Harnessing the DayâNight Rhythm of Humidity and ... - ACS Publications

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Applications of Polymer, Composite, and Coating Materials

Harnessing the Day-Night Rhythm of Humidity and Sunlight into Mechanical Work Using Recyclable and Reprogrammable Soft Actuators Qiaomei Chen, Xiaojie Qian, Yan-Shuang Xu, Yang Yang, Yen Wei, and Yan Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09324 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Applied Materials & Interfaces

Harnessing the Day-Night Rhythm of Humidity and Sunlight into Mechanical Work Using Recyclable and Reprogrammable Soft Actuators Qiaomei Chen,a Xiaojie Qian,a Yanshuang Xu,a Yang Yang,a Yen Wei*a,b and Yan Ji*a a

MOE Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology, Department

of

Chemistry,

Tsinghua

University,

Beijing

100084,

China

E-mail: [email protected]; [email protected] b

Department of Chemistry, Center for Nanotechnology and Institute of Biomedical Technology,

Chung-Yuan Christian University, Chung-Li 32023, Taiwan, China KEYWORDS: soft actuators, energy-saving, vitrimer-based bilayers, humidity and sunlight responsivity, complex 3D structures

ABSTRACT: Towards a sustainable society, soft actuators driven by environmentally friendly energy from nature are of great social and economic significance. Meanwhile, recyclability,

ACS Paragon Plus Environment

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repeated reconfiguration for other use and complex 3D geometries are also essential for mitigating the energy crisis and practical application demands. Here, we integrate all the above features in one actuator using vitrimers with exchangeable disulfide links. By reconfiguration, welding, patterning and kirigami techniques, complex 3D actuators can be easily fabricated, which can be repeatedly reconfigured for other applications to save cost in new materials preparation. These actuators operate synergistically with the day-night rhythm of humidity and sunlight without the need of extra energy input.

INTRODUCTION Every day, the humidity of the atmosphere decreases as the sun rises while it increases as the sun sets. To address the shortage of traditional power supplies, harnessing the hidden ecofriendly and inexhaustible energy from nature has been attractive subject. Is it possible to directly convert the ubiquitous day-night change of both humidity and sunlight into mechanical work? Even though this idea has been seldom explored, for other purpose, there are already more than a dozen reports on actuators which can respond to both light and humidity. Based on the isomerization of azobenzene groups, humidity driven actuators made of liquid crystalline network and agarose hydrogels can also respond to UV light.1-3 Acidochromic fluorophore and trans-to-cis isomerization (similar to azobenzene groups) 1,4-bis(para-hydroxystyryl)benzene (BHSB) was introduce into the humidity responsive agarose film, getting a smart pH-responsive film that can simultaneously convert light and aerial humidity gradients into mechanical work.4 Made of carbon nitride polymer, actuators driven by the fluctuations in ambient humidity can jump upon UV irradiation.5 Taking the advantage of the photo-thermal effect of carbon-based materials such


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as graphene, graphite, graphene oxide and carbon nanotubes, infrared light has been employed to make actuators both humidity and light responsive.6-11 Utilizing the temperature response of NIPAAm and light response of spiropyran, a spiropyran-NIPAAm hydrogel functionalized cotton fabric was capable of dimensional changes upon irradiation with visible light or temperature changes in water.12 In addition, sunlight has been able to regulate the humidity response of TiO2 patterned agarose@CNT/agarose and sodium alginate/polyvinylidene fluoride bilayers.13-14 Here, we bring forth a robust platform to fabricate reversible shape-morphing actuators which can harness the daily sunlight and humidity change cooperatively using recyclable polymers. These actuators are reprogrammable and can easily be reconfigured into complex 3D shapes. Moreover, the actuators can be durably reused by repeatedly reconfiguring into required 3D actuators, without disposal of the waste actuators or new materials fabrication, which is much energy-saving and economical. The platform we bring forth here is based on vitrimers.15-16 Vitrimers are covalently cross-linked polymer networks. They can be reprocessed and recycled for repeated utilization like thermoplastics. The reprocessability is attributed to the involved exchangeable dynamic bonds such as transesterification,17-19 dynamic boronic ester,20 olefin metathesis,21 vinylogous transamination,22 silyl ether23 and disulfide rearrangements.24-25 Since they were brought forward in 2011, upsurging research efforts have made it possible to fabricate various types of vitrimers by traditional thermoset chemistry such as epoxy26-29 and polyurethane.30-33 Different kinds of vitrimer actuators have been designed and fabricated. However, vitrimer actuators responsive to humidity or driven by sunlight have been barely reported, needless to say to simultaneously use both natural available green energies as efficient energy resources.


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Our design of the vitrimer actuators is based on the widely used bilayer structure. It is easy to make two vitrimers with different hygroscopicity just by incorporating more hydrophilic groups into one vitrimer than the other. To allow harnessing both humidity and sunlight at the same time, the effect of exposing to sunlight and the low humidity should make the bilayer move to the same direction, otherwise, the effect of sunlight and humidity change of the day-night rhythm will cancel each other, resulting in either energy lost or even no actuation at all. Our strategy to make sunlight and humidity work together is to add certain substance with photothermal effect into the layer which is more hydrophilic. Therefore, when sunlight is on, the temperature of the bilayer increases, thus the bilayer can lose the absorbed water quickly, which has the same effect like low humidity. There are a lot of photo-thermal agents readily available, such as CNTs,34-35 graphenes,36-37 carbon black,38 polydopamine nanoparticles39 and gold nanorods.40 We choose amino-capped aniline trimer (ACAT), which has shown very excellent photo-thermal effect in our previous works41-43 and can be covalently linked to the polymer chains without aggregation like inorganic nanoparticles. Due to the intrinsic nature of vitrimers, the bilayer can easily be assembled by welding, reconfigured into new geometries and reprogrammed into new actuation modes. For the currently available systems which can respond to both sunlight and humidity, permannent complex 3D structures are hard to be processed. Meanwhile, 3D to new 3D transformation is extremely hard to be achieved for sophisticated 3D actuations demand spatial folding of different areas. Here, to get complex 3D structures, we combine reconfiguration, patterning, welding and kirigami techniques together, which can not only construct complex 3D structures at will, but also allow easy modifcation or durable reusing of the actuators. In practical applications, a little modification of the shapes or the folding modes may enable the prior unsuitable devices to


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function properly. And reshaping the wasted actuators into required structures for new applications without disposal and new materials preparation will save much time, cost and energy.

RESULTS AND DISCUSSION Design, fabrication and characterization of vitrimers involved

As a proof-of-concept, we design and synthesize the two following vitrimers to make a bilayer actuator. They are all vitrimers with exchangeable disulfide crosslinks (Figure 1a). Layer A (Vitrimer-A) is a humidity-inert layer (rigid passive layer with smaller shrinkage ratio upon dehydration). Layer B (Vitrimer-B) is a humidity-sensitive layer (soft active layer with bigger shrinkage ratio upon dehydration). The relatively long length of poly(ethylene glycol) diglycidyl ether (average Mn~500) and photo-thermal amino-capped aniline trimer (ACAT) are incorporated in Vitrimer-B to offer higher hygroscopicity than Vitrimer A and the photo-thermal effect, respectively. 4-Aminophenyl disulfide is introduced into both the two layers to make the synthesized polymers become vitrimers due to disulfide exchange reaction. Fourier transform infrared spectra (FTIR) confirmed the completion of the curing reactions in both Vitrimer-A and Vitrimer-B (Figure S2). We further used X-ray photoelectron spectroscopy (XPS) to confirm the chemical structures of Vitrimer A and Vitrimer B (Figure S3,4). According to differential scanning calorimetry (DSC) (Figure S5,6), the glass transition temperature Tg of Vitrimer-A and Vitrimer-B upon heating are about 57ºC and -15ºC, respectively. Thermogravimetric analysis (TGA) (Figure S7) shows that both the layers have excellent thermal stability, of which the onset of decomposition in air are about 275ºC and 234ºC, respectively. The Modulus-temperature curves (Figure S8,9) reveal that both the layers behave like classical thermosets, exhibiting a


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major relaxation related to Tg and one rubbery plateau with modulus of about 19.1 MPa and 5.2 MPa. The glassy-state storage modulus for Vitrimer-A and Vitrimer-B are about 2.1 GPa and 2.3 GPa, respectively. The exchange reaction allows the vitrimers to relax stresses. Stress relaxation experiments are individually conducted on the Vitrimer-A and Vitrimer-B films in the linear region (Figure S10,11) within a temperature range. Relaxation times τ* follow an Arrhenius law at temperature above the topology-freezing transition temperature (Tv). Tv values of Vitrimer-A and Vitrimer-B were about 67ºC and 35ºC (Figure S12,13), respectively, determined using the Maxwell equation according the method previously reported.27 It should be noted that the relatively low Tv values of Vitrimer-A and Vitrimer-B do not mean their poor service stability. On one hand, at Tv,

τ*

is quite a long time (43.6 h and 159.9 h for Vitrimer-A and Vitrimer-B,

respectively). On the other hand, external force must be applied to induce macroscopic deformations (demonstrations of the stability of the samples can be found in the reconfiguration section below).

Actuations regulated by the day-night synergistical change of humidity and sunlight

The two polymers have different response to humidity and sunlight. Vitrimer-B can absorb more water than Vitrimer-A. TGA was conducted on Vitrimer-A and Vitrimer-B, which had been swelled in water for 1 hour and then measured by being holded at 120ºC for 2 h. As shown in Figure 1b, both the weight of Vitrimer-A and Vitrimer-B reach equilibrium within 4 min and the relevant weight losses are about 4% and 23%, respectively. To verify the photo-thermal effect, we measured the temperature of Vitrimer-A and Vitrimer-B under 1 sun illumination (100 mW cm-2). Interestingly, it is found that Vitrimer-A also has the capacity of photo-thermal conversion, though which is not as excellent as that of Vitrimer-B. As shown in the infrared


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thermal images (Figure 1c), the temperature of Vitrimer-B rises to 53.1ºC within just 10 s, which is about 13ºC higher than that of Vitrimer-A (40.1ºC, about 6.5ºC higher than the surrounding temperature, Figure S14).

The bilayer actuator made from Vitrimer A and Vitrimer B can respond to humidity and sunlight in a synergistic way. It is very easy to prepare inseparable self-bending bilayer actuators using vitrimers (The detailed preparation of the bilayer actuators is described in the MATERIALS AND METHODS section and detailed characterizations of the bilayer actuators such and scanning electron microscopy (SEM) images, swelling tests, stress-strain tests and fatigue tests can be found in the Supporting Information). As shown in Figure 1d, when exposed to high humidity and low/no light intensity, the side of Vitrimer-B can absorb water to swell, leading to bending towards the side of Vitrimer-A, which will dehydrate to shrink upon high sunlight intensity or low humidity. The photo-thermal effect of both the Vitrimer-A and Vitrimer-B sides of the bilayer was also investigated. The results show that the temperatures of both the two sides are higher than that of the single Vitrimer-B film at the same light intensity and the temperature of the Vitrimer-A side is slightly higher than that of the Vitrimer-B side (Figure S15). We speculate that the reason for the higher temperature of the Vitrimer-A side and Vitrimer-B side than the single Vitrimer-B film may be that the thicker bilayer (compared to the single Vitrimer-B) endows the bilayer with the more excellent thermal insulation against the test desk that the samples contact. And the higher temperature of the Vitrimer-A side than the Vitrimer-B side may be resulted from the aforementioned relatively weaker photo-thermal effect of Vitrimer-A. To test the actuations of the bilayer regulated by the day-night synergistical change of humidity and sunlight, we first investigated the curvature-time profiles of the straight bilayer strip when treated with a constant relative humidity (RH) of 80% and recovered by


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sunlight illumination (100 mW cm-2). As shown in Figure 1e, the results reveal that the curvature increases gradually with time and reaches 0.248 mm-1 within 10 min at 80% RH. With sunlight illumination at 80% RH, the curvature decreases quickly and nearly recovered to the original straight shape within 30 s (Figure 1f). The effect of humidity and sunlight change on the curvature was studied in detail. The curvature of the strip increases gradually with increasing RH (without sunlight, Figure 1g). The shape-change of the strip upon both the varied humidity and sunlight intensity was also investigated. As shown in Figure 1h, the curvature of the strip decreases with decreasing RH and increasing sunlight intensity. Moreover, when the strip is exposed to sunlight irradiation under 80% RH, the curvature decreases with increasing sunlight intensity and nearly unchanged above 80 mW cm−2 (Figure 1i). All above reveal that the bilayer actuators can be used in nearly all the conditions such as cloudy day or night (similar to Figure 1g), sunny day (similar to Figure 1h) and humid weather (similar to Figure 1i).


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Figure 1. Synthesis and characterization of the bilayer film. (a) Synthesis of the Vitrimer-A and Vitrimer-B film. (b) TGA curves of the Vitrimer-A and Vitrimer-B after swelled in water for 1 h (heating rate: 20ºC min-1). (c) The temperatures of the Vitrimer-A and Vitrimer-B after illuminated by sunlight (100 mW cm-2) for 10 s. (d) Schematic illustration of preparation and actuation mechanism of the bilayer film. (e) Curvature-time profiles of the bilayer strip hold at 80% RH (strip dimension: 17 mm × 2 mm × 135 µm). (f) Curvature-time profiles of the bilayer strip (sample in Figure 1e, after treated with 80% RH for 10 min) illuminated by sunlight (100 mW cm-2) at 80% RH. (g) Change in curvature of bilayer strip with increasing RH (without sunlight, for 10 min). (h) Shape-change of bilayer strip with both the varied humidity and sunlight intensity (for 10 min). (i) Effect of light intensity (for 30 s) on curvature of bilayer strip held at a constant RH of 80%. Reconfiguration enabled flexible designs of reprogrammable complex 3D actuators Owing to the exchangeable disulfide reaction at the temperature above Tv, flat bilayer actuators can be readily reshaped into 3D configurations without molds. And such well-shaped 3D configurations can be reprogrammed into other structures when exposed to high humidity. As shown in Figure S20, a strip of bilayer film (30*2 mm2) is first reconfigured into concentric circles by heating at 180ºC for 10 min with external force, where the layer of Vitrimer-B is inside. When exposed to high humidity, the layer of Vitrimer-B absorbs water to swell, leading to the gradual unfolding of circles. Up to our expectations, it recovers its original concentric circles quickly with sunlight illumination (the intensity here and below used is 100 mW cm-2). The above reshaping strategy offer great flexibility to achieve permanent complex 3D structures, which are difficult to access by previously reported thermosets containing bilayers.


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Here, we take the helical actuator as an example. Helical motion (such as winding and unwinding) is very important. In nature, plants’ tendrils rely on this to find a support or to track the sunlight. In robotics, this offers a unique movement to increase the motion flexibility. However, soft helical actuators are few, and most of them are difficult to prepare and of highcost.19, 44-46 Meanwhile, their movement modes and structures are limited. With the bilayer here, various helical actuators can be easily obtained. As Figure 2 shows, the flat strip of bilayer film (75*2 mm2) is firstly reconfigured into a spring-like permanent shape by winding the sample around a slender Teflon rod with the two ends fixed and heating at 180ºC for 10 min, where Vitrimer-B is inside (Spring-1). When exposed to high humidity, the layer of Vitrimer-B absorbs water to drive the unwinding deformation of the spring (Spring-1’), similar to the extension process of common springs. It should be noted that the spring here has some difference with the common springs. The spring diameter increases and number of active coils decreases during the deformation process, which of the common springs maintain constantly. Moreover, the above spring can be repeatedly reconfigured. It can be directly reconfigured using the aforementioned method for Spring-1 without flattening of the sample. First, it is reconfigured to make a new spring with Vitrimer-B outside (Spring-2). The new spring winds at RH of 80% accompanied with decreasing of the spring diameter and increasing of the number of active coils (Spring-2’), showing a totally contrary behavior with Spring-1. The above wellshaped spring can be further reconfigured into a conical spring with Vitrimer-B outside and the pitch decreases along with the diameter decreasing (Spring-3). When exposed to high humidity, it behaves like Spring-2 (Spring-3’). They all recover their original spiral shapes when irradiated with sunlight (the illumination time here and below for 3D structures is about 60 s). To verify the durability of the reconfigurable vitrimer bilayer actuators, we cyclically reconfigured a bilayer


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between straight and curved shape for 20 times (the details can be found in the Supporting Information). As shown in Figure S21a , the curling curvature of the curved bilayer maintains excellent stability during the whole reconfiguration process (0.276 ± 0.003 mm-1) and the humidity and sunlight responsivity is substantially retained after reconfiguration for 20 times (Figure S21b). The above helical motion offers a broad new way to do mechanical works. As shown in Figure 3, the humidity treated spring (similar to Spring-1’ and with a diameter of about 8 mm) can grab up a cylinder (with a diameter of about 5 mm and a weight of more than 40 times of the spring itself) and move it away. Because the diameter of the spring decreases quickly with sunlight illumination and the new diameter (about 4 mm) is less than that of the cylinder.

Figure 2. Repeatedly reconfiguring of the bilayer film into helical springs with different responsive behaviours.


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Figure 3. (a) Schematic illustration and (b) photographs of utilizing the humidity treated helical spring to do mechanical work via sunlight illumination. Similar to reconfiguration, the separate springs can be welded together via the exchangeable disulfide reaction at the interface to obtain more variations. For example, to fully mimic a plant tendril, oppositely handed helices (left-handed and right-handed) can be assembled in one single piece, which is also a challenging task for ordinary actuators. Moreover, the parameters of absolute construction and actuation performance can be flexibly adjusted. As shown in Figure 4a, the two oppositely handed helical springs are welded together, with Vitrimer-B of the lefthanded spring inside and the right-handed one outside. When exposed to high humidity, the spring with mixed-helicity displays simultaneously unwinding and winding. Furthermore, the spring pitch of mixed-helicity spring can be flexibly adjusted. As shown in Figure 4b, the pitch of the left-handed spring decreases regularly and that of the right-handed one decreases first and then increases. All the above results reveal greater flexibility and advantage of the bilayer here than the reported ones. The welding strength is quantitatively measured by 90° peeling test and


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the calculated adhesion energy is about 152 J m-2, revealing a relatively strong covalent connection of the welded actuators and also reflecting the robustness of the bilayer itself (the details can be found in the Supporting Information). Moreover, not any obvious interfacial delamination was observed in the welding interface or the bilayer itself during continuous actuations driven by humidity and sunlight. To verify the shape stability of the 3D actuators at T>Tv, we hold the helical actuator (cut from the oppositely handed helices in Figure 4b) at 85ºC for 48 h. As shown in Figure S23, the shape and actuations to humidity and sunlight were almost totally reserved after heating.

Figure 4. The mixed-helicity springs by welding two oppositely handed helical springs together. (a) With the similar pitch of each helix. (b) With the pitch change of each helix. Reprogrammable complex 3D actuators fabricated via kirigami, patterning and welding techniques The technique of “kirigami” (paper art by cutting)47-48 can also be applied to the bilayer actuators to increase local flexibility and fabricate 3D structures with more sophisticated shapes. Most of the reported kirigami-based actuators can only display 2D to 3D transformations and the transformations are always irreversible.30,

48-49

However, the bilayer here can be directly

constructed into 3D structures by welding, patterning and kirigami techniques and perform


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reversible 3D to new 3D transformations. As Figure 5a shows, a vase like structure is fabricated by welding the edges of a flat patterned bilayer film together, of which a narrow strip of Vitrimer-B film has been hot-pressed outside (the details can be obtained from Figure S24). The vase can flexibly adjust its shape with the changes of ambient humidity and sunlight intensity. When exposed to high humidity, the layer of Vitrimer-B absorbs water to swell, leading to outward expansion of the vase’s wall companied with decreasing of the vase’s height. It recovers the original shape when the humidity decreases or illuminated with sunlight. The Vitrimer-B film also can be hot-pressed inside (Figure 5b). The vase becomes slimmer and slimmer when the humidity increases and converts to its original shape when treated with sunlight or humidity decreases. Furthermore, the deformation can be easily amplified by proper designations. As shown in Figure 5c, three strips of Vitrimer-B are hot-pressed inside, outside and inside from the top to bottom of Vitrimer-A, which results in inward, outward and inward bending at high humidity. Thus, the top and bottom inward bending will contribute to the middle outward bending, leading to larger deformation of the vase than that in Figure 5a. Similarly, three strips of Vitrimer-B are hot-pressed outside, inside and outside from top to bottom of Vitrimer-A (Figure 5d), generating a slimmer vase than the one in Figure 5b. Moreover, separating the above three strips of Vitrimer-B by Vitrimer-A, evident inward and outward bending can be individually integrated on the same vases (Figure 5e,f).


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Figure 5. Humidity and sunlight responsive vase-like structures fabricated by kirigami, patterning and welding technique. (a) Vase-like structure with one strip of Vitrimer-B outside. (b) Vase-like structure with one strip of Vitrimer-B inside. (c) Vase-like structure with three strips of Vitrimer-B inside, outside and inside from the top to bottom. (d) Vase-like structure with three strips of Vitrimer-B outside, inside and outside from the top to bottom. (e) Vase-like structure with three strips of Vitrimer-B inside, outside and inside from the top to bottom, where the three strips of Vitrimer-B are separated from each other by Vitrimer-A. (f) Vase-like structure with three strips of Vitrimer-B outside, inside and outside from the top to bottom, where the three strips of Vitrimer-B are separated from each other by Vitrimer-A. CONCLUSIONS In summary, we have presented here a kind of actuator which can be operated without the involvement of extra energy input due to the collaborated humidity and sunlight change in nature. Recyclability, complex 3D geometries and green energy triggered actuations can be united together into one system. The introduction of reconfiguration, patterning, welding and kirigami techniques make it possible to fabricate sophisticated actuating 3D structures that are


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hard to be realized in the past, which might have potential applications in areas such as micromachine systems, soft robotics and biomimetic devices. Due to the convenience to acquire numerous vitrimers by different exchangeable reactions and diverse range of monomer selections, optical, mechanical, electronical and other physical properties can be easily accommodated as required. For example, if porous vitrimer is used, the response speed will be greatly improved. This opens up the possibility of fabricating various actuators with on-demand performances to meet a wide range of practical requirements. MATERIALS AND METHODS Materials 1,4-Butanediol diglycidyl ether (J&K), 4-aminophenyl disulfide (TCI), decanedioicacid (Aladdin) and poly(ethylene glycol) diglycidyl ether (Sigma-Aldrich, average Mn~500) were all used as received. Amino-capped aniline trimer (ACAT) was synthesized according to the method reported in the literature.50 Vitrimer-A Preparation The Vitrimer-A film was prepared by first melting 1,4-butanediol diglycidyl ether (1 mmol), 4-aminophenyl disulfide (0.4 mmol) and decanedioicacid (0.2 mmol) in a Teflon mold at 80ºC with manually stirring for about 1 h. Then the mixture was slowly heated up to 100ºC with manually stirring until it was too viscous to flow. The pre-polymer was transferred into a mold and cured at 120ºC for 2 h then 160ºC for 2 h under a pressure of 5 MPa. Vitrimer-B Preparation


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The preparation of Vitrimer-B is a little different from that of Vitrimer-A. Poly(ethylene glycol) diglycidyl ether (1 mmol), ACAT (0.1 mmol) and N,N-dimethylformamide (DMF, 4 mL) were added into a 25 mL flask. After sonicating for 5 minutes to dissolve all the chemicals, the mixture was reacted at 120ºC for 2 h with stirring. The pre-reacted mixture was transferred to a watch glass covered with Teflon film to evaporate the solvent at 120ºC with manually stirring. When most of the solvent had gone, 4aminophenyl disulfide (0.4 mmol) was added into the mixture. Then the mixture was continuous to react until it was too viscous to flow. The mixture was further cured as the same procedure with that of Vitrimer-A. The bilayer actuators preparation The bilayer actuators were fabricated by simply connecting Vitrimer-A and Vitrimer-B films via hot-pressing, under a moderate pressure of a foldback clip at 180ºC for 10 min. ASSOCIATED CONTENT Supporting Information NMR characterization of ACAT; FTIR spectra, XPS spectra, DSC curves, TGA curves, mechanical properties, stress relaxation curves and Arrhenius plots of the measured relaxation times of Vitrimer-A and Vitrimer-B; photo-thermal effect of Vitrimer-A, Vitrimer-B and the vitrimer bilayer; the cross-sectional SEM images, stress-strain tests and fatigue tests of the vitrimer bilayer; reconfigured concentric circle and cyclic reconfiguration tests of the vitrimer bilayer; interfacial strength of the welded actuators; investigation of 3D shape stability at T>Tv; and demonstration of the fabrication process for the vase-like 3D structures.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Author Contributions Y.J. and Q.C. developed the concept. Y.J., Q.C. and Y.W. arranged the funding and infrastructure for the project. Q.C. carried out the experiments. Q.C. and Y.J. wrote the paper. All the authors contributed to scientific discussion of the article. Funding Sources This research was supported by the National Natural Science Foundation of China (nos 51722303 and 21674057) and China Postdoctoral Science Foundation (2019M650624). Notes The authors declare no competing financial interest. REFERENCES (1) Wani, O. M.; Verpaalen, R.; Zeng, H.; Priimagi, A.; Schenning, A., An Artificial Nocturnal Flower via Humidity-Gated Photoactuation in Liquid Crystal Networks. Adv. Mater. 2019, 31, e1805985. (2) Zhang, L.; Liang, H.; Jacob, J.; Naumov, P., Photogated Humidity-Driven Motility. Nat. Commun. 2015, 6, 7429. (3) Liu, Y.; Xu, B.; Sun, S.; Wei, J.; Wu, L.; Yu, Y., Humidity- and Photo-Induced Mechanical Actuation of Cross-Linked Liquid Crystal Polymers. Adv. Mater. 2017, 29, 1604792. (4) Zhang, L.; Naumov, P., Light- and Humidity-Induced Motion of an Acidochromic Film. Angew. Chem. Int. Edit. 2015, 54, 8642-8647. (5) Arazoe, H.; Miyajima, D.; Akaike, K.; Araoka, F.; Sato, E.; Hikima, T.; Kawamoto, M.; Aida, T., An Autonomous Actuator Driven by Fluctuations in Ambient Humidity. Nat. Mater. 2016, 15, 1084-1089.


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