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Skin is the largest organ of many animals. Its protective function against hostile environments and predatorial attack makes high mechanical strength a vital characteristic. Here, we measured the mechanical properties of bass fish skins and found that fish skins are highly ductile with a rupture strain of up to 30–40% and a rupture strength of 10–15 MPa. The fish skins exhibit a strain-stiffening behavior. Stretching can effectively eliminate the stress concentrations near the pre-existing holes and edge notches, suggesting that the skins are highly damage tolerant. Our measurement determined a flaw-insensitivity length that exceeds those of most engineering materials. The strain-stiffening and damage tolerance of fish skins are explained by an agent-based model of a collagen network in which the load-bearing collagen microfibers assembled from nanofibrils undergo straightening and reorientation upon stretching. Our study inspires the development of artificial skins that are thin, flexible, but highly fracture-resistant and widely applicable in soft robots. [ABSTRACT FROM AUTHOR]

Double network (DN) hydrogels have been recognized as new tough materials for several industries due to their precise structural platforms and significant properties. However, a comprehensive understanding of microstructural changes of DN hydrogels under large deformations is required to extend their applications. In this work, we use the large amplitude oscillatory shear (LAOS) technique to study the nonlinear response of a thermoresponsive κ-carrageenan/polyacrylamide DN system and its nanocomposite containing graphene oxide (GO) in comparison to its single network components. The results show a combination of strain stiffening and shear thickening nonlinear responses. The elastic intracycle strain stiffening was mainly attributed to the shear-induced increase in the elasticity of network chains and non-Gaussian stretching of individual chains. In addition, the orientation of the κ-carrageenan double helix segments and their enhancing effect on molecular orientation could be proposed as another possible mechanism of strain stiffening. The viscous intracycle shear thickening is also interpreted by two mechanisms of shear-induced temporary structure formation and reformation of dissociated physical interactions. It is also found that the GO nanosheets could contribute to the viscoelastic response by increasing the molecular interactions and, thus, amplification of energy dissipation. Furthermore, temperature dependency of the DN hydrogel owing to the conformational changes of the κ-carrageenan network at sufficiently high temperatures is used to investigate the effect of temperature on nonlinear behaviors. Increasing the temperature is found to have a significant decreasing effect on viscous nonlinearity, while its effect on the elastic nonlinearity was strongly dependent on the strain amplitude. This study provides a better understanding of the correlation between the microstructure and viscoelastic properties for designing tough hydrogels. [ABSTRACT FROM AUTHOR]

Fiber networks are the primary structural components of many biological structures, including the cell cytoskeleton and the extracellular matrix. These materials exhibit global nonlinearities, such as stiffening in extension and shear, during which the fibers bend and align with the direction of applied loading. Precise details of deformations at the scale of the fibers during strain stiffening are still lacking, however, as prior work has studied fiber alignment primarily from a qualitative perspective, which leaves incomplete the understanding of how the local microstructural evolution leads to the global mechanical behavior. To fill this gap, we studied how axial forces are transmitted inside the fiber network along paths called force chains, which continuously evolve during the course of deformation. We performed numerical simulations on two-dimensional networks of random fibers under uniaxial extension and shear, modeling the fibers using beam elements in finite element software. To quantify the force chains, we identified all chains of connected fibers for which the axial force was larger than a preset threshold and computed the total length of all such chains. To study the evolution of force chains during loading, we computed the derivative of the total length of all force chains with respect to the applied engineering strain. Results showed that the highest rate of evolution of force chains coincided with the global critical strain for strain stiffening of the fiber network. Therefore, force chains are an important factor connecting understanding of the local kinematics and force transmission to the macroscale stiffness of the fiber network. [ABSTRACT FROM AUTHOR]

Hydrogels are often used to study the impact of biomechanical and topographical cues on cell behavior. Conventional hydrogels are designed a priori , with characteristics that cannot be dynamically changed in an externally controlled, user-defined manner. We developed a composite hydrogel, termed an acoustically-responsive scaffold (ARS), that enables non-invasive, spatiotemporally controlled modulation of mechanical and morphological properties using focused ultrasound. An ARS consists of a phase-shift emulsion distributed in a fibrin matrix. Ultrasound non-thermally vaporizes the emulsion into bubbles, which induces localized, radial compaction and stiffening of the fibrin matrix. In this in vitro study, we investigate how this mechanism can control the differentiation of fibroblasts into myofibroblasts, a transition correlated with substrate stiffness on 2D substrates. Matrix compaction and stiffening was shown to be highly localized using confocal and atomic force microscopies, respectively. Myofibroblast phenotype, evaluated by α-smooth muscle actin (α-SMA) immunocytochemistry, significantly increased in matrix regions proximal to bubbles compared to distal regions, irrespective of the addition of exogenous transforming growth factor-β1 (TGF-β1). Introduction of the TGF-β1 receptor inhibitor SB431542 abrogated the proximal enhancement. This approach providing spatiotemporal control over biophysical signals and resulting cell behavior could aid in better understanding fibrotic disease progression and the development of therapeutic interventions for chronic wounds. Hydrogels are used in cell culture to recapitulate both biochemical and biophysical aspects of the native extracellular matrix. Biophysical cues like stiffness can impact cell behavior. However, with conventional hydrogels, there is a limited ability to actively modulate stiffness after polymerization. We have developed an ultrasound-based method of spatiotemporally-controlling mechanical and morphological properties within a composite hydrogel, termed an acoustically-responsive scaffold (ARS). Upon exposure to ultrasound, bubbles are non-thermally generated within the fibrin matrix of an ARS, thereby locally compacting and stiffening the matrix. We demonstrate how ARSs control the differentiation of fibroblasts into myofibroblasts in 2D. This approach could assist with the study of fibrosis and the development of therapies for chronic wounds. [Display omitted] [ABSTRACT FROM AUTHOR]

Transdermal therapeutic systems (TTS) based on pressure-sensitive adhesives (PSAs) allow for the application of pharmaceutical substances via diffusion through the skin. The rheological performance of PSA is largely investigated within small amplitude oscillatory shear (typically up to 1 %), although the skin motions exceed strains beyond 40 %. In this paper, amine-compatible (AC) and non-amine-compatible (NAC) silicone-based PSA compounds differing in the resin content were subjected to strain amplitude sweeps in a twin drive rheometer. Carreau-Yasuda-like fitting of storage and loss moduli curves intercepts the substantial effect of resin content on both compounds; up to four times higher, moduli of AC compounds were determined in SAOS, and their higher molecular mass combined with enhanced interactions contributed to an earlier transition to the nonlinear viscoelastic region. In the nonlinear range, elastic and viscous properties are affected by strains in a different manner with the trend favorable for the PSA application as TTS. The third relative higher harmonic from Fourier transformation I3/1 as well as intra-cycle strain stiffening and shear thickening ratios provide information relevant for an optimization of PSA subjected to large deformations. [ABSTRACT FROM AUTHOR]

Vitrimer is a polymer network with dynamic covalent bonds, which can be dynamically broken and reformed. Thanks to the dynamic covalent bonds, vitrimer behaves like an elastomer at high deformation rate or low temperature but a viscous fluid at low deformation rate or high temperature. In this article, we study the cavitation dynamics in a vitrimer, which is known to be an important damaging mechanism in it. In the formulation, a nonlinear three-dimensional rate-dependent constitutive model for the vitrimer is adopted, in which both of the strain stiffening and the kinetics of dynamic exchanging reactions are considered. The theory predicts strong dependence of the cavitation dynamics on the loading rate. The computational results may help to reveal some critical insights into the rate-dependent fracture in vitrimer. [ABSTRACT FROM AUTHOR]

Basement membrane (BM) is a thin layer of extracellular matrix that surrounds most animal tissues, serving as a physical barrier while allowing nutrient exchange. Although they have important roles in tissue structural integrity, physical properties of BMs remain largely uncharacterized, which limits our understanding of their mechanical functions. Here, we perform pressure-controlled inflation and deflation to directly measure the nonlinear mechanics of BMs in situ. We show that the BMs behave as a permeable, hyperelastic material whose mechanical properties and permeability can be measured in a model-independent manner. Furthermore, we find that BMs exhibit a remarkable nonlinear stiffening behavior, in contrast to the reconstituted Matrigel. This nonlinear stiffening behavior helps the BMs to avoid the snap-through instability (or structural softening) widely observed during the inflation of most elastomeric balloons and thus maintain sufficient confining stress to the enclosed tissues during their growth. [ABSTRACT FROM AUTHOR]

Metamaterials show elaborate mechanical behavior such as strain stiffening, which stems from their unit cell design. However, the stiffening response is typically programmed in the design step and cannot be adapted postmanufacturing. Here, we show hydrogel metamaterials with highly programmable strain-stiffening responses by exploiting the out-of-plane buckling of integrated pH-switchable hydrogel actuators. The stiffening upon reaching a certain extension stems from the initially buckled active hydrogel beams. At low strain, the beams do not contribute to the mechanical response under tension until they straighten with a resulting step-function increase in stiffness. In the hydrogel actuator design, the acrylic acid concentration hard-codes the configuration of the metamaterial and range of possible stiffening onsets, while the pH soft-codes the exact stiffening onset point after fabrication. The utilization of out-of-plane buckling to realize subsequent stiffening without the need to deform the passive structure extends the application of hydrogel actuators in mechanical metamaterials. Our concept of out-of-plane buckled active elements that stiffen only under tension enables strain-stiffening metamaterials with high programmability before and after fabrication.

In order to accurately predict the cellular response, it is necessary, along with other factors, to consider the effect of the cell spreading on the substrate. Also, the core tensions, due to the cell spreading, play a crucial role in the fate of a stem cell. Therefore, the exact prediction of these tensions is of particular importance. The effect of the strain stiffening of a mesenchymal cell, in a two-dimensional model, was investigated numerically using finite element method, by exerting a time function displacement, to the cytoplasm boundary. Utilizing Schwartz-Christoffel transformation, a model for cell-spreading was proposed that can be used to achieve accurate cellular responses. Three different models are considered. In the first model, the cell is treated as a non-alive material. That is, the mechanical properties remain constant on the substrate. Two other models, the linear and exponential strain-stiffening, are active models. By comparing the results of these models with the experimental results, it was found that the assumption that the cell is inactive departs the response from the exact amount. Therefore, considering the cell’s living nature, in both linear and exponential models, leads to more similarity of the results, both the tension value and the slope of the variations, with the experimental observations. Furthermore, by increasing the amount of the cell spreading, the difference in the amount of the nucleus stress in active models with the inactive model increases, so that the predicted tension by the linear model reaches 2.3 times that predicted by the non-alive model.

The structural organization of peripheral nerves enables them to adapt to different body postures and movements by varying their stiffness. Indeed, they could become either compliant or stiff in response to the amount of external solicitation. In this work, the global response of nerves to axial stretch was deterministically derived from the interplay between the main structural constituents of the nerve connective tissue. In particular, a theoretical framework was provided to explicitly decouple the action of the ground matrix and the contribution of the collagen fibrils on the macroscopic stiffening of stretched nerves. To test the overall suitability of this approach, as a matter of principle, the change of the shape of relevant curves was investigated for changes of numerical parameters, while a further sensitivity study was performed to better understand the dependence on them. In addition, dimensionless stress and curvature were used to quantitatively account for both the matrix and the fibril actions. Finally, the proposed framework was used to investigate the stiffening phenomenon in different nerve specimens. More specifically, the proposed approach was able to explicitly and deterministically model the nerve stiffening of porcine peroneal and canine vagus nerves, closely reproducing ( R 2 > 0.997 ) the experimental data. [ABSTRACT FROM AUTHOR]

We study experimentally the rheological behavior of carbon black (CB) suspensions in water at different ionic strengths and concentrations. We show by means of standard rheometry completed by local magnetic resonance imaging-rheometry that these suspensions first appear to be thixotropic yield stress fluids: they exhibit a yield stress increasing with the time of rest, their apparent viscosity decreases under shear, and a viscosity bifurcation occurs around the yield stress, the fluid evolving either towards stoppage or to steady flow at a large shear rate for a small stress change. Then, an original effect appears when we follow the mechanical state of the material in the solid regime by measuring its apparent elastic modulus at small deformation during a creep test under various stresses. In contrast with various other yield stress fluids for which the elastic modulus under small deformation appears to be constant for any deformation in the solid regime, for CB suspensions, this modulus widely increases while deformation increases up to yielding. We suggest that this strain stiffening effect finds its origin in the specificities of the (van der Waals) interactions and of the (rough) structure (aggregates) of the particles: the slight relative rotation of particles in contact due to deformation would, on average, tend to increase the net area of contact between particles, which stiffens the whole material structure. This is supported by the observation that the relative increase of elastic modulus is approximately proportional to sample deformation, whatever the material characteristics (ionic strength, concentration) and whatever the deformation history. [ABSTRACT FROM AUTHOR]

Hybrid biomimetic hydrogels with enhanced reactive oxygen species (ROS)‐generation efficiency under 600 nm light show high antibacterial activity. The hybrid gels are composed of helical tri(ethylene glycol)‐functionalized polyisocyanides (PICs) and a conformation‐sensitive conjugated polythiophene, poly(3‐(3′‐N,N,N‐triethylammonium‐1′‐propyloxy)‐4‐methyl‐2,5‐thiophene chloride) (PMNT). The PIC polymer serves as a scaffold to trap and align the PMNT backbone into a highly ordered conformation, resulting in redshifted, new sharp bands in the absorption and fluorescence spectra. Similar to PIC, the hybrid closely mimics the mechanical properties of biological gels, such as collagen and fibrin, including the strain stiffening properties at low stresses. Moreover, the PMNT/PIC hybrids show much higher ROS production efficiency under red light than PMNT only, leading to an efficient photodynamic antimicrobial effect towards various pathogenic bacteria. [ABSTRACT FROM AUTHOR]

• We provide a quantitative framework able to reproduce the evolution with strain of the instantaneous stiffness. • We provide a quantitative framework able to reproduce the evolution with strain of the rate of change of the instantaneous stiffness. • We compare the strain stiffening phenomenon in two different nerves and we account for the main features of each specimen through quantitative indexes. The mechanical response of peripheral nerves is crucial to understand their physiological and pathological conditions. However, their response to external mechanical solicitations is still partially unclear, since peripheral nerves could behave in a quite complex way. In particular, nerves react to longitudinal strains increasing their stiffness to keep axons integrity and to preserve endoneural structures from overstretch. In this work, the strain stiffening of peripheral nerves was investigated in vitro through a recently introduced computational framework, which is able to theoretically reproduce the experimental behaviour of excised tibial and sciatic nerves. The evolution and the variation of the tangent modulus of tibial and sciatic nerve specimens were quantitatively investigated and compared to explore how stretched peripheral nerves change their instantaneous stiffness. [ABSTRACT FROM AUTHOR]

Due to their outstanding physical and mechanical properties, double-network (DN) hydrogels have gained a most attention among various synthetic tough hydrogels. The Mullins effect and buckling instability are two frequent phenomena that may occur simultaneously in slender DN hydrogel structures as high load-bearing candidates. The swelling/deswelling degree of the DN hydrogel affects the coupling phenomena. It is essential to understanding the interplay between these behaviors. In this work, a physically based damage constitutive model for a DN polymer under water-diffusion equilibrium and cyclic loadings is developed. This theory of coupled swelling and load-induced deformations show good capability in capturing the Mullins effect of swollen DN hydrogels and the corresponding swelling ratio. The model parameters are determined by fitting experimental data of a freely swollen DN hydrogel under cyclic compression. Based on the constitutive model and determined parameters, the buckling instability of DN hydrogel beams is studied via the analytical formula of incremental modulus. The stability diagrams of the DN hydrogel beams under virgin loading and reloading are presented. The influences of stress softening, strain stiffening and chemical potential on buckling conditions for compressive stress and slenderness ratio are thoroughly analyzed. It demonstrates that the stress softening is dramatically against the stability of the beam, but the strain-stiffening effect would conversely help widen the stable range. Besides, it is found that a DN gel beam immersed in a sufficient low chemical potential environment has better buckling stability. These theoretical results are valuable in the preparation and structural design of DN hydrogels for repeated use purpose. • A constitutive model for double-network (DN) polymers under swelling and cyclic loads. • Global instability diagrams of DN hydrogel beams with various damage degrees. • Analysis of impacts of the Mullins effect and water content on buckling conditions. [ABSTRACT FROM AUTHOR]

The gelation procedure and the gelation time of gelatin gels may lead to apparently similar materials, however, with different rheological fingerprints under small and large oscillatory shear deformations. Here, in the first paper of this series, gelation of 3 and 5% w/w gelatin solutions at 5 °C was performed in situ on the rheometer plate and in custom-made casting modules to obtain individual gel layers. Large amplitude oscillatory shear (LAOS) tests were performed. The nonlinear deformation regime was qualitatively analyzed using normalized Lissajous-Bowditch curves. The MITlaos software was employed to decompose the total intracycle stress response and to calculate the Chebyshev coefficients ratios. The dynamic moduli of the fresh gels were measured directly after the in situ preparation and within a time frame until 1.5 h. In the strain sweeps, we observed intense stiffening followed by yielding above 200% strain. However, the individual gel layers aged for 24 and 48 h show different LAOS fingerprints. The extensive loops in the viscous Lissajous-Bowditch curves indicate an elastoplastic material response. Based on the overall nonlinear rheological response, we propose a structural formation that describes the behavior of the gels for the conditions studied here. In the second paper of this series, we give the impact of hard micro-fillers (glass beads) and we describe the nonlinear characteristics of the filled gels. [ABSTRACT FROM AUTHOR]

This work discusses the first set of rheometric measurements carried out on commercially accessible juvenile and aged skin models under large amplitude oscillatory shear deformations. The results were compared with those of native male whole human and dermis-only foreskin specimens, catering to a few ages from 0.5 to 68 years, including specimens from a 23-year-old male abdomen. At large strains, strain thinning was more pronounced for the dermis of the young skins and for their whole skin counterparts. An inverse qualitative tendency was observed for the adult skins and the skin models. This can be explained by the high dermal collagen compactness associated with an incomplete epidermal proliferation. The qualitative Lissajous plots as well as the quantitative dimensionless indices analyzed using the MITlaos software indicated predominant nonlinear intracycle elastic strain stiffening and viscous shear thinning for all the native specimens at the maximum deformation. For the full thickness models, we have evidence of structure collapse and yielding under similar conditions. The whole skin specimen from the 68-year-old male showed smaller age-dependent nonlinear elastic contributions than the dermis, which we relate to the epidermal degeneration taking place during aging. Regardless of the age group, the models manifested more pronounced intercycle and intracycle elastic nonlinearities, and their magnitudes were significantly larger. The nonlinear elastic trends will serve as advanced standards for understanding and delineating the mechanical limits of destructive and non-destructive deformations of such unique biomaterials. Image 1 • Skin models show stronger intercycle elastic strain thinning than native skins. • Specimen independent intracycle elastic strain stiffening at high strains. • Native skin nonlinearity greatly affected by age and epidermis. • Native skins exhibit higher deformation-resisting abilities at maximum strain. • LAOS fingerprints of 3D skin models do not comply with those of native skins. [ABSTRACT FROM AUTHOR]

We present an approach to understand geometric-incompatibility– induced rigidity in underconstrained materials, including subisostatic 2D spring networks and 2D and 3D vertex models for dense biological tissues. We show that in all these models a geometric criterion, represented by a minimal length ℓmin, determines the onset of prestresses and rigidity. This allows us to predict not only the correct scalings for the elastic material properties, but also the precise magnitudes for bulk modulus and shear modulus discontinuities at the rigidity transition as well as the magnitude of the Poynting effect. We also predict from first principles that the ratio of the excess shear modulus to the shear stress should be inversely proportional to the critical strain with a prefactor of 3. We propose that this factor of 3 is a general hallmark of geometrically induced rigidity in underconstrained materials and could be used to distinguish this effect from nonlinear mechanics of single components in experiments. Finally, our results may lay important foundations for ways to estimate ℓmin from measurements of local geometric structure and thus help develop methods to characterize large-scale mechanical properties from imaging data. [ABSTRACT FROM AUTHOR]

The structural organization of peripheral nerves enables them to adapt to different body postures and movements by varying their stiffness. Indeed, they could become either compliant or stiff in response to the amount of external solicitation. In this work, the global response of nerves to axial stretch was deterministically derived from the interplay between the main structural constituents of the nerve connective tissue. In particular, a theoretical framework was provided to explicitly decouple the action of the ground matrix and the contribution of the collagen fibrils on the macroscopic stiffening of stretched nerves. To test the overall suitability of this approach, as a matter of principle, the change of the shape of relevant curves was investigated for changes of numerical parameters, while a further sensitivity study was performed to better understand the dependence on them. In addition, dimensionless stress and curvature were used to quantitatively account for both the matrix and the fibril actions. Finally, the proposed framework was used to investigate the stiffening phenomenon in different nerve specimens. More specifically, the proposed approach was able to explicitly and deterministically model the nerve stiffening of porcine peroneal and canine vagus nerves, closely reproducing (R2>0.997) the experimental data.

When a confined elastic layer is under tension, undulations can occur at its exposed surfaces, giving the fingering or fringe instability. These instabilities are of great concern in the design of robust adhesives, since they not only initiate severe local deformations in adhesive layers but also cause non-monotonic overall stress vs. stretch relations of the layers. Here, we show that the strain stiffening of soft elastic materials can significantly delay and even suppress the fringe and fingering instabilities, and give monotonic stress vs. stretch relations. Instability development requires local large deformation, which can be inhibited by material-stiffening. We provide a quantitative phase diagram to summarize the stiffening's effects on the instabilities and stress vs. stretch relations in confined elastic layers. We further use numerical simulations and experiments to validate our findings. [ABSTRACT FROM AUTHOR]

The mechanical properties of tissues are important indicators of tissue “health”. Diseased tissues due to cancer and other causes tend to stiffen with increase in strain, exhibiting a nonlinear stress–strain behavior. The hyperelastic mechanical behavior of soft tissues can be characterized by an exponential model with two material parameters, namely the shear modulus μ and a nonlinearity parameter γ . A variety of methods and techniques have been developed to solve inverse problems in nonlinear elasticity to determine these properties given the mechanical response of the tissues. Reconstruction of the nonlinear parameter from noisy measurements of displacement response is a difficult problem, and obtaining a well-recovered solution is challenging. This article is directed towards the improvement in the reconstruction of the nonlinear parameter, γ , by introducing a new parameter, which is a combination of γ and the first invariant of the Cauchy–Green deformation tensor. Comparative study is carried out between reconstructions of γ directly from previously existing formulations and the reconstruction of γ from the new parameter, for 2D problems. The new method is compared with previous methods using numerical experiments in terms of shape of the stiff regions, the contrast in γ and robustness under different loading conditions. We find that the new method is a considerable improvement to previous methods and could be a valuable tool in biomedical applications. [ABSTRACT FROM AUTHOR]

• Effects of micro/nanostructures on the strain stiffening behaviors of hydrogels. • Helmholtz free energy capturing the potential energy of physical crosslink. • Fiber orientation model correlating nanofiber orientation with mechanical stretch. • Evolution of nanofiber orientation in nanocellulose hydrogels under stretching. Nanocellulose hydrogels that consist of physically crosslinked nanofibrous networks with large amounts of interstitial water have many potential applications as structural materials in biomedical engineering. Quantifying the relationship between micro/nanostructures and their mechanical responses can provide some guidelines for the high-performance mechanical design of counterparts. To this end, we develop a modified physically based constitutive model for examining the effects of the micro/nanostructures of a nanofibrous network on the strain stiffening behaviors of nanocellulose hydrogels under stretching. In addition to the configurational entropy of cellulose nanofiber and mixing energy of water molecule, this theoretical model is particularly concerned with the contribution of the potential energy of hydrogen bond in a thermodynamics framework. Increasing the physical crosslinks between cellulose nanofibers or decreasing the content of water molecules in a certain range can enhance the strain stiffening behaviors of nanocellulose hydrogels. Relatively long and thin cellulose nanofibers have increased tensile strengths. The theoretical predictions are in good agreement with the relevant experimental results. We propose a fiber orientation model for quantifying the relationship between the nanofiber orientation degree and mechanical stretching. This model provides a theoretical foundation for fabricating anisotropic nanofibrous hydrogels via cold drawing. [ABSTRACT FROM AUTHOR]

Mechanical characterization of living cells undergoing substantial external strain promises insights into material properties and functional principles of mechanically active tissues. However, due to the high strains that occur in physiological situations (up to 50%) and the complex structure of living cells, suitable experimental techniques are rare. In this study, we introduce a new system composed of an atomic force microscope (AFM), a cell stretching system based on elastomeric substrates, and light microscopy. With this system, we investigated the influence of mechanical stretch on monolayers of keratinocytes. In repeated indentations at the same location on one particular cell, we found significant stiffening at 25% and 50% strain amplitude. To study the contribution of intermediate filaments, we used a mutant keratinocyte cell line devoid of all keratins. For those cells, we found a softening in comparison to the wild type, which was even more pronounced at higher strain amplitudes.

Excellent elasticity and thermal stability are both essential for the applications of ultra-high temperature sealing materials. Since strain-stiffening behavior guarantees the materials can sustain large strains without initiation of irreversible defects, seeking for strain-stiffening materials with good thermal stability is significant for the development of thermal sealing at ultra-high temperatures. Strain-stiffening behaviors of crystalline Y 2 B 3 C 2 , a new member of ultra-high temperature ceramics, are predicted under (010)[001] and (010)[100] shear deformation. The moduli against these two shears are extremely low at first, but gradually increase to handsome values of 62 and 101 GPa. The ultra-high ideal shear strain and considerable ideal stress indicate the promising applications of Y 2 B 3 C 2 in thermal sealing at ultra-high temperatures. It is noticed that the strain-stiffening effect is originated from the deformable planar B-C network. The special bonding features in the CBC-chain are found to be responsible for the easy distortion of the covalent network. [ABSTRACT FROM AUTHOR]

Polymer nanocomposites--materials in which a polymer matrix is blended with nanoparticles (or fillers)--strengthen under sufficiently large strains. Such strain hardening is critical to their function, especially for materials that bear large cyclic loads such as car tires or bearing sealants. Although the reinforcement (i.e., the increase in the linear elasticity) by the addition of filler particles is phenomenologically understood, considerably less is known about strain hardening (the nonlinear elasticity). Here, we elucidate the molecular origin of strain hardening using uniaxial tensile loading, microspectroscopy of polymer chain alignment, and theory. The strain-hardening behavior and chain alignment are found to depend on the volume fraction, but not on the size of nanofillers. This contrasts with reinforcement, which depends on both volume fraction and size of nanofillers, potentially allowing linear and nonlinear elasticity of nanocomposites to be tuned independently [ABSTRACT FROM AUTHOR]

The structure, mechanical properties, and optical density of gels prepared from collagen in a manner that induces the dynamical arrest of the constituent polymers before substantial fibrillogenesis can take place have been investigated. Small angle X-ray scattering and confocal laser scanning fluorescence microscopy reveal that these gels exhibit substantially different network structures, over length scales ranging from a few nanometers to many microns, when compared with traditional collagen networks in which fibrillogenesis is intentionally induced. The highly associated arrangements of the more flexible structural components found in the arrested network yield a considerably lower optical density and higher viscoelastic storage modulus when compared to a “conventional” collagen gel; while the small amount of fibrils that do manage to form still yield strain stiffening and account for the fact that at high strains, moduli from both systems fall onto the same master curve. [ABSTRACT FROM AUTHOR]

We investigate the linear and nonlinear viscoelastic properties as well as the reversibility of strain-stiffening behavior of silk fibroin gels. The gels are prepared from 4.2 w/v% fibroin solution in the presence of butanediol diglycidyl ether and N , N , N' , N' -tetramethylethylenediamine (TEMED) as a cross-linker and catalyst, respectively. By changing the concentration of TEMED in the gelation system, fibroin gels exhibiting a storage modulus G’ between 10 −1 –10 5 Pa and a loss factor tan δ between 10 −2 and 10° could be obtained. We observe a strong stiffening (up to 900%) in fibroin gels with increasing strain above 10% deformation, but reversibly if the strain is removed, the gel recovers its initial viscoelastic properties. The strain induced formation of transient intermolecular domains acting as reversible cross-links are responsible for the stiffening behavior of fibroin gels. These additional cross-links formed in the hardened fibroin gels have a temporary nature with lifetimes of the order of seconds. The nonlinear behavior of fibroin gels can be reproduced by a wormlike chain model taking into account the entropic elasticity of fibroin molecules and the strain induced increase in the cross-link density of fibroin gels. [ABSTRACT FROM AUTHOR]

Highly stretchable and conductive ionogels have great potential in flexible electronics and soft robotic skins. However, current ionogels are still far from being able to accurately duplicate the mechanically responsive behavior of real human skin. Furthermore, durable robotic skins that are applicable under harsh conditions are still lacking. Herein, a strong noncovalent interaction, ionic clusters, is combined with hydrogen bonds to obtain a physically cross-linked ionogel (PCI). Benefiting from the strong ionic bonding of the ionic cluster, the PCI shows strain-stiffening behavior similar to that of human skin, thus enabling it to have a perception-strengthening ability. Additionally, the strong ionic clusters can also ensure the PCI remains stable at high temperatures. Even when the temperature is raised to 200 °C, the PCI can maintain the gel state. Moreover, the PCI exhibits high transparency, recyclability, good adhesion, and high conductivity. Such excellent features distinguish the PCI from ordinary ionogels, providing a new way to realize skin-like sensing in harsh environments for future bionic machines.

Elastomers are indispensable materials due to their flexible, stretchable, and elastic nature. However, the polymer network structure constituting an elastomer is generally inhomogeneous, limiting the performance of the material. Here, a highly stretchable elastomer with unprecedented strain-stiffening capability is developed based on a highly homogeneous network structure enabled by a module assembly strategy. The elastomer is synthesized by efficient end-linking of a star-shaped aliphatic polyester precursor with a narrow molecular-weight distribution. The resulting product shows high strength (≈26 MPa) and remarkable stretchability (stretch ratio at break ≈1900%), as well as good fatigue resistance and notch insensitivity. Moreover, it shows extraordinary strain-stiffening capability (>2000-fold increase in the apparent stiffness) that exceeds the performance of any existing soft material. These unique properties are due to strain-induced ordering of the polymer chains in a uniformly stretched network, as revealed by in situ X-ray scattering analyses. The utility of this great strain-stiffening capability is demonstrated by realizing a simple variable stiffness actuator for soft robotics., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Two examples of self-assembled polymer systems of which the properties or functions can be tuned by solvent interactions are presented. The first involves polymers of isocyanopeptides with ethylene glycol side chains, which form gels with strain stiffening properties in water. The second deals with block copolymers of styrene and ethylene glycol, which self-assemble in water to give polymersomes. Depending on the composition of the solvent mixture these nano-objects can undergo shape transformations yielding bowl-shaped architectures (stomatocytes). These stomatocytes can entrap platinum particles or enzymes and act as nanomotors. [ABSTRACT FROM AUTHOR]

This article presents a novel model to describe the nonlinear relationships between shear force and displacement/velocity in a magnetorheological elastomer base isolator. The proposed model, containing a strain stiffening element, is able to portray the distinct dynamic behaviors of magnetorheological elastomer base isolator. To identify the model parameters, an enhanced particle swarm optimization is used on force–displacement/velocity data sampled under different loading conditions. In this algorithm, a self-adaptive inertia weight replaces the general linear weight, enhancing the convergence rate of iteration process. Besides, the mutation operator in genetic algorithm is adopted for finding global optimum. Testing data of the device displacement, velocity and force from magnetorheological elastomer base isolator are utilized to validate the proposed model and corresponding parameter identification algorithm. [ABSTRACT FROM AUTHOR]

Nondestructive and noninvasive assessments of the mechanical properties of biological tissues in vivo are essential in tissue engineering studies, for example, for designing scaffolds and monitoring tissue growth and degeneration. Agarose is widely used in biomaterial and tissue engineering studies to develop phantoms and tissue-mimicking materials because of its high biocompatibility, high stability, and low toxicity. This study aimed to provide an alternate method for nondestructive and noninvasive assessment of mechanical properties of agarose hydrogels by employing the more basic method without additional instruments compared to the standard method used in magnetic resonance imaging (MRI) measurements. In this study, the mechanical properties of agarose hydrogels of various concentrations were assessed noninvasively using quantitative T 2 MRI to obtain the relaxation rate r 2 , and their results were compared to those from rheological measurements using the amplitude sweep method to obtain the shear modulus. The comparisons showed that the shear modulus and r 2 increased exponentially with agarose concentration; however, agarose with concentrations lower than 2% had a different exponential factor than those at higher concentrations, supporting the previous cascade model theory that suggested the concentration limit for forming percolating networks in agarose hydrogels and agarose–solvent interactions. The results of this study show that the shear modulus can be assessed noninvasively by quantitative T 2 MRI measurements; both methods characterize hydrogen bonding of the agarose. However, the factors related to agarose–water interactions and agarose network chains that contribute to the modulus must be considered, especially at agarose of low concentrations. [ABSTRACT FROM AUTHOR]