Elastomers Grow into Actuators

Rejuvenating liquid crystal elastomers for self-growth

To date, only one polymer can self-grow to an extended length beyond its original size at room temperature without external stimuli or energy input. This breakthrough paves the way for significant advancements in untethered autonomous soft robotics, eliminating the need for the energy input or external stimuli required by all existing soft robotics systems. However, only freshly prepared samples in an initial state can self-grow, while non-fresh ones cannot. The necessity of synthesizing from monomers for each use imposes significant limitations on practical applications. Here, we propose a strategy to rejuvenate non-fresh samples to their initial state for on-demand self-growth through the synergistic effects of solvents and dynamic covalent bonds during swelling. The solvent used for swelling physically transforms the non-fresh LCEs from the liquid crystal phase to the isotropic phase. Simultaneously, the introduction of the transesterification catalyst through swelling facilitates topological rearrangements through exchange reactions of dynamic covalent bonds. The rejuvenation process can also erase growth history, be repeated several times, and be regulated by selective swelling. This strategy provides a post-modulation method for the rejuvenation and reuse of self-growing LCEs, promising to offer high-performance materials for cutting-edge soft growing robotics.

Magnetic-field induced shape memory hydrogels for deformable actuators

Magnetic hydrogel actuators exhibit promising applications in the fields of soft robotics, bioactuators, and flexible sensors owing to their inherent advantages such as remote control capability, untethered deformation and motion control, as well as easily manipulable behavior. However, it is still a challenge for magnetic hydrogels to achieve adjustable stiffness and shape fixation under magnetic field actuation deformation. Herein, a simple and effective approach is proposed for the design of magnetic shape memory hydrogels to accomplish this objective. The magnetic shape memory hydrogels, consisting of methacrylamide, methacrylic acid, polyvinyl alcohol and Fe3O4 magnetic particles, which crosslinked by hydrogen bonds, are facilely prepared via one-pot polymerization. The dynamic nature of noncovalent bonds offers the magnetic hydrogels with excellent mechanical properties, precisely controlled stiffness, and effective shape fixation. The presence of Fe3O4 particles renders the hydrogels soft when subjected to an alternating current field, facilitating their deformation under the influence of an actuation magnetic field. After the elimination of the alternating current magnetic field, the hydrogels stiffen and attain a fixed actuated shape in the absence of any external magnetic field. Moreover, this remarkable magnetic shape memory hydrogel is effectively employed as an underwater soft gripper for lifting heavy objects. This work provides a novel strategy for fabricating magnetic hydrogels with non-contact reversible actuation deformation, tunable stiffness and shape locking.

"Cloth-to-Clothes-Like" Fabrication of Soft Actuators

Inspired by adaptive natural organisms and living matter, soft actuators appeal to a variety of innovative applications such as soft grippers, artificial muscles, wearable electronics, and biomedical devices. However, their fabrication is typically limited in laboratories or a few enterprises since specific instruments, strong stimuli, or specialized operation skills are inevitably involved. Here a straightforward "cloth-to-clothes-like" method to prepare soft actuators with a low threshold by combining the hysteretic behavior of liquid crystal elastomers (LCEs) with the exchange reaction of dynamic covalent bonds, is proposed. Due to the hysteretic behavior, the LCEs (resemble "cloth") effectively retain predefined shapes after stretching and releasing for extended periods. Subsequently, the samples naturally become soft actuators (resemble "clothes") via the exchange reaction at ambient temperatures. As a post-synthesis method, this strategy effectively separates the production of LCEs and soft actuators. LCEs can be mass-produced in bulk by factories or producers and stored as prepared, much like rolls of cloth. When required, these LCEs can be customized into soft actuators as needed. This strategy provides a robust, flexible, and scalable solution to engineer soft actuators, holding great promise for mass production and universal applications.

Programmable, Self-Healable, and Photochromic Liquid Crystal Elastomers Based on Dynamic Hindered Urea Bonds for Biomimetic Flowers

Recently, researchers have been exploring the use of dynamic covalent bonds (DCBs) in the construction of exchangeable liquid crystal elastomers (LCEs) for biomimetic actuators and devices. However, a significant challenge remains in achieving LCEs with both excellent dynamic properties and superior mechanical strength and stability. In this study, a diacrylate-functionalized monomer containing dynamic hindered urea bonds (DA-HUB) is employed to prepare exchangeable LCEs through a self-catalytic Michael addition reaction. By incorporating DA-HUB, the LCE system benefits from DCBs and hydrogen bonding, leading to materials with high mechanical strength and a range of dynamic properties such as programmability, self-healing, and recyclability. Leveraging these characteristics, bilayer LCE actuators with controlled reversible thermal deformation and outstanding dimensional stability are successfully fabricated using a simple welding method. Moreover, a biomimetic triangular plum, inspired by the blooming of flowers, is created to showcase reversible color and shape changes triggered by light and heat. This innovative approach opens new possibilities for the development of biomimetic and smart actuators and devices with multiple functionalities.

Flexible Organic Chiral Crystals with Thermal and Excitation Modulation of the Emission for Information Transmission, Writing, and Storage

Organic single crystals quickly emerge as dense yet light and nearly defect-free media for emissive elements. Integration of functionalities and control over the emissive properties is currently being explored for a wide range of these materials to benchmark their performance against organic emissive materials diluted in powders or films. Here, we report mechanically flexible emissive chiral organic crystals capable of an unprecedented combination of fast, reversible, and low-fatigue responses. UV-excited single crystals of both enantiomers of the material, 4-chloro-2-(((1-phenylidene)imino)methyl)phenol, exhibit a drastic yet reversible change in the emission color from green to orange-yellow within a second and can be cycled at least 2000 times. The photoresponse was found to depend strongly on the excitation intensity and temperature. Combining chirality, mechanical compliance, rapid emission switching, multiple responses, and writability by UV light, this material provides a unique and versatile platform for developing organic crystal-based materials for on-demand signal transfer, information storage, and cryptography.

Electro-optics of confined systems

Confinement in microenvironments occurs in many natural systems and technological applications. However, little is known about the behaviour of the immersed nanoparticles. In this work we show that their diffusion, electro-orientation and electric field induced polarization can be determined through electric birefringence experiments. We analyze aqueous dispersions of silver nanowires and clay particles confined inside microdroplets. We have observed that confinement reduces the amount of particles that can be oriented by the external electric field. However, the polarizability of the oriented particles is not affected by the presence of the oil/water boundary, and it is the same as in unbounded media, which agrees with the fact that the electric polarization and related phenomena are short-ranged.

Self-Growing Organic Materials

The growth of living systems is ubiquitous. Living organisms can continually update their sizes, shapes, and properties to meet various environmental challenges. Such a capability is also demonstrated by emerging self-growing materials that can incorporate externally provided compounds to grow as living organisms. In this Minireview, we summarize these materials in terms of six aspects. First, we discuss their essential characteristics, then describe the strategies for enabling crosslinked organic materials to self-grow from nutrient solutions containing polymerizable compounds. The developed examples are grouped into five categories based on their molecular mechanisms. We then explain the mechanism of mass transport within polymer networks during growth, which is critical for controlling the shape and morphology of the grown products. Afterwards, simulation models built to explain the interesting phenomena observed in self-growing materials are discussed. The development of self-growing materials is accompanied by various applications, including tuning bulk properties, creating textured surfaces, growth-induced self-healing, 4D printing, self-growing implants, actuation, self-growing structural coloration, and others. These examples are then summed up. Finally, we discuss the opportunities brought by self-growing materials and their facing challenges.

Fracture and mechanical properties of an impact toughened polypropylene composite: modification for automotive dashboard-airbag application

Thermoplastic olefin (TPO) is the principal material for automotive instrument panels and is prone to fracture especially under heavy airbag deployment, which can prevent the airbag deploying properly. Thus, polyolefin elastomer (POE) was introduced to improve impact properties and fracture resistance. Fundamental methods to characterise TPO with the addition of POE are proposed. The influence of POE content on the mechanical properties was examined. With increasing POE content, the storage modulus and glass transition temperature values decreased, and the damping increased due to the POE increasing the polymer chain mobility. The tensile modulus, ultimate tensile strength and yield stress decreased with increasing POE content, while the ductility of the blends significantly increased. Furthermore, the POE reduced hardness and increased energy absorption during impact. In the fracture analysis, the addition of POE content increased the fracture resistance by increasing the crack energy and decreasing the resistance to crack initiation. Fractographic analysis showed how stretched microfibrils in the blends increase the fracture resistance. These results gave a significant indication of the utility of the elastomer in improving some mechanical properties and impact toughness of the interior automotive material to resist an undesired failure or over-fracture in airbag deployment.