Three-dimensional Microstructures Research Articles

Effect of multiple laser peening on the wear resistance of 304 stainless steel

Purpose The purpose of this paper is to investigate the effect of laser peening (LP) on mechanical and wear properties of 304 stainless steel sheet. Design/methodology/approach Three-dimensional morphology, micro-hardness and micro-structure of shocked samples were tested. The wear amount, wear track morphology and wear mechanism were also characterized under dry sliding wear using Al2O3 ceramics ball. Findings The LP treatment generates deformation twins that contribute to the grain refinement and hardness increase. The wear test displays that the wear mechanism of samples is mainly abrasive wear and oxidation wear at 10 N load. While at 30 N, the delamination and adhesion areas of treated sample are reduced visibly compared to untreated ones. Originality/value This study specifically investigates the mechanical and wear properties of 304 stainless steel after the direct action of LP on its surface, which shows an effective improvement on the wear resistance. For example, the wear loss of processed sample is reduced by 19% at 30 N, the friction coefficient decreases from 0.4714 to 0.4308 and the groove depth is reduced from 78.1 to 74.4 µm under same condition. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-01-2024-0007/

Freeze-Casting of Alumina and Permeability Analysis Based on a 3D Microstructure Reconstructed Using Generative Adversarial Networks.

In this study, alumina ceramics with hierarchical pores were successfully fabricated using freeze casting. Experimental studies show that both the solid loading of the slurry and the thermal insulation layer at the interface of the slurry and cooling plate can influence the pore characteristics of cast samples. In order to examine the pore characteristics and evaluate the permeability of the freeze-cast samples fabricated under different conditions, a generative adversarial network (GAN) method was employed to reconstruct the three-dimensional (3D) microstructure from two-dimensional (2D) scanning electron microscopy (SEM) images of the samples. Furthermore, GAN 3D reconstruction was validated against X-ray tomography 3D reconstruction results. Based on the GAN reconstructed microstructures, the permeability and pore distribution of the various samples were analyzed. The sample cast with 35 wt.% solid loading shows an optimal permeability.

Reliability-Based Design Optimization of Additive Manufacturing for Lithium Battery Silicon Anode

Abstract With the blooming of the electric vehicle market and the advancement in the lithium-ion battery industry, silicon anode has shown great potential for the next-generation battery. Using the state-of-the-art additive manufacturing technique (three-dimensional holographic lithography), researchers have demonstrated that silicon anode can be fabricated as a three-dimensional bicontinuous porous microstructure. However, the volume fluctuation of the silicon anode caused by lithiation during the discharging process causes continuous capacity decay and poor cycling life. Besides, uncertainties are inherent in the manufacturing and usage processes, making it crucial to systematically consider them in the silicon anode design to improve its performance and reliability. To fill the gap between current silicon anode research and future industrial need, this study established a digital twin to investigate the optimal design for silicon anode under the uncertainties of additive manufacturing and battery usage. This study started with developing multiphysics finite element models of the silicon anode lithiation process to investigate the volume fluctuation of silicon. Then, surrogate models were built based on the results from the finite element models to reduce computational cost. The reliability-based design optimization was employed to find the best design point for the silicon anode, in which an outer optimization loop maximized the objective function and an inner loop dedicated to reliability analysis. Finally, the Pareto optimal front of the silicon anode designs was obtained and validated, which shows over 10% improvements in the silicon anode's total capacity and rate capability.

Spatiotemporal Characterization of Microstructure Morphology, Mechanical Properties and Bone Remodeling of Rat Tibia Under Uniaxial Compressive Overload Loading.

Bone tissue is subjected to increased mechanical stress during high-intensity work. Inadequate bone remodeling reparability can result in the continuous accumulation of microdamage, leading to stress fractures. The aim of this work was to investigate the characteristics and repair mechanisms of tibial microdamage under several degrees of overload. Also, we aimed at better understanding the effects of overload on the multi-scale structure and mechanical properties of bone. Sixty 5-month female rats were divided into three groups with different time points. Micro-CT was used to evaluate the three-dimensional microstructure, and three-point bending, quasi-static fracture toughness and creep mechanical test were carried out to evaluate the mechanical properties. SEM was used to observe the morphological characteristics of fracture surfaces. Section staining was used to count the microdamage parameters and numbers of osteoblasts and osteoclasts. The microarchitectures of cancellous and cortical bones in the three overload groups showed different degrees of damage. Overload led to a messy crystal structure of cortical bone, with slender microcracks mixed in, and a large number of broken fibers of cancellous bone. The properties associated with the elastic plasticity, fracture toughness, and viscoelasticity of cortical bone reduced in three groups, with that corresponding to day 30 presenting the highest damage. The accumulation of microdamage mainly occurred in the first 14days, that is, the crack density peaked on day 14. Peak-targeted bone remodeling of cortical and cancellous bones occurred mainly between days 14 and 30. The influence of overload mechanical environment on bone quality at different time points was deeply investigated, which is of great significance for the etiology and treatment of stress fractures.

Multi-plane denoising diffusion-based dimensionality expansion for 2D-to-3D reconstruction of microstructures with harmonized sampling

Acquiring reliable microstructure datasets is a pivotal step toward the systematic design of materials with the aid of integrated computational materials engineering (ICME) approaches. However, obtaining three-dimensional (3D) microstructure datasets is often challenging due to high experimental costs or technical limitations, while acquiring two-dimensional (2D) micrographs is comparatively easier. To deal with this issue, this study proposes a novel framework called ‘Micro3Diff’ for 2D-to-3D reconstruction of microstructures using diffusion-based generative models (DGMs). Specifically, this approach solely requires pre-trained DGMs for the generation of 2D samples, and dimensionality expansion (2D-to-3D) takes place only during the generation process (i.e., reverse diffusion process). The proposed framework incorporates a concept referred to as ‘multi-plane denoising diffusion’, which transforms noisy samples (i.e., latent variables) from different planes into the data structure while maintaining spatial connectivity in 3D space. Furthermore, a harmonized sampling process is developed to address possible deviations from the reverse Markov chain of DGMs during the dimensionality expansion. Combined, we demonstrate the feasibility of Micro3Diff in reconstructing 3D samples with connected slices that maintain morphologically equivalence to the original 2D images. To validate the performance of Micro3Diff, various types of microstructures (synthetic or experimentally observed) are reconstructed, and the quality of the generated samples is assessed both qualitatively and quantitatively. The successful reconstruction outcomes inspire the potential utilization of Micro3Diff in upcoming ICME applications while achieving a breakthrough in comprehending and manipulating the latent space of DGMs.

Acoustofluidics enables direct ink writing of vascular scaffolds with intrinsically patterned porous microstructures

Vascular scaffolds with well-organized pore characteristics are crucial for promoting infiltration, adhesion, growth, tissue-specific arrangement, and substance-exchanges of biological cells. However, their controllable fabrication is extremely challenging, especially for those with specially designed three-dimensional (3D) tubular and porous microstructures. In this paper, a new methodology based on acoustofluidics-assisted direct ink writing (DIW) technique is developed to fabricate vascular scaffolds with controlled porosity distributions and well-defined porous microstructures. Calcium carbonate (CaCO3) particles and alginate are uniformly mixed as the printing ink, which is printed along a rotating rod to form a vessel-like structure. Distributions of these CaCO3 particles in the printed structure are precisely controlled using bulk acoustic waves. The particles within the vessel-like structure are dissolved in situ using hydrochloric acid, resulting in the formation of 3D vascular scaffolds with intrinsically patterned porous microstructures. The scaffolds printed by the acoustofluidics-assisted DIW method exhibit a 69% improvement in pore connectivity compared to conventional 3D-printed scaffolds. Fibroblast cells are successfully seeded onto these scaffolds, achieving distinctive distributions along the patterned porous microstructures. This innovative methodology, with its excellent microfabrication capabilities, holds great promise for applications in tissue engineering, drug release, composite formation, microfluidic chips, and biosensors.

3-aminopropyltriethoxysilane modified MXene on three-dimensional nonwoven fiber substrates for low-cost, stable, and efficient solar-driven interfacial evaporation desalination

Recently, the solar-driven interfacial evaporation desalination has attracted more and more attentions due to the advantages of low cost, zero energy consumption, and high water purification rate, etc. One of the bottlenecks of this emerging technique lies in a lack of simple and low-cost ways to construct three-dimensional (3D) hierarchical microstructures for photothermal membranes. To this end, a two-step strategy is carried out by combining surface functionalization with substrate engineering. Firstly, a silane coupling agent 3-aminopropyltriethoxysilane (APTES) is grafted onto an ideal photothermal material of Ti3C2Tx MXene, to improve the nanochannel sizes and hydrophilicity, which are attributed to enlarged interspaces of MXene and introduced hydrophilic group e.g., –NH2 and –OH, respectively. Secondly, a low-cost and robust nonwoven fiber (NWF) substrate, which has a 3D micron-sized mesh structure with interlaced fiber stacks, is employed as the skeleton to load enough APTES-grafted MXene by a simple soaking method. Benefited from above design, the Ti3C2Tx-APTES/NWF composite membrane with a 3D hierarchical structure shows enhanced light scattering and utilization, water transport and vapor escape. A remarkable evaporation rate of 1.457 kg m−2 h−1 and an evaporation efficiency of 91.48 % are attained for a large-area (5 × 5 cm2) evaporator, and the evaporation rate is further increased to 1.672 kg m−2 h−1 for a small-area (2 × 2 cm2) device. The rejection rates of salt ions and heavy metal ions are higher than 99 % and 99.99 %, respectively, and the removal rates of organic dye molecules are nearly to 100 %. Besides, the composite photothermal membrane exhibits great stabilities in harsh conditions such as high salinities, long cycling, large light intensities, strong acid/alkali environments, and mechanical bending. Most importantly, the photothermal membrane shows a considerable cost-effectiveness of 89.4 g h−1/$. Hence, this study might promote the commercialization of solar-driven interfacial evaporation desalination by collaboratively considering surface modification and substrate engineering for MXene.

Three-dimensional layered multifunctional carbon aerogel for energy storage and pressure sensors

Carbon aerogel has a broad application prospect in various intelligent electronic devices. However, due to the complex preparation process and poor mechanical properties of carbon aerogel, its practical application was seriously affected. Therefore, in this study, a low-energy bidirectional freezing method was designed, using natural renewable cellulose nanofibers (CNF) and chitosan (CS) as biological matrix, and graphene oxide (GO) as conductive filler to successfully prepare a high-performance multi-functional carbon aerogel. Its ordered three-dimensional sheet microstructure is conducive to stress transfer and electron transfer. The test results show that the carbon aerogel has a large specific surface area (622.79 m2/g) and can withstand 80 % high compressive strain. The carbon aerogel electrode material showed high specific capacitance in 3 M KCl electrolyte in the three-electrode system (299 F/g at 1A/g). The assembled symmetric solid-state supercapacitors were tested in a two-electrode system with excellent area-specific capacitance (381.8 mF/cm2 at 1.8 mA/cm2) and cycle stability (10,000 cycles, capacitance retention rate of 86.2 %). In addition, the carbon aerogel exhibits an excellent linear sensitivity of 25.2 kPa−1, which can be assembled into pressure sensors to monitor human biological signals such as fingers, wrists and throat in real time. These excellent properties enable carbon aerogels to be used in energy storage devices and wearable electronics.

Synchrotron micro-computed tomography analysis of neutron-irradiated U-Mo fuel

The three-dimensional (3D) microstructure of neutron-irradiated uranium-10 wt.% molybdenum (U-10Mo) fuel with a burn-up of 9.8 × 1021 fissions/cm3 was characterized using a novel, multi-modal synchrotron micro-computed tomography approach combining propagation-based phase-contrast enhanced and absorption contrast techniques. The porosity development, porosity interconnectedness, swelling, composition, local thickness of the zirconium (Zr) diffusion barrier, and the influence of the fuel–cladding interaction on the local composition and pore morphology, were uniquely determined in 3D. Two cuboids were produced using a focused ion beam-scanning electron microscope at the Zr diffusion barrier–fuel interface and in the bulk fuel. The bulk fuel sample swelled by 53.3 [+9.7/−3.1]%, while the fuel near the Zr–fuel interface swelled by 63.3 [+14.7/−7.2]%. The average local thickness of the Zr diffusion barrier decreased by 53 %, compared to the expected pre-irradiated thickness. Four pore morphology regions were identified initiating parallel to the fuel–Zr interaction region: (1) an interaction layer of suppressed porosity, (2) a layer of elongated and interconnected porosity, (3) a transition zone of low porosity, and (4) a layer of unoriented porosity representative of the bulk fuel behavior. The increase in porosity near the diffusion barrier corresponded to a higher U concentration compared to that in the bulk fuel. The interconnected porosity in the fuel near the diffusion barrier was extensive and oriented parallel to the diffusion barrier, while the bulk fuel had more compact and isolated pore networks. The interaction layer, despite having suppressed porosity, was nearly 100 wt.% U. Porosity suppression at the diffusion barrier corresponds to the expected reduction in radiation-driven diffusion of Xe at the interface despite the anticipated increase in fission product nucleation originating from a higher U concentration. The novel 3D insights of the porosity, swelling, and compositional variations characterized herein can improve the fidelity of fuel performance codes for proliferation-resistant fuels for research and test reactors.

All-aqueous droplets-templated tailorable core-shell alginate microspheres for constructing vascularized intestinal mucosa in vitro models

Recently, in vitro models of intestinal mucosa have become important tools for drug screening and studying the physiology and pathology of the intestine. These models enable the examination of cellular behavior in diseased states or in reaction to alterations in the microenvironment, potentially serving as alternatives to animal models. One of the major challenges in constructing physiologically relevant in vitro models of intestinal mucosa is the creation of three-dimensional microstructures that accurately mimic the integration of intestinal epithelium and vascularized stroma. Here, core–shell alginate (Alg) microspheres were generated to create the compartmentalized extracellular matrix microenvironment needed to simulate the epithelial and vascularized stromal compartments of the intestinal mucosa. We demonstrated that NIH-3T3 and human umbilical vein endothelial cells embedded in the core of the microspheres can proliferate and develop a vascular network, while human colorectal adenocarcinoma cells (Caco-2) can form an epithelial monolayer in the shell. Compared to Caco-2 monolayer encapsulated within the shell, the presence of the vascularized stroma enhances their proliferation and functionality. As such, our core–shell Alg microspheres provide a valuable method for generating in vitro models of vascularized intestinal mucosa with epithelial and vascularized stroma arranged in a spatially relevant manner and demonstrating near-physiological functionality.

Effects of Nafion content in the catalyst layer of PEMFC on the transport phenomenon among nanoscale particles

Nafion, as a key component in the catalyst layer, plays a crucial role in the mass transport and electrochemical reactions within the catalyst layer. In this study, a numerical method is employed to reconstruct the three-dimensional nanoscale microstructure of the catalyst layer, including pores, Nafion, carbon particles, and Pt phases. A pore-scale model in which the Knudsen effect is taken into account is used to simulate the impact of Nafion contents on the interparticle transport and electrochemical reactions within the catalyst layer. The results show that the pore-scale simulation based on the reconstructed model can clearly reveal the mass transport process and highly uneven distribution of each physical quantity in the nanoscale particles and pores; When the Nafion content is relatively higher, the proton current distribution at the Pt/Nafion interface is relatively more uniform and the proton supply is sufficient. However, there is an inverse correlation between the oxygen mole concentration on the Pt surface and the Nafion content, which is the dominant factor affecting the electrochemical reactions on the Pt particle surface. When the Nafion content is lower, the proton supply becomes the dominant factor in the electrochemical reaction on the surface of Pt particles due to the weak connectivity of Nafion between the agglomerates. The combined effect of oxygen and proton transport result in the optimal Nafion volume fractions of 0.296 and 0.262 at voltages of 0.8 V and 0.7 V, respectively. The optimal Nafion volume fraction ranges between 0.224 and 0.262 at 0.6 V. As the voltage decreases, the effect of oxygen transport becomes more significant, and the optimal Nafion content decreases accordingly.

Self-Assembly for Creating Vertically-Aligned Graphene Micro Helices with Monolayer Graphene as Chiral Metamaterials.

Graphene's emergence enables creating chiral metamaterials in helical shapes for terahertz applications, overcoming material limitations. However, practical implementation remains theoretical due to fabrication challenges. This paper introduces a dual-component self-assembly technique that enables creating vertically-aligned continuous monolayer graphene helices at microscale with great flexibility and high controllability. This assembly process not only facilitates the creation of three-dimensional (3D) microstructures, but also positions the 3D structures from a horizontal to a vertical orientation, achieving an aspect ratio (height/width) of approximately 2700. As a result, an array of vertically-aligned graphene helices is formed, reaching up to 4mm in height, which is equivalent to 4 million times the height of monolayer graphene. The benefit of these 3D chiral structures made from graphene is their capability to infinitely extend in height, interacting with light in ways that are not possible with traditional 2D layering methods. Such an impressive height elevates a level of interaction with light that far surpasses what is achievable with traditional 2D layering methods, resulting in a notable enhancement of optical chirality properties. This approach is applicable to various 2D materials, promising advancements in innovative research and diverse applications across fields. This article is protected by copyright. All rights reserved.

Stochastic reconstruction and performance prediction of cathode microstructures based on deep learning

The effective properties of lithium-ion battery (LIB) cathode are determined by both the volume fractions of constituents and the morphological features of microstructure. However, it is difficult to establish an accurate quantitative relationship between the macroscopic effective properties and microstructural features. Deep learning techniques, due to their exceptional nonlinear fitting capabilities, have been widely applied in various complex fields. Our study presents a generation scheme of numerous three-dimensional (3D) digital microstructures of cathode, using a deep convolutional neural network (CNN)-based stochastic reconstruction algorithm combining with the scanning electron microscope (SEM) images. The reconstructed samples are substituted with the corresponding finite element (FE) models, and the effective mechanical and electrochemical properties are assessed through the FE-based homogenization theory. Finally, the generated cathode samples and their effective properties are used to train the 3D CNN for performance prediction. This study demonstrates that the deep learning approaches can accurately and rapidly reconstruct the microstructure of cathode and predict their effective properties. Furthermore, the established framework can be extended to other heterogeneous materials.

Multiscale characterisation study on the effect of heat treatment on the microstructure of additively manufactured 316L stainless steel

Additively manufactured 316L components are known to exhibit complex three-dimensional microstructures on multiple length-scales. The effect of heat treatment on the stability of grains and dislocation cell structures within the microstructure has previously been investigated but there are appreciable disagreements surrounding the nature of the texture that is present in the specimens, and there is also limited published work on the compositional homogeneity of specimens, particularly on the atomic scale, after heat treatments. In this study, we investigate the effect of applying two different heat treatments to 316L components produced by laser powder bed fusion before characterising them using a combination of electron microscopy and atom probe tomography. The importance of using multiple characterisation techniques, which span from the nanometre to micrometre scale, in addition to carefully and accurately describing analysis methods when investigating the evolution of the microstructure of additively manufactured 316L steels is demonstrated. Our EBSD (Electron Back Scatter Diffraction) results show the presence of a strong texture in the build direction of the samples, and a reduction in morphological texture as a result of heat treatment. TEM (Transmission Electron Microscopy) results indicate a dissolution of the dislocation cell structure that forms during solidification. Atom probe tomography was used to investigate compositional homogeneity in the samples and indicated that there are regions enriched in Cr, Mn, Mo, and Ni in the As-Printed samples that are likely associated with the dislocation cell walls. The atom probe results also reveal the presence of impurities, such as Co, which were not detected in the feedstock powder.

Three-Dimensional Columnar Microstructure Representation Using 2D Electron Backscatter Diffraction Data for Additive-Manufactured Haynes®282®.

This study provides a methodology for exploring the microstructural and mechanical properties of the Haynes®282® alloy produced via the Powder Bed Fusion-Electron Beam (PBF-EB) process. Employing 2D Electron Backscatter Diffraction (EBSD) data, we have successfully generated 3D representations of columnar microstructures using the Representative Volume Element (RVE) method. This methodology allowed for the validation of elastic properties through Crystal Elasticity Finite Element (CEFE) computational homogenization, revealing critical insights into the material behavior. This study highlights the importance of accurately representing the grain morphology and crystallographic texture of the material. Our findings demonstrate that created virtual models can predict directional elastic properties with a high level of accuracy, showing a maximum error of only ~5% compared to the experimental results. This precision underscores the potential of our approach for predictive modeling in Additive Manufacturing (AM), specifically for materials with complex, non-homogeneous microstructures. It can be concluded that the results uncover the intricate link between microstructural features and mechanical properties, underscoring both the challenges encountered and the critical need for the accurate representation of grain data, as well as the significance of achieving a balance in EBSD area selection, including the presence of anomalies in strongly textured microstructures.

Microstructure reconstruction of 2D/3D random materials via diffusion-based deep generative models

Microstructure reconstruction serves as a crucial foundation for establishing process–structure–property (PSP) relationship in material design. Confronting the limitations of variational autoencoder and generative adversarial network within generative models, this study adopted the denoising diffusion probabilistic model (DDPM) to learn the probability distribution of high-dimensional raw data and successfully reconstructed the microstructures of various composite materials, such as inclusion materials, spinodal decomposition materials, chessboard materials, fractal noise materials, and so on. The quality of generated microstructure was evaluated using quantitative measures like spatial correlation functions and Fourier descriptor. On this basis, this study also achieved the regulation of microstructure randomness and the generation of gradient materials through continuous interpolation in latent space using denoising diffusion implicit model (DDIM). Furthermore, the two-dimensional microstructure reconstruction was extended to three-dimensional framework and integrated permeability as a feature encoding embedding. This enables the conditional generation of three-dimensional microstructures for random porous materials within a defined permeability range. The permeabilities of these generated microstructures were further validated through the application of the lattice Boltzmann method. The above methods provide new ideas and references for material reverse design.

Characterizing the microstructures of mammalian enamel by synchrotron phase contrast microCT

The enamel of mammalian teeth is a highly mineralized tissue that must endure a lifetime of cyclic contact and is inspiring the development of next-generation engineering materials. Attempts to implement enamel-inspired structures in synthetic materials have had limited success, largely due to the absence of a detailed understanding of its microstructure. The present work used synchrotron phase-contrast microCT imaging to evaluate the three-dimensional microstructure of enamel from four mammals including Lion, Gray Wolf, Snow Leopard, and Black Bear. Quantitative results of image analysis revealed that the decussation pattern of enamel consists of discrete diazone (D) and parazone (P) bands of rods organized with stacking arrangement of D+/P/D-/P in all mammals evaluated; the D+ and D- refer to distinct diazone bands with juxtaposed rod orientations from the reference plane. Furthermore, the rod orientations in the bands can be described in terms of two principal angles, defined here as the pitch and yaw. While the pitch angle increases from the outer enamel to a maximum (up to ≈ 40°) near the dentin enamel junction, minimal spatial variations are observed in yaw across the enamel thickness. There are clear differences in the decussation parameters of enamel across species that are interpreted here with respect to the structural demands placed on their teeth. The rod pitch and band width of enamel are identified as important design parameters and appear to be correlated with the bite force quotient of the four mammals evaluated. Statement of significanceThe multi-functionality of tooth enamel requires both hardness and resistance to fracture, properties that are generally mutually exclusive. Ubiquitous to all mammalian teeth, the enamel is expected to have undergone adaptations in microstructure to accommodate the differences in diet, body size and bite force across animals. For the first time, we compare the complex three-dimensional microstructure of enamel from teeth of multiple mammalian species using synchrotron micro-computed tomography. The findings provide new understanding of the “design” of mammalian enamel microstructures, as well as how specific parameters associated with the decussation of rods appear to be engineered to modulate its fracture resistance.

Atomistic insights into the mechanical properties of cross-linked Poly(N-isopropylacrylamide) hydrogel

Poly(N-isopropylacrylamide) (PNIPAM) is a highly versatile thermo-responsive hydrogel with immense potential for applications in biomedicine and tissue engineering. While PNIPAM has been extensively researched, there remains a significant gap in understanding its mechanical behavior at the atomistic scale. To address this challenge, this study employs molecular dynamics (MD) simulations to delve into the mechanical response of cross-linked PNIPAM hydrogels. The uniqueness of this research lies in our emphasis on providing atomic-level insights into the influence of chemical cross-linking and physical entanglements, as well as quantifying their effects during the application of strain. This represents a novel contribution to the study of hydrogels. MD simulations enable us to scrutinize the behavior of individual polymer chains and their intricate interactions in unprecedented detail, shedding light on microscale phenomena that are often inaccessible through experiments alone. To tackle the complexities associated with atomistic simulations of cross-linked hydrogels, we introduce a dynamic cross-linking algorithm. This innovative approach incorporates a nonreactive force field to construct realistic three-dimensional hydrogel microstructures, employing a stepwise bond formation strategy. Our findings underscore the remarkable impact of increasing the Degree of Cross-linking (DoC) and/or the Degree of Polymerization (DoP) on enhancing the robustness and elastic modulus of PNIPAM hydrogels. Additionally, we uncover the stochastic nature of chemical cross-linking, leading to the formation of anisotropic super-fragments when DoC exceeds a critical threshold. Furthermore, this research highlights the pivotal role of low strain rates on the order of 106 s−1 in accurately modeling the mechanical properties of hydrogels using MD simulations. This approach enables us to obtain meaningful quantitative mechanical properties that align with experimental results reported in the existing literature. In summary, this paper offers an unprecedented level of insight into the intricate interplay of chemical cross-linking and physical entanglements, providing a foundation for future research in this domain.

Uniaxial mechanical stretch properties correlated with three-dimensional microstructure of human dermal skin.

The intact and healthy skin forms a barrier to the outside world and protects the body from mechanical impact. The skin is a complex structure with unique mechano-elastic properties. To better direct the design of biomimetic materials and induce skin regeneration in wounds with optimal outcome, more insight is required in how the mechano-elastic properties emerge from the skin's main constituents, collagen and elastin fibers. Here, we employed two-photon excited autofluorescence and second harmonic generation microscopy to characterize collagen and elastin fibers in 3D in 24 human dermis skin samples. Through uniaxial stretching experiments, we derive uni-directional mechanical properties from resultant stress-strain curves, including the initial Young's modulus, elastic Young's modulus, maximal stress, and maximal and mid-strain values. The stress-strain curves show a large variation, with an average Young's modules in the toe and linear regions of 0.1 MPa and 21 MPa. We performed a comprehensive analysis of the correlation between the key mechanical properties with age and with microstructural parameters, e.g., fiber density, thickness, and orientation. Age was found to correlate negatively with Young's modulus and collagen density. Moreover, real-time monitoring during uniaxial stretching allowed us to observe changes in collagen and elastin alignment. Elastin fibers aligned significantly in both the heel and linear regions, and the collagen bundles engaged and oriented mainly in the linear region. This research advances our understanding of skin biomechanics and yields input for future first principles full modeling of skin tissue.

Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement.

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.