Magnetic Driving Flowerlike Soft Platform: Biomimetic Fabrication and

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Magnetic Driving Flowerlike Soft Platform: Biomimetic Fabrication and External Regulation Wei Gao,† Lanlan Wang,§,‡ Xingzhe Wang,*,† and Hongzhong Liu‡ †

Key Laboratory of Mechanics on Western Disaster and Environment, Ministry of Education, College of Civil Engineering and Mechanic, Key Laboratory of Special Function Materials and Structure Design of Ministry of Education, Lanzhou University, Lanzhou 730000, People’s Republic of China § Food Equipment Engineering and Science, Xi'an Jiaotong University, Xi'an 710049, People's Republic of China ‡ State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China S Supporting Information *

ABSTRACT: Nature-inspired actuators that can be driven by various stimuli are an emerging application in mobile microrobotics and microfluidics. In this study, a soft and multiple-environment-adaptive robotic platform with ferromagnetic particles impregnated in silicon-based polymer is adopted to fabricate microrobots for minimally invasive locomotion and control interaction with their environment. As an intelligent structure of platform, the change of its bending, deformation, and flapping displacement is rapid, reversible, and continuously controllable with sweeping and multicycle magnetic actuation. The bending angle of the soft platform (0.2 mm in thickness and 8.5 mm in length) can be deflected up to almost 90° within 2.7 s. Experiments demonstrated that the flexible platform of human skin-like material in various shapes, that is, flowerlike shapes, can transport a cargo to targeted area in air and a variety of liquids. It indicates excellent magnetic-actuation ability and good controllability. The results may be helpful in developing a magnetic-driven carrying platform, which can be operated like a human finger to manipulate biological objects such as single cells, microbeads, or embryos. Especially, it is likely to be used in harsh chemical and physical circumstances. KEYWORDS: magnetic particle-filled composites, stimuli-responsive materials, flexible gripper, biomimetics, magnetic actuation, magneto-elastic coupling offering the possibility to mimic biological movements.1 Due to their intriguing shape or physicomechanical properties, several smart polymeric composites,2−8 such as electroactive polymers,6 liquid crystalline elastomers,7 and shape-memory polymers,8 have been widely applied in wireless and remote actuation fields to realize movement in a reversible way. Depending on the properties of additives dispersed in a soft polymer matrix, various types of actuation principles, including electric,9,10 heat,11,12 light,3,13 and pneumatic stimulus,14,15 have been successfully explored to manipulate artificial muscles,16,17 sensor and actuator,18,19 and some other biomimetic or bioinspired microrobotic systems. Although these driving methods have certain advantages, they sometimes have auxiliary devices that increase their weight and complexity or cause unwanted temperature effects and other adverse effects20 compared with magnetic-driven systems.2,4,5,16,21,22 For instance, electrostatic actuation is difficult to operate in liquid

1. INTRODUCTION For automatic manipulation in complex environments, research in micro/nanomedicine and nanotechnology has drawn growing interest in design of microrobotics or biomimetic micromotors, especially the realization of functionalities mimicking biological systems with lifelike motions by varying external stimuli. During the driven process, its reliability and noninvasiveness toward targets are the most important requirement for microrobotics. This is mainly decided by the actuator that is directly in contact with target objects. Thus, to fabricate the key part in microrobots, stimuli-responsive materials have drawn enormous attention for creating mechanical motion rapidly and mildly under different external stimuli. Conventional robotics usually has some characteristics such as sophisticated structure and control system, high power consumption, and noise of the campaign objective because of its component composed of stiff materials. As compared to hard robotics, soft robotics has many advantages of less mass, lower cost, noiseless locomotion, and stronger adaptability to any irregular object, as well as lower modulus of elasticity © XXXX American Chemical Society

Received: March 16, 2016 Accepted: May 16, 2016

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DOI: 10.1021/acsami.6b03218 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces environments and for large displacements,23 while actuators based on the shape-memory alloy are hard to control and cannot transmit motions to adapt to a shape change of cargo because of their thermomechanical nonlinearities and stiff external characteristics.24,25 In order to provide electrical power or gas sources for pneumatically actuated platforms,14,15 some auxiliary equipment have been presented, which led to an increase in weight and complexity of operating systems. Moreover, limited by the driving mechanism, chemical regulation26 and heat actuation11,12 (i.e., hydrogel) usually work in some specific solution and need to change the characteristics of the whole environment to realize the expected trajectory. For light actuation,3,13 the manipulation method can be attributed to contraction inconsistent with special structure with different thermal expansion, that produced by lightthermal conversion of inclusion phase. On the other hand, the limitations of thermal-driven platforms11,12 may cause some unwanted temperature effect or other adverse effects to objects, such as strong damage during the manipulation, and prevent a quick return to its original state due to the characteristic of thermal hysteresis.5 Thereby, a responsive material of microrobots that is flexible and autoperforming without derivative heat and electricity may be pursued, especially for environments without unique physicochemical properties. Magnetic actuation result in an interesting candidate for the principles mentioned above: it does not require special environment properties (i.e., temperature or conductivity), has fast response to an external magnetic field, and possesses good biocompatibility with various human cells even at relatively high field strengths.13 Many magnetically actuated platforms have been employed as a potential approach to the realization of transmission, vibration absorption, and damping components in the engineering field, and particularly as attractive candidates for a remotely actuated robotic platform to transport and manipulate cargo.4,16,21,27 Fusco et al.13 reported a self-folding, soft microrobotic platform driven by a combination of magnetic field and near-infrared light. This magnetic manipulation system can transport magnetic microbeads encapsulated by near-infrared light, which need the cooperation of two different drive models. Likewise, Nguyen and co-workers20,28 fabricated a thermomagnetically responsive soft microgripper, which uses the deformation characteristic of hydrogel in varying thermal environments to capture an object. With embedded iron oxide nanoparticles, the microgripper can be remotely steered by magnetic field. In fact, these actuators essentially were carried by means of light and thermal stimuli to realize the folding behavior. Using multiple permanent magnetic materials with specific magnetization directions, Diller and Sitti29 developed an untethered magnetic submillimeter microgripper applied to three-dimensional assembly of heterogeneous materials, which possesses the characteristic of complex process of preparation and control and especially can be applied only to objects with a certain shape (i.e., square object).30 Regarding permanent magnetism platform, there exist some limitations: for example, the permanent magnet is more difficult to realize and microfabricate than soft magnetic materials, and some extra force is required to separate them efficiently due to the demagnetizing field.31 Nowadays, researchers are seeking new fabrication methods to pave the way to applications; for example, the responsive platforms should be broadened to adapt to more universal working environments, rather than just air or a particular environment most studied in previous works;

the manipulatable objects should be more extensive; and so on. In recent years, a kind of magnetoresponsive elastomer consisting of poly(dimethylsiloxane) (PDMS) and various concentrations of ferromagnetic particles has been fabricated for sensors, drug delivery, and remote magnetic manipulation.2,4,21,22,32−34 However, to the best of our knowledge, soft composites reinforced with ferromagnetic particles, also called carbonyl iron/silicon rubber composites, have not been applied to the field of transporting cargo to a targeted area, which requires the characteristics of responding quickly during the loading and unloading stage and showing significant fieldcontrolled performance without changing the circumstances of a given system. Herein, an effective method for fabrication of a polymeric biomimetic platform is demonstrated to remotely achieve actuation both in air and in various liquid solutions. Inspired by the photoresponsive behavior of flowers, flowerlike microrobots driven by controllable magnetic strength were fabricated by three-dimensional (3D) laser printing. The platform constituted by silicon rubber matrix filled with ferromagnetic particles can be deflected when the applied external magnetic field gradient is increased and resumed its original shape in the absence of any external magnetic field, which can be attributed to magnetic force acting on the arm and the intrinsic elasticity of the carbonyl iron/silicon rubber composite, as Figure 1

Figure 1. Closing and opening process of a designed flowerlike soft platform with a permanent magnet closer to and then farther from the platform, which can simulate the movement of flowers expanding and closing under the action of light.

shows. This process has been happening similarly to petals of many flowers expanding in the sunshine and closing during the night by really simple control strategies. We prove that the deflection behavior of the petal consisting of the same ingredients is dependent on the imposed magnetic intensity and the structure (e.g., size, length, and thickness). Furthermore, the platform with entirely flexible structures can be used as a gripper that can grasp objects of widely varying shapes in air or complex environments, as shown in this paper.

2. EXPERIMENTAL SECTION 2.1. Materials. Essil 296 (from Axson Technologies Shanghai Co. Ltd.) was selected as the host matrix for preparation of the flexible platform. It is a kind of two-component solvent-free flexible silicon organic polymer in the form of a base compound with a separate hydrosilane curing agent that acts as a cross-linker. The filler particles used were carbonyl iron particles (type CM, from BASF Co.) with a mean size of 7 μm. A cube-shaped neodymium permanent magnet with dimensions 50 × 50 × 50 mm was purchased from Ningbo Permanent Magnetic Materials and used as the driving device. HT201 gaussmeter with accuracy better than 1% was purchased from Hengtong Magnetoelectricity Co. Ltd.. B


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ACS Applied Materials & Interfaces 2.2. Fabrication of Carbonyl Iron/Silicon Rubber Composite. Carbonyl iron/silicon rubber composite was fabricated by mixing carbonyl iron particles into the silicone elastomer at a desired mass fraction. Subsequently, the hybrid system of prepolymer and carbonyl iron particles was stirred thoroughly with a spatula for 10 min at room temperature to ensure uniform distribution of the particles and also to avoid clustering. Then the PDMS-based compound was added at a ratio of 10:1 to the PDMS cross-linker and mixed for about 10 min. To remove trapped air pockets, the resulting mixture was degassed for 30 min in a vacuum oven. In this way, uncured homogeneous composites containing 30, 50, and 70 wt % carbonyl iron particles with respect to the final composite were prepared. During the membraneforming stage, the low-viscosity compound was poured on silicon substrate (50 mm diameter) and then spin-coated for 20 s at 600 rpm (thickness of the petal is 0.5 mm) or for 6 s at 600 rpm and then 30 s at 2000 rpm (thickness of the petal is 0.2 mm). To further eliminate air bubbles, the entire structure including silicon substrate and carbonyl iron/silicon rubber composites was degassed again for the vulcanizing process for about 10 min. Afterward, the sample was cured in a drying oven for 2 h at 60 °C. After vulcanization, the resulting magnetic elastomer membrane was analyzed by scanning electron microscopy (SEM) to recognize the distribution of filler particles. 2.3. Biomimetic Fabrication of Flowerlike Flexible Platform. As soon as the magnetic elastomer membrane was achieved, a laser mark instrument was applied to cut the flowerlike soft pattern with maximum diameter of 9, 13, or 17 mm and inner diameter of 1 mm, as designed by a computer-aided design (CAD) program and shown in Figure S1. From the view of preparation process, the technique is facile and convenient, as it only includes fabrication of carbonyl iron/silicon rubber composite membrane and shape definition by the laser mark instrument. Inspired by the flower opening and closing movement, the flowerlike pattern magnetic membrane was mounted on a conelike epoxy rod (hammerhead of pipettes of 100 μL) to form a lightweight arm. Figure 2 schematically shows the fabrication process of the flexible platform.

indicates that the iron particles were homogeneously distributed within the matrix, compared with the SEM image of pure silicone matrix operated at an acceleration voltage of 20 kV (Figure S2). In order to evaluate the contribution of ferromagnetic particles impregnated in the polymer matrix in the absence of magnetic field, the stress−strain relationships for all samples were measured on a material testing machine, as shown in Figure 3b. It was found that the slope of the stress− strain curve increases with increasing magnetic filler concentration, which means the improvement in strength of such composites can be achieved by incorporation of a few weight percent iron particles. The reinforcement effect is usually ascribed to specific interactions between polymer and reinforcing fillers. Upon addition of rigid particles to a polymer matrix, the filler acted as the cross-linking point and can accelerate the formation of complex network structure. During the deformation process, the movement of molecular chain and the stress transformed is limited.35 Therefore, the composite modulus consistently increases with increasing particle loading since many interactions between polymer segments and inorganic fillers are produced. It is also concluded that these materials exhibited similar human skin-like behavior, as the elastic modulus (E0) was measured to be 1.13, 1.5, 1.83, and 2.17 MPa for PDMS and 30, 50, and 70 wt % carbonyl iron materials, respectively.36 The main feature of using human skinlike material as soft actuators is not to leave any marks or damage to objects during the manipulation and to achieve stable grasping due to area contact.36 In addition, the polymer composite materials have not only the certain intensity but also the high capability of elastic distortion. Figure 4 is the photo of folding behavior of the platform with 70 wt % carbonyl iron concentration. When the pressure is removed, the petal will revert to its original shape from the intricate structures. The left-hand side of Figure 4 shows the folding state constrained by the force acting on the petal, and the corresponding initial state is displayed on the right-hand side, revealing softness and good flexibility when folded. 3.2. Magnetic Actuation of Flowerlike Platform. In order to evaluate the magnetic-responsive behavior of our flowerlike soft platform, one of the petals was selected for measurement and simulation because of its symmetry feature. In fact, the principles of physics that determine bending shape of a petal are rather simple: evenly distributed magnetic force due to the magnetization in an external magnetic field gradient. The magnetic field drives and controls the displacement, and the final shape is set by the balance of magnetic and elastic interaction. Figure 5a shows the deflection of one petal of the platform in the presence of magnetic field. When the permanent magnet approaches, the petal curves sideways to balance magnetic torque; otherwise the petal will revert to its original form in the intrinsic elasticity of carbonyl iron/silicon rubber composite while the field is away. The corresponding real equipment system is shown in Figure S3. Magnetic intensity around the petal is controlled through changing the vertical component of the permanent magnet, and the intensity is monitored by the probe of a gaussmeter positioned at the same height with the petal. At the same time, deformation of the petal is video-recorded by a microscope camera, and the distance change of the tips from the relative initial state is measured by the analysis software. For simplicity, only partial results from some experiments were selected to interpret the impact of magnetoelastic interaction. Figure 5b shows similar deflection curves of the

Figure 2. Fabrication steps for the flowerlike soft platform. 2.4. Mechanical Performance of Carbonyl Iron/Silicon Rubber Composite. Meanwhile, striplike samples with dimensions of 20 × 6 × 3 mm were cut from the sheet of obtained membrane, and the mechanical properties and performance were measured on a material testing machine. During testing, the loading speed of the transverse beam was 10 mm/min by a displacement control, and the maximum tensile displacement was 20 mm. Hooke’s law was introduced to calculate the elastic modulus as E0 = qL/ΔL. Here L is initial sample length and ΔL is the change of sample length as a result of applied tension stress q.

3. RESULTS AND DISCUSSION 3.1. Characterization of Carbonyl Iron/Silicon Rubber Composite. Figure 3a illustrates the SEM micrograph of composite loaded with 50 wt % carbonyl iron particles, which C


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ACS Applied Materials & Interfaces

Figure 3. (a) SEM image of composite samples with 50 wt % carbonyl iron particles, showing magnetic microparticles homogeneously distributed inside the matrix. (b) Engineering stress−strain relationships for samples with different mass fractions of carbonyl iron in the absence of magnetic field.

equals the length of the petal will be attained. Now the bending angle of each petal can reach up to almost 90°. On the other hand, it is evident that the hysteretic effect appears during the loading/unloading process and the areas of reverse circle increase with increasing petal thickness. This could be mainly because of the Mullins effect of the particlefilled rubberlike composite materials, which causes an appreciable change in their mechanical properties resulting from the first deformation.37 However, when the magnet moves away, the petal will recover its original shape quickly to show the carbonyl iron/silicon rubber composite has better recoverability in response to the intrinsic elasticity. Obviously, the response time of the petal is dependent on the rate of change of magnetic field. In our experiment, when the magnet was moving at 500 mm/min, the fastest response time of petal with particle mass fraction of 70% (0.2 mm in thickness and 8.5 mm in length) is 2.7 s, influenced by characteristics of the permanent magnet. It is well-known that the longer the lateral length, the more torque is produced by the external force. As shown in Figure 5c, it can be concluded that the sample length also has a large impact on the properties of magnetic response. It indicates that longer petal is more sensitive to the magnetic field. To further explain the magnetoelastic response of the platform from physics mechanism, a theoretical model is proposed based on linear elastic theory of the cantilevered ferromagnetic beam. For the sake of simplicity in succeeding discussions, only one petal of the flowerlike platform is considered for explanation of the observed behavior, as Figure 5a indicates. In our experiment, a cube-shaped neodymium permanent magnet was used to provide the magnetic energy. The corresponding field intensity as a function of the distance from the magnetic pole was measured with a gaussmeter and calculated with a simpler analytical expression:38

Figure 4. Photo of the platform with 70 wt % carbonyl iron concentration, revealing good flexibility when folded.

petal induced by gradient distribution of the applied magnetic field. These curves can be divided into the following regimes: in the first stage, the deflection increases slowly with increasing magnetic field strength except for the relatively thin petal with a higher concentration of ferromagnetic particles. This phenomenon can be explained by the ability to resist deformation caused by magnetic field. When the magnetic field is applied to the sample, force in the direction perpendicular to the surface is generated and increases with increasing concentration of fine particles. It can be seen, from Figure 5b, that the petals contain more particles to generate a larger deformation at the same magnetic field. Combined with the lower bending stiffness, the relatively thin sample will become even more sensitive to deformation than others. That is, the thinner petal prepared by composite with relatively higher particle content has a quicker response time. With further increase of the magnetic field, the sample undergoes an abrupt shape change in the second stage. As the magnet approaches closely, the magnitude of magnetic field surrounding the sample shows nonlinear growth. The magnetic intensity as a function of the distance from the magnetic pole is shown in Figure S4. Hence, the force acting on the sample is correspondingly increased and the nonlinear distribution may cause the biggest deformation. After this transition, the flat stage where maximum deformation nearly

B (y ) =

⎡ Br ⎢ AB arctan π ⎢⎣ 2(Y − y) 4(Y − y)2 + A2 + B2

− arctan

⎤ ⎥ 2 2 2⎥ 2(C + Y − y) 4(C + Y − y) + A + B ⎦ AB

(1) D


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the magnetization intensity M of the particle can be determined by the Fröhlich−Kennely equation:40 M=

Ms(μr − 1)H Ms + (μr − 1)H

(2)

where Ms and μr are the saturation magnetization and relative magnetic permeability of the iron particles, respectively. Typically μr = 1000 and μ0Ms = 2 T, where μ0 is permeability of the vacuum. The applied magnetic field H can be obtained by H = B(0)/μ0. On the basis of a simple dipole model of ferromagnetic particles filling a rubberlike composite,27,39 the increase in shear modulus respond to an applied magnetic field is given by ⎛ R ⎞3 ΔG = 4.808μ0 ϕ⎜ ⎟ M2 ⎝d⎠

(3)

where R and d are average particle radius and particle distance, 4πR3

respectively, and volume fraction ϕ is given by ϕ = 3d3 . So the magnetic-dependent Young’s modulus is related to the shear modulus by ΔE = 2ΔG(1 + ν) ≈ 3ΔG, where ν (=0.5) is Poisson’s ratio of the composite.21 For a transverse magnetic field, the equivalent magnetic force per unit area exerted on the sample can be expressed as41,42 Fm =

μ0 μm (μm − 1) 2

2 2 + + {[H0z(h)] − [H0z( −h)] }

(4)

where H+0z(h) and H+0z(−h) are the normal components of magnetic intensity of upper and lower surfaces of the sample and h is the half-thickness of the sample. The parameter μm is the effective relative magnetic permeability of the composite, which depends on the material and the volume of the 3ϕ(4 + ϕ) ferromagnetic particle,43 that is, μm = 1 + 4(1 − ϕ) . Due to the relatively short length, the forces that act on the petal at steady state in an external magnetic field gradient can be considered to have uniform distribution in the initial phase. According to the Bernoulli−Euler theory of beams, the deflection w of the beam with thickness 2h, width b, and length L (Figure 5a), is given as follows:44 w= Figure 5. (a) Deflection of one petal of the flowerlike soft platform in the presence of magnetic field. (b) Relationship between deflection and applied magnetic field for the petal with different particle volume fractions and thicknesses. (c) Deflection induced by gradient distribution of the magnetic field for petals with different lengths and thicknesses.

FmbL4 8EI

(5)

where E = E0 + ΔE is Young’s modulus of the specimen in magnetic field, E0 represents Young’s modulus of carbonyl iron/silicon rubber composite in the absence of any magnetic field, and I = 2bh3/3 is the moment of inertia of beam while EI is called the bending stiffness of the beam. It is noted that eq 5 is obtained on the basis of uniform distribution of magnetic force for initial conditions. However, the force will be changed as the sample is deformed, which is the so-called magnetoelastic coupling. To account for these changes, an iterative algorithm was used to solve eq 5 with increasing deformation. The initial deformation w1 was calculated on the basis of the original hypothesis, and then the distribution of magnetic field and the magnetic-dependent modulus induced by the change of distance between sample and magnetic pole was recalculated. Finally, maximum deflection of the beam is achieved through comparing the difference of deformation in a permitted range (ε < 10−6). A detailed flowchart for calculating maximum deformation of the petals in consideration of the magnetic field gradient distribution is shown in Figure S5.

where A, B, and C are the length, width, and thickness of the permanent magnet, respectively. Residual induction of the permanent magnet, Br, is a material characteristic value. The comparison result shows that the expression could be used to correctly simulate the field distribution along its axial line at Br set as 1.3 T, as shown in Figure S4. Actually, the mechanical properties of these composites are variable in an external magnetic field due to interactions among micrometer-sized magnetically permeable particles in the matrix.21,27,39 In this model, all magnetic particles are assumed to be homogeneous spheres that can be treated as identical dipoles and the distances among particles are equal. In this case, E


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ACS Applied Materials & Interfaces Figure 6a−c shows the comparison between experimental data during the loading stage and predictions by the theoretical

composite, etc. If the ferromagnetic particle content is too high, a complicated microstructure can be formed, which may result in inaccuracies of magnetic-dependent Young’s modulus and effective relative magnetic permeability of the composite when the simplified model is used.27,39,43 Hence, even more discrepancy appears for specimens with a relatively higher concentration, as shown in Figure 6b,c. Fortunately, the proposed model is efficient for interpretating the bending deformation mechanism of the petal and prediction of deflection under low curvature conditions, which is very useful for the design of intelligent materials and structure as desired. 3.3. Flexible Gripper: Remotely Driven by Magnet. For microrobotics, it is more interesting if there are prospects of extensive application in various environments. In this section, we will show the remote actuation used as a flexible gripper. At first, the gripper is placed on the top of a cargo with opening wide, and then the magnet is applied and fixed below the experimental bench, as shown in Figure S6. Under the external electromagnetic load, the petals bend to the side of the magnet to grasp the object. Figure 7 briefly shows the flowerlike soft platform used as a gripper to transport cargo in the presence of magnetic field. Each column represents an entire process (gripping, moving, releasing) of trying to manipulate an object from top to bottom. Figure 7a,b shows the gripper can transport a cotton ball with relatively softer surface and a resin plate with cylindrical outer surface. It can be seen that the petals are deformed according to the 3D surface information on an object. This also means that the gripper possesses the characteristic of self-tuning capabilities without introducing excessive stress to the profile of objects. Additionally, the gripper can transfer cargo in various liquid environments, such as water at different temperatures, alkaline or acidic solutions, and so on. Figure 7c reveals the moving of a pill, which measures 6.96 mm in diameter and weighs 0.164g, in surrounding water. However, the weight of corresponding soft gripper is 0.0383 g. From these it could be seen that the gripper made by carbonyl iron/silicon rubberlike composite can transport cargos with different shapes anywhere in the presence of magnetic field. Videos for gripping and transporting cargo in Supporting Information show the platform used as a gripper to transport a cotton ball (Movie 1), a resin plate (Movie 2), and a pill in liquid environment (Movie 3). Movie 4 shows the gripper used to transform a stone weighing 0.214 g in sodium hydroxide solution (pH = 12). We can see that these materials, which combine the advantages of polymers such as flexibility, elasticity, chemical resistance, and biocompatibility with the magnetic properties of additives, have broad application prospects.

Figure 6. Comparison between experimental data and model predictions for the petal: (a) with particle mass fraction 30% and various lengths, (b) with thickness 0.2 mm and various particle mass fractions, and (c) with particle mass fraction 70% and various lengths.

4. CONCLUSION In this work, we present the fabrication of a flowerlike soft platform that can be deflected by magnetic force acting on the petal and resumes its original shape in the absence of any external magnetic field, due to the intrinsic elasticity of the carbonyl iron/silicon rubber composite, like the real locomotion of a flower with pulse and recovery processes. Vertical bending displacement of the petal under various magnetic intensity stimuli was measured and simulated, which can be used to understand the mechanisms behind these associations and for further design of intelligent material and structure as desired. Meanwhile, we imagine that the platform will be available for applications in developing novel large displacement-driven actuators or special holding devices. To

model without any adjustable parameter. It can be seen that the prediction is in qualitative agreement with experimental data to indicate that the proposed model is capable of describing the magnetoelastic response to some extent, especially for composites with low concentrations of particles, as shown in Figure 6a,b. The model could not predict the flat stage because it was based on small deformation assumption without consideration of the effects of axial deformation, variable width of the beam, particle distribution profile in the F


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Figure 7. Demonstration of the flowerlike soft platform used as a gripper to transport cargo (throughout the entire process, the permanent magnet is fixed below the experimental bench). (a) Cotton ball with relatively soft surface; (b) resin plate with outer cylindrical surface; (c) pill in water.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (11172117), Doctoral Fund of Ministry of Education of China (20120211110005), and the Foundation for Innovative Research Groups of the NNSFC (11421062) is acknowledged.

further illustrate the actuation ability, the platform with human skin-like material can be used for soft manipulations, which can transport a cargo with irregular shape or relatively softer surface in various kinds of working environments, as shown. It indicated great potential applications in controllable delivery ability by remote control. We believe the magnetically driven soft platform provides an efficient actuation technology that can be used in many applications over a range of operational environments, such as biomimetic research, drug delivery, micro/nanorobotics, flexible drive structure, and so on.

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(1) Martinez, R. V.; Branch, J. L.; Fish, C. R.; Jin, L.; Shepherd, R. F.; Nunes, R. M.; Suo, Z.; Whitesides, G. M. Robotic Tentacles with Three-Dimensional Mobility Based on Flexible Elastomers. Adv. Mater. 2013, 25 (2), 205−212. (2) Lazarus, N.; Meyer, C. D.; Bedair, S. S.; Slipher, G. A.; Kierzewski, I. M. Magnetic Elastomers for Stretchable Inductors. ACS Appl. Mater. Interfaces 2015, 7 (19), 10080−10084. (3) Jiang, W.; Niu, D.; Liu, H.; Wang, C.; Zhao, T.; Yin, L.; Shi, Y.; Chen, B.; Ding, Y.; Lu, B. Photoresponsive Soft-Robotic Platform: Biomimetic Fabrication and Remote Actuation. Adv. Funct. Mater. 2014, 24 (48), 7598−7604. (4) Fuhrer, R.; Schumacher, C. M.; Zeltner, M.; Stark, W. J. Soft Iron/Silicon Composite Tubes for Magnetic Peristaltic Pumping: Frequency-Dependent Pressure and Volume Flow. Adv. Funct. Mater. 2013, 23 (31), 3845−3849. (5) Wang, S.; Xuan, S.; Wang, Y.; Xu, C.; Mao, Y.; Liu, M.; Bai, L.; Jiang, W.; Gong, X. Stretchable Polyurethane Sponge Scaffold Strengthened Shear Stiffening Polymer and Its Enhanced Safeguarding Performance. ACS Appl. Mater. Interfaces 2016, 8 (7), 4946−4954. (6) Okuzaki, H.; Kuwabara, T.; Funasaka, K.; Saido, T. HumiditySensitive Polypyrrole Films for Electro-Active Polymer Actuators. Adv. Funct. Mater. 2013, 23 (36), 4400−4407. (7) Ohm, C.; Brehmer, M.; Zentel, R. Liquid Crystalline Elastomers as Actuators and Sensors. Adv. Mater. 2010, 22 (31), 3366−3387. (8) Serrano, M. C.; Carbajal, L.; Ameer, G. A. Novel Biodegradable Shape-Memory Elastomers with Drug-Releasing Capabilities. Adv. Mater. 2011, 23 (19), 2211−2215. (9) Deng, M.; Kumbar, S. G.; Nair, L. S.; Weikel, A. L.; Allcock, H. R.; Laurencin, C. T. Biomimetic Structures: Biological Implications of Dipeptide-Substituted Polyphosphazene-Polyester Blend Nanofiber

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03218. Six figures, showing designed flowerlike soft platform, SEM image of pure silicone matrix, equipment systems, magnetic intensity as a function of distance from the magnetic pole and comparison between experimental and simulated data, and flowchart for simulating bending performance of the petal (PDF) Movie 1, showing the platform used as a gripper to transport a cotton ball (MPG) Movie 2, showing the platform used as a gripper to transport a resin plate (MPG) Movie 3, showing the platform used as a gripper to transport a pill in liquid environment (MPG) Movie 4, showing the gripper used to transform a stone in sodium hydroxide solution (MPG)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. G


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