Rotational multimaterial printing of filaments with subvoxel control.

Helical structures are ubiquitous in nature and impart unique mechanical properties and multifunctionality1. So far, synthetic architectures that mimic these natural systems have been fabricated by winding, twisting and braiding of individual filaments1-7, microfluidics8,9, self-shaping1,10-13 and printing methods14-17. However, those fabrication methods are unable to simultaneously create and pattern multimaterial, helically architected filaments with subvoxel control in arbitrary two-dimensional (2D) and three-dimensional (3D) motifs from a broad range of materials. Towards this goal, both multimaterial18-23 and rotational24 3D printing of architected filaments have recently been reported; however, the integration of these two capabilities has yet to be realized. Here we report a rotational multimaterial 3D printing (RM-3DP) platform that enables subvoxel control over the local orientation of azimuthally heterogeneous architected filaments. By continuously rotating a multimaterial nozzle with a controlled ratio of angular-to-translational velocity, we have created helical filaments with programmable helix angle, layer thickness and interfacial area between several materials within a given cylindrical voxel. Using this integrated method, we have fabricated functional artificial muscles composed of helical dielectric elastomer actuators with high fidelity and individually addressable conductive helical channels embedded within a dielectric elastomer matrix. We have also fabricated hierarchical lattices comprising architected helical struts containing stiff springs within a compliant matrix. Our additive-manufacturing platform opens new avenues to generating multifunctional architected matter in bioinspired motifs.

Bioinspired shape shifting of liquid-infused ribbed sheets.

The recent emergence of stimuli-responsive, shape-shifting materials offers promising applications in fields as different as soft robotics, aeronautics, or biomedical engineering. Targeted shapes or movements are achieved from the advantageous coupling between some stimulus and various materials such as liquid crystalline elastomers, magnetically responsive soft materials, swelling hydrogels, etc. However, despite the large variety of strategies, they are strongly material dependent and do not offer the possibility to choose between reversible and irreversible transformations. Here, we introduce a strategy applicable to a wide range of materials yielding systematically reversible or irreversible shape transformations of soft ribbed sheets with precise control over the local curvature. Our approach-inspired by the spore-releasing mechanism of the fern sporangium-relies on the capillary deformation of an architected elastic sheet impregnated by an evaporating liquid. We develop an analytical model combining sheet geometry, material stiffness, and capillary forces to rationalize the onset of such deformations and develop a geometric procedure to inverse program target shapes requiring fine control over the curvature gradient. We finally demonstrate the potential irreversibility of the transformation by UV-curing a photosensitive evaporating solution and show that the obtained shells exhibit enhanced mechanical stiffness.

A molecular interpretation of the toughness of multiple network elastomers at high temperature.

SignificanceSoft materials can be toughened by creating dissipative mechanisms in stretchy matrixes. Yet using them over a wide range of temperatures requires dissipative mechanisms independent of stretch rate or temperature. We show that sacrificial covalent bonds in multiple network elastomers are most useful in toughening elastomers at high temperature and act synergistically with viscoelasticity at lower temperature. We do not attribute this toughening mechanism only to the scission of bonds during crack propagation but propose that the highly stretched network diluted in a stretchy matrix acts by simultaneously stiffening the elastomer and delaying the localization of bond scission and the propagation of a crack. Such a toughening mechanism has never been proposed for elastomers and should guide network design.

Degradable biomedical elastomers: paving the future of tissue repair and regenerative medicine.

Degradable biomedical elastomers (DBE), characterized by controlled biodegradability, excellent biocompatibility, tailored elasticity, and favorable network design and processability, have become indispensable in tissue repair. This review critically examines the recent advances of biodegradable elastomers for tissue repair, focusing mainly on degradation mechanisms and evaluation, synthesis and crosslinking methods, microstructure design, processing techniques, and tissue repair applications. The review explores the material composition and cross-linking methods of elastomers used in tissue repair, addressing chemistry-related challenges and structural design considerations. In addition, this review focuses on the processing methods of two- and three-dimensional structures of elastomers, and systematically discusses the contribution of processing methods such as solvent casting, electrostatic spinning, and three-/four-dimensional printing of DBE. Furthermore, we describe recent advances in tissue repair using DBE, and include advances achieved in regenerating different tissues, including nerves, tendons, muscle, cardiac, and bone, highlighting their efficacy and versatility. The review concludes by discussing the current challenges in material selection, biodegradation, bioactivation, and manufacturing in tissue repair, and suggests future research directions. This concise yet comprehensive analysis aims to provide valuable insights and technical guidance for advances in DBE for tissue engineering.

Highly Stretchable, Self-Healing, and Sensitive E-Skins at -78 °C for Polar Exploration.

Ultralow temperature-tolerant electronic skins (e-skins) can endow polar robots with tactile feedback for exploring in extremely cold polar environments. However, it remains a challenge to develop e-skins that enable sensitive touch sensation and self-healing at ultralow temperatures. Herein, we describe the development of a sensitive robotic hand e-skin that can stretch, self-heal, and sense at temperatures as low as -78 °C. The elastomeric substrate of this e-skin is based on poly(dimethylsiloxane) supramolecular polymers and multistrength dynamic H-bonds, in particular with quadruple H-bonding motifs (UPy). The structure-performance relationship of the elastomer at ultralow temperatures is investigated. The results show that elastomers with side-chain UPy units exhibit higher stretchability (∼3257%) and self-healing efficiency compared to those with main-chain UPy units. This is attributed to the lower binding energy variation and lower potential well. Based on the elastomer with side-chain UPy and man-made electric ink, a sensitive robotic hand e-skin for usage at -78 °C is constructed to precisely sense the shape of objects and specific symbols, and its sensation can completely self-recover after being damaged. The findings of this study contribute to the concept of using robotic hands with e-skins in polar environments that make human involvement limited, dangerous, or impossible.

Wearable and Recyclable Water-Toleration Sensor Derived from Lipoic Acid.

Flexible wearable sensors recently have made significant progress in human motion detection and health monitoring. However, most sensors still face challenges in terms of single detection targets, single application environments, and non-recyclability. Lipoic acid (LA) shows a great application prospect in soft materials due to its unique properties. Herein, ionic conducting elastomers (ICEs) based on polymerizable deep eutectic solvents consisting of LA and choline chloride are prepared. In addition to the good mechanical strength, high transparency, ionic conductivity, and self-healing efficiency, the ICEs exhibit swelling-strengthening behavior and enhanced adhesion strength in underwater environments due to the moisture-induced association of poly(LA) hydrophobic chains, thus making it possible for underwater sensing applications, such as underwater communication. As a strain sensor, it exhibits highly sensitive strain response with repeatability and durability, enabling the monitoring of both large and fine human motions, including joint movements, facial expressions, and pulse waves. Furthermore, due to the enhancement of ion mobility at higher temperatures, it also possesses excellent temperature-sensing performance. Notably, the ICEs can be fully recycled and reused as a new strain/temperature sensor through heating. This study provides a novel strategy for enhancing the mechanical strength of poly(LA) and the fabrication of multifunctional sensors.

Confining Nano-ZrO2 in Nanodomains Leads to Electroactive Artificial Muscle with Large Deformation.

Dielectric elastomers generate muscle-like electroactive actuation, which is applicable in soft machines, medical devices, etc. However, the actuation strain and energy density of most dielectric elastomers, in the absence of prestretch, have long been limited to ∼20% and ∼10 kJ m-3, respectively. Here, we report a dielectric elastomer with ZrO2 nanoparticles confined in nanodomains, which achieves an actuation strain >100% and an energy density of ∼150 kJ m-3 without prestretch. We decorate the surface of each nanoparticle with a layer of a diblock oligomer, poly(acrylic acid-b-styrene). The surface-decorated nanoparticles coassemble with a triblock copolymer elastomer, poly(styrene-b-(2-ethylhexyl acrylate)-b-styrene) during cosolvent casting. Consequently, the nanoparticles are confined in the polystyrene nanodomains, which results in a dielectric elastomer nanocomposite with a low modulus, high breakdown strength, and intense strain-hardening behavior. During the actuation, the nanocomposite avoids the snap-through instability that most elastomers would suffer and achieves a superior actuation performance.

Design of 3D-Photoprintable, Bio-, and Hemocompatible Nonisocyanate Polyurethane Elastomers for Biomedical Implants.

Polyurethanes (PUs) have adjustable mechanical properties, making them suitable for a wide range of applications, including in the biomedical field. Historically, these PUs have been synthesized from isocyanates, which are toxic compounds to handle. This has encouraged the search for safer and more environmentally friendly synthetic routes, leading today to the production of nonisocyanate polyurethanes (NIPUs). Among these NIPUs, polyhydroxyurethanes (PHUs) bear additional hydroxyl groups, which are particularly attractive for derivatizing and adjusting their physicochemical properties. In this paper, polyether-based NIPU elastomers with variable stiffness are designed by functionalizing the hydroxyl groups of a poly(propylene glycol)-PHU by a cyclic carbonate carrying a pendant unsaturation, enabling them to be post-photo-cross-linked with polythiols (thiol-ene). Elastomers with remarkable mechanical properties whose stiffness can be adjusted are obtained. Thanks to the unique viscous properties of these PHU derivatives and their short gel times observed by rheology experiments, formulations for light-based three-dimensional (3D) printing have been developed. Objects were 3D-printed by digital light processing with a resolution down to the micrometer scale, demonstrating their ability to target various designs of prime importance for personalized medicine. In vitro biocompatibility tests have confirmed the noncytotoxicity of these materials for human fibroblasts. In vitro hemocompatibility tests have revealed that they do not induce hemolytic effects, they do not increase platelet adhesion, nor activate coagulation, demonstrating their potential for future applications in the cardiovascular field.

Linear Block Copolymer Synthesis.

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Exchangeable Liquid Crystalline Elastomers and Their Applications.

This Review presents and discusses the current state of the art in "exchangeable liquid crystalline elastomers", that is, LCE materials utilizing dynamically cross-linked networks capable of reprocessing, reprogramming, and recycling. The focus here is on the chemistry and the specific reaction mechanisms that enable the dynamic bond exchange, of which there is a variety. We compare and contrast these different chemical mechanisms and the key properties of their resulting elastomers. In the conclusion, we discuss the most promising applications that are enabled by dynamic cross-linking and present a summary table: a library of currently available materials and their main characteristics.

Materials for Smart Soft Actuator Systems.

In contrast to conventional hard actuators, soft actuators offer many vivid advantages, such as improved flexibility, adaptability, and reconfigurability, which are intrinsic to living systems. These properties make them particularly promising for different applications, including soft electronics, surgery, drug delivery, artificial organs, or prosthesis. The additional degree of freedom for soft actuatoric devices can be provided through the use of intelligent materials, which are able to change their structure, macroscopic properties, and shape under the influence of external signals. The use of such intelligent materials allows a substantial reduction of a device's size, which enables a number of applications that cannot be realized by externally powered systems. This review aims to provide an overview of the properties of intelligent synthetic and living/natural materials used for the fabrication of soft robotic devices. We discuss basic physical/chemical properties of the main kinds of materials (elastomers, gels, shape memory polymers and gels, liquid crystalline elastomers, semicrystalline ferroelectric polymers, gels and hydrogels, other swelling polymers, materials with volume change during melting/crystallization, materials with tunable mechanical properties, and living and naturally derived materials), how they are related to actuation and soft robotic application, and effects of micro/macro structures on shape transformation, fabrication methods, and we highlight selected applications.

Separating Charge Centers of Chain Segments in Dielectric Elastomer through Steric Hindrance Engineering.

Theoretically, separating the positive and negative charge centers of the chain segments of dielectric elastomers (DEs) is a viable alternative to the conventional decoration of chain backbone with polar handles, since it can dramatically increase the dipole vector and hence the dielectric constant (ε') of the DEs while circumvent the undesired impact of the decorated polar handles on the dielectric loss (tan δ). Herein, a novel and universal method is demonstrated to achieve effective separation of the charge centers of chain segments in homogeneous DEs by steric hindrance engineering, i.e., by incorporating a series of different included angle-containing building blocks into the networks. Both experimental and simulation results have shown that the introduction of these building blocks can create a spatially fixed included angle between two adjacent chain segments, thus separating the charge center of the associated region. Accordingly, incorporating a minimal amount of these building blocks (≈5 mol%) can lead to a considerably sharp increase (≈50%) in the ε' of the DEs while maintaining an extremely low tan δ (≈0.006@1 kHz), indicating that this methodology can substantially optimize the dielectric performance of DEs based on a completely different mechanism from the established methods.

Transparent and Soft Crack-Resistant Bouligand Elastomers Inspired by Fish Scales.

Nature with its abundant source offers numerous inspirations for structural and engineering designs. The oriented membranes stacked with bouligand structures in the fish scales show an outstanding combination of high strength and crack resistance. Although the applications of hard biomimetic composites are reported, the structures are rarely utilized in soft materials. Inspired by the scales of various fishes, electrospun membranes are used and stacked to fabricate bouligand elastomers, including orthogonal-plywood, single-bouligand, and double-bouligand structures. The effects of different structures on the properties of elastomers are systematically investigated and possible mechanism is explained using finite element analysis (FEA). The stiffness and fatigue characteristics of these biomimetic elastomers with the above structures are improved compared with the original membranes, especially the elastomers with a single-bouligand structure, which can undergo 5 000 cycles at a maximum strain of 35% without complete failure. The crack only propagates to half of the width of the elastomer with remaining strength of 50% of its original strength. Moreover, the mechanical performance can be adjusted by regulating the proportion of the components. The excellent crack-resistant properties and transparency promote its various potential applications.

Highly Stretchable, Transparent, Self-Healing Ion-Conducting Elastomers for Long-Term Reliable Human Motion Detection.

The flexible electronic sensor is a critical component of wearable devices, generally requiring high stretchability, excellent transmittance, conductivity, self-healing capability, and strong adhesion. However, designing ion-conducting elastomers meeting all these requirements simultaneously remains a challenge. In this study, a novel approach is presented to fabricate highly stretchable, transparent, and self-healing ion-conducting elastomers, which are synthesized via photo-polymerization of two polymerizable deep eutectic solvents (PDESs) monomers, i.e., methacrylic acid (MAA)/choline chloride (ChCl) and itaconic acid (IA)/ChCl. The as-prepared ion-conducting elastomers possess outstanding properties, including high transparency, conductivity, and the capability to adhere to various substrates. The elastomers also demonstrate ultra-stretchability (up to 3900%) owing to a combination of covalent cross-linking and noncovalent cross-linking. In addition, the elastomers can recover up to 3250% strain and over 94.5% of their original conductivity after self-healing at room temperature for 5 min, indicating remarkable mechanical and conductive self-healing abilities. When utilized as strain sensors to monitor real-time motion of human fingers, wrist, elbow, and knee joints, the elastomers exhibit stable and strong repetitive electrical signals, demonstrating excellent sensing performance for large-scale movements of the human body. It is anticipated that these ion-conducting elastomers will find promising applications in flexible and wearable electronics.

Fibrous Conductive Metallogels with Hybrid Electron/Ion Networks for Boosted Extreme Sensitivity and High Linearity Strain Sensor.

Fibrous strain sensing materials with both high sensitivity and high linearity are of significant importance for wearable sensors, yet they still face great challenges. Herein, a photo-spun reaction encapsulation strategy is proposed for the continuous fabrication of fibrous strain sensor materials (AMGF) with a core-sheath structure. Metallogels (MOGs) formed by bacterial cellulose (BC) nanofibers and Ag nanoparticles (AgNPs), and thermoplastic elastomers (TPE) are employed as the core and sheath, respectively. The in situ ultraviolet light reduction of Ag+ ensured AgNPs to maintain the interconnections between the BC nanofibers and form electron conductive networks (0.31 S m-1 ). Under applied strain, the BC nanofibers experience separation, bringing AMGF a high sensitivity (gauge factor 4.36). The concentration of free ions in the MOGs uniformly varies with applied deformation, endowing AMGF with high linearity and a goodness-of-fit of 0.98. The sheath TPE provided AMGF sensor with stable working life (>10 000 s). Furthermore, the AMGF sensors are demonstrated to monitor complex deformations of the dummy joints in real-time as a wearable sensor. Therefore, the fibrous hybrid conductive network fibers fabricated via the photo-spun reaction encapsulation strategy provide a new route for addressing the challenge of achieving both high sensitivity and high linearity.

Highly Stretchable Conjugated Polymer/Elastomer Blend Films with Sandwich Structure.

The physical blending of high-mobility conjugated polymers with ductile elastomers provides a simple way to realize high-performance stretchable films. However, how to control the morphology of the conjugated polymer and elastomer blend film and its response to mechanical fracture processes during stretching are not well understood. Herein, a sandwich structure is constructed in the blend film based on a conjugated polymer poly[(5-fluoro-2,1,3-benzothiadiazole-4,7-diyl)(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b″]dithiophene-2,6-diyl)(6-fluoro-2,1,3-benzothiadiazole-4,7-diyl)(4,4-dihexadecyl-4H-cyclopenta[2,1-b:3,4-b″]dithiophene-2,6-diyl)] (PCDTFBT) and an elastomer polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The sandwich structure is composed of a PCDTFBT:SEBS mixed layer laminated with a PCDTFBT-rich layer at both the top and bottom surfaces. During stretching, the external strain energy can be effectively dissipated by the deformation of the crystalline PCDTFBT domains and amorphous SEBS phases and the recrystallization of the PCDTFBT chains. This endows the blend film with excellent ductility, with a large crack onset strain exceeding 1100%, and minimized the electrical degradation of the blend film at a large strain. This study indicates that the electrical and mechanical performance of conjugated polymer/elastomer blend films can be improved by manipulating their microstructure.

Effects of Operating Mechanical Conditions and Polymer Networks of Nematic Elastomers on Photo-Induced Mechanical Performances.

Photoresponsive liquid-crystalline elastomers (LCEs) are promising candidates for light-controlled soft actuators. Photoinduced stress/strain originates from the changes in mechanical properties after light irradiation. However, the correlation between the photoinduced mechanical performance and in-use conditions such as stress/strain states and polymer network properties (such as effective crosslink density and dangling chain density) remains unexplored for practical applications. Here, isometric photo-induced stress or isotonic strain is investigated at different operating strains or stresses, respectively, on LCEs with polymer network variations, produced by different amounts of solvent during polymerization. As the solvent volume increases, the moduli and photoinduced stresses decrease. However, the photo-induced strain, fracture strain, fracture stress, and viscosity increase. The optical response performance initially increases with the operating strain/stress, peaks at a higher actuation strain/stress, and then, decreases depending on the polymer network. The maximum work densities, which also depend on the operating stress, are in the range of ≈200-300 kJm-3. These findings, highlighting the significant variations in the mechanical performance with the operating stress/strain ranges and amount of solvent used in the synthesis, are critical for designing LCE-based mechanical devices.

A Molecular Rheology Dynamics Study on 3D Printing of Liquid Crystal Elastomers.

This work presents a rheological study of a biocompatible and biodegradable liquid crystal elastomer (LCE) ink for three dimensional (3D) printing. These materials have shown that their structural variations have an effect on morphology, mechanical properties, alignment, and their impact on cell response. Within the last decade LCEs are extensively studied as potential printing materials for soft robotics applications, due to the actuation properties that are produced when liquid crystal (LC) moieties are induced through external stimuli. This report utilizes experiments and coarse-grained molecular dynamics to study the macroscopic rheology of LCEs in nonlinear shear flow. Results from the shear flow simulations are in line with the outcomes of these experimental investigations. This work believes the insights from these results can be used to design and print new material with desirable properties necessary for targeted applications.

Fabrication of Triblock Elastomer Foams and Gelation Studies for Oil Spill Remediation.

Polymeric foamed materials are among the most widely utilized technologies for oil spill accidents and releases of oil-contaminated wastewater oil due to their porosity to absorb and separate oil/water effectively. However, a major limitation of traditional polymeric foams is their reliance on an ad/absorption mechanism as the sole method of oil capture, leading to potential oil leakage once their saturation point is exceeded. Tri-block polymer styrene-ethylene-butylene-styrene (SEBS) is a fascinating absorbent material that can bypass this limitation by both capturing oil and providing a sealing mechanism via gelation to prevent oil leakage due to its unique chemical structure. SEBS foams are produced via simultaneous crosslinking and foaming that results in an impressive expansion ratio of up to 15.2 with over 93% porosity. Most importantly, the SEBS foams show great potential as oil absorbents in spill remediation, demonstrating rapid and efficient oil absorption coupled with superhydrophobic properties. Moreover, the unique interaction between the oil and SEBS enables the formation of a physical gel, acting as an effective barrier against oil leakage. These findings indicate the potential for commercializing SEBS foam as a viable option for geotextiles to mitigate oil spill concerns from infrastructures.

Investigating the Effect of Chain Extender on the Phase Separation and Mechanical Properties of Polybutadiene-Based Polyurethane.

The thermodynamic incompatibility between the soft and hard segments of thermoplastic polyurethane (TPU) results in a microphase-separated behavior and excellent mechanical properties. However, the effect of the chain extender on the degree of microphase separation (DMS) and the resultant mechanical properties of TPU have not been well studied because of the complex interactions between the soft and hard segments. Herein, hydroxyl-terminated polybutadiene-based TPUs(HTPB-TPUs) without hydrogen bonding between the soft and hard segments are synthesized using hydroxyl-terminated polybutadiene, toluene diisocyanate, and four different chain extenders, and the effect of the chain extender structure on DMS is analyzed experimentally using a combination of analytical techniques. Furthermore, the solubility parameters of the soft and hard segments, glass transition temperatures, and hydrogen-bond density of the HTPB-TPUs, are computed using all-atom molecular dynamics simulations. The results clearly reveal that the chain extender significantly affects the DMS and thus the mechanical properties of HTPB-TPUs. This study paves the way for studying the relationship between the structure and properties of TPU.