Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft

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Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft Robotics Haitao Yang, Bok Seng Yeow, Ting-Hsiang Chang, Kerui Li, Fanfan Fu, Hongliang Ren, and Po-Yen Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00144 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft Robotics Haitao Yang†,⊥, Bok Seng Yeow‡,⊥, Ting-Hsiang Chang†, Kerui Li†, Fanfan Fu†, Hongliang Ren‡*, Po-Yen Chen†*

†Department

of Chemical and Biomolecular Engineering, National University of Singapore

(NUS), 117585, Singapore. ‡ Department

of Biomedical Engineering, National University of Singapore (NUS), 117585,

Singapore. ⊥These

authors contributed equally to this work.

KEYWORDS: graphene oxide templating, metal oxide origamis, reconfigurable metamaterials, multifunctional robotic backbones, soft robotics. * Email: [email protected] (H. Ren); [email protected] (P.-Y. Chen)

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ABSTRACT: Origami structures have been widely applied in various technologies especially in the fields of soft robotics. Metal oxides (MOs) have recently emerged as unconventional backbone materials for constructing complex origamis with distinct functionalities. However, the MO origami structures reported in the literature were rigid and not deformable, thus limiting their applications from soft robotics. Herein, we reported a graphene oxide (GO)-enabled templating synthesis to produce complex MO origami structures from their paper origami templates with high structural replication. The MO origami structures were next stabilized with elastomer, and the MOelastomer origamis were able to be adapted into multiple actuation systems (including magnetic fields, shape-memory alloys, and pneumatics) for the fabrication of MO origami robots. Compared with conventional paper origami robots, the MO robots were lightweight, mechanically compliant, fire-retardant, magnetic responsive, and power-efficient. We further demonstrated that the legendary phoenix-fire-reborn concept in the soft robotic fields: a paper origami robot sacrificed itself in a fire scene and transformed itself into a downsized Al2O3 robot; the Al2O3 robot was able to crawl through a narrow tunnel where the original paper robot was unfit. These MO reconfigurable origamis provide an expanded material library for building soft robotics, and the functionalities of MO robots can be systematically engineered via the intercalation of various metal ions during the GO-enabled synthesis.


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Origami folding/unfolding process can turn a two-dimensional (2D) planar sheet into a complex three-dimensional (3D) structure via a sequence of pre-programmed folds.1 The deployable transformation in an object’s shape, size, and motion makes the origami structures widely applicable to biomedical devices,2,3 deformable electronics,4,5 and autonomous machines.6 Various backbone materials, including papers,5 plastics,7 DNA,8 composites of Nd-Fe-B particles,9 and 2D materials (e.g., graphene),10 have been utilized to construct complex origamis. Recently, metal oxides (MOs) and their composites, such as TiO211 and ZrO2,12 have emerged as new backbone materials to fabricate complex origamis,13 which exhibit intrinsic fire retardancy,14 high chemical resistance,15 and distinct mechanical properties.11 Different from conventional papers and plastics, the MO origamis are expected to facilitate the fabrication of multifunctional soft robotics6 and allow the robots to work in a harsh environment (e.g., fire scene, chemical accident).16 However, most of the reported MO origami structures were rigid, and the fabricated folding patterns were not compliant and deformable, limiting their applications from mechanically dynamic actuators and robotics.11,13 To the best of our knowledge, the prototypical soft robots with compliant MO bodies have not been demonstrated. Therefore, it is favorable to fabricate the reconfigurable MO origamis that can be integrated with multiple actuation systems, which provide an expanded material library for building soft robotics with unconventional functions and abilities. The confined assembly within the interlayer galleries of 2D materials provides an alternative route for the creation of ultrathin MO origami architectures. In particular, the multilayer of graphene oxide (GO) nanosheets was introduced with various metal ion precursors through intercalation.12,17-20 The interlayer spaces can guide the conversion of precursors into lamellar MO structures during the high-temperature oxidation of GO templates. This intercalation templating approach offered excellent chemistry control for the growth of various MO nanocrystals (e.g., ZnO,


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Al2O3, Mn2O3, Fe2O3) and reproduced multiple higher dimensional GO structures into the MO replicas, such as planar multilayers,20 wrinkled or crumpled textures,17 and freestanding strands.12 However, the challenges remain in folding the brittle, freestanding GO templates into complex origami structures,10 so the GO templating methods so far were only able to synthesize the MO products with low structural complexity. Additionally, the templated MO structures exhibited low fracture resistance and mechanical stability, which also refrained their applications from the dynamic and stretchable devices such as soft actuators and robots.12,17 In this article, we reported a GO-enabled templating synthesis to produce complex MO origami structures from their paper origami templates with high structural replication. The MO origami structures were further stabilized with thin elastomer, and the MO-elastomer origamis enabled the fabrication of MO robots with tunable functionalities as well as power-efficient actuation. The synthesis of MO robotic backbones is illustrated in Scheme 1. The procedure includes (i) deposition of GO nanosheets onto cellulose paper origamis, (ii) spontaneous intercalation of hydrated metal ions, (iii) assembly of origami units by metalized GO glue, (iv) removal of carbonaceous templates via calcination, and (v) stabilization of templated MO replicas with elastomer. The resulting MO-elastomer origamis were reproduced with identical folding patterns of their paper templates and exhibited large deformability (180° folding, 180° twisting, 60% stretching). The MO-based origamis were able to undergo locomotion by different actuation systems (including magnetic fields, shape-memory alloys (SMAs), and pneumatics) for the fabrication of MO robots. Compared with conventional paper origami robots, the MO robots were downsized, lightweight, mechanically compliant, fire-retardant, and magnetic responsive. In addition, due to reduced body weight and low backbone stiffness, the MO robots were more powerefficient than the paper robots (93% and 70% lower energy consumption in SMAs and pneumatics,


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respectively). We further demonstrated that the legendary phoenix-fire-reborn concept in the soft robotic fields: a paper origami robot sacrificed itself in a fire scene and transformed itself into a downsized Al2O3 robot; the Al2O3 robot was able to crawl through a narrow tunnel where the original paper robot was unfit. These MO reconfigurable origamis provide an expanded material library for constructing soft robotics, and the functionalities of MO robots can be systematically engineered via the intercalation of various metal ions during the GO-enabled synthesis.

RESULTS AND DISCUSSION Figure S1 illustrates the GO-enabled templating synthesis for converting paper origamis into a variety of downsized MO replicas. We first used a cellulose paper with a simple star shape for the preliminary investigation (Figure 1a, i), and the paper was then immersed into GO dispersion. After air-dried, the GO-coated cellulose paper (abbreviated as GO-cellulose) was utilized as a sacrificial template for the growth of various MO nanocrystals (Figure 1a, ii). The deposition of GO nanocoating is about 16.1 wt.% of final GO-cellulose composite. The GOcellulose template was then soaked in the precursor solution containing metal ions (Mn+, 0.1 M), and the negative charges of GO nanocoatings (ζ ~−45 mV) along with the concentration gradients offered significant driving force to recruit Mn+ from bulk solution to the GO-cellulose templates.21 The hydrated GO nanocoatings exhibited expanded interlayer spacings (>12 Å),22 so the ions (e.g., Ho3+) were unimpededly intercalated into the stacked GO films.23,24 The intercalated metal ions were coordinated with one or multiple carboxylate group(s) on the GO nanosheets (m–RCOO− + Mn+ ↔ –(RCOO)mM(n-m)+),17,25 resulting in a metal ion-intercalated GO-cellulose complex (abbreviated as Mn+-GO-cellulose). Additionally, the metal ions were stabilized by means of the


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complexation with other oxygen-containing groups of GO (e.g., hydroxyl and epoxy groups) and other electrostatic interaction with aromatic rings.26 The Mn+-GO-cellulose complex underwent a calcination process at above 500 °C in air to burn off the carbonaceous templates, producing the MO structures with morphological replication of both macroscale shape and microfiber structures. Figure 1a, iii shows the digital and scanning electron microscope (SEM) images of the Ho2O3 product, which preserves the star shape of GOcellulose template and exhibits similar microfiber networks. The templating synthesis is highly versatile. We have synthesized 11 MO replicas, which involved alkaline earth metals (e.g., Mg), transition metals (e.g., Mn, Fe, Co, Ni, Cu, Zn), lanthanide metals (e.g., Gd, Tb, Ho), and aluminum (Al) (digital images in Figure S2). Several other metal elements were also tried; as expected, the alkali metal salts exhibited high thermal stability and did not undergo any reactions during the templating synthesis. Several noble metal ions (e.g., Ag+, Au3+) were also intercalated into the GO-cellulose templates, and these noble metal ions would form the noble metal oxides at lower annealing temperature, but the oxides were unstable and further decomposed under annealing at high temperature (above 500 °C). (e.g., Ag2O (decompose at 260 °C),27 Au2O3 (decompose at 250 °C)28). Their X-ray diffraction (XRD) analysis, energy-dispersive X-ray spectroscopy (EDS) results, and the SEM images of templated microfiber networks are shown in Figure S3, S4 and S5, respectively. To better elucidate the sequence of template decomposition and the mass/size change during the calcination, thermogravimetric analysis (TGA) experiments (Figure 1b) and direct video recording (Figure 1c) were conducted. The GO nanocoatings were first thermally reduced at 150 °C (FTIR result in Figure S6), and the bound metal ions (e.g., Al3+) were liberated and formed the MO nanocrystals (Al2O3) within the reduced GO (rGO) multilayers.17,29 The decomposition of cellulose paper was completed by 400 °C, and large


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volumetric shrinkage was observed (Movie S1). The rGO oxidation occurred later and was completely removed at above 750 °C (Figure 1b), and the color of samples gradually turned from black to white, indicating that the Al2O3 replica was synthesized after the removal of rGO templates. The weight of resulting Al2O3 replica was only 1.7 wt.% of the Al3+-GO-cellulose template, and the volumetric shrinkage at 98% was observed (92% areal shrinkage and 4 times thinner than the paper template). Similar sample shrinkage was also observed in other MOs after the calcination (Figure S7).17 In addition, the thickness and shrinkage percentage of MO replica can be controlled by the metal ion concentration of precursor solution. For example, as the concentration of metal ion solution (i.e., Al(NO3)3) increased from 0.01 M to 0.2 M, the weight of templated MO product (Al2O3) increased (Figure S8), leading to thicker templated Al2O3 films from 6 to 33 µm (Figure S9) and reduced areal shrinkage (Figure S10). In the following studies, the metal ion concentration was kept at 0.1 M. Next, we demonstrated that the GO nanosheets were necessary to achieve 3D MO origami structures during the templating synthesis. We first folded a planar paper into a wavy structure with four folds as our origami template. With the incorporation of nanoscale GO templates, we were able to produce a set of downsized wavy replicas in Al2O3 (Figure 1d), Mn2O3/Mn3O4, Fe2O3, TbO2, and Ho2O3 (Figure S11), and the number of folds were identical to the paper templates. We carried out a similar templating experiment without the deposition of GO nanocoatings in which metal ions were directly absorbed into the wavy paper template followed by the annealing and oxidation processes. The resulting Al2O3 products did not resemble the original wavy structures (inset of Figure 1d). The unsuccessful replication results from multiple reasons. First, without the GO nanosheets, fewer metallic precursors were involved in the templating synthesis, causing insufficient MO nanocrystals to construct 3D origami structures. For instance, the weight of the


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Al2O3 product synthesized from the Al3+-cellulose template (without GO) was ~70% lower than the one from the Al3+-GO-cellulose template. The Al2O3 structure synthesized from the celluloseonly template was more porous, and the microfibers were with smaller diameter (2-3 µm) than the GO-enabled Al2O3 samples (about 5 µm) (Figure S12). Second, the GO nanosheets served as the secondary templates to direct the MO growth at nanoscale. Transmission electron microscope (TEM) analyses were performed on two Al2O3 nanostructures synthesized from the Al3+-GO-cellulose and Al3+-cellulose templates, as shown in Figure 1e and 1f, respectively. The presence of GO nanosheets can guide the mobility of Al2O3 nanocrystals along the 2D gallery spaces during the templating synthesis. The sintering of Al2O3 nanoparticles (~5 nm) resulted in the formation of 2D interconnected particle arrays as the basic nanoscale building blocks (Figure 1e). On the other hand, without the GO nanosheets, the Al2O3 nanoclusters exhibited extensive mobility, yielding a randomly-distributed Al2O3 matrix (Figure 1f), which were not able to replicate the folding patterns of wavy origamis accurately. With the effective templating synthesis, we reproduced various 3D origami structures in Al2O3, including airplane, boat, bird, and auxetic hexagonal honeycomb (Figure 1g and closer-view images in Figure S13). In comparison with their original paper templates, the spatially organized origami patterns were well preserved in the Al2O3 replicas, and the areal shrinkage at ~90% was observed. The folding procedures of each paper origami template are presented in Figure S14 and Movie S2. Although the GO-enabled templating synthesis produced complex MO origami structures, it remained challenging to synthesize the enclosed MO origamis and to assemble multiple MO units together. One strategy is to use commercial glue to connect the MO products after the synthesis, but the fragile MO structures would break during the assembly. Another approach is to


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glue the paper origamis together prior to the templated synthesis, yet the glue would degrade during the calcination (Figure S15). To address this challenge, we developed a metalized GO glue (abbreviated as a Mn+-GO glue) that created a thin MO layer to connect two sides of MO unit(s) after the calcination (Figure 2a). The Mn+-GO glue contained GO nanosheets, polyvinyl alcohol (PVA), and metal salts. The purpose of adding PVA is to stabilize the Mn+-GO nanosheets and to increase the dispersion viscosity from 0.02 to 1.63 Pa·s (Figure S16). After applied onto the paper origami templates, the Mn+-GO glue was infiltrated into the cellulose network and provided strong adhesion (Figure S17). After the calcination and annealing processes, a thin layer of MO nanoparticles was synthesized to connect two MO units. For instance, a Ho3+-GO glue resulted in a stable Ho2O3 joint for the fabrication of Ho2O3-MgO connector (Figure 2b, i), and the interconnected Ho2O3 nanoparticles were observed in both MO films from the energy-dispersive X-ray spectroscopy (EDS) analysis (Figure 2b, ii). With the development of metalized GO glues, we were able to transform the origami assemblies, such as circular and tubular structures, from cellulose paper into various MOs. For example, we applied the Al3+-GO glue to enclose one or two Al3+-GO-cellulose strip(s) into a circular ring, a Möbius wavy strip, and interlocked rings, as shown in Figure 2c, i-iii, respectively. After the calcination, the Al3+-GO glue produced stable Al2O3 joints to enclose the Al2O3 strip(s), and comparable ring structures in Al2O3 were synthesized (Figure 2d, i-iii). The metalized glue was applied onto two sides of the auxetic hexagonal honeycomb and bellow origamis of Al3+-GOcellulose, and the complexes were enclosed into the tubular structures (Figure 2c, iv-v). After the templating synthesis, the tubular Al2O3 origami structures were produced, and the honeycomb and bellow patterns were replicated on the side of Al2O3 tubes (Figure 2d, iv-v). In comparison with the paper templates, the tubular Al2O3 replicas exhibited ~95% weight loss and ~90% volumetric


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shrinkage. To investigate the smallest MO origamis we could produce, we folded a small auxetic hexagonal tubular origami in cellulose paper with 5 mm  8 mm dimension (Figure S18), which approached the extreme of hand folding (the finger width is 1.6 to 2 cm). After the GO-enabled synthesis, the auxetic hexagonal patterns were preserved in the Al2O3 replica, and the dimension of origami patterns was shrunk into 2 mm × 3 mm. We believe such size should be close to the limit of produced MO origamis. The mechanical stability of templated MO origami structures was improved by the incorporation of elastomer, and the elastomer-stabilized MO origamis exhibited large deformability and were able to undergo repeated actuation. In the following studies, we focused on the Al2O3 structures due to its light weight, high material stability, and high structural replication. Dilute PDMS solution (in dichloromethane (DCM)) was gradually infiltrated into the Al2O3 auxetic hexagonal structures. Followed by the curing process at 70 °C, the Al2O3 microfibers were conformally coated with thin elastomer (SEM images in Figure 3a, i and ii). By controlling the PDMS concentration, the thickness of Al2O3-elastomer film was able to be adjusted (Figure S19). Here we used the PDMS concentration at 300 mg mL-1 for further investigations. As shown in Figure 3a, the thickness of elastomer-stabilized Al2O3 composites was observed at 50 µm, about four times thinner and 86% lighter (0.5 g cm-3) than the cellulose paper (~200 µm, 7.0 g cm-3). In addition, the Al2O3-elastomer thin film was more compliant (Young’s modulus at 8.4 MPa) than the as-templated Al2O3 film (740 MPa) and the original cellulose paper (582 MPa); the stressstrain curves are shown in Figure S20-S22. With the replicated auxetic hexagonal patterns, the Al2O3-elastomer origamis were able to sustain large and repeated deformations (180° bending, 180° twisting, and 60% stretchability) (Figure 3b). As shown in Figure 3c, the auxetic hexagonal


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origami of Al2O3-elastomer was able to undergo the compressing/stretching processes to relieve the strains in both horizontal (x-axis, -30% ~ 65%) and vertical directions (y-axis, -10% ~ 20%). The MO-elastomer origamis can be categorized as reconfigurable metamaterials for the fabrication of soft robotics. The in-situ SEM images in Figure 4a showed that, while the auxetic hexagonal origami of Al2O3-elastomer was stretched, the replicated folding patterns were flattened to relieve the uniaxial strains. The strain-stress curves of Al2O3-elastomer origami are shown in Figure 4b, and no strain-induced fractures were observed after a 75-cycle fatigue test. The mechanically stable Al2O3-elastomer origamis were next used for the fabrication of soft robotics. For instance, we used the Al3+-GO glue to assemble three Al3+-GO-cellulose units together (one bellow origami and two flat films) (Figure S23a). After the GO-enabled synthesis, a fully enclosed Al2O3 bellow tube was produced (~10 vol.% of the original paper template, Figure S23b). The downsized Al2O3 tube was further stabilized with thin elastomer coating and connected to an air pump for the fabrication of a pneumatic origami robot (abbreviated as the Al2O3 robot). As the pressure increased, the bellow patterns of Al2O3 robot were gradually unfolded, leading to the axial elongation of the robotic body. The robotic shape and length recovered as the pressure decreased (Figure 4c). This cyclic axial motion enabled the Al2O3 robot to crawl forward at a speed of 1.1 cm s-1. The unconventional MO backbones introduced several distinct functionalities into soft robotics. First, since the calcination process removed the carbonaceous components (i.e., cellulose and GO), the Al2O3-elastomer film was 4 times thinner (50 µm) and 14 times lighter (0.5 g cm-3) than the cellulose paper predecessor (200 µm, 7.0 g cm-3). Secondly, the elastomer-stabilized Al2O3 film (8.4 MPa) was more compliant than the cellulose paper (582 MPa) and still provided sufficient mechanical stability for reversible actuation. Thirdly, the intrinsic flame resistance of


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MO backbones (e.g., Al2O3) allowed the templated origami structures (Figure S24) and pneumatic robots (Figure 4d) to sustain direct flame contact from an ethanol burner (flame temperature at ~800 °C). The pneumatic origami robot with a paper-elastomer backbone was easily damaged by fire and ignited after 90-second burning, while the Al2O3 robot was able to crawl through the fire scene and remained intact (Movie S3). Lastly, the functionalities of MO backbones can be tuned by intercalating different metal ions into the GO-cellulose templates. For instance, the intercalation of paramagnetic metal ions (e.g., Ho3+) produced the Ho2O3 origami structures with strong paramagnetism (volume susceptibility 8.8×10−6, Figure 4e). Followed by the elastomer stabilization, the Ho2O3-elastomer backbones (volume susceptibility 7.5×10−6) were mechanically stable, lightweight, and can be driven by an Nd-Fe-B permanent magnet. For instance, a roll of Ho2O3-elastomer was able to follow the trajectory of a magnet to climb a hill, jump into a water channel, and sail across an aqueduct without submerging (Movie S4). On the other hand, the heavier paper-PDMS device exhibited no magnetic response and sunk during the journey (Figure S25). Another demonstration showed the lightweight Ho2O3 wings were actuated upwards and downwards via ultrasound pulses (Figure S26, Movie S5). The multifunctionality of Al2O3 robotic backbones in terms of lightweight, large deformability, and intrinsic flame retardancy advanced the development of soft robotics actuated by the SMA wires (e.g., nitinol). A fire safety concern has been raised in the use of nitinol wires to actuate the origami robots composed of conventional paper (or plastic) backbones.30,31 While the nitinol wires were charged constantly to maintain the robotic configuration, the heat generated by nitinol wires raised the surrounding temperature up to 250-270 °C and carbonized the paper or plastic backbone, which may lead to fire risks (Figure 5a). The challenge can be addressed by using the Al2O3 backbones, which were not damaged by the high heat generated by the nitinol


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wires. Additionally, the thin, lightweight, and highly compliant Al2O3-elastomer backbones (thickness: 50 µm, weight: 0.2 g, Young’s modulus: 8.4 MPa) were easily actuated by the nitinol wires with a smaller diameter at 150 µm. The Al2O3 robots exhibited a worm-like gait with a speed of 0.08 mm s-1, as recorded in Figure 5b and Movie S6. On the other hand, the predecessor paper robot was thicker (200 µm), heavier (2.1 g), and stiffer (582 MPa) than the Al2O3 robot, so the paper robot required the nitinol wires with a thicker diameter (over 500 µm) to actuate the paper robot with the same locomotion. No visible actuation can be observed in the paper robot by using the nitinol wires at 150 µm. The MO soft robotics were more power-efficient and energy-saving in compared with the robot with traditional paper backbones. We fabricated two SMA-actuated robots by using cellulose paper and Al2O3-elastomer composite as the backbone materials. Figure 5c monitored the displacement and energy consumption of both robots in one actuation cycle. As we mentioned earlier, 500-µm nitinol wires were adopted for the paper robot to achieve visible actuation, and 150 µm wires were used for the Al2O3 robot. Since the Al2O3-elastomer backbones were lighter, thinner, and more complaint than the cellulose papers, the Al2O3 robot demonstrated larger displacement (2.3 mm per actuation) and shorter response time (0.2 second after the current applied to the nitinol wires) than the paper robot (1.8 mm/actuation and 2.2 seconds). Moreover, the energy consumption of an Al2O3 robot was 2.5 J for one actuation of nitinol wires, which was 93% lower than the one of a paper robot (34 J). We next compared two pneumatic robots with cellulose paper and Al2O3-elastomer backbones (both robots were with the diameter 1 cm and length 2.5 cm), and the pressure required to actuate the robots was recorded in Figure 5d. The results showed that the pressure required to actuate the Al2O3 robot fluctuated from -1 kPa to 1 kPa during the deflation and inflation cycles (1.5 cycle s-1), which was 70% lower than the one needed for the paper robot


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with the same actuation frequency (from -2.9 kPa to 3.4 kPa). These power-efficient manners of Al2O3 robotics were mainly owing to the lightweight and highly compliant Al2O3-elastomer backbones which needed less force to induce the repeated deformation. We finally demonstrated the material transformation concept in the soft robotic fields by using the GO-enabled templating synthesis (Figure 5e, Movie S7). Although the robotic functionalities were reported to be improved or altered by engineering the surface with functional materials or equipping with new exoskeletons,32,33 the robotic backbones and their physiochemical properties remained unchanged. Therefore, we were inspired by the myth where the phoenix burns in flames and regenerates from the ashes of its predecessor, and the newborn phoenix revives with a smaller size and stronger vitality. Comparably, in terms of the material transformation from a fire scene, the synthesis of MO structures utilizing the GO-cellulose templates demonstrates an analogy to the fire-reborn phoenix in the material science fields. In our demonstration, an enclosed bellow origami tube was first assembled by using the Al3+-GO-cellulose complex and the Al3+-GO glue. The paper tubular structure was then sealed with thin PDMS coating and connected to an air pump for the fabrication of a pneumatic origami robot. The paper robot (height ~3 cm) exhibited the inchworm motions down an inclined plane until the robot was blocked by a 2-cm-height tunnel. The dilemma can be solved by undergoing a calcination process where the paper robot was burned off in a fire scene, and a downsized Al2O3 origami replica was produced from the ashes of its predecessor. Followed by the re-stabilization with thin elastomer, an Al2O3 robot with replicated origami patterns was re-assembled and connected with the air pump. The fire-reborn Al2O3 pneumatic robot exhibited similar inchworm actuation and was small enough (height ~1 cm) to crawl through the narrow tunnel. The showcase resembled the legendary phoenix: the predecessor robot burned itself in flames, and the Al2O3


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robot was fire-reborn from the ashes of its predecessor, exhibiting a smaller size and distinct abilities to accomplish the tasks unfit for the predecessor robot.


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CONCLUSIONS In this article, the transformation of robotic materials from cellulose paper to various soft MO composites provides an expanded material library for the fabrication of soft robotics. The GOenabled templating synthesis combined with the metalized GO glue realized the production of complex MO origamis and origami assemblies with high structural replication of their paper templates. The reproduced MO origami structures were further stabilized with thin elastomeric. The MO-elastomer origamis were reconfigurable and served as the functional backbones of soft robotics. The functionalities of MO backbones can be systematically tuned by intercalating different metal ion precursors into the GO-cellulose template. Compared with traditional paper and plastic materials for building origami robots, the MO robots demonstrated distinct functionalities (e.g., lightweight, highly deformable, fire-retardant, magnetic responsive) and were actuated by multiple systems (including SMAs, pneumatics, and magnetic fields). In addition, the lightweight, compliant, and deformable MO-elastomer backbones largely reduced the required energy and mechanical forces to power the MO robots. The fire-reborn-phoenix concept were finally demonstrated in the robotic fields by using the GO-enabled templating synthesis: a predecessor origami robot was burned in a fire scene and transformed itself into a downsized Al2O3 robot that was able to crawl through a narrow tunnel where the predecessor robot was unfit. We expect that the MO robots are advantageous for a wide range of applications, such as the robots that can work in high-risk environments (e.g., chemical spills, fire disaster),34,35 pneumatic artificial muscles,36 and humanoid robotic arms.37 In addition, multiple MO backbones (Fe2O3 and Mn2O3) are electrochemically active,17,38 providing the opportunities to fabricate the energy-storage and sensing devices on the robotic backbone. The


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development of such multifunctional, compliant, and energy-efficient MO backbones enriches the material library for the fabrication of soft robotics toward high functional integration.


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EXPERIMENTAL SECTION Materials Magnesium chloride (MgCl2), copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), terbium(III) nitrate pentahydrate (Tb(NO3)3·5H2O), holmium(III) nitrate pentahydrate (Ho(NO3)3·5H2O), gadolinium(III) nitrate hexahydrate (Gd(NO3)3·6H2O), cobalt(II) chloride (CoCl2), iron(III) chloride (FeCl3), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O), manganese(II) nitrate tetrahydrate (Mn(NO3)2·4H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), and polyvinyl alcohol (PVA) (Mw ~67,000) were purchased from Sigma Aldrich. The chromatography cellulose paper (size: 46 cm × 57 cm × 200 µm) was purchased from Fisher Scientific. Graphene oxide (GO) dispersion (10 mg mL-1) was purchased from Tokyo Chemical Industry Co., Ltd. DCM (99.9%) was purchased from J. T. Baker. Polydimethylsiloxane (PDMS) (Sylgard®184) base and curing agent were purchased from Dow Corning Corporation. All reagents were used as received without further purification. Preparation of Mn+-GO-cellulose complexes The cut or folded paper samples were first soaked into the diluted GO dispersion (5 mg mL-1) for 12 hours, and the samples were then air-dried to achieve the GO-cellulose templates. The GOcellulose templates were soaked into various metal salt solution for another 12 hours and air-dried to obtain the Mn+-GO-cellulose complexes. The concentration was kept at 0.1 M to synthesize MO replica if not declare. Preparation of metalized GO glue The metalized GO glue was prepared by mixing PVA (200 mg mL-1), GO dispersion (1 mg mL1),

and metal salt (0.2 M) in water followed by the ultrasonication (Sonica 5200) for 5 hours. The


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metalized glue was applied on the edges of Mn+-GO-cellulose templates to assemble and enclose the origami unit(s). Calcination process for the removal of templates The Mn+-GO-cellulose origami was then put into a tube furnace (Thermal Craft, Polaris Science Pte., Ltd.) and calcined in air under a heating program as follows: set temperature: 800 °C, ramp time: 1 hour, hold time: 3 hours. The furnace was cooled down to room temperature without control. Stabilization of synthesized MO origami structure with thin elastomer The dilute PDMS solution was prepared by mixing PDMS curing agent and base in a 1-to-10 weight ratio and further diluting the mixture with DCM. The PDMS solution was slowly dripped onto the surface of MO origami structures followed by the curing process at 75 °C for 3 hours in an oven. The PDMS concentration was set as 300 mg mL-1 to prepare samples if not declare. Tensile testing The cellulose paper, templated Al2O3 structures, planar film and auxetic hexagonal origami of Al2O3-elastomer were secured onto a tensile tester (Instron 5543, Instron, USA) with a 500 N load cell. The load cell was calibrated before the testing. The sample was pulled with an extension rate of 5 mm min-1 until the samples were visually broken. Calculation of volume magnetic susceptibility When a paramagnetic material is placed in a magnetic field, a magnetization (magnetic moment per unit volume) (M) is induced in the material and is related to magnetic field intensity (H) by the following equation:


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𝜒𝑣 =

𝑀(𝑒𝑚𝑢 𝑔 ―1) 𝐻(𝑂𝑒)

× 𝜌(𝑔 𝑐𝑚

) = 4𝜋 ×

―3

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𝑀(𝐴 𝑚2 𝑔 ―1) 𝐻(𝐴 𝑚 ―1)

× 𝜌(𝑔 𝑚 ―3)

, where 𝜒𝑣 is the volume magnetic susceptibility (unitless in SI units). According to the slopes of magnetic hysteresis curves and materials densities (Ho2O3 film: 0.07 g cm-3; Ho2O3-PDMS film: 0.1 g cm-3), the volume susceptibilities were calculated. The detailed unit transformation is shown in dimensionless volume magnetic susceptibility.

𝜒𝑣(𝐶𝐺𝑆) =

𝑀(𝑒𝑚𝑢 𝑔 ―1) 𝐻(𝑂𝑒)

× 𝜌(𝑔 𝑐𝑚 ―3) =

𝑀(0.001 𝐴 𝑚2 𝑔 ―1)

(

)

1000 𝐻 𝐴 𝑚 ―1 4𝜋

× 𝜌(10 ―6𝑔 𝑚 ―3) = 𝜒𝑣(𝑆𝐼)

1 𝑒𝑚𝑢 𝑂𝑒 ―1𝑐𝑚 ―3 = 4𝜋 Fabrication of shape memory alloy (SMA)-actuated Al2O3 robot The SMA wires (150-µm-diameter nitinol wire, Flexinol®, DYNALLOY, Inc.) were shaped with a 3D printed mold at 300 °C. The wires were bent into a “U” shape and glued to the surface of the Al2O3 backbone with dilute PDMS solution. The SMA wires at the martensite state was compliant and conformed to the curvature of Al2O3 backbone. When we applied current to heat up the SMA wire, it reverted to the austenite state and straightened out, causing the robotic backbone to inch forward. When current was cut, the wire was cooled down and reverted to the martensite state, pulling the rest of the Al2O3 robot forward. Fabrication of pneumatic Al2O3 robot An Al3+-GO-cellulose bellow origami was assembled with two planar Al3+-GO-cellulose complexes with the Al3+-GO glue (Figure S23). The assembled Al3+-GO-cellulose origami underwent the calcination process at above 500 °C to produce the replicated Al2O3 bellow tube. The Al2O3 bellow tube was further stabilized with a thin layer of PDMS, and a pneumatic channel


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was connected to the Al2O3 tube. The connection between the air channel and the Al2O3 tube was further sealed with a thin layer of PDMS to ensure no air leaks/pores. The pneumatic Al2O3 robot was actuated by an air pump, and the actuating behaviors were recorded by a camera.

Characterization and measurements X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer (Bruker, D8 Advance X-ray Powder Diffractometer, Cu K ( = 0.154 nm) radiation) at a scan rate of 4 degrees min−1. The morphologies of cellulose paper and templated MO structures were characterized by a scanning electron microscope (SEM, FEI Quanta 600) and a high-resolution transmission electron microscopy (HRTEM, JEOL 2010F). Magnetic hysteresis curves were obtained through a vibrating sample magnetometer (VSM, Lake Shore 7304). Thermogravimetric analysis (TGA) was performed using a Shimadzu DTG–60AH Thermal Analyzer with a heating rate of 10 °C min-1. The viscosity of glue was tested by rheometer (AR G2, RESEARCH INSTRUMENTS). The sample size and microfiber diameter were characterized by using ImageJ to quantify the gray-scale line profiles of the optical and SEM images.


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Scheme 1. The GO-enabled templating synthesis for transforming the backbone material of origami robots from cellulose paper to reconfigurable MO metamaterials.


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Figure 1. GO-enabled templating synthesis of MO origami structures. (a) i. Star-shaped cellulose paper and its SEM image; ii. Ho3+-GO-cellulose complex and its SEM image; iii. astemplated Ho2O3 film and its SEM image. The templated Ho2O3 product exhibited high structural replication of both star shape and microfiber network. The sample size and fiber diameter were both reduced after the calcination. (b) TGA curves of i. cellulose paper, ii. GO-cellulose paper, iii. Al3+-cellulose, and iv. Al3+-GO-cellulose under air condition. (c) Photos representing the calcination process at different stages for transforming an Al3+-GO-cellulose auxetic hexagonal origami into a downsized Al2O3 replica. (d) Photo of the four-fold paper origami template and the Al2O3 products synthesized from Al3+-GO-cellulose (w/ GO, red square) and Al3+-cellulose (w/o GO, blue square). (e) TEM image of the Al2O3 nanostructure synthesized from Al3+-GO-cellulose.


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(f) TEM image of the Al2O3 matrix synthesized from Al3+-cellulose. (g) Photos of Al3+-GOcellulose origamis and corresponding downsized replicas in Al2O3, including airplane, boat, bird, and auxetic hexagonal honeycomb.

Figure 2. Metalized GO glue for the assembly of MO origami structures. (a) Purpose of metalized GO glue (Mn+-GO glue). The Mn+-GO glue synthesized a thin MO layer to connect two sides of MO unit(s) after the calcination. (b) i. Side-view photo of a Ho2O3-MgO connector with a stable Ho2O3 joint. Inset is the front view. ii. SEM image with EDS analysis at the joint of Ho2O3MgO film. (c) Photos of the Al3+-GO-cellulose origami assemblies. (d) Photos of the replicated Al2O3 origami structures. Five paper origami assemblies were reproduced into downsized Al2O3


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replicas, including i. ring, ii. Möbius wavy strip, iii. interlocked rings (the Al2O3 rings were colored with blue and yellow for better visualization), iv. auxetic hexagonal tube, and v. bellow tube.

Figure 3. Turning MO origami replicas into deformable MO-elastomer metamaterials. (a) Infiltration of dilute elastomer solution into Al2O3 replica enables the fabrication of Al2O3elastomer metamaterial. i. Top-down SEM image and ii. cross-sectional SEM image of Al2O3elastomer composite. The thickness of Al2O3-elastomer thin film is about 50 µm. iii. The photo of elastomer-stabilized Al2O3 auxetic hexagonal origami. (b) Large deformability of Al2O3-elastomer


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auxetic hexagonal origami (180° bending, 180° twisting, 60% stretching). (c) The patterns of Al2O3-elastomer auxetic hexagonal origami were strain-dependent during the uniaxial compressing (marked with -1, -2, -3) and stretching processes (marked with 1, 2, 3, 4, 5, 6). The figure at lower-left corner represented the initial state.


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Figure 4. MO origami robots with unconventional functionalities. (a) in situ SEM images with EDS mapping of an Al2O3-elastomer origami fold under 60% uniaxial stretching. (b) Fatigue test of an Al2O3-elastomer auxetic hexagonal origami. No significant degradation of stress-strain properties was observed under repeated stretching/relaxation cycles. (c) Pneumatic actuation of an Al2O3 bellow origami robot. (d) Paper pneumatic robot was ignited after the 90-second burning, while the Al2O3 pneumatic robot remained intact. (e) Magnetic hysteresis of cellulose paper, Ho3+GO-cellulose complex, as-synthesized Ho2O3 film, and Ho2O3-PDMS film under an in-plane magnetic field. The Ho2O3-PDMS film showed high volume susceptibility of 7.5×10−6.


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Figure 5. Properties and applications of MO origami robots. (a) The nitinol wires generated high heat (temperature ~250-270 °C) that damaged the paper backbone. The Al2O3-ealstomer backbone exhibited high thermal resistance and remained intact when the nitionol wires were continuouly charged over 5 minutes. (b) Motion recording of the paper and Al2O3 robots actuated by the nitinol wires (150 µm in diameter). Only the Al2O3 robot was actuated by the 150-µmdiameter nitinol wires and demonstrated periodic undulating motions. The paper robot showed nearly no motions. (c) Motion and power input recordings of the paper and Al2O3 robots actuated


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by the nitinol wires (500-µm wires for the paper robot and 150-µm wires for the Al2O3 robot) in one actuation cycle. The current was applied to the nitinol wires at t = 0. Compared with the paper robot, the Al2O3 robot showed much faster response time and 93% lower power consumption. (d) Gas pressure recording of the pneumatic robots with cellulose paper and Al2O3-elastomer backbones at the actuation frequency of 1.5 cycle s-1. The sizes of both robots were the same (diameter 1 cm, length 2.5 cm). (e) A fire-reborn origami robot. A pneumatic paper robot sacrificed itself in a fire scene, and a downsized Al2O3 robot was reproduced and able to crawl through a narrow tunnel where the original robot unfitted. This process was recorded in Movie S7, and the displayed time was listed.


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ACKNOWLEDGMENT We thank Y. Tian, J.-Y. Mi, Z.-J. Tan, and C. Lim for discussion and samples preparation. The authors acknowledge the financial support provided by the Faculty Research Committee (FRC) Start-Up Grant of University of Singapore R-279-000-515-133, the Ministry of Education (MOE) Academic Research Fund (AcRF) R-279-000-532-114, R279-000-551-114, R-397-000-227-112, NMRC Bedside & Bench grant R-397-000-245-511, and the AME Young Investigator Research Grant R-279-000-546-305.

ASSOCIATED CONTENT The following files are available free of charge on the ACS Publications website at DOI: Detailed processes of the GO-enabled templating synthesis; photos of the GO-cellulose template and its replicas in various MOs; XRD patterns of GO and as-templated MO products; EDS results of and atomic ratio table of 11 as-templated MO products; SEM images of the GO-cellulose template and its replicas in various MOs; FTIR spectra of GO-Al3+ film before and after annealing at 150 °C; areal shrinkage of each MO product after calcination; weight of templated Al2O3 products after templated synthesis; thickness of templated Al2O3 films after templated synthesis; areal shrinkage of Al2O3 square replica synthesized from different concentration of Al(NO3)3 solution; templating synthesis of wavy origami structures in various MOs; SEM images of the cellulose template and its replicas in various MOs (no GO templates involved in the templating synthesis); closer view of downsized Al2O3 replicas of boat, airplane, and bird; programmed origami folding patterns used in this work; the commercial glue did not connect two MO units after the calcination; viscoelastic measurement of GO dispersion and metalized GO glue; viscous Ho3+-GO glue to adhere two cellulose papers; photos of a small-sized paper tubular predecessor


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and its Al2O3 replica; thickness of Al2O3-PDMS films fabricated from different concentration of PDMS solution; stress-strain curves of Al2O3-elastomer planar film and auxetic hexagonal origami; stress-strain curve of cellulose paper; stress-strain curve of as-templated Al2O3 structure; assembly of an Al2O3-elastomer bellow tube; flame retardancy of paper-elastomer and Al2O3elastomer origamis; a paramagnetic roll of Ho2O3-PDMS; motion recording of the Ho2O3elastomer wings actuated by ultrasonic pulses. Movie S1. The transformation process of auxetic hexagonal origami of Al3+-GO-cellulose into downsized Al2O3 replica though the high-temperature calcination. Movie S2. The folding procedures of boat, airplane, bird, auxetic hexagonal honeycomb, and bellow origamis. Movie S3. A pneumatic paper robot was damaged and ignited under the burning of ethanol flame, while the pneumatic Al2O3 robot was able to sustain the direct flame contact and still crawled through the fire scene. Movie S4. A paramagnetic Ho2O3-PDMS roll can climb the hill, jump into a water channel, and sail across the channel. Movie S5. Two Ho2O3-PDMS wings flapped under ultrasonic pulses. Movie S6. An Al2O3 robot was actuated by SMA wires and exhibited an inchworm motion. Movie S7. A paper robot sacrificed itself in a fire scene and produced a downsized Al2O3 robot that was able to crawl through a narrow tunnel.

AUTHOR CONTRIBUTIONS P.-Y. C., H. R., H. Y., and B.-S. Y. designed the research. H. Y. carried out the material synthesis


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and characterizations. H. Y., T.-H. C., and K. L. conducted tension test and magnetic measurements. B.-S. Y. and H. Y. designed the origami robots and performed actuation demonstrations and analyses. P.-Y. C., H. Y., and H. R. co-wrote the manuscript. B.-S. Y., T.-H. C., K. L. and F. F. were involved in the discussion and manuscript modification. P.-Y. C and H. R. supervised the project.


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Graphene Oxide-Enabled Synthesis of Metal Oxide Origamis for Soft

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