J-shaped Stress-strain Curves Research Articles

3D printing sequentially strengthening high-strength natural polymer hydrogel bilayer scaffold for cornea regeneration.

3D printing of high-strength natural polymer biodegradable hydrogel scaffolds simultaneously resembling the biomechanics of corneal tissue and facilitating tissue regeneration remains a huge challenge due to the inherent brittleness of natural polymer hydrogels and the demanding requirements of printing. Herein, concentrated aqueous solutions of gelatin and carbohydrazide-modified alginate (Gel/Alg-CDH) are blended to form a natural polymer hydrogel ink, where the hydrazides in Alg-CDH are found to form strong hydrogen bonds with the gelatin. The hydrogen-bonding-strengthened Gel/Alg-CDH hydrogel demonstrates an appropriate thickened viscosity and shear thinning for extrusion printing. The strong hydrogen bonds contribute to remarkably increased mechanical properties of Gel/Alg-CDH hydrogel with a maximum elongation of over 400%. In addition, sequentially Ca2+-physical crosslinking and then moderately chemical crosslinking significantly enhance the mechanical properties of Gel/Alg-CDH hydrogels that ultimately exhibit an intriguing J-shaped stress-strain curve (tensile strength of 1.068 MPa and the toughness of 677.6 kJ/m2). The dually crosslinked Gel-Alg-CDH-Ca2+-EDC hydrogels demonstrate a high transparency, physiological swelling stability and rapid enzymatic degradability, as well as suturability. The growth factor and drug-loaded biomimetic bilayer hydrogel scaffold are customized via a multi-nozzle printing system. This bioactive bilayer hydrogel scaffold considerably promotes regeneration of corneal epithelium and stroma and inhibits cornea scarring in rabbit cornea keratoplasty.

Modeling the nonlinear responses of soft network materials assisted by masked-fusion artificial neural network

Soft network materials (SNMs) have attracted intensive attentions due to the merits of lightweight, high stretchability, and highly tunable biomechanical properties and responses. In this context, the SNMs have been widely used in the fields of flexible electronics, health monitoring, and tissue engineering etc. in the last decade. To assist the modeling of nonlinear J-shaped stress-strain curve of SNMs with complex curved filaments, a phenomenological framework assisted by machine learning (ML) approach was developed in the last year. However, the overfitting problem affected the computational efficiency and numerical stability. Herein, a novel masked-fusion artificial neural network (ANN) has proposed to replace the conventional feed-forward ANN in ML approach in this research. Compared to the conventional strategy, the masked-fusion ANN has fewer hidden layer which leads to smaller test loss and less computational burden. The risk of overfitting can also be avoided. Besides, the predicted J-shaped stress-strain curves of randomly generated SNMs from masked-fusion ANN are closer to those obtained from finite element (FE) analyses compared to the conventional ANN.

Designing Viscoelastic Gelatin-PEG Macroporous Hybrid Hydrogel with Anisotropic Morphology and Mechanical Properties for Tissue Engineering Application

The mechanical properties of scaffolds play a vital role in regulating key cellular processes in tissue development and regeneration in the field of tissue engineering. Recently, scaffolding material design strategies leverage viscoelasticity to guide stem cells toward specific tissue regeneration. Herein, we designed and developed a viscoelastic Gel-PEG hybrid hydrogel with anisotropic morphology and mechanical properties using a gelatin and functionalized PEG (as a crosslinker) under a benign condition for tissue engineering application. The chemical crosslinking/grafting reaction was mainly involved between epoxide groups of PEG and available functional groups of gelatin. FTIR spectra revealed the hybrid nature of Gel-PEG hydrogel. The hybrid hydrogel showed good swelling behavior (water content > 600%), high porosity and pore interconnectivity suitable for tissue engineering application. Simple unidirectional freezing followed by a freeze-drying technique allowed the creation of structurally stable 3D anisotropic macroporous architecture that showed tissue-like elasticity and was capable of withstanding high deformation (50% strain) without being damaged. The tensile and compressive modulus of Gel-PEG hybrid hydrogel were found to be 0.863 MPa and 0.330 MPa, respectively, which are within the range of normal human articular cartilage. In-depth mechanical characterizations showed that the Gel-PEG hybrid hydrogel possessed natural-tissue-like mechanics such as non-linear and J-shaped stress-strain curves, stress softening effect, high fatigue resistance and stress relaxation response. A month-long hydrolytic degradation test revealed that the hydrogel gradually degraded in a homogeneous manner over time but maintained its structural stability and anisotropic mechanics. Overall, all these interesting features provide a potential opportunity for Gel-PEG hybrid hydrogel as a scaffold in a wide range of tissue engineering applications.

Designing Hierarchical Soft Network Materials with Developable Lattice Nodes for High Stretchability.

Soft network materials (SNMs) represent one of the best candidates for the substrates and the encapsulation layers of stretchable inorganic electronics, because they are capable of precisely customizing the J-shaped stress-strain curves of biological tissues. Although a variety of microstructures and topologies have been exploited to adjust the nonlinear stress-strain responses of SNMs, the stretchability of most SNMs is hard to exceed 100%. Designing novel high-strength SNMs with much larger stretchability (e.g., >200%) than existing SNMs and conventional elastomers remains a challenge. This paper develops a class of hierarchical soft network materials (HSNMs) with developable lattice nodes, which can significantly improve the stretchability of SNMs without any loss of strength. The effects of geometric parameters, lattice topologies, and loading directions on the mechanical properties of HSNMs are systematically discussed by experiments and numerical simulations. The proposed node design strategy for SNMs is also proved to be widely applicable to different constituent materials, including polymers and metals.

Bionic artificial penile Tunica albuginea

Bionic artificial penile Tunica albuginea

A Biomimetic Nonwoven-Reinforced Hydrogel for Spinal Cord Injury Repair.

In clinical trials, new scaffolds for regeneration after spinal cord injury (SCI) should reflect the importance of a mechanically optimised, hydrated environment. Composite scaffolds of nonwovens, self-assembling peptides (SAPs) and hydrogels offer the ability to mimic native spinal cord tissue, promote aligned tissue regeneration and tailor mechanical properties. This work studies the effects of an aligned electrospun nonwoven of P11-8—enriched poly(ε-caprolactone) (PCL) fibres, integrated with a photo-crosslinked hydrogel of glycidylmethacrylated collagen (collagen-GMA), on neurite extension. Mechanical properties of collagen-GMA hydrogel in compression and shear were recorded, along with cell viability. Collagen-GMA hydrogels showed J-shaped stress–strain curves in compression, mimicking native spinal cord tissue. For hydrogels prepared with a 0.8-1.1 wt.% collagen-GMA concentration, strain at break values were 68 ± 1–81 ± 1% (±SE); maximum stress values were 128 ± 9–311 ± 18 kPa (±SE); and maximum force values were 1.0 ± 0.1–2.5 ± 0.1 N (±SE). These values closely mimicked the compression values for feline and porcine tissue in the literature, especially those for 0.8 wt.%. Complex shear modulus values fell in the range 345–2588 Pa, with the lower modulus hydrogels in the range optimal for neural cell survival and growth. Collagen-GMA hydrogel provided an environment for homogenous and three-dimensional cell encapsulation, and high cell viability of 84 ± 2%. Combination of the aligned PCL/P11-8 electrospun nonwoven and collagen-GMA hydrogel retained fibre alignment and pore structure, respectively, and promoted aligned neurite extension of PC12 cells. Thus, it is possible to conclude that scaffolds with mechanical properties that both closely mimic native spinal cord tissue and are optimal for neural cells can be produced, which also promote aligned tissue regeneration when the benefits of hydrogels and electrospun nonwovens are combined.

A phenomenological framework for modeling of nonlinear mechanical responses in soft network materials with arbitrarily curved microstructures

Soft network materials (SNMs), consisting of rationally designed filamentary microstructures distributed periodically in lattice patterns, offer high air permeability, excellent defect insensitivity and highly tunable mechanical properties that could match precisely those of biological tissues. Owing to these excellent physical properties, SNMs have been widely used in stretchable electronics, biomedical engineering, soft robotics, and other areas. Although a few theoretical models were developed previously to predict nonlinear J-shaped stress–strain curves of SNMs with horseshoe microstructures, these models are not applicable to SNMs with arbitrarily curved microstructures. Herein, we establish a phenomenological framework capable of predicting nonlinear mechanical responses of SNMs with arbitrarily curved microstructures and various lattice topologies. The phenomenological framework incorporates a two-segment model that exploits simple, explicit expressions to capture the J-shaped stress–strain relationship, and a machine learning (ML) approach that enables the determination of the single phenomenological parameter in the model. Both finite element analysis (FEA) and experimental measurements on a diversity of SNMs have been conducted to validate the phenomenological model. The developed model enables a rapid inverse design of biomimetic SNMs for reproducing the target J-shaped stress–strain curves of soft biological tissues, and can be also extended to model anisotropic mechanical responses of SNMs with horseshoe microstructures. The versatile applicability of the phenomenological framework suggests that it can serve as a reliable design tool of SNMs for uses in tissue engineering and bioelectronics.

Mechanics of Three-Dimensional Soft Network Materials With a Class of Bio-Inspired Designs

Abstract Inspired by the helix-shaped microstructures found in many collagenous tissues, a class of three-dimensional (3D) soft network materials that incorporate similar helical microstructures into periodic 3D lattices was reported recently. Owing to their high stretchability, high air permeability, defect-insensitive behavior, and capabilities of reproducing anisotropic J-shaped stress–strain curves of real biological tissues (e.g., heart muscles), these 3D soft network materials hold great promise for applications in tissue engineering and bio-integrated devices. Rapid design optimization of such soft network materials in practical applications requires a relevant mechanics model to serve as the theoretical basis. This paper introduces a nonlinear micromechanics model of soft 3D network materials with cubic and octahedral lattice topologies, grounded on the development of finite-deformation beam theory for the 3D helical microstructure (i.e., the building-block structure of 3D network materials). As verified by finite element analysis (FEA) and experimental measurements, the developed model can well predict the anisotropic J-shaped stress–strain curves and deformed configurations under large levels of uniaxial stretching. The theoretical model allows a clear understanding of different roles of microstructure parameters on the J-shaped stress–strain curve (that is characterized by the critical strain of mode transition, as well as the stress and the tangent modulus at the critical strain). Furthermore, we demonstrate the utility of the theoretical model in the design optimization of 3D soft network materials to reproduce the target isotropic/anisotropic stress–strain curves of real biological tissues.

Characterization of Cryopreserved Canine Amniotic Membrane.

Amniotic membrane is an effective corneal reconstruction material in veterinary surgery. Cryopreserved amniotic membrane is widely used in practice. Properties of cryopreserved canine amniotic membranes are currently not well studied. This study aimed to compare three properties between canine amniotic membranes cryopreserved for 7 days and 30 days, including tensile strength, transparency, and cell viability. After their respective cryopreservation time, stress–strain curves of the cryopreserved membranes’ tensile strength were assessed using a universal testing machine. Both groups produced J-shaped stress–strain curves with statistically comparable parameters, including maximum stress, strain, and Young’s modulus. The percentage of cell viability was observed by trypan blue staining under a light microscope. Membrane transparency was tested with a spectrophotometer. Transparency tests showed high levels of light transmission and low haze, with no statistical difference between groups. Cell viability was statistically lower in the 30-day cryopreserved group. Tensile strength and transparency of cryopreserved CAM were not significantly impeded for up to 30 days. For CAM to be used as an alternative corneal transplant material in veterinary and regenerative medicine, further research on cell biology, biomechanical properties of the membrane, and cell viability should be conducted.

Effect of multi-armed chain extender on microphase morphology, stress-strain behavior and electromechanical properties of polyurethane elastomers

High dielectric constant and low modulus of thermoplastic polyurethane elastomers (PUEs) are required for various applications in actuators, sensors and energy harvestings. In this work, we controlled the microphase separated morphology of PUEs by regulation of the content of hydroxyl-terminated four-armed polycaprolactone (4amPCL) chain extender to achieve J-shaped stress-strain curve with low modulus and high electromechanical properties. Our results showed that the hydrogen bonding between hard segments (HSs) and crystallization of soft segment (SS) are significantly impeded by long-chain branched structure when added lower molar fraction (<20% of chain extenders) of 4amPCL as chain extender, whereas the branched long-chains are embedded in microphase separated hard and soft domains (HD and SD) to form strip shaped interconnected hard domain (HD) rich regions when added higher molar fraction (>20% of chain extenders) of 4amPCL as chain extender. The PUE with 29% of 4amPCL (PU-4amPCL-0.29) showed lower modulus (1.5 MPa), higher tensile strength (17.5 MPa) at 1048% of elongation, and 3.9% of actuation strain at 20 kV/mm, and higher contribution (48.1%) of Maxwell stress on actuation strain at 20 kV/mm. We found that the microphase separation of branched long-chains induced semi-interpenetration polymer network like structure of PUEs played an important role in exhibiting J-shaped stress-strain behavior. Our results can provide new insight on fabrication of high electromechanical performance of dielectric polyurethane elastomers.

Mechanics of unusual soft network materials with rotatable structural nodes

Soft network materials consisting of filamentary horseshoe microstructures represent a class of architected materials of practical interest in flexible, bio-integrated electronics, because they can be tailored to reproduce accurately nonlinear stress−strain responses of biological tissues. The previous studies of these bio-inspired soft network materials focused mostly on the intriguing mechanical responses (e.g., J-shaped stress-strain curves and negative Poisson's ratios) before their fracture, and paid little attention on their utmost strength and stretchability. The development of soft network materials that can offer both a high stretchability (e.g., > 100%) and a relatively high strength (e.g., > 50 MPa) remains challenging. This paper presents the design, mechanics modeling and experimental measurement of a novel type of soft network materials that introduce rotatable structural lattice nodes (either with ring or disk shapes) to offer an enhanced deformability. Based on the analyses of the periodic building-block structure, we develop a nonlinear mechanics model of unusual soft network materials made of elastic constituent materials. Validated by finite element analyses (FEA) and experiments, this theoretical model can well predict the J-shaped stress-strain curves and deformed configurations of unusual soft network materials with a variety of geometric parameters. The results based on the developed model provide insights into the underlying relationship between nonlinear mechanical properties and key geometric parameters. For unusual soft network materials made of metals, an Ashby plot in terms of the strength and the stretchability is presented, highlighting the advantages of developed network materials over the existing network materials and typical soft materials. Moreover, computational models based on representative unit cells with periodic boundary conditions allow precise predictions of the utmost strength and stretchability, which can thereby serve as a reliable design tool. Due to the combined mechanical attributes of high stretchability, relatively high strength, and negative Poisson's ratio, the proposed unusual soft network materials can be used in various device systems of bio-integrated electronics.

Soy Protein Nanofiber Scaffolds for Uniform Maturation of Human Induced Pluripotent Stem Cell-Derived Retinal Pigment Epithelium.

Retinal pigment epithelium (RPE) differentiated from human induced pluripotent stem cells, called induced retinal pigment epithelium (iRPE), is being explored as a cell-based therapy for the treatment of retinal degenerative diseases, especially age-related macular degeneration. The success of RPE implantation is linked to the use of biomimetic scaffolds that simulate Bruch's membrane and promote RPE maturation and integration as a functional tissue. Due to difficulties associated with animal protein-derived scaffolds, including sterility and pro-inflammatory responses, current practices favor the use of synthetic polymers, such as polycaprolactone (PCL), for generating nanofibrous scaffolds. In this study, we tested the hypothesis that plant protein-derived fibrous scaffolds can provide favorable conditions permissive for the maturation of RPE tissue sheets in vitro. Our natural, soy protein-derived nanofibrous scaffolds exhibited a J-shaped stress-strain curve that more closely resembled the mechanical properties of native tissues than PCL with significantly higher hydrophilicity of the natural scaffolds, favoring in vivo implantation. We then demonstrate that iRPE sheets growing on these soy protein scaffolds are equivalent to iRPE monolayers cultured on synthetic PCL nanofibrous scaffolds. Immunohistochemistry demonstrated RPE-like morphology and functionality with appropriate localization of RPE markers RPE65, PMEL17, Ezrin, and ZO1 and with anticipated histotypic polarization of vascular endothelial growth factor and pigment epithelium-derived growth factor as indicated by enzyme-linked immunosorbent assay. Scanning electron microscopy revealed dense microvilli on the cell surface and homogeneous tight junctional contacts between the cells. Finally, comparative transcriptome analysis in conjunction with principal component analysis demonstrated that iRPE on nanofibrous scaffolds, either natural or synthetic, matured more consistently than on nonfibrous substrates. Taken together, our studies suggest that the maturation of cultured iRPE sheets for subsequent clinical applications might benefit from the use of nanofibrous scaffolds generated from natural proteins. Impact statement Induced retinal pigment epithelium (iRPE) from patient-derived induced pluripotent stem cells (iPSCs) may yield powerful treatments of retinal diseases, including age-related macular degeneration. Recent studies, including early human clinical trials, demonstrate the importance of selecting appropriate biomaterial scaffolds to support tissue-engineered iRPE sheets during implantation. Electrospun scaffolds show particular promise due to their similarity to the structure of the native Bruch's membrane. In this study, we describe the use of electroprocessed nanofibrous soy protein scaffolds to generate polarized sheets of human iPSC-derived iRPE sheets. Our evaluation, including RNA-seq transcriptomics, indicates that these scaffolds are viable alternatives to scaffolds electrospun from synthetic polymers.

Toward Imperfection-Insensitive Soft Network Materials for Applications in Stretchable Electronics.

Development of stretchable devices with mechanical responses that mimic those of biological tissues/organs is of particular importance for the long-term biointegration, as the discomfort induced by the mechanical mismatch can be minimized. Recent works have established the bioinspired designs of soft network materials that can precisely reproduce the unconventional J-shaped stress-strain curves of human skin at different regions. Existing studies mostly focused on the design, fabrication, and modeling of perfect soft network materials. When utilized as the substrates of biointegrated electronics, the soft network designs, however, often need to incorporate deterministic holes, a type of imperfection, to accommodate hard, inorganic electronic components. Understanding of the effect of hole imperfections on the mechanical properties of soft network materials is thereby essential in practical applications. This paper presents a combined experimental and computational study of the stretchability and elastic modulus of imperfect soft network materials consisting of circular holes with a variety of diameters. Both the size and location of the circular-hole imperfections are shown to have profound influences on the stretchability. Based on these results, design guidelines of imperfection-insensitive network materials are introduced. For the imperfections that result in an evident reduction of stretchability, an effective reinforcement approach is presented by enlarging the width of horseshoe microstructures at strategic locations, which can enhance the stretchability considerably. A stretchable and imperfection-insensitive integrated device with a light-emitting diode embedded in the network material serves a demonstrative application.

Fracture toughness of human amniotic membranes.

Amnion is a membrane that surrounds and structurally protects the developing fetus during pregnancy. The rupture of amniotic membranes prior to both normal and preterm deliveries involves stretch forces acting on a biochemically triggered weak zone of the membranes. Fracture toughness is an important mechanical property describing how the membranes containing a defect resist fracture, but this property has never been investigated in amniotic membranes. In this work, the fracture toughness of many samples cut from four pieces of amniotic membrane from different mothers was examined by uniaxial and pure shear (mode I) fracture tests. The measurement was checked for dependence on the sample geometry and notch length. Results from the uniaxial tensile test show J-shaped stress-strain curves and confirm that the amniotic membrane is a nonlinear material. The measured fracture toughness of four amniotic membranes ranged from 0.96 ± 0.11 to 1.83 ± 0.18 kJ m-2. Despite considering the effect of the presence of the defect on mechanical property measurement, similar fracture behaviour was observed for pre-notched and unnotched specimens, indicating that the membranes were extremely tolerant to defects. This defect-tolerant characteristic provides insight into the understanding of fetal membrane rupture.

Customised multiphasic nucleus/annulus scaffold for intervertebral disc repair/regeneration

ABSTRACTBackground: In the case of a degenerated intervertebral disc (IVD), even though spinal fusion has provided good short-term clinical results, an alteration of the spine stability has been demonstrated by long-term studies. In this context, different designs of IVD prostheses have been proposed as alternative to spinal fusion. However, over the past few years, much of the recent research has been devoted to IVD tissue engineering, even if several limitations related to the complex structure of IVD are still presented.Purpose/Aim: Accordingly, the aim of the current paper was to develop a strategy in designing customised multiphasic nucleus/annulus scaffolds for IVD tissue engineering, benefiting from the great potential of reverse engineering, additive manufacturing and gels technology.Materials and Methods: The device consisted of a customised additive-manufactured poly(ε-caprolactone) scaffold with tailored architectural features as annulus and a cell-laden collagen-low molecular weight hyaluronic acid-based material as nucleus with specific rheological and functional properties. To this aim, injectability and viscoelastic properties of the hydrogel were analyzed. Furthermore, a mechanical and biological characterization of cell-laden multiphasic nucleus/annulus scaffold was performed.Results and Conclusions: Analyses on the developed devices demonstrated appropriate viscoelastic and mechanical properties. As evidenced by rheological tests, the hydrogel showed a shear-thinning behaviour, supporting the possibility to inject the material. The mechanical characterization highlighted a compressive modulus which falls in the range of lumbar discs, with the typical initial J-shaped stress–strain curve of natural IVDs. Furthermore, preliminary biological tests showed that human mesenchymal stem cells were viable over the culture period.

Out-of-Plane Impact Resistance Enhancement in Plane Lattice With Curved Links

In the past decades, various novel functions (i.e., negative Poisson's ratio, zero thermal expansion) have been obtained by tailoring the microstructures of the cellular structures. Among all the microstructures, the horseshoe topology shows a J-shaped stress–strain curve, which is quite different from the conventional materials. It can be inferred that the 2D lattice structure with horseshoe microstructure will also exhibit excellent out-of-plane impact resistance since the spider silk also exhibits the J-shaped stress–strain curve. In this paper, the out-of-plane sphere impact of 2D truss lattice structure is conducted using finite element method (FEM) simulation. The point has been made that, by replacing the direct-line beam to horseshoe curved beam, the out-of-plane impact resistance has been greatly improved. The most curved beam structure is found to have the best out-of-plane performs with the maximum energy absorption and the minimum passing through velocity.

Gradient porous fibrous scaffolds: a novel approach to improving cell penetration in electrospun scaffolds

Electrospinning is an increasingly popular technique to generate 3D fibrous tissue scaffolds that mimic the submicron sized fibers of extracellular matrices. A major drawback of electrospun scaffolds is the small interfibrillar pore size, which normally prevents cellular penetration in between fibers. In this study, we introduced a novel process, based on electrospinning, to manufacture a unique gradient porous fibrous (GPF) scaffold from soy protein isolate (SPI). The pore sizes in the GPF scaffolds gradually increase from one side of the scaffold to the other, ranging from 7.8 ± 2.5 μm in the small pore side, 21.4 ± 10.3 μm in the mid layer to 58.0 ± 23.6 μm in the large pore side. The smallest pores of the GPF scaffolds appeared to be somewhat larger than those in conventionally electrospun SPI scaffolds (4.2 ± 1.3 μm). Hydrated GPF scaffolds exhibited J-shaped stress–strain curves, reminiscent of those for soft biological scaffolds. Attachment, spreading, and proliferation of human dermal fibroblasts (HDFB) were supported on both the small and the large pore sides of the GPF scaffolds. Cultured HDFB and murine RAW 264.7 macrophages penetrated significantly deeper (98.7 ± 24.2 μm and 53.3 ± 9.6 μm, respectively) into the larger pores than when seeded onto the small pore side of GPF scaffolds (22.8 ± 6.2 μm and 25.7 ± 7.3 μm) and control SPI scaffolds. (11.3 ± 3.8 μm and 15.3 ± 3.1 μm). This study introduces a novel fabrication technique, which, by convergence of several biofabrication technologies, produces scaffolds with enhanced cellular penetration.

A Computational Model of Bio-Inspired Soft Network Materials for Analyzing Their Anisotropic Mechanical Properties

Soft network materials constructed with horseshoe microstructures represent a class of bio-inspired synthetic materials that can be tailored precisely to match the nonlinear, J-shaped, stress–strain curves of human skins. Under a large level of stretching, the nonlinear deformations associated with the drastic changes of microstructure geometries can lead to an evident mechanical anisotropy, even for honeycomb and triangular lattices with a sixfold rotational symmetry. Such anisotropic mechanical responses are essential for certain targeted applications of these synthetic materials. By introducing appropriate periodic boundary conditions that apply to large deformations, this work presents an efficient computational model of soft network materials based on the analyses of representative unit cells. This model is validated through comparison of predicted deformed configurations with full-scale finite element analyses (FEA) for different loading angles and loading strains. Based on this model, the anisotropic mechanical responses, including the nonlinear stress–strain curves and Poisson's ratios, are systematically analyzed for three representative lattice topologies (square, triangular and honeycomb). An analytic solution of the geometry-based critical strain was found to show a good correspondence to the critical transition point of the calculated J-shaped stress–strain curve for different network geometries and loading angles. Furthermore, the nonlinear Poisson's ratio, which can be either negative or positive, was shown to depend highly on both the loading angle and the loading strain.

Preparation and properties of high performance gelatin-based hydrogels with chitosan or hydroxyethyl cellulose for tissue engineering applications

ABSTRACTHigh performance gelatin-based biocompatible hybrid hydrogels are developed using functionalized polyethylene glycol as a cross-linker in presence of chitosan or hydroxyethyl cellulose. Tensile test shows robust and tunable mechanical properties and reveals non-linear and J-shaped stress-strain curves similar to those found for native extracellular matrix. Degradation study demonstrates that the mass loss and change in mechanical properties are dependent on hydrogel composition and cross-linking density. Structural features of the hydrogels are confirmed by infrared spectroscopy. A preliminary biological evaluation is carried out using rat myoblasts and human fibroblasts cell lines. The results show that all hydrogels allow cell adhesion and proliferation during four days culture, hence, they might have a great potential for use in the biomedical applications.

Mechanics of Fractal-Inspired Horseshoe Microstructures for Applications in Stretchable Electronics

Fractal-inspired designs represent an emerging class of strategy for stretchable electronics, which have been demonstrated to be particularly useful for various applications, such as stretchable batteries and biointegrated electrophysiological electrodes. The fractal-inspired constructs usually undergo complicated, nonlinear deformations under mechanical loading, because of the highly complex and diverse microstructures inherent in high-order fractal patterns. The underlying relations between the nonlinear mechanical responses and microstructure geometry are essential in practical applications, which require a relevant mechanics theory to serve as the basis of a design approach. Here, a theoretical model inspired by the mechanism of ordered unraveling is developed to study the nonlinear stress–strain curves and elastic stretchability for a class of fractal-inspired horseshoe microstructures. Analytic solutions were obtained for some key mechanical quantities, such as the elastic modulus and the tangent modulus at the beginning of each deformation stage. Both the finite-element analyses (FEA) and experiments were carried out to validate the model. Systematic analyses of the microstructure–property relationship dictate how to leverage the various geometric parameters to tune the multistage, J-shaped stress–strain curves. Moreover, a demonstrative example shows the utility of the theoretical model in design optimization of fractal-inspired microstructures used as electrophysiological electrodes, aiming to achieve maximum elastic stretchability for prescribed filling ratios. The results indicate a substantial enhancement (e.g., &gt;4 times) of elastic stretchability by using fractal designs, as compared to traditional horseshoe designs. This study can serve as design guidelines of fractal-inspired microstructures in different stretchable electronic systems.