Gradient Of Mechanical Properties Research Articles

Cell-Laden Gradient Hydrogel Scaffolds for Neovascularization of Engineered Tissues.

Gradients in mechanical properties, physical architecture and biochemical composition exist in a variety of complex tissues, yet 3D in vitro models that enable investigation of these cues on cellular processes, especially those contributing to vascularization of engineered tissues are limited. Here, a photopolymerization approach to create cell-laden hydrogel biomaterials with decoupled and combined gradients in modulus, immobilized cell adhesive peptide (RGD) concentration, and proteolytic degradation enabling spatial encapsulation of vascular spheroids is reported to elucidate their impact on vascular sprouting in 3D culture. Vascular spheroids encapsulated in these gradient scaffolds exhibit spatial variations in total sprout length. Scaffolds presenting an immobilized RGD gradient promote biased vascular sprouting toward increasing RGD concentration. Importantly, biased sprouting is found to be dependent on immobilized RGD gradient characteristics, including magnitude and slope, with increases in these factors contributing to significant enhancements in biased sprouting responses. Conversely, reduction in biased sprouting responses is observed in combined gradient scaffolds possessing opposing gradients in RGD and modulus. The presented work is the first to demonstrate the use of a cell-laden biomaterial platform to systematically investigate the role of multiple scaffold gradients as well as gradient slope, magnitude and orientation on vascular sprouting responses in 3Dculture.

Criterion for microcrack resistance of multi-phase steels based on property gradient maps

A novel criterion for microcrack resistance of multi-phase steels based on property gradient maps is proposed. Two industrial sheets of steel were processed to obtain dual-phase and complex-phase microstructures with exactly the same chemical composition. Experimental investigations showed characteristic differences for the tensile tests, hole expansion and the local plastic behavior during deformation. An innovative full-field modeling approach that explicitly predicts mechanical property gradients as a function of microstructural gradients during forming was developed and validated. This allowed to form a new criterion for evaluation of structure–property relationship in nano-structured multi-phase steels and can reveal the formability limitations.

In vivo Neural Crest Cell Migration Is Controlled by "Mixotaxis".

Directed cell migration is essential all along an individual’s life, from embryogenesis to tissue repair and cancer metastasis. Thus, due to its biomedical relevance, directed cell migration is currently under intense research. Directed cell migration has been shown to be driven by an assortment of external biasing cues, ranging from gradients of soluble (chemotaxis) to bound (haptotaxis) molecules. In addition to molecular gradients, gradients of mechanical properties (duro/mechanotaxis), electric fields (electro/galvanotaxis) as well as iterative biases in the environment topology (ratchetaxis) have been shown to be able to direct cell migration. Since cells migrating in vivo are exposed to a challenging environment composed of a convolution of biochemical, biophysical, and topological cues, it is highly unlikely that cell migration would be guided by an individual type of “taxis.” This is especially true since numerous molecular players involved in the cellular response to these biasing cues are often recycled, serving as sensor or transducer of both biochemical and biophysical signals. In this review, we confront literature on Xenopus cephalic neural crest cells with that of other cell types to discuss the relevance of the current categorization of cell guidance strategies. Furthermore, we emphasize that while studying individual biasing signals is informative, the hard truth is that cells migrate by performing a sort of “mixotaxis,” where they integrate and coordinate multiple inputs through shared molecular effectors to ensure robustness of directed cell motion.

Applying Bio-Inspired hierarchical design to jamming technology: Improving density-efficient mechanical properties and opening application spaces

In the soft-robotics field, bio-inspiration is often cited, pointing to the animal-like forms created—however, the concept of hierarchical architecture common to biological materials has yet to be applied effectively. Here, it is shown how that by considering the hierarchical structure of the medium (primary level), the organization of jamming media (secondary level), and the organization of jammers (tertiary level) new functionalities not possible with conventional jamming technology can be obtained. This is accomplished at the three layers enumerated above. At the primary level, optimal compositions of fibrous flakes and grains are identified to improve stiffness and strength per unit weight; fish-inspired ganoid scales are used to create flexible armors. At the secondary level, layers and grains are combined in the tensile and compressive faces of beams to maximize mechanical properties, while ganoid scales of different compositions are layered to create mechanical gradients, among other combinations of jamming media. Finally, at the tertiary level, the isotropy of triadically woven jammers is demonstrated relative to traditional biaxial jammers; a cylindrical “finger-trap” weave with adjustable radius is shown. The improved mechanical weight-efficiency, anisotropy control, mechanical property gradients, and other features enabled by considering hierarchical design in jamming promise new application spaces for an established field.

Entrainment and Dynamics of Ocean‐Derived Impurities Within Europa's Ice Shell

AbstractCompositional heterogeneities within Europa's ice shell likely impact the dynamics and habitability of the ice and subsurface ocean, but the total inventory and distribution of impurities within the shell are unknown. In sea ice on Earth, the thermochemical environment at the ice‐ocean interface governs impurity entrainment into the ice. Here, we simulate Europa's ice‐ocean interface and bound the impurity load (1.053–14.72 g/kg [parts per thousand weight percent, or ppt] bulk ice shell salinity) and bulk salinity profile of the ice shell. We derive constitutive equations that predict ice composition as a function of the ice shell thermal gradient and ocean composition. We show that evolving solidification rates of the ocean and hydrologic features within the shell produce compositional variations (ice bulk salinities of 5–50% of the ocean salinity) that can affect the material properties of the ice. As the shell thickens, less salt is entrained at the ice‐ocean interface, which implies Europa's ice shell is compositionally homogeneous below ~1 km. Conversely, the solidification of water filled fractures or lenses introduces substantial compositional variations within the ice shell, creating gradients in mechanical and thermal properties within the ice shell that could help initiate and sustain geological activity. Our results suggest that ocean materials entrained within Europa's ice shell affect the formation of geologic terrain and that these structures could be confirmed by planned spacecraft observations.

Physical and Technical Foundations of the Use of Alternating Free Bending for Producing Long-length Semi-products from Metals and Alloys with Improved Mechanical Properties

In this work, we give a theoretical justification of the non-monotonic character of strain in the process of refinement and formation of a grain-type structure during the deformation processing of low-alloyed copper heat-hardenable alloys. A regularity is revealed, suggesting the alternating behavior of the strain state in the process of the continuous bending of billets. It is found that the continuous free bending process in the conditions of multi-cycle (four cycles) processing increases the strength and produces a gradient-type structure and a gradient of mechanical properties. Using fabrication of the experimental samples of the contact wire as an example, it is shown that the obtained results can serve as a scientific and technical foundation for the development of complex methods for the processing of axis-symmetrical long-length objects from copper heat-hardenable alloys in the form of wires and shaped products with improved mechanical properties, with potential adaptability for manufacturing.

Thermal gradient of in-flight polymer particles during cold spraying

The manufacture of polymer coatings via the cold-spray process remains challenging owing to the viscoelastic-viscoplastic behavior exhibited by polymers. One crucial step to improve cold-spray polymer coating is to determine the particles’ thermal history during their flight from inside the nozzle to their impact on the substrate. In this study, we propose estimating the velocity and temperature of an isolated polymer particle traveling through a nozzle with a sharp change in its cross-section. The preliminary results show that the geometric discontinuity constricts the flow, thereby increasing the particle velocity. Moreover, a significant thermal gradient is expected inside the particle, which in turn leads to a gradient of mechanical properties of the polymeric particle during impact.

Modeling the mechanics, kinetics, and network evolution of photopolymerized hydrogels

Photopolymerized hydrogels are critical to soft devices, mechanobiology, regenerative medicine, and next generation drug delivery. However, the optimization of processing protocols for all of these applications of photopolymerized hydrogels has been at least semi-empirical due to the lack of a comprehensive predictive framework. Herein, we developed the first comprehensive predictive framework for how the chemical kinetics, optical properties, and mechanical properties of a photopolymerized hydrogel emerge from a precursor solution as the solution is illuminated, and of how these processing parameters relate to the final mechanics of the hydrogel. We validated the model experimentally using an eosin Y-initiated di-acrylate system. The model revealed that processing kinetics were dominated by photobleaching and crosslinking, and that network mechanics were dominated by chain growth and loop formation. We demonstrated the utility of the model by using it to design and then synthesize hydrogels with specified gradients in mechanical properties. The modeling framework is general and enables design of a broad range of hydrogels.

Distribution of mechanical strain in equine distal metacarpal subchondral bone: A microCT-based finite element model

Microdamage accumulation and adaptation of subchondral bone subjected to intensive cyclic loading are important processes associated with catastrophic bone failure, and joint degeneration in athletic humans and racehorses. At the tissue-level, they lead to a spatial variation in bone tissue mineral density (TMD) which affects the response of the bone to mechanical load. Quantifying the spatial distribution of mechanical load within the subchondral bone is critical for understanding the mechanism of the joint failure. Previously, a gradient of TMD and mechanical properties has been reported under unconfined compression in osteochondral plugs. In the present study, we used micro computed tomography (μCT)-based finite element (FE) models of cartilage-bone to investigate the gradient of strain in the subchondral bone (SCB) from the third metacarpal (MC3) condyle of racehorses under simulated in situ compression. Non-destructive mechanical testing of specimens under high-rate compression provided the apparent-level modulus of SCB. FE models were analysed using unconfined and confined boundary conditions. Unconfined FE-predicted apparent-level gradient of modulus across the SCB thickness correlated well with the experimental results (R2 ​= ​0.72, p ​< ​0.05). The highest strain occurred in the most superficial SCB (0.5–2.5 ​mm deep to the cartilage-bone interface) under the simulated in-situ compression through articular cartilage. The findings of this study provide an estimation for the spatial distribution of mechanical strain within SCB in-situ in the presence of heterogeneous bone tissue which is commonly observed in joints subjected to intensive cyclic loading.

Modular 4D Printing Assisted by Dynamic Chemical Bonds

Modular 4D printing is achieved by assembling and welding the modules of printed self-folding structures containing dynamic chemical bonds, enabling multifunctional shape-shifting structures and devices.

Microstructure effect of weld joint heat affected zone on the temperature velocity factor of ultrasound

The effect of temperature on the longitudinal wave speed in the heat-affected zone of the weld joint of structural steel 20 is investigated. The heat-affected zone is the most dangerous place where the stress-strain state is realized and a gradient of physical and mechanical properties is observed. As a rule, it is in the heat-affected zone that the structures are destroyed. The structural state of the heat-affected zone has a significant effect on the temperature dependence of the elastic wave velocity. As a result of the research, numerical values of the temperature-velocity factor for longitudinal ultrasonic waves in steel 20 for the heat-affected zone and the base metal were obtained. In the future, the dependencies can be used to develop methods for determining material damage by the ultrasonic method.

Erosion–Corrosion Resistance of Laser Surface Alloying of NbC Thermal Spray Coatings on AISI 304L Steel

NbC coatings were flame sprayed on AISI 304L substrates and further remelted using a 400-W power ytterbium-doped fiber laser (YLR-500-MM-AC-Y11), inserted into a chamber for argon atmosphere control. For the selection of the remelting parameters, isolated beads with remelting of the coatings were performed using three focal lengths and three remelting speeds. Using the selected laser parameters, layers were remelted with eight passes and 40% superposition. Erosion–corrosion tests were performed in a solution of NaCl in distilled water with SiO2. Surfaces and cross sections of the coatings were characterized by optical and scanning electron microscopy with dispersive energy microanalysis and x-ray diffraction. Microhardness measurements were taken in the cross section of the coatings. Modified surfaces 800 μm dense without pores or other defects and interfaces with the substrate of excellent metallurgical bonding were obtained. It was found that the dilution of the coating with the substrate formed a gradient of chemical composition and mechanical properties. The erosive–corrosive wear resistance of the laser-remelted coatings was higher for the eroding impact angle of 90°. When the impact angle was 30°, the weight loss was higher and the ductile wear mechanism was conditioned by the formation of pitting on the modified surface.

Evaluation of contact fatigue risk of a carburized gear considering gradients of mechanical properties

Carburized gears are widely used in geared machines such as wind turbines. Contact fatigue problems occur in engineering practice, reducing reliabilities of machines. Contact fatigue failures are related to many factors, such as gradients of mechanical properties of the hardening layer. In this work, an elastic-plastic contact model of a carburized gear is developed based on the finite element method to evaluate contact fatigue failure risk, considering variations in hardness and strength. The Dang Van multiaxial equivalent stress is calculated via Python coding within the Abaqus framework. The gradient of yield strength along the depth from case to core is considered. The concept of local material fatigue failure risk is defined to evaluate the probability of pitting failure. The effects of design factors, such as the case hardening depth (CHD), surface hardness, and contact pressure on fatigue failure risk, are studied. As the CHD increases or the surface hardness decreases, the risk of deep spalling failure reduces. The increase in surface hardness leads to a decreased risk of pitting failure, while the variation in CHD hardly affects the pitting failure risk.

Method and Mathematical Modeling of Formation of Gradually and Homogeneously Oriented Linear Polymers

Previously, we developed several technical solutions for the conversion of isotropic polymers into materials of a new type—gradually oriented polymers with a gradient of physical and mechanical properties, thereby materializing for the first time the unused possibilities inherent in the polymeric nature of the substance. The scientific basis of these developments is the concept of a new structural state of linear polymers—a gradually oriented (stretched) state (GOS), the essence of which is set out briefly. An algorithm and a mathematical model for controlling the process of uniaxial, zonal stretching of linear polymers are proposed, which allow the formation of gradually or homogeneously oriented polymers (polymer composites). At the same time, specified values of quantitative parameters are ensured, in particular, the selected profile of distribution of the relative elongation (linear, sinusoidal, etc.) along the length of the gradually oriented polymer sample and the specified constant relative elongation along the length of the homogeneously oriented polymer. Mathematical expressions for calculating the conditions to obtain gradually oriented polymers with given geometric dimensions (length, width) and with a given distribution of relative elongation along the length of the resulting sample were also derived. The description of method and principles of operation of the appropriate device is given. Experimental data illustrating the possibilities of the proposed method and the efficiency of mathematical modeling are presented. The issues of creating functionally graded materials with a gradient of microstructure (gradient of degree of orientation/relative elongation) or chemical composition (volume fraction of functional fillers), respectively, based on thermoplastic polymer/copolymer or polymer composite (containing functional fillers) by method of uniaxial, zonal graded stretching are discussed.

In situ Wear Study Reveals Role of Microstructure on Self-Sharpening Mechanism in Sea Urchin Teeth

Summary Animals' teeth have evolved to provide food procurement, mastication, and protection. These functions, directly linked to survival of many living animal species, require superior hardness and abrasion resistance in the animal's dentition system. Such resistance typically emerges from damage tolerance and sharpness preservation during the organism’s life span. An example is the sea urchin tooth, which through gradients in mechanical properties together with exploitation of microstructural features achieves such functionality. Using contact mechanics, dimensional analysis, and a novel in situ scanning electron microscopy experimental methodology, conditions for tooth deformation and wear, via a self-sharpening mechanism consisting in plate chipping, were imaged and quantified. Nonlinear finite-element modeling of the self-sharpening mechanism provided insight into the synergy between constituent material properties and tooth microstructural elements. The findings reported here should inspire the design of novel tools used in machining operations, e.g., cutting and grinding, as well as in mining and tunnel boring.

Asymmetric local strain, microstructure and superelasticity of friction stir welded Nitinol alloy

Thus far, the joining of Nitinol alloys has been studied extensively using laser welding. Here, we studied the weldability of powder metallurgy (PM) Ti–51 at %Ni alloy by friction stir welding (FSW) via microstructural, superelasticity and mechanical property evaluations. The FSW welds were produced by a constant welding speed and different rotational speeds of 250 rpm, 300 rpm, and 350 rpm utilizing the cemented carbide tool. No Ti3Ni4 precipitate was observed after FSW welding, which agreed with XRD analysis. Additionally, it was found that FSW triggered the dynamically re-crystallized fine grains in which tangles of dislocations existed. The crystallographic orientations in the weld zone revealed locally high texture evolution along with diverse Schmid factors measured for active slip systems of Nitinol in different locations of the welds. These heterogeneous crystallographic orientations, dislocation density and Schmid factor variations caused a gradient in local mechanical properties across the welds during the uniaxial tensile test performed by digital image correlation (DIC). The 350 rpm weld exhibited the excellent mechanical properties, yielding at ~765 MPa (~37% higher than the base metal and all previously reported Nitinol welds) and reaching an ultimate strength of ~870 MPa and 8.1% elongation. However, the superelasticity of the FSW-welded joints curtailed upon cyclic loading-unloading; hence it is suggested that post-heat treatment is essential to recovering their functionality.

A microstructure sensitive contact fatigue model of a carburized gear

Rolling contact fatigue (RCF) issues of carburized gears are not fully understood and relate to many factors, such as gradients of mechanical properties, the multiaxial stress state and the complex stress-strain response due to the polycrystalline anisotropy. In this work, a numerical model for RCF performance evaluation of a carburized gear is proposed by considering the anisotropy of carburized material microstructure and the hardness gradient. The crystal plasticity framework is adopted by using a user material subroutine. The effects of mechanical property gradients induced by carburizing are considered and introduced on the grain level. The Fatemi-Socie fatigue criterion is implemented to capture the RCF performance on crystallographic slip systems during the gear meshing process. The RCF performance of carburized gear is evaluated and compared with a non-carburized gear. The simulation results reveal that the influence of microstructure anisotropy on the maximum contact pressure is the most significant at the pitch point meshing position. The plasticity accumulates at a certain depth of non-carburized gear, and this accumulated plasticity leads to more scattered and larger values of a fatigue indicator parameter. Several sets of random grain orientations are developed, and the result shows that the RCF indicator parameter of non-carburized gear is more sensitive to the grain orientation.

Ductile cast irons: Microstructure influence on the fatigue initiation mechanisms

AbstractIn the last decades, the combination of high mechanical performances and low production costs increased the industrial interest on ductile cast irons. These grades are often used for applications where the fatigue resistance can be a critical issue (eg, machine frames for the wind‐power industry or crankshaft used in trucks) and the investigation of the main damaging mechanisms during both the crack initiation and the crack propagation stage could offer new perspectives about these alloys.Ductile cast irons can be considered as a natural composite, being characterized by graphite elements (nodules) embedded in a more or less ductile matrix (ranging from fully ferritic to pearlitic, from martensitic to austempered). In this work, the fatigue crack initiation mechanisms were investigated considering different matrix microstructure and the presence of a mechanical properties gradient in the graphite nodules.

Facile tuning of the mechanical properties of a biocompatible soft material

Herein, we introduce a method to locally modify the mechanical properties of a soft, biocompatible material through the exploitation of the effects induced by the presence of a local temperature gradient. In our hypotheses, this induces a concentration gradient in an aqueous sodium alginate solution containing calcium carbonate particles confined within a microfluidic channel. The concentration gradient is then fixed by forming a stable calcium alginate hydrogel. The process responsible for the hydrogel formation is initiated by diffusing an acidic oil solution through a permeable membrane in a 2-layer microfluidic device, thus reducing the pH and freeing calcium ions. We characterize the gradient of mechanical properties using atomic force microscopy nanoindentation measurements for a variety of material compositions and thermal conditions. Significantly, our novel approach enables the creation of steep gradients in mechanical properties (typically between 10–100 kPa/mm) on small scales, which will be of significant use in a range of tissue engineering and cell mechanosensing studies.

Biofilm compressibility in ultrafiltration: A relation between biofilm morphology, mechanics and hydraulic resistance

Poroelastic fluid-structure interaction models were coupled to experimental data to determine the effects of biofilm spatial distribution of mechanical and hydraulic properties on the biofilm hydraulic resistance and compressibility in membrane filtration processes. Biofilms were cultivated on ultrafiltration membranes for 20 and 30 days under high (0.28 bar) and low (0.06 bar) transmembrane pressure (TMP), in dead-end filtration mode. Subsequently, biofilms were subjected to a compression/relaxation cycles by step-wise TMP changes. Structural deformation of biofilms during compression was observed in-situ using optical coherence tomography.Experimental results show that the observed increase in the biofilm hydraulic resistance during compression is not necessarily accompanied by a detectable biofilm thickness reduction. A dual-layer biofilm model with a dense base and porous top layer could explain these observed results. Because porosity controls indirectly the mechanical response of biofilms under compression, results could be described without assuming a gradient in mechanical properties within the biofilm. The biofilm surface roughness did not significantly influence the water flux in this study. However, the fraction of biofilm base layer directly exposed to bulk liquid could be a good indicator in the determination of water flux. The main implications of this study for the design and operation of low-pressure membrane systems (e.g., MF and UF with fouling layer being the main filtration resistance) lays in the selection of favorable operational TMP and biofilm morphology.