Light-driven Actuators Research Articles

Photo Actuation Performance of Nanotube Sheet Incorporated Azobenzene Crosslinked Liquid Crystalline Polymer Nanocomposite.

Crosslinked liquid crystalline polymers (CLCPs) containing azobenzene (AZO-CLCPs) are a type of promising material due to their significance in the design of light-driven smart actuators. Developing AZO-CLCP composites by incorporating AZO-CLCPs with other materials is an effective way of enhancing their practicability. Herein, we report an AZO-CLCP/CNT nanocomposite prepared by the in situ polymerization of diacrylates containing azobenzene chromophores on carbon nanotube (CNT) sheets. The liquid crystal phase structure of CLCP matrix was evidenced by the two-dimensional X-ray scattering. The prepared pure AZO-CLCP films and AZO-CLCP/CNT nanocomposite films demonstrated strong reversible photo-triggered deformation under the irradiation of UV light at 366 nm of wavelength, as a result of photo-induced isomerization of azobenzene moieties in the polymer network. But compared to pure AZO-CLCP films, the AZO-CLCP/CNT nanocomposite films could much more rapidly return to their initial shapes after the UV light irradiation was removed due to the elasticity effect of CNT sheets. The deformation behavior of AZO-CLCP/CNT nanocomposite films under the light irradiation was also different from that of the pure AZO-CLCP films due to the interfacial interaction between a polymer network and CNT sheet. Furthermore, incorporation of a CNT sheet remarkably increased the mechanical strength and robustness of the material. We also used this AZO-CLCP/CNT nanocomposite as a microvalve membrane actuator, which can be controlled by light, for a conceptual device of a microfluidic system. The results showed that this AZO-CLCP/CNT nanocomposite may have great potential in smart actuator applications for biological engineering, medical treatment, environment detection and microelectromechanical systems (MEMS), etc.

A Cut-and-Weld Process to 3D Architectures from Multiresponsive Crosslinked Liquid Crystalline Polymers.

Crosslinked liquid crystalline polymers (CLCPs) have garnered extensive attention in recent years for their significant values in the design of light-driven soft actuators. However, poor processabilities due to the insoluble and infusible crosslinked networks prevent their practical applications severely. In this study, a weldable azobenzene-containing CLCP is designed with photo- and humidity-responsive actuations, which enables a cut-and-weld process to 3D CLCP architectures. The tensile properties and stability are almost unchanged after welding, much better than those of the films pasted by common adhesive tapes. Meanwhile, the mechanisms of the welding process are clarified on the base of surface hydrogen bonding and further crosslinking. By taking advantage of the cut-and-weld process, a 3D "claw" integrated into a robotic arm is realized for grabbing millimeter-scale objects by remote control. This work enhances significantly not only the processability of CLCP films but also the utilization of leftover pieces, which provides an efficient approach to create functional 3D structures from film precursors for the potential application in the smart materials.

Determination of the material characteristics of the light-driven actuators from the kinetics of photo-mechanical response

The methodology of the quantification of photoinduced bending of crystals is described and illustrated by the case study of [Co(NH3)5NO2]Cl(NO3) crystal bending caused by the reversible photochemical isomerization. The measurement technique is combined with the quantitative model to give an effective method for assessing of the key material properties defining the characteristics of the potential light-driven actuators.

Gel-based soft actuators driven by light

Gels that display light-induced motile, life-like actions are reviewed and their potential applications as light-driven soft actuators are also discussed.

Light- and humidity-driven actuators with programmable complex shape-deformations

Programmable complex shape-deformation, multi-responsive property and convenient fabrication are all crucial for the development of actuators. However, a simultaneous realization of all these advantages has not been reported. Here, we report a new type of light- and humidity-driven actuator with programmable complex shape-deformations. The fabrication employs a laser printing technology. The light-driven actuation is based on a dual-mode actuation mechanism which utilizes the water adsorption/desorption properties. When irradiated by near infrared light, the actuator shows a large bending actuation with a curvature up to 2.1 cm−1. More importantly, programmable complex shape-deformations can be realized by printing patterns with different/gradient grayscale distributions on the actuators. The bending rates and amplitudes of actuators can be programmed and controlled. Complex 3D shapes, such as an anomalous tube and a helical cylinder, are obtained. In addition, the actuator can also perform a curvature of 1.3 cm−1 when driven by humidity. Finally, a series of smart biomimetic devices with programmable complex shape-deformations are demonstrated, including a self-adjustment iris responding to incident light, a biomimetic hand demonstrating a complex “OK” gesture, and a lotus with folding petals of different bending rates. This new-type actuator will have great potential in robotics and biomimetic applications.

2-Amino-6-methylpyridinium nitrophenolate nitrophenol

Organic co-crystals fashioned with two or more different constituents through intermolecular non-covalent interactions (hydrogen and halogen bonds) gain cumulative research considerations as of their applications in ambipolar charge transport, nonlinear optics (NLO), light-driven actuator, liquid crystal material and electronics industry. In this contribution, charge-transfer (CT) interactions and the corresponding physicochemical and nonlinear properties of bulk organic, CT co-crystal of 2-amino-6-methylpyridinium nitrophenolate nitrophenol (2A6MPNN) were comprehensively investigated. CT crystals were fabricated by the facile solvent evaporation strategy and crystallized with an extensive hydrogen bonding network existed between the aromatic donor (D) and acceptor (A). The organic salt formation of phenol–pyridine co-crystal was investigated by using 1H NMR, 13C NMR and FTIR. More importantly, we show that the CT interactions in co-crystals are related to their molecular packing which eventually leads to distinct optoelectronic properties. The convincing evidence for multiple CT was found by UV–Vis spectral measurements, i.e., both π–π and n–π* interactions (between 4-nitrophenol and 2-amino-6-methylpyridine) are simultaneously present and additionally new feature band arises at 408 nm. Remarkably, upon photoexcitation at 374 nm in the solid state, CT displays an unusual emission around 572 nm, which is probably attributed for the shallow traps of the ion pairs, along with a usual phenolate-centered green emission. Thermal analysis was performed using TG/DTA/DSC and Modulated DSC studies. NLO response on CT powder (for diverse particle sizes) indicates a phase matchability and NLO coefficient about 1.8-fold larger than that of urea.

Chemical/Light-Powered Hybrid Micromotors with "On-the-Fly" Optical Brakes.

Hybrid micromotors capable of both chemically powered propulsion and fuel-free light-driven actuation and offering built-in optical brakes for chemical propulsion are described. The new hybrid micromotors are designed by combining photocatalytic TiO2 and catalytic Pt surfaces into a Janus microparticle. The chemical reactions on the different surfaces of the Janus particle hybrid micromotor can be tailored by using chemical or light stimuli that generate counteracting propulsion forces on the catalytic Pt and photocatalytic TiO2 sides. Such modulation of the surface chemistry on a single micromotor leads to switchable propulsion modes and reversal of the direction of motion that reflect the tuning of the local ion concentration and hence the dominant propulsion force. An intermediate Au layer (under the Pt surface) plays an important role in determining the propulsion mechanism and operation of the hybrid motor. The built-in optical braking system allows "on-the-fly" control of the chemical propulsion through a photocatalytic reaction on the TiO2 side to counterbalance the chemical propulsion force generated on the Pt side. The adaptive dual operation of these chemical/light hybrid micromotors, associated with such control of the surface chemistry, holds considerable promise for designing smart nanomachines that autonomously reconfigure their propulsion mode for various on-demand operations.

Humidity- and light-driven actuators based on carbon nanotube-coated paper and polymer composite.

Multi-responsive actuators driven by different stimuli (e.g. light, humidity, electricity) have attracted intense attention recently for the advantages of being used in various environments and show enormous actuation. In this work, we propose humidity- and light-driven actuators based on carbon nanotube (CNT)-coated paper and a biaxially oriented polypropylene (BOPP) composite. The CNT-paper/BOPP actuator shows large bending actuation when driven by humidity change (curvature of 1.2 cm-1) and near infrared (NIR) light irradiation (curvature up to 1.6 cm-1). The great actuation performances outperform most other paper-based actuators. Finally, a smart gripper, of which the initial opening width can be enlarged, is fabricated on the basis of the CNT-paper/BOPP actuators. By utilizing the bidirectional bending motion of the actuator, the opening width of the gripper can increase to a width that is 4 times larger than its initial width, so as to grasp a large object. The gripper is also able to raise and move an object that is 20 times heavier than one actuator of the gripper. We assume that this new type of actuator has great potential in artificial muscle, soft robotics and biomimetic applications.

Graphene oxide/poly (N-isopropylacrylamide) hybrid film-based near-infrared light-driven bilayer actuators with shape memory effect

Abstract Smart actuators with fast and reversible changes towards external stimuli have attractive potentials in diverse applications. Here, we present the rational design and facile fabrication of graphene oxide (GO)/poly (N-isopropylacrylamide) (PANIPAM) hybrid film-based bilayer actuators, which display efficient and reversible bending/unbending behaviors in response to repeated cycles of near-infrared (NIR) light irradiations. Our bilayer actuator comprises a layer of GO, and a layer of GO/PNIPAM (GP) hybrid, taking advantages of the NIR absorbance and photothermal conversion capability of GO, along with the thermo-responsiveness of PNIPAM. Upon NIR light irradiation, GO converts absorbed light into heat, which subsequently triggers the shrinkage of PANIPAM chains, leading to the bending of the bilayer actuator. Owning to the shape inertness of GO to the NIR light, repeated irradiations of NIR light cause the bilayer to shrink/relax in an asymmetric manner, consequently leading to reversible bending/unbending behaviors of the bilayer actuator. Remarkably, our actuator shows templated bending orientation once exposed to NIR light, and exhibits shape memory effect due to the incomplete recovery of PNIPAM chains. This shape memory effect is retained in rectangular strips cut from the original disk-shaped bilayer and is exploited to selectively achieve either bending or twisting deformation. As an interesting example for potential practical utilization, a smart NIR light-driven forklift capable of lifting goods has been built by pinning one side of the bilayer film.

Tunable Photocontrolled Motions Using Stored Strain Energy in Malleable Azobenzene Liquid Crystalline Polymer Actuators.

A new strategy for enhancing the photoinduced mechanical force is demonstrated using a reprocessable azobenzene-containing liquid crystalline network (LCN). The basic idea is to store mechanical strain energy in the polymer beforehand so that UV light can then be used to generate a mechanical force not only from the direct light to mechanical energy conversion upon the trans-cis photoisomerization of azobenzene mesogens but also from the light-triggered release of the prestored strain energy. It is shown that the two mechanisms can add up to result in unprecedented photoindued mechanical force. Together with the malleability of the polymer stemming from the use of dynamic covalent bonds for chain crosslinking, large-size polymer photoactuators in the form of wheels or spring-like "motors" can be constructed, and, by adjusting the amount of prestored strain energy in the polymer, a variety of robust, light-driven motions with tunable rolling or moving direction and speed can be achieved. The approach of prestoring a controllable amount of strain energy to obtain a strong and tunable photoinduced mechanical force in azobenzene LCN can be further explored for applications of light-driven polymer actuators.

Photothermal Transport of DNA in Entropy-Landscape Plasmonic Waveguides.

The ability to handle single, free molecules in lab-on-a-chip systems is key to the development of advanced biotechnologies. Entropic confinement offers passive control of polymers in nanofluidic systems by locally asserting a molecule's number of available conformation states through structured landscapes. Separately, a range of plasmonic configurations have demonstrated active manipulation of nano-objects by harnessing concentrated electric fields. The integration of these two independent techniques promises a range of sophisticated and complementary functions to handle, for example, DNA, but numerous difficulties, in particular, conflicting requirements of channel size, have prevented progress. Here, we show that metallic V-groove waveguides, embedded in fluidic nanoslits, form entropic potentials that trap and guide DNA molecules over well-defined routes while simultaneously promoting photothermal transport of DNA through the losses of plasmonic modes. The propulsive forces, assisted by in-coupling to propagating channel plasmon polaritons, extend along the V-grooves with a directed motion up to ≈0.5 μm·mW-1 away from the input beam and λ-DNA velocities reaching ≈0.2 μm·s-1·mW-1. The entropic trapping enables the V-grooves to be flexibly loaded and unloaded with DNA by variation of transverse fluid flow, a process that is selective to biopolymers versus fixed-shape objects and also allows the technique to address the challenges of nanoscale interaction volumes. Our self-aligning, light-driven actuator provides a convenient platform to filter, route, and manipulate individual molecules and may be realized wholly by wafer-scale fabrication suitable for parallelized investigation.

Photochromic Torsional Switch (PTS): a light-driven actuator for the dynamic tuning of π-conjugation extension.

Here we present a molecular architecture that can reversibly change the geometric conformation of its π-system backbone via irradiation with two different wavelengths. The proposed 'molecular actuator' consists of a photoswitchable azobenzene orthogonally connected to a π-conjugated bithiophene by both direct and aliphatic linker-assisted bonding. Upon exposure to 350 nm light, the trans azobenzene moiety isomerizes to its cis form, causing the bithiophene to assume a semiplanar anti conformation (extended π-conjugation). Exposure to 254 nm light promotes the isomerization of the azobenzene unit back to its initial extended trans conformation, thus forcing the bithiophene fragment to twist out of coplanarity (restricted π-conjugation). The molecular conformation of the bithiophene was characterized using steady-state UV-vis and nuclear magnetic resonance spectroscopy, as well as ab initio computations. The proposed molecular design could be envisaged as a π-conjugation modulator, which has potential to be incorporated into extended linear π-systems, i.e. via the terminal α-thiophene positions, and used to tune their optical and electronic properties.

Photoinduced deformation of crosslinked liquid crystalline polymers and soft actuators

Photoresponsive polymeric materials are a type of functional materials that can absorb photo energy and undergo intra- or inter-molecular physical or chemical reactions. As a result, the materials exhibit macroscopic property changes when exposed to light, such as shape, color or refractive index. They attract more and more attention for the reason that light is an environmental-friendly and remotely controllable stimulus source. Liquid crystals exhibit cooperative motion of molecules in changing the molecular alignment by external stimuli. That means that only a small amount of energy is needed to bring about the alignment change in the whole system. Crosslinked liquid crystalline polymers (CLCPs) can be prepared by crossliking conventional liquid crystal polymers into a network, which possess both the order of liquid crystals and the elasticity of elastomers. The cooperative effect of liquid crystals coupled with polymer network structure gives rise to the characteristic properties of CLCPs. The initial ordering of mesogens can be fixed by the crosslinkers in CLCPs, which might cause a quick change in shape due to a fast order-disorder transition. CLCPs have been a hot topic because large deformation can be induced by changing the alignment of mesogens in CLCPs by external stimuli such as electric fields, changes in temperature, humidity and light. Incorporation of photochromic moieties into CLCPs provides the materials with photo controllable properties. Photoresponsive CLCPs can generate sophisticated movements including contraction/expansion, bending and twisting which convert photo energy into mechanical energy directly. Therefore, the CLCPs can be fabricated into various soft smart actuators by rational design, which brings about broad applications in artificial muscles, microrobots, microvalves, sensors and so on. In this review, the photoresponsive mechanism is firstly elucidated involving photochemical phase transition and photo realignment. Then, we detail the historical development of photoresponsive CLCPs, with emphasis on light- driven mechanical responses concerning two dimensional and three dimensional movements and surface property changes caused by deformation. We also summarize the recent progress of light-driven soft actuators made of photoresponsive CLCPs. We end with an outlook of existing challenges and opportunities.

Photoresponsive Liquid Crystal Elastomers as Feedback Controlled Light-driven Actuators – Theory, Real-time Behaviour, Limitations

Liquid Crystal Elastomers constitute a class of intelligent materials and actuators. External stimulation induces an internal phase-change that results in a mechanical motion of the Liquid Crystal Elastomer. External stimuli can be humidity, thermal energy but also radiation with an appropriate wavelength. In this paper we use the photomechanic response of Liquid Crystal Elastomers as a driving force for a controlled actuator, operating in feedback constellation with a tuned cascade-compensator. To accomplish this, we go the methodical route of dynamic system investigation consisting of an analysis of the phenomenological system-properties, the identification of the dynamic behaviour and the overall synthesis of the feedback-control loop. Since we also take practical considerations into account, we present a coordinated hard- and software concept to realize the application of the Liquid Crystal Elastomer as a controlled actuator. An application guide complements the paper and discusses the limits of this class of actuators.

A supramolecular assembly of cross-linked azobenzene/polymers for a high-performance light-driven actuator

Light-driven flexible actuators based on a photo-responsive polymer draw much attention due to their great ability for rapid and reversible light-to-work transduction based on a large deformation.

A light-driven supramolecular nanowire actuator.

A single photomechanical supramolecular nanowire actuator with an azobenzene-containing 1,3,5-tricarboxamide derivative is developed by employing a direct writing method. Single nanowires display photoinduced reversible bending and the bending behavior follows first-order kinetics associated with azobenzene photoisomerization. A wireless photomechanical nanowire tweezers that remotely manipulates a single micro-particle is also demonstrated.

A reactive azobenzene liquid-crystalline block copolymer as a promising material for practical application of light-driven soft actuators

The freestanding fiber and film comprised of a reactive liquid-crystalline block copolymer were fabricated by facile techniques such as dip-drawing or casting and thereafter immersion to undergo the post-crosslinking. Both of the CLCP fiber and film showed reversible bending and unbending behaviors in response to UV and visible light, respectively.

Reversible Actuation of Polyelectrolyte Films: Expansion-Induced Mechanical Force Enables cis–trans Isomerization of Azobenzenes

Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzene-containing polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)-poly(acrylic acid) (PAA) complexes (noted as PAZO-PAA) with poly(allylamine hydrochloride) (PAH) produces azobenzene-containing PAZO-PAA/PAH films. UV irradiation induces trans-cis isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the cis-trans back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the cis-trans back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for cis-trans azobenzene isomerization can be helpful for designing novel polymeric actuators.

Photomechanical energy conversion based on cross-linked liquid-crystalline polymers

Cross-linked liquid-crystalline (LC) polymers with a photochromic moiety show photoinduced deformation with change in molecular shape and alignment of photochromic compounds. Molecular-level photoisomerization of the photochromic moieties can give rise to macroscopic deformation of the materials, allowing one to convert light energy directly into mechanical work. The photomechanical effects extend the applicability of azobenzene-containing polymers towards light-driven actuators and artificial muscles. Recently, the effect of structure–property relationships and crosslinking density on the photomechanical property of photochromic polymers was investigated. Various motions based on the photoinduced deformation of the LC polymers were achieved by forming the polymer materials. This chapter summarizes the recent progress in photoinduced movements and light-driven actuation property of the LC polymers, in particular cross-linked LC polymers with a photochromic property.

Chiral meta-atoms rotated by light

We study the opto-mechanical properties of coupled chiral meta-atoms based on a pair of twisted split-ring resonators. By using a simple analytical model in conjunction with the Maxwell stress tensor, we capture insight into the mechanism and find that this structure can be used as a general prototype of subwavelength light-driven actuators over a wide range of frequencies. This coupled structure can provide a strong and tunable torque, and can support different opto-mechanical modes, including uniform rotation, periodically variable rotation and damped oscillations. Our results suggest that chiral meta-atoms are good candidates for creating sub-wavelength motors or wrenches controlled by light.