Biomaterial systems for mechanosensing and actuation

Impact of halogen⋯halogen interaction on the mechanical motion of a 3D Pb(II) coordination polymer of elusive topology

Herein, we report the synthesis of a Pb(II) based three-dimensional coordination polymer (3D CP), [Pb(DCTP)]n (1) [H2DCTP = 2,5-dichloroterephthalic acid] with an unprecedented topology, which exhibits a photomechanical effect wherein crystals show jumping upon UV irradiation. The Pb(II) CP forms a type II Cl⋯Cl interaction, which weakens further upon UV irradiation to resolve the anisotropic mechanical strain. The work presented here could be a beacon to the nascent field of photoactuating smart materials.

Crossing length scales: X-ray approaches to studying the structure of biological materials

Biological materials have outstanding properties. With ease, challenging mechanical, optical or electrical properties are realised from comparatively `humble' building blocks. The key strategy to realise these properties is through extensive hierarchical structuring of the material from the millimetre to the nanometre scale in 3D. Though hierarchical structuring in biological materials has long been recognized, the 3D characterization of such structures remains a challenge. To understand the behaviour of materials, multimodal and multi-scale characterization approaches are needed. In this review, we outline current X-ray analysis approaches using the structures of bone and shells as examples. We show how recent advances have aided our understanding of hierarchical structures and their functions, and how these could be exploited for future research directions. We also discuss current roadblocks including radiation damage, data quantity and sample preparation, as well as strategies to address them.

Specialized structure and function of the apical extracellular matrix at sense organs

Apical extracellular matrix (aECM) covers every surface of the body and exhibits tissue-specific structures that carry out specialized functions. This is particularly striking at sense organs, where aECM forms the interface between sensory neurons and the environment, and thus plays critical roles in how sensory stimuli are received. Here, we review the extraordinary adaptations of aECM across sense organs and discuss how differences in protein composition and matrix structure assist in sensing mechanical forces (tactile hairs, campaniform sensilla, and the tectorial membrane of the cochlea); tastes and smells (uniporous gustatory sensilla and multiporous olfactory sensilla in insects, and salivary and olfactory mucus in vertebrates); and light (cuticle-derived lenses in arthropods and mollusks). We summarize the power of using C. elegans, in which defined sense organs associate with distinct aECM, as a model for understanding the tissue-specific structural and functional specializations of aECM. Finally, we synthesize results from recent studies in C. elegans and Drosophila into a conceptual framework for aECM patterning, including mechanisms that involve transient cellular or matrix scaffolds, mechanical pulling or pushing forces, and localized secretion or endocytosis.

Water training initiates spatially regulated microstructures with competitive mechanics in hydroadaptive polymers

The strategy using water as a medium for dynamic modulation of competitive plasticity and viscoelasticity provides a unique perspective to attain adaptive materials. We reveal sustainable polymers, herein cellulose phenoxyacetate as a typical example, with unusual water-responsive dual-mechanic functionalities addressed via a chronological water training strategy. The temporal significance of such water-responsive mechanical behaviors becomes apparent considering that a mere 3-minute exposure or a prolonged 3-hour exposure to water induced different types of mechano-responsiveness. This endows the materials with multiple recoverable shape-changes during water and air training, and consequently even underlines the switchability between the pre-loaded stable water shapes (> 20 months) and the sequentially fixed air shapes. Our discovery exploits the competitive mechanics initiated by water training, enabling polymers with spatially regulated microstructures via their inherently distinct mechanical properties. Insights into the molecular changes represents a considerable fundamental innovation, can be broadly applicable to a diverse array of hydroadaptive polymers.

Photomechanical properties in metal-organic crystals

The emergence of materials that can effectively convert photon energy (light) into motion (mechanical work) and change their shapes on command is of great interest for their potential in the fabrication of devices (powered by light) that will revolutionize the technologies of optical actuators, smart medical devices, soft robotics, artificial muscles and flexible electronics. Recently, metal-organic crystals have emerged as desirable smart hybrid materials that can hop, split and jump. Thus, their incorporation into polymer host objects can control movement from molecules to millimetres, opening up a new world of light-switching smart materials. This feature article briefly summarizes the recent part of the fast-growing literature on photomechanical properties in metal-organic crystals, such as coordination compounds, coordination polymers (CPs), and metal-organic frameworks (MOFs). The article highlights the contributions of our group along with others in this area and aims to provide a consolidated idea of the engineering strategies and structure-property relationships of these hybrid materials for such rare phenomena with diverse potential applications.

Pushing Trap-Controlled Persistent Luminescence Materials toward Multi-Responsive Smart Platforms: Recent Advances, Mechanism, and Frontier Applications

Smart stimuli-responsive persistent luminescence materials, combining the various advantages and frontier applications prospects, have gained booming progress in recent years. The trap-controlled property and energy storage capability to respond to external multi-stimulations through diverse luminescence pathways make them attractive in emerging multi-responsive smart platforms. This review aims at the recent advances in trap-controlled luminescence materials for advanced multi-stimuli-responsive smart platforms. The design principles, luminescence mechanisms, and representative stimulations, i.e., thermo-, photo-, mechano-, and X-rays responsiveness, are comprehensively summarized. Various emerging multi-responsive hybrid systems containing trap-controlled luminescence materials are highlighted. Specifically, temperature dependent trapping and de-trapping performance is discussed, from extreme-low temperature to ultra-high temperature conditions. Emerging applications and future perspectives are briefly presented. It is hoped that this review would provide new insights and guidelines for the rational design and performance manipulation of multi-responsive materials for advanced smart platforms.

Voxel Design of Grayscale DLP 3D-Printed Soft Robots

Grayscale digital light processing (DLP) printing is a simple yet effective way to realize the variation of material properties by tuning the grayscale value. However, there is a lack of available design methods for grayscale DLP 3D-printed structures due to the complexities arising from the voxel-level grayscale distribution, nonlinear material properties, and intricate structures. Inspired by the dexterous motions of natural organisms, a design and fabrication framework for grayscale DLP-printed soft robots is developed by combining a grayscale-dependent hyperelastic constitutive model and a voxel-based finite-element model. The constitutive model establishes the relationship between the projected grayscale value and the nonlinear mechanical properties, while the voxel-based finite-element model enables fast and efficient calculation of the mechanical performances with arbitrarily distributed material properties. A multiphysics modeling and experimental method is developed to validate the homogenization assumption of the degree of conversion (DoC) variation in a single voxel. The design framework is used to design structures with reduced stress concentration and programmable multimodal motions. This work paves the way for integrated design and fabrication of functional structures using grayscale DLP 3D printing.

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.

Digital Light Processing of 19F MRI-Traceable Gelatin-Based Biomaterial Inks towards Bone Tissue Regeneration

Gelatin-based photo-crosslinkable hydrogels are promising scaffold materials to serve regenerative medicine. They are widely applicable in additive manufacturing, which allows for the production of various scaffold microarchitectures in line with the anatomical requirements of the organ to be replaced or tissue defect to be treated. Upon their in vivo utilization, the main bottleneck is to monitor cell colonization along with their degradation (rate). In order to enable non-invasive visualization, labeling with MRI-active components like N-(2,2-difluoroethyl)acrylamide (DFEA) provides a promising approach. Herein, we report on the development of a gelatin-methacryloyl-aminoethyl-methacrylate-based biomaterial ink in combination with DFEA, applicable in digital light processing-based additive manufacturing towards bone tissue regeneration. The fabricated hydrogel constructs show excellent shape fidelity in line with the printing resolution, as DFEA acts as a small molecular crosslinker in the system. The constructs exhibit high stiffness (E = 36.9 ± 4.1 kPa, evaluated via oscillatory rheology), suitable to serve bone regeneration and excellent MRI visualization capacity. Moreover, in combination with adipose tissue-derived stem cells (ASCs), the 3D-printed constructs show biocompatibility, and upon 4 weeks of culture, the ASCs express the osteogenic differentiation marker Ca2+.

Janus structure hydrogels: recent advances in synthetic strategies, biomedical microstructure and (bio)applications

Janus structure hydrogels (JSHs) are novel materials. Their primary fabrication methods and various applications have been widely reported. JSHs are primarily composed of Janus particles (JNPs) and polysaccharide components. They exhibit two distinct physical or chemical properties, generating intriguing characteristics due to their asymmetric structure. Normally, one side (adhesive interface) is predominantly constituted of polysaccharide components, primarily serving excellent adhesion. On the other side (functional surface), they integrate diverse functionalities, concurrently performing a plethora of synergistic functions. In the biomedical field, JSHs are widely applied in anti-adhesion, drug delivery, wound healing, and other areas. It also exhibits functions in seawater desalination and motion sensing. Thus, JSHs hold broad prospects for applications, and they possess significant research value in nanotechnology, environmental science, healthcare, and other fields. Additionally, this article proposes the challenges and future work facing these fields.

Programming Motion into Materials Using Electricity-Driven Liquid Crystal Elastomer Actuators

As thermally driven smart materials capable of large reversible deformations, liquid crystal elastomers (LCEs) have great potential for applications in bionic soft robots, artificial muscles, controllable actuators, and flexible sensors due to their ability to program controllable motion into materials. In this article, we introduce conductive LCE actuators using a liquid metal electrothermal layer and a polyethylene terephthalate substrate. Our LCE actuators can be stimulated at low currents from 2 to 4 A and produce a maximum work density of 9.4 k J ∕ m 3 . We illustrate the potential applications of this system by designing a palm-activated artificial muscle gripper, which can be used to grasp soft objects ranging from 5 to 55 mm in size, and even ring-shaped workpieces with precise external or internal support. Furthermore, inspired by the movement of fruit fly larvae, we designed a new soft robot capable of bioinspired crawling and turning by inducing anisotropic friction with an asymmetric design. Finally, we illustrate advanced motional control by designing an autonomously rotating wheel based on the asymmetric contraction of its spokes. To assist in the production of autonomously moving robots, we provide a thorough characterization of its motion dynamics.

Pinecone-Inspired Humidity-Responsive Paper Actuators with Bilayer Structure

Many plant materials in nature have the ability to change their shape to respond to external stimuli, such as humidity or moisture, to ensure their survival or safe seed release. A well-known example for this phenomenon is the pinecone, which is able to open its scales at low humidity due to the specific bilayer structures of the scale. Inspired by this, we developed a novel humidity-driven actuator based on paper. This was realized by the lamination of untreated paper made from eucalyptus fibers to a paper-carboxymethyl cellulose (CMC) composite. As observed, the hygroexpansion of the composite can be easily controlled by the amount of CMC in the impregnated paper sheet, which, thus, controls the morphologic deformation of the paper bilayer. For a more detailed understanding of these novel paper soft robots, we also studied the dynamic water vapor adsorption, polymer distribution and hygroexpansion of the paper-polymer composites. Finally, we applied a geometrically nonlinear finite element model to predict the bending behavior of paper bilayers and compared the results to experimental data. From this, we conclude that due to the complexity of structure of the paper composite, a universal prediction of the hygromorphic behavior is not a trivial matter.

Effect of Porosity on the Colonization of Digital Light-Processed 3D Hydrogel Constructs toward the Development of a Functional Intestinal Model

With the rapid increase of the number of patients with gastrointestinal diseases in modern society, the need for the development of physiologically relevant in vitro intestinal models is key to improve the understanding of intestinal dysfunctions. This involves the development of a scaffold material exhibiting physiological stiffness and anatomical mimicry of the intestinal architecture. The current work focuses on evaluating the scaffold micromorphology of gelatin-methacryloyl-aminoethyl-methacrylate-based nonporous and porous intestinal 3D, intestine-like constructs, fabricated via digital light processing, on the cellular response. To this end, Caco-2 intestinal cells were utilized in combination with the constructs. Both porous and nonporous constructs promoted cell growth and differentiation toward enterocyte-like cells (VIL1, ALPI, SI, and OCLD expression showed via qPCR, ZO-1 via immunostaining). The porous constructs outperformed the nonporous ones regarding cell seeding efficiency and growth rate, confirmed by MTS assay, live/dead staining, and TEER measurements, due to the presence of surface roughness.

Untethered soft actuators for soft standalone robotics

Soft actuators produce the mechanical force needed for the functional movements of soft robots, but they suffer from critical drawbacks since previously reported soft actuators often rely on electrical wires or pneumatic tubes for the power supply, which would limit the potential usage of soft robots in various practical applications. In this article, we review the new types of untethered soft actuators that represent breakthroughs and discuss the future perspective of soft actuators. We discuss the functional materials and innovative strategies that gave rise to untethered soft actuators and deliver our perspective on challenges and opportunities for future-generation soft actuators.

An all-optical multidirectional mechano-sensor inspired by biologically mechano-sensitive hair sensilla

Mechano-sensitive hair-like sensilla (MSHS) have an ingenious and compact three-dimensional structure and have evolved widely in living organisms to perceive multidirectional mechanical signals. Nearly all MSHS are iontronic or electronic, including their biomimetic counterparts. Here, an all-optical mechano-sensor mimicking MSHS is prototyped and integrated based on a thin-walled glass microbubble as a flexible whispering-gallery-mode resonator. The minimalist integrated device has a good directionality of 32.31 dB in the radial plane of the micro-hair and can detect multidirectional displacements and forces as small as 70 nm and 0.9 μN, respectively. The device can also detect displacements and forces in the axial direction of the micro-hair as small as 2.29 nm and 3.65 μN, respectively, and perceive different vibrations. This mechano-sensor works well as a real-time, directional mechano-sensory whisker in a quadruped cat-type robot, showing its potential for innovative mechano-transduction, artificial perception, and robotics applications.

Viscoelastic Response in Hydrous Polymers: The Role of Hydrogen Bonds and Microstructure

Water responsive polymers represent a remarkable group of soft materials, acting as a laboratory for diverse water responsive physical phenomena and cutting-edge biology-electronics interfaces. We report on peculiarly distinctive viscoelastic behaviors of the biobased water responsive polymer cellulose 10-undecenoyl ester, while biobased regenerated cellulose displays stronger hydroplastic behaviors. We discovered a novel hydrous deformation mechanism involving the stretching of hydrogen bonds mediated by hydroxyl groups and water molecules, serving as a crucial factor in accommodating deformations. In parallel, the microstructure of cellulose 10-undecenoyl ester with unique coexisting nanoparticles and a continuous phase of entangled chains is mechanically resilient in the anhydrous state but enhances structural stiffness in the hydrous state. This variation arises from a different hydration level within the hydrous microstructure. Such a fundamental discovery offers valuable insights into the connection between the microscopic physical properties that can be influenced by water and the corresponding viscoelastic responses, extending its applicability to a wide range of hygroscopic materials.

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.

Harvesting Energy from Atmospheric Water: Grand Challenges in Continuous Electricity Generation

Atmospheric water is ubiquitous on earth and extensively participates in the natural water cycle through evaporation and condensation. This process involves tremendous energy exchange with the environment, but very little of the energy has so far been harnessed. The recently emerged hydrovoltaic technology, especially moisture-induced electricity, shows great potential in harvesting energy from atmospheric water and gives birth to moisture energy harvesting devices. The device performance, especially the long-term operational capacity, has been significantly enhanced over the past few years. Further development; however, requires in-depth understanding of mechanisms, innovative materials, and ingenious system designs. In this review, beginning with describing the basic properties of water, the key aspects of the water-hygroscopic material interactions and mechanisms of power generation are discussed. The current material systems and advances in promising material development are then summarized. Aiming at the chief bottlenecks of limited operational time, advanced system designs that are helpful to improve device performance are listed. Especially, the synergistic effect of moisture adsorption and water evaporation on material and system levels to accomplish sustained electricity generation is discussed. Last, the remaining challenges are analyzed and future directions for developing this promising technology are suggested.

Static micromechanical measurements of the flexural modulus and strength of micrometre-diameter single fibres using deflecting microcantilever techniques

The mechanical properties of natural and man-made fibres ultimately govern the robustness of products. Examples range from textiles to composite materials for mechanical parts in emerging technological applications. An accurate determination of the mechanical properties of microscopic single fibres is therefore important. Today, macroscopic mechanical techniques, such as tensile testing, are commonly employed to obtain this information. However, a relatively high dispersion of results is often encountered due to a relatively long sample size. As an alternative to tensile methods, we demonstrate here micromechanical techniques to accurately measure the flexural modulus and strength of micrometre-sized diameter fibres without the need of force sensing. To demonstrate our ideas, we use the example of single natural fibres (Linum Usitatissimum). The flexural modulus of the single fibres is first accurately measured in the low deflection regime of an inclined bending cantilever in an original setup. The flexural strength of the single fibres is then measured in the high deflection regime of a bending cantilever. Interestingly, the novel measurements have allowed the authors to quantify the flexural strength of two different failure modes in flax fibre, enabling a contribution to plant mechanics.

Liquid Crystal Elastomer Artificial Tendrils with Asymmetric Core-Sheath Structure Showing Evolutionary Biomimetic Locomotion

The sophisticated and complex haptonastic movements in response to environmental-stimuli of living organisms have always fascinated scientists. However, how to fundamentally mimic the sophisticated hierarchical architectures of living organisms to provide the artificial counterparts with similar or even beyond-natural functions based on the underlying mechanism remains a major scientific challenge. Here,  liquid crystal elastomer (LCE) artificial tendrils showing evolutionary biomimetic locomotion are developed following the structure-function principle that is used in nature to grow climbing plants. These elaborately designed tendril-like LCE actuators possess an asymmetric core-sheath architecture which shows a higher-to-lower transition in the degree of LC orientation from the sheath-to-core layer across the semi-ellipse cross-section. Upon heating and cooling, the LCE artificial tendril can undergo reversible tendril-like shape-morphing behaviors, such as helical coiling/winding, and perversion. The fundamental mechanism of the helical shape-morphing of the artificial tendril is revealed by using theoretical models and finite element simulations. Besides, the incorporation of metal-ligand coordination into the LCE network provides the artificial tendril with reconfigurable shape-morphing performances such as helical transitions and rotational deformations. Finally, the abilities of helical and rotational deformations are integrated into a new reprogrammed flagellum-like architecture to perform evolutionary locomotion mimicking the haptonastic movements of the natural flagellum.

A dual-functional 2D coordination polymer exhibiting photomechanical and electrically conductive behaviours

A photoactive two-dimensional coordination polymer (2D CP) [Zn2(4-spy)2(bdc)2]n (1) [4-spy = 4-styrylpyridine and H2bdc = 1,4-benzendicarboxylic acid] undergoes a photochemical [2 + 2] cycloaddition reaction upon UV irradiation. Interestingly, the crystals of 1 show different photomechanical effects, such as jumping, swelling, and splitting, during UV irradiation. In addition, the CP was employed for conductivity measurements before and after UV irradiation via current density-voltage characteristics and impedance spectroscopy, which suggest that they are semiconducting in nature and can be used as Schottky diodes. Thus, this work demonstrates the potential dual applications of a 2D CP based on photosalient and conductivity properties.

Single-Crystal-to-Single-Crystal Topochemical Synthesis of a Collagen-inspired Covalent Helical Polymer

There is much demand for crystalline covalent helical polymers. Inspired by the helical structure of collagen, we synthesized a covalent helical polymer wherein the repeating dipeptide Gly-Pro units are connected by triazole linkages. We synthesized an azide and alkyne-modified dipeptide monomer made up of the repeating amino acid sequence of collagen. In its crystals, the monomer molecules aligned in head-to-tail fashion with proximally placed azide and alkyne forming supramolecular helices. At 60 °C, the monomer underwent single-crystal-to-single-crystal (SCSC) topochemical azide-alkyne cycloaddition polymerization, yielding a covalent helical polymer as confirmed by single-crystal X-ray diffraction (SCXRD) analysis. Compared to the monomer crystals, the polymer single-crystals were very strong and showed three-fold increase in Young's modulus, which is higher than collagen, many synthetic polymers and other materials. The crystals of this covalent helical polymer could bear loads as high as 1.5 million times of their own weight without deformation. These crystals could also withstand high compression force and did not disintegrate even at an applied force of 98 kN. Such light-weight strong materials are in demand for various technological applications.

Surface Segregation Methods toward Molecular Separation Membranes

As a highly promising approach to solving the issues of energy and environment, membrane technology has gained increasing attention in various fields including water treatment, liquid separations, and gas separations, owing to its high energy efficiency and eco-friendliness. Surface segregation, a phenomenon widely found in nature, exhibits irreplaceable advantages in membrane fabrication since it is an in situ method for synchronous modification of membrane and pore surfaces during the membrane forming process. Meanwhile, combined with the development of synthesis chemistry and nanomaterial, the group has developed surface segregation as a versatile membrane fabrication method using diverse surface segregation agents. In this review, the recent breakthroughs in surface segregation methods and their applications in membrane fabrication are first briefly introduced. Then, the surface segregation phenomena and the classification of surface segregation agents are discussed. As the major part of this review, the authors focus on surface segregation methods including free surface segregation, forced surface segregation, synergistic surface segregation, and reaction-enhanced surface segregation. The strategies for regulating the physical and chemical microenvironments of membrane and pore surfaces through the surface segregation method are emphasized. The representative applications of surface segregation membranes are presented. Finally, the current challenges and future perspectives are highlighted.

Ions-Silica Percolated Ionic Dielectric Elastomer Actuator for Soft Robots

Soft robotics systems are currently under development using ionic electroactive polymers (i-EAP) as soft actuators for the human-machine interface. However, this endeavor has been impeded by the dilemma of reconciling the competing demands of force and strain in i-EAP actuators. Here, the authors present a novel design called "ions-silica percolated ionic dielectric elastomer (i-SPIDER)", which exhibits ionic liquid-confined silica microstructures that effectively resolve the chronic issue of conventional i-EAP actuators. The i-SPIDER actuator demonstrates remarkable electromechanical conversion capacity at low voltage, thanks to improved ion accumulation facilitated by interpreting electrode polarization at the electrolyte-electrode interface. This approach concurrently enhances both strain (by approximately 1.52%) and force (by roughly 1.06 mN) even at low Young's modulus (merely 5.9 MPa). Additionally, by demonstrating arachnid-inspired soft robots endowed with user-desired tasks through control of various form factors, the development of soft robots using the i-SPIDER that can concomitantly enhance strain and force holds promise as a compelling avenue for ushering in the next generation of miniaturized, low-powered soft robotics.

Polydopamine-Modified MXene-Integrated Poly(N-isopropylacrylamide) to Construct Ultrafast Photoresponsive Bilayer Hydrogel Actuators with Smart Adhesion

In nature, living organisms, such as octopuses, cabrito, and frogs, have already evolved admirable adhesive abilities for better movement and predation in response to the surroundings. Inspired by biological structures, researchers have made enormous efforts in developing actuators that can respond to external stimuli, while such adhesive property is very desired, yet there is still limited research in responsive hydrogel actuators. Here, a bilayer actuator with high stretchability and robust interface bonding is presented, which has a smart adhesion and thermoreception function. The system consists of an adhesive passive layer copolymerized of amphoteric ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl), SBMA) and acrylic acid (AA), and an active layer hydrogel composed of poly(N-isopropylacrylamide) (PNIPAm) containing polydopamine-modified MXene (P-MXene) and calcium chloride (CaCl2). The coordination of carboxylate and Ca2+ at the interface of the two layers enhances the interfacial bonding from 14 to 30 N m-1, which facilitates withstanding large strain and preventing stratification. The resulting hydrogel actuator can bend approximately 360° in a mere 10 s, exhibiting excellent photothermal effect, a large angle bending deformation, and ultrafast photoresponsive ability. As a proof of concept, the photothermal actuators are programmed to present various shapes and grab objects. Importantly, the hydrogel actuator exhibits remarkable adhesion capabilities toward diverse substrates, with a maximum peel force of up to 280 N m-1. Relying on their own adhesion and the photoresponse properties, these flexible adhesion actuators show outstanding gripping capability, enabling them to grip and release objects of different shapes and weights. More interestingly, the hydrogel exhibits a smart adjustable adhesion capability at different temperatures, which enables it as a gripper to recognize temperature signals through real-time different feedback actions based on its own adhesion. This study presents innovative insights into biomimetic hydrogel actuators, providing new opportunities for developing intelligent soft robots with multiple functions.

Naked-Eye Thiol Analyte Detection via Self-Propagating, Amplified Reaction Cycle

We present an approach for detecting thiol analytes through a self-propagating amplification cycle that triggers the macroscopic degradation of a hydrogel scaffold. The amplification system consists of an allylic phosphonium salt that upon reaction with the thiol analyte releases a phosphine, which reduces a disulfide to form two thiols, closing the cycle and ultimately resulting in exponential amplification of the thiol input. When integrated in a disulfide cross-linked hydrogel, the amplification process leads to physical degradation of the hydrogel in response to thiol analytes. We developed a numerical model to predict the behavior of the amplification cycle in response to varying concentrations of thiol triggers and validated it with experimental data. Using this system, we were able to detect multiple thiol analytes, including a small molecule probe, glutathione, DNA, and a protein, at concentrations ranging from 132 to 0.132 μM. In addition, we discovered that the self-propagating amplification cycle could be initiated by force-generated molecular scission, enabling damage-triggered hydrogel destruction.

Light-Based 3D Printing of Gelatin-Based Biomaterial Inks to Create a Physiologically Relevant In Vitro Fish Intestinal Model

To provide prominent accessibility of fishmeal to the European population, the currently available, time- and cost-extensive feeding trials, which evaluate fish feed, should be replaced. The current paper reports on the development of a novel 3D culture platform, mimicking the microenvironment of the intestinal mucosa in vitro. The key requirements of the model include sufficient permeability for nutrients and medium-size marker molecules (equilibrium within 24 h), suitable mechanical properties (G' < 10 kPa), and close morphological similarity to the intestinal architecture. To enable processability with light-based 3D printing, a gelatin-methacryloyl-aminoethyl-methacrylate-based biomaterial ink is developed and combined with Tween 20 as porogen to ensure sufficient permeability. To assess the permeability properties of the hydrogels, a static diffusion setup is utilized, indicating that the hydrogel constructs are permeable for a medium size marker molecule (FITC-dextran 4 kg mol-1 ). Moreover, the mechanical evaluation through rheology evidence a physiologically relevant scaffold stiffness (G' = 4.83 ± 0.78 kPa). Digital light processing-based 3D printing of porogen-containing hydrogels results in the creation of constructs exhibiting a physiologically relevant microarchitecture as evidenced through cryo-scanning electron microscopy. Finally, the combination of the scaffolds with a novel rainbow trout (Oncorhynchus mykiss) intestinal epithelial cell line (RTdi-MI) evidence scaffold biocompatibility.

Biomimetic Silk Architectures Outperform Animal Horns in Strength and Toughness

Structural biomimicry is an intelligent approach for developing lightweight, strong, and tough materials (LSTMs). Current fabrication technologies, such as 3D printing and two-photon lithography often face challenges in constructing complex interlaced structures, such as the sinusoidal crossed herringbone structure that contributes to the ultrahigh strength and fracture toughness of the dactyl club of peacock mantis shrimps. Herein, bioinspired LSTMs with laminated or herringbone structures is reported, by combining textile processing and silk fiber "welding" techniques. The resulting biomimetic silk LSTMs (BS-LSTMs) exhibit a remarkable combination of lightweight with a density of 0.6-0.9 g cm-3 , while also being 1.5 times stronger and 16 times more durable than animal horns. These findings demonstrate that BS-LSTMs are among the toughest natural materials made from silk proteins. Finite element simulations further reveal that the fortification and hardening of BS-LSTMs arise primarily from the hierarchical organization of silk fibers and mechanically transferable meso-interfaces. This study highlights the rational, cost-effective, controllable mesostructure, and transferable strategy of integrating textile processing and fiber "welding" techniques for the fabrication of BS-LSTMs with advantageous structural and mechanical properties. These findings have significant implications for a wide range of applications in biomedicine, mechanical engineering, intelligent textiles, aerospace industries, and beyond.

Plant-inspired rearrangement of liquid in a porous structure for controlled swelling

Soft robots can adapt to dynamic environments without prior knowledge of their properties. Plants inspire mechanisms for counterbalancing dynamic loads by locally modulating compliance through anisotropic humidity-responsive materials and structures. In addition to well-known passive bilayers, plants may also actively control swelling. The combination of robust hygroscopic material-level response and simple electrical control makes active swelling particularly attractive for technological implementation. However, dynamic swelling demands the development and optimisation of congruent pumping solutions. This work suggests electrohydrodynamic pumping, enabled by highly reversible ion immobilisation at capacitive electrodes, as a particularly suitable low-pressure, high-area liquid displacement solution for active swelling. Local pore fill ratio (PFR) modulation is used as a measure for dynamic liquid displacement and swelling. A method for highly localised (10μm membrane thickness) assessment of the dynamic variation of PFR in a 400μm laminate undergoing cross-plane electrokinetic liquid displacement is developed. Two modes for transient PFR modulation were identified: electrokinetic ion transfer and diffusive solvent redistribution, pronounced at high and low voltage scan rates, respectively. The strategic combination of these modes enables various compliance-modulation scenarios. The system contains (within a cycle) a constant amount of liquid in an open network of liquid-filled pores. 30%-75% PFR yielded the highest dynamic PFR modulation: a high amount of empty pores is beneficial, yet a too-low PFR compromises the continuous liquid pathway necessary for electrokinetic pumping. The dynamic nature of internal liquid rearrangement was characterised by relatively fast electrokinetics-driven fluxes (6.3% PFR change in 80 s), followed by a slow equilibration of concentration and PFR. At high scan rates, PFR decreased at positive polarisation, while both positive and negative polarity yielded a similar decrease at low scan rates (5 mV s-1). Localised control over the swelling gradient enables the design of systems that morphologically adapt to complex dynamic loading conditions.

A Programmable Actuator as Synthetic Earthworm

Natural earthworm with the ability to loosen soils that favors sustainable agriculture has inspired worldwide interest in the design of intelligent actuators. Given the inability to carry heavy loads and uncontrolled deformation, the vast majority of actuators can only perform simple tasks by bending, contraction, or elongation. Herein, a degradable actuator with the ability to deform in desired ways is presented, which successfully mimics the burrowing activities of earthworms to loosen soils with increased soil porosity by digging, grabbing, and lifting the soil when it receives rains. Such a scarifying actuator is made of degradable cellulose acetate and uncrosslinked polyacrylamide via the swelling-photopolymerizing method. The water absorption of polyacrylamide in moisture conditions causes rapid and remarkable bending. Such mechanical bending can be controlled in specific areas of the cellulose acetate film if polyacrylamide is polymerized in a patterned way, so as to generate complicated deformations of the whole cellulose acetate. Patterning polyacrylamide within cellulose acetate is achieved based on reversible surface protection by means of pen writing, rather than the traditional masking techniques. The water-induced deformation of programmable cellulose-based actuators is well preserved in soil, which is appropriate for promoting rain diffusion as well as root breath.

Construction of Wearable Touch Sensors by Mimicking the Properties of Materials and Structures in Nature

Wearable touch sensors, which can convert force or pressure signals into quantitative electronic signals, have emerged as essential smart sensing devices and play an important role in various cutting-edge fields, including wearable health monitoring, soft robots, electronic skin, artificial prosthetics, AR/VR, and the Internet of Things. Flexible touch sensors have made significant advancements, while the construction of novel touch sensors by mimicking the unique properties of biological materials and biogenetic structures always remains a hot research topic and significant technological pathway. This review provides a comprehensive summary of the research status of wearable touch sensors constructed by imitating the material and structural characteristics in nature and summarizes the scientific challenges and development tendencies of this aspect. First, the research status for constructing flexible touch sensors based on biomimetic materials is summarized, including hydrogel materials, self-healing materials, and other bio-inspired or biomimetic materials with extraordinary properties. Then, the design and fabrication of flexible touch sensors based on bionic structures for performance enhancement are fully discussed. These bionic structures include special structures in plants, special structures in insects/animals, and special structures in the human body. Moreover, a summary of the current issues and future prospects for developing wearable sensors based on bio-inspired materials and structures is discussed.

Bioinspired humidity-responsive liquid crystalline materials: from adaptive soft actuators to visualized sensors and detectors

Inspired by nature, humidity-responsive materials and devices have attracted significant interest from scientists in multiple disciplines, ranging from chemistry, physics and materials science to biomimetics. Owing to their superiorities, including harmless stimulus and untethered control, humidity-driven materials have been widely investigated for application in soft robots, smart sensors and detectors, biomimetic devices and anticounterfeiting labels. Especially, humidity-responsive liquid crystalline materials are particularly appealing due to the combination of programmable and adaptive liquid crystal matrix and humidity-controllability, enabling the fabrication of advanced self-adaptive robots and visualized sensors. In this review, we summarize the recent progress in humidity-driven liquid crystalline materials. First, a brief introduction of liquid crystal materials, including liquid crystalline polymers, cholesteric liquid crystals, blue-phase liquid crystals and cholesteric cellulose nanocrystals is provided. Subsequently, the mechanisms of humidity-responsiveness are presented, followed by the diverse strategies for the fabrication of humidity-responsive liquid crystalline materials. The applications of humidity-driven devices will be presented ranging from soft actuators to visualized sensors and detectors. Finally, we provide an outlook on the development of humidity-driven liquid crystalline materials.

Hydration solids

Hygroscopic biological matter in plants, fungi and bacteria make up a large fraction of Earth's biomass1. Although metabolically inert, these water-responsive materials exchange water with the environment and actuate movement2-5 and have inspired technological uses6,7. Despite the variety in chemical composition, hygroscopic biological materials across multiple kingdoms of life exhibit similar mechanical behaviours including changes in size and stiffness with relative humidity8-13. Here we report atomic force microscopy measurements on the hygroscopic spores14,15 of a common soil bacterium and develop a theory that captures the observed equilibrium, non-equilibrium and water-responsive mechanical behaviours, finding that these are controlled by the hydration force16-18. Our theory based on the hydration force explains an extreme slowdown of water transport and successfully predicts a strong nonlinear elasticity and a transition in mechanical properties that differs from glassy and poroelastic behaviours. These results indicate that water not only endows biological matter with fluidity but also can-through the hydration force-control macroscopic properties and give rise to a 'hydration solid' with unusual properties. A large fraction of biological matter could belong to this distinct class of solid matter.

Bioinspired Tactile Sensation Based on Synergistic Microcrack-Bristle Structure Design toward High Mechanical Sensitivity and Direction-Resolving Capability

Natural tactile sensation is complex, which involves not only contact force intensity detection but also the perception of the force direction, the surface texture, and other mechanical parameters. Nevertheless, the vast majority of the developed tactile sensors can only detect the normal force, but usually cannot resolve shear force or even distinguish the directions of the force. Here, we present a new paradigm of bioinspired tactile sensors for resolving both the intensity and the directions of mechanical stimulations via synergistic microcrack-bristle structure design and cross-shaped configuration engineering. The microcrack sensing structure gives high mechanical sensitivity to the tactile sensors, and the synergistic bristle structure further amplifies the sensitivity of the sensors. The cross-shaped configuration engineering of the synergistic microcrack-bristle structure further endows the tactile sensors with good capability to detect and distinguish the directions of the applied mechanical forces. The as-fabricated tactile sensors exhibit a high sensitivity (25.76 N^-1), low detection limit (5.4 mN), desirable stability (over 2,500 cycles), and good capability to resolve both mechanical intensity and directional features. As promising application scenarios, surface texture recognition and biomimetic path explorations are successfully demonstrated with these tactile sensors. This newly proposed tactile sensation strategy and technology have great potential applications in ingenious tactile sensation and construction of various robotic and bionic prostheses with high operational dexterity.

Biomimetic Superhydrophobic Materials through 3D Printing: Progress and Challenges

Superhydrophobicity, a unique natural phenomenon observed in organisms such as lotus leaves and desert beetles, has inspired extensive research on biomimetic materials. Two main superhydrophobic effects have been identified: the "lotus leaf effect" and the "rose petal effect", both showing water contact angles larger than 150°, but with differing contact angle hysteresis values. In recent years, numerous strategies have been developed to fabricate superhydrophobic materials, among which 3D printing has garnered significant attention due to its rapid, low-cost, and precise construction of complex materials in a facile way. In this minireview, we provide a comprehensive overview of biomimetic superhydrophobic materials fabricated through 3D printing, focusing on wetting regimes, fabrication techniques, including printing of diverse micro/nanostructures, post-modification, and bulk material printing, and applications ranging from liquid manipulation and oil/water separation to drag reduction. Additionally, we discuss the challenges and future research directions in this burgeoning field.

Hygrothermic Wood Actuated Robotic Hand

Stimulus-responsive actuators play a vital role in the new generation of intelligent systems. However, poor mechanical performance, complicated fabrication processes, and the inability to complex deformation limit their practical applications. Herein, these challenges are overcome via designing a strong hygrothermic wood actuator with asymmetric water affinity. The actuator is readily constructed by sandwiching polypyrrole-coated wood with a Ni complex hygroscopic gel top layer for moisture absorption and a polyimide bottom layer as the water barrier. The resulting hygrothermic wood spontaneously stretches and bends itself in response to moisture and thermal/light stimulation. A robotic hand and a series of grippers made of hygrothermic wood demonstrate dexterous object-hand interactions during grasping and holding, while the reversible hygrothermic property allows the actuator to be potentially applied in fire rescue scenarios to rescue trapped objects. A combination of good mechanical properties, multi-stimulus-response, complex deformation, wide working temperature range, low manufacturing cost, and biocompatibility are simultaneously realized by one device. It is thus believed that such a strong wood actuator will open up a new avenue for building intelligent robotic hand systems.

Soft Functionally Gradient Materials and Structures - Natural and Manmade: A Review

Functionally gradient materials (FGM) have gradual variations in their properties along one or more dimensions due to local compositional or structural distinctions by design. Traditionally, hard materials (e.g., metals, ceramics) are used to design and fabricate FGMs; however, there is increasing interest in polymer-based soft and compliant FGMs mainly because of their potential application in the human environment. Soft FGMs are ideally suitable to manage interfacial problems in dissimilar materials used in many emerging devices and systems for human interaction, such as soft robotics and electronic textiles and beyond. Soft systems are ubiquitous in everyday lives; they are resilient and can easily deform, absorb energy, and adapt to changing environments. Here, the basic design and functional principles of biological FGMs and their manmade counterparts are discussed using representative examples. The remarkable multifunctional properties of natural FGMs resulting from their sophisticated hierarchical structures, built from a relatively limited choice of materials, offer a rich source of new design paradigms and manufacturing strategies for manmade materials and systems for emerging technological needs. Finally, the challenges and potential pathways are highlighted to leverage soft materials' facile processability and unique properties toward functional FGMs.

Engineered Living Materials For Sustainability

Recent advances in synthetic biology and materials science have given rise to a new form of materials, namely engineered living materials (ELMs), which are composed of living matter or cell communities embedded in self-regenerating matrices of their own or artificial scaffolds. Like natural materials such as bone, wood, and skin, ELMs, which possess the functional capabilities of living organisms, can grow, self-organize, and self-repair when needed. They also spontaneously perform programmed biological functions upon sensing external cues. Currently, ELMs show promise for green energy production, bioremediation, disease treatment, and fabricating advanced smart materials. This review first introduces the dynamic features of natural living systems and their potential for developing novel materials. We then summarize the recent research progress on living materials and emerging design strategies from both synthetic biology and materials science perspectives. Finally, we discuss the positive impacts of living materials on promoting sustainability and key future research directions.

Multifunctional Dual Cross-Linked Bioadhesive Patch with Low Immunogenic Response and Wet Tissues Adhesion

The development of bioadhesives is an important, yet challenging task as seemingly mutually exclusive properties need to be combined in one material, that is, strong adhesion, water resistance, and high biocompatibility. Here, a biocompatible and biodegradable protein-based bioadhesive patch (PBP) with high adhesion strength and low immunogenic response is reported. PBP exists as a strong adhesion for biological surfaces, which is higher than some conventional bioadhesives (i.e., polyethylene glycol and fibrin). Robust adhesion and strength are realized through the removal of interfacial water and fast formation of multiple supramolecular interactions induced by metal ions. The PBP's high biocompatibility is evaluated and immunogenic response in vitro and in vivo is neglected. The strong adhesion on soft biological tissues qualifies the PBP as biomedical glue outperforming some commercial products for applications in hemostasis performance, accelerated wound healing, and sealing of defected organs, anticipating to be useful as a tissue adhesive and sealant.

Recent Progress on Hydrogel-Based Piezoelectric Devices for Biomedical Applications

Flexible electronics have great potential in the application of wearable and implantable devices. Through suitable chemical alteration, hydrogels, which are three-dimensional polymeric networks, demonstrate amazing stretchability and flexibility. Hydrogel-based electronics have been widely used in wearable sensing devices because of their biomimetic structure, biocompatibility, and stimuli-responsive electrical properties. Recently, hydrogel-based piezoelectric devices have attracted intensive attention because of the combination of their unique piezoelectric performance and conductive hydrogel configuration. This mini review is to give a summary of this exciting topic with a new insight into the design and strategy of hydrogel-based piezoelectric devices. We first briefly review the representative synthesis methods and strategies of hydrogels. Subsequently, this review provides several promising biomedical applications, such as bio-signal sensing, energy harvesting, wound healing, and ultrasonic stimulation. In the end, we also provide a personal perspective on the future strategies and address the remaining challenges on hydrogel-based piezoelectric electronics.

Photomechanical Motions in Organoboron-Based Phosphorescent Molecular Crystals Driven by a Crystal-State [2 + 2] Cycloaddition Reaction

Photoluminescent molecular crystals integrated with the ability to transform light energy into macroscopic mechanical motions are a promising choice of materials for both actuating and photonic devices. However, such dynamic photomechanical effects, based on molecular organoboron compounds as well as phosphorescent crystalline materials, are not yet known. Here we present an intriguing example of photomechanical molecular single crystals of a newly synthesized organoboron containing Lewis acid-base molecular adduct (BN1, substituted triphenylboroxine and 1,2-di(4-pyridyl)ethylene) having a capsule shape molecular geometry. The single crystals of BN1 under UV light exhibit controllable rapid bending-shape recovery, delamination, violent splitting-jumping, and expanding features. The detailed structural investigation by single-crystal X-ray diffraction and ¹H NMR spectroscopy reveals that the photosalient behavior of the BN1 single crystals is driven by a crystal-to-crystal [2 + 2] cycloaddition reaction, supported by four donor-acceptor type B←N bonds. The instant photomechanical reaction in the BN1 crystals occurs under UV on account of sudden release of stress associated with the strained molecular geometry, significant solid-state molecular movements (supramolecular change), and cleavage of half intermolecular B←N linkages to result in a complete photodimerized single-crystalline product via the existence of two other intermediate photoproducts. In addition, the BN1 crystals display short-lived room temperature phosphorescence, and the photodynamic events are accompanied by the enhancement of their phosphorescence intensity to yield the photoproduct. Interestingly, the molecular crystals of the final photoproduct polymerize at ambient conditions when recrystallized from the solution forming a 2D supramolecular crystalline polymer stabilized by the retention of all B←N coordination modes.

Unperceivable motion mimicking hygroscopic geometric reshaping of pine cones

The hygroscopic deformation of pine cones, featured by opening and closing their scales depending on the environmental humidity, is a well-known stimuli-responsive model system for artificial actuators. However, it has not been noted that the deformation of pine cones is an ultra-slow process. Here, we reveal that vascular bundles with unique parallelly arranged spring/square microtubular heterostructures dominate the hygroscopic movement, characterized as ultra-slow motion with the outer sclereids. The spring microtubes give a much larger hygroscopic deformation than that of the square microtubes along the longitudinal axis direction, which bends the vascular bundles and consequently drives the scales to move. The outer sclereids with good water retention enable the vascular-bundle-triggered deformation to proceed ultra-slowly. Drawing inspiration, we developed soft actuators enabling controllable yet unperceivable motion. The motion velocity is almost two orders of magnitude lower than that of the same-class actuators reported, which made the as-developed soft actuators applicable in camouflage and reconnaissance.

An ultrasensitive and stretchable strain sensor based on a microcrack structure for motion monitoring

Flexible strain sensors are promising candidates for intelligent wearable devices. Among previous studies, although crack-based sensors have attracted a lot of attention due to their ultrahigh sensitivity, large strain usually causes fractures in the conductive paths. Because of the unstable crack structure, the tradeoff between sensitivity and workable strain range is still a challenge. As carbon nanotubes (CNTs) and silver nanowires (AgNWs) can form a strong interface with the thermoplastic substrate and strengthen the conductive network by capillary force during water evaporation, CNTs and AgNWs were deposited on electrospun TPU fiber mats via vacuum-assisted filtration in this work. The prestretching treatment constructed a microcrack structure that endowed the sensor with the combined characteristics of a wide working range (0~171% strain), ultrahigh sensitivity (a gauge factor of 691 within 0~102% strain, ~2 × 10⁴ within 102~135% strain, and >11 × 10⁴ within 135~171% strain), a fast response time (~65 ms), small hysteresis, and superior durability (>2000 cycles). Subsequently, the sensing mechanism of the sensor was studied. Distributed microcrack propagation based on the "island-bridge" structure was explained in detail, and its influence on the strain-sensing behavior of the sensor was analyzed. Finally, the sensor was assembled to monitor various vibration signals and human motions, demonstrating its potential applications in the fields of electronic skin and human health monitoring.

Bioresorbable Bilayered Elastomer/Hydrogel Constructs with Gradual Interfaces for the Fast Actuation of Self-Rolling Tubes

In the biomedical field, self-rolling materials provide interesting opportunities to develop medical devices suitable for drug or cell encapsulation. However, to date, a major limitation for medical applications is the use of non-biodegradable and non-biocompatible polymers that are often reported for such applications or the slow actuation witnessed with degradable systems. In this work, biodegradable self-rolling tubes that exhibit a spontaneous and rapid actuation when immersed in water are designed. Photo-crosslinkable hydrophilic and hydrophobic poly(ethylene glycol)-poly(lactide) (PEG-PLA) star-shaped copolymers are prepared and used to prepare bilayered constructs. Thanks to the discrete mechanical and swelling properties of each layer and the cohesive/gradual nature of the interface, the resulting bilayered films are able to self-roll in water in less than 30 s depending on the nature of the hydrophilic layer and on the shape of the sample. The cytocompatibility and degradability of the materials are demonstrated and confirm the potential of such self-rolling resorbable biomaterials in the field of temporary medical devices.

Flexible Equivalent Strain Sensor with Ordered Concentric Circular Curved Cracks Inspired by Scorpion

Slit sensillum, a unique sensing organ on the scorpion's legs, is composed of several cracks with curved shapes. In fact, it is just its particular morphological distribution and structure that endows the scorpions with ultrasensitive sensing capacity. Here, a scorpion-inspired flexible strain sensor with an ordered concentric circular curved crack array (CCA) was designed and fabricated by using an optimized solvent-induced and template transfer combined method. The morphology of the cracks can be effectively controlled by the heating temperature and the lasting time. Instead of the nonuniform stress distribution induced by disordered cracks, ordered concentric circle curved structures are introduced to generate a uniform stress distribution and larger deformation, which can significantly improve the performance of the strain sensor. Thus, the CCA sensor exhibits ultrahigh sensitivity (GF ∼ 7878.6), excellent stability (over 16 000 cycles), and fast response time (110 ms). Furthermore, the CCA sensor was demonstrated to be feasible for monitoring human motions and detecting noncontact vibration signals, indicating its great potential in human-health monitoring and vibration signal detection applications.

Stiffness Variable Polymers Comprising Phase-Changing Side-Chains: Material Syntheses and Application Explorations

Stiffness variable materials have been applied in a variety of engineering fields that require adaptation, automatic modulation, and morphing because of their unique property to switch between a rigid, load-bearing state and a soft, compliant state. Stiffness variable polymers comprising phase-changing side-chains (s-SVPs) have densely grafted, highly crystallizable long alkyl side-chains in a crosslinked network. Such a bottlebrush network-like structure gives rise to rigidity modulation as a result of the reversible crystallization and melting of the side chains. The corresponding modulus changes can be more than 1000-fold within a narrow temperature span, from ≈10²  MPa to ≈10²  kPa or lower. Other important properties of the s-SVP, such as stretchability, optical transmittance, and adhesion, can also be altered. This work reviews the underlying molecular mechanisms in the s-SVP's, discusses the material's structure-property relationship, and summarizes important applications explored so far, including reversible shape transformation, bistable electromechanical transduction, optical modulation, and reversible adhesion.

Recrystallization solvent-dependent elastic/plastic flexibility of an n-dodecyl-substituted tetrachlorophthalimide

A long alkyl-chained organic molecule, 4,5,6,7-tetrachloro-2-dodecylisoindoline-1,3-dione (1), was crystallized into needle-like crystals in dichloromethane (1DCM) or plate-like ones in tetrahydrofuran (1THF) depending on the recrystallisation solvent. X-ray crystallography analyses revealed the alkyl chains of the molecules, in which they were assembled differently, with the former responding flexibly bendable and elastic deformation, and the later being a permanent plastic one by external mechanical stress. The elastic modulus (E) and hardness (H) indicating both compliant and soft nature, reflecting their weak interaction in crystals, were quantified from the nano-indentation test.