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polyacrylamide hydrogels with supramolecular shape memory properties
Carbohydrate Polymers 231 (2020) 115736
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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Continuous liquid interface production of alginate/polyacrylamide hydrogels with supramolecular shape memory properties
T
Bingxue Huanga, Rui Hua, Zhouhang Xuea, Jiangqi Zhaoa, Qingye Lia, Tian Xiaa, Wei Zhanga,b,*, Canhui Lua,b,* a b
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, Chengdu, 610065, China Advanced Polymer Materials Research Center of Sichuan University, Shishi, 362700, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Continuous liquid interface production Hydrogel Toughness Conductivity Supramolecular shape memory
A recently developed three-dimensional (3D) gel-printing technology, namely continuous liquid interface production (CLIP), was utilized to fabricate supramolecular shape memory hydrogels with high resolutions and complex 3D geometries. The UV-curable ink for CLIP was composed of hydrogel precursors (alginate and acrylamide) and a photo-initiator (ammonium persulfate). As expected, the double network formed from ionically crosslinked alginate and covalently crosslinked polyacrylamide endowed the printed hydrogels with excellent mechanical properties. Meanwhile, due to the reversible metal-ligand coordination interaction, the hydrogel could be temporarily immobilized into an optional shape after introducing calcium ions and return to its original shapes upon ion removal, exhibiting ion-triggered shape memory effect. Moreover, the presence of ions greatly improved the conductivity of the resultant hydrogels. Such 3D printed versatile hydrogels with complex geometries demonstrated the potential for selected applications, particularly in load-bearing materials and flexible electronic devices.
1. Introduction As a water-rich stimuli-responsive material (Bilici & Okay, 2013; Dai et al., 2015; Gulyuz & Okay, 2014; Li, Dunn, Zhang, Deng, & Qi, 2017; Xu et al., 2015), shape memory (SM) hydrogels have the capability to stabilize a temporary shape and then return to their permanent geometry in response to external stimuli such as heat, light and chemicals (Inomata et al., 2012; Miyamae, Nakahata, Takashima, & Harada, 2015; Si et al., 2017; Torbati & Mather, 2016; Zhao, Huang, Wang, Sun, & Tong, 2017; Zhao, Zhang, Liu, Zhou, & Liu, 2017). In recent years, SM hydrogels have experienced impressive progress of SM hydrogels in the field of biomedical and smart materials (Hu, Guo, Kahn, Aleman‐Garcia, & Willner, 2016; Zhao, Huang et al., 2017; Zhao, Zhang et al., 2017). Thermo-responsive SM hydrogels triggered by heat are mostly investigated. However, they often encounter obstacles when applied under ambient conditions where heat sources are not easily available, such as soft robotics, textile and biomedical area (Lu, Le, Zhang, Huang, & Chen, 2017). To address this issue, supramolecular SM hydrogels commonly based on supramolecular interactions and dynamic covalent bonds have been introduced thanks to their multi-stimuli sensitivity and recyclability at room temperature (Zhao, Huang
⁎
et al., 2017; Zhao, Zhang et al., 2017; Meng et al., 2014). For example, Tang et al. designed a triple supramolecular SM hydrogel utilizing the supramolecular interaction between Ca2+ and alginate, a thiol–disulfide interchange reaction, and the hydrogen-bonding effect (Tang, Wen, Xu, Pi, & Wen, 2018). Such supramolecular SM hydrogels are well suited for more complicated applications like drug carriers to achieve sequential release of various payloads. In spite of those impressive successes, further development of SM hydrogels has been largely hampered by their simple geometries the conventional manufacturing methods can only produce (Odent et al., 2017). For example, in tissue engineering, a precise and complex 3D structure of a hydrogel scaffold is particularly desired. Consequently, more and more researchers attempted to fabricate complex-structured hydrogels with various 3D printing technologies, such as direct ink writing (DIW), stereolithography apparatus (SLA) and digital light processing (DLP) (Huang et al., 2017; Pawar et al., 2016; Yuk & Zhao, 2018). For instance, Li et al. obtained novel double network hydrogels with different geometries via DIW from sodium alginate, acrylamide and acrylic acid (Li et al., 2017). Martinez et al. employed SLA to fabricate drug-loaded hydrogels (Martinez, Goyanes, Basit, & Gaisford, 2017). Zhang et al. utilized DLP to print biostructures and tissues from
Corresponding authors at: State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, Chengdu, 610065, China. E-mail addresses: [email protected] (W. Zhang), [email protected] (C. Lu).
https://doi.org/10.1016/j.carbpol.2019.115736 Received 2 October 2019; Received in revised form 22 November 2019; Accepted 11 December 2019 Available online 20 December 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of a CLIP printer and its working mechanism.
tetramethylethylenediamine (TEMED) were supplied by Sigma-Aldrich (Shanghai, China). All the regents and ingredients were used without further purification. Distilled water was used throughout the experiment.
the acrylamide ink (Zhang et al., 2018). Nonetheless, the layer-by-layer processing of DIW induces obvious staircasing effect on the prints, leading to the low resolution of the resultant 3D structures (Hong et al., 2015). Whereas SLA and DLP are also layer-by-layer printing processes, which are not continuous and can hardly achieve a high print speed and excellent print accuracy synchronously. Fortunately, an innovative stereolithography 3D printing technology, namely continuous liquid interface production (CLIP), was developed in 2015, which well overcome these shortcomings in traditional 3D printing (Tumbleston et al., 2015). As demonstrated in Fig. 1, the CLIP printer is able to transform the 2D model files into 3D entity structures when the UV-curable liquid resin is illuminated by patterned UV radiation. On the principle of oxygen-inhibited photopolymerization to create a dead zone throughout the printing process, CLIP allows layerless and monolithic fabrication, resulting in the prints with superb resolution at a rather high printing speed. Specifically, it took less than 10 min to create a layerless 3D part with a height of 5 cm (Tumbleston et al., 2015). Alginate/polyacrylamide hydrogels have been widely investigated owing to their excellent mechanical properties and biocompatibility, and so on (Bahrami, Akbari, & Eftekhari-Sis, 2019; Morelle et al., 2018). And alginate/acrylamide hydrogels with arbitrary shapes and complicated structures may find a wide range of applications. Herein, the CLIP technology was employed for the first time to fabricate supramolecular SM alginate/polyacylamide hydrogels with various geometries. It is worth to note that the currently available inks for CLIP are almost oilbased ones (Janusziewicz, Tumbleston, Quintanilla, Mecham, & DeSimone, 2018) making the 3D prints not suitable for many advanced applications, such as biomedical and tissue engineering. The prints were subsequently immersed in a calcium chloride solution to introduce ionic crosslinking. The resultant two interpenetrating polymers inside the hydrogel with both ionic and covalent crosslinking created strong inter-molecular interactions, giving rise to its excellent mechanical properties. The highest compressive strength reached 547.2 kPa when the compressive strain was 75 %, and the hydrogel could still maintain its integrity even at 90 % compressive strain. Moreover, due to the supramolecular interactions between the calcium ions and the carboxyls on sodium alginate, the shape of the hydrogel could be temporally fixed when immersed in a calcium chloride solution in 30 s. And it could return to its original shape after the removal of calcium ions. In addition, the existence of a large amount of movable metal ions provided the hydrogel with prominent ionic conductivity, which was also highly stress-sensitive.
2.2. Preparation of the ink for CLIP Sodium alginate (0.36 g, 2 wt%), AAm (2.89 g, 0.04 mol), 7.32 mg TEMED (0.15 mol%, molar ratio of TEMED/AAm), MBA (1.40 mol%, molar ratio of MBA/AAm) were dissolved in deionized water (18 g). Then, ammonium persulfate (0.09 mol%, molar ratio of AP/AAm) was added to initiate the radical photo-polymerization of AAm monomers under UV exposure. In order to improve the resolution of the prints, crystal violet and methylene blue were incorporated to minimize the UV-light scattering. 2.3. CLIP 3D printing The printing models were designed using the 3D MAX software and exported as STL files. The printing process was conducted on the CLIP 3D printer (Laichuang 3D Science & Technology Co. Ltd, Chengdu, China) equipped with an oxygen-permeable window (60 × 70 mm2) and a UV-laser of 405 nm. The working mechanism for this printer can be found in the literature (Tumbleston et al., 2015). The exposure time for each layer (10 μm) was set to 0.1 s with a light power density of 10 mW/cm2 3. Characterization 3.1. Rheological properties Rheological measurements on the printing ink were performed with a rheometer (AR2000ex, TA Instruments Ltd., Crawley, UK) on the steady-shear-rate scanning mode at room temperature. A circular parallel plate (diameter = 35 mm) with a gap of 0.5 mm was used. 3.2. FTIR analysis The chemical structures of the samples (alginate, polyacrylamide and CLIP-printed hydrogel with ionic crosslinking) were examined by a Fourier transform infrared (FTIR) spectrometer (Nicolet Magna-IR 550, USA) in the wavenumber range from 600 to 4000 cm−1 with a resolution of 2 cm-1. For the printed hydrogel sample with ionic crosslinking, it was freeze-dried until the mass remained unchanged and then ground into fine powder for FTIR analysis.
2. Experimental section 2.1. Materials
3.3. Micro-morphology and pore structure characterization Sodium alginate, calcium chloride, methylene blue, and ammonium persulfate (AP) were all purchased from Kelong Chemicals Reagent Company (Chengdu, China). Acrylamide (AAm), N, N′-Methylenebis (acrylamide) (MBA), crystal violet, N, N, N′, N′-
The printed hydrogel was freeze-dried for 24 h before the SEM observation. The obtained samples were coated with gold for 60 s using a vacuum sputter coater (Quorum Q150 T ES, UK) and then observed 2
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early studies (Sun et al., 2017) they can also serve as the photo-initiator and the crosslinking accelerator for polyacrylamide, respectively. The UV radiation is able to break down the peroxy bond of AP to generate more free radicals. In this sense, UV can promote the polymerization of AAm, making the gelation speed compatible with the 3D printing speed (during CLIP 3D printing, the exposure time for each slice was only 0.1 s). Moreover, the photo absorbers of crystal violet and methylene blue were added to absorb excess refracted light, thereby inhibiting photo-polymerization of the resin outside the projection area. Note that the photo absorber was one of the key ingredients to ensure high printing resolution (Zhang et al., 2018). The as-prepared ink was rather flowable at ambient conditions and exhibited a stable viscosity (η) of around 970 mPa•s at a shear rate ranged from 0.01 to 10 s−1 (see Fig. S1a and b, and the discussion in the Supporting Information). Subsequently, the CLIP 3D printer was employed to run the printing of alginate-polyacrylamide hydrogel (see Video S1 in the Supporting Information). During CLIP 3D printing, a computer-aided design (CAD) file made from the 3D MAX software was sliced into two-dimensional renderings with 10 μm for each slice and the exposure time of each layer was set to 0.1 s. The patterned UV projection from the CLIP printer triggered the photopolymerization of the flowable aqueous ink in a defined area to solidify into a hydrogel by opening the double bonds on AAm and MBA (covalent crosslinker) to form polyacrylamide with a covalently crosslinked network structure (Fig. 2b). Due to the existence of oxygen entering from the oxygen-permeable window, there was an uncured area (dead zone) between the window and the cured part (see Fig. 1), which ensured a continuous printing process, leading to a high printing speed as well as an amazing apparent resolution of the product. Consequently, a series of hydrogels with complex structures, such as gear, star and letters (Fig. 2f–h), were successfully fabricated. Note that the hydrogel samples might appear in various colors, owing to the different dyes used and/or soaking times in the solution. Next, the 3D printed hydrogel with a covalent crosslinking structure was immersed in a calcium chloride solution to ionically crosslink its alginate part (Fig. 2c). And the soaking time was optimized to 4 h (see Fig. S2 and the discussion in the Supporting Information).
with SEM (LEO 1530, Germany) at 10 kV. The specific surface area and the pore structure of the freeze-dried hydrogel were determined from nitrogen adsorption-desorption experiments at −196 °C using an accelerated surface area and porosimetry system (NOVA-2000E, Quantachrome, USA). 3.4. Mechanical properties For tensile testing, the samples were fabricated by replica-molding from cuboids (width = 15 mm, thickness = 4 mm and gauge length = 50 mm), while column shaped molds (diameter = 25 mm, height = 15 mm) were used to produce samples for compressive testing. Photo-polymerization of the hydrogel was completed after exposure to the UV light of the CLIP printer for 3 h. All the samples were then soaked in a calcium chloride solution (0.5 wt%) for 4 h to stabilize the physicochemical properties of the hydrogels. The mechanical properties were measured on a universal testing machine (Instron 5567, USA) in ambient conditions. For both compressive and tensile tests, the crosshead speed was set at 2 mm/min. 3.5. Swelling ratio measurements To test the swelling ratio before and after ionic crosslinking, samples were put into an oven at 70 °C until the weight remained unchanged and then soaked in deionized water for a specified time until the equilibrium was attained. The sample weight was measured every few hours. The swelling ratio of the hydrogel was calculated as follows:
Swelling ratio =
Wt − Wo Wo
where Wt is the sample weight after swelling and Wo is the dry weight. 3.6. Shape memory effect To evaluate the shape memory properties, the hydrogel sample was re-shaped under external force in 0.5 wt% calcium chloride solution to fix the temporary geometry. Subsequently, the hydrogel sample was soaked in 0.1 M EDTA•2Na solution to recover its original shape.
4.2. FTIR analysis 3.7. Ionic conductivity measurements The FTIR spectra were depicted in Fig. 3. There were a broad peak near 3400 cm−1 for OeH stretching, two sharp peaks at 1620 and 1410 cm−1 for asymmetric and symmetric COOe stretching, a peak at 1320 cm−1 for C–H deformation with secondary alcohols, and three peaks at 1120, 1090, and 1030 cm−1 for asymmetric CeOeC stretching, CeO stretching in CHeOH, and CeO stretching in CeOeC structures in the spectrum of alginate. For polyacrylamide, the spectrum showed two peaks at 3410 and 3200 cm−1 representing the stretching vibration of NeH, and a sharp peak at 1670 cm−1 for C]O stretching. There were also bands at 1620 cm−1 (NeH for primary amine), 1450 cm−1 (CH2 in-plane scissoring), 1420 cm−1 (CeN for primary amide), 1350 cm−1 (C–H), and 1120 cm−1 (NH2 in-plane rocking). In the spectrum of ion-crosslinked hydrogel, the characteristic peaks of both alginate and polyacrylamide could be observed. However, the intensity of the bands related to OeH (3400 cm−1), primary amide (1620 and 1420 cm−1), CeO in CHeOH structure (1090 cm−1), symmetric CeO stretching in CeOeC structure (1030 cm−1) decreased. Notably, compared with alginate, the wavenumber for asymmetric −COO- stretching of alginate in the ion-crosslinked alginate/polyacrylamide hydrogel decreased, while that for symmetric −COOstretching increased. These results consistently revealed the interaction between calcium ions and the carboxyl groups on sodium alginate according to the literature (Nakamoto, 2006). Hence, it could be concluded that the hydrogel with assumed structure in Fig. 2 had been successfully prepared.
The changes of ionic conductivity for the hydrogels under various compressive strains were detected using a multimeter (UNI-T 61E, China) and a universal testing machine (Instron 5567, USA) at a compressive speed of 2 mm/min. The testing circuit was consisted of a cylindrical hydrogel (diameter = 25 mm, height = 15 mm), conductive copper wire and the multimeter. 4. Results and discussion 4.1. CLIP 3D printing of the alginate-polyacrylamide hydrogels The formulation of the liquid resin and the detailed photo-polymerization process for CLIP 3D printing were provided in Fig. 2a–f. The UV-curable ink was composed of water, sodium alginate, AAm monomer and other functional additives (photoinitiator, covalent crosslinker, and dye). And Fig. 2a showed the chemical structures of the main ingredients of the ink. Two crosslinkable polymers, polyacrylamide and sodium alginate, were selected as the main components of the hydrogels. And MBA and calcium chloride were used as covalent and ionic crosslinkers for polyacrylamide and sodium alginate, respectively. The photo-polymerization of AAm was triggered by AP under UV irradiation. The optimum photoinitiator concentration was at 0.09 % (molar ratio of AP/AAm) as demonstrated in Fig. S1c in the Supporting Information. Note that AP and TEMED are widely used as the redox initiator pair for polymerization. However, according to some 3
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Fig. 2. (a)–(e) Scheme of the chemical composition of liquid resin and the mechanism of photo-polymerization and crosslinking; (f)–(h) 3D printed gear, star and letters.
concentrations (0.2 wt%, 0.5 wt% and 0.8 wt%) were measured to be 0.519 ± 0.107, 0.052 ± 0.009 and 0.035 ± 0.011 mm, respectively (the pores were randomly selected from the SEM images for size measurement). Obviously, the pore size of hydrogels decreased when soaked in a calcium ion solution with a higher concentration. Moreover, the BET tests also suggested a similar trend in pore size as well as the
4.3. Morphologies and pore structures of CLIP printed hydrogel SEM was employed to intuitively observe the micro morphologies of the 3D printed hydrogel ionically crosslinked with different content of Ca2+. From the SEM images in Fig. 4, the average pore sizes for hydrogels ionically crosslinked in CaCl2 solutions with different
Fig. 3. FTIR spectra of alginate, polyacrylamide (a), and 3D printed alginate/polyacrylamide hydrogels after ionic crosslinking (b). 4
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Fig. 4. SEM images of 3D printed hydrogels ionically crosslinked in CaCl2 solutions with different concentrations (0.2 wt% (a), 0.5 wt% (b) and 0.8 wt% (c)).
4.5. Mechanical properties
specific surface area upon ion concentration changes (see Table S1 and Fig. S3 in the Supplementary Information for details). This is normal because a higher crosslinking density will induce a smaller pore size of the hydrogel (Zhang, Zhang, Lu, & Deng, 2012). Notably, the existence of abundant interconnected pores could endow the hydrogel with a large specific surface area and provide a medium for the transport of functional species (such as conductive ions, nutrients and wastes).
Both tensile and compressive mechanical properties of the hydrogels were examined, and the results were presented in Fig. 6a and b. The ionic crosslinking with calcium ions greatly improved the hydrogels’ tensile strength (60.8 ± 1.7 kPa) and elongation at break (508.3 ± 21.8 %), corresponded to the enhancement of 396.8 % in strength and 229.1 % in elongation as compared with those without ion cross-linking. In the meantime, the compressive strength of the hydrogel was also significantly increased after ionic crosslinking (from 156.4 ± 17.6 to 468.7 ± 97.6 kPa at a compressive strain of 75 %). The improvement of mechanical properties should be possibly attributed to the following reasons. First, the unzipping of ionic crosslinks facilitated energy dissipation. Second, the long molecular chains of polyacrylamide with covalent crosslinks were favorable for the maintenance of hydrogels’ structural integrity when their ionic crosslinks were damaged by external stress. In order to further explore the anti-fatigue and anti-compression properties of hydrogels, a compression recovery test (five cycles, 2 mm/ s) was performed on the samples both with and without ionic crosslinking. For the sample without ionic crosslinking, the five compression curves overlapped almost completely (Fig. 6c). And the dissipation energy corresponding to the cyclic compression curve also had little difference (around 7278.9 J/m3). It indicated that the mechanical integrity of hydrogel was well maintained after five compressive cycles. In contrast, for the ionically crosslinked hydrogel, its compression strength was greatly improved. Except for the larger area of the first hysteresis loop, the later four test curves almost coincided (Fig. 6d). And the dissipation energy of ion-crosslinked hydrogel was distinctly higher than that of the sample without ionic crosslinking (Fig. 6e and f). Fig. 6g intuitively demonstrated the anti-compression performance of a CLIP printed cylindrical hydrogel subjected to 90 % compressive deformation on a universal testing machine. The hydrogel was not crushed and could restore to the original shape rapidly when the pressure
4.4. Effect of ionic crosslinking on the swelling behaviors of hydrogels Hydrogels are a kind of water-rich materials with 3D network structures. Hydrogel will swell and its volume will expand when soaked in water. The infiltration of water into a polymer matrix results in the weakening of intermolecular force and the increase of molecular distance. However, the volume expansion is always accompanied by the contraction of the hydrogel network structure. When such two opposing trends become balanced, a swelling equilibrium state is reached. Thereby, the swelling behaviors are important features of hydrogels highly associated with their internal structures. Generally, the equilibrium swelling ratio is governed by the chemical composition as well as the crosslinking density of hydrogel. Herein, the effect of ionic crosslinking on the swelling ratio of dehydrated hydrogel in deionized water was measured, and the results were illustrated in Fig. 5. The hydrogels had been ionically crosslinked in calcium chloride solutions with different concentrations. All the hydrogels swelled right after immersed in water. And the water absorption rate obviously decreased as the Ca2+ concentration of the soaking solution increased. After 170 h, the hydrogels reached their equilibrium swelling ratios of 18.3 ± 0.1 (without ionic crosslinking), 13.0 ± 0.6 (0.2 wt%), 10.7 ± 0.1 (0.5 wt%) and 9.2 ± 0.3 (0.8 wt %), respectively. The decreased equilibrium swelling ratio for the hydrogels ionically crosslinked in a higher concentration Ca2+ solution agreed well with their higher crosslinking density that restricted more extensively the molecular chain extension during swelling.
Fig. 5. (a) The swelling ratios of hydrogels ionically crosslinked in calcium chloride solutions with different concentrations at different swelling time; (b) The equilibrium swelling ratios for different hydrogel samples. 5
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Fig. 6. Tensile (a) and compressive (b) stress-strain curves for hydrogels with and without ionic crosslinking. Cyclic compressive stress-strain curves for hydrogels without (c) and with (d) ionic crosslinking at a maximum strain of 75 % for 5 compression-release cycles, respectively. Histograms of hysteresis energy corresponding to cyclic compressive stress-strain curves of hydrogels without (e) and with (f) ionic crosslinking. (g) A cylindrical hydrogel sample subjected to a compressive strain of 90 % could return to its original shape after the release of external force.
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shown in Fig. 7c, the triangular hydrogel could be fixed into a torch-like shape with Ca2+ crosslinking (time = 0 h). And when soaked in EDTA•2Na, the hydrogel took 6 ± 1 h to gradually return to its initial state. Furthermore, the recovered hydrogel was subjected to the second shape memory test. It could still be fixed into a temporary torch-like shape and recover the initial shape after EDTA treatment, suggesting its excellent cyclability in SM.
was released, suggesting its excellent anti-fatigue and anti-compression properties.
4.6. Shape memory effect Sodium alginate is a natural polysaccharide extracted from seaweed, whose molecule chains are formed by β-D-mannuronate (M) blocks and α-L-guluronate (G) blocks. Various metal ions can form temporary supramolecular interactions with carboxyl groups on the molecular chain of G blocks due to the high inclusiveness of the interaction. Also, the supramolecular interaction is highly responsive to external stimuli such as pH changes and the exclusion of metal ions, which endows the alginate hydrogel with fantastic shape memory effect. To evaluate the ion-triggered shape memory effect, a strip-shaped hydrogel sample was deformed into various shapes (such as helix and U shapes) by external force, and then soaked in 0.1 M CaCl2 or FeCl3 solution. As expected, the sample could ‘memorize’ its temporary shape very rapidly (∼30 s) as a result of strong supramolecular interactions between sodium alginate and metal ions (see Fig. 1). Afterward, by using EDTA•2Na which has a much higher chelating constant, the metal ions could be removed from the hydrogel. And ultimately it recovered its initial strip shape at room temperature (Fig. 7a and b). The shape memory properties were also verified on a triangle-shaped hydrogel. As
4.7. Ionic conductivity Owing to the presence of abundant metal ions and the porous structures, the hydrogels could be used as a conductive material. To visually demonstrate the ionic conductivity of the hydrogel, a complete circuit was designed, which consisted of a cylindrical hydrogel, a blue light-emitting diode (LED) and a dry battery. As shown in Fig. 8a and b, the LED was lighted up when the circuit was switched on. In addition, when the hydrogel was compressed, the brightness of the LED was remarkably increased, suggesting its potential applications in pressure sensors. Moreover, Fig. 8c quantitatively showed the resistance (R) of different hydrogels as a function of compressive strain (from 0 to 50 %). Obviously, the R value would decrease under compression and the hydrogel with a higher metal ion concentration exhibited lower R at the same compressive strain.
Fig. 7. (a, b) Shape memory effect of the hydrogel induced by metal ions (calcium and iron ions); (c) Photographs that demonstrated the shape recovery process of the 3D-printed hydrogel in EDTA and the second SM behavior of hydrogel. 7
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Fig. 8. (a, b) Photographs and corresponding schematics that visually demonstrate the electrical conductivity changes of hydrogels under compression; (c) the R for hydrogels fabricated at different metal ion concentrations as a function of compressive strain.
5. Conclusion
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In this work, we report a mechanically strong and ionically conductive 3D double network hydrogel of alginate/polyacrylamide with fascinating supramolecular SM properties, which was 3D printed for the first time by the novel CLIP technology. This technology allowed a rapid and continuous production of complex 3D structured hydrogels without sacrificing their resolution and fidelity. Moreover, due to the chelation of carboxyl groups of sodium alginate with metal ions, the hydrogel could fix a temporary shape in a short period of time in metal ion solutions, and recover its original shape after metal ion removal. Given that the hydrogel was highly compressible/stretchable and had a high content of water to host movable ions, it exhibited a prominent ionic conductivity which was sensitive to external forces. It was envisaged that these CLIP printed hydrogels with complex 3D geometries and attractive properties would play important roles in various advanced applications, such as in load-bearing materials and flexible electronic devices. Author contribution Bingxue Huang finished completed the literature writing and analyzed experimental results. Rui Hu and Zhouhang Xue completed the picture production in this paper together. Jiangqi Zhao, Tian Xia and Qingye Li carried out experiments. Wei Zhang and Canhui Lu conceived and designed the experiments. Acknowledgements This work was supported by the National Natural Science Foundation of China (51433006 and 51861165203), Sichuan Science and Technology Program (2019YJ0125), State Key Laboratory of Polymer Materials Engineering (sklpme2019-2-19), and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2019.115736. References Bahrami, Z., Akbari, A., & Eftekhari-Sis, B. (2019). Double network hydrogel of sodium alginate/polyacrylamide cross-linked with POSS: Swelling, dye removal and mechanical properties. International Journal of Biological Macromolecules, 129, 187–197. Bilici, C., & Okay, O. (2013). Shape memory hydrogels via micellar copolymerization of acrylic acid and n-octadecyl acrylate in aqueous media. Macromolecules, 46(8), 3125–3131. Dai, X., Zhang, Y., Gao, L., Bai, T., Wang, W., Cui, Y., et al. (2015). A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel. Advanced Materials, 27, 3566–3571. Gulyuz, U., & Okay, O. (2014). Self-healing poly (acrylic acid) hydrogels with shape memory behavior of high mechanical strength. Macromolecules, 47(19), 6889–6899.
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Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Continuous liquid interface production of alginate/polyacrylamide hydrogels with supramolecular shape memory properties
T
Bingxue Huanga, Rui Hua, Zhouhang Xuea, Jiangqi Zhaoa, Qingye Lia, Tian Xiaa, Wei Zhanga,b,*, Canhui Lua,b,* a b
State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, Chengdu, 610065, China Advanced Polymer Materials Research Center of Sichuan University, Shishi, 362700, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Continuous liquid interface production Hydrogel Toughness Conductivity Supramolecular shape memory
A recently developed three-dimensional (3D) gel-printing technology, namely continuous liquid interface production (CLIP), was utilized to fabricate supramolecular shape memory hydrogels with high resolutions and complex 3D geometries. The UV-curable ink for CLIP was composed of hydrogel precursors (alginate and acrylamide) and a photo-initiator (ammonium persulfate). As expected, the double network formed from ionically crosslinked alginate and covalently crosslinked polyacrylamide endowed the printed hydrogels with excellent mechanical properties. Meanwhile, due to the reversible metal-ligand coordination interaction, the hydrogel could be temporarily immobilized into an optional shape after introducing calcium ions and return to its original shapes upon ion removal, exhibiting ion-triggered shape memory effect. Moreover, the presence of ions greatly improved the conductivity of the resultant hydrogels. Such 3D printed versatile hydrogels with complex geometries demonstrated the potential for selected applications, particularly in load-bearing materials and flexible electronic devices.
1. Introduction As a water-rich stimuli-responsive material (Bilici & Okay, 2013; Dai et al., 2015; Gulyuz & Okay, 2014; Li, Dunn, Zhang, Deng, & Qi, 2017; Xu et al., 2015), shape memory (SM) hydrogels have the capability to stabilize a temporary shape and then return to their permanent geometry in response to external stimuli such as heat, light and chemicals (Inomata et al., 2012; Miyamae, Nakahata, Takashima, & Harada, 2015; Si et al., 2017; Torbati & Mather, 2016; Zhao, Huang, Wang, Sun, & Tong, 2017; Zhao, Zhang, Liu, Zhou, & Liu, 2017). In recent years, SM hydrogels have experienced impressive progress of SM hydrogels in the field of biomedical and smart materials (Hu, Guo, Kahn, Aleman‐Garcia, & Willner, 2016; Zhao, Huang et al., 2017; Zhao, Zhang et al., 2017). Thermo-responsive SM hydrogels triggered by heat are mostly investigated. However, they often encounter obstacles when applied under ambient conditions where heat sources are not easily available, such as soft robotics, textile and biomedical area (Lu, Le, Zhang, Huang, & Chen, 2017). To address this issue, supramolecular SM hydrogels commonly based on supramolecular interactions and dynamic covalent bonds have been introduced thanks to their multi-stimuli sensitivity and recyclability at room temperature (Zhao, Huang
⁎
et al., 2017; Zhao, Zhang et al., 2017; Meng et al., 2014). For example, Tang et al. designed a triple supramolecular SM hydrogel utilizing the supramolecular interaction between Ca2+ and alginate, a thiol–disulfide interchange reaction, and the hydrogen-bonding effect (Tang, Wen, Xu, Pi, & Wen, 2018). Such supramolecular SM hydrogels are well suited for more complicated applications like drug carriers to achieve sequential release of various payloads. In spite of those impressive successes, further development of SM hydrogels has been largely hampered by their simple geometries the conventional manufacturing methods can only produce (Odent et al., 2017). For example, in tissue engineering, a precise and complex 3D structure of a hydrogel scaffold is particularly desired. Consequently, more and more researchers attempted to fabricate complex-structured hydrogels with various 3D printing technologies, such as direct ink writing (DIW), stereolithography apparatus (SLA) and digital light processing (DLP) (Huang et al., 2017; Pawar et al., 2016; Yuk & Zhao, 2018). For instance, Li et al. obtained novel double network hydrogels with different geometries via DIW from sodium alginate, acrylamide and acrylic acid (Li et al., 2017). Martinez et al. employed SLA to fabricate drug-loaded hydrogels (Martinez, Goyanes, Basit, & Gaisford, 2017). Zhang et al. utilized DLP to print biostructures and tissues from
Corresponding authors at: State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute at Sichuan University, Chengdu, 610065, China. E-mail addresses: [email protected] (W. Zhang), [email protected] (C. Lu).
https://doi.org/10.1016/j.carbpol.2019.115736 Received 2 October 2019; Received in revised form 22 November 2019; Accepted 11 December 2019 Available online 20 December 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Schematic of a CLIP printer and its working mechanism.
tetramethylethylenediamine (TEMED) were supplied by Sigma-Aldrich (Shanghai, China). All the regents and ingredients were used without further purification. Distilled water was used throughout the experiment.
the acrylamide ink (Zhang et al., 2018). Nonetheless, the layer-by-layer processing of DIW induces obvious staircasing effect on the prints, leading to the low resolution of the resultant 3D structures (Hong et al., 2015). Whereas SLA and DLP are also layer-by-layer printing processes, which are not continuous and can hardly achieve a high print speed and excellent print accuracy synchronously. Fortunately, an innovative stereolithography 3D printing technology, namely continuous liquid interface production (CLIP), was developed in 2015, which well overcome these shortcomings in traditional 3D printing (Tumbleston et al., 2015). As demonstrated in Fig. 1, the CLIP printer is able to transform the 2D model files into 3D entity structures when the UV-curable liquid resin is illuminated by patterned UV radiation. On the principle of oxygen-inhibited photopolymerization to create a dead zone throughout the printing process, CLIP allows layerless and monolithic fabrication, resulting in the prints with superb resolution at a rather high printing speed. Specifically, it took less than 10 min to create a layerless 3D part with a height of 5 cm (Tumbleston et al., 2015). Alginate/polyacrylamide hydrogels have been widely investigated owing to their excellent mechanical properties and biocompatibility, and so on (Bahrami, Akbari, & Eftekhari-Sis, 2019; Morelle et al., 2018). And alginate/acrylamide hydrogels with arbitrary shapes and complicated structures may find a wide range of applications. Herein, the CLIP technology was employed for the first time to fabricate supramolecular SM alginate/polyacylamide hydrogels with various geometries. It is worth to note that the currently available inks for CLIP are almost oilbased ones (Janusziewicz, Tumbleston, Quintanilla, Mecham, & DeSimone, 2018) making the 3D prints not suitable for many advanced applications, such as biomedical and tissue engineering. The prints were subsequently immersed in a calcium chloride solution to introduce ionic crosslinking. The resultant two interpenetrating polymers inside the hydrogel with both ionic and covalent crosslinking created strong inter-molecular interactions, giving rise to its excellent mechanical properties. The highest compressive strength reached 547.2 kPa when the compressive strain was 75 %, and the hydrogel could still maintain its integrity even at 90 % compressive strain. Moreover, due to the supramolecular interactions between the calcium ions and the carboxyls on sodium alginate, the shape of the hydrogel could be temporally fixed when immersed in a calcium chloride solution in 30 s. And it could return to its original shape after the removal of calcium ions. In addition, the existence of a large amount of movable metal ions provided the hydrogel with prominent ionic conductivity, which was also highly stress-sensitive.
2.2. Preparation of the ink for CLIP Sodium alginate (0.36 g, 2 wt%), AAm (2.89 g, 0.04 mol), 7.32 mg TEMED (0.15 mol%, molar ratio of TEMED/AAm), MBA (1.40 mol%, molar ratio of MBA/AAm) were dissolved in deionized water (18 g). Then, ammonium persulfate (0.09 mol%, molar ratio of AP/AAm) was added to initiate the radical photo-polymerization of AAm monomers under UV exposure. In order to improve the resolution of the prints, crystal violet and methylene blue were incorporated to minimize the UV-light scattering. 2.3. CLIP 3D printing The printing models were designed using the 3D MAX software and exported as STL files. The printing process was conducted on the CLIP 3D printer (Laichuang 3D Science & Technology Co. Ltd, Chengdu, China) equipped with an oxygen-permeable window (60 × 70 mm2) and a UV-laser of 405 nm. The working mechanism for this printer can be found in the literature (Tumbleston et al., 2015). The exposure time for each layer (10 μm) was set to 0.1 s with a light power density of 10 mW/cm2 3. Characterization 3.1. Rheological properties Rheological measurements on the printing ink were performed with a rheometer (AR2000ex, TA Instruments Ltd., Crawley, UK) on the steady-shear-rate scanning mode at room temperature. A circular parallel plate (diameter = 35 mm) with a gap of 0.5 mm was used. 3.2. FTIR analysis The chemical structures of the samples (alginate, polyacrylamide and CLIP-printed hydrogel with ionic crosslinking) were examined by a Fourier transform infrared (FTIR) spectrometer (Nicolet Magna-IR 550, USA) in the wavenumber range from 600 to 4000 cm−1 with a resolution of 2 cm-1. For the printed hydrogel sample with ionic crosslinking, it was freeze-dried until the mass remained unchanged and then ground into fine powder for FTIR analysis.
2. Experimental section 2.1. Materials
3.3. Micro-morphology and pore structure characterization Sodium alginate, calcium chloride, methylene blue, and ammonium persulfate (AP) were all purchased from Kelong Chemicals Reagent Company (Chengdu, China). Acrylamide (AAm), N, N′-Methylenebis (acrylamide) (MBA), crystal violet, N, N, N′, N′-
The printed hydrogel was freeze-dried for 24 h before the SEM observation. The obtained samples were coated with gold for 60 s using a vacuum sputter coater (Quorum Q150 T ES, UK) and then observed 2
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early studies (Sun et al., 2017) they can also serve as the photo-initiator and the crosslinking accelerator for polyacrylamide, respectively. The UV radiation is able to break down the peroxy bond of AP to generate more free radicals. In this sense, UV can promote the polymerization of AAm, making the gelation speed compatible with the 3D printing speed (during CLIP 3D printing, the exposure time for each slice was only 0.1 s). Moreover, the photo absorbers of crystal violet and methylene blue were added to absorb excess refracted light, thereby inhibiting photo-polymerization of the resin outside the projection area. Note that the photo absorber was one of the key ingredients to ensure high printing resolution (Zhang et al., 2018). The as-prepared ink was rather flowable at ambient conditions and exhibited a stable viscosity (η) of around 970 mPa•s at a shear rate ranged from 0.01 to 10 s−1 (see Fig. S1a and b, and the discussion in the Supporting Information). Subsequently, the CLIP 3D printer was employed to run the printing of alginate-polyacrylamide hydrogel (see Video S1 in the Supporting Information). During CLIP 3D printing, a computer-aided design (CAD) file made from the 3D MAX software was sliced into two-dimensional renderings with 10 μm for each slice and the exposure time of each layer was set to 0.1 s. The patterned UV projection from the CLIP printer triggered the photopolymerization of the flowable aqueous ink in a defined area to solidify into a hydrogel by opening the double bonds on AAm and MBA (covalent crosslinker) to form polyacrylamide with a covalently crosslinked network structure (Fig. 2b). Due to the existence of oxygen entering from the oxygen-permeable window, there was an uncured area (dead zone) between the window and the cured part (see Fig. 1), which ensured a continuous printing process, leading to a high printing speed as well as an amazing apparent resolution of the product. Consequently, a series of hydrogels with complex structures, such as gear, star and letters (Fig. 2f–h), were successfully fabricated. Note that the hydrogel samples might appear in various colors, owing to the different dyes used and/or soaking times in the solution. Next, the 3D printed hydrogel with a covalent crosslinking structure was immersed in a calcium chloride solution to ionically crosslink its alginate part (Fig. 2c). And the soaking time was optimized to 4 h (see Fig. S2 and the discussion in the Supporting Information).
with SEM (LEO 1530, Germany) at 10 kV. The specific surface area and the pore structure of the freeze-dried hydrogel were determined from nitrogen adsorption-desorption experiments at −196 °C using an accelerated surface area and porosimetry system (NOVA-2000E, Quantachrome, USA). 3.4. Mechanical properties For tensile testing, the samples were fabricated by replica-molding from cuboids (width = 15 mm, thickness = 4 mm and gauge length = 50 mm), while column shaped molds (diameter = 25 mm, height = 15 mm) were used to produce samples for compressive testing. Photo-polymerization of the hydrogel was completed after exposure to the UV light of the CLIP printer for 3 h. All the samples were then soaked in a calcium chloride solution (0.5 wt%) for 4 h to stabilize the physicochemical properties of the hydrogels. The mechanical properties were measured on a universal testing machine (Instron 5567, USA) in ambient conditions. For both compressive and tensile tests, the crosshead speed was set at 2 mm/min. 3.5. Swelling ratio measurements To test the swelling ratio before and after ionic crosslinking, samples were put into an oven at 70 °C until the weight remained unchanged and then soaked in deionized water for a specified time until the equilibrium was attained. The sample weight was measured every few hours. The swelling ratio of the hydrogel was calculated as follows:
Swelling ratio =
Wt − Wo Wo
where Wt is the sample weight after swelling and Wo is the dry weight. 3.6. Shape memory effect To evaluate the shape memory properties, the hydrogel sample was re-shaped under external force in 0.5 wt% calcium chloride solution to fix the temporary geometry. Subsequently, the hydrogel sample was soaked in 0.1 M EDTA•2Na solution to recover its original shape.
4.2. FTIR analysis 3.7. Ionic conductivity measurements The FTIR spectra were depicted in Fig. 3. There were a broad peak near 3400 cm−1 for OeH stretching, two sharp peaks at 1620 and 1410 cm−1 for asymmetric and symmetric COOe stretching, a peak at 1320 cm−1 for C–H deformation with secondary alcohols, and three peaks at 1120, 1090, and 1030 cm−1 for asymmetric CeOeC stretching, CeO stretching in CHeOH, and CeO stretching in CeOeC structures in the spectrum of alginate. For polyacrylamide, the spectrum showed two peaks at 3410 and 3200 cm−1 representing the stretching vibration of NeH, and a sharp peak at 1670 cm−1 for C]O stretching. There were also bands at 1620 cm−1 (NeH for primary amine), 1450 cm−1 (CH2 in-plane scissoring), 1420 cm−1 (CeN for primary amide), 1350 cm−1 (C–H), and 1120 cm−1 (NH2 in-plane rocking). In the spectrum of ion-crosslinked hydrogel, the characteristic peaks of both alginate and polyacrylamide could be observed. However, the intensity of the bands related to OeH (3400 cm−1), primary amide (1620 and 1420 cm−1), CeO in CHeOH structure (1090 cm−1), symmetric CeO stretching in CeOeC structure (1030 cm−1) decreased. Notably, compared with alginate, the wavenumber for asymmetric −COO- stretching of alginate in the ion-crosslinked alginate/polyacrylamide hydrogel decreased, while that for symmetric −COOstretching increased. These results consistently revealed the interaction between calcium ions and the carboxyl groups on sodium alginate according to the literature (Nakamoto, 2006). Hence, it could be concluded that the hydrogel with assumed structure in Fig. 2 had been successfully prepared.
The changes of ionic conductivity for the hydrogels under various compressive strains were detected using a multimeter (UNI-T 61E, China) and a universal testing machine (Instron 5567, USA) at a compressive speed of 2 mm/min. The testing circuit was consisted of a cylindrical hydrogel (diameter = 25 mm, height = 15 mm), conductive copper wire and the multimeter. 4. Results and discussion 4.1. CLIP 3D printing of the alginate-polyacrylamide hydrogels The formulation of the liquid resin and the detailed photo-polymerization process for CLIP 3D printing were provided in Fig. 2a–f. The UV-curable ink was composed of water, sodium alginate, AAm monomer and other functional additives (photoinitiator, covalent crosslinker, and dye). And Fig. 2a showed the chemical structures of the main ingredients of the ink. Two crosslinkable polymers, polyacrylamide and sodium alginate, were selected as the main components of the hydrogels. And MBA and calcium chloride were used as covalent and ionic crosslinkers for polyacrylamide and sodium alginate, respectively. The photo-polymerization of AAm was triggered by AP under UV irradiation. The optimum photoinitiator concentration was at 0.09 % (molar ratio of AP/AAm) as demonstrated in Fig. S1c in the Supporting Information. Note that AP and TEMED are widely used as the redox initiator pair for polymerization. However, according to some 3
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Fig. 2. (a)–(e) Scheme of the chemical composition of liquid resin and the mechanism of photo-polymerization and crosslinking; (f)–(h) 3D printed gear, star and letters.
concentrations (0.2 wt%, 0.5 wt% and 0.8 wt%) were measured to be 0.519 ± 0.107, 0.052 ± 0.009 and 0.035 ± 0.011 mm, respectively (the pores were randomly selected from the SEM images for size measurement). Obviously, the pore size of hydrogels decreased when soaked in a calcium ion solution with a higher concentration. Moreover, the BET tests also suggested a similar trend in pore size as well as the
4.3. Morphologies and pore structures of CLIP printed hydrogel SEM was employed to intuitively observe the micro morphologies of the 3D printed hydrogel ionically crosslinked with different content of Ca2+. From the SEM images in Fig. 4, the average pore sizes for hydrogels ionically crosslinked in CaCl2 solutions with different
Fig. 3. FTIR spectra of alginate, polyacrylamide (a), and 3D printed alginate/polyacrylamide hydrogels after ionic crosslinking (b). 4
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Fig. 4. SEM images of 3D printed hydrogels ionically crosslinked in CaCl2 solutions with different concentrations (0.2 wt% (a), 0.5 wt% (b) and 0.8 wt% (c)).
4.5. Mechanical properties
specific surface area upon ion concentration changes (see Table S1 and Fig. S3 in the Supplementary Information for details). This is normal because a higher crosslinking density will induce a smaller pore size of the hydrogel (Zhang, Zhang, Lu, & Deng, 2012). Notably, the existence of abundant interconnected pores could endow the hydrogel with a large specific surface area and provide a medium for the transport of functional species (such as conductive ions, nutrients and wastes).
Both tensile and compressive mechanical properties of the hydrogels were examined, and the results were presented in Fig. 6a and b. The ionic crosslinking with calcium ions greatly improved the hydrogels’ tensile strength (60.8 ± 1.7 kPa) and elongation at break (508.3 ± 21.8 %), corresponded to the enhancement of 396.8 % in strength and 229.1 % in elongation as compared with those without ion cross-linking. In the meantime, the compressive strength of the hydrogel was also significantly increased after ionic crosslinking (from 156.4 ± 17.6 to 468.7 ± 97.6 kPa at a compressive strain of 75 %). The improvement of mechanical properties should be possibly attributed to the following reasons. First, the unzipping of ionic crosslinks facilitated energy dissipation. Second, the long molecular chains of polyacrylamide with covalent crosslinks were favorable for the maintenance of hydrogels’ structural integrity when their ionic crosslinks were damaged by external stress. In order to further explore the anti-fatigue and anti-compression properties of hydrogels, a compression recovery test (five cycles, 2 mm/ s) was performed on the samples both with and without ionic crosslinking. For the sample without ionic crosslinking, the five compression curves overlapped almost completely (Fig. 6c). And the dissipation energy corresponding to the cyclic compression curve also had little difference (around 7278.9 J/m3). It indicated that the mechanical integrity of hydrogel was well maintained after five compressive cycles. In contrast, for the ionically crosslinked hydrogel, its compression strength was greatly improved. Except for the larger area of the first hysteresis loop, the later four test curves almost coincided (Fig. 6d). And the dissipation energy of ion-crosslinked hydrogel was distinctly higher than that of the sample without ionic crosslinking (Fig. 6e and f). Fig. 6g intuitively demonstrated the anti-compression performance of a CLIP printed cylindrical hydrogel subjected to 90 % compressive deformation on a universal testing machine. The hydrogel was not crushed and could restore to the original shape rapidly when the pressure
4.4. Effect of ionic crosslinking on the swelling behaviors of hydrogels Hydrogels are a kind of water-rich materials with 3D network structures. Hydrogel will swell and its volume will expand when soaked in water. The infiltration of water into a polymer matrix results in the weakening of intermolecular force and the increase of molecular distance. However, the volume expansion is always accompanied by the contraction of the hydrogel network structure. When such two opposing trends become balanced, a swelling equilibrium state is reached. Thereby, the swelling behaviors are important features of hydrogels highly associated with their internal structures. Generally, the equilibrium swelling ratio is governed by the chemical composition as well as the crosslinking density of hydrogel. Herein, the effect of ionic crosslinking on the swelling ratio of dehydrated hydrogel in deionized water was measured, and the results were illustrated in Fig. 5. The hydrogels had been ionically crosslinked in calcium chloride solutions with different concentrations. All the hydrogels swelled right after immersed in water. And the water absorption rate obviously decreased as the Ca2+ concentration of the soaking solution increased. After 170 h, the hydrogels reached their equilibrium swelling ratios of 18.3 ± 0.1 (without ionic crosslinking), 13.0 ± 0.6 (0.2 wt%), 10.7 ± 0.1 (0.5 wt%) and 9.2 ± 0.3 (0.8 wt %), respectively. The decreased equilibrium swelling ratio for the hydrogels ionically crosslinked in a higher concentration Ca2+ solution agreed well with their higher crosslinking density that restricted more extensively the molecular chain extension during swelling.
Fig. 5. (a) The swelling ratios of hydrogels ionically crosslinked in calcium chloride solutions with different concentrations at different swelling time; (b) The equilibrium swelling ratios for different hydrogel samples. 5
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Fig. 6. Tensile (a) and compressive (b) stress-strain curves for hydrogels with and without ionic crosslinking. Cyclic compressive stress-strain curves for hydrogels without (c) and with (d) ionic crosslinking at a maximum strain of 75 % for 5 compression-release cycles, respectively. Histograms of hysteresis energy corresponding to cyclic compressive stress-strain curves of hydrogels without (e) and with (f) ionic crosslinking. (g) A cylindrical hydrogel sample subjected to a compressive strain of 90 % could return to its original shape after the release of external force.
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shown in Fig. 7c, the triangular hydrogel could be fixed into a torch-like shape with Ca2+ crosslinking (time = 0 h). And when soaked in EDTA•2Na, the hydrogel took 6 ± 1 h to gradually return to its initial state. Furthermore, the recovered hydrogel was subjected to the second shape memory test. It could still be fixed into a temporary torch-like shape and recover the initial shape after EDTA treatment, suggesting its excellent cyclability in SM.
was released, suggesting its excellent anti-fatigue and anti-compression properties.
4.6. Shape memory effect Sodium alginate is a natural polysaccharide extracted from seaweed, whose molecule chains are formed by β-D-mannuronate (M) blocks and α-L-guluronate (G) blocks. Various metal ions can form temporary supramolecular interactions with carboxyl groups on the molecular chain of G blocks due to the high inclusiveness of the interaction. Also, the supramolecular interaction is highly responsive to external stimuli such as pH changes and the exclusion of metal ions, which endows the alginate hydrogel with fantastic shape memory effect. To evaluate the ion-triggered shape memory effect, a strip-shaped hydrogel sample was deformed into various shapes (such as helix and U shapes) by external force, and then soaked in 0.1 M CaCl2 or FeCl3 solution. As expected, the sample could ‘memorize’ its temporary shape very rapidly (∼30 s) as a result of strong supramolecular interactions between sodium alginate and metal ions (see Fig. 1). Afterward, by using EDTA•2Na which has a much higher chelating constant, the metal ions could be removed from the hydrogel. And ultimately it recovered its initial strip shape at room temperature (Fig. 7a and b). The shape memory properties were also verified on a triangle-shaped hydrogel. As
4.7. Ionic conductivity Owing to the presence of abundant metal ions and the porous structures, the hydrogels could be used as a conductive material. To visually demonstrate the ionic conductivity of the hydrogel, a complete circuit was designed, which consisted of a cylindrical hydrogel, a blue light-emitting diode (LED) and a dry battery. As shown in Fig. 8a and b, the LED was lighted up when the circuit was switched on. In addition, when the hydrogel was compressed, the brightness of the LED was remarkably increased, suggesting its potential applications in pressure sensors. Moreover, Fig. 8c quantitatively showed the resistance (R) of different hydrogels as a function of compressive strain (from 0 to 50 %). Obviously, the R value would decrease under compression and the hydrogel with a higher metal ion concentration exhibited lower R at the same compressive strain.
Fig. 7. (a, b) Shape memory effect of the hydrogel induced by metal ions (calcium and iron ions); (c) Photographs that demonstrated the shape recovery process of the 3D-printed hydrogel in EDTA and the second SM behavior of hydrogel. 7
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Fig. 8. (a, b) Photographs and corresponding schematics that visually demonstrate the electrical conductivity changes of hydrogels under compression; (c) the R for hydrogels fabricated at different metal ion concentrations as a function of compressive strain.
5. Conclusion
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In this work, we report a mechanically strong and ionically conductive 3D double network hydrogel of alginate/polyacrylamide with fascinating supramolecular SM properties, which was 3D printed for the first time by the novel CLIP technology. This technology allowed a rapid and continuous production of complex 3D structured hydrogels without sacrificing their resolution and fidelity. Moreover, due to the chelation of carboxyl groups of sodium alginate with metal ions, the hydrogel could fix a temporary shape in a short period of time in metal ion solutions, and recover its original shape after metal ion removal. Given that the hydrogel was highly compressible/stretchable and had a high content of water to host movable ions, it exhibited a prominent ionic conductivity which was sensitive to external forces. It was envisaged that these CLIP printed hydrogels with complex 3D geometries and attractive properties would play important roles in various advanced applications, such as in load-bearing materials and flexible electronic devices. Author contribution Bingxue Huang finished completed the literature writing and analyzed experimental results. Rui Hu and Zhouhang Xue completed the picture production in this paper together. Jiangqi Zhao, Tian Xia and Qingye Li carried out experiments. Wei Zhang and Canhui Lu conceived and designed the experiments. Acknowledgements This work was supported by the National Natural Science Foundation of China (51433006 and 51861165203), Sichuan Science and Technology Program (2019YJ0125), State Key Laboratory of Polymer Materials Engineering (sklpme2019-2-19), and the Fundamental Research Funds for the Central Universities. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2019.115736. References Bahrami, Z., Akbari, A., & Eftekhari-Sis, B. (2019). Double network hydrogel of sodium alginate/polyacrylamide cross-linked with POSS: Swelling, dye removal and mechanical properties. International Journal of Biological Macromolecules, 129, 187–197. Bilici, C., & Okay, O. (2013). Shape memory hydrogels via micellar copolymerization of acrylic acid and n-octadecyl acrylate in aqueous media. Macromolecules, 46(8), 3125–3131. Dai, X., Zhang, Y., Gao, L., Bai, T., Wang, W., Cui, Y., et al. (2015). A mechanically strong, highly stable, thermoplastic, and self-healable supramolecular polymer hydrogel. Advanced Materials, 27, 3566–3571. Gulyuz, U., & Okay, O. (2014). Self-healing poly (acrylic acid) hydrogels with shape memory behavior of high mechanical strength. Macromolecules, 47(19), 6889–6899.
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