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Fe3+-, pH‑, Thermoresponsive Supramolecular Hydrogel with Multishape Memory Effect Xiaoxia Le, Wei Lu, He Xiao, Li Wang, Chunxin Ma, Jiawei Zhang,* Youju Huang, and Tao Chen* Division of Polymer and Composite Materials, Key Laboratory of Marine Materials and Related Technologies, Ningbo Institute of Material Technology and Engineering, Chinese Academy of Science, Ningbo 315201, P. R. China S Supporting Information *

ABSTRACT: Poor, nontunable mechanical properties as well as finite shape memory performance pose a barrier to shape memory hydrogels to realize practical applications. Here, a new shape memory hydrogel with tunable mechanical properties and multishape memory effect was presented. Three programmable reversible systems including PBA-diol ester bonds, AAc-Fe3+, and coil−helix transition of agar were applied to memorize temporary shapes and endow the hydrogel with outstanding multishape memory functionalities. Moreover, through changing the cross-linking densities, the mechanical properties of the as-prepared hydrogel can be adjusted.

KEYWORDS: multiresponsive, multishape memory effect, tunable mechanical properties, supramolecular hydrogel, reversible switches

1. INTRODUCTION As one kind of smart polymers, shape memory polymers (SMPs), which can fix temporary shapes and recover to original shapes under external stimuli,1,2 have aroused tremendous attention because of their promising application in textiles, the biomedical field, and so on.3−5 Usually, there are two different cross-links, one for keeping the permanent shape, and the other one is reversible, the formation and breakage of the reversible cross-links contribute to the shape memory and shape recovery process. The traditional SMPs are usually polymers with thermoresponsiveness, in which the vitrification or crystallization of switching domains is the main driving force for shape memory. The follow-up development of SMPs are focused on changing external stimuli from direct heat to light, electricity, magnetic field by using photothermal effect, electro-thermal effect, and magneto-thermal effect, which are actually still applying the morphology transformation of polymers through indirectly heat.6−9 Because of the reversible and dynamical nature, reversible interactions,10−14 such as hydrogen bonds,15−17 host−guest recognition,18,19 metal−ligand coordination,20−24 etc., are now utilized to realize shape memory performance.25 By introducing reversible switches, SMPs, most of them are shape memory hydrogels (SMHs), can response to much more stimuli such as light,19,26 heat,16,27 chemical,28,29 ultrasound,30 and so forth, which broaden the way for memorizing temporary shapes and would expand the potential applications in other fields.31 For example, Liu et al. have reported an unprecedented CO2triggered shape memory behavior on the basis of diaminotriazine hydrogen bonds.32 Willner et al. have developed DNA © 2017 American Chemical Society

hydrogels with shape memory performance by regulating pH value.33,34 In addition, shape memory effect can be realized by applying host−guest interactions as temporary cross-links.18,35 Since the number of fixed temporary shapes normally has a great impact on the potential application, constructing multiSMPs, which could keep two or more temporary shapes, has attracted broad attention. In our previous works, triple shape memory effect has been achieved by combining two noninterfering switchable interactions such as Alg-Ca2+ and PBAdiol ester bonds,36 CS-Mn+, and Schiff base bonds.37 However, there are still some challenges to develop supramolecular shape memory hydrogels with tunable mechanical strength as well as multishape memory effect for real promising applications. Herein, we present a novel Fe3+-, pH-, thermoresponsive hydrogel with tunable mechanical properties also multishape memory effect. Briefly, mechanical properties can be tuned by changing the cross-linking densities. Moreover, the coordination interactions between acrylic acid (AAc) and Fe3+ ions, the reversible PBA-diol ester bonds formed by phenylboronic acid groups (PBA) and adjacent hydroxyl groups of glucosamine, and the coil−helix transition of agar can all be employed to stabilize the temporary shapes to realize shape memory behavior. In addition, by combining the three reversible switches, programmable multishape memory effect can be realized. The presented strategy could enrich the construction Received: January 5, 2017 Accepted: February 21, 2017 Published: February 21, 2017 9038

DOI: 10.1021/acsami.7b00169 ACS Appl. Mater. Interfaces 2017, 9, 9038−9044

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of Multi-Responsive Shape Memory Hydrogel on the Basis of Three Different Crosslinks, Including AAc-Fe3+, Dynamic PBA-Diol Ester Bonds and Helix−Coil Transition of Agar: (A−D) Preparation Process of Hydrogel; (E−G) Reversible Interactions Can Be Formed in the As-Prepared Hydrogel, through Which the Hydrogel Shows Shape Memory Behavior

of the crude product with dichloromethane/methanol as eluent (ratio 4:1). White powder was obtained after dried overnight in 35 °C vacuum oven. The structure of AGA was confirmed by the 400 MHz 1 HNMR (D2O) spectrum (Figure S2). 2.4. Preparation of Hydrogels. Agar powder (50 mg), AM monomers (450 mg), AAc monomers (15 wt % of AAm), AAPBA monomers (10 wt % of AAm) and AGA monomers (10 wt % of AAm) were first dissolved in 2.5 mL H2O at 90 °C for 10 min, the polymerization of monomers may not happen at this stage because of the absence of initiator. Then, APS solution (1 wt % of AAm) was added and oscillated rapidly. After that, the transparent, low-viscosity solution was transfused into corresponding molds, including a homemade mold consisting of two glass plates (2 mm) and one silica plate (1 mm or 2 mm), syringes with diameter of 1 cm. After cooling in refrigerator for 30 min, agar formed the first physical-linked network by the agar helix bundles. Upon thermal initiation (60 °C, APS), all monomers were copolymerized. For better vision, the hydrogels were dyed with rhodamine B (red). 2.5. Preparation of Hydrogels with Tunable Mechanical Properties. The mechanical properties of the hydrogels were adjusted by changing the immersing time from 0 to 20 min in Gly-NaOH buffer (pH 10. 6) or Fe3+ solution (0.1 M) to obtain PBA-diol ester bonds and AAc-Fe3+ coordination. 2.6. Mechanical Measurements. The tensile and compression tests were conducted on a tensile-compressive tester (Instron 5567). Samples with a dumbbell shape (length: 50 mm, width: 4 mm, thickness: 2 mm) were prepared for tensile testing. And the tensile tests were conducted at a constant rate of 50 mm/min. Samples with a cylinder shape (length: 1 cm, diameter: 1 cm) were prepared for compression tests (compression rate: 10% of original height/min, maximum compressive deformation: 95%). 2.7. Evaluation of Shape Memory Behaviors. All experiments were conducted at room temperature. Straight strip of hydrogels with size of 60 mm× 2 mm × 1 mm were first bended into temporary shapes, and then were soaked into Gly-NaOH buffer (pH 10.6) or Fe3+ solution (0.1 M) for a certain time to fix the temporary shapes. By submerging the gels in Gly solution (pH 6) or 0.3 M EDTA solution, the shape recovery process was investigated. The thermal-induced

as well as application of surpamolecular shape memory hydrogels.

2. EXPERIMENTAL SECTION 2.1. Materials. Agar powder (gel strength of 700−900 g/cm2), 3Aminophenylboronic acid (APBA), D-(+)-glucosamine hydrochloride (GA), potassium carbonate (K2CO3), acryloyl chloride, acrylic acid (AAc), Glycine(≥99.0%), NaOH, FeCl3·6H2O, EDTA·2Na, and rhodamine B (red dye) were purchased from Aladdin. Ammonium persulfate (APS), acrylamide (AAm), NaHCO3, methyl alcohol, methylene dichloride, tetrahydrofuran, and ethyl acetate were commercially offered by Sinopharm Chemical Reagent Co., Ltd. Chemical reagents such as APS and AAm were used through recrystallization, and the others were used directly. 2.2. Synthesis of N-Acryloyl-3-aminophenylboronic Acid (AAPBA). AAPBA was synthesized in accordance with previous report and the synthetic route was shown in Scheme S1.38 3Aminophenylboronic acid (APBA) and NaHCO3 were dissolved in the mixed solution containing H2O and THF (v/v = 2:1), then put into ice bath. After that, acryloyl chloride was cautiously added dropwise to the solution under intensive stirring over 1 h. Then the solution was stirred for another 2 h. The above solution was evaporated to obtain a crude product after being extracted with ethyl acetate. Eighty milliliters of H2O was added for recrystallization at 90 °C. Subsequently, the acicular crystals were filtered, washed and dried. The structure of AAPBA was characterized by the 400 MHz 1HNMR (d6-DMSO) spectrum (Figure S1). 2.3. Synthesis of N-Acryloyl Glucosamine (AGA).39 According to previous report, AGA was synthesized and the synthetic route was shown in Scheme S2. Five grams of D-(+)-Glucosamine hydrochloride (GA) and 3.2 g of potassium carbonate (K2CO3) were dissolved in 125 mL of methanol. The mixed solution was cooled using an ice-salt bath. Acryloyl chloride was then added into above solution and reacted for 0.5 h under the rotational speed of 1500 r/min. The reaction was kept for another 4 h at 25 °C, and then was filtrated to move away solid impurity. After concentrating by rotary evaporation, white slurry was obtained. Column chromatography was then used for purification 9039

DOI: 10.1021/acsami.7b00169 ACS Appl. Mater. Interfaces 2017, 9, 9038−9044

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Figure 1. Prepared hydrogels are quite tough and flexible and can withstand different large deformations. (A) Finger compression, (B) bending, (C) stretching ordinarily, (D) stretching after being knotted, (E) stretching after being twisted, (F) tensile stress−strain curves, and (G) compressive stress−strain curves of the as-prepared hydrogels.

content.18,24,29,40 The hydrogels were tough enough to endure high-level deformations such as compression (Figure 1A), bending (Figure 1B), ordinary stretching (Figure 1C), knotted stretching (Figure 1D), twisted stretching (Figure 1E) without any obvious damage. In particular, the hydrogels will recover quickly after removing the external force. More specifically, the hydrogels could achieve a maximum tensile stress of 0.1 MPa with the elongation was almost 800%, and at compressive strain of 90%, the compressive stress can reach almost 6 MPa. As coordination interactions between carboxylic acid and Fe3+, PBA-diol ester bonds could be formed in our hydrogels, the mechanical properties of our hydrogels can be adjusted as expected. As shown in Figure 2, the impact of cross-linking density of two different cross-links was investigated by altering the soaking time in Gly-NaOH buffer solution (to form crosslink1) or FeCl3 solution (to form cross-link2). With increasing immersing time in Gly-NaOH buffer solution, the tensile stress, tensile strain as well as elastic modulus (Figure 2A, Figure 2B) of the hydrogel enhance at first, and then decrease. The reason

shape memory phenomenon was performed by placing a hydrogel sample with temporary shape in an oven (10 min) and then cooling in a refrigerator (4 °C) for 6 min to memorize the temporary shapes. At last, the shape recovery process was achieved in hot water (60 °C).

3. RESULTS AND DISCUSSION The preparation of hydrogel with multishape memory effect is illustrated in Scheme 1. The first network was constructed by agar through heat−cool procedure, and the second network was formed by the copolymerization of all the monomers (Scheme 1A−D). The coordination interactions between carboxylic acid and Fe3+, the reversible PBA-diol ester bonds and the coil−helix transition of agar could be employed for stabilizing temporary shapes and realizing shape memory performance. As shown in Figure 1, our as-prepared hydrogels (existing only agar helix as cross-linking points) exhibit outstanding mechanical properties compared with previously reported supramolecular shape memory hydrogels with similar water 9040

DOI: 10.1021/acsami.7b00169 ACS Appl. Mater. Interfaces 2017, 9, 9038−9044

Research Article

ACS Applied Materials & Interfaces

Figure 2. Tunable mechanical properties of hydrogels with different cross-links. (A) Tensile stress−strain curves and (B) the variation of elasticity modulus of hydrogels with different cross-linking degree of dynamic PBA-diol ester bonds by immersing in Gly-NaOH buffer solution (0−20 min). (C) Tensile stress−strain curves and (D) the variation of elasticity modulus of hydrogels with different cross-linking degree of PAAc-Fe3+ by immersing in FeCl3 solution (0.1 M, 0−20 min). (E) Tensile and (F) compressive stress−strain curves of hydrogels with different cross-links (each for 10 min).

Compared to hydrogels with cross-link1 (10 min immersing in Gly-NaOH buffer) and cross-link2 (10 min immersing in FeCl3 solution), a much tougher hydrogel with 0.3 MPa of tensile stress and 1050% of tensile strain can be obtained by combining both cross-link1 and cross-link2 (cross-link1 + 2, through soaking into Gly-NaOH buffer and FeCl3 solution for 10 min, respectively). As the two different reversible interactions coexisting in the network, they can have a synergistic effect for energy dissipation, thus the hydrogels show better mechanical properties comparing with single crosslinked hydrogels. Moreover, with the increasing of cross-link density, the compressive strength presents an uptrend (Figure 3F), and cross-link through AAc-Fe3+ seems to have more obvious enhancement to the compressive strength. It is worth mentioning that after compression test, all samples can recover to their initial status and the recovery rates are all greater than

may be that most dynamic PBA-diol ester bonds were formed with increasing immersing time, which are more like covalent bonds at pH 10.6 and leads to the loss of energy dissipation capacity. As to the coordination interactions between carboxylic acid and Fe3+, the tensile stress was improved from 0.1 to 0.28 MPa, then drops to 0.24 MPa with prolonging immersing time. In the meanwhile, the tensile strain also shows a similar trend, in which the maximum elongation can reach 1350%. A possible explanation to the above mechanical behavior is that the Fe3+ could form a mixture of mono-, bi-, and tridentates with AAc,41 the longer immersing time leading to more Fe3+ enter into the internal hydrogel and also form more mono- or bidentates with AAc. As a result, for the hydrogels, the capacity of dissipating energy first rising and then has a downward trend. The strain range used to estimate the modulus is from 0% to 100% and the elastic modulus was increased with longer immersion time. 9041

DOI: 10.1021/acsami.7b00169 ACS Appl. Mater. Interfaces 2017, 9, 9038−9044

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Figure 3. Dual shape memory effect based on Fe3+, pH, and thermal, respectively. (A, B) Heart shape can be memorized and recovered through the chelation and damage between carboxylate and Fe3+. (A, C) On the basis of dynamic PBA-diol ester bonds, the hydrogel can memorize spiral shape and recover to permanent shape. (A, D) Attributed to agar’s responsiveness of temperatures, the hydrogel can get temporary “N” shape fixed as well as recover to the original shape.

Figure 4. Fe3+-, pH-, thermo-induced multishape memory effect. (A, B) Temporary shape I was fixed by coil−helix transition of agar upon changing the temperature. (B, C) Cross-links of dynamic PBA-diol ester bonds can be applied for memorizing temporary shape II. (C, D) Through the chelation between Fe3+ and carboxylic groups, temporary shape III can be fixed. (E)Mechanism of programmed multishape memory process.

slowly recover (Figure S7A−C). In addition, temporary shapes can also be fixed by the formation of dynamic PBA-diol ester bonds (Figure 3C, Figure S7D−F). The hydrogel with spiral temporary shape was put into Gly-NaOH buffer solution (pH 10.6) for 10 min. Once the fixed shapes were immersed into Gly solution (pH 6), the recovery to the original state can be achieved through the disassociation of PBA-diol ester bonds (Figure S7D−F). In addition, the coil−helix transition of agar upon temperature changing can also be utilized to accomplish shape memory performance (Figure 3D, Figure S7G−I). A straight strip of hydrogel was first put into an oven (60 °C) for 10 min to become soft and could be shaped into many shapes. Then the hydrogel with a deformed “N” shape was cooled in the refrigerator (4 °C) to induce the formation of helix structure of agar, and the temporary shapes could be memorized. After putting into hot water (60 °C), the hydrogel

90% (Figures S3 and S4), which further elucidate that our hydrogels have good self-recovery performance. Our hydrogel networks contain three different kinds of functional groups as well as agar, among which PBA groups can integrate with glucosamine groups under basic condition to form dynamic PBA-diol ester bonds, carboxylate groups can chelate Fe3+, and agar can realize coil−helix transition upon changing the temperature.42,43 All the reversible cross-links could be employed to keep the temporary shapes of the hydrogel, and achieve shape memory performance (Figure S6 shows the shape fixity ratios and shape recovery ratios). Figure 3 shows that a straight strip of hydrogel was shaped into a heart shape and then immersed into FeCl3 solution (0.1 M). Through the AAc-Fe3+ cross-links, the final temporary heart shape could be well fixed. After immersing into EDTA (0.3 M) solution, Fe3+ will be extracted by EDTA, the hydrogel can 9042

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could recover to the initial shape in a short time as agar transfer to the coil state. A multishape memory material which could remember more than two temporary shapes could meet more specific and complicated applications, but not easy to realize. As our hydrogel could respond to three different stimuli, which urge us to further explore the multishape memory effect. As shown in Figure 4, hydrogel was first put into an oven at 60 °C for 10 min, the hydrogel was bent and put into a refrigerator at 4 °C to fix temporary shape I. Then the hydrogel was shaped again and immersed into Gly-NaOH buffer solution (pH 10.6) to fix temporary shape II, finally the hydrogel was bent into a “W” shape and stabilized by the complexation of AAc and Fe3+. The recovery of “W” shape hydrogel from temporary shape III to temporary shape II was conducted by immersing in the competition ligand solution (EDTA, 0.3 M) to break the AAcFe3+ interaction (Figure S8B). By destroying the PBA-diol ester bonds in acidic condition, the hydrogel can recover to temporary shape I (Figure S8C). It will finally become straight because of the helix−coil transition of agar (Figure S8D). Here, we point out that the programmable steps cannot be changed for the following reasons: first, Fe3+ would be deposited in GlyNaOH buffer (pH 10.6), therefore gels was first immersed into Gly-NaOH buffer; second, all the reversible switches may be damaged by heating, so heat was put at last during the recovery process. In addition, the strategy introduced here selects three different reversible interactions, by which multishape memory properties in a programmatic way can be realized and the procedure could also be repeated.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J-W. Z.). *E-mail: [email protected] (T. C.). ORCID

Jiawei Zhang: 0000-0002-3182-9239 Youju Huang: 0000-0001-5815-9784 Tao Chen: 0000-0001-9704-9545 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21644009), Natural science foundation of Zhejiang (LY17B040003, LY17B040004), Key Research Program of Frontier Science, Chinese Academy of Sciences (QYZDB-SSW-SLH036), Ningbo Science and Technology Bureau (2016C50009), China Postdoctoral Science Foundation (2016M590556), and Youth Innovation Promotion Association of Chinese Academy of Sciences (2017337, 2016268).

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CONCLUSIONS In conclusion, through integrating various reversible interactions, we present a novel supramolecular hydrogel with both tunable mechanical properties and multishape memory effect. The hydrogel contains a physically cross-linked agar network and a supramolecular network cross-linked by reversible PBAdiol ester bonds and AAc-Fe3+ coordination. The hydrogel exhibits good mechanical properties and the mechanical performance could be tuned by adjusting the cross-linking density. The coil−helix transition of agar, PBA-diol ester bonds, and AAc-Fe3+ coordination could all be applied to fix temporary shapes and realize dual shape memory behavior. Furthermore, multishape memory effect can be realized through programmable integration of three reversible interactions. The presented strategy could broaden the list of shape memory hydrogels and promote the design of novel shape memory systems for real applications.

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Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00169. Synthetic route of AAPBA; 1H NMR (d6-DMSO) spectrum of AAPBA; the synthetic route of AGA; 1H NMR (D2O) spectrum of AGA; the photographs of compressive tests; the graph of recovery rate of different samples after being compressed; the water contents of hydrogels with different cross-links; the shape fixity ratios and shape recovery ratios of three reversible interactions; the pictures of dual-shape memory and shape recovery process; the recovery of multishape memory effect (PDF) 9043

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DOI: 10.1021/acsami.7b00169 ACS Appl. Mater. Interfaces 2017, 9, 9038−9044