Multi-Responsive Kinematics and Robotics of Surface-Patterned

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Multi-Responsive Kinematics and Robotics of Surface-Patterned Polymer Film Shumin Liang, Xiaxin Qiu, Jun Yuan, Wei Huang, Xuemin Du, and Lidong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04829 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Applied Materials & Interfaces

Multi-Responsive Kinematics and Robotics of Surface-Patterned Polymer Film Shumin Liang,† Xiaxin Qiu,† Jun Yuan,† Wei Huang,† Xuemin Du,‡ Lidong Zhang*

†

Department of Chemistry and Molecular Engineering, East China Normal University, Shanghai,

200241, People’s Republic of China *Corresponding author: Lidong Zhang: E-mail: [email protected] ORCID: Lidong Zhang: 0000-0002-0501-6162 ‡

Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen, 518055,

People’s Republic of China KEY WORDS: Smart materials, polymer films, surface patterning and modification, soft robotics, multi-responsive kinematics

ACS Paragon Plus Environment

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ABSTRACT Soft robots, sensors and energy harvesters require materials that are capable of converting external stimuli to visible deformations, and especially shape-programmable deformations are desired. Herein, we develop a polymer film that can reversibly respond to humidity, heating and acetone vapors with generation of shape-programmable large deformations. Polyvinylidene fluoride (PVDF) film, capable of providing acetone responsiveness, is designed with microchannel patterns on its one side by using templates, and the microchannels-patterned side is then treated with hygroscopic 3-aminopropyltriethoxysilane (APTES) to give humidity/heatingresponsive elements. The APTES-modified microchannels lead to anisotropic flexural modulus and hygroscopicity in the film, resulting in the shape-programmed kinematics depending on the orientations of surface microchannels. As the microchannels align at oblique/right angles with respect to the long axis of the film strips, the coiling/curling motions can be generated in response to the stimuli, and the better motion performances are found in humidity- and heatingdriven systems. This material utilized in self-adaptive soft robots exhibits prominent toughness, powerful strength and long endurance for converting humidity and heating to mechanical works including transportation of lightweight objects, automatic sensing cap and mimicking crawling in nature. We thus believe that this material with shape-programmable multi-sensing capability might be suitable for soft machines and robotics.

1. INTRODUCTION Multiple responsiveness of smart materials is capable of endowing actuators with more operational features which may broaden the application potentials. To attain multiple responsiveness, it normally requires combination of materials that have own responsive


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properties.1,2 Photosensitive elements are integrated into a hygroscopic matrix, resulting in the composite actuator with responses to light and humidity.3 Combination of graphite, paper and polymer composite gives rise to a multi-responsive actuating system which can be driven by humidity, near infrared light and electricity for generation of large-shape deformations.1 These multi-responsive actuating materials have demonstrated promising applications for soft robotics,4,5 artificial muscles6,7 and optical devices.8−10 Multi-responsive materials are expected to accurately express their kinematic nature in a shape-programmed manner.8,11,12 Thus, controllable elements are normally introduced in the system to result in the shape-programmed responses such as directional curling, bending, twisting, and self-folding. The common protocols for introduction of these controllable elements include direct printing,13,14 multilayer assembling,15,16 surface patterns17−19 and light crosslinking.20−23 Among them, the surfacepatterning technique is preferred owing to its facile yet versatile features to direct shapeprogrammed changes of soft materials in response to external stimuli.24−29 Ideal surface patterning techniques should be capable of programming materials’ kinematics, but not reducing the responsive activities and mechanical properties. Especially in multi-responsive systems, the surface patterns must have a synergistic effect that can modulate the kinematics of materials in response to various stimuli. This requires considered design of surface patterns including the patterning materials and methods which is still a big challenge. In multi-responsive systems, the stimuli are expected to be able to bring the materials into broader applications. Humidity is of great importance as one of environment-dependent factors, which is a type of eco-friendly, inexhaustible and ready energy source to trigger hygroscopic materials with large shape deformation.30−36 Thus humidity-responsive materials have attracted more and more attentions. In earlier research, people demonstrated the humidity-responsive


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materials in a humidity-driven single system.37,38 The materials absorbed humidity rapidly, but took long time for the dehydration. As a result, the materials were quickly saturated with humidity and became inactive. To address this issue, this single system was operated accompanying with heating effect, in which the shape changes caused by humidity sorption could be rapidly recovered by the heating-induced rapid dehydration. The combination of humidity and heating can thus dramatically improve the reversibility and activity of materials.39 With the assistance of heating effect, humidity-driven soft robots can perform better in the conversion of humidity energy to mechanical work.40 Acetone vapor is also an ideal energy source in polymer actuating systems. In turn, the materials that respond to acetone vapors can be designed into sensing devices which have huge potentials for acetone-leaking detection in chemical plants and breath acetone detection for type 2 diabetes diagnosis.41 For development of acetone-sensitive devices, Zhao and Yuan reported a type of porous actuator that could be driven by acetone vapors to generate rapid curling, however, shape-programmed behaviors were not done at that time.42 Later on, they introduced aligned carbon nanotubes into a porous film, successfully attaining shape-programmed actuating devices driven by acetone vapors.43 To continue the much-needed developments of these multi-responsive actuating systems, we report a microchannels-patterned polymer film composed of polyvinylidene fluoride (PVDF) and 3-aminopropyltriethoxysilane (APTES) which responds to humidity, heating and acetone vapors by the generation of shape-programmed motility. We focus on the systematic research on the kinematics and robotics of the surface-patterned film strips of PVDF/APTES. We carried out these kinematic investigations in air and in a sealed glovebox with a consideration of the parameters that may affect multi-responsive motility. Through accurately controlling over the


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external stimuli, the motility of surface-patterned film strips might be modulated to satisfy application requirements. These kinematic features are then involved in the design of various soft robots and devices to further demonstrate the robotic features of this multi-responsive material. Not only would this research reveal fundamental theories in the preparation of multi-responsive materials, but also inspire the design of various sensing devices in the future.

2. EXPERIMENTAL SECTION 2.1. Fabrication of the microchannel-patterned silica template: The epoxy resin mold for fabricating the patterned template was prepared by ultraviolet (UV) lithography as follows. A silica wafer was sonicated in acetone and isopropanol, and finally dried in a stream of nitrogen gas. Then, the wafer was treated with oxygen plasma (Weike PDC-M, China). A defined quantity of photoreactive epoxy resin polymer (SU-8 3050; Microchem, U.S.A.) was coated onto the pretreated wafer by spin coating at 500 rpm for 20 s and then at 3000 rpm for 30 s. The wafer was heated for 10 min at 95 °C. A specially designed photomask was attached to the wafer, which was then irradiated with UV light (450 – 450 nm) at 195 mJ/cm2 for 30 s. After irradiation, the wafer was heated at 65 °C for 1 min followed by 4 min at 95 °C, and then rinsed in an SU-8 developer (Microchem) for 2 min to dissolve the unreacted epoxy resin polymer. Finally, the SU-8 mold was obtained after being washed with acetone and dried in a stream of nitrogen gas. The template was obtained with the channel width of 0.5 mm and the spacing of 1.5 mm. The template size was 4 cm x 4 cm. 2.2. Preparation of the microchannel-patterned single-layer PVDF film: Pre-designed silica template was washed with acetone before use. A certain amount of PVDF/DMF solution was homogeneously casted onto the microchannel-patterned side of the template and dried in a


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natural convection drying oven at 70 °C until complete evaporation of DMF solvent. The template was removed to obtain surface-patterned PVDF film. 2.3. Preparation of the microchannel-patterned PVDF/APTES film: The surface-patterned side of PVDF film (PVDF thickness: 83 µm at ridged area; 75 µm at channeled area) was treated with oxygen plasma for 20 s to introduce hydroxyl groups for the better combination with APTES element. After casting APTES solution to the surface-patterned side of PVDF film, the hybrid film was carefully shifted into natural convection drying oven, drying at 30 °C for 8 hours. After removal of the template, the APTES-modified PVDF film was achieved (APTES thickness: 30 µm at ridged area; 50 µm at channeled area). 2.4. Response to humidity in a sealed environment: The processes were operated in a sealed glovebox. A mini humidifier was placed inside the glovebox to change the relative humidity and a cameral was equipped for recording the shape-changing processes of the film strip. The relative humidity was monitored with a hygrometer (Extech, RH350) that recorded the relative humidity and output the results as a textfile. As the relative humidity was slowly increased from 35 to 97%, the surface-patterned film strip of PVDF/APTES exhibited gradually-enhanced curling/coiling. These processes were recorded and analyzed by measuring the curling/coiling angles with time. This sealed environment allowed to establish a linear relationship between relative humidity and curling/coiling angles of the film strips. 2.5. Response to acetone vapors in a sealed environment: The operational processes were similar to that in humidity-driven motion in a sealed environment. Here a mini acetone gas detector (Model: AP-S-C3H60-S; Manufacturer: Shenzhen Empaer Technology Co. LTD.) was equipped inside the glovebox that recorded concentration of acetone vapors and output the


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results as a textfile. The acetone vapor was supplied through a soft pipe. 2.6. Design and assemblies of soft robots: All soft robots were designed and prepared based on the film strips of surface-patterned PVDF/APTES. The excavator-like soft robot and the selfadaptive soft robot gripper were prepared by cutting the surface-patterned film to strips having the microchannels aligned perpendicular to the long axis and assembling them together with double faced adhesive tape. A square plastic foam was utilized as a lightweight object (0.6 g) to demonstrate the working capability of the soft robots of PVDF/APTES. 2.7. Assembling of fully automatic mechanical device: The film strips were designed into a soft motor that was connected to a paper rotor (black disk) at the left side by using a capillary tube and double faced adhesive tape. The right side of the paper rotor was connected to an alumina foil-based lid through the capillary tube and double faced adhesive tape. As the soft motor generated upward/downward movements, the paper rotor would be powered to move with clockwise/anticlockwise rotations to convert humidity and heating to mechanical work.

3. RESULTS AND DISSCUSION 3.1. Preparation and characterizations of the multi-responsive polymer film We prepared microchannels-patterned PVDF single layer by using a silicon template, and the microchannels-patterned side was then modified with hygroscopic APTES (Figure 1a). The lowmolecular-weight APTES was selected, which was deposited on the channeled surface capable of penetrating into the porous PVDF but did not form a membrane at the PVDF surface as confirmed by the microscopic observation (Figure S1 in Supporting Information, SI). As a result, APTES-modified PVDF was still a single-layer film but with respective different responsiveness at the two sides. The neat PVDF film was composed of the porous structures (Figure S2a in SI),


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which is good for the absorption of acetone vapors through the interaction with fluorine groups.42,44 After coating with APTES from the microchannels-patterned side, the coated surface became smooth which hindered adsorption of acetone vapors and reduced the sensitivity to acetone vapors (Figure S2b in SI). As a result, the film adsorbed acetone molecules and expanded from uncoated PVDF side which caused the film to deform unidirectionally toward the APTES side. Prior to the coating, microchannels-patterned side was treated with oxygen plasma to introduce hydroxyl groups at the surface for enhancing the interactions with APTES molecules. The appropriate treating time was 20 s to attain enough surface hydrophilicity as determined with water contact angle (WCA) measurements, in which WCA has been decreased from 95 to 55° by plasma treatment for 20 s (Figure S3 in SI). The APTES-coated surface was analyzed with XPS measurements. The binding energy at 531.88 eV represents the chemical state of organic C-O which indicates that the APTES was grafted onto the PVDF surface (Figure S4b in SI). The binding energy of silicon was detected at 101.68 eV which came from the chemical state of organic silicon, further confirming the successful coating of the APTES element (Figure S4c in SI). The hygroscopic APTES element endowed the film with high hygroscopicity (Figure S5 in SI). Upon exposure to humidity, the film adsorbed humidity and expanded from APTES side and deformed toward PVDF side. This shape deformation recovered as the film dehydrated by heating.


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Figure 1. Preparation of silicon template and PVDF/APTES film strips. (a) The template of silicon wafer which contains microchannel structures on the surface was designed through micro-electro-mechanical system (MEMS) technique. The microchannel structures were transferred on to the PVDF film by solution-casting. The patterned side of PVDF film was then treated with oxygen plasma and coated with APTES solution along the channels. (b) Sketch of top view of patterned PVDF/APTES film that was cut to various strips with microchannels aligned at angles of 90°, -θ and +θ with respect to the L. The aligned angles of microchannels highly directed the kinematics of the strips of PVDF/APTES in response to external stimuli. (c) As the strip was cut out with the surface microchannels aligned at 90° to the L, it was capable of


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absorption of acetone vapors from PVDF side that induced the strip to curve towards the APTES-coated side. Upon exposure to humidity, only APTES-coated side expanded by humidity sorption. This asymmetric humidity sorption resulted in the strip curling to the PVDF side. Humidity-caused curling could be recovered by heating. (d) The microchannels aligned at an angle of +θ with respect to the L of the strip which induced the strip coiling in left handedness by humidity sorption and in right handedness in response to heating and acetone vapors. (e) While the angle was at +θ to the L, the strip showed completely-opposite coiling motility in response to humidity, heating and acetone vapors respectively. This surface modification with APTES did not reduce the mechanical strength of PVDF element (Figure S6 in SI). In the meantime, visible interfacial failure and shape breakage were not observed in the PVDF/APTES film during continual shape deformations (Figure S7 in SI). In stimuli-responsive polymer bilayer system, the interfacial failure would happen if the bilayer is combined physically.39 On the other hand, modification at channeled side of the PVDF film instead of the non-patterned side provided the modified film with reversible and shapeprogrammed motility in response to humidity, acetone vapors and heating (Movie S1 in SI). To quantitatively demonstrate the responsiveness to humidity, heating and acetone vapors, the weight changes of the PVDF/APTES films upon stimuli were monitored. The film increased in weight upon exposure to humidity and acetone vapors and decreased after the stimuli were terminated (Figure S8 in SI). The humidity-desorption process was highly affected by the surrounding temperatures. The higher temperature would result in more rapid humidity desorption in the film (Figure S9 in SI). 3.2. Multi-responsive kinematics analysis


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Driven by humidity, heating and acetone vapors, the kinematics of microchannels-patterned PVDF/APTES film depends on the orientation of microchannels. Figure 1b shows the sketch from top view of the film, demonstrating how to prepare the strips with different orientations of microchannels. As the film was cut to strip with the surface microchannels aligned perpendicular to the long axis L, the strip deformed with rapid unidirectional bending driven by humidity, heating and acetone vapors (Figure 1c). It bent towards the APTES-modified side caused by asymmetric expansion by adsorption of acetone vapors (p = 19 kPa, 15 °C). This deformed shape recovered to its original state by spontaneous release of acetone molecules as the strip was retracted from acetone vapors. This shape recovery process proceeded rapidly and could be finished in seconds owing to high volatility of acetone.

The acetone responsiveness of

PVDF/APTES film was mainly from PVDF element which could interact with acetone molecules.42 Upon exposure to humidity, the strip bent towards the PVDF side with adsorption of humidity from the APTES side. Upon heating, the strip dehydrated rapidly and contracted towards the APTES side. All above kinematic processes supplemented each other and thus could perform repeatedly. The film strips generated more complex mechanical actuations as they were cut out with microchannels aligned at an angle to the long axis, L. As the microchannels aligned at the angle of +θ (0°< θ

Multi-Responsive Kinematics and Robotics of Surface-Patterned

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