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Cite this: DOI: 10.1039/c0xx00000x
ARTICLE
Glucose-responsive Hydrogels Based on Dynamic Covalent Chemistry and Inclusion Complexation Ting Yang, a Ran Ji, a Xin-Xing Deng, a Fu-Sheng Dua and Zi-Chen Li*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 2013, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A novel glucose-responsive hydrogel system based on dynamic covalent chemistry and inclusion complexation was described. Hydrogels are formed by simply mixing the solutions of three components: poly (ethylene oxide)-b-poly vinyl alcohol (PEO-b-PVA) diblock polymer, α-cyclodextrin (α-CD) and phenylboronic acid (PBA)-terminated PEO crosslinker. Dynamic covalent bonds between PVA and PBA provide sugar-responsive crosslinking, and the inclusion complexation between PEO and α-CD can promote hydrogel formation and enhance hydrogel stability. The ratios of the three components have a remarkable effect on the gelation time and the mechanical properties of the final gels. In rheological measurements, the hydrogels are demonstrated to possess solid-like behaviour and good structural recovery ability after yielding. The sugar-responsiveness of the hydrogels was examined by protein loading and release experiments, and the results indicate that this property is also dependent on the compositions of the gels, at a proper component ratio, a new glucose-responsive hydrogel system operating at physiological pH can be obtained. Combination of good biocompatibility of the three components and the easy preparation of hydrogels with tunable glucose-responsiveness may enable an alternative design of hydrogel system that finds potential applications in biomedical and pharmaceutical fields, such as, treatment of diabetes.
Introduction
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Hydrogels are three-dimensional polymeric networks swollen by large amounts of water. Due to their high water content and good biocompatibility, hydrogels have been studied extensively in biomedical fields as drug delivery systems and tissue engineering.1-3 The networks in hydrogels can either be formed by covalent crosslinking or various non-covalent interactions.4 Owing to the irreversibility of the covalent bonds, the chemical gels thus obtained possess high stability. However, they can only be implanted into subcutaneous, and drug-loading may be timeconsuming and less efficient. Physical gels from noncovalent interactions can accomplish drug and protein loading in the gelation process under moderate conditions, but these gels suffer from low stability when practical use is considered. Dynamic covalent chemistry5 provides another choice to construct robust hydrogels with stimuli-responsiveness. Gels based on reversible covalent bonds can simultaneously achieve robustness and dynamically restructuring properties, moreover, 6-8 they can undergo a sol-gel transition in response to external stimuli. Stimuli-responsive hydrogels are deemed as intelligent materials due to their ability to sense environmental changes,9 such as temperature,10 pH,11 light,12-15 redox,16, 17 and biomolecules,18 etc. In the absence of a cure for diabetes, plenty of research work has been devoted to developing an artificial closed-loop system which is capable of conducting insulin release in response to glucose concentration under physiological conditions. Glucose-responsive hydrogels which are susceptible to glucose concentration are very attractive for developing such This journal is © The Royal Society of Chemistry 2013
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closed-loop systems.18, 19 Owing to the ability to bind with diols through dynamic covalent bonds, phenylboronic acid (PBA) containing polymers have gained much attentions in constructing glucose-responsive molecular assemblies and hydrogels.20,21 The initial work of utilizing PBA as glucose sensor in hydrogel systems was accomplished by Kataoka et al.22-24 In recent years, to achieve faster kinetics, glucose-responsive microgels,25-34 micelles35-41 and polymersomes 42-44 either for glucose-sensing or for insulin delivery have also been widely studied. PBA can form dynamic complex with a polyol, such as poly (vinyl alcohol) (PVA). The complex can be dissociated in the presence of another competing polyol, such as glucose. However, this glucose-responsive complex is not suitable to be used under physiological conditions due to its instability at neutral pH.45 Incorporation of amino22, 46 or carboxyl47 groups to the complex system and lowering the pKa of PBA by changing the substituents48 on the benzene ring are two effective methods to stabilize the complex between PBA and a polyol. Cyclodextrins (CDs) are a series of cyclic oligosaccharide composed of 6, 7, or 8 (D+)-glucose units connected by α-1, 4 glucosidic bonds, and named α-, β-, or γ-CD, respectively. They are widely employed to construct supramolecular architectures and hydrogels based on host-guest interactions.49-51 The ability of α-CD to form inclusion complex with PEO was first described by Harada et al.52 Later on, CD-based polyrotaxanes53 and supramolecular hydrogels54 were also reported. It is worth mentioning that these hydrogels are thixotropic and reversible, which endows them with the potential to be used as injectable Soft Matter, 2013, [vol], 00–00 | 1
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method.58 Synthesis of PEO114-b-PVA339 was reported in our previous paper. 59 50
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2.2 Synthesis of tosylate-terminated PEO (1) 60 To the solution of PEO (30 g, 30 mmol) and triethylamine (9.5 g, 93.9 mmol) in 200 mL of CH2Cl2, tosyl chloride (18 g, 94.4 mmol) in 100 mL of CH2Cl2 was added dropwise at low temperature (ice bath). The mixture was stirred at room temperature for 12 h. The insoluble salts were removed by filtration and the solvent was removed under reduced pressure. The residue was re-dissolved in water (500 mL) and washed with diethyl ether (3 x 100 mL), and then extracted with CH2Cl2 (3 x 150 mL). The combined organic fractions were dried over Na2SO4 and evaporated to dryness to yield a yellow liquid (36.5 g, yield: 93%). 1H NMR (300 MHz, CDCl3, δ, ppm): 2.46 (s, 6H), 3.59-3.72 (m, 84H), 4.16 (t, 4H), 7.35 (d, 4H), 7.80 (d, 4H). 2.3 Synthesis of azido-terminated PEO (2) 61
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2.1 Materials 3-Aminophenylboronic acid (97%, Beijing Pure Chem. Co., Ltd.), fluorescein isothiocyanate (FITC, 95%, Alfa Aesar), αcyclodextrin (α-CD, 98%, TCI), N, N-diisopropyl-N-ethylamine (DIPEA, 98 %, Aldrich), triphenylphosphine (99%, Acros) and sodium azide (NaN3, 98%, Zhejiang Dongyang Kaiming Chem. Co.) were used as received. Poly (ethylene oxide) (OH-PEO-OH, Mn=1000, Polyscience) was dissolved in CH2Cl2, dried with MgSO4, recovered after removal of MgSO4 by filtration and removal of CH2Cl2 by evaporation. It was further dried under high vacuum at 45oC for 24 h. Tosyl chloride was recrystallized from petroleum ether and dried under vacuum. Triethylamine was refluxed with acetic anhydride, distilled, dried with CaH2 over night, and then redistilled. THF was refluxed with sodium for 8 h, and distilled. Acryloyl chloride was synthesized from the reaction between acrylic acid and benzoyl chloride.57 3-(Acrylamido) phenylboronic acid was obtained by the reaction of acryloyl chloride and 3-aminophenylboronic acid according to a literature
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A mixture of 1 (36.5 g, 27.9 mmol), NaN3 (5.85 g, 90 mmol) was stirred in DMF (350 mL) at 80 oC for 14 h. After removal of the insoluble salts by filtration, DMF was removed under reduced pressure. The crude product was then poured into water (400 mL) and extracted with CHCl3 (3 x 200 mL). After drying the combined organic fractions over Na2SO4 and removal of the solvent, a liquid product was obtained (25.5 g, yield: 91%).1H NMR (300 MHz, CDCl3, δ, ppm): 3.37 (t, 4H), 3.49-3.71 (m, 84H). 2.4 Synthesis of amino-terminated PEO (3) 61 A mixture of 2 (25.5 g, 25.5 mmol), triphenylphosphine (19 g, 72 mmol), H2O (1 mL) and THF (300 mL) was stirred at room temperature for 14 h. After most of the THF was removed under reduced pressure, a yellow liquid was obtained. Then, 100 mL of water was added, and the pH of the solution was adjusted to 1 with hydrochloric acid, the white precipitate formed was removed by filtration. The aqueous solution was washed with ethyl acetate (3 x 50 mL) and the aqueous phase was combined. NaOH was then added to the aqueous solution until an oil layer appeared. The solution was extracted with CH2Cl2 (3 x 150 mL), the combined organic fractions were dried with NaOH and Na2SO4 over night, evaporated to dryness to get a white solid (22 g, yield: 78 %). 1H NMR (400 MHz, CDCl3, δ, ppm): 2.89 (t, 4H), 3.55 (t, 4H), 3.60-3.66 (m, 80H). 2.5 Synthesis of PBA-terminated PEO crosslinker (4)
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A mixture of 3 (4 g, 4 mmol), 3-(acrylamino) phenylboronic acid (2.29 g, 12 mmol), DIPEA (1.55 g , 12 mmol) and methanol (30 mL) was degassed by two freeze/thaw circles, sealed under reduced pressure, and then heated at 50 oC for 72 h. The product was obtained after pouring the reaction mixture into diethyl ether, then it was dried under reduced pressure at 35 oC for 24 h to give a pale solid (4.52 g, yield: 82%). 1H NMR (400 MHz, D2O, NaOD, δ, ppm): 2.72-2.75 (m, 5.4H), 2.94 (t, 4H), 3.07-3.13 (m, 5.4H), 3.76-3.82 (m, 84H), 7.37-7.55 (m, 10.8H). 2.6 Preparation of hydrogels
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Scheme 1 Schematic representation of the hydrogel composed of PEO-bPVA, crosslinker and α-CD and its glucose-responsive mechanism.
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Stock aqueous solutions of PEO114-b-PVA339 (10 wt%), the crosslinker 4 (6 wt%) and α-CD (12 wt%) were prepared in 50 mmol phosphate buffer (pH=7.4). Then, a predetermined amount of polymer and α-CD solutions was mixed by shaking for 5 min., followed by the addition of a known amount of crosslinker solution. After slightly shaking, the mixture was kept at room This journal is © The Royal Society of Chemistry 2013
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drug delivery systems. In recent years, several stimuli-responsive hydrogel systems based on inclusion complexation between PEO and α-CD have been developed.55, 56 To the best of our knowledge, there has been no report of glucose-responsive hydrogel systems based on inclusion complexation and dynamic covalent chemistry. In this work, we describe a glucose-responsive hydrogel system based on dynamic covalent bonds and inclusion complexation. PEO-b-PVA, α-CD and PBA-terminated aminecontaining PEO crosslinker were used to construct the hydrogel system. The introduction of inclusion complexation between PEO and α-CD and the utilization of a secondary amine-containing crosslinker were intended to strengthen the hydrogel network. The amino groups in the PEO crosslinker can stabilize the complex between PVA and PBA at pH 7.4, and also enable a larger binding constant for PBA and glucose, thus endowing the hydrogel with tunable glucose- responsiveness at physiological pH. As depicted in Scheme 1, the hydrogel network arises from two cooperative driving forces: the dynamic covalent bonds between PVA and PBA, and the inclusion complexation between PEO and α-CD. The hydrogel structure can be disrupted in the presence of glucose and other competing polyols which can replace PVA from the dynamic complex structure and accelerate the hydrogel dissolution process. The effect of the compositions on the mechanical properties of the hydrogels was studied by rheological measurements. The glucose-responsiveness of the hydrogels was evaluated by protein loading and release experiments.
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DOI: 10.1039/C3SM53059K
temperature for 24 h before rheological measurements and other tests were made. The formulations of the gels are listed in Table 1.
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2.7 Rheological measurements
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Dynamic rheological measurements were carried out on a Physica MCR 301 Rheometer (Anton Paar) equipped with a parallel plate geometry (25 mm diameter) at a gap of 0.5 mm. The edge of the gel sample was covered with low viscous oil to prevent water evaporation during the measurements. After being loaded on the measuring plate, all the gel samples were allowed to stabilize for 40 min under a small stress (3-10 Pa) at a frequency of 0.1 Hz. Oscillatory frequency sweeps were performed from 100-0.1 rad/s at a constant strain of 0.1%. Oscillatory strain sweeps were performed from 0.01%-3000% at 1 rad/s. After a 3000 % deformation, gel samples were allowed to recover under a small stress (1-10 Pa) at 1 rad/s. 2.8 Wide-angle X-ray diffraction
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boronic acid group can stabilize the complex between PBA group and polyols at lower pH as widely accepted in literatures.22, 42 The 1 H NMR spectrum of the crosslinker is shown in Fig. 1. The spectrum is consistent with the proposed structure. The degree of substitution (the number of PBA per PEO chain) was determined from the integral ratio of the phenyl protons (peak e, 7.3-7.6 ppm) of PBA groups to (-CH2-CH2-O-) (peak n, 3.7-3.9 ppm) of PEO. The calculated value was 2.7, which suggested that some of the end amino groups added two PBA groups and made the PBA functionalized PEO as an effective crosslinker.
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Scheme 2 Synthetic routes of the crosslinker: a) tosyl chloride, triethylamine, r.t., 12 h. b) NaN3, DMF, 80 oC, 14 h. c) triphenylphosphine, THF, H2O, r.t., 12 h. d) 3-(acrylamino)phenylboronic acid, DIPEA, MeOH, 50 oC, 72 h.
XRD patterns of the lyophilized gel samples were recorded on a Rigaku DMAX-2400 power diffractmeter with Cu Kα (1.54) radiation (40 kV, 100 mA). Samples were scanned at a rate of 4 o /min over the range 2θ=3-60o at room temperature. 2.9 Morphology of the lyophilized hydrogels
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The morphology of the hydrogels was studied with a scanning electron microscope (FEI NanoSEM 430, Landing E: 10.0 keV). Pieces of the lyophilized hydrogel were carefully stuck onto an aluminium plate with double-sided adhesive before measurements. 2.10 Loading and release of protein
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Take the Gel 3 (Table 1) as an example, the protocol is as follows: α-CD solution (0.3 g) containing BSA-FITC (5 mg) was added to a 15 mL tube, a crosslinker solution (0.15 g) was then added to the tube soon after the addition of the polymer solution (0.15 g). The mixture was slightly shaken for 5 min. and then kept at room temperature for 24 h. Afterwards, phosphate buffer (10 mL, pH=7.4) was carefully added on top of the formed hydrogels and the tube was placed into a shaker. The release experiments were carried out at 25 oC at an oscillation frequency of 100 rad/min. At determined time intervals, 1.5 mL of the buffer was withdrawn to determine the content of the released FITC-labeled BSA by measuring the absorbance at 455 nm (ε=1326 L/g·cm) on a UVVis spectrophotometer. After each measurement, the withdrawn buffer solution was added back to the releasing system.
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Fig. 1 1NMR spectrum of crosslinker 4 in D2O (with NaOD).
3.2 Hydrogel preparation
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PEO114-b-PVA339 diblock copolymer, the PEO based crosslinker (4) and α-CD were used to construct hydrogels. The formulations are listed in Table 1. Table 1 Formulations of hydrogels based on PEO114-b-PVA339. Crosslinker 4 (% w/w)
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3 Results and Discussion
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The crosslinker 4 was synthesized in a four-step procedure starting from PEO as shown in Scheme 2. The overall yield is 30%. The first three steps are followed literature procedures, and the final step is the Michael addition reaction between the bisamino terminated PEO and 3-(acrylamido) phenyl boronic acid to render the final crosslinker with two secondary amines attached to a phenyl boronic acid respectively. We consider such a design has two advantages: first, PEO (Mn=1000) was used as the starting material, this will render the crosslinker water-soluble at neutral pH; second, the presence of secondary amine close to the
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Fig. 2 XRD patterns of lyophilized pure polymer, Gel 3, Gel 6 and Gel 10.
The viscoelastic properties of the gel samples were characterized by dynamic rheological measurements. The effect of compositions was first studied. Fig. 3 showed the dynamic frequency sweeps of Gel 3, Gel 6 and Gel 10. For all the three samples, G′ exceeded G′′ over the entire measured frequency and G′ was independent of angular frequency, which is indicative of elastic solid behaviour.50 Gel 10 with the highest α-CD content possessed the largest G′ and G′′, while Gel 3 with the lowest αCD possessed the minimum G′ and G′′. The higher α-CD content led to larger G′ and G′′, indicating that an increase in α-CD to EO ratio could increase the crosslinking density more effectively. The G′ values (at 68.7 rad/s) for Gel 3, Gel 6 and Gel 10 were 8.8, 38.9, 127 kPa, respectively, which are comparable with those of bone tissue engineering materials.64 It has been reported that hydrogels based on PVA and PBA-containing crosslinker do not behave as elastic solid and have an easily detectable relaxation time even at pH around PBA’s pKa.65 In our system, inclusion complexation between the PEO segments and α-CD strengthened the hydrogel network effectively to obtain solid-like hydrogels at neutral pH.
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Fig. 3 Dynamic storage modulus and loss modulus as a function of angular frequency at 25 oC for Gel 3, Gel 6 and Gel 10.
The influences of α-CD and crosslinker content on the hydrogel properties were further studied. Gel 7 possessed higher α-CD content than that of Gel 6, it had much higher G′ and G′′, as shown in Fig. 4A, indicating increased crosslinking density. Although the ratio of α-CD to EO had exceeded 1:2 for Gel 6, a stiffer and stronger hydrogel network could be obtained by increasing the α-CD content. However, different results were obtained for Gel 10 and Gel 11, as shown in Fig. 4B. They had similar G′ and G′′ even that Gel 11 possessed higher α-CD content. This is probably due to the high ratio of α-CD to EO in Gel 10 (0.85:1). Although further increasing the α-CD content could shorten the gelation time, it barely contributed to the crosslinking density of hydrogel networks. The effect of crosslinker content was shown in Fig. 4C. Gel 12 with higher crosslinker content possessed higher G′ and G′′ than Gel 10. This result implied that the elevated dynamic covalent interactions between PVA and PBA which resulted from increased crosslinker content could contribute to the hydrogel network significantly. Since inclusion complexation between PEO and α-CD was indispensable in our hydrogel system, we wanted to know whether the hydrogels displayed thixotropic property. This test has been carried out by Li et al.,63 and the process is described This journal is © The Royal Society of Chemistry 2013
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3.3 Rheological measurements
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First we checked the solubility of the diblock copolymer PEO114-b-PVA339 in phosphate buffer solution, and found that even when the concentration reached 20 wt%, no gelation occurred. This is understandable, since both PEO and PVA are water soluble. Then, we studied the behaviour of mixture of two components at different concentrations: PEO114-b-PVA339 and crosslinker; PEO114-b-PVA339 and α-CD. In the first series (sample 2, 5, 9), the mixtures are also clear solution with no gel formation. In this system, the interaction of the crosslinker with PVA block exists, but the PEO block may hinder gel formation at such lower concentration. In the second series (sample 1, 4, 8), white precipitates are obtained after the mixtures being kept at room temperature for about 30 min. This is due to the formation of low soluble complex of PEO and α-CD.62 To our pleasure, we obtained opaque free-standing hydrogel for the mixture of the three components. This suggests that the coexistence of two driving forces, dynamic covalent bonds between PVA and PBA, and the inclusion complexation between PEO and α-CD is essential for gel formation at low concentrations. We then systematically investigated the influence of each component on the gelation time. The total amount of solute was fixed at 10 wt % for sample 3, 6, 10, with the weight ratio of the block copolymer and the crosslinker being 5:3. Gel 10 (αCD:EO= 1.14:1) with the highest -CD to polymer ratio had the shortest gelation time (4.5 h), while it took 12 h for Gel 3 (αCD:EO= 0.52:1) (the lowest -CD content) to form a gel. For Gel 6 (α-CD:EO= 0.85:1) with the medium -CD content, the gelation time is 5 h, in-between of the above two. If the -CD content was further increased to 8.5% (Gel 11), the gelation time could be shortened to 3 h as compared to Gel 10. In these three component solution system, two kinds of interactions exist, one is the dynamic covalent bond between PVA and the PBA group, and the other is the inclusion complexation between PEO and α-CD. At lower polymer concentration, neither of these two interactions is strong enough to ensure gel formation; they must work cooperatively for the hydrogel formation. Since the interaction of PVA and PBA has been confirmed in aqueous solution, we want to know whether inclusion complex of α-CD and PEO was formed during the gelation. Therefore, we prepared dried samples by freeze-drying the gels, and measured the XRD patterns of the three dried gels as shown in Fig. 2. We observed typical peaks at 2θ around 19.8o, indicating the formation of the hexagonal columnar channel-type crystallization which arose from the inclusion complexation between PEO and α-CD.62,63 In the control experiments for pure polymer or α-CD sample, such patterns could not be detected.
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structural recovery ability. Generally, our hydrogels showed remarkably better abilities to recover the networks when compared with hydrogels based on PEG5000 and α-CD which needed about 30 min to regain the solid-like behaviour.63 These results also showed that our hydrogel system retained thixotropic nature; the similar results have been reported by Li et al.63 and Dong et al.53 in the study of hydrogel systems based on inclusion complexation and another driving force (hydrophobic interactions and hydrogen bonding, respectively).
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The morphology of the hydrogels was studied with SEM on lyophilized hydrogel samples. As shown in Fig. 6, all the three lyophilized hydrogels possessed porous structures and the pore sizes were dependent on the crosslinking density which was determined by the gel compositions. Gel 10 had the smallest pore sizes among all the three gel samples which is consistent with its highest crosslinking density, while Gel 3 with the lowest crosslinking density showed the largest pore sizes. The porous structure could serve as channels in release experiments and the pore sizes may affect release kinetics.
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briefly as following: a gradually increased strain was imposed to all the three gel samples in order to deform the hydrogel networks. After the strain value reached 3000%, the high shear stress was removed and replaced by a small stress (≤ 10 Pa). As shown in Fig. 5A, all the three Gels showed good structural recovery ability which was characterized by G′ exceeding G′′ immediately after the removal of high shear stress. The recovery extent was assessed from the percentages of G′ values soon after the removal of high shear stress with respect to the G′ values from linear viscoelastic regime. As shown in Fig. 5B, the instantaneous recovery of G′ after yielding of Gel 3, Gel 6 and Gel 10 were 11.4%, 70.7%, and 93.5%, respectively. For Gel 10, the G′ was fully recovered in less than 5 min, while it took 15 min for Gel 6 to achieve one hundred percent recovery of G′. For Gel 3, the recovery percentage gradually increased to 20.2% after 60 min. restoration. Gels with higher α-CD content possessed better
Fig. 6 SEM images of Gel 3 (A), Gel 6 (B) and Gel 10 (C).
3.5 Loading and releasing of FITC- labeled BSA
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We prepared FITC-labeled BSA hydrogels to examine the sugarresponsiveness of the hydrogels. Protein was loaded in the gelation process, and the addition of protein didn’t have a significant influence on the formation of hydrogels. As shown in Fig. 7A and 7B, the release of BSA was in a relative mild manner at room temperature, and no burst release was observed at the beginning in the absence or presence of sugar, indicating that most of the protein was encapsulated inside the gels instead of being adsorbed on the surfaces. In the absence of sugar, the hydrogels can also be dissolved gradually under shaking, which is due to the reversible nature of the crosslinking points. As competing diols, glucose and fructose could replace PVA from the hydrogel network due to their larger binding constants with PBA; as a result, the dissolution of hydrogel as well as the release of protein was accelerated in the presence of sugar. Fructose had a more significant effect on the dissolution rate of the hydrogel, which was in agreement with the larger binding constant between PBA and fructose.66,67
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Fig. 5 (A) Dynamic storage modulus and loss modulus as a function of time for Gel 3, Gel 6 and Gel 10 at 25 oC in the deformation-recovery measurement. (B) Recovery percentage of storage modulus of Gel 3, Gel 6 and Gel 10 immediately after 3000% deformation.
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Fig. 7 Release profiles of BSA-FITC from (A) Gel 6 and (B) Gel 3 at 25 oC in 50 mmol phosphate buffer (pH=7.4) containing 0 g/L glucose , 30 g/L glucose and 10 g/L fructose respectively.
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For Gel 6, the release of BSA was accelerated in the presence of 10 g/L fructose; however, the effect of glucose on the release kinetics was negligible even when the glucose concentration reached 30 g/L. For Gel 3, the release of BSA was in a faster manner in the presence of 30 g/L glucose, indicating that Gel 3 possessed better glucose-responsiveness, and the release rate of protein from Gel 3 was higher than that from Gel 6 in all the three cases. The portion of α-CD of Gel 6 was higher than that of Gel 3, which resulted in higher hydrophobicity of the complex, higher crosslinking density and smaller pore size for Gel 6. We speculated that the enhanced hydrophobicity and crosslinking density could probably retard the diffusion of saccharides and dissolution of hydrogel networks, therefore leading to a lower protein release rate. The hydrogel system was responsive to glucose and fructose under physiological pH, however, the glucose concentration was much higher than normal blood concentration. There may be two reasons: (1) the inclusion complex of PEO and α-CD was relatively hydrophobic, which may hinder the diffusion of saccharide molecule into the hydrogel; (2) in the release experiments, placing the hydrogels at the bottom of the tube led to limited contact area with solution. 4 Conclusions
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We prepared a series of hydrogels based on biocompatible materials, PEO-b-PVA diblock polymer, PBA modified PEO and α-CD. The gelation process was driven by two forces, the inclusion complexation and the dynamic covalent bonds. With the assistance of amino groups from the crosslinker and inclusion complexation between PEO and α-CD, a series of stiff hydrogels with good structural recovery abilities was obtained. The introduction of dynamic covalent chemistry provided the hydrogels with glucose-responsive characteristics. Protein loading could be achieved in the gelation process, and the release manner was responsive to glucose and fructose under physiological pH. The hydrogel system may have the potential to be applied as glucose-responsive materials for insulin delivery. Extending the hydrogel system to microgel systems by miniaturizing their size and increasing the portion of PVA and PBA in the hydrogel system may be two effective ways to optimize the glucose-responsive properties.
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This work was financially supported by the National Natural Science Foundation of China (No. 20534010, 50973002).
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Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Fax: +86-10-62751708; Tel: +86-10-62755543; E-mail: [email protected]
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TOC (SM-ART-12-2013-053059)
Ting Yang, Ran Ji, Xin-Xing Deng, Fu-Sheng Du and Zi-Chen Li*
A novel glucose-responsive hydrogel system based on dynamic covalent chemistry and inclusion complexation was described.
Soft Matter Accepted Manuscript
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Glucose-responsive Hydrogels Based on Dynamic Covalent Chemistry and Inclusion Complexation
Soft Matter Accepted Manuscript
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ARTICLE
Glucose-responsive Hydrogels Based on Dynamic Covalent Chemistry and Inclusion Complexation Ting Yang, a Ran Ji, a Xin-Xing Deng, a Fu-Sheng Dua and Zi-Chen Li*a 5
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Received (in XXX, XXX) Xth XXXXXXXXX 2013, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x A novel glucose-responsive hydrogel system based on dynamic covalent chemistry and inclusion complexation was described. Hydrogels are formed by simply mixing the solutions of three components: poly (ethylene oxide)-b-poly vinyl alcohol (PEO-b-PVA) diblock polymer, α-cyclodextrin (α-CD) and phenylboronic acid (PBA)-terminated PEO crosslinker. Dynamic covalent bonds between PVA and PBA provide sugar-responsive crosslinking, and the inclusion complexation between PEO and α-CD can promote hydrogel formation and enhance hydrogel stability. The ratios of the three components have a remarkable effect on the gelation time and the mechanical properties of the final gels. In rheological measurements, the hydrogels are demonstrated to possess solid-like behaviour and good structural recovery ability after yielding. The sugar-responsiveness of the hydrogels was examined by protein loading and release experiments, and the results indicate that this property is also dependent on the compositions of the gels, at a proper component ratio, a new glucose-responsive hydrogel system operating at physiological pH can be obtained. Combination of good biocompatibility of the three components and the easy preparation of hydrogels with tunable glucose-responsiveness may enable an alternative design of hydrogel system that finds potential applications in biomedical and pharmaceutical fields, such as, treatment of diabetes.
Introduction
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Hydrogels are three-dimensional polymeric networks swollen by large amounts of water. Due to their high water content and good biocompatibility, hydrogels have been studied extensively in biomedical fields as drug delivery systems and tissue engineering.1-3 The networks in hydrogels can either be formed by covalent crosslinking or various non-covalent interactions.4 Owing to the irreversibility of the covalent bonds, the chemical gels thus obtained possess high stability. However, they can only be implanted into subcutaneous, and drug-loading may be timeconsuming and less efficient. Physical gels from noncovalent interactions can accomplish drug and protein loading in the gelation process under moderate conditions, but these gels suffer from low stability when practical use is considered. Dynamic covalent chemistry5 provides another choice to construct robust hydrogels with stimuli-responsiveness. Gels based on reversible covalent bonds can simultaneously achieve robustness and dynamically restructuring properties, moreover, 6-8 they can undergo a sol-gel transition in response to external stimuli. Stimuli-responsive hydrogels are deemed as intelligent materials due to their ability to sense environmental changes,9 such as temperature,10 pH,11 light,12-15 redox,16, 17 and biomolecules,18 etc. In the absence of a cure for diabetes, plenty of research work has been devoted to developing an artificial closed-loop system which is capable of conducting insulin release in response to glucose concentration under physiological conditions. Glucose-responsive hydrogels which are susceptible to glucose concentration are very attractive for developing such This journal is © The Royal Society of Chemistry 2013
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closed-loop systems.18, 19 Owing to the ability to bind with diols through dynamic covalent bonds, phenylboronic acid (PBA) containing polymers have gained much attentions in constructing glucose-responsive molecular assemblies and hydrogels.20,21 The initial work of utilizing PBA as glucose sensor in hydrogel systems was accomplished by Kataoka et al.22-24 In recent years, to achieve faster kinetics, glucose-responsive microgels,25-34 micelles35-41 and polymersomes 42-44 either for glucose-sensing or for insulin delivery have also been widely studied. PBA can form dynamic complex with a polyol, such as poly (vinyl alcohol) (PVA). The complex can be dissociated in the presence of another competing polyol, such as glucose. However, this glucose-responsive complex is not suitable to be used under physiological conditions due to its instability at neutral pH.45 Incorporation of amino22, 46 or carboxyl47 groups to the complex system and lowering the pKa of PBA by changing the substituents48 on the benzene ring are two effective methods to stabilize the complex between PBA and a polyol. Cyclodextrins (CDs) are a series of cyclic oligosaccharide composed of 6, 7, or 8 (D+)-glucose units connected by α-1, 4 glucosidic bonds, and named α-, β-, or γ-CD, respectively. They are widely employed to construct supramolecular architectures and hydrogels based on host-guest interactions.49-51 The ability of α-CD to form inclusion complex with PEO was first described by Harada et al.52 Later on, CD-based polyrotaxanes53 and supramolecular hydrogels54 were also reported. It is worth mentioning that these hydrogels are thixotropic and reversible, which endows them with the potential to be used as injectable Soft Matter, 2013, [vol], 00–00 | 1
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method.58 Synthesis of PEO114-b-PVA339 was reported in our previous paper. 59 50
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2.2 Synthesis of tosylate-terminated PEO (1) 60 To the solution of PEO (30 g, 30 mmol) and triethylamine (9.5 g, 93.9 mmol) in 200 mL of CH2Cl2, tosyl chloride (18 g, 94.4 mmol) in 100 mL of CH2Cl2 was added dropwise at low temperature (ice bath). The mixture was stirred at room temperature for 12 h. The insoluble salts were removed by filtration and the solvent was removed under reduced pressure. The residue was re-dissolved in water (500 mL) and washed with diethyl ether (3 x 100 mL), and then extracted with CH2Cl2 (3 x 150 mL). The combined organic fractions were dried over Na2SO4 and evaporated to dryness to yield a yellow liquid (36.5 g, yield: 93%). 1H NMR (300 MHz, CDCl3, δ, ppm): 2.46 (s, 6H), 3.59-3.72 (m, 84H), 4.16 (t, 4H), 7.35 (d, 4H), 7.80 (d, 4H). 2.3 Synthesis of azido-terminated PEO (2) 61
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2.1 Materials 3-Aminophenylboronic acid (97%, Beijing Pure Chem. Co., Ltd.), fluorescein isothiocyanate (FITC, 95%, Alfa Aesar), αcyclodextrin (α-CD, 98%, TCI), N, N-diisopropyl-N-ethylamine (DIPEA, 98 %, Aldrich), triphenylphosphine (99%, Acros) and sodium azide (NaN3, 98%, Zhejiang Dongyang Kaiming Chem. Co.) were used as received. Poly (ethylene oxide) (OH-PEO-OH, Mn=1000, Polyscience) was dissolved in CH2Cl2, dried with MgSO4, recovered after removal of MgSO4 by filtration and removal of CH2Cl2 by evaporation. It was further dried under high vacuum at 45oC for 24 h. Tosyl chloride was recrystallized from petroleum ether and dried under vacuum. Triethylamine was refluxed with acetic anhydride, distilled, dried with CaH2 over night, and then redistilled. THF was refluxed with sodium for 8 h, and distilled. Acryloyl chloride was synthesized from the reaction between acrylic acid and benzoyl chloride.57 3-(Acrylamido) phenylboronic acid was obtained by the reaction of acryloyl chloride and 3-aminophenylboronic acid according to a literature
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A mixture of 1 (36.5 g, 27.9 mmol), NaN3 (5.85 g, 90 mmol) was stirred in DMF (350 mL) at 80 oC for 14 h. After removal of the insoluble salts by filtration, DMF was removed under reduced pressure. The crude product was then poured into water (400 mL) and extracted with CHCl3 (3 x 200 mL). After drying the combined organic fractions over Na2SO4 and removal of the solvent, a liquid product was obtained (25.5 g, yield: 91%).1H NMR (300 MHz, CDCl3, δ, ppm): 3.37 (t, 4H), 3.49-3.71 (m, 84H). 2.4 Synthesis of amino-terminated PEO (3) 61 A mixture of 2 (25.5 g, 25.5 mmol), triphenylphosphine (19 g, 72 mmol), H2O (1 mL) and THF (300 mL) was stirred at room temperature for 14 h. After most of the THF was removed under reduced pressure, a yellow liquid was obtained. Then, 100 mL of water was added, and the pH of the solution was adjusted to 1 with hydrochloric acid, the white precipitate formed was removed by filtration. The aqueous solution was washed with ethyl acetate (3 x 50 mL) and the aqueous phase was combined. NaOH was then added to the aqueous solution until an oil layer appeared. The solution was extracted with CH2Cl2 (3 x 150 mL), the combined organic fractions were dried with NaOH and Na2SO4 over night, evaporated to dryness to get a white solid (22 g, yield: 78 %). 1H NMR (400 MHz, CDCl3, δ, ppm): 2.89 (t, 4H), 3.55 (t, 4H), 3.60-3.66 (m, 80H). 2.5 Synthesis of PBA-terminated PEO crosslinker (4)
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A mixture of 3 (4 g, 4 mmol), 3-(acrylamino) phenylboronic acid (2.29 g, 12 mmol), DIPEA (1.55 g , 12 mmol) and methanol (30 mL) was degassed by two freeze/thaw circles, sealed under reduced pressure, and then heated at 50 oC for 72 h. The product was obtained after pouring the reaction mixture into diethyl ether, then it was dried under reduced pressure at 35 oC for 24 h to give a pale solid (4.52 g, yield: 82%). 1H NMR (400 MHz, D2O, NaOD, δ, ppm): 2.72-2.75 (m, 5.4H), 2.94 (t, 4H), 3.07-3.13 (m, 5.4H), 3.76-3.82 (m, 84H), 7.37-7.55 (m, 10.8H). 2.6 Preparation of hydrogels
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Scheme 1 Schematic representation of the hydrogel composed of PEO-bPVA, crosslinker and α-CD and its glucose-responsive mechanism.
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Stock aqueous solutions of PEO114-b-PVA339 (10 wt%), the crosslinker 4 (6 wt%) and α-CD (12 wt%) were prepared in 50 mmol phosphate buffer (pH=7.4). Then, a predetermined amount of polymer and α-CD solutions was mixed by shaking for 5 min., followed by the addition of a known amount of crosslinker solution. After slightly shaking, the mixture was kept at room This journal is © The Royal Society of Chemistry 2013
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drug delivery systems. In recent years, several stimuli-responsive hydrogel systems based on inclusion complexation between PEO and α-CD have been developed.55, 56 To the best of our knowledge, there has been no report of glucose-responsive hydrogel systems based on inclusion complexation and dynamic covalent chemistry. In this work, we describe a glucose-responsive hydrogel system based on dynamic covalent bonds and inclusion complexation. PEO-b-PVA, α-CD and PBA-terminated aminecontaining PEO crosslinker were used to construct the hydrogel system. The introduction of inclusion complexation between PEO and α-CD and the utilization of a secondary amine-containing crosslinker were intended to strengthen the hydrogel network. The amino groups in the PEO crosslinker can stabilize the complex between PVA and PBA at pH 7.4, and also enable a larger binding constant for PBA and glucose, thus endowing the hydrogel with tunable glucose- responsiveness at physiological pH. As depicted in Scheme 1, the hydrogel network arises from two cooperative driving forces: the dynamic covalent bonds between PVA and PBA, and the inclusion complexation between PEO and α-CD. The hydrogel structure can be disrupted in the presence of glucose and other competing polyols which can replace PVA from the dynamic complex structure and accelerate the hydrogel dissolution process. The effect of the compositions on the mechanical properties of the hydrogels was studied by rheological measurements. The glucose-responsiveness of the hydrogels was evaluated by protein loading and release experiments.
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DOI: 10.1039/C3SM53059K
temperature for 24 h before rheological measurements and other tests were made. The formulations of the gels are listed in Table 1.
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2.7 Rheological measurements
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Dynamic rheological measurements were carried out on a Physica MCR 301 Rheometer (Anton Paar) equipped with a parallel plate geometry (25 mm diameter) at a gap of 0.5 mm. The edge of the gel sample was covered with low viscous oil to prevent water evaporation during the measurements. After being loaded on the measuring plate, all the gel samples were allowed to stabilize for 40 min under a small stress (3-10 Pa) at a frequency of 0.1 Hz. Oscillatory frequency sweeps were performed from 100-0.1 rad/s at a constant strain of 0.1%. Oscillatory strain sweeps were performed from 0.01%-3000% at 1 rad/s. After a 3000 % deformation, gel samples were allowed to recover under a small stress (1-10 Pa) at 1 rad/s. 2.8 Wide-angle X-ray diffraction
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boronic acid group can stabilize the complex between PBA group and polyols at lower pH as widely accepted in literatures.22, 42 The 1 H NMR spectrum of the crosslinker is shown in Fig. 1. The spectrum is consistent with the proposed structure. The degree of substitution (the number of PBA per PEO chain) was determined from the integral ratio of the phenyl protons (peak e, 7.3-7.6 ppm) of PBA groups to (-CH2-CH2-O-) (peak n, 3.7-3.9 ppm) of PEO. The calculated value was 2.7, which suggested that some of the end amino groups added two PBA groups and made the PBA functionalized PEO as an effective crosslinker.
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Scheme 2 Synthetic routes of the crosslinker: a) tosyl chloride, triethylamine, r.t., 12 h. b) NaN3, DMF, 80 oC, 14 h. c) triphenylphosphine, THF, H2O, r.t., 12 h. d) 3-(acrylamino)phenylboronic acid, DIPEA, MeOH, 50 oC, 72 h.
XRD patterns of the lyophilized gel samples were recorded on a Rigaku DMAX-2400 power diffractmeter with Cu Kα (1.54) radiation (40 kV, 100 mA). Samples were scanned at a rate of 4 o /min over the range 2θ=3-60o at room temperature. 2.9 Morphology of the lyophilized hydrogels
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The morphology of the hydrogels was studied with a scanning electron microscope (FEI NanoSEM 430, Landing E: 10.0 keV). Pieces of the lyophilized hydrogel were carefully stuck onto an aluminium plate with double-sided adhesive before measurements. 2.10 Loading and release of protein
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Take the Gel 3 (Table 1) as an example, the protocol is as follows: α-CD solution (0.3 g) containing BSA-FITC (5 mg) was added to a 15 mL tube, a crosslinker solution (0.15 g) was then added to the tube soon after the addition of the polymer solution (0.15 g). The mixture was slightly shaken for 5 min. and then kept at room temperature for 24 h. Afterwards, phosphate buffer (10 mL, pH=7.4) was carefully added on top of the formed hydrogels and the tube was placed into a shaker. The release experiments were carried out at 25 oC at an oscillation frequency of 100 rad/min. At determined time intervals, 1.5 mL of the buffer was withdrawn to determine the content of the released FITC-labeled BSA by measuring the absorbance at 455 nm (ε=1326 L/g·cm) on a UVVis spectrophotometer. After each measurement, the withdrawn buffer solution was added back to the releasing system.
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Fig. 1 1NMR spectrum of crosslinker 4 in D2O (with NaOD).
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PEO114-b-PVA339 diblock copolymer, the PEO based crosslinker (4) and α-CD were used to construct hydrogels. The formulations are listed in Table 1. Table 1 Formulations of hydrogels based on PEO114-b-PVA339. Crosslinker 4 (% w/w)
α-CD (% w/w)
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3 Results and Discussion
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3.1 Synthesis of crosslinker
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The crosslinker 4 was synthesized in a four-step procedure starting from PEO as shown in Scheme 2. The overall yield is 30%. The first three steps are followed literature procedures, and the final step is the Michael addition reaction between the bisamino terminated PEO and 3-(acrylamido) phenyl boronic acid to render the final crosslinker with two secondary amines attached to a phenyl boronic acid respectively. We consider such a design has two advantages: first, PEO (Mn=1000) was used as the starting material, this will render the crosslinker water-soluble at neutral pH; second, the presence of secondary amine close to the
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Fig. 2 XRD patterns of lyophilized pure polymer, Gel 3, Gel 6 and Gel 10.
The viscoelastic properties of the gel samples were characterized by dynamic rheological measurements. The effect of compositions was first studied. Fig. 3 showed the dynamic frequency sweeps of Gel 3, Gel 6 and Gel 10. For all the three samples, G′ exceeded G′′ over the entire measured frequency and G′ was independent of angular frequency, which is indicative of elastic solid behaviour.50 Gel 10 with the highest α-CD content possessed the largest G′ and G′′, while Gel 3 with the lowest αCD possessed the minimum G′ and G′′. The higher α-CD content led to larger G′ and G′′, indicating that an increase in α-CD to EO ratio could increase the crosslinking density more effectively. The G′ values (at 68.7 rad/s) for Gel 3, Gel 6 and Gel 10 were 8.8, 38.9, 127 kPa, respectively, which are comparable with those of bone tissue engineering materials.64 It has been reported that hydrogels based on PVA and PBA-containing crosslinker do not behave as elastic solid and have an easily detectable relaxation time even at pH around PBA’s pKa.65 In our system, inclusion complexation between the PEO segments and α-CD strengthened the hydrogel network effectively to obtain solid-like hydrogels at neutral pH.
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Fig. 3 Dynamic storage modulus and loss modulus as a function of angular frequency at 25 oC for Gel 3, Gel 6 and Gel 10.
The influences of α-CD and crosslinker content on the hydrogel properties were further studied. Gel 7 possessed higher α-CD content than that of Gel 6, it had much higher G′ and G′′, as shown in Fig. 4A, indicating increased crosslinking density. Although the ratio of α-CD to EO had exceeded 1:2 for Gel 6, a stiffer and stronger hydrogel network could be obtained by increasing the α-CD content. However, different results were obtained for Gel 10 and Gel 11, as shown in Fig. 4B. They had similar G′ and G′′ even that Gel 11 possessed higher α-CD content. This is probably due to the high ratio of α-CD to EO in Gel 10 (0.85:1). Although further increasing the α-CD content could shorten the gelation time, it barely contributed to the crosslinking density of hydrogel networks. The effect of crosslinker content was shown in Fig. 4C. Gel 12 with higher crosslinker content possessed higher G′ and G′′ than Gel 10. This result implied that the elevated dynamic covalent interactions between PVA and PBA which resulted from increased crosslinker content could contribute to the hydrogel network significantly. Since inclusion complexation between PEO and α-CD was indispensable in our hydrogel system, we wanted to know whether the hydrogels displayed thixotropic property. This test has been carried out by Li et al.,63 and the process is described This journal is © The Royal Society of Chemistry 2013
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First we checked the solubility of the diblock copolymer PEO114-b-PVA339 in phosphate buffer solution, and found that even when the concentration reached 20 wt%, no gelation occurred. This is understandable, since both PEO and PVA are water soluble. Then, we studied the behaviour of mixture of two components at different concentrations: PEO114-b-PVA339 and crosslinker; PEO114-b-PVA339 and α-CD. In the first series (sample 2, 5, 9), the mixtures are also clear solution with no gel formation. In this system, the interaction of the crosslinker with PVA block exists, but the PEO block may hinder gel formation at such lower concentration. In the second series (sample 1, 4, 8), white precipitates are obtained after the mixtures being kept at room temperature for about 30 min. This is due to the formation of low soluble complex of PEO and α-CD.62 To our pleasure, we obtained opaque free-standing hydrogel for the mixture of the three components. This suggests that the coexistence of two driving forces, dynamic covalent bonds between PVA and PBA, and the inclusion complexation between PEO and α-CD is essential for gel formation at low concentrations. We then systematically investigated the influence of each component on the gelation time. The total amount of solute was fixed at 10 wt % for sample 3, 6, 10, with the weight ratio of the block copolymer and the crosslinker being 5:3. Gel 10 (αCD:EO= 1.14:1) with the highest -CD to polymer ratio had the shortest gelation time (4.5 h), while it took 12 h for Gel 3 (αCD:EO= 0.52:1) (the lowest -CD content) to form a gel. For Gel 6 (α-CD:EO= 0.85:1) with the medium -CD content, the gelation time is 5 h, in-between of the above two. If the -CD content was further increased to 8.5% (Gel 11), the gelation time could be shortened to 3 h as compared to Gel 10. In these three component solution system, two kinds of interactions exist, one is the dynamic covalent bond between PVA and the PBA group, and the other is the inclusion complexation between PEO and α-CD. At lower polymer concentration, neither of these two interactions is strong enough to ensure gel formation; they must work cooperatively for the hydrogel formation. Since the interaction of PVA and PBA has been confirmed in aqueous solution, we want to know whether inclusion complex of α-CD and PEO was formed during the gelation. Therefore, we prepared dried samples by freeze-drying the gels, and measured the XRD patterns of the three dried gels as shown in Fig. 2. We observed typical peaks at 2θ around 19.8o, indicating the formation of the hexagonal columnar channel-type crystallization which arose from the inclusion complexation between PEO and α-CD.62,63 In the control experiments for pure polymer or α-CD sample, such patterns could not be detected.
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structural recovery ability. Generally, our hydrogels showed remarkably better abilities to recover the networks when compared with hydrogels based on PEG5000 and α-CD which needed about 30 min to regain the solid-like behaviour.63 These results also showed that our hydrogel system retained thixotropic nature; the similar results have been reported by Li et al.63 and Dong et al.53 in the study of hydrogel systems based on inclusion complexation and another driving force (hydrophobic interactions and hydrogen bonding, respectively).
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The morphology of the hydrogels was studied with SEM on lyophilized hydrogel samples. As shown in Fig. 6, all the three lyophilized hydrogels possessed porous structures and the pore sizes were dependent on the crosslinking density which was determined by the gel compositions. Gel 10 had the smallest pore sizes among all the three gel samples which is consistent with its highest crosslinking density, while Gel 3 with the lowest crosslinking density showed the largest pore sizes. The porous structure could serve as channels in release experiments and the pore sizes may affect release kinetics.
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briefly as following: a gradually increased strain was imposed to all the three gel samples in order to deform the hydrogel networks. After the strain value reached 3000%, the high shear stress was removed and replaced by a small stress (≤ 10 Pa). As shown in Fig. 5A, all the three Gels showed good structural recovery ability which was characterized by G′ exceeding G′′ immediately after the removal of high shear stress. The recovery extent was assessed from the percentages of G′ values soon after the removal of high shear stress with respect to the G′ values from linear viscoelastic regime. As shown in Fig. 5B, the instantaneous recovery of G′ after yielding of Gel 3, Gel 6 and Gel 10 were 11.4%, 70.7%, and 93.5%, respectively. For Gel 10, the G′ was fully recovered in less than 5 min, while it took 15 min for Gel 6 to achieve one hundred percent recovery of G′. For Gel 3, the recovery percentage gradually increased to 20.2% after 60 min. restoration. Gels with higher α-CD content possessed better
Fig. 6 SEM images of Gel 3 (A), Gel 6 (B) and Gel 10 (C).
3.5 Loading and releasing of FITC- labeled BSA
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Fig. 7 Release profiles of BSA-FITC from (A) Gel 6 and (B) Gel 3 at 25 oC in 50 mmol phosphate buffer (pH=7.4) containing 0 g/L glucose , 30 g/L glucose and 10 g/L fructose respectively.
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For Gel 6, the release of BSA was accelerated in the presence of 10 g/L fructose; however, the effect of glucose on the release kinetics was negligible even when the glucose concentration reached 30 g/L. For Gel 3, the release of BSA was in a faster manner in the presence of 30 g/L glucose, indicating that Gel 3 possessed better glucose-responsiveness, and the release rate of protein from Gel 3 was higher than that from Gel 6 in all the three cases. The portion of α-CD of Gel 6 was higher than that of Gel 3, which resulted in higher hydrophobicity of the complex, higher crosslinking density and smaller pore size for Gel 6. We speculated that the enhanced hydrophobicity and crosslinking density could probably retard the diffusion of saccharides and dissolution of hydrogel networks, therefore leading to a lower protein release rate. The hydrogel system was responsive to glucose and fructose under physiological pH, however, the glucose concentration was much higher than normal blood concentration. There may be two reasons: (1) the inclusion complex of PEO and α-CD was relatively hydrophobic, which may hinder the diffusion of saccharide molecule into the hydrogel; (2) in the release experiments, placing the hydrogels at the bottom of the tube led to limited contact area with solution. 4 Conclusions
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We prepared a series of hydrogels based on biocompatible materials, PEO-b-PVA diblock polymer, PBA modified PEO and α-CD. The gelation process was driven by two forces, the inclusion complexation and the dynamic covalent bonds. With the assistance of amino groups from the crosslinker and inclusion complexation between PEO and α-CD, a series of stiff hydrogels with good structural recovery abilities was obtained. The introduction of dynamic covalent chemistry provided the hydrogels with glucose-responsive characteristics. Protein loading could be achieved in the gelation process, and the release manner was responsive to glucose and fructose under physiological pH. The hydrogel system may have the potential to be applied as glucose-responsive materials for insulin delivery. Extending the hydrogel system to microgel systems by miniaturizing their size and increasing the portion of PVA and PBA in the hydrogel system may be two effective ways to optimize the glucose-responsive properties.
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This work was financially supported by the National Natural Science Foundation of China (No. 20534010, 50973002).
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Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Fax: +86-10-62751708; Tel: +86-10-62755543; E-mail: [email protected]
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TOC (SM-ART-12-2013-053059)
Ting Yang, Ran Ji, Xin-Xing Deng, Fu-Sheng Du and Zi-Chen Li*
A novel glucose-responsive hydrogel system based on dynamic covalent chemistry and inclusion complexation was described.
Soft Matter Accepted Manuscript
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Glucose-responsive Hydrogels Based on Dynamic Covalent Chemistry and Inclusion Complexation
