Advances in 4D printing of liquid crystalline elastomers: materials, techniques, and applications

Extrusion-based 3D printing of soft active materials

Active materials are capable of responding to external stimuli, as observed in both natural and synthetic systems, from sensitive plants to temperature-responsive hydrogels. Extrusion-based 3D printing of soft active materials facilitates the fabrication of intricate geometries with spatially programmed compositions and architectures at various scales, further enhancing the functionality of materials. This Feature Article summarizes recent advances in extrusion-based 3D printing of active materials in both non-living (i.e., synthetic) and living systems. It highlights emerging ink formulations and architectural designs that enable programmable properties, with a focus on complex shape morphing and controllable light-emitting patterns. The article also spotlights strategies for engineering living materials that can produce genetically encoded material responses and react to a variety of environmental stimuli. Lastly, it discusses the challenges and prospects for advancements in both synthetic and living composite materials from the perspectives of chemistry, modeling, and integration.

3D Printable Active Hydrogels with Supramolecular Additive-Driven Adaptiveness

Smart hydrogels are a promising candidate for the development of next-generation soft materials due to their stimuli-responsiveness, deformability, and biocompatibility. However, it remains challenging to enable hydrogels to actively adapt to various environmental conditions like living organisms. In this work, supramolecular additives are introduced to the hydrogel matrix to confer environmental adaptiveness. Specifically, their microstructures, swelling behaviors, mechanical properties, and transparency can adapt to external environmental conditions. Moreover, the presence of hydrogen bonding provides the hydrogel with applicable rheological properties for 3D extrusion printing, thus allowing for the facile preparation of thickness-dependent camouflage and multistimuli responsive complex. The environmentally adaptive hydrogel developed in this study offers new approaches for manipulating supramolecular interactions and broadens the capability of smart hydrogels in information security and multifunctional integrated actuation.

Specific Condis Crystal-like Mesophase of Poly(butylene succinate-co-butylene adipate)

Understanding the properties of polymers, such as their crystallinity, is crucial for their material performance and predicting their behavior during and after use, especially in the case of environmentally friendly (bio)degradable polymers, enabling optimized design. In this work, for the first time, a pressure-induced condis crystal-like mesophase of poly(butylene succinate-co-butylene adipate) (PBSA) is presented. The phase behavior of pressed films obtained from commercial PBSA with 25% butylene adipate units is investigated at various processing temperatures from room temperature to 100 °C, pressed at a pressure of the press jaws and at 2-5 t for 1-5 min. The characterization and quantification evaluation of the condis crystal-like mesophase of pressed PBSA formed at temperatures above the glass transition is investigated by X-ray diffraction, polarized optical microscopy (POM), and differential scanning calorimetry (DSC) methods. Our results demonstrate that pressed PBSA films at 60 °C show a condis crystal-like mesophase, characterized by the presence of reflections at wide angles, birefringence by POM, as well as a higher melting point (endotherm) by DSC. The resulting oriented mesomorphic green polymer can, in a sustainable manner, expand further technological applications of (bio)degradable polymers, especially in the medical field, and open up opportunities for further research that could provide such polymers with tailored persistence and degradation, thus changing the shelf life.

Toward Monodomain Nematic Liquid Crystal Elastomers of Arbitrary Thickness through PET-RAFT Polymerization

Liquid crystal elastomers (LCEs) are polymeric materials that are proposed for a range of applications. However, to reach their full potential, it is desirable to have as much flexibility as possible in terms of the sample dimensions, while maintaining well-defined alignment. In this work, photoinduced electron/energy transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization is applied to the synthesis of LCEs for the first time. An initial LCE layer (∼100 μm thickness) is partially cured before a second layer of the precursor mixture is added. The curing reaction is then resumed and is observed by FTIR to complete within 15 min of irradiation, yielding samples of increased thickness. Monodomain samples that exhibit an auxetic response and are of thickness 250-300 μm are consistently achieved. All samples are characterized thermally, mechanically, and in terms of their order parameters. The LCEs have physical properties comparable to those of analogous LCEs produced via free-radical polymerization.

Fiber Actuators Based on Reversible Thermal Responsive Liquid Crystal Elastomer

Soft actuators inspired by the movement of organisms have attracted extensive attention in the fields of soft robotics, electronic skin, artificial intelligence, and healthcare due to their excellent adaptability and operational safety. Liquid crystal elastomer fiber actuators (LCEFAs) are considered as one of the most promising soft actuators since they can provide reversible linear motion and are easily integrated or woven into complex structures to perform pre-programmed movements such as stretching, rotating, bending, and expanding. The research on LCEFAs mainly focuses on controllable preparation, structural design, and functional applications. This review, for the first time, provides a comprehensive and systematic review of recent advances in this important field by focusing on reversible thermal response LCEFAs. First, the thermal driving mechanism, and direct and indirect heating strategies of LCEFAs are systematically summarized and analyzed. Then, the fabrication methods and functional applications of LCEFAs are summarized and discussed. Finally, the challenges and technical difficulties that may hinder the performance improvement and large-scale production of LCEFAs are proposed, and the development opportunities of LCEFAs are prospected.

Sustainable 3D printing by reversible salting-out effects with aqueous salt solutions

Achieving a simple yet sustainable printing technique with minimal instruments and energy remains challenging. Here, a facile and sustainable 3D printing technique is developed by utilizing a reversible salting-out effect. The salting-out effect induced by aqueous salt solutions lowers the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) solutions to below 10 °C. It enables the spontaneous and instant formation of physical crosslinks within PNIPAM chains at room temperature, thus allowing the PNIPAM solution to solidify upon contact with a salt solution. The PNIPAM solutions are extrudable through needles and can immediately solidify by salt ions, preserving printed structures, without rheological modifiers, chemical crosslinkers, and additional post-processing steps/equipment. The reversible physical crosslinking and de-crosslinking of the polymer through the salting-out effect demonstrate the recyclability of the polymeric ink. This printing approach extends to various PNIPAM-based composite solutions incorporating functional materials or other polymers, which offers great potential for developing water-soluble disposable electronic circuits, carriers for delivering small materials, and smart actuators.

Modification of the Dielectric and Thermal Properties of Organic Frameworks Based on Nonterminal Epoxy Liquid Crystal with Silicon Dioxide and Titanium Dioxide

A nonterminal liquid crystal epoxy monomer is used to create an epoxy-amine network with a typical diamine 4,4'diaminodiphenylmethane. The plain matrix is compared to matrices modified with inorganic fillers: TiO2 or SiO2. Conditions of the curing reaction and glass transition temperatures in the cured products are determined through differential scanning calorimetry and broadband dielectric spectroscopy. The curing process is also followed through optical and electrical observations. The dielectric response of all investigated networks reveals a segmental α-process related to structural reorientation (connected to the glass transition). In all products, a similar process associated with molecular motions of polar groups also appears. The matrix modified with TiO2 exhibits two secondary relaxation processes (β and γ). Similar processes were observed in the pure monomer. An advantage of the network with the TiO2 filler is a shorter time or lower temperature required for optimal curing conditions. The physical properties of cured matrices depend on the presence of a nematic phase in the monomer and nonterminal functional groups in the aliphatic chains. In effect, such cured matrices can have more flexibility and internal order than classical resins. Additional modifiers used in this work shift the glass transition above room temperature and influence the fragility index in both cases.

Adaptive nanotube networks enabling omnidirectionally deformable electro-driven liquid crystal elastomers towards artificial muscles

Artificial muscles that can convert electrical energy into mechanical energy promise broad scientific and technological applications. However, existing electro-driven artificial muscles have been plagued with problems that hinder their practical applications: large electro-mechanical attenuation during deformation, high-driving voltages, small actuation strain, and low power density. Here, we design and create novel electro-thermal-driven artificial muscles rationally composited by hierarchically structured carbon nanotube (HS-CNT) networks and liquid crystal elastomers (LCEs), which possess adaptive sandwiched nanotube networks with angulated-scissor-like microstructures, thus effectively addressing above problems. These HS-CNT/LCE artificial muscles demonstrate not only large strain (>40%), but also remarkable conductive robustness (R/R0 < 1.03 under actuation), excellent Joule heating efficiency (≈ 233 °C at 4 V), and high load-bearing capacity (R/R0 < 1.15 at 4000 times its weight loaded). In addition, our artificial muscles exhibit real-muscle-like morphing intelligence that enables preventing mechanical damage in response to excessively heavyweight loading. These high-performance artificial muscles uniquely combining omnidirectional stretchability, robust electrothermal actuation, low driving voltage, and powerful mechanical output would exert significant technological impacts on engineering applications such as soft robotics and wearable flexible electronics.

3D-Printed Biomimetic Structural Colors

Resolution control and expansibility have always been challenges to the fabrication of structural color materials. Here, a facile strategy to print cholesteric liquid crystal elastomers (CLCEs) into complex structural color patterns with variable resolution and enhanced expansibility is reported. A volatile solvent is introduced into the synthesized CLC oligomers, modifying its rheological properties and allowing direct-ink-writing (DIW) under mild conditions. The combination of printing shear flow and anisotropic deswelling of ink drives the CLC molecules into an ordered cholesteric arrangement. The authors meticulously investigate the influence of printing parameters to achieve resolution control over a wide range, allowing for the printing of multi-sized 1D or 2D patterns with constant quality. Furthermore, such solvent-cast direct-ink-writing (DIW) strategy is highly expandable and can be integrated easily into the DIW of bionic robots. Multi-responsive bionic butterfly and flower are printed with biomimetic in both locomotion and coloration. Such designs dramatically reduced the processing difficulty of precise full-color printing and expanded the capability of structural color materials to collaborate with other systems.

Direct 4D printing of ceramics driven by hydrogel dehydration

4D printing technology combines 3D printing and stimulus-responsive materials, enabling construction of complex 3D objects efficiently. However, unlike smart soft materials, 4D printing of ceramics is a great challenge due to the extremely weak deformability of ceramics. Here, we report a feasible and efficient manufacturing and design approach to realize direct 4D printing of ceramics. Photocurable ceramic elastomer slurry and hydrogel precursor are developed for the fabrication of hydrogel-ceramic laminates via multimaterial digital light processing 3D printing. Flat patterned laminates evolve into complex 3D structures driven by hydrogel dehydration, and then turn into pure ceramics after sintering. Considering the dehydration-induced deformation and sintering-induced shape retraction, we develop a theoretical model to calculate the curvatures of bent laminate and sintered ceramic part. Then, we build a design flow for direct 4D printing of various complex ceramic objects. This approach opens a new avenue for the development of ceramic 4D printing technology.

Mechanochromic and ionic conductive cholesteric liquid crystal elastomers for biomechanical monitoring and human-machine interaction

Cholesteric liquid crystal elastomers (CLCEs) that combine rubbery elasticity with structural colour from self-assembled helical nanostructures are of paramount importance for diverse applications such as biomimetic skins, adaptive optics and soft robotics. Despite great advances, it is challenging to integrate electrical sensing and colour-changing characteristics in a single CLCE system. Here, we report the design and synthesis of an ionic conductive cholesteric liquid crystal elastomer (iCLCE) through in situ Michael addition and free-radical photopolymerization of CLCE precursors on silane-functionalized polymer ionic liquid networks, in which robust covalent chemical bonding was formed at the interface. Thanks to superior mechanochromism and ionic conductivity, the resulting iCLCEs exhibit dynamic colour-changing and electrical sensing functions in a wide range upon mechanical stretching, and can be used for biomechanical monitoring during joint bending. Importantly, a capacitive elastomeric sensor can be constructed through facilely stacking iCLCEs, where the optical and electrical dual-signal reporting performance allows intuitive visual localization of pressure intensity and distribution. Moreover, proof-of-concept application of the iCLCEs has been demonstrated with human-interactive systems. The research disclosed herein can provide new insights into the development of bioinspired somatosensory materials for emerging applications in diverse fields such as human-machine interaction, prostheses and intelligent robots.

Stretching-Induced 2D-to-3D Shape Transformation of an Elastic Composite for Sensitivity-Tailorable Soft Electronics

The shapes of rubbers and elastomers are challenging to alter, and current methods relying on permanent plasticity and dynamic cross-linking strategies are usually laborious and can inevitably compromise the network elasticity. Here, we report a photoresponsive elastic composite that can be programmed into 3D shapes by first UV light irradiation and then stretching. The composite comprises ethylene propylene rubber as the elastic substrate and photoliquefiable azobenzene small molecules as the responsive filler. Upon UV light irradiation, the liquefication of the filler induces the destruction of the crystalline aggregates near the irradiated surface, and after stretching and subsequent stress release, the irradiated part bends to the irradiated side based on a gradient network orientation mechanism. The position and amplitude of bending deformation can be controlled to realize a 2D-to-3D shape transformation. We further show that the resulting 3D-shaped elastomer can integrate with silver conductive paste to develop soft conductive lines with tailorable strain-sensitive conductivities. This study may open a new door for the development of shape-tailorable elastomers and soft electronics with designable strain-sensitive conductivities.

Deep Learning in Mechanical Metamaterials: From Prediction and Generation to Inverse Design

Mechanical metamaterials are meticulously designed structures with exceptional mechanical properties determined by their microstructures and constituent materials. Tailoring their material and geometric distribution unlocks the potential to achieve unprecedented bulk properties and functions. However, current mechanical metamaterial design considerably relies on experienced designers' inspiration through trial and error, while investigating their mechanical properties and responses entails time-consuming mechanical testing or computationally expensive simulations. Nevertheless, recent advancements in deep learning have revolutionized the design process of mechanical metamaterials, enabling property prediction and geometry generation without prior knowledge. Furthermore, deep generative models can transform conventional forward design into inverse design. Many recent studies on the implementation of deep learning in mechanical metamaterials are highly specialized, and their pros and cons may not be immediately evident. This critical review provides a comprehensive overview of the capabilities of deep learning in property prediction, geometry generation, and inverse design of mechanical metamaterials. Additionally, this review highlights the potential of leveraging deep learning to create universally applicable datasets, intelligently designed metamaterials, and material intelligence. This article is expected to be valuable not only to researchers working on mechanical metamaterials but also those in the field of materials informatics.

From Static to Dynamic: Smart Materials Pioneering Additive Manufacturing in Regenerative Medicine

The emerging field of regenerative medicine holds immense promise for addressing complex tissue and organ regeneration challenges. Central to its advancement is the evolution of additive manufacturing techniques, which have transcended static constructs to embrace dynamic, biomimetic solutions. This manuscript explores the pivotal role of smart materials in this transformative journey, where materials are endowed with dynamic responsiveness to biological cues and environmental changes. By delving into the innovative integration of smart materials, such as shape memory polymers and stimulus-responsive hydrogels, into additive manufacturing processes, this research illuminates the potential to engineer tissue constructs with unparalleled biomimicry. From dynamically adapting scaffolds that mimic the mechanical behavior of native tissues to drug delivery systems that respond to physiological cues, the convergence of smart materials and additive manufacturing heralds a new era in regenerative medicine. This manuscript presents an insightful overview of recent advancements, challenges, and future prospects, underscoring the pivotal role of smart materials as pioneers in shaping the dynamic landscape of regenerative medicine and heralding a future where tissue engineering is propelled beyond static constructs towards biomimetic, responsive, and regenerative solutions.

A systematic review of the role of 4D printing in sustainable civil engineering solutions

This systematic review, not financially supported by any funding body, aims to synthesize the current knowledge on the applications, potential benefits, and challenges of 4D printing in civil engineering, with a focus on its role in sustainable solutions. Comprehensive searches were conducted in Scopus, Web of Science, and Google Scholar using related keywords. Articles that discussed 4D printing within civil engineering and construction contexts, encompassing both conceptual and empirical studies, were included. The findings suggest that 4D printing, with its time-responsive transformation feature, can enhance design freedom, improve structural performance, and increase environmental efficiency in construction. However, challenges persist in material performance, scalability, and cost. Despite these, ongoing advancements signal potential future developments that could widen the opportunities for large-scale applications of 4D printing in civil engineering. The potential use of renewable, bio-based materials could also lead to more sustainable construction practices. This review highlights the transformative potential of 4D printing, underlining the need for further research to fully leverage its capabilities and address current limitations. 4D printing emerges as a promising avenue for sustainable civil engineering solutions, offering a transformative approach that calls for continued exploration and development.

Three/Four-Dimensional Printed PLA Nano/Microstructures: Crystallization Principles and Practical Applications

Compared to traditional methods, three/four-dimensional (3D/4D) printing technologies allow rapid prototyping and mass customization, which are ideal for preparing nano/microstructures of soft polymer materials. Poly (lactic acid) (PLA) is a biopolymer material widely used in additive manufacturing (AM) because of its biocompatibility and biodegradability. Unfortunately, owing to its intrinsically poor nucleation ability, a PLA product is usually in an amorphous state after industrial processing, leading to some undesirable properties such as a barrier property and low thermal resistance. Crystallization mediation offers a most practical way to improve the properties of PLA products. Herein, we summarize and discuss 3D/4D printing technologies in the processing of PLA nano/microstructures, focusing on crystallization principles and practical applications including bio-inspired structures, flexible electronics and biomedical engineering mainly reported in the last five years. Moreover, the challenges and prospects of 3D/4D printing technologies in the fabrication of high-performance PLA materials nano/microstructures will also be discussed.

Additive Manufacturing of Liquid Crystal Elastomer Actuators Based on Knitting Technology

Liquid crystal elastomer (LCE) exhibits large and reversible deformability originating from the alignment of liquid crystal mesogens. Additive manufacturing provides high controllability in the alignment and shaping process of LCE actuators. However, it still remains a challenge to customize LCE actuators with both diverse 3D deformability and recyclability. In this study, a new strategy is developed to exploit knitting technique to additively manufacture LCE actuators. The obtained LCE actuators are fabric-structured with designed geometry and deformability. By accurately adjusting the parameters of the knitting patterns as modules, diverse geometry is pixel-wise designed, and complex 3D deformations including bending, twisting, and folding are quantitatively controlled. In addition, the fabric-structured LCE actuators can be threaded, stitched, and reknitted to achieve advanced geometry, integrated multi-functions and efficient recyclability. This approach allows the fabrication of versatile LCE actuators with potential applications in smart textiles and soft robots.

Four-Dimensional-Printed Microrobots and Their Applications: A Review

Owing to their small size, microrobots have many potential applications. In addition, four-dimensional (4D) printing facilitates reversible shape transformation over time or upon the application of stimuli. By combining the concept of microrobots and 4D printing, it may be possible to realize more sophisticated next-generation microrobot designs that can be actuated by applying various stimuli, and also demonstrates profound implications for various applications, including drug delivery, cells delivery, soft robotics, object release and others. Herein, recent advances in 4D-printed microrobots are reviewed, including strategies for facilitating shape transformations, diverse types of external stimuli, and medical and nonmedical applications of microrobots. Finally, to conclude the paper, the challenges and the prospects of 4D-printed microrobots are highlighted.

Light-triggered multi-joint microactuator fabricated by two-in-one femtosecond laser writing

Inspired by the flexible joints of humans, actuators containing soft joints have been developed for various applications, including soft grippers, artificial muscles, and wearable devices. However, integrating multiple microjoints into soft robots at the micrometer scale to achieve multi-deformation modalities remains challenging. Here, we propose a two-in-one femtosecond laser writing strategy to fabricate microjoints composed of hydrogel and metal nanoparticles, and develop multi-joint microactuators with multi-deformation modalities (>10), requiring short response time (30 ms) and low actuation power (<10 mW) to achieve deformation. Besides, independent joint deformation control and linkage of multi-joint deformation, including co-planar and spatial linkage, enables the microactuator to reconstruct a variety of complex human-like modalities. Finally, as a proof of concept, the collection of multiple microcargos at different locations is achieved by a double-joint micro robotic arm. Our microactuators with multiple modalities will bring many potential application opportunities in microcargo collection, microfluid operation, and cell manipulation.

Recent Advances in 4D Printing of Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are renowned for their large, reversible, and anisotropic shape change in response to various external stimuli due to their lightly cross-linked polymer networks with an oriented mesogen direction, thus showing great potential for applications in robotics, bio-medics, electronics, optics, and energy. To fully take advantage of the anisotropic stimuli-responsive behaviors of LCEs, it is preferable to achieve a locally controlled mesogen alignment into monodomain orientations. In recent years, the application of 4D printing to LCEs opens new doors for simultaneously programming the mesogen alignment and the 3D geometry, offering more opportunities and higher feasibility for the fabrication of 4D-printed LCE objects with desirable stimuli-responsive properties. Here, the state-of-the-art advances in 4D printing of LCEs are reviewed, with emphasis on both the mechanisms and potential applications. First, the fundamental properties of LCEs and the working principles of the representative 4D printing techniques are briefly introduced. Then, the fabrication of LCEs by 4D printing techniques and the advantages over conventional manufacturing methods are demonstrated. Finally, perspectives on the current challenges and potential development trends toward the 4D printing of LCEs are discussed, which may shed light on future research directions in this new field.

Four-Dimensional Printing of Temperature-Responsive Liquid Crystal Elastomers with Programmable Shape-Changing Behavior

Liquid crystal elastomers (LCEs) are polymer networks that exhibit anisotropic liquid crystalline properties while maintaining the properties of elastomers, presenting reversible high-speed and large-scale actuation in response to external stimuli. Herein, we formulated a non-toxic, low-temperature liquid crystal (LC) ink for temperature-controlled direct ink writing 3D printing. The rheological properties of the LC ink were verified under different temperatures given the phase transition temperature of 63 °C measured by the DSC test. Afterwards, the effects of printing speed, printing temperature, and actuation temperature on the actuation strain of printed LCEs structures were investigated within adjustable ranges. In addition, it was demonstrated that the printing direction can modulate the LCEs to exhibit different actuation behaviors. Finally, by sequentially conforming structures and programming the printing parameters, it showed the deformation behavior of a variety of complex structures. By integrating with 4D printing and digital device architectures, this unique reversible deformation property will help LCEs presented here apply to mechanical actuators, smart surfaces, micro-robots, etc.

A cold-responsive liquid crystal elastomer provides visual signals for monitoring a critical temperature decrease

Critical temperature indicators have been extensively utilized in various fields, ranging from healthcare to food safety. However, the majority of the temperature indicators are designed for upper critical temperature monitoring, indicating when the temperature rises and exceeds a predefined limit, whereas stringently demanded low critical temperature indicators are scarcely developed. Herein, we develop a new material and system that monitor temperature decrease, e.g., from ambient temperature to the freezing point, or even to an ultra-low temperature of -20 °C. For this purpose, we create a dynamic membrane which can open and close during temperature cycles from high temperature to low temperature. This membrane consists of a gold-liquid crystal elastomer (Au-LCE) bilayer structure. Unlike the commonly used thermo-responsive LCEs which actuate upon temperature rise, our LCE is cold-responsive. This means that geometric deformations occur when the environmental temperature decreases. Specifically, upon temperature decrease the LCE creates stresses at the gold interface by uniaxial deformation due to expansion along the molecular director and shrinkage perpendicular to it. At a critical stress, optimized to occur at the desired temperature, the brittle Au top layer fractures, which allows contact between the LCE and material on top of the gold layer. Material transport via cracks enables the onset of the visible signal for instance caused by a pH indicator substance. We apply the dynamic Au-LCE membrane for cold-chain applications, indicating the loss of the effectiveness of perishable goods. We anticipate that our newly developed low critical temperature/time indicator will be shortly implemented in supply chains to minimize food and medical product waste.

Ultrasonic-Excited Ultrafast Seamless Integration of Heterostructured Liquid Crystalline Elastomers for Multi-responsive Soft Actuators

Multicomponent/heterostructured liquid crystalline elastomers (LCEs) have recently garnered extensive attention for the design of soft robots with high dexterity and flexibility. However, the reported integration strategies of LCEs seriously suffer from high welding temperature, long processing time, and poor joint quality. Herein, the high-efficiency seamless ultrasonic welding (UW) of reprogrammable silver nanowire-LCE composites (AgNW-LCEs) have been realized without any auxiliary reagents based on the dynamic silver-disulfide coordination interactions. The elaborate combination of silver-disulfide coordination interactions and UW technology establishes an effective double-network welding mechanism of AgNWs and dynamic LC networks due to the high-frequency vibration at the welding interface. During the UW process, monolithic AgNW-LCEs can be integrated into heterostructured actuators at room temperature for 0.68 s. Furthermore, the welded AgNW-LCEs demonstrate an exceptional strain healing efficiency of ∼100%, a stress healing efficiency of ∼85%, and a maintained orientation of the LC alignment. Taking advantage of the high-efficiency UW technology, the heterostructured AgNW-LCE actuators with different LC alignments or LC monomers have been successfully implemented for a multi-degree-of-freedom soft robotic arm and a time-modulated flower-mimic actuator. This work provides an efficient approach toward the development of multi-responsive entirely soft actuators based on smart polymers.

3D printing programmable liquid crystal elastomer soft pneumatic actuators

Soft pneumatic actuators (SPAs) rely on anisotropic mechanical properties to generate specific motions after inflation. To achieve mechanical anisotropy, additional stiff materials or heterogeneous structures are typically introduced in isotropic base materials. However, the inherent limitations of these strategies may lead to potential interfacial problems or inefficient material usage. Herein, we develop a new strategy for fabricating SPAs based on an aligned liquid crystal elastomer (LCE) by a modified 3D printing technology. A rotating substrate enables the one-step fabrication of tubular LCE-SPAs with designed alignments in three dimensions. The alignment can be precisely programmed through printing, resulting in intrinsic mechanical anisotropy of the LCE. With a specially designed alignment, LCE-SPAs can achieve basic motions-contraction, elongation, bending, and twisting-and accomplish diverse tasks, e.g., grabbing objects and mixing water. This study provides a new perspective for the design and fabrication of SPAs.

Controlled Diels-Alder "Click" Strategy to Access Mechanically Aligned Main-Chain Liquid Crystal Networks

Aligned liquid crystal polymers are materials of interest for electronic, optic, biological and soft robotic applications. The manufacturing and processing of these materials have been widely explored with mechanical alignment establishing itself as a preferred method due to its ease of use and widespread applicability. However, the fundamental chemistry behind the required two-step polymerization for mechanical alignment has limitations in both fabrication and substrate compatibility. In this work we introduce a new protection-deprotection approach utilizing a two-stage Diels-Alder cyclopentadiene-maleimide step-growth polymerization to enable mild yet efficient, fast, controlled, reproducible and user-friendly polymerizations, broadening the scope of liquid crystal systems. Thorough characterization of the films by DSC, DMA, POM and WAXD show the successful synthesis of a uniaxially aligned liquid crystal network with thermomechanical actuation abilities.

A Power Compensation Strategy for Achieving Homogeneous Microstructures for 4D Printing Shape-Adaptive PNIPAM Hydrogels: Out-of-Plane Variations

In the last decade, 3D printing has attracted significant attention and has resulted in benefits to many research areas. Advances in 3D printing with smart materials at the microscale, such as hydrogels and liquid crystalline polymers, have enabled 4D printing and various applications in microrobots, micro-actuators, and tissue engineering. However, the material absorption of the laser power and the aberrations of the laser light spot will introduce a decay in the polymerization degree along the height direction, and the solution to this problem has not been reported yet. In this paper, a compensation strategy for the laser power is proposed to achieve homogeneous and high aspect ratio hydrogel structures at the microscale along the out-of-plane direction. Linear approximations for the power decay curve are adopted for height steps, discretizing the final high aspect ratio structures. The strategy is achieved experimentally with hydrogel structures fabricated by two-photon polymerization. Moreover, characterizations have been conducted to verify the homogeneity of the printed microstructures. Finally, the saturation of material property is investigated by an indirect 3D deformation method. The proposed strategy is proved to be effective and can be explored for other hydrogel materials showing significant deformation. Furthermore, the strategy for out-of-plane variations provides a critical technique to achieve 4D-printed homogeneous shape-adaptive hydrogels for further applications.

Surface-Enforced Alignment of Reprogrammable Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are stimuli-responsive materials capable of undergoing large deformations. The thermomechanical response of LCEs is attributable to the coupling of polymer network properties and disruption of order between liquid crystalline mesogens. Complex deformations have been realized in LCEs by either programming the nematic director via surface-enforced alignment or localized mechanical deformation in materials incorporating dynamic covalent chemistries. Here, the preparation of LCEs via thiol-Michael addition reaction is reported that are amenable to surface-enforced alignment. Afforded by the thiol-Michael addition reaction, dynamic covalent bonds are uniquely incorporated in chemistries subject to surface-enforce alignment. Accordingly, LCEs prepared with complex director profiles are able to be programmed and reprogrammed by (re)activating the dynamic covalent chemistry to realize distinctive shape transformations.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Scalable Fabrication of Metallic Conductive Fibers from Rheological Tunable Semi-Liquid Metals

Conductive polymer fibers/wires (CPFs) are important materials in modern technologies due to their unique one-dimension geometry, electrical conductivity, and flexibility. However, the advanced applications of current CPFs are limited by their low electrical conductivities (<500 S/m) and poor interfacial interactions between conductive fillers (e.g., graphite) and polymers. Therefore, in current electrical applications, metal wires/foils like copper and aluminum are the most frequently utilized conductive fibers/wires instead of the inferior conductive CPFs. This work successfully addresses the heavy phase segregation between polymers and conductive inorganic materials to obtain semiliquid metal polymer fibers (SLMPFs) which exhibit an ultrahigh electrical conductivity (over 10⁶ S/m), remarkable thermal processability, and considerable mechanical performance (Young's modulus: ~300 MPa). Semiliquid metal (gallium-tin alloy) with tunable viscosities is the key to achieve the excellent miscibility between metals and polymers. Both the rheological results and numerical simulations demonstrate the critical viscosity matching for the successful preparation of the fibers. More importantly, the fibers are adapted with classic polymer melt-processing like melt injection, which indicates the scalable production of the highly conductive fibers. The SLMPFs are highly promising substitutes for metal wires/fibers in modern electrical applications such as electricity transmission, data communication, and underwater works.