Composites Science and Technology 188 (2020) 107973

Available online 28 December 20190266-3538/© 2020 Elsevier Ltd. All rights reserved.

3D printed shape-programmable magneto-active soft matter for biomimetic applications

Song Qi a,b, Hengyu Guo c, Jie Fu a, Yuanpeng Xie a, Mi Zhu a, Miao Yu a,*

a Key Lab for Optoelectronic Technology and Systems, Ministry of Education, College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, PR China b Postdoctoral Station of Optical Engineering, College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044, PR China c Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences, Beijing, 100083, PR China

A R T I C L E I N F O

Keywords: Shape-programmable 3D printing Magneto-active soft materials Biomimetic materials Soft robots

A B S T R A C T

Imitating from natural biology, shape-programmable materials play a significant role in biomimetic applications. Magneto-active soft materials (MASMs) with programmable shapes, which can assume desired shapes under external magnetic actuation as well as potential for applications in actuators, soft robotics, medical care, and bionics. Here, we propose a shape-programming strategy that can quickly design the desired magnetic moment and actuating magnetic fields for MASMs with fast, reversible, programmable, and stable shape transformation properties. This method allows us to program the magnetic moment in the soft matrix by printing diverse magnetic structural elements. The flexible matrix and soft-magnetic 3D printing filament enable the high- performance deformation of MASMs. With these capabilities, various biomimetic structures (inchworm, manta ray, and soft gripper) can be easily fabricated with walking, swimming and snatching functions. The proposed shape-programming strategy would provide an efficient way to fully capitalize the potential of MASMs, allowing researchers to develop a wide range of soft actuators that are critical in soft robotics, medical care, and bionics applications.

1. Introduction

Today, with nature as a source of inspiration, materials scientists have developed ingenious material manufacturing methods to provide soft materials with the deformation behavior of soft organisms. Among them, shape-programmable soft materials belong to a class of active materials whose geometry can be controlled by external stimuli; they are capable of transforming between complex shapes and are applied in actuators, soft robotics, medical care, and bionics [1]. With a wide range of actuations such as electric fields [2–4], chemicals [5,6], heat [7,8], light [9,10], pressure [11,12] or magnetic fields [13–19], soft active materials have the potential to create mechanical functionalities beyond those of traditional machines [20]. In particular, magnetic fields can easily and harmlessly penetrate most biological and synthetic materials and offer a safe and effective actuation method. With the advantages of magnetic field control, magneto-active soft materials(MASMs), which are typically formed by incorporating magnetic particles or discrete magnets into a soft compound, has great application potential for soft sensors and actuators [21–26].

Previously, most researches on MASMs rely on human intuition and

trial and error to get the desired programming shapes. Lacking of instructive programming strategy highly restrict the designing of ver-satile functional structures, moreover, the reversibility and stability in shape transformation also need to be improved. With the development of additive manufacturing technology, 3D printing technology com-bined with magnetic field has emerged in recent years. By using this “3D magnetic printing”, researchers can print desired anisotropic composite materials and magnets [27,28]. Recently, some highlight works also have reported significant progress of shape-programmable MASMs based on advanced manufacturing techniques and novel design concepts [29–35]. In general, the non-uniform magnetic moment that causes magneto-deformation can be generated by gradient magnetic fields or anisotropic magnetization profiles. Designing and controlling magneti-zation profiles is a necessary step toward the programmed shape of MASMs. Generally, the anisotropic magnetization profiles are generated based on non-uniform particle distribution or anisotropic hard magnetic domains. The materials could have anisotropic physical properties by applying a magnetic field during the curing process to form a chain structure in the polymeric matrix [36–39]. By combining the magnetic self-assembly of superparamagnetic nanoparticles and rapid

* Corresponding author. E-mail address: yumiao@cqu.edu.cn (M. Yu).

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https://doi.org/10.1016/j.compscitech.2019.107973 Received 5 October 2019; Received in revised form 1 December 2019; Accepted 24 December 2019

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photopolymerization, Kim et al. have prepared an MASM with aniso-tropic magnetization profiles [29]. The non-uniform magnetic moment of MASM causes the magnetic micro-actuators to actuate in pre-programmed directions under magnetic actuation. Kimura et al. have prepared MASM strips composed of short steel wires and carbon fiber, and with different directions of the fibers, the non-uniform torque causes the strips to bend under the magnetic field [16,40]. Many re-searchers developed MASMs with hard magnetic particles and designed the magnetization profiles by adjusting the hard magnetic domains [30, 31]. These hard magnetic particles dispersed in soft matrix can be magnetized by prestructured magnetic field, and the generated aniso-tropic domains enabled the complex shape-changes of MASMs under low magnetic fields. Lum et al. proposed a universal programming methodology that can automatically generate the required magnetiza-tion profiles and actuating fields for the MASMs to achieve new time-varying shapes [32]. Hu et al. have fabricated a millimeter-scale soft robot with multimodal locomotion by incorporating neodymium iron boron (NdFeB) particles into silicon rubber (SR) [33]. Kim et al. 3D printed programmed hard magnetic domains in MASMs that enable fast transformations between complex 3D shapes via magnetic actuation [34]. Xu et al. reported an UV lithography based method for pattern permanent magnetic particles in polymer matrix [35]. This method can fabricate actuators that have programmable 3D magnetization profiles, complex geometries, and thus new types of mechanical motions.

Generally, hard magnetic particles have a large hysteresis loop, which means that the magnetic field strength H and magnetization M have significant nonlinearities. That is, for a certain magnetic field strength H, the values of magnetization M and magnetic flux density B cannot be uniquely determined, it also related to the magnetization history. On the other hand, magnetized particles could rotate and shift easily in the soft matrix [41], and that the strong magnetic field applied

can cause the magnetic domains to change. Therefore, magnetic-controlled mechanical properties based on the hard magnetic domain exhibit hysteresis and non-linear behaviors [42,43]. It’s difficult to obtain an accurate linear relationship between the magnetic field and the deformation, which bring challenges to controlling the robustness of soft robots and actuators.

In this paper, we propose a novel shape-programming strategy that can quickly design the desired magnetic moment and actuating mag-netic fields for MASMs to yield fast, reversible, programmable, and stable shape transformations. With the advantages of 3D printing, this manufacturing approach enables the magnetic structural elements with any shape, distribution, and orientation to generate anisotropic magnetization profiles. This method allows us to program magnetic moment in the soft matrix, enabling the desired actuation capabilities of the MASMs. The deformation property of shape-programmable MASMs have been studied synthetically, and the related physical mechanism has been proposed. With these excellent capabilities, various biomimetic structures (inchworm, manta ray, and soft gripper) can be easily fabri-cated with walking, swimming and snatching functions under the uni-form magnetic field (UMF). This proposed approach can make up for the shortcomings of the existing programming methods, and opens new avenues to fully capitalize the potential of MASMs.

2. Experiment

2.1. Materials

Polylactic acid (PLA) was purchased from Nature Works. Carbonyl iron particles (CIPs, type: CN, 1-8μm) provided by BASF. Silicone rubber (SR, Ecoflex 0020) provided by Smooth-On, Inc. KH570 and Si69 were provided by Dongfang Chemical Co., Ltd,were employed as silane

Fig. 1. Shape-programming strategy and manufacturing process of MASMs. (a) The flow chart of shape-programming strategy and a programming case for imitation of inchworm. The deformations are simulated by COMSOL, and direction of the UMF is indicated by the red arrows. (b) Schematic illustrations of 3D printing and encapsulation. 3D printing is employed for manufacturing the various magnetic structural elements, which are encapsulated by silicone rubber. (c) The photos of the MASM samples with oriented magnetic structural elements. The radius of the disk samples is 10 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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coupling agent. Materials mentioned above were used as received without further purification.

2.2. Fabrication of the magnetic filament for 3D printing

Fig. S1a illustrated the preparation process of magnetic filament. The mixture was formulated with 6:1 vol ratio of PLA particle and CIPs, after mechanical blending for 30 min, mixture was added into the mixing chamber of hot melt extruder. Silane coupling agent (KH570, 1 wt%) is used to improve the particle dispersion in PLA. With mechanical mixing, the mixture was melt-blended at 200 �C for 30 min. Finally, the mixture was extruded from the nozzle into a filament with a diameter of 1.7 mm.

2.3. Fabrication of MASMs with programmed shape

According to the designed structure, printing the oriented magnetic structural elements and encapsulated mold by a 3D printer with Fused Deposition Modelling (FDM). The highest printing accuracy is 0.1 mm

and it can be adjusted by temperature, discharge rate and print speed. Before potting SR, 2 wt% Si69 is used to modify the surface of the magnetic structural elements for improving interface between magnetic structural elements and SR. After then, pouring the liquid SR into the mold and cured at room temperature. To ensure that the magnetic structure is completely encased, after the SR is cured, remove the MASM and turn the face over to place it in the mold for a secondary sealing. The thickness of the upper and lower sealing skins is about 0.5 mm. Finally, remove the supporting mold and assemble the MASMs into the desired biomimetic body.

2.4. Characterization of MASMs

The magnetic properties (M� H curve) of magnetic filament and CIPs were determined by a vibrating sample magnetometer (Lake Shore 7407) at laboratory temperature. Morphologies of the magnetic filament and MASM samples were examined using a scanning electron micro-scope (SEM; MIRA3 TESCAN) employing 5–10 kV of accelerating

Fig. 2. Shape-change mechanisms and deformation performance of MASM strip. (a) Finite element analysis of the oriented magnetic structural elements evaluated by COMSOL. The strength of the applied UMF is 300 mT, and the direction is indicated by the red arrow. The magnetic flux density is indicated by the color legend, and the direction and magnitude of Maxwell’s surface stress tensor are indicated by the direction and logarithmic size of the red arrows. (b) Magnetization curves (magnetic moment per unit mass) of magnetic filament and CIPs. The inserted photos are SEM images of CIPs and magnetic filament, and the scale bars are 2 μm and 0.5 mm respectively. (c) Photographs of the bending deformation of the MASM strip under 0, 100, and 200 mT, and the scale bars are 5 mm. In this case, the magnetic structural elements with a length of 1.5 mm, width of 0.3 mm, and thickness of 4 mm have distributions in SR. (d) Curve of the bending deformation of the MASM strip under increasing and decreasing magnetic flux density, which is controlled by the current in the electromagnet. The deformation is evaluated by the displacement of the central point of the MASM strip. The test schematic is illustrated in Fig. S1e. (e) Cycle test of deformation performance of the MASM strip under transient magnetic field. The time for each cycle is 1.5 s, and it includes the transient increase and decrease of the magnetic field. The transient magnetic field is generated on the basis of the motion of the permanent magnet controlled by a linear motor, as shown in Fig. S1f. (f) Single-cycle curves of bending deformation and transient magnetic field. The response and reset times are marked. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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voltage. Schematic diagram of the measurement system for detecting the deformation of MASM strip is shown in Fig. S1e. The UMF was generated with a double yoke electromagnet, and magnetic field strength can be controlled by the current I in the coil. The circular pole face has a diameter of 200 mm and the adjustable air gap is up to 180 mm. The magnetic flux density B could be changed from 0 to approximately 2 T by varying the driving current I in the coil of the electromagnet in the range of 0–25 A. The displacement of the center point in the MASM strip was detected by the laser rangefinder. The time-varying magnetic field has been monitored by a real-time monitoring teslameter, and the data acquisition instrument collected the dates with a frequency of 5000 Hz. For the cycle tests, the transient magnetic field is generated based on the motion of the permanent magnet controlled by a linear motor, as Fig. S1f illustrated.

3. Results and discussion

3.1. Shape-programming strategy of MASMs

Fig. 1a illustrates the flow chart of our shape-programming strategy and provide a programming case for imitation of inchworm. The pro-gramming principle is mainly derived from the coupling relationship between magnetic moment, actuation magnetic field and the shape of the MASMs. First, inspired by nature, we can define desired shapes and chose an appropriate UMF actuation. Then, according to the magnetic analysis of MASMs (see Note S1 of Supporting Information), we just need to adjust the direction of the magnetic moment is parallel to the magnetic field, in order to minimize the overall magnetic energy of deformed MASMs. We can program the magnetic moment by designing the shape and distribution of the magnetic structural elements, on this basis, we can determine the structural distribution of deformed MASMs. Next, the mechanical parameters of each structural model are con-structed in the finite element simulation software (COMSOL in this paper). It should be emphasized that the elastic action of the matrix need to be considered to avoid excessive local strain in MASM when pro-gramming the shape of MASMs by simulation. Finally, using the solid mechanics module, the MASM can be restored to its original shape based on the principle of elasticity. Therefore, the structure of the undeformed MASM can be determined, which provides a structural model for sub-sequent 3D printing. In the case of the inchworm, based on different actuation magnetic field, we can program the magnetic moments by adjusting the orientation of the fibrous magnetic structural elements. Here, it should be noted that printing magnetic structural elements is just one of the methods for the design of magnetic moment. Adjusting the magnetic moment can also depend on the particle distribution and magnetic domain.

3.2. Morphology and mechanical characterization

Fig. 1b illustrates the fabrication process of shape-programmable MASMs based on 3D printing. The geometric model of the MASMs is imported into a 3D printer and the magnetic structural elements are hot melt deposited under suitable printing conditions. To obtain excellent deformation property, the magnetic structural elements are encapsu-lated by flexible SR and assembled into different shape-programmable MASMs. The SR must possess the requisite rheological properties for encapsulation and the appropriate mechanical properties upon curing. The storage modulus (G’) of the cured SR in this study is approximately 42 kPa. Fig. S1c shows the optical photographs of oriented magnetic structural elements. The SEM images of the stack layer and particles of the magnetic structural elements are shown in Fig. S1d. The uniform dispersion of particles in PLA enable the printed magnetic structural elements to have isotropic ferromagnetic properties. To investigate the influence of the magnetic structural elements on the dynamic mechan-ical properties of MASMs, samples with different oriented magnetic structural elements are prepared (Fig. 1c). The schematic of the shear

oscillation testes based on the parallel plate rheometer is shown in Fig. S2a. Test results reveal that the dynamic mechanical properties of the MASMs are obviously affected by the orientation of the magnetic structural elements (see Figs. S2b–c and Note S2 of Supporting Information).

3.3. Shape-change mechanisms and deformation performance

To show the feasibility of our concept, an MASM strip containing oriented magnetic structural elements for imitation of inchworm is fabricated. Fig. 2 shows the shape-change mechanisms and deformation performance of the MASM strip. In this case, the magnetic structural elements have a length of approximately 1.5 mm and a width of 0.3 mm. The magnetizing curves of the magnetic filament and CIPs are shown in Fig. 2b, which verifies the excellent soft magnetic properties of the printed magnetic structural elements. The saturation magnetization of the magnetic filament is lower than the CIPs, which can be attributed to the dominant non-magnetic PLA matrix in the unit volume. For simplicity of simulation, the magnetic structural elements are regarded as a homogeneous isotropic magnetic material in finite element modelling. The simulation results of COMSOL show that the magneti-zation of the magnetic structural elements is inversely proportional to the angle with the direction of the UMF, which is indicated by the color legend in Fig. 2a. The direction and magnitude of Maxwell’s surface stress tensor are indicated by the size and direction of the red arrows. Obviously, Maxwell’s surface stress tensor of magnetic structural ele-ments is also inversely proportional to the angle with the direction of the UMF. However, the deflection torque caused by the resultant force of the magnetic force increases first and then decreases with the angle between the element and the magnetic field. This is consistent with the analysis in the Note S1 of supporting information. We can easily infer that the resultant torque deflects the magnetic structural element in a direction parallel to the UMF. For the entire MASM strip, the non-uniform torque of each part comes from the oriented magnetic structural elements that would cause bending deformation. This is the core concept of designing the anisotropic magnetization profiles by printing magnetic structural elements.

Fig. 2c shows the deformation of the MASM strip under different magnetic flux densities and reveals that the bending strip approximates the arc of different circles. The curves of bending deformation of the MASM strip under the increasing and decreasing magnetic flux density are shown in Fig. 2d. The loading and unloading curves are nearly symmetrical, indicating that the MASM strip has no hysteresis under different time-dependent magnetic fields. It can be attributed to the excellent soft magnetic properties of the magnetic structural elements and the elasticity of SR. When the magnetic structural elements are nearly parallel to the direction of the magnetic field, the deformation will saturate under a high magnetic field. A theoretical model, in terms of the magnetic and elastic energies, is applied to analyse the bending angle (β) qualitatively (see Note S3 of Supporting Information). A continuous test is carried out to investigate the stability of the bending performance of the MASM strip, and no obvious change can be observed in the displacement curve, as shown in Fig. 2e. Fig. 2f shows the single- cycle curves of bending deformation and transient magnetic field. The bending response is slightly delayed in magnetic field response, and the response and reset times of the bending deformation are approximately 120 and 140 ms (including the response and reset times of the transient magnetic field), respectively. The above results show that the MASMs prepared by this method have a fast, stable, and accurate shape-change response, ensuring the excellent actuation performance of designed soft robots and actuators.

3.4. Biomimetic imitation of shape-programmable MASMs

Inspired by the shapes of snake and mollusks, we fabricate MASMs capable of simulating their shapes. Fig. 3 shows the biomimetic

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imitations of snake, starfish, and brittle star. In these cases, we also select the simplest UMF as the magnetic actuation. With finite element simulation by COMSOL, we design and print the oriented magnetic structural elements in SR and assemble them into a biomimetic body. The simulation results indicate that the magnetization of the magnetic structural elements depends not only on its own orientation but also on the distribution and orientation of adjacent magnetic structural ele-ments. Therefore, not only should the shape and orientation of the in-dividual magnetic structural elements be considered, but the overall structure of the magnetic structural elements should ensure that they

can be magnetized efficiently. In addition, the experimental verifica-tions are consistent with the simulation results. Fig. S3 supplements several designed MASMs on the basis of different UMF or magnetic structural elements. The video (Supplementary Video 1) of the shape change indicates that the biomimetic imitation can be continuously, quickly, and reversibly controlled by magnetic actuation. Notably, this design method is also applicable to gradient magnetic actuation and 3D programmed shapes.

Supplementary video related to this article can be found at http s://doi.org/10.1016/j.compscitech.2019.107973.

Fig. 3. Various shape-programmable MASMs with two-dimensional shape change under applied magnetic fields for biomimetic imita-tion. (a) Photographs of snake, starfish, and brittle star. (b) Finite element simulation of programmed shapes via COMSOL. The magnetic flux density is indicated by the color. (c) Two-dimensional shape changes of MASMs under applied magnetic fields. The intensity of the horizontal UMF is 200 mT, and the direction is indicated by the arrow. For different programmed shapes, the mag-netic structural elements with a length of 1.5 mm, width of 0.3 mm, and thickness of 4 mm have different distributions in SR. All scales are 10 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4. Design of soft robots and actuator on the basis of shape-programmable MASMs. (a) Walking motion of the inchworm-like soft robot on serration plate. The bending of the robot is driven by the UMF, and it is restored by the coupling of gravity and elastic forces. The white scale bar is 10 mm. (b) Swimming of the manta ray-like soft robot under water. The swing of the swimming robot is also driven by the UMF. The white scale bar is 10 mm. (c) Grab and release of the soft gripper. The gripping action is driven by the UMF, and the weight of the cylindrical object is 15.3 g. The white scale bar is 20 mm. In these cases, the intensity of the UMF is 300 mT, and the direction is as indicated by the arrow.

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3.5. Performance of soft actuators and robots

Given the low density, excellent flexibility, and actuation properties of MASMs, we attempt to assemble them into soft robots and actuators. The walking motion of the inchworm involves bending and stretching its soft body. We design a serration plate for the walking motion of the inchworm-like soft robot and control the bending and stretching of the robot by time-varying UMF. Under the coupling of gravity and elastic force, the inchworm-like soft robot can be oriented to walk on the serration plate (Fig. 4a and Supplementary Video 2). A manta ray swims by flapping its oblate soft bodies, and many researchers have developed robots on the basis of the its swimming style [44,45]. In this study, owing to the low CIP content per unit volume, this manufacturing approach enables the MASMs to have a low density similar to pure SR, bringing great prospects for MASMs to manufacture underwater robots. Here, we fabricate a manta ray-like soft robot whose muscles and wingspan are composed of MASMs and pure SR, respectively. By adjusting the magnetic actuation, the manta ray-like soft robot can swim under water (Fig. 4b and Supplementary Video 3). Soft gripper is an important actuator for the soft robot and has the potential to create mechanical functionalities beyond those of the traditional manipulator. As shown in Fig. 4c, we design a soft gripper on the basis of the MASMs. The grab and release movement can be controlled by magnetic actuation accurately (Supplementary Video 4).

Supplementary video related to this article can be found at http s://doi.org/10.1016/j.compscitech.2019.107973.

4. Conclusions

In this paper, we report a shape-programming strategy that can quickly design the desired magnetic moment and actuating magnetic fields for MASMs with programmable shape transformation properties. This method allows us to program the magnetic moment in the soft matrix by printing diverse magnetic structural elements. The test results show that the flexible SR and soft-magnetic 3D printing filament enable the fast, reversible, and stable deformation properties of MASMs. With these capabilities, various biomimetic structures (inchworm, manta ray, and soft gripper) with walking, swimming and snatching functions have been successfully fabricated and performed. This work simply uses UMF as the actuation only, but more complex actuation behaviors can also be generated by using the gradient magnetic field. The proposed approach opens new avenues to fully capitalize the potential of MASMs, allowing researchers to develop a wide range of soft actuators that are critical in soft robotics, medical care, and bionics applications.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Song Qi: Conceptualization, Methodology, Writing - original draft, Formal analysis, Investigation, Visualization, Project administration. Hengyu Guo: Methodology, Formal analysis, Visualization. Jie Fu: Supervision, Resources, Writing - review & editing. Yuanpeng Xie: Validation, Data curation. Mi Zhu: Investigation, Validation. Miao Yu: Conceptualization, Resources, Supervision.

Acknowledgments

The research is supported by NSFC (Grant NO. 51775064, 51875056,11902055), the China Postdoctoral Science Foundation (Grant NO. 2018M643403), and the Chongqing Artificial Intelligence Technology Innovation Major Topic Special Key R&D Project (Grant No.

cstc2017rgzn-zdyfX0011).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2019.107973.

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