Alginate Tough Composite

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Ionically Cross-Linked Silk Microfibers/Alginate Tough Composite Hydrogels with Hierarchical Structures Lei Meng, Changyou Shao, and Jun Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04055 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 24, 2018

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ACS Sustainable Chemistry & Engineering

Ionically Cross-Linked Silk Microfibers/Alginate Tough Composite Hydrogels with Hierarchical Structures

Lei Meng,

Changyou Shao,

Jun Yang*

Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China. *E-mail:

[email protected]

Tel: 86-10-62337223.

Keywords: Hydrogels, Alginate, Silk Fibroin, Reinforcement, Hierarchical Structures

Abstract Developing hydrogels with enhanced mechanical properties have attracted broad attention in recent years. In this work, we propose a facile procedure to prepare tough composite hydrogels by incorporating silk microfibers (mSF) into alginate ionically cross-linked network. The mSF gives rise to ionic bonds with Ca2+ and interfacial hydrogen bonds because of the carboxyl groups on the surface of mSF, and the neighboring alginate chains are interlinked by mSF, which synergistically lead to efficient energy dissipation and prevention of stress concentration. The attained composite hydrogels show superior elastic modulus (1.58 MPa), tensile strength (1.60 MPa) and unique adaptive interface response. Moreover, the mechanical properties of the composite hydrogels can be tailored by the concentration of mSF and post-crosslinking time by immersing the composite hydrogels in CaCl2 solution. Intriguingly, the mechanical properties can be further improved through prestretching methodology to align the alginate chains along the stress direction, where the oriented hierarchical structures are formed and well-retained in the prestretched composite hydrogels. We envisage that this study provides a general strategy for designing composite hydrogels with both excellent and tunable mechanical properties, which enriches the route of alginate hydrogels for promising applications where high-loading requirement needed.

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INTRODUCTION Hydrogels with three dimensional hydrophilic networks that cross-linked by physical or chemical crosslinking1-4, are being developed for many applications, such as tissue engineering5, 6, drug delivery7, sensors8-10 and actuators11, 12. Derived from natural algae, alginate has been proved to be a promising candidate for the hydrogel matrix, which exhibits the desirable advantages of being favorable biodegradability, biocompatibility and wound healing effects13, 14. Alginate is a nature polymer of (1-4)β-D-mannuronic acid (M) and (1-4)-α-L-guluromic acid (G) units, forming alginate ionic network by virtue of coordination interaction in the presence of calcium and other divalent metal in aqueous mediu ions15-18. However, owing to poor mechanical performance, low thermal stability, and uncontrollable structure degradation, the practical application of neat alginate hydrogels are limited. To solve these drawbacks, a common practice is incorporating various nanofillers to prepare composite hydrogels, such as carbon nanotubes (CNTs)19, graphene20,

21,

hydroxyapatite22, silicates23, silk fibroin24 and

nanocellulose25, 26. Silk fibroin, being one of the ideal candidates for biomedical applications, is derived from Bombyx mori cocoons that composed of nanocrystalline domain and amorphous domain27, showing significant potential due to its combined excellent mechanical properties and high biocompatibility28-32. Silk microfibers (mSF) can be directly prepared from silk fibroin by alkali hydrolysis, which leads to the removal of amorphous domain of silk fibroin and the nanocrystalline domain with higher resistance to alkali attack remain intact33, 34. Due to its highly nanocrystalline domain, mSF has been extensively utilized as reinforcing agent in polymer matrices. For example, Kaplan et al. added mSF into silk scaffolds to enhance human bone marrow-derived mesenchymal stem cells (hMSCs) differentiation via mimicking the mechanical features of native bone33. Shi et al. in situ assembled of SF-based hydrogels with shear-thinning and autonomous self-healing characteristics through dynamic metal-ligand coordination chemistry in physiological conditions34. The obtained hydrogels could support stem cell proliferation in vitro and accelerated bone regeneration in cranial critical size defects without additional


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morphogenes delivered. The dynamics of molecular assembly are the core of natural material generation and result in structural hierarchy ranging from the nano- to the macroscale, such as bone, nacre, and wood that possess a heterogeneous and porous structure35, 36. These architectures have been a source of inspiration for the preparation of synthetic counterparts in biological systems37, 38. Recently, Gong et al applied a top-down manufacturing technique to direct the self-assembly process of alginate and generated hierarchically well-defined nanofibrillar hydrogels39. Due to the anisotropic architectures of alginate hydrogels that endowed by the properties of flexibility and elastic extensibility, the backbone of linear polysaccharide straightened out along the direction of external force and formed the aligned architecture from initial random arrangements. This stress directed assembly method can induce spontaneous mechanical signals and tune supramolecular interactions among alginate to create hydrogels with highly aligned self-templated hierarchical fibrillar structures, and the attained well-defined fibrous networks were well-retained in the reswollen hydrogels40. In this study, we attempt a facile approach to design mSF/alginate composite hydrogels with reinforced mechanical properties by incorporating mSF into alginate ionically cross-linked network. The attained composite gels demonstrate superior elastic modulus (1.58 MPa) and ultrahigh tensile strength (1.60 MPa) at the mSF concentration as low as 0.1 wt%, which are 2.43-fold and 6.15-fold of pristine alginate gels, respectively. Besides, the mechanical performance of composite hydrogels can be easily regulated by varying the concentration of mSF and post-crosslinking time via immersion of the composite hydrogels in CaCl2 solution. The excellent mechanical performance stems from the carboxyl groups on the surface of mSF that can give rise to ionic bonds with Ca2+ and interfacial hydrogen bonds. Furthermore, the mechanical properties can be further improved by prestretching methodology. Under the uniaxial tension, the alginate chains are aligned along the stress direction to build strength field and the oriented structures are formed in the prestretched composite hydrogels. Meanwhile, the mechanical reinforcement can be controlled in a wide range by prestretching strain levels and the oriented fibrillar networks are well-retained even immersing in water for 4 d. It is expected that the super mechanical properties combined with desirable biodegradability and biocompatibility of mSF/alginate composite



hydrogels may provide great potential in tissue engineering and other biomedical fields41.

EXPERIMENTAL SECTION Chemical and Materials. Silk cocoons of silkworm Bombyx mori were provided by Shandong Hanxing Biotechnology Co., Ltd. China. Sodium alginate (G/M = 0.3, Mw =1.44×106 Da), sodium hydroxide (NaOH), calcium chloride (CaCl2) and sodium carbonate (Na2CO3) were obtained from Xilong Chemical Reagent Company, China. All others reagents were analytical grade and used as received without further purification. Deionized water was used in all the experiments. Preparation of silk microfibers (mSF). In brief, silkworm cocoons (10 g) were cut into small pieces and then degummed twice in 2 L of sodium carbonate aqueous solution (0.02 M) at 100 ºC for 30 min followed by rinsing thoroughly with distilled water to remove the sericin from silk fibroin. The collected degummed silk fibroin was then immersed in 400 mL of sodium hydroxide solution (1 M) and stirred at 40 ºC for 5 h. In this alkali hydrolysis process, the amorphous domain of silk fibroin was preferentially hydrolyzed, whereas nanocrystalline domain with the higher resistance to alkali attack remained intact. The alkaline hydrolysis was stopped by removing alkaline liquid using centrifugation (10000 rpm, 10 min), which was followed by washing the microfibers with distilled water until pH neutrality33. Finally, the microfibers from wet silk slurry were obtained by drying at an air circulation oven at 60 ºC for 24 h. Preparation of mSF/alginate hydrogels. The mSF/alginate hydrogels were prepared by a soaking process from mixture solution of mSF and alginate. Typically, to prepare 2.5 wt% alginate solutions, sodium alginate (5 g) was added into 195 ml of deionized water and stirred at 60 ºC for 4 h to ensure that the alginate was completely dissolved. After cooling to room temperature, the mSF (the mass ratio of mSF against alginate solution at 0.05, 0.1, 0.3, 0.5 wt%) was added to the above uniform mSF/alginate aqueous suspension. The above suspension was degassed under vacuum and poured into a PTFE mold (80 mm × 80 mm × 8 mm), then 15 mL of calcium chloride solution (0.5 M) was


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introduced from the upper empty part of the PTFE mold (0.4 mm in thickness) and left for 7 d to complete the cross-linking process. Finally, the attained composite hydrogels were thoroughly washed with distilled water to remove superfluous cations. To prepare prestretched mSF/alginate hydrogels, the pristine composite gels were fixed with clamps at both ends and stretched to a certain strain (10%, 20%, 40%, 80%, 120%) under CaCl2 solution (0.5 M) at room temperature for 2 days. To assess the aligned structure stability, the prestretched gels were left in deionized water at room temperature for 4 d, and the mechanical properties were measured as a function of immersing time. Mechanical tests. The mechanical properties of the gels were measured using the Zwick Z005 with a 100 N load cell at room temperature. The rectangular samples (35 mm in length, 10 mm in width, and 0.4 mm in thickness) coated with silicone oil were mounted between the clamps where the initial distance was 15 mm, and the stretching rate was set at 25 mm/min. The nominal tensile stress (σ) was obtained by dividing the force by the initial cross-sectional area of the specimen. The tensile strain (ε) was defined as the ratio of gauge length to the initial gauge length.The elastic modulus (E) was calculated as the initial linear range from the stress-strain curves and the energy-to-break (T) of hydrogels was defined as the area under stress-strain curves. To assess the mechanical stability of composite gels, the mechanics retention coefficient of the swollen composite hydrogels immersing in phosphatic buffer solution (PBS, 0.1 M, pH = 6.8) for 2 days after equilibrium swelling was calculated as the ratio between the tensile strength (σ), fracture strain (ε), elastic modulus (E), and energy-to-break (T) of the swollen composite hydrogels and as-prepared hydrogels. For self-recovery experiment, the samples were initially stretched to a predetermined strain (25%) at a stretching rate of 10 mm/min and unloaded at the same rate. After each cycle, the samples were relaxed at room temperature for a certain waiting time (0, 30, 60 min) before the next loading process. The dissipated energy was calculated as the area between the loading-unloading curves, and the energy dissipation ratio was defined as the ratio between dissipated energy and the elastic energy stored in the materials. For stress relaxation, the specimens were extended to a predetermined strain (50%), and the time-dependent relaxation of stress was recorded. Morphological analysis. Atomic force microscopy (AFM) test was conducted on AFM Bruker



Multimode 8 (Bruker, American) in ScanAsyst mode. The mSF aqueous suspension (0.05 wt%) was dropped onto a fresh mica substrate and allowed to dry in a fume hood at room temperature before observation. The polarizing microscope (POM) images were captured using Olympus LV320 polarizing microscope. The surface morphology of samples was examined under a field-emission scanning electron microscope (FESEM, Hitachi SU8010) at an accelerating voltage of 15 kV. The samples were frozen by plunging into liquid nitrogen followed by freeze-dried and gold-coated for 60 s using a plasma coater under high vacuum. Fourier transformed infrared spectroscopy. Infrared spectra of gel samples were recorded on an infrared spectrophotometer (Nicolet iN10-MX, Thermo Scientific) using the KBr disc method mode in the range of 4000 ~ 400 cm-1. Thermal analysis. The thermal degradation behavior of aerogels (5 mg) was investigated using TA Instruments-Waters LLC under nitrogen flow and heated from 40 to 800 ºC at a heating rate of 10 ºC/min in a platinum pan.

RESULT AND DISCUSSION

Figure 1. Characterization of mSF (a) FTIR of alginate, mSF, Ca2+-alginate gel and mSF/alginate gel. (b) The differential thermogravimetry (DTG) curves of mSF, silk fibroin, alginate, alginate gel and mSF/alginate gel.


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The mSF with length about 300 nm and diameter around 50-200 nm was prepared from nature silk fibroin through alkali hydrolysis34(Figure S1). During the hydrolysis process, alkali initiated the hydrolysis of amide bonds to form the carboxylic group and amide, leading to the preferential hydrolysis of amorphous domain of silk fibroin, whereas the nanocrystalline domains with higher resistance to alkali attack remained intact. Indeed, the 1621, 1516, and 1224 cm-1peaks appeared in fourier transform infrared spectroscopy (FTIR) after hydrolysis, which corresponded to the crystalline domains of silk fibroin functional groups of amide Ι (C=O stretching), amide ΙΙ (C-N stretching and N-H deformation), and amide ΙΙΙ (C-N stretching and N-H deformation) (Figure 1a), respectively. Besides, the maximum degradation temperature increased from 305 ºC for silk fibroin to 325 ºC for mSF after alkali hydrolysis in differential thermogravimetry (DTG), which further indicated the remaining of crystalline domains from silk fibroin after alkali hydrolysis treatment (Figure 1b).

Figure 2. Schematic illustration of the fabrication process for the flexible mSF/alginate composite hydrogels.

Figure 2 demonstrated the two-step preparation process of mSF/alginate gels. Briefly, the mSF was firstly added to the alginate solution to obtain uniformly mixed mSF/alginate aqueous suspension. Then, calcium chloride solution was introduced to the above suspension to produce ionic cross-linking between Ca2+ and mSF and alginate. In this process, Ca2+ ions diffused into suspension to form the ionic



bonds with COO- of mSF and alginate, which acted as cross-links for random coil chains of silk and alginate via metal–chelate complexes. Motivated by the strong ionic interaction and hydrogen bonding, as well as the interlinks between mSF and alginate, the alginate chains formed the ionic cross-linked network interpenetrated with the mSF to achieve tough mSF/alginate composite gels. With the addition of Ca2+, the absorption peaks of amid Ι, ΙΙ, ΙΙΙ of mSF at 1621, 1516, and 1224 cm-1 were shifted to 1617, 1500, 1256 cm-1, respectively, and the typical alginate adsorption bands also moved from 1603 to 1600 cm-1, 1360 to 1450 cm-1 and 1009 to 1113 cm-1, respectively (Figure 1a), inferring the formation of ionic interaction between Ca2+ ions and the carboxyl groups on the mSF and alginate4, 34. Besides, compared with mSF, the amide Ι band of mSF/alginate composite gels (C=O stretching) shifted to lower frequency (from 1621 to 1617 cm-1) verified the formation of hydrogen bonds between mSF and alginate. The improvement of thermal stability of mSF/alginate composites was shown by differential thermogravimetry curves (Figure 2b), where the maximum degradation temperature for the mSF/alginate gels shifted toward the higher temperature (311 ºC) compare with that of neat alginate gels (287 ºC). This result was related to the dissociation of ionic bonds and hydrogen bonds42, 43.

Figure 3. POM images of (a) alginate gel, (b) 0.1 wt% and (c) 0.3 wt% mSF/alginate composite gels.

The distribution of the composite gels observed by polarizing microscope (POM) was shown in Figure 3. It is well known that the compatibility between polymer matrix and reinforcing phase and the dispersion of reinforcing phase within matrix are essential to attain high-performance composites44. The neat alginate gels exhibited a plenty of interconnecting networks, which was consistent with previous reports45 (Figure 3a). Figure 3b indicated that when a small amount of mSF was introduced in alginate (0.1 wt%), the homogeneous dispersion of mSF in alginate matrix was achieved, and no apparent self-


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aggregation or microphase separation in the gels was observed. However, the self-aggregation of mSF appeared in the composite gels as the concentration of mSF increased to 0.3 wt%, which resulted in deteriorated mechanical properties of composite gels (Figure 3c).

Figure 4. (a) Tensile stress-strain curves, (b) elastic modulus and energy-to-break of composite hydrogels with different weight ratios of mSF.

As shown in Figure S2, the 0.1 wt% mSF/alginate composite gels exhibited excellent mechanical strength that can easily bear 500 g of weight (corresponding to 600 times of its own weight). The strainstress curves showed that mSF content had a significant influence on the strength and extensibility of the gels. For example, when the mSF concentration increased from 0.05 to 0.1 wt% (Figure 4a), the tensile strength and fracture strain of composite gels increased from 0.26 to 1.6 MPa and 98 to 184%, respectively. The elastic modulus and energy-to-break of gels displayed the similar trend, and the tensile elastic modulus and energy-to-break of 0.1 wt% mSF/alginate composite gels achieved 1.58 MPa and 128.48 MJ/m3, respectively, which were 3 and 10 times higher than that of the pristine alginate gels (Figure 4b). However, when the concentration of mSF exceeded to 0.3 wt%, the tensile strength and strain decreased to 0.91 MPa and 135%, respectively. At a relative low content, the mSF could be uniformly dispersed in the alginate matrix, the carboxyl groups participated the building of ionic bonds and interacted with alginate chains through hydrogen bonds, resulting in the formation of robust and flexible network structure. However, the excessive mSF would aggregate in the alginate matrix and result in stress concentration, which led to the structural homogeneity and low energy dissipation



efficiency under large deformation.

Figure 5. (a) Tensile stress-strain curves of 0.1 wt% mSF/alginate composite gels at different strain rate. (b) Stress relaxation curves of mSF/alginate gels composites.

As shown in Figure 5a, the uniaxial tensile mechanical properties of composite gels were found to be strain-rate dependent. The result indicated that the increase in loading rate from 0.083 to 1.333 s-1 led to 2.91 times increase in tensile strength, whereas the fracture strain decreased from 224% to 156%. In addition, to examine the time-dependent stress evolution of the deformed physical network, the stressrelaxation measurement was performed by holding a prestrain at 50% to stretch the gels (Figure 5b). The results indicated the stress level of composite gels released much larger in comparison with that of neat alginate gels, which was mainly attributed to the synergetic relaxation of hydrogen bonds and ionic bonds between Ca2+ and carboxyl groups of mSF and alginate. The disruption and reassociation of Ca2+coordinated ionic bonds and hydrogen bonds between mSF and alginate occurred in the stretched composite gels during the stress-relaxation process, thus allowing the ionic network of composite gels to adopt to the applied forces. These results are consistent with the result of tensile tests. Moreover, to assess the mechanical stability of swollen composite gels, the retention coefficient of tensile strength (σ), fracture strain (ε), elastic modulus (E), and energy-to-break (T) of the swollen composite gels in PBS for 2 days were shown in Figure S3. The results indicated that the mechanical properties did not show significant decrease in aqueous environment and the mechanics retention coefficient exceeded


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90%, implying high structure stability of the swollen composite gels.

Figure 6. Mechanical behaviors of 0.1 wt% mSF/alginate composite hydrogels with different soaking times in 0.5 M CaCl2 solution: (a) Tensile stress-strain curves and (b) the corresponding elastic modulus and energy-to-break.

For the mSF/alginate composite hydrogels, the increase in the soaking time in calcium chloride solution led to a substantial increase in the mechanical properties (Figure 6). One can note that the tensile strength, strain, elastic modulus, and energy-to-break of the composite gels increased at the initial 7 d, then leveled off when the post-crosslinking time exceeded to 14 d. This result can be explained that the ionic cross-linking between Ca2+ and carboxyl groups of mSF and alginate was formed at the initial immersing time and then most metal-chelate sites were saturated with further extended immersion time, which led to the almost constant mechanical properties.



Figure 7. Hysteresis and self-recovery properties of 0.1 wt% mSF/alginate composite hydrogels. (a) Fatigue resistance by fifty successive loading-unloading cycles and the (b) corresponding dissipated energy and maximal stress. (c) Cyclic tensile loading-unloading curves at 25% strain with different resting times (0, 30, 60 min). (d) Time-dependent recovery of hysteresis and elastic modulus.

The hysteresis of composite gels can be measured by loading-unloading test to reflect the energy dissipation in a cycle, and the area enclosed by the hysteresis curves represented the energy dissipation per unit volume upon deformation46. As shown in Figure 7a, the 0.1 wt% mSF/alginate composite gels with maximum stretch of 25% displayed a large hysteresis loop in the first loading-unloading cycle and then the hysteresis loop appeared a substantial decrease in the following cycles. After fifty continuous cycles, the dissipated energy and the maximum stress still remained to be more than 1.65 kJ/m3 and 0.1 MPa, respectively (Figure 7b). For the mSF/alginate composite hydrogels, the strong ionic coordination bonds acted as the reversible sacrificial bonds and disrupted to dissipate energies when the external loading was applied. When the external loading was removed, the temporarily dissociated ionic coordinate bonds could be reconstructed without any external stimuli at room temperature, leading to


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the excellent fatigue resistance of mSF/alginate composite gels. This reversible mSF and alginate ionic networks empowered the composite gels with excellent antifatigue ability, which would be benefit to the composite gels being the substitution of soft tissue scaffolds. To further examine the time-dependent recovery behaviors of composite gels, the loadingunloading tests were performed at a predetermined strain of 25% with different resting time (Figure 7c). The results indicated that the tensile strength of gels without resting between two consecutive loadings decreased obviously compared to the original sample. Interestingly, as the sample rest for 30 min, the composite gels achieved an even higher stress (0.21 MPa) than its original stress (0.18 MPa), and the tensile strength further increased to 0.23 MPa when the tensile test was measured after 60 min resting time. Meanwhile, the dissipated energy and elastic modulus of composite gels recovered to be 3.89 MJ/m3 and 1.49 MPa after 60 min waiting time, respectively, which were also higher than the original sample (3.48 MJ/m3 and 1.03 MPa) (Figure 7d). This improvement in tensile strength, dissipated energy and elastic modulus may be attributed to the stiffness of individual mSF and reversible adsorptiondesorption between the flexible alginate chain and adjacent rigid mSF, leading to adaptive interface response. According to Akcora et al, periodic deformation process led to unique stiffening of the composites and this adaptive interface response was related to the enhancement in the entanglement of chains at interface47. In this regard, this self-stiffening of mSF/alginate composite gels with attractive mSF-alginate interaction under deformation can occur through reversible adsorption-desorption of alginate chains at mSF surface under repeated cyclic deformation48-50.



Figure 8. (a) Schematic illustration of prestretching methodology for creating highly aligned fibrous hydrogels. Optical microscope images of prestretching 0.1 wt% mSF/alginate composite hydrogels with a certain strain (b) 0%, (c) 10%, (d) 20%, (e) 40%, (f) 80%, and (g) 120%. SEM images of 0.1 wt% mSF/alginate composite gels before (h) and after (i) prestretching deformation.

To further enhance the mechanical properties of composite gels, we adopted the prestretching


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methodology to form oriented structure under stress. Considering the excellent mechanical properties of 0.1 wt% mSF/alginate composite gels, we choose this gel as the model to demonstrate the capacity of mechanical force directed assembly. As schemed in Figure 8a, a rectangular specimen were clamped at both long ends and stretched to a certain strain (10%-120%). Then, the gels were allowed to immerse them into CaCl2 (0.5 M) solution for 2 days to prepare the prestretched composite gels. Optical images showed the evolution of oriented fibrous structure formation process as a function of prestretching strain (Figure 8b-g), where a thick fibrous structure aligned along axial direction was observed with increase of strain. This result suggested the alignment of alginate chains can be controlled by varying the degree of mechanical stress. Furthermore, the SEM images also showed that the remarkable oriented structure was formed after prestretching treatment, which further indicated the relatively rigid alginate chains aligned easily along the tensile stress direction through the redistribution of physical interactions to form the oriented superstructure (Figure 8h, i). It should be noted that the oriented architecture was only ascribed to the alginate chains and no orientation was found for mSF in the prestretched composite gels (Figure S4).

Figure 9. (a) Tensile stress-strain curves of prestretched composite hydrogels with a certain strain (0, 10%, 20%, 40%, 80%, 120%) and (b) the corresponding elastic modulus and energy-to-break. (c) The variation in tensile strength of 120% prestretching mSF/alginate composite hydrogels after immersing



in water for 4 d at room temperature and the corresponding strength retention coefficient, and (d) the optical images of aligned fibrillar structure with increased immersion time.

The tensile strength of prestretched composite gels was found to be strongly dependent on the degree of prestretching level (Figure 9a). When the degree of prestretching level increased from 20 to 120%, the tensile strength can increase from 1.60 to 19.13 MPa, which was 12 times higher than that of the original composite gels. Besides, the elastic modulus and energy-to-break of the prestretched composite gels also increased significantly relative to the initial composite gels(Figure 9b), where the elastic modulus and energy-to-break of composite gels with prestretching of 120% can attain as high as 8.05 MPa and 359.35 MJ/m3, respectively. At a fine scale, alginate chains within the composites gels displayed highly aligned and dense packed conformation manner, which led to a high degree of alignment of alginate chains and thus drastically increased the interfacial area among neighboring mSF. Thus, at the molecular scale, owning to the rich hydroxyl groups in alginate chains, relative sliding among densely packed composite gels involved an enormous number of repeating events of hydrogen bond formation, disruption, and reformation, consequently, the total energy needed to fracture prestretched gels was much higher than that of pristine gels. Moreover, this well-ordered oriented fibrillar architecture was largely maintained even the prestretched gels were immersed in water for 4 d. As shown in Figure 9c, the tensile strength of prestretched gels still remained as high as 17.2 MPa after immersing in water for 4 d, which corresponded to 90% of the original strength. This mechanical stability is mainly attributed to the formation of permanent hierarchical structure where the engineered mechanical tension generated in the prestretching process were translated into the oriented nanofibrillar networks, and they did not change with time (Figure 9d).

CONCLUSIONS In summary, we propose a facile procedure to prepare composite hydrogels with integrated mechanical properties by incorporating mSF into alginate ionically network. The composite hydrogels reinforced with mSF display the higher tensile strength, elastic modulus and energy-to-break than that


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of pristine alginate hydrogels, which can be ascribed to the rigid mSF and dissipate energy mechanisms that the mSF gives rise to ionic bonds with Ca2+ and interfacial hydrogen bonds. The mechanical properties of composite hydrogels can be readily tuned in an extensive range by varying the concentration of mSF and immersing time in CaCl2 solution. Meanwhile, because of their unique ionically reversible network structures, the composite hydrogels could reconstruct their network structures, resulting in excellent antifatigue ability. Furthermore, by adopting the prestretching strategy, the oriented structures are formed in the prestretched composite hydrogels, where the aligned structure can be controlled by prestretching level and well retained even in the swollen state. It is expected that integrated with the excellent mechanical properties, the alginate hydrogels with desirable biodegradability and biocompatibility would expand their applications in tissue scaffolds and other biomedical fields.

ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (21674013, 21404011).

SUPPORTING INFORMATION Morphology of the mSF (a) digital image, (b) AFM image and (c) POM image; load-bearing capacity of the 0.1 wt% mSF/alginate composite gel under stress; mechanics retention coefficient of tensile strength (σ), fracture strain (ε), elastic modulus (E), and energy-to-break (T) of the swollen composite hydrogels; POM images of 0.1 wt% mSF/alginate composite gels before (a) and after (b) prestretching deformation with 120% strain.

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TOC:

SYNOPSIS: We report a facile procedure to prepare silk microfibers/alginate tough composite



hydrogels with hierarchical fibrillar structures that can be aligned by mechanical stress.


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Alginate Tough Composite

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