Dual-Mechanism and Multimotion Soft Actuators Based on

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Applications of Polymer, Composite, and Coating Materials

Dual-mechanism and multi-motion soft actuators based on commercial plastic film Linpeng Li, Junxing Meng, Chengyi Hou, Qinghong Zhang, Yaogang Li, Hao Yu, and Hongzhi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00396 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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ACS Applied Materials & Interfaces

Dual-mechanism and multi-motion soft actuators based on commercial plastic film Linpeng Li†, Junxing Meng†, Chengyi Hou*,†, , Qinghong Zhang‡, Yaogang Li‡, Hao Yu*,†, and Hongzhi Wang† †

State Key Laboratory for Modification of Chemical Fibres and Polymer Materials,

College of Material Science and Engineering, Donghua University, Shanghai 201620, China ‡

Engineering Research Center of Advanced Glasses Manufacturing Technology,

MOE, Donghua University, Shanghai 201620, China

KEYWORDS: soft actuators, polyethylene, anisotropy, multi-response, dual-mechanism

ABSTRACT: Soft actuators have attracted a lot of attention owing to its biomimetic performance. However, the development of soft actuators that are easily prepared from readily available raw materials, conveniently utilized and cost-efficient is still a challenge. Here, we present a simple method to fabricate a polyethylene-based soft actuator. It has controllable anisotropic structure and can realize multiple motions including bi-directional bending and twisting based on dual mechanisms, which is a rare phenomenon. Especially, the soft actuators can response at a very small 1 ACS Paragon Plus Environment

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temperature difference (∆T≥2.3 °C), therefore even skin touch can quickly drive the actuator, which greatly broadens its applications in daily life. The soft actuator could demonstrate a curvature up to 7.8 cm-1 accompanied by powerful actuation. We have shown that it can lift an object 27 times its own weight. We also demonstrate the application of this actuator as intelligent mechanical devices.

INTRODUCTION

Soft actuator is a nascent environmental responsive device aiming to provide safer, more bio-friendly soft robot system that is more concerned with wearability compared to traditional rigid actuator.1-5 Soft actuators are usually made of highly deformable materials or composites that can convert chemical or physical energy into mechanical work in response to various environmental stimuli. They can exhibit flexible motion through shape or volume changes due to accumulation and integration of microscopic conformational changes at the molecular level into a macroscopic large deformation of the actuator materials.6

The soft actuators can be classified by materials those always have their own advantages and disadvantages. Gel actuator can be driven by salt,7,8 solvents,9 heat,10,11 electricity and ionization,12,13 but most have merely single responsive form, i.e., bending. Liquid crystalline gels can achieve multiple forms of deformation, yet the high-cost and complex synthesis process restrict the large-scale application.14 Conductive polymer shows favorable dimensional change resulting from the electrochemical doping,15 but an electrolyte environment is always required. 2 ACS Paragon Plus Environment

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Electro-responsive elastomer is voltage responsive polymer in which high efficiency and durability are expected, while the drawbacks are obvious such as high driving voltages (>1000 V).16 Super-aligned carbon nanotubes (SACNT)/polymer actuator, whose actuating behavior is mainly relay on the orientation of CNT, would be attractive because of rapid and large deformation,17,18 but it is difficult to deploy as daily robotic tools considering the complicated SACNT fabrication technology. Overall, low-cast and multi-responsive actuator is still an urgent need for practical applications.

The goal of this work is to convert inexpensive polymers into soft robot components that achieve the performance of large, rapid deformation and the ability of lifting heavy load, and are also suitable for large scale fabrication. With the development of living economy, polyethylene based materials, such as plastic wrap and zip-lock bag, play an important role in daily life. As these commercial polymer products are processed and shaped by blow molding, the shear stress fields induce preferred orientations in the molecules, making the anisotropic physical properties that vary significantly with direction, which inspires us to employ them in soft actuator.

Here, we present a simple and cost-effective method for fabrication of a polyethylene-based bilayer actuator, which can realize bi-directional and multiple motions that are driven at a very small temperature difference (∆T≥2.3 °C). The actuator simultaneously meets the requirement for practical application and shows 3 ACS Paragon Plus Environment


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powerful actuation (lifting a 27 times own weight object) under various stimuli including heat, electricity, near infrared light (NIR) and solvents. This actuator is expected to have a promising prospect in soft robots and wearable devices.

RESULTS AND DISCUSSION

Design and fabrication of the soft actuator. A low-cost and multiple-responsive soft actuator is designed based on following principles: 1) Simple structure and actuation mechanism: A bilayer film that is made up of two materials having large-differential coefficient of thermal expansion (CTE). 2) Home-made: Materials are easily obtained in daily life and processed using common tools. 3) Multi-function: One of two materials is responsive to multiple stimuli. The spray coating method is much plainer than wet chemical reactions reported previously19,20.

Figure 1a shows CTE of several common materials, such as low density polyethylene (LDPE), polypropylene (PP), etc., among which LDPE have a maximum CTE value. In contrast, electrical and thermal functional materials including copper, aluminum and carbon nanotube (CNT) typically show much lower CTE, which are good candidates for our actuator design. Figure 1b depicts the method for fabricating a bilayer film with CTE differential. Typically, commercial LDPE films that are made through blow molding process is used without further treatment to keep the orientation of polymer chains. Functional layer with hundred-nanometer thick is prepared by spraying different functional material on LDPE layer. Using this easily accessible way, a film actuator with a typical size of 1.3 m × 0.5 m was accomplished 4 ACS Paragon Plus Environment

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showing the possibility for large-scaled manufacture.

Figure 1. (a) Coefficient of thermal expansion (CTE) of typical common materials with decreasing values along the arrow direction , including low density polyethylene (LDPE), polyvinylidene fluoride (PVDF), polypropylene (PP), polycarbonate (PC), polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polydimethylsiloxane (PDMS). (b) Schematic illustration of the fabrication process of the soft actuator and digital photograph of the actuator with a large scale (1.3 m × 0.5 m). (c) Actuator consists of a polymer layer, and a spraying layer containing the functional group. Imaginary lines represent the interaction force between acidized single-walled carbon nanotube (a-SWCNT) and polymer molecule. (d) Overlying image shows dramatic and fast response of the actuator on a hand.

However, different functional materials show various performances during fabrication. Electric conductive materials including acidized single-walled carbon 5 ACS Paragon Plus Environment


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nanotube (a-SWCNT), silver nanowire, and reduced graphite oxide (rGO) were used to coat LDPE film, respectively. It is notable that a-SWCNTs have a better mechanical contact to the polymer film, while silver nanowire and rGO coatings are fragile and easily fall off from the base. The critical factor for stablizing the interface of the bilayer film is the surface engineering of the function materials. X-ray photoelectron spectroscopy (XPS) spectra of silver nanowires, a-SWCNT and rGO are shown in Figure S2. their oxygen contents are calculated to be 6.85%, 33.7%, 7.08%, while hydroxyl contents are 1.66%, 5.06%, 0.85%, respectively. This result indicates that a-SWCNT contains more oxygen-containing functional group, owing to that acidification can graft a lot of functional groups, such as hydroxyl and carboxyl.21 The key to improving binding force lies on these functional groups, which facilitate the formation of hydrogen bond at the interface.22,23 In addition, CTE of LDPE (>200 ppm K-1) is much larger than that of inter-tube CNT (42 ppm K-1).24 Therefore, a stable and thermal-responsive a-SWCNT/LDPE thin-film actuator is expected.

Surprisingly, we found that even body temperature can drive the actuator, which greatly broadens the application in real life. Once the actuator is put on the hand, it responses immediately to reach a large bending angle (90 degrees) within 1 second. To eliminate the effect of moisture differential in the microenvironment of naked hand, we tested it on a gloved hand and obtained the same results. (Movie S1)

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Figure 2. (a) Schematic image of the strips cut from one actuator film. Black lines represent the polymer orientation direction. Three strips were cut along the orientation direction, perpendicular to the orientation direction and 45 degrees to the orientation direction. (b) Schematic diagram of the molecular structure in different states. In state 1, the polymer is after blow molding, which is commercial product state. In state 2, the polymer is further cold-drawn. The polymer expands and shrink in state 1 and 2 under heating, respectively. Inset shows 2D-XRD and AFM images of the LDPE film in State 1 (upper panel) and 2 (lower panel). Scale bar: 500 nm. (c) Actuators fabricated from a-SWCNT and LDPE in State 1 and 2 bend toward opposite directions under heat.

Multi-motion and its mechanism. Commercial LDPE fabricated by blow molding has a certain degree of molecular orientation (0.55), therefore it is not surprising that the actuating behavior of the bilayer actuator is strictly relative to the polymer layer 7 ACS Paragon Plus Environment


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orientation. As shown in Figure 2a, strip-shaped actuators with different polymer alignment (0°, 90°, and 45°) can be obtained, their thermal-introduced deformation could thus vary from bending along the length and width to twisting into helix. (Movie S2)

The actuating mechanim is understood as follows: During blow molding process of LDPE film, the longitudinal stretching (along the blowing direction) is relatively larger than transverse stretching, which results in the higher orientation degree of molecular chains in longitudinal direction, defined as State 1. As shown in Figure 2b, two dimensional X-ray diffraction (2D-XRD) pattern proves a certain orientation degree in State 1, which indicates the anisotropic molecular structure. The anisotropic structure further leads to the anisotropic CTE.25 CTE along the longitudinal direction (MD, 479.71 ppm/K) is much larger than the transverse direction (TD, 145.50 ppm/K), which are tested on thermal mechanical analyzer (TMA) under the same condition. As for the bilayer structure in State 1, the LDPE layer has a larger CTE, leading to bending the actuator toward the CNT layer which has a much smaller CTE.

However, a dramatic change in bending direction is found if LDPE is cold drawn after blow bolding (State 2). Figure 2c shows totally opposite bending motions of States 1 and 2. 2D-XRD images indicate that the film in State 2 has a larger orientation index (0.90) than State 1 (0.55), as calculated in Note S1. As a result, CTE of the film along drawing direction changes to -978.50 ppm/K, whereas the


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crystallites along the chain (∥ ) axis only have -130 ppm/K.26,27 So the amorphous region of polymer chains may plays an important role in the negative CTE.

Under heating, State 2 shows a negative elongation differing from the positive one in State 1. Before cold drawing, the lamellae will not form a periodic structure in axial direction and grow laterally outward, according to the AFM picture (Figure 2b), which is consistent with the reference.28 The intertwined lamella constitutes an interlocked lamella assembly instead of well-separated rows.29 But when cold drawing is applied, the structure forms row nuclei, and the lamella grows perpendicular to them30 (see AFM image in Figure 2b), resulting a periodic arrangement. Besides, the original crystalline lamellae break, and subsequently rearrange into new blocks which align along the draw direction31. This process promotes row nucleation28. So the amorphous region contributes the most to the film contraction. With increasing draw ratio, the fraction of the tie molecules increases and further promotes the contraction of the amorphous regions.32 More detailed information can be found in Note S2. The above results promise changeable bending and multiple motions of the bilayer actuator.



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Figure 3. Actuating performance of the actuator under electricity or NIR. (a) Digital photographs showing the a-SWCNT/LDPE actuator under DC voltage of 9V (upper panel) and 250 mW/cm2 NIR (lower panel). (b) Temperature and curvature as a function of time with a DC voltage of 9V and 250 mW/cm2 NIR. (c) Photographs showing the response of actuators with different orientations under NIR. (Scale bar: 1 cm)

Actuation performance of the bilayer actuator under electricity or NIR. For further study, an actuator with a typical thickness of 44 µm (42.5 µm LDPE layer and 1.5 µm a-SWCNT layer, see also Figure S3) was cut into U-shape strip to form a circuit loop, in which the orientation of LDPE is parallel to the longitudinal direction of the strip. Figure 3a shows typical motions of the actuator triggered by electricity and NIR. The upper panel presents the electrical actuating under a DC voltage (9 V) with two electrodes connected to the actuator. The sheet resistance of the a-SWCNT layer is 4.73 Ω/◻. During the test, the current and power generated are mearsured as 10 ACS Paragon Plus Environment

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214 mA and 1.926 W, respectively. It shows excellent actuating behavior that can form a tubular shape within 1 s, with a maximum curvature exceeds 7.8 cm-1 and a bending angle more than 1440° (Movie S3). This performance is superior to most previously reported electro-thermal polymer actuators. Similarly, the stripe actuator shows fast response under NIR irradiation as well. According to the equation:

= ∆ ∆

(1)

where ∆α is coefficient difference of thermal expansion, ∆T is the temperature variation, h is the polymer film thickness, L0 is the initial length, and θ is the bending angle. The actuation performance θ is proportional to polymer film length, CTE, temperature differential, and inversely proportional to film thickness (see also Note S3).

Above performance can be attributed to the Joule-heating effect and NIR absorption of the CNT layer. As shown in Figure 3b, the temperature of the actuator increases immediately when stimulus on. The heating effect under electricity (9 V) and NIR (250 mW/cm2) are almost the same, and the saturated temperature (slightly lower than 55 °C when ambient temperature is around 26 °C) is reached within 3 seconds. Notably, the actuator can exhibit large deformation under small temperature variation. A bending degree larger than 360o was observed when the surface temperature of the actuator increased less than 10 °C in 1 second. Owing to the low energy requirement, the driving voltage can thus be further reduced. The actuation performance under 3, 4, and 5 V applied voltage was measured (Figure S4 and S5). 11 ACS Paragon Plus Environment


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Under 3 V, The temperature increased only 2.3 °C but the actuator achieved a curvature of 0.5 cm-1. When applying 9 V, bending degree exceeded 1440o, while the curvature reached 7.8 cm-1 in 2 seconds. More importantly, multiple motions can be achieved. Figure 3c shows the multiple responsive behaviors of the stripe actuators under NIR. The stripe actuators are able to bend along the length, width directions, or twist into helix as they are cutted along different molecular orientation.

Figure 4. Actuating performance of the actuator in solvents. (a) Schematic illustration of the polymer swelling process. The color depth represents the concentration of the solvent. (b) Digital photographs showing multiple motions of a-SWCNTs/LDPE actuator in chloroform vapors. (c) The length changes of the soft actuator in different solvents. (d) Solubility parameter (δ) of different solvents and PE.


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Actuation performance of the bilayer actuator in solvents. Interestingly, owing to the inherent swelling/deswelling property of LDPE film, the bilayer actuator can also be triggered by solvent molecules. As illustrated in Figure 4a, solvent molecules can enter into polymer amorphous region and weaken intermolecular force, resulting in the macroscopic expansion. The actuation can be easily triggered by altering the atmosphere using solvent vapor. As shown in Figure S6, a 150 mL beaker containing 25 mL chloroform was used. Once the strip actuator that cutted along the orientation direction was put above the solvent, it immediately bent toward the a-SWCNT layer. The motions can be clearly observed, as shown in Figure S7. The actuator bent nearly 360 degrees in 1 second and recover to its original shape once the solvent was removed, in which process a maximum curvature of over 12 cm-1 was achieved (Figure S8).

Control experiments were conducted to confirm the swelling effect of LDPE. A piece of rolled LDPE film was put in different media, e.g., chloroform and water, as shown in Figure S9 and Movie S4. After 30 seconds, it expanded in chloroform but maintains its original shape in water (even for 3 hours). This result suggests that the LDPE swells in chloroform. Depending on the cutting directions, the strip-shaped actuators can bend along the length/width or twist into helix (Figure 4b), similar to its thermal-responsive performance.

In addition, we have demonstrated that the soft actuator is responsive to various solvents. As shown in Figure 4c, we tested its response in n-hexane, cyclohexane, 13 ACS Paragon Plus Environment


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chloroform, dichloromethane, acetone, N,N-dimethylformamide (DMF), and dimethylacetamide (DMAC). The actuator shows fastest response in chloroform, then n-hexane and cyclohexane. Its response period for reaching 360 bending degrees is shown in Figure S10. However, in DMAC and dichloromethane, slight shape changes were observed, while almost no changes occurred in acetone and DMF.

According to the Hildebrand equation, ⁄

∆ = [

⁄

− ]

(2)

where ∆H is the enthalpy change in mixing. φ1, φ2 are the volume fraction of polymer and solvent, respectively. V is the volume of the mixture. ε1, ε2 are cohesive energy density of polymer and solvent, respectively. = ⁄ is the solubility parameter of polymer or solvent.

Solubility parameter of solvents and polymer presents the intermolecular force, including hydrogen bonding, dispersion interactions and polar cohesive energy.33 During the swelling process, the stronger the interaction between molecules, the ⁄

easier it is to swell. In equation (2), the closer the

⁄

and , the smaller the

enthalpy changes, and it is beneficial to polymer swelling. As shown in Figure 4d, the δ of chloroform, n-hexane and cyclohexane are closer to δ(PE),34 therefore the interaction between these three solvents and PE is relatively stronger compared to other solvents.


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Figure 5. Mechanical property of the actuator. (a) Digital photograph of the actuator (0.018 g) lifting a 0.5 g loading.. (b) Optical images showing actuator with a cross structure holding an object driven under 3.6 Sun. (c) Photographs of the walking device under 250 mW cm-2 NIR. (d) Optical photos showing the actuating process of two-layer (LDPE/ a-SWCNT) and three-layer (LDPE/ a-SWCNT /BOPP) ribbon under NIR. (The illustration above represents their structure.)

Mechanical performance. The application of this actuator as intelligent mechanical devices is finally illustrated based on its multi-responsive and versatile actuating behaviors. First, the load-carrying capacity of the actuator was measured. As shown in Figure 5a, a PDMS block with a weight of 0.50 g (more than 27 times of the actuator) is placed on the the actuator film (0.018 g, 4 cm × 1 cm). When applying NIR, the actuator can lift the heavy object upward nearly 0.7 cm (Movie S5). The output, work efficiency (0.0019 %) and power density (1.06 W/kg) of this actuator are 15 ACS Paragon Plus Environment


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also studied (see Figure S11 and Note S4). More importantly, the ability of actuating materials to resist damage is investigated in this work. We compared the maximum tensile stress and corresponding strain of a series of actuating materials18,19,35-41 before and after being deliberately damaged at a certain degree (Figures S12 and S13). It is found that the LDPE used in this work have durability advantages over other previously reported CTE-based actuating materials. Besides, it is also found that this actuator has a good UV-resistant property (Figure S14).

Versatile mechanial performance of this soft actuator are further investigated. As shown in Figure 5b, two strips of 5.5 cm × 1.1 cm × 50 µm were glued together to form a cross structure. Under 3.6 Sun, the actuator can lift and move object that 3.5 times its own weight. Furthermore, actuator can be fabricated into a walking device (see also Figure S15), as shown in Figure 5c.

The actuator in ribbon shape can deform into a three dimensional structure, i.e., helical structure (Figure 5d, left panel). Owing to that very low driving energy is required, the bilayer soft actuator has a quick and reversible response, e.g., spiraling 12 laps in 2 s (Movie S6). The helical structure is also alterable and found to be relative to the thickness of the actuator.

As is known to all, material stiffness is described as

= =

(3)


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where E is elasticity modulus, I is second moment of area, b is width, and h is the thickness of the thin-film actuator. It is clear that even a slight increase in film thickness can greatly improve the stiffness of the actuator. As a result of which, the diameter and height of the as-deformed helical structure change remarkably. For instance, when a bi-axially oriented polypropylene (BOPP) is selected (based on its low cost and suitable CTE) to cover the a-SWCNT layer, the helical actuator contracts further to give a large strain (57.1%) (Figure 5d, right panel, and Movie S7).

CONCLUSIONS

In summary, we utilized the basic physical property of polymer material to develop high-performance actuators through simple, scalable, and cost-effective approaches. The LDPE/a-SWCNT actuator could be driven at a very low temperature difference (∆T≥2.3 °C), even body temperature could quickly drive it due to the large CTE difference between LDPE and CNT. Furthermore, we used the anisotropic property of LDPE to endow the actuator with multiple motions. The actuators exhibit reversible shape-changing behavior with curvature up to 7.8 cm-1 and bending angle of 1440°. The actuation is powerful that can lift an object 27 times its own weight. This actuator is expected to have a promising prospect in soft robots and wearable devices.

EXPERIMENTAL SECTION

Synthesis of a-SWCNT Dispersion: 500 mg SWCNT and mixed acid (H2SO4:HNO3 = 30ml:10ml) were put in three-neck flask and reflux reacted at 70 °C for 8 hours. After cooling, the mixture was poured into 400 ml deionized water and 17 ACS Paragon Plus Environment


stirred overnight, then filtered and washed. The wet production was freeze drying for several days, then cut into pieces and disperse in ethanol. The a-SWCNT dispersion used for spray-coating was 0.5 mg/ml.

Fabrication of LDPE/a-SWCNT Actuator: The fabrication process of the LDPE/ a-SWCNT is illustrated in Figure 1b. Commercial LDPE films were used as received. A single layer of LDPE was put in the heating stage which surface temperature was 70 °C, then certain amount of a-SWCNT dispersion was sprayed onto it to make a conductive layer. The obtained LDPE/ a-SWCNT film was dried at room temperature. And different orientation films were obtained through cutting the stripes along different direction. (Figure 2a)

Fabrication of helical structure actuator: BOPP films were commercial products. The total thickness of the BOPP film is 40 µm, coated with acrylic as adhesive that can combine the LDPE/ a-SWCNT film and BOPP film tightly. The LDPE/ a-SWCNT /BOPP stripe was cut along the molecluar orientation.

Fabrication of walking device: The LDPE/ a-SWCNT /BOPP actuator strip was cut along the molecular orientation. Two different lengths of paper covered two ends of the strip, thus making the middle actuating part being exposed to the air.

Characterization of the actuators: The twisting and bending motions of the actuators were taken with a charged coupled device (CCD) video camera (EOS D7000, Nikon). Electro-thermal actuation was carried out using Keithley 2400 as the power supply. The surface temperature of the actuator was measured through a FLIR 18 ACS Paragon Plus Environment

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Automation & Science Cameras A300 infrared thermometer. An IR light source (250Wmax, Philips BR125) was used to produce NIR. 2D-XRD patterns and spectroscopy were characterized on a Bruker D8 diffractometer with Cu Kα irradiation (λ = 1.5406 Å) and a VÅNTEC-500 Area Detector. The operating voltage and current were 40 kV and 40 mA. CTE measurement was carried out on thermal mechanical analyzer from METTLER TOLEDO. Mechanical property was tested through Instron Model 5969. AFM images were recorded using an AFM (Nanoscope IV SPM, Digital Instruments). The sheet resistance was measured by a portal four-point probe system (MCP-T360, Mitsubishi Chemical, Japan). The stress generated by the actuator and the ability to resist damage were measured on a universal testing machine (Instron Model 5969, Instron). An UV light (80 mW/cm2) was used in UV-resistant tests.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website.

XPS spectra; FESEM images; characterization; actuation performance; walking device; damage resistance ability; calculations; analysis of the thermal contraction of LDPE (PDF)

Actuation behavior on a gloved hand (AVI) 19 ACS Paragon Plus Environment


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Strip-shaped actuators with different polymer alignment (AVI)

Actuation performance of the actuator under 9 V (AVI)

Rolled LDPE film put in chloroform (AVI)

Load-carrying capacity (AVI)

Actuation performance of LDPE/a-SWCNT driven by NIR (AVI)

Actuation performance of LDPE/a-SWCNT/BOPP driven by NIR (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

*E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge the financial support by Natural Science Foundation of China (No. 51672043, 61674028), The Shanghai Natural Science Foundation 20 ACS Paragon Plus Environment

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(15ZR1401200), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Program of Shanghai Academic Research Leader (16XD1400100), Science and Technology Commission of Shanghai Municipality (16JC1400700), Innovation Program of Shanghai Municipal Education Commission (2017-01-07-00-03-E00055), and the Program of Introducing Talents of Discipline to Universities (No.111-2-04). Dr.Chengyi Hou thanks the Shanghai ChenGuang Program (15CG33), the Shanghai Natural Science Foundation (16ZR1401500), the Shanghai Sailing Program (16YF1400400), and Young Elite Scientists Sponsorship Program by CAST (2017QNRC001).

REFERENCES 1.

Bartolozzi, C.; Natale, L.; Nori, F.; Metta, G. Robots With A Sense of Touch.

Nature Mater. 2016, 15, 921-925.

2. Kazerooni, H. Human Augmentation and Exoskeleton Systems in Berkeley. Int. J.

Human. Robot., 2007, 4, 575-605.

3.

Bellingham, J. G.; Rajan, K. Robotics in Remote and Hostile Environments.

Science 2007, 318, 1098-1102.

4.

Chin, S. Y.; Poh, Y. C.; Kohler, A. C.; Compton, J. T.; Hsu, L. L.; Lau, K. M.;

Kim, S.; Lee, B. W.; Lee, F. Y.; Sia, S. K. Additive Manufacturing of Hydrogel-Based Materials for Next-Generation Implantable Medical Devices. Science

Robotics 2017, 2, eaah6451.



5.

Page 22 of 28

Laschi, C.; Mazzolai, B.; Cianchetti, M. Soft robotics: Technologies and Systems

Pushing the Boundaries of Robot Abilities. Science Robotics 2016, 1, eaah3690.

6.

Asaka, K.; Okuzaki, H. Soft actuators: Materials, Modeling, Applications, and

Future Perspectives. Springer: Tokyo, 2014; pp 3-5.

7.

Steinberg, I. Z.; Oplatka, A.; Katchalsky, A. Mechanochemical Engines. Nature

1966, 210, 568-571.

8. Sussman, M. V.; Katchalsky, A. Mechanochemical Turbine: A New Power Cycle.

Science 1970, 167, 45-47.

9.

Zhao, Q.; Dunlop, J. W. C.; Qiu, X.; Huang, F.; Zhang, Z.; Heyda, J.; Dzubiella,

J.; Antonietti, M.; Yuan, J. An Instant Multi-Responsive Porous Polymer Actuator Driven by Solvent Molecule Sorption. Nat. Commun. 2014, 5, 4293.

10. Yoshida, R.; Takahashi, T.; Yamaguchi, T.; Ichijo, H. Self-Oscillating Gel. J. Am.

Chem. Soc. 1996, 118, 1014-1016.

11. Chen, S.; Hu, J.; Zhuo, H. Properties and Mechanism of Two-Way Shape Memory Polyurethane Composites. Compos. Sci. Technol. 2010, 70, 1437-1443.

12. Hirai, T.; Ogiwara, T.; Fujii, K.; Ueki, T.; Kinoshita, K.; Takasaki, M. Electrically Active Artificial Pupil Showing Amoeba-Like Pseudopodial Deformation.

Adv. Mater. 2009, 21, 2886-2888.


Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


13. Kuhn, W.; Hargitay, B.; Katchalsky, A.; Eisenberg, H. Reversible Dilation and Contraction by Changing the State of Ionization of High-Polymer Acid Networks.

Nature 1950, 165, 514-516.

14. Wang, M.; Lin, B.-P.; Yang, H. A Plant Tendril Mimic Soft Actuator With Phototunable Bending and Chiral Twisting Motion Modes. Nat. Commun. 2016, 7, 13981.

15. Baughman, R. H.; Shacklette, L. W.; Elsenbaumer, R. L.; Plichta, E.; Becht, C.

Conducting Polymer Electromechanical Actuators. Springer: Netherlands, 1990; pp 161-170.

16. Pelrine, R. High-Speed Electrically Actuated Elastomers with Strain Greater Than 100%. Science 2000, 287, 836-839.

17. Li, Q.; Liu, C.; Lin, Y.; Liu, L.; Jiang, K.; Fan, S. Large-Strain, Multiform Movements from Designable Electrothermal Actuators Based on Large Highly-Anisotropic Carbon Nanotube Sheets. ACS Nano 2015, 9, 409-418.

18. Chen, L.; Weng, M.; Zhou, Z.; Zhou, Y.; Zhang, L.; Li, J.; Huang, Z.; Zhang, W.; Liu, C.; Fan, S. Large-Deformation Curling Actuators Based on Carbon Nanotube Composite: Advanced-Structure Design and Biomimetic Application. ACS Nano 2015,

9, 12189-12196.



Page 24 of 28

19. Meng, J.; Mu, J.; Hou, C.; Zhang, Q.; Li, Y.; Wang, H. A Flexible Metallic Actuator Using Reduced Graphene Oxide as A Multifunctional Component.

Nanoscale 2017, 9, 12963-12968.

20. Mu, J.; Hou, C.; Wang, H.; Li, Y.; Zhang, Q.; Zhu, M. Origami-Inspired Active Graphene-Based Paper for Programmable Instant Self-Folding Walking Devices. Sci.

Adv. 2015, 1, e1500533.

21. Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 17, 1290-1295.

22. Morokuma, K. Molecular Orbital Studies of Hydrogen Bonds. III. C=O···H–O Hydrogen Bond in H2CO···H2O and H2CO···2H2O. J. Chem. Phys. 1971, 55, 1236-1244.

23. Taylor, R.; Kennard, O. Crystallographic Evidence for the Existence of C-H···O, C-H···N, and C-H···C1 Hydrogen-Bonds. J. Am. Chem. Soc. 1982, 104, 5063-5070.

24. Maniwa, Y.; Fujiwara, R.; Kira, H.; Tou, H.; Kataura, H.; Suzuki, S.; Achiba, Y.; Nishibori, E.; Takata, M.; Sakata, M.; Fujiwara, A.; Fujiwara, A. Thermal Expansion of Single-Walled Carbon Nanotube (SWNT) Bundles: X-Ray Diffraction Studies.

Phys. Rev. B 2001, 64, 241402.

25. Broer, D.; Mol, G. Anisotropic Thermal Expansion of Densely Cross◻Linked Oriented Polymer Networks. Polym. Eng. Sci. 1991, 31, 625-631. 24 ACS Paragon Plus Environment

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26. Kobayashi, Y.; Keller, A. The Temperature Coefficient of The c Lattice Parameter of Polyethylene; An Example of Thermal Shrink̄age Along the Chain Direction. Polymer 1970, 11, 114-117.

27. Davis, G. T.; Eby, R. K.; Colson, J. P. Thermal Expansion of Polyethylene Unit Cell: Effect of Lamella Thickness. J. Appl. Phys. 1970, 41, 4316-4326.

28. Pazur, R. J.; Prudhomme, R. E. X-ray Pole Figure and Small Angle Scattering Measurements on Tubular Blown Low-Density Poly(ethylene) Films.

Macromolecules 1996, 29, 119-128.

29. Zhang, X. M.; Elkoun, S.; Ajji, A.; Huneault, M. A. Oriented Structure and Anisotropy Properties of Polymer Blown Films: HDPE, LLDPE and LDPE. Polymer 2004, 45, 217-229.

30. Keller, A.; Machin, M. J. Oriented Crystallization in Polymers. J.Macromol.Sci.

B 1967, 1, 41-91.

31. Séguéla, R. On the Natural Draw Ratio of Semi-Crystalline Polymers: Review of the Mechanical, Physical and Molecular Aspects. Macromol. Mater. Eng. 2007, 292, 235-244.

32. Choy, C. L.; Chen, F. C.; Young, K. Negative Thermal Expansion in Oriented Crystalline Polymers. J. Polym. Sci., Part B: Polym. Phys. 1981, 19, 335–352.



Page 26 of 28

33. Hansen, C. M. Hansen Solubility Parameters : A User's Handbook; CRC Press: Boca Raton, FL, 2007.

34. Barton, A. F. M. CRC Handbook of Polymer-Liquid Interaction Parameters and

Solubility Parameters; CRC Press: Boca Raton, FL, 1990.

35. Hu, Y.; Liu, J.; Chang, L.; Yang, L.; Xu, A.; Qi, K.; Lu, P.; Wu, G.; Chen, W.; Wu, Y. Electrically and Sunlight◻Driven Actuator with Versatile Biomimetic Motions Based on Rolled Carbon Nanotube Bilayer Composite. Adv. Funct. Mater. 2017, 27, 1704388.

36. Amjadi, M.; Sitti, M. High-Performance Multiresponsive Paper Actuators. ACS

Nano 2016, 10, 10202-10210.

37. Sugino, T.; Kiyohara, K.; Takeuchi, I.; Mukai, K.; Asaka, K. Improving The Actuating Response of Carbon Nanotube/Ionic Liquid Composites by The Addition of Conductive Nanoparticles. Carbon 2011, 49, 3560-3570.

38. Hara, S.; Zama, T.; Ametani, A.; Takashima, W.; Kaneto, K. Enhancement in Electrochemical Strain of a Polypyrrole–Metal Composite Film Actuator. J. Mater.

Chem. 2004, 14, 2724-2725.

39. Taccola, S.; Greco, F.; Sinibaldi, E.; Mondini, A.; Mazzolai, B.; Mattoli, V. Toward a New Generation of Electrically Controllable Hygromorphic Soft Actuators.

Adv. Mater. 2015, 27, 1668-1675.


Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


40. Weng, M.; Zhou, P.; Chen, L.; Zhang, L.; Zhang, W.; Huang, Z.; Liu, C.; Fan, S. Multiresponsive Bidirectional Bending Actuators Fabricated by a Pencil-on-Paper Method. Adv. Funct. Mater. 2016, 26, 7368-7368.

41. Cheng, H.; Zhao, F.; Xue, J.; Shi, G.; Jiang, L.; Qu, L. One Single Graphene Oxide Film for Responsive Actuation. Acs Nano 2016, 10, 9529-9535.



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Table Of Contents


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