Polymer solutions and solids that contain light-sensitive molecules can undergo photo-contraction, whereby light energy is converted into mechanical energy. Here we show that a single film of a liquid-crystal network containing an azobenzene chromophore can be repeatedly and precisely bent along any chosen direction by using linearly polarized light. This striking photomechanical effect results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale or nanoscale applications, for example in microrobots in medicine or optical microtweezers.

The manipulation of small amounts of liquids has applications ranging from biomedical devices to liquid transfer. Direct light-driven manipulation of liquids, especially when triggered by light-induced capillary forces, is of particular interest because light can provide contactless spatial and temporal control. However, existing light-driven technologies suffer from an inherent limitation in that liquid motion is strongly resisted by the effect of contact-line pinning. Here we report a strategy to manipulate fluid slugs by photo-induced asymmetric deformation of tubular microactuators, which induces capillary forces for liquid propulsion. Microactuators with various shapes (straight, 'Y'-shaped, serpentine and helical) are fabricated from a mechanically robust linear liquid crystal polymer. These microactuators are able to exert photocontrol of a wide diversity of liquids over a long distance with controllable velocity and direction, and hence to mix multiphase liquids, to combine liquids and even to make liquids run uphill. We anticipate that this photodeformable microactuator will find use in micro-reactors, in laboratory-on-a-chip settings and in micro-optomechanical systems.

Muscle is a transducer that can convert chemical energy into mechanical motion. To construct artificial muscles, it is desirable to use soft materials with high mechanical flexibility and durability rather than hard materials such as metals. For effective muscle-like actuation, materials with stratified structures and high molecular orders are necessary. Liquid-crystalline elastomers (LCEs) are superior soft materials that possess both the order of liquid crystals and the elasticity of elastomers (as they contain polymer networks). With the aid of LCEs, it is possible to convert small amounts of external energy into macroscopic amounts of mechanical energy. In this Review, we focus on light as an energy source and describe the recent progress in the area of soft materials that can convert light energy into mechanical energy directly (photomechanical effect), especially the photomechanical effects of LCEs with a view to applications for light-driven LCE actuators.

As light is a good energy source that can be controlled remotely, instantly, and precisely, light-driven soft actuators could play an important role for novel applications in wideranging industrial and medical fields. Liquid-crystalline elastomers (LCEs) are unique materials having both properties of liquid crystals (LCs) and elastomers, [1][2][3] and a large deformation can be generated in LCEs, such as reversible contraction and expansion, and even bending, by incorporating photochromic molecules, such as an azobenzene, with the aid of photochemical reactions of these chromophores. [4][5][6][7][8][9][10][11][12] Herein we demonstrate new sophisticated motions of LCEs and their composite materials: a plastic motor driven only by light.If materials absorb light and change their shape or volume, they can convert light energy directly into mechanical work (the photomechanical effect) and could be very efficient as a single-step energy conversion. Furthermore, these photomobile materials would be widely applicable because they can be controlled remotely just by manipulating the irradiation conditions. LCEs show an anisotropic order of mesogens with a cooperative effect, which leads them to undergo an anisotropic contraction along the alignment direction of mesogens when heated above their LC-isotropic(I) phase transition temperatures (T LC-I ) and an expansion by lowering the temperature below T LC-I . [1,[13][14][15][16][17][18] The expansion and contraction is due to the microscopic change in alignment of mesogens, followed by the significant macroscopic change in order through the cooperative movement of mesogens and polymer segments.It is well known that when azobenzene derivatives are incorporated into LCs, the LC-I phase transition can be induced isothermally by irradiation with UV light to cause trans-cis photoisomerization, and the I-LC reverse-phase transition by irradiation with visible light to cause cis-trans back-isomerization. [19] This photoinduced phase transition (or photoinduced reduction of LC order) has led successfully to a reversible deformation of LCEs containing azobenzene chromophores just by changing the wavelength of actinic light. [4][5][6][7][8][9][10][11][12] Although the photoinduced deformation of LCEs previously reported is large and interesting, it is limited to contraction/expansion and bending, preventing them from being used for actual applications. Herein we report potentially applicable rotational motions of azobenzene-containing LCEs and their composite materials, including a first lightdriven plastic motor with laminated films composed of an LCE film and a flexible polyethylene (PE) sheet.The LCE films were prepared by photopolymerization of a mixture of an LC monomer containing an azobenzene moiety (molecule 1 shown in Scheme 1) and an LC diacrylate with an azobenzene moiety (2 in Scheme 1) with a ratio of 20/ 80 mol/mol, containing 2 mol % of a photoinitiator in a glass cell coated with rubbed polyimide alignment layers. The photopolymerization was conducted at a temperatur...

crosslinker are controlled, it is technically possible to prepare a gel membrane reflecting the specific color at a certain temperature. This method to prepare the interconnecting porous gels will give us the potential to study appropriate smart gels that may have interesting applications, such as in tunable optical filters, actuators, and sensors. Studies to confirm this are underway. ExperimentalSynthesis of Colloidal Crystals as Templates: To prepare the closest-packed colloidal crystals, a 15 wt.-% aqueous solution of silica spheres, having a diameter of 291 nm as determined from SEM measurements, was used. The growth of the crystal by the gravity sedimentation method was conducted in a flat Petri dish at 20 C. The stable dried crystals were obtained within 1 week as the water evaporated, and then completely dried in vacuo at 60 C. It is generally accepted that the crystalline arrays produced by the gravity sedimentation method have a cubic close-packed structure containing polycrystalline domains, similar to that of a natural opal. The thickness of the crystal can be easily controlled by the colloidal concentration and the iterative treatment. On the other hand, fine ordered crystals were created by the solvent evaporation method as follows: the colloidal suspension (ca. 15 wt.-%) was dropped onto a clean microscope slide and was placed in a thermostatic chamber at 90 C, in which the solvent gradually evaporated. A high quality crystal can be prepared with a thickness of up to 1 mm.Synthesis of Porous Hydrogels and Cylindrical Hydrogels: The thermosensitive gels were prepared by free-radical polymerization as follows. First, N-isopropylacrylamide (NIPA, 11.3 g), N, N¢-methylene bis(acrylamide) (0.513 g) as a crosslinker, and benzoylperoxide (0.048 g), the initiator, were dissolved in degassed and nitrogen-saturated 1,4-dioxane to a final volume of 50 mL. The solution was then infiltrated into the colloidal crystals in a Petri dish, and the polymerization was conducted at 60 C for 40 h. Afterwards, the samples were immersed in a 5 wt.-% HF aqueous solution to remove the SiO 2 . The gels for a swelling measurement were prepared in micropipettes of 100 lm diameter. The resulting porous gels and the cylindrical gels were washed carefully with distilled water for 1 week.Measurements: The swelling measurement was carried out by monitoring the diameter of the cylindrical gel in water. The temperature was controlled by using a temperature control system with circulating water. The reflection spectra were obtained by an Ocean Optics USB2000 fiber optic spectrometer [13] We did not use a macroporous gel to determine the swelling behavior.Therefore, a small difference in swelling size between the porous gel and cylindrical gel seems to be present, because the quantitatively different values were obtained in the wavelength of the peak of the reflection spectra from a porous gel as a function of temperature in Fig. 4b. Nevertheless, the experimental results qualitatively prove our description.

Photodeformable liquid crystal polymers (LCPs) that adapt their shapes in response to light have aroused a dramatic growth of interest in the past decades, since light as a stimulus enables the remote control and diverse deformations of materials. This review focuses on the growing research on photodeformable LCPs, including their basic actuation mechanisms, the various deformation modes, the newly designed molecular structures, and the improvement of processing techniques. Special attention is devoted to the novel molecular structures of LCPs, which allow for easy processing and alignment. The soft actuators with various deformation modes such as bending, twisting, and rolling in response to light are also covered with the emphasis on their photo‐induced bionic functions. Potential applications in energy harvesting, self‐cleaning surfaces, sensors, and photo‐controlled microfluidics are further illustrated. The existing challenges and future directions are discussed at the end of this review.

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When upconversion nanophosphors were incorporated into an azotolane-containing cross-linked liquid-crystal polymer film, the resulting composite film generated fast bending upon exposure to continuous-wave near-IR light at 980 nm. This occurs because the upconversion luminescence of the nanophosphors leads to trans-cis photoisomerization of the azotolane units and an alignment change of the mesogens. The bent film completely reverted to the initial flat state after the light source was removed.