Reversing Redox Responsiveness of Hydrogels due to

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Reversing Redox Responsiveness of Hydrogels Due to Supramolecular Interactions by Utilizing Double-Network Structures. Shingo Tamesue, Shingo Noguchi, Yuko Kimura, and Takuo Endo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10001 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Applied Materials & Interfaces

Reversing Redox Responsiveness of Hydrogels Due to Supramolecular Interactions by Utilizing Double-Network Structures. Shingo Tamesue,*,†,‡ Shingo Noguchi,‡ Yuko Kimura,‡ and Takuo Endo‡ †Department of Material and Environmental Chemistry, Graduate School of Engineering, Utsunomiya University, 7-1-2, Yoto, Utsunomiya, Tochigi 321-8585, Japan ‡Department of Material Science and Technology, Faculty of Engineering, Niigata University, 2-8050, Ikarashi, Nishi-ku, Niigata 950-2181, Japan Keywords: Host-guest, Hydrogel, Softmaterials, Stimuli-responsive Materials, Supramolecule ABSTRACT: Stimuli responsive hydrogels have been actively researched, and some of them have been put into practical use. When we create and use stimuli responsive hydrogel materials, controlling stimuli responsiveness of hydrogels is very import issue. In this research, we prepared hydrogels having single-network (SN) or double-network (DN) gel structures with the host–guest interaction groups cyclodextrin and methylviologen and evaluated their stimuli responsiveness. The results of the tensile and compression tests showed that the hydrogels with SN and DN structures exhibited opposite stimuli responsiveness in response to the redox reaction of methylviologen through the association and dissociation of the host molecule, -cyclodextrin, and the guest molecule, methylviologen. Spectroscopic measurements and rheological studies all indicated that this difference in stimuli responsiveness originated from the polymer-network structures. In addition, a chemically cross-linked DN gel was prepared and its redox responsiveness was evaluated.

1. Introduction Recently, the creation of materials containing supramolecular bonds1-3 has become very attractive because almost all supramolecular bonds are reversible, and these materials exhibit unique properties such as self-healing4-7 and shape memory.8-10 Meanwhile, these characteristics have become sought after in various fields, such as medicine, engineering, and materials science. There are many supramolecular materials exhibiting stimuli (e.g., photo and redox.) responsiveness,11 such as sol– gel transitions12-15 or stimuli responsive macroscopic selforganizations,16 because of the ease of reforming the stimuliresponsive supramolecular bond in response to stimuli. However, it is difficult to create materials that show opposite stimuli responsiveness toward the same stimulus from the same components. That is to say, almost all supramolecular materials formed from the same components exhibit similar macroscopic responsiveness when they are exposed to a particular stimulus. If it was possible to prepare supramolecular materials formed from the same constituents that exhibit opposite responsiveness toward the same stimulus, the potential of supramolecular stimuli responsive materials would increase enormously. This would also be desirable from an industrial chemistry perspective because the cost of components forming the stimuli responsive materials could be greatly decreased. It has been reported that network structures, such as dendritic structures and block copolymers, affect the macroscopic properties of materials. For example, the kinetics of the formation of hydrogels can be tuned as they are dependent on the polymer structures utilized as cross-linkers bridging the inorganic clay nanosheets.17 Hydrogel materials with a high mechanical

Figure 1. (a) Molecular structures of the monomers forming hydrogels, (b) chemical structure of the single network (SN) gel, and (c) the first and the second double-network (DN) gel structures containing the same concentrations of the β-cyclodextrin group and the methylviologen group ([CD] = 15 mM, [MV] = 1.7 mM).

strength due to their unique polymeric structures, such as nanocomposite gels,18-20 slide-ring gels21-23 and tetra-PEG gels,24-26 have been reported. Among them are the double network (DN) gels reported by Gong et al27-30, which show a high

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mechanical strength because of two isolated polymer networks, formed from soft and hard networks, which delocalize the stress on the gel network effectively. To obtain supramolecular gel materials exhibiting opposite stimuli responsiveness from the same components, we utilized this DN structure, and demonstrated the reversing of the effects of the supramolecular bonds to the stimuli responsiveness of hydrogel materials. Some researchers reported stimuli responsive DN gels due to host–guest interactions.31-33 However, there are still few examples of hydrogel materials, which two separated polymer networks can be bridged by host–guest interactions, and change their stimuli responsiveness. In this research, we used an inclusion complex of βcyclodextrin (CD) and methylviologen (MV) as a stimuli responsive supramolecular bond cross-linking the gel networks. CD is a cyclic oligosaccharide which is well-known for its use as a host molecule, entrapping various functional guest molecules.34-37 MV is a redox responsive molecule and reduced MV (MV0) is strongly entrapped into the cavity of CD.38-39 To prepare the gel materials utilizing these molecules, polymerizing groups were modified to CD and MV (CD monomer and MV monomer) as shown in Figure 1a and according to the synthetic procedure detailed in the supporting information. 2. Results and Discussion Firstly, two hydrogels were prepared from the same components using different preparation procedures. The first hydrogel was created via a single step. All the components, acrylamide (AAM), N,N’-methylenebis(acrylamide) (MBAM), and the CD and MV monomers shown in Figure 1a, were mixed in water and polymerized by radical polymerization. The resultant hydrogel (SN gel) was formed from a single-polymer network shown in Figure 1b. The other hydrogel was prepared in a step-wise manner. First, the gel network formed from AAM, MBAM, and the CD monomer by radical polymerization was immersed into an aqueous solution of monomers consisting of AAM, MBAM, and MV monomers for three days. After immersion, the monomers entrapped inside the first network gel were polymerized by in-situ radical polymerization. The resultant hydrogel (DN gel) had two isolated polymer networks, the first was composed of CD units, and the second was composed of MV units as shown in Figure 1c. In this way, two types of hydrogel, SN and DN gels, were prepared from the same components with the same concentration of CD and MV units. A tensile test was carried out on the SN gel and DN gel before and after repeated redox reactions. Both the original hydrogels could be expanded approximately three times their original length (tensile tenacity = 300%) as shown in Figure 2a and 2b. After the reduction of MV groups with sodium borohydride (NaBH4), the tensile tenacity of the SN gel increased (400%) because the reduced MV (MV0) groups were entrapped into the cavities of the CD groups because of the hydrophobic interaction between CD and MV0 and they formed reversible supramolecular cross-linking, serving as sacrificial bonds to relax tensile stress across the gel network. In addition, it was confirmed that the responsiveness of the SN gel decreased when the concentrations of the CD and MV monomer were decreased, as shown in Figure 3. In contrast, the increasing tensile tenacity of the DN gel (300%) decreased after the reduction (185%) of the MV groups, which is

Figure 2. (a) Photographs of the SN and DN gels just before breaking by tensile elongation and schematic representations of polymer network structures of the SN and DN gels before and after redox reactions, (b) tensile tenacity of the respective hydrogels (SN gel: blue line, DN gel: red line) and error bars before and after repeated redox reactions calculated from the results of three samples, and (c) strain-stress curves of SN and DN gels before and after the redox reaction obtained from compression test.

shown with a red line in Figure 2a and 2b. Furthermore, as with the SN gel, the responsiveness of the DN gel decreased as the concentrations of CD and MV decreased. It is believed that the two separated polymer networks existing inside the DN gel were cross-linked to each other by the formation of an inclusion complex between CD and MV. This cross-linked network structure, which is like a pseudo-SN, could not sufficiently delocalize the tensile stress over the gel network. Although the DN structure was able to localize the stress onto the first network, utilizing the second network like a cushion. In addition, it is suggested that this could be because the second gel network had cross-linked with the first gel network due to the host–guest interaction, and that this became rigid after the reduction reaction. This stimuli responsiveness of the DN gel is in accordance with the results of the tensile test on the truly isolated DN gel and the covalently cross-linked DN gel reported by Gong et al.40 According to the literature, when the second network of the DN gel had a measurable amount of MBAM, the mechanical strength decreased as the connection between the first and second network became stronger. In this way, in spite of the fact that these two hydrogels were formed


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Figure 3. Tensile strains of original SN and DN gels and the hydrogels including half as high concentrations of CD and MV as original ones after repeated redox reactions: SN gel and DN gel ([CD] = 15 mM, [MV] = 1.7 mM) and SN0.5 gel and DN0.5 gel ([CD] = 7.5 mM, [MV] = 0.84 mM).

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Figure 4. Changes in the elongation tenacity of (a) SN and (b) DN gels without CD (light blue line) or MV groups (green line), and (c) SN and (d) DN gels containing free β-CD (15 mM, light blue line) as a competitive host molecule or AdNa (15 mM, green line) as a competitive guest molecule after the repeated redox reaction of MV.

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Figure 5. Absorption spectra of MV (0.08 mM) in water with or without β-CD (0 to 4.0 mM) (a) before and (b) after reduction reaction utilizing NaBH4 (80 mM).

from the same components (AAM, MBAM, CD, and MV), these supramolecular soft materials exhibited the opposite redox responsiveness of tensile tenacity depending on the polymer network structures. The investigation into the difference between the redox responsiveness of SN and DN gels also included a compression

Figure 6. 1H NMR (400 MHz, D2O, 25˚C) spectra of MV monomer with various concentrations of CD in the state of (a) MV2+ and (b) MV0. (c) CD concentration dependency of 1H NMR peak shift obtained from (b).

test, as shown in Figure 2c. The compressive strength of the SN gel (maximum strain and force were 47% and 18 kPa, respectively) increased after the reduction of MV groups (57%, 27 kPa), and recovered again after the oxidation reaction (44%, 17 kPa). In the case of the DN gel, the changes were reversed. The maximum strain and force of the DN gel before reduction reaction were 42% and 62 kPa, respectively, and decreased to 31% and 40 kPa, respectively, after the reduction reaction. As with the SN gel, the compressive strength of the DN gel recovered again after the re-oxidation reaction (41%, 64 kPa).



Figure 7. 2D ROESY NMR (400 MHz, D2O, 30˚C) spectra of β-CD (18 mM) and MV monomer (6.0 mM) before and after reduction with NaBH4 (60 mM). Correlation peaks between inner protons of CD and MV unit were found after reduction. Peaks were emphasized with squares.

As reference experiments, SN and DN gels without CD or MV substituents were prepared and their tensile tenacity before and after the redox reaction was measured, as shown in Figure 4a and 4b. The tensile tenacity of these hydrogels hardly changed compared to that of the SN and DN gel. In addition, competitive experiments were carried out utilizing SN and DN gels containing free β-CD as a competitive host molecule and an adamantine carboxylate sodium salt (AdNa) as a competitive guest molecule, of which the association constant with CD is 5.0 × 104,41 as shown in Figure 4c and 4d. Because these competitive molecules interacted with the CD and MV units in the polymer network, respectively, and hindered the formation of an association complex between CD and MV within the gel

networks, both the SN and DN gel containing free β-CD or AdNa exhibited a dramatic decrease in redox responsiveness. These results clearly support the postulate that the redox responsiveness of the hydrogels originates from the association and dissociation of the CD and MV groups. UV-vis spectra of aqueous solutions of MV2+ with respective concentrations of β-CD are shown in Figure 5. The spectra indicated no significant changes, regardless of the concentration of β-CD. However, after the reduction of MV groups, the absorption intensity of MV0 at 230 nm increased with the increase in the concentration of β-CD due to the interaction between MV0 and CD.


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2D ROESY NMR spectra of the mixture of MV and β-CD in D2O before and after the redox reaction were measured to confirm the redox responsive interaction between the CD and MV monomers, as shown in Figure 7. Before the reduction of

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Figure 10. (a) Scheme of the chemically cross-linked DN gel, the DN structure of which was cross-linked by covalent bonds. (b) Chemical structures of chemically cross-linked DN gel. (c) Schematic illustration of the chemically cross-linked DN gel. (d) Tensile tenacity of the chemically cross-linked DN gel (purple line) and DN gel (red line) before and after the repeated redox reactions. (b)

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Figure 9. (a) Photographs and schematic representations of an aqueous polymer mixture formed from CD monomers, MV monomers, and AAM ([CD] = 10 mM, [MV] = 1.1 mM, and [AAM] = 2.0 M) before and after redox reactions, and (b) their Gʹ and tan d values obtained from rheological measurements (shear rate = 0.5%, frequency = 0.1 rad s−1, temperature = 10 °C).

H NMR spectra of the MV monomer with respective concentrations of β-CD in deuterium oxide (D2O) were measured as shown in Figure 6a and 6b. The 1H NMR peaks of MV2+ did not move significantly before MV reduction, regardless of the concentration of β-CD. In contrast, after MV reduction, the peaks corresponding to MV0 moved to a higher magnetic field with increasing concentration of β-CD because the MV0 was entrapped inside the β-CD cavity by hydrophobic interactions. The association constant of the MV monomer and β-CD was estimated to be 1.7 × 104 from the peak shift of the MV0 groups depending on the concentration of β-CD as shown in Figure 6c using Equation S1 in the Supporting Information.

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Figure 8. Circular dichroism spectra of aqueous solutions of (a) MV2+, and MV2+ with β-CD, (b) MV0, MV0 with β-CD by the addition of NaBH4 (0.80 mM), (c) MV0 with β-CD, MV0 with AdNa (0.72 mM), and (d) MV0 with β-CD after addition of NaOCl (0, 0.80 and 40 mM). Concentrations of MV monomer and β-CD of the samples were 0.08 mM and 0.24 mM, respectively.

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Figure 11. (a) Chemical structure of the resultant chemically cross-linked DN gels. (b) Strain-stress curves and (c) tensile tenacity of the chemically cross-linked DN gels.

the MV groups, no significant correlation peaks were observed between the protons of β-CD and MV, whereas the correlation peaks arising from the inclusion complex of β-CD (inner protons C3H and C5H observed at 3.36 ppm and 3.57 ppm) and MV (observed at 6.91 ppm and 7.15 ppm) appeared after the reduction of MV with NaBH4. From the results, it was believed that the MV had become entrapped in the cavity of CD after the reduction. Circular dichroism (Cd) spectra of MV monomer in water were measured, as shown in Figure 8. Before MV reduction, no significant circular dichroism bands were observed in spite of the existence of β-CD. In contrast, an induced circular di-



chroism (ICd) band at 250 nm appeared after the reduction of the MV groups. The ICd band disappeared again after the addition of AdNa which interacted strongly with CD in the solution of reduced MV and β-CD. Furthermore, the addition of NaOCl to the solution also caused a decrease in the ICd band because of the dissociation of β-CD and MV. In this way, it was indicated that the association and dissociation of the inclusion complex formed from β-CD and MV could be regulated via the redox reaction of the MV groups. We also measured Cd spectra of DN and SN gels. As shown in Figure 8, we confirmed increase of Cd intensity after reduction reactions, and decrease after re-oxidation reactions from both DN and SN gels. From the results of Cd spectroscopy, it was found that CD and MV unit interact in response to redox reaction inside gel networks. In this way, the results obtained from the spectroscopy clearly indicate the formation of a redox responsive inclusion complex between β-CD and MV. To confirm that association and dissociation between CD and MV in the polymer chains occurs in response to a redox reaction that affects the macroscopic properties such as viscosity, rheological measurements were carried out. CD and MV monomers were polymerized with AAM in water by radical polymerization, and a viscous solution was obtained. Figure 9a shows photographs of the solution before and after the redox reaction. After MV reduction, the viscosity of the solution increased and the solution did not flow, even when the sample tube was inverted. After re-oxidation of the MV units, the viscosity of the solution decreased again. As shown in Figure 9b, the G’ value of the solution increased to over 10 times that of the original solution after the reduction reaction from 0.24 Pa to 2.9 Pa. In contrast, the G’ value decreased again to 0.36 Pa after the re-oxidation reaction. Additionally, tan d (loss elastic modulus Gʹʹ/Gʹ) of the solution also clearly changed in response to the redox reaction. From these results, it is thought that CD and MV inside polymer chains interact with each other, and polymer chains connected after the reduction reaction because of the MV0 entrapped by CD by hydrophobic interactions. Moreover, it was confirmed that the redox responsive interaction between CD and MV in polymer chains causes changes in macroscopic properties. The hydrogel with a chemically cross-linked DN structure was prepared as shown in Figure 10a and the supporting information, and its elongation tenacity was measured to ensure the two separated gel networks were required to reverse the redox responsiveness of hydrogel. The two polymer networks inside the chemically cross-linked hydrogel were bridged by amide bonds, as shown in Figure 10a-c. The formation of amide bond bridging between the first and second polymer networks was confirmed using IR spectroscopy. The C=O absorption band of poly(acryl acid) gel at 1715 cm−1 decreased and the broad IR absorption band at 1635 cm−1, ascribed to the overlapping absorption bands of amide I and amide II, was observed after the formation of amide bonds in Figure S19. Interestingly, the chemically cross-linked DN gel indicated a change in tensile tenacity similar to an SN gel in response to the redox reactions, as shown in Figure 10d. The tensile tenacity of the cross-linked DN gel decreased after reduction of the MV groups and re-increased after oxidation. It is thought that these characteristics, which originated from the existence of two isolated polymer networks, were lost because of crosslinking by amide bonds and that the supramolecular cross-

linking just served as a sacrificial bond to relax the tensile stress after the redox reaction. The chemically cross-linked DN gel exhibited the same behaviors toward the redox reaction of MV groups as the SN gel. To confirm that cross-linking density between two gel networks of DN gels affects tenacity of hydrogels, the tensile tenacity of chemically cross-linked DN hydrogels having different cross-linking density detailed in the Supporting Information (Table S1) were measured as shown in Figure 11. As chemically cross-linking density increased, the tensile tenacity decreased gradually. These results indicate that the softer second gel networks could not disperse tensile stress well as they were cross-linked by the stiffer first gel networks. 4. Conclusion Network structures as well as reversible supramolecular bonds are involved in the stimuli responsiveness inside living organisms. Our research reported the control of stimuli responsiveness as a result of supramolecular interactions via the polymer-network structures, similar to the interactions in living organisms. We prepared two types of hydrogels (SN and DN gels) from the same components. Nevertheless, they exhibited the opposite response toward the same redox reaction, as a result of the supramolecular bonds depending on the polymer-network structures. The knowledge obtained from this research will be useful in creating stimuli responsive supramolecular polymeric materials, demonstrating a finely tunable stimuli-responsiveness, and modelling the stimuli responsive systems in living organisms.

EXPERIMENTAL SECTION General β-cyclodextrin was purchased from Wako Pure Chemicals Co., Inc. All other chemicals were purchased from KANTO Chemical Co., Inc. and used as received. All solvents utilized were reagent grade (99.9 %). 1H NMR spectra were recorded on a Varian model 400-MR spectrometer and NMR system700 spectrometer. 13C NMR spectra were recorded on a Varian model NMR System700 spectrometer. ESI-Ms spectra were collected on a mass spectrometer (Water/Micromass, ZQ-4000). Tensile test was recorded on Imada MX2500N-FA equipped with Imada, ZTA-50N and compressive test was recorded on Imada MX2-500N-FA equipped with Imada, ZTA-500N. Shapes of samples used for tensile test were thin sheet shaped (thickness/ width/ height = 2 mm/ 10 mm/ 3 mm), and these used for compression test were cylinder (thickness/ radius= 12 mm/ 10 mm). The rates of extending and compressing samples were 10 mm/sec. Error bars of tensile test were calculated from the results of three samples. Redox reaction of hydrogels for tensile test and compression test was conducted by immersing hydrogels into aqueous solution of NaBH4 or NaOCl for 3 min and 30 min, respectively. To keep the concentrations of CD and MV unit, all the hydrogels for tensile test and compression test were kept in air after redox reaction till the weight of the gels were back to the original weight. UV/vis spectra were measured by using SHIMADSU UV-1800, and the optical path length was 5 mm. Cd spectra were measured by using JASCO J-820 Cd spectrometer, and the optical path length was 10 mm. Attenuated total reflection infrared (ATR-IR) spectroscopy was conducted by using Shimadzu FTIR-8400S. Rheology data was measured by using AntonPaar MCR-301. Synthesis of monomers The monomers forming gel networks were synthesized according to the Supporting Information. Preparation of single network (SN) gel containing β-CD and methyl viologen (MV) groups CD monomer (44.1 mg, 3.75 × 10-2 mmol, 15.0 mM), MV monomer (1.90 mg, 4.20 × 10-3 mmol, 1.68 mM), AAM (533 mg, 7.50


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ACS Applied Materials & Interfaces mmol, 3.00 M), and MBAM (1.16 mg, 7.52 × 10-3 mmol, 3.01 mM) were mixed in water, and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) was added as a radical initiator. After N2 bubbling to the solution for 2 hours, this mixture was heated at 65˚C for 12 hours, and the gel was obtained as a single network (SN) gel containing β-CD and methyl viologen groups as shown in Figure S4. Preparation of CD gel (1st network gel) CD monomer (44.1 mg, 3.75 × 10-2 mmol, 15.0 mM), acrylamide (AAM, 164 mg, 2.31 mmol, 924 mM), and N,N’methylenebisacrylamide (MBAM, 13.5 mg, 8.76 × 10-2 mmol, 35.0 mM) were mixed in water (2.5 mL), and ammonium peroxodisulfate (APS, 5.73 mg, 2.51 × 10-2 mmol, 10.0 mM) was added as a radical initiator. After N2 bubbling for 2 hours, this aqueous mixture was heated at 65˚C for 12 hours, and the hydrogel was obtained as a CD gel as shown in Figure S5. Preparation of methyl viologen gel MV monomer (30.4 mg, 6.74 × 10-2 mmol, 1.69 mM), AAM (8.53 g, 1.20 × 102 mmol, 3.00 M), and MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM) were mixed in H2O (2.5 mL), and 2, 2’-azobis[2-(2imidazolin-2-yl)propane] (ABIP, 12.5 mg, 4.99 × 10-2 mmol, 20.0 mM) was added as a radical initiator. After N2 bubbling for 2 hours, this mixture was heated at 65˚C for 12 hours. In the result, gel was obtained as a MV gel as shown in Figure S6. In this way, it was confirmed that the second gel network could be constructed on these reaction conditions. Preparation of DN hydrogel containing CD and MV groups CD gel was immersed into an aqueous solution of MV monomer (30.4 mg, 6.74 × 10-2 mmol, 1.69 mM), AAM (8.53 g, 1.20 × 102 mmol, 3.00 M), MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) at 5˚C for 3 days. After that, the hydrogel was heated at 65˚C for 12 hours. In this way, 2nd gel network was formed inside the 1st gel network as shown in Figure S7. Preparation of DN hydrogel containing MV groups Poly(acrylamide) gel was prepared from AAM (164 mg, 2.31 mmol, 924 mM) and MBAM (13.5 mg, 8.76 × 10-2 mmol, 35.0 mM) in the same procedure as CD gel. The resultant Poly(acrylamide) gel was immersed into an aqueous solution of MV monomer (30.4 mg, 6.74 × 10-2 mmol, 1.69 mM), AAM (8.53 × 103 mg, 1.20 × 102 mmol, 3.00 M), MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) at 5˚C for 3 days. After that, the hydrogel was heated at 65˚C for 12 hours. In this way, 2nd gel network was formed inside the 1st gel network as shown in Figure S8. Preparation of DN hydrogel containing CD groups CD gel was immersed into an aqueous solution of AAM (8.53 g, 1.20 × 102 mmol, 3.00 M), MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) at 5˚C for 3 days. After that, the hydrogel was heated at 65˚C for 12 hours. In this way, 2nd gel network was formed inside the 1st gel network as shown in Figure S9. Preparation of SN hydrogel containing competitive host molecules CD monomer (44.1 mg, 3.75 × 10-2 mmol, 15.0 mM), MV monomer (1.90 mg, 4.20 × 10-3 mmol, 1.68 mM), AAM (533 mg, 7.50 mmol, 3.00 M), MBAM (1.16 mg, 7.52 × 10-3 mmol, 3.01 mM), and free β-CD (42.9 mg, 3.78 × 10-2 mmol, 15.1 mM) were mixed in water, and ABIP (12.5 mg, 4.99 × 10-2 mmol, 20.0 mM) was added as a radical initiator. After N2 bubbling to the solution for 2 hours, this mixture was heated at 65˚C for 12 hours, and the gel was obtained as a single network (SN) gel containing a competitive host molecule, free β-CD, as shown in Figure S10. Preparation of DN hydrogel containing competitive host molecules CD gel was immersed into an aqueous solution of free β-CD as a competitive host molecule (688 mg, 0.607 mol, 15.2 mM), MV monomer (30.4 mg, 6.74 × 10-2 mmol, 1.69 mM), AAM (8.53 × 103 mg, 1.20 × 102 mmol, 3.00 M), MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) at 5˚C for 3 days. After that, the hydrogel was heated at 65˚C for 12 hours, and 2nd gel network was formed inside the 1st gel network. In this way, DN hydrogel containing competitive host molecules was prepared as shown in Figure S11.

Preparation of SN hydrogel containing competitive guest molecules CD monomer (44.1 mg, 3.75 × 10-2 mmol, 15.0 mM), MV monomer (1.90 mg, 4.20 × 10-3 mmol, 1.68 mM), AAM (533 mg, 7.50 mmol, 3.00 M), MBAM (1.16 mg, 7.52 × 10-3 mmol, 3.00 mM), and adamantine carboxylate sodium salt (AdNa, 7.64 mg, 3.78 × 10-2 mmol, 15.1 mM) were mixed in water, and ABIP (12.5 mg, 4.99 × 102 mmol, 20.0 mM) was added as a radical initiator. After N2 bubbling to the solution for 2 hours, this mixture was heated at 65 ˚C for 12 hours, and the gel was obtained as a single network (SN) gel containing a competitive guest molecule, AdNa, as shown in Figure S12. Preparation of DN hydrogel containing competitive guest molecules CD gel was immersed into an aqueous solution of adamantane carboxylate sodium salt as a competitive guest molecule (123 mg, 6.07 × 10-1 mmol, 15.2 mM), MV monomer (30.4 mg, 6.74 × 10-2 mmol, 1.69 mM), AAM (8.53 × 103 mg, 1.20 × 102 mmol, 3.00 M), MBAM (18.5 mg, 1.20 × 10-1 mmol, 3.00 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-1 mmol, 20.0 mM) at 10˚C for 3 days. After that, the hydrogel was heated at 65˚C for 12 hours, and 2nd gel network was formed inside the 1st gel network. In this way, DN hydrogel containing competitive guest molecules was prepared as shown in Figure S13. Preparation of chemically cross-linked double network gel 1st network gel containing acrylic acid was prepared from acrylic acid (1.76 mg, 2.44 × 10-2 mmol, 9.75 mM), AAM (533 mg, 7.50 mmol, 3.00 mM), MBAM (1.16 mg, 7.52 × 10-3 mmol, 3.01 mM), and APS (5.73 mg, 2.51 × 10-2 mmol, 10.0 mM). This gel was immersed into an aqueous mixture of compound 1 (4.77 mg, 2.68 × 10-2 mmol, 8.05 mM) and DMT-MM (7.24 mg, 2.68 × 10-2 mmol, 8.05 mM) for 4 hours. The hydrogel was taken out from the solution, and immersed into an aqueous solution, in which 2nd network monomers (AAM (8.53 × 103 mg, 1.20 × 102 mmol, 3.00 mM), MBAM (11.6 mg, 7.52 × 10-2 mmol, 1.88 mM), and ABIP (2.00 × 102 mg, 7.99 × 10-2 mmol, 20.0 mM)) were dissolved for 3 days. By heating at 65˚C for 12 hours, 2nd network gel was synthesized inside the polymer network of 1st network gel.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Details of synthesis of the monomers, details of preparing the gels, 1H NMR spectra, test curves of tensile test, calculation of the association constant, and IR spectra (PDF).

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Prof. Dr. Harada A. and Dr. Takashima Y., Osaka University, and Prof. Dr. Yamauchi T., Niigata University, for their kind experimental help. This work was supported by JSPS KAKENHI (Grant Number: 17K14537).

REFERENCES



(1) Lehn, J. M. Supramolecular Chemistry-Molecular Information and the Design of Supramolecular Materials, Macromol. Symp. 1993, 69, 1-13. (2) Faul, C. F. J.; Antonietti, M. Ionic Self-Assembly: Facile Synthesis of Supramolecular Materials, Adv. Mater. 2003, 15, 673-683. (3) Aida, T.; Meijer, E. W.; Stupp, S. I. Functional Supramolecular Polymers, Science 2012, 335, 813-817. (4) White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S. Autonomic Healing of Polymer Composites, Nature 2001, 409, 794-797. (5) Wojtecki, R. J.; Meador, M. A.; Rowan, S. J.; Using the Dynamic Bond to Access Macroscopically Responsive Structurally Dynamic Polymers, Nat. Mater. 2011, 10, 14-27. (6) Nakahata, M.; Takashima, Y.; Yamaguchi, H.; Harada, A. Redox-Responsive Self-healing Materials Formed from Host–guest Polymers, Nat Comm. 2011, 2, 511. (7) Zhang, M.; Xu, D.; Chen, J.; Dong, S.; Zheng, B.; Huang, F. Self-healing Supramolecular Gels Formed by Crown Ether Based Host-guest Interactions, Angew. Chem. Int. Ed. 2012, 51, 7011–7015. (8) Li, J.; Viveros, J. A.; Wrue, M. H.; Anthamatten, M. ShapeMemory Effects in Polymer Networks Containing Reversibly Associating Side-Groups, Adv. Mater. 2007, 19, 2851-2855. (9) Kumpfer, J. R.; Rowan, S. J. Thermo-, Photo-, and ChemoResponsive Shape-Memory Properties from Photo-Cross-Linked Metallo-Supramolecular Polymers, J. Am. Chem. Soc. 2011, 133, 12866-12874. (10) Ware, T.; Hearon, K.; Lonnecker, A.; Wooley, K. L.; Maitland, D. J.; Voit, W. Triple-Shape Memory Polymers Based on SelfComplementary Hydrogen Bonding, Macromolecules 2012, 45, 10621069. (11) Wang, H.; Ji, X.; Li, Z.; Huang, F. Fluorescent Supramolecular Polymeric Materials, Adv. Mater. 2017, 29, 1606117. (12) Weng, W.; Beck, B.; Jamieson, A. M.; Rowan, S. J. Understanding the Mechanism of Gelation and Stimuli-Responsive Nature of a Class of Metallo-Supramolecular Gels, J. Am. Chem. Soc. 2006, 128, 11663-11672. (13) Zhao, Y. L.; Stoddart, J. F. Azobenzene-Based LightResponsive Hydrogel System, Langmuir 2009, 25, 8442-8446. (14) Tamesue, S.; Takashima, Y.; Yamaguchi, H.; Shinkai, S.; Harada, A. Photoswitchable Supramolecular Hydrogels Formed by Cyclodextrins and Azobenzene Polymers, Angew. Chem. Int. Ed. 2010, 49, 7461-7464. (15) Yan, X.; Xu, D.; Chi, X.; Chen, J.; Dong, S.; Ding, X.; Yu, Y.; Huang, F. A Multiresponsive, Shape-Persistent, and Elastic Supramolecular Polymer Network Gel Constructed by Orthogonal SelfAssembly, Adv. Mater. 2012, 24, 362-369. (16) Yamaguchi, H.; Kobayashi, Y.; Kobayashi, R.; Takashima, Y.; Hashidzume, A.; Yamaguchi, H. Photoswitchable Gel Assembly Based on Molecular Recognition, Nat. Commun. 2012, 3, 603. (17) Tamesue, S.; Ohtani, M.; Yamada, K.; Ishida, Y.; Spruell, J. M.; Lynd, N. A.; Hawker, C. J.; Aida, T. Linear Versus Dendritic Molecular Binders for Hydrogel Network Formation with Clay Nanosheets: Studies with ABA Triblock Copolyethers Carrying Guanidinium Ion Pendants., J. Am. Chem. Soc. 2013, 135, 1565015655. (18) Haraguchi, K.; Takehisa, T. Nanocomposite Hydrogels: A Unique Organic-Inorganic Network Structure with Extraordinary Mechanical, Optical, and Swelling/De-swelling Properties, Adv. Mater. 2002, 14, 1120-1124. (19) Shibayama, M.; Suda, J.; Karino, T.; Okabe, S.; Takehisa, T.; Haraguchi, K. Structure and Dynamics of Poly(Nisopropylacrylamide)-Clay Nanocomposite Gels, Macromolecules 2004, 37, 9606-9612. (20) Haraguchi, K.; Li, H.-J. Mechanical Properties and Structure of Polymer Clay Nanocomposite Gels with High Clay Content, Macromolecules 2006, 39, 1898-1905. (21) Okumura, Y.; Ito, K. The Polyrotaxane Gel: A Topological Gel by Figure-of-Eight Cross-links, Adv. Mater. 2001, 13, 485-487.

Page 8 of 9

(22) Ito, K. Novel Cross-Linking Concept of Polymer Network: Synthesis, Structure, and Properties of Slide-Ring Gels with Freely Movable Junctions, Polym. J. 2007, 39, 489-499. (23) Mayumi, K.; Tezuka, M.; Bando, A.; Ito, K. Mechanics of Slide-ring Gels: Novel Entropic Elasticity of a Topological Network Formed by Ring and String, Soft. Mater. 2012, 8, 8179-8183. (24) Sakai, T.; Matsunaga, T.; Yamamoto, Y.; Ito, C.; Yoshida, R.; Suzuki, S.; Sasaki, N.; Shibayama, M.; Chung, U. Design and Fabrication of a High-Strength Hydrogel with Ideally Homogeneous Network Structure from Tetrahedron-like Macromonomers, Macromolecules 2008, 41, 5379-5384. (25) Matsunaga, T.; Sakai, T.; Akagi, Y.; Chung, U.; Shibayama, M. Structure Characterization of Tetra-PEG Gel by Small-Angle Neutron Scattering, Macromolecules 2009, 42, 1344-1351. (26) Kurakazu, M.; Katashima, T.; Chijiishi, M.; Nishi, K.; Akagi, Y.; Matsunaga, T.; Shibayama, M.; Chung, U.; Sakai, T. Evaluation of Gelation Kinetics of Tetra-PEG Gel, Macromolecules 2010, 43, 3935-3940. (27) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. DoubleNetwork Hydrogels with Extremely High Mechanical Strength, Adv. Mater. 2003, 15, 1155-1158. (28) Na, Y.-H.; Kurokawa, T.; Katsuyama, Y.; Tsukeshiba, H.; Gong, J. P.; Osada, Y.; Okabe, S.; Karino, T.; Shibayama, M. Structural Characteristics of Double Network Gels with Extremely High Mechanical Strength, Macromolecules 2004, 37, 5370-5374. (29) Kaneko, D.; Tada, T.; Kurokawa, T.; Gong, J. P.; Osada, Y. Mechanically Strong Hydrogels with Ultra-Low Frictional Coefficients, Adv. Mater. 2005, 17, 535-538. (30) Yang, W.; Furukawa, H.; Gong, J. P. Highly Extensible Double Network Gels with Self-Assembling Anisotropic Structure, Adv. Mater. 2008, 20, 4499-4503. (31) Rodell, C. B.; Dusaj, N. N.; Highley, C. B.; Burdick, J. A. Injectable and Cytocompatible Tough Double-Network Hydrogels through Tandem Supramolecular and Covalent Crosslinking, Adv. Mater. 2016, 28, 8419-8424. (32) Li, C.; Rowland, M. J.; Shao, Y.; Cao, T.; Chen, C.; Jia, H.; Zhou, X.; Yang, Z.; Scherman, O. A.; Liu, D. Responsive Double Network Hydrogels of Interpenetrating DNA and CB[8] Host–Guest Supramolecular Systems, Adv. Mater. 2015, 27, 3298-3304. (33) Sun, W.; Xue, B.; Li, Y.; Qin, M.; Wu, J.; Lu, K.; Wu, J.; Cao, Y.; Jiang, Q.; Wang, W. Polymer-Supramolecular Polymer DoubleNetwork Hydrogel, Adv. Funct. Mater. 2016, 26, 9044-9052. (34) Connors, K. A. The Stability of Cyclodextrin Complexes in Solution, Chem. Rev. 1997, 97, 1325-1357. (35) Chen, G.; Jiang, M. Cyclodextrin-based Inclusion Complexation Bridging Supramolecular Chemistry and Macromolecular Selfassembly, Chem. Soc. Rev. 2011, 40, 2254-2266. (36) Hu, Q.-D.; Tang, G.-P.; Chu, P. K. Cyclodextrin-Based HostGuest Supramolecular Nanoparticles for Delivery: From Design to Applications, Acc. Chem. Res. 2014, 47, 2017-2025. (37) Xu, M.; Shuizhu, Z.; Fang Y. Cyclodextrin Supramolecular Complex as a Water-Soluble Ratiometric Sensor for Ferric Ion Sensing, Langmuir 2010, 26, 4529-4534. (38) Mirzoian, A.; Kaifer, A. E. Reactive Pseudorotaxanes: Inclusion Complexation of Reduced Viologens by the Hosts βCyclodextrin and Heptakis(2,6-di-o-Methyl)-β-Cyclodextrin, Chem. Eur. J. 1997, 3, 1052-1058. (39) Matsue, T.; Kato, T.; Akiba, U.; Osa, T. Inclusion, Solbilization, and Stabilization of Two-electron Reduced Species of Methyl Viologen by Cyclodextrins, Chem. Lett. 1985, 14, 1825-1828. (40) Nakajima, T.; Furukawa, H.; Tanaka, Y.; Kurokawa, T.; Osada, Y.; Gong, J. P. True Chemical Structure of Double Network Hydrogels, Macromolecules 2009, 42, 2184-2189. (41) Harries, D.; Rau, D. C.; Parsegian, V. A. Solutes Probe Hydration in Specific Association of Cyclodextrin and Adamantane, J. Am. Chem. Soc. 2005, 127, 2184-2190.


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Single network gel + +

+

Single Network

+

Single Network

Reduction

Weak

Tough

Oxidation 1st Network

1st Network

+

2nd Network 2nd Network

+

+

+

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Weak

Tough Double network gel


9

Reversing Redox Responsiveness of Hydrogels due to

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