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High strength dual-crosslinked hydrogels with photo-switchable color changing behavior
Accepted Manuscript High Strength Dual-crosslinked Hydrogels with Photo-switchable Color Changing Behavior Beibei Wang, Xiaozhen Xiao, Yuhuan Zhang, Liqiong Liao PII: DOI: Reference:
S0014-3057(19)30355-6 https://doi.org/10.1016/j.eurpolymj.2019.04.035 EPJ 8999
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
22 February 2019 12 April 2019 17 April 2019
Please cite this article as: Wang, B., Xiao, X., Zhang, Y., Liao, L., High Strength Dual-crosslinked Hydrogels with Photo-switchable Color Changing Behavior, European Polymer Journal (2019), doi: https://doi.org/10.1016/ j.eurpolymj.2019.04.035
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High Strength Dual-crosslinked Hydrogels with Photo-switchable Color Changing Behavior Beibei Wanga, Xiaozhen Xiaob, Yuhuan Zhangc, Liqiong Liaob,* a
College of Chemistry and Molecular Science, Wuhan University, Wuhan, Hubei,
China. b
Guangdong Provincial Key Laboratory of Construction and Detection in Tissue
Engineering, Biomaterials Research Center, School of Biomedical Engineering, Southern Medical University, Guangzhou, Guangdong, China c
Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University,
Chongqing, China. * Correspondence should be addressed to: Prof. Liqiong Liao (Ph.D), E-mail: [email protected]
1
Abstract Herein, high strength dual-crosslinked hydrogels with photo-switchable color changing behavior were designed and fabricated. In the mixture solution of α-cyclodextrin (α-CD), α-cyclodextrin-spiropyran (α-CD-SP), and poly(ethylene glycol) diacrylate (PEGDA), the PEGDA chains first threaded into the cavity of α-CD/α-CD-SP and form inclusion complexes to generate the physical gel; then, the as-prepared physical gels were subjected to photopolymerization to form a covalently crosslinked network. With the contribution of both no-covalent and covalent bond, the dual-crosslinked hydrogels possess high tensile strength, which is about fifteen times that of the pure PEG hydrogel. And due to the sacrifice of no-covalent bond, hydrogels showed high energy dissipation ratio, up to 75.3% and high fatigue resistance during loading-unloading tests. While the ingenious introduction of SP moieties, enables the as-prepared gels photo-switchable spatiotemporal color changing behavior reversibly. Erasable patterns on hydrogels could be written using UV light and erased by Vis light for multiple cycles. This flexible hydrogel with good physical performance shows high potential in photochromic flexible wearable devices. Keyword: Hydrogel; dual-crosslinked; high strength; spiropyran
2
Introduction Flexible and wearable devices, such as supercapacitors1, 2, electronic skin3, force4 and strain5 sensors, health monitor6, light detector7,8, have attracted extensive attentions for their unique advantages of portability, sensitivity, and real-time. In the fabrication/construction of these devices, stimuli-responsive hydrogels with appreciable mechanical properties (tensile strength ranged from tens of KPa to tens of MPa)9 would be ideal candidate materials. Color is an important feature of materials, and the color changing of materials in respond to external stimuli (such as pH, temperature or light) may be harnessed in camouflage, display/storage devices10, sensors11, and packaging materials12. In particular, hydrogel possessing such color-change capability are highly desirable in the flexible wearable devices. One of the efficient strategies to prepare hydrogels with color changing property is to incorporate stimuli responsive moiety into the hydrogel backbone. For instant, hydrogel incorporated with indigo dye showed color changing behavior in the presence of specific enzyme13, or hydrogel consisting of anthocyanin moiety could change its color under different external pH12. Recently, interests have been aroused in the field of photo-switchable color changing hydrogel in response to light irradiation, which is characterized by unique advantages of real-time spatiotemporal precision14 and flexible adjustability of wavelength15. Various photo-chromic dyes such as azobenzenes16, 17, spiropyrans, diarylethenes, and fulgides have been utilized in the constructing of photo-responsive color changing hydrogels18,19. 3
As one of the extensively studied photochromic dyes, spiropyran (SP), undergoes isomerization between the closed-ring colorless SP form and the open-ring colored merocyanine (MC) form which possess different physicochemical properties, and has gained specific attentions in the construction of color changing hydrogels owing to its high fatigue resistant and multi-stimuli responsive properties20. Generally, the SP moiety can be introduced into the hydrogel network by copolymerization of water insoluble SP derivatives with water soluble monomer in organic/aqueous mixture solutions21, 22. Several SP hydrogels have been prepared through this strategy, such as the
poly(N-isopropylacrylamide)
hydrogel23,
(PNIPAM)-SP
poly(2-vinyl-4,6-diamino-1,3,5-triazine)
(PVDT)-SP
hydrogel
24
,
and
poly(acrylamide-co-methyl acrylate) (PAA-PMA)-SP hydrogels25. However, most hydrogels are characterized by inferior mechanical properties, which hinder their application in many fields including flexible and wearable devices. To address this concern, efforts have been devoted to construct SP hydrogels with high mechanical strength. Wang et al. prepared a SP hydrogel through the copolymerization of acrylamide (AAm, hydrophilic hydrogen bonding monomer), 2-vinyl-4,6-diamino-1,3,5-triazine (VDT, hydrophobic hydrogen bonding monomer), and SP containing monomer (SPAA) in the presence of poly(ethylene glycol) (PEG) diacrylate (Mn = 575 g/mol) as the crosslinker. Due to the double hydrogen bondings from AAm-AAm and diaminotriazine-diaminotriazine, the hydrogels possessed high tensile
strength
(around
250
KPa)26.
Chen
et
al.
constructed
a
polyacrylamide-co-methyl acrylate/spiropyran (SP) hydrogel with a high tensile 4
strength of 1.5 MPa, the hydrogels exhibited color change from yellow to purple upon force, UV light, and heat stimuli and color reversion from purple to yellow upon white light25. Although several high strength SP hydrogels have been constructed, it is still attractive and challenging to afford a facile method to prepare high strength SP hydrogels with excellent photo-switchable color changing property. Several strategies have been very successful in the fabrication of high strength hydrogels, for example, the topological hydrogel (slide-ring hydrogel)27, double network hydrogel28, nanocomposite hydrogel29, and dual-crosslinked hydrogel30. Among them, dual-crosslinked hydrogel combines the advantages of physical and chemical crosslinking and could achieve enhanced toughness/strength; particularly, the physical crosslinking, which usually utilizes reversible weak interaction such as hydrogen bond31, hydrophobic interaction32, and Van der Waals force33, could dissipate the stress energy effectively and consequently enhance the toughness of the hydrogels34. Various dual-crosslinked hydrogel with high strength (tensile fracture stress: 0.3-10 MPa), self-healing properties, as well as shape-memory behavior have been reported35, 36. Different approaches have been used to construct dual-crosslinked hydrogels and a facile method is through the host-guest interaction between cyclodextrins (CDs) and guest molecules. For instance, physical hydrogels could be prepared from CDs and high molecular diacrylated poly(ethylene glycol) (PEGDA) trough host-guest interaction37, 38, after that, dual-crosslinked hydrogel could form through the chemical crosslinking of the PEG chains33. In this work, host-guest interaction was used to introduce SP moiety into a 5
dual-crosslinked hydrogel; briefly, α-CD and SP-modified α-CD (α-CD-SP) were first mixed with PEGDA to produce physical hydrogel through the host-guest interaction; subsequently, a robust dual-crosslinked gel can be produced by photopolymerization of the PEGDA. With the aid of photomask, different patterns can be “drawn” or “erased” on the hydrogels under specific light irradiation, revealing the potential of this hydrogel in the fabrication of wearable devices with photo-switchable color changing property (Scheme 1).
Scheme 1. Schematic illustration of the preparation of dual-crosslinked hydrogel with photo-switchable color changing behavior.
Experimental section Materials 3-Iodopropionic acid (99%, J&K Chemicals, China), 2,3,3-trimethylindolenine (98%, Energy Chemical, China), 5-nitrosalicylaldehyde (98%, Energy Chemical, China), α-CD (98%, Aladdin, China), N,N′-dicyclohexylcarboiimide (DCC, 95%, Sinopharm, 6
China), 4-(dimethylamino) pyridine (DMAP, 98%, Sinopharm, China), piperidine (98%, Sinopharm, China), PEG (20000 g/mol, Aladdin, China), acryloyl chloride (96%, Aladdin, China), and Irgacure-2959 (98%, Sigma, USA) were of analytical and used as received. All the solvents were of analytical grade and used without purification unless stated elsewhere. Synthesis of poly(ethylene glycol) diacrylate (PEGDA) PEGDA was synthesized according to the literature39. Proton nuclear magnetic resonance spectroscopy (1H NMR) was used to characterize the chemical structure of the product (Fig S1, ESI†). 1H NMR (400 MHz, CDCl3): δ 6.32 (dd, J = 17.3, 1.0 Hz, 2H), 6.09 (dd, J = 17.3, 10.5 Hz, 2H), 5.87 (dd, J = 10.5, 1.0 Hz, 2H), 4.21 (m, 4H), 3.57 (m, 2019H). The substitution degree (DS) of the acrylate groups was 90%, which was calculated though the ratio of relative peak integral value of the acrylate and methylene group. Preparation
of
l′-(β-Carboxyethyl)-3′,3′-dimethyl-6-nitrosipiro(indoline-2′,2–chromane) (SPCOOH) SPCOOH was synthesized according to the literature40 (71.6% yield). The 1H NMR spectrum of the product is shown in Figure S2 (ESI†). 1H NMR (400 MHz, DMSO-d6): δ 12.24 (s, 1H), 8.22 (d, J = 2.8 Hz, 1H), 8.00 (dd, J = 9.0, 2.8 Hz, 1H), 7.21 (d, J = 10.4 Hz, 1H), 7.17-7.08 (m, 2H), 6.87 (d, J = 9.0 Hz, 1H), 6.83-6.76 (m, 1H), 6.70-6.63 (m, 1H), 6.00 (d, J = 10.4 Hz, 1H), 3.49 (m, 1H), 3.39 (m, 1H), 2.62-2.54 (m, 1H), 2.48-2.40 (m, 1H), 1.16 (s, 3H), 1.07 (s, 3H). 7
Synthesis of α-CD-SP α-CD-SP was synthesized by esterification of SPCOOH and α-CD (Scheme S1, ESI†)41, 42. SPCOOH (200 mg, 0.53 mmol) was added to anhydrous DMF (30 mL) in the presence of DCC (109.4 mg, 0.53 mmol) and DMAP (6.5 mg, 0.053 mmol). The solution was stirred at room temperature for 30 minutes and then α-CD (250 mg, 0.26 mmol) was added and stirred at room temperature for 24 h. After that, the solvent was evaporated under reduced pressure and the solid was washed by acetone and ethanol. The solid product was collected and dried under vacuum (173.6g, 50% yield). The product was characterized by 1H NMR,
13
C NMR, and HRMS (Figure S2, S3, and
S4, ESI†). 1H NMR (400 MHz, DMSO-d6): δ 8.24-8.17 (m, 1H), 8.04-7.97 (m, 1H), 7.15 (m, 3H), 6.89-6.76 (m, 2H), 6.64 (m, 1H), 5.98 (dd, J = 10.4, 2.6 Hz, 1H), 5.64-5.34 (m, 12H), 4.78 (d, J = 3 Hz, 6H), 4.48 (t, J = 5.7 Hz, 5H), 3.94-3.54 (m, 24H), 3.52-3.45 (m, 2H), 3.45-3.37 (m, 6H), 3.29-3.24 (m, 6H), 2.71-2.53 (m, 2H), 1.17 (s, 3H), 1.06 (s, 3H).
13
C NMR (400 MHz, DMSO-d6): δ 171.3, 159.1, 156.7,
153.3, 146.1, 140.6, 135.7, 128.2, 127.7, 125.7, 122.8, 121.8, 119.4, 118.9, 115.5, 106.6, 101.9 (C1 of α-CD), 82.1 (C4 of α-CD), 73.3 (C5 of α-CD), 72.1 (C2 and C3 of α-CD), 60.0 (C6 of α-CD), 52.5, 33.4, 25.3, 24.5, 19.5. HRMS (ESI) m/z calcd. for C57H79N2O34 (M+H)+ 1335.4509, found 1335.4506. It was estimated that one spiropyran molecule was attached to per α-CD molecule by 1H NMR and HRMS. Preparation of dual-crosslinked hydrogels 1 mL of 20% (wt/vol) PEGDA aqueous solution was mixed with 1 mL of α-CD aqueous solution with varied concentration (120, 160, 200, and 240 mg/mL). Then, 8
200 μL of 0.5% (wt/vol) Irgacure-2959 aqueous solution was added to this mixture solution. The solution was sonicated for one minute and stored at 4 oC for 5 minutes to allow the formation of the inclusion complex/physical gel. Subsequently, the obtained physical hydrogel was exposed to 365 nm UV irradiation for 5 minutes to produce the dual-crosslinked hydrogel. The as-prepared gel was named as Gel x, where x is the mass of α-CD per 100 μL of 10% (wt/vol) PEGDA aqueous solution. PEG hydrogels were prepared by the photopolymerization of a mixture solution containing 100 μL of 10% (wt/vol) PEGDA aqueous solution and 10 μL of 0.5% (wt/vol) Irgacure-2959 aqueous solution under 365 nm UV irradiation for 5 minutes. Similarly, PEGDA/α-CD/α-CD-SP dual-crosslinked hydrogels were prepared. The precursors’ solution was obtained by mixing 0.6 mL of 33.3% (wt/vol) PEGDA aqueous solution, 0.4 mL of α-CD-SP (4 mg, 8 mg, 12 mg, 16 mg and 20 mg) aqueous solution, 1 mL of 240 mg α-CD aqueous solution, and 200 μL 0.5% (wt/vol) Irgacure-2959 photoinitiator aqueous solution. The prepared gel was named as Gel 12-y, where y is the mass of α-CD-SP per 12 mg of α-CD and 100 μL of 10% (wt/vol) PEGDA aqueous solution. Nuclear magnetic resonance spectroscopy (NMR) NMR spectra were acquired by Bruker Avance ċ HD 400 MHz spectrometer at room temperature with 10 mg (30 mg for 13C NMR spectra) simple dissolved in 600 μL deuterium generation reagent. Solvent peaks in 1H NMR spectra were referenced to δH 7.26 ppm for CDCl3, δH 2.5 ppm for DMSO-d6, and in 13C NMR spectra was referenced to δC 39.5 ppm for DMSO-d6. 9
High resolution mass spectroscopy (HRMS) HRMS (ESI) were obtained using positive or negative ionization mode by Thermo Scientific™ Orbitrap Fusion™ Tribrid™ mass spectrometer. X-ray powder diffraction X-ray powder diffraction (XRD) patterns were collected on a Bruker D8 Advance equipment (BRUKER, Germany) under the following conditions: a voltage of 40 kV, a current of 40 mA, a scanning speed of 5◦/min, and a scan range of 2θ from 4◦ to 40◦ was applied. The sample of inclusion complexes was prepared as follow: PEGDA (20 mg) and α-CD-SP (20 mg) were dissolved in water, after the precipitate formed, filtrated and dried, the sample of inclusion complexes was obtained. Rheology analysis Rheometer (HAAKE MARS III) was employed to study gelation process of the physical gel. 400 μL of the precursors’ solution was measured on a 20 mm parallel plate. Time sweep tests were performed at temperature of 4 °C. A strain of 1% and a frequency of 1 Hz were applied to the measurement. Morphologies The interior morphologies of the hydrogels were observed by scanning electron microscopy (SEM, Hitachi S-3000N, Japan). The swollen hydrogels were frozen in liquid nitrogen and then lyophilized for 48 h, and sputter-coated with gold before observation. Equilibrium swelling ratios The equilibrium swelling ratios (ESR) of the hydrogels were measured at room 10
temperature. The as-prepared hydrogels were immersed in deionized water to remove the unreacted precursors and dried to a constant weight (mdry). Then the hydrogels were immersed in deionized water until they reached swelling equilibrium. Then the fully swollen hydrogels were wiped using filter paper to remove water on the surface and weighted (mwet). ESR is defined as: ESR = (mwet - mdry)/mdry Triplicate samples were measured for each point and the results were presented as mean ± SD. Tensile test The tensile properties of the hydrogels were measured on Material Testing System Model 5967 (LLOYD, Britain) at room temperature. The tensile strength and elongation at break are obtained directly by material testing machine and Young's modulus is derived from the slope of first 15% of stress-strain curve. The hydrogel strips were prepared in a 20 mm (length) ×6 mm (width) ×1 mm (thickness) mold and used directly for the tensile test. The rate was set at 10 mm/min for the tensile test. Triplicate samples were measured for each point and the results were presented as mean ± SD. Hysteresis was observed by performing the tensile loading-unloading cycle in which the sample was initially stretched to a predetermined strain and immediately unloaded at a fixed velocity (10 mm/min). The total toughness was obtained by integrating the area under the stress-strain curve, and the dissipated toughness was calculated from the internal area of hysteresis loops. Photo-isomerization 11
The photo-isomerization of the spiropyran moiety was characterized by a UV-Vis spectrophotometer (UV3600 230 VCE, Japan). The hydrogel films were prepared in a 30 mm × 10 mm × 0.3 mm mold and immersed in deionized water for remove the unreacted precursors. The sample was first exposed to visible light irradiation for 3 min, the UV-Vis spectrum of the sample was collected. And the sample was irradiated by UV light for different time intervals and the corresponding UV-Vis spectra of the sample were collected. Then the sample was irradiated by visible light for different time intervals and the corresponding UV-Vis spectra of the sample were also collected.
Results and Discussions Preparation of dual-crosslinked hydrogels After α-CD, α-CD-SP and PEGDA were mixed in solution, the PEGDA chains are threaded into the cavity of α-CD and, α-CD-SP, the CD rings on PEG chains aggregate through intermolecular hydrogen bonding between them to form α-CD-PEG inclusion complexes; then the α-CD/PEG inclusion complexes form aggregates through the hydrophobic interaction, which acted as physical cross-linking domains of the gel38. The formation of inclusion complex between PEGDA and α-CD-SP was confirmed by powder XRD patterns (Figure 1). It is clear that the diffractogram of the PEGDA/α-CD-SP inclusion complexes was different from the pattern of the mixture, which is the superposition of the individual patterns of PEGDA and α-CD-SP. The disappearance of the characteristic peaks of PEGDA at 19.2o and 23.4o and the appearance of a new peak appeared at 18.2o, which was a characteristic 12
peak for the channel-type of α-CD inclusion complexes43, strongly indicated the formation of PEGDA/α-CD-SP inclusion complexes.
Figure 1. Powder X-ray diffraction patterns of PEGDA, α-CD-SP, the PEGDA/α-CD-SP mixture, and PEGDA/α-CD-SP inclusion complexes. Physical gels could be obtained by simply tuning the concentrations of α-CD/α-CD-SP and PEGDA in the solution (Table 1). Since spiropyran itself is insoluble in aqueous solution, it is difficult to prepare hydrogel containing SP moiety without the use of water/organic mixture solvent21. In this work, the grafting of SP onto the CD ring facilitated the dissolution of SP moiety and physical gels could be fabricated in aqueous, avoiding the utilization of organic solvent and post-purification and may benefit possible biomedical applications. Upon the addition of α-CD or α-CD/α-CD-SP (Figure 2a, d) was to the PEGDA solution, the solution became turbid (Figure 2b, e) but still was of relatively low viscosity, indicating the formation of inclusion complexes; and the turbid solution became semi-solid after being stored at 4 oC for 5 minutes, suggesting a sol-gel transition owing to the formation of 13
aggregates (Figure 2c, f)11. Table 1. Preparation and equilibrium swelling ratios of dual-crosslinked hydrogels. Molar ratio of Equilibrium α-CDa α-CD-SPa Irgacure-2959a Hydrogels α-CD/α-CD-SP/ swelling (mg) (mg) (mg) PEGDA ratios Gel 6 6 0 12.3/0/1 0.05 10.7 ± 0.1 Gel 8 8 0 16.4/0/1 0.05 8.1 ± 0.3 Gel 10 10 0 20.6/0/1 0.05 7.2 ± 0.4 Gel 12 12 0 24.7/0/1 0.05 6.0 ± 0.1 Gel 12-0.2 12 0.2 24.7/0.3/1 0.05 6.5 ± 0.4 Gel 12-0.4 12 0.4 24.7/0.6/1 0.05 6.9 ± 0.1 Gel 12-0.6 12 0.6 24.7/0.9/1 0.05 6.5 ± 0.3 Gel 12-0.8 12 0.8 24.7/1.2/1 0.05 6.3 ± 0.4 Gel 12-1 12 1 24.7/1.5/1 0.05 6.1 ± 0.2 a mass of per 100 μL 10% (wt/vol) PEGDA aqueous solution.
Figure 2. Photographs of the formation process of the physical hydrogels: (a, b, and c) the photographs of the mixture of PEGDA/α-CD (Gel 12) at different time; (d, e, and f) the photographs of the mixture of PEGDA/α-CD/α-CD-SP (Gel 12-1) at different time. The gelation kinetics of the physical gels was further investigated by the rheological analysis (Figure 3). It was found in all cases that the storage modulus (G′) and loss modulus (G′′) increased slightly and plateaued off corresponding to the formation of α-CD/PEG inclusion complexes; then the G′ and G′′ underwent rapid 14
increase and reached to another plateau, which can be attributed to the sol-gel transition44. Further increase of the concentrations of α-CD could reduce the time for the moduli of the precursors to reach constant, i.e., the gelation time. The gelation time of Gel 8, Gel 10, and Gel 12 was 1000 s, 500 s, and 300 s, respectively, which decreased gradually with the increase of the α-CD concentrations. In addition, the final moduli of the physical hydrogels increased from 104 to 105 Pa, as the concentrations of α-CD increased from 80 (Gel 8) to 120 mg/mL (Gel 12). These results can be attributed to the higher number of physical crosslinks at higher concentration of α-CD. However, the addition of α-CD-SP showed a negligible effect on the formation of the α-CD-PEG inclusion complexes. Compared with Gel 12 (Figure 3c), Gel 12-1 needs a longer gelation time, which might be explained by the weakening of the hydrogen bonds between the α-CDs in the presence of SP moiety.
Figure 3. Storage (G′) and loss (G′′) moduli of the precursors of (a) Gel 8, (b) Gel 10, 15
(c) Gel 12, and (d) Gel 12-1 versus time at 4 °C. After the formation of inclusion complex, the physical gel was further subjected to photopolymerization of the PEGDA to obtain the dual-crosslinked hydrogel. By tuning the concentrations of α-CD and α-CD-SP, dual-crosslinked hydrogels with different ESRs were prepared (Table 1). The ESRs of the hydrogels decreased gradually with the increase of the concentrations of α-CD, for instant, the ESRs of the hydrogel range from 10.7 ± 0.1 (Gel 6) to 6.0 ± 0.1 (Gel 12). The addition of α-CD resulted in higher cross-link density, leading to lower swelling ratio. While for Gel 12-0.4 to Gel 12-1, the ESR decreased slightly with the increase of the concentrations of α-CD-SP. Compared with Gel 12, the Gel 12-y with the addition of α-CD-SP showed higher ESRs. Since the PEGDA chains were packed closely by α-CDs aggregates and then fixed by chemical crosslinks, the Gel 12 possessed relatively dense porous structure and small pore size. For example, the pore size of Gel 12 is about 4 μm, and the pore sizes of Gel 12-0.4, Gel 12-0.6 and Gel 12-1 are around 6-10 μm, indicating that the addition of α-CD-SP enlarged the pore size of the hydrogel (Figure 4).
16
Figure 4. SEM images of morphologies of freeze-dried hydrogels (a) Gel 12, (b) Gel 12-0.4, (c) Gel 12-0.6, and (d) Gel 12-1 (Scale: 20 μm). Tensile properties The tensile properties of the dual-crosslinked hydrogels were examined (Table 2). Compared with the pure PEG hydrogel, the tensile strength as well as Young’s modulus and the elongation at break of the dual-crosslinked hydrogels was improved obviously. The PEGDA/α-CD/α-CD-SP (Gel 12-y series) hydrogels exhibited good mechanical properties with tensile strength of 440-520 KPa, Young’s modulus of 360-380 KPa, and elongation at break of 330-550% (Table 2). The gels could withstand large deformation without breaking after stretching, twisting, and knotting (Figure 5a, b), and also possessed good load-bearing capacity (Figure 5c). Table 2. Tensile properties of the dual-crosslinked hydrogels. Hydrogel Tensile strength Young’s modulus Elongation at (KPa) (KPa) break (%)
17
Gel 6 Gel 8 Gel 10 Gel 12 Gel 12-0.2 Gel 12-0.4 Gel 12-0.6 Gel 12-0.8 Gel 12-1 PEG
247.43 ± 0.35 307.92 ± 3.95 438.83 ± 14.88 458.08 ± 17.83 519.49 ± 11.59 491.28 ± 12.76 466.61 ± 4.19 441.73 ± 9.57 439.11 ± 13.95 29.60 ± 2.11
93.48 ± 11.99 188.87 ± 15.03 258.29 ± 38.39 343.18 ± 21.22 364.37 ± 39.47 385.38 ± 32.17 372.30 ± 23.81 374.91 ± 27.70 366.21 ± 17.73 12.51 ± 0.80
543.87 ± 80.07 402.19 ± 13.89 481.81 ± 25.60 402.94 ± 19.00 554.28 ± 21.08 442.26 ± 13.36 439.75 ± 11.68 359.87 ± 21.39 334.00 ± 10.04 333.98 ± 20.56
Figure 5. (a), (b), and (c) Photographs of Gel 12-1 undergoing stretching, twisting, knotting, and load-bearing; (d) Loading-unloading tests of gels at 300% of strain; (e) The calculated total toughness and the dissipated toughness of gels during the loading-unloading tests at 300% of strain; (f) Loading-unloading tests of Gel 12-1 18
at different strains (100%, 200%, and 300%); (g) The calculated total toughness and the dissipated toughness of Gel 12-1 during the loading-unloading tests at different strains; (h) Five successive loading-unloading cycles of Gel 12-1. The mechanical properties of the dual-crosslinked hydrogels could be tuned by the concentrations of α-CDs at fixed concentration of PEGDA. As for the PEGDA/α-CD hydrogel, the tensile strength of the hydrogel increased with the concentrations of α-CD. For example, the tensile strength of Gel 6 was 247 KPa and increased to 458 KPa for Gel 12 at higher concentration of α-CD. At the same time, the Young’s modulus showed similar trend as well. The results might be attributed to the higher number of physical crosslinks, i.e. higher crosslink density, formed at higher concentration of α-CD, which could not only contribute to the enhanced modulus but also help dissipate the energy and thus strengthen the hydrogels. Unlike α-CDs, α-CD-SP had a different effect on the mechanical properties of the PEGDA/α-CD/α-CD-SP hydrogel. When small amount (0.2 mg per 100 μL) of α-CD-SP was added, the tensile strengths of the PEGDA/α-CD/α-CD-SP hydrogel were improved as compared with the PEGDA/α-CD hydrogel, for instance, the tensile strength of Gel 12-0.2 were higher than that of Gel 12. However, further increasing the concentration of α-CD-SP led to gradual decrease of the tensile strength of the hydrogels. The lower tensile strength of the hydrogels at higher α-CD-SP concentration could be explained by the deteriorated hydrogen bonding between the CDs in the presence of hydrophobic SP moiety with increasing concentration. In such dual-crosslinked hydrogel, it is generally believed that the presence of 19
physical crosslinks could contribute to the relaxation of locally applied stress and dissipation of the crack energy45. The cyclic loading-unloading tests of the as-prepared gels were measured to evaluate their energy dissipation (Figure 5). In the loading-unloading curves of different dual-crosslinked gels and PEG hydrogel at 300% strains (Figure 5d), characteristic hysteresis loops were recorded, indicating the dual-crosslinked hydrogel could dissipate energy effectively30. As for the pure PEG gel with only chemical crosslinks, no hysteresis loops were presented in the curve and the hydrogel experienced approximately elastic deformation (Figure 5d). These results demonstrated that the energy dissipation process of the dual-crosslinked hydrogels was related directly to the physical crosslinks. The dissipated energy of hydrogels was evaluated as a function of concentrations of α-CD (Figure 5e). With the increase of the concentrations of α-CDs, the dissipated energy, the total energy, as well as the ratio of the dissipated energy to the total energy increased. From Gel 6 to Gel 12, the ratio of the dissipated energy increased from 61.2%, 71.2%, and 72.2% to 78.7%, respectively. Since the chemical crosslinking points were presumably the same for the tested hydrogels, the increase in the dissipated/total energy could largely be attributed to the higher number of physical crosslinks at higher concentration of α-CDs. This result further demonstrated that the physical crosslinks played a vital role during the energy dissipation process and thus improvement of the mechanical properties of the gels. In the presence of α-CD-SP, similar energy dissipation was observed (Figure 5f, g). For example, the dissipated energy of Gel 12-1 was about 509.1 KJ/m3 at the strain 20
of 300%, achieving 75% of the total energy. The dissipated energy of the Gel 12-1 was lower than that of Gel 12, resulting in lower tensile strength of the Gel 12-1 hydrogel. When the gel was stretched, the weak physical crosslinks ruptured first to dissipate energy, followed by the rupture of the strong chemical crosslinks to resist high levels of deformation46. At higher strain, more physical crosslinks were sacrificed, leading to higher dissipate energy and energy dissipation ratio. From 100% to 300% strain, the dissipated energy of the gels increased, and the energy dissipation ratio also increased from 68.6% to 75.2%. Good fatigue resistance property was also observed with the hydrogel containing SP moiety (Figure 5h). For example, the hysteresis loop of Gel 12-1 decreased dramatically after the first cyclic tensile test, and the dissipated energy decreased from 509.1 KJ/m3 (1st) to 53.8 KJ/m3 (2nd). The significantly lower dissipated energy at 2nd circle could be mainly attributed to the irreversible fracture of the chemical crosslinks at high strain (300%). In the following circles (3rd to 5th), the dissipated energy mainly originated from the physical crosslinks, which almost kept constant. The 2nd to 5th curves were almost identical with no obvious decrease in the tensile strength at 300% strain, indicating the gels possess good fatigue resistance47. Flexibility, high strength and good fatigue resistance, these results present that ability of this dual-crosslinked hydrogel as wearable devices. Photo-switchable color changing behavior of the hydrogels SP could exist in two stable isomerization structures: the closed-ring SP form and the open-ring MC form. When SP is exposed to UV light, SP can be isomerized into 21
the colored, hydrophilic MC; irradiated by Vis light, the MC can be isomerized into the colorless, hydrophobic SP48. The isomerization of the SPoMC and MCoSP can be detected by the appearance and disappearance of the adsorption peak (at around 540 nm) of MC in the UV-Vis spectrum. When Gel 12-1 was exposed to 365 nm UV light for one minute, a new absorbance peak appeared at 540 nm, indicating the SP form was gradually converted into MC form49 (Figure 6a). At about five minutes’ UV irradiation, the isomerization of the SPoMC completed as the intensity of 540 nm reached maximum. Then the Gel 12-1 was exposed to visible light, the peak at 540 nm disappeared within two minutes (Figure 6b). In practice, the reversibility of the photo-isomerization of the hydrogel is of essential importance. The hydrogel was irradiated with alternative UV light and Vis light to evaluate its photo-switchable reversibility. When the Gel 12-1 was irradiated by Vis light for two minutes, the absorbance intensity at 540 nm decreased resulting from the photo-isomerization from MC to SP isomer. Following with subsequent five minutes of UV irradiation, the absorbance intensity at 540 nm increased owing to photo-isomerization from SP to MC. After that, another seven Vis-UV irradiations circles were carried out on the same sample. First, the absorbance intensity (at 540 nm) decreased in the following two UV/Vis irradiations; then the absorbance intensity (at 540 nm) showed a slightly decrease for the left five UV/Vis irradiations. The reduction of absorbance intensity could be attributed to the gradual photo-degradation of the spiropyran moiety, which may be caused by irreversible oxidation20 or aggregation of MC isomer50. Even so, the above results demonstrated that the Gel 22
12-1 possessed impressive photo-switchable reversibility and is an idealized photo-switchable color changing material (Figure 6c). As a unique stimulus, light can achieve real-time spatiotemporal precision. In this case, the spatiotemporal photo-switchable color changing behavior of the hydrogel was further investigated. The photographs of the Gel 12-1 upon Vis-UV light irradiation through photomask were presented in Figure 6d-6f. When the swollen hydrogel was irradiated by UV light for 1 min through a “butterfly” photomask, the pattern was recorded on the hydrogel clearly (Figure 6d). After irradiated by Vis light for 5 min, the “butterfly” pattern was erased (Figure 6e). Then the same hydrogel was subsequently irradiated by UV light from another “snowflake” photomask, and a “snowflake” pattern could be created on the hydrogel (Figure 6f). This phenomenon
demonstrated the real-time reversible photo-switchable color changing property of the hydrogel containing SP moieties. And photo-switchable patterned hydrogel may have promising application in the field of self-erasing and display/storage devices.
23
Figure 6. (a),(b) UV-vis absorbance spectra of Gel 12-1 under UV and Vis light; (c) The ultraviolet absorption at 540 nm of reversibility of photo-isomerization of Gel 12-1 irradiated by alternative UV-Vis light; (d),(e),(f) Photo-switchable hydrogel pattern under UV light with photomask. Conclusions In
summary,
a
high
strength
dual-crosslinked
hydrogel
with
excellent
photos-switchable color changing behavior was fabricated in aqueous in a simple, convenient, and environmental friendly manner. The dual-crosslinked hydrogels possessed prominent high strength and superior Young’s modulus, which can be ascribed to the interactions of two networks and the efficient energy dissipation of physical crosslinks. The host-guest interaction between α-CD and PEG not only 24
provided the hydrogel with high strength, but also afforded a simple way to incorporate photochromic moiety (i.e. spiropyran) into the hydrogel. The as-prepared hydrogel possessed real-time reversible, precision spatiotemporal photo-switchable color changing property. The hydrogel showed great potential in information storage, optical sensor, and wearable devices. Acknowledgment: This work is supported by Start-up Funding of Southern Medical University and the Natural Science Foundation of Guangdong Province (No. 2018A030313454). References 1.
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29
Graphical abstract
30
Highlights: Preparing a novel dual-crosslinked hydrogel using polyethylene glycol diacrylate and photochromic α-cyclodextrin derivatives as the starting materials. The as-prepared hydrogel possessed high tensile strength. The as-prepared hydrogel exhibited real-time reversible, precision spatiotemporal photo-switchable color changing property. Providing a facile and convenient method for incorporating photochromic compound (i.e. spiropyran) into the hydrogel in aqueous.
31
S0014-3057(19)30355-6 https://doi.org/10.1016/j.eurpolymj.2019.04.035 EPJ 8999
To appear in:
European Polymer Journal
Received Date: Revised Date: Accepted Date:
22 February 2019 12 April 2019 17 April 2019
Please cite this article as: Wang, B., Xiao, X., Zhang, Y., Liao, L., High Strength Dual-crosslinked Hydrogels with Photo-switchable Color Changing Behavior, European Polymer Journal (2019), doi: https://doi.org/10.1016/ j.eurpolymj.2019.04.035
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High Strength Dual-crosslinked Hydrogels with Photo-switchable Color Changing Behavior Beibei Wanga, Xiaozhen Xiaob, Yuhuan Zhangc, Liqiong Liaob,* a
College of Chemistry and Molecular Science, Wuhan University, Wuhan, Hubei,
China. b
Guangdong Provincial Key Laboratory of Construction and Detection in Tissue
Engineering, Biomaterials Research Center, School of Biomedical Engineering, Southern Medical University, Guangzhou, Guangdong, China c
Institute for Clean Energy & Advanced Materials (ICEAM), Southwest University,
Chongqing, China. * Correspondence should be addressed to: Prof. Liqiong Liao (Ph.D), E-mail: [email protected]
1
Abstract Herein, high strength dual-crosslinked hydrogels with photo-switchable color changing behavior were designed and fabricated. In the mixture solution of α-cyclodextrin (α-CD), α-cyclodextrin-spiropyran (α-CD-SP), and poly(ethylene glycol) diacrylate (PEGDA), the PEGDA chains first threaded into the cavity of α-CD/α-CD-SP and form inclusion complexes to generate the physical gel; then, the as-prepared physical gels were subjected to photopolymerization to form a covalently crosslinked network. With the contribution of both no-covalent and covalent bond, the dual-crosslinked hydrogels possess high tensile strength, which is about fifteen times that of the pure PEG hydrogel. And due to the sacrifice of no-covalent bond, hydrogels showed high energy dissipation ratio, up to 75.3% and high fatigue resistance during loading-unloading tests. While the ingenious introduction of SP moieties, enables the as-prepared gels photo-switchable spatiotemporal color changing behavior reversibly. Erasable patterns on hydrogels could be written using UV light and erased by Vis light for multiple cycles. This flexible hydrogel with good physical performance shows high potential in photochromic flexible wearable devices. Keyword: Hydrogel; dual-crosslinked; high strength; spiropyran
2
Introduction Flexible and wearable devices, such as supercapacitors1, 2, electronic skin3, force4 and strain5 sensors, health monitor6, light detector7,8, have attracted extensive attentions for their unique advantages of portability, sensitivity, and real-time. In the fabrication/construction of these devices, stimuli-responsive hydrogels with appreciable mechanical properties (tensile strength ranged from tens of KPa to tens of MPa)9 would be ideal candidate materials. Color is an important feature of materials, and the color changing of materials in respond to external stimuli (such as pH, temperature or light) may be harnessed in camouflage, display/storage devices10, sensors11, and packaging materials12. In particular, hydrogel possessing such color-change capability are highly desirable in the flexible wearable devices. One of the efficient strategies to prepare hydrogels with color changing property is to incorporate stimuli responsive moiety into the hydrogel backbone. For instant, hydrogel incorporated with indigo dye showed color changing behavior in the presence of specific enzyme13, or hydrogel consisting of anthocyanin moiety could change its color under different external pH12. Recently, interests have been aroused in the field of photo-switchable color changing hydrogel in response to light irradiation, which is characterized by unique advantages of real-time spatiotemporal precision14 and flexible adjustability of wavelength15. Various photo-chromic dyes such as azobenzenes16, 17, spiropyrans, diarylethenes, and fulgides have been utilized in the constructing of photo-responsive color changing hydrogels18,19. 3
As one of the extensively studied photochromic dyes, spiropyran (SP), undergoes isomerization between the closed-ring colorless SP form and the open-ring colored merocyanine (MC) form which possess different physicochemical properties, and has gained specific attentions in the construction of color changing hydrogels owing to its high fatigue resistant and multi-stimuli responsive properties20. Generally, the SP moiety can be introduced into the hydrogel network by copolymerization of water insoluble SP derivatives with water soluble monomer in organic/aqueous mixture solutions21, 22. Several SP hydrogels have been prepared through this strategy, such as the
poly(N-isopropylacrylamide)
hydrogel23,
(PNIPAM)-SP
poly(2-vinyl-4,6-diamino-1,3,5-triazine)
(PVDT)-SP
hydrogel
24
,
and
poly(acrylamide-co-methyl acrylate) (PAA-PMA)-SP hydrogels25. However, most hydrogels are characterized by inferior mechanical properties, which hinder their application in many fields including flexible and wearable devices. To address this concern, efforts have been devoted to construct SP hydrogels with high mechanical strength. Wang et al. prepared a SP hydrogel through the copolymerization of acrylamide (AAm, hydrophilic hydrogen bonding monomer), 2-vinyl-4,6-diamino-1,3,5-triazine (VDT, hydrophobic hydrogen bonding monomer), and SP containing monomer (SPAA) in the presence of poly(ethylene glycol) (PEG) diacrylate (Mn = 575 g/mol) as the crosslinker. Due to the double hydrogen bondings from AAm-AAm and diaminotriazine-diaminotriazine, the hydrogels possessed high tensile
strength
(around
250
KPa)26.
Chen
et
al.
constructed
a
polyacrylamide-co-methyl acrylate/spiropyran (SP) hydrogel with a high tensile 4
strength of 1.5 MPa, the hydrogels exhibited color change from yellow to purple upon force, UV light, and heat stimuli and color reversion from purple to yellow upon white light25. Although several high strength SP hydrogels have been constructed, it is still attractive and challenging to afford a facile method to prepare high strength SP hydrogels with excellent photo-switchable color changing property. Several strategies have been very successful in the fabrication of high strength hydrogels, for example, the topological hydrogel (slide-ring hydrogel)27, double network hydrogel28, nanocomposite hydrogel29, and dual-crosslinked hydrogel30. Among them, dual-crosslinked hydrogel combines the advantages of physical and chemical crosslinking and could achieve enhanced toughness/strength; particularly, the physical crosslinking, which usually utilizes reversible weak interaction such as hydrogen bond31, hydrophobic interaction32, and Van der Waals force33, could dissipate the stress energy effectively and consequently enhance the toughness of the hydrogels34. Various dual-crosslinked hydrogel with high strength (tensile fracture stress: 0.3-10 MPa), self-healing properties, as well as shape-memory behavior have been reported35, 36. Different approaches have been used to construct dual-crosslinked hydrogels and a facile method is through the host-guest interaction between cyclodextrins (CDs) and guest molecules. For instance, physical hydrogels could be prepared from CDs and high molecular diacrylated poly(ethylene glycol) (PEGDA) trough host-guest interaction37, 38, after that, dual-crosslinked hydrogel could form through the chemical crosslinking of the PEG chains33. In this work, host-guest interaction was used to introduce SP moiety into a 5
dual-crosslinked hydrogel; briefly, α-CD and SP-modified α-CD (α-CD-SP) were first mixed with PEGDA to produce physical hydrogel through the host-guest interaction; subsequently, a robust dual-crosslinked gel can be produced by photopolymerization of the PEGDA. With the aid of photomask, different patterns can be “drawn” or “erased” on the hydrogels under specific light irradiation, revealing the potential of this hydrogel in the fabrication of wearable devices with photo-switchable color changing property (Scheme 1).
Scheme 1. Schematic illustration of the preparation of dual-crosslinked hydrogel with photo-switchable color changing behavior.
Experimental section Materials 3-Iodopropionic acid (99%, J&K Chemicals, China), 2,3,3-trimethylindolenine (98%, Energy Chemical, China), 5-nitrosalicylaldehyde (98%, Energy Chemical, China), α-CD (98%, Aladdin, China), N,N′-dicyclohexylcarboiimide (DCC, 95%, Sinopharm, 6
China), 4-(dimethylamino) pyridine (DMAP, 98%, Sinopharm, China), piperidine (98%, Sinopharm, China), PEG (20000 g/mol, Aladdin, China), acryloyl chloride (96%, Aladdin, China), and Irgacure-2959 (98%, Sigma, USA) were of analytical and used as received. All the solvents were of analytical grade and used without purification unless stated elsewhere. Synthesis of poly(ethylene glycol) diacrylate (PEGDA) PEGDA was synthesized according to the literature39. Proton nuclear magnetic resonance spectroscopy (1H NMR) was used to characterize the chemical structure of the product (Fig S1, ESI†). 1H NMR (400 MHz, CDCl3): δ 6.32 (dd, J = 17.3, 1.0 Hz, 2H), 6.09 (dd, J = 17.3, 10.5 Hz, 2H), 5.87 (dd, J = 10.5, 1.0 Hz, 2H), 4.21 (m, 4H), 3.57 (m, 2019H). The substitution degree (DS) of the acrylate groups was 90%, which was calculated though the ratio of relative peak integral value of the acrylate and methylene group. Preparation
of
l′-(β-Carboxyethyl)-3′,3′-dimethyl-6-nitrosipiro(indoline-2′,2–chromane) (SPCOOH) SPCOOH was synthesized according to the literature40 (71.6% yield). The 1H NMR spectrum of the product is shown in Figure S2 (ESI†). 1H NMR (400 MHz, DMSO-d6): δ 12.24 (s, 1H), 8.22 (d, J = 2.8 Hz, 1H), 8.00 (dd, J = 9.0, 2.8 Hz, 1H), 7.21 (d, J = 10.4 Hz, 1H), 7.17-7.08 (m, 2H), 6.87 (d, J = 9.0 Hz, 1H), 6.83-6.76 (m, 1H), 6.70-6.63 (m, 1H), 6.00 (d, J = 10.4 Hz, 1H), 3.49 (m, 1H), 3.39 (m, 1H), 2.62-2.54 (m, 1H), 2.48-2.40 (m, 1H), 1.16 (s, 3H), 1.07 (s, 3H). 7
Synthesis of α-CD-SP α-CD-SP was synthesized by esterification of SPCOOH and α-CD (Scheme S1, ESI†)41, 42. SPCOOH (200 mg, 0.53 mmol) was added to anhydrous DMF (30 mL) in the presence of DCC (109.4 mg, 0.53 mmol) and DMAP (6.5 mg, 0.053 mmol). The solution was stirred at room temperature for 30 minutes and then α-CD (250 mg, 0.26 mmol) was added and stirred at room temperature for 24 h. After that, the solvent was evaporated under reduced pressure and the solid was washed by acetone and ethanol. The solid product was collected and dried under vacuum (173.6g, 50% yield). The product was characterized by 1H NMR,
13
C NMR, and HRMS (Figure S2, S3, and
S4, ESI†). 1H NMR (400 MHz, DMSO-d6): δ 8.24-8.17 (m, 1H), 8.04-7.97 (m, 1H), 7.15 (m, 3H), 6.89-6.76 (m, 2H), 6.64 (m, 1H), 5.98 (dd, J = 10.4, 2.6 Hz, 1H), 5.64-5.34 (m, 12H), 4.78 (d, J = 3 Hz, 6H), 4.48 (t, J = 5.7 Hz, 5H), 3.94-3.54 (m, 24H), 3.52-3.45 (m, 2H), 3.45-3.37 (m, 6H), 3.29-3.24 (m, 6H), 2.71-2.53 (m, 2H), 1.17 (s, 3H), 1.06 (s, 3H).
13
C NMR (400 MHz, DMSO-d6): δ 171.3, 159.1, 156.7,
153.3, 146.1, 140.6, 135.7, 128.2, 127.7, 125.7, 122.8, 121.8, 119.4, 118.9, 115.5, 106.6, 101.9 (C1 of α-CD), 82.1 (C4 of α-CD), 73.3 (C5 of α-CD), 72.1 (C2 and C3 of α-CD), 60.0 (C6 of α-CD), 52.5, 33.4, 25.3, 24.5, 19.5. HRMS (ESI) m/z calcd. for C57H79N2O34 (M+H)+ 1335.4509, found 1335.4506. It was estimated that one spiropyran molecule was attached to per α-CD molecule by 1H NMR and HRMS. Preparation of dual-crosslinked hydrogels 1 mL of 20% (wt/vol) PEGDA aqueous solution was mixed with 1 mL of α-CD aqueous solution with varied concentration (120, 160, 200, and 240 mg/mL). Then, 8
200 μL of 0.5% (wt/vol) Irgacure-2959 aqueous solution was added to this mixture solution. The solution was sonicated for one minute and stored at 4 oC for 5 minutes to allow the formation of the inclusion complex/physical gel. Subsequently, the obtained physical hydrogel was exposed to 365 nm UV irradiation for 5 minutes to produce the dual-crosslinked hydrogel. The as-prepared gel was named as Gel x, where x is the mass of α-CD per 100 μL of 10% (wt/vol) PEGDA aqueous solution. PEG hydrogels were prepared by the photopolymerization of a mixture solution containing 100 μL of 10% (wt/vol) PEGDA aqueous solution and 10 μL of 0.5% (wt/vol) Irgacure-2959 aqueous solution under 365 nm UV irradiation for 5 minutes. Similarly, PEGDA/α-CD/α-CD-SP dual-crosslinked hydrogels were prepared. The precursors’ solution was obtained by mixing 0.6 mL of 33.3% (wt/vol) PEGDA aqueous solution, 0.4 mL of α-CD-SP (4 mg, 8 mg, 12 mg, 16 mg and 20 mg) aqueous solution, 1 mL of 240 mg α-CD aqueous solution, and 200 μL 0.5% (wt/vol) Irgacure-2959 photoinitiator aqueous solution. The prepared gel was named as Gel 12-y, where y is the mass of α-CD-SP per 12 mg of α-CD and 100 μL of 10% (wt/vol) PEGDA aqueous solution. Nuclear magnetic resonance spectroscopy (NMR) NMR spectra were acquired by Bruker Avance ċ HD 400 MHz spectrometer at room temperature with 10 mg (30 mg for 13C NMR spectra) simple dissolved in 600 μL deuterium generation reagent. Solvent peaks in 1H NMR spectra were referenced to δH 7.26 ppm for CDCl3, δH 2.5 ppm for DMSO-d6, and in 13C NMR spectra was referenced to δC 39.5 ppm for DMSO-d6. 9
High resolution mass spectroscopy (HRMS) HRMS (ESI) were obtained using positive or negative ionization mode by Thermo Scientific™ Orbitrap Fusion™ Tribrid™ mass spectrometer. X-ray powder diffraction X-ray powder diffraction (XRD) patterns were collected on a Bruker D8 Advance equipment (BRUKER, Germany) under the following conditions: a voltage of 40 kV, a current of 40 mA, a scanning speed of 5◦/min, and a scan range of 2θ from 4◦ to 40◦ was applied. The sample of inclusion complexes was prepared as follow: PEGDA (20 mg) and α-CD-SP (20 mg) were dissolved in water, after the precipitate formed, filtrated and dried, the sample of inclusion complexes was obtained. Rheology analysis Rheometer (HAAKE MARS III) was employed to study gelation process of the physical gel. 400 μL of the precursors’ solution was measured on a 20 mm parallel plate. Time sweep tests were performed at temperature of 4 °C. A strain of 1% and a frequency of 1 Hz were applied to the measurement. Morphologies The interior morphologies of the hydrogels were observed by scanning electron microscopy (SEM, Hitachi S-3000N, Japan). The swollen hydrogels were frozen in liquid nitrogen and then lyophilized for 48 h, and sputter-coated with gold before observation. Equilibrium swelling ratios The equilibrium swelling ratios (ESR) of the hydrogels were measured at room 10
temperature. The as-prepared hydrogels were immersed in deionized water to remove the unreacted precursors and dried to a constant weight (mdry). Then the hydrogels were immersed in deionized water until they reached swelling equilibrium. Then the fully swollen hydrogels were wiped using filter paper to remove water on the surface and weighted (mwet). ESR is defined as: ESR = (mwet - mdry)/mdry Triplicate samples were measured for each point and the results were presented as mean ± SD. Tensile test The tensile properties of the hydrogels were measured on Material Testing System Model 5967 (LLOYD, Britain) at room temperature. The tensile strength and elongation at break are obtained directly by material testing machine and Young's modulus is derived from the slope of first 15% of stress-strain curve. The hydrogel strips were prepared in a 20 mm (length) ×6 mm (width) ×1 mm (thickness) mold and used directly for the tensile test. The rate was set at 10 mm/min for the tensile test. Triplicate samples were measured for each point and the results were presented as mean ± SD. Hysteresis was observed by performing the tensile loading-unloading cycle in which the sample was initially stretched to a predetermined strain and immediately unloaded at a fixed velocity (10 mm/min). The total toughness was obtained by integrating the area under the stress-strain curve, and the dissipated toughness was calculated from the internal area of hysteresis loops. Photo-isomerization 11
The photo-isomerization of the spiropyran moiety was characterized by a UV-Vis spectrophotometer (UV3600 230 VCE, Japan). The hydrogel films were prepared in a 30 mm × 10 mm × 0.3 mm mold and immersed in deionized water for remove the unreacted precursors. The sample was first exposed to visible light irradiation for 3 min, the UV-Vis spectrum of the sample was collected. And the sample was irradiated by UV light for different time intervals and the corresponding UV-Vis spectra of the sample were collected. Then the sample was irradiated by visible light for different time intervals and the corresponding UV-Vis spectra of the sample were also collected.
Results and Discussions Preparation of dual-crosslinked hydrogels After α-CD, α-CD-SP and PEGDA were mixed in solution, the PEGDA chains are threaded into the cavity of α-CD and, α-CD-SP, the CD rings on PEG chains aggregate through intermolecular hydrogen bonding between them to form α-CD-PEG inclusion complexes; then the α-CD/PEG inclusion complexes form aggregates through the hydrophobic interaction, which acted as physical cross-linking domains of the gel38. The formation of inclusion complex between PEGDA and α-CD-SP was confirmed by powder XRD patterns (Figure 1). It is clear that the diffractogram of the PEGDA/α-CD-SP inclusion complexes was different from the pattern of the mixture, which is the superposition of the individual patterns of PEGDA and α-CD-SP. The disappearance of the characteristic peaks of PEGDA at 19.2o and 23.4o and the appearance of a new peak appeared at 18.2o, which was a characteristic 12
peak for the channel-type of α-CD inclusion complexes43, strongly indicated the formation of PEGDA/α-CD-SP inclusion complexes.
Figure 1. Powder X-ray diffraction patterns of PEGDA, α-CD-SP, the PEGDA/α-CD-SP mixture, and PEGDA/α-CD-SP inclusion complexes. Physical gels could be obtained by simply tuning the concentrations of α-CD/α-CD-SP and PEGDA in the solution (Table 1). Since spiropyran itself is insoluble in aqueous solution, it is difficult to prepare hydrogel containing SP moiety without the use of water/organic mixture solvent21. In this work, the grafting of SP onto the CD ring facilitated the dissolution of SP moiety and physical gels could be fabricated in aqueous, avoiding the utilization of organic solvent and post-purification and may benefit possible biomedical applications. Upon the addition of α-CD or α-CD/α-CD-SP (Figure 2a, d) was to the PEGDA solution, the solution became turbid (Figure 2b, e) but still was of relatively low viscosity, indicating the formation of inclusion complexes; and the turbid solution became semi-solid after being stored at 4 oC for 5 minutes, suggesting a sol-gel transition owing to the formation of 13
aggregates (Figure 2c, f)11. Table 1. Preparation and equilibrium swelling ratios of dual-crosslinked hydrogels. Molar ratio of Equilibrium α-CDa α-CD-SPa Irgacure-2959a Hydrogels α-CD/α-CD-SP/ swelling (mg) (mg) (mg) PEGDA ratios Gel 6 6 0 12.3/0/1 0.05 10.7 ± 0.1 Gel 8 8 0 16.4/0/1 0.05 8.1 ± 0.3 Gel 10 10 0 20.6/0/1 0.05 7.2 ± 0.4 Gel 12 12 0 24.7/0/1 0.05 6.0 ± 0.1 Gel 12-0.2 12 0.2 24.7/0.3/1 0.05 6.5 ± 0.4 Gel 12-0.4 12 0.4 24.7/0.6/1 0.05 6.9 ± 0.1 Gel 12-0.6 12 0.6 24.7/0.9/1 0.05 6.5 ± 0.3 Gel 12-0.8 12 0.8 24.7/1.2/1 0.05 6.3 ± 0.4 Gel 12-1 12 1 24.7/1.5/1 0.05 6.1 ± 0.2 a mass of per 100 μL 10% (wt/vol) PEGDA aqueous solution.
Figure 2. Photographs of the formation process of the physical hydrogels: (a, b, and c) the photographs of the mixture of PEGDA/α-CD (Gel 12) at different time; (d, e, and f) the photographs of the mixture of PEGDA/α-CD/α-CD-SP (Gel 12-1) at different time. The gelation kinetics of the physical gels was further investigated by the rheological analysis (Figure 3). It was found in all cases that the storage modulus (G′) and loss modulus (G′′) increased slightly and plateaued off corresponding to the formation of α-CD/PEG inclusion complexes; then the G′ and G′′ underwent rapid 14
increase and reached to another plateau, which can be attributed to the sol-gel transition44. Further increase of the concentrations of α-CD could reduce the time for the moduli of the precursors to reach constant, i.e., the gelation time. The gelation time of Gel 8, Gel 10, and Gel 12 was 1000 s, 500 s, and 300 s, respectively, which decreased gradually with the increase of the α-CD concentrations. In addition, the final moduli of the physical hydrogels increased from 104 to 105 Pa, as the concentrations of α-CD increased from 80 (Gel 8) to 120 mg/mL (Gel 12). These results can be attributed to the higher number of physical crosslinks at higher concentration of α-CD. However, the addition of α-CD-SP showed a negligible effect on the formation of the α-CD-PEG inclusion complexes. Compared with Gel 12 (Figure 3c), Gel 12-1 needs a longer gelation time, which might be explained by the weakening of the hydrogen bonds between the α-CDs in the presence of SP moiety.
Figure 3. Storage (G′) and loss (G′′) moduli of the precursors of (a) Gel 8, (b) Gel 10, 15
(c) Gel 12, and (d) Gel 12-1 versus time at 4 °C. After the formation of inclusion complex, the physical gel was further subjected to photopolymerization of the PEGDA to obtain the dual-crosslinked hydrogel. By tuning the concentrations of α-CD and α-CD-SP, dual-crosslinked hydrogels with different ESRs were prepared (Table 1). The ESRs of the hydrogels decreased gradually with the increase of the concentrations of α-CD, for instant, the ESRs of the hydrogel range from 10.7 ± 0.1 (Gel 6) to 6.0 ± 0.1 (Gel 12). The addition of α-CD resulted in higher cross-link density, leading to lower swelling ratio. While for Gel 12-0.4 to Gel 12-1, the ESR decreased slightly with the increase of the concentrations of α-CD-SP. Compared with Gel 12, the Gel 12-y with the addition of α-CD-SP showed higher ESRs. Since the PEGDA chains were packed closely by α-CDs aggregates and then fixed by chemical crosslinks, the Gel 12 possessed relatively dense porous structure and small pore size. For example, the pore size of Gel 12 is about 4 μm, and the pore sizes of Gel 12-0.4, Gel 12-0.6 and Gel 12-1 are around 6-10 μm, indicating that the addition of α-CD-SP enlarged the pore size of the hydrogel (Figure 4).
16
Figure 4. SEM images of morphologies of freeze-dried hydrogels (a) Gel 12, (b) Gel 12-0.4, (c) Gel 12-0.6, and (d) Gel 12-1 (Scale: 20 μm). Tensile properties The tensile properties of the dual-crosslinked hydrogels were examined (Table 2). Compared with the pure PEG hydrogel, the tensile strength as well as Young’s modulus and the elongation at break of the dual-crosslinked hydrogels was improved obviously. The PEGDA/α-CD/α-CD-SP (Gel 12-y series) hydrogels exhibited good mechanical properties with tensile strength of 440-520 KPa, Young’s modulus of 360-380 KPa, and elongation at break of 330-550% (Table 2). The gels could withstand large deformation without breaking after stretching, twisting, and knotting (Figure 5a, b), and also possessed good load-bearing capacity (Figure 5c). Table 2. Tensile properties of the dual-crosslinked hydrogels. Hydrogel Tensile strength Young’s modulus Elongation at (KPa) (KPa) break (%)
17
Gel 6 Gel 8 Gel 10 Gel 12 Gel 12-0.2 Gel 12-0.4 Gel 12-0.6 Gel 12-0.8 Gel 12-1 PEG
247.43 ± 0.35 307.92 ± 3.95 438.83 ± 14.88 458.08 ± 17.83 519.49 ± 11.59 491.28 ± 12.76 466.61 ± 4.19 441.73 ± 9.57 439.11 ± 13.95 29.60 ± 2.11
93.48 ± 11.99 188.87 ± 15.03 258.29 ± 38.39 343.18 ± 21.22 364.37 ± 39.47 385.38 ± 32.17 372.30 ± 23.81 374.91 ± 27.70 366.21 ± 17.73 12.51 ± 0.80
543.87 ± 80.07 402.19 ± 13.89 481.81 ± 25.60 402.94 ± 19.00 554.28 ± 21.08 442.26 ± 13.36 439.75 ± 11.68 359.87 ± 21.39 334.00 ± 10.04 333.98 ± 20.56
Figure 5. (a), (b), and (c) Photographs of Gel 12-1 undergoing stretching, twisting, knotting, and load-bearing; (d) Loading-unloading tests of gels at 300% of strain; (e) The calculated total toughness and the dissipated toughness of gels during the loading-unloading tests at 300% of strain; (f) Loading-unloading tests of Gel 12-1 18
at different strains (100%, 200%, and 300%); (g) The calculated total toughness and the dissipated toughness of Gel 12-1 during the loading-unloading tests at different strains; (h) Five successive loading-unloading cycles of Gel 12-1. The mechanical properties of the dual-crosslinked hydrogels could be tuned by the concentrations of α-CDs at fixed concentration of PEGDA. As for the PEGDA/α-CD hydrogel, the tensile strength of the hydrogel increased with the concentrations of α-CD. For example, the tensile strength of Gel 6 was 247 KPa and increased to 458 KPa for Gel 12 at higher concentration of α-CD. At the same time, the Young’s modulus showed similar trend as well. The results might be attributed to the higher number of physical crosslinks, i.e. higher crosslink density, formed at higher concentration of α-CD, which could not only contribute to the enhanced modulus but also help dissipate the energy and thus strengthen the hydrogels. Unlike α-CDs, α-CD-SP had a different effect on the mechanical properties of the PEGDA/α-CD/α-CD-SP hydrogel. When small amount (0.2 mg per 100 μL) of α-CD-SP was added, the tensile strengths of the PEGDA/α-CD/α-CD-SP hydrogel were improved as compared with the PEGDA/α-CD hydrogel, for instance, the tensile strength of Gel 12-0.2 were higher than that of Gel 12. However, further increasing the concentration of α-CD-SP led to gradual decrease of the tensile strength of the hydrogels. The lower tensile strength of the hydrogels at higher α-CD-SP concentration could be explained by the deteriorated hydrogen bonding between the CDs in the presence of hydrophobic SP moiety with increasing concentration. In such dual-crosslinked hydrogel, it is generally believed that the presence of 19
physical crosslinks could contribute to the relaxation of locally applied stress and dissipation of the crack energy45. The cyclic loading-unloading tests of the as-prepared gels were measured to evaluate their energy dissipation (Figure 5). In the loading-unloading curves of different dual-crosslinked gels and PEG hydrogel at 300% strains (Figure 5d), characteristic hysteresis loops were recorded, indicating the dual-crosslinked hydrogel could dissipate energy effectively30. As for the pure PEG gel with only chemical crosslinks, no hysteresis loops were presented in the curve and the hydrogel experienced approximately elastic deformation (Figure 5d). These results demonstrated that the energy dissipation process of the dual-crosslinked hydrogels was related directly to the physical crosslinks. The dissipated energy of hydrogels was evaluated as a function of concentrations of α-CD (Figure 5e). With the increase of the concentrations of α-CDs, the dissipated energy, the total energy, as well as the ratio of the dissipated energy to the total energy increased. From Gel 6 to Gel 12, the ratio of the dissipated energy increased from 61.2%, 71.2%, and 72.2% to 78.7%, respectively. Since the chemical crosslinking points were presumably the same for the tested hydrogels, the increase in the dissipated/total energy could largely be attributed to the higher number of physical crosslinks at higher concentration of α-CDs. This result further demonstrated that the physical crosslinks played a vital role during the energy dissipation process and thus improvement of the mechanical properties of the gels. In the presence of α-CD-SP, similar energy dissipation was observed (Figure 5f, g). For example, the dissipated energy of Gel 12-1 was about 509.1 KJ/m3 at the strain 20
of 300%, achieving 75% of the total energy. The dissipated energy of the Gel 12-1 was lower than that of Gel 12, resulting in lower tensile strength of the Gel 12-1 hydrogel. When the gel was stretched, the weak physical crosslinks ruptured first to dissipate energy, followed by the rupture of the strong chemical crosslinks to resist high levels of deformation46. At higher strain, more physical crosslinks were sacrificed, leading to higher dissipate energy and energy dissipation ratio. From 100% to 300% strain, the dissipated energy of the gels increased, and the energy dissipation ratio also increased from 68.6% to 75.2%. Good fatigue resistance property was also observed with the hydrogel containing SP moiety (Figure 5h). For example, the hysteresis loop of Gel 12-1 decreased dramatically after the first cyclic tensile test, and the dissipated energy decreased from 509.1 KJ/m3 (1st) to 53.8 KJ/m3 (2nd). The significantly lower dissipated energy at 2nd circle could be mainly attributed to the irreversible fracture of the chemical crosslinks at high strain (300%). In the following circles (3rd to 5th), the dissipated energy mainly originated from the physical crosslinks, which almost kept constant. The 2nd to 5th curves were almost identical with no obvious decrease in the tensile strength at 300% strain, indicating the gels possess good fatigue resistance47. Flexibility, high strength and good fatigue resistance, these results present that ability of this dual-crosslinked hydrogel as wearable devices. Photo-switchable color changing behavior of the hydrogels SP could exist in two stable isomerization structures: the closed-ring SP form and the open-ring MC form. When SP is exposed to UV light, SP can be isomerized into 21
the colored, hydrophilic MC; irradiated by Vis light, the MC can be isomerized into the colorless, hydrophobic SP48. The isomerization of the SPoMC and MCoSP can be detected by the appearance and disappearance of the adsorption peak (at around 540 nm) of MC in the UV-Vis spectrum. When Gel 12-1 was exposed to 365 nm UV light for one minute, a new absorbance peak appeared at 540 nm, indicating the SP form was gradually converted into MC form49 (Figure 6a). At about five minutes’ UV irradiation, the isomerization of the SPoMC completed as the intensity of 540 nm reached maximum. Then the Gel 12-1 was exposed to visible light, the peak at 540 nm disappeared within two minutes (Figure 6b). In practice, the reversibility of the photo-isomerization of the hydrogel is of essential importance. The hydrogel was irradiated with alternative UV light and Vis light to evaluate its photo-switchable reversibility. When the Gel 12-1 was irradiated by Vis light for two minutes, the absorbance intensity at 540 nm decreased resulting from the photo-isomerization from MC to SP isomer. Following with subsequent five minutes of UV irradiation, the absorbance intensity at 540 nm increased owing to photo-isomerization from SP to MC. After that, another seven Vis-UV irradiations circles were carried out on the same sample. First, the absorbance intensity (at 540 nm) decreased in the following two UV/Vis irradiations; then the absorbance intensity (at 540 nm) showed a slightly decrease for the left five UV/Vis irradiations. The reduction of absorbance intensity could be attributed to the gradual photo-degradation of the spiropyran moiety, which may be caused by irreversible oxidation20 or aggregation of MC isomer50. Even so, the above results demonstrated that the Gel 22
12-1 possessed impressive photo-switchable reversibility and is an idealized photo-switchable color changing material (Figure 6c). As a unique stimulus, light can achieve real-time spatiotemporal precision. In this case, the spatiotemporal photo-switchable color changing behavior of the hydrogel was further investigated. The photographs of the Gel 12-1 upon Vis-UV light irradiation through photomask were presented in Figure 6d-6f. When the swollen hydrogel was irradiated by UV light for 1 min through a “butterfly” photomask, the pattern was recorded on the hydrogel clearly (Figure 6d). After irradiated by Vis light for 5 min, the “butterfly” pattern was erased (Figure 6e). Then the same hydrogel was subsequently irradiated by UV light from another “snowflake” photomask, and a “snowflake” pattern could be created on the hydrogel (Figure 6f). This phenomenon
demonstrated the real-time reversible photo-switchable color changing property of the hydrogel containing SP moieties. And photo-switchable patterned hydrogel may have promising application in the field of self-erasing and display/storage devices.
23
Figure 6. (a),(b) UV-vis absorbance spectra of Gel 12-1 under UV and Vis light; (c) The ultraviolet absorption at 540 nm of reversibility of photo-isomerization of Gel 12-1 irradiated by alternative UV-Vis light; (d),(e),(f) Photo-switchable hydrogel pattern under UV light with photomask. Conclusions In
summary,
a
high
strength
dual-crosslinked
hydrogel
with
excellent
photos-switchable color changing behavior was fabricated in aqueous in a simple, convenient, and environmental friendly manner. The dual-crosslinked hydrogels possessed prominent high strength and superior Young’s modulus, which can be ascribed to the interactions of two networks and the efficient energy dissipation of physical crosslinks. The host-guest interaction between α-CD and PEG not only 24
provided the hydrogel with high strength, but also afforded a simple way to incorporate photochromic moiety (i.e. spiropyran) into the hydrogel. The as-prepared hydrogel possessed real-time reversible, precision spatiotemporal photo-switchable color changing property. The hydrogel showed great potential in information storage, optical sensor, and wearable devices. Acknowledgment: This work is supported by Start-up Funding of Southern Medical University and the Natural Science Foundation of Guangdong Province (No. 2018A030313454). References 1.
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Graphical abstract
30
Highlights: Preparing a novel dual-crosslinked hydrogel using polyethylene glycol diacrylate and photochromic α-cyclodextrin derivatives as the starting materials. The as-prepared hydrogel possessed high tensile strength. The as-prepared hydrogel exhibited real-time reversible, precision spatiotemporal photo-switchable color changing property. Providing a facile and convenient method for incorporating photochromic compound (i.e. spiropyran) into the hydrogel in aqueous.
31