Elastic Composites Research Articles

Ligand presentation controls collective MSC response to matrix stress relaxation in hybrid PEG-HA hydrogels

Cell interactions with the extracellular matrix (ECM) influence intracellular signaling pathways related to proliferation, differentiation, and secretion, amongst other functions. Herein, bone-marrow derived mesenchymal stromal cells (MSCs) are encapsulated in a hydrazone crosslinked hyaluronic acid (HA) hydrogel, and the extent of stress relaxation is controlled by systemic introduction of irreversible triazole crosslinks. MSCs form elongated multicellular structures within hydrogels containing RGD peptide and formulated with elastic composition slightly higher than the hydrogel percolation threshold (12 % triazole, 88 % hydrazone). A scaling analysis is presented (<RgStructure2>12 ∼Nα) to quantify cell-material interactions within these structures with the scaling exponent (α) describing either elongated (0.66) or globular (0.33) structures. Cellular interactions with the material were controlled through peptides to present integrin binding ECM cues (RGD) or cadherin binding cell-cell cues (HAVDI) and MSCs were observed to form highly elongated structures in RGD containing hydrogels (α=0.56±0.05), whereases collapsed structures were observed within HAVDI containing hydrogels (α=0.39±0.04). Finally, cytokine secretion was investigated, and a global increase in secreted cytokines was observed for collapsed structures compared to elongated. Taken together, this study presents a novel method to characterize cellular interactions within a stress relaxing hydrogel where altered cluster morphology imparts changes to cluster secretory profiles.

Fourier Transform Approach to Boundary Domain Integral Equations for Elastic Composites With Domain Decomposition and Multi Reference Parameters

ABSTRACTIn this article, displacement and strain periodic boundary domain integral equations for homogenization problems of elastic composites are derived in the context of FFT homogenization methods. The resolution methods based on regular grids and discrete Green's tensors are presented. The displacement based equations can be used to solve problems in arbitrary domains under periodic and non‐periodic boundary conditions. The strain based integral equation is obtained from the combination of the displacement based equations for different domains, each one having its own reference elasticity tensor. In the latter, the strain values inside every phases are connected to material mismatch parameters on the phase boundary. It was shown that by decomposing suitably domains by stiffness and using adapted reference parameters, the iteration schemes converge faster.

An alternative stress boundary condition in small deformations and its application to soft elastic composites and structures

Linear elasticity theory has been used extensively in the study of the elastic behavior of various perforated structures and composite materials requiring the accompaniment of appropriate boundary conditions to derive qualitatively correct and quantitatively referential solutions. When incorporating conventional boundary conditions, however, linear elasticity theory fails to predict certain essential phenomena associated with perforate structures and composite materials even when they undergo small deformations. For example, a soft elastic porous medium is appreciably stiffened when inflated despite the fact that the internal air pressure is significantly lower than the modulus of the medium itself. In this paper, we propose an improved stress boundary condition by simply incorporating a small change in the normal to the boundary during deformation. We show via numerical examples that in the context of linear elasticity theory, the use of this improved boundary condition offers the possibility of predicting the influence of initial or residual stress in a perforated structure on the elastic response of the structure to external loadings (which can never be captured with the use of conventional boundary conditions). We perform also large-deformation-based finite element simulations to verify the accuracy of the closed-form results obtained from the improved boundary condition for a soft elastic perforated structure with initial internal pressure. We believe that the idea presented in this paper will extend the applicability of linear elasticity theory and yield more accurate referential analytic results for soft elastic structures and composites.

Effects of interfacial debonding on the stability of finitely strained periodic microstructured elastic composites.

Predicting failure initiation in nonlinear composite materials, often referred to as metamaterials, is a fundamental challenge in nonlinear solid mechanics. Microstructural failure mechanisms encompass fracture, decohesion, cavitation, compression-induced contact and instabilities, affecting their unconventional static and dynamic performances. To fully take advantage of these materials, especially in extreme applications, it is imperative to predict their nonlinear behaviour using reliable, accurate and computationally efficient numerical methodologies. This study presents an innovative nonlinear homogenization-based theoretical framework for characterizing the failure behaviour of periodic reinforced hyperelastic composites induced by reinforcement/matrix decohesion and interaction between contact mechanisms and microscopic instabilities. Debonding and unilateral contact between different phases are incorporated by employing an enhanced cohesive/contact model, which features a special nonlinear interface constitutive law and an accurate contact formulation within the context of finite strain continuum mechanics. The theoretical formulation is demonstrated using periodically layered composites subjected to macroscopic compressive loading conditions along the lamination direction. Numerical results illustrate the ways in which debonding phenomena, in conjunction with fibre microbuckling, may influence the critical loads of the examined composite solid. The sensitivity of the results obtained through the proposed contact-cohesive model at finite strain with respect to its implementation is also explored. This article is part of the theme issue 'Current developments in elastic and acoustic metamaterials science (Part 1)'.

Elastic shear-stiffening composites with locally tunable mechanics for protection and damping

Shear-stiffening gels are flexible materials whose modulus is significantly increased upon rapid impact. They have applications in protective and other devices but are generally limited by difficult processability and poor shape retention. Here we demonstrate a simple and scalable process for making elastic shear-stiffening composites with locally controllable and complex geometries. We construct elastic shear-stiffening composites combining mechanical integrity with shear-stiffening behaviour and elasticity. Shear-stiffening gels were 3D-printed as thin fibres with interstitial spaces filled with polydimethylsiloxane elastomer to hold the gels in place. The composite exhibits strong impact-resistance and shape recovery, which may be due to synergistic energy absorption and dissipation at the composite interface, as well as to the elastomer architecture. Composite mechanics can also be locally modulated by tuning the infill percentages to selectively vary part stiffness and therefore aid motion and wearer comfort. Similarly, a composite hinge exhibits excellent damping, shown in a robotic demonstration.

Investigation of the intelligent anti-aging bitumen using microcapsule clean-product secreting waste cooking oil through polyacrylamide-gel in porous-shell structure

The aging of bitumen is an urgent problem in pavement engineering materials. The objective of this work was to fabricate an intelligent anti-aging bitumen using a clean microcapsule product that secretes waste cooking oil through polyacrylamide (PAM)-gel in a porous shell structure. The microcapsule had a composite shell with cross-linked hexamethoxymethylmelamine as a skeleton with a porous structure and gel PAM filled in pores as the channels of the liquid waste cooking oil as core material. Aged bitumen was mixed with various microcapsules, and their physical and chemical properties were investigated. The testing results indicated that shells had successfully microencapsulated the oily rejuvenator without defects and damages. FT-IR results showed that the HMMM had been cross-linked and PAM also deposited in shells. SEM morphology showed that PAM was removed from the shell material after alcohol elution, and the shell still kept the spherical shape with a porous structure. Penetration experiments have demonstrated that PAM in the porous structure of the shell material can serve as a secretion channel for core material. Microcapsules had perfect thermal stability and homogeneous dispersion in bitumen, and there was no interface separation phenomenon. The test results showed that the bitumen/microcapsules samples gradually became softer and increased viscosity based on the viscosity changes, softening point, and penetration parameters. The tensile test also proved that adding microcapsules reduced the tensile strength of aged bitumen and increased the tensile elongation with the extension of time, which was entirely attributed to the action of the secreted rejuvenator molecules. The complex shear modulus (G*) values of bitumen/microcapsules samples gradually decreased with anti-aging time increasing at the same temperature. The phase angle values (δ) show that its value gradually decreases after the anti-aging process, indicating that the elastic composition of bitumen increases.

Experimental and theoretical analysis of mechanical properties of FRP laminated rubber bearing

Fiber reinforced polymer (FRP) laminated rubber bearing is a kind of high-performance isolator with simple structure, low cost, light weight and excellent mechanical properties. The mechanical properties of FRP laminated rubber bearings are investigated by experimental and theoretical methods. Based on the pseudo-static vertical compression test and compression-shear test experiment, the vertical and horizontal mechanical properties of the bearings under different vertical pressures are investigated. According to the deformation compatibility and boundary conditions of elastic composites, the modified calculation equations for the vertical and horizontal stiffness of FRP laminated rubber bearings are derived, and the theoretical analysis is verified by the results of laboratory tests. The influence law of characteristic parameters such as shape factor on the mechanical properties of bearings has been investigated. Tests and theoretical analysis show that the increase of vertical pressure causes a certain degree of increase in the vertical stiffness and a significant reduction in the horizontal stiffness of the bearing. By studying the effect of horizontal and vertical stiffness, the manuscript provides a useful reference for the design of FRP laminated rubber bearings.

Tough and Elastic Cellulose Composite Hydrogels/Films for Flexible Wearable Sensors.

Cellulose and its composites, despite being abundant and sustainable, are typically brittle with very low flexibility/stretchability. This study reports a solution processing method to prepare porous, amorphous, and elastic cellulose hydrogels and films. Native cellulose dissolved in a water-ZnCl2 mixture can form ionic gels through in situ polymerization of acrylic acid (AA) to poly(acrylic acid) (PAA). The addition of up to 30 vol % AA does not change the solubility of cellulose in the water-ZnCl2 mixture. After polymerization, the formation of interpenetrated networks, resulting from the chemical cross-linking of PAA and the ionic/coordination binding among cellulose/PAA and ZnCl2, gives rise to strong, transparent, and ionically conductive hydrogels. These hydrogels can be used for wearable sensors to detect mechanical deformation under stretching, compression, and bending. Upon removal of ZnCl2 and drying the gels, semitransparent amorphous cellulose composite films can be obtained with a Young's modulus of up to 4 GPa. The rehydration of these films leads to the formation of tough, highly elastic composites. With a water content of 3-10.5%, cellulose-containing films as strong as paper also show typical characteristics of elastomers with an elongation of up to 1300%. Such composite films provide an alternative solution to resolving the material sustainability of natural polymers without compromising their mechanical properties.

Flexible Sensors Based on Conductive Polymer Composites.

Elastic polymer-based conductive composites (EPCCs) are of great potential in the field of flexible sensors due to the advantages of designable functionality and thermal and chemical stability. As one of the popular choices for sensor electrodes and sensitive materials, considerable progress in EPCCs used in sensors has been made in recent years. In this review, we introduce the types and the conductive mechanisms of EPCCs. Furthermore, the recent advances in the application of EPCCs to sensors are also summarized. This review will provide guidance for the design and optimization of EPCCs and offer more possibilities for the development and application of flexible sensors.

INVESTIGATION ON THE DYNAMICS OF MOVEMENT OF CYLINDRICAL CLEANING PIGS THROUGH THE BENDS OF PIPELINE SYSTEMS FOR FLUID TRANSPORTATION

Development and implementation of methods for cleaning the internal cavity of pipelines in the process of transporting fluids is important to ensure their energy efficiency and safe operation. The dynamics of the movement of one-piece cylindrical cleaning pistons made of various materials (silicone compound, self-destructive elastic polymer composition, polyurethane foam assembly foam) in the pipelines of pipeline systems intended for fluid transportation were studied by modelling and experimentally. The dynamics of movement of solid cylindrical cleaning pigs made from various materials (silicone compound, self-destructive elastic-polymer composition, polyurethane foam) in bends of pipeline systems intended for fluid transportation were studied through modelling and experiments. The analysis of forces acting on the pig during its movement in the pipeline bend was conducted, and the equations of pig motion based on Newton's second law were formulated. Contact forces of normal reaction, arising due to pig bending in the pipeline bend and determining the magnitude of frictional force between the pig and the inner wall of the bend, were determined based on classical bending theory. The impact of the material's modulus of elasticity, from which the pig is made, on the magnitude of contact forces of normal reaction at the pig pressing points against the inner wall of the pipeline bend was established. A method for determining the conditions under which pigs made from different materials will pass through pipeline bends without stopping and getting stuck was proposed. An equation for calculating the minimum allowable pig movement speed at the entrance to the bend to ensure its passage without stopping and getting stuck was derived.

A novel strategy and characteristics of PVA/citric acid cross-linked hydrophilic elastic sponge of cellulose based on medicinal herb residue

A new ultra-hydrophilic elastic sponge composite has been proposed. Medicinal herbs, commonly used in herbal medicine and subsequently discarded, are rich in natural polymer substances, making them promising candidates for various material industries. TEMPO-oxidized cellulose was extracted from medicinal herb residue, and the physicochemical properties of an ultra-hydrophilic elastic sponge, prepared through a PVA and CA impregnate cross-linking process, were investigated. The fabricated composite sponge exhibited an increase in compressive stress-strain proportional to the PVA cross-linking concentration, and its water retention capability was assessed through retention tests. Swelling tests for various solvents were conducted to evaluate the potential use of the sponge in diverse industries, revealing the highest swelling ratio in water. Pressure distribution measurements using prescale film indicated that the sponge's shock absorption capacity was enhanced by PVA cross-linking, leading to improved pressure dispersion.

Analysis of micropolar elastic multi-laminated composite and its application to bioceramic materials for bone reconstruction.

The asymptotic homogenization method is applied to characterize the effective behaviour of periodic multi-laminated micropolar elastic heterogeneous composites under perfect contact conditions. The local problem formulations and the analytical expressions for the effective stiffness and torque coefficients are derived for the centrosymmetric case. One of the main findings in this work is the analysis of the rotations effect of the layers' constitutive properties on the mechanical response of bi-laminated composites. The effects of microstructure and interfacial interactions on the composite's mechanical behaviour are captured through the independent effective moduli. Comparisons with the classical elastic case show the approach validation. Some numerical examples are shown. Furthermore, considering the micropolar media's prevalence in bio-inspired systems, the model's applicability is evaluated for reconstructing bone fractures using multi-laminated biocomposites. An important finding in this bio-inspired simulation is related to the analysis of a periodic bi-laminated micropolar composite whose isotropic constituents are a bioceramic material and a compact bone. This artificial bio-inspired material should integrate with host tissue to support cell growth and be stable and compatible. These characteristics are crucial in the enhancement of the fractured bone.

A Macroscopic Interpretation of the Correlation between Electrical Percolation and Mechanical Properties of Poly-(Ethylene Vinyl Acetate)/Zn Composites.

Elastic composites were prepared using a procedure involving hot plates and zinc powder that was directly dispersed into an EVA matrix. The correlation between the zinc content and the conductive properties of the material was studied via impedance spectroscopy, the thermal properties of the material were studied via differential calorimetry and the mechanical properties of the composites were studied via tensile strength curves, representing an important advancement in the characterization of this type of composite material. The composites' tensile strength and elongation at break decrease with the addition of filler since zinc particles act as stress-concentrating centres, while the composites' hardness and Young's modulus increase because of an increase in the stiffness of the material. The AC perturbation across the EVA/Zn composites was characterized using an RC parallel equivalent circuit that allowed us to easily measure their resistivity (ρp) and permittivity (εp). The dependence of these electrical magnitudes on the zinc content is correlated with their mechanical properties across the characteristic time constant τp = ρp·εp of this equivalent circuit. The dependence of the mechanical and electrical magnitudes on the zinc content is consistent with the formation of percolation clusters. The addition of graphite particles increases their potential performance. Three possible mechanisms for the electrical transport of the ac-perturbation across the EVA/Zn composites have been identified. Chemical corrosion in acid media causes the loss of zinc surface particles, but their bulk physical properties practically remain constant.

A Low-Cost Soft Gripper for Automated Pick-and-Place Systems

The study introduces a pioneering low-cost soft gripper designed for the automation industry, specifically targeting sensitive handling requirements in the food and health sectors. Utilizing rubber or elastic composites, the gripper combines improved sensitivity and adaptability to handle delicate materials safely and efficiently, aligning with stringent industry standards. Central to the design is a pneumatic control system that ensures precise manipulation, allowing the gripper to adapt to varying object contours without causing damage. This innovation not only addresses the mechanical aspects of soft robotics but also integrates seamlessly with Industry 4.0 technologies through smart sensors and AI algorithms, enhancing operational intelligence and versatility. By leveraging additive manufacturing techniques, the design also achieves significant cost reductions, facilitating broader adoption. This study's soft gripper represents a critical step forward in the development of robotic systems that can perform complex tasks with high sensitivity and economic efficiency, promising transformative impacts on the automation capabilities of sensitive industrial sectors.

Phase-separated porous nanocomposite with ultralow percolation threshold for wireless bioelectronics.

Realizing the full potential of stretchable bioelectronics in wearables, biomedical implants and soft robotics necessitates conductive elastic composites that are intrinsically soft, highly conductive and strain resilient. However, existing composites usually compromise electrical durability and performance due to disrupted conductive paths under strain and rely heavily on a high content of conductive filler. Here we present an in situ phase-separation method that facilitates microscale silver nanowire assembly and creates self-organized percolation networks on pore surfaces. The resultant nanocomposites are highly conductive, strain insensitive and fatigue tolerant, while minimizing filler usage. Their resilience is rooted in multiscale porous polymer matrices that dissipate stress and rigid conductive fillers adapting to strain-induced geometry changes. Notably, the presence of porous microstructures reduces the percolation threshold (Vc = 0.00062) by 48-fold and suppresses electrical degradation even under strains exceeding 600%. Theoretical calculations yield results that are quantitatively consistent with experimental findings. By pairing these nanocomposites with near-field communication technologies, we have demonstrated stretchable wireless power and data transmission solutions that are ideal for both skin-interfaced and implanted bioelectronics. The systems enable battery-free wireless powering and sensing of a range of sweat biomarkers-with less than 10% performance variation even at 50% strain. Ultimately, our strategy offers expansive material options for diverse applications.

An FFT-based adaptive polarization method for infinitely contrasted media with guaranteed convergence

We propose an FFT-based iterative algorithm for solving the Lippmann–Schwinger equation in the context of periodic homogenization of infinitely contrasted linear elastic composites. Our work initially reformulates the Moulinec–Suquet, Eyre–Milton and Monchiet–Bonnet schemes using a residual formulation. Subsequently, we introduce an enhanced scheme, termed Adaptive Eyre–Milton (AEM), as a natural extension of the EM scheme where we optimize a relaxation parameter to minimize the residual. We demonstrate the unconditional linear convergence of the AEM scheme, regardless of initialization and the chosen reference material. The paper further extends the AEM scheme to handle composites with both pores and infinitely rigid inclusions. Practical implementation aspects and illustrative applications in two- and three-dimensional settings are discussed, highlighting the efficiency of the proposed AEM scheme. We particularly emphasize the scheme’s robustness for materials with infinitely large contrasts in elastic properties.

Self-Assembly Enabled Printable Asymmetric Self-Insulated Stretchable Conductor for Human Interface.

Soft and stretchable conductors with high electrical conductivity and tissue-like mechanical properties are crucial for both on-skin and implantable electronic devices. Liquid metal-based conductors hold great promise due to their metallic conductivity and minimal stiffness. However, the surface oxidation of liquid metal particles in polymeric matrices poses a challenge in forming a continuous pathway for highly conductive elastic composites. Here, it is reported a printable composite material based on liquid metal and conducting polymer that undergoes a self-assembly process, achieving high conductivity (2089 S cm-1) in the bottom surface while maintaining an insulated top surface, high stretchability (>800%), and a modulus akin to human skin tissue. This material is further applied to fabricate skin-interfaced strain sensors and electromyogram sensors through 3D printing.

Accounting for spatial distribution in mean-field homogenization of particulate composites

Several analytical mean-field homogenization methods, which take into account the particle volume fraction, shape and orientation are readily available to estimate the effective properties of particulate composites. Models have also been proposed to account for the spatial distribution of the particles. The classical Ponte-Castañeda and Willis (PCW) model is based on a parametrization of the statistical distribution law, while the Interaction Direct Derivative (IDD) model associates a matrix cell to each inclusion, representative of close interactions. In the literature, the use of the IDD is commonly reduced to the particular case of the classical Mori and Tanaka (MT) scheme or to the aforementioned PCW model. The present study introduces an original approach to calibrate the IDD model, for 2D linear conductivity, based on representative images of the microstructure. The links between the models and the range of validity of the IDD model are discussed. Besides, an “IDD-based” PCW model and a two-step scheme are proposed for situations where the IDD estimate is inconsistent (lack of major symmetry). Finally, an image analysis method using Voronoï diagrams is implemented to define the cells associated to each inclusion and supply the models. The method is validated by comparisons between the obtained IDD and PCW estimates, the Mori–Tanaka (MT) model and benchmark full-field numerical simulations. Accounting for the inclusion distribution is seen to lead to better estimates, both qualitatively (by capturing anisotropic behaviors due to the sole distribution) and quantitatively. Possible extensions to elastic composites are discussed.

3D Printing Bio‐Inspired Micro Soft Robot with Programming Magnetic Elastic Composites

AbstractBio‐inspired micro soft robots mimic biologically specific body structures and movement mechanisms by utilizing bionic principles. Because of its soft body, it can adapt to complex external environments. However, the existing polymer material system is single and the fabrication process is limited. How to further reduce soft robot size has become a key issue. In this work, a Programming Magnetic Elastic Composites (PMEC) is proposed for 3D printing to manufacture micro soft robots. The PMEC has the advantage of changing the direction of the internal magnetic field in response to changes in the external magnetic field. A Microsoft robot is designed and fabricated using PMEC inspired by an inchworm. Numerical simulations are also used to study the effect of soft robot size parameters on local stresses and optimize the structure. The results show that the microscale soft robot can crawl with a load of 2 times its weight at a speed of 6.67 mm s−1, crawl on slopes from 0° to 90°, and crawl over obstacles with a maximum height of 7 mm. In addition, active adaptation of soft robots to complex tunnel models based on external stimuli is achieved by magnetic field control.

Interfacial Triboelectricity Lights Up Phosphor-Polymer Elastic Composites: Unraveling the Mechanism of Mechanoluminescence in Zinc Sulfide Microparticle-Embedded Polydimethylsiloxane Films.

Composites comprising copper-doped zinc sulfide phosphor microparticles embedded in polydimethylsiloxane (ZnS:Cu-PDMS) have received significant attention over the past decade because of their bright and durable mechanoluminescence (ML); however, the underlying mechanism of this unique ML remains unclear. This study reports empirical and theoretical findings that confirm this ML is an electroluminescence (EL) of the ZnS:Cu phosphor induced by the triboelectricity generated at the ZnS:Cu microparticle-PDMS matrix interface. ZnS:Cu microparticles that exhibit bright ML are coated with alumina, an oxide with strong positive triboelectric properties; the contact separation between this oxide coating and PDMS, a polymer with strong negative triboelectric properties, produces sufficient interfacial triboelectricity to induce EL in ZnS:Cu microparticles. The ML of ZnS:Cu-PDMS composites varies on changing the coating material, exhibiting an intensity that is proportional to the amount of interfacial triboelectricity generated in the system. Finally, based on these findings, a mechanism that explains the ML of phosphor-polymer elastic composites (interfacial triboelectric field-driven alternating-current EL model) is proposed in this study. It is believed that understanding this mechanism will enable the development of new materials (beyond ZnS:Cu-PDMS systems) with bright and durable ML.