Soft Composites Research Articles

Designing Supertough and Ultrastretchable Liquid Metal-Embedded Natural Rubber Composites for Soft-Matter Engineering.

Functional elastomers with incredible toughness and stretchability are indispensable for applications in soft robotics and wearable electronics. Furthermore, coupled with excellent electrical and thermal properties, these materials are at the forefront of recent efforts toward widespread use in cutting-edge electronics and devices. Herein, we introduce a highly deformable eutectic-GaIn liquid metal alloy-embedded natural rubber (NR) architecture employing, for the first time, industrially viable solid-state mixing and vulcanization. Standard methods of rubber processing and vulcanization allow us to fragment and disperse liquid metals into submicron-sized droplets in cross-linked NR without compromising the elastic properties of the base matrix. In addition to substantial boosts in mechanical (strain at failure of up to ∼650%) and elastic (negligible hysteresis loss) performances, the tearing energy of the composite was enhanced up to 6 times, and a fourfold reduction in the crack growth rate was achieved over a control vulcanizate. Moreover, we demonstrate improved thermal conductivity and dielectric properties for the resulting composites. Therefore, this work provides a facile and scalable pathway to develop liquid metal-embedded soft elastomeric composites that could be instrumental toward potential applications in soft-matter engineering.

Novel boron nitride filler polymer matrix dielectric elastomer composites with enhanced actuation performance

This paper describes novel soft dielectric elastomer composites that exhibits interesting features from the perspective of dielectric-elastomer transducer development, with enhanced actuation performance under an applied electric field that are potentially suitable for actuator applications. Materials solutions described are a combination of ceramic BN platelets, used at low loading levels, and a plasticizer (Naphthenic Oil) in a polymer matrix and synergistic effects between them that leads to enhancement in actuation strain by as high as 328% under applied electric field. Such functional dielectric composites would alleviate several issues such as size, functionality, and performance for many real-world actuator applications.

Fluid interfacial energy drives the emergence of three-dimensional periodic structures in micropillar scaffolds

Structures that are periodic on a microscale in three dimensions are abundant in nature, for example, in the cellular arrays that make up living tissue. Such structures can also be engineered, appearing in smart materials1–4, photonic crystals5, chemical reactors6, and medical7 and biomimetic8 technologies. Here we report that fluid–fluid interfacial energy drives three-dimensional (3D) structure emergence in a micropillar scaffold. This finding offers a rapid and scalable way of transforming a simple pillar scaffold into an intricate 3D structure that is periodic on a microscale, comprising a solid microscaffold, a dispersed fluid and a continuous fluid. Structures generated with this technique exhibit a set of unique features, including a stationary internal liquid–liquid interface. Using this approach, we create structures with an internal liquid surface in a regime of interest for liquid–liquid catalysis. We also synthesize soft composites in solid, liquid and gas combinations that have previously not been shown, including actuator materials with temperature-tunable microscale pores. We further demonstrate the potential of this method for constructing 3D materials that mimic tissue with an unprecedented level of control, and for microencapsulating human cells at densities that address an unresolved challenge in cell therapy. The revelation that fluid–fluid interfacial energy can drive structure formation in micropillar scaffolds offers a scalable way of synthesizing soft composites, which may have applications in building materials that mimic biological tissue.

Construction of synergistic Toughening, Self-Healing Puncture-Resistant soft composites by using Fabric-Reinforced Pluronic/PMEA hydrogel

Recycling of puncture-resistant soft composites is challenging. In this paper, we report a new strategy of designing puncture-resistant composites with self-healing and synergistic toughening efficacy for secondary utilization after puncture damage. Puncture-resistant soft composites are prepared with Pluronic/PMEA hydrogel and aramid fabrics. The composites exhibit extremely high toughness and puncture resistance that are superior to those of their neat components. This result proves that the tough hydrogel and aramid fabrics have a synergistic effect, which provides the composites with a maximal spike-resistant peak force of 155 N and a maximal knife- resistant peak force of 182 N. The peak force of the composites is about 12 times that of neat fabric. In addition, the Pluronic/PMEA hydrogel matrix attains 85% self-healing efficiency at 100℃ in 3 h. The proposed soft composites with effective puncture- resistant performance can serve as an important guidance for future design of recyclable puncture-resistant composites after damage.

A 3D analytical ion transport model for ionic polymer metal composite actuators in large bending deformations

Ionic polymer metal composites (IPMCs) are a kind of soft electroactive polymer composites. An IPMC strip commonly has a thin polymer membrane coated with a noble metal as electrodes on both sides. Whenever an electric voltage is applied to the IPMC, it bends and whenever it is deformed, a low voltage is measurable between its electrodes, hence IPMC is an actuator as well as a sensor. They are well known for their promising features like low density, lightness, high toughness and remarkable stimulus strain, also, they have the potential for low-voltage operation while exhibiting acceptable large bending deformation. In this paper, a three-dimensional (3D), dynamic and physics-based model is presented analytically and experimentally for IPMC actuators. The model combines the ion transport dynamics within the IPMC and the bending dynamics of it as a beam under an electrical stimulation. In particular, we present an analytical model to create a relation between the input voltage and the output tip displacement of an IPMC actuator for large bending deformations. Experimental results show that the proposed model captures well the tip displacement.

Crack tip fields in a neo-Hookean sheet reinforced by nonlinear fibers

In this paper, we derive the asymptotic crack tip fields in a neo-Hookean material reinforced by nonlinear fibers. The fiber are characterized by a power law model which exhibits significant stiffening behavior at large stretch. A specific case of the model yields the so-called standard reinforcing model (Triantafyllidis and Abeyaratne, 1983). A series of transformations is used to set the stage for the asymptotic solution. We begin with a coordinate scaling (Liu and Moran, 2020b) to account for fiber orientation aspects. This is followed by a hodograph transformation to the strain plane to deal with nonlinearities. Finally, an additional nonlinear algebraic transformation is employed to render the equations suitable for a separable solution. The asymptotic solutions are compared with and agree well with finite element solutions for different fiber modulus ratios, fiber orientation angles, and loading modes. We find that, due to the stiffening effects of the fibers, the deformation fields can be divided into three regions, where the deformation in the two regions near to the crack faces is significantly larger than in the middle region. Stress peaks are observed at the interfaces between regions. The solutions in the paper may provide insight into damage modes and crack initiation at a crack tip in fiber-reinforced soft composites.

Prediction of mechanical and thermal properties of particle reinforced hydrogel composites using the structural genome approach

In this paper, the structural genome approach is used for multiscale analyses to predict the mechanical and thermal properties of particle reinforced hydrogel composites. First, the structure genome model of particle reinforced hydrogel composites is created by the random sequential adsorption algorithm. Then the mechanical properties and equivalent thermal conductivity of hydrogel composites are numerically studied by the structural genome approach. The effects of particles with different volume fractions and material properties on their mechanical and thermal properties are investigated. From the simulation results, it can be found that within a certain range of volume fraction, the mechanical properties and equivalent thermal conductivity of hydrogel composites are positively correlated with the volume fractions of particles. We also find that with the increase of the mechanical properties and thermal conductivity of particles, the properties of hydrogel can be improved and eventually reach stabilization. The structural genome approach shows excellent efficiency in multiscale structure analysis. It is a convenient method for the simulation of complex soft material composites.

Elastic composites with PDMS matrix and polysulfone-supported silver nanoparticles as filler

Polysulfone-supported silver nanoparticles (Ag@PSF) were obtained by a combination of polymer nanoprecipitation and redox synthesis of silver, in the presence of two siloxane-based stabilizing agents, and were incorporated into a PDMS matrix. Soft elastic composites (PDMS-Ag@PSF) were obtained and were investigated by dielectric relaxation spectroscopy and tensile measurements, in comparison with similar materials containing either PSF particles or AgNPs. The mechanical properties of the composites are tunable by composition, depending quasi-linearly on the Ag content. A PDMS-Ag@PSF film with 0.8 wt% Ag, having Y = 0.12 MPa, maximum strain 550% and EBD = 36 V μm−1 exhibited electrical actuation of 8% lateral strain at 22 V μm−1. Apparent piezoelectric coefficient d33 values of ~15 pm V−1 were measured by piezoresponse force microscopy for the composites with Ag@PSF, showing a synergistic effect of the fillers’ components. In addition, the materials exhibited antibacterial and antifungal activity and low bioadhesion. Based on their properties, the highly stretchable composites can be regarded as candidates for applications like actuators, sensors or micro/nanogenerators in biomedical field.

Study on the heterogeneous material coupling connection characteristics and mechanical strength of Oratosquilla oratoria mantis shrimp saddle.

The microstructure, chemical composition and mechanical strength of heterogeneous materials of mantis shrimp (Oratosquilla oratoria) saddle were studied. As the key component of the striking system, the saddle comprised two distinct layers including outer layer and inner layer. The outer layer contained blocky microtubules and exhibited compact appearance. The inner layer presented a typical periodic lamellar structure. Due to the change of the thickness of the mineralized outer layer, the organic multilamellar structure became the foundation and enhanced the connection strength (4.55 MPa) at the connect regions between the saddle and merus exoskeleton and membrane, respectively. In the process of fracture, the lamellar structure dispersed the stress effectively by the change of the crack deflection direction and the microfibrils ordered arrangement. The exploration of mantis shrimp saddle region is beneficial to understand the striking system and provided the possibility for the stable connection of heterogeneous materials in engineering fields. The microstructure, heterogeneous material connection characteristics and high mechanical strength of saddle provide bionic models for the preparation of fiber-reinforced resin composites and soft composites.

A review on soft materials utilized for the manufacturing of soft robots

Since the revolution of soft robotic technology, researchers are focused on the development of the soft robotic bodies which can replicate the motion and the functions of biological bodies. The aim is to develop the soft materials which have the ability to show the deformability and mechanical compliance of biological tissues. This paper focuses on the developed soft robotic materials utilized either for the development of the robotic body or for the actuation techniques. Utilization of various soft materials based on the different actuation techniques such as fluids, gels, soft elastomers, soft polymer composites, and liquid metal-embedded elastomers (LMEE) are reviewed in this work. Thus, the paper provides an insight about the soft materials, which is useful for the new researchers interested in the field of soft robotics. Motivated from the nature or biology, researchers started to design and develop soft robotic bodies using soft and compliant materials for different types of real life applications.

Cost-effective hard–soft carbon composite anodes with promising potassium ions storage performance

Potassium-ion batteries (KIBs) are promising candidates as alternatives to lithium-ion batteries (LIBs) for large-scale energy storage due to the abundant potassium reserves and low cost. However, many well-established anode materials in LIBs are rarely suitable for KIBs. Herein, amorphous carbons including hard carbon, soft carbon and hard–soft carbon composites as KIBs anodes were systematically investigated. The synergy between hard carbon and soft carbon results in structure of relatively large interlayer distance, moderate defects and ordered carbon clusters, leading to excellent electrochemical performance. The hard–soft carbon composite with only 20 wt% hard carbon exhibits high reversible capacity of 376.8 mA h g−1 with a remarkable initial coulombic efficiency (ICE) of 71.04%, an excellent rate capability and a stable cycling performance for KIBs. In addition, full cells assembled with the obtained hard–soft composite as anode and Potassium Prussian blue (KPB) as cathode also exhibit an initial discharge capacity of 200.3 mA h g−1 at 100 mA g−1 and stable cycling performance (146.1 mA h g−1 after 100 cycles at a current density of 100 mA g−1). Owing to the great potential for practical applications, this work may appeal the development of cost-effective and sustainable carbonaceous materials for KIBs.

Integration of a soft dielectric composite into a cantilever beam for mechanical energy harvesting, comparison between capacitive and triboelectric transducers

Flexible dielectrics that harvest mechanical energy via electrostatic effects are excellent candidates as power sources for wearable electronics or autonomous sensors. The integration of a soft dielectric composite (polydimethylsiloxane PDMS-carbon black CB) into two mechanical energy harvesters is here presented. Both are based on a similar cantilever beam but work on different harvesting principles: variable capacitor and triboelectricity. We show that without an external bias the triboelectric beam harvests a net density power of 0.3 upmu mathrm{W}/{mathrm{cm}}^{2} under a sinusoidal acceleration of 3.9g at 40 Hz. In a variable capacitor configuration, a bias of 0.15 mathrm{V}/upmu mathrm{m} is required to get the same energy harvesting performance under the same working conditions. As variable capacitors’ harvesting performance are quadratically dependent on the applied bias, increasing the bias allows the system to harvest energy much more efficiently than the triboelectric one. The present results make CB/PDMS composites promising for autonomous portable multifunctional systems and intelligent sensors.

Temperature dependence of the conductivity of filled diblock copolymers.

We demonstrate that an insulating diblock copolymer system (DBC) filled with conductive fillers can be used as an electrically responsive soft composite that changes its conductivity in response to temperature-induced changes in its morphology. By combining the phase field model describing the morphology of the DBC system with the Monte Carlo simulations and the resistor network model describing electrical properties of the filler network, we calculate the conductivity of this composite. The calculated conductivity is found to essentially depend, in particular, on the temperature of the composite. Changing the temperature is shown to result in morphological changes in the DBC system causing the structural changes in the filler network. In particular, the order-disorder transition in the host DBC system is found to be accompanied by the conductor-insulator transition in the filler network. The effect of the difference between the affinities of the fillers for dissimilar copolymer blocks on the composite conductivity, as well as the effect of the repulsive and attractive interaction between fillers, is considered in detail.

Exfoliation Chemistry of Soft Layered Materials toward Tailored 2D Materials

Abstract Exfoliation of layered materials provides nanosheets, such as monolayers and few-layers. In recent years, nanosheets have attracted much interest as two-dimensional (2D) materials for their diverse properties and applications originating from the anisotropic characteristic structures. Exfoliation methods have been developed depending on the types of interactions between the layers, such as van der Waals and electrostatic interactions. The present review focuses on exfoliation chemistry based on rigid and soft natures of the layered materials. The rigid inorganic layered compounds are converted to the soft layered materials with interaction of organic guests. The surface-functionalized nanosheets are obtained by exfoliation of the soft layered composites with dispersion in organic media. The exfoliation behavior is governed by the flexibility of the precursor layered materials, i.e. interaction between the interlayer guest and dispersion medium. Although exfoliation is generally an uncontrollable top-down process, materials informatics on our own experimental small data assists elucidation of the control factors toward tailored 2D materials. The exfoliation schemes are applied to a variety of layered materials. The present review shows potential new insights for exfoliation chemistry of soft layered materials.

Mechanically Cloaked Multiphase Magnetic Elastomer Soft Composites for Wearable Wireless Power Transfer.

Wearable electronics allow for new and immersive experiences between technology and the human body, but conventional devices are made from rigid functional components that lack the necessary compliance to safely interact with human tissue. Recently, liquid inclusions have been incorporated into elastomer composites to produce functional materials with high extensibility and ultrasoft mechanical responses. While these materials have shown high thermal and electrical conductivity, there has been an absence of research into compliant magnetic materials through the incorporation of magnetic fluids. Compliant magnetic materials are important for applications in soft matter engineering including sensing, actuation, and power transfer for soft electronics and robotics. In this work, we establish a new class of highly functional soft materials with advanced magnetic and mechanical properties by dispersing magnetic colloidal suspensions as compliant fluid inclusions into soft elastomers. Significantly, the rigid magnetic particles are encapsulated by the fluid. This mechanically cloaks the solid particles and enables a fluid-like mechanical response while imparting high magnetic permeability to the composite. This microstructure reduces the modulus of the composite below that of the initial elastomer to <40 kPa while increasing the permeability by over 100% to greater than 2. We demonstrate the functionality of these materials through conformable magnetic backplanes, which enables a completely soft, coupled inductor system capable of transferring power up to 100% strain and wearable devices for wireless power transfer.

Microscopic and long-wave instabilities in 3D fiber composites with non-Gaussian hyperelastic phases

We investigate the microscopic and long-wave (or macroscopic) instabilities in fiber composites (FCs) with hyperelastic phases. To study the influence of the phase stiffening behavior, we employ the phase constitutive models that are developed based on the non-Gaussian statistics of polymeric molecular chains. These non-Gaussian models accurately predict the non-linear behavior of soft materials. Moreover, they can capture the essential soft material stiffening behavior arising due to the finite extensibility of the polymer chains. In turn, the non-linear behavior of the composite phases significantly influences the elastic instabilities and the buckling patterns. Here, we illustrate the phenomenon by the example of FCs with Gent phases. We derive an explicit closed-form expression – in terms of the phase properties and composition – for the onset of long-wave instabilities. To predict the onset of finite length scale (or microscopic) instabilities, we employ the Bloch-Floquet analysis superimposed on finite deformations. We find that the matrix stiffening behavior stabilizes the composites, and can even result in an absolutely stable scenario. Remarkably, Gent FCs with identical phase stiffening characteristics are found to be more stable than their neo-Hookean counterparts for morphologies at which the microscopic instabilities are to develop first. The stiffening behavior of the phases dictates the interplay between the long-wave and microscopic instabilities, and defines the wavelength of the buckling patterns. Thus, the pre-designed phase properties can be used in tailoring the instability-induced patterns in soft fiber composites.

Conceptual design and multifield coupling behavior of magnetically propelled fish-like swimmers

Composites reinforced with ferromagnetic particles have been widely used in various engineering areas and have been extensively attracted more attention for the past decade because of their magnetic dependent properties can be dramatically altered by applying a magnetic field. In this study, a conceptual model based on the soft composites filled with magnetoactive particles was proposed, and the numerical research of magneto-fluid-solid coupling behaviours for a soft biomimetic robot fish driven magnetically was carried out. Under the action of magnetic field, the magnetic sensitive materials distributed in specific parts of the robot fish body will deform and cause the whole body of the fish swinging, to flap the liquid and generate forward thrust, resulting in pushing the fish forward. The corresponding multifield coupling equations for the mechanism and responses of magnetically propelled fish-like swimmers were developed and simulated numerically using the COMSOL multiphysics software. The effects of the tail length, the modulus of magnetic sensitive materials and spatial arrangement feature on the swimming ability of the fish-like structure were analyzed in detail. The relevant results are expected to provide a theoretical guidance for the potential applications of such biomimetic devices in intelligent far-field driving, drug delivery and other fields.

Analytical transient phase change heat transfer model of wearable electronics with a thermal protection substrate

As thermal protection substrates for wearable electronics, functional soft composites made of polymer materials embedded with phase change materials and metal layers demonstrate unique capabilities for the thermal protection of human skin. Here, we develop an analytical transient phase change heat transfer model to investigate the thermal performance of a wearable electronic device with a thermal protection substrate. The model is validated by experiments and the finite element analysis (FEA). The effects of the substrate structure size and heat source power input on the temperature management efficiency are investigated systematically and comprehensively. The results show that the objective of thermal management for wearable electronics is achieved by the following thermal protection mechanism. The metal thin film helps to dissipate heat along the in-plane direction by reconfiguring the direction of heat flow, while the phase change material assimilates excessive heat. These results will not only promote the fundamental understanding of the thermal properties of wearable electronics incorporating thermal protection substrates, but also facilitate the rational design of thermal protection substrates for wearable electronics.

Strain stiffening and positive piezoconductive effect of liquid metal/elastomer soft composites

Liquid metal (LM) droplets have been demonstrated as an effective additive to transform elastomers into multifunctional composites that remain highly conductive under large mechanical deformation. Herein, we present a new phenomenon of strain hardening of polydimethylsiloxane (PDMS) composites with Galinstan LM droplet inclusions. Small LM inclusions increase the stiffness of PDMS matrix due to the surface tension effect at the liquid/solid interface after the droplet size becomes less than the elastocapillary length during stretching. The tensile strength was 0.60 MPa, 1.10 MPa and 1.72 MPa for PDMS composites with LM contents of 4.8 vol%, 7.5 vol% and 11.0 vol%, respectively, which represents 58%, 189%, 353% increase in comparison with the PDMS matrix. The electrical conductivity of the PDMS/LM composites with 4.8 vol% and 7.5 vol% LM droplets increased with the strain of the specimens, overcoming the shortcomings of rising resistance of conventional polymer/conductive filler composites under mechanical deformation. When subjected to tensile cyclic loading, the PDMS/LM composites demonstrated excellent durability after 2000 cycles with only 13% decrease (with 10% strain amplitude) in relative resistance for the composite with 7.5 vol% LM droplets and 7% reduction (with 20% strain amplitude) for that with 11.0 vol% of LM. The hybridization of carbon nanofibres (CNFs) however affected the distribution of LM droplets in the matrix and only showed enhanced mechanical property (a 52% increase in tensile strength and 31% increase in Young's modulus in PDMS4.8LM0.1CNF). This new class of soft materials create new opportunities of designing wearable electronics, soft robotics, stretchable heater and biomedical implants.

Controlled Assembly of Liquid Metal Inclusions as a General Approach for Multifunctional Composites.

Soft composites that use droplets of gallium-based liquid metal (LM) as the dispersion phase have the potential for transformative impact in multifunctional material engineering. However, it is unclear whether percolation pathways of LM can support high electrical conductivity in a wide range of matrix materials. This issue is addressed through an approach to LM composite synthesis that focuses on the interrelated effects of matrix curing/solidification and droplet formation. The combined influence of LM concentration, particle size, and sedimentation is explored. By developing this approach, the functionalities that have been demonstrated with LM composites can be generalized to other matrix materials that impart additional functionality. Specifically, composites are synthesized using a biodegradable/reprocessable plastic (polycaprolactone), a hydrogel (poly(vinyl alcohol)), and a processable rubber (a styrene-ethylene-butylene-styrene derivative) to demonstrate wide applicability. This method enables synthesis of composites: i) with high stretchability and negligible electromechanical coupling (>600% strain); ii) with Joule-heated healing and reprocessability; iii) with electrical and mechanical self-healing; and iv) that can be printed. This approach to controlled assembly represents a widely applicable technique for creating new classes of LM composites with unprecedented multifunctionality.