 mm away from the A. The uniform-thickness strip curled into a uniform circle in 300 s, while the non-uniform-thickness strip deformed with forming a non-uniform curling shape in 100 s (Figure S7c, d, and e). The curvatures in A and B segments first decreased and then increased as a same rate for the uniform-thickness strip;


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however, for the non-uniform-thickness strip of Film-5, the curvature of A segment changed more rapidly than that of B segment (Figure S7f). Such a variation was caused by the nonuniform curling owing to the non-uniform thickness along its long axis. This non-uniform curling allowed design of polymer soft actuators that required non-uniform curling along the long axis. Humidity is an important environmental factor; it is an eco-friendly, inexhaustible, and readily available energy source. Humidity-driven soft robots and actuators have shown promising performances in energy conversion.2,13,32,54-61 There have been numbers of reports on soft materials that can convert humidity to mechanical motions, where the mechanism is normally supported by the sorption of humidity that induces asymmetric expansion in the materials.12,18,26,37-40 However, there is still a big challenge regarding the energy conversion processes. The confusing point is that what energy has been converted or transduced, and how this energy conversion proceeds. To get insights into this confusing issue, we designed a humidity-driven polymer-film-based bending device, and its kinematic parameters such as curling or bending were recorded. Through the building of theoretical models, these kinematic parameters were included for calculation of energy conversion from humidity to mechanical work, which would provide significant foundations for future study on humidity-responsive materials.


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Figure 4. The sketch of a simple motion model. The mechanism on energy conversion from humidity to mechanical work was revealed. It was considered that the driving force was from the reduced chemical potential of humidity after being absorbed into the film. It is well known that humidity in gaseous state would turn into liquid upon sorption by soft polymer films, and the chemical potential in the both states was different, which might be the driving force to induce the kinematic motions. We thus started this study with exploration of the chemical potential of humidity before and after being absorbed into soft polymer actuators. We created a composite bilayer film actuator based on agarose (AG) and polystyrene (PS), since the rigid actuator of AG@PS was capable of generating strong plane stress with giving rise to a slow bending motion in response to external humidity. As a weight was loaded at its one of the ends, the actuator absorbed humidity and could bend with lifting the weight up (Figure 4). The mechanical energy generated in this process was


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from the variation of chemical potential, and this variation was subject to the environmental parameters such as temperatures and pressures. The process of humidity sorption by the AG@PS actuator could be regarded through a special chemical reaction as follows: (AG@PS)–OH + H2O ＝ (AG@PS)–OH----H2O

(1)

The chemical potential of water vapor (H2O) was recorded as μv*, and after being adsorbed into the film actuator, the chemical potential in liquid state (water) was defined as

μw. Then the difference between two kinds of chemical potential could be calculated as: Δμ = μw – μv*

(2)

Based on Gibbs free energy (G) formula, we have the following formula: dG = -SdT + VdP + ∑μidni

(3)

where the S, T, V, P represent entropy, temperature, volume, and intensity of pressure respectively. Assuming that the temperature and pressure was constant during the energy conversion. Then the equation (3) was rewritten as follows: dGT,P = ∑μidni

(4)

For the adsorbed humidity, we have the follow equation:


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ΔGT,P = nΔμ

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(5)

where n is the molar mass of humidity adsorbed into the film As the film actuator reached saturation with sorption of humidity, then: -dGT,P = dW or - ΔGT,P = W

(6)

where W is the energy generated by the actuator during the sorption of humidity. Therefore： W = -nΔμ

(7)

To measure the stored energy by sorption of water vapor, we measured the energy that was released from the AG@PS film. The released energy was converted into mechanical work such as gravitational energy that could be calculated as: W = mgh

(8)

where m, g, and h were the mass of small weight, the acceleration of gravity, the height of the small weigh rose, respectively. Combining (8) with equation (7), we obtained: mgh = -nΔμ

(9)

It should be noted that the stored energy by AG@PS film could not be completed converted to gravitational potential energy. Therefore, the energy in equation (9) reflected partial output of the stored energy by AG@PS film.


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To investigate the harvested energy and efficiency, AG@PS film was designed, and cut into a strip of 2.5 cm × 1.0 cm × 60.0 m. Upon humidity sorption, the strip (28.9 mg) bended with lifting the weight (130.5 mg) to a height of 3.0 cm. In this course, the energy generated by sorption of humidity was 48.3 µJ, and the output for lifting object was 39.2 µJ. Therefore, Δμ was calculated to be 0.3 J/mol, and the energy conversion efficiency from humidity to mechanical work was 81.2% at 25 °C. The difference between the chemical potential of humidity before and after being absorbed into the film was 0.3 J/mol that was the energy source for the mechanical work.


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Figure 5. Motility of sophisticated shapes of Film-5. (a) The schematic diagram showing the motility of arrow-shaped Film-5 in response to humidity. (b) Humidity-driven twisting motion of octopus-shaped Film-5 with arrow-shaped arms. (c) Bellflower-like shape of Film5 holding a small object and releasing it upon exposure to humidity. (d) A dendritic shape was cut out from a non-uniform-thickness Film-5, which bent and fixed a locomotive object by humidity. To demonstrate the energy conversion for the sophisticated deformations, the Film-5 was cut to a octopus-like shape. The sketch in Figure 5a represented the kinematic deformation of one arm of the octopus, which first bent to the AP5 side and then bent back with further twisting toward AP1 side upon being approached with the water-saturated cotton. When the six arms were approached by the cotton one-by-one, they showed similar shape-change patterns (Figure 5b, and Movie S5). More sophisticated deformations were demonstrated by cutting Film-5 to a bellflower-like shape. It was initially closed with holding a small object by dehydration. As it approached to the water surface, the sorption of humidity resulted in its opening motion and releasing the small object out, showing an effective energy conversion from humidity to mechanical work (Figure 5c, and Movie S6). Moreover, a nonuniform-thickness Film-5 was cut to a dendritic shape, where the branching parts were thinner than the root segment. Driven by the sorption of humidity, the dendritic shape bent with grabbing a locomotive object and could hold it for 2 min with continuous sorption of


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humidity (Figure 5d, and Movie S7). This process was reversible. As the humidity was terminated, the dendritic shape dehydrated and released the small object.

Figure 6. Applications of humidity-responsive laminated films. (a) Preparation of CoCl2functionalized laminated film. The composite film/CoCl2 was capable of changing its color in response to external humidity. (b) Quantitative detection of color change with time upon humidity adsorption. (c) A warning sign from the Film-1/CoCl2 that changed its color to


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remind people to prevent slipping or falling during the high-humidity environment. (d) A circuit diagram of the laminated film-based bidirectional switch. (e) Snapshot of the laminated film-based bidirectional switch. (f) Upon the environmental relative humidity (RH) higher than 40%, the bidirectional switch bent and made connection to turn blue LED on, while the red LED was switched on at the RH below 40%. As the laminated SA@PVA composite film was coated with CoCl2 aqueous solution and then dried at 40 °C for 48 h, it deformed with visible color change from red to blue upon again exposure to humidity; after dehydration, the color retuned from blue to red (Figure 6a). This was caused by the reversible hydration/dehydration of CoCl2. 62-64 For the equalthickness Film-1 and Film-5 containing the same content of CoCl2, the Film-1 changed its color more rapidly than the Film-5, where the Film-1 has become red color while the Film-5 was still in blue color as they were exposed to the same humid environment for 2 min (Figure 6b). This result further reflected that the Film-1 has the higher sensitivity to humidity while the Film-5 has the higher water swellability. Therefore, the Film-1 was selected to prepare a warning sign that could spontaneously change its color between red and blue in response to variation of external humidity (Figure 6c). Such a warning sign was simple yet helpful, which might remind people to prevent slipping or falling by conspicuous red sign during the high-humidity environment (e.g. rainy day). This reminding sign appeared by humidity and did not require any other energy sources and special assistances. In sunny day, the sign recovered to the blue color that matched with its substrate to hide


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itself. To further demonstrate the humidity-responsive potential, the Film-5 was used to design a humidity-controlled bidirectional switch as shown in Figure 6d and c, where the humidity-driven movement of Film-5 could be delivered to a soft electrode 1 to trigger the connecting with electrode 2 or 3. Upon the external humidity above 40%, the Film-5 bent to AP5 side to make the connection of electrode 1 and 2 which turned on the blue LED; as the relative humidity was decreased to below 40%, the Film-5 bent to AP1 side, which induced the disconnection of the electrode 1 and 2, but re-built the connection between the electrode 1 and 3 to turn on the red LED (Figure 6f). If this bidirectional switch was involved in ventilating device, it would maintain the environmental humidity in a relatively-constant level by automatically running or shutting down the ventilating device. In summary, inspired by the structure of the stem of Selaginella lepidophylla and its functions, we reported a layer-by-layer assembling protocol for the preparation of soft polymer films that were similar to multilayer polymer films in structures and functions, but resembled single-layered films in structural integrity during the repeated shape deformations. The layer-by-layer assembling resulted in the films with non-uniform hygroscopicity in thickness and strong interfacial interaction between layers. The high structural integrity was attributed to the inter/intramolecular hydrogen bonding interactions from two kinds of hydrophilic polymer materials, SA and PVA. The non-uniform hygroscopicity in thickness endowed the films capability to respond to humidity in a directional manner, while the


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strong interfacial interaction combined all layers into an inseparable unit that enabled the films to keep its structural integrity during the continuous and long-term shape deformation. The combination of non-uniform hygroscopicity in thickness and strong interfacial interaction between layers enabled synergistic action of the films in response to the gradients of humidity motion, similar to the movements of Selaginella lepidophylla, showing a controllable energy conversion process. For the energy conversion from humidity to mechanical work, we proposed that the reduction of chemical potential of humidity from gaseous fluid to liquid state might provide the power for the mechanical work of the film actuator. Therefore, a universal mathematic model was created to demonstrate the variation of chemical potential during the energy conversion. When a humidity-responsive strip (28.9 mg) bended with lifting a weight of 130.5 mg to a height of 3 cm, the energy generated by sorption of humidity was capable of reaching 48.285 µJ, and the its energy output for lifting object got up to 39.2 µJ. In this course, the difference of the chemical potential of humidity before and after being adsorbed by the film was 0.315 J/mol. The reduced chemical potential was transferred to the film to induce its motion, and the energy conversion efficiency from humidity to mechanical work was 81.2% at 25 °C. This experimental model not only verifies the theory of chemical potential change of water molecules before and after being absorbed into soft film actuators, but also demonstrates the powerful potential of humidity to be converted into mechanical


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energy. Various examples were demonstrated to show the capability of energy conversion by designing Film-5 into sophisticated shapes and exposure to humidity. Finally, we demonstrated the potential of the SA@PVA composite film by coating with CoCl2 and designing it into a warning sign that could provide a reminding information to prevent people from slipping or falling by conspicuous red sign during the high-humidity environment. We further revealed its potential by involving the film in a humidity-driven bidirectional switch to get control over the LEDs, which showed a promising potential for maintaining the environmental humidity in a relatively-constant level by automatically running or shutting down a ventilating device. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. Author Contributions H. T. contributed to all aspects of this study. X. Y. contributed to the preparation of the SA@PVA films and SA@PVA/CoCl2 films and energy harvest. Y. T. contributed to the


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application. L. D. Z. conceived the study. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant No. 51873064, 51603068), and the Natural Science Foundation of Shanghai (Grant No. 17ZR1440600). Supporting Information Available: Supplementary materials of experimental methods on this subject. (PDF) Figure S1. A standard 90° peeling test. (PDF) Figure S2. Mechanical tests of various multilayer films. (PDF) Figure S3. Humidity-driven bending tests. (PDF) Figure S4. Tip deflection of thickness-different Film-5 strips driven by humidity. (PDF) Figure S5. Bending cycling capability of Film-5 strip. (PDF) Figure S6. Contractile stress-time profiles of various films. (PDF) Figure S7. Humidity-driven curling motion of the Film-5 strips with uniform/non-uniform thickness. (PDF)


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Movie S1. Humidity-driven bending motion of a uniform-thickness strip of Film-5. (MPG) Movie S2. Bending motion of various laminated films. (MPG) Movie S3. Comparison of the humidity-driven motility of SA/PVDF and PVA/PVDF bilayers. (MPG) Movie S4. Humidity-driven bending motion of a non-uniform-thickness strip of Film-5. (MPG) Movie S5. Humidity-driven twisting motion of octopus-like shape of Film-5. (MPG) Movie S6. Humidity-driven unfolding motion of bellflower-like shape of Film-5. (MPG) Movie S7. Humidity-driven bending motion of a dendritic shape cut from the non-uniformthickness Film-5. (MPG)

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Humidity-Driven Soft Actuator Built up Layer-by-Layer and

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