Making waves in a photoactive polymer film

Photoinitiated Transient Self-Assembly in a Catalytically Driven Chemical Reaction Cycle

Chemically fueled chemical reaction networks (CRNs) are essential in controlling dissipative self-assembly. A key challenge in the field is to store chemical fuel-precursors or "pre-fuels" in the system that are converted into activating or deactivating fuels in a catalytically controlled CRN. In addition, real-time control over catalysis in a CRN by light is highly desirable, but so far not yet achieved. Here we show a catalytically driven CRN that is photoinitiated with 450 nm light, producing activated monomers that go on to perform transient self-assembly. Monomer activation proceeds via photoredox catalysis, converting the monomer alcohol groups into the corresponding aldehydes that self-assemble into large supramolecular fibers. Monomer deactivation is achieved by organometallic catalysis that relies on pre-fuel hydrolysis to release formate (i.e. the deactivating fuel). Additionally, irradiation with 305 nm light accelerates the release of formate by photo-uncaging the pre-fuel, leading to a factor of ca. 2 faster deactivation of the monomer. Overall, we show transient self-assembly upon visible light photoactivation, and tunable life-times by ultraviolet light.

Shape Memory Polymers with Patternable Recovery Onset Regulated by Light

Shape memory polymers (SMPs) show attractive prospects in emerging fields such as soft robots and biomedical devices. Although their typical trigger-responsive character offers the essential shape-changing controllability, having to access external stimulation is a major bottleneck toward many applications. Recently emerged autonomous SMPs exhibit unique stimuli-free shape-shifting behavior with its controllability achieved via a delayed and programmable recovery onset. Achieving multi-shape morphing in an arbitrary fashion, however, is infeasible. In this work, a molecular design that allows to spatio-temporally define the recovery onset of an autonomous shape memory hydrogel (SMH) is reported. By introducing nitrocinnamate groups onto an SMH, its crosslinking density can be adjusted by light. This affects greatly the phase separation kinetics, which is the basis for the autonomous shape memory behavior. Consequently, the recovery onset can be regulated between 0 to 85 min. With masked light, multiple recovery onsets in an arbitrarily defined pattern which correspondingly enable multi-shape morphing can be realized. This ability to achieve highly sophisticated morphing without relying on any external stimulation greatly extends the versatility of SMPs.

3D Directional Assembly of Liquid Crystal Molecules

The precise construction of hierarchically long-range ordered structures using molecules as fundamental building blocks can fully harness their anisotropy and potential. However, the 3D, high-precision, and single-step directional assembly of molecules is a long-pending challenge. Here, a 3D directional molecular assembly strategy via femtosecond laser direct writing (FsLDW) is proposed and the feasibility of this approach using liquid crystal (LC) molecules as an illustrative example is demonstrated. The physical mechanism for femtosecond (fs) laser-induced assembly of LC molecules is investigated, and precise 3D arbitrary assembly of LC molecules is achieved by defining the discretized laser scanning pathway. Additionally, an LC-based Fresnel zone plate array with polarization selection and colorization imaging functions is fabricated to further illustrate the potential of this method. This study not only introduces a 3D high-resolution alignment method for LC-based functional devices but also establishes a universal protocol for the precise 3D directional assembly of anisotropic molecules.

Visible-Light-Activated Heteroaryl Azoswitches: Toward a More Colorful Future

Azobenzenes (Ph-N═N-Ph) are known as the most widely studied molecular photoswitches, and the recent rise of azoheteroarenes (Het-N═N-Ph or Het-N═N-Het) offers great opportunities to advance this already mature field. A common limitation is that azo-switches generally require harmful UV light for activation, which hinders their application across various fields. Despite great efforts in developing visible-light azobenzenes over the past few decades, the potential of visible-light heteroaryl azoswitches remains largely unexplored. This Perspective summarizes the state-of-the-art advancements in visible-light heteroaryl azoswitches, covering molecular design strategies, the structure-property relationship, and potential applications. We highlight the distinctive advantages of azoheteroarenes over azobenzenes in the research and development of visible-light switches. Furthermore, we discuss the opportunities and challenges in this emerging field and propose potential solutions to address crucial issues such as spectral red-shift and thermal half-life. Through this Perspective paper, we aim to provide inspiration for further exploration in this field, in anticipation of the growing prosperity and bright future of visible-light azoheteroarene photoswitches.

Liquid crystal polymer actuators with complex and multiple actuations

Deformable liquid crystal polymers (LCPs), which exhibit both entropic elasticity of polymer networks and anisotropic properties originating from ordered mesogens, have gained more and more interest for use as biomedical soft actuators. Especially, LCP actuators with controllable mesogen alignment, sophisticated geometry and reprogrammability are a rising star on the horizon of soft actuators, since they enable complex and multiple actuations. This review focuses on two topics: (1) the regulation of mesogen alignment and geometry of LCP actuators for complex actuations; (2) newly designed reprogrammable LCP materials for multiple actuations. First, basic actuation mechanisms are briefly introduced. Then, LCP actuators with complex actuations are demonstrated. Special attention is devoted to the improvement of fabrication methods, which profoundly influence the available complexity of the mesogen alignment and geometry. Subsequently, reprogrammable LCP actuators featuring dynamic networks or shape memory effects are discussed, with an emphasis on their multiple actuations. Finally, perspectives on the current challenges and potential development trends toward more intelligent LCP actuators are discussed, which may shed light on future investigations in this field.

Solar-Powered Molecular Crystal Motor Based on an Anthracene-Thiazolidinedione Photoisomerization Reaction

Assembling molecular machines into crystals provides a way to harness their power on large length scales, but the development of a crystal analogue to a molecular motor remains a challenge. The molecule (Z)-5-(anthracen-9-ylmethylene)-3-butylthiazolidine-2,4-dione (C4-ATD) has E and Z isomers with strongly overlapping absorption spectra. This spectroscopic property allows both Z → E and E → Z photoisomerization reactions to be driven by a single light source, and simulations indicate this property can provide a route to robust oscillatory motion. Reprecipitation in an aqueous surfactant enables the growth of single crystal microwires that exhibit continuous mechanical oscillations under a wide range of illumination conditions, including ambient solar irradiation. Molecular crystal motors provide a new approach for transforming continuous light into oscillatory mechanical motion.

Photochemically Induced Propulsion of a 4D Printed Liquid Crystal Elastomer Biomimetic Swimmer

Underwater organisms exhibit sophisticated propulsion mechanisms, enabling them to navigate fluid environments with exceptional dexterity. Recently, substantial efforts have focused on integrating these movements into soft robots using smart shape-changing materials, particularly by using light for their propulsion and control. Nonetheless, challenges persist, including slow response times and the need of powerful light beams to actuate the robot. This last can result in unintended sample heating and potentially necessitate tracking specific actuation spots on the swimmer. To tackle these challenges, new azobenzene-containing photopolymerizable inks are introduced, which can be processed by extrusion printing into liquid crystalline elastomer (LCE) elements of precise shape and morphology. These LCEs exhibit rapid and significant photomechanical response underwater, driven by moderate-intensity ultraviolet (UV) and green light, being the actuation mechanism predominantly photochemical. Inspired by nature, a biomimetic four-lapped ephyra-like LCE swimmer is printed. The periodically illumination of the entire swimmer with moderate-intensity UV and green light, induces synchronous lappet bending toward the light source and swimmer propulsion away from the light. The platform eliminates the need of localized laser beams and tracking systems to monitor the swimmer's motion through the fluid, making it a versatile tool for creating light-fueled robotic LCE free-swimmers.

Transforming patterned defects into dynamic poly-regional topographies in liquid crystal oligomers

We create high-aspect-ratio dynamic poly-regional surface topographies in a coating of a main-chain liquid crystal oligomer network (LCON). The topographies form at the topological defects in the director pattern organized in an array which are controlled by photopatterning of the alignment layer. The defect regions are activated by heat and/or light irradiation to form reversible topographic structures. Intrinsically, the LCON is rubbery and sensitive to temperature changes, resulting in shape transformations. We further advanced such system to make it light-responsive by incorporating azobenzene moieties. Actuation reduces the molecular order of the LCON coating that remains firmly adhered to the substrate which gives directional shear stresses around the topological defects. The stresses relax by deforming the surfaces by forming elevations or indents, depending on the type of defects. The formed topographies exhibit various features, including two types of protrusions, ridges and valleys. These poly-regional structures exhibit a large modulation amplitude of close to 60%, which is 6 times larger than the ones formed in liquid crystal networks (LCNs). After cooling or by blue light irradiation, the topographies are erased to the initial flat surface. A finite element method (FEM) model is adopted to simulate structures of surface topographies. These dynamic surface topographies with multilevel textures and large amplitude expand the application range, from haptics, controlled cell growth, to intelligent surfaces with adjustable adhesion and tribology.

Closed Twisted Hydrogel Ribbons with Self-Sustained Motions under Static Light Irradiation

Self-sustained motions are widespread in biological systems by harvesting energy from surrounding environments, which inspire scientists to develop autonomous soft robots. However, most-existing soft robots require dynamic heterogeneous stimuli or complex fabrication with different components. Recently, control of topological geometry has been promising to afford soft robots with physical intelligence and thus life-like motions. Reported here are a series of closed twisted ribbon robots, which exhibit self-sustained flipping and rotation under constant light irradiation. Both Möbius strip and Seifert ribbon robots are devised for the first time by using an identical hydrogel, which responds to light irradiation on either side. Experiment and simulation results indicate that the self-regulated motions of the hydrogel robots are related to fast and reversible response of muscle-like gel, self-shadowing effect, and topology-facilitated refresh of light-exposed regions. The motion speeds and directions of the hydrogel robots can be tuned over a wide range. These closed twisted ribbon hydrogels are further applied to execute specific tasks in aqueous environments, such as collecting plastic balls, climbing a vertical rod, and transporting objects. This work presents new design principle for autonomous hydrogel robots by benefiting from material response and topology geometry, which may be inspirative for the robotics community.

Reversible Snapping of Constrained Anisotropic Hydrogels Upon Light Stimulations

Creatures, such as Venus flytrap and hummingbirds, capable of rapid predation through snap-through transition, provide paradigms for the design of soft actuators and robots with fast actions. However, these artificial "snappers" usually need contact stimulations to trigger the flipping. Reported here is a constrained anisotropic poly(N-isopropylacrylamide) hydrogel showing fast snapping upon light stimulation. This hydrogel is prepared by flow-induced orientation of nanosheets (NSs) within a rectangular tube. The precursor containing gold nanoparticles is immediately exposed to UV light for photopolymerization to fix the ordered structure of NSs. Two ends of the slender gel are clamped to form a buckle with bistability nature, which snaps to the other side upon laser irradiation. Systematic experiments are conducted to investigate the influences of power intensity and irradiation angle of the laser, as well as thickness and buckle height of the gel, on the snapping behaviors. The fast snapping is further used to kick a plastic bead and control the switch state. Furthermore, synergetic or oscillated snapping of the gel with two buckles of opposite directions is realized by inclined irradiation of a laser or horizontal irradiation with two lasers, respectively. Such light-steered snapping of hydrogels should merit designing soft robots, energy harvests, etc.

Responsive Liquid Crystal Network Microstructures with Customized Shapes and Predetermined Morphing for Adaptive Soft Micro-Optics

Stimuli-responsive materials have garnered substantial interest in recent years, particularly liquid crystal networks (LCNs) with sophisticatedly designed structures and morphing capabilities. Extensive efforts have been devoted to LCN structural designs spanning from two-dimensional (2D) to three-dimensional (3D) configurations and their intricate morphing behaviors through designed alignment. However, achieving microscale structures and large-area preparation necessitates the development of novel techniques capable of facilely fabricating LCN microstructures with precise control over both overall shape and alignment, enabling a 3D-to-3D shape change. Herein, a simple and cost-effective in-cell soft lithography (ICSL) technique is proposed to create LCN microstructures with customized shapes and predesigned morphing. The ICSL technique involves two sequential steps: fabricating the desired microstructure as the template by using the photopolymerization-induced phase separation (PIPS) method and reproducing the LCN microstructures through templating. Meanwhile, surface anchoring is employed to design and achieve molecular alignment, accommodating different deformation modes. With the proposed ICSL technique, cylindrical and spherical microlens arrays (CMLAs and SMLAs) have been successfully fabricated with stimulus-driven polarization-dependent focusing effects. This technique offers distinct advantages including high customizability, large-area production, and cost-effectiveness, which pave a new avenue for extensive applications in different fields, exemplified by adaptive soft micro-optics and photonics.

The Use of Photocycloaddition Reactions to Drive Mechanical Motions Resembling Humanoid Movements

With the aim of producing a photomechanical material for incorporation in soft microrobots, a one-dimensional diene coordination polymer (CP) [Cd(F-bpeb)(3-CBA)2]n (CP1, F-bpeb=4,4'-((1E,1'E)-(2,5-difluoro-1,4-phenylene)bis(ethene-2,1-diyl))dipyridine, 3-HCBA=3-chlorobenzoic acid) was synthesized and characterized. Irradiation of CP1 with ultraviolet (UV) or visible light causes [2+2] photocycloaddition reactions resulting in the introduction of crystal strain which triggers various types of crystal movements. Composite films of CP1-PVA (SC) fabricated by dispersing CP1 crystals into polyvinyl alcohol (PVA) solution allow amplification of the crystal movement so that the film strips exhibit fast and flexible curling upon photoirradiation. The composite films may be cut into long rectangular strips and folded to simulate soft microrobots which exhibit a variety of fast, flexible and continuous photomechanical movements resembling a human performing various gymnastic exercises.

Theoretical Analysis of Light-Actuated Self-Sliding Mass on a Circular Track Facilitated by a Liquid Crystal Elastomer Fiber

Self-vibrating systems obtaining energy from their surroundings to sustain motion can offer great potential in micro-robots, biomedicine, radar systems, and amusement equipment owing to their adaptability, efficiency, and sustainability. However, there is a growing need for simpler, faster-responding, and easier-to-control systems. In the study, we theoretically present an advanced light-actuated liquid crystal elastomer (LCE) fiber-mass system which can initiate self-sliding motion along a rigid circular track under constant light exposure. Based on an LCE dynamic model and the theorem of angular momentum, the equations for dynamic control of the system are deduced to investigate the dynamic behavior of self-sliding. Numerical analyses show that the theoretical LCE fiber-mass system operates in two distinct states: a static state and a self-sliding state. The impact of various dimensionless variables on the self-sliding amplitude and frequency is further investigated, specifically considering variables like light intensity, initial tangential velocity, the angle of the non-illuminated zone, and the inherent properties of the LCE material. For every increment of π/180 in the amplitude, the elastic coefficient increases by 0.25% and the angle of the non-illuminated zone by 1.63%, while the light intensity contributes to a 20.88% increase. Our findings reveal that, under constant light exposure, the mass element exhibits a robust self-sliding response, indicating its potential for use in energy harvesting and other applications that require sustained periodic motion. Additionally, this system can be extended to other non-circular curved tracks, highlighting its adaptability and versatility.

Dual-Responsive Gradient Structured Actuator via Photopolymerization-Induced Diffusion

Stimuli-responsive materials have recently gained significant attention in the field of soft robotics, sensors, and biomimetic devices. The most facile way for the fabrication of such materials remains to endow bilayer structures which are fabricated with the combination of active and passive layers. Although, easily fabricated, these structures suffer from the generation of stress points between connection areas. In this work we develop a method to create a thin film with controlled cross-link variation across its thickness. The cross-link gradient is achieved through polymerization induced diffusion of dithiol molecules in thiol-ene network. As a result, the film exhibits bending deformation upon illumination with light or exposure to a chemical solvent, thereby demonstrating dual responsiveness. Light actuation of the film is achieved via photothermal effects due to the incorporation of dye into the system which can absorb UV light and heat the network. While solvent induced actuation is due to anisotropic swelling. Furthermore, the straightforward fabrication procedure allows for the creation of more complex deformations by patterning the film using a photomask during photopolymerization.

Bis-azopyrazole Photoswitches for Efficient Solar Light Harvesting

Although natural sunlight is one of the most abundant and sustainable energy resources, only a fraction of its energy is currently harnessed and utilized in photoactive systems. The development of molecular photoswitches that can be directly activated by sunlight is imperative for unlocking the full potential of solar energy and addressing the growing energy demands. Herein, we designed a series of 2-amino-1,3-bis-azopyrazoles featuring a coupled πn system, resulting in a pronounced redshift in their spectral absorption, reaching up to 661 nm in the red region. By varying the amino substituents of these molecules, highly efficient E→Z photoisomerization under unfiltered sunlight can be achieved, with yields of up to 88.4 %. Moreover, the Z,Z-isomers have high thermal stability with half-lives from days to years at room temperature. The introduction of ortho-amino substitutions and meta-bisazo units leads to a reversal of the n-π* and πn-π* transitions on the energy scale. This change provides a new perspective for further tuning the visible absorption of azo-switches by utilizing the πn-π* band instead of the conventional n-π* band. These results suggest that photoresponsive systems can be powered by sunlight instead of traditional artificial lights, thereby paving the way for sustainable smart materials and devices.

Multimodal Autonomous Locomotion of Liquid Crystal Elastomer Soft Robot

Self-oscillation phenomena observed in nature serve as extraordinary inspiration for designing synthetic autonomous moving systems. Converting self-oscillation into designable self-sustained locomotion can lead to a new generation of soft robots that require minimal/no external control. However, such locomotion is typically constrained to a single mode dictated by the constant surrounding environment. In this study, a liquid crystal elastomer (LCE) robot capable of achieving self-sustained multimodal locomotion, with the specific motion mode being controlled via substrate adhesion or remote light stimulation is presented. Specifically, the LCE is mechanically trained to undergo repeated snapping actions to ensure its self-sustained rolling motion in a constant gradient thermal field atop a hotplate. By further fine-tuning the substrate adhesion, the LCE robot exhibits reversible transitions between rolling and jumping modes. In addition, the rolling motion can be manipulated in real time through light stimulation to perform other diverse motions including turning, decelerating, stopping, backing up, and steering around complex obstacles. The principle of introducing an on-demand gate control offers a new venue for designing future autonomous soft robots.

Light-Responsive Hydrogel Microcrawlers, Powered and Steered with Spatially Homogeneous Illumination

Sub-millimeter untethered locomoting robots hold promise to radically change multiple areas of human activity such as microfabrication/assembly or health care. To overcome the associated hurdles of such a degree of robot miniaturization, radically new approaches are being adopted, often relying on soft actuating polymeric materials. Here, we present light-driven, crawling microrobots that locomote by a single degree of freedom actuation of their light-responsive tail section. The direction of locomotion is dictated by the robot body design and independent of the spatial modulation of the light stimuli, allowing simultaneous multidirectional motion of multiple robots. Moreover, we present a method for steering such robots by reversibly deforming their front section, using ultraviolet (UV) light as a trigger. The deformation dictates the robot locomotion, performing right- or left-hand turning when the UV is turned on or off respectively. The robots' motion and navigation are not coupled to the position of the light sources, which enables simultaneous locomotion of multiple robots, steering of robots and brings about flexibility with the methods to deliver the light to the place of robot operation.

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.

Reversible Photochromism of 4,4'-Disubstituted 2,2'-Bipyridine in the Presence of SO3

We report an unusual photochromic behavior of 4,4'-disubstituted-2,2'-bipyridine. It was found that in the presence of a SO3 source and HCl, 2,2'-bipyridine-4,4'-dibutyl ester undergoes a color change from yellow to magenta in solution with maximum absorbance at 545 nm upon irradiation with 395 nm light. The photochromism is thermally reversible in solution. Different from the known bipyridine-based photoswitching pathways, the photo response does not involve any metal which form colored complexes or the formation of colored free radical cations like the photo-reduction of viologens. A combination of experimental and computational analysis was used to probe the mechanism. The results suggest the colored species to be a complex formed between N-oxide of the 2,2'-bipyridine-4,4'-dibutyl ester and SO2; the N-oxide and SO2 are formed from photoactivated oxidation of the bipyridine with SO3 serving as the oxygen source. This complex represents a new addition to the library of photoswitches that is easy to synthesize, reversible in solution, and of high fatigue resistance, making it a promising candidate for applications in photo-switchable materials and SO3 detection. We also demonstrated experimentally similar photochromic behaviors with 2,2'-bipyridine-containing polymers.

Advancing interactive systems with liquid crystal network-based adaptive electronics

Achieving adaptive behavior in artificial systems, analogous to living organisms, has been a long-standing goal in electronics and materials science. Efforts to integrate adaptive capabilities into synthetic electronics traditionally involved a typical architecture comprising of sensors, an external controller, and actuators constructed from multiple materials. However, challenges arise when attempting to unite these three components into a single entity capable of independently coping with dynamic environments. Here, we unveil an adaptive electronic unit based on a liquid crystal polymer that seamlessly incorporates sensing, signal processing, and actuating functionalities. The polymer forms a film that undergoes anisotropic deformations when exposed to a minor heat pulse generated by human touch. We integrate this property into an electric circuit to facilitate switching. We showcase the concept by creating an interactive system that features distributed information processing including feedback loops and enabling cascading signal transmission across multiple adaptive units. This system responds progressively, in a multi-layered cascade to a dynamic change in its environment. The incorporation of adaptive capabilities into a single piece of responsive material holds immense potential for expediting progress in next-generation flexible electronics, soft robotics, and swarm intelligence.

Photo-steered rapid and multimodal locomotion of 3D-printed tough hydrogel robots

Hydrogels are an ideal material to develop soft robots. However, it remains a grand challenge to develop miniaturized hydrogel robots with mechanical robustness, rapid actuation, and multi-gait motions. Reported here is a facile strategy to fabricate hydrogel-based soft robots by three-dimensional (3D) printing of responsive and nonresponsive tough gels for programmed morphing and locomotion upon stimulations. Highly viscoelastic poly(acrylic acid-co-acrylamide) and poly(acrylic acid-co-N-isopropyl acrylamide) aqueous solutions, as well as their mixtures, are printed with multiple nozzles into 3D constructs followed by incubation in a solution of zirconium ions to form robust carboxyl-Zr4+ coordination complexes, to produce tough metallo-supramolecular hydrogel fibers. Gold nanorods are incorporated into ink to afford printed gels with response to light. Owing to the mechanical excellence and small diameter of gel fibers, the printed hydrogel robots exhibit high robustness, fast response, and agile motions when remotely steered by dynamic light. The design of printed constructs and steering with spatiotemporal light allow for multimodal motions with programmable trajectories of the gel robots. The hydrogel robots can walk, turn, flip, and transport cargos upon light stimulations. Such printed hydrogels with good mechanical performances, fast response, and agile locomotion may open opportunities for soft robots in biomedical and engineering fields.

Intrinsically Multistable Soft Actuator Driven by Mixed-Mode Snap-Through Instabilities

Actuators utilizing snap-through instabilities are widely investigated for high-performance fast actuators and shape reconfigurable structures owing to their rapid response and limited reliance on continuous energy input. However, prevailing approaches typically involve a combination of multiple bistable actuator units and achieving multistability within a single actuator unit still remains an open challenge. Here, a soft actuator is presented that uses shape memory alloy (SMA) and mixed-mode elastic instabilities to achieve intrinsically multistable shape reconfiguration. The multistable actuator unit consists of six stable states, including two pure bending states and four bend-twist states. The actuator is composed of a pre-stretched elastic membrane placed between two elastomeric frames embedded with SMA coils. By controlling the sequence and duration of SMA activation, the actuator is capable of rapid transition between all six stable states within hundreds of milliseconds. Principles of energy minimization are used to identify actuation sequences for various types of stable state transitions. Bending and twisting angles corresponding to various prestretch ratios are recorded based on parameterizations of the actuator's geometry. To demonstrate its application in practical conditions, the multistable actuator is used to perform visual inspection in a confined space, light source tracking during photovoltaic energy harvesting, and agile crawling.

Soft Robots with Plant-Inspired Gravitropism Based on Fluidic Liquid Metal

Plants can autonomously adjust their growth direction based on the gravitropic response to maximize energy acquisition, despite lacking nerves and muscles. Endowing soft robots with gravitropism may facilitate the development of self-regulating systems free of electronics, but remains elusive. Herein, acceleration-regulated soft actuators are described that can respond to the gravitational field by leveraging the unique fluidity of liquid metal in its self-limiting oxide skin. The soft actuator is obtained by magnetic printing of the fluidic liquid metal heater circuit on a thermoresponsive liquid crystal elastomer. The Joule heat of the liquid metal circuit with gravity-regulated resistance can be programmed by changing the actuator's pose to induce the flow of liquid metal. The actuator can autonomously adjust its bending degree by the dynamic interaction between its thermomechanical response and gravity. A gravity-interactive soft gripper is also created with controllable grasping and releasing by rotating the actuator. Moreover, it is demonstrated that self-regulated oscillation motion can be achieved by interfacing the actuator with a monostable tape spring, allowing the electronics-free control of a bionic walker. This work paves the avenue for the development of liquid metal-based reconfigurable electronics and electronics-free soft robots that can perceive gravity or acceleration.

Multimodal Soft Robotic Actuation and Locomotion

Diverse and adaptable modes of complex motion observed at different scales in living creatures are challenging to reproduce in robotic systems. Achieving dexterous movement in conventional robots can be difficult due to the many limitations of applying rigid materials. Robots based on soft materials are inherently deformable, compliant, adaptable, and adjustable, making soft robotics conducive to creating machines with complicated actuation and motion gaits. This review examines the mechanisms and modalities of actuation deformation in materials that respond to various stimuli. Then, strategies based on composite materials are considered to build toward actuators that combine multiple actuation modes for sophisticated movements. Examples across literature illustrate the development of soft actuators as free-moving, entirely soft-bodied robots with multiple locomotion gaits via careful manipulation of external stimuli. The review further highlights how the application of soft functional materials into robots with rigid components further enhances their locomotive abilities. Finally, taking advantage of the shape-morphing properties of soft materials, reconfigurable soft robots have shown the capacity for adaptive gaits that enable transition across environments with different locomotive modes for optimal efficiency. Overall, soft materials enable varied multimodal motion in actuators and robots, positioning soft robotics to make real-world applications for intricate and challenging tasks.

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.

Tunable Light-Responsive Polyurethane-urea Elastomer Driven by Photochemical and Photothermal Coupling Mechanism

Light-driven soft actuators based on photoresponsive materials can be used to mimic biological motion, such as hand movements, without involving rigid or bulky electromechanical actuations. However, to our knowledge, no robust photoresponsive material with desireable mechanical and biological properties and relatively simple manufacture exists for robotics and biomedical applications. Herein, we report a new visible-light-responsive thermoplastic elastomer synthesized by introducing photoswitchable moieties (i.e., azobenzene derivatives) into the main chain of poly(ε-caprolactone) based polyurethane urea (PAzo). A PAzo elastomer exhibits controllable light-driven stiffness softening due to its unique nanophase structure in response to light, while possessing excellent hyperelasticity (stretchability of 575.2%, elastic modulus of 17.6 MPa, and strength of 44.0 MPa). A bilayer actuator consisting of PAzo and polyimide films is developed, demonstrating tunable bending modes by varying incident light intensities. Actuation mechanism via photothermal and photochemical coupling effects of a soft-hard nanophase is demonstrated through both experimental and theoretical analyses. We demonstrate an exemplar application of visible-light-controlled soft "fingers" playing a piano on a smartphone. The robustness of the PAzo elastomer and its scalability, in addition to its excellent biocompatibility, opens the door to the development of reproducible light-driven wearable/implantable actuators and lightweight soft robots for clinical applications.

Smart materials for light control of droplets

Droplet manipulation plays a critical role in both fundamental research and practical applications, especially when combined with smart materials and external fields to achieve multifunctional droplet manipulation. Light control of droplets has emerged as a significant and widely used strategy, driven primarily by photochemistry, photomechanics, light-induced Marangoni effects, and light-induced electric effects. This approach allowing for droplet manipulation with high spatial and temporal resolution, all while maintaining a remote and non-contact mode of operation. This review aims to provide a comprehensive overview of the mechanisms underlying light control of droplets, the design of smart materials for this purpose, and the diverse range of applications enabled by this technique. These applications include merging, splitting, releasing, forwarding, backward movement, and rotation of droplets, as well as chemical reactions, droplet robots, and microfluidics. By presenting this information, we aim to establish a unified framework that guides the sustainable development of light control of droplets. Additionally, this review addresses the challenges associated with light control of droplets and suggests potential directions for future development.

Reprogramming Photoresponsive Liquid Crystalline Elastomer via Force-Directed Evaporation

Incorporating photothermal agents into thermoresponsive liquid crystalline elastomers (LCEs) offers remote and spatio-temporal control in actuation. Typically, both the light responsiveness and actuation behaviors are fixed since the agent doping and mesogen alignment are conducted before network formation. Here, we report an approach that enables programming photoresponsive LCEs after synthesis via force-directed evaporation. Different photothermal agents can be doped or removed by swelling the fully cross-linked LCEs in a specific solution, achieving the introduction and erasing of the photoresponsiveness. Moreover, the network swelling deletes the registered alignment, which allows for redefining the molecular order via re-evaporating the solvent with force imposed. This "one stone, two birds" strategy paves the way to simultaneously program/reprogram the actuation mode and responsiveness of LCEs, even in a spatio-selective manner to achieve complex actuations. Our approach is expandable to three-dimensional (3D) printed LCEs to access geometrically sophisticated shape-changing.

Modeling of a light-fueled self-paddling boat with a liquid crystal elastomer-based motor

Active materials possess unique properties of being able to respond autonomously to external stimuli, yet realizing and regulating the motion behavior of active machines remains a major challenge. Conventional control approaches, including sensor control and external device control, are both complex and difficult to implement. In contrast, active materials-based self-oscillators offer distinct properties such as periodic motion and ease of regulation. Inspired by paddle boats, we have proposed a conceptual light-fueled self-paddling boat with a photothermally responsive liquid crystal elastomer (LCE)-based motor that operates under steady illumination and incorporates an LCE fiber. Based on the well-established dynamic LCE model and rotation dynamics, the dynamic equations for governing the self-paddling of the LCE-steered boat are derived, and the driving torque of the LCE-based motor and the paddling velocity of the LCE-steered boat are formulated successively. The numerical results show that two motion modes of the boat under steady illumination: the static mode and the self-paddling mode. The self-paddling regime arises from the competition between the light-fueled driving torque and the frictional torque. Moreover, the critical conditions required to trigger the self-paddling are quantitatively examined as well as the significant system parameters affecting the driving torque, angular velocity, and paddling velocity. The proposed conceptual light-fueled self-paddling LCE-steered boat exhibits benefits including customizable size and being untethered and ambient powered, which provides valuable insights into the design and application of micromachines, soft robotics, energy harvesters, and beyond.

A light-powered self-rotating liquid crystal elastomer drill

Self-oscillating systems can directly convert ambient energy to mechanical work, and new type self-oscillating systems are worth designing for applications in energy harvesters, engines, and actuators. Taking inspiration from the hand drill, we have developed a novel self-rotating drill system, which is consist of a turnplate and a liquid crystal elastomer (LCE) fiber under steady illumination. To investigate the self-rotating behaviors of the LCE drill, we have proposed a nonlinear theoretical model of the LCE drill under steady illumination based on the well-established dynamic LCE model. Numerical calculation reveals that the LCE drill can undergo a supercritical Hopf bifurcation between the static regime and the self-rotation regime. The self-rotation of drill originates from the contraction of winding portion of LCE fiber in illumination at winding state, and its continuous periodic motion is sustained by the interrelation between light energy and damping dissipation. The Hopf bifurcation conditions are also investigated in detail, as well as the vital system parameters affecting its frequency and amplitude. In contrast to the abundant existing self-oscillating systems, this self-rotating drill stands out due to its simple and lightweight structure, customizable dimensions, and high speed, and thus facilitates the design of compact and integrated systems, enhancing their applicability in microdevices and systems. This bears great significance in fields like micro-robotics, micro-sensors, and medical instruments, enabling the realization of smaller and higher-performance devices.

Optical deformations of azobenzene polymers: orientation approach vs. other concepts

It has been 30 years since the discovery of surface restructuring in thin azopolymer films by two independent research groups. A wide variety of topographical structures have been created by the application of two-/four-beam interference patterns, space light modulators and even helical beams. There are a number of comprehensive reviews which describe in detail the advances in superficial photopatterning of azopolymer films and macroscopic deformations of azonetworks. The theoretical approaches are only briefly touched on in these reviews and often are accompanied by the remark that the phenomenon is far from being understood. In this review, we would like to present the polymer theoretist's point of view on this intriguing problem. We begin by describing a multitude of theoretical approaches and commenting on the pluses and drawbacks of each. Importantly, we show that in most cases the presence of an azopolymer matrix is either ignored or limited to a specific class of azopolymers (liquid-crystalline or elastomeric). We then move to early orientation approaches based on the hypothesis that reorientation of azo-chromophores by modulated polarized light is the sole cause of superficial patterning. At the end of the review a modern orientation approach, as proposed by our own group, is presented. This approach has high predictive power because it can explain a large pool of experimental data for different classes of azopolymers including glassy and liquid-crystalline materials. This is made possible by taking into account both the light-induced orientation process and the change of anisotropic interactions between the chromophores upon their isomerization. Last but not least, this is the only approach that provides an estimate of the light-induced stress large enough to cause plastic deformations of glassy azopolymers. Recent finite element modeling results show remarkable similarity to real patterns and even time-dependent data are well explained. With this, we claim that the puzzle is finally understood and the orientation approach is ready for its implementation for major azopolymer classes.

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

Light-Responsive Shape- and Color-Changing Block Copolymer Particles with Fast Switching Speed

Polymer particles capable of dynamic shape changes in response to light have received substantial attention in the development of intelligent multifunctional materials. In this study, we develop a light-responsive block copolymer (BCP) particle system that exhibits fast and reversible shape and color transitions. The key molecular design is the integration of spiropyran photoacid (SPPA) molecules into the BCP particle system, which enables fast and dynamic transformations of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) particles in response to light. The SPPA photoisomerization, induced by 420 nm light irradiation, lowers the pH of the aqueous surroundings from 5.5 to 3.3. The protonated P4VP block substantially increases in domain size from 14 to 39 nm, resulting in significant elongation of the BCP particles (i.e., an increase in the aspect ratio (AR) of the particles from 1.8 to 3.4). Moreover, SPPA adsorbed onto the P4VP surface induces significant changes in the luminescent properties of the BCP particles via photoisomerization of SPPA. Notably, the BCP particles undergo fast, dynamic shape and color transitions within a period of 10 min, maintaining high reversibility over multiple light exposures. Functional dyes are selectively incorporated into different domains of the light-responsive BCP particles to achieve different ranges of color responses. Thus, this study showcases a light-responsive hydrogel display capable of reversible and multicolor photopatterning.

Self-sustainable autonomous soft actuators

Self-sustainable autonomous locomotion is a non-equilibrium phenomenon and an advanced intelligence of soft-bodied organisms that exhibit the abilities of perception, feedback, decision-making, and self-sustainment. However, artificial self-sustaining architectures are often derived from algorithms and onboard modules of soft robots, resulting in complex fabrication, limited mobility, and low sensitivity. Self-sustainable autonomous soft actuators have emerged as naturally evolving systems that do not require human intervention. With shape-morphing materials integrating in their structural design, soft actuators can direct autonomous responses to complex environmental changes and achieve robust self-sustaining motions under sustained stimulation. This perspective article discusses the recent advances in self-sustainable autonomous soft actuators. Specifically, shape-morphing materials, motion characteristics, built-in negative feedback loops, and constant stimulus response patterns used in autonomous systems are summarized. Artificial self-sustaining autonomous concepts, modes, and deformation-induced functional applications of soft actuators are described. The current challenges and future opportunities for self-sustainable actuation systems are also discussed.

Collective Molecular Machines: Multidimensionality and Reconfigurability

Molecular machines are key to cellular activity where they are involved in converting chemical and light energy into efficient mechanical work. During the last 60 years, designing molecular structures capable of generating unidirectional mechanical motion at the nanoscale has been the topic of intense research. Effective progress has been made, attributed to advances in various fields such as supramolecular chemistry, biology and nanotechnology, and informatics. However, individual molecular machines are only capable of producing nanometer work and generally have only a single functionality. In order to address these problems, collective behaviors realized by integrating several or more of these individual mechanical units in space and time have become a new paradigm. In this review, we comprehensively discuss recent developments in the collective behaviors of molecular machines. In particular, collective behavior is divided into two paradigms. One is the appropriate integration of molecular machines to efficiently amplify molecular motions and deformations to construct novel functional materials. The other is the construction of swarming modes at the supramolecular level to perform nanoscale or microscale operations. We discuss design strategies for both modes and focus on the modulation of features and properties. Subsequently, in order to address existing challenges, the idea of transferring experience gained in the field of micro/nano robotics is presented, offering prospects for future developments in the collective behavior of molecular machines.

Halloysite nanotubes as nano-support matrix for programming the photo/H2O dual triggered reversible gel actuator

Gel actuators are a kind of soft intelligent material that can convert external stimuli into deformations to generate mechanical responses. The development of gel actuators with advanced structures to integrate multiple responsiveness, programmability, and fast deformation ability is urgently needed. Here, we explored a poly(7-(2-methacryloyloxyethoxy)-4-methylcoumarin-co-acrylic acid-co-glycol) ternary gel network as an actuator with reprogrammable photo/H2O dual responsibilities. In such a design, [2 + 2] photodimerization and photocleavage reactions of coumarin moieties can be realized under 365 and 254 nm light irradiation, respectively, affording reversible photodriven behaviour of the gels. The abundant carboxylic acid in the backbone has the capacity to form additional crosslinks to assist and accelerate the photodriven behaviour. The incorporation and orientation of halloysite nanotubes (HNTs) in gel matrices support an axial direction force and result in a more controllable and programmable actuating behaviour. The synergistic response enables fast grasping-releasing of 5-times the weight of the object in water within 10 min by fabricating HNT-incorporated gels as a four-arm gripper.

Light-Fueled Nonreciprocal Self-Oscillators for Fluidic Transportation and Coupling

Light-fueled self-oscillators based on soft actuating materials have triggered novel designs for small-scale robotic constructs that self-sustain their motion at non-equilibrium states and possess bioinspired autonomy and adaptive functions. However, the motions of most self-oscillators are reciprocal, which hinders their use in sophisticated biomimetic functions such as fluidic transportation. Here, an optically powered soft material strip that can perform nonreciprocal, cilia-like, self-sustained oscillation under water is reported. The actuator is made of planar-aligned liquid crystal elastomer responding to visible light. Two laser beams from orthogonal directions allow for piecewise control over the strip deformation, enabling two self-shadowing effects coupled in one single material to yield nonreciprocal strokes. The nonreciprocity, stroke pattern and handedness are connected to the fluidic pumping efficiency, which can be controlled by the excitation conditions. Autonomous microfluidic pumping in clockwise and anticlockwise directions, translocation of a micro-object by liquid propulsion, and coupling between two oscillating strips through liquid medium interaction are demonstrated. The results offer new concepts for non-equilibrium soft actuators that can perform bio-like functions under water.

Modeling the light-powered self-rotation of a liquid crystal elastomer fiber-based engine

Self-oscillating systems possess the ability to convert ambient energy directly into mechanical work, and new types of self-oscillating systems are worth designing for practical applications in energy harvesters, engines and actuators. Taking inspiration from the four-stroke engine. A concept for a self-rotating engine is presented on the basis of photothermally responsive materials, consisting of a liquid crystal elastomer (LCE) fiber, a hinge and a turnplate, which can self-rotate under steady illumination. Based on the photo-thermal-mechanical model, a nonlinear theoretical model of the LCE-based engine under steady illumination is proposed to investigate its self-rotating behaviors. Numerical calculations reveal that the LCE-based engine experiences a supercritical Hopf bifurcation between the static regime and the self-rotation regime. The self-rotation of the LCE-based engine originates from the photothermally driven strain of the LCE fiber in illumination, and its continuous periodic motion is sustained by the correlation between photothermal energy and damping dissipation. The Hopf bifurcation conditions are also explored in detail, as well as the vital system parameters affecting self-rotation frequency. Compared to the abundant existing self-oscillating systems, this conceptual self-rotating LCE-based engine stands out due to its simple and lightweight structure, customizable dimensions and high speed, and it is expected to offer a broader range of design concepts applicable to soft robotics, energy harvesters, medical instruments, and so on.

Dynamic Shape Change of Liquid Crystal Polymer Based on An Order-Order Phase Transition

Liquid crystal actuators conventionally undergo shape changes across an order-disorder phase transition between liquid crystal (LC) and isotropic phases. In this study, we introduce an innovative Liquid Crystal Polymer (LCP) actuator harnessing an order-order LC phase transition mechanism. The LCP film is easily stretchable within the LC phase, facilitated by the π-π stacking of phenyl groups serving as robust physical crosslinking points, and thereby transforms to a stable monodomain structure. The resultant monodomain LCP actuator shows a distinctive reversible dynamic shape change, exhibiting extension followed by contraction along the LC director on cooling. The extension is propelled by the reversible smectic C to smectic A phase transition, and the contraction is attributed to the re-entry to the smectic C phase from smectic A phase. Thermal annealing temperature determines this peculiar dynamic shape change, which occurs during both heating and cooling processes. This pivotal attribute finds manifestation in gripper and flower-shaped actuators, adeptly executing grabbing and releasing as well as blooming and closure motions within a single thermal stimulation. In essence, our study introduces an innovative approach to the realm of LCP actuators, ushering in a new avenue for the design and fabrication of versatile and dynamically responsive LCP actuators.

Embedded Physical Intelligence in Liquid Crystalline Polymer Actuators and Robots

Responsive materials possess the inherent capacity to autonomously sense and respond to various external stimuli, demonstrating physical intelligence. Among the diverse array of responsive materials, liquid crystalline polymers (LCPs) stand out for their remarkable reversible stimuli-responsive shape-morphing properties and their potential for creating soft robots. While numerous reviews have extensively detailed the progress in developing LCP-based actuators and robots, there exists a need for comprehensive summaries that elucidate the underlying principles governing actuation and how physical intelligence is embedded within these systems. This review provides a comprehensive overview of recent advancements in developing actuators and robots endowed with physical intelligence using LCPs. This review is structured around the stimulus conditions and categorizes the studies involving responsive LCPs based on the fundamental control and stimulation logic and approach. Specifically, three main categories are examined: systems that respond to changing stimuli, those operating under constant stimuli, and those equip with learning and logic control capabilities. Furthermore, the persisting challenges that need to be addressed are outlined and discuss the future avenues of research in this dynamic field.

Synchronous behaviors of three coupled liquid crystal elastomer-based spring oscillators under linear temperature fields

Self-oscillating coupled systems possess the ability to actively absorb external environmental energy to sustain their motion. This quality endows them with autonomy and sustainability, making them have application value in the fields of synchronization and clustering, thereby furthering research and exploration in these domains. Building upon the foundation of thermal responsive liquid crystal elastomer-based (LCE-based) spring oscillators, a synchronous system comprising three LCE-based spring oscillators interconnected by springs is established. In this paper, the synchronization phenomenon is described, and the self-oscillation mechanism is revealed. The results indicate that by varying system parameters and initial conditions, three synchronization patterns emerge, namely, full synchronous mode, partial synchronous mode, and asynchronous mode. For strongly interacting systems, full synchronous mode always prevails, while for weak interactions, the adjustment of initial velocities in magnitude and direction yields the three synchronization patterns. Additionally, this study explores the impact of several system parameters, including LCE elasticity coefficient and spring elasticity coefficient, on the amplitude, frequency, and synchronous mode of the system. The findings in this paper can enhance our understanding of the synchronization behavior of multiple mutually coupled LCE-based spring oscillators, with promising applications in energy harvesting, soft robotics, signal monitoring, and various other fields.

Defected twisted ring topology for autonomous periodic flip-spin-orbit soft robot

Periodic spin-orbit motion is ubiquitous in nature, observed from electrons orbiting nuclei to spinning planets orbiting the Sun. Achieving autonomous periodic orbiting motions, along circular and noncircular paths, in soft mobile robotics is crucial for adaptive and intelligent exploration of unknown environments-a grand challenge yet to be accomplished. Here, we report leveraging a closed-loop twisted ring topology with a defect for an autonomous soft robot capable of achieving periodic spin-orbiting motions with programmed circular and re-programmed irregular-shaped trajectories. Constructed by bonding a twisted liquid crystal elastomer ribbon into a closed-loop ring topology, the robot exhibits three coupled periodic self-motions in response to constant temperature or constant light sources: inside-out flipping, self-spinning around the ring center, and self-orbiting around a point outside the ring. The coupled spinning and orbiting motions share the same direction and period. The spinning or orbiting direction depends on the twisting chirality, while the orbital radius and period are determined by the twisted ring geometry and thermal actuation. The flip-spin and orbiting motions arise from the twisted ring topology and a bonding site defect that breaks the force symmetry, respectively. By utilizing the twisting-encoded autonomous flip-spin-orbit motions, we showcase the robot's potential for intelligently mapping the geometric boundaries of unknown confined spaces, including convex shapes like circles, squares, triangles, and pentagons and concaves shapes with multi-robots, as well as health monitoring of unknown confined spaces with boundary damages.

Animating hydrogel knotbots with topology-invoked self-regulation

Steering soft robots in a self-regulated manner remains a grand challenge, which often requires continuous symmetry breaking and recovery steps for persistent motion. Although structural morphology is found significant for robotic functions, geometric topology has rarely been considered and appreciated. Here we demonstrate a series of knotbots, namely hydrogel-based robots with knotted structures, capable of autonomous rolling and spinning/rotating motions. With symmetry broken by external stimuli and restored by self-regulation, the coupling between self-constraint-induced prestress and photothermal strain animates the knotbots continuously. Experiments and simulations reveal that nonequilibrium processes are regulated dynamically and cooperatively by self-constraints, active deformations, and self-shadowing effect of the photo-responsive gel. The active motions enable the knotbots to execute tasks including gear rotation and rod climbing. This work paves the way to devise advanced soft robots with self-regulated sustainable motions by harnessing the topology.

Photo-Responsive Dynamic Organic Room-Temperature Phosphorescence Materials Based on a Functional Unit Combination Strategy

A rational molecular design strategy facilitates the development of a purely organic room-temperature phosphorescence (RTP) material system with precisely regulated luminescence properties, which surely promotes its functional integration and intelligent application. Here, a functional unit combination strategy is proposed to design novel RTP molecules combining a folding unit with diverse luminescent cores. The different luminescent cores are mainly responsible for tunable RTP properties, while the folding unit contributes to the spin-orbit coupling (SOC) enhancement, which makes the RTP material design as workable as the building block principle. By this strategy, a series of color/lifetime-tunable RTP materials is achieved with unique photo-responsive RTP enhancement when subjected to UV irradiation, which expands their application scenarios in reusable privacy tags, advanced "4D" encryption, and phase separation analysis of blended polymers. This work suggests a simple and effective strategy to design purely organic RTP materials with tunable color and lifetime, and also provides new application options for photo-responsive dynamic RTP materials.

Self-Sustained Chaotic Jumping of Liquid Crystal Elastomer Balloon under Steady Illumination

Self-sustained chaotic jumping systems composed of active materials are characterized by their ability to maintain motion through drawing energy from the steady external environment, holding significant promise in actuators, medical devices, biomimetic robots, and other fields. In this paper, an innovative light-powered self-sustained chaotic jumping system is proposed, which comprises a liquid crystal elastomer (LCE) balloon and an elastic substrate. The corresponding theoretical model is developed by combining the dynamic constitutive model of an LCE with Hertz contact theory. Under steady illumination, the stationary LCE balloon experiences contraction and expansion, and through the work of contact expansion between LCE balloon and elastic substrate, it ultimately jumps up from the elastic substrate, achieving self-sustained jumping. Numerical calculations reveal that the LCE balloon exhibits periodic jumping and chaotic jumping under steady illumination. Moreover, we reveal the mechanism underlying self-sustained periodic jumping of the balloon in which the damping dissipation is compensated through balloon contact with the elastic substrate, as well as the mechanism involved behind self-sustained chaotic jumping. Furthermore, we provide insights into the effects of system parameters on the self-sustained jumping behaviors. The emphasis in this study is on the self-sustained chaotic jumping system, and the variation of the balloon jumping modes with parameters is illustrated through bifurcation diagrams. This work deepens the understanding of chaotic motion, contributes to the research of motion behavior control of smart materials, and provides ideas for the bionic design of chaotic vibrators and chaotic jumping robots.

Tunable Molecular Sieving by Hierarchically Assembled Porous Organic Cage Membranes with Solvent-Responsive Switchable Pores

Molecular separations involving solvents and organic impurities represent great challenges for environmental and water-intensive industries. Novel materials with intrinsic nanoscale pores offer a great choice for improvement in terms of energy efficiency and capital costs. Particularly, in applications where gradient and ordered separation of organic contaminants remain elusive, smart materials with switchable pores can offer efficient solutions. Here, we report a hierarchically networked porous organic cage membrane with dynamic control over pores, elucidating stable solvent permeance and tunable dye rejection over different molecular weights. The engineered cage membrane can spontaneously modulate its geometry and pore size from water to methanol and DMF in a reversible manner. The cage membrane exhibits ≥585.59 g mol-1 molecular weight cutoff preferentially in water and is impeded by methanol (799.8 g mol-1) and DMF (≈1017 g mol-1), reflecting 36 and 73% change in rejection due to self-regulation and the flexible network, respectively. Grazing incidence X-ray diffraction illustrates a clear peak downshift, suggesting an intrinsic structural change when the cage membranes were immersed in methanol or DMF. We have observed reversible structural changes that can also be tuned by preparing a methanol/DMF mixture and adjusting their ratio, thereby enabling gradient molecular filtration. We anticipate that such cage membranes with dynamic selectivity could be promising particularly for industrial separations and wastewater treatment.

Self-Rotation of Electrothermally Responsive Liquid Crystal Elastomer-Based Turntable in Steady-State Circuits

Self-excited motions, characterized by their ability to harness energy from a consistent environment and self-regulate, exhibit significant potential in micro-devices, autonomous robotics, sensor technology, and energy generation. This study introduces an innovative turntable system based on an electrothermally responsive liquid crystal elastomer (LCE). This system facilitates self-rotation within a steady-state circuit. Employing an electrothermal LCE model, we have modeled and numerically analyzed the nonlinear dynamics of an LCE-rope within steady-state circuits, utilizing the four-order Runge-Kutta method for calculations. The numerical results reveal the emergence of two distinct motion patterns in the turntable system under steady-state conditions: a self-rotation pattern and a static pattern. The self-rotation is initiated when the system's absorbed energy surpasses the energy lost due to damping effects. Furthermore, this paper delves into the critical conditions necessary for initiating self-rotation and examines the influence of various key dimensionless parameters on the system's rotation amplitude and frequency. These parameters include gravitational acceleration, the initial position of the mass ball, elastic stiffness of the LCE and spring, limiting temperature, heating zone angle, thermal shrinkage coefficient, and damping factor. Our computational findings establish that these parameters exert a modulatory impact on the rotation amplitude and period. This research enhances the understanding of self-excited motions and offers promising avenues for applications in energy harvesting, monitoring, soft robotics, medical devices, and micro- and nano-devices.

Functional PDMS Elastomers: Bulk Composites, Surface Engineering, and Precision Fabrication

Polydimethylsiloxane (PDMS)-the simplest and most common silicone compound-exemplifies the central characteristics of its class and has attracted tremendous research attention. The development of PDMS-based materials is a vivid reflection of the modern industry. In recent years, PDMS has stood out as the material of choice for various emerging technologies. The rapid improvement in bulk modification strategies and multifunctional surfaces has enabled a whole new generation of PDMS-based materials and devices, facilitating, and even transforming enormous applications, including flexible electronics, superwetting surfaces, soft actuators, wearable and implantable sensors, biomedicals, and autonomous robotics. This paper reviews the latest advances in the field of PDMS-based functional materials, with a focus on the added functionality and their use as programmable materials for smart devices. Recent breakthroughs regarding instant crosslinking and additive manufacturing are featured, and exciting opportunities for future research are highlighted. This review provides a quick entrance to this rapidly evolving field and will help guide the rational design of next-generation soft materials and devices.

Artificial Molecular Ratchets: Tools Enabling Endergonic Processes

Non-equilibrium chemical systems underpin multiple domains of contemporary interest, including supramolecular chemistry, molecular machines, systems chemistry, prebiotic chemistry, and energy transduction. Experimental chemists are now pioneering the realization of artificial systems that can harvest energy away from equilibrium. In this tutorial Review, we provide an overview of artificial molecular ratchets: the chemical mechanisms enabling energy absorption from the environment. By focusing on the mechanism type-rather than the application domain or energy source-we offer a unifying picture of seemingly disparate phenomena, which we hope will foster progress in this fascinating domain of science.

Inducing Motions of Polymers in Liquid Nitrogen with Light

Polymer materials that show macroscopic deformation in response to external stimuli are feasible for novel soft actuators including microactuators. Incorporation of photochromic moieties, such as azobenzenes, into polymer networks enables macroscopic deformation under irradiation with light through photoisomerization. Under cryogenic conditions, however, it has been difficult to induce macroscopic deformation as polymers lose their soft nature due to the severe restrictions of molecular motions. Here, activation of molecular motions and macroscopic deformation in liquid nitrogen only with light for polymers containing photochromic moieties is reported. Photoinduced bending of polymer networks with normal azobenzenes in liquid nitrogen is enabled by preliminary UV irradiation at room temperature to produce cis-isomers. To realize photoinduced deformation directly in liquid nitrogen, polymer networks are functionalized with bridged azobenzenes, which exist as cis-isomers in thermodynamic equilibrium. The films with bridged azobenzenes exhibit reversible photoisomerization and bending upon irradiation with light in liquid nitrogen without the need of preliminary irradiation, implying that the change in conformation of polymer chains can be isothermally induced even under cryogenic conditions. Achievement of flexible motions under cryogenic conditions through isothermal processes will greatly expand the operating temperature range of soft actuators.