Synergistic fluorescent hydrogel actuators with selective spatial shape/color‐changing behaviors via interfacial supramolecular assembly

Biomimetic intelligent polymeric hydrogel actuators with cooperative fluorescence‐color switchable behaviors are expected to find great potential applications in soft robotics, visual detection/display, and camouflage applications. However, it remains challenging to realize the spatial manipulation of synergistic shape/color‐changing behaviors. Herein, we report an interfacial supramolecular assembly (ISA) approach that enables the construction of robust fluorescent polymeric hydrogel actuators with spatially anisotropic structures. On the basis of this ISA approach, diverse 2D/3D soft fluorescent hydrogel actuators, including chameleon‐ and octopi‐shaped ones with spatially anisotropic structures, were facilely assembled from two different fluorescent hydrogel building blocks sharing the same physically cross‐linked agar network. Spatially control over synergistic shape/color‐changing behaviors was then realized in one single anisotropic hydrogel actuator. The proposed ISA approach is universal and expected to open promising avenues for developing powerful bioinspired intelligent soft actuators/robotics with selective spatial shape/color‐changing behaviors.

2][3][4][5] Such synergistic color/shapechangeable behaviors are believed to be essential survival traits for communication, camouflage, attraction, warning, and so on in their environments.8][19][20][21][22][23][24][25][26][27][28][29][30][31][32] For example, Tang, Li, and colleagues recently incorporated the responsive aggregation-induced emissive luminogens into the hydrogel actuators to achieve the simultaneous fluorescence color/brightness changes and complex shape deformation of hydrogel flower actuators in response to one single pH signal change. 33We have recently reported a series of responsive multicolor fluorescent polymer hydrogels, which could be assembled with other soft materials (e.g., nonfluorescent hydrogel, elastomer) to produce bilayer soft actuators/robotics that could behave like natural chameleons and octopi to exhibit synergistic shape/color changes in dynamic environments. 34,35hese recent advances have verified the feasibility to develop synergistic hydrogel actuators and demonstrated their potential importance for many potential uses in multifunctional sensors, smart display, and so on. 36ompared with these intense advances in which the shape and color of the whole hydrogel actuators are globally regulated, few attempts have been conducted to achieve the spatial control of such multifunctional behaviors, that is, to selectively achieve the synergistic color/shape changes of its certain part, while keeping the skin color and shape of other body parts unchanged.This makes the reported color-changing soft actuators/robots far inferior to natural chameleons and octopi in terms of spatial control capacity.For example, octopi are known to have the marvelous ability to selectively regulate the skin color and gesture of their palms while keeping the skin color and gesture of other body parts unchanged. 4herefore, such amazing spatial control of synergistic shape/color-changing behaviors, if implemented in artificial systems, would greatly promote the development of bioinspired hydrogel actuators and soft robots.However, the related study has been largely ignored.
The recent development of interfacial supramolecular assembly (ISA) is expected to be a promising solution to do this because diverse 2D/3D hydrogel structures could be facilely assembled from limited basic building blocks via a supramolecular selfassembly process. 37,38As a proof of concept, we herein take the advantage of the ISA approach to prepare the fluorescent hydrogel actuators with spatially anisotropic structures, which can be used to achieve the bioinspired synergistic fluorescence-color/ shape-changeable behaviors in a spatially controlled manner.As illustrated in Scheme 1, two doublenetwork fluorescent hydrogels, Agar/poly(potassium 6-acrylamidopicolinate-co-acrylamide) (Agar/P6A) and Agar/poly(acrylic acid-co-acrylamide-co-4-(dimethylamino)ethoxy-N-allyl-1,8-naphthalimide) (Agar/PAAD), were designed and used as the hydrogel building blocks for ISA.In both of these two hydrogels, hydrogen bond-associated agar helix bundles were involved as the physically cross-linked network.Because of the excellent reversibility of hydrogen bonds, the involved agar helix bundles could be dissociated at elevated temperature and reformed after being cooled to room temperature, 39 indicating the excellent self-gluing property of these hydrogels.On the basis of the ISA approach, several basic building blocks of Agar/P6A and Agar/PAAD hydrogel could be facilely assembled into various 2D/ 3D hydrogel actuators with spatially anisotropic structures.Owing to the differential swelling capacities of these two hydrogel layers as well as the lanthanide luminescence sensitization effect of the grafted picolinate moieties, 1 the hydrogel chameleon with spatially controlled shape/color-changing behaviors were achieved in response to the subtle interplay between different environmental stimuli (e.g., pH, Eu 3+ ions).

| Preparation of the bilayer actuator via the ISA strategy
The Agar/P6A hydrogel was prepared by radical copolymerization of acrylamide (AAm), potassium 6-acrylamidopicolinate (K6APA), methylene diacrylamide crosslinker, and potassium persulfate (KPS, the initiator) in the presence of the agar polymer according to our previous report. 40As illustrated in Figure 1A, Agar/P6A hydrogel has a double-network structure, consisting of one hydrogen-bonded agar network and one chemically crosslinked poly(K6APA-co-AAm) (P6A) network.The presence of this physically cross-linked agar network via the reversible hydrogen bonds provided a promising possibility to glue two or more hydrogel films into the laminates.To demonstrate, two identical Agar/P6A stripes were first cut by a laser cutting machine and then sealed together side by side in the mold at 80 °C (Figure 1A).At the elevated temperature, the hydrogen bonds between agar polymer chains become weakened and dissociated, making the agar polymer capable of moving across the hydrogel border of two stripes.After cooling to room temperature again, hydrogen bond-associated agar helix bundles would be reformed to glue these two hydrogel stripes together.Its scanning electron microscope image shows the distinct "self-glued" layer (Figure 1B and Supporting Information: S1), indicating the tight bonding of two hydrogel stripes without gaps.The peeling test demonstrates the maximum peeling strength exceeding 100 N/m (Figure 1C), which is large enough to stably bond two Agar/P6A hydrogel films together.Additionally, owing to the reversible nature of supramolecular hydrogen bonds between agar polymer chains, two Agar/P6A hydrogel films can be separated by the peeling operation at 60 °C and then rebonded together for several cycles (Supporting Information: Figure S2 and Movie S1).These results clearly demonstrated the high efficiency of the proposed ISA approach.
To prove the generality of this ISA approach via hydrogen bond-associated agar helix bundles, another aggregation-induced emission-active polymeric hydrogel, Agar/poly(acrylic acid-co-acrylamide-co-4-(dimethylamino) ethoxy-N-allyl-1,8-naphthalimide) (Agar/PAAD), 41 was prepared and utilized to assemble with the Agar/P6A hydrogel film.As shown in Scheme 1, the Agar/PAAD hydrogel also features a double-network structure, one chemically crosslinked PAAD network and the other physically cross-linked agar network.By using the ISA approach, stable interfacial bonding of the Agar/PAAD and Agar/P6A hydrogel stripes was realized to produce bilayer hydrogel actuators.Supporting Information: Figure S3 shows the peeling test result and the maximum peeling strength was above 150 N/ m, indicating the strong bonding between Agar/PAAD and Agar/P6A hydrogel layers.As shown in Figure 1D, there are two rectangular bilayer actuating units, one with a top S C H E M E 1 Preparation of fluorescent hydrogel actuators with spatial control of their synergistic shape/color changeable behaviors.(A) Illustration showing the fabrication of bilayer fluorescent hydrogel actuators via interfacial supramolecular assembly, as well as the realization of the spatially controlled shape/color switchable behaviors.Note that the two fluorescent hydrogel building blocks, Agar/P6A and Agar/PAAD, share the same physical cross-linked network that is stabilized by hydrogen bond-associated agar helix bundles.Because of the excellent reversibility of hydrogen bonds, the involved agar helix bundles could be dissociated at elevated temperatures and reformed after being cooled to room temperature.Therefore, several Agar/P6A and Agar/PAAD hydrogel blocks could be facilely assembled into various 2D/3D hydrogel actuators with spatially anisotropic structures, leading to spatially controlled shape/color-changing behaviors.(B) Chemical structures of the designed hydrogels.
Agar/PAAD layer and bottom Agar/P6A layer and the other with a top Agar/P6A layer and bottom Agar/PAAD layer.Although these two actuating units are actually identical, they are still specified as the Obverse (O) and Reverse (R) bonding units, respectively, to better describe the following origami-like shape deformation results.When putting this straight bilayer actuator into an aqueous Eu 3+ solution (0.01 mol/L), it gradually bent toward the Agar/P6A hydrogel layer, accompanied by remarkable emission color change (Figure 1D,E) for the following reasons.For one thing, the Agar/PAAD layer contained a high acrylic acid (molar acrylamide/acrylic acid ratio ~1) content to form high-density acrylic acid-Eu 3+ (AAc-Eu 3+ ) coordinated cross-links (Scheme 1), while the content of K6APA in the Agar/P6A layer is much less (molar acrylamide/K6APA ratio ~99), leading to much less K6APA-Eu 3+ coordinated cross-links.As a result of their differential crosslinking densities, the formed Agar/PAAD-Eu and Agar/P6A-Eu layers have quite different swelling capacities (Supporting Information: Figure S4), transforming the straight bilayer straight actuator to the circle-shaped one.For another, since the grafted K6APA moieties were known to sensitize the lanthanide fluorescence via antenna effect, simultaneous blue-to-red fluorescence color change was also noticed for the Agar/P6A-Eu hydrogel layer, resulting in synergistic shape/color changeable behaviors (Figure 1D and Supporting Information: S5).To further investigate how the length of agar-bonded bilayer actuators influences their deforming curvedness, three bilayer actuators with the length of 20, 30, and 40 mm and the same width/thickness (2 mm) were prepared and their deforming kinetics in 0.1 mol/L Eu 3+ solution were recorded.As shown in Supporting Information: Figure S6, all of these three hydrogel stripes were deformed into the circle shape with different diameters.But their response times varied considerably.It was found that the deforming speed is in a positive correlation with the actuator length.Additionally, bioinspired synergistic fluorescence-color/shape-changing behaviors can also be achieved in the Ca 2+ /Eu 3+ (molar ratio 99/1) or Al 3+ /Eu 3+ (molar ratio 99/1) solution, because common metal ions such as Ca 2+ and Al 3+ can strongly coordinate with the COO -groups in the Agar/P6A and Agar/PAAD hydrogel layers to induce the differential swelling (Supporting Information: Figure S7).
Based on their actuating performance, the O/R-type bilayer units were then utilized to further develop functional structures that enable origami-like programmable shape deformation.We first prepared a series of 1D hydrogel structures containing one to three O/R-type bilayer units, which were depicted in Figure 2A.When placing them in an aqueous Eu 3+ solution, the differential swelling extent between the Agar/P6A-Eu and Agar/PAAD-Eu layers would transform the bilayer parts of these straight actuators to the circle-shaped ones while leaving the monolayer hydrogel parts unchanged.In other words, spatial control over shape changes was realized.Besides, similar shape deformation behaviors were also observed in aqueous Tb 3+ or mixed Tb 3+ /Eu 3+ solutions but accompanied by different emission color changes (Supporting Information: Figure S8).This is because Tb 3+ has different-colored (green) intrinsic emission albeit similar coordination capacity with Eu 3+ .Therefore, by facilely varying the Tb 3+ /Eu 3+ ratio in the mixed solution, various-shaped hydrogel structures with tunable fluorescence colors would be obtained by using the developed ISA strategy.Further, a series of linear hydrogel structures containing specific sequences of four O-or R-type actuating units were prepared.As shown in Figure 2B, these as-prepared hydrogel actuators kept identical flat geometry but exhibited different 3D bending and folding deformation when placed in an aqueous Eu 3+ solution.The observed origami-like shape deformation was clearly dictated by pre-set permutations of the four O-or R-type actuating units.Following a similar line, the initial two-dimensional Hshaped hydrogel structures with specific sequences of eight O-or R-type actuating units were then designed (Figure 2C), which was found to exhibit more complex 2D-to-3D shape changes to produce well-defined 3D hydrogel configurations that are difficult to be achieved by traditional methods.Such well-demonstrated programmability of the developed ISA strategy makes it possible to predict the shape transformations by the predetermined permutations of these actuating units, holding great potential to enrich our capacity to construct complex 3D hydrogel structures.

| Bioinspired synergistic fluorescent-color-changing polymer actuators
Having demonstrated the capacity to obtain hydrogel actuators with origami-like shape changes via the ISA strategy, we next explored to develop the bioinspired multicolor fluorescence switchable polymer actuators.As illustrated in Figure 3A, the multicolor fluorescence color change of the Agar/P6A layer was derived from the stimuli-responsive transformation from the red fluorescent Eu 3+ -K6APA complexes to green fluorescent Tb 3+ -K6APA complexes via lanthanide ion replacement in an aqueous Tb 3+ solution.To showcase the synergistic multicolor fluorescence and shape changes, one flowershaped hydrogel actuator was prepared through the interfacial bonding between the Agar/P6A hydrogel layer and Agar/PAAD hydrogel layer according to the configuration schemed in Figure 3B.The as-prepared hydrogel actuator with flat geometry was blue fluorescent under 254 nm UV light.To mimic the simultaneous blooming and color-changing process of natural flowers, the actuator was first placed into 0.1 mol/L aqueous Eu 3+solution.As can be seen from Figure 3C, the differential swelling capacities between the hydrogel petals and scape led to the gradual flower blooming accompanying blue-to-red fluorescence color change.Further multifluorescence-color changes from red to yellow and then green were also realized by treating this hydrogel flower in 0.1 mol/L aqueous Tb 3+ solution.
Not only multicolor-changing systems but the colorfading hydrogel actuators could also be achieved because these lanthanide coordinated fluorescent centers are dynamic and sensitive to the environmental pH variation. 5The circle-shaped bilayer hydrogel actuator consisting of one red fluorescent Agar/P6A-Eu layer and one blue fluorescent Agar/PAAD-Eu layer was taken as an example (Figure 4). Figure 4A depicts its shape/ color-changing process in response to the environmental stimulus of NaOH.It was found that the circle-shaped hydrogel actuator was gradually transformed into a straight shape.Simultaneously, the red fluorescence of the Agar/P6A-Eu hydrogel layer became weakened and finally changed to blue.Such a shape/color-changing process was believed to originate from the NaOHtriggered chemical structure changes of both hydrogel layers.For one thing, the high-density AAc-Eu 3+ crosslinks in the Agar/PAAD-Eu hydrogel layer were gradually destroyed by NaOH, making the inner hydrogel layer capable of absorbing more water to swell.For another, the red fluorescent K6APA-Eu 3+ complexes were also not stable in alkaline NaOH solutions, leading to the red fluorescence degradation of the outer hydrogel layer.To facilitate the quantitative study, a parameter, the project length d of two ends of the actuator, was specially defined (Figure 4B).Obviously, as the actuating process proceeded, the project length d gradually increased.Therefore, we think it is a good parameter to quantitatively describe the actuating extent of the bilayer actuator.At the initial state (circle shape), when two ends of the actuator were placed together, d is about 0. The d value slowly increased within the first 800 s but rose very quickly to nearly equal the length of the straight actuator (Figure 4C).As expected, this pHcontrolled shape/color changeable speed could be adjusted by facilely varying the concentration of NaOH (Supporting Information: Figure S9).Furthermore, more complex bioinspired hydrogel actuators with NaOHtriggered synergistic shape-changing and fluorescencecolor fading behavior were demonstrated by using our ISA strategy, suggesting the universal and general ability of the proposed strategy (Supporting Information: Figures S10 and S11).

| Bioinspired hydrogel actuators with spatially controlled color/shape-changing performance
Many natural animals are well-known to have amazing control over both their skin appearance and gesture in their environments when responding to certain environmental changes.The most famous examples include the chameleons and octopi, which are capable of displaying simultaneous skin color change and body gesture deformation for better self-disguise.We tried to replicate such interesting synergistic shape/color switchable behaviors by using our fluorescent polymeric actuators.To realize the efficient spatial control, a proof of concept soft hydrogel chameleon actuator was prepared by bonding the Agar/PAAD tail onto the tail part of the Agar/P6A hydrogel body (Figure 5A and Supporting Information: Movie S2).As expected, after being immersed in an aqueous Eu 3+ solution, the chameleon body part gradually turned from weak blue to red but exhibited nearly no shape deformation.Interestingly, simultaneous shape and fluorescence-color changes were observed over time in its tail part with the bilayer hydrogel structure albeit the red fluorescence color changes happened a little earlier than the shape change of the tail parts at the beginning.Furthermore, similar synergistic color/shape-changeable behaviors of the tail part were also noticed for the subsequent actuating experiments in NaOH solution, while only the colorchanging behavior was noticed in the body part.Following a similar procedure, we also demonstrated a soft hydrogel octopus actuator, whose legs could display the bioinspired synergistic color/shape-changeable behaviors in response to the sequential stimuli of lanthanide ions (Tb 3+ /Eu 3+ mixtures) and NaOH (Figure 5B and Supporting Information: Movie S3).In this way, the spatial control over the synergistic color/ shape-changeable behaviors was successfully realized in the specially designed soft hydrogel chameleon and octopus actuators.In summary, we developed smart fluorescent polymeric hydrogel actuators with spatially anisotropic structures via the ISA.The basic hydrogel building blocks are two double-network fluorescent polymeric hydrogels, the multicolor fluorescent Agar/P6A-Eu/Tb with pHtriggered emission color change and the blue fluorescent Agar/PAAD-Eu with pH-responsive swelling/shrinking properties.They share the same physical cross-linked agar network stabilized by hydrogen bond-associated agar helix bundles.Owing to the temperature-responsive reversible nature of hydrogen bonds, different building blocks of Agar/P6A-Eu/Tb and Agar/PAAD-Eu hydrogels could be facilely assembled into various anisotropic bilayer actuators via a supramolecular assembly process.On the basis of these findings, chameleon-and octopishaped soft hydrogel actuators with spatially anisotropic structures were prepared, which enabled the spatial control of synergistic shape/color-changing behaviors for the first time.In view of its facile operation and modular design principle, the established strategy is expected to be a starting point for the development of bioinspired intelligent soft actuators/robotics with spatially controlled shape/color-changing behaviors.
The synergistic hydrogel actuators developed in this study were able to have bifunctional responsiveness (both shape deformation and fluorescence color change) in response to a single stimulus and are thus expected to provide many extra advantages compared with the classic shape-deforming materials.For example, when environmental pH was varied, the developed actuator was able to not only sense the pH change by displaying fluorescence discoloration but also deform into different shapes (Figures 4 and 5).This means that our synergistic hydrogel actuator may potentially serve as an interesting dualchannel pH sensor that can output simultaneous optical and shape signals.Similarly, many different types of dualchannel sensors would be envisaged by engineering the actuators to become sensitive to other external stimuli (e.g., solvent, temperature, light, electricity).Moreover, if the high-level multifunctional cooperative behaviors of natural organisms could be efficiently replicated, a large number of next-generation synergistic materials would be produced for wide potential uses, including soft robotics, smart display, and biomimetic camouflage.

F I G U R E 1
Interfacial hydrogel bonding via interfacial supramolecular assembly.(A) Photos and illustrations showing the interfacial bonding of two identical Agar/P6A stripes.Scale bar = 1 cm.(B) Scanning electron microscope image of the freeze-dried bilayer Agar/P6A hydrogel stripe.(C) Interfacial peeling strength of the bilayer Agar/P6A hydrogen stripe as a function of strain.(D) Schemes of the Obverse (O) and Reverse (R) bonding actuating units and their actuating behavior in an aqueous Eu 3+ solution, as well as the time-dependent actuating kinetics in a 0.01 mol/L Eu 3+ solution (E).

F I G U R E 2
Preparation of hydrogel actuators with origami-like shape deformation.(A) Illustration and photos of various bilayer hydrogel actuators prepared from Agar/PAAD and Agar/P6A building blocks via the interfacial supramolecular assembly approach, as well as the photos showing the spatial control over their color/shape switchable behaviors.Construction of a (B) series of linear hydrogel structures containing specific sequences of four O-or R-type actuating units and (C) two-dimensional H-shaped hydrogel structures with specific sequences of eight O-or R-type actuating units, which exhibit differential shape deforming behaviors in 0.1 mol/L Eu 3+ solution.Scale bars = 1 cm.

F
I G U R E 3 Construction of multifluorescence-color changeable hydrogel actuators.(A) Schemes showing the multifluorescence-color tunable mechanism via lanthanide ion replacement.(B, C) Schemes and photos showing the synergistic shape/color switchable process of one hydrogel flower in 0.1 mol/L aqueous Eu 3+ and Tb 3+ solutions.Scale bar = 1 cm.

F
I G U R E 4 Responsive shape/color-changing process of the bilayer fluorescent hydrogel actuator consisting of blue fluorescent Agar/ PAAD-Eu and red fluorescent Agar/P6A-Eu hydrogels.(A) Photos showing the time-dependent actuating process of the bilayer actuator in NaOH solutions (0.01 mol/L).(B) Illustration showing the definition of the project length d of two ends of the actuator.(C) Curve showing the project length d change as a function of time.

F I G U R E 5
Spatial control over the synergistic shape/color switchable behaviors.(A) Simultaneous shape and fluorescence colorchanging process of the chameleon-shaped soft hydrogel actuator in response to the sequential stimuli of Eu 3+ and NaOH.Its tail part consisted of both the Agar/PAAD and Agar/P6A hydrogel layers, while the body part was made of the monolayer Agar/P6A hydrogel.(B) Synergistic shape/color-changing process of the octopi-shaped soft hydrogel actuator in response to the sequential stimuli of the Eu 3+ / Tb 3+ (75/25) mixture and NaOH.Its palms consisted of both the Agar/PAAD and Agar/P6A hydrogel layers, while the body part was made of the monolayer Agar/P6A hydrogel.