Editing the Shape Morphing of Monocomponent Natural Polysaccharide Hydrogel Films

Shape-morphing hydrogels can be widely used to develop artificial muscles, reconfigurable biodevices, and soft robotics. However, conventional approaches for developing shape-morphing hydrogels highly rely on composite materials or complex manufacturing techniques, which limit their practical applications. Herein, we develop an unprecedented strategy to edit the shape morphing of monocomponent natural polysaccharide hydrogel films via integrating gradient cross-linking density and geometry effect. Owing to the synergistic effect, the shape morphing of chitosan (CS) hydrogel films with gradient cross-linking density can be facilely edited by changing their geometries (length-to-width ratios or thicknesses). Therefore, helix, short-side rolling, and long-side rolling can be easily customized. Furthermore, various complex artificial 3D deformations such as artificial claw, horn, and flower can also be obtained by combining various flat CS hydrogel films with different geometries into one system, which can further demonstrate various shape transformations as triggered by pH. This work offers a simple strategy to construct a monocomponent hydrogel with geometry-directing programmable deformations, which provides universal insights into the design of shape-morphing polymers and will promote their applications in biodevices and soft robotics.


Introduction
In nature, shape-morphing behaviors of organisms are vital for their survival [1]. Typically, pinecones and peasecod can adjust their morphologies in response to humidity for releasing their seeds at appropriate periods [2]. This intriguing shape morphing results from the mismatch strain/stresses in local swelling behaviors which are determined by anisotropic tissue compositions and micro-/nanostructures within the organisms [3][4][5]. Inspired by these smart behaviors in nature, efforts have been focused on the development of artificial materials with programmable shape-morphing capabilities [6], such as electroactive polymers [7], liquid crystal elastomers [8], shape memory polymers [9], and stimuli-responsive hydrogels [10], which hold great promises to develop artificial muscles [11], reconfigurable tissue engineering scaffolds [12][13][14], and soft robotics [15][16][17] for a broad range of applications. Among these, hydrogels are one of the most attractive candidates, because they not only can swell and shrink in response to certain stimuli (thus changing their volumes reversibly) but also can exhibit unique superiorities in high similarity (water-rich structures and mechanical properties) to soft biological tissues, making hydrogels the most promising candidates in biomedical and robotic fields [18][19][20].
In recent years, shape-morphing hydrogels have gained extensive attention [20,21]. Conventionally, the shapemorphing hydrogels are realized by introducing diverse components with different swelling/shrinking behaviors upon exposure to specific stimuli. For example, bilayer hydrogels consisting of active and passive components are usually used to realize deformations based on the mismatch strain triggered by the swelling/shrinking of the active layer [22,23]. However, the introduction of diverse components may suffer from intrinsic limitations, including unstable interfaces, time-consuming fabrication, and limited opportunities for biomedical applications [24,25]. Therefore, it is greatly significant to prepare monocomponent hydrogel films enabling precisely controlled deformations. Typically, anisotropic structures and cross-linking densities either in plane or out of plane (across the thickness) are rationally designed into monocomponent hydrogel films to induce programmable shape morphing [26,27]. Nevertheless, the complex anisotropic designs are highly dependent on various advanced manufacturing techniques, such as photomaskassisted photopolymerization [28], and 3D/4D printing [29][30][31], to tune the mismatch strain/stresses, resulting in high cost and low versatility. In view of the intrinsic influence of the geometry effect (i.e., length-to-width ratio and thickness) on the mismatch strain/stresses [32][33][34], we therefore hypothesize that the shape morphing of monocomponent natural polysaccharide hydrogel films could be edited solely by modulating their geometries.
To confirm this hypothesis, we develop an unprecedented strategy to integrate gradient cross-linking density and geometry effect into a monocomponent natural polysaccharide hydrogel film, chitosan (CS), which has remarkable advantages of excellent biocompatibility and functionalities [35,36]. Upon swelling or shrinking of CS hydrogel films, the gradient cross-linking density across the thickness results in different swelling/shrinking ratios, which further leads to the mismatch strain/stress. The out-of-plane strain/stress finally induces bending deformations of CS hydrogel films, which can be modulated via the geometry effect, leading to controllable three-dimensional (3D) deformations in water or solutions with various pH values. Due to the synergistic effect, the shape morphing of CS hydrogels can be facilely edited by changing their geometries (length-to-width ratios or thicknesses). Therefore, helix, short-side rolling, and long-side rolling can be easily customized. Furthermore, various complex artificial 3D deformations such as artificial claw, horn, and flower can also be obtained by combining various planar CS hydrogel films with different geometries into one system, of which shapes can further be altered by pH. Our studies not only offer a simple yet effective strategy for precise controlling over the deformations of monocomponent hydrogel films (e.g., CS and sodium alginate (SA) hydrogels) but also present universal insights into the geometry-directing programmable deformations, which will provide valuable guidelines for the design and developments of shape-morphing polymers, soft actuators, and reconfigurable biodevices.

Fabrication of CS Hydrogel
Films with Gradient Cross-Linking Density. CS hydrogel films with gradient cross-linking density across the thickness are fabricated by top-down diffusion of glutaraldehyde (GA) molecules within a CS pregel solution. In a typical experiment (Figure 1(a)), CS powders were firstly dissolved in an acetic acid solution (2.0 wt%) to prepare CS pregel solution (2.2 wt%), which was then poured into a home-made device, and GA solution was subsequently dripped onto the top of the pregel solution. Thus, CS hydrogel was cross-linked via Schiff-base reaction between the amino groups of CS and the aldehyde groups of GA [37,38]. After being placed at room temperature for 24 hours, the dried CS hydrogel film was peeled off and then cut into specific sizes, which could quickly change their shapes from planar to 3D structures after immersing into water in seconds (Figures 1(b)-1(d) and Figure S1). The deformed shape and swelling ratio can be kept in deionized water stably after complete removal of the residual acetic acid by a 0.01 M sodium bicarbonate solution ( Figure S2). The shape morphing of CS hydrogel film is induced by the synergistic effect of the gradient cross-linking density across the thickness and geometry effect.
We first investigate the effect of the gradient cross-linking density across the thickness on the swelling property of CS hydrogel film. As shown in Figure 2(a), the amount of GA molecules diffused into CS hydrogel films determines the network density of CS chains. Consequently, CS chains form the tightest networks at the top surface owing to their complete exposure to the GA solution, leading to the highest cross-linking density. On the contrary, CS chains form looser networks at the bottom surface due to the decreasing GA molecules diffusible into the deeper sites of the hydrogel film, thus resulting in a lower cross-linking density and looser structure at the bottom surface than at the top surface ( Figure 2(b)). Therefore, the gradient cross-linking density results from the GA concentration gradient across the thickness of the CS hydrogel film, which is further verified by the results of Young's modulus mapping. As shown in Figures 2(c)-2(e), Young's modulus at the top surface of the CS hydrogel film is always higher than that at the bottom surface, regardless of the changes in the thickness of the film (Table S1). Notably, Young's modulus of both the top and bottom surfaces decreases with increase of the thickness of the CS hydrogel film due to the increase of CS molecules involved in the cross-linking (Figure 2(e)). Specifically, for a thin hydrogel film, the GA molecules can reach the bottom surface of the hydrogel rapidly due to the short diffusion distance and thus have enough time to react with the CS hydrogel film on both sides. Consequently, the thin CS hydrogel film is saturated with the GA solution, resulting in high Young's modulus at both sides (for a 20 μm thick hydrogel film, Young's modulus of the top surface: 6:41 ± 0:49 MPa, and Young's modulus of the bottom surface: 1:06 ± 0:13 MPa). In comparison, for a thick hydrogel film, the GA molecules need a long time to reach the bottom surface due to the long diffusion distance and thus have short time to react with the increased CS molecules across the thickness, resulting in a low Young's modulus at both sides (for an 80 μm thick hydrogel film, Young's modulus of the top surface: 0:94 ± 0:05 MPa, and Young's modulus of the bottom surface: 0:08 ± 0:01 MPa). These distinct differences in stiffness between the top and bottom surfaces of the hydrogel films further confirm their different cross-linking densities across the thickness [39]. Moreover, these cross-linking density differences induce different swelling/shrinking ratios of CS hydrogel films (Figure 2(e) and Figure S2), which further leads to the mismatch strain/stress (out-of-plane strain/stress).

Mechanical
Analysis of Shape-Morphing Hydrogels. As reported in our previous work [26,28], solely out-of-plane strain/stress can lead to nonuniform bending of a planar hydrogel film upon swelling. It means that programming the bending directions of the hydrogel films into welldefined 3D shapes still requires precise tuning of the mismatch strain/stress at specific directions, which is highly relevant to not only the gradient cross-linking density across the thickness of the hydrogel films but also the original geometries (e.g., length-to-width ratio and thickness) of the hydrogel films [32]. To further understand the synergistic effect of the gradient cross-linking density and geometry effect on the shape-morphing behaviors, we analyze the bending moments induced by the internal stresses of the swelling or shrinking hydrogel films. The basic mechanics of the hydrogel film with gradient cross-linking density across the thickness and specific geometry (e.g., length-towidth ratio and thickness) can be simply described by the bimetallic strip model (Equation (1)) [40].
where EI is the bending rigidity of a hydrogel layer, and ρ is the radius of the bending curvature. The M is positively related to the length difference before and after swelling of one side (Δl) and square to the thickness of the hydrogel film (h) (Equation (1)), and k is a Young's modulus-related parameter. Unlike in a simple beam-like bilayer, the swelling of a hydrogel film with gradient cross-linking density across the thickness triggers a biaxial expansion inside the hydrogel, which creates mismatch strain/stress [32]. The resultant mismatch strain/stress further induces shape transformation of a  ). By changing the geometry of a hydrogel film, e.g., initial length-to-width ratio (initial length/initial width: L 0 /W 0 ) or thickness (h), the bending moments along the x-axis and/or y-axis will change due to the changing Δl and h or the changing Young's modulus of the top and bottom surfaces of the hydrogel film, leading to the change of the resultant bending moment (M xy ). Notably, the shape morphing of a hydrogel film is determined by the changed resultant bending moment, while the shape deformations always meet the request of lowest energy principle [34, 40-43].

Programmable Shape Morphing via Changing Length-to-Width Ratios.
To reveal the geometry effect on the shape morphing of CS hydrogel films, we first investigate the influences of the length-to-width ratios (L 0 /W 0 ) on the shape-morphing behaviors of CS hydrogel films with a certain thickness (80 μm). As shown in Figure S3, three types of shape-morphing behaviors (diagonal rolling, helix, and short-side rolling) are observed. When L 0 /W 0 is 1 (L 0 : 12 mm, 24 mm, and 36 mm), the hydrogel films prefer to demonstrate diagonal rolling (Movie S1). It results from the resultant bending moment (M xy ) being along the diagonal direction, where the moment along the x-axis (M x ) is close to the moment along the y-axis (M y ). When the L 0 /W 0 ratio increases from 2 to 12 via decreasing the initial width (W 0 ) of CS hydrogel films, the hydrogel films mainly demonstrate helix structures, which have the lowest energy. Specifically, as the L 0 /W 0 ratio increases slightly (<6), the diffusion of water inside the hydrogel layer is similar along both the length (L 0 ) and width (W 0 ) directions, resulting in bending simultaneously at these two sides. Afterwards, the bending along the length direction (L 0 ) eventually dominated due to the slightly larger moment along this direction (M y ), in agreement with energetic considerations, leading to a final helix shape. Comparatively, as the L 0 /W 0 ratio increases largely (≥6), the diffusion of water inside the hydrogel layer is faster along the width direction (W 0 ) than the length direction (L 0 ), resulting in bending along the width direction (W 0 ) first and then along the length direction. The bending (M x ) along the width direction (W 0 ) firstly occurs due to the fast swelling along this direction, and the bending (M y ) along the length direction (L 0 ) starts later due to the edge effects [34]. Thus, the collaborative bending (M xy ) along these two directions leads to helix structures (Movie S2 and Movie S3). Notably, the edge effects become stronger as the L 0 /W 0 increases largely, which would change the shape-morphing path. As shown in Figure S3, the hydrogel films (L 0 : 12 mm, L 0 /W 0 ≥ 8) show short-side rolling as the W 0 decreases to~1 mm, which results from the much faster diffusion of water inside the hydrogel layer along the width direction (W 0 ) than the length direction (L 0 ) due to the strong edge effects, leading to initial bending along the width direction (Movie S4). Moreover, the initial bending of the hydrogel film determines the shapemorphing path. Even though a helix shape can be obtained in  a lower energy state, the bending along the length direction is hindered due to the difficulty of overcoming the high energy barrier to reverse the short-side rolling. In addition, diagonal rolling, helix, and short-side rolling are still observed as the L 0 /W 0 ratio increases from 1 to 12, where the initial length (L 0 ) of CS hydrogel films increases from 1 mm to 60 mm. The edge effects are further confirmed experimentally by using CS hydrogel films with both L 0 and W 0 close to millimeter size as shown in Figure S4, which increases the probability of short-side rolling. Therefore, various shape-morphing behaviors can be facilely obtained via changing the lengthto-width ratios (L 0 /W 0 ) of CS hydrogel films.
To confirm the effect of length-to-width ratios (L 0 /W 0 ) on shape-morphing behaviors, we further explore the relationships between the length-to-width ratios of the swollen CS hydrogel films (L d /W d ; L d : the length of the swollen hydrogel film, and W d : the width of the swollen hydrogel film) and various shapes ( Figure 3). According to our experimental results (Figure 3), for both diagonal rolling and short-side rolling, the length-to-width ratios (L d /W d ) of hydrogel films are close to 1. For the helix, the length-towidth ratios (L d /W d ) of hydrogel films are greater than 1. Moreover, the shape evolution of hydrogel films can be indicated via investigating the relationship between the L d /W d and the L 0 /W 0 . As shown in Figure 3(c) and Figure S3, it is clear that the L d /W d ratios (≥1) increase with further increases of L 0 /W 0 ratios, indicating that the planar hydrogel films transform into diagonal rolling firstly and then helix mainly. Specially for the hydrogel films with L 0 = 12 mm, the L d /W d ratios increase with increasing L 0 /W 0 ratios firstly and then decrease (Figure 3(c)). Correspondingly, the planar hydrogel film evolves from diagonal rolling, helix, to final short-side rolling. Further decreasing the W 0 (from 5 mm to 1 mm), the shape deformations show differences in the same range of L 0 /W 0 ratios from 1 to 12 (Movie S4 and Movie S5). As shown in Figure 3(d) and Figure S4, the L d /W d (≥1) increases with further increasing L 0 /W 0 , showing that the planar hydrogel films transform into diagonal rolling firstly, then short-side rolling (increasing probability) and helix mainly. It is worth noting that strong edge effects are particularly significant for the diagonal rolling and short-side rolling (L d /W d :~1, very small L 0 or/and W 0 ) [34]. Although we can obtain various shape deformations including diagonal rolling, helix, and short-side rolling via solely changing the lengthto-width ratios (Movie S1, Movie S6, and Movie S7), the shape-morphing rules still need to be expounded via studying the geometry effect systematically.

Controllable Shape Morphing via Changing the Thickness.
In order to enhance the shape-morphing editing capability, we further investigate the influences of hydrogel film thickness on shape-morphing behaviors, which is of equal importance for shape-morphing regulation [40,41]. As discussed previously, for CS hydrogel films with certain initial length and width (L 0 : 5 mm, W 0 : 5 mm), Young's modulus of both the top and bottom surfaces of the CS hydrogel film decreases with increasing thickness from 20 μm, 40 μm, 60 μm, to 80 μm. Conversely, the decreasing Young's modulus (decreasing cross-linking density) will increase the swelling ratio of the hydrogel film on the one hand, resulting in a larger swelling difference between the top and bottom surfaces of CS hydrogel films. On the other hand, the low Young's modulus of the hydrogel film indicates high deformability according to our previous work [26]. Therefore, increasing the thickness (h) of CS hydrogel films would increase the swelling difference (thus increasing Δl) between the top and bottom surfaces of CS hydrogel films, resulting in a large bending moment (M xy ) according to Equation (1) that determines the shape morphing of CS hydrogel films (Figure 4(a)). This effect is further experimentally confirmed by employing CS hydrogel films (certain initial length and width: L 0 : 9 mm, W 0 : 3 mm, L 0 /W 0 = 3 : 1) with various thicknesses from 10 μm to 80 μm as shown in Figure S5, which show no bending (10 μm thickness), long-side bending (20 μm thickness), helix (30 μm and 40 μm thicknesses), and short-side bending (50 μm to 80μm thickness) (Figure 4(b), Movie S8-S11). The shape evolution also agrees with the change of L d /W d as shown in Figure S5. To reveal the geometry effect (including both L 0 /W 0 and h) on the shape-morphing behaviors systematically, the effects of the both length-to-width ratio and thickness on the deformation behaviors of CS hydrogel films are described in a phase diagram (Figure 4(b) and Figure S6). Guided by the phase diagram, we can precisely program diverse deformations (i.e., no bending, diagonal rolling, long-side rolling, helix, and short-side rolling) of CS hydrogel films by tuning the thicknesses and/or the length-to-width ratios (Figures 4(c) and 4(d)). Moreover, this robust strategy is not only suitable for CS hydrogel films but also appropriate for sodium alginate (SA) hydrogel films. All shape-morphing behaviors (diagonal rolling, shortside rolling, helix, and long-side rolling) can be facilely achieved by tuning the gradient cross-linking density and geometries of SA hydrogel films ( Figure S7), which provides a universal and efficient strategy to precisely control the shape-morphing behaviors of various hydrogels.

Versatile Cooperative Shape Morphing and Actuation.
Combining both length-to-width-and thickness-determining effects on programmable shape morphing of CS hydrogel films, we further prepare some complex actuators from planar CS hydrogel films assisted by the art of "kirigami" (Figures 5(a)-5(d)). For cross CS hydrogel films with certain thicknesses (80 μm) but different length-to-width ratios, they quickly change their planar shapes into an artificial claw ( Figure 5(a), L 0 = 6 mm, W 0 = 1 mm) and an artificial windmill ( Figure 5(b), L 0 = 12 mm, W 0 = 5 mm) after immersing in water for 3 min. Furthermore, an artificial horn can also be obtained upon swelling of a U-shaped hydrogel film ( Figure 5(c)), where the feet and bridge of the U-shaped hydrogel film are deformed into helix and short-side rolling, respectively, due to the different length-to-width ratios. In addition, through controlling the thicknesses and the lengthto-width ratios of CS hydrogel films precisely, an artificial flower with various parts undergoing distinct deformation modes (i.e., long-side rolling, short-side rolling, and helix) can be facilely achieved ( Figure 5(d)).
Besides, owing to the existence of massive cations in CS, the hydrogel films also exhibit pH responsiveness [44]. The deformation of CS hydrogel films can therefore be adjusted by different pH values. Compared with shape morphing in deionized water (pH 7), the swelling degrees of CS hydrogel films are enhanced in an acid condition (pH 3) and reduced in an alkaline condition (pH 11) due to the protonation (increased swelling) and deprotonation (shrinking) of CS chains, respectively (Movie S12 and Movie S13). As shown in Figure S8(a), CS hydrogel films show only two types of shape morphing (diagonal rolling and helix) due to the sharply increased swelling of CS hydrogel films in an aqueous solution at pH 3. On the contrary, the shape morphing of CS hydrogel films changes little in an aqueous solution at pH 11 solution, because CS hydrogel films show slight shrinking ( Figure S8(b)). Based on this, we realize dynamic transformations of the underwater artificial claw ( Figure 5 angles of the pH-responsive CS hydrogel fingers. And the bending degrees of CS hydrogel films are quantitively described by the curve of bending angles with respect to pH values ( Figure 5(f)). These pH-responsive programmable deformation behaviors of CS hydrogel films make them promising candidates for soft actuators and drug delivery.

Discussion
In summary, we demonstrate an unprecedented strategy to edit the shape morphing of monocomponent natural poly-saccharide hydrogel films via integrating the gradient crosslinking density and geometry effect. Distinct from previous approaches that depend on diverse components or complex manufacturing techniques (photomask-assisted fabrication or 3D printing), the cooperative effect enriches our capabilities to edit the shape morphing of CS hydrogel films solely by tuning their geometries (length-to-width ratios or thicknesses). Moreover, various simple (helix, short-side rolling, and long-side rolling) and complex 3D (artificial claw, horn, and flower) deformations can be easily customized, of which shapes can be further altered by pH. It can be envisioned that this universal strategy can be extended for guiding the design of shape-morphing polymers (for example, hydrogels, liquid crystals, and shape memory polymers) in a simple manner, which also opens new avenues for employing shape-editing polymers for reconfigurable biodevices, soft robotics, and beyond [45][46][47][48][49][50].

Data Availability
The data that support the findings of this study are available from the paper and/or Supplementary Materials. Additional data related to this paper may be requested from the authors.

Supplementary Materials
Supplementary 1. Figure S1: the bending angles of CS hydrogel films (width: 3 mm, length: 21 mm, thickness: 80 μm) cross-linked with different concentrations of GA. The scale bars are 10 mm. Figure S2: the curve of the swelling ratio of CS hydrogel films with swelling time after removal of residual acetic acid by a 0.01 M NaHCO 3 solution. The size of the CS hydrogel film is 5 mm × 5 mm × 80 μm. Figure S3: the digital photos of the shape-morphing behaviors of CS hydrogel films (certain thickness: 80 μm) with different L 0 (36 mm, 24 mm, and 12 mm) in the same range of L 0 /W 0 ratios (from 1 to 12). L 0 and W 0 are the initial length and width of the dried CS hydrogel film, respectively. L d and W d are the length and width of the swollen hydrogel, respectively. The scale bar is 10 mm. Figure S4: the digital photos of the shape-morphing behaviors of CS hydrogel films (80 μm thickness) with different W 0 (1 mm, 2 mm, 3 mm, 4 mm, and 5 mm) in the same range of L 0 /W 0 ratios (from 1 to 12). L 0 and W 0 represent the initial length and width of the dried CS hydrogel film, respectively. L d and W d represent the length and width of the swollen hydrogel, respectively. The scale bars are 10 mm. Figure S5: the shape morphing of CS hydrogel films (certain length and width: L 0 = 9 mm, W 0 = 3 mm) changes as the thickness of CS hydrogels increases, resulting in swelling differences (as shown in the inserted pictures). The scale bars are 10 mm. Figure S6: the shape-morphing behaviors of various CS hydrogel films with different L 0 /W 0 ratios (from 1, 2, 3, 4, 5, 6, 8, 10, to 12) and thicknesses (from 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, to 80 μm) in the phase diagram. The scale bar is 10 mm. Figure Figure S8: the phase diagrams of the swollen CS hydrogel films with the same width (W 0 : 3 mm.), different L 0 /W 0 ratios (from 1, 2, 3, 4, 5, 6, 8, 10, to 12), and thicknesses (from 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, to 80 μm) after immersing in acid (pH 3) and alkaline solutions (pH 11). Supplementary 11. Movie S10: the helical deformation of CS hydrogel film (9 mm × 3 mm × 30 μm).