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Shape Morphing of Hydrogels in Alternating Magnetic Field Jingda Tang, Qianfeng Yin, Yancheng Qiao, and Tiejun Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Materials & Interfaces

Shape Morphing of Hydrogels in Alternating Magnetic Field Jingda Tang, Qianfeng Yin, Yancheng Qiao, Tiejun Wang* State Key Lab for Strength and Vibration of Mechanical Structures, Department of Engineering Mechanics, Xi’an Jiaotong University, Xi’an 710049, China KEYWORDS: hydrogels, shape morphing, magnetic materials, origami, magnetic navigation

ABSTRACT Shape morphing hydrogels have found myriad of applications in biomimetics, soft robotics and biomedical engineering. Magnetic field is favorable for specific applications of hydrogels, since it is non-contact and biocompatible at high field strengths. However, most magnetosensitive shape morphing structures are made of elastomers rather than hydrogels, because the magnetization of magnetic hydrogels is usually too low to be actuated under static magnetic field. Here we propose a strategy to achieve the shape morphing of magnetic hydrogels.We actuate magneto-thermal sensitive hydrogels by alternating magnetic field (AMF), where magnetic poly(Nisopropylacrylamide) (PNIPAm) hydrogels can be heated by AMF and undergo giant volume shrinkage under high temperature. We design the distributing pattern of magnetic hydrogels strips on an elastomer substrate to realize various two dimensional (2D) and three dimensional (3D) shapes such as heart-shape, truss, tube and helix. 1

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Complex three-dimensional origami structures have been demonstrated using the elastomer-magnetic hydrogels as hinges. We further demonstrate the combination of magnetic navigation and magnetic shape morphing, by applying both direct magnetic field and alternating magnetic field. The strategy may open new opportunities for the shape morphing of functional hydrogels.

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1. INTRODUCTION Shape morphing of materials is ubiquitous in nature and engineering. For instance, Mimosa will closes leaves when gentlely touched to protect it from stormy weather. Since most biological tissues are typically soft (Elastic moduli, 1 kPa ~ 1 MPa), soft shape morphing materials recently draw more and more attention in biomimetics, soft robotics, tissue engineering and biomedical engineering1-2. Hydrogels, molecular aggregates of polymers and water, are an important class of soft shape morphing materials due to their material versatility and giant shape change1. A general approach for the shape morphing of hydrogels is to introduce anisotropy into the structure, such that the anisotropic hydrogel can evolve into the designed shapes upon swelling or mechanical loading3-7. To expand the scope of the shape morphing of hydrogels, functional additives are always added into the hydrogel matrix to render it responsible to various environmental stimuli (i.e., light, electric and magnetic field)8-14. The most widely used strategy is called photothermal heating, where light is used to heat a temperature-sensitive hydrogel such as poly(N-isopropylacrylamide) (PNIPAm) doped with light-absorbing nanoparticles2, 8-9. Despite the recent proliferation of new hydrogels, still few other stimuli are demonstrated to trigger the shape morphing of functional hydrogels2. Magnetic field is a preferred stimulus for specific applications, since it can trigger remote and local actuation, and is biocompatible even at high field strengths.15-16 Recently developed soft magnetic machines include cargo-carrying robots,17 3D printed robots 18, microswimmers,19 microgrippers 12 and lifter, accordion and vlave11. Most of 3



these shape morphing structures are made of magnetoactive elastomers rather than magnetic hydrogels, since the magnetization of magnetic hydrogels is usually too low to be actuated under normal magnetic field strength.20 The magnetization of magnetic hydrogels is 0.15~0.8 kA/m,21 two orders of magnitudes lower than that of magnetoactive elastomers of 20~80 kA/m18. However, magnetic hydrogels are more favorable than magnetoactive elastomers for in vivo applications such as drug release and muscle regeneration,22-23 due to their good biocompatibility and wet nature. Thus an effective strategy is needed to achieve the shape morphing of magnetic hydrogels. Recently, we proposed an alternative way to realize the shape morphing of magnetic PNIPAm hydrogels through alternating magnetic field (AMF)13. AMF actuation has been used for shape memory polymers before, 24-26 but seldom used for hydrogels27-28. These early attempts have shown the possibility in the development of AMF induced shape morphing of magnetic hydrogels through the geometry design. To advance this field, here we show a facile strategy to remotely trigger the shape morphing of the hybrid structure of magnetic PNIPAm hydrogel and an elastomer. Magnetic PNIPAm hydrogels can be heated by AMF and shrink under high temperature. The isotropic volume collapse of magnetic PNIPAm hydrogels can be transformed to unidirectional bending, with the help of the strip geometry of large aspect ratio. We investigate the bending behavior of the strip bilayer and find that the curvature increases abruptly above a critical temperature. This critical phenomenon for the curvaturetemperature relation always holds, irrelevant to the magnetic field strength of AMF, the material composition of magnetic hydrogels or the thickness ratio of the two layers. We 4


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design the distributing pattern of magnetic hydrogels strips on the elastomer substrate to show various 2D and 3D shapes such as heart-shape, truss, tube, helix, etc. The bilayer strips have been further used as hinges to morph a planar structure into a pyramid or a box. To our best knowledge, it is the first time to use the AMF actuation to realize remotely triggered origami structures of magnetic hydrogels. We further demonstrate the possible combination of magnetic navigation and magnetic shape morphing, by applying both direct magnetic field and alternating magnetic field. The AMF actuating strategy has many advantages compared to the existing methods, such as remote triggering, no solvent involved and magnetic navigation. The AMF actuating strategy may emerge as a new mechanism for the shape morphing of functional hydrogels.

2. EXPERIMENTAL SECTION Materials VHB 4905, 4910 (3M, USA), N-isopropylacrylamide (Shanghai Macklin Biochemical, Co. Ltd, China), clay Laponite® XLG (BYK Additives and Instruments, Germany), N,N,N’,N’-tetramethylethylenediamine (TEMED) (Sigma Aldrich, USA) and ammonium persulfate (APS) (Sigma Aldrich, USA). To prepare Fe3O4 nanoparticles, ferric chloride (FeCl3·6H2O) and ferrous chloride (FeCl2·4H2O) were used as iron sources and sodium hydroxide (NaOH) was used as a reductant (Shanghai Macklin Biochemical, Co. Ltd, China). Superglue (Loctite 406, Henkel, Germany), ethyl acetate (Guangdong Guanghua Sci-Tech CO., Ltd.). All chemicals were used as received 5



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without any further purification. Preparation of magnetic PNIPAm hydrogels Firstly,

PNIPAm

nanocomposite

hydrogels

were

prepared

by

free-radical

polymerization. The composition of the hydrogel precursor is as follows: H2O/NIPAm/Clay/TEMED/APS=30 g: 3.39 g: 1.14 g: 30 μL: 0.02 g. We mixed clay (1.14 g), N-isopropylacrylamide (3.39 g) and distilled water (30 mL) by magnetic stirring for 1 h to make a uniform suspension. TEMED (30 μL) and APS (0.02 g) were then added and stirred for 10 min. The aqueous precursor was injected into an acrylic mold of 50 × 40 × 1mm3, sandwiched between two glass plates. After 12 h polymerization, the aqueous precursor formed a hydrogel. Secondly, we used the in situ co-precipitation method to prepare magnetic PNIPAm hydrogels. We immersed the asprepared PNIPAm nanocomposite hydrogel into a mixed solution of FeCl3·6H2O and FeCl2·4H2O at a constant molar ratio of Fe2+: Fe3+ = 1:2 for 12 h to load iron ions. The used concentration of FeCl3·6H2O was 0.2 M or 0.3 M, and most cases were 0.2M. The Fe2+/Fe3+ ions loaded hydrogels were further immersed into a NaOH aqueous solution for 12 h to precipitate Fe3O4 magnetic nanoparticles into the hydrogel matrix. The prepared magnetic PNIPAm hydrogels were transferred into deionized (DI) water to reach the fully swollen state. According to our previous study, the weight content of Fe3O4 nanoparticles is determined to be ~45wt% in the dehydrated magnetic hydrogels when the concentration of FeCl3·6H2O was 0.2 M, independent of the type of monomers. Considering the measured water content of the swollen magnetic hydrogels is 93.72 wt%, the content of Fe3O4 nanoparticles is ~ 2.83 wt%. 6


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Fabrication of magnetic hydrogel / elastomer hybrid For the fabrication of the magnetic hydrogel / elastomer hybrid, we first placed a sheet of magnetic hydrogel on a cut pad (acrylic sheet) and used a laser cutter to cut the designed pattern of hydrogel strips (length: 20 mm, width: 2 mm). We peeled off the residual parts of the hydrogel sheet and left the patterned strips on the cut pad. To bond the VHB layer to the hydrogels, we prepared a specific adhesive. Superglue was mixed with ethyl acetate with a volume ratio of 1:10, where ethyl acetate can dilute Superglue and diffuse into VHB. After preparing the adhesive, we used a brush to spread it on the surface of a sheet of VHB and immediately pressed VHB onto the hydrogel strips. The adhesive can realize instant bonding between VHB and magnetic hydrogels. After ~1 min, the glued hybrid was carefully peeled off from the cut pad and stored in a plastic bag for the AMF actuating shape morphing. For the cases where the magnetic hydrogel and elastomer have the same geometry, we just glued a sheet of VHB to a sheet of magnetic hydrogel and cut the bilayer into desired pattern with the laser cutter. AMF actuated shape morphing The fabricated magnetic hydrogel / VHB hybrid was placed into/onto the solenoids to realize shape morphing functions through AMF. AMF is generated by a commercial induction heating system (Easyheat 224, Cheltenham Induction Heating). The magnetic hydrogel could be heated by AMF. An IR camera (Flir A6703sc 470) was used for the thermal imaging of the heated hybrid. A video camera (Cannon GI 890, Cannon) was used to capture real-time images and videos of the shape morphing hybrid. The bending process of the bilayer in AMF was videoed and the photos of the bent 7



bilayer was screen captured every an interval. We fitted the shape of the bilayer using a circle with a radius of r and calculated the curvature by κ=1/ r. Magnetic navigation and magnetic shape morphing We cut a star-like bilayer of magnetic hydrogel / VHB hybrid. The bilayer was placed into a curved glass tube filled with pink water. We used a magnet to guide the bilayer from one end of the tube to the other end surrounded by a coil. The bilayer could bend upward under the AMF.

3. RESULTS AND DISCUSSION 3.1 Shape morphing mechanism using alternating magnetic field

Figure 1. The concept of shape morphing driven by alternating magnetic field. (a) Schematic of magnetothermal effect of magnetic hydrogels. (b) At initial state, the 8


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bilayer of magnetic gel and elastomer is straight; when the AMF is applied, the magnetic gel is heated and collapsed, induces the bending of the bilayer. Left: schematic. Right: photos. (c) The temperature increase of the magnetic hydrogel in the deformation process. Insets are the infrared images of the bilayer at 0 s, 120 s and 240 s. (d) The curvature of the bilayer increases sharply around a narrow temperature range. Insets are photos of the bilayer at 0 s, 120 s and 240 s. The magnetic field strength H=11.86 kA/m and frequency =187 kHz. Figure 1 shows the concept of AMF driven shape morphing of magnetic hydrogels. We first illustrate the magnetothermal effect of magnetic hydrogels in AMF generated by a coil (Figure 1a). The magnetic hydrogel is composed of magnetic nanoparticles and a thermosensitive polymer network. When placed in AMF, magnetic hydrogels can be heated due to Néel relaxation of magnetic moments.29 The heated magnetic nanoparticles will continuously transmit heat to the thermosensitive polymer network of the hydrogel. Once the temperature reaches a critical temperature, the polymer network will collapse. The magnetothermal effect of magnetic hydrogels has been throughly described in our previous papers.13, 16 Figure 1b demonstrates the bilayer design for the shape morphing of soft materials through magnetothermal effect. For the shape morphing of hydrogels, the hydrogel-hydrogel composite systems are prevailing, since the seminar work of Hu et al, where the polyacrylamide-PNIPAm hydrogel bilayers were used.30 However, the hydrophilic-hydrophobic hybrids made of hydrogelelastomer composite draw more and more attention recently, especially for the fabrication of ionic devices. It is significant to investigate the programmed deformation 9



of the hydrogel-elastomer system. Here we design a bilayer consisting of a magnetic hydrogel and an elastomer. We choose the temperature-sensitive PNIPAm as the matrix of magnetic hydrogels and an adhesive VHB (3M, USA) as the elastomer. Magnetic PNIPAm hydrogels can be heated in AMF and undergo giant volume shrinkage, once the internal temperature exceeds the lower critical solution temperature (LCST) of PNIPAm.31 The isotropic volume shrinkage of the hydrogel layer will cause the bending of the whole bilayer, since the aspect ratio of the strip is very large (~10). If the width is comparable to the length, the bilayer will bend diagonally rather than unidirectionally (Figure S1), similar to the phenomenon reported in previous literature32. As shown in the right photos, magnetic PNIPAm hydrogel is black in color and VHB is transparent. In initial state, the bilayer is straight; while in the actuated state, the bilayer is bent. It is noted that the strong adhesion between hydrogel and elastomer is vital for the bending of the structure, since the intrinsic adhesion between the two materials is very weak.33 To achieve good adhesion, we have prepared a specific adhesive by mixing cyanoacrylate based superglue with an organic solvent (ethyl acetate), according to the strategy developed by Wirthl et al.34 The prepared adhesive may form physical entanglement, van der Waals and hydrogen bonds between the two polymer networks to realize a high adhesion energy. We monitor the temperature change of the magnetic hydrogel during the shape morphing process with an infrared camera. The temperature of the magnetic gel can increase from 20.5 °C to 38.0 °C within 240 s (Figure 1c). Insets show the infrared images of the magnetic hydrogels at 0 s, 120 s and 240 s. We can see that the two processes, temperature increase and shape transforming, occur 10


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simultaneously. We observe another significant phenomenon in the shape morphing process as shown in Figure 1d: the curvature of the bilayer increases smoothly in the initial temperature range, while rises dramatically around a critical temperature. The curvature increases slowly from 0 to 0.076 when the temperature changes from 20.5 °C to 35.5 °C (∆κ/∆T = 5.1×10-3 /°C); while increases sharply from 0.076 to 0.23 when the temperature changes from 35.5 °C to 38°C (∆κ/∆T = 6.2×10-2 /°C). The speed of the temperature increase shows a difference of one order of magnitude. The sharp curvature increase is consistent to the critical collape of PNIPAm hydrogels at LCST. Insets are the photos of the deformed bilayer at 0 s, 120 s and 240 s. 3.2 Basic parameters affecting the shape morphing In the following section, we will investigate the parameters governing the shape morphing of the soft bilayers. We focus on three parameters: the magnetic field strength H, the thickness ratio of the hydrogel to the elastomer and the concentration CFe3+ used for preparing magnetic hydrogels. Figure 2a shows the profile of the curvature increase with different magnetic field strengths H. It is seen that the curvature increases faster at a larger magnetic field strength, because of a faster temperature increases (Figure S2). In spite of this, the final curvature for the three cases is almost the same (~0.20). The bending deformation of the bilayer is determined by the geometrical and mechanical properties of the two layers. We keep the thickness of the magnetic hydrogel strip to be 1.55 mm, while changing the thickness of the elastomer. We choose two appropriate types of the commercial product VHB for our experiments: VHB 4905 and 4910 with a thickness of 0.5 mm and 1 mm, respectively. In the bending process, 11



the magnetic hydrogel is the active layer, while the elastomer is the passive layer. Thus the thicker the elastomer, the smaller the curvature, as shown in Figure 2b. We also change the ferric concentration CFe3+ when preparing magnetic hydrogels. It is seen that a higher CFe3+ induces a faster increase of the curvature (Figure 2c). This is because a higher CFe3+ will cause more magnetic nanoparticles precipitated in magnetic hydrogels, inducing a faster conversion of magnetic energy to heat.16 We collect all the temperature-curvature data and replot them in Figure 2d. It is found that all the data collapse together, revealed by the shaded area. This implies that all the soft bilayers will suddenly bend up once the temperature reaches a critical value, irrelevant of the magnetic field strength, the thickness ratio and the material composition. The critical temperature corresponding to the sharp curvature increase is ~34 °C, quite close to the LCST of undoped PNIPAm hydrogel.35 From the above evidence, we can conclude that the shape transformation of the elastomer-magnetic hydrogel bilayer activated by AMF is essentially a temperature controlling process.

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Figure 2. Basic parameters involved in the shape morphing. The effect of (a) the magnetic field strength H, (b) the thickness ratio of the magnetic hydrogel to the elastomer and (c) the concentration of CFe3+, on the curvature increase profile of the bilayer. (d) All the curvature-temperature data collapse together, indicating that this is a temperature controlling process. The thickness of magnetic hydrogel strip is kept as 1.55 mm. In (a), the thickness ratio is 3.1 and CFe3+ = 0.2 M. In (b), H = 11.86 kA/m and CFe3+ = 0.2 M. In (c), H = 11.86 kA/m and the thickness ratio is 3.1. 3.3 Complex 2D and 3D shapes The elastomer-magnetic hydrogel bilayer offers numerous opportunities to design the shape morphing structures. By arranging the pattern of the magnetic hydrogel on one dimensional (1D) elastomer strip or 2D elastomer sheets, various 2D and 3D shapes 13



can be realized in the alternating magnetic field. Figure 3a shows the transformation of the elastomer-magnetic hydrogel bilayer from 1D strips to 2D shapes. The schematics are the upper view of the bilayer designs. The location of the magnetic hydrogel strip can be tuned to change the final shape. When the magnetic hydrogel is located on one end of the elastomer, a “?” like shape is realized, since the bonded side bends toward to the gel while the un-bonded side keeps straight (Figure 3-a1). The finite element analysis (FEA) simulation confirms the deformed shape. When the magnetic hydrogel is located in the center of the elastomer, a tilted “C” like shape is obtained (Figure 3-a2). When two separate magnetic hydrogel strips are bonded to the two ends of the elastomer substrate, both the two ends bend downward to form a “bow” like structure (Figure 3-a3). If we further add the number of the magnetic hydrogel strips and tune their distribution, we can get a shape of “Golden hoop” of the Chinese Monkey king and a “heart” shape (Figure 3-a4-a5). To realize complex 3D shapes, we propose two different strategies in Figure 3b and Figure 3c. In Figure 3b, the 2D bilayers are composed of magnetic hydrogel and elastomer of identical geometry. An ellipse and an ellipse circle bend from the long axis, because of the lower bending stiffness in this direction (Figure 3-b1-b2). A circle bends upward almost in all directions, since no one axis is easier than the other one to bend (Figure 3-b3). A cross bends its four arms to form a four-fingered hand (Figure 3-b4). Extending a single cross to a grid, we can get a complex truss structure (Figure 3-b5). In Figure 3c, patterned magnetic hydrogel strips are distributed on the elastomer sheet. The elastomer sheet acts as a passive substrate, while the magnetic hydrogel strips act 14


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as active elements for deformation. When the center of a triangle or a square is connected to its corners with magnetic hydrogel strips, the planar sheet can roll up into complex surfaces (Figure 3-c1-c2). Arranging parallel strips of magnetic hydrogels onto the elastomer sheet, vertical or tilted to (angle, 45°) the long axis, we can obtain a tube (Figure 3-c3) or a helix (Figure 3-c4, Movie S1). We carry out FEA simulation for all the above designs and the simulated shapes match the experiments well.

Figure 3. Shape morphing structures (2D and 3D) in alternating magnetic field. (a) 1D to 2D shapes. The schematics are the upper view of the bilayer designs. (b) 2D to 3D shapes, where the magnetic hydrogel and the elastomer have the same geometry. (c) The magnetic hydrogel strips are distributed on the elastomer substrate to realize 15



complex 3D shapes. All the deformed shapes are reproduced by the finite element analysis (FEA). All scale bars are 5mm. The used magnetic field strength is 17.76 kA/m and the frequency is 203 kHz. 3.4 Origami structures In the above sections, the substrate is always used as a continuous film. Here we adopt the design of hinges to realize folded origami structures.36 The magnetic hydrogel strips are used as hinges to connect separate plastic films (thickness, 0.1mm). After triggered by the alternating magnetic field, the hydrogel hinge will close and drive the plastic films to fold. We use three magnetic hydrogel strips to connect four separate triangular plastic films. It evolves into a pyramid after 5 min treatment of alternating magnetic field (Figure 4a). Similarly, we use four magnetic hydrogel strips to connect five separate square plastic films. It folds into an uncovered box in 6 min (Figure 4b). More complex origami structures can be realized using this strategy.

Figure 4. Origami structures driven by alternating magnetic field. (a) A pyramid16


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like structure and (b) an uncovered box, with hinges of elastomer-magnetic hydrogel bilayer strips and stiff plastic films. After triggered by alternating magnetic field, the bilayer will bend and drive the plastic films to form folded structures. The FEA simulations show the initial and final shapes of these origami structures. The used magnetic field strength is 17.76 kA/m and the frequency is 203 kHz. All scale bars are 5 mm. 3.5 Magnetic navigation and magnetic shape morphing One of the outstanding characteristics for magnetic hydrogels is that they can be navigated by magnetic field12, 28, 37. In this section, we attempt to combine magnetic navigation and magnetic shape morphing, by applying both direct magnetic field and alternating magnetic field. We guide the movement of the magnetic hydrogel-elastomer bilayer using a magnet and trigger its shape morphing by the alternating magnetic field (Figure 5a). We cut a star-like bilayer and place it into a curved glass tube filled with water. We use a magnet to guide the bilayer from one end of the tube to the other end surrounded by a coil within 12 s. After the bilayer is guided to the center of the coil, the alternating magnetic field is applied. Within 4.2 min, the planar bilayer can bend upward substantially (Figure 5b, Movie S2).

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Figure 5. Magnetic navigation and magnetic shape morphing. (a) The schematic for the magnetic navigation of a star-like bilayer and its shape morphing in alternating magnetic field. (b) The bilayer is guided by a magnet from one end of the tube to the other end surrounded by a coil within 12 s. After the bilayer is guided to the center of the coil, the alternating magnetic field is applied. Within 4.2 min, the planar bilayer can bend upward. All scale bars are 10mm. The used magnetic field strength is 12.83 kA/m and the frequency is 158 kHz.

4. CONCLUSIONS In summary, we have developed an approach to achieve the shape morphing of 18


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magnetic hydrogels in alternating magnetic field. We have designed the hybrid structure of elastomer and magnetic PNIPAm hydrogels to realize various 2D, 3D and origami structures. We further demonstrate the combination of magnetic navigation and magnetic shape morphing. The actuation speed of this approach can be improved by adding magnetic microparticles with strong magnetization (e.g., NdFeB particles) into the hydrogel precursors. Moreover, the approach can be readily combined with 3D printing technology to achieve more complex and practical shapes. For instance, the ink of magnetic hydrogels can be printed on the elastomer substrate via the direct ink writing strategy. It is hoped that the approach can find broad applications in soft robotics and biomedical engineering.

ASSOCIATED CONTENT Supporting Information. The effect of the aspect ratio of the elastomer-magnetic hydrogel bilayer on the final shapes. The temperature increase curves of the bilayer under different magnetic field strengths. The procedure of the FEA simulation. Supporting movies. Movie 1: the physical and infrared videos of the shape morphing process to achieve a helix structure. Movie 2: the navigation of a star-like bilayer using a magnet and its shape morphing in alternating magnetic field.

AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected]. Notes The authors declare no competing financial interest. 19



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ACKNOWLEDGMENTS The work was supported by the NSFC (No. 11702208), China Postdoctoral Science Foundation (No. 2018M643620), the Program for Postdoctoral Innovative Talents (No. BX201700192).

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Shape Morphing of Hydrogels in Alternating Magnetic Field | ACS

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