Structural characterization of lateral phase separation in polymer-lipid hybrid membranes.

Hierarchic self-assembly is the main mechanism used to create diverse structures using soft materials. This is a case for both synthetic materials and biomolecular systems, as exemplified by the non-covalent organization of lipids into membranes. In nature, lipids often assemble into single bilayers, but other nanostructures are encountered, such as bilayer stacks and tubular and vesicular aggregates. Synthetic block copolymers can be engineered to recapitulate many of the structures, forms, and functions of lipid systems. When block copolymers are amphiphilic, they can be inserted or co-assembled into hybrid membranes that exhibit synergistic structural, permeability, and mechanical properties. One example is the emergence of lateral phase separation akin to the raft formation in biomembranes. When higher-order structures, such as hybrid membranes, are formed, this lateral phase separation can be correlated across membranes in the stack. This chapter outlines a set of important methods, such as X-ray Scattering, Atomic Force Microscopy, and Cryo-Electron Microscopy, that are relevant to characterizing and evaluating lateral and correlated phase separation in hybrid membranes at the nano and mesoscales. Understanding the phase behavior of polymer-lipid hybrid materials could lead to innovative advancements in biomimetic membrane separation systems.

Soft Actuators and Actuation: Design, Synthesis, and Applications.

Soft actuators are one of the most promising technological advancements with potential solutions to diverse fields' day-to-day challenges. Soft actuators derived from hydrogel materials possess unique features such as flexibility, responsiveness to stimuli, and intricate deformations, making them ideal for soft robotics, artificial muscles, and biomedical applications. This review provides an overview of material composition and design techniques for hydrogel actuators, exploring 3D printing, photopolymerization, cross-linking, and microfabrication methods for improved actuation. It examines applications of hydrogel actuators in biomedical, soft robotics, bioinspired systems, microfluidics, lab-on-a-chip devices, and environmental, and energy systems. Finally, it discusses challenges, opportunities, advancements, and regulatory aspects related to hydrogel actuators.

Complex Pathways Drive Pluripotent Fmoc-Leucine Self-Assemblies.

Nature uses complex self-assembly pathways to access distinct functional non-equilibrium self-assemblies. This remarkable ability to steer same set of biomolecules into different self-assembly states is done by avoiding thermodynamic pit. In synthetic systems, on demand control over 'Pathway Complexity' to access self-assemblies different from equilibrium structures remains challenging. Here we show versatile non-equilibrium assemblies of the same monomer via alternate assembly pathways. The assemblies nucleate using non-classical or classical nucleation routes into distinct metastable (transient hydrogels), kinetic (stable hydrogels) and thermodynamic structures [(poly)-crystals and 2D sheets]. Initial chemical and thermal inputs force the monomers to follow different assembly pathways and form soft-materials with distinct molecular arrangements than at equilibrium. In many cases, equilibrium structures act as thermodynamic sink which consume monomers from metastable structures giving transiently formed materials. This dynamics can be tuned chemically or thermally to slow down the dissolution of transient hydrogel, or skip the intermediate hydrogel altogether to reach final equilibrium assemblies. If required this metastable state can be kinetically trapped to give strong hydrogel stable over days. This method to control different self-assembly states can find potential use in similar biomimetic systems to access new materials for various applications.

Self-Healing Materials for Bioelectronic Devices.

Though the history of self-healing materials stretches far back to the mid-20th century, it is only in recent years where such unique classes of materials have begun to find use in bioelectronics-itself a burgeoning area of research. Inspired by the natural ability of biological tissue to self-repair, self-healing materials play a multifaceted role in the context of soft, wireless bioelectronic systems, in that they can not only serve as a protective outer shell or substrate for the internal electronic circuitry-analogous to the mechanical barrier that skin provides for the human body-but also, and most importantly, act as an active sensing safeguard against mechanical damage to preserve device functionality and enhance overall durability. This perspective presents the historical overview, general design principles, recent developments, and future outlook of self-healing materials for bioelectronic devices, which integrates topics in many research disciplines-from materials science and chemistry to electronics and bioengineering-together.

Complexity of crack front geometry enhances toughness of brittle solids.

Brittle solids typically fail by growth and propagation of a crack from a surface flaw. This process is modelled using linear elastic fracture mechanics, which parameterizes the toughness of a material by the critical stress intensity factor, or the prefactor of the singular stress field. This widely used theory applies for cracks that are planar, but cracks typically are not planar, and instead are geometrically complex, violating core tenets of linear elastic fracture mechanics. Here we characterize the crack tip kinematics of complex crack fronts in three dimensions using optical microscopy of several transparent, brittle materials, including hydrogels of four different chemistries and an elastomer. We find that the critical strain energy required to drive the crack is directly proportional to the geodesic length of the crack, which makes the sample effectively tougher. The connection between crack front geometry and toughness has repercussions for the theoretical modelling of three-dimensional cracks, from engineering testing of materials to ab-initio development of novel materials, and highlights an important gap in the current theory for three-dimensional cracks.

Analytical model of friction at low shear rates for soft materials in 3D printing.

BACKGROUND: The biological properties of silicone elastomers such as polydimethylsiloxane (PDMS) have widespread use in biomedicine for soft tissue implants, contact lenses, soft robots, and many other small medical devices, due to its exceptional biocompatibility. Additive manufacturing of soft materials still has significant challenges even with major advancements that have occurred in development of these technologies for customized medical devices and tissue engineering. OBJECTIVE: The aim of this study was to develop a mathematical model of tangential stress in relation to shear stress, shear rate, 3D printing pressure and velocity, for non-Newtonian gels and fluids that are used as materials for 3D printing. METHOD: This study used FENE (finitely extensible nonlinear elastic model) model, for non-Newtonian gels and fluids to define the dependences between tangential stress, velocity, and pressure, considering viscosity, shear stress and shear rates as governing factors in soft materials friction and adhesion. Experimental samples were fabricated as showcases, by SLA and FDM 3D printing technologies: elastic polymer samples with properties resembling elastic properties of PDMS and thermoplastic polyurethane (TPU) samples. Experimental 3D printing parameters were used in the developed analytical solution to analyse the relationships between governing influential factors (tangential stress, printing pressure, printing speed, shear rate and friction coefficient). Maple software was used for numerical modelling. RESULTS: Analytical model applied on a printed elastic polymer, at low shear rates, exhibited numerical values of tangential stress of 0.208-0.216 N m - 2 at printing velocities of 0.9 to 1.2 mm s - 1, while the coefficient of friction was as low as 0.09-0.16. These values were in accordance with experimental data in literature. Printing pressure did not significantly influence tangential stress, whereas it was slightly influenced by shear rate changes. Friction coefficient linearly increased with tangential stress. CONCLUSION: Simple analytical model of friction for elastic polymer in SLA 3D printing showed good correspondence with experimental literature data for low shear rates, thus indicating possibility to use it for prediction of printing parameters towards desired dimensional accuracy of printed objects. Further development of this analytical model should enable other shear rate regimes, as well as additional soft materials and printing parameters.

Rigorous Progress in Coarse-Graining.

Low-resolution coarse-grained (CG) models provide remarkable computational and conceptual advantages for simulating soft materials. In principle, bottom-up CG models can reproduce all structural and thermodynamic properties of atomically detailed models that can be observed at the resolution of the CG model. This review discusses recent progress in developing theory and computational methods for achieving this promise. We first briefly review variational approaches for parameterizing interaction potentials and their relationship to machine learning methods. We then discuss recent approaches for simultaneously improving both the transferability and thermodynamic properties of bottom-up models by rigorously addressing the density and temperature dependence of these potentials. We also briefly discuss exciting progress in modeling high-resolution observables with low-resolution CG models. More generally, we highlight the essential role of the bottom-up framework not only for fundamentally understanding the limitations of prior CG models but also for developing robust computational methods that resolve these limitations in practice.

Soft Self-Regulating Heating Elements for Thermoplastic Elastomer-Based Electronic Skin Applications.

Resistive heating elements can be of particular interest for many applications, such as e-skin. In this study, soft heating elements were developed by combining thermoplastic polyurethane (TPU) with carbon black. In contrast to previous studies on thermoplastic polymer-based thermistors, the heating elements could endure elongations above 100%. Due to the high melting point of the TPU and the carbon filler, the thermistors could be heated up to 180°C without significant deformation. The heating elements were extruded on TPU substrates using material extrusion additive manufacturing in one-step process. Self-regulating behavior to control the maximum temperature was achieved with the application of two different voltages (20 and 25 V) and different current thresholds, between 100 and 800 mA. The heating performance was adjusted by changing the geometry of the sensing elements; an increase in cross section resulted in a lower current density and lower temperature. For the heating elements, variation of the additive manufacturing parameters such as offset, layer height, nozzle speed, and extrusion multiplier resulted in a different width/height aspect ratio of the cross section of the extruded lines, affecting the initial resistivity of the thermistor. Orientation of the carbon filler during extrusion process is one reason for the small change of the longitudinal conductivity of the heating elements. The resulting skin with the integrated heating elements allowed the possibility to perform the in situ heating for the localized healing of structural damage, while maintaining the softness required for the application of soft robotic electronic skin.

Brittle and ductile yielding in soft materials.

Many soft materials yield under mechanical loading, but how this transition from solid-like behavior to liquid-like behavior occurs can vary significantly. Understanding the physics of yielding is of great interest for the behavior of biological, environmental, and industrial materials, including those used as inks in additive manufacturing and muds and soils. For some materials, the yielding transition is gradual, while others yield abruptly. We refer to these behaviors as being ductile and brittle. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a steep increase in the loss modulus during oscillatory amplitude sweeps. In this work, we show how this spectrum of yielding behaviors may be accounted for in a continuum model for yield stress materials by introducing a parameter we call the brittility factor. Physically, an increased brittility decreases the contribution of recoverable deformation to plastic deformation, which impacts the rate at which yielding occurs. The model predictions are successfully compared to results of different rheological protocols from a number of real yield stress fluids with different microstructures, indicating the general applicability of the phenomenon of brittility. Our study shows that the brittility of soft materials plays a critical role in determining the rate of the yielding transition and provides a simple tool for understanding its effects under various loading conditions.

Dielectric Elastomer Actuators with Enhanced Durability by Introducing a Reservoir Layer.

A Dielectric Elastomer Actuator (DEA) consists of electrodes with a dielectric layer between them. By controlling the design of the electrodes, voltage, and frequency, the operating range and speed of the DEA can be adjusted. These DEAs find applications in biomimetic robots, artificial muscles, and similar fields. When voltage is applied to the DEA, the dielectric layer undergoes compression and expansion due to electrostatic forces, which can lead to electrical breakdown. This phenomenon is closely related to the performance and lifespan of the DEA. To enhance stability and improve dielectric properties, a DEA Reservoir layer is introduced. Here, stability refers to the ability of the DEA to perform its functions even as the applied voltage increases. The Reservoir layer delays electrical breakdown and enhances stability due to its enhanced thickness. The proposed DEA in this paper is composed of a Reservoir layer and electrode layer. The Reservoir layer is placed between the electrode layers and is independently configured, not subjected to applied voltage like the electrode layers. The performance of the DEA was evaluated by varying the number of polymer layers in the Reservoir and electrode designs. Introducing the Reservoir layer improved the dielectric properties of the DEA and delayed electrical breakdown. Increasing the dielectric constant through the DEA Reservoir can enhance output characteristics in response to electrical signals. This approach can be utilized in various applications in wearable devices, artificial muscles, and other fields.

Crack-Resistant and Tissue-Like Artificial Muscles with Low Temperature Activation and High Power Density.

Constructing soft robotics with safe human-machine interactions requires low-modulus, high-power-density artificial muscles that are sensitive to gentle stimuli. In addition, the ability to resist crack propagation during long-term actuation cycles is essential for a long service life. Herein, a material design is proposed to combine all these desirable attributes in a single artificial muscle platform. The design involves the molecular engineering of a liquid crystalline network with crystallizable segments and an ethylene glycol flexible spacer. A high degree of crystallinity can be afforded by utilizing aza-Michael chemistry to produce a low covalent crosslinking density, resulting in crack-insensitivity with a high fracture energy of 33 720 J m-2 and a high fatigue threshold of 2250 J m-2. Such crack-resistant artificial muscle with tissue-matched modulus of 0.7 MPa can generate a high power density of 450 W kg-1 at a low temperature of 40 °C. Notably, because of the presence of crystalline domains in the actuated state, no crack propagation is observed after 500 heating-cooling actuation cycles under a static load of 220 kPa. This study points to a pathway for the creation of artificial muscles merging seemingly disparate, but desirable properties, broadening their application potential in smart devices.

Magnetically Actuated Trigger Transient Soft Actuators Comprising On-Demand Photo-Initiated and Thermo-Degradable Polypropylene Carbonate-Photo-Acid Generator.

Lifetime-reconfigurable soft robots have emerged as a new class of robots, emphasizing the unmet needs of futuristic sustainability and security. Trigger-transient materials that can both actuate and degrade on-demand are crucial for achieving life-reconfigurable soft robots. Here, we propose the use of transient and magnetically actuating materials that can decompose under ultraviolet light and heat, achieved by adding photo-acid generator (PAG) and magnetic particles (Sr-ferrite) to poly(propylene carbonate) (PPC). Chemical and thermal analyses reveal that the mechanism of PPC-PAG decomposition occurs through PPC backbone cleavage by the photo-induced acid. The self-assembled monolayer (SAM) encapsulation of Sr-ferrite preventing the interaction with the PAG allowed the transience of magnetic soft actuators. We demonstrate remotely controllable and degradable magnetic soft kirigami actuators using blocks with various magnetized directions. This study proposes novel approaches for fabricating lifetime-configurable magnetic soft actuators applicable to diverse environments and applications, such as enclosed/sealed spaces and security/military devices.

Development and application of a 3D image analysis strategy for focused ion beam - Scanning electron microscopy tomography of porous soft materials.

In recent years, the potential of porous soft materials in various device technologies has increased in importance due to applications in fields, such as wearable electronics, medicine, and transient devices. However, understanding the 3-dimensional architecture of porous soft materials at the microscale remains a challenge. Herein, we present a method to structurally analyze soft materials using Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) tomography. Two materials, polymethyl methacrylate (PMMA) membrane and pine wood veneer were chosen as test-cases. FIB-SEM was successfully used to reconstruct the true topography of these materials in 3D. Structural and physical properties were subsequently deduced from the rendered 3D models. The methodology used segmentation, coupled with optimized thresholding, image processing, and reconstruction protocols. The 3D models generated pore size distribution, pore inter-connectivity, tortuosity, thickness, and curvature data. It was shown that FIB-SEM tomography provides both an informative and visual depiction of structure. To evaluate and validate the FIB-SEM reconstructions, porous properties were generated from the physical property analysis techniques, gas adsorption analysis using Brunauer-Emmett-Teller (BET) surface area analysis and mercury intrusion porosimetry (MIP) analysis. In general, the data obtained from the FIB-SEM reconstructions was well-matched with the physical data. RESEARCH HIGHLIGHTS: Porous specimens of both synthetic and biological nature, a poly(methyl methacrylate) membrane and a pine veneer respectively, are reconstructed via FIB-SEM tomography without resin-embedding. Different thresholding and reconstruction methods are explored whereby shadowing artifacts are present with the aid of free open-source software. Reconstruction data is compared to physical data: MIP, gas adsorption isotherms which are analyzed via BET and Barrett-Joyner-Halenda (BJH) analysis to yield a full picture of the materials.

Piezoionic Elastomers by Phase and Interface Engineering for High-Performance Energy-Harvesting Ionotronics.

Piezoionic materials play a pivotal role in energy-harvesting ionotronics. However, a persistent challenge lies in balancing the structural requirements for voltage generation, current conduction, and mechanical adaptability. The conventional approach of employing crystalline heterostructures for stress concentration and localized charge separation, while effective for voltage generation, often compromises the stretchability and long-range charge transport found in homogeneous quasisolid states. Herein, phase and interface engineering strategy is introduced to address this dilemma and a piezoionic elastomer is presented that seamlessly integrates ionic liquids and ionic plastic crystals, forming a finely tuned microphase-separated structure with an intermediate phase. This approach promotes charge separation via stress concentration among hard phases while leveraging the high ionic charge mobility in soft and intermediate phases. Impressively, the elastomer achieves an extraordinary piezoionic coefficient of about 6.0 mV kPa-1, a more than threefold improvement over current hydrogels and ionogels. The resulting power density of 1.3 µW cm-3 sets a new benchmark, exceeding that of state-of-the-art piezoionic gels. Notably, this elastomer combines outstanding stretchability, remarkable toughness, and rapid self-healing capability, underscoring its potential for real-world applications. This work may represent a stride toward mechanically robust energy harvesting systems and provide insights into ionotronic systems for human-machine interaction.

Network physics of attractive colloidal gels: Resilience, rigidity, and phase diagram.

Colloidal gels exhibit solid-like behavior at vanishingly small fractions of solids, owing to ramified space-spanning networks that form due to particle-particle interactions. These networks give the gel its rigidity, and with stronger attractions the elasticity grows as well. The emergence of rigidity can be described through a mean field approach; nonetheless, fundamental understanding of how rigidity varies in gels of different attractions is lacking. Moreover, recovering an accurate gelation phase diagram based on the system's variables has been an extremely challenging task. Understanding the nature of colloidal clusters, and how rigidity emerges from their connections is key to controlling and designing gels with desirable properties. Here, we employ network analysis tools to interrogate and characterize the colloidal structures. We construct a particle-level network, having all the spatial coordinates of colloids with different attraction levels, and also identify polydisperse rigid fractal clusters using a Gaussian mixture model, to form a coarse-grained cluster network that distinctly shows main physical features of the colloidal gels. A simple mass-spring model then is used to recover quantitatively the elasticity of colloidal gels from these cluster networks. Interrogating the resilience of these gel networks shows that the elasticity of a gel (a dynamic property) is directly correlated to its cluster network's resilience (a static measure). Finally, we use the resilience investigations to devise [and experimentally validate] a fully resolved phase diagram for colloidal gelation, with a clear solid-liquid phase boundary using a single volume fraction of particles well beyond this phase boundary.

Magnetoactive Millirobots with Ternary Phase Transition.

Magnetoactive soft millirobots have made significant advances in programmable deformation, multimodal locomotion, and untethered manipulation in unreachable regions. However, the inherent limitations are manifested in the solid-phase millirobot as limited deformability and in the liquid-phase millirobot as low stiffness. Herein, we propose a ternary-state magnetoactive millirobot based on a phase transitional polymer embedded with magnetic nanoparticles. The millirobot can reversibly transit among the liquid, solid, and viscous-fluid phases through heating and cooling. The liquid-phase millirobot has elastic deformation and mobility for unimpeded navigation in a constrained space. The viscous-fluid phase millirobot shows irreversible deformation and large ductility. The solid-phase millirobot shows good shape stability and controllable locomotion. Moreover, the ternary-state magnetoactive millirobot can achieve prominent capabilities including stiffness change and shape reconfiguration through phase transition. The millirobot can perform potential functions of navigation in complex terrain, three-dimensional circuit connection, and simulated treatment in a stomach model. This magnetoactive millirobot may find new applications in flexible electronics and biomedicine.

Piperidinium surfactants functionalized with carbamate fragment: Aggregation, antimicrobial activity and cytotoxicity.

The biomimetic nature of supramolecular systems, the structural similarity of synthetic surfactants to biomolecules (lipids, proteins), provide them with high membranotropy, the ability to overcome biological barriers, and affinity towards biosubstances. Despite rather high toxicity cationic surfactants are of importance as antimicrobial agents, gene nanocarriers and mitochondria targeted ligands. To minimize this limitation, cationic amphiphilic matrix undergoes modification with various functional groups. In this work, new piperidinium cationic surfactants containing one or two carbamate fragments were prepared; their aggregation behavior was systematically studied by tensiometery, spectrophotometry and fluorimetry. The presence of a carbamate fragment leads to a 2-3-fold decrease in critical micelle concentration and to a significant increase in solubilization capacity compared to unsubstituted analogue. Evaluation of the antimicrobial effect showed that all compounds exhibit high bactericidal and fungicidal activity against a wide range of pathogenic microorganisms, including their resistant forms. Importantly, the introducing carbamate moiety allows of decreasing hemolytic activity of cationic surfactants. The data obtained make it possible to recommend carbamate piperidinium surfactants as effective biocompatible and biodegradable nanocontainers for hydrophobic probes with high antimicrobial effect and moderate hemolytic activity.

Reprogrammable Metamaterial Processors for Soft Machines.

Soft metamaterials have attracted extensive attention due to their remarkable properties. These materials hold the potential to program and control the morphing behavior of soft machines, however, their combination is limited by the poor reprogrammability of metamaterials and incompatible communication between them. Here, printable and recyclable soft metamaterials possessing reprogrammable embedded intelligence to regulate the morphing of soft machines are introduced. These metamaterials are constructed from interconnected and periodically arranged logic unit cells that are able to perform compound logic operations coupling multiplication and negation. The scalable computation capacity of the unit cell empowers it to simultaneously process multiple fluidic signals with different types and magnitudes, thereby allowing the execution of sophisticated and high-level control operations. By establishing the laws of physical Boolean algebra and formulating a universal design route, soft metamaterials capable of diverse logic operations can be readily created and reprogrammed. Besides, the metamaterials' potential of directly serving as fluidic processors for soft machines is validated by constructing a soft latched demultiplexer, soft controllers capable of universal and customizable morphing programming, and a reprogrammable processor without reconnection. This work provides a facile way to create reprogrammable soft fluidic control systems to meet on-demand requirements in dynamic situations.

4D Printing of Ultrastretchable Magnetoactive Soft Material Architectures for Soft Actuators.

Magnetoactive soft materials (MSMs) comprising magnetic particles and soft matrices have emerged as smart materials for realizing soft actuators. 4D printing, which involves fabricating 3D architectures that can transform shapes under external magnetic fields, is an effective way to fabricate MSMs-based soft actuators with complex shapes. The printed MSMs must be flexible, stretchable, and adaptable in their magnetization profiles to maximize the degrees of freedom for shape morphing. This study utilizes a facile 4D printing strategy for producing ultrastretchable (stretchability > 1000%) MSM 3D architectures for soft-actuator applications. The strategy involves two sequential steps: (i) direct ink writing (DIW) of the MSM 3D architectures with ink composed of NdFeB and styrene-isoprene block copolymers (SIS) at room temperature and (ii) programming and reconfiguration of the magnetization profiles of the printed architecture using an origami-inspired magnetization method (magnetization field, Hm = 2.7 T). Various differently shaped MSM 3D architectures, which can be transformed into desired shapes under an actuation magnetic field (Ba = 85 mT), are successfully fabricated. In addition, two different soft-actuator applications are demonstrated: a multifinger magnetic soft gripper and a Kirigami-shaped 3D electrical switch with conductive and magnetic functionalities. Our strategy shows potential for realizing multifunctional, shape-morphing, and reprogrammable magnetoactive devices for advanced soft-actuator applications.

Ultrashort Cationic Peptide Fmoc-FFK as Hydrogel Building Block for Potential Biomedical Applications.

Fmoc-diphenylalanine (Fmoc-FF) is a low-molecular-weight peptide hydrogelator. This simple all-aromatic peptide can generate self-supporting hydrogel materials, which have been proposed as novel materials for diagnostic and pharmaceutical applications. Our knowledge of the molecular determinants of Fmoc-FF aggregation is used as a guide to design new peptide-based gelators, with features for the development of improved tools. Here, we enlarge the plethora of Fmoc-FF-based hydrogelated matrices by studying the properties of the Fmoc-FFK tripeptide, alone or in combination with Fmoc-FF. For multicomponent matrices, the relative weight ratios between Fmoc-FFK and Fmoc-FF (specifically, 1/1, 1/5, 1/10, and 1/20 w/w) are evaluated. All the systems and their multiscale organization are studied using different experimental techniques, including rheology, circular dichroism, Fourier transform infrared spectroscopy, and scanning electron microscopy (SEM). Preliminary profiles of biocompatibility for the studied systems are also described by testing them in vitro on HaCaT and 3T3-L1 cell lines. Additionally, the lysine (K) residue at the C-terminus of the Fmoc-FF moiety introduces into the supramolecular material chemical functions (amino groups) which may be useful for modification/derivatization with bioactive molecules of interest, including diagnostic probes, chelating agents, active pharmaceutical ingredients, or peptide nucleic acids.