Moisture‐Driven Cellulose Actuators with Directional Motion and Programmable Shapes

The hygroscopic motion of plants has inspired the development of moisture‐activated soft actuators. These actuators driven by ambient moisture sources are of great research interest in robotics and self‐regulating textiles. However, these actuators often have slow motion and can only perform bending and twisting motions. Herein, a cellulose film‐based fast‐morphing and motion‐programmable soft actuator is presented that can generate caterpillar‐like movement. The cellophane films reported here bend almost instantaneously under changing humidity, with a large bending curvature, high repeatability, and negligible hysteresis. Different actuation modes are studied using both coated and uncoated cellophane films. The uncoated cellophane film can continuously move on a moist substrate through autonomous bending–rolling–flipping (or oscillating) cycles. A facile strategy is used here to control the rolling direction and facilitate the flipping motion by offsetting its center of gravity during deformation by adding appropriate weights on the end of the actuator. The coated cellophane film is used to fabricate motion‐programmable actuators through heat‐laminating. Several actuator structures are designed and fabricated and their diverse moisture‐induced motions are demonstrated.


Introduction
In nature, moisture is the primary driving force for passive actuation in plants.Examples include the opening of pea pods [1] and pinecones [2] for seed disposal, as well as the locomotion of wheat awns, [3] all due to the change in environmental humidity.The passive motion relies on the moisture absorption and desorption-induced volume change of the hygroscopic cellulose.Different movements such as bending, twisting, and coiling are achieved through the response of the moisture-active and inactive materials present in the structures; for example, the opening of pinecone under decreasing humidity results from the differential deswelling of the bilayered scale. [2]The various motions of plants have given insight into the design of soft artificial actuators, [4] which play a crucial role in emerging technologies such as humanmachine interfaces [5] and smart textiles. [6]oft actuators produce mechanical work under a variety of energy inputs.The active materials employed in soft actuators include dielectric elastomers (DE), [7] photoresponsive polymers, [8] thermoresponsive polymers, [9] hygroscopic materials, [10] carbon nanotubes (CNTs), [11] etc. The actuation performance such as the large actuation strain of DEs over 200%, [12] the high output stress of twisted and coiled polymer over 22 MPa, [13] the high energy density of CNT actuators of 160 J kg À1 , [14] and the wide range of mechanical flexibility and multistimuli responsiveness [15] point to the potential of soft actuators to overcome the limits of conventional actuators (e.g., motors) and possibly surpass the performance of natural muscles.
Among various stimuli used to power the soft actuators, moisture can be rated as the greenest energy that is abundantly available in the environment.[18][19] However, the slow response speed is a common issue in moisture-driven actuation primarily due to the time needed for moisture diffusion, absorption, and desorption. [20,21]Therefore, efforts are underway to increase the actuation speed by creating porous morphologies to facilitate diffusion, [19,22] accelerating the desorption rate by heating, [16] and altering the absorption and desorption rate of the material by other means. [23]urrent moisture-driven soft actuators are limited by the types of motion they can produce due to the global nature of moisture variation and the lack of sufficient exploration of structural design.][25] A few reported self-walking robots with directional friction legs [18] and selfoscillating actuators on a moist surface [25,26] are made by DOI: 10.1002/aisy.202300638 The hygroscopic motion of plants has inspired the development of moistureactivated soft actuators.These actuators driven by ambient moisture sources are of great research interest in robotics and self-regulating textiles.However, these actuators often have slow motion and can only perform bending and twisting motions.Herein, a cellulose film-based fast-morphing and motion-programmable soft actuator is presented that can generate caterpillar-like movement.The cellophane films reported here bend almost instantaneously under changing humidity, with a large bending curvature, high repeatability, and negligible hysteresis.Different actuation modes are studied using both coated and uncoated cellophane films.The uncoated cellophane film can continuously move on a moist substrate through autonomous bending-rolling-flipping (or oscillating) cycles.A facile strategy is used here to control the rolling direction and facilitate the flipping motion by offsetting its center of gravity during deformation by adding appropriate weights on the end of the actuator.The coated cellophane film is used to fabricate motion-programmable actuators through heatlaminating.Several actuator structures are designed and fabricated and their diverse moisture-induced motions are demonstrated.
patterning moisture-inert materials on hygroscopic films [27] and are capable of programmable deformations.However, directional (or asymmetric) friction of the robotic legs, often necessary for directional motion, is difficult to achieve and is sensitive to the ground surface.The movement of the self-oscillating actuators reported so far is arbitrary.
Here, we report proof-of-concept of a cellulose film-based fast-morphing and a motion-programmable soft actuator that can generate caterpillar-like recoil-and-roll motion. [28]The regenerated cellulose film (cellophane) is a biodegradable, low-cost, commercially available film produced from sodium cellulose xanthate. [29]Cellophane is commonly used in packaging and specialty applications such as batteries and medicine (hemodialysis) as a semipermeable membrane where its hydrophilicity and selective diffusion characteristics are of value. [30]The outstanding actuation performance of the cellophane is due to its large moisture absorption and associated dimensional change, with fast absorption and desorption rate. [31]wo types of cellophane, i.e., uncoated and one side-coated, were characterized for their material properties and moistureinduced bending behavior.The uncoated cellophane deforms continuously in an environment with varying moisture gradients (e.g., the palm of a hand and near the water surface) through a series of bending, rotational, and flipping motions. [23,26,32,33]The coated cellophane, having a thin layer of moisture-blocking polyvinyl dichloride (PVdC) on the surface, undergoes enhanced bending motion due to the gradient in moisture absorption and resulting swelling.A self-walking robot with directionally controlled rotation and rapid flipping is fabricated from the uncoated cellophane by shifting its center of gravity (CG) with optimally attached masses.While shifting CG with moving masses has been used to control motion in airborne vehicles [34] and spherical robots, [35] to our knowledge, this is the first report on controlling the motion of soft actuators using this principle.Additionally, We demonstrate several programmable movements of the cellophane actuators, including rotation, snake-like, and lifting motions.

Actuator Fabrication
Coated films were used to construct the shape-programmable actuators through heat-lamination.The films were cut into designed shapes along the desired directions and then laminated with the coated sides in contact with each other at 120 °C for 2 min using a heat press.

Cellophane Characterization
The degree of crystallinity and crystalline orientation was measured using a wide-angle X-ray diffractometer (WXRD) (Rigaku SmartLab X-ray diffractometer).The hygral expansion of the uncoated cellophane in three dimensions, i.e., machine (MD), cross (CD), and thickness (TD) directions, were measured at ambient temperature, with the in-plane dimensions (MD and CD) measured with a ruler, and the thickness measured with a digital thickness gauge (Mitutoyo, Model 517-890).The mechanical properties of the uncoated cellophane in the ambient environment and wet conditions were evaluated using a tensile testing machine (MTS-30G) with appropriate load cells.The coating thickness was measured on the micrographs captured using a scanning electron microscope (SEM TM4000, Hitachi).

Measurement of Moisture-Induced Actuation
The moisture-induced deformation of the cellophane was characterized under an ambient environment and in a custom-designed humidity chamber, with a probe set up to monitor the humidity close to the actuator surface.While the environmental moisture sources were the palm of a hand and near-water-surface, the actuation was performed in a glove box to avoid air turbulence, and the motion was recorded with a video camera for further analysis.The customized humidity chamber was used for humidity-controlled actuation, and the RH within was modulated by tuning the wet and dry air influx (Figure 3a).The bending angle and curvature of the bent film were then measured frame by frame using an image editor (Photoshop by Adobe).A humidity and temperature sensor, SHTC3 (Sensirion, Switzerland), continuously recorded the relative humidity and temperature near the sample.

Moisture-Induced Bending
Soft bending actuators are generally fabricated from films. [23,36]hile the extent of their deformation is enhanced by constructing multilayer structures, [15,18] the bending direction is often controlled by introducing mechanical anisotropy. [37,38]In this study, we used cellophane films that are inherently anisotropic and are available with ultrathin hydrophobic coatings.
The anisotropy of cellulose films comes from the extrusion and roll-to-roll fabrication process, where the molecules are aligned preferentially in-plane along the machine direction, forming a semicrystalline structure.The degree of crystallinity of 43% and preferential crystal orientation in ½110 plane (in-plane) was determined by the WXRD for our films (Figure 1a).This microstructural feature manifests in the film's anisotropic differential swelling and tensile behavior.Both are essential properties in influencing the moisture response of the actuator.The measured differential swelling of the films in three principal directions, i.e., MD, CD, and TD, are shown in Figure 1b,c.The swelling is primarily observed in the TD (97%), with minor swelling in the CD (8.7%) and negligible swelling in the MD.The mechanical anisotropy is also illustrated by the tensile behavior of the films in Figure 1e.The measured Young's modulus (E) in the MD (E MD ¼ 5.76GPa) is almost twice of that in the CD ðE CD ¼ 3.2GPaÞ in the ambient environment (50%RH, 22 °C).After dipping the films in water, E decreases drastically due to the softening effect of water on cellulose, [39] while the degree of anisotropy increases ðE MD ¼ 0.38GPa, E CD ¼ 0.14GPaÞ.While the cellophane is highly sensitive to moisture, the coated cellophane film is widely used in commercial for moisture-blocking and heat-sealing purposes.Here, we utilize both features to fabricate moisturesensitive soft actuators with enhanced bending response and programmable motions.An ultrathin layer of hydrophobic PVdC (3.3 μm, Figure 1d) coated on the cellophane forms a bilayered film with differential swelling behavior, enhancing the bending deformation.
To study the effect of film thickness and coating on the hygroscopic bending performance, we compared the uncoated cellophane films of three different thicknesses (21, 28, 42 μm) and one-side-coated cellophane having 24 μm thickness.
Unless otherwise specified, all samples for moisture-induced bending actuation were cut along CD in 20 mm lengths and 10 mm widths.Samples were placed on a polyester screen mesh that neither blocks nor absorbs the moisture significantly and then in a glove box.The coated cellophane samples were placed on the mesh with coated side up.
The cellophane samples instantaneously bent upward as soon as a water bath (moisture source) was brought beneath the mesh fabric holding the film and then recovered when the moisture source was removed (Video S1, Supporting Information).The curvature (κ) of the test samples and the RH, as well as the temperature near the film's surface as a function of time, are shown in Figure 2c,d.In general, thicker films show lower curvature and more extended response time because of the higher bending stiffness and prolonged diffusion of water molecules into the film. [31]The coated cellophane generates almost twice the curvature of the uncoated cellophane under similar exposure to moisture, suggesting that the bilayer structure with differential swelling can improve the response significantly.A similar response was observed upon placing the film on the palm of the hand (Video S2, Supporting Information).However, the films bend faster with higher curvature due to the higher RH (þ20%) on the palm of the hand than on the water surface (Figure 2c,d).
Based on our observations thus far, we focused on a more detailed study of the moisture-induced bending behavior of the coated cellophane using a customized humidity chamber, see Figure 3a.The RH of the humidity chamber can be reliably and reversibly increased from 10 to 90% in around 40 s by controlling the influx of dry and humid air.The bending deformation of the coated film is measured as a function of increasing RH by placing it on the polyester mesh fabric in the RH chamber, see Figure 3a.As the RH increases from ambient (≈55%) to 90%, the film bends toward the coated side with a maximum curvature of 2.8 cm À1 and then gradually recovers as the RH passively (by turning off the humid air) returns to the ambient condition (Figure 3b).When the RH decreases from ambient to 10%, the film bends in the opposite direction with an equilibrium κ of 4 cm À1 and flattens as RH recovers to ambient.The change in curvature closely follows the RH changes.However, the speed of RH change is limited by the chamber size, which in turn restricts the actuation speed of the cellophane.
The response of the coated cellophane was further evaluated by subjecting it to cyclic change in RH between 20% and 80%.This range was chosen for faster actuation speed due to the limitations of the experimental set-up that allowed for more rapid change within this range.In the cyclic actuation at 30 s cycle À1 , curvature changes between À3 and 2 cm À1 in a cyclic manner (Figure 3d, Video S3, Supporting Information).Interestingly, the curvature changes almost linearly with RH during one cycle of absorption and desorption, see Figure 3e.The result of multiple (five) actuation cycles demonstrates good repeatability of deformation with small hysteresis (Figure 3f ).The film recovers to nearly zero curvature during each cycle when RH returns to the ambient level.

Self-Walking Actuators
The published reports on the moisture-driven self-walking actuators are primarily designed based on two working principles: 1) oscillatory bending motion with asymmetric frictional legs/substrate for directional motion under changing RH [18,40] and 2) continuous rolling and flipping locomotion on a moist surface. [23,26,32,33]Construction of self-walking actuators using the second mechanism is promising; first, there is no need for tedious RH tuning; second, the rotation is much faster than bending and can be used to enhance locomotion speed.
In our work, we also observed the uncoated cellophane's continuous bending, rotation, and flipping motion on moist surfaces.The mechanism of such locomotion has been described in other reports [32,41] and is shown schematically in Figure 4a.In brief, when a film is placed on a surface with a moisture gradient (e.g., near the water/wet surface, the palm of a hand, etc., State1), it bends upward to a specific curvature due to the differential moisture absorption (State 2) that leads to instability and the bent film rotates (State 3).The rotating film can either flip (State 4) or oscillate back.(State 4').If the film flips, it flattens (State 5) and bends upward due to higher moisture (State 6) at the bottom.The motion continues afterward.However, such movement is almost random, with no control of the rotational direction and certainty of flipping.
Based on the observations thus far, the actuator was modified to control the rotational direction by attaching weights to the top of one end of the film and the bottom of the other to offset the CG as the film bends in a gradient RH environment, see Figure 4c.The bent shape of uncoated cellophane without additional masses is shown in Figure 4b, with the Cartesian coordinate (X, Y, Z) used to track the deformation and motion.Assuming that the bent shape has a uniform curvature and is symmetric about the Z-axis, the CG of the film should lie on the Z-axis.By attaching the masses at the top and bottom at the two ends of the film, the bending of the film shifts the CG of the bent shape to one side of the Z-axis due to the asymmetric mass distribution, see Figure 4c.The shifted CG generates a torque on the bent film, leading to its rotation in a particular direction.After flipping, followed by flattening and bending, the CG of the film is still on the same side of the Z-axis, resulting in a continuous motion in the desired direction.
The added masses accelerate the flipping motion as larger potential energy (U p ), which is stored in the bent shape before rotation.Specifically, the increase of the mass (m) and the height of the CG (h) of the bent film both contribute to a larger potential   Assuming the CG lies at the midpoint of the two added masses (assuming the weight of cellophane is negligible), the conversion of U p to U k , during rotation, is determined by the bending angle (θ) of the film, see Figure 4d.When θ < 90°, ω first increases and then decreases, as h decreases and then increases, leading to the possibility of oscillation.When θ ≥ 90°, ω increases due to the continuous decrease of h (see Figure 4d), resulting in the actuator flipping.The bending angle can be simply tuned by changing the film's length, and in this experiment, we used an appropriate film dimension of 20 mm in length and 10 mm in width.
The newly designed end-loaded flipping actuator (ELF-A), having two masses (13 mg each) attached asymmetrically at the top and bottom of the cellophane (8 mg) at the two ends, was fabricated for further evaluation, see Figure 5a.The ELF-A was placed on a mesh fabric on top of a water bath (water T = 34 AE 0.5 °C).Due to the moisture gradient on the substrate, the actuator bent upward with θ ≥ 90°, and the CG shifted to the left of the Z-axis, resulting in the rolling to the left followed by flipping.Then, the flipped ELF-A flattened and bent upward again with the CG still on the left of the Z-axis, leading to a continuous bending-rolling-flipping toward the left.Figures 5a,b and Video S4, Supporting Information, show the movement sequence during self-walking.During repeated experiments, this version of the ELF-A flipped and rotated in the designed direction 93% and 78% of the time at a frequency of 7 rotation min À1 (Figure 5c).Ideally, the asymmetric cellophane actuator should rotate in the direction of the mass farther away from the Z-axis.However, there are still chances of rotation in the opposite direction, possibly due to: 1) instability caused by air turbulence and surface roughness of the substrate, 2) inconsistency of the mass due to trimming and assembly.

Programmable Actuation Potential of ELF-A
The cellophane's moisture response and structural anisotropy present many opportunities for various deformation modes for useful applications.Here, we demonstrate the possibilities by using the one-side-coated cellophane to fabricate actuators with programmable motions.We designed several structures to convert the bending of the film into various motion types for potential applications in robotic locomotion and morphing.
A tripod actuator fabricated by heat-laminating three-coated cellophane strips, each trimmed in the bias direction (at 45°t o MD) and attached at one end at 120°to each other, is shown in Figure 6b and Video S5, Supporting Information.The trimming of the film along the bias direction was necessary to generate the helical deformation of the film.As the strips deform into spiral shapes under changing RH, they exert rotational forces on the laminate, leading to its rotational locomotion.Another two-tier actuator designed to generate bending in opposite directions, resulting in lifting motion, is shown in Figure 6c and Video S6, Supporting Information.In this design, two cellophane strips trimmed along the TD were thermally cross-laminated with the coating side facing each other.Upon exposure to a humidity gradient, the strips bend in opposite directions, leading to the reversible lifting motion during increasing and decreasing RH.Another design utilized the bidirectional bending of the coated cellophane to generate a snake-like movement by alternatively heat laminating the cellophane strips in a linear manner (Figure 6d).The adjacent strips bend in opposite directions as RH changes, leading to an overall wavy deformation.Cycling the RH between low and high levels leads to repetitive changes in the direction of curvature of the individual strips (Figure 6d, Video S7, Supporting Information).

Conclusion
In this study, we demonstrate a cellulose film-based fastmorphing and motion-programmable soft actuator based on hygroscopic bending of low-cost and commercially available coated and uncoated cellophane.The actuator can generate a caterpillar-like recoil-and-roll motion, reported as a reverse gallop in the literature.The large and anisotropic swelling behavior and decent moisture absorption and desorption rate make cellophane a good candidate as an actuator stimulated by abundantly and naturally available moisture.Remarkably, pure cellophane can move continuously on a moist substrate, converting the chemical potential (moisture gradient) into the mechanical energy that powers its motion.We enhanced the self-walking movement by increasing the angular momentum for flipping and shifting the center of mass for directional locomotion.The one-sidecoated cellophane films showed larger bending curvature than the uncoated ones due to the differential swelling of the two layers.The coating promotes the bending motion and enables the heat-laminating ability, which was a convenient method for building different actuator structures for diversified motions.We further demonstrated the potential for this approach through several structural designs, including rotational, lifting, and snake-like movements, for future exploration of motionprogrammable actuators.

Figure 1 .
Figure 1.Material properties of the cellophane.a) WXRD spectrum of the films, b) images showing water-induced dimensional changes along the MD and CD, c) anisotropic swelling along MD, CD, and TD, d) SEM images of the cross-section of the coated cellophane with the inset highlighting the coating, and e) anisotropic tensile properties of the uncoated film along MD and CD, in ambient and wet conditions.

Figure 2 .
Figure 2. Actuation of cellophane under natural moisture gradient.a) Schematic of moisture-induced bending of the pure and coated films.b) Images of moisture-induced bending of cellophane cut along (clockwise) MD, TD, and the bias direction (45°to the MD).c,d) Curvature (κ) change of films with changing RH as a function of time when (c) a water bath, and (d) the palm of the hand is moved closer and farther from underneath the film.

Figure 3 .
Figure 3. Actuation of the one-side-coated cellophane in an RH-controlled chamber.a) Schematic of the customized RH control unit (inset shows the view of a bent film within the unit), b) deformation (curvature) of a film in the RH chamber during increasing (↗) and decreasing (↘) RH, shown as a function of time.c) One actuation cycle from d) several cycles within a selected RH range.e) One actuation cycle from f ) cyclic actuation plotted as a function of RH.

Figure 4 .
Figure 4. Schematic of the self-walking actuator design.a) Locomotion sequence of uncoated cellophane on a moist surface, b,c) shifting of the CG (Â) of the bent film with asymmetric masses attached for directional motion, d) influence of the bending angle (θ) on the probability of flipping.From left to right: θ < 180°, θ = 180°, θ > 180°.The size of the rhombus (♦) represents larger and smaller torque due to the asymmetric location of the added mass.

Figure 5 .
Figure 5. Directional self-walking actuators.a) Schematic showing the sequence and b) experimental images of the directional motion of an ELF-A.c) Comparison of the self-walking performances between the actuators with and without attached masses.

Figure 6 .
Figure 6.The potential of the ELF-A design.a) The coated cellophane film shows varying trimming angles, b-d) schematic of the assembled tripod, lifting, and snake-like actuators, and e) corresponding images of their actuation under moisture gradient.