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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
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Facile Access to Guar Gum Based Supramolecular
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Hydrogels with Rapid Self-Healing Ability and Multi-Stimuli
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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
57
three-dimensional network structure. Because of their high hydrophilicity, nontoxicity,
58
biodegradability and biocompatibility properties, hydrogels have wide utilizations in
59
various areas, such as waste water treatment 1, drug delivery 2, tissue engineering 3,
60
artificial articular cartilage 4,5 and electronic devices 6.
61
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
72
called supramolecular hydrogels 23. At present, many different types of covalently and
73
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
76
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
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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
83
increasing environmental issues, products based on abundant naturally-occurring
84
polysaccharides will be desirable. Thus, the motivation for this work was the absence
85
of a low-cost, facile method to prepare fast self-healable, multi-stimuli responsible
86
natural-based hydrogels. To develop this kind of supramolecular hydrogel system, we
87
tried a kind of “smart” naturally-occurring polysaccharide --- guar gum.
88
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-
91
d-galactopyranosyl units (G) with hydroxyl groups
92
polysaccharides, guar gum has many advantages such as high molecular weight and
93
plentiful side chains, which is believed to help improve mechanical strength of
94
hydrogels by increasing the intramolecular cross-linking and the entanglement degree
95
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
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the automatically self-healing ability as well as multi-stimuli responsive property into
108
one system to obtain multifunctional smart guar gum-based hydrogels.
29,37
. Herein, it would be interesting to make
109
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-
111
stimuli responsive functionalities, which uses cheap and commercially available
112
polymer with no need for any other chemical modification. As expected, the resulting
113
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
115
have great significance in broadening their practical applications. We believe that this
116
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
119
2.1 Materials
120
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
129
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
131
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
143
and was placed on a plate and compressed to 85% of its original height with speed of
144
10 mm∙min-1. The displacement and load data were collected from the compressive
145
measurements.
146
The rheological behaviors of guar gum hydrogels were investigated by using a
147
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
154
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.
159
3. RESULTS AND DISCUSSIONS
160
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
171
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.
176
The hydrogel became liquid when acid solutions was added and the gel could be
177
reformed after the addition of alkaline solutions.
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a
b
c Heating
H+
Cooling
OH-
178 179
Scheme 1. a) One possible mechanism of the formation of borax-guar gum
180
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.
184
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.
196
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.
261
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
272
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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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 60C
20C
60C
20C 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
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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-
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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
Journal of Agricultural and Food Chemistry
360
Corresponding Author
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*E-mail:
[email protected];
[email protected] 362
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
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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).
372 373
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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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