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Multi-responsive hydrogel actuator with photo-switchable color changing behaviors
Dyes and Pigments xxx (xxxx) xxx
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Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Multi-responsive hydrogel actuator with photo-switchable color changing behaviors Xin Zhang a, 1, Xiaolei Xu a, 1, Lishan Chen a, Chao Zhang b, Liqiong Liao a, * a
Guangdong Provincial Key Laboratory of Construction and Detection in Tissue Engineering, School of Biomedical Engineering, Southern Medical University, Guangzhou, 510515, China b School of Biomedical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(N-isopropylacrylamide) Spiropyran Hydrogel actuator Photo-switchable
Hydrogel actuators, in addition to their programmable shape deformations triggered by external stimuli, are expected to integrate multi-functionality in response to multiple stimuli from the environment and to mimic the biology systems. In this work, inspired by animals (i.e. cephalopods or chameleon) that can change the color of the skin to mimic the environment, a hydrogel actuator was designed to possess both solvent and thermoresponsive bending properties as well as photo-switchable color changing behaviors. This hydrogel actuator consists of a poly (N-isopropylacrylamide) (PNIPAAm) layer and a PNIPAAm layer with spiropyran moiety (PNIPAAm-SP). Spiropyran, a hydrophobic and photochromic compound, endowed the bilayer gel with solvent and temperature triggered reversible bending behaviors as well as the reversible displaying of different patterns that can be “drawn” or “erased” by light. Moreover, accompanying with its shape deformation, the bilayer gel showed a change in fluorescent behavior, i.e. its fluorescent intensity increased upon switching the solvent from DI water to EtOH or increasing the temperature. The potential applications of this bilayer hydrogel in biomimetic devices, gripper, and information storage were demonstrated. With complicated 3D shape-morphing as well as color changing ability integrated in one bilayer gel system, this bilayer hydrogel may find its broad application including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors.
1. Introduction Hydrogel actuators with programmable shape deformations in responding to external stimuli have arisen tremendous interests due to their great potential in multiple applications [1–3], such as wearable devices [4], nanomechanical devices [5], biocompatible/medical de vices [6], transparent ionic conductors [7], and micro-biorobots [8]. Usually, the design of hydrogel actuators could be based on the homo geneous expansion/contraction with special structures [9] or inhomo geneous structures with different swelling/shrinkage degree in different directions. Under external stimuli, the former shows movement toward specific direction and the later exhibits bending/unbending behavior [10,11]. Due to the advantages of good operability and controllability, anisotropic hydrogels with inhomogeneous structures have shown to be good candidates as actuators, among which the bilayer hydrogel that can switch to complicated 2D and 3D shapes under external stimuli have attracted great interests [12,13]. Planar-to-3D shape transitions are
achieved by modulating a local concentration and/or cross-linking density of the two layers [14]. For example, a bilayer hydrogel made from thermo-responsive precursor with varied concentrations may eventually undergo inhomogeneous deswelling upon heating, and consequently adopt a particular 3D morphology to minimize the elastic energy [15]. Utilizing the above strategy, bilayer hydrogel actuators in response to temperature [16], pH [17], solvent [18], light [19], and electric field [20] have been successfully constructed. In practical applications, multi-functional actuators may be highly desirable to respond to multiple stimuli from the environment and biology systems [21–23]. Inspired by animal tissues/organs, actuators that can camouflage (i.e. color change) by sensing the environment stimuli were developed [24–26]. Until now, only a few hydrogel actu ators with color change functionality were reported [21,22,27,28]. For example, Chen et al. developed a temperature stimulated bilayer hydrogel actuator with pH switchable fluorescence behavior, which consisted of a graphene oxide-poly (N-isopropylacrylamide)
* Corresponding author. E-mail address: [email protected] (L. Liao). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.dyepig.2019.108042 Received 15 September 2019; Received in revised form 5 November 2019; Accepted 11 November 2019 Available online 11 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xin Zhang, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108042
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afford the bilayer gel with shape morphing ability accompanied by fluorescent color changing. Taking the advantage of the real-time spatiotemporal precision of light stimulus, the bilayer hydrogel, namely, PNIPAAm-SP/PNIPAAm, can reversibly display different pat terns that are “drawn” or “erased” by light (Scheme 1c). The above new function will broaden the application of the bilayer hydrogel actuators in a wide range fields, including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors. 2. Experimental section 2.1. Materials N-isopropylacrylamide (NIPAAm, 98%, Sigma-Aldrich), N, N0 methylenebisacrylamide (BIS, 99%, Sigma-Aldrich), and phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (PBPO, 97%, Sigma-Aldrich) were used as received. Dioxane, ethanol, methylene blue, and acetone were of analytical grade and used without further purification. 2-(30 ,30 dimethyl-6-nitrospiro [chromene-2,20 -indoline]-10 -yl) ethyl methacry late (SP) was synthesized according to literature [33] (Schemes S1 and SI) and its chemical structure was characterized by proton nuclear magnetic resonance spectroscopy (1H NMR) (Figs. S1 and SI). 2.2. Preparation of monolayer gels To prepare a typical monolayer PNIPAAm gel sheet, a precursor solution containing 2.35 M NIPAAm, 0.77 mol% PBPO, and pre determined amount of BIS was first prepared using a mixture solvent of dioxane and deionized water (DI) (4:1 in volume). The precursor solu tion was then transferred to a mold, which consists of a transparent quartz upper layer, a thin polytetrafluoroethylene (PTFE) frame (0.3 mm thickness), and a PTFE bottom layer. Then the precursor solu tion was irradiated under blue light (490 nm, 18 W) for 5 min to obtain a PNIPAAm gel sheet. The gel sheet was washed with acetone, and equilibrated in DI water for one week to remove unreacted chemicals. Then the gel was dyed with 1 M methylene blue aqueous solution. The gels were named as PNIPAAm-y (y ¼ 1, 2, 3, 4, and 5) for the gel pre pared from 0.04, 0.12, 0.16, 0.20, and 0.24 M BIS, respectively. In the case of PNIPAAm-SP gel sheet, 2 mol % of SP and 0.04 M of BIS was added to the precursor solution and then the same protocol was followed subsequently.
Scheme 1. Schematic illustration of the multi-responsive PNIPAAm-SP/PNI PAAm bilayer hydrogel actuator with photo-switchable color changing behav iors. (a) Solvent and temperature actuating of the bilayer gel, (b) Photoisomerization of the SP moiety in the PNIPAAm-SP layer, (c) 3D-morphing and photo-pattern of the bilayer hydrogels.
2.3. Preparation of bilayer hydrogels
(GO-PNIPAAm) hydrogel layer with a pH responsive perylene bisimide-functionalized hyperbranched polyethylenimine (PBI-HPEI) hydrogel layer [28]. Generally, it is still a challenge to construct hydrogel actuators with integrated multi-stimuli responsive shape morphing capability as well as color changing properties. Herein, we report the design and fabrication of bilayer hydrogel as solvent and temperature triggered actuator with photo-switchable color changing behaviors (Scheme 1). Briefly, a hydrophobic and photo chromic moiety (spiropyran (SP)), was incorporated into one layer of the PNIPAAm bilayer hydrogel, which gave distinct affinity of two layers toward water/organic solvent owing to their different hydrophilicity/ hydrophobicity, and thus help achieve anisotropic swelling of each layer and deformation of the bilayer hydrogel upon solvent and temperature change (Scheme 1a). Moreover, the spiropyran (SP) moiety will bring photo-switchable color changing properties to the bilayer gels, owing to the photo-isomerization between the “colorless” SP isomer and the “colored” merocyanine (MC) isomer (Scheme 1b) [29]. Due to the different physicochemical properties of the two isomers, such as polar ity, fluorescence, and electronic properties, photo-responsive SP hydrogels have been constructed with applications in photo-tunable microvalves [30] and photoactuators [31]. In addition, the fluorescent behavior of MC was found to be responsive to polarity [32], which might
Upon the completion of the preparation of the monolayer PNIPAAmSP gel sheet, a second PTFE spacer of 0.3 mm thick was placed between the first spacer and the PTFE bottom. Then, a precursor solution of the PNIPAAm gel was injected into the mold and irradiated under blue light for additional 3 min. The obtained bilayer gels were named PNIPAAmSP/PNIPAAm-y (y ¼ 1, 2, 3, 4, and 5) in the following context. Varied shapes of the bilayer hydrogel, such as “strip”, “flower”, and “hand” were fabricated using custom-made molds. 2.4. Swelling ratio of the gels The fully swollen PNIPAAm and PNIPAAm-SP monolayer gel were weighed at 4 � C and dried under vacuum. The swelling ratio (SR) for the hydrogels was calculated using the following equation: The swelling ratio in water: SR
DI water ¼
Ws water Wd Wd � ρwater
(1)
The swelling ratio in ethanol: SR
2
Ethanol ¼
Ws ethanol Wd Wd � ρethanol
(2)
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Fig. 1. Solvent responsive bending behavior of the bilayer gels. (a) The swelling ratios of the gels in H2O and EtOH, (b) Bending degree of the bilayer gels as a function of time in EtOH (A–E: PNIPAAm-SP/PNIPAAm-y (y is from 1 to 5), F: PNIPAAm-1/PNIPAAm-4), (c) Photographs of the PNIPAAm-SP/PNIPAAm-y (y ¼ 1–5) and PNIPAAm-1/PNIPAAm-4 bilayer gels in H2O and EtOH, (d) Bending degrees of the PNIPAAm-SP/PNIPAAm-4 bilayer hydrogel in EtOH, acetone, and dioxane, (e) Bending degrees of the PNIPAAm-SP/PNIPAAm-4 bilayer hydrogel after soaking to reach equilibrium alternately in EtOH and H2O for three cycles.
where Ws-water is the weight of gel swollen in water and Ws-ethanol is the weight of gel swollen in ethanol; Wd is the dry weight of the gel, ρwater and ρethanol are the density of the water and ethanol, respectively.
where Wx is the weight of the gels at predetermined temperature (26 � C–40 � C), W4 is the weight of the full swollen gel at 4 � C. 2.6. Bending degree of the bilayer gels
2.5. De-swelling ratio of gels
The bending process of a bilayer gel strip (20 mm � 1 mm � 0.6 mm) soaking in solvents was recorded using a digital camera. The instanta neous radius of the bending bilayer gel strip (R) was measured on the images using Image J software (1.44p, 2014, National Institutes of Health, USA), and the bending degree Κ was derived from R:
The relative de-swelling ratio of the PNIPAAm and PNIPAAm-SP monolayer gels in DI water under different temperatures was deter mined. The de-swelling ratio of the gel at 4 � C and predetermined temperature (26 � C–40 � C) was measured. The de-swelling ratio (De-SR) was calculated by the following equation: De
SR ¼
W4
Wx W4
K ¼ 1=R
(3)
(4)
In this work, the bending toward the PNIPAAm layer is designated a positive bending degree, and a negative bending degree represents the 3
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Fig. 2. Thermo-responsive bending behavior of the bilayer gels. (a) De-swelling ratio of the PNIPAAm-1, PNIPAAm-4, and PNIPAAm-SP hydrogel at various tem peratures, (b) Bending degree of the PNIPAAm-SP/PNIPAAm-4 and PNIPAAm-1/PNIPAAm-4 bilayer gels at 35 � C in water, (c) Bending degree of the PNIPAAm-SP/ PNIPAAm-4 bilayer gel after soaking to reach equilibrium alternately at 35 � C and 4 � C aqueous solution for three cycles.
direction of the bending toward the PNIPAAm-SP layer.
would re-swell to some extent when being transferred from DI water to ethanol, and the PNIPAAm-SP gel exhibited much higher SR change than that of PNIPAAm gel. Usually, the layer with larger volume change acts as the driven/active layer to induce the bending upon the exposure to stimuli [35]. Thus, the PNIPAAm-SP layer that experienced larger volume phase change acted as the active layer to drive the bilayer gels to bend/unbend in the studied cases. Generally, larger difference in SRs in response to stimulus could produce a stronger driving force to cause a larger deformation of the bilayer gels. Since the SR of two layers could be tuned by varying the BIS concentration, the bilayer gels also showed different bending behaviors (i.e. bending degree) in response to solvent. When the bilayer gels were immersed in ethanol to reach equilibrium, the bending degree increased (Fig. 1b, A-E) and the bilayer gels trans formed to different shapes (Fig. 1c, A-E). The PNIPAAm-SP/PNIPAAm-y (y ¼ 1 and 2) bent to the PNIPAAm-SP layer side (Fig. 1c, A-B); the PNIPAAm-SP/PNIPAAm-y (y ¼ 3, 4, and 5) bilayer gels bent to the PNIPAAm layer side (Fig. 1c, C-E). In addition, it was observed that the bilayer gel from PNIPAAm layer with higher concentration of BIS responded faster upon the solvent change. In detail, the PNIPAAm-SP/PNIPAAm-y (y ¼ 3, 4, and 5) gels reached the maximum deformation in 8 min. However, it took nearly 12 min for the PNIPAAm-SP/PNIPAAm-y (y ¼ 1 and 2) to reach equilibrium. The bending behavior of PNIPAAm-1/PNIPAAm-4 bilayer gel was also examined to evaluate the role of the SP moiety. In DI water or ethanol, the PNIPAAm-1/PNIPAAm-4 bilayer gel bends to the PNIPAAm-4 layer owing to the different SRs of the two layers. The gel showed slight difference in bending degree after being transferred from DI water to ethanol (Fig. 1b, F), with no obvious change of shape (Fig. 1c, f-F). In contrast, the PNIPAAm-SP/PNIPAAm-4 bilayer gel possesses excellent solvent triggered bending behavior with a “straightto-curve” change of shape (Fig. 1b, D and Fig. 1c, d-D). The above result demonstrates that PNIPAAm bilayer gel with excellent solvent respon sive bending behavior can be fabricated by incorporation of the hy drophobic SP moiety into one of the layers. The bilayer gel also exhibited reversible solvent responsive bending behavior triggered by other organic solvent like acetone and dioxane (Fig. 1d). For instance, the bending degree of the PNIPAAm-SP/ PNIPAAm-4 bilayer gel increased when it was transferred from DI water to the solvents, and then decreased when it was put back into DI water. The response time of the bilayer gels depended on the solvent, i.e. the PNIPAAm-SP/PNIPAAm-4 bilayer gel needs longer response time in dioxane than that in ethanol and acetone, which may be caused by the different diffusion rates of the solvents. The deformation repeatability is one of the important parameters for actuating materials. Here, the bending degrees of the bilayer gels (in DI water and ethanol) in cyclic tests were recorded (Fig. 1e). The bilayer gel showed bending-unbending behavior after being immersed in ethanol and water alternatively for three cycles. The bending degree of the
2.7. Photo-switchable color changing behavior The PNIPAAm-SP gel sheets were fully swollen in DI water and ethanol, respectively. After being irradiated by UV (UV-black Ray 365 nm, 30W) or white light (LED, 40 W), the UV–Vis absorption spectra of the gel sheets were measured at room temperature. The measure ments were performed on a Shimadzu UV-3600 230VCE UV–Vis spec trometer. The fluorescence spectra of the gel sheets were recorded on fluorescence spectrophotometers (HORIBA Scientific, FluoroMax-4) with an excitation wavelength of 560 nm and a combined fluorescence lifetime and steady state spectrometer (EDINBURGH, FLS920) with an excitation wavelength of 540 nm. The fluorescent photographs of the gel sheets were taken by a fluorescence imager (Alpha, USA). 3. Results and discussions 3.1. Solvent responsive bending behavior A series of PNIPAAm-SP/PNIPAAm-y bilayer hydrogels were pre pared with varied crosslink density by tuning the concentration of the crosslinker (BIS). The interior morphology of the bilayer gel (i.e. PNIPAAm-SP/PNIPAAm-4) was examined by SEM (Figs. S2 and SI), and it was found that the both layers possess characteristic porous structure and no obvious phase separation between the two layers was discern ible, which is of essentially importance to achieve appreciable actuation upon stimuli. Spiropyran (contact angle of 92� ) is far more hydrophobic than NIPAAm monomer (contact angle of 28� ) (Figs. S3 and SI), such difference in the hydrophobicity/hydrophilicity of the precursors would no doubt bring anisotropic swelling behavior to the bilayer hydrogel in response to solvents with different hydrophobicity/hydrophilicity. The SR of the PNIPAAm-y (y ranges from 1 to 5) and PNIPAAm-SP gel sheets in DI water and ethanol was evaluated (Fig. 1a). The SR-DI water of the PNIPAAm-y gel decreased at higher BIS concentration. Since the Hil debrand solubility parameter of PNIPAAm is closer to ethanol than to DI water [33], the PNIPAAm chains preferred to be more relaxed in ethanol and the gels underwent further swelling when transferred from DI water to ethanol. In addition, SP moiety may also facilitate the swelling of the gels in ethanol; it was observed that the SR-Ethanol of PNIPAAm-SP is 1.6 times that in DI water, which is also significantly higher than those of the PNIPAAm-y gels (1.1–1.3 times) without SP moiety. Usually, het erogeneous volume change upon the diffusion of solvent into actuators may result in macroscopic shape deformations and/or adaptive move ments [34]. The different swelling ratios of the two layers in DI water/ethanol open the opportunity for designing bilayer gels with controlled solvent-responsive bending behaviors. It was noted that both PNIPAAm and PNIPAAm-SP monolayer gels 4
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Fig. 3. Photo-switchable color changing behaviors. (a) UV–visible spectra of PNIPAAm-SP hydrogel under UV/Vis irradiation in H2O at 25 � C, (b) Fluorescence emission spectra of the PNIPAAm-SP hydrogel under UV/Vis irradiation in H2O at 25 � C, (c) Fluorescence intensity of the PNIPAAm-SP hydrogel under alternative UV/Vis irradiation in H2O at 25 � C, (d) Fluorescence emission spectra of the PNIPAAm-SP hydrogel in H2O and EtOH at 25 � C under UV irradiation, (e) Fluorescence emission spectra of the PNIPAAm-SP hydrogel in H2O at 25 � C and 35 � C under UV irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
bilayer gel reached maximum in 7 min in ethanol, and then dropped to zero within 13 min in DI water. The rate for the unbending is lower than for the bending, which may be attributed to the slower rate of the diffusion of ethanol out of the gel [34].
7 min at 35 � C, and then dropped to zero within 10 min at 4 � C. 3.3. Color changing behavior The isomerization of spiropyran upon Vis/UV light irradiation not only changes the hydrophobicity/hydrophilicity, but also would lead to different fluorescence properties: the MC isomer has intense red fluo rescence, while the SP virtually shows weak or no fluorescence [36]. In this case, incorporation of the SP moiety may endow the hydrogel actuator with reversible photo-chromic and photo-switchable fluores cence properties. The photo-chromic and photo-switchable fluorescence properties of PNIPAAm-SP gel were studied in detail. As evidenced by the UV–Vis spectra (Fig. 3a), there was no obvious absorption peak in the investigated range of wavelength after irradiation under white light, indicating the spiropyran moiety in the hydrogel exists as the SP struc ture; when the gel was irradiated under UV light, an absorption peak at around 550 nm belonging to opening MC isomer was observed and the color of the gel changed from pale yellow to red [36]. Using rhodamine B as the reference, the quantum yield of the PNIPAAm-SP gel was calculated as 0.10 (Figs. S4 and SI). Owing to the photo-isomerization of the spiropyran, the PNIPAAm-SP hydrogel shows photo-switchable “on/off” fluorescence behavior. Being irradiated under white light in H2O, the gel may exhibit weak fluorescence emission at 610 nm, which is mainly the fluorescence generated by the ring-closing form [36]; when exposed to UV light, the gel shows strong emission at 665 nm with a strong red fluorescence (Fig. 3b). After eight UV/white light irradiation cycles (Fig. 3c), the fluorescence intensity at 665 nm of the gel showed no obvious change, indicating that the gel possesses excellent revers ibility and anti-fading properties. The PNIPAAm-SP gel also showed solvent and temperature respon sive fluorescent behavior. Compared with that in H2O, the fluorescent intensity of the PNIPAAm-SP gel in EtOH exhibited an obvious increase (Fig. 3d), indicating that the fluorophore MC is polarity-sensitive and its
3.2. Thermo-responsive bending behavior The volume phase transition temperature (VPTT) of the gels in DI water were derived from the de-swelling curves (Fig. 2a). Incorporation of a hydrophobic moiety into the PNIPAAm gel matrix may decrease the VPTT of the gel. As expected, the VPTT of the PNIPAAm-1 and PNIPAAm-4 gel is about 32 � C, which decreases to 30 � C in the presence of SP moiety (PNIPAAm-SP hydrogel). Significant difference of the deswelling ratio between PNIPAAm-SP and PNIPAAm-y (y ¼ 1 or 4) gel was observed in the range of 28–35 � C, thus 35 � C was selected as the triggering temperature in the following study. The bending degrees of the PNIPAAm-SP/PNIPAAm-4 and PNIPAAm-1/PNIPAAm-4 bilayer gels at 35 � C in water were evaluated (Fig. 2b). At 35 � C, both of the two layers shrunk, while the PNIPAAm-SP with lower VPTT may shrink faster than the PNIPAAm-4 layer. Thus, the PNIPAAm-SP can act as the active layer and the difference in the deswelling ratio drove the gel to bend to the PNIPAAm-SP side. The bilayer gel bends to the PNIPAAm-SP layer and form a circular shape in 2 min and reaches maximum in 7 min. On the contrary, a different phenomenon was observed for the PNIPAAm-1/PNIPAAm-4 bilayer gel. Since there was no obvious difference in the de-swelling ratios between the two layers, the original curly PNIPAAm-1/PNIPAAm-4 gel strip shrunk at 35 � C with no obvious change of the shape. With the incor poration of SP moiety, the PNIPAAm-based bilayer gel can achieve planar-to-3D shape transitions in response to the temperature change. The bilayer hydrogel also shows excellent repeatability of bending/un bending behavior at 4 � C and 35 � C in DI water for three cycles (Fig. 2c). The bending degree of the bilayer gel reached maximum (K ¼ 5.2) in 5
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Fig. 4. Demonstration of (a) capture and (b) release of a silica gel bead by the cross-shaped PNIPAAm-SP/PNIPAAm-4 bilayer gel as temperature-controlled gripper.
fluorescence intensity increases with the decrease of the polarity of the solvent. It is expected that the PNIPAAm-SP gel might exhibit enhanced fluorescence intensity above its VPTT resulting from the formation of hydrophobic domain near the polarity-responsive MC fluorophore [37–40]. As shown in the variable-temperature fluorescence spectra, the fluorescent intensity of the PNIPAAm-SP increased when the tempera ture increased from 25 � C to 35 � C (Fig. 3e).
shrunk at 35 � C in water and at the same time, the petals rolled upward and deformed to extreme in 12 min. When the gel “flower” was put back into water at 4 � C, the gel flower re-swelled and it took 20 min for the petals to become flat. As expected, the gel “flower” blossoms fall toward the opposite direction triggered by solvent and temperature. In addition to the shape deformation of the gel “flower”, the color of the gel “flower” can also be tuned by the external stimulus (light, solvent, and temper ature) (Fig. 5d). The PNIPAAm-SP/PNIPAAm-4 hydrogel “flower” shows reversible photochromic properties with its color changed between colorless and red when it was irradiated by visible and UV light in water. Correspondingly, upon UV irradiation, the fluorescent emission of the gel “flower” was switched “on” with a strong reddish color, and then it was turned “off” upon irradiation by white light. Moreover, the fluo rescent emission intensity of the gel flower changed during the solvent or temperature-triggered blooming/blossom falling process. Similarly to the PNIPAAm-SP gel, the fluorescent emission of the blossom falling gel “flower” became stronger in EtOH or at 35 � C. Furthermore, inspired by the different bending-unbending behaviors of the bilayer gels in DI water and ethanol, a biomimetic “hand” was also assembled from the PNIPAAm-SP/PNIPAAm-y bilayer gel (Fig. 5e). The composition of the fingers of the biomimetic “hand” was as follows: the PNIPAAm-SP/PNIPAAm-2 thumb and forefinger, the (PNIPAAm-SP/ PNIPAAm-4) middle finger, ring finger, and little finger. In DI water, the biomimetic “hand” showed a gesture of the “F” letter in American Sign Language with bent thumb/forefinger and straight middle/ring/little fingers. In ethanol, the PNIPAAm-SP/PNIPAAm-2 gels became straight, and at the same time the PNIPAAm-SP/PNIPAAm-4 bent, in this case, the biomimetic “hand” showed a different gesture of the “L” letter. The gels with various bending/unbending behaviors may also be designed to each joints of the hand to meet the critical need of the complicated soft robot. Taking the advantage of the real-time spatiotemporal precision of light stimulus, the bilayer hydrogel could reversibly display different patterns. Moreover, with their planar-to-3D shape morphing ability, patterns could be easily “drawn” in a 3D way. For instance, as shown in Fig. 6a, firstly, the bilayer gel film was covered with photo-mask and exposed to UV light for 2 min, resulting in a “snowflake” pattern on the gel film; then the photo-patterned gel was transferred to EtOH for 2 min, the gel film “self-rolled” and a gel tube formed with “snowflake” pat terns on the outer surface; after being exposed to Vis light for 1 min, the pattern was “erased”. On the second and third cycle, different patterns (the “star” and “windmill” patterns) on the gel could be reversibly “drawn” and “erased” by light (Fig. 6b and c). Moreover, the
3.4. Applications Due to its thermo-responsive bending property, the PNIPAAm-SP/ PNIPAAm-4 gel exhibited rapid, reversible, and repeatable bending/ unbending properties upon heating and cooling. A cross-shaped bilayer gel was designed as a temperature-controlled gripper. The PNIPAAm-1/ PNIPAAm-4 bilayer gel (without the SP moiety), the gel stayed as the gripping state both at 25 � C and 35 � C (Figs. S5 and SI). When the PNIPAAm-SP/PNIPAAm-4 bilayer gel was immersed in a water bath at 35 � C, it bent rapidly and captured, i.e., gripped, the silica gel bead within 30 s (Fig. 4a). When the temperature of the water decreased to 25 � C (lower than the VPTT), the bilayer hydrogel unfolded and released the bead within 180 s (Fig. 4b). The gripping and releasing of targeted objects by the bilayer hydrogel could be simply controlled by the temperature. The applications of the bilayer gel as biomimetic devices were then testified. A solvent and thermo-triggered blooming/blossom falling of a gel “flower” accompanying with fluorescent color changing was demonstrated using the PNIPAAm-SP/PNIPAAm-4 bilayer gel (Fig. 5). During the solvent and thermo-triggered bending process, the SP moiety played different roles on the bilayer gel. Due to its hydrophobic nature, the SP moiety facilitated the swelling of the PNIPAAm gel in organic solvents as well as its de-swelling at VPTT. Consequently, the PNIPAAmSP layer acted as the active layer to drive the bilayer gel to bend to bidirection in response to solvent and temperature as illustrated in Fig. 5a. Since the PNIPAAm-SP layer possesses solvent and temperature responsive fluorescent behaviors, the gel “flower” might appear a color changing behavior during the blooming/blossom falling process. Fig. 5b demonstrated the blooming/blossom falling of a PNIPAAm-SP/ PNIPAAm-4 bilayer gel “flower” triggered by solvent. When the fully swollen gel was transferred from water to ethanol, the petals bent downward and deformed to extreme in 16 min. When the gel “flower” was put back into water, it took 20 min for the gel to recover and the petals became flat. The blooming/blossom falling of the gel “flower” can also be driven by the change of temperature (Fig. 5c). The actuator 6
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Fig. 5. Bilayer gels as biomimetic devices. (a) Illustration of the solvent and temperature triggered blooming/blossom falling of the gel “flower”, (b) Photos of PNIPAAm-SP/PNIPAAm-4 bilayer gel “flower” in H2O and EtOH (The top layer of the gel is PNIPAAm-SP), (c) Photos of PNIPAAm-SP/PNIPAAm-4 bilayer gel “flower” in H2O at 35 � C and 4 � C (The top layer of the gel is PNIPAAm-SP), (d) Fluorescent images of PNIPAAm-SP/PNIPAAm-4 gel “flower” in water (25 � C and 35 � C) and EtOH, (e) Gestures of a gel “hand” simulated by H2O and EtOH.
corresponding fluorescence images with the reddish colored patterns can also be detected after the UV irradiation, from pale red to strong reddish switched by Vis and UV light both in aqueous and EtOH. With the aid of photolithography and computer-aided design, variable complicated information or patterns would be “encrypted” or “decryp ted” on the bilayer gel with different geometric configurations. Taking advantages of this unique characteristic, the bilayer gel can be utilized in diverse fields, especially for soft robotics or wearable devices which need display or encrypt/decrypt information.
hydrogel can be stimulated to bend to bi-direction in response to solvent and temperature, i.e. the gel “flower” blossom fall toward the opposite direction triggered by solvent and temperature. By adjusting the con centrations of cross-linker of the PNIPAAm layer, bilayer gel with var iable bending/unbending behaviors could be constructed. Furthermore, owing to the multi-stimuli responsive color changing characteristic of the spiropyran moiety, the bilayer gel actuator can realize color changing behaviors accompanying with its shape deformation process. Soft devices with complicated 3D structures and patterns can also be integrated together using the bilayer gel. Taking the above advantages, the multifunctional gel actuator showed potential applications as gripper, biomimetic devices, and information storage/encryption devices.
4. Conclusions Solvent and thermo-responsive PNIPAAm-based bilayer hydrogel actuators with photo-switchable color changing behaviors are designed by simply incorporation of spiropyran moiety into one of the layers. Spiropyran played different roles in the swelling/deswelling of the PNIPAAm gel, which is not only favorable of the swelling of the PNI PAAm gel in organic solvent (i.e. ethanol), but also benefit of the shrinking of the PNIPAAm gel above VPTT. In this case, the bilayer
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Fig. 6. Planar-to-3D shape morphing of bilayer hydrogel with different patterns “drawn” and “erased” by light. (a), (b), and (c) The photographs of the actuating and photo-patterning of the same bilayer gel for three cycles; Insets: fluorescence images of the corresponding patterns on the gel films or the tubes.
Acknowledgments
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Multi-responsive hydrogel actuator with photo-switchable color changing behaviors Xin Zhang a, 1, Xiaolei Xu a, 1, Lishan Chen a, Chao Zhang b, Liqiong Liao a, * a
Guangdong Provincial Key Laboratory of Construction and Detection in Tissue Engineering, School of Biomedical Engineering, Southern Medical University, Guangzhou, 510515, China b School of Biomedical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(N-isopropylacrylamide) Spiropyran Hydrogel actuator Photo-switchable
Hydrogel actuators, in addition to their programmable shape deformations triggered by external stimuli, are expected to integrate multi-functionality in response to multiple stimuli from the environment and to mimic the biology systems. In this work, inspired by animals (i.e. cephalopods or chameleon) that can change the color of the skin to mimic the environment, a hydrogel actuator was designed to possess both solvent and thermoresponsive bending properties as well as photo-switchable color changing behaviors. This hydrogel actuator consists of a poly (N-isopropylacrylamide) (PNIPAAm) layer and a PNIPAAm layer with spiropyran moiety (PNIPAAm-SP). Spiropyran, a hydrophobic and photochromic compound, endowed the bilayer gel with solvent and temperature triggered reversible bending behaviors as well as the reversible displaying of different patterns that can be “drawn” or “erased” by light. Moreover, accompanying with its shape deformation, the bilayer gel showed a change in fluorescent behavior, i.e. its fluorescent intensity increased upon switching the solvent from DI water to EtOH or increasing the temperature. The potential applications of this bilayer hydrogel in biomimetic devices, gripper, and information storage were demonstrated. With complicated 3D shape-morphing as well as color changing ability integrated in one bilayer gel system, this bilayer hydrogel may find its broad application including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors.
1. Introduction Hydrogel actuators with programmable shape deformations in responding to external stimuli have arisen tremendous interests due to their great potential in multiple applications [1–3], such as wearable devices [4], nanomechanical devices [5], biocompatible/medical de vices [6], transparent ionic conductors [7], and micro-biorobots [8]. Usually, the design of hydrogel actuators could be based on the homo geneous expansion/contraction with special structures [9] or inhomo geneous structures with different swelling/shrinkage degree in different directions. Under external stimuli, the former shows movement toward specific direction and the later exhibits bending/unbending behavior [10,11]. Due to the advantages of good operability and controllability, anisotropic hydrogels with inhomogeneous structures have shown to be good candidates as actuators, among which the bilayer hydrogel that can switch to complicated 2D and 3D shapes under external stimuli have attracted great interests [12,13]. Planar-to-3D shape transitions are
achieved by modulating a local concentration and/or cross-linking density of the two layers [14]. For example, a bilayer hydrogel made from thermo-responsive precursor with varied concentrations may eventually undergo inhomogeneous deswelling upon heating, and consequently adopt a particular 3D morphology to minimize the elastic energy [15]. Utilizing the above strategy, bilayer hydrogel actuators in response to temperature [16], pH [17], solvent [18], light [19], and electric field [20] have been successfully constructed. In practical applications, multi-functional actuators may be highly desirable to respond to multiple stimuli from the environment and biology systems [21–23]. Inspired by animal tissues/organs, actuators that can camouflage (i.e. color change) by sensing the environment stimuli were developed [24–26]. Until now, only a few hydrogel actu ators with color change functionality were reported [21,22,27,28]. For example, Chen et al. developed a temperature stimulated bilayer hydrogel actuator with pH switchable fluorescence behavior, which consisted of a graphene oxide-poly (N-isopropylacrylamide)
* Corresponding author. E-mail address: [email protected] (L. Liao). 1 These two authors contributed equally to this work. https://doi.org/10.1016/j.dyepig.2019.108042 Received 15 September 2019; Received in revised form 5 November 2019; Accepted 11 November 2019 Available online 11 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Xin Zhang, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108042
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afford the bilayer gel with shape morphing ability accompanied by fluorescent color changing. Taking the advantage of the real-time spatiotemporal precision of light stimulus, the bilayer hydrogel, namely, PNIPAAm-SP/PNIPAAm, can reversibly display different pat terns that are “drawn” or “erased” by light (Scheme 1c). The above new function will broaden the application of the bilayer hydrogel actuators in a wide range fields, including bio-mimetic robotics, wearable devices, biocompatible/medical devices, and environmental sensors. 2. Experimental section 2.1. Materials N-isopropylacrylamide (NIPAAm, 98%, Sigma-Aldrich), N, N0 methylenebisacrylamide (BIS, 99%, Sigma-Aldrich), and phenylbis (2,4,6-trimethylbenzoyl) phosphine oxide (PBPO, 97%, Sigma-Aldrich) were used as received. Dioxane, ethanol, methylene blue, and acetone were of analytical grade and used without further purification. 2-(30 ,30 dimethyl-6-nitrospiro [chromene-2,20 -indoline]-10 -yl) ethyl methacry late (SP) was synthesized according to literature [33] (Schemes S1 and SI) and its chemical structure was characterized by proton nuclear magnetic resonance spectroscopy (1H NMR) (Figs. S1 and SI). 2.2. Preparation of monolayer gels To prepare a typical monolayer PNIPAAm gel sheet, a precursor solution containing 2.35 M NIPAAm, 0.77 mol% PBPO, and pre determined amount of BIS was first prepared using a mixture solvent of dioxane and deionized water (DI) (4:1 in volume). The precursor solu tion was then transferred to a mold, which consists of a transparent quartz upper layer, a thin polytetrafluoroethylene (PTFE) frame (0.3 mm thickness), and a PTFE bottom layer. Then the precursor solu tion was irradiated under blue light (490 nm, 18 W) for 5 min to obtain a PNIPAAm gel sheet. The gel sheet was washed with acetone, and equilibrated in DI water for one week to remove unreacted chemicals. Then the gel was dyed with 1 M methylene blue aqueous solution. The gels were named as PNIPAAm-y (y ¼ 1, 2, 3, 4, and 5) for the gel pre pared from 0.04, 0.12, 0.16, 0.20, and 0.24 M BIS, respectively. In the case of PNIPAAm-SP gel sheet, 2 mol % of SP and 0.04 M of BIS was added to the precursor solution and then the same protocol was followed subsequently.
Scheme 1. Schematic illustration of the multi-responsive PNIPAAm-SP/PNI PAAm bilayer hydrogel actuator with photo-switchable color changing behav iors. (a) Solvent and temperature actuating of the bilayer gel, (b) Photoisomerization of the SP moiety in the PNIPAAm-SP layer, (c) 3D-morphing and photo-pattern of the bilayer hydrogels.
2.3. Preparation of bilayer hydrogels
(GO-PNIPAAm) hydrogel layer with a pH responsive perylene bisimide-functionalized hyperbranched polyethylenimine (PBI-HPEI) hydrogel layer [28]. Generally, it is still a challenge to construct hydrogel actuators with integrated multi-stimuli responsive shape morphing capability as well as color changing properties. Herein, we report the design and fabrication of bilayer hydrogel as solvent and temperature triggered actuator with photo-switchable color changing behaviors (Scheme 1). Briefly, a hydrophobic and photo chromic moiety (spiropyran (SP)), was incorporated into one layer of the PNIPAAm bilayer hydrogel, which gave distinct affinity of two layers toward water/organic solvent owing to their different hydrophilicity/ hydrophobicity, and thus help achieve anisotropic swelling of each layer and deformation of the bilayer hydrogel upon solvent and temperature change (Scheme 1a). Moreover, the spiropyran (SP) moiety will bring photo-switchable color changing properties to the bilayer gels, owing to the photo-isomerization between the “colorless” SP isomer and the “colored” merocyanine (MC) isomer (Scheme 1b) [29]. Due to the different physicochemical properties of the two isomers, such as polar ity, fluorescence, and electronic properties, photo-responsive SP hydrogels have been constructed with applications in photo-tunable microvalves [30] and photoactuators [31]. In addition, the fluorescent behavior of MC was found to be responsive to polarity [32], which might
Upon the completion of the preparation of the monolayer PNIPAAmSP gel sheet, a second PTFE spacer of 0.3 mm thick was placed between the first spacer and the PTFE bottom. Then, a precursor solution of the PNIPAAm gel was injected into the mold and irradiated under blue light for additional 3 min. The obtained bilayer gels were named PNIPAAmSP/PNIPAAm-y (y ¼ 1, 2, 3, 4, and 5) in the following context. Varied shapes of the bilayer hydrogel, such as “strip”, “flower”, and “hand” were fabricated using custom-made molds. 2.4. Swelling ratio of the gels The fully swollen PNIPAAm and PNIPAAm-SP monolayer gel were weighed at 4 � C and dried under vacuum. The swelling ratio (SR) for the hydrogels was calculated using the following equation: The swelling ratio in water: SR
DI water ¼
Ws water Wd Wd � ρwater
(1)
The swelling ratio in ethanol: SR
2
Ethanol ¼
Ws ethanol Wd Wd � ρethanol
(2)
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Fig. 1. Solvent responsive bending behavior of the bilayer gels. (a) The swelling ratios of the gels in H2O and EtOH, (b) Bending degree of the bilayer gels as a function of time in EtOH (A–E: PNIPAAm-SP/PNIPAAm-y (y is from 1 to 5), F: PNIPAAm-1/PNIPAAm-4), (c) Photographs of the PNIPAAm-SP/PNIPAAm-y (y ¼ 1–5) and PNIPAAm-1/PNIPAAm-4 bilayer gels in H2O and EtOH, (d) Bending degrees of the PNIPAAm-SP/PNIPAAm-4 bilayer hydrogel in EtOH, acetone, and dioxane, (e) Bending degrees of the PNIPAAm-SP/PNIPAAm-4 bilayer hydrogel after soaking to reach equilibrium alternately in EtOH and H2O for three cycles.
where Ws-water is the weight of gel swollen in water and Ws-ethanol is the weight of gel swollen in ethanol; Wd is the dry weight of the gel, ρwater and ρethanol are the density of the water and ethanol, respectively.
where Wx is the weight of the gels at predetermined temperature (26 � C–40 � C), W4 is the weight of the full swollen gel at 4 � C. 2.6. Bending degree of the bilayer gels
2.5. De-swelling ratio of gels
The bending process of a bilayer gel strip (20 mm � 1 mm � 0.6 mm) soaking in solvents was recorded using a digital camera. The instanta neous radius of the bending bilayer gel strip (R) was measured on the images using Image J software (1.44p, 2014, National Institutes of Health, USA), and the bending degree Κ was derived from R:
The relative de-swelling ratio of the PNIPAAm and PNIPAAm-SP monolayer gels in DI water under different temperatures was deter mined. The de-swelling ratio of the gel at 4 � C and predetermined temperature (26 � C–40 � C) was measured. The de-swelling ratio (De-SR) was calculated by the following equation: De
SR ¼
W4
Wx W4
K ¼ 1=R
(3)
(4)
In this work, the bending toward the PNIPAAm layer is designated a positive bending degree, and a negative bending degree represents the 3
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Fig. 2. Thermo-responsive bending behavior of the bilayer gels. (a) De-swelling ratio of the PNIPAAm-1, PNIPAAm-4, and PNIPAAm-SP hydrogel at various tem peratures, (b) Bending degree of the PNIPAAm-SP/PNIPAAm-4 and PNIPAAm-1/PNIPAAm-4 bilayer gels at 35 � C in water, (c) Bending degree of the PNIPAAm-SP/ PNIPAAm-4 bilayer gel after soaking to reach equilibrium alternately at 35 � C and 4 � C aqueous solution for three cycles.
direction of the bending toward the PNIPAAm-SP layer.
would re-swell to some extent when being transferred from DI water to ethanol, and the PNIPAAm-SP gel exhibited much higher SR change than that of PNIPAAm gel. Usually, the layer with larger volume change acts as the driven/active layer to induce the bending upon the exposure to stimuli [35]. Thus, the PNIPAAm-SP layer that experienced larger volume phase change acted as the active layer to drive the bilayer gels to bend/unbend in the studied cases. Generally, larger difference in SRs in response to stimulus could produce a stronger driving force to cause a larger deformation of the bilayer gels. Since the SR of two layers could be tuned by varying the BIS concentration, the bilayer gels also showed different bending behaviors (i.e. bending degree) in response to solvent. When the bilayer gels were immersed in ethanol to reach equilibrium, the bending degree increased (Fig. 1b, A-E) and the bilayer gels trans formed to different shapes (Fig. 1c, A-E). The PNIPAAm-SP/PNIPAAm-y (y ¼ 1 and 2) bent to the PNIPAAm-SP layer side (Fig. 1c, A-B); the PNIPAAm-SP/PNIPAAm-y (y ¼ 3, 4, and 5) bilayer gels bent to the PNIPAAm layer side (Fig. 1c, C-E). In addition, it was observed that the bilayer gel from PNIPAAm layer with higher concentration of BIS responded faster upon the solvent change. In detail, the PNIPAAm-SP/PNIPAAm-y (y ¼ 3, 4, and 5) gels reached the maximum deformation in 8 min. However, it took nearly 12 min for the PNIPAAm-SP/PNIPAAm-y (y ¼ 1 and 2) to reach equilibrium. The bending behavior of PNIPAAm-1/PNIPAAm-4 bilayer gel was also examined to evaluate the role of the SP moiety. In DI water or ethanol, the PNIPAAm-1/PNIPAAm-4 bilayer gel bends to the PNIPAAm-4 layer owing to the different SRs of the two layers. The gel showed slight difference in bending degree after being transferred from DI water to ethanol (Fig. 1b, F), with no obvious change of shape (Fig. 1c, f-F). In contrast, the PNIPAAm-SP/PNIPAAm-4 bilayer gel possesses excellent solvent triggered bending behavior with a “straightto-curve” change of shape (Fig. 1b, D and Fig. 1c, d-D). The above result demonstrates that PNIPAAm bilayer gel with excellent solvent respon sive bending behavior can be fabricated by incorporation of the hy drophobic SP moiety into one of the layers. The bilayer gel also exhibited reversible solvent responsive bending behavior triggered by other organic solvent like acetone and dioxane (Fig. 1d). For instance, the bending degree of the PNIPAAm-SP/ PNIPAAm-4 bilayer gel increased when it was transferred from DI water to the solvents, and then decreased when it was put back into DI water. The response time of the bilayer gels depended on the solvent, i.e. the PNIPAAm-SP/PNIPAAm-4 bilayer gel needs longer response time in dioxane than that in ethanol and acetone, which may be caused by the different diffusion rates of the solvents. The deformation repeatability is one of the important parameters for actuating materials. Here, the bending degrees of the bilayer gels (in DI water and ethanol) in cyclic tests were recorded (Fig. 1e). The bilayer gel showed bending-unbending behavior after being immersed in ethanol and water alternatively for three cycles. The bending degree of the
2.7. Photo-switchable color changing behavior The PNIPAAm-SP gel sheets were fully swollen in DI water and ethanol, respectively. After being irradiated by UV (UV-black Ray 365 nm, 30W) or white light (LED, 40 W), the UV–Vis absorption spectra of the gel sheets were measured at room temperature. The measure ments were performed on a Shimadzu UV-3600 230VCE UV–Vis spec trometer. The fluorescence spectra of the gel sheets were recorded on fluorescence spectrophotometers (HORIBA Scientific, FluoroMax-4) with an excitation wavelength of 560 nm and a combined fluorescence lifetime and steady state spectrometer (EDINBURGH, FLS920) with an excitation wavelength of 540 nm. The fluorescent photographs of the gel sheets were taken by a fluorescence imager (Alpha, USA). 3. Results and discussions 3.1. Solvent responsive bending behavior A series of PNIPAAm-SP/PNIPAAm-y bilayer hydrogels were pre pared with varied crosslink density by tuning the concentration of the crosslinker (BIS). The interior morphology of the bilayer gel (i.e. PNIPAAm-SP/PNIPAAm-4) was examined by SEM (Figs. S2 and SI), and it was found that the both layers possess characteristic porous structure and no obvious phase separation between the two layers was discern ible, which is of essentially importance to achieve appreciable actuation upon stimuli. Spiropyran (contact angle of 92� ) is far more hydrophobic than NIPAAm monomer (contact angle of 28� ) (Figs. S3 and SI), such difference in the hydrophobicity/hydrophilicity of the precursors would no doubt bring anisotropic swelling behavior to the bilayer hydrogel in response to solvents with different hydrophobicity/hydrophilicity. The SR of the PNIPAAm-y (y ranges from 1 to 5) and PNIPAAm-SP gel sheets in DI water and ethanol was evaluated (Fig. 1a). The SR-DI water of the PNIPAAm-y gel decreased at higher BIS concentration. Since the Hil debrand solubility parameter of PNIPAAm is closer to ethanol than to DI water [33], the PNIPAAm chains preferred to be more relaxed in ethanol and the gels underwent further swelling when transferred from DI water to ethanol. In addition, SP moiety may also facilitate the swelling of the gels in ethanol; it was observed that the SR-Ethanol of PNIPAAm-SP is 1.6 times that in DI water, which is also significantly higher than those of the PNIPAAm-y gels (1.1–1.3 times) without SP moiety. Usually, het erogeneous volume change upon the diffusion of solvent into actuators may result in macroscopic shape deformations and/or adaptive move ments [34]. The different swelling ratios of the two layers in DI water/ethanol open the opportunity for designing bilayer gels with controlled solvent-responsive bending behaviors. It was noted that both PNIPAAm and PNIPAAm-SP monolayer gels 4
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Fig. 3. Photo-switchable color changing behaviors. (a) UV–visible spectra of PNIPAAm-SP hydrogel under UV/Vis irradiation in H2O at 25 � C, (b) Fluorescence emission spectra of the PNIPAAm-SP hydrogel under UV/Vis irradiation in H2O at 25 � C, (c) Fluorescence intensity of the PNIPAAm-SP hydrogel under alternative UV/Vis irradiation in H2O at 25 � C, (d) Fluorescence emission spectra of the PNIPAAm-SP hydrogel in H2O and EtOH at 25 � C under UV irradiation, (e) Fluorescence emission spectra of the PNIPAAm-SP hydrogel in H2O at 25 � C and 35 � C under UV irradiation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
bilayer gel reached maximum in 7 min in ethanol, and then dropped to zero within 13 min in DI water. The rate for the unbending is lower than for the bending, which may be attributed to the slower rate of the diffusion of ethanol out of the gel [34].
7 min at 35 � C, and then dropped to zero within 10 min at 4 � C. 3.3. Color changing behavior The isomerization of spiropyran upon Vis/UV light irradiation not only changes the hydrophobicity/hydrophilicity, but also would lead to different fluorescence properties: the MC isomer has intense red fluo rescence, while the SP virtually shows weak or no fluorescence [36]. In this case, incorporation of the SP moiety may endow the hydrogel actuator with reversible photo-chromic and photo-switchable fluores cence properties. The photo-chromic and photo-switchable fluorescence properties of PNIPAAm-SP gel were studied in detail. As evidenced by the UV–Vis spectra (Fig. 3a), there was no obvious absorption peak in the investigated range of wavelength after irradiation under white light, indicating the spiropyran moiety in the hydrogel exists as the SP struc ture; when the gel was irradiated under UV light, an absorption peak at around 550 nm belonging to opening MC isomer was observed and the color of the gel changed from pale yellow to red [36]. Using rhodamine B as the reference, the quantum yield of the PNIPAAm-SP gel was calculated as 0.10 (Figs. S4 and SI). Owing to the photo-isomerization of the spiropyran, the PNIPAAm-SP hydrogel shows photo-switchable “on/off” fluorescence behavior. Being irradiated under white light in H2O, the gel may exhibit weak fluorescence emission at 610 nm, which is mainly the fluorescence generated by the ring-closing form [36]; when exposed to UV light, the gel shows strong emission at 665 nm with a strong red fluorescence (Fig. 3b). After eight UV/white light irradiation cycles (Fig. 3c), the fluorescence intensity at 665 nm of the gel showed no obvious change, indicating that the gel possesses excellent revers ibility and anti-fading properties. The PNIPAAm-SP gel also showed solvent and temperature respon sive fluorescent behavior. Compared with that in H2O, the fluorescent intensity of the PNIPAAm-SP gel in EtOH exhibited an obvious increase (Fig. 3d), indicating that the fluorophore MC is polarity-sensitive and its
3.2. Thermo-responsive bending behavior The volume phase transition temperature (VPTT) of the gels in DI water were derived from the de-swelling curves (Fig. 2a). Incorporation of a hydrophobic moiety into the PNIPAAm gel matrix may decrease the VPTT of the gel. As expected, the VPTT of the PNIPAAm-1 and PNIPAAm-4 gel is about 32 � C, which decreases to 30 � C in the presence of SP moiety (PNIPAAm-SP hydrogel). Significant difference of the deswelling ratio between PNIPAAm-SP and PNIPAAm-y (y ¼ 1 or 4) gel was observed in the range of 28–35 � C, thus 35 � C was selected as the triggering temperature in the following study. The bending degrees of the PNIPAAm-SP/PNIPAAm-4 and PNIPAAm-1/PNIPAAm-4 bilayer gels at 35 � C in water were evaluated (Fig. 2b). At 35 � C, both of the two layers shrunk, while the PNIPAAm-SP with lower VPTT may shrink faster than the PNIPAAm-4 layer. Thus, the PNIPAAm-SP can act as the active layer and the difference in the deswelling ratio drove the gel to bend to the PNIPAAm-SP side. The bilayer gel bends to the PNIPAAm-SP layer and form a circular shape in 2 min and reaches maximum in 7 min. On the contrary, a different phenomenon was observed for the PNIPAAm-1/PNIPAAm-4 bilayer gel. Since there was no obvious difference in the de-swelling ratios between the two layers, the original curly PNIPAAm-1/PNIPAAm-4 gel strip shrunk at 35 � C with no obvious change of the shape. With the incor poration of SP moiety, the PNIPAAm-based bilayer gel can achieve planar-to-3D shape transitions in response to the temperature change. The bilayer hydrogel also shows excellent repeatability of bending/un bending behavior at 4 � C and 35 � C in DI water for three cycles (Fig. 2c). The bending degree of the bilayer gel reached maximum (K ¼ 5.2) in 5
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Fig. 4. Demonstration of (a) capture and (b) release of a silica gel bead by the cross-shaped PNIPAAm-SP/PNIPAAm-4 bilayer gel as temperature-controlled gripper.
fluorescence intensity increases with the decrease of the polarity of the solvent. It is expected that the PNIPAAm-SP gel might exhibit enhanced fluorescence intensity above its VPTT resulting from the formation of hydrophobic domain near the polarity-responsive MC fluorophore [37–40]. As shown in the variable-temperature fluorescence spectra, the fluorescent intensity of the PNIPAAm-SP increased when the tempera ture increased from 25 � C to 35 � C (Fig. 3e).
shrunk at 35 � C in water and at the same time, the petals rolled upward and deformed to extreme in 12 min. When the gel “flower” was put back into water at 4 � C, the gel flower re-swelled and it took 20 min for the petals to become flat. As expected, the gel “flower” blossoms fall toward the opposite direction triggered by solvent and temperature. In addition to the shape deformation of the gel “flower”, the color of the gel “flower” can also be tuned by the external stimulus (light, solvent, and temper ature) (Fig. 5d). The PNIPAAm-SP/PNIPAAm-4 hydrogel “flower” shows reversible photochromic properties with its color changed between colorless and red when it was irradiated by visible and UV light in water. Correspondingly, upon UV irradiation, the fluorescent emission of the gel “flower” was switched “on” with a strong reddish color, and then it was turned “off” upon irradiation by white light. Moreover, the fluo rescent emission intensity of the gel flower changed during the solvent or temperature-triggered blooming/blossom falling process. Similarly to the PNIPAAm-SP gel, the fluorescent emission of the blossom falling gel “flower” became stronger in EtOH or at 35 � C. Furthermore, inspired by the different bending-unbending behaviors of the bilayer gels in DI water and ethanol, a biomimetic “hand” was also assembled from the PNIPAAm-SP/PNIPAAm-y bilayer gel (Fig. 5e). The composition of the fingers of the biomimetic “hand” was as follows: the PNIPAAm-SP/PNIPAAm-2 thumb and forefinger, the (PNIPAAm-SP/ PNIPAAm-4) middle finger, ring finger, and little finger. In DI water, the biomimetic “hand” showed a gesture of the “F” letter in American Sign Language with bent thumb/forefinger and straight middle/ring/little fingers. In ethanol, the PNIPAAm-SP/PNIPAAm-2 gels became straight, and at the same time the PNIPAAm-SP/PNIPAAm-4 bent, in this case, the biomimetic “hand” showed a different gesture of the “L” letter. The gels with various bending/unbending behaviors may also be designed to each joints of the hand to meet the critical need of the complicated soft robot. Taking the advantage of the real-time spatiotemporal precision of light stimulus, the bilayer hydrogel could reversibly display different patterns. Moreover, with their planar-to-3D shape morphing ability, patterns could be easily “drawn” in a 3D way. For instance, as shown in Fig. 6a, firstly, the bilayer gel film was covered with photo-mask and exposed to UV light for 2 min, resulting in a “snowflake” pattern on the gel film; then the photo-patterned gel was transferred to EtOH for 2 min, the gel film “self-rolled” and a gel tube formed with “snowflake” pat terns on the outer surface; after being exposed to Vis light for 1 min, the pattern was “erased”. On the second and third cycle, different patterns (the “star” and “windmill” patterns) on the gel could be reversibly “drawn” and “erased” by light (Fig. 6b and c). Moreover, the
3.4. Applications Due to its thermo-responsive bending property, the PNIPAAm-SP/ PNIPAAm-4 gel exhibited rapid, reversible, and repeatable bending/ unbending properties upon heating and cooling. A cross-shaped bilayer gel was designed as a temperature-controlled gripper. The PNIPAAm-1/ PNIPAAm-4 bilayer gel (without the SP moiety), the gel stayed as the gripping state both at 25 � C and 35 � C (Figs. S5 and SI). When the PNIPAAm-SP/PNIPAAm-4 bilayer gel was immersed in a water bath at 35 � C, it bent rapidly and captured, i.e., gripped, the silica gel bead within 30 s (Fig. 4a). When the temperature of the water decreased to 25 � C (lower than the VPTT), the bilayer hydrogel unfolded and released the bead within 180 s (Fig. 4b). The gripping and releasing of targeted objects by the bilayer hydrogel could be simply controlled by the temperature. The applications of the bilayer gel as biomimetic devices were then testified. A solvent and thermo-triggered blooming/blossom falling of a gel “flower” accompanying with fluorescent color changing was demonstrated using the PNIPAAm-SP/PNIPAAm-4 bilayer gel (Fig. 5). During the solvent and thermo-triggered bending process, the SP moiety played different roles on the bilayer gel. Due to its hydrophobic nature, the SP moiety facilitated the swelling of the PNIPAAm gel in organic solvents as well as its de-swelling at VPTT. Consequently, the PNIPAAmSP layer acted as the active layer to drive the bilayer gel to bend to bidirection in response to solvent and temperature as illustrated in Fig. 5a. Since the PNIPAAm-SP layer possesses solvent and temperature responsive fluorescent behaviors, the gel “flower” might appear a color changing behavior during the blooming/blossom falling process. Fig. 5b demonstrated the blooming/blossom falling of a PNIPAAm-SP/ PNIPAAm-4 bilayer gel “flower” triggered by solvent. When the fully swollen gel was transferred from water to ethanol, the petals bent downward and deformed to extreme in 16 min. When the gel “flower” was put back into water, it took 20 min for the gel to recover and the petals became flat. The blooming/blossom falling of the gel “flower” can also be driven by the change of temperature (Fig. 5c). The actuator 6
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Fig. 5. Bilayer gels as biomimetic devices. (a) Illustration of the solvent and temperature triggered blooming/blossom falling of the gel “flower”, (b) Photos of PNIPAAm-SP/PNIPAAm-4 bilayer gel “flower” in H2O and EtOH (The top layer of the gel is PNIPAAm-SP), (c) Photos of PNIPAAm-SP/PNIPAAm-4 bilayer gel “flower” in H2O at 35 � C and 4 � C (The top layer of the gel is PNIPAAm-SP), (d) Fluorescent images of PNIPAAm-SP/PNIPAAm-4 gel “flower” in water (25 � C and 35 � C) and EtOH, (e) Gestures of a gel “hand” simulated by H2O and EtOH.
corresponding fluorescence images with the reddish colored patterns can also be detected after the UV irradiation, from pale red to strong reddish switched by Vis and UV light both in aqueous and EtOH. With the aid of photolithography and computer-aided design, variable complicated information or patterns would be “encrypted” or “decryp ted” on the bilayer gel with different geometric configurations. Taking advantages of this unique characteristic, the bilayer gel can be utilized in diverse fields, especially for soft robotics or wearable devices which need display or encrypt/decrypt information.
hydrogel can be stimulated to bend to bi-direction in response to solvent and temperature, i.e. the gel “flower” blossom fall toward the opposite direction triggered by solvent and temperature. By adjusting the con centrations of cross-linker of the PNIPAAm layer, bilayer gel with var iable bending/unbending behaviors could be constructed. Furthermore, owing to the multi-stimuli responsive color changing characteristic of the spiropyran moiety, the bilayer gel actuator can realize color changing behaviors accompanying with its shape deformation process. Soft devices with complicated 3D structures and patterns can also be integrated together using the bilayer gel. Taking the above advantages, the multifunctional gel actuator showed potential applications as gripper, biomimetic devices, and information storage/encryption devices.
4. Conclusions Solvent and thermo-responsive PNIPAAm-based bilayer hydrogel actuators with photo-switchable color changing behaviors are designed by simply incorporation of spiropyran moiety into one of the layers. Spiropyran played different roles in the swelling/deswelling of the PNIPAAm gel, which is not only favorable of the swelling of the PNI PAAm gel in organic solvent (i.e. ethanol), but also benefit of the shrinking of the PNIPAAm gel above VPTT. In this case, the bilayer
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 7
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Fig. 6. Planar-to-3D shape morphing of bilayer hydrogel with different patterns “drawn” and “erased” by light. (a), (b), and (c) The photographs of the actuating and photo-patterning of the same bilayer gel for three cycles; Insets: fluorescence images of the corresponding patterns on the gel films or the tubes.
Acknowledgments
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