Hydrogen Bond Enhances Photomechanical Swing of Liquid-Crystalline Polymer Bilayer Films

Bioinspired Hydro- and Hydrothermally Responsive Tubular Soft Actuators

Soft actuators made of thermoresponsive polymers have great potential for intelligent robotics and biomedical devices due to their reversible deformation capability in response to temperature fluctuations. However, they are constrained by a predefined phase transition temperature, limited directional deformation, and nonbiocompatible formulations, thereby restricting their practical utility. Herein a new biomimicry approach is presented to overcome these limitations by developing hydro- and hydrothermally responsive soft actuators made of biocompatible and pliable materials i.e. cotton yarn and polyurethane. We mimic the tubular shape of elephant trunks with their unique muscle orientation by embedding a helical cotton yarn within a hydrophilic polyurethane tube, followed by targeted surface patterning. Unlike the narrow-range shape morphing across the phase transition temperature boundary of typical thermoresponsive hydrogel actuators, we harness hydrothermal stiffness variations in polyurethane to obtain consistent morphing capabilities over a much wider temperature range. The developed actuators can perform versatile activities such as linear, bending, curvilinear, and rotating movements, overcoming the unidirectional motion limitations of conventional soft actuators. The cell viability assay on the building block materials also confirms the high biocompatibility of the actuators. The reported facile fabrication strategy provides new insights for designing complex yet free-standing soft actuators from readily available supple materials.

Liquid crystalline elastomer self-oscillating fiber actuators fabricated from soft tubular molds

The self-oscillation of objects that perform continuous and periodic motions upon unchanging and constant stimuli is highly important for intelligent actuators, advanced robotics, and biomedical machines. Liquid crystalline elastomer (LCE) materials are superior to traditional stimuli-responsive polymeric materials in the development of self-oscillators because of their reversible, large and anisotropic shape-changing ability, fast response ability and versatile structural design. In addition, fiber-shaped oscillators have attracted much interest due to their agility, flexibility and diverse oscillation modes. Herein, we present a strategy for fabricating fiber-shaped LCE self-oscillators using soft tubes as molds. Through the settlement of different configuration states of the soft tubes, the prepared fiber-shaped LCE oscillators can perform continuous rotational self-oscillation or up-and-down shifting self-oscillation under constant light stimuli, which are realized by photoinduced repetitive self-winding motion and self-waving motion, respectively. The mechanism of self-oscillating movements is attributed to the local temperature oscillation of LCE fibers caused by repetitive self-shadowing effects. LCE self-oscillators can operate stably over many oscillating cycles without obvious performance attenuation, revealing good robustness. Our work offers a versatile way by which LCE self-oscillators can be conveniently designed and fabricated in bulk and at low cost, and broadens the road for developing self-oscillating materials for biological robotics and health care machines.

Light-driven soft microrobots based on hydrogels and LCEs: development and prospects

In the daily life of mankind, microrobots can respond to stimulations received and perform different functions, which can be used to complete repetitive or dangerous tasks. Magnetic driving works well in robots that are tens or hundreds of microns in size, but there are big challenges in driving microrobots that are just a few microns in size. Therefore, it is impossible to guarantee the precise drive of microrobots to perform tasks. Acoustic driven micro-nano robot can achieve non-invasive and on-demand movement, and the drive has good biological compatibility, but the drive mode has low resolution and requires expensive experimental equipment. Light-driven robots move by converting light energy into other forms of energy. Light is a renewable, powerful energy source that can be used to transmit energy. Due to the gradual maturity of beam modulation and optical microscope technology, the application of light-driven microrobots has gradually become widespread. Light as a kind of electromagnetic wave, we can change the energy of light by controlling the wavelength and intensity of light. Therefore, the light-driven robot has the advantages of programmable, wireless, high resolution and accurate spatio-temporal control. According to the types of robots, light-driven robots are subdivided into three categories, namely light-driven soft microrobots, photochemical microrobots and 3D printed hard polymer microrobots. In this paper, the driving materials, driving mechanisms and application scenarios of light-driven soft microrobots are reviewed, and their advantages and limitations are discussed. Finally, we prospected the field, pointed out the challenges faced by light-driven soft micro robots and proposed corresponding solutions.

Multifunctional UV-NIR Dual Light-Responsive Soft Actuators from a Main-Chain Azobenzene Semi-Crystalline Poly(ester-amide) Doped with Polydopamine Nanoparticles

The development of soft photoactuators with multifunctionality and improved performance is highly important for their broad applications. Herein, we report on a facile and efficient strategy for fabricating such photoactuators with UV-NIR dual light-responsivity, room-temperature 3D shape reprogrammability and reprocessability, and photothermal healability by doping polydopamine (PDA) nanoparticles into a main-chain azobenzene semi-crystalline poly(ester-amide) (PEA). The PEA/PDA nanoparticle composite was readily processed into free-standing films with enhanced mechanical and photomechanical properties compared with the blank PEA films. Its physically crosslinked uniaxially oriented films showed rapid and highly reversible photochemically induced bending/unbending under the UV/visible light irradiation at room temperature in both the air atmosphere and water. When exposed to the NIR light, they (and their bilayer films formed with a polyimide film) exhibited photothermally induced bending even at a temperature much lower than their crystalline-to-isotropic phase transition temperature based on a unique mechanism (involving photothermally induced polymer chain relaxation due to the disruption of their hydrogen bonds). The room-temperature 3D shape reprogrammability and reprocessability and photothermal healability of the composite polymer films were also demonstrated. Such multifunctional dual light-responsive photoactuators with well-balanced mechanical robustness, actuation stability, 3D shape reprogrammability/reprocessability and photothermal healability hold much promise in various photoactuating applications.

A Bionic Venus Flytrap Soft Microrobot Driven by Multiphysics for Intelligent Transportation

With the continuous integration of material science and bionic technology, as well as increasing requirements for the operation of robots in complex environments, researchers continue to develop bionic intelligent microrobots, the development of which will cause a great revolution in daily life and productivity. In this study, we propose a bionic flower based on the PNIPAM-PEGDA bilayer structure. PNIPAM is temperature-responsive and solvent-responsive, thus acting as an active layer, while PEGDA does not change significantly in response to a change in temperature and solvent, thus acting as a rigid layer. The bilayer flower is closed in cold water and gradually opens under laser illumination. In addition, the flower gradually opens after injecting ethanol into the water. When the volume of ethanol exceeds the volume of water, the flower opens completely. In addition, we propose a bionic Venus flytrap soft microrobot with a bilayer structure. The robot is temperature-responsive and can reversibly transform from a 2D sheet to a 3D tubular structure. It is normally in a closed state in both cold (T < 32 °C) and hot water (T > 32 °C), and can be used to load and transport objects to the target position (magnetic field strength < 1 T).

Geometry Controlled Oscillations in Liquid Crystal Polymer Films Triggered by Thermal Feedback

Light-induced oscillatory behavior of liquid crystal polymer network (LCN) films has been demonstrated by several researchers in the past decade. Similarly, oscillations in LCN films under constant thermal stimulus have been reported recently, although the mechanism and the factors that govern the oscillatory behavior are not well understood. In this work, we study the dynamics of self-sustained oscillations exhibited by LCN films under a constant thermal stimulus through experiments and simulations. Geometrically asymmetric films such as a right triangle and an equilateral triangle are obtained from a twisted nematic square film. A multiphysics computational framework using the finite element method is developed to simulate the oscillatory behavior of the LCN films kept on a hot plate. The framework accounts for a coupling between heat transfer and mechanical deformations during the oscillations. Small temperature fluctuations (≈ 1 °C) coupled with gravity induced torque are shown to drive the oscillatory behavior at a specific plate temperature. We show for the first time that self-sustained oscillations can also be achieved in symmetric shapes, such as square films, by creating a thickness tapering between two opposite edges. The frequency of the oscillations is found to be in the range of 0.5 to 2.5 Hz for different geometries studied. The oscillation temperature depends on the mean thickness, size, and thickness profile of the films. As a possible application, we demonstrate a thermally actuated optical chopper using the oscillatory response of the films.

Light-Powered Liquid Crystal Polymer Network Actuator Using TiO2 Nanoparticles as an Inorganic Ultraviolet-Light Absorber

Recently, the design and fabrication of light-powered actuators have attracted immense attention because of the manufacturing of intelligent soft robots and innovative self-regulating devices. Accordingly, a liquid crystal polymer network (LCN) provides a promising platform due to its reversible and multistimulus-responsive shape-changing behaviors. In particular, doping nanoparticles with exclusive properties into the LCN can produce interesting results. In this work, we investigated a TiO₂ nanoparticle-based LCN polymer light-powered actuator. TiO₂ nanoparticles as an inorganic ultraviolet (UV)-light absorber can substantially affect the LCN polymer's oscillatory behavior. Our results demonstrate that the oscillation characteristics are directly influenced by the presence of nanoparticles, and we studied the influencing factors. The effectiveness of the elastic modulus, thermomechanical force, and curvature was investigated using different weight percentages of TiO₂ nanoparticles. Our results show that, in the presence of TiO₂ nanoparticles, the polymer chain order and inter-chain interactions in the polymer matrix as well as the structural deformation of relevant polymer surfaces are changed.

Optical wavelength selective actuation of dye doped liquid crystalline elastomers by quasi-daylight

We explore obtaining different photo responses of liquid crystalline elastomer (LCE) materials through modulating the optical wavelengths in order to promote the development of precise photocontrol on LCE actuators, and thus study the effect of light-absorbing dyes with different absorption bands on the selective actuation of LCE materials. The dye-doped LCEs were prepared by incorporating special visible absorber dyes into thiol-acrylate main chain LCE (MC-LCE) matrices. The dyes showed photo actuation performance to LCEs due to the photothermal effects. But, every dye-doped LCE could be effectively actuated by light irradiation whose wavelength was inside its absorption band, but could not be effectively actuated by the light whose wavelength was beyond its absorption band. Wavelength selective actuation effects, no matter actuating deformation or actuating force, could be remarkably demonstrated by these dye-doped LCEs through filtering the same quasi-daylight source to be different wavelength bands. Our work opens up a significant way for the precise and convenient photo actuation of LCE actuators, while expanding the utilization potential of quasi-daylight, and further natural sunlight.

Self-regulating electrical rhythms with liquid crystal oligomer networks in hybrid circuitry

Self-regulation is an essential aspect in the practicality of electronic systems, ranging from household heaters to robots for industrial manufacturing. In such devices, self-regulation is conventionally achieved through separate sensors working in tandem with control modules. In this paper, we harness the reversible actuating properties of liquid crystal oligomer network (LCON) polymers to design a self-regulated oscillator. A dynamic equilibrium is achieved by applying a thermally-responsive and electrically-functionalized LCON film as a dual-action component, namely as a combined electrical switch and composite actuating sensor, within a circuit. This hybrid circuit configuration, consisting of both inorganic and organic material, generates a self-regulated feedback loop which cycles regularly and indefinitely. The feedback loop cycle frequency is tunable between approximately 0.08 and 0.87 Hz by altering multiple factors, such as supplied power or LCON chemistry. Our research aims to drive the material-to-device transition of stimuli-responsive LCONs, striving towards applications in electronic soft robotics.

Light-Powered Self-Sustained Oscillators of Graphene Oxide/Liquid Crystalline Network Composites Showing Amplitude and Frequency Superposition

Light-activated self-oscillators have drawn enormous attention for their potential applications in mobile machines, energy harvesting, signal modulation, etc. Herein, we report one graphene oxide (GO)/liquid crystalline network (LCN) actuator that presents a unique light-activated oscillation with amplitude and frequency superposition. The GO/LCN composite film is prepared by the one-step polymerization of LC monomers, which favors a splay orientation in LC cells made by gluing together two glass sheets, one coated with photothermal agent GO and the other coated with a rubbed polyimide alignment layer. Owing to the asymmetric contraction/expansion, changing the cutting direction gives rise to notably different actuation behaviors for GO/LCN composite films. Moreover, it twists a little during the deflection process as a result of experimental error during the cutting process, which may cause the strip to be cut inaccurately. When the composite film is embedded in a self-shadowing system, it produces an unconventional hybrid oscillation mode upon near-infrared light irradiation, i.e., bending and twisting oscillation coupled. Furthermore, when the aspect ratio of the film decreases, the twisting mode is suppressed and the actuator changes from a coupled mode to a single bending mode. The proposed strategy may extend the application of GO/LCN composite materials and enrich light-activated self-oscillating behaviors.

Design, Regulation, and Applications of Soft Actuators Based on Liquid-Crystalline Polymers and Their Composites

Soft actuators designed from stimuli-responsive polymers often possess a certain amount of bionic functionality because of their versatile deformation. Liquid-crystalline polymers (LCPs) and their composites are among the most fascinating materials for soft actuators due to their great advantages of flexible structure design and easy regulation. In this Spotlight on Applications, we mainly focus on our group's latest research progress in soft actuators based on LCPs and their composites. Some representative research findings from other groups are also included for a better understanding of this research field. Above all, the essential principles for the responsive behavior and reconfigurable performance of the soft actuators are discussed, from the perspective of material morphology and structure design. Further on, we analyze recent work on how to precisely regulate the responsive modes and quantify the operating parameters of soft actuators. Finally, some application examples are given to demonstrate well-designed soft actuators with different functions under varied working environments, which is expected to provide inspiration for future research in developing more intelligent and multifunctional integrated soft actuators.

Electrically Controlled Aquatic Soft Actuators with Desynchronized Actuation and Light-Mediated Reciprocal Locomotion

Soft-bodied aquatic invertebrates can overcome hydrodynamic resistance and display diverse locomotion modes in response to environmental cues. Exploring the dynamics of locomotion from bioinspired aquatic actuators will broaden the perspective of underwater manipulation of artificial systems in fluidic environments. Here, we report a multilayer soft actuator design based on a light-driven hydrogel and a laser-induced graphene (LIG) actuator, minimizing the effect of the time delay by a monolithic hydrogel-based system while maintaining shape-morphing functionality. Moreover, different time scales in the response of actuator materials enable a real-time desynchronization of energy inputs, holding great potential for applications requiring desynchronized stimulation. This hybrid design principle is ultimately demonstrated with a high-performance aquatic soft actuator possessing an underwater walking speed of 0.81 body length per minute at a relatively low power consumption of 3 W. When integrated with an optical sensor, the soft actuator can sense the variation in light intensity and achieve mediated reciprocal motion. Our proposed locomotion mechanism could inspire other multilayer soft actuators to achieve underwater functionalities at the same spatiotemporal scale. The underwater actuation platform could be used to study locomotion kinematics and control mechanisms that mimic the motion of soft-bodied aquatic organisms.

Biobased Composites with High Lignin Content and Excellent Mechanical Properties toward the Ingenious Photoresponsive Actuator

The fabrication of biobased smart materials from renewable biomasses is of great importance for sustainable development. Although lignin possesses photothermal conversion potential, the development of lignin-based actuators with large contraction and fast photoresponse has various hurdles. Herein, simply by blending with castor oil-derived polyamide elastomers, a lignin-based photoresponsive actuator can be obtained, which accomplishes up to 18% light-driven contraction under loading within 3 s. The crystals in polymer matrix serve as switch segments, firmly locking the stress-induced strain energy, which is swiftly released due to photothermal processes and induced a huge contraction. The composite, LP₄-50, can contract and induce dynamic bending in multiple directions when irradiated locally with a near-infrared 808 nm laser. Furthermore, at standard 1 sun irradiation (100 mW/cm²), LP₄-50 was successfully employed to power a thermoelectric generator. This strategy establishes the groundwork for further research into the photothermal characteristics of lignin and encourages new applications in stimulus-responsive actuators.

Enhancing the performances of physically cross-linked photodeformable main-chain azobenzene poly(ester-amide)s via chemical structure engineering

The development of physically cross-linked photodeformable main-chain azobenzene poly(ester-amide)s with enhanced performances via chemical structure engineering and obtention of their detailed structure–property relationship are first described.

Photoinduced deformation of amorphous polyimide enabled by an improved azobenzene isomerization efficiency

A newly designed azo-PI, without pre-stretching or polarized-light irradiation, exhibits reversible bending behaviors under alternate UV and visible light irradiation, providing a facile route to deformable 2D/3D structure actuators.

Photoresponsive nanostructures of azobenzene-containing block copolymers at solid surfaces

An azobenzene-containing block copolymer self-assembled into island-like nanostructures. The island-like nanostructures fused into chain-like nanostructures under UV irradiation based on photoinduced solid-to-liquid transitions at the nanoscale.

The photoinduced back-and-forth deformation behavior of poly(arylene ether)s containing bis-azobenzene groups in the main chain

A novel series of poly(arylene ether)s containing various bis-azobenzene groups in the main chain were synthesized and showed photoinduced back-and-forth deformation behavior.

Reprocessable Photodeformable Azobenzene Polymers

Photodeformable azobenzene (azo) polymers are a class of smart polymers that can efficiently convert light energy into mechanical power, holding great promise in various photoactuating applications. They are typically of crosslinked polymer networks with highly oriented azo mesogens embedded inside. Upon exposure to the light of appropriate wavelength, they experience dramatic order parameter change following the configuration change of the azo units. This could result in the generation and accumulation of the gradient microscopic photomechanical force in the crosslinked polymer networks, thus leading to their macroscopic deformation. So far, a great number of photodeformable azo polymers have been developed, including some unoriented ones showing photodeformation based on different mechanisms. Among them, photodeformable azo polymers with dynamic crosslinking networks (and some uncrosslinked ones) have aroused particular interest recently because of their obvious advantages over those with stable chemical crosslinking structures such as high recyclability and reprocessability. In this paper, I provide a detailed overview of the recent progress in such reprocessable photodeformable polymers. In addition, some challenges and perspectives are also presented.

High-Speed, Heavy-Load, and Direction-Controllable Photothermal Pneumatic Floating Robot

Light-fueled actuators are promising in many fields due to their contactless, easily controllable, and eco-efficiency features. However, their application in liquid environments is complicated by the existing challenges of rapid deformation in liquids, light absorption of the liquid media, and environmental contamination. Here, we design a photothermal pneumatic floating robot (PPFR) using a boat-paddle structure. Light energy is converted into thermal energy of air by an isolated photothermal composite, which is then converted into mechanical energy of liquid to drive the movement of PPFRs. By understanding and controlling the photothermal actuation, the PPFR can achieve an average velocity of 13.1 mm s^-1 in water and can be modified for remote on-demand differential steering and self-sustained oscillation. The PPFR may be modified to provide a lifting mechanism, capable of moving 4 times the PPFR mass. Various shapes and materials are suitable for the PPFR, providing a platform for liquid surface transporting, water sampling, pollutant collecting, underwater photography, and photocontrol robots in shallow water.