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A 3D Printable and Mechanically Robust Hydrogel based on Alginate and Graphene Oxide Sijun Liu, Anil Kumar Bastola, and Lin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13534 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 8, 2017

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ACS Applied Materials & Interfaces

A 3D Printable and Mechanically Robust Hydrogel based on Alginate and Graphene Oxide Sijun Liu, Anil Kumar Bastola, Lin Li* School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

ABSTRACT Sodium alginate (SA) was used for the first time to non-covalently functionalize amino-graphene oxide (aGO) to produce the SA-functionalized GO, A-aGO.

A-aGO was then filled into a double

network (DN) hydrogel consisting of an alginate network (SA) and a polyacrylamide (PAAm) network. Before UV curing, A-aGO was able to provide the SA-PAAm DN hydrogel with a remarkable thixotropic property, which is desirable for 3D printing. Thus, the A-aGO-filled DN hydrogel could be nicely used as an “ink” of a 3D printer to print complicated 3D structures with a high stackability and a high shape fidelity. After UV curing, the 3D printed A-aGO filled DN hydrogel showed the robust mechanical strength and great toughness. For the function of A-aGO, it was considered that A-aGO acted as a secondary but physical crosslinker, not only to give the hydrogel a satisfactory thixotropic property but also to increase the energy dissipation by combining the physical SA network and the chemical PAAm network. As an exciting result, we have successfully developed a 3D printable and mechanically robust hydrogel.

Keywords: hydrogel, 3D printing, functionalized graphene oxide, alginate, multi-strengthening

*

Corresponding author. E-mail address: [email protected] (L. Li).

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INTRODUCTION The stretchable hydrogels have been known to be useful for applications such as tissue engineering and robotics.1-2 Elasticity, toughness and anti-fatigue are the main mechanical properties of hydrogels. Various strategies have been exploited to improve these properties, focusing mainly on controlling the mechanism of energy dissipation through a double network (DN)3-6 and the interfacial bonding between nanofiller and hydrogel matrix.7-12 However, these properties are, in general, mutually exclusive and hard to attain simultaneously. Specifically, increasing energy dissipation can greatly improve the stiffness and strength of a DN hydrogel, but often leads to a low elongation at break.13-14 The in situ grafting polymerization initiated by a free radical initiator immobilized on the surface of inorganic particles could strongly increase the interfacial interaction between polymer and inorganic particles, which resulted in a nanocomposite hydrogel with an extremely high elongation (5,300 %).15 However, the lack of an effective energy dissipation mechanism usually leads to a notch sensitivity of resulting nanocomposite hydrogels with a low stiffness (elastic modulus is lower than 100 kPa).16 Therefore, it is still a challenge for conventional hydrogels to achieve comprehensive mechanical performance for practical applications such as prosthetic tendons. The rational and smart design of microstructure is necessary for achieving distinguished mechanical properties for hydrogels. Although a great success in developing stretchable hydrogels has been achieved, it is hard to construct stretchable hydrogels into complex 3D structures because of multiple steps and rigorous conditions needed in the fabrication of stretchable hydrogels. 3D printing is an additive manufacturing process aimed at rapid production of complex 3D structures. Although it was first proposed by Hull in 1986,17 3D printing of hydrogels mainly focus on weak hydrogels for tissue

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engineering in the past three decades. The introduction of stretchable hydrogels to 3D printing

happened recently, for example, Wei et al. fabricated the stretchable agar/polyacrylamide (PAAm) hydrogels into meniscus, by using a fused deposition 3D printing technique based on thermal sensitivity of agar.18 However, to ensure the semi-gelled state of agar after extrusion, a thermal controller with a syringe heating pad has to be used to strictly control temperature of the syringe. By adding nanoclay into an alginate/polyethylene glycol (SA/PEG) pre-gel solution to control the viscosity, Hong et al. printed the tough SA/PEG hydrogels into complicated 3D structures such as pyramid and human nose, using an extrusion-based 3D printing technique.19 Wu’s group systematically studied the 3D printing of ultratough polyion complex hydrogels and constructed a spider-web-like gel structure with folded domains via 3D printing with multiple nozzles. Their hydrogels showed remarkably enhanced toughness compared to that of a normal web.20-22 It is worth noting that the printability of a pre-gel solution mainly attributes to its rheological properties, such as high viscosity and thixotropy, which endow high stackability and shape fidelity of printed constructs. For practical applications with diverse and complex patterns, such as those in biomedical devices and artificial tissues, hydrogels with good mechanical performance are desired. However, studies

on stretchable hydrogels with high strength and toughness obtained via the design of microstructure have not included 3D printing so far. Graphene, as a single atomic sheet of carbon atoms densely packed in a honeycomb crystal lattice,23-24 has attracted enormous interests in the area of composite materials because of its excellent mechanical properties and biocompatibility.25-28 Nevertheless, many of these interesting and unique properties greatly depend on the noncovalent or covalent functionalization of the basal plane or the edge of graphene sheets.29-30 Those modifications enable physical or chemical bonding

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between graphene and organic materials of interest, and also make graphene as an ideal platform to anchor various molecules to further improve the mechanical properties of the final nanocomposites and enlarge the application range of graphene.31-32 In this study, we first synthesized alginate noncovalently functionalized amino-graphene oxide, A-aGO, by a facile one-pot approach. Subsequently, we constructed the A-aGO filled alginate/polyacrylamide nanocomposite hydrogel, A-aGO/SA/PAAm, where A-aGO acted as a cocrosslinker, not only to increase the energy dissipation but also make a hierarchy microstructure by combining the physical SA network and the chemical PAAm network. As expected, the strength and toughness of the resulting A-aGO/SA/PAAm nanocomposite hydrogel were simultaneously improved. At the same time, the presence of A-aGO endows a remarkable shear-thinning behavior and thixotropic property of the A-aGO/SA hydrogel, so that this new hydrogel, A-aGO/SA/AAm, can be used as an “ink” of 3D printer to print complicated 3D structures, and the 3D printed patterns also showed the excellent mechanical properties after UV-curing.

EXPERIMENTAL SECTION Materials Graphene oxide (GO) was the product of XF NANO (Nanjing, China), and the thickness of single sheet ranged from 0.8 to 1.2 nm. Sodium alginate (SA) was purchased from Sigma-Aldrich (Singapore), and its molecular weight ranged from 100,000 to 150,000 g/mol and G block content was 50 – 60 % according to the supplier. Before use, the SA powder was dried and kept in a desiccator to avoid the absorption of moisture at room temperature. The other reagents such as ammonia (NH3), sodium bisulfite (NaHSO3), calcium chloride (CaCl2), acrylamide (AAm), N,N’-

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methylenebisacrylamide (MBA), and 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone as a photo-initiator were purchased from Sigma-Aldrich (Singapore) and used as received.

Synthesis of A-aGO Amination and alginate noncovalently functionalization of GO (A-aGO) was performed by onepot approach on the basis of a modified Bucherer reaction. First, SA powder (0.1 g) was dissolved in deionized water (20 ml) and added dropwise to a suspension of GO in deionized water (0.05 g GO, 50 ml) under vigorous stirring at room temperature for 5 h, and then the GO/SA aqueous solution was sonicated for 1 h. After that, 40 mmol NH3 and 80 mmol NaHSO3 were added to the GO/SA solution in a closed reactor and it was kept at 120 oC for about 10 h. After cooling to ambient temperature, the suspension was filtered over a 0.2 μm nylon microporous membrane and thoroughly washed with a large amount of deionized water to remove SA, NaHSO3 and unreacted NH3. Thereafter, the collected solid was dispersed and dialyzed against deionized water by a dialysis membrane [molecular weight cut-off (MWCO) = 150 kDa] at room temperature, where the water was renewed every 12 h for 4 days. The final product was lyophilized for the characterization and desired experiments. The amination of GO in the aqueous solution without SA (designated as aGO) was produced with the same method. Furthermore, SA noncovalently functionalization of aGO (denoted as A-aGO-2 to differ from A-aGO) was also prepared as follows: first of all, the obtained aGO was dispersed in deionized water, and then the aqueous solution of SA (0.1 g) was added into the aGO suspension under vigorous stirring for 12 h and sonication for 1 h; subsequently, the above solution was filtered and dialyzed, and the A-aGO-2 was obtained after freeze drying.

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Fabrication of A-aGO/SA/PAAm Nanocomposite Hydrogels The A-aGO/SA/PAAm nanocomposite hydrogels were prepared by solution polymerization from the initial solutions consisting of A-aGO, SA, AAm, MBA and a photo-initiator. Firstly, the desired amount of dried A-aGO was dispersed in 20 mL of deionized water by sonication for 1 h, and then SA and AAm were added in the A-aGO suspension of 20 mL under stirring at room temperature for 5 h, where the concentrations of SA and AAm were fixed at 4 wt% and 16 wt%, respectively. Subsequently, MBA and the photo-initiator (0.03 wt% and 5 wt% based on the weight of AAm) were added into the dispersion of A-aGO/SA/AAm, and the mixture was stirred over 2 h to obtain a homogeneous dispersion. Secondly, a certain amount of Ca2+ aqueous solution was added into the above solution and vigorously stirred at room temperature for over 5 h, and then degassed. Thereafter, the dispersion was transferred into a plastic tube (diameter D = 25 mm) and a glass mold (length × width × thickness = 120.0 mm × 120.0 mm × 2.5 mm) covered with a glass plate of a thickness of 2 mm, and then cooled at 5 °C for 2 days to produce a physically crosslinked AaGO/SA/AAm hydrogel. Thirdly, the plastic tube and the glass mold were placed under a UV lamp (wavelength of 365 nm and intensity of 8 mW/cm2) to photo-crosslink AAm at room temperature for 1 h to produce the nanocomposite hydrogel, A-aGOm/SACa-n/PAAm, where m and n are the weight percentages of A-aGO and CaCl2 respectively based on SA. For comparison, the A-aGO/PAAm nanocomposite hydrogels without and with a chemical crosslinker (MBA) were also fabricated using the same method, where the concentration of AAm was fixed at 16 wt%. At the same time, we also prepared the A-aGO/SA hydrogels to investigate the effect of A-aGO on the viscoelasticity of SA hydrogel, where the concentration of SA was fixed at 4 wt%.

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Rheological Measurement The rheological measurements were performed on a rotational rheometer (DHR, TA Instruments, USA) with a parallel plate geometry of 40 mm in diameter and a gap of 0.55 mm. The samples for rheological measurement were transferred directly from a glass bottle to the bottom plate of the rheometer using a small spoon. A low-viscosity silicone oil was placed to the sample’s edge to prevent water evaporation during a measurement. Strain sweeps in the range of 0.1 – 100 % at frequencies of 0.1 – 2 Hz were carried out to determine the linear viscoelastic range of the samples. Four kinds of rheological experiments were adopted for the SA and A-aGO/SA hydrogels: (1) frequency sweeps in the angular frequency range of 0.1 – 100 rad/s and at a constant strain of 1.0 %. (2) Steady-state shear flow from 0.01 to 500 s-1 of shear rate, (3) Strain sweeps with the oscillation strain amplitudes of 0.01 – 500 % at a constant frequency of 1 Hz, and (4) Time sweeps with alternant oscillation strains of 2 % and 300 % at a constant frequency of 1 Hz. All experiments were performed at a fixed temperature (T = 20 oC).

Mechanical Tests Compression and tensile tests were carried out using an Instron machine (Model 5567) with a 500 N load cell. The cylindrical samples with a height of 12 mm and a diameter of 25 mm were used for compression tests at 50 mm/min. The compressive strain was estimated as h/h0, where h is the height under compression and h0 is the original height. For the tensile tests, the hydrogel samples were cut into a dumbbell shape with a gauge length of 30 mm, a width of 5 mm, and a thickness of 2.5 mm. The stress-elongation curve was recorded at 100 mm/min at room temperature. The tensile strength was obtained from the failure point, and the elastic modulus was determined by the

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average slope over 20 – 70 % of elongation from the stress-elongation curve. In a hysteresis measurement, a dumbbell-shaped sample was first stretched to a predetermined elongation and then unloaded to zero force at the same speed of 100 mm/min. The energy dissipation was calculated by the area under a cycle of loading-unloading. The fracture energy, Γ, was determined as described in our previous paper.33

Characterization of A-aGO/SA/PAAm nanocomposite gels The chemical characteristics of the A-aGO/SA/PAAm nanocomposite gels were investigated using a Shimadzu IR Prestige 21 FTIR spectrometer with the ATR mode in the wavelnumber range of 4000 – 500 cm−1. The morphology of A-aGO/SA/PAAm nanocomposite gels was observed using a field emission scanning electron microscope (FESEM, JSM-7600F, JEOL, Japan). The samples for FTIR and FESEM were prepared by freeze drying as follows. The hydrogel samples with a thickness of 1 mm were frozen under a refrigerator of −20 oC for about 24 hours. After then the frozen samples were quickly transferred into a vacuum freeze dryer (Telstar cryodos-80, Telstar industrial, Spain) and dried at −80 oC for about one week. For FESEM, the freeze dried samples were fractured in liquid nitrogen and sputtered with gold in vacuum and then mounted vertically on a sample holder. Subsequently, they were observed using FESEM with an accelerating voltage of 2 KV.

3D Printing A 3D bioprinter (Biofactory® by regenHU Ltd., Switzerland) was used for printing of the AaGO/SA/PAAm nanocomposite hydrogel. The A-aGO/SA/AAm hydrogel was loaded as the ink into a printing cartridge and the printing was conducted using an extrusion-based print-head. During

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printing, the A-aGO/SA/AAm hydrogel underwent shear thinning inside the extrusion needle, and quickly regained its viscoelasticity upon exiting. The pre-defined structures were input into BioCAD (regenHU Ltd., Switzerland). Because the viscosity of SA hydrogel was enhanced by adding A-aGO, the resulting A-aGO/SA/AAm hydrogel was able to be printed into various shapes almost free from a vertical limitation. To demonstrate its ability to fabricate a multi-layered hydrogel shape, a hollow pentagon pattern was constructed by printing the hydrogel layer by layer using an optimal combination of moving rate of the bottom platform (500 mm/min) and printing pressures (1.2 bar). The printed patterns can be chemically crosslinked by UV-exposure to form the A-aGO/SA/PAAm nanocomposite hydrogel.

RESULTS AND DISCUSSION The A-aGO was synthesized by simultaneous amination and noncovalent functionalization of GO in the aqueous solution of SA on the basis of the Bucherer reaction.34-35 The reaction mechanism is illustrated in Figure 1, and the characterization of A-aGO was described previously.36 The Bucherer reaction is known as a reversible reaction of a naphthol to a naphthylamine in the presence of ammonia and sodium bisulfite.37 The initial reactants could be dispersed well in an aqueous solution, the exfoliated GO sheets were surrounded by SA chains. During ammonia functionalization of GO in the presence of SA, the phenolic oxygen groups located at the surface and the edge of GO were partially replaced by –NH2 groups in the reaction. After amination, -NH2 groups were protonated into -NH3+ and able to absorb SA chains onto the surface of aGO through the electrostatic interactions, which prevented A-aGO sheets from agglomeration. A-aGO displayed a nice dispersability in water, and no precipitates were observed even when stored at room temperature for 1 month. Similar

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experiments have also been reported for a system of GO/sodium carboxymethyl cellulose (SCMC), where SCMC was used as a polymeric dispersant that showed a remarkable ability to stabilize the reduced graphene sheets.38

Figure 1. Reaction mechanism of simultaneous amination and noncovalent functionalization of GO in the aqueous solution of SA.

The A-aGOm/SACa-n/PAAm nanocomposite hydrogels (m and n refer to the weight percentages of A-aGO and CaCl2 relative to SA) were prepared using a two-step approach. In the first step, the aqueous solution of CaCl2 with a fixed content was dropped into an initial solution consisting of AaGO, SA, acrylamide (AAm), chemical crosslinker (MBA) and photo-initiator to form the first physical network. In the subsequent step, the system was exposed to UV light (wavelength = 365 nm) to produce the final nanocomposite hydrogel. This method can exactly control the composition in the nanocomposite hydrogel to eliminate the uncontrollable swelling process and unnecessary diffusion process as reported in the literature.39-41 As shown in Figure 2a, the colors of the nanocomposite hydrogels gradually darkened with increasing the content of A-aGO. Furthermore, the A-aGO0.4/SACa6/PAAm

nanocomposite hydrogel with a cylindrical shape could resist the slicing with the cutter even

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at a strain of up to 85 % (Figure 2b), and the hydrogel sheet (dimension of 120.0 mm × 120.0 mm × 2.5 mm) with the same A-aGO content could be inflated into a balloon under the air pressure of 2 bar (Figure 2c), indicating that our nanocomposite hydrogel possesses the excellent mechanical strength and deformability. A series of tests were carried out to quantitatively examine mechanical properties of AaGO/SA/PAAm nanocomposite hydrogels. It was observed from compressive tests (Figure 2d) that the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel achieved a compressive stress of 33.2 MPa, which are 47.4 and 2.7 times higher than those of the PAAm SN hydrogel and the SACa-6/PAAm double network (DN) hydrogel, respectively. The strength and elongation at break of the AaGO0.2/SACa-6/PAAm nanocomposite hydrogel reached to 862.7 kPa and 3,324 %, which are 35.2 and 3.3 times as well as 4.0 and 2.1 times those of the PAAm SN hydrogel and the SACa-6/PAAm DN hydrogel (Figure 2e), respectively. In addition, we also compared our A-aGO/SA/PAAm nanocomposite hydrogel with the GO filled SA/PAAm nanocomposite hydrogel (GO/SA/PAAm),42 and found the tensile strength and elongation at break of A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel are larger than those (tensile strength = 201.7 kPa and elongation at break = 592 %) of GO/SA/PAAm nanocomposite hydrogel filled with 5 wt% GO, suggesting that our nanocomposite hydrogel filled by functionalized GO possesses a remarkable mechanical property. The dependence of mechanical properties of A-aGO/SACa-6/PAAm nanocomposite hydrogel on A-aGO was further displayed by the simultaneous increases in the elastic modulus and fracture energy with increasing A-aGO content (Figure 2f; Figure S1, Supporting Information), and the highest elastic modulus and fracture energy were respectively found to be 280 kPa and 12.5 kJ·m-2. Remarkably, the A-aGO/SA/PAAm nanocomposite hydrogel possesses the great fracture energy that is very much superior to the most

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developed DN hydrogels reported in the literature.14, 43 Furthermore, this result is also different from the SA/PAAm DN hydrogel whose stiffness increased but ductility decreased with increasing the concentration of physical crosslinker, Ca2+ ions (Figure S2, Supporting Information). The increases in both stiffness and toughness imply that A-aGO not only participated in the formation of SA physical gel network but also effectively connected with PAAm chains.

Figure 2. (a) Photographs of the A-aGO/SACa-6/PAAm nanocomposite hydrogels with different weight percentages of A-aGO. For the A-aGO0.4/SACa-6/PAAm nanocomposite hydrogel, it can resist slicing with a cutter even at a strain of 80 % (b) and is able to inflate into a large balloon upon applying an air pressure (c). (d) Stress-strain curves under uniaxial compression and (e) stress-elongation curves under uniaxial stretching for the PAAm SN hydrogel, the SAca-6/PAAm DN hydrogel, and the AaGO0.2/SAca-6/PAAm nanocomposite hydrogel, respectively. (f) Dependence of elastic modulus and fracture energy on A-aGO contents in the A-aGO/SAca-6/PAAm nanocomposite hydrogels.

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To understand the function of A-aGO in the A-aGO/SA/PAAm nanocomposite hydrogel, the AaGO/SA hydrogels and the A-aGO0.2/PAAm hydrogels without and with a chemical crosslinker (MBA) were fabricated. It was found that storage modulus G’ and loss modulus G’’ of the A-aGO0.2/SACa-6 hydrogel respectively increased by 2.6 and 2.0 times as compared to the SACa-6 hydrogel at 0.1 rad/s (Figure 3a). This result suggests that there would exist another physical interaction, i.e. electrostatic interaction between –COO– on the SA chains and –NH3+ on the surface of A-aGO, which contributed to the increase of G’ and G’’ in the A-aGO0.2/SACa-6 hydrogel. For the A-aGO/PAAm hydrogel, it was surprised to find that A-aGO was able to produce, in the absence of MBA, an A-aGO/PAAm nanocomposite hydrogel with a wonderful ductility (Figure 3b). For the A-aGO0.2/PAAm nanocomposite hydrogel synthesized with MBA, the stress-elongation curve showed the increased strength and elongation at break as compared to the A-aGO0.2/PAAm hydrogel prepared without MBA. This result implies that the unreduced oxygen-containing groups of A-aGO sheets participated in the radical chain transfer reactions during the free radical polymerization of AAm monomers as reported,42 which led to a fact that the A-aGO sheets acted as a platform for PAAm macromolecules to be grafted onto and as a result the mechanical properties of the nanocomposite hydrogel could be further improved. The FTIR and FESEM experiments further prove our inferences. In the FTIR spectra of the PAAm, SACa-6/PAAm and A-aGO0.2/SACa-6/PAAm gels (Figure 3c), the characteristic peaks of PAAm in the SACa-6/PAAm were almost kept unchanged in contrast to pure PAAm. After introduction of A-aGO, however, the characteristic peaks of PAAm shifted to the lower wavelengths in the A-aGO0.2/SACa6/PAAm,

indicating that the radical chain transfer reactions took place in the A-aGO0.2/SACa-6/PAAm

nanocomposite hydrogel. On the other hand, as compared to the SACa-6/PAAm DN hydrogel (Figure

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3d), a small pore size and thin pore wall were observed in the A-aGO/SACa-6/PAAm nanocomposite hydrogels, and the pore size and the wall thickness decreased with increasing A-aGO contents (Figure 3e and 3f). More interestingly, the formation of an embedded micro-network structure (red circle) in the pores was observed. That is to say, A-aGO acted as “bridges” to respectively connect the physical SA network and the chemical PAAm network, which produced the unique embedded micro-network structure. During deformation, the dissociation of SA aggregates and desorption of SA chains from the surface of A-aGO would enable an effective distribution of an applied load, reduce the crack tip stress and slow down the crack propagation. With further increasing deformation, the applied load was transferred from the SA network to the PAAm network through A-aGO, which greatly enhanced the stretching of the hydrogel. Thus, the embedded micro-network is critical for energy dispersion and force transmission, and accounts for the high mechanical strength and superior toughness of the resulting A-aGO/SA/PAAm nanocomposite hydrogels.

Figure 3. (a) Dependence of G' and G" on angular frequency for the SACa-6 and A-aGO0.2/SACa-6 hydrogels. (b) Stress-elongation curves for the PAAm SN hydrogel, and the A-aGO0.2/PAAm hydrogels with and without the chemical crosslinker, MBA. (c) FTIR spectra of the PAAm, SACa-6/PAAm and A-

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aGO0.2/SACa-6/PAAm gels. Cross-sectional morphologies of (d) the SACa-6/PAAm gel, (e) the AaGO0.2/SACa-6/PAAm gel and (f) the A-aGO0.4/SACa-6/PAAm gel prepared by freeze drying.

The schematic diagrams for the fabrication of the A-aGO/SA/PAAm nanocomposite hydrogel were proposed as shown in Figure 4. First, a dual energy dissipation mechanism through the Ca2+ ions and A-aGO sheets crosslinked SA network is considered. Second, the introduction of A-aGO into a PAAm network by a radical chain transfer reaction constructed a hybrid PAAm network, which allows for crack bridging and retaining of a mechanical integrity once the first and physical crosslinks are broken. Third, secondary crosslinks might be formed between alginate and PAAm chains by AaGO, which may facilitate force transfer between the two networks and then greatly contribute to the improvement of mechanical properties.

Figure 4. Schematic diagrams for the fabrication of the A-aGO/SA/PAAm nanocomposite hydrogel, (a) a homogeneous aqueous solution of SA, A-aGO, AAm, MBA and photo-initiator, (b) Ca2+ ions inducing the ionic association of SA chains, and (c) UV-exposure resulting in the final A-aGO/SA/PAAm

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nanocomposite hydrogel.

The loading and unloading tensile tests were performed to further illustrate the internal fracture process and toughening mechanism of the A-aGO/SA/PAAm nanocomposite hydrogels. At a constant elongation (Figure 5a), the SACa-6/PAAm DN hydrogel exhibited a small hysteresis loop and energy dissipation of 2.4 MJ·m-3, while the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel consumed a larger amount of energy of 5.6 MJ·m-3, suggesting that a lot of energy was needed to detach SA chains from A-aGO surfaces. The dissipated energy was also found to increase with increasing elongation (Figure 5b), suggesting that the internal fracture of the nanocomposite hydrogel is a gradual process. Eight successive loading and unloading tensile tests without pause between two neighboring cycles were carried out to evaluate the anti-fatigue property of our nanocomposite hydrogel (Figure S3, Supporting Information). After an obvious hysteresis in the first cycle, the overlapping for the cyclic loading-unloading curves was observed for the both hydrogels (AaGO0.2/SACa-6/PAAm and SACa-6/PAAm), indicating an excellent mechanical stability. At the same time, in contrast to the SACa-6/PAAm DN hydrogel, the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel exhibited a larger energy dissipation from the second loading-unloading cycle (Figure 5c), suggesting that the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel possesses a more outstanding anti-fatigue capacity because of the dual energy dissipation mechanism. In addition, because the high crosslinking density and small pore size restrict the dispersion of water molecules, the A-aGO/SACa6/PAAm

nanocomposite hydrogels show a lower swelling rate and smaller equilibrium swelling ratio

as compared to the SACa-6/PAAm DN hydrogel (Figure 5d).

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Figure 5. (a) Loading-unloading curves at a constant elongation for the PAAm SN hydrogel, the SACa6/PAAm

DN hydrogel and the A-aGO0.2/SA Ca-6/PAAm nanocomposite hydrogel. (b) The A-aGO0.2/SACa-

6/PAAm

nanocomposite hydrogels were subjected to a loading-unloading cycle with various

elongations. (c) Energy dissipation in eight successive loading-unloading cycles for the SACa-6/PAAm DN hydrogel and the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel. (d) Changes of the swelling ratio (q) with time for the A-aGO/SACa-6/PAAm nanocomposite hydrogels and the SACa-6/PAAm DN hydrogel, and the swelling ratio was calculated by the following equation, q = (Ws-Wd)/Wd, where Ws and Wd are the weights of the swollen hydrogel and the corresponding dried gel, respectively. For the PAAm SN hydrogel, the amount of AAm was kept the same as that in the nanocomposite hydrogel.

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As there are two kinds of physical interactions, namely ionic association of G blocks of SA chains and electrostatic interaction between –COO– on the SA chains and –NH3+ on the surface of AaGO, it is expected that the strong A-aGO/SA hydrogel will exhibit a shear-thinning behavior. As comparison, the GO/SA hydrogel was also fabricated. As shown in Figure 6a, the A-aGO0.2/SACa-6 hydrogel shows an obvious shear-thinning property upon increasing the shear rate from 0.01 to 500 s−1. At a fixed shear rate, viscosity of the A-aGO0.2/SACa-6 hydrogel is higher than that of the GO0.2/SACa-6 and SACa-6 hydrogels, indicating an enhancement effect from A-aGO. At the same time, the A-aGO0.2/SACa-6 hydrogel also shows a high sensitivity to oscillation strain as compared to the GO0.2/SACa-6 and SACa-6 hydrogels. From Figure 6b, it can be found that G' rapidly decreased when the strain increased to 3 % and intersected with G" at 23.5 % for the A-aGO0.2/SACa-6 hydrogel. If the gelsol transition point was used as an indication of the start of the collapse of the gel network structure into a quasi-liquid, it is obvious that the apparent gel-sol transition strain (23.5 %) of the AaGO0.2/SACa-6 hydrogel is smaller than those (34.7 % and 51.1 %) of the GO0.2/SACa-6 and SACa-6 hydrogels, suggesting that the A-aGO0.2/SACa-6 hydrogel is much sensitive to oscillation strain. Furthermore, the transition from the gel state to the quasi-liquid state is reversible when the AaGO0.2/SACa-6 hydrogel was subjected to the alternant oscillation strains. For example, G' and G" recovered to 92 % and 80 % of their initial values when the applied strain was decreased from 300 % to 2 % (Figure 6c), while G' and G" just recovered to 77 % and 64 % of the initial values for the SACa-6 hydrogel that experienced the same alternant strains (Figure S4, Supporting Information). These results tell us that the introduction of electrostatic interaction between SA and A-aGO can greatly improve the shear-thinning behavior and thixotropic property of the A-aGO/SA hydrogels because of the quick absorption and desorption of SA chains from the surface of A-aGO. This kind of sensitive

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shear-thinning and quick recovery properties are favored for an extrusion or injection-based 3D printing process.

Figure 6. (a) Apparent viscosity as a function of shear rate for the A-aGO0.2/SACa-6 and SACa-6 hydrogels as well as the SA solution without Ca2+ ions. (b) Storage modulus G' and loss modulus G" as a function of oscillation strain at an angular frequency of 1 Hz for the A-aGO0.2/SACa-6 and SACa-6 hydrogels as well as the SA solution without Ca2+ ions. (c) Changes of G' and G" with time at alternant oscillation strains of 2 and 300 % and at an angular frequency of 1 Hz for the A-aGO0.2/SACa-6 hydrogel, where the red numbers represent the average modulus at the oscillation strain of 2 %.

Next, we demonstrate the capability of printing A-aGO/SA/PAAm nanocomposite hydrogels into various complicated 3D structures. Because A-aGO significantly enhances the viscosity of SA hydrogel and improves its shear-thinning properties, we chose the A-aGO0.2/SACa-6/AAm system as the ink for our 3D printer (Biofactory® by regenHU Ltd., Switzerland). It has been verified that the A-aGO0.2/SACa6/PAAm

hydrogel could be printed into various shapes, such as the meshes with various pitches

(Figure 7a) and the hollow pentagon of a height of ~8 mm with a self-weight supporting ability (Figure 7b). In contrast to the A-aGO0.2/SACa-6/PAAm hydrogel, the printed hollow pentagon structure quickly collapsed when the SACa-6/PAAm hydrogel was used as the ink of 3D printer (Figure S5, Supporting Information). Furthermore, the printed patterns of the A-aGO0.2/SACa-6/PAAm hydrogel

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were also highly deformable after UV curing. As shown in Figure 7c, even though the mechanical properties of the printed dumbbell shape of the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel are lower than those of the molded one, it could still be stretched to 26 times of its initial length, which is larger than that of the corresponding DN hydrogel made by a mold. For the printed pyramid compressed to a strain of 90 %, the sample could still regain 94 % of its original height within 10 min after unloading (Figure 7d; Video S1, Supporting Information).

Figure 7. 3D printed A-aGO0.2/SACa-6/PAAm nanocomposite hydrogels. (a) Meshes with various pitches. (b) Hollow pentagon (fifteen layers) viewed from the top and side. (c) A dumbbell shape with its stress-elongation curve where a molded sample is compared, and (d) a pyramid undergoing a compressive strain of 90 % (see the left two photos) and it recovered to its original shape after the test. The printed dumbbell-shaped and pyramid samples were sealed and cured under UV light (wavelength of 365 nm) for 1 hour before testing.

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CONCLUSIONS In summary, a facile one-pot method was designed and used to synthesize alginate (SA) noncovalently functionalized amino-graphene, A-aGO, which possesses an excellent dispersibility in water. Subsequently, A-aGO filled SA/polyacrylamide nanocomposite hydrogel, A-aGO/SA/PAAm, was fabricated. As compared to the SA/PAAm DN hydrogel without A-aGO, the A-aGO/SA/PAAm nanocomposite hydrogels exhibited high tensile strength and ultra-high toughness. The simultaneously enhanced mechanical properties of the A-aGO/SA/PAAm nanocomposite hydrogels originated from the dual energy dissipation mechanism and the hierarchy structure, wherein the dissociation of SA aggregates and desorption of SA chains from A-aGO surfaces effectively dissipated energy while the secondary crosslinks by A-aGO allowed for force transfer between the SA network and the PAAm network. This multi-strengthening mechanism is a promising strategy for the design of high strength and super tough hydrogels. In addition, the presence of A-aGO endows the A-aGO/SA hydrogel with a remarkable shear-thinning behavior and thixotropic property, so that this new hydrogel (A-aGO/SA/AAm hydrogel) can be used as an ideal ink to print out complicated 3D structures with a high stackability, a high shape fidelity, and good mechanical properties after UVexposure. This novel strategy greatly enriches the construction of advanced hydrogels with good mechanical properties and complicated shapes, and expands its application in the area of tissue engineering.

ACKNOWLEDGMENT This work was supported by the Academic Research Fund Tier 1 (RG100/13) from the Ministry of Education, Singapore.

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Supporting Information

Effects of various A-aGO contents on the stress-elongation curves of the nanocomposite hydrogels; Effects of various CaCl2 contents on the mechanical properties of the SA/PAAm DN hydrogels; Eight successive loading-unloading cycles of the SACa-6/PAAm DN hydrogel and the A-aGO0.2/SACa-6/PAAm nanocomposite hydrogel; Changes of G' and G" with time in alternant oscillation strains of 2 and 300 % for the SACa-6 hydrogel; 3D printed hollow pentagon pattern when the SACa-6/AAm and GO0.2/SACa6/AAm

hydrogels were used as the ink of 3D printer.

Video S1. Compression of 3D printed nanocomposite pyramid sample after UV-curing.

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