Zero‐waste emission design of sustainable and programmable actuators

Moisture‐responsive actuators are widely used as energy‐harvesting devices due to their excellent ability to spontaneously and continuously convert external energy into kinetic energy. However, it remains a challenge to sustainably synthesize moisture‐driven actuators. Here, we present a sustainable zero‐waste emission methodology to prepare soft actuators using carbon nano‐powders and biodegradable polymers through a water evaporation method. Due to the water solubility and recyclability of the matrixes employed here, the entire synthetic process achieves zero‐waste emission. Our composite films featured strong figures of merit and capabilities with a 250° maximum bending angle under 90% relative humidity. Programmable motions and intelligent bionic applications, including walkers, smart switches, robotic arms, flexible excavators, and hand‐shaped actuators, were further achieved by modulating the geometry of the actuators. This sustainable method for actuators’ fabrication has great potential in large‐scale productions and applications due to its advantages of zero‐waste emission manufacturing, excellent recyclability, inherent adaptive integration, and low cost.


INTRODUCTION
In nature, many creatures possess innate abilities to recognize and respond to external surrounding stimuli via dynamic biological behavior, such as the deformation of body configurations, conversion of skin colors, and mechanical movements. Bio-inspired by these natural intelligent behaviors, stimuli-responsive biomimetic actuators that could implement an action in response to external stimuli, such as temperature, 1,2 pH, 3,4 light, 5,6 atmospheric humidity gradient a potentially enormous source of energy. 21 To date, a variety of humidity-sensitive materials have been developed to enhance moisture swelling and design constructions for a predetermined deformation. Among these appealing stimuli-responsive materials that include hydrogels, liquid crystal elastomers, shape memory polymers, carbon-based materials, and metal nanomaterials, carbon-based materials are superior candidates for soft actuators due to their outstanding flexibility and stability, extraordinary electrical conductivity, superior photothermal conversion capability, and simple functionalization with other materials. [22][23][24][25] Additionally, by the virtue of the synergistic roles of additional components, the actuating performance of carbon-based materials can be enhanced with the modification of functional soft species and the formation of composites. Specifically, in comparison with other carbon-based actuating materials, such as graphene and carbon nanotubes, carbon nano-powders have remarkable advantages in environmental friendliness, low production cost, and unique adsorption capacity. 26 In this regard, alginate is a linear polysaccharide found in part of the cell wall and intracellular material in seaweed. In the past decades, sodium alginate (SA) materials have been explored as stimuliresponsive candidates due to high hygroscopicity, 27 convenient processing, 28 low cost, multi-stimulus response 29 and environmental friendliness. 30 Despite the considerable progress made recently in terms of the sophistication of the shapes and the functional diversity of SA materials, research on SA-based film actuators is still in its infancy. 31 The existing SA-based film actuators still suffer from shortcomings, including slow response rate and weak mechanical properties. Additionally, polyvinyl alcohol (PVA) has been widely investigated as another promising candidate for humidity-responsive smart materials because of its excellent flexibility, extreme biocompatibility, and rapid water absorption properties. 20,32 Both SA and PVA are nontoxic and biodegradable hydrophilic materials with plenty of hydrophilic functional groups such as hydroxyl and carboxyl groups. Hence, atmospheric water molecules can be spontaneously captured and released by intermolecular/intramolecular hydrogen bonding interactions, inducing potential humidity responses. In this context, An et al. 33 developed graphene oxide (GO)/alginate hydrogel fiber with improving mechanical properties by constructing hierarchically arranged helical structures. Maity et al. 34 prepared PVA/multiwall carbon nanotube composites on fabrics for ethanol detection, showing good sensitivity and selectivity. In addition, Wang et al. 35 designed a twisted GO/SA fiber with rapid, reversible, and multi-responsive performance for applications in intelligent clothing and artificial muscles. Nevertheless, to the best of our knowl-edge, few works have focused on the sustainability of humidity-responsive materials, including the hazardousness of derivatives, the toxicity of reagents, their biodegradability, and recyclability. 27,36 For example, as an appealing multi-responsive actuating material, two-dimensional MXene (Ti 3 C 2 T x ) exhibited promising potential in soft actuators. 37 However, its synthesis process involves highly corrosive reagents and produces a large number of harmful derivatives, such as HF and AlF 3 . Consequently, developing a sustainable methodology of general applicability for the synthesis of actuating materials is of great interest to the field of soft biomimetic robots. Zero-waste emission is an attractive sustainable strategy that emphasizes the sustainability of raw materials, reducing waste and energy consumption in the synthesis process and facilitating the reuse and recycling of products. A zero-waste manufacturing process for actuating materials with robust performance can have significant implications for industrial production and environmental protection, which is highly desirable.
Herein, we report a sustainable zero-waste emission strategy for the production of soft actuators based on a homogeneous composite ink of SA, PVA, and carbon nano-powders. The composite film was formed via a simple water evaporation process and, due to its excellent hygroscopicity, was extremely sensitive to ambient humidity changes, undergoing anisotropic shape transformation under a humidity gradient. The developed single-layer construction film can not only overcome the limitation of interfacial bonding on the scale of deformation but also be beneficial to integrated processing. Different geometries were constructed to demonstrate various practical applications, such as smart switches, actuators, and soft robots, upon alternative on/off moisture gradients. In addition, the obtained composite film can be recycled using consistent water dissolution and evaporation procedures without the production of any harmful derivatives. The entire synthetic process achieves zero-waste emission, with only the evaporation of water; an outstanding greenness degree which is confirmed by the analytical GREEnness (AGREE) assessment.

RESULTS AND DISCUSSION
Inspired by the ubiquitous biological behavior of nature, smart soft actuators are designed for mimicking these spontaneous and continuous motions under the stimulation of external stimuli. In consideration of sustainability and compatibility, carbon nano-powders and biodegradable raw materials, including SA and PVA, with excellent water absorption and flexibility are integrated in this work to construct an intelligent artificial film actuator F I G U R E 1 Fabrication, actuation mechanism, and characterization of sustainable actuator (SusAct) composite film: (A) the schematic of the elaborate synthesis process and hygroscopic actuating behavior of recyclable SusAct composite film; (B) schematic illustration of the actuating mechanism; (C) surface scanning electron microscopy (SEM) images and a cross-sectional SEM image (inset) of SusAct films; (D) a representative topographical atomic force microscope (AFM) images of SusAct films and (E) the corresponding height profile. triggered by surrounding moisture gradient. Herein, a sustainable strategy is presented for preparing the freestanding composite film by water evaporation, namely, the sustainable actuator (SusAct). The predetermined motions of SusAct can be manipulated by the design of geometric construction and by regulating the moisture gradient. Freestanding SusAct composite films were synthesized by the direct drying method. The preparation process of the recyclable SusAct films is illustrated in Figure 1A. First, carbon nano-powders, SA, and PVA were dispersed in water and stirred to prepare a homogeneous mixture.
Subsequently, the quantitatively obtained suspension was poured into a Petri dish and evaporated under dehydration to form the composite films. The thickness of the films is controlled by regulating the weight of the mixed suspension. Finally, the composite films can be easily peeled off from the substrate of the petri dish due to obvious interfacial distinction, as shown in Figure S1. Here, the plane exposed to the air is the top surface, and the plane in contact with the Petri dish is the bottom surface. The weight loss ratio of films after water evaporation is shown in Figure S2. The water loss ratio of composite films containing carbon throughout the drying process was approximately 98.5%, which is slightly lower than that of one-component films. The abundant hydrophilic functional groups such as -OH and -COO − endow the hygroscopic actuating capability of SusAct films. Figure 1B shows the responsive mechanism of dynamic hydrogen bond interaction between functional groups and water molecules under surrounding humidity fluctuation. When exposed to high moisture, the hydrophilic functional groups can capture water molecules in the air and form hydrogen bonds, resulting in the volume expansion of the hygroscopic sites. Conversely, for the dehydration stage under low humidity, the hydrogen bonds are dissociated, and those previously captured water molecules are released to maintain the dynamic equilibrium with the surrounding environment. The photographical imaging shown in the middle of Figure 1B highlights the excellent flexibility of the films, which have the potential for constructing soft actuators with large-scale deformation capabilities.
The hygroscopic actuating behavior is mostly caused by the absorption and desorption of water molecules, water absorption volume expansion, and conversely water loss volume shrinkage. In this work, the heterogeneous deformation of SusAct films is due to imposing the inhomogeneous humidity gradient. The SusAct film maintained its initial state and the humidity exposed to each point was consistent. When applying a humidity gradient to a certain position, the film would swell dramatically due to the adsorption of water molecules, resulting in the heterogeneous deformation of the integral shape. After the removal of the external humidity input, the SusAct film recovered its original shape. Notably, the responsive and recovery processes of the SusAct film were found to be reversible, determined by the reversibility of hydration and dehydration when exposed to high and low humidity.
The composite films were further investigated via Fourier transform infrared (FTIR) spectroscopic analysis. Figure S3 shows the FTIR spectra of pure films and composite films that were dried at 20 • C. The peak between 3000 and 3700 cm −1 in the spectra can be assigned to the stretching of hydroxyl groups. 38 Stretching vibrations of aliphatic C-H were observed at 2945 cm −1 . COO − stretching is split into asymmetric and symmetric C=O vibrations. The two band peaks at 1596 and 1409 cm −1 were identified as representing the asymmetric and symmetric stretching vibrations of C=O, respectively. However, there no peaks were found at 1596 cm −1 , indicating the absence of C=O vibrations in the PVA films. Besides, it is worth noting that the appearance of peaks at 1415 and 1329 cm −1 for pure PVA film is ascribed to the bending and the wagging vibration of -CH 2 , respectively. 39 The peak at 1293 cm −1 was attributed to the C-O stretching vibration, whereas the band peak at 1128 cm −1 was assigned to the stretching vibration of glycoside bonds in the polysaccharide (asymmetric C-O-C). In addition, the strong and sharp peaks at 1086 and 1024 cm −1 are related to the stretching vibrations of the C-C and the C-O bonds of the glycosidic linkage. 40 The composite films with the modification of carbon nanopowders have no significant absorption peaks due to the lack of functional groups and the highly absorbent nature of carbon nano-powders. 41 Raman spectroscopy was performed to identify the presence of carbon nano-powders in the composite films and the chemical state of carbon. Two broad bands were identified at 1340 cm −1 (D-band) and 1596 cm −1 (G-band) in the Raman spectrum ( Figure S4), corresponding to disorder-induced sp 3 -hybridized carbon and two-dimensional hexagonal lattice sp 2 -hybridized carbon, respectively. [42][43][44][45] Furthermore, the structural order and graphitization degree were evaluated by the intensity ratio of the D-band and G-band (I D /I G ). The calculated result was 1.09, implying that the content of sp 3 -hybridization is higher than that of sp 2 -hybridization in our carbon nano-powders. Thermogravimetric analysis (TGA) was performed to characterize the thermal stability of the composite films ( Figure S5). A weight loss of the films was observed at two different steps. The initial weight loss was caused by the expulsion of moisture in the range of 30-110 • C. The major weight loss originated from the formation of H 2 O and CH 4 , and the release of CO 2 between 200 and 280 • C was ascribed to the destruction of glycosidic bonds and the decomposition of composite films. 46,47 In order to identify the crystal structure of the composite materials, X-ray diffraction (XRD) analysis was performed on the films: see Figure S6. The result revealed two broad diffraction peaks at near 2θ = 24 • and 2θ = 43.2 • , respectively, corresponding to the presence of amorphous carbon in association with hexagonal graphite lattice. [48][49][50] In addition, the XRD pattern of PVA films showed the peak at 19.5 • , indexing the semicrystalline nature of the PVA membrane. 51 The semicrystalline structure was constructed by intra-and intermolecular hydrogen bonds. Conversely, the broad peak at 13.5 • with low intensity in the XRD patterns of SA films indicates an amorphous structure for SA. 52 Scanning electron microscopy (SEM) imaging ( Figure 1C) shows the topography features of the bottom view of the as-prepared SusAct composite films containing 0.1 g of carbon nano-powders with the largest humidity-responsive bending angle. The inset cross-sectional SEM images indicated that the thickness of the film was ∼20 μm. Nanoparticles with ∼100 nm particle size contacted each other at high filler contents. Moreover, the abundant carbon nanoparticles provide more active sites for the adsorption of water molecules. 53,54 The morphology and roughness of SusAct composite films were characterized by an atomic force microscope (AFM), as shown in Figure 1D. The depth profile ( Figure 1E) illustrates that the surface was smooth within 30 nm fluctuations, and the overall roughness of the bottom surface was 10.2 nm.
The SusAct composite film (diameter of 25 mm, the thickness of 0.052 mm) was fabricated by a direct water evaporation process and maintained its initial flat state ( Figure 2A) under ambient conditions of 20 • C and 40% relative humidity (RH). When placed on the gloved palm, the SusAct composite film exhibited a slow bending tendency, as shown in Figure 2B. In comparison, the SusAct composite film had an autonomous large-scale bending motion when being put on the palm without gloves ( Figure 2C), indicating that the deformation of films is mainly caused by the humidity rather than the heat from the hand. Figure 2D confirms the reversible bending behavior of SusAct composite films under finger triggering. The ribbon-shaped film bent in the opposite direction toward the finger. The RH of the surface approaching the finger was higher, inducing a higher level of expansion, and thereby, it caused a quick bending behavior of the composite film. Furthermore, the folding and unfolding deformations of the cross-shaped films are shown in Figure S7A-C and Movie S1, when they were alternately approaching and moving away from the palm. Due to the variation of humidity gradients across the palm, the ribbon-shaped films spontaneously twisted and stretched like a worm in nature ( Figure S7D-F and Movie S2). In addition, these actuation behaviors also confirmed the excellent humidity sensitivity of the actuator because of the finger triggering. To further investigate the humidityresponsive behavior of composite films, the circle-shaped film (diameter of 35 mm, thickness of 0.032 mm) was placed on the palm of a hand to observe the deformation process induced by the humidity fluctuations ( Figure 2E and Movie S3). The composite film flipped spontaneously and continuously on the palm in a regular manner. First, the bottom side close to the palm absorbed more water molecules, causing the bending and movement of the film. Then, the large-scale deformation raised the barycenter of the film to a mechanically unstable state and turned the film over when reaching the critical position. Subsequently, the top side got close to the palm and swelled due to the absorption of water, and the bottom side contracted because of the desorption of water; thus, the film stretched and subsequently bent in the opposite direction of the palm. Finally, the film continued to deform and restarted a new flipping process.
To quantitatively evaluate performance, the bending angle comparison between composite films and other single films was conducted. Herein, the bending angle is defined by the supplementary angle of two straight lines formed by the ribbon boundary, 37,55-58 as shown in Figure  S8. The bend radius is obtained by considering the osculating circle formed by the bending position. Besides, one humidification system was established with one humidifier and an extension tube to control the humidity level and create a humid atmosphere ( Figure S9). The humidity level can be adjusted through the distance between the tube and actuators. Furthermore, the humidifier can also regulate the power of humid air generation. The humid atmosphere created by the humidification system is measured and calibrated by a humidity meter. The RH level as a function of the distance between the mist outlet and the responsive composite films is shown in Figure  S10A. The result indicates that the RH value decreases almost linearly with distance. Figure S10B demonstrates the humidity level dependence of the time at a distance of 1 cm between the mist outlet and responsive composite films when the humidifier is switched on/off. The result indicates that the humidity level could rise dramatically and subsequently remain stable when the humidifier was turned on. After switching off the supply of moist air, the surrounding humidity dropped immediately at the beginning and then slowly returned to the initial humidity level. Figure 3A shows the bending angle results of different films when exposed to an identical humidity level (90% RH). The SusAct composite films had the maximum bending angle of ∼250 • . Figure 3B exhibits the real-time bending angle change of the composite films with and without stimulation at 90% RH. Under 90% RH for a few seconds, the flat SusAct composite film bent with an angle change of ∼250 • . In comparison, the carbon nano-powders and SA composite (CSA) films and carbon nano-powders and PVA composite (CPVA) films present slower bending speeds and smaller bending angles. When the humidity was switched off, the bending angle decreased dramatically to a certain value and then slowly recovered to the original position. The absorption and desorption capabilities of SusAct film could be reflected by measuring the weight change ratio when exposed to and excluded from the humid air ( Figure S11, black line). Herein, the weight change ratio ( ) was calculated through the following formula: = − , where is the initial weight of SusAct film, and is the weight of SusAct film under a certain humidity level. The result indicates that the weight of SucAct film increased by 4.8% of its initial weight due to the absorption of water molecules with the accumulation of humid air, which is induced by the formation of hydrogen bonds between internal functional groups and external water molecules. On the contrary, the weight of SusAct film could recover to its initial weight when the humidity dropped to the same level as the ambient humidity. The corresponding humidity level is shown in Figure S11 (blue line). A mechanical test was used to confirm the flexibility of SusAct composite films and the role of carbon in the mixed films, as shown in Figure 3C. The mechanical test demonstrates that the existence of carbon can increase the tensile strain, which means that the flexibility of the films is enhanced. However, the tensile strength decreases at the same time due to the introduction of carbon nano-powders. This is because the higher content of carbon nano-powders reduces filler-polymer contacts and consequently reduces the interactions between polymers. 59 Even though the tensile strength decreases slightly after adding carbon nano-powders, the value can still exceed 30 MPa.
Generally, the thickness of a composite film plays a significant role in its actuating performance, determining the bending angle and response speed. The relationship between the thickness of films and the weight of composite suspension with different concentrations of carbon nano-powders is shown in Figure S12, where the concentrations of SA and PVA solutions were both 0.5% in the composite suspension with a total weight of 200 g. It is worth mentioning that when the concentration exceeds 1%, the SA and PVA solutions became more viscous, which is not conducive to the dispersion of carbon nanopowders. The result shows that the thickness of composite films increases with the weight of the suspension and the carbon concentration. Furthermore, the effect of carbon contents and thickness of composite films on the humidity-actuating performance was further investigated by measuring the maximum bending angle of strip composite films ( Figure 3D). The maximum bending angle is generally inversely proportional to the thickness that is dominated by carbon contents and masses of the mixture due to the significant increase in stiffness. The prepared composite films with more than 0.07 mm of thickness have slow and negligible deformation with a bending angle of 60 • , which is ascribed to the larger bending force required for deformation. Notably, the formed films containing more than 2% of carbon nano-powders cannot remain intact due to the effect of large internal stress from water loss. In contrast, the composite films with lower carbon content and thickness have an intact surface and higher deformation capability, with bending angles exceeding 200 • . However, the block of thinner films is extremely soft and not enough to support and sustain for practical applications. Hence, the thickness was tuned by altering the weight of the mixture suspension from 4 to 6 g when the composite film was prepared throughout the experiments. To intuitively learn the humidity response performance of composite films with different SA and PVA contents (same weight of mixture 6 g), the maximum bending angles of composite films were investigated under 90% RH stimulation, as shown in Figure 3E. The composite films prepared from 0.5% carbon, 0.5% SA, and 0.5% PVA mixture could achieve the optimal bending angle as large as nearly 221 • , further demonstrating the excellent moisture response properties. Furthermore, the bending angles of composite films with different masses under different humidity levels were investigated ( Figure S13). The maximum bending angles are positively correlated with the applied RH level, which demonstrates a high sensitivity to moisture, even with higher thickness. In addition, the curve of bending angle versus time was measured to validate the stability of SusAct composite film (thickness of 41 μm) during five continuous humidity on-and-off cycles. According to the result shown in Figure 3F, the maximum bending angle and response curve for each cycle remained nearly consistent under 79% RH stimulation, confirming the outstanding reliability.
A mechanical model (see the "Supporting Information" section and Figure S14) was employed to rationalize the moisture-driven bending-dominated deformation of a homogeneous actuator, which can be characterized by a bilayer beam 60 with the length of l and thickness of h. Driven by moisture content change , the curvature of the beam can be defined as where is the coefficient of moisture linear expansion, and is the ratio of the thickness of the expansible layer to that of the whole beam. This shows that the curvature is proportional to the combined term ∕ℎ, indicating that the higher coefficient of moisture expansion and larger humidity load lead to a greater curvature and extraordinary actuating capability for a thinner actuator. The finite analysis result was verified to be consistent with the experimental observation ( Figure S15). Additionally, the humidity-responsive performance of triangular shapes (isosceles triangle, base: 18 mm, height: 10 mm, thickness: 0.040 mm) and corresponding real-time infrared (IR) thermal images of the actuation process are shown in Figure 3G,H and Movie S4. Obviously, the shape change of the composite film under moisture triggering is accompanied by a temperature change. The triangular-shaped film deformed rapidly in 3 s after being continuously stimulated by moisture and the temperature of the composite film increased from 21 to 27 • C ( Figure 3G). After switching off the external moisture stimulus, the triangular-shaped film recovered to its initial state, and the temperature dropped from 27 to 21 • C ( Figure 3H). The deformation process and corresponding IR images revealed the moisture actuating mechanism with the potential energy of the moisture gradients captured by the composite film and subsequently converted into internal chemical energy. 61 The accumulation of internal energy is further converted into mechanical energy, which macroscopically manifests as film bending. To further investigate humidity-driven process, the finite element analysis of triangular SusAct composite film was carried out, as shown in Figure 3I. With the Gaussian distribution and 70% RH at the center, a large deformation at the humidified zone can be observed, with the rotation at ends leading to an archlike configuration, while three corners are free to move in the plane, but with the out-of-plane displacement is prohibited. Our numerical solution is consistent with the experimental result: High stress with a maximum of 21.8 MPa occurs in the central zone, whereas nearly zero-stress can be found at the corners.
The excellent humidity-responsive capability provides a broad range of potential applications for our SusAct composite films as smart actuators. Inspired by the crawling motion of the inchworm, a biomimetic directional controllable soft walker (length of 23 mm, width of 5 mm, thickness of 0.040 mm) triggered by moisture gradient was designed, by mimicking the principle of the crawling, as shown in Figure 4. The inchworm in nature has the ability of directional crawling through advancing and anchoring its tail with the claws and stretching its head ( Figure 4A). Figure 4B shows the schematic diagram of the walking principle of the walker, and Figure 4C shows its directional locomotion driven by humidity. First, the strip-shaped walker maintained the flat stationary state, and then, the external humidity stimulation was applied to one end of the walker as the tail, causing the bending of film. Meanwhile, the tail of the walker moved forward and formed an asymmetric curved shape. Herein, the curvature of the tail is larger than that of the head, which means that the frictional force of the tail is much higher than that of the head; thereby, the tail was recognized as a stationary point to avoid the tail from going backward. When the humidity stimulation was switched off, water was desorbed from the composite film, and it had a tendency to return to its initial flat state. At the same time, the head stretched forward, and the tail was anchored due to the bondage of friction. Finally, by alternating the humidity stimulation on and off, continuous directional locomotion can be realized for the soft walker (Movie S5). Finite element model was established to simulate the directional locomotion of a soft walker ( Figure 4D and Movie S6). The real-time length fluctuation as a function of the time of soft walker is shown in Figure 4E; after five walking cycles, the displacement of the walker was about 24 mm in 36.5 s ( Figure 4F).
Based on the fast humidity actuating performance of SusAct composite film, a soft robotic arm was designed for "weightlifting," as shown in Figure 5A-D and Movie S7. When exposed to humid air, the soft robotic arm bent and lifted a foam (6.0 mg), imitating the motion of a human arm lifting a dumbbell ( Figure 5E). After removing the humidity air input, the soft robotic arm recovered to its original horizontal state. Moreover, as shown in Figure 5F, the middle region and edge region of a soft robotic arm exhibit almost identical abilities to lift objects that are heavier than its own weight (4.5 mg), which validates the potential for the construction of an artificial muscle. Furthermore, a smart excavator ( Figure 5G and Movie S8) was designed by utilizing the bending and recovery behavior of the films, with and without moisture, respectively. When the composite film was set away from the moisture tube, the circular film remained unbent; the circular composite film was curled on the foam stick (44.0 mg) after approaching the moisture tube. Subsequently, the foam stick was loaded and moved with the composite film. When the composite film moved away from the moisture tube, the curled film returned to a flat state, and the foam stick was released and dropped, realizing the functions of the smart excavator.
Compared with single films, for composite films, the presence of carbon nano-powders not only enhances the moisture-responsive performance but also broadens the application prospects due to its excellent electrical conductivity. A smart switch that is dominated by the humidity level was designed in this work, as shown in Figure 6A and Movie S9. This switch system was constructed by a strip of composite film (width of 5 mm, length of 20 mm, and thickness of 0.060 mm) and an integrated circuit with a light-emitting diode and one power supply. The smart switch was controlled by the humidity stimulus. When the composite strip kept the straight status, the circuit was closed and the indicator light was on. On the contrary, when the composite strip bent away from the conductor wire upon a high humidity level, the circuit became an open electrical circuit, and the indicator light was turned off. After removing the external moisture, the indicator was turned on again when the composite film recovered to the initial straight state. Interestingly, a hand-shaped actuator was designed for demonstrating the programmable deformation ability, as shown in Figure 6B. The optimal images (blue gloves) show the bending status of the real fingers and the separate movements of the five fingers. Under the manipulation of moisture, the handshaped actuator can controllably perform the corresponding joint-like flexing motions and even complex gestures where multiple fingers are bent simultaneously. Generally, SA-based actuators have been constructed by the cross-linking interaction with Ca 2+ ions, 29,35,62 whereas the cross-linking reaction is irreversible. Therefore, the cross-linked actuators could not be recycled. Differently, our SusAct composite films were fabricated by water evaporation rather than cationic crosslinking, making them recyclable. A recycling experiment was executed to confirm this. The original composite films were immersed in the water again by the virtue of the excellent water solubility of SA and PVA. After sonication and subsequent magnetic stirring for 12 h, a new homogeneous suspension was formed. Finally, the composite films were further remolded through the identical water evaporation process. Figure 6C,D shows the time dependence of the bending curve and recovery curve of original and recyclable SusAct film, respectively. It is evident that there is no significant attenuation in the response bending angle, except for the slower response and recovery time, demonstrating the excellent recyclability of SusAct composite films.
To the best of our knowledge, there is no suitable or comprehensive metric tool to quantitatively assess the greenness of the synthesis and measurement process of actuating materials. As an emerging metric system for green analytical chemistry (GAC), the AGREE calculator was developed to evaluate the greenness of analytical methodologies based on 12 principles of GAC. 63 We employed this existing metric system to systematically measure the degree of greenness of our films compared to other typical actuating materials, although some assessment criteria are not fully applicable and precise. To accurately evaluate the greenness of actuating materials through the AGREE software, the input parameters should consider the whole experimental procedures, including the synthesis and measurement rather than merely analytical methodologies. Therefore, the output score refers to the quality and quantity of product, the complexity of synthesis, waste generation, energy consumption, and hazardousness of reagents throughout the experimental period. The criteria corresponding to 12 GAC principles and evaluation parameters for greenness score calculation in AGREE software are listed in Table S1. As shown in Figure 7, the results show that the SusAct composite film in this work exhibits the highest score of 0.85 compared with other actuating materials, which indicates the optimal greenness degree in the process of synthesis and measurement.

CONCLUSIONS
In conclusion, humidity-responsive SusAct composite films were prepared using a facile colloidal dispersion and a subsequent water evaporating method. The composite films and their geometry featured strong figures of merit and capabilities with a 250 • maximum bending angle under 90% RH. Inspired by the species in nature, programmable and smart actuators, such as walkers, smart switches, flexible excavators, soft robotic arms, and hand-shaped actuators, were further fabricated by a rational design. This demonstrated the general applicability of our SusAct films and their inherent adaptive integration and low-cost manufacturability. In addition, the composite films exhibit excellent recyclability; thus, there is zero-waste emission throughout the synthesis except for water evaporation. An AGREE calculator was employed to demonstrate the excellent greenness degree of SusAct composites. This sustainable synthetic method makes it accessible to mass-produce and integrate the actuators at low cost.

Materials
SA, carbon nano-powders (nanoparticle size < 100 nm), and PVA (weight-average molecular weight value M w = 89,000-98,000, 99+% hydrolyzed) were purchased from Sigma-Aldrich Company Ltd. These reagents were analytical grade and used without further purification.

Preparation of SA and PVA films
The films in this work were prepared using a facile colloidal dispersion and subsequently water evaporating method. SA powders with gradient mass fractions (0.5, 1, 1.5, and 2 wt.%) were dissolved into deionized water and heated at 90

Preparation of CSA and CPVA films
The following procedure was applied to prepare carbon nano-powders and SA composite (CSA) films. First, 1.0 g of carbon nano-powders and 2.0 g of SA powders were dispersed into 200.0 g of deionized water and heated to 90 • C for 1 h under continuous mechanical stirring and then stirred at 20 • C for 24 h to prepare a colloidally homogeneous suspension. Subsequently, the suspension was divided into different masses (3.0, 4.0, 5.0, 6.0, and 7.0 g), and then, they were poured into the Petri dishes with an identical diameter of 50 mm. Finally, these Petri dishes were placed in a chamber with constant temperature and humidity (20 • C, 37% RH), and then, the suspension was evaporated at 20 • C for 72 h to form uniform and desiccative films. The carbon nano-powders and PVA composite (CPVA) films were prepared following the same procedure of that for the CSA film, where SA was replaced by using PVA powders.

Preparation of SusAct composite film
First, 1.0 g of carbon nano-powders, 1.0 g of SA powders, and 1.0 g of PVA powders were dispersed into 200.0 g of deionized water and heated to 90 • C for 1 h under continuous mechanical stirring and then were stirred at 20 • C for 24 h to prepare a colloidally homogeneous suspension. Subsequently, the suspension was divided into different masses (3.0, 4.0, 5.0, 6.0, and 7.0 g), and then, they were poured into the Petri dishes with an identical diameter of 50 mm. Finally, these Petri dishes were placed in a chamber with constant temperature (20 • C, 37% RH) and humidity, and then, the suspension was evaporated at 20 • C for 72 h to form uniform and desiccative films (SusAct films).

Characterization
FTIR spectra were recorded using a PerkinElmer Spectrum 65 spectrometer within a scan range of 4000-600 cm −1 and a resolution of 1 cm −1 . Raman spectroscopy was tested by Renishaw inVia Reflex spectrometer system under 633 nm laser excitation in the range of 800-2000 cm −1 . TGA (TGA 5500) was performed to characterize the thermal stability of our samples. The crystal structures were characterized using XRD, which is monochromatic Cu-K α radiation on a Siemens D5000 X-Ray powder diffractometer fitted with a Ge monochromator. The microtopography, including the cross-section morphology of the films, was observed by SEM with FEI Inspect F. The topography and roughness analysis (Image Rq, Root mean square average of height deviations) of the film surface was characterized by a Bruker Dimension Icon AFM with SCANASYST AIR tips. The thickness and dimension of the films were measured by RS Pro external micrometer and digital caliper, respectively. The temperature distributions of the surface were taken by a thermal imaging camera (RS Pro Thermal Imager, RS-9875). In addition, humidity-driven performance tests were carried out in the laboratory with stable temperature and humidity levels, where the ambient temperature and RH were approximately 20 • C and 37%, respectively. The ambient temperature and humidity level were recorded by a multifunctional moisture meter (RS Pro moisture meter, DT-229). A humidity system was established using one humidifier and an extension tube to control the humidity level and create a humid atmosphere. The humidity level can be regulated by adjusting the distance between the tube and actuators and the power of humid air generation. Mechanical properties were tested with Instron 3442 machine at a loading rate of 1 mm min −1 . The photographs and videos of actuating motions were captured by a digital camera (Sony, DSC-HX400V).

A C K N O W L E D G M E N T S
We acknowledge that this work was financially supported by the Royal Society Research Grant (RGS\R1\201071) and QMUL-CSC (China Scholarship Council) scholarship.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.