Facile Access to Guar Gum Based Supramolecular Hydrogels with

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Facile Access to Guar Gum Based Supramolecular Hydrogels with Rapid Self-Healing Ability and Multi-Stimuli Responsive Gel-Sol Transitions Nan Li, Chuanjie Liu, and Wei Chen J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b05130 • Publication Date (Web): 20 Dec 2018 Downloaded from http://pubs.acs.org on December 22, 2018

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Journal of Agricultural and Food Chemistry

1

Facile Access to Guar Gum Based Supramolecular

2

Hydrogels with Rapid Self-Healing Ability and Multi-Stimuli

3

Responsive Gel-Sol Transitions

4 5

Nan Li a, 1,*, Chuanjie Liu a, 1, Wei Chen a, b,*

6 7 8

a

9

b

10

College of Engineering, Qufu Normal University, Rizhao, 276826, China Key Laboratory of Pulp and Paper Science & Technology of Ministry of

Education/Shandong Province, Qilu University of Technology, Jinan, 250353, China

11 12 13

1

Nan Li and Chuanjie Liu are the first authors.

14 15 16 17 18 19 20

KEYWORDS: Guar gum; Borax; Hydrogels; Self-healing; Thermal and pH

21 22 23 24 25 26

responsive

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ABSTRACT: In this work, we prepare guar gum-based supramolecular hydrogel

28

through the formation of borate/didiol bonds. This dynamic and reversible noncovalent

29

borate/didiol interaction is critical for the multi-functional properties of supramolecular

30

hydrogel. The FT-IR and XRD analysis verified the existence of boronate ester

31

interactions between borax and guar gum. Moreover, the viscoelastic and mechanical

32

behaviors of the hydrogels with different guar gum concentrations showed that the

33

storage modulus and compressive stress were highest at guar gum concentration of 2

34

wt%. Besides, due to the dynamic and reservable properties of boronate ester, these

35

guar gum-based hydrogels had excellent self-healing property, outstanding reformable

36

and injectable capability. In addition, hydrogels also exhibited reversible gel-sol

37

transformations by the application of physicochemical stimuli such as thermal and pH

38

value. The coupling of these multifunctional properties made from low-cost,

39

environment friendly and readily available materials could have potential applications

40

in many biomedical areas in the future. We expect that this simple strategy of

41

fabricating the self-healing guar gum hydrogels with multi-stimuli responsive

42

properties may enrich the avenue in the exploration of multifunctional guar gum-based

43

hydrogels to expand their potential applications in various fields.

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1. INTRODUCTION

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Hydrogels are “soft and wet” materials that can absorb and retain water in their

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three-dimensional network structure. Because of their high hydrophilicity, nontoxicity,

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biodegradability and biocompatibility properties, hydrogels have wide utilizations in

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various areas, such as waste water treatment 1, drug delivery 2, tissue engineering 3,

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artificial articular cartilage 4,5 and electronic devices 6.

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Although the traditional hydrogels exhibit superior performances, their mechanical

62

properties and 3D network structure are often significantly deteriorated or even lost

63

when suffer from interior or exterior cracks, which limiting their lifetime 7. As a class

64

of smart hydrogels, self-healing hydrogels could be able to automatic self-healed after

65

fracture and thus have drawn tremendous attention in various fields

66

of efforts have been made to engineering hydrogels with self-healing capabilities either

67

by introducing the dynamic covalent bonds or noncovalent bonds. The former deals

68

with imine bond 11, C-C bond 12, phenylboronate ester bond

69

acylhydrazone bond 15, etc. Whereas the later involves boronate esters 16, hydrophobic

70

association

71

20,21

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called supramolecular hydrogels 23. At present, many different types of covalently and

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noncovalently linked supramolecular hydrogels have been developed that respond to

74

external stimuli, including changes in light, pH and temperature 24-27. When exposed to

75

external stimuli, covalent hydrogels cannot exhibit gel-sol transition behavior, while

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noncovalently cross-linked supramolecular hydrogels can be designed to have multi-

77

stimuli responsive gel-sol transitions. Recently, significant efforts have been focused

78

on research into gels that are crosslinked by reversible covalent interactions. Among

79

them, boronate esters interaction has been developed as one of the most well-known

17

, electrostatic interaction

18

13

8-10

. To date, lots

, disulfide bond

14

and

, hydrogen bonding 19, host-guest interaction

, ionic bonding 22, etc. Notably, hydrogels formed via noncovalent bonds were also



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28,29

. However, these

80

noncovalent reactions for designing self-healing hydrogels

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multifunctional hydrogels still have some limitations, such as complicated chemical

82

modification or non-naturally occurring macromolecular components. Because of the

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increasing environmental issues, products based on abundant naturally-occurring

84

polysaccharides will be desirable. Thus, the motivation for this work was the absence

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of a low-cost, facile method to prepare fast self-healable, multi-stimuli responsible

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natural-based hydrogels. To develop this kind of supramolecular hydrogel system, we

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tried a kind of “smart” naturally-occurring polysaccharide --- guar gum.

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Guar gum (GG) is a crucial kind of naturally-occurring polysaccharides that

89

extracted from the seeds of cyamopsis tetragonalobus plant. It consists of a linear

90

backbone of 1,4-linked β-d-mannopyranosyl units (M) and side chains of 1,6-linked α-

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d-galactopyranosyl units (G) with hydroxyl groups

92

polysaccharides, guar gum has many advantages such as high molecular weight and

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plentiful side chains, which is believed to help improve mechanical strength of

94

hydrogels by increasing the intramolecular cross-linking and the entanglement degree

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of the networks. Notably, thanks to the existence of hydroxyl bonds, sodium borate

96

(Na2B4O7•10H2O) cross-link guar gum solution by borate/didiol interactions, resulting

97

in a network in water

98

and multi-stimuli responsive properties of borax cross-linked guar gum hydrogels have

99

not yet been systematic investigated.

32-34

30,31

. Compared with many other

. Nevertheless, in these studies, the self-healing, injectable

100

The borax-guar gum cross-linking mechanism can be explained as the followings:

101

when sodium borate (Na2B4O7•10H2O) dissolved in water, it could hydrolyze into boric

102

acid and borate ions, which subsequently form a buffer solution of boric/borate 24,33,35.

103

Guar gum solution can form hydrogels easily through borate/didiol complexations

104

between borate ions and hydroxyl groups

28,36

(Scheme 1a). Moreover, boronate ester


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bond is a multi-stimuli responsive bond

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use of the dynamic and reservable properties of the boronate ester bonds to integrate

107

the automatically self-healing ability as well as multi-stimuli responsive property into

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one system to obtain multifunctional smart guar gum-based hydrogels.

29,37

. Herein, it would be interesting to make

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In this work, we describe a more general and facile approach to prepare

110

multifunctional borax-guar gum hydrogels possessing both self-healing and multi-

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stimuli responsive functionalities, which uses cheap and commercially available

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polymer with no need for any other chemical modification. As expected, the resulting

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hydrogel exhibit extremely rapid and repeatable self-healing capability without external

114

stimulus as well as outstanding thermal and pH triggered gel-sol transaction, which will

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have great significance in broadening their practical applications. We believe that this

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biopolymer low-costs, non-toxic and environmentally friendly guar gum-based

117

hydrogel has great potential applications in a broad range of bio-related applications.

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2. EXPERIMENTAL SECTION

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2.1 Materials

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Guar gum (viscosity: 5000-5500 mpa•s), Sodium tetraborate decahydrate

121

(Na2B4O7•10H2O, borax) were obtained from Aladdin Company (Shanghai, China). All

122

the solutions used in this work were made with deionized (DI) water. All of the reagents

123

were analytical grade.

124

2.2 Preparation of hydrogels

125

Borax-Guar gum hydrogels were prepared as the following procedures. A certain

126

amount of guar gum was dissolved in 25 mL of deionized water and were stirred for 2

127

h at room temperature. Subsequently, 5 mL 10% borax aqueous suspension was added

128

to guar gum solution slowly. The mixed solution was stirred uniformly and left at room

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temperature until a homogeneously stable borax-guar gum hydrogel was formed. In this 5 ACS Paragon Plus Environment


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study, the hydrogels with 0.5 g, 1.0 g, 1.5 g and 2.0 g guar gum were designated as

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Borax-Guar-0.5, Borax-Guar-1.0, Borax-Guar-1.5 and Borax-Guar-2.0, respectively.

132

The total volume of the mixture was fixed at 30 mL with deionized water.

133

2.3 Characterization

134

Hydrogel samples were dried in an oven at 60 °C for 24 h to remove water. FT-IR

135

spectra of dried guar gum samples were recorded with FT-IR spectrometer (Themo

136

Nicolet Nexus 470, Nicolet, USA) range from 4000 to 400 cm−1. XRD spectra of the

137

samples were recorded in reflection mode at a range of 2 = 5-80 (MiniFlex 600,

138

Rjgaku, Japan). Morphologies of the freeze-dried guar gum hydrogel samples were

139

observed by scanning electron microscopy (SEM). The freeze-dried samples were

140

sputtered with gold in a vacuum, and then they were observed using SEM.

141

Compressive tests were performed using an Instron Machine 5300 with 100N load

142

cell. A cylindrical hydrogel sample with a height of 20 mm and a diameter of 10 mm

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and was placed on a plate and compressed to 85% of its original height with speed of

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10 mm∙min-1. The displacement and load data were collected from the compressive

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measurements.

146

The rheological behaviors of guar gum hydrogels were investigated by using a

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stress-controlled rheometer (Ares G2, TA INSTRUMENTS, USA) with a parallel plate

148

geometry of a diameter of 25 mm. The storage (G´) modulus and loss (G˝) modulus of

149

borax-guar gum samples were obtained as functions of angular frequency ω between

150

the range of 0.1 rad/s and 100 rad/s.

151

Borax-guar gum hydrogels were subjected to the rheological measurements before

152

and after self-healing process. Besides, optical microscopy images (IX73-DP 80,

153

Olympus, Japan) were used to record the self-healing process of supramolecular guar

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gum hydrogels at set intervals. Moreover, the recovery property of borax-guar gum 6 ACS Paragon Plus Environment

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hydrogels in response to applied shear forces were performed at ω = 1.0 Hz as the

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following procedure: 10% (300 s) → 100% (300 s) → 10% (300 s) → 100% (300 s) →

157

10% (300 s). To avoid the evaporation of water, silicone oil was coating on the edge of

158

hydrogel.

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3. RESULTS AND DISCUSSIONS

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3.1 Borax-guar gum supramolecular hydrogel formation

Each measurement was repeated three times.

161

The formation procedure and self-healing capability of the supramolecular borax-

162

guar gum hydrogels are demonstrated in Scheme 1b. Guar gum which consists hydroxyl

163

groups were first completely dissolved in water and form a homogeneous solution.

164

Subsequently, borax aqueous suspension (10%) added to guar gum solution to

165

hydrolyzed B(OH)4− slowly. It is known that borate ion (B(OH)4−) could effectively

166

cross-linked hydroxyl groups of guar gum via borate/didiol bonds. Therefore, a

167

supramolecular guar gum hydrogel was obtained (Scheme 1a). Because of the

168

reversible borate/didiol interaction between borax and guar gum, our supramolecular

169

guar gum-based hydrogels could “autonomous” self-heal multiple cycles without any

170

need of external stimulus. In addition to self-healing property, borax-guar gum hydrogel

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also exhibited interesting thermal and pH responsive gel-sol transition properties

172

(Scheme 1c). Due to the exothermic and dynamic reversible property of the reaction

173

between B(OH)4− and hydroxyl groups of guar gum, borax-guar gum hydrogel could

174

be easily disintegrated into sol state when heating and could be reformed after cooling.

175

Furthermore, the obtained borax-guar gum hydrogel can also respond to pH variations.

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The hydrogel became liquid when acid solutions was added and the gel could be

177

reformed after the addition of alkaline solutions.



a

b

c Heating

H+

Cooling

OH-

178 179

Scheme 1. a) One possible mechanism of the formation of borax-guar gum

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supramolecular hydrogels; b) Preparation and self-healing process of borax-guar gum

181

hydrogels; c) Thermal and pH responsive gel-sol transition of borax-guar gum

182

hydrogels.

183

3.2 Characterization of guar gum based supramolecular hydrogels.

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The FT-IR spectra of native guar gum, Borax-G-0.5, Borax-G-1.0, Borax-G-1.5,

185

and Borax-G-2.0 hydrogels are shown in Figure 1a. Guar gum powder has a broad

186

absorption band at 3420 cm−1 because of O-H stretching vibrations and the band at 2906

187

cm−1 can be assigned to C-H stretching vibrations32,38,39. Peaks at 1405 cm−1 and 1150

188

cm−1 can be assigned to C-H bending and C-O-C stretching vibration, respectively.

189

Moreover, band at 1660 cm−1 is associated with hydroxyl bending. In comparation with

190

the absorption peaks of native guar gum, all of the borax-guar gum hydrogels did not

191

show any new peak, suggesting that there were no new chemical bonds formed between

192

borax and guar gum. Nevertheless, the broad peak at 3428 cm−1 reduced to sharp peaks

193

with a small shift to longer wavelength in FT-IR spectrum of borax-guar gum hydrogels. 8 ACS Paragon Plus Environment

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This probably because the intermolecular hydrogen bonds are formed between

195

hydroxyl groups of guar gum and borax in the formation of covalent borate/diol bonds.

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Figure 1b displays the X-ray diffraction patterns of native guar gum powder and

197

guar gum hydrogels. It can be seen that native guar gum powder shows a very small

198

crystallinity, similar phenomenon has been reported in previous literatures38,40. When

199

guar gum solution cross-linked with borax, a pronounced crystallinity reduction could

200

be observed in the XRD patterns of borax-guar gum hydrogels because of the

201

consumption of the hydroxyl groups of guar gum with borax. It is well known that

202

hydrogen bonds help maintain the stability of guar gum crystals, when the hydrogen

203

bonds are broken, the crystallinity of guar gum reducing. Figure 1c and d showed the

204

morphology and porosity of the freeze-dried Borax-G-2.0 supramolecular hydrogels. It

205

can be seen that the Borax-G-2.0 hydrogel exhibited a uniform interconnected

206

macroporous structures with the average pore size of about 200 μm.

b 3428

GG Borax-G-0.5 Borax-G-1.0 Borax-G-1.5 Borax-G-2.0

Relative intensity

Relative transmittance

a

GG Borax-G-0.5 Borax-G-1.0 Borax-G-1.5 Borax-G-2.0

4000 3500 3000 2500 2000 1500 1000 500

5

Wavenumber (cm-1)

c

15 25 35 45 55 65 75 2 (degree)

d

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Figure 1. a) FT-IR spectra; b) X-ray diffraction patterns of guar gum powder and borax-

209

guar gum hydrogels; c) SEM microimage of Borax-G2.0 hydrogel (scale bar: 400 m)

210

and d) enlarged image (scale bar: 100 m).

211

3.3 Mechanical properties of supramolecular guar gum hydrogel.

212

To gain insight into the properties of the hydrogel process, dynamic mechanical

213

measurements were taken to investigate the rheological behaviors of borax-guar gum

214

supramolecular hydrogels with different concentrations of guar gum powder. Figure 2a

215

shows the changes of storage (G´) modulus and loss (G˝) modulus of guar gum

216

supramolecular hydrogels in 0.01-100 Hz frequency range. It was observed that when

217

the concentration of guar gum exceeds 0.5%, storage (G´) modulus was always higher

218

than loss (G˝) modulus over the experimental frequency range. This result suggested

219

that Borax-G-1.0, Borax-G-1.5 and Borax-G-2.0 exhibited gel-like character, which

220

contributed to the formation of borax cross-linked networks in these supramolecular

221

guar gum gels. Whereas, as to Borax-G-0.5 hydrogel, it could be observed that storage

222

(G´) modulus nearly equals loss (G˝) modulus at relatively low frequency region,

223

indicating a weak gel. More importantly, the amount of guar gum introduced into the

224

gel had a great influence on the rheological properties. As demonstrated in Figure 2a,

225

the storage (G´) modulus of borax cross-linked supramolecular hydrogels at guar gum

226

concentration of 2.0% w/v were about 130 folds higher than those at 0.5% w/v, which

227

indicated that the hydrogels prepared at higher guar gum concentrations were tougher.

228

Compressive property is an important part in evaluation of mechanical properties of

229

hydrogel. In order to study mechanical properties of guar gum supramolecular

230

hydrogels, compressive stress-strain tests were conducted and the experimental results

231

are depicted in Figure 2b. When the content of guar gum was 0.5% the gel was too

232

weak for compressive testing. The Borax-G-2.0 hydrogel obtained by adding higher 10 ACS Paragon Plus Environment

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amount of guar gum powder shown better mechanical property, which could be due to

234

the higher cross-linked density. More specifically, with the increase concentration of

235

guar gum, compressive stress increased from 3.5 KPa for Borax-G-1.0 hydrogel to 9.4

236

KPa for Borax-G-2.0 hydrogel. These results are consistent with the result of the

237

rheological properties test.

a

b

G,G (Pa)

100 10 1 G Borax-G-0.5 G Borax-G-1.0 G Borax-G-1.5 G Borax-G-2.0

0.1 0.01 0.1

1

G Borax-G-0.5 G Borax-G-1.0 G Borax-G-1.5 G Borax-G-2.0

10

100

Commpressive Stress (KPa)

233

10 8 6 4

Borax-G-2.0 Borax-G-1.5 Borax-G-1.0

2 0 0

20

Frequency (Hz)

40

60

80

100

Strain (%)

238 239 240

Figure 2. a) Rheological properties of borax cross-linked supramolecular hydrogel with

241

different guar gum concentrations; b) Compression tests for the hydrogels of Borax-G-

242

1.0, Borax-G-1.5, and Borax-G-2.0.

243

3.4 Self-healing ability.

244

Due to the dynamic property of the borax-diol ester bonds that drive the formation

245

of Borax-G hydrogels, our guar gum hydrogels had excellent self-healing ability. As

246

shown in Figure 3, hydrogels with and without dying were cut into two pieces. Two

247

halves pieces from different borax-guar gum hydrogels were brought into contact

248

(Figure 3a-c). During the self-healing process, the dye diffusion from one piece to the

249

other piece of hydrogel, resulting in a blurred contact interface between two pieces

250

hydrogels (Figure 3d). After 2 mins, the damaged pieces adhered and autonomously

251

healed to one integral hydrogel without using any external force or stimulus (Movie

252

S1). More remarkably, this self-healed hydrogel could withstand all kinds of

253

mechanical forces such as bending (Figure 3d) or stretched by tweezers without crack 11 ACS Paragon Plus Environment


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(Figure 3e), implying that the 3D structure as well as mechanical strength of the

255

hydrogels recovered.

a

d

c

b

e

256 257

Figure 3. Photos showing the self-healing behavior of borax-guar gum hydrogels: a)

258

original hydrogels with and without dying by methylene blue; b) Two pieces cut from

259

original hydrogel; c) Two hydrogel pieces contact at room temperature for 2 min; d)

260

Bending and e) stretching the self-healed hydrogels.

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To further observe self-healing performance of guar gum hydrogel, optical

262

microscopy was used to observe the self-healing process. As expected, when brought

263

into contact, two fragments of hydrogel could adhere to each other rapidly and could

264

instantly self-healed into one single piece at room temperature (Figure 4a-c). Besides,

265

self-healing capacity of this supramolecular guar gum hydrogels were also investigated

266

by rheological measurements. The elastic (G´) modulus and loss (G˝) modulus of

267

supramolecular guar gum hydrogels depend on different frequency were tested. It could

268

be seen that the G´ and G˝ values of healed guar gum supramolecular hydrogel was

269

almost the same as the initial state of the guar gum hydrogel (Figure 4d), demonstrating

270

the recovery of inner 3D structure of guar gum supramolecular hydrogel. Furthermore, 12 ACS Paragon Plus Environment

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elastic response of supramolecular guar gum hydrogel was studied through the

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continuous step strain measurements (Figure 4e). Initially, the guar gum hydrogels were

273

deformed under small amplitude oscillatory force (frequency = 1.0 Hz, γ = 10.0%),

274

Borax-G-2.0 hydrogel shows solid nature with G´ of about 120 Pa and G˝ of about 37

275

Pa (tan δ = G˝ / G´≈ 0.31). When exerting larger amplitude oscillatory force (frequency

276

=1.0 Hz, γ = 100%) on guar gum hydrogels, the G´ value decrease from 120 Pa to about

277

80 Pa and at the meantime, the tan δ value increases from 0.31 to 0.88, implying that

278

the hydrogel lost its parts of mechanical stability. After decreasing the amplitude

279

oscillatory force (frequency = 1.0 Hz, γ = 10%), G´ and G˝ could returned to its original

280

values immediately, which implying the rapid recovery of the internal network and

281

mechanical properties of guar gum supramolecular hydrogel. The above process could

282

be repeated for multiple cycles without any distinct degradation.

a

b

400 µm

0s

c

400 µm

60s

e

d

10%

100%

10%

100%

10%

G,G(Pa)

100

G,G (Pa)

400 µm

120 s

100

10 G Original gel G Self-healed gel

G G

G Original gel G Self-healed gel

10

1 0.1

1

10

100

0

200 400 600 800 1000 1200 1400 1600

Time (s)

Frequency (Hz)

283 284

Figure 4. Optical microscopy images and rheological measurements of the self-healing

285

Borax-G-2.0 supramolecular hydrogel. a-c) Optical microscopy images show changes

286

of the incision of two pieces Borax-G-2.0 hydrogel (one half piece was dyed with 13 ACS Paragon Plus Environment


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methylene blue) over time at room temperature; d) G´ and G˝ of original and after self-

288

healed Borax-G-2.0 hydrogels; e) The self-healing property of Borax-G-2.0 hydrogel

289

verified via continuous step strain (10% strain → 100% strain → 10% strain → 100%

290

strain → 10% strain) measurements.

291

The excellent self-healing behavior of the supramolecular guar gum hydrogel,

292

endows the borax-guar gum hydrogel excellent reformable properties. As depicted in

293

Figure 5a-b, guar gum hydrogel was cut into halves, then remodeled by assembling the

294

hydrogel pieces and directly reshaped the hydrogel by various modulus. As expected,

295

the pieces of the hydrogel could be remoulded into any desired shape by gently pressing

296

it for less than 5 min (Figure 5b). Furthermore, the injectable property of borax-guar

297

gum hydrogel was confirmed by extruding the damaged guar gum hydrogel pieces

298

through a manual syringe. Continuous extrusion of methylene dye incorporated

299

hydrogel pieces was easily achieved as shown in Figure 5c and d, indicating the

300

promising application in biomedical applications.


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a

b Remodeling Self-healing

Reshaped hydrogel

Hydrogel pieces

Cut into pieces

c

d Injecting

301

Reshaped pieces

Hydrogel pieces

302

Figure 5. Reformable and injectable properties of the guar gum based self-healing

303

hydrogels. The hydrogels dyed with methylene blue.

304

3.5 Thermal and pH-responsive performance.

305

In addition to self-healing property, borax-guar gum hydrogels also exhibited

306

interesting thermal and pH responsive gel-sol transitions. The guar gum hydrogels

307

underwent gel-sol transitions during cycles of heating-cooling process. As depicted in

308

Figure 6a the gel state of borax cross-linked guar gum became liquid when heated to

309

60 °C and could reform gel state after cooled down to 20 C. This thermal responsive

310

capability was due to reversible and exothermic reactions between hydroxyl groups of

311

guar gum and B(OH)4− .

312

For pH responsive gel-sol transitions performance, diluted HCl (0.1 M) and NaOH

313

(0.1 M) were used as pH regulators. Adding diluted HCl solution into the original

314

borax-guar gum hydrogel leads to the transformation of gel state into solvent state 15 ACS Paragon Plus Environment


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315

rapidly, while the adding an equivalent amount of diluted NaOH solution give rise to

316

gel state reformation of guar gum hydrogel (Figure 6b). These thermal and pH

317

responsive gel-sol transition processes of borax-guar gum hydrogel were repeated ten

318

times, showing borax-guar gum hydrogels has good repeatability and dynamically

319

reversible behavior. During the and pH responsive gel-sol transition processes, there

320

was a little increase of the volume among each sol-gel conversion process because of

321

the addition of diluted HCl and NaOH solution during each gel-sol transition

322

experiment. Basic condition could stimulate a gel state formed by stabilizing the

323

tetrahedral borate efficiently, while acid condition trigger the dissociation of the “di-

324

diol” complexation, which was formed between diol units of guar gum and borate ions.

325

The dissociation and recombine of the reversible “borate/didiol” bonds between

326

hydroxyl groups of guar gum and borate ions was the main reason of the pH-triggered

327

gel-sol transition performance.

a 60C

20C

60C

20C after 10 cycles

b

H+

OH-

H+

OHafter 10 cycles

328 329

Figure 6. Ten cycles of gel-sol transitions. a) thermal-responsive performance of borax-

330

guar gum hydrogel: heating to 60 °C to obtain the sol state, while cooling to 20 °C to

331

reform hydrogel; b) pH-responsive performance of borax-guar gum hydrogel: adding

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diluted HCl solution to gel state to obtain sol state, while adding diluted NaOH

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solution to the sol state to obtain gel state.

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4. CONCLUSIONS 16 ACS Paragon Plus Environment

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In summary, a facile method is presented to prepare supramolecular hydrogels by

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using naturally occurring polysaccharide guar gum as raw material and borax as a cross-

337

linker with the help of dynamic covalent bond between hydroxyl groups and borax

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through the covalent borate/didiol bonds. The effect of the concentration of guar gum

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on the mechanical properties of guar gum-based hydrogel was evaluated by using

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dynamic mechanical and compressive measurement. The G´ of borax cross-linked

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supramolecular hydrogels at guar gum concentration of 2.0% w/v were about 130 folds

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higher than those at 0.5% w/v. In addition, with the increase concentration of guar gum,

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the compressive stress increased from 3.5 KPa for Borax-G-1.0 hydrogel to 9.4 KPa for

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Borax-G-2.0 hydrogel. Both of these results indicating that the hydrogels prepared at a

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higher concentration was tougher. As expected, the reversible and dynamic nature of

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the borax-diol linkages give rise to hydrogels excellent self-healing capability which

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was further verified by microscopic self-healing and rheological recovery test.

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Furthermore, the excellent self-healing ability of guar gum-based hydrogel, endows the

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Borax-G hydrogel excellent reformable and injectable properties. Remarkably, these

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hydrogels have shown stimuli responsiveness towards pH due to the pH responsive

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dissociation and recombine of the reversible borate/didiol complexations between

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borate ions and hydroxyl groups of guar gum. This self-healing, injectable and pH

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responsive hydrogels from low-cost, environment friendly and readily available

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materials could have potential applications in many biomedical areas in the future.

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ASSOCIATED CONTENT

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Supporting Information

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Video of the self-healing process of guar gum based supramolecualr hydrogels.

358 359

AUTHOR INFORMATION 17 ACS Paragon Plus Environment


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Corresponding Author

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*E-mail: [email protected]; [email protected]

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Notes

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The authors declare no competing ﬁnancial interest.

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ACKNOWLEDGEMENTS

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The authors are extremely grateful to financial support from National Natural Science

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Foundation of China (No. 21808126), Science and Technology Planning Project of

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Higher Education of Shandong Province (No. J14LD51), Science and Technology

368

Planning Project of Qufu Normal University (No. xkj201413), Doctoral Starting up

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Foundation of Qufu Normal University (No. BSQD2012058) and the Foundation of

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Key Laboratory of Pulp and Paper Science and Technology of Ministry of

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Education/Shandong Province of China (No. KF201702).

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TABLE OF CONTENTS GRAPHICS

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Synopsis:

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Our guar gum-based supramolecular hydrogel had excellent self-healing property,

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and reversible gel-sol transformations by the application of physicochemical stimuli

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such as thermal and pH value.

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Facile Access to Guar Gum Based Supramolecular Hydrogels with

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