Enabling liquid crystal elastomers with tunable actuation temperature

Vanillin-Derived Degradable and Reprocessable Liquid-Crystalline Epoxy Resins with High Intrinsic Thermal Conductivity

The demand for high-functionality and miniaturized consumer electronics is driving the development of polymer packaging materials with intrinsically high thermal conductivity. Herein, a novel vanillin-based liquid-crystalline epoxy monomer (BEP) and three curing agents (ICA-1, ICA-2, and ICA-3) containing conjugated aromatic imine structures were synthesized. The results of X-ray diffraction measurements show that the structural order of the epoxy resins based on BEP and ICAs increases with the number of conjugated benzene rings in ICAs. Compared with the conventional epoxy reference (0.23 W m-1 K-1), the prepared liquid-crystalline epoxy resins exhibit enhanced intrinsic thermal conductivity (0.28-0.38 W m-1 K-1) due to the synergistic effect from the liquid-crystalline phase structure in BEP and the additional ordered structure (via π-π stacking) in ICAs. Both experimental and molecular dynamics calculation results show that the thermal conductivity of the epoxy resins is proportional to the length of the conjugated structures in ICAs. Owing to the incorporation of dynamic aromatic imine bonds, the three cured epoxy resins based on BEP and ICAs demonstrate excellent reprocessibility through imine metathesis and are chemically degradable in the amine solution.

Should I stay or should I flow? An exploration of phase-separated metallosupramolecular liquid crystal polymers

Dynamic liquid crystalline polymers (dLCPs) incorporate both liquid crystalline mesogens and dynamic bonds into a single polymeric material. These dual functionalities impart order-dependent thermo-responsive mechano-optical properties and enhanced reprocessability/programmability enabling their use as soft actuators, adaptive adhesives, and damping materials. While many previous works studying dynamic LCPs utilize dynamic covalent bonds, metallosupramolecular bonds provide a modular platform where a series of materials can be accessed from a single polymeric feedstock through the variation of the metal ion used. A series of dLCPs were prepared by the addition of metal salts to a telechelic 2,6-bisbenzimidazolylpyridine (Bip) ligand endcapped LCP to form metallosupramolecular liquid crystal polymers (MSLCPs). The resulting MSLCPs were found to phase separate into hard and soft phases which aids in their mechanical robustness. Variations of the metal salts used to access these materials allowed for control of the thermomechanical, viscoelastic, and adhesive properties with relaxations that can be tailored independently of the mesogenic transition. This work demonstrates that by accessing phase separation through the incorporation of metallosupramolecular moieties, highly processable yet robust MSLCP materials can be realized. This class of materials opens the door to LCPs with bulk flow behavior that can also be utilized as multi-level adhesives.

Luminescent Liquid Crystalline Elastomer Promoted Self-Adaptive Smart Active Optical Waveguide with Ultra-Low Optical Loss

Currently, optical waveguides show extensive application in photonics and optoelectronic devices due to their high information capacity and transmission capabilities. However, developing self-adaptive, smart optical waveguide materials with ultra-low optical loss remains a significant challenge. To address this issue, luminescent liquid crystalline elastomers (LLCEs) with remarkable flexibility and minimal optical loss through one-pot synthetic method is synthesized, marking the first example of such an approach. The resultant organic optical waveguide materials (OOWMs) demonstrate exceptional mechanical performance and low optical loss, even under significant deformation. An optical loss coefficient of 0.0375 dB mm-1 has been achieved in LLCE-based OOWMs through synergistic Förster resonance energy transfer. Additionally, these flexible OOWMs can endure large deformations and be shaped into arbitrary forms within macro-scale dimensions. Notably, LLCE-based OOWMs demonstrate smart, self-adaptive behavior with ultra-low optical loss when exposed to heat or light. Consequently, these OOWMs can be used to fabricate photo switches of various shapes. This work provides a feasible approach to achieving integrated photonic systems with low optical loss for intelligent high-speed data transmission.

Dynamic Liquid Crystal Elastomers for Body Heat- and Sunlight- Driven Self-Sustaining Motion via Material-Structure Synergy

Self-sustained actuators powered by natural, low-energy sources based on liquid crystal elastomers (LCEs) are attractive as they offer high safety, abundant energy availability, and practicality in applications. However, achieving stable self-sustaining motion with low-energy sources requires high actuation strain rates within a narrow temperature range near ambient conditions - a great challenge as LCEs with low nematic-to-isotropic transition temperatures (Tni) generally exhibit reduced actuation strain and strain rates. To address this, we synthesized a carbon nanotube-doped LCE with a low Tni and reversible Diels-Alder crosslinks, termed DALCE, and readily (re)fabricated it into specific structures (e.g., twisted-and-coiled or bimorph shapes). By leveraging material-structure synergy, we achieved both low Tni and high actuation strain rates, enabling self-rolling, self-breathing and autonomous twisting-untwisting movements powered by ambient/body temperature or natural sunlight. This low-energy, self-sustained actuator design opens new possibilities for LCE-based biomedical applications and naturally powered automatic devices.

Tunable photo-responsive liquid crystal elastomer fibers via disperse dyeing for smart textiles

Active fibers, responding autonomously to environmental changes, are the basis of the development of smart textiles. However, there are still challenges in achieving responsive specificity and self-resilience of these fibers, which restrict the implementation of precise and complex actuation behaviors. Herein, an efficient strategy with a combination of a two-step crosslinking and disperse dyeing method was proposed to integrate multiple independent and non-interfering photo-thermal conversion nanoparticles into liquid crystal elastomer fibers (LCEFs). Three dyed LCEFs that selectively respond to 532 nm, 808 nm, and 980 nm wavelengths of light have been achieved. Based on this, a Delta robot was constructed with the capability of identifying specific light. The dyed LCEFs were also successfully incorporated into functional textiles through different fabrication technologies, demonstrating an embroidered anti-counterfeit logo, a 2D to 3D transformable disc-woven bionic flower, and an adaptive breathing knitted fabric. This work may facilitate the development of untethered soft robots with tunable and complex actuation, as well as the advancement of novel smart fabrics.

Auxetic Liquid Crystal Elastomers: Overcoming Barriers to Scale-Up

The observation of auxetic behavior (i.e., negative Poisson's ratio) in liquid crystal elastomers (LCEs) presents an exciting opportunity to explore application areas previously inaccessible to LCEs. Since its initial discovery, research has focused on improving understanding of the underpinning physics that drives the auxetic response, the structure-property relationships that enable the response to be tuned, and LCE properties such as the refractive index. However, the auxetic LCE materials reported to date have made use of either mechanical strain during fabrication, or unreactive 'templates' to stabilize the nematic ordering in the precursors. The latter approach provides excellent monodomain films, but there is unavoidable anisotropic shrinkage of the LCE. Both processes previously employed create complications toward manufacturing and scale-up. In this article, we report the first example of an auxetic LCE synthesized through surface alignment without the use of a nonreactive 'template' and thus without the need for a washout. The LCE includes both terminally and laterally attached mesogens, presents an auxetic threshold of 76% strain, and displays a comparable dependence of auxetic behavior on its glass transition temperature as that reported in the literature. This work presents an exciting milestone in the journey toward realizing applications for auxetic LCEs.

Dynamic Programme Locking of Isomerization Behavior of Molecular Switch in Liquid Crystal Elastomers

The reversible isomerization behavior of molecular switches in liquid crystal elastomers (LCEs) usually only can be monotonically repeated, because the molecular motion environment is the same for each isomerization cycle in a permanently cross-linked polymer network. Therefore, achieving a tunable photostationary state (PSS) in the same LCE material system is a significant challenge. Herein, a spiropyran-based material (SPBM) as the molecular switch is introduced into a LCE system, which constructed a typical photo-responsive material with reversible isomerization behavior. Furthermore, dynamic cross-linked polymer networks via diselenide bonds endow the SPBM in this system with a tunable molecular motion environment, which switches freely or restrictedly depending on the size of the free volume. Thus, the molecular switch can endow the LCE with programmable photo-response, and a program locking or unlocking is enabled by tuning the free volume. This post-programming locking (PPL) strategy may offer a new sight for promoting the higher controllability of the stimulus-responsive behavior of the smart materials.

Soft Materials and Devices Enabling Sensorimotor Functions in Soft Robots

Sensorimotor functions, the seamless integration of sensing, decision-making, and actuation, are fundamental for robots to interact with their environments. Inspired by biological systems, the incorporation of soft materials and devices into robotics holds significant promise for enhancing these functions. However, current robotics systems often lack the autonomy and intelligence observed in nature due to limited sensorimotor integration, particularly in flexible sensing and actuation. As the field progresses toward soft, flexible, and stretchable materials, developing such materials and devices becomes increasingly critical for advanced robotics. Despite rapid advancements individually in soft materials and flexible devices, their combined applications to enable sensorimotor capabilities in robots are emerging. This review addresses this emerging field by providing a comprehensive overview of soft materials and devices that enable sensorimotor functions in robots. We delve into the latest development in soft sensing technologies, actuation mechanism, structural designs, and fabrication techniques. Additionally, we explore strategies for sensorimotor control, the integration of artificial intelligence (AI), and practical application across various domains such as healthcare, augmented and virtual reality, and exploration. By drawing parallels with biological systems, this review aims to guide future research and development in soft robots, ultimately enhancing the autonomy and adaptability of robots in unstructured environments.

Controlling Self-Oscillation of a Single-Layer Liquid Crystal Elastomer at the Air-Water Interface via Light Programming for Water Strider-Inspired Aquatic Robots

Biomimicking aquatic organisms offers many opportunities for designing intelligent robots that can freely move on water. However, most works were focused on multilayered materials or assembled structures and faced limitations in stability, versatility, and motion navigation. Here, we develop an assembly-free water-strider-like aquatic robot using a single layer of light-programmable liquid-crystal elastomer (LCE) that could be used to create asymmetric structures. The LCE strider mimics both the shape and functions of natural water striders; it is designed with four legs, with the fore and hind legs being programmed respectively via light. Consequently, the LCE strider shows self-oscillation and self-propulsion behaviors on low-grade thermal water with a temperature gradient at the air-water interface, owing to unbalanced changes in the contact areas and tensions between the legs and water. Furthermore, the trajectories of the LCE strider are manipulated by NIR light after selectively depositing polydopamine with photothermal conversion. In this way, path navigation is realized, that is, moving straight and on-demand turning, similar to the movement of natural water striders. This study should inspire the development of soft intelligent robots using shape-morphing materials.

Thermal Actuators Relying on Elastomer-Dispersed Liquid Crystals: From Imines with Supramolecular Chirality and Ferroelectricity to Soft Robots

The locomotion of various organisms relies on the alternated elongation-contraction of their muscles or bodies. Such biomimicry can offer a promising approach to developing soft robotic devices with improved mobility and efficiency. Most strategies to mimic such motions rely on reversible size modifications of some materials upon exposure to external stimuli. An example is the combination of liquid crystals (LCs) with elastomers that afford materials with reversible and programmable shape morphing upon heat treatment. This strategy is supposed to involve mainly liquid crystalline elastomers or liquid crystalline networks, but low molecular weight LCs were disregarded. Unlike the previous routes, we utilized a new type of thermal actuator, i.e., elastomer-dispersed LCs (EDLCs), where the LCs rely on small organic molecules, i.e., salicylaldimines with 1,3,4-thiadiazole core and silane or siloxane as mobility units. The individual components of EDLC are not chemically bound and have the advantage of retaining their intrinsic properties. By combining their particularities, herein we highlighted: rare molecules with supramolecular chirality and piezo-/ferroelectricity, new thermal actuators with >340% strain actuation, programmable twisting actuation through helical patterning of elastomers with cholesteric LCs, and crawler and walker soft robots, which show bidirectional gait with high speeds up to 2 mm s-1.

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.

Highly Thermally Conductive Liquid Crystalline Epoxy Resin Vitrimers with Reconfigurable, Shape-Memory, Photo-Thermal, and Closed-Loop Recycling Performance

The low thermal conductivity, poor toughness, and non-reprocessability of thermosetting epoxy resins severely restrict their applications and sustainable development in flexible electronics. Herein, liquid crystalline epoxy (LCE) and dynamic ester and disulfide bonds are introduced into the cured network of bisphenol A epoxy resin (E-51) to construct highly thermally conductive flexible liquid crystalline epoxy resin (LCER) vitrimers. LCER vitrimers demonstrate adjustable mechanical properties by varying the ratio of LCE to E-51, allowing it to transition from soft to strong. Typically, a 75 mol% LCE to 25 mol% E-51 ratio results in an in-plane thermal conductivity (λ) of 1.27 W m-1 K-1, over double that of pure E-51 vitrimer (0.61 W m-1 K-1). The tensile strength and toughness increase 2.88 folds to 14.1 MPa and 2.45 folds to 20.1 MJ m-3, respectively. Besides, liquid crystalline phase transition and dynamic covalent bonds enable triple shape memory and three-dimensional shape reconstruction. After four reprocessing cycles, λ and tensile strength remain at 94% and 72%, respectively. Integrating carbon nanotubes (CNTs) imparts photo-thermal effect and enables "on" and "off" switch under near-infrared light to LCER vitrimer. Furthermore, the CNTs/LCER vitrimer displays light-induced actuation, self-repairing, and self-welding besides the closed-loop recycling and rapid degradation performance.

Visible-light-programmed patterning in dynamically bonded cholesteric liquid crystal elastomer

Optical properties of cholesteric liquid crystal elastomers (CLCEs) can be tuned by an external field, however, it will spontaneously restore to the original state after the field is removed. Here, we introduce diselenide dynamic covalent bonds (DCBs) into CLCEs, whose optical properties can be reversibly and precisely tuned under the combined action of force and light. The tuned optical properties will be written into and remembered by the CLCEs, thus a programming effect is achieved. The prepared dynamical diselenide bonded CLCE films have the typical reversibly mechanochromism property, and high-resolution colourful patterning can be programmed by adjusting exposure time and intensity of masked visible-light under different tensile or compressive strain states. The DCB-CLCEs combine the novel anisotropy of CLCEs and the dynamic chain exchangeable ability of DCBs, which endows the materials with reprogrammable optical properties. We demonstrate a simple strategy of writing naked-eye high-resolution colourful patterning into a film with mechanochromism property by thermal or visible-light, it shows great potential in display devices, anticounterfeiting labels, sensors, optical films and smart materials.

Carbohydrate-Based Reprocessable and Healable Covalent Adaptable Biofoams

Polymeric foams derived from bio-based resources and capable of self-healing and recycling ability are of great demand to fulfill various applications and address environmental concerns related to accumulation of plastic wastes. In this article, a set of polyester-based covalent adaptable biofoams (CABs) synthesized from carbohydrates and other bio-derived precursors under catalyst free conditions to offer a sustainable alternative to conventional toxic isocyanate-based polyurethane foams is reported. The dynamic β-keto carboxylate linkages present in these biofoams impart self-healing ability and recyclability to these samples. These CABs display adequate tensile properties especially compressive strength (≤123 MPa) and hysteresis behavior. The CABs swiftly stress relax at 150 °C and are reprocessable under similar temperature conditions. These biofoams have displayed potential for use as attachment on solar photovoltaics to augment the output efficiency. These CABs with limited swellability in polar protic solvents and adequate mechanical resilience are suitable for other commodity applications.

Cluster-Triggered Self-Luminescence, Rapid Self-Healing, and Adaptive Reprogramming Liquid Crystal Elastomers Enabled by Dynamic Imine Bond

The integration of advanced functions and diverse practical applications calls for multifunctional liquid crystal elastomers (LCEs); however, the structure-intrinsic luminescence and excellent mechanical properties of LCEs have not yet been explored. In this study, clusteroluminescence (CL)-based LCEs (CL-LCEs) are successfully fabricated without depending on large conjugated structures, thereby avoiding redundant organic synthesis and aggregation-caused quenching. The experimental and theoretical results reveal that secondary amine (-NH-) and imine (-C = N-) groups play vital roles in determining the presence of fluorescence in CL-LCEs. Based on the above observation, the strategy universalization and a molecular library for constructing CL-LCEs are further demonstrated. Meanwhile, the dynamic bond of imine bonds endows the CL-LCE system with rapid self-healing under mild conditions (70 °C in 10 min), excellent stretchability, and adaptive programmable characteristics. Furthermore, the self-luminescent performance enables visual detection of the self-healing process. Finally, CL-based information storage and anticounterfeiting are successfully realized and their applications in fiber actuators and fluorescent textiles are demonstrated. The distinctive luminescence and dynamic chemistry presented in this work has significant implications in elucidating the mechanism of CL and providing new strategies for the rational design of novel multifunctional LCE materials.

New Prospects Arising from Dynamically Crosslinked Polymers: Reprogramming Their Properties

Dynamically crosslinked polymers (DCPs) have gained significant attention owing to their applications in fabricating (re)processable, recyclable, and self-healable thermosets, which hold great promise in addressing ecological issues, such as plastic pollution and resource scarcity. However, the current research predominantly focuses on redefining and/or manipulating their geometries while replicating their bulk properties. Given the inherent design flexibility of dynamic covalent networks, DCPs also exhibit a remarkable potential for various novel applications through postsynthesis reprogramming their properties. In this review, the recent advancements in strategies that enable DCPs to transform their bulk properties after synthesis are presented. The underlying mechanisms and associated material properties are overviewed mainly through three distinct strategies, namely latent catalysts, material-growth, and topology isomerizable networks. Furthermore, the mutual relationship and impact of these strategies when integrated within one material system are also discussed. Finally, the application prospects and relevant issues necessitating further investigation, along with the potential solutions are analyzed.

Multimode opto-magnetic dual-responsive actuating fibers and fabrics programmed via direct ink writing

Current methods lack one-step actuation programming for weave structures that can achieve multimodal motions in fiber and fabric actuators. Fiber and fabric actuators with dual-response to magnetic fields and near-infrared (NIR) light were fabricated via direct ink writing (DIW) in this work, and have 105.3 J kg-1 energy density, enabling multimodal motions including rolling, grasping, and transportation.

Programming actuation onset of a liquid crystalline elastomer via isomerization of network topology

Tuning actuation temperatures of liquid crystalline elastomers (LCEs) achieves control of their actuation onsets, which is generally accomplished in the synthesis step and cannot be altered afterward. Multiple actuation onsets in one LCE can be encoded if the post-synthesis regulation of actuation temperature can be spatiotemporally achieved. This would allow realizing a logical time-evolution of actuation, desired for future soft robots. Nevertheless, this task is challenging given the additional need to ensure mesogen alignment required for actuation. We achieved this goal with a topology isomerizable network (TIN) of LCE containing aromatic and aliphatic esters in the mesogenic and amorphous phases, respectively. These two ester bonds can be distinctly activated for transesterification. The homolytic bond exchange between aliphatic esters allows mechanically induced mesogen alignment without affecting the mesogenic phase. Most importantly, the heterolytic exchange between aromatic and aliphatic esters changes the actuation temperature under different conditions. Spatial control of the two mechanisms via a photo-latent catalyst unleashes the freedom in regulating actuation temperature distribution, yielding unusual controllability in actuation geometries and logical sequence. Our principle is generally applicable to common LCEs containing both aromatic and aliphatic esters.

Innovations in Food Packaging for a Sustainable and Circular Economy

Packaging is fundamental to maintaining the quality of food, but its contribution with a negative footprint to the environment must be completely changed worldwide to reduce pollution and climate change. Innovative and sustainable packaging and new strategies of reutilization are necessary to reduce plastic waste accumulation, maintain food quality and safety, and reduce food losses and waste. The purpose of this chapter is to present innovations in food packaging for a sustainable and circular economy. First, to present the eco-design packaging approach as well as new strategies for recycled or recyclable materials in food packaging. Second, to show current trends in new packaging materials developed from the use of agro-industrial wastes as well as new methods of production, including 3D/4D printing, electrostatic spinning, and the use of nanomaterials.

Programmable and Self-Healable Liquid Crystal Elastomer Actuators Based on Halogen Bonding

Shape-changing polymeric materials have gained significant attention in the field of bioinspired soft robotics. However, challenges remain in versatilizing the shape-morphing process to suit different tasks and environments, and in designing systems that combine reversible actuation and self-healing ability. Here, we report halogen-bonded liquid crystal elastomers (LCEs) that can be arbitrarily shape-programmed and that self-heal under mild thermal or photothermal stimulation. We incorporate halogen-bond-donating diiodotetrafluorobenzene molecules as dynamic supramolecular crosslinks into the LCEs and show that these relatively weak crosslinks are pertinent for their mechanical programming and self-healing. Utilizing the halogen-bonded LCEs, we demonstrate proof-of-concept soft robotic motions such as crawling and rolling with programmed velocities. Our results showcase halogen bonding as a promising, yet unexplored tool for the preparation of smart supramolecular constructs for the development of advanced soft actuators.