Soft Tendril-Inspired Grippers: Shape Morphing of ... - ACS Publications

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Applications of Polymer, Composite, and Coating Materials

Soft tendril-inspired grippers: shape-morphing of programmable polymer-paper bilayer composites Wei Wang, Chenzhe Li, Maenghyo Cho, and Sung-Hoon Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18079 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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ACS Applied Materials & Interfaces

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, Seoul National University, Seoul, 151742, Republic of Korea. ‡ Institute of Advanced Machines and Design, Seoul National University, Seoul, 151-742, Republic of Korea. ABSTRACT Botanic nastic movements that occur in response to environmental stimuli have inspired many man-made shape-morphing systems. Tendril is an exemplification serving the climbing plants as their parasitic grasping component through transforming from a straight shape into a coiled configuration via the asymmetric contraction of internal stratiform plant tissues. Inspired by tendrils, this study using three-dimensional (3D) printing approach developed a class of soft gripper with pre-programmed 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 polymer on the paper substrate with different patterns. The rough and porous paper surface provides printed polymer well adhesion to the paper substrate which serves as a passive strain-limiting layer. During printing, the melted polymer filament is stretched enabling the internal strain is 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 obtainable 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 the tendril-like twistless spring-like structure. This study further helps to develop soft robots using active polymer materials for engineered systems. KEYWORDS: biomimetics, tendril, shape-shifting, 3D/4D-printing, soft robotics, soft gripper 1. INTRODUCTION

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Tendril is clasping structure that used by climbing plants to anchor and support their vining stems by coiling around suitable hosts, providing the plant its superior approach to sunlight and numerous ecological niches. The general fundamental motion modes of plant tendril are bending, spiral and helical distortions which occurs via asymmetric longitudinal contraction 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 of multiple layers combined with activation components have been developed showing the significant merits of various large continuous deformation, being lightweight and compact, and being controllable with a wide range of stimuli. Based on these advantages, shape-morphing systems offer new solutions to the functionality-induced devices in a broad range of engineering applications including biotechnology, 2,3,4 electronics,5 aerospace,6 robotics,7,8,9 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 hard-bodied counterparts.11,12 Soft tendril-like manipulators have been developed based on different actuation techniques such as pneumatic actuators,13,14,15 cable-driven systems16 and shape memory alloy (SMA) actuators.17 Among these techniques, pneumatic actuators powered by compressed gas and motor-based 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 actuator 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 , 20 , 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 process of these structures in previous studies always includes a series of specific steps which always involves special devices and tedious manual operations. 22 , 23 , 24 , 25 , 26 Meanwhile, polymer-activated shape-morphing structures fabricated


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through 3D printing named as four-dimensional (4D) printing have been investigated with the dominating advantages of being capable of synchronously fabricating and programming of polymer structures. 27,28,29 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 being able to respond to a wide range of stimuli such as heat,9, water,31 light 32 , 33 , 34

and magnetic field 35 can be used for designing untethered polymer-activated shape-

morphing mechanisms with contactless remote control. Additionally, these shape-morphing structures have the excellent shape-retained capability that they could maintain their deformed shapes in the high stiffness when the stimulus was removed. 36,37 Based on the advantages of 3D printing and polymer-activated shape-morphing structures, this research aims to develop a more versatile and economical approach to fabricate the tendril-like grasping mechanisms utilizing 3D printing as a single manufacturing step that requires only a 3D printer and inexpensive off-theshelf materials. This study described a class of 3D printing based soft gripper with pre-programmed grasping deformations being capable of imitating the general motions of plant tendrils including bending, spiral and helical distortions (Figure 1A). All the structures of the gripper were initially in flat configurations tailored from a thin composite sheet which 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 in a single-step printing process where the printed polymer is serving as the stimuli-triggered active layer and the paper serving as the passive strain-limiting layer. The polymer structure was fabricated that each printed layer was printed in identical pattern and 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 where the contracting direction of the polymer layer is parallel to or makes an angle with the length of the paper layer, respectively. The basic deformable mechanisms were then used for designing tendril-inspired grasping devices with bending, spiral and helical deformation. 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 the tendril-like twistless spring-like structure.

2. RESULTS AND DISCUSSION


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2.1 Materials and fabrication All the specimens are fabricated by using an FDM based 3D printer. Before printing polymer material, a sheet of paper is first flat placed and fixed on the platform of the printer. During printing, the melted polymer filament is stretched enabling the internal strain is 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 show the schematic of the corresponding changes of the PE monomer structure during these processes. The polymer is first printed on the paper sheet, and then after printing, it will be tailored to the different shapes as the specimens with desired deformations. The specimens are designed with uniform printed polymer pattern on the entire paper substrate for achieving the 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 high stiffness state to low stiffness state. And 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 due to the resistance of the paper layer against 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 achieving 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 achieved (Figure 1, C and D). In addition, all the specimens and designs presented in this work were designed for achieving the permanent shape deformation. Both self-bending and self-twisting 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 due to its stable elastic modulus and negligible volume change for a large range of temperature change. Polyester (PE), a shape memory polymer with a melting temperature >200 °C is chosen as the 3D printing material, but a number of other polymers also work. 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 chain tends to maintain high polymer crystallinity and alignment (Figure 1B). The images of the printed PE film are taken at 0° and 45°


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between the cross-polarizer angles by using polarizing optical microscopy (POM) where the arrows indicate the printed direction (Figure 1E). The image shows the homogeneous alignment of the stretched PE molecular chains along the printed direction where a dark image was obtained 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 optimizations of the printing and activating parameters are not considered in this work. The elastic modulus of the printed PE varies with the change of 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 PE’s modulus will be too low to achieve a better deformation. The activated time was determined as 15 minutes by placing the specimens in the oven at 90 °C until there is no further movement can 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 minutes are around 99.3% (1447.5 MPa and 10.1 MPa) and 0.5% (2236.5 MPa 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. That is because the copy-paper is composed of intertwined cellulose fibers and their constituent of lignin ensures the thermal stability of the paper structure.38 It is also noticed that the strain of PE structure increases in the first 4 minutes due to 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 (see Supporting Information).

2.2 Proof of concept: bending deformation Experiments were conducted to evaluate the effect of different thickness of the printed PE on the maximum bending deformation of the specimen. To do so, the dimensions of all the specimens


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was 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 mm to 2.0 mm in increments of 0.1 mm, and the thickness of 2.5 mm and 3.0 mm. Scanning electron microscopy (SEM) images of both the copy-paper surface and the specimen cross-section are taken to verify the bonding conditions between the PE and paper subtract (Figure 2A, top-right). It can be discovered that the paper’s rough and porous surface provides printed polymer well adhesion to the paper substrate with multi-groove structures. The schematic of bending deformation of the specimen is described (Figure 2A, middle, and Figure S3). All the activated specimens are placed on 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 on different thickness of the printed PE are illustrated (Figure 2A). According to the results, one 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 first stage of curve AB, the maximum bending curvature increases as the thickness of the printed PE layer increased. That is because during 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 the increase of the thickness of the PE layer, it can be seen that the bending curvature gradually decreases due to the increased distance between the neutral surfaces of the PE layer and the paper sheet (see Supporting Information for modeling). To demonstrate the mentioned bending specimen with immediate potential for use, a gripper with cruciform shape is constructed where each branch is a self-bending specimen with the dimension of 50 mm in length, 4 mm in width and 1 mm in thickness of the printed PE layer (Figure 2B). Under activation, the actuated branches of the gripper curled about axes perpendicular to the long axes of the branches. The gripper was used to grip a ping pong ball in the same oven with the initial interior temperature is 90 °C. The grasping process is described (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 to a universal testing machine to measure the force during an upward motion generated by the gripper on the ping pong ball fixed on the


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ground (Figure 2D, 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 fast 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 obtained results of 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-section strip can obtain homogeneous bending deformation, and then a variable cross-section strip with a gradually varied thickness of PE layer can generate the tendril-like spiral deformation. In this study, the variable cross-section specimen with the dimensions of (t1, t2, Ls) is designed and fabricated where the PE layer has the linearly varied thickness changing from t1 to t2 on a total length of Ls is printed on the paper substrate while keeping a constant width of 2 mm (Figure 3A and Figure 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 that t2 is 2 mm, 3 mm and 4 mm, and Ls is 50 mm, 75 mm and 100 mm, respectively. The experimental and modeling configurations of all the activated specimens are described (Figure 3, C and D, and see Supporting Information for modeling). From the results, it can be seen that the specimen with a smaller t2 or a larger Ls can generate larger spiral angle subtended by the full length. The specimen with the dimensions of (1, 3, 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 with the same conditions and the grasping process is recorded (Figure 3E and Video S2). To test the loading capacity of the tentacle-type gripper, the activated gripper with spiral deformation is installed to the same universal testing machine to measure the


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force during an upward motion generated by the gripper on the cylindrical object fixed on the ground (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 fast increases first and then gradually decreases, due to 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 the helical deformation in a single structure, the printed PE layer serves as the anisotropic layer where all the PE filaments were printed in parallel with a determined angle 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 constant the thickness of the PE layer at 1 mm and by varying the PE filament orientation. Three samples were fabricated for each configuration with the same overall dimension of 75 mm in length, 10 mm in width and 1.1 mm in thickness, but with filament orientation ranging from 15° to 75° in an increment of 15°. All the samples are 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 are measured. The measured and modeling results are described (Figure 4A, Figure S5, and see Supporting Information for modeling). The insets (Figure 4A) show the pictures 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 leading 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 that is 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 the diameter of 10 mm. The experiment was conducted in the same oven with 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 to the same universal testing machine to measure the force during an upward motion generated by the gripper on the cylindrical object fixed on the ground


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(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 fast increases first and then gradually increases, due to the gradually deformed shape of the gripper resulting in a decreasing 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 tendril-inspired gripper have been described being capable of generating pre-programmed bending, spiral and helical deformation, respectively. 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 bilayer fabricated via rapid prototyping method in mechanical engineering to understand the adaptability of biological tendril structures. The pulling force (i.e., the grasping force) of each type gripper from caging the object to separating it has been evaluated, and the results are summarized in Figure 5. The self-weight of these three grippers with bending, spiral and helical deformation are 0.97 g, 0.53 g and 0.55 g and their generated maximum grasping forces are 0.69 N, 8.1 N 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 N/g, 15.3N/g and 14.9 N/g. Based on these results, it can be seen that grasping an object with spiral and helical deformation is a more efficient approach in comparison to the one with bending deformation. From the tested results, it also can be seen that both the grippers with spiral and helical deformation 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 is 2 mm) after reaching the maximum force reduced fast that is because the large plastic deformation results in the failure of gripper structure. Differently, the pulling force of the gripper with helical deformation (thickness is 1 mm) rises rapidly and then maintain a steady increase until the gripper separated from the object. The main reason for the difference is the different thickness of the gripper that is 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


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comparison to 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 plant puts less resource into its structure and more into active growth.

2.6 Biomimetic robotic tendril The plant tendril seems like the pre-programmed 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 1mm in width 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 through 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 the segment-3 was first activated for twisting around the object and its deformed shape was retained through subsequent cooling to accomplish firmly attaching and grasping. Afterwards, the segment-2 and segment-1 are successively activated to form the


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twistless spring-like structure. The whole sequential coiling motion of the specimen is described (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. 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 to the same universal testing machine to measure the force generated from the springlike structure during 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 go down to the original place, and the result of the vertical pulling force generated by the spring-like structure on the time scale is measured (Figure 6E). From the curve drawn in 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 for 100 times in a row with the same up-and-down pulling distance (Figure 6F). One can be seen that both the maximum force and the minimum force in one cycle gradually decreases under repeated tests. The test results of the first cycle and the one-hundredth cycle are compared (Figure 6E). From the results, one can be seen that the maximum and minimum forces of the first cycle and the one-hundredth cycle are 8.24 N and 0 N, and 6.51 N and -2.48 N, which corresponds to the stiffness of the spring-like structure are 0.41 N/mm and 0.45N/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, due to the elongation of the coiled spring-like structure with the gradually accumulated plastic strain of the printed PE layer during the repeated measurements. The length of the spring-like structure was measured before and after the repeatability test are 153.8 mm and 157.6 mm corresponding to an overall length elongation of around 2.5%.

3. CONCLUSION


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This work has demonstrated the design and fabrication of plant tendril-inspired grippers being capable of grasping objects with pre-programmed deformation including bending, spiral and helical distortions. The shape-morphing specimens were tailored from the flat polymer-paper composite fabricated by 3D printing polymer material on the paper substrate. This work successively introduced the design of the specimens with specific pre-programmed deformation. Meanwhile, different grippers based on these different types of specimen were then developed capable of grasping weight varying from dozens of times to more than one thousand times of their weight. This work also reproduced a fully equipped bio-mimetic robotic tendril with three segments to imitate the sequential coiling motion of the plant tendril. The robotic tendril started in the straight configuration, using one segment to twist around the object for attaching and grasping, and then, using the other two segment to form the 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 tendril 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 lowcost which only requires a 3D printer and inexpensive off-the-shelf materials. 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, based on the different thermal deformation coefficients, other materials or the same materials with different physical properties could also be used for designing layered soft-morphing structures.27,40 Also, based on other possible materials responding to different stimuli, different types of activation can be used for deforming the structure including water, light and magnetic field. The active material employed in this study is PE instead of the polylactic acid (PLA) filaments used in the several previous studies. 10 This change of the filament materials is critical to this study since the π-π interaction between the polymer chain tends to maintain high polymer crystallinity and alignment resulting in the directly 3D printed cold-draw polymer structure can generate more than 10% strain upon activation and similar elastic moduli comparing to the real plants,41 which is critical to achieving the shape-morphing of plant tendril. This study provides a novel method with short experiential cycle and high repeatability for the understanding the plant structure, which in return could be beneficial for future design of bio-inspired materials and structures.


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The actuation speed of the plant tendril-inspired structures demonstrated in this work is slower with the comparison to the reported ones made of hydrogels or liquid crystal elastomers (LCEs). That 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)

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leading to a

longer time for absorbing heat energy. Secondly, the applied actuation temperature is 90°C which is slightly higher than the Tg of PE resulting in a slowly convective heat transfer. Both 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 not only provides a new direction for fabricating of the polymer-activated soft robots but also provides new solutions for the development of printable robotic systems. Besides, the design of the proposed work is illustrated at centimeter-scale, however, by exploiting the scale-free geometric of the specimens, the approach in this work can be extended to the fabrication of transformable materials and structures at a very small scale.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Full experimental details, including 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)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M.C.), [email protected] (S.H.A.).


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ORCID Wei Wang: 0000-0003-0416-390X 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.

ACKNOWLEDGEMENTS 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) (NRF-2016R1A5A1938472) and (MSIP) (2012R1A3A2048841). The authors would like to thank the reviewers for their insightful feedback.

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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 its subsequent effects of self-contracting once 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 is parallel to or makes 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 DMA test of printed PE film and paper sheet on their elastic modulus and linear strain.


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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 the different thickness 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 paper subtract, scale bars are 30 ߤm. Middle and bottom insets show the schematic and images of activated bending specimen with different printed polymer thickness. (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) The gripper hung using rope grasping a ping pong ball. (D) Pull force of the gripper from caging the ball to separating it as shown in the bottom inset. The top-right inset shows the schematic of experimental setup. All dimensions are in mm and unspecified scale bars are 10 mm.


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Figure 3. Specimens with spiral deformation and application to the tentacle-type gripper. (A) Schematic of the strip with spiral deformation and its dimensions are defined as (t1, t2, Ls). (B) The image of one fabricated specimen with the dimensions of (1, 3, 75). The experimental results (C) and the modeling results (D) of the activated configurations of the specimens with different geometric parameters. (E) The gripper hung using rope grasping a cylindrical object. (F) Pull force of the gripper from caging the cylindrical object to separating it as shown in the bottom


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inset. The top-right inset shows the schematic of experimental setup. All dimensions are in mm and all the scales are 10 mm.

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) The process of the hung twining-type gripper grasping a cylindrical object. (C) Pull force of the gripper from twining the cylindrical object to separating it as shown in the bottom inset. The top-left inset shows the schematic of experimental setup. All scales are 20 mm.


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


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Figure 6. Design and performance evaluation of biomimetic robotic tendril. (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 are 30°, -30° and 15°, respectively. (C) The sequential deformation of each segment activated by a heating gun. The segment-3 is first activated for twining the object and then the segment-2 and segment-1 are successively activated to form the twistless spring-like structure. (D) The 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 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 in red solid line and red dashed line, respectively. All the scales are 30 mm.


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Soft Tendril-Inspired Grippers: Shape Morphing of ... - ACS Publications

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