An actuator is a transducer that can convert external energy into mechanical motion. The most familiar example of an actuator is a muscle, which can convert chemical energy into mechanical

motion. The materials of useful actuators should possess softness and strength in response to external stimuli. Among the various materials used for developing soft actuators, polymers are

promising materials owing to their high processability and softness. Therefore, the potential of various types of polymer materials, such as polymer gels1 and conducting polymers,2 have been

investigated. Liquid-crystalline elastomers (LCEs) have been determined to be suitable for making soft actuators because of their molecular order and elasticity.3, 4 An LCE can convert

fabricate practical photoresponsive actuators, no effective material architecture that addresses these aspects has yet been reported. It is expected that developing high-strength

photomechanical materials will have a broad range of applications. Moreover, considering radiation sources, most of the photomechanical LCEs have been manipulated by irradiation with

ultraviolet or visible light. In contrast to short-wavelength radiation, infrared (IR) irradiation as the stimulus light is useful for developing practical actuators because IR light is a

lower-energy light source for biological molecules.9, 10, 11 Herein, we report a photomechanical material composed of a double-network hydrogel (DNG)12, 13 that exhibits high mechanical

strength, and we focus on the photothermal effect, a change in molecular alignment in the gel induced by IR irradiation. The sample gels were synthesized by multistep radical polymerization.

In the first gel preparation, we poured the cationic monomer _N_-[3-(_N_,_N_-dimethylamino)propyl]acrylamide methyl chloride quaternary (DMAPAA-Q; 2.0 M), the crosslinking agent

_N,N_′-methylenebis(acrylamide) (MBAA; 10 mM) and the radical initiator 2,2′-azobis(2-methylpropionamidine)dihydrochloride (10 mM) into a glass mold measuring 25 × 25 × 1.5 mm3. The glass

cells containing the mixture were heated to 60 °C for 24 h and then immersed in acrylamide (2.0 M), MBAA and KPS (1.0 mM) aqueous solution at 5 °C for 1 week. After swelling, the gel was

heated to 60 °C for 24 h, and the gel was then peeled from the mold. The gel was glued to two glass clamps for mechanical tests, and its size was 30 × 20 × 0.5 mm3. First, we investigated

the effect of the crosslinker concentration, MBAA, on the mechanical properties because this element affects the strength and elasticity of the gel (Supplementary Figure S1).14 The critical

stretch length of the gel increases with decreasing crosslinker concentration in the second gel preparation (Supplementary Figure S2). However, the gel containing low crosslinker

concentration (<0.2 mM) showed little elasticity even while exhibiting high stretchability of beyond 20 times the original length. Based on the results, a crosslinker concentration of 0.2

 mM was chosen for the second gel preparation. In the preparation condition, the gel showed elastic elongation to >11 times its original length without rupture (Figure 1a). Systematic

loading and unloading experiments at uniaxial tension showed a significant hysteresis during the first loading cycles, and the hysteresis increased strongly with the applied maximum

deformation (Figure 1b). However, this hysteresis was not observed after the second loading cycle (Figure 1c). Similar hysteresis has also been reported to occur in tough DNGs.9 This

phenomenon is explained by the effect of the first ionic network on the mechanical property. That is, the first fragile network formed by an ionic interaction is broken into small clusters

by elongation, and the clusters act as crosslinkers of long polyacrylamide chains. The crosslinkers of the first network clusters slide on the stretching chains of the second networks,

resulting in high stretchablility.15 In this case, the sliding network clusters are also formed by ionic polyDMAPAA-Q (Supplementary Figure S2), and they give rise to high stretchability in

the gel. A photomechanical LCE should possess molecular anisotropy for effective mechanical actuation along the preferential direction of deformation.16 Generally, LCE properties are used to

add molecular order to a material. In this study, we give molecular order to the DNG by stretching along a specified direction. Figure 2 shows a polarizing image of the stretched sample. As

viewed through crossed polarizers, the transmittance of the backlight through the DNG was the brightest when the angle between the analyzer and the stretching direction was at 45 and 135°.

This result indicates that the polymer networks were aligned along the stretching direction. Next, to investigate the IR response of the DNG system, we used an IR light with an intensity of

130 mW cm−2 at 1000 nm. In this study, we used low-intensity IR light to limit water evaporation. The DNG samples were fixed at both ends using mechanical clips, and the IR light source was

focused on the central region of the sample. Figure 3a shows the photomechanical behavior of DNG upon irradiation with IR light. When IR light was used, the nominal stress increased,

indicating photoinduced contraction of the gel. The photoinduced mechanical change reverted after the irradiation stopped. Furthermore, this actuation behavior occurred repeatedly. The

results indicate that the high-strength photomechanical material made using DNG without additives, such as CNTs to enhance absorption, can mechanically respond to IR irradiation. Next, we

investigated the photothermal effect on the actuation by monitoring the temperature profile of the irradiated area (Figure 3b). Upon IR irradiation, the temperatures of the irradiated area

increased to approximately 60 °C due to absorption of IR light by the water component in the DNG. The change in nominal stress was induced by IR irradiation and yields good agreement with

the increase in temperature. The change in nominal stress increased gradually after irradiation for 180 s, even when a maximum temperature of 60 °C was maintained. If the actuation behavior

arose solely from the volume change caused by water vaporization, the change in nominal stress would continue to increase. Therefore, the decrease in the nominal stress indicates minimal

response of the actuation by volume change with vaporization of the water component. To clarify the mechanism of the photoinduced actuation, we then investigated the effect of molecular

order on the photomechanical motion. Figure 4 shows the photoinduced mechanical effect at various degrees of stretching (_λ_=2, 3, 4, 5) under IR irradiation. All of the stretched DNGs

contracted upon IR irradiation and exhibited relaxation after irradiation stopped. Apparently, samples that stretched more experienced an increased change in the nominal stress (Figure 5),

indicating the importance of molecular alignments in the DNG for the actuation behavior. That is, the actuation behavior is induced by a change in the molecular alignment as a result of the

photothermal effect of IR absorption. In this study, we showed actuation behavior in high-strength DNGs induced by the photothermal effect. In the photomechanical DNG system, the water

component acts as an absorber of IR light and converts light energy into heat energy to facilitate mechanical actuation. The heat energy induces a cooperative change in the aligned molecular

structure with stretching. The gel is not only a high-strength material but is also prepared using commercial reagents with simple reaction steps, consists largely of water (approximately

90 wt%) and responds to IR light. Therefore, we believe that this material system has the potential to provide practical photomechanical actuators for a broad range of applications.

REFERENCES * Osada, Y., Okuzaki, H. & Hori, H. A polymer gel with electrically driven mobility. _Nature_ 355, 242–244 (1992). Article  CAS  Google Scholar  * Baughman, R. H. Conducting

polymer artificial muscles. _Synth. Met._ 78, 339–353 (1996). Article  CAS  Google Scholar  * De, Jeu & Wim, H. (ed.) _Liquid Crystal Elastomers: Materials and Applications_, (Springer,

2012). Google Scholar  * Zentel, R. Liquid crystalline elastomers. _Adv. Mater._ 1, 321–329 (1989). Article  Google Scholar  * Finkelmann, H., Nishikawa, E., Pereira, G. G. & Warner, M.

A new opto-mechanical effect in solids. _Phys. Rev. Lett._ 87, 015501 (2001). Article  CAS  Google Scholar  * Hogan, P. M., Tajbakhsh, A. R. & Terentjev, E. M. UV manipulation of order

and macroscopic shape in nematic elastomers. _Phys. Rev. E._ 65, 041720 (2002). Article  CAS  Google Scholar  * Natansohn, A & Rochon, P. Photoinduced motions in azo-containing polymers.

_Chem. Rev._ 102, 4109 (2002). Article  Google Scholar  * Ikeda, T. Photomodulation of liquid crystal orientations for photonic applications. _J. Mater. Chem._ 13, 2037 (2003). Article  CAS

  Google Scholar  * Koerner, H., Price, G., Pearce, N. A., Alexander, M. & Vaia, R. A. Remotely actuated polymer nanocomposites stress-recovery of carbonnanotube-filled thermoplastic

elastomers. _Nat. Mater._ 3, 115–120 (2004). Article  CAS  Google Scholar  * Ahir, S. V. & Terentjev, E. M. Photomechanical actuation in polymer-nanotube compsites. _Nat. Mater._ 14,

491–495 (2005). Article  Google Scholar  * Ji, Y., Huang, Y., Rungsawang, R. & Terentjev, E. M. Photomechanical actuation in polymer-nanotuve composites. _Adv. Mater._ 22, 3436–3440

(2010). Article  CAS  Google Scholar  * Gong, J. P. Why are double network hydrogels so tough? _Soft Matter_ 6, 2583–2590 (2010). Article  CAS  Google Scholar  * Gong, J. P., Katsuyama, Y.,

Kurokawa, T. & Osada, Y. Double network hydrogels with extremely high mechanical strength. _Adv. Mater._ 15, 1155–1158 (2003). Article  CAS  Google Scholar  * Sun, J. Y., Zhao, X.,

Illeperuma, W. R. K., Chaudhuri, O., Oh, K. H. & Mooney, D. J. Highly stretchable and tough hydrogels. _Nature_ 489, 133–136 (2012). Article  CAS  Google Scholar  * Na, Y.H., Tanaka, Y.,

Kawaguchi, Y., Furukawa, H., Sumiyoshi, T., Gong, J. P. & Osada, Y. Necking phenomenon of double-network gel. _Macromolecules_ 39, 4641–4645 (2006). Article  CAS  Google Scholar  *

Ikeda, T., Mamiya, J. & Yu, Y. Photomechanics of liquid-crystalline elastomers and other polymers. _Angew. Chem. Int. Ed._ 46, 506–528 (2006). Article  Google Scholar  Download

references ACKNOWLEDGEMENTS This work was supported by CLUSTER (the second stage) of the Ministry of Education, Culture, Sports, Science and Technology, Japan. AUTHOR INFORMATION AUTHORS AND

AFFILIATIONS * Department of Applied Chemistry, Keio University, Yokohama, Kanagawa, Japan Kunihiko Okano, Aya Nogami & Kouichi Asakura Authors * Kunihiko Okano View author publications

You can also search for this author inPubMed Google Scholar * Aya Nogami View author publications You can also search for this author inPubMed Google Scholar * Kouichi Asakura View author

publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHOR Correspondence to Kunihiko Okano. ADDITIONAL INFORMATION Supplementary Information accompanies

the paper on Polymer Journal website SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION (DOC 180 KB) RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE

Okano, K., Nogami, A. & Asakura, K. High-strength gel actuator driven by a photothermal effect. _Polym J_ 46, 827–830 (2014). https://doi.org/10.1038/pj.2014.78 Download citation *

Published: 17 September 2014 * Issue Date: December 2014 * DOI: https://doi.org/10.1038/pj.2014.78 SHARE THIS ARTICLE Anyone you share the following link with will be able to read this

content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided by the Springer Nature SharedIt content-sharing initiative