Adhesive Tough Magnetic Hydrogels with High Fe3O

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Applications of Polymer, Composite, and Coating Materials 3

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Adhesive Tough Magnetic Hydrogels with High FeO Content Xiaocheng Hu, Guodong Nian, Xueya Liang, Lei Wu, Tenghao Yin, Haotian Lu, Shaoxing Qu, and Wei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20937 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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ACS Applied Materials & Interfaces

Adhesive Tough Magnetic Hydrogels with High Fe3O4 Content Xiaocheng Hu a, Guodong Nian*,a, Xueya Liang a, Lei Wu a, Tenghao Yin a, Haotian Lu b, Shaoxing Qu*,a, and Wei Yang a a. State Key Laboratory of Fluid Power & Mechatronic System, Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Center for X-Mechanics, Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, China b. College of Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States * Corresponding authors: [email protected]; [email protected] Keywords: magnetic hydrogel, fracture, adhesion, water proof, artificial muscle Abstract Magnetic hydrogels have promising applications in flexible electronics, biomedical devices, and soft robots. However, most existing magnetic hydrogels are fragile and suffer insufficient magnetic response. In this paper, we present a new approach to fabricate a strong, tough, and adhesive magnetic hydrogel with non-toxic PAAm hydrogel as the matrix and the functional additive (3-(Trimethoxysilyl) propyl methacrylate (TMSPMA) coated Fe3O4) as the inclusions. This magnetic hydrogel not 1

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only offers a relatively high modulus and toughness compared to the pure hydrogel, but also responds to the magnetic field rapidly due to a high magnetic particle content (up to 60%, with respect to the total weight of the polymers and water). The hydrogel can be bonded to hydroxyl-rich hard and soft surfaces. Magnetic hydrogel with PDMS coating exhibits excellent underwater performance. The bonding between magnetic hydrogel and PDMS is very stable even under cyclic loading. An artificial muscle and its magnetomechanical coupling performance are demonstrated using this hydrogel. The adhesive tough magnetic hydrogel will open up extensive applications in many fields, such as controlled drug delivery systems, coating of soft devices, microfluidics. The strategy is applicable to other functional soft materials.

1.

Introduction

Smart hydrogels can respond to stimuli including temperature, pH, light, electric, and magnetic fields1-3, and have applications in diverse areas such as soft robotics, biomedicine4, and flexible electronics5-7. Among all the stimuli-responsive hydrogels, magnetic hydrogel is considered as a promising and attractive material because of its unique properties, such as prompt response and remote controlling ability, which are required especially in biomedical applications. Magnetic hydrogel is a composite made of hydrogel matrix and magnetic inclusions. The highly stretchable hydrogel matrix such as polyacrylamide8, polyacrylic acid9, and poly vinyl alcohol2, 10-12 makes the composite stretchable and soft. The magnetic inclusions such as iron oxide 2


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nanoparticles (NPs) make the composite capable of deforming, moving, and transforming in response to remotely controlled magnetic fields. The magnetic manipulation method is considered as one of the remote controlling (RC) methods13, and enables the hydrogel devices to work without an electronic source. The method allows the devices to be actuated by the energy supplied from the external magnetic field, so the devices become more compact. The required deformation14 and motion15 of the magnetic hydrogel can be achieved by designing the distribution of magnetic NPs and controlling the magnetic field16. Most existing magnetic hydrogels are fragile and have insufficient magnetic response17, 18. Zhong et al. fabricated a magnetic hydrogel by uniformly dispersing NPs into the hydrogel precursor, but the concentration of the NPs cannot exceed 4.5 mg mL−1, which makes it difficult to achieve remote control19. Haider et al. prepared a strong and tough double network magnetic hydrogel, but the weight ratio of NPs is less than 20%, which provides insufficient magnetic force20. Tang et al. synthesized a super tough magnetic hydrogel using in-situ precipitation method, while the concentration of magnetic particles cannot be precisely assigned18. The goals to achieve both high magnetic particle content and good dispersion of NPs in hydrogels are often contradictory. In most existing studies on Fe3O4 NPs dispersing system, the content of magnetic powders is typically on the order of mg mL−1, which is too low though the dispersion can be very good. Meanwhile, as the content of magnetic powders increases, the nanoscale magnetic powders tend to cluster and settle down to 3



the bottom of the container. It remains a challenge to synthesize magnetic hydrogel with both uniform distribution and high content of magnetic NPs. In this paper, we present a new strategy to fabricate tough magnetic hydrogels that can adhere to diverse materials and achieve extremely high content of magnetic NPs. PAAm is adopted as the matrix because it is widely used as one synthetic polymer with excellent mechanical property and biocompatibility20. We add magnetic NPs modified by silane coupling agents into the hydrogel precursor and use the dispersing agent to prevent the NPs from clustering. Free polymer chains are used as the thickener to prevent the NPs from settling down to the bottom. After polymerization, the silane coupling agents are copolymerized into the polymer network and make the magnetic hydrogels stronger. The silane coupling agents on the surfaces of the magnetic NPs will condensate with silanol group or hydroxyl group on the surfaces of other materials to achieve strong bonding. By optimizing the recipe, the magnetic hydrogel with high modulus and toughness is achieved. Meanwhile, the content of magnetic NPs reaches to 60%, and the distribution is extremely uniform. Due to the surface modification of Fe3O4 NPs, the obtained hydrogel can adhere to different hydroxyl-rich surfaces, showing a superior bonding property. By coating PDMS on the surface of the magnetic hydrogel as a waterproof layer, the magnetic NPs are prevented from leakage when used in the aqueous environment under the cycling load, so it is avoided contaminating the surrounding environment. Due to its prompt response and the ability to deform largely under controllable magnetic fields2, 4


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the magnetic hydrogel is used as an artificial muscle that can contract up to 80% of its original length when carrying a weight of 50 grams. Current applications of magnetic hydrogels include drug delivery systems21, tissue engineering12, coatings of tablets1, 17, implantable medical devices15, soft microgrippers22-24, artificial muscle25 and other promising fields. The proposed strategy to achieve tough magnetic hydrogels with high NPs concentration and bonding property can be adapted to other hydrogel systems, and thus will greatly broaden the frontier of fields and applications in these soft smart materials.

2.

Results and discussions

2.1. Characterization of the Fe3O4 NPs Dispersion

Magnetic NPs possess a large specific surface area, and the free energy of the entire system is very high. According to thermodynamics, magnetic NPs in the hydrogel precursors tend to self-aggregate to reduce the surface energy, resulting in settlement. After polymerization, the hydrogels exhibit poor mechanical and magnetic properties. The more the magnetic NPs are added into the hydrogel precursor without uniform distribution, the worse the properties are. It is difficult to make magnetic hydrogels with high content of magnetic NPs and excellent mechanical properties. We present herein an approach to tackle this challenge. The additions of dispersing agents and thickeners into the precursors make the magnetic NPs disperse uniformly and stably in the precursor during polymerization. The mechanisms of improving the 5



dispersion and stability of NPs in the precursors are described below. Dispersing Mechanism. NPs can be dispersed into water by mechanical stirring, but they tend to aggregate due to the intermolecular force. Dispersant improves the separation of NPs through a double layer structure (Figure 1a) consisting of a stern layer and a diffusion layer. The stern layer forms when ions from the dispersant are closely adsorbed on the surfaces of the NPs due to electrostatic attraction and Van der Waals forces, while the oppositely charged ions that do not attach on the surfaces of the NPs form a diffusion layer. So the surface charges of NPs mainly come from hydrolysis and ion adsorption. For Fe3O4 NPs, water, and ammonium citrate dispersion system, acetate ions and ammonium ions are derived from the dissolved ammonium citrate. Due to the mutual attraction of ions with opposite charges, acetate ions will adsorb to the surface of Fe3O4 NPs, forming a stern layer. The acetate ions in the diffusion layer exhibit a Poisson distribution. Repulsion occurs when NPs carrying the same charge are nearby (Figure S1). Stability of the System. In the suspension that contains magnetic NPs and hydrogel precursors, the stability of the system is an important criterion to evaluate a dispersion26. However, if the stability of the hydrogel precursor is poor, NPs will settle due to gravity before the polymerization is done, and thus the distribution of NPs in the hydrogel will be strongly non-uniform and the physical properties of hydrogels will be anisotropic. The magnetic NPs in the dispersion medium are 6


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affected by the gravity, the buoyancy, and the resistance from the dispersion medium. The gravity acts vertically downwards, while the directions of medium resistance and buoyancy are opposite. Due to these three forces, a magnetic NP first accelerates downward in the medium, and when the force is balanced, it moves in uniform velocity. The densely aggregated NPs gradually block their way to descend, and eventually stabilize the settlement. Since both gravity and buoyancy are constant, the sedimentation velocity of a magnetic NP is only related to the resistance of the medium, which depends on the viscosity. Within a certain range, the dispersion time of the system is proportional to the viscosity of the solvent. It has been found that 2% of polyacrylamide thickener shows a very good stability (details in supporting information). For a dispersion system containing g ml-1 NPs, the uniform distribution of NPs lasts several hours.

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Figure 1. Illustration of the magnetic hydrogel. (a) Double layer structure for particle dispersion system. (b) FT-IR spectra of Fe3O4 before and after modification. (c) Magnetic hydrogel dispersion system. (d) Hydrogel polymerization principle.

2.2. Mechanism of Bonding Magnetic Hydrogels to Diverse Materials

The FT-IR spectra of the Fe3O4 NPs before and after the modification with the 8


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silane coupling agent are shown in Figure 1b. The absorption peak at 3429 cm-1 corresponds to the stretching vibration peak of the hydroxyl group on the surface of the Fe3O4 NPs. The 1629 cm-1 corresponds to the bending vibration peak of hydroxyl group on the surface of the Fe3O4 NPs, and the peak at 587 cm-1 corresponds to the absorption peak of the ferrous oxide. The absorption peak at 1705cm-1 is the stretching vibration peak of the carbon-oxygen double bond in the silane coupling agent. Therefore, it can be determined that the silane coupling agents are successfully coated on the surface of the Fe3O4 NPs. There are three types of bonding in the magnetic hydrogel (Figure 1c). It is well known that the outermost surface of an oxide is covered with a layer of hydroxyl groups27. In silane coupling agents, a silicon atom links to three hydrolysable groups and an organofunctional group28, and one example is 3-(Trimethoxysilyl)propyl methacrylate. In the presence of water, the alkoxy groups hydrolyze into silanol groups, which can react with hydroxyl groups on the surfaces of the magnetic NPs to form strong covalent bonds. During the formation of a polymer network, the organofunctional group covalently incorporates the silane coupling agents into the network (Figure 1d). Also, it is possible that the silanol groups on the surface of the NPs condensate to form a siloxane bond which connects two NPs. Silane-modified magnetic particles make the hydrogels capable of bonding to surfaces with hydroxyl groups and enrich the crosslink density inside the hydrogel network, which will influence its mechanical properties. 9



Comparison of SEM observations between pure PAAm hydrogels and magnetic hydrogels shows that the freeze-dried hydrogels have loose and porous structures (Figure 2a, b). In magnetic hydrogels, the magnetic NPs uniformly distribute in the hydrogel network without aggregation, so the mechanical and magnetic properties are isotropic (Figure 2c). From Figure 2d, it is observed that the average diameter of the NPs is around 200 nm.

Figure 2. SEM images. (a) SEM image of pure PAAm hydrogel at 1000 times magnification. (b) SEM image of pure PAAm hydrogel at 10000 times magnification. (c) SEM image of magnetic hydrogel at 1000 times magnification. (d) SEM image of magnetic hydrogel at 10000 times magnification. The red circular frame is a partial magnified image of 50,000 times, and the Fe3O4 particles can be seen clearly.

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2.3. Mechanical Properties

Tensile Tests. First, we prepared a 20 × 10 × 5 mm silane-modified Fe3O4 NPs-PAAm hydrogel (Fe3O4 = 38 wt%) strip, which can be stretched to 8 times of its original length without rupture, while most of magnetic hydrogels failed to achieve high NPs concentration and stretchability simultaneously

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Figure 3a shows the

relationship between NPs concentration and tensile modulus of magnetic hydrogel. The Fe3O4 NPs serve as additional crosslinkers and enhance the stability of the polymer network29. The tensile tests indicate that the tensile modulus depends on the concentration of NPs: for hydrogel with 60% NPs, the tensile modulus is 5 times larger than that of the pure hydrogel. Meanwhile, even though PAAm is only 2 wt% of the whole system, it raises the tensile modulus from 4 kPa (for the pure PAAm hydrogel) to 10 kPa. Figure 3b illustrates how the surface modification of magnetic NPs affects the tensile modulus of magnetic hydrogels. Since the surfaces of the nanoparticles are modified by silane coupling agents that can be incorporated into the hydrogel network during polymerization, each nanoparticle connects to several polymer chains and serves as a chemical cross-linker, so the crosslinking density of the network increases. The loading-unloading curve (Figure 3c) shows that the magnetic NPs act as crosslinking agents that form physical crosslinking between macromolecules and introduce the dissipation. The hysteresis curves are used to describe the energy dissipating capability of magnetic hydrogels and characterize the toughness20. The more addition of NPs, the larger the area enclosed by the hysteresis 11



curve. Energy dissipation is shown in Figure 3d. For the pure PAAm hydrogel, the hysteresis curve is almost a straight line, and the energy dissipation is 6.13 kJ m-3. However, for hydrogel contains 20% NPs, the energy dissipation increases to 23 kJ m-3. By further increasing the content of magnetic NPs, the dissipation of energy increases dramatically to 118 kJ m-3.

Figure 3. The modulus of the magnetic hydrogel varies with different factors. (a) The tensile modulus of the hydrogel vs. the content of magnetic NPs. (b) Effect of silane-modification on the tensile modulus of hydrogels. (c) Uniaxial tensile curves of the hydrogels with various contents of magnetic NPs. (d) The area enclosed by uniaxial tensile hysteresis curve of the hydrogel vs. the content of magnetic NPs.

Fracture Tests. Experiments have shown that magnetic NPs act as crosslinking 12


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agents in hydrogels (Figure 4). The fracture energy of the magnetic hydrogels with 0%, 20% and 40% NPs is 434  90 J/m2, 1208  78 J/m2, and 1584  99 J/m2, respectively. Magnetic NPs added to the hydrogels increase the tensile modulus and stretchability. The large energy dissipation and extended elongation mainly result from the sliding and pulling out of nanoparticles in the hydrogel networks

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However, the stretchability decreases when the content of NPs increases to a relatively high level. For silane-modified magnetic hydrogels, the fracture energy of the hydrogels with 20% and 40% magnetic NPs are 1077  80 J/m2 1783  189 J/m2, respectively. Compared with unmodified magnetic hydrogel, the fracture energy does not change significantly, which implies the fracture energy does not depend on the surface treatment of magnetic NPs but depend on the concentration of NPs.

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Figure 4. Fracture tests on magnetic hydrogels. (a) Stress-stretch curves of magnetic hydrogels with 20% magnetic NPs. (b) Stress-stretch curves without magnetic NPs. (c) Stress-stretch curves of magnetic hydrogels with 40% magnetic NPs. (d) Effect of the surface treatment on the fracture energy of magnetic hydrogels.

Peeling Tests. We cured magnetic hydrogels on glass substrates and measure the adhesion energy using the 90-degree peeling test (Figure 5a, d). The adhesion energy of the unmodified magnetic hydrogel is 3J /m2, which is independent of the concentration of NPs. However, if the surface of NPs is modified by silane coupling agents, the adhesion energy increases. In the peeling test, the hydrogels adhered firmly to the glass substrate, and the hydrogel-glass interface remained intact. The failure occurred in the hydrogel-backing layer (PET film) interface, and the measured adhesion energy is 118 J/m2. However, the adhesion energy may be larger than that of the hydrogel-PET interface (using cyanoacrylate glue). More information can be found in Figure S5-6. Waterproof property is achieved by coating a layer of PDMS on the surface of the magnetic hydrogel (Figure 5b and 5e). If the magnetic NPs are not modified, the coating just contact the unmodified magnetic hydrogel without bonding, so in the experiment the coating easily debonds from the hydrogel and breaks. The modified magnetic hydrogel and PDMS have a strong interface, so the sample hardly debonds and breaks under cyclic loading. Figure 5c and 5f show the difference of contact angle between the water droplet and magnetic hydrogel (Magnetic hydrogel in Figure 14


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5c has a layer of PDMS coating). The coating layer reduces the rate of evaporation of water in the hydrogel so that the hydrogel can retain its original properties for a long time (more information can be found in supporting materials).

Figure 5 Magnetic hydrogel bonding on different surfaces. (a) Peeling test of modified magnetic hydrogel on glass. (b) The modified magnetic hydrogel is adhered to PDMS coating and rinsed for one minute without leaking magnetic NPs. (c) The water droplet on the magnetic hydrogel coated with PDMS has a large contact angle. (d) Peeling test for unmodified magnetic hydrogel on glass substrate. (e) Unmodified magnetic hydrogel was rinsed for one minute and magnetic NPs leaked out. (f) The water droplet on the magnetic hydrogel without PDMS coating has a small contact angle.

Fatigue Tests. Actuators made of magnetic hydrogels may be employed under cyclic loadings or in aqueous environment. So it is necessary to study the fatigue behaviors of magnetic hydrogels in aqueous environment. We obtained the stress-stretch curves for fatigue fracture under the pure shear tests (Figure 6a). After 2000 cycles of loading, there are no visible cracks on the surfaces of the magnetic 15



hydrogel (Figure 6b), and no leakage of magnetic NPs. Figure 6c shows that the maximum stress decreases slowly as the number of cycles increases. We also observed the stress decreases after the first cycle. Large portions of the energy dissipation, material softening, and residual strain mostly occur in the first cycle30. During the first loading cycle, the randomly oriented long chains of PAAm are elongated and straightened and cannot return to the original state during the unloading process, leading to a reduction in modulus. As the number of cycles increases, the curve gradually converges, and the stress-stretch curve eventually reaches a steady state.

Figure 6. Underwater fatigue test of magnetic hydrogel. (a) Schematic of the fatigue test. (b) Magnetic hydrogel coated with PDMS under water after fatigue test. (c) Maximum stress vs. the number of cycles. (d) Stress-stretch curves of sample subjected to cyclic pure shear test. 16


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2.4. Results of Magnetic Test

In previous works, the saturation magnetization of magnetic hydrogels with 20% and 22% magnetic NPs are 17 emu/g20 and 13 emu/g31, respectively, while the saturation magnetization of pure Fe3O4 powders could reach 65 emu/g32. Accordingly, the easiest way to improve the performance of magnetic hydrogels is to increase the content of magnetic powders. The saturation magnetization of magnetic hydrogel with 60% magnetic NPs reaches to 42.3 emu/g (Figure 7a), which is more than twice of that in the previous work20. As the content of the magnetic NPs increases, the saturation magnetization increases. However, it cannot be higher than that of the pure magnetic NPs. In order to test the magneto-mechanical coupling behavior of the magnetic hydrogels, we used an electromagnet to generate magnetic fields by applying different voltages. Figure 7b shows a prototype of the artificial muscle. The maximum magnetic field intensity of the electromagnet was 335mT, and the magnetic field disappeared at 400mm above the surface of the electromagnet (Figure 7d). Since the distribution of the magnetic field was not uniform, hydrogels contract along the axial direction. A magnetic hydrogel with 60% magnetic NPs was stretched by a weight of 50g, and the top surface of the magnetic hydrogel was glued to the bottom surface of the electromagnet through the double-sided adhesive. The deformation of the hydrogel is calculated as 22% by comparing the length of the hydrogel (Figure 7c). That is, a hydrogel cylinder of 10 cm can lift the weight of 50 g upward by 2.2 cm 17



(Figure 7e and 7f). Due to the large gradient of the magnetic field, the deformation of the magnetic hydrogel cylinder is not uniform, and the closer to the electromagnet the more it contracts. After contracting, the cross-section area of the cylinder became larger, and the cross-section of the hydrogel varied along the axial direction.

Figure 7. Performance of the magnetic hydrogel as artificial muscle. (a) M-H curve of dried magnetic hydrogel. (b) Schematic of artificial muscle. (c) Stretch vs. voltage. (d) The magnetic field strength vs. the distance. (e) Initial configuration of the magnetic hydrogel without magnetic field. (f) 18


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Current configuration of the magnetic hydrogel with applied magnetic field.

There are several ways to improve the magneto-mechanical coupling behavior of magnetic hydrogels: first, decrease the tensile modulus of magnetic hydrogels to make them soft; second, increase the magnetic powder content, because the higher the magnetic powder content is, the larger the magnetic force is; third, adopt the magnetic powder of higher saturation magnetization. It is our next work to optimize the magnetic hydrogel recipe and manufacturing process to improve its mechanical and magnetic properties to meet various requirements of applications.

3.

Conclusion

In summary, we report a new strategy to synthesize a tough magnetic hydrogel with high magnetic NPs content (up to 60%,with respect to the total weight of the polymers and water), which has the ability to adhere firmly to a variety of hydroxyl-rich surfaces. The uniform distribution of highly concentrated magnetic NPs in the hydrogel is achieved. Mechanical properties (tensile strength, fracture toughness, etc.) of magnetic hydrogels significantly change with the content of the magnetic NPs. The surface treatment of magnetic NPs improves the mechanical performance of the hydrogel and makes the magnetic hydrogel capable of bonding to diverse materials. PDMS coatings can function as seals to prevent magnetic hydrogels from dehydration. The magnetic hydrogel coated with PDMS shows excellent performance in aqueous environment. Due to the tough adhesion, the PDMS-coated 19



magnetic hydrogel is reliable even under fatigue loading. This new type of magnetic hydrogel has great potentials in applications for artificial muscles, implantable medical devices, and other emerging needs. In addition, our approach can be readily adopted to other soft functional materials with inclusions such as laponite nanoclay or fumed silica NPs for 3D printing and microparticles of neodymium-iron-boron for shape-programming.

4.

Experiment Section

Materials. Acrylamide (AAm) was purchased from Aladdin Co Ltd. (China). N, N’-Methylenebisacrylamide (MBAA), N, N, N’, N’-Tetramethyl ethylenediamine (TEMED), ammonium peroxydisulphate (APS) were bought from Sigma Co Ltd (U.S.A.). Ammonium citrate and glacial acetic acid were bought from Hu Shi Co Ltd (China). Iron oxide (II, III) (Fe3O4), 3-(Trimethoxysilyl) propyl methacrylate (TMSPMA) and Polyacrylamide (PAAm) (Molecular weight:1200w) were purchased from Macklin Co Ltd (China). Chemical Modification of NPs. We used a universal method to coat NPs32. First, 20 ml of TMSPMA, 40ml of anhydrous ethanol, and 20ml of deionized water were mixed in a beaker. Next acetic acid was added to adjust the pH value to about 6. Then 5g of Fe3O4 NPs were added into the mixture, and sealed in a container. The mixture was vigorously stirred for 2 minutes to uniformly disperse NPs, and then container was put in water bath (60 degrees Celsius for 4 hours). After that, the container was 20


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taken out from the water bath. We removed the upper clear liquid and put the lower sediments precipitation into a centrifuge tube with 30ml of alcohol as the detergent. Next, we put the tube into the centrifuge, and set up corresponding parameters (1200 rpm/min for 3 minutes). The tube was then removed from the centrifuge, with supernatant being aspirated to leave the precipitation. We repeated this step for several times until the precipitation was cleaned. This step is necessary since the hydrolysis of the silane coupling agent generates toxic methanol. Methanol and excess silane coupling agents were cleaned by ethanol. Finally, the precipitation was put into an oven (65 degrees Celsius for 4 hours), and then the Fe3O4 NPs coated with TMSPMA were obtained. Synthesis of Magnetic Hydrogel. We synthesized magnetic hydrogel by the following methods: first, powders of AAM were dissolved in the deionized water, in which the total amount of AAM was 14 wt%. A crosslinking agent (MBAA) (0.0608 wt% of AAm) were dissolved into the solution, and then 20%, 40%, 60% (with respect to the total weight of polymer and water) of coated Fe3O4 NPs were added into the solution, followed by manual stirring for 30s. After that, 2% (with respect to the total weight of polymer and water) PAAm as thickener and 1% (with respect to the total weight of polymer and water) dispersant (ammonium citrate) were mixed into the dispersion stirring for 2h (600 rpm). Only by mechanically stirring, the PAAm powders can be totally dissolved in the solution. After that the thermal initiator APS (0.029 wt% of AAm) and the accelerator TEMED (0.199 wt% of AAm) were 21



added into the mixture. Then a syringe was used to inject the mixture into the mold. After sealing the opening, the mold was placed into a box in which the air was expelled by a constant nitrogen flux for two minutes. After that, the mold was left at room temperature for 24 hours. Structure and Composition Analysis. Fourier infrared spectrometer (Nicolet 5700, USA) were used to analyze the sample, and the Fe3O4 powders with and without the modification were analyzed to determine whether the organic functional groups were grafted onto the surface of the granules. Morphological Analysis. The size and morphology of the Fe3O4 NPs embedded in the silane-modified Fe3O4-PAAm hydrogels were observed using field emission scanning electron microscopy (ZEISS). The silane-modified Fe3O4-PAAm hydrogels were freeze-dried in a vacuum freeze-dryer (Alpha 1-4/L Dplus, Germany) for 12 h. Tensile Tests. To perform the Tensile tests, the gel was cured in a mold of dog-bone shape. Tensile tests of silane-modified Fe3O4-PAAm hydrogels were performed using a testing machine (Li Gao, China) with 50N load cell. Sizes of the examples were measured using a caliper. The strain rate negligibly affects the hysteresis of tension or compression[20]. More data can be found in the supplementary information. Fracture Tests. We measured the fracture energy of the magnetic hydrogel using a method introduced by Rivlin and Thomas33. The width and thickness of the sample are a0 = 50 mm and b0 = 3mm, respectively, and the distance between two clamps is L0 = 10 mm. We prepared two kinds of samples, i.e., the unnotched one and the other 22


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one with a 10mm notch cut by a razor blade. When the notched one was stretched to fracture, the critical distance Lc between clamps was recorded. Peeling Tests. The samples of the hydrogels were prepared in the size of 80  20  3 mm, and the glass substrate was cut into the size of 110  60  2 mm. A film (35  40 mm) was put between the silane-modified magnetic hydrogel and the glass at one end, which prevents the bonding in the corresponding area and serves as a pre-crack. The whole mold was a sandwich structure and clamped by clips. After the hydrogel precursor was injected into the mold, the opening of the mold was sealed by white Vaseline. The mold was placed into a box in which the air was expelled by a constant nitrogen flux. After that, the mold was left at room temperature for 24 hours. The sample with glass substrate and PET backing layer is loaded to a mechanical testing machine (Instron) using the 90-degree peeling fixture with a peeling rate of 10 mm/min. The plateau value of the force-displacement curve gives the adhesion energy. Fatigue Tests. The sample of the hydrogel was prepared in the size of 50  50  3mm, and the surface were coated with a layer of PDMS. The sample was clamped by two clips at two sides, and then put into water for fatigue test at the rate of 150 mm/minute and stretched for 1.5 times of its original length. VSM Tests. The magnetic properties of the silane-modified Fe3O4-PAAm hydrogel and Fe3O4 NPs were investigated by a vibrating sample magnetometer (J3426, UK) with a maximum applied magnetic field of 10000 Oe at room temperature. 23



5.

Supporting Information

Sedimentation experiment, details of 90-degree peeling test, washing experiment and supporting Movie. These materials are available free of charge via the Internet at http://pubs.acs.org.

6.

Author information

Corresponding author: *E-mail: [email protected] *E-mail: [email protected] Notes: The authors declare no competing financial interest.

7.

Author contributions

S.X.Q., G.D.N., and W.Y. guided the entire work and provided useful suggestions. X.C.H. did the mechanical and magnetic experiment. X.Y.L., L.W. and H.T.L. helped to design the mechanical experiment. T.H.Y. provided help in the data processing. All authors discussed and analyzed the results.

8.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos. 11525210, 11621062, 91748209), and the Fundamental Research Funds for the Central Universities. 24


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9.

Reference

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10. Graphic for Table of Contents

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Adhesive Tough Magnetic Hydrogels with High Fe3O

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