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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

Soft Tendril-Inspired Grippers: Shape Morphing of Programmable Polymer−Paper Bilayer Composites Wei Wang,†,‡,§ Chenzhe Li,†,‡,§ Maenghyo Cho,*,†,‡ and Sung-Hoon Ahn*,†,‡ †

Department of Mechanical and Aerospace Engineering and ‡Institute of Advanced Machines and Design, Seoul National University, Seoul 151-742, Republic of Korea

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S Supporting Information *

ABSTRACT: Nastic movements in plants that occur in response to environmental stimuli have inspired many man-made shape-morphing systems. Tendril is an exemplification serving as a parasitic grasping component for the climbing plants by transforming from a straight shape into a coiled configuration via the asymmetric contraction of internal stratiform plant tissues. Inspired by tendrils, this study using a three-dimensional (3D) printing approach developed a class of soft grippers with preprogrammed deformations being capable of imitating the general motions of plant tendrils, including bending, spiral, and helical distortions for grasping. These grippers initially in flat configurations were tailored from a polymer−paper bilayer composite sheet fabricated via 3D printing a polymer on the paper substrate with different patterns. The rough and porous paper surface provides a printed polymer that is well-adhered to the paper substrate which in turn serves as a passive strain-limiting layer. During printing, the melted polymer filament is stretched, enabling the internal strain to be stored in the printed polymer as memory, and then it can be thermally released, which will be concurrently resisted by the paper layer, resulting in various transformations based on the different printed geometries. These obtained transformations were then used for designing grippers to grasp objects with corresponding motions. Furthermore, a fully equipped robotic tendril with three segments was reproduced, where one segment was used for grasping the object and the other two segments were used for forming a tendril-like twistless spring-like structure. This study further helps in the development of soft robots using active polymer materials for engineered systems. KEYWORDS: biomimetics, tendril, shape shifting, 3D/4D printing, soft robotics, soft gripper memory alloy (SMA) actuators.17 Among these techniques, pneumatic actuators powered by compressed gas and motorbased cable-driven mechanisms being concomitant with the cumbersome appendages have limitations to making compact and lightweight autonomous robotic systems. Meanwhile, the main limitation of the SMA-based soft actuators is their inherent softness, resulting in a small actuation force with limited applications. SMA-based manipulators also need constant energy consumption to maintain a specific temperature of the SMA to keep the desired deformation.18 Moreover, it is impractical to fabricate these structures composed of different functional components in a single step resulting in the fabrication process always involves significant knowledge.19−21 On the other hand, the poor repeatability and low actuation force of the polymers including shape memory polymer (SMP) make them rarely be directly used for designing grasping devices. However, polymer-based manipulators with their characteristic properties provide the possibility to overcome the drawbacks of the mentioned actuation systems. To produce the general motions of plant tendrils using polymer-based structures, the fabricating and programming

1. INTRODUCTION Tendril is a clasping structure used by climbing plants to anchor and support their vining stems by coiling around suitable hosts, providing the plant maximum exposure to sunlight and numerous ecological niches. The general fundamental motion modes of a plant tendril are bending, spiral, and helical distortions, which occur via asymmetrical longitudinal contractions of an internal fiber ribbon of specialized cells between the ventral side and the dorsal side of the tendril.1 The alternative shape-morphing materials or mechanisms made up of multiple layers combined with activation components have been developed, showing the significant merits of various large continuous deformations, being lightweight and compact and being controllable with a wide range of stimuli. On the basis of these advantages, shapemorphing systems offer new solutions to the functionalityinduced devices in a broad range of engineering applications, including biotechnology,2−4 electronics,5 aerospace,6 robotics,7−9 and metamaterials.10 Recent advances in soft robotics exploit the materials’ flexibility and compliance to create highly adaptive robots for soft interactions with environments compared with their hardbodied counterparts.11,12 Soft tendril-like manipulators have been developed based on different actuation techniques such as pneumatic actuators,13−15 cable-driven systems,16 and shape © 2018 American Chemical Society

Received: November 28, 2017 Accepted: March 5, 2018 Published: March 5, 2018 10419

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

Research Article

ACS Applied Materials & Interfaces

Figure 1. Design and fabrication of polymer−paper based shape-morphing structures. (A) Images of the tendril of Cucumis with bending, spiral, and helical distortions. (B) As the filament is extruded during the printing process, it stretches and adheres to the paper substrate, and it exhibits the subsequent effects of self-contracting after being heated again. Insets show the schematic of the corresponding PE monomer structure. (C,D) Basic self-bending and self-twisting mechanisms with the direction of the printed filament being parallel to or making an angle with the length of the paper layer. (E) POM images of printed PE films taken at 0° and 45° between the polarizer angles, where the arrows indicate the printed direction. Scale bar is 50 μm. (F) Results of the DMA test of the printed PE film and paper sheet on their elastic modulus and linear strain.

in a single-step printing process, where the printed polymer serves as the stimuli-triggered active layer and the paper serves as the passive strain-limiting layer. The polymer structure was fabricated such that each printed layer was printed in an identical pattern and that all the layers were stacked in both the same pattern and the same orientation. The polymer−paper composite with fundamental bending or twisting deformation is achieved, in which the contracting direction of the polymer layer is parallel to or makes an angle with the length of the paper layer. The basic deformable mechanisms were then used for designing tendril-inspired grasping devices with bending, spiral, and helical deformations. Furthermore, a fully equipped robotic tendril with three segments was reproduced, where one segment was used for grasping the object and the other two segments were used for forming a tendril-like twistless springlike structure.

process of these structures in previous studies always includes a series of specific steps which always involve special devices and tedious manual operations.22−26 Meanwhile, polymer-activated shape-morphing structures fabricated through three-dimensional (3D) printing named as four-dimensional (4D) printing have been investigated with the dominating advantages of being capable of synchronously fabricating and programming polymer structures.27−29 Direct printing technology could be easily adapted for producing the fundamental modes of tendril deformation such as bending and twisting.10,27,30 Meanwhile, polymer materials that are able to respond to a wide range of stimuli such as heat,9 water,31 light,32−34 and magnetic field35 can be used for designing untethered polymer-activated shapemorphing mechanisms with contactless remote control. Additionally, these shape-morphing structures have the excellent shape-retaining capability that they could maintain their deformed shapes in the high stiffness when the stimulus was removed.36,37 On the basis of the advantages of 3D printing and polymer-activated shape-morphing structures, this research aims to develop a more versatile and economical approach to fabricate tendril-like grasping mechanisms utilizing 3D printing as a single manufacturing step that requires only a 3D printer and inexpensive off-the-shelf materials. This study described a class of 3D printing-based soft grippers with preprogrammed grasping deformations that are capable of imitating the general motions of plant tendrils, including bending, spiral, and helical distortions (Figure 1A). All the structures of the grippers were initially in flat configurations tailored from a thin composite sheet that was fabricated through 3D printing polymer materials on the paper substrate with different patterns. The polymer was printed on the paper substrate to form a bilayer polymer−paper composite

2. RESULTS AND DISCUSSION 2.1. Materials and Fabrication. All the specimens are fabricated by using a fused deposition modeling-based 3D printer. Before printing a polymer material, a sheet of paper is first placed flat and fixed on the platform of the printer. During printing, the melted polymer filament is stretched, enabling the internal strain to be stored in the printed polymer as memory after cooling, and then it can be thermally released when heated above its Tg, resulting in a large contracting strain in the filament length (Figure 1B). The insets of Figure 1B show the schematic of the corresponding changes in the polyester (PE) monomer structure during these processes. The polymer is first printed on the paper sheet, and then after printing, it is tailored to different shapes as specimens with desired deformations. The specimens are designed with a uniform printed polymer pattern 10420

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

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Figure 2. Specimens with bending deformation and application to the cruciform gripper. (A) Comparison of experimental and modeling results of maximum bending curvature for bending specimen with different thicknesses of the printed PE layer on the paper substrate. Upper-right insets show the SEM image of the paper surface and the interface between the printed PE and the paper substrate; scale bars are 30 μm. Middle and bottom insets show the schematic and images of the activated bending specimen with different printed polymer thicknesses. (B) Configuration of the cruciform gripper composed of four pure bending specimens. The inset shows that the direction of the printed filament is parallel to the length of the paper layer. (C) Gripper is hung using a rope grasping a ping-pong ball. (D) Pull force of the gripper from caging the ball to its separation is shown in the bottom inset. The top-right inset shows the schematic of the experimental setup. All dimensions are in millimeters and unspecified scale bars are 10 mm.

when the printed direction was perpendicular to the analyzer and a bright image was obtained when the sample was rotated by 45° relative to the polarizers. Actually, the shrinking percentage of the printed PE structure was determined by the printing parameters such as nozzle temperature, layer thickness, printing speed, and activation temperature (Figure S1), and in this work, such printing parameters were experimentally determined as 240 °C, 0.1 mm, 40 mm/s, and 90 °C, respectively. The optimization of the printing and activating parameters is not considered in this work. The elastic modulus of the printed PE varies with the change in temperature (Figure S2), and the activated temperature was experimentally chosen as 90 °C (above its Tg) to maintain a suitable modulus of the printed PE; otherwise, with a higher temperature, the modulus of PE will be too low to achieve a better deformation. The activated time was determined to be 15 min by placing the specimens in an oven at 90 °C until no further movement could be observed. Throughout this work, all the specimens and structures without specific indication are activated under these conditions. Dynamic mechanical analysis (DMA) tests show that the loss of the elastic modulus of the printed PE and the paper sheet when heated at 90 °C for 15 min are around 99.3% (1447.5 and 10.1 MPa) and 0.5% (2236.5 and 2225.4 MPa), and their corresponding contracted strains are around 10.7 and 0.1%, respectively (Figure 1F). Results show that the thermal effects on the properties of paper are relatively small and negligible. This is because the copy paper is composed of intertwined cellulose fibers and their constituent lignin ensures the thermal stability of the paper structure.38 It is also noticed that the strain of the PE structure increases in the first 4 min because of the heat expansion of the polymer material and then decreases rapidly when the temperature of the structure achieves its Tg. A mechanical model was developed based on the measured properties of the material to predict the maximum deformation of the specimens (see the Supporting Information). 2.2. Proof of Concept: Bending Deformation. Experiments were conducted to evaluate the effect of different

on the entire paper substrate to obtain a uniform deformation. For this purpose, all the extruded filaments are aligned in the same direction layer-by-layer during the fabrication process. When the specimen is heated above the Tg of the polymer, the printed polymer will change from a high stiffness state to a low stiffness state. Then, the printed polymer with low stiffness placed eccentrically from the neutral plane of the paper sheet will start to shrink along the filament orientation where the shrinking amount of the printed polymer is much larger than that of the paper layer, that is, the deformation of the specimen occurs because of the resistance of the paper layer against the contracting deformation of the polymer layer. The maximum deformation of the specimens can be achieved once the force provided by the PE layer and the resistance of the paper layer reach an equilibrium state. If the contracting direction of the polymer layer is parallel to or makes an angle with the length of the paper layer, a self-bending or self-twisting specimen will be obtained (Figure 1C,D). In addition, all the specimens and designs presented in this work were designed for achieving the permanent shape deformation. Both self-bending and selftwisting mechanisms were then used as basic mechanisms in the following shape-morphing designs of the tendril-inspired structures. In this work, the copy paper (thickness is ∼0.1 mm) is used as the paper substrate because of its stable elastic modulus and negligible volume change for a large range of temperature changes. PE, an SMP with a melting temperature >200 °C, is chosen as the 3D printing material, but a number of other polymers can also be used. PE was selected due to its significant thermally activated contracting strain of the printed PE filament once heated above its Tg since the π−π interaction between the polymer chains tends to maintain high polymer crystallinity and alignment (Figure 1B). The images of the printed PE film are obtained at 0° and 45° between the cross-polarizer angles by using polarized optical microscopy (POM), where the arrows indicate the printed direction (Figure 1E). The image shows a homogeneous alignment of the stretched PE molecular chains along the printed direction, where a dark image was obtained 10421

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

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Figure 3. Specimens with spiral deformation and application to the tentacle-type gripper. (A) Schematic of the strips with spiral deformation and its dimensions are defined as t1, t2, Ls. (B) Image of one fabricated specimen with the dimensions of 1, 3, 75. Experimental results (C) and modeling results (D) of the activated configurations of the specimens with different geometric parameters. (E) Gripper hung using a rope grasping a cylindrical object. (F) Pull force of the gripper from caging the cylindrical object to its separation, as shown in the bottom inset. The top-right inset shows the schematic of the experimental setup. All dimensions are in millimeters, and all scale bars are 10 mm.

first stage of curve AB, the maximum bending curvature increases with an increase in the thickness of the printed PE layer. This is because at this stage, a thicker PE layer will provide a larger contraction force to produce a larger bending deformation. In the second stage of curve BC, with an increase in the thickness of the PE layer, it can be seen that the bending curvature gradually decreases because of the increased distance between the neutral surfaces of the PE layer and the paper sheet (see the Supporting Information for modeling). To demonstrate the above-mentioned bending specimen with immediate potential for use, a gripper with a cruciform shape is constructed, in which each branch is a self-bending specimen with the dimensions of 50 mm in length, 4 mm in width, and 1 mm in thickness of the printed PE layer (Figure 2B). When activated, the actuated branches of the gripper curled about the axes perpendicular to the long axes of the branches. The gripper was used to grip a ping-pong ball in the same oven at an initial interior temperature of 90 °C. The grasping process is described in Figure 2C and Video S1 where the caging time of grasping is around 600 s, and then the oven is powered off; the gripper’s deformed shape will be fixed once the interior temperature reduced to around 70 °C. The activated gripper is installed on a universal testing machine to measure the force generated by the gripper on the ping-pong ball fixed on the ground during an upward motion (Figure 2D,

thicknesses of the printed PE on the maximum bending deformation of the specimen. To do so, the dimensions of all the specimens were fixed as 15 mm in length and 2 mm in width, and three samples were fabricated for each configuration, with the thickness of the printed PE ranging from 0.3 to 2.0 mm in increments of 0.1 mm and the thickness of 2.5 and 3.0 mm. Scanning electron microscopy (SEM) images of both the copy-paper surface and the cross section of the specimen are obtained to verify the bonding conditions between the PE and the paper substrate (Figure 2A, top right). It can be discovered that the paper’s rough and porous surface provides a printed polymer that is well-adhered to the paper substrate with multigroove structures. The schematic of bending deformation of the specimen is described in Figure 2A (middle) and Figure S3. All the activated specimens are placed on a flat ground, filmed using a digital camera, and then their final bending deformations are measured visually from the images (Figure 2A, bottom). Both the experimental and modeling results of the maximum bending curvature of the bending specimens with different thicknesses of the printed PE are illustrated in Figure 2A. According to the results, it can be seen that the proposed model is able to predict well for the maximum bending curvature where there is also an optimal point B for the thickness of the printed PE layer approximately between 0.8 and 1.0 mm to achieve the maximum bending curvature. In the 10422

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

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Figure 4. Specimens with twisting deformation and application to the twining-type gripper. (A) Effect of filament orientation on the pitch and diameter of a turn of the obtained helical structures shown in the insets. (B) Process of the hung twining-type gripper grasping a cylindrical object. (C) Pull force of the gripper from twining the cylindrical object to its separation, as shown in the bottom inset. The top-left inset shows the schematic of the experimental setup. All scale bars are 20 mm.

To test the loading capacity of the tentacle-type gripper, the activated gripper with spiral deformation is installed on the same universal testing machine to measure the force generated by the gripper on the cylindrical object fixed on the ground during an upward motion (Figure 3F, top right). The gripper is moved upward using the universal testing machine, and the vertical pulling force generated by the gripper is measured (Figure 3F). From the curve, it can be seen that the pulling force rapidly increases first and then gradually decreases because of the destructive plastic deformation of the gripper (Figure 3F, bottom). It is notable that the maximum grasping weight of the tentacle-type gripper is around 8.1 N, that is, ∼1557.7 times its own weight of 5.2 mN. 2.4. Helical Deformation. To generate helical deformation in a single structure, the printed PE layer serves as the anisotropic layer where all the PE filaments are printed with a determined angle in parallel to the length of the paper substrate. The effect of the PE filament orientation on the twisting capability of the specimen was analyzed by keeping the thickness of the PE layer constant at 1 mm and by varying the PE filament orientation. Three samples were fabricated for each configuration with the same overall dimensions of 75 mm in length, 10 mm in width, and 1.1 mm in thickness but with the filament orientation ranging from 15° to 75° in an increment of 15°. All the samples were activated under the same conditions as described for the bending specimens, and then the pitch and the diameter of a turn (paper layer) of the obtained helical structures were measured. The measured and modeling results are described (Figures 4A, S5, and see the Supporting Information for modeling). The insets (Figure 4A) show the images of the obtained helical structures with different filament orientations. The results show that a smaller angle (θ in Figure 1D) between the directions of the printed filament and the specimen length led to a smaller pitch, resulting in a more compact configuration. However, regardless of the filament orientation, the diameter of a turn of the twisted specimen is nearly constant because the diameter is mainly determined by the thickness of the printed PE layer. The specimen with the filament orientation of 45° is adapted as a twining-type gripper utilizing twisting deformation to grasp a cylindrical object with a diameter of 10 mm. The experiment

top right). Then, the gripper is moved upward using the universal testing machine, and the vertical pulling force generated by the gripper is measured (Figure 2D). From the curve, it can be seen that the pulling force rapidly increases first and then gradually decreases, depending on the positions of the contacting point between the shape-retained stiff branches of the gripper and the spherical surface (Figure 2D, bottom). It is notable that the maximum grasping weight of this gripper is around 0.7 N, that is, ∼73.7 times its own weight of 9.5 mN. 2.3. Spiral Deformation. The results obtained from the maximum bending curvature (Figure 2A) were utilized to arrive at the design of the specimen with spiral deformation. It was found that the thickness of the printed PE layer significantly affects the final bending deformation and therefore could be used to adjust the activated behavior of the self-bending structures. That is, a constant cross-sectional strip can obtain homogeneous bending deformation, and then a variable crosssectional strip with a gradually varied thickness of the PE layer can generate tendril-like spiral deformation. In this study, a specimen with variable cross section with the dimensions of t1, t2, and Ls is designed and fabricated, where the PE layer with a linearly varied thickness ranging from t1 to t2 on a total length of Ls is printed on the paper substrate, while maintaining a constant width of 2 mm (Figures 3A and S4). The printing direction of PE is parallel to the length of the paper substrate, and Figure 3B shows the image of one fabricated specimen. To avoid deformation interference and to ensure the endpoint of the strip that PE with the thinnest thickness has the maximum bending curvature, t1 was determined as 1 mm. Nine specimens were fabricated and tested with varying design parameters such that t2 is 2, 3, and 4 mm and Ls is 50, 75, and 100 mm, respectively. The experimental and modeling configurations of all the activated specimens are described (Figure 3C,D, and see the Supporting Information for modeling). From the results, it can be seen that the specimen with a smaller t2 or a larger Ls can generate a larger spiral angle subtended by the full length. The specimen with the dimensions of 1, 3, and 75 is adapted as a tentacle-type gripper using spiral deformation to grasp a cylindrical object with a diameter of 10 mm. The experiment was conducted in the same oven under the same conditions, and the grasping process is recorded (Figure 3E and Video S2). 10423

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

Research Article

ACS Applied Materials & Interfaces

Differently, the pulling force of the gripper with helical deformation (thickness of 1 mm) rises rapidly and then maintains a steady increase until the gripper separates from the object. The main reason for the difference is the different thickness of the gripper because a thinner thickness of the structure can withstand a larger elastic deformation. Besides, the gripper with helical deformation has a large contact area with the object, leading to a more stable grasping in comparison with the one with spiral deformation. To sum up, grasping with helical deformation is an optimal approach which is also used by climbing plants for support and attachment, indicating that the climbing plants are designed to put less resource into its structure and more into active growth. 2.6. Biomimetic Robotic Tendril. The plant tendril seems like a preprogrammed shape-morphing structure with sequential shape transformation, changing from a straight state into a helically coiled shape. During climbing, an initially straight planet tendril first finds a suitable object and twists around it for grasping and anchoring. Once tethered, the planet tendril will contract by internal coiling around the perversion point with an equal number of oppositely handed coils on each side, resulting in a spring-like structure (Figure 6A), with the purpose of lifting the plant body and providing increased flexibility in response to the external disturbances.39 To imitate the functional deformation of the plant tendril, a ribbon-shaped specimen with three segments is reproduced (Figure 6B), where the printed PE filament orientations of the three segments concerning the specimen length are 30° for segment 1, −30° for segment 2, and 15° for segment 3, respectively. The overall structure of the specimen could be divided into two main parts: the first, segment 1 and segment 2, with symmetric design devoted to forming internal coiling and pulling phase and the second, segment 3, was in charge of coiling to create a firm grasp. Meanwhile, the PE with a width of 1 mm was also printed along each side of the ribbon-shaped specimen to enhance the stiffness of the whole structure. The three segments have equal length, and the adjacent segments are connected using the printed PE being parallel to the specimen length. The overall dimensions of the ribbon-shaped specimen are 240 mm in length and 12 mm in width. To mimic the sequential coiling motion of the plant tendril and its grasping behavior, the specimen was activated using a heating gun by locally heating the individual segment to accomplish the sequential shape transformations of the whole structure. The working temperature of the heating gun was set as 150 °C, which is much lower than the 3D printing temperature of PE (∼240 °C) to avoid the heat-induced damages of the specimen. Before activation, a cylindrical object with a diameter of 10 mm was placed close to the tip of the specimen (segment 3), and then segment 3 was first activated for twisting around the object and its deformed shape was retained through subsequent cooling to accomplish firm attaching and grasping. Afterward, segment 2 and segment 1 were successively activated to form a twistless spring-like structure. The whole sequential coiling motion of the specimen is described in Figure 6C and Video S4. Moreover, the object is then lifted up by the coiled structure (Figure 6D), and the insets in the figure show the coiling directions of each segment. It is noticed that the spring-like structure has an equal number of coils of opposite handedness on each side of the perversion point, based on the symmetric design of segment 1 and segment 2.

was conducted in the same oven under the same conditions, and the grasping process is recorded (Figure 4B and Video S3). To test the loading capacity of the twining-type gripper, the activated gripper is installed on the same universal testing machine to measure the force generated by the gripper on the cylindrical object fixed on the ground during an upward motion (Figure 4C, top left). Then, the gripper is moved upward using the universal testing machine, and the vertical pulling force generated by the gripper is measured (Figure 4C). From the results, it can be seen that the pulling force rapidly increases first and then gradually increases because of the gradually deformed shape of the gripper, resulting in a decreased contact area (Figure 4C, bottom). It is notable that the maximum grasping weight of the twining-type gripper is around 8.2 N, that is, ∼1518.5 times its own weight of 5.4 mN. 2.5. Performance Comparison. Three different types of the tendril-inspired gripper have been described to be capable of generating preprogrammed bending, spiral, and helical deformations. These grippers were fabricated in the same way utilizing 3D printing as a single manufacturing step that requires only a 3D printer and inexpensive off-the-shelf materials. Then, this study also aims to take the first step to using such kind of bilayers fabricated via a rapid prototyping method in mechanical engineering to understand the biological tendril structures. The pulling force (i.e., the grasping force) of each type of gripper from caging the object up to its separation has been evaluated, and the results are summarized in Figure 5.

Figure 5. Pulling force of three different types of grippers from caging the object to its separation. The insets show the separation process of the grippers with bending deformation (B1−B4), spiral deformation (S1−S4), and helical deformation (H1−H4), respectively. All scale bars are 20 mm.

The self-weights of these three grippers with bending, spiral, and helical deformations are 0.97, 0.53, and 0.55 g, and the maximum grasping forces generated from them are 0.69, 8.1, and 8.2 N, respectively. Then the force density (i.e., the grasping force generated per unit mass) of each tested gripper is approximately calculated as 0.7, 15.3, and 14.9 N/g. On the basis of these results, it can be seen that grasping an object with spiral and helical deformations is a more efficient approach when compared with the one with bending deformation. From the tested results, it can also be seen that the grippers with both spiral and helical deformations after the pulling test generate different degrees of plastic deformation (S2−S4, H4 in Figure 5). It is also noticed that the pulling force of the gripper with spiral deformation (average thickness of 2 mm) after reaching the maximum force reduced rapidly because the large plastic deformation results in the failure of the gripper structure. 10424

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

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Figure 6. Design and performance evaluation of biomimetic robotic tendrils. (A) Time-lapsed images of the nastic movements of Cucumis tendril while encountering a support. (B) Schematic of the specimen with three segments, with the filament orientations of 30°, −30°, and 15°, respectively. (C) Sequential deformation of each segment activated by a heating gun. Segment 3 is first activated for twining the object, and then segment 2 and segment 1 are successively activated to form a twistless spring-like structure. (D) Coiled robotic tendril lifts up the object where the insets show the coiling directions of each segment. (E) Pull force of the spring-like structure for the first cycle and the one-hundredth cycle. The inset shows the schematic of the experimental setup. (F) Results of repeatability performance of the spring-like structure where the maximum forces and the minimum forces of each cycle are indicated by a red solid line and a red dashed line, respectively. All scale bars are 30 mm.

spring-like structure with the gradually accumulated plastic strain of the printed PE layer during the repeated measurements. The lengths of the spring-like structure measured before and after the repeatability test are 153.8 and 157.6 mm corresponding to an overall length elongation of around 2.5%.

To evaluate the mechanical performance of the formed twistless spring-like structure, one end of the structure is tethered to the object fixed on the ground and the other end of the structure is installed on the same universal testing machine to measure the force generated from the spring-like structure during the tensile and contracting process corresponding to the up-and-down motion of the load cell of the testing machine (Figure 6E, right). The load cell is moved upward by 20 mm (∼13% of the length of the coiled spring-like structure) and then move down to the original place, and the result of the vertical pulling force generated by the spring-like structure on the timescale is measured (Figure 6E). From the curve drawn in a solid line, it can be seen that the pulling force increases around linearly concerning the pulling distance. To evaluate the spring-like structure’s capability of resisting the external disturbances, its performance of repeatability was also tested 100 times in a row with the same up-and-down pulling distance (Figure 6F). It can be seen that both the maximum force and the minimum force in one cycle gradually decrease under repeated tests. The test results of the first cycle and the onehundredth cycle are compared (Figure 6E). From the results, it can be seen that the maximum and minimum forces of the first cycle and the one-hundredth cycle are 8.24 and 0 and 6.51 and −2.48 N, respectively, which correspond to the values of the stiffness of the spring-like structure, which are 0.41 and 0.45 N/ mm, respectively. It can be seen that the repeated measurements do not appear to have an evident impact on the stiffness change of the spring-like structure and the resultant force (maximum force minus minimum force), where its average change in one cycle is 0.75 × 10−2 N. However, the repeated measurements decrease the maximum force and the minimum force synchronously because of the elongation of the coiled

3. CONCLUSIONS This work has demonstrated the design and fabrication of plant tendril-inspired grippers being capable of grasping objects with preprogrammed deformations, including bending, spiral, and helical distortions. The shape-morphing specimens were tailored from the flat polymer−paper composite fabricated by 3D printing a polymer material on the paper substrate. This work successively introduced the design of the specimens with specific preprogrammed deformations. Meanwhile, different grippers based on these different types of specimens were then developed capable of grasping weights varying from dozens of times to more than one thousand times of their weight. This work also reproduced a fully equipped biomimetic robotic tendril with three segments to imitate the sequential coiling motion of the plant tendril. The robotic tendril started with the straight configuration, using one segment to twist around the object for attaching and grasping, and then, using the other two segments to form a twistless spring-like structure with an equal number of oppositely handed coils relative to the perversion point. These polymer−paper composite-based soft grippers and tendrils have shown their significant advantages of being simple and compact in configuration, being controllable through contactless remote control, being capable of retaining deformed shapes in high stiffness, and being easily fabricated in low cost, which only requires a 3D printer and inexpensive off-the-shelf materials. 10425

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ACS Applied Materials & Interfaces

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The choice of using paper as the substrate in this study was motivated mainly by the fact that paper is ubiquitous, inexpensive, and lightweight. Moreover, on the basis of the different thermal deformation coefficients, other materials or the same material with designed anisotropic properties could also be used for designing layered soft-morphing structures.27,40 Also, on the basis of other possible materials responding to different stimuli, such as water, light, and magnetic field, different types of activation can be used for deforming the structure. The active material employed in this study is PE instead of the polylactic acid filaments used in several previous studies.10 This change of the filament materials is critical to this study because the π−π interaction between the polymer chains tends to maintain high polymer crystallinity and alignment, resulting in a directly 3D printed polymer structure that can generate more than 10% contracting strain upon activation and similar elastic moduli compared to the real plants,41 which is critical to achieving the shape-morphing structure of plant tendrils. This study provides a novel method with short experiential cycle and high repeatability for understanding the plant structure, which in return could be beneficial for the future design of bioinspired materials and structures. The actuation speed of the plant tendril-inspired structures demonstrated in this work is slower when compared with the reported ones made of hydrogels or liquid crystal elastomers (LCEs). This is because the designs in this work have an average thickness of 1 mm, which is much thicker than that of the hydrogels or LCE-based structures (≲0.1 mm),2,22−24,42 leading to a longer time for absorbing heat energy. Second, the applied actuation temperature is 90 °C, which is slightly higher than the Tg of PE, resulting in a slow convective heat transfer. These two reasons result in the necessity of a long time to generate motions. However, a specimen in this study with thinner thickness and applied higher actuation temperature can achieve a much faster motion. In addition, although the specimens and their corresponding grippers presented in this work were designed for achieving the permanent shape deformation, composite polymers based on multiple responsive materials could be used to accomplish reversible shape transformations.27 The proposed method in this study provides not only a new direction for the fabrication of the polymeractivated soft robots but also new solutions for the development of printable robotic systems. Besides, the design of the proposed work is illustrated in a centimeter-scale; however, by exploiting the scale-free geometry of the specimens, the approach in this work can be extended to the fabrication of transformable materials and structures at a very small scale.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.C.). *E-mail: [email protected] (S.-H.A.). ORCID

Wei Wang: 0000-0003-0416-390X Maenghyo Cho: 0000-0003-3942-9261 Sung-Hoon Ahn: 0000-0002-1548-2394 Author Contributions §

W.W. and C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Industrial Strategic Technology Development Program (10049258) funded by the Ministry of Knowledge Economy (MKE), Korea, and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF2015R1A2A1A13027910), (MSIP) (NRF2016R1A5A1938472), and (MSIP) (2012R1A3A2048841). The authors would like to thank the reviewers for their insightful feedback.

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REFERENCES

(1) Gerbode, S. J.; Puzey, J. R.; McCormick, A. G.; Mahadevan, L. How the Cucumber Tendril Coils and Overwinds. Science 2012, 337, 1087−1091. (2) Breger, J. C.; Yoon, C.; Xiao, R.; Kwag, H. R.; Wang, M. O.; Fisher, J. P.; Nguyen, T. D.; Gracias, D. H. Self-Folding ThermoMagnetically Responsive Soft Microgrippers. ACS Appl. Mater. Interfaces 2015, 7, 3398−3405. (3) Randall, C. L.; Gultepe, E.; Gracias, D. H. Self-Folding Devices and Materials for Biomedical Applications. Trends Biotechnol. 2012, 30, 138−146. (4) Ghosh, A.; Yoon, C.; Ongaro, F.; Scheggi, S.; Selaru, F. M.; Misra, S.; Gracias, D. H. Stimuli-Responsive Soft Untethered Grippers for Drug Delivery and Robotic Surgery. Front. Mech. Eng. 2017, 3, 7. (5) Sundaram, S.; Kim, D. S.; Baldo, M. A.; Hayward, R. C.; Matusik, W. 3D-Printed Self-Folding Electronics. ACS Appl. Mater. Interfaces 2017, 9, 32290−32298. (6) Wang, W.; Rodrigue, H.; Ahn, S.-H. Deployable soft composite structures. Sci. Rep. 2016, 6, 20869. (7) Huang, H.-W.; Sakar, M. S.; Petruska, A. J.; Pané, S.; Nelson, B. J. Soft Micromachines with Programmable Motility and Morphology. Nat. Commun. 2016, 7, 12263. (8) Hawkes, E. W.; Blumenschein, L. H.; Greer, J. D.; Okamura, A. M. A Soft Robot that Navigates Its Environment Through Growth. Sci. Robot. 2017, 2, eaan3028. (9) Felton, S.; Tolley, M.; Demaine, E.; Rus, D.; Wood, R. A Method for Building Self-Folding Machines. Science 2014, 345, 644−646. (10) Zhang, Q.; Zhang, K.; Hu, G. Smart Three-Dimensional Lightweight Structure Triggered from a Thin Composite Sheet via 3D Printing Technique. Sci. Rep. 2016, 6, 22431. (11) Hughes, J.; Culha, U.; Giardina, F.; Guenther, F.; Rosendo, A.; Iida, F. Soft Manipulators and Grippers: A Review. Front. Robot. AI 2016, 3, 69. (12) Laschi, C.; Mazzolai, B.; Cianchetti, M. Soft Robotics: Technologies and Systems Pushing the Boundaries of Robot Abilities. Sci. Robot. 2016, 1, eaah3690. (13) Paek, J.; Cho, I.; Kim, J. Microrobotic Tentacles with Spiral Bending Capability based on Shape-Engineered Elastomeric Microtubes. Sci. Rep. 2015, 5, 10768. (14) Martinez, R. V.; Branch, J. L.; Fish, C. R.; Jin, L.; Shepherd, R. F.; Nunes, R. M. D.; Suo, Z.; Whitesides, G. M. Robotic Tentacles with

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18079. Characterization of the described materials, methods, additional data, figures, and movies (PDF) Tendril-inspired gripper with bending deformation (AVI) Tendril-inspired gripper with spiral deformation (AVI) Tendril-inspired gripper with helical deformation (AVI) A fully equipped biomimetic robotic tendril (AVI) 10426

DOI: 10.1021/acsami.7b18079 ACS Appl. Mater. Interfaces 2018, 10, 10419−10427

Research Article

ACS Applied Materials & Interfaces Three-Dimensional Mobility based on Flexible Elastomers. Adv. Mater. 2013, 25, 205−212. (15) Sun, Y.; Yap, H. K.; Liang, X.; Guo, J.; Qi, P.; Ang, M. H., Jr.; Yeow, C.-H. Stiffness Customization and Patterning for Property Modulation of Silicone-based Soft Pneumatic Actuators. Soft Robot. 2017, 4, 251−260. (16) Calisti, M.; Giorelli, M.; Levy, G.; Mazzolai, B.; Hochner, B.; Laschi, C.; Dario, P. An Octopus-Bioinspired Solution to Movement and Manipulation for Soft Robots. Bioinspiration Biomimetics 2011, 6, 036002. (17) Cianchetti, M.; Calisti, M.; Margheri, L.; Kuba, M.; Laschi, C. Bioinspired Locomotion and Grasping in Water: the Soft Eight-Arm Octopus Robot. Bioinspiration Biomimetics 2015, 10, 035003. (18) Wang, W.; Rodrigue, H.; Kim, H.-I.; Han, M.-W.; Ahn, S.-H. Soft Composite Hinge Actuator and Application to Compliant Robotic Gripper. Composites, Part B 2016, 98, 397−405. (19) Morin, S. A.; Kwok, S. W.; Lessing, J.; Ting, J.; Shepherd, R. F.; Stokes, A. A.; Whitesides, G. M. Elastomeric Tiles for the Fabrication of Inflatable Structures. Adv. Funct. Mater. 2014, 24, 5541−5549. (20) Wang, W.; Kim, N.-G.; Rodrigue, H.; Ahn, S.-H. Modular Assembly of Soft Deployable Structures and Robots. Mater. Horiz. 2017, 4, 367−376. (21) Han, M.-W.; Ahn, S.-H. Blooming Knit Flowers: Loop-Linked Soft Morphing Structures for Soft Robotics. Adv. Mater. 2017, 29, 1606580. (22) Wang, M.; Lin, B.-P.; Yang, H. A Plant Tendril Mimic Soft Actuator with Phototunable Bending and Chiral Twisting Motion Modes. Nat. Commun. 2016, 7, 13981. (23) Iamsaard, S.; Aßhoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. J. L. M.; Fletcher, S. P.; Katsonis, N. Conversion of Light into Macroscopic Helical Motion. Nat. Chem. 2014, 6, 229−235. (24) de Haan, L. T.; Verjans, J. M. N.; Broer, D. J.; Bastiaansen, C. W. M.; Schenning, A. P. H. J. Humidity-Responsive Liquid Crystalline Polymer Actuators with An Asymmetry in the Molecular Trigger that Bend, Fold, and Curl. J. Am. Chem. Soc. 2014, 136, 10585−10588. (25) Janbaz, S.; Hedayati, R.; Zadpoor, A. A. Programming the Shape-Shifting of Flat Soft Matter: from Self-Rolling/Self-Twisting Materials to Self-Folding Origami. Mater. Horiz. 2016, 3, 536−547. (26) de Espinosa, L. M.; Meesorn, W.; Moatsou, D.; Weder, C. Bioinspired Polymer Systems with Stimuli-Responsive Mechanical Properties. Chem. Rev. 2017, 117, 12851−12892. (27) Ding, Z.; Yuan, C.; Peng, X.; Wang, T.; Qi, H. J.; Dunn, M. L. Direct 4D Printing via Active Composite Materials. Sci. Adv. 2017, 3, e1602890. (28) van Manen, T.; Janbaz, S.; Zadpoor, A. A. Programming 2D/3D Shape-Shifting with Hobbyist 3D Printers. Mater. Horiz. 2017, 4, 1064−1069. (29) Khare, V.; Sonkaria, S.; Lee, G.-Y.; Ahn, S.-H.; Chu, W.-S. From 3D to 4D PrintingDesign, Material and Fabrication for MultiFunctional Multi-Materials. Int. J. Precis. Eng. Manuf.-Green Tech. 2017, 4, 291−299. (30) Zhang, Q.; Yan, D.; Zhang, K.; Hu, G. Pattern Transformation of Heat-Shrinkable Polymer by Three-Dimensional (3D) Printing Technique. Sci. Rep. 2015, 5, 8936. (31) Fang, Z.; Kuang, Y.; Zhou, P.; Ming, S.; Zhu, P.; Liu, Y.; Ning, H.; Chen, G. Programmable Shape Recovery Process of WaterResponsive Shape-Memory Poly (vinyl alcohol) by Wettability Contrast Strategy. ACS Appl. Mater. Interfaces 2017, 9, 5495−5502. (32) Wang, L.; Liu, Y.; Cheng, Y.; Cui, X.; Lian, H.; Liang, Y.; Chen, F.; Wang, H.; Guo, W.; Li, H.; Zhu, M.; Ihara, H. A Bioinspired Swimming and Walking Hydrogel Driven by Light-Controlled Local Density. Adv. Sci. 2015, 2, 1500084. (33) Li, C.; Moon, J.; Yun, J.-H.; Kim, H.; Cho, M. Influence of External Loads on Structure and Photoactuation in Densely Crosslinked Azo-Incorporated Liquid Crystalline Polymers. Polymer 2017, 129, 252−260. (34) Li, C.; Yun, J.-H.; Kim, H.; Cho, M. Light Propagation and Photoactuation in Densely Cross-linked Azobenzene-Functionalized

Liquid-Crystalline Polymers: Contribution of Host and Concerted Isomerism. Macromolecules 2016, 49, 6012−6020. (35) Schmauch, M. M.; Mishra, S. R.; Evans, B. A.; Velev, O. D.; Tracy, J. B. Chained Iron Microparticles for Directionally Controlled Actuation of Soft Robots. ACS Appl. Mater. Interfaces 2017, 9, 11895− 11901. (36) Hubbard, A. M.; Mailen, R. W.; Zikry, M. A.; Dickey, M. D.; Genzer, J. Controllable curvature from planar polymer sheets in response to light. Soft Matter 2017, 13, 2299−2308. (37) Wang, W.; Ahn, S.-H. Shape Memory Alloy-based Soft Gripper with Variable Stiffness for Compliant and Effective Grasping. Soft Robot. 2017, 4, 379−389. (38) Hamedi, M. M.; Campbell, V. E.; Rothemund, P.; Güder, F.; Christodouleas, D. C.; Bloch, J.-F.; Whitesides, G. M. Electrically Activated Paper Actuators. Adv. Funct. Mater. 2016, 26, 2446−2453. (39) Smyth, D. R. Helical Growth in Plant Organs: Mechanisms and Significance. Development 2016, 143, 3272−3282. (40) Gladman, A. S.; Matsumoto, E. A.; Nuzzo, R. G.; Mahadevan, L.; Lewis, J. A. Biomimetic 4D Printing. Nat. Mater. 2016, 15, 413− 418. (41) Hesse, L.; Wagner, S. T.; Neinhuis, C. Biomechanics and Functional Morphology of a Climbing Monocot. AoB Plants 2016, 8, plw005. (42) Gelebart, A. H.; Mulder, D. J.; Varga, M.; Konya, A.; Vantomme, G.; Meijer, E. W.; Selinger, R. L. B.; Broer, D. J. Making Waves in a Photoactive Polymer Film. Nature 2017, 546, 632−636.

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