Adhesive Hydrogel System Based on the Intercalation of Anionic

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Applications of Polymer, Composite, and Coating Materials

Adhesive Hydrogel System Based on the Intercalation of Anionic Substituents into Layered Double Hydroxides Shingo Tamesue, Kento Yasuda, and Takuo Endo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09136 • Publication Date (Web): 08 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Materials & Interfaces

Adhesive Hydrogel System Based on the Intercalation of Anionic Substituents into Layered Double Hydroxides Shingo Tamesue*,†, Kento Yasuda‡, 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: adhesion, hydrogels, intercalation, layered inorganic compounds, soft materials ABSTRACT: Hydrogels comprising anionic substituents in their polymer network were synthesized and adhered to each other following application of layered double hydroxides (LDHs) onto their surfaces. The resulting systems displayed high adhesive strength and tolerance for changes in parameters like solvent, salt concentration, and temperature. In experiments involving hydrogels with bulky anionic substituents, it was confirmed that the efficiency of the intercalation of the anionic groups into the layered inorganic compound LDH determines the strength of the adhesion. Moreover, intercalation-based adhesive joints connecting anionic hydrogels displayed higher tolerance for saline solutions than adhesive joints relying on electrostatic interactions between anionic and cationic hydrogels, even though, due to the electrostatic repulsion between charges with the same sign, one would expect that polymer networks comprising opposite charges would tolerate better the disruption caused by high saline concentration.

1. INTRODUCTION Hydrogels, which are composed of water and tiny amounts of organic or inorganic compounds, are very attractive to many researchers, as these species can be used as environmentally friendly materials to generate artificial muscles as well as artificial versions of other living tissues.1-4 Two important requirements exist for the use of hydrogels as artificial tissues. First, they need to display sufficient mechanical strength to enable them to withstand considerable shape deformations. Recently, various types of hydrogel materials characterized by high mechanical strength have been reported.5,6 For instance, double-network gels, which are formed by two distinct gel networks with different stiffness, have been developed. Through their structure, these materials effectively defuse the stress force produced by deformation.7,8 Nanocomposite gels also have excellent mechanical strength. They are composed of mixtures of inorganic nanosheets and polymers. Many of them are created by in situ polymerization in aqueous dispersions of inorganic compounds such as laponite.9,10 The high mechanical strength of nanocomposite hydrogels is due to the co-existence within them of stiff inorganic compounds and soft polymeric networks. A high level of homogeneity in polymer networks also gives hydrogels brilliant mechanical strength. For instance, tetraPEG gels, which are composed of tetra-branched PEG units, have high mechanical strength, despite not including in their structures either inorganic compounds or two separate polymer networks.11,12 In fact, their polymer networks, which are formed by cross-linked tetra-PEG units, are characterized

Scheme 1. Synthetic schemes of (a) SMPS gel and (b) layered double hydroxide–nitrate (LDH-NO3). AAM: acrylamide; APS: ammonium peroxodisulfate; MBAM: N,N’methylenebisacrylamide; SMPS: 3-sulfopropyl methacrylate potassium salt.

by high homogeneity, which helps tetra-PEG gels disperse pressure effectively. The second important requirement for using hydrogels in artificial tissues is the development of adhesive systems that are easy to generate and bring about strong hydrogel–hydrogel or hydrogel–living tissue interactions. For example, hydrogels used as artificial cartilages need to adhere strongly to bones, and developing systems that cause hydrogels and skin to strongly adhere to each other are important when hydrogels are employed as artificial skin or injury-resistant coating agents.

ACS Paragon Plus Environment


(a) 105 105 100 100 Weight lost 39.6 %

80 80

SMPS

60 60

Weight / %

Weight / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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LDH-NO3 LDH-SMPS

40 40

Weight lost 7.2 % Weight lost 22.8 %

20 20

0025 100 200 300 400 500 600 700 800 900 25 100 200 300 Temperature 400 500/ ˚C600 700 800 900 Temperature / ˚C 20000$

LDH$NO3(

(b)

LDH$SMPS(

15000$

20000$ 10000$

d = 0.942 nm

15000$ 5000$

LDH-NO3

10000$ 0$ 5000$ !5000$ !1$ 0$ !5000$ !1$

Figure 1. (a) Schematic depiction of SMPS gels ([SMPS] = 20 mM) adhering to each other owing to the intercalation properties of layered double hydroxides (LDHs). (b) Summary of the results obtained with the reference samples. (c) Photographs of the reference samples. The volume and concentration of the applied dispersion of LDH-NO3 were 40 µL and 4.0 wt%, respectively. The calculated density of LDH-NO3 applied onto the surface of the hydrogels was 2.67 mg/m2.

d = 2.05 nm 1$

3$

2 1$

5$

7$

4 3$

6

LDH-SMPS

9$

8

11$

10

7$2θ / deg. 9$

5$

13$

12 11$

15$

14 13$

15$

Figure 2. (a) Thermogravimetric analysis (TGA) data of the LDH–SMPS composite (green), LDH-NO3, and SMPS monomer. (b) Powder X-ray diffraction (pXRD) data of LDH-NO3 and LDH–SMPS composite.

Table 1. Sulfur ratio of LDH-NO3, LDH–SMPS, and LDHAMPS obtained from elemental analysis. LDH-NO3 LDH-SMPS LDH-AMPS

Various adhesive systems for hydrogels have been reported recently.13-16 For instance, hydrogel materials can adhere to each other via host–guest interactions, such as, for example, those that arise between cyclodextrins and their guest moleucles, such as adamantans.17,18 As another example, hydrogel adhesion can be achieved by knotting gel networks with inorganic nanoparticles.19 This strategy can be applied to polymerized hydrogels as well as to living tissues, such as the liver, with a structure similar to that of hydrogels. Some layered inorganic compounds can entrap various compounds in their interlayer area by way of exchange with ions or other materials originally intercalated between layers.20-22 We have recently reported that hydrogels with cationic substituents in their polymer network can become involved in adhesive interactions due to intercalation into the inorganic compound mica.23 These adhesive systems can adhere cationic hydrogels easily and strongly. However, since the surfaces of most living tissues are negatively charged, it is important to employ anionic hydrogels to use the mentioned adhesion system on living tissues.

Sulfur ratio (wt%)

0

4.57

0.25

Hence, in this report, we focus on adhesion interactions between anionic hydrogels and layered inorganic compounds that are based on the intercalation properties of the latter. We also evaluate how to enhance the strength of these interactions, investigating how to structurally improve the polymer net works to favor their intercalation into the layered inorganic compounds.

2. RESULTS AND DISCUSSION Layered double hydroxides (LDHs) are a class of layered inorganic compounds that comprise metal ions.24,25 LDHs are studied and used, among other things, as electrolytes and absorbents.26,27 The interiors of LDH layers are positively charged due to the metal ions, magnesium and aluminum that LDHs incorporate. These systems can entrap various anions in their interlayer area via intercalation.28,29 We focused on this intercalation property of LDHs and decided to use them as cross-linkers between anionic hydrogel networks.


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(a)

Sample names

[SMPS] (M)

[AAM] (M)

[MBAM] (mM)

[APS] (mM)

with SMPS without SMPS

0.80 0

1.70 2.50

10.0 10.0

3.00 3.00

SMPS0,LDH0.5' SMPS0,LDH0.5' SMPS0,LDH0.5' SMPS0,LDH1' SMPS0,LDH1' SMPS0,LDH1' without SMPS with SMPS SMPS0,LDH2' SMPS0,LDH2' SMPS0,LDH2' [LDH-NO ] / wt% 3 (b) SMPS0,LDH0.5' SMPS0,LDH3' SMPS0,LDH0.5' SMPS0,LDH3' 0.047 SMPS0,LDH0.5' SMPS0,LDH3' 0.28 SMPS0,LDH1' SMPS0,LDH4' SMPS0,LDH0.5'10 SMPS8+LDH0.5$ 0.094 SMPS0,LDH1' SMPS0,LDH4' with LDH-NO3 0.38 SMPS0,LDH1' SMPS0,LDH4' SMPS0,LDH1' SMPS8+LDH1$ SMPS0,LDH2' SMPS0,LDH5' 0.188 0.47 SMPS0,LDH0.5' 10,000$ SMPS0,LDH2' SMPS0,LDH5' 10,000$ 10,000$ SMPS0,LDH0.5' SMPS0,LDH2' SMPS0,LDH5' 10,000$ SMPS0,LDH2' SMPS8+LDH2$ SMPS0,LDH0.5' SMPS0,LDH3' SMPS0,LDH1' SMPS0,LDH3' SMPS0,LDH1' SMPS0,LDH3' SMPS0,LDH3' SMPS8+LDH3$ SMPS0,LDH1' SMPS0,LDH4' SMPS0,LDH2' SMPS0,LDH4' 10,000$ 10,000$ SMPS8+LDH4$ SMPS0,LDH4' SMPS0,LDH2' SMPS0,LDH4' 10,000$ 10,000$ [LDH-NO wt% SMPS0,LDH5' 10,000$ 10,000$ 3] / SMPS0,LDH2' 5.00E+03' SMPS0,LDH0.5' SMPS0,LDH5' 5.00E+03'SMPS0,LDH3' SMPS8+LDH5$ SMPS0,LDH5' SMPS0,LDH0.5' SMPS0,LDH3' SMPS0,LDH5' 0.1' 1' 10' 100' 0$ 5.00E+03' 0.047 SMPS0,LDH0.5' SMPS0,LDH3' 0.28 0$ 0.1' 1' 10' 100' SMPS0,LDH1' SMPS0,LDH4' 0.1' 1' 10' 100' 0.1$ 0.094 SMPS0,LDH1' SMPS0,LDH4' 0.38 0.1$ SMPS0,LDH1' SMPS0,LDH4' SMPS0,LDH2' SMPS0,LDH5' 0.188 0.2$ 0.47 SMPS0,LDH2' 0.2$ SMPS0,LDH5' [SMPS] / M SMPS0,LDH2' SMPS0,LDH5' 0$ 5,000$ 5.00E+03'SMPS0,LDH3' 5.00E+03' 0$ 0.5$ 5.0 5.0 0$ 5.00E+03' 0.5$ 0 0.5 0.1$-1 10$ 100$ 2 2 0.1'-1 1' 10' 100' 0.1' 10 1' 10' 10 100' 1 5.00E+03' 0.1$ 10 11$ SMPS0,LDH3' 10 10 10 SMPS0,LDH3' 0.1$ 1$1.0 0.1' 1' 10' 100' 0.1 SMPS0,LDH4' 0.1$ 1$ 0.1' / rad s-1 1' 10' 100' Frequency / rad s-1 Frequency 0.2$ SMPS0,LDH4' 0.2 0.2$ SMPS0,LDH4' 0.2$ SMPS0,LDH5' 2,500% 2.5 0.5$ SMPS0,LDH5' 0.5$ 10% (e) (d) 1' 0.1%-1 100%2 0.5$ 10 SMPS0,LDH5' 10' 100' 10 11% 10 1$ 1' 10' 100' 1$ 1' 10' 100' 30 30 Frequency1$/ rad s-1

30

with SMPS

5.0

0

0.1 0.2 0.3 0.4 0.5 0.6 [LDH-NO3] / wt%

G’×10-3 / Pa

1$ 1$

(d)

10$ 10$

10

5.0 0

1,000$ 1,000$ 1,000$ 0.1$ 0.1$ 0.1$

1$ 1$ 1$

10$ 10$ 10$

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with LDH-NO1,000$ 3 1,000$ 100$ 100$ 100$

1,000$ 0.1$ 0.1$ 0.1$

0.1$ 0.1$

0.1 0.2 0.3 0.4 0.5 0.6 [LDH-NO3] / wt%

Figure 3. (a) Table of ingredients forming the gels used for rheological measurement. Each of the gels, which volumes were 4.8 mL, was mixed with 0.5 mL of LDH-NO3 dispersions, and used for the measurement. (b) G’ values of the mixtures of LDH-NO3 and SMPS gel at various concentrations of LDH-NO3 (0.047−0.47 wt%) and strain γ = 0.5%. (c) G’ values of the mixtures of LDHNO3 and AAM gels at various concentrations of LDH-NO3 (0.047−0.47 wt%) and strain γ = 0.5%. (d) G’ values as a function of LDH-NO3 concentration of the samples evaluated in (b) at frequency ω = 1.0 rad s-1. (e) G’ values as a function of LDH-NO3 concentration of the samples evaluated in (c) at frequency ω = 1.0 rad s-1.

An LDH incorporating NO3- anions between its layers (LDHNO3) was synthesized as summarized in Scheme 1b, according to the details reported in the experimental section. The diameter of the average LDH particle synthesized was estimated to be 0.251 µm, based on the results of the grain size distribution measurement whose results are reported in Figure S1. Initially, the as-synthesized LDH had H2O molecules and NO3anions in the interlayer area. The NO3- groups can, however, be exchanged with different anions (intercalation). Hydrogels having sulfonate substituents, like those obtained from 3sulfopropyl methacrylate potassium salt (SMPS gels, Scheme 1a) were prepared via the free radical polymerization reaction detailed in the experimental section and were used for the adhesion.

2.5

1.00 0.500 0.200 0.100 0

2.50 2.50 2.50 2.50 2.50

3.00 3.00 3.00 3.00 3.00

3.00 3.00 3.00 3.00 3.00

(c) 10

without LDH-NO3

0%

0%

0.1%

0.1%

0$ 0$ 0.1$ 0.1$ 0.2$ 0.2$ 0.5$ 0.5$ 1$ 1$

0.2% 0.5% 1%

2.5 0.1%-1 10

2,500%

(e)

10 10$ 10$ 10$

0.5%

[SMPS] /M 0$ 0$ 0$ 0 0.5 0.1$ 0.1$ 0.1 1.0 0.1$ 0.2$ 0.2 0.2$ 0.2$ 0.5$

0.5$ 10% 0.5$ 10 11% 1$ 1$ Frequency1$ / rad s-1

1$ 1$

1$ 1$ 1$

0.2%

10$ 10$

2 100% 10

100$ 100$

without LDH-NO3 100$ 100$ 100$

G’×10-3 / Pa

100' 100' 100'

0.1$ 0.1$

G’×10-3 / Pa

10' 10' 10'

1,000$ 1,000$

without SMPS

G’×103 / Pa

1' 1' 1'

G’×103 / Pa

G’×103 / Pa

30

5.00E+03' 5.00E+03' 5.00E+03' 0.1' 0.1' 0.1'

1 M SMPS 0.5 M SMPS 0.2 M SMPS 0.1 M SMPS 0 M SMPS

(c)

(b)

[SMPS] [AAM] [MBAM] [APS] (mM) (M) (M) (mM)

Sample names

G’×10-3 / Pa

(a)

G’×103 / Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 E+03' E+03' 0.1' E+03'20 0.1' 0.1' 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0.2

0.4 0.6 0.8 [SMPS] / M

1.0

2.5 0

0.2

0.4 0.6 0.8 [SMPS] / M

1.0

Figure 4. (a) Data on the various SMPS gels utilized. G’ values of the SMPS gels reported in (a) when mixed with LDH-NO3 (0.19 wt%) (b) or by themselves (c) at strain γ = 0.5%. (d) and (e) shows the G’ values as a function of SMPS concentration in the SMPS gels of the samples reported in (b) and (c) at frequency ω = 1.0 rad s-1, respectively.

We also prepared hydrogels having sulfonate substituents that were shorter and bulkier than those of SMPS gels (i.e., AMPS gels, prepared from 2-acrylamido-2-methylpropanesulfonic acid). An aqueous dispersion of LDH-NO3 (4.0 wt%, 40 µL) was applied onto the surface of the anionic SMPS hydrogel (30 mm × 20 mm), and two pieces of SMPS hydrogel were brought into close contact with each other by putting 100 g on the fixed hydrogels, as depicted in Figure 1a and S2. After 12 h, the hydrogel pieces were observed to adhere to each other, as can be evinced from Figure 1c. Moreover, the adhering hydrogels did not separate even when researchers used their fingers to pull them apart. To clarify the mechanism of adhesion, reference samples were prepared. In Figure 1b–1c are summarized the results of the adhesion tests conducted, including those on samples prepared in the absence of LDH-NO3 and in the absence of anionic substituents in the gel networks, as detailed in the experimental section. No hydrogels except for the SMPS gels adhered to


1%


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25"

25"

20"

20"

15"

(a)

Hydrogels

25

20"

20

15"

15

72.10%"

10"

20

62.10%"

5"

5"

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68.0%.1" Water content ratio / wt% 72.90%" 62.10%" 72.9 62.1 72.10%" 61.50%" 72.1 61.5 68.0%.1" 68.0

10"

0" 15"

20"

25"

30"

35"

10"

61.50%"

40"

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F/w / N m-1

15"

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25"

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45"

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72.90%" 72.10%" 68.0%.1"

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62.10%" 61.50%"

0" 15"

20"

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30"

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.2 0.4 [LDH-NO3] / wt%

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25"

30" 35" 10 15 Stroke / mm

40" 20

0 45" 74

72 70 68 66 64 62 Water content ratio / wt%

60

(c)

10 8

(d)

6 4

SMPS gel

Steel wire

2 0

5

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 [SMPS] / M

Figure 5. (a) Schematic illustration of the 90˚ peeling test. (b) Adhesive strength of the anionic hydrogels ([SMPS] = 0.5 M, [AAm] = 2.5 M, [MBAM] = 3.0 mM) as a function of the concentrations of LDH-NO3 (0.05–0.5 wt%, 200 µL). (c) Adhesive strength of the anionic hydrogels as a function of the number of sulfonate groups incorporated in the SMPS gels ([SMPS] = 0.1– 0.6 M) (on whose surfaces had been applied 200 µL of 4.0 wt% LDH-NO3). The width and height of the hydrogels utilized in 90˚ peeling test were 30 mm × 2.0 mm. The calculated densities of LDH-NO3 applied onto the surface of the hydrogels were (b) 41.7 – 417 mg/m2 and (c) 3.33 g/m2.

LDH-NO3 SMPS gel

Steel plate (10 kg)

Figure 6. (a) Adhesive strength as a function of the water content ratio of the hydrogels (61.5–72.9 wt%), with adhesion achieved with 4.0 wt % of LDH-NO3 (400 µL), and (b) average adhesive strength of the adhesive joints calculated from the data reported in (a). (c) Photograph of steel plates weighing 10 kg being lifted using a high-concentration adhesive joint. (d) Schematic depiction of the adhesive joint utilized in the experiment photographed for (c). The dashed lines in (c) indicate steel wires used to pull the weight by hand. (a)

each other after LDH-NO3 had been applied onto their surface. It is suggested that the layered inorganic compound LDH and the anionic substituents in the hydrogel are necessary for the adhesion to occur. As another reference sample, an anionic hydrogel containing sulfonate anions not bonded to the gel network was used in the adhesion experiment, instead of the original anionic hydrogel. In this case, the hydrogels did not adhere to one another because the free sulfonate ions competed with the anionic substituents in the gel network for the interaction with LDH. Similarly, even when a highconcentration LDH dispersion was applied in advance on the surfaces of the anionic gels, the hydrogels still did not adhere to each other. It is assumed that almost no anionic substituents of the gels interacted with LDH, so they could not act effectively as cross-linkers to bridge between hydrogels. Based on these results, it is suggested that the anionic substituents of the SMPS gels intercalated between LDH layers, thus acting as cross-linkers between the hydrogel surfaces. On the other hand, when AMPS gels were used in the tests detailed in the experimental section, these hydrogels hardly adhered to each other, and were easily separated, even after LDH-NO3 was applied on their surfaces using the same approach implemented with SMPS gels. It was suggested that the structural differences between the sulfonate substituents of SMPS and AMPS gels affected the intercalation efficiency, leading AMPS gels not to adhere to each other (Figure S3). To confirm intercalation of the SMPS-gel-based sulfonate groups between the layers of LDHs, thermogravimetric analy

+

(b)

+

SMPS gel

LDH-NO3

(Intercalation)

(c)

Initiator Water, 65˚C 12 h

MVBIM gel

(Electrostatic Interaction)

Mica （Intercalation)

Saturated NaCl aq.

Figure 7. (a) Synthetic scheme of the cationic MVBIM hydrogels. (b) Schematic depiction of SMPS hydrogels adhering to each other via an LDH-NO3-based joint and of MVBIM gels adhering to each other via a mica-based joint. The adhesive joint between the SMPS and MVBIM gels is based on electrostatic interactions. (c) Adhesive joints depicted in (b) prepared with a saturated brine solution. The electrostatic interaction between the SMPS and MVBIM gels weakened as a consequence of the high ionic strength, and the two gels separated. The cationic gels were dyed in red for the sake of clarity.


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sis (TGA) and powder X-ray diffraction (pXRD) patterns were studied as shown in Figure 2. The TGA patterns of SMPS monomer (Figure 2a) showed two points where weight decreased, at 300˚C and 600˚C, which were ascribed to the decomposition of 3-sulfopropyl methacrylate potassium anions. The TGA pattern of LDH-NO3 indicated weight decreases at 100˚C and 300˚C, which are associated with the evaporation of water and NO3- present in the layers, respectively. LDHintercalated SMPS monomers (LDH-SMPS) were characterized by three weight-loss temperatures at 100˚C, 300˚C, and 600˚C. As mentioned above, the weight loss at 100˚C is associated with the evaporation of water. On the other hand, the weight loss at 600˚C was observed only in the case of LDHSMPS composites and of the SMPS monomer alone, although the weight decreases at 300˚C was observed in all samples. Hence, evidence suggests that the weight loss occurring at 600˚C is due to degradation of the 3-sulfopropyl methacrylate anions. The weight-loss percentage at 600˚C was 7.2%. Consequently, the amount of 3-sulfopropyl methacrylate anion was estimated to be 0.29 mmol per 1 g of LDH. On the other hand, the TGA data of LDH-AMPS, reported in Figure S4, did not indicate any significant weight loss at 500˚C, at which only AMPS decomposes. This evidence indicates that the AMPS gel does not have as high affinity for LDH as the SMPS gel does. pXRD patterns of LDH-NO3 and LDH-NO3 mixed with SMPS monomer were measured (Figure 2b). A diffraction peak at 0.942 nm observed with the sample of LDH-NO3 alone was correlated with the interlayer distance of LDH-NO3. After LDH-NO3 was mixed with the SMPS monomer, the interlayer distance increased to 2.05 nm as a consequence of the intercalation of SMPS anions between the layers of LDH (Figure S5). In this way, pXRD patterns also provided evidence of the intercalation of the sulfonate groups of the SMPS gel into the interlayer regions of LDHs. On the other hand, evidence indicated that the interlayer distance hardly changed when the AMPS monomer was used instead of the SMPS monomer (Figure S6). Elemental analysis data are reported in Table 1. As expected, no sulfur mass fraction was observed in LDH-NO3. On the other hand, a 4.57 wt% sulfur mass fraction was observed in the case of LDH-SMPS, and the proportion of SMPS anions in the composite calculated from the elemental analysis was 29.5 wt%. However, the sulfur mass fraction observed in the elemental analysis of LDH-AMPS was much smaller (0.25 wt%), and the amount of AMPS anions entrapped in LDH was estimated to be 1.6 wt%. The difference in entrapment efficiency between the two gels is assumed to derive from the molecular structures of their anionic substituents. The sulfonate groups of SMPS hydrogels are not as bulky as those of AMPS hydrogels, and within SMPS hydrogels, the alkyl linker between the main chains and the anionic groups is longer and more flexible than that within AMPS hydrogels. These structural differences are assumed to favor sulfonate intercalation between LDH layers. The rheological data reported in Figure 3 and 4 were collected to confirm that the sulfonate anions in the gel networks were cross-linked as a consequence of intercalation into LDH. The SMPS gels shown in Figure 3a showed higher elastic modulus (G’) in water by mixing with higher concentrations of LDH as shown in Figure 3b and 3d. Also, the G’ value increased as the

amount of SMPS in the gels increased after mixed with LDH by the data reported in Figure 4b and 4d. As can be evinced from the data reported in Figure 4d, the G’ value almost plateaued when the concentration of SMPS in the gel preparation was 1.0 M. By contrast, the mixture of poly(acrylamide) gel and LDH did not show any significant change in G’ value as the concentration of LDH increased as shown in Figure 3c and 3e. Also, the G’ value of the gels containing SMPS and acrylamide not mixed with LDH did not change as the concentration of SMPS increased as shown in Figure 4c and 4e. The tolerance of the adhesive joints to various environments, that is, different solvents, acidic, alkaline, and saturated brine solutions, was evaluated. The prepared adhesive joints were soaked into various solvents (polar organic, non-polar organic, or water) for 2 h (Figure S7). In spite of the fact that soaking the joints in DMSO and water deformed them significantly, as the joints absorbed the said solvents, none of them broke apart. As reported before,23 adhesive systems based on intercalation have high tolerance for acidic, alkaline, and salty solutions. However, interestingly, the adhesive joints prepared in this study broke apart as a result of being soaked in acidic (pH = 1.68) or alkaline solutions (pH = 13.0) for 24 h (Figure S8). The hydrogel side chains, consisting of sulfonate anions, were connected to the main chain via ester bonds, as opposed to the imidazole cations of the cationic gels reported by our research group previously,23 which were connected via amide bonds. With respect to amide bonds, ordinary ester bonds are known to be easily hydrolyzed and cleaved in acidic and alkaline conditions. It is suggested that the structural differences between the linkers connecting main chains to side chains induced the difference in tolerance to acidic and alkaline conditions observed to exist between the adhesion system described herein and that described in our previous report. As shown in Figure S9, we confirmed acidic (pH = 1.68) or alkaline (pH= 13.0) hydrolysis of the ester bond in SMPS monomer utilizing IR spectroscopy. The adhesive strength of the hydrogels was measured conducting 90° peeling tests, whose results are not affected by the softness of hydrogels (Figure 5a).30 As can be evinced from the data in Figure 5b, the strength of the adhesive joints clearly increased as the concentration of the LDH dispersions applied onto the hydrogel pieces increased from 0.05 wt% to 0.5 wt%. The adhesive strength almost plateaued at a dispersion concentration of 0.5 wt%. Also, as the concentration of SMPS-based groups incorporated in the hydrogels increased, so did the adhesive strength (Figure 5c). Moreover, interestingly, the hydrogels characterized by higher SMPS concentrations (above 0.6 M) were torn off before the adhesive joints peeled. In other words, in this case, the adhesive strength of the joints could not be measured, because it was higher than the mechanical strength of the hydrogels. Dependency of water content ratio of hydrogels on adhesive strength was shown in Figure 6a and 6b. As can be seen, the adhesive strength increased as the water content ratio of hydrogels lowered. Hence, we prepared very low water content ratio hydrogels (water content ratio: 20 wt%) were prepared as depicted in Figure 6c–6d. In this case, the adhesive strength of the hydrogels was such that lifting a 10-kg weight did not cause the hydrogels to get separated although only 1.2 mg of LDH were used in the adhesive joint.


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Two pieces of the cationic hydrogel developed previously23 (Figure 7a) and two pieces of the anionic hydrogel SMPS were bridged using mica and LDH, as can be seen in Figure 7b–7c. As we reported previously, mica and cationic hydrogels adhere to each other through intercalation between the layers of mica of the cationic substituents of the gel networks.23 The two cationic hydrogel pieces adhered to each other through intercalation into mica, whereas the two anionic hydrogel pieces adhered to each other through intercalation into LDH. On the other hand, the cationic and anionic hydrogels adhered to each other via electrostatic interactions between the imidazolium cations and the sulfonate anions. In the condition that extra salt was not added when the hydrogels were prepared, all the pieces adhered strongly. To confirm the hydrogels’ tolerance to brine, we mixed saturated sodium chloride inside the hydrogels in advance. After the gels were made to adhere to each other, they were incubated in a saturated brine solution for 24 h. Although one would expect that adhesive interactions between ionic hydrogels with charges of the same sign would not be stronger than those between ionic hydrogels with opposite charges, the cationic hydrogels separated from their anionic counterparts before the hydrogels with the same charge separated from each other. In particular, the electrostatic interaction between cationic and anionic hydrogels was weakened by soaking in brine solution, causing the adhesive joint between these hydrogels to break apart. On the other hand, in the case of the adhesive joint between the hydrogels with charges of the same sign (cationic–cationic or anionic–anionic adhering hydrogels), which were based on cross-linking intercalations, the negatively charged or positively charged substituents of the hydrogels were protected by the layered inorganic compounds LDH and mica. This difference would result in a higher tolerance for salty conditions of the adhesive joints based on intercalation-type interactions than of adhesive joints based on ordinary electrostatic interactions.

3. CONCLUSION In conclusion, we reported an adhesive hydrogel system that can be easily prepared and has high adhesive strength, owing to the intercalation property of the inorganic compound LDH, which is able to entrap anionic substituents of gel networks. This system displays high tolerance for various solvents and salty conditions. Evidence indicates that both cationic and anionic hydrogel pieces adhering to each other via the intercalation of gel-based ionic substituents into the layers of suitable inorganic compounds (mica and LDH) remained joined together, even after being soaked in saturated brine solution. This result was made possible by the protection provided to the intercalated ionic hydrogel moieties by the layered structure of the inorganic compounds. Furthermore, we evaluated how the polymeric structure of the hydrogels affected the systems’ adhesive strength and tolerance for acidic and alkaline conditions. Due to the adhesive strength of these hydrogel systems based on the intercalating properties of layered inorganic compounds and the ease of their preparation, these adhesive systems have the potential to become very useful research tools. In particular, the adhesive system at the center of the present study could in the future be used to generate self-healing hydrogels; in fact, we are currently trying to develop such materials. Also,

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we expect that in the near future these systems will be able to affect the adhesion between living tissues and hydrogels.

4. EXPERIMENTAL SECTION 1. General 3-Sulfopropylmethacrylate potassium salt (SMPS), acrylamide, and N,N’-methylenebisacrylamide were purchased from Wako Pure Chemicals Co., Inc. Buffer solutions at pH 1.68 (oxalate) and 13.0 (KCl/NaOH) were purchased from Kanto Chemical Co., Inc, and Tokyo Chemical Industry Co., LTD, respectively. All other chemicals were purchased from Kanto Chemical Co., Inc. and were used as received. All solvents utilized in this research were reagent grade (99.9% purity). When the adhesive joints were prepared, dispersions of LDH-NO3 were applied onto the surfaces of hydrogels uniformly using spatula, which width was 5 mm. Ninety-degree peeling tests and shear adhesive strength tests were carried out using Imada force tester MX2–500N–FA equipped with Imada, force gauge ZTA–50N. All the samples on which the 90° peeling test was conducted had a width of 30 mm and a thickness of 2.0 mm. The rate at which the samples were extended was set to 10 mm/sec. Rheology data were collected by using Anton–Paar, rheometer MCR–301 with a 25-mm diameter parallel plate attached to a transducer. The gap was set at 1.0 mm. Thermogravimetric analysis (TGA) was conducted using a Shimadzu Manufacturing Co., Ltd. thermal gravity analyzer TGA–50. Xray diffraction (XRD) patterns were obtained using Bruker, Xray diffraction analyzer D2 Phaser. Elemental analysis was conducted using a Perkin Elmer Elemental analyzer 2000II. The average particle size distribution data was measured using a Shimadzu SALD-7100 nanoparticle size analyzer. The highconcentration hydrogels depicted in Figure 6 were prepared by evaporation in the air. 2. Preparation of LDH-NO3 Under nitrogen atmosphere, 200 mL of an aqueous mixture of magnesium nitrate hexahydrate (10.3 g, 40 mmol) and aluminum nitrate nonahydrate (7.5 g, 20 mmol) were added dropwise through a dropping funnel to a separable flask containing an aqueous NaOH solution (40 mM, 500 mL). During titration, the pH of the resulting mixture was adjusted to 10.0 ± 0.2 using a pH controller. After titration, the reaction mixture was left to stand for 24 h at room temperature. Subsequently, the precipitate that had formed was collected by filtration, washed with ion-exchanged water, and lyophilized. Consequently, LDH incorporating NO3- in the interlayer area (LDH-NO3) was obtained as a white powder. 3. Preparation of LDH-SMSP and LDH-AMPS. The synthesized LDH-NO3 (40 mg) and SMPS monomer or AMPS monomer (1.0 mmol) were mixed in 10 mL of water using magnetic stirrer for 2 days. The resultant LDH-SMPS or LDH-AMPS was collected by centrifugation and washed with water repeatedly. Finally, the aqueous suspension of LDH-SMPS or LDH-AMPS was lyophilized, and LDH-SMPS or LDH-AMPS was obtained. 4. Preparation of SMPS gels Acrylamide (AAM, 2.13 g, 30.0 mmol), N,N’methylenebisacrylamide (MBAM, 4.62 mg, 3.00 × 10-2 mmol), and 3-sulfopropyl methacrylate potassium salt (SMPS, 46.9 mg, 2.00 × 10-1 mmol) were dissolved in 10 mL of water. Af-


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ter forcing N2 gas to bubble into the solution for 1 h, ammonium peroxodisulfate (APS, 5.01 mg, 2.00 × 10-2 mmol) was added as a radical initiator, and the solution was heated to 65˚C and kept at that temperature for 12 h. 5. Preparation of AAM gels AAM (2.13 g, 30.0 mmol) and MBAM (4.62 mg, 3.00 × 10-2 mmol) were dissolved in 10 mL of water. After forcing N2 gas to bubble into the solution for 1 h, APS (5.01 mg, 2.00 × 10-2 mmol) was added as a radical initiator, and the solution was heated to 65˚C and kept at that temperature for 12 h. 6. Preparation of AMPS gels AAM (2.13 g, 30.0 mmol) and 2-acrylamido-2methylpropanesulfonic acid (AMPS, 46.9 mg, 2.00 × 10-1 mmol) were dissolved in 10 mL of water. After forcing N2 gas to bubble into the solution for 1 h, APS (5.01 mg, 2.00 × 10-2 mmol) was added as a radical initiator, and the solution was heated to 65˚C and kept at that temperature for 12 h. 7. Preparation of the cationic gels (MVBIM gels) The cationic gels (MVBIM gels) were prepared as reported previously.23

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: . Particle size distribution data of LDH-NO3, detail of preparing the adhesive samples, photograph of adhesion test of AMPS gel, TGA data of AMPS gel, tolerance test of the adhesive joint, simulation of interlayer distance obtained from pXRD, and rheological data (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. Takeshi Yamauchi, Niigata University, for his experimental support. This work was supported by JSPS KAKENHI, Grant Number 17K14537.

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Adhesive Hydrogel System Based on the Intercalation of Anionic

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