Dual Ionically Crosslinked Double Network Hydrogels with High

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Biological and Medical Applications of Materials and Interfaces

Dual Ionically Crosslinked Double Network Hydrogels with High Strength, Toughness, Swelling-resistance and Improved 3D Printing Processability Xuefeng Li, Hui Wang, Dapeng Li, Shijun Long, Gaowen Zhang, and Zi Liang Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13038 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

Dual Ionically Crosslinked Double Network Hydrogels with High Strength, Toughness, Swelling-resistance and Improved 3D Printing Processability Xuefeng Li*ab, Hui Wanga, Dapeng Lic, Shijun Longb, Gaowen Zhanga, ZiLiang Wud

a. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, P. R. China b. Collaborative Innovation Center of Green Light-weight Materials and Processing, Hubei University of Technology, Wuhan 430068, P. R. China c. Bioengineering Department, College of Engineering, University of Massachusetts Dartmouth, North Dartmouth, MA 02747-2300, USA d. Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

* To whom correspondence should be addressed. E-mail: [email protected] (X LI)

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Figure 1. Synthesis scheme of hydrogel.

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Figure 2. (a) Typical tensile stress-strain profiles and (b) elastic modulus and toughness of p-, f-, and e-hydrogels. Table 1. Mechanical properties of hydrogels*. Σb (MPa)

εb (%)

Wb (MJ/m3)

Eb (MPa)

SA/AAm/AAc(1wt%/7M/5mol%)

2.154

1108.3

14.264

0.474

SA/AAm/AAc(1.5wt%/7M/5mol%)

2.386

1424.9

19.368

0.460


3.237

1228.0

25.097

0.940


3.440

1012.1

23.997

1.380


2.653

868.1

16.476

1.300


1.573

963.4

9.215

0.127


2.550

1117.5

19.004

0.823


3.055

1062.3

22.260

1.115


2.915

1065.0

20.168

0.887

Samples

* The concentration of the iron solution for soaking was 0.06 mol/L. 3



Figure 3. Tensile hysteresis loops of e-hydrogels in (a) small and (b) large strains measured in a cycle tensile test.

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Figure 4. (a) Dissipated energy Uhys , (b) The ratio of dissipated energy and work of extension, Uhys /W, as a function of strain.

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Figure 5. Dynamic mechanical behavior of the e-hydrogels. (a) Frequency dependence of the storage modulus G′, loss modulus G″, and loss factor tan δ of the 6


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hydrogels. The frequency sweeps were performed from 0.1 to 100 Hz at different temperatures and at strain amplitude 0.1%; the master curves were obtained by time-temperature superposition shifts at a reference temperature of 24 °C. (b) Arrhenius plot for the temperature-dependent shift factors. The apparent activation energy was calculated from the slop of the curve.

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Figure 6. (a) Cyclic loading curves and (b) toughness recovery ratio of e-hydrogel at different amounts of rest time.

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Figure 7. Healing properties of e-hydrogels. (a) Photos to show the healing process at room temperature for 48h and, (b) tensile behavior of the intact and healed e-hydrogel samples.

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Figure 8. Photos to show the shape memory behavior of e-hydrogel mediated by pH. (a) The e-hydrogel incubated in acidic solution (pH 1) and (b) i) The e-hydrogel was shaped in acidic solution (pH 1) by wrapping it around a glass rod, ii) the shaped e-hydrogel was swelled in 0.06 M Fe(NO3)3·9H2O solution for 1 h and followed by, iii) swelled in water (pH 7) for 4 h. The structure maintained after removal of external force. The shaped hydrogel (with a spiral shape) recovered to the original flat shape after being swelled in pH 1 solution for 2 h.

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Figure 9. Freshly 3D printed hydrogels in rectangular shape (a), circular (e) and circular ring (g), turned into yellow after immersed in ferric solution and DI water and UV irradiation (b), (f), (h). Microphotograph of freshly 3D printed hydrogels (c) and after immersed in ferric solution and DI water (d). Photo in the printing process (i).

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Figure 10. Rheological behavior of 3D printable 1) Pre-solution (a solution that contains SA, AAm, and AAc), 2) Transition solution (e-hydrogel fully swelled in pH 14 solution) and, 3) e-hydrogel. The frequency sweeps were performed from 0.1 to 100 Hz at different temperatures and at strain amplitude 0.5%, Insets showed the appearance of the three samples.

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Table of Contents (TOC) Graphic

Synergistic dual ionic crosslinking of Fe3+ with carboxyls from both alginate and poly(acrylamide-co-acrylic acid) produced double network hydrogels with high strength and toughness, high water content, and high swelling resistance in aqueous environment. The hydrogels also exhibited good 3D printing processability because of the dynamic nature of metal-ligand coordination.

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Dual Ionically Crosslinked Double Network Hydrogels with High Strength, Toughness, Swelling-resistance and Improved 3D Printing Processability Xuefeng Li*ab, Hui Wanga, Dapeng Lic, Shijun Longb, Gaowen Zhanga, ZiLiang Wud

a. Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan 430068, P. R. China b. Collaborative Innovation Center of Green Light-weight Materials and Processing, Hubei University of Technology, Wuhan 430068, P. R. China c. Bioengineering Department, College of Engineering, University of Massachusetts Dartmouth, North Dartmouth, MA 02747-2300, USA d. Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

* To whom correspondence should be addressed. E-mail: [email protected] (X LI)

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Abstract We report a dual ionic crosslinking approach for preparation of robust, high strength

and

toughness

double

network

hydrogels,

sodium

alginate/poly(acrylamide-co-acrylic acid)/Fe3+ (SA/P(AAm-co-AAc)/Fe3+), in a facile “one-step” dual ionic crosslinking method. We take advantage of the abundant carboxyl groups on alginate molecules and the copolymer chains and their high coordination capacity with multivalent metal ions to obtain high strength and toughness. The optimal SA/P(AAm-co-AAc)/Fe3+ (SA 2 wt% and AAc 5 mol%) hydrogels showed remarkable mechanical performance with 3.24 MPa tensile strength and 1228% strain, both of which remained stable with 76% water content and were highly swelling resistant in aqueous environment. The hydrogels possessed high fatigue resistance, self-recovery, pH-triggered healing capability, shape memory and reversible gel-sol transition facilitated by pH regulation. Moreover, it has 3D printing processability by properly adjusting the solution viscosity. The approach may provide a convenient way of obtaining high strength and toughness hydrogels with a number of desirable properties for a broad range of biomedical applications. Keywords: dual ionically crosslinked hydrogel, high strength, toughness, swelling-resistance, shape memory, 3D printing processability 1.

Introduction Hydrogels, with low fracture energies and/or low elastic moduli, typically 10

Jm-2 and 10 kPa, respectively, are generally mechanically inadequate for applications 2



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as load-bearing soft tissue replacements, such as cartilages,1,2 tendons,3 and ligaments.4 Many attempts have been made to develop high strength and toughness hydrogels to address the challenge, including double network (DN) hydrogels,

5-7

nanocomposite hydrogels,8 sliding-ring hydrogels,9 macromolecular microsphere composite hydrogels,10 tetra-PEG hydrogels,11 and physical interaction enhanced hydrogels.12-15 Among these efforts, Gong et al.16 developed a two-step sequential free-radical polymerization method and synthesized the first chemically crosslinked DN hydrogels consisting of poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first network and polyacrylamide (PAAm) as the second. DN hydrogels have been demonstrated to have extremely high mechanical strength and toughness (typical fracture tensile stress > 1 MPa and strain in the 1000 – 2000% range), both of which were comparable to those of cartilage or rubber.17 Gong et al.18 pointed out the principles of designing tough chemically linked DN hydrogels, (i) a rigid and brittle polymer component, such as polyelectrolyte, to form the first network, while a soft and ductile neutral polymer the second network; (ii) the molar concentration of the polymer component for the second network is 20 – 30 times of that of the first and; (iii) the first network is tightly crosslinked while the second network loosely to achieve a strong asymmetric DN structure. Recently, non-covalent and reversible metal ion-ligand coordination has attracted much attention as a means of physical crosslinking to develop hydrogels with high strength and toughness, along with many other desired mechanical properties including resilience, processability,19 and dynamic adaptability.20 Zhou et al.21 3


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synthesized strong and tough hydrogels using Fe3+ ions to cross-link a chemically cross-linked network containing carboxyl motifs. Although high strength was successfully achieved with this approach, the hydrogels exhibited poor toughness (typical strain < 1000%). Further, the existence of permanent or quasi-permanent network in such hydrogels has led to poor processability, recyclability, and self-healing ability. Zheng et al.22 demonstrated a different strategy of taking advantage of ionic association as mechanical strength enhancer by swelling a cast film of poly(acrylamide-co-acrylic acid) (P(AAm-co-AAc)) in FeCl3 aqueous solution to form supramolecular networks crosslinked by carboxyl-Fe3+ coordination bonds. The resulting hydrogels with 35 − 85 wt % water content showed excellent mechanical properties, such as 0.1 − 80 MPa tensile modulus, 150 − 1100% breaking strain, 0.5 − 18 MPa breaking stress, and 100 − 1300 J/m2 fracture energy. The strong metal-ligand coordinate bonds did provide the hydrogels with high strength, yet there were two problems associated with such hydrogels, (i) there was a mismatch between high strength and high toughness, when stress was > 1 MPa, strain was smaller than 1000%. and (ii) entanglement/disentanglement of AAm chains was the only contributor to energy dissipation, which provided limited toughening to the hydrogels. Incorporating a second physically crosslinked network into the hydrogel structure was attempted as a potential solution to the problem. Zheng et al.23 prepared Agar/PAMAAc-Fe3+ DN hydrogels consisting of a secondary hydrogen bond-based physically crosslinked agar network, on top of the primary chemically-physically 4



crosslinked PAMAAc-Fe3+ network with hybrid Fe3+-carboxyl coordination interactions and covalent bonds. High toughness of such Agar/PAMAAc-Fe3+ DN hydrogels was achieved and attributed to sufficient energy dissipation through a combined effect of (i) unzipping of Fe3+-carboxyl coordination interactions in the secondary network and (ii) pulling out of the agar chains in the primary network. Despite of remarkably improved toughness, the high strength and high toughness mismatch issue still existed, with stress/strain 1.6 MPa/1400% at 5% AAc concentration and 2.4 MPa/900% at 10% AAc. Further, the chemically crosslinked network largely limited the hydrogel processibility. Suo et al.24 synthesized extremely stretchable and tough hydrogels by combining ionically crosslinked alginate and covalently crosslinked polyacrylamide. The alginate chain comprised mannuronic acid (M unit) and guluronic acid (G unit), with three distinct G-, M-, and alternative G/M-rich blocks. In aqueous solutions, the G blocks in different alginate chains formed ionic crosslinks through divalent cations (Ca2+ for example), resulting in an alginate hydrogel network that could effectively dissipate energy by unzipping the ionically cross-linked points. Extremely high strain (> 2000%) was observed from such hydrogels, yet still the strength was less than 0.2 MPa, presumably due to the poor swelling resistance of the alginate component, a serious strength weakening issue that has been well recognized.25, 26 Hence, it is critical to create an effective energy dissipation mechanism to adequately toughen the dynamic metal-coordination bond strengthened hydrogels, which, according to our view, can be achieved through deliberate molecular and 5


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structural design of the hydrogel components and synergistically mutually enhanced manner. Here we report our design, synthesis, and characterization of high strength and toughness DN hydrogels, sodium alginate/poly(acrylamide-co-acrylic acid)/Fe3+ (SA/P(AAm-co-AAc)/Fe3+), using a simple “one-step” dual ionic crosslinking method by which the crosslinking of Fe3+ with the carboxyls from both SA and P(AAm-co-AAc) take place simultaneously. Ionic crosslinking of Fe3+ with the carboxyls from both alginate and poly(acrylamide-co-acrylic acid) simultaneously occurred in a “one-step” soaking operation in an aqueous Fe(NO3)3·9H2O solution to form strong and tough supramolecular networks. This work, to the best of our knowledge, is the very first study of developing high strength and toughness hydrogels with the dual ionic crosslinking approach and in a “one-step” dual ionic crosslinking method. The best tensile strength and strain achieved from the optimal SA/P(AAm-co-AAc)/Fe3+ (SA 2wt% and AAc 5mol%) were 3.24 MPa and 1228%, respectively, the highest values reported so far for hydrogels without any other means of mechanical property enhancement such as nanoclay.27 In addition, the dual ionic crosslinking approach developed in this study produces no permanent or quasi-permanent chemical network in the resulting hydrogel structure. Lastly, because of the dynamic nature of metal-ligand coordination, the hydrogels exhibited pH triggered healing capability and good 3D printing processability. 2.

Materials and methods

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2.1. Materials All chemicals and solvents purchased were of the highest available purity and used as received, unless otherwise stated. Sodium alginate (SA) and acrylamide(AAm) was obtained from TCI Shanghai Inc, China. Acrylic acid (AAc) and iron nitrate nonahydrate (Fe(NO3)3·9H2O) were purchased from Sinopharm Chemical Reagents Co., Ltd, China. 2-hydroxy-4’-(2-hydoxyethoxy)-2-methylpropiophenone (Irgacure 2959, KA) and other metal ions salts were purchased from Aladdin Inc (Shanghai, China ). 2.2. Preparation of hydrogels. Iron ion dual crosslinked SA/P(AAm-co-AAc) double network hydrogels were prepared with a “one-step” dual ionic crosslinking method. Firstly, various amounts of SA were added to DI water at 50 °C to form a series of homogeneous aqueous solutions of various SA concentrations (1, 1.5, 2, 2.5, 3 wt%). Secondly, the monomers AAm (7 mol/L, with respect to the volume of water) and AAc (3 mol% ,5 mol%, 7 mol%, 9 mol%, molar ratio of AAc/AAm) and ultraviolet (UV)-light initiator KA (0.1 mol%, molar ratio with respect to the total amount of monomers, used for preparing all the hydrogel samples in this study) were added to the SA solutions and the mixture solutions thus formed were gently stirred by a magnetic stirrer at 50 °C for 30 mins to remove any air bubbles. Hydrogel samples in rectangular shape were prepared by pouring the mixture solutions into a home-made, rectangular reaction-cell consisting of a pair of glass plates (100 mm × 100 mm) with a hollow silicone-rubber spacer of ~1.2 mm wide. The assembly was then sealed with plastic wrap and 7


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irradiated by UV light at room temperature for 6 hours during which polymerization occurred to form the preliminary hydrogels (coded as p-hydrogels). The p-hydrogel was subsequently soaked in Fe(NO3)3·9H2O solution (0.02, 0.04, 0.06, 0.08, or 0.1 mol/L) for 3 h when iron ion dual crosslinking took place due to Fe3+-COOcoordination, producing the transition state Fe3+-crosslinked hydrogels (coded as f-hydrogels). The f-hydrogels were yellow in color, different from the transparent appearance of the p-hydrogels and indicating the incorporation of Fe3+ into the hydrogel networks. Finally, the f-hydrogels were soaked in water for 48 h to obtain the equilibrium hydrogels, coded as e-hydrogels, with the removal of superfluous Fe3+ in the meantime. All the tests carried out in this study were for the e-hydrogels, unless mentioned otherwise. Fig. S1 showed the appearance and water content of the p-, f-, and e-hydrogel. SA-Fe3+/PAAm hydrogels were prepared in the same way described above but without AAc. Copolymer P(AAm-co-AAc)/Fe3+ hydrogels were simply prepared by adding the AAm, AAc and ultraviolet (UV)-light initiator KA to DI water at room temperature to form homogeneous aqueous solutions and irradiated by UV light at room temperature for 6 hours, then soaking in Fe3+ and in DI water. 2.3. 3D Printing of hydrogels Presolution has a high viscosity, and then additional meteorological silica viscosity adjustment, can be 3D printing. The 3D printing uses a robotic dispenser provided by the Sistema Dosificador Ultra 2800. The instrument has two components: 8



drive

unit

and

computer

software

package

(MTASC).

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Dispensing

was

computer-controlled using the automatic dispenser in conjunction with the MTASK software of the Mutronicm CNC drive unit. Preferred the silicone was 10wt% of total mixture solution to adjust the viscosity. 2.4. Water contents test The water contents of the hydrogels were measured by the weight change upon drying using a vacuum oven. The complete dry sample was obtained by evaporating the water of the hydrogels at high temperature of 120 °C.28 The water content C (wt %) is defined as the ratio in percentage between the weight of water in the hydrogels to the total weight of the hydrogels. C(wt) =

weight of water total weight of hydrogel

×100%

(1)

The swelling ratios of the hydrogels were measured by the weight change upon swelling in solution for 48 h at room temperature. The swelling ratio S is defined as the ratios between the weight of hydrogels after swelling (m1) to the weight of the hydrogels before swelling (m0). S (g/g) =

m1

(2)

m0

2.5. Mechanical test Tensile test Uniaxial tensile tests were carried out with rectangular hydrogel samples (40 mm × 10 mm × 1.2 mm) and a universal tensile testing machine equipped with a 1 kN 9


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load cell and at a constant stretch velocity of 100 mm/min and at room temperature. Elastic modulus E was calculated from the slope of the initial linear region of the stress-strain curve (within 5%~10%). Toughness was obtained by integrating the area under the stress-strain curve until sample fracture. For successive loading-unloading tests, the loading-unloading operations were repeated for the same sample with gradually increased strains until the sample failed.29 λmax denotes the maximum extension ratio that the sample experienced in the test. The nominal stress σ was calculated from the tensile force and the initial cross-sectional area of the sample. The strain rate, ε, is defined as the ratio of stretch velocity to the original gauge length. Dissipated energy (Uhys) was estimated from the area below the stress-strain curve or between the loading-unloading curves. Three duplicated measurements were performed for each sample unless mentioned otherwise. Tearing tests Tearing testing was performed using a commercial test machine with a 1 kN load cell. Rectangular hydrogel samples (40 mm × 10 mm × 1.2 mm) were cut into a trousers shape with an initial notch of 20 mm long. The two arms of the samples were nipped, and the lower clamp was stretched at a constant velocity (50 mm/min) until the crack advanced through the entire sample, while the other remained stationary. The tearing energy (Γ ) is defined as the work required to tear a unit area,30 and estimated by the following equation, Γ = 2 F/w

(3) 10



Where w is the thickness of the sample and F is the peak force obtained from the steady-state tear test. Three duplicated measurements were performed for each sample unless mentioned otherwise. 2.6. Rheological test The rheological behavior of e-hydrogel was characterized with a DHR-2 rheometer (TA Instruments, USA). The disk-shaped sample with a diameter of 25.0 mm and a thickness of 1.2 mm was fixed between two metal plates using super glue.31-34 To prevent water evaporation, the sample was surrounded by water throughout the test. Strain sweeps were set from 0.01 to 10%, at frequency 1 Hz and temperature 16 or 80 °C. Temperature sweeps were performed from 20 to 80 °C (heating rate: 5 °C/min), at strain 0.1% and frequency 1 Hz. Frequency sweeps were performed at different temperatures at 0.1% strain. Master curves of storage modulus G′, loss modulus G″, and loss factor tan δ were obtained by time-temperature superposition shifts (TTS) at the 24 ℃ reference temperature. The temperature of water, controlled by a thermal bath, was increased stepwise from low to high. Before each measurement, the sample was held at the set temperature for 300 s to reach equilibrium. Based on the Arrhenius plot of temperature-dependent shift factors, the apparent activation energy Ea was calculated from the slope of the curve. For Pre-solution and transition solution, the e-hydrogel fully swelled in pH 14 solution, the frequency sweeps were performed at room temperature and 0.5% strain. 2.7. Healing behavior measurement 11


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A hydrogel strip (60 mm × 10 mm × 1.2 mm) was cut in two pieces (30 mm × 10 mm × 1.2 mm). The two freshly cut surfaces were made in contact to each other with a few drops of pH 14 alkaline solution placed at the interface to join them together; the two pieces thus joint were remained in contact for 24 hours for healing. To avoid water evaporation, the sticked hydrogels were wrapped in polyethylene films and stored in a sealed polyethylene bags at room temperatures over specific times. The hydrogel strip was then soaked in Fe3+ solution for 3 hours and in water for 24 hours to obtain the healed hydrogel sample which was subject to mechanical measurements at room temperature in the same way described above. 3.

Result and discussion

3.1. Synthesis hydrogels The route of synthesizing dual ionically crosslinked DN hydrogels, SA/P(AAm-co-AAc)/Fe3+, using the facile “one-step” dual ionic crosslinking is illustrated by Figure 1. In brief, the carboxyls from both the SA molecules and P(AAm-co-AAc) copolymer chains were simultaneously crosslinked with iron ions upon the addition of iron nitrate to the mixture, forming SA/P(AAm-co-AAc)/Fe3+ double network hydrogels, with p-, f-, and e-hydrogels obtained at different stages of synthesis. Specifically, a mixture solution that contained SA, AAm and AAc, and initiator KA was subject to light irradiation to form the preliminary p-hydrogel. The p-hydrogel was then soaked in Fe(NO3)3·9H2O solution, where the simultaneous ionic coordination took place, to form dual Fe3+ crosslinked f-hydrogel. Finally, the 12



equilibrium e-hydrogel was obtained by immersing the f-hydrogel in DI water for an extensive amount of time. The detailed synthesis procedures were described in section 2.2. Preparation of hydrogels.

Figure 1. Synthesis scheme of hydrogel. 3.2. Mechanical properties Shown in Figure 2 are the tensile stress-strain curves, elastic modulus, and toughness of the p-, f-, and e-hydrogels. The tensile strength, fracture strain, elastic modulus, and toughness of typical p-hydrogels were only ca. 0.10 MPa, 960.2%, 0.10 MPa, and 1.05 MJ/m3, respectively. In contrast, those of the e-hydrogels were ca. 3.24 MPa, 1228.0%, 0.94 MPa, and 25.10 MJ/m3, respectively, or 32.6, 1.3, 9.3, and 23.8 times higher than the p-hydrogels. Apparently, the introduction of ionic coordinate interactions into the hydrogel has resulted in significantly improved mechanical strength, extensibility, stiffness, and toughness. Besides, comparing to f-hydrogels, e-hydrogels had larger E, σb, and Wb, but smaller εb, which was likely due to the 13


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formation of a more compact network in the e-hydrogels due to the equilibrating process. Further, the water content of e-hydrogels is lower than that of f-hydrogels, Fig. S1, indicating the crosslinking density of e-hydrogels is higher than that of f-hydrogels. Further, the water turned into slightly yellow after the equilibrating treatment, indicating that the excessive amount of iron ions that did not participate in coordination had released from the network and diffused out of the hydrogel. It should be noted that uptake of an excessive amount of iron ions into the f-hydrogels created the possibility of single, double, and triple ligand crosslinking.35 The soaking of f-hydrogels in DI water helped to remove any excessive Fe3+ and balance the hydrogel networks, resulting in equilibrium e-hydrogels. This process promoted the desired tri-ligand crosslinking in the e-hydrogels for the best possible mechanical performance.

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Figure 2. (a) Typical tensile stress-strain profiles and (b) elastic modulus and toughness of p-, f-, and e-hydrogels. 15


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One of the key factors that affect the mechanical properties of a hydrogel is its composition, i.e. the ratio of the components that are used to design the hydrogel. The highest mechanical properties can potentially be achieved when the dual ionic crosslinking occurs synergistically, which was attempted by controlling the amount of Fe3+ crosslinked SA and P(AAm-co-AAc) in the double network through adjusting the contents of the SA and AAc components. Nine types of hydrogels with various contents of SA and AAc were prepared for this study and their mechanical properties (εb, σb, Wb, Eb) were summarized in Table 1.

With a gradual increase in SA

concentration from 1 wt% to 3 wt%, the hydrogel strength increased first and then decreased, with a maximal stress reached 3.44 MPa at 2.5 wt% SA concentration. The corresponding strain, 1012.1%, however, was not the highest at this SA concentration; instead, the sample prepared with 1.5 wt% SA resulted in the highest strain 1424.9%, with a moderate stress 2.39 MPa. Further, the work of extension at fracture, 25.10 MJ / m3, was best at 2 wt% SA concentration. The effect of AAc composition on the mechanical properties of the hydrogels showed a similar pattern to that of SA, i.e. an initial increase followed by a decrease. On the other hand, the optimal stress and strain were both achieved at 5 mol% AAc. Overall, the e-hydrogels exhibited the best mechanical performance at 2 wt% SA and 5 mol% AAc. Such conditions were therefore used to prepare hydrogel samples for further tests. The detailed tensile results for the e-hydrogels were summarized in Figure S2 in the Supporting Information.

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Table 1. Mechanical properties of e-hydrogels*. Σb (MPa)

εb (%)

Wb (MJ/m3)

Eb (MPa)


2.154

1108.3

14.264

0.474


2.386

1424.9

19.368

0.460


3.237

1228.0

25.097

0.940


3.440

1012.1

23.997

1.380


2.653

868.1

16.476

1.300


1.573

963.4

9.215

0.127


2.550

1117.5

19.004

0.823


3.055

1062.3

22.260

1.115


2.915

1065.0

20.168

0.887

Samples

* The concentration of the iron solution for soaking was 0.06 mol/L. Another key factor that affects the mechanical properties of e-hydrogels is Fe3+ concentration which determines the level of ionic association intensity. Shown in Figure S3a are the tensile results of the hydrogels at different Fe3+ concentrations. With increasing Fe3+ concentrations from 0.02 to 0.10 mol/L, the stress increased at the beginning and then started to decrease, with the maximal value of 3.28 MPa at 0.08 mol/L, on the other hand, strain was constantly decreasing over the entire concentration range. Elastic modulus increased with the increase of Fe3+ concentration and plateaued in the 0.08 – 0.1 mol/L range; toughness, on the other hand, reached peak value at 0.06 mol/L and then started to decrease (Figure S3b). These results led us to a conclusion that the highest mechanical performance was achieved in the Fe3+ concentration range of approximately 0.06 – 0.08 mol/L.

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The poor mechanical properties seen at low Fe3+concentrations were presumably due to insufficient cross-linking, whereas excessive amount of Fe3+ resulted in inhomogeneous crosslinking, less compact coordinate complexes, and small stability constant due to the decrease in pH.36 As a result, unbalanced stress and strain were observed from e-hydrogels with too high or too low Fe3+ concentrations. It appeared that a balanced high strength (3.24 MPa) and high extensibility (1228.0%) was achieved at 0.06 mol/L Fe3+ concentration. We also investigated the effects of other cation ions (Ag+, Na+, Ca2+, Cu2+, Al3+) on the mechanical properties of e-hydrogels (Figure S4). However, none of the hydrogels prepared with the five types of cations showed any better performance than that with Fe3+. Comparing to low-valent cations, trivalent cations are able to crosslink three carboxyl groups from the two different molecular chains to form a more compact three-dimensional structure with a larger coordination number. Hence, the hydrogels crosslinked with trivalent cations showed better performance than those with single or divalent cations. On the other hand, same trivalent Al3+ and Fe3+ resulted in hydrogels of rather different strengths. This may be due primarily to the ionic radius difference between the two, 0.645 nm for Fe3+ and 0.535 nm for Al3+, with the larger Fe3+ having stronger charge effect for a stronger crosslinked network.[37] To understand the role of dual ionic crosslinking plays in determining the overall mechanical performance of the e-hydrogels, we further fabricated and tested two types of single ionic crosslinked hydrogels, SA-Fe3+/PAAm and P(AAm-co-AAc)/Fe3+ (Figure S5). The SA-Fe3+/PAAm hydrogels showed an outstanding 1300.0% 18



elongation at break, but an extraordinary low break stress, 0.60 MPa; in comparison, the strain of the P(AAm-co-AAc)/Fe3+ hydrogels was only 800.0%, but had much larger stress, 1.80 MPa (Figure S5a). Such results pointed out that the P(AAm-co-AAc)/Fe3+ network was the major contributor to the enhanced strength of the e-hydrogel, while SA-Fe3+/PAAm to strain, which was confirmed by the elastic modulus, toughness, and tearing energy results shown in Figure S5b. The fact that neither P(AAm-co-AAc)/Fe3+ nor SA-Fe3+/PAAm showed a comparable performance to e-hydrogel indicated the significance and effectiveness of iron ion dual crosslinking as an mechanical property enhancer. 3.3. Hysteresis measurements To gain insights into the internal fracture mechanism of e-hydrogels, successive loading-unloading tests with no resting time between any two consecutive loading cycles were conducted. As seen in Figure 3, obvious hysteresis loops developed from the stress-strain curves at different stain levels before and after yield, besides, as strain increased, the size of the hysteresis loops gradually increased, suggesting that the internal fracture started far below the yield point and continued to spread with increasing elongations. We speculate that fracture occurred in the primary network at extensive stains. It also can be seen that there were overlapped regions between any adjacent hysteresis loops and that the degree of overlapping was dependent upon strain level, both of which indicated the self-recovery ability of e-hydrogels. Further, much larger hysteresis loop areas were observed at high strains than those at low, suggesting that damage propagated to the P(AAm-co-AAc) network 19


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when hydrogels were subject to large strains.

Figure 3. Tensile hysteresis loops of e-hydrogels in (a) small and (b) large strains 20



measured in a cycle tensile test. 3.4. Fracture mechanism To further understand energy dissipation upon facture of e-hydrogels, the relationship between dissipation energy Uhys and strain Ɛn and that between Uhys/W and strain Ɛn were plotted and presented in Figure 4. A larger Uhys/W indicates more damage to the hydrogel. The consistently gradual increase of Uhys during the entire deformation process (Figure 4a) indicated that there was no phase transition from continuous to discontinuous networks within the hydrogels. Further, two stages were observed in Figure 4a, the first being an exponential U-Ɛn relationship in the 0~250% strain range, whereas the second a linear correlation for strains at and above 250%. The two distinctive stages can be seen clearly in Figure 4b. At small strain levels (0~250%), the net disassociation of SA-Fe3+ network was responsible for the gradual increase of Uhys seen in the initial stage of the Uhys -Ɛn curve. When strains were 250%, at which the e-hydrogel began to yield, and beyond, stress started to transfer from the SA-Fe3+ network to the P(AAm-co-AAc)/Fe3+ network through entanglement among the P(AAm-co-AAc) chains and initiated fracture of the P(AAm-co-AAc)/Fe3+ network. In the meantime of the rupture of the double networks, Fe3+-COOre-crosslinking and therefore re-constructing of the networks were taking place. In the competition, the breaking outplayed re-bonding at increasing levels of strain, ultimately leading to structural failure by undergoing a slowly increased Uhys/W stage, i.e. the second, linear stage of the Uhys/W-Ɛn curve as seen in Figure 4b. Overall, the double ionic crosslinking provided the e-hydrogels outstanding fatigue resistance and 21


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self-recovery capability.

Figure 4. (a) Dissipated energy Uhys , (b) The ratio of dissipated energy and work of extension, Uhys /W, as a function of strain. 22



The linear dynamic mechanical behaviors of the e-hydrogels were observed from the rheological measurements. Figure S6. Frequency sweeps were performed in the linear region of strains and temperatures, and the spectra appeared to follow the time-temperature superposition shifts (TTS) principle (Figure 5a), validating the use of TTS curve for calculating activation energy. The Arrhenius plot for the shift factor αT of the master curve is shown in Figure 5b, along with two distinct apparent activation energies Ea = 112.7 and 206.6 kJ/mol, corresponding to the two types of ionic bonding, the weaker Fe3+-COO- (from SA) and the stronger Fe3+-COO- (from P(AAm-co-AAc)). During stretching, the weaker associations are the first to rupture to dissipate energy, followed by rupture of the stronger ones to resist higher levels of deformation.

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Figure 5. Dynamic mechanical behavior of the e-hydrogels. (a) Frequency dependence of the storage modulus G′, loss modulus G″, and loss factor tan δ of the hydrogels. The frequency sweeps were performed from 0.1 to 100 Hz at different temperatures and at strain amplitude 0.1%; the master curves were obtained by time-temperature superposition shifts at a reference temperature of 24 °C. (b) Arrhenius plot for the temperature-dependent shift factors. The apparent activation energy was calculated from the slop of the curve. 3.5. Self-recovery and healing performance test Self-recovery and healing were expected from e-hydrogels with their reversible, Fe3+-COO- ionically crosslinked double networks. Cyclic loading tests were conducted at different rest times and with the maximum strain λmax = 6 being used in the first loading cycle. With no rest time between the first and second loading cycle, 24



the second hysteresis loop was much smaller than the first (black and red curves in Figure 6a). With increasing amounts of rest time, the size of the hysteresis loops gradually grew and became closer to that of the original, indicating that time was needed for the local networks to rebuild and the global mechanical properties to recover. Shown in Figure 6b is the recovery of dissipated energy calculated with the data from Figure 6a. ~40% toughness were recovered with 15 min rest time, whereas in 4h, the hydrogel recovered ~64%.

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Figure 6. (a) Cyclic loading curves and (b) toughness recovery ratio of e-hydrogel at different amounts of rest time. The pH influences metal-ligand coordination mode and stability constants of the coordinate complexes,37,38 and was taken advantage of in this study as a dissociation/re-association trigger for attaining hydrogel healing. As e-hydrogels in form of discs of 10 mm diameter were immersed in solutions of different pH’s, at pH 1, the samples changed color from yellow to colorless, suggesting that ionic crosslinking was destructed as a result of Fe3+-COO- bond dissociation and Fe3+ ions being freed and released from the hydrogels; at pH 12~14, the samples were significantly softened and became sticky. Such a soft and sticky state of hydrogels turned out to be the point at which healing started to take place. Figure 7a showed the 26



healing of two rectangular e-hydrogel samples, with one being dyed with methyl blue for easy visualization. The initial distinctive blue/yellow border became blur after the two pieces stayed in natural contact for 48h at room temperature, indicating the joining of two separated pieces into one. The healed sample was then subject to sequential soaking in Fe(NO3)3·9H2O aqueous solution for re-establishment of Fe3+-COO- bonding and then in DI water for removal of excessive ions and regained approximately 56% tensile strength and 17% strain of the intact sample (Figure 7b)

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Figure 7. Healing properties of e-hydrogels. (a) Photos to show the healing process at room temperature for 48h and, (b) tensile behavior of the intact and healed e-hydrogel samples. 3.6. Functions and 3D printing processability Besides triggering healing, pH manipulation helped to develop a variety of functions in e-hydrogels and create desired elastic properties for easy processing for various possible applications. The swelling behavior of e-hydrogel was closely related to pH change and developed a “J-shape” profile as pH varied from 1 to 14, Figure S7a. While the swelling ratio remained low and there was not much change of it in the range of pH 2 to 11, high and significantly higher levels of swelling were observed at pH 1 and 12 28



and above, respectively. Strong acidic conditions (pH equal or lower than 1) suppressed ionization of polymers and produced only handful amounts of -COO- groups to coordinate with Fe3+; as a result, the majority of Fe3+ ions were released from the networks, leading to transparent appearance of the hydrogel (Inset picture for pH 1 in Figure S7a). Under extremely basic conditions, the vast availability of OH- became a competitor with the -COO- for coordination with Fe3+, resulting in a great amount of unbound -COO- on the polymer chains repulsive to one another and weakening the polymer networks. The mechanical properties of the hydrogels swelled under different pH conditions were shown in Figure S7b. The hydrogels immersed in the pH 1 solution had poor mechanical properties (black curve), whereas at pH 12-14, the hydrogels completely lost integrity for testing. While the Stress-Strain profiles under other pH’s were similar to each other, pH 7 treatment resulted in the best performance. The pH responsive mechanical behavior satisfied our expectation on such e-hydrogels in their initial design and inspired us to further testing and validating pH-regulated tunable functions of the hydrogels. As shown in Figure 8, the tough e-hydrogels could be softened by an acidic solution, with the fading of color as an indicator. The softened hydrogel sample was deformed and turned into a spiral geometry (Movie S1) when soaked in 0.06 M Fe(NO3)3·9H2O solution and DI water, which could be reversed by immersing it in acidic solution (pH 1). 29


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Figure 8. Photos to show the shape memory behavior of e-hydrogel mediated by pH. (a) The e-hydrogel incubated in acidic solution (pH 1) and (b) i) The e-hydrogel was shaped in acidic solution (pH 1) by wrapping it around a glass rod, ii) the shaped e-hydrogel was swelled in 0.06 M Fe(NO3)3·9H2O solution for 1 h and followed by, iii) swelled in water (pH 7) for 4 h. The structure maintained after removal of external force. The shaped hydrogel (with a spiral shape) recovered to the original flat shape after being swelled in pH 1 solution for 2 h. Such hydrogels are 3D printable. A video clip (Movie S2) has been included in the Supporting Materials to show the printing process. Two freshly 3D printed hydrogel samples are presented in Figure 9a. The samples turned from colorless and transparent into yellow and opaque after 10-min UV radiation followed by soaking in Fe(NO3)3·9H2O solution for 3 hours and DI water for 48 hours, Figure 9c. Individual lines could be seen from microscopic photographs for the freshly 3D printed hydrogels, Figure 9b, and the lines remained distinctive after treatment with ferric solution and water, even though with slightly swelling, Figure 9d. 30



Figure 9. Freshly 3D printed hydrogels in rectangular shape (a), circular (e) and circular ring (g), turned into yellow after immersed in ferric solution and DI water and UV irradiation (b), (f), (h). Microphotograph of freshly 3D printed hydrogels (c) and after immersed in ferric solution and DI water (d). Photo in the printing process (i). From the rheological behaviors shown in Figure 10, we observed that the loss modulus G″ of the Pre-solution was larger than the storage modulus G′ in the low 0.1~10 Hz frequency range, which indicated that the liquid state was favorable for printing. The fully swelled hydrogel state may also be taken advantage of for 3D printing purposes with a simple treatment of the hydrogel in pH 14 solution to satisfy the viscosity and gelation rate requirements.

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Figure 10. Rheological behavior of 3D printable 1) Pre-solution (a solution that contains SA, AAm, and AAc), 2) Transition solution (e-hydrogel fully swelled in pH 14 solution) and, 3) e-hydrogel. The frequency sweeps were performed from 0.1 to 100 Hz at different temperatures and at strain amplitude 0.5%, Insets showed the appearance of the three samples. 4. Conclusions Double network SA/P(AAm-co-AAc)/Fe3+ hydrogels were prepared via a dual Fe3+-COO- crosslinking approach with high strength and toughness (σf of 3.24 MPa, Strain of 1228.00%, E of 0.94 MPa, and W of 25.10 MJ m-3 ). The hydrogel was capable of effectively dissipating energy through Fe3+-COO- interactions and SA and P(AAm-co-AAc) polymer chains entanglement. Due to the reversible nature of ionic 32



crosslinking, the hydrogels could heal with a healing power of up to 56%. In addition, the hydrogels were successfully 3D printed with properly adjusted viscosity and gelation rate. In summary, the dual ionically crosslinked SA/P(AAm-co-AAc)/Fe3+ hydrogels exhibited a number of impressive mechanical properties, including high strength, toughness, self-recovery , healing, and 3D printing processibality, which are desirable for a variety of biomedical applications and facilitate multifunctional hydrogel development.

Acknowledgements The authors acknowledge with gratitude the support from the National Natural Science Foundation of China (NSFC) (Contract no. 51603065) and from the Open Fund of Hubei Provincial Key Laboratory of Green Materials for Light Industry (Contract no. 201611A04). Supporting Information Figures S1−S7 (PDF) Shape retention of the robust equilibrated hydrogel with a helical structure (AVI) 3D printing process (AVI)

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Table of Contents (TOC) Graphic

Synergistic dual ionic crosslinking of Fe3+ with carboxyls from both alginate and poly(acrylamide-co-acrylic acid) produced double network hydrogels with high strength and toughness, high water content, and high swelling resistance in aqueous environment. The hydrogels also exhibited good 3D printing processability because of the dynamic nature of metal-ligand coordination.

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Dual Ionically Crosslinked Double Network Hydrogels with High

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