Wall Lamella Research Articles

Strong and tough 3D-(SiC–Si3N4)/Al co-continuous composite with enhanced interfaces

Materials with high specific strength and toughness are urgently needed in the fields of electronic packaging, armor, and aerospace. SiC/Al composites show good potential for these applications but the problem of mutual exclusion of strength and toughness still needs to be solved. Herein, we propose a new strategy to simultaneously enhance its strength and toughness from the perspective of structural optimization and interface enhancement. Inspired by the nacre structure, a 3D-(SiC–Si3N4)/Al co-continuous composite was prepared by freeze-casting combined with the pressure infiltration method. During the process, elongated Si3N4 grains were introduced into the ceramic framework to design the internal lamella wall structure and increase the contact area with the aluminum substrate. Moreover, a favorable interfacial bonding was designed between Si3N4 and Al to improve interface wettability. The reliable microstructure of porous 3D-(SiC–Si3N4) preform was tailored through the adjustment of initial solid loading and the α-Si3N4 component. The effects of initial solid loading and the α-Si3N4 component on the microstructure, interfacial wettability, and mechanical properties of the final composites were investigated. The results showed that the SiC–Si3N4/Al composites possess high flexural strength (769 MPa) and fracture toughness (15.1 MPa m1/2), which is superior to data reported in general literature. Numerous elongated β-Si3N4 grains are produced on the ceramic skeleton, forming an interlocking structure with the aluminum matrix, which increases the contact area with the aluminum substrate. A chemical reaction layer was formed at the interface of the composite, greatly improving the interfacial bonding strength. This work provides a promising design guideline for developing and optimizing high-strength and tough composite.

Enhanced piezo-photocatalytic degradation of organic pollutants by cambered wall lamellar structure of porous tubular g-C3N4

The piezoelectric polarization electric field induced by the non-centrosymmetric triangular holes of g-C3N4 was beneficial to promote the separation and transport of photogenerated carrier. The porous structure of porous tubular g-C3N4 (PTCN) facilitated the absorption of visible light, and its thin and highly extended cambered tube wall lamellae and large specific surface area per unit cross-sectional area facilitated the response of mechanical vibration, thus improving the piezo-photocatalytic performance. The piezo-photocatalytic degradation rates of tetracycline hydrochloride (TCH), ciprofloxacin (CIP), rhodamine B (RhB) and methylene blue (MB) by PTCN were 89%, 51%, 100% and 100% within 30 min, respectively. The yield of H2O2 reached 196.6 μM within 120 min. Photocurrent intensity, finite element simulation, DFT calculation, PFM and free radical quantification experiments were conducted to explain the reason for the high piezoelectric performance of PTCN, and the reason for the higher piezoelectric properties of the tubular structure than the bulk structure was also analyzed in depth. This work filled the research gap of tubular g-C3N4 in piezo-photocatalysis and made up the research gap of the influence of material morphology structure on piezoelectric properties, which provided a new reference and incentive for future piezoelectric photocatalysis research.

Changes in Density of Cellulose Microfibrils in Cell Wall Lamellae during Cell Growth in the Giant-Celled Green Alga <i>Valonia utricularis</i>

Cell walls in the giant-celled green alga Valonia are composed of multiple lamellae and designated as “crossed-fibril” type wherein each lamella contains cellulose microfibrils (CMFs) arranged in a uniform orientation. Outer, older lamellae are thinner than inner, younger lamellae, probably because the former are extended during cell growth more so than the latter without deposition of new cell wall materials. If this hypothesis is correct, the density of CMFs in the outer lamellae should be lower than that in the inner lamellae. In the present study, the density of CMFs in the outermost lamellae of the cell wall, in various sizes of Valonia utricularis growing cells, was calculated using images obtained with an atomic force microscope, and the densities were compared among the cells of different sizes. The CMF density ranged from 0.5 to 4.0 µm−1, which is lower than that previously recorded for the innermost lamellae (more than 10 µm−1). Moreover, the CMF density was negatively correlated with the cell size indicating that spacing between neighboring CMFs was increased by the cell surface extension due to cell growth. These findings may support the abovementioned hypothesis that the outer cell wall lamellae are extended by cell growth without the deposition of new CMFs.

The walnut shell network: 3D visualisation of symplastic and apoplastic transport routes in sclerenchyma tissue

Main conclusionHigh symplastic connectivity via pits was linked to the lignification of the developing walnut shell. With maturation, this network lessened, whereas apoplastic intercellular space remained and became relevant for shell drying.The shell of the walnut (Juglans regia) sclerifies within several weeks. This fast secondary cell wall thickening and lignification of the shell tissue might need metabolites from the supporting husk tissue. To reveal the transport capacity of the walnut shell tissue and its connection to the husk, we visualised the symplastic and apoplastic transport routes during shell development by serial block face-SEM and 3D reconstruction. We found an extensive network of pit channels connecting the cells within the shell tissue, but even more towards the husk tissue. Each pit channel ended in a pit field, which was occupied by multiple plasmodesmata passing through the middle lamella. During shell development, secondary cell wall formation progressed towards the interior of the cell, leaving active pit channels open. In contrast, pit channels, which had no plasmodesmata connection to a neighbouring cell, got filled by cellulose layers from the inner cell wall lamellae. A comparison with other nut species showed that an extended network during sclerification seemed to be linked to high cell wall lignification and that the connectivity between cells got reduced with maturation. In contrast, intercellular spaces between cells remained unchanged during the entire sclerification process, allowing air and water to flow through the walnut shell tissue when mature. The connectivity between inner tissue and environment was essential during shell drying in the last month of nut development to avoid mould formation. The findings highlight how connectivity and transport work in developing walnut shell tissue and how finally in the mature state these structures influence shell mechanics, permeability, conservation and germination.

Deformation mechanisms in ice-templated alumina–epoxy composites for the different directions of uniaxial compressive loading

The ice-templating technique enables the fabrication of multilayered ceramic-based composite materials. Very little is known on the inelastic deformation mechanisms that evolve in this class of composite materials under compressive loading conditions and cause macroscopic failure. The current investigation is motivated by a recent study by the authors, which revealed that the uniaxial compressive response of ice-templated ceramic–polymer composites is strongly dependent on the loading direction relative to the layer orientation. The current investigation reveals that the inelastic deformation mechanisms in ice-templated alumina–epoxy composites are strongly influenced by the compressive loading orientation relative to the growth direction of ice crystals. The deformation mechanisms were investigated for the loading directions of 0° (parallel to the growth direction), 45° (to the growth direction), and 90° (to the growth direction). For 0°, kink band formation and longitudinal splitting were observed to be the primary strength limiting mechanisms. Kink band formation could be the primary strength limiting factor and responsible for the catastrophic-type compressive failure response. For the loading directions of 45° and 90°, interface delamination and fracture within the lamella walls and across the alumina–epoxy interfaces were the main deformation mechanisms. These mechanisms significantly reduced the compressive strength but attributed progressive-type failure behavior in ice-templated composites. The knowledge of the inelastic deformation mechanisms in ice-templated ceramic–polymer composites under compressive loading is vital for an improved understanding of structure–mechanical property relationships and hierarchical materials design.

Role of Microstructure on Impact Response and Damage Morphology of Ice-Templated Porous Ceramics

The central focus of this study is to investigate the influence of microstructure and direction of impact (relative to the growth direction of ice crystals) on the impact behavior of ice-templated sintered alumina materials and understand the relationship between dynamic compressive strength and impact response. All materials were fabricated using alumina suspensions of the same solid loading but at three different freezing front velocity (FFV) regimes; very-high FFV, moderately-high FFV, and low FFV. Lamella wall thickness, pore size, and pore aspect ratio decreased, and wall connectivity increased with FFV. Materials also exhibited a structural gradient along the growth direction of ice crystals. As the templated microstructure became finer with FFV, the impact resistance of the materials increased, and radius of damage crater, depth-of-penetration, and mass loss decreased. Materials also exhibited radial cracking, and the materials fabricated at very-high FFV showed a greater propensity for radial cracking for impact along the growth direction. The impact process evolved in three phases; penetration phase, dwell phase and rebound phase. Analysis of high-speed videos revealed that modifying the microstructure affected not only the impact resistance but also the duration of these phases. Variation in microstructure caused a change in the mechanism of damage evolution during impact. Dynamic compressive strength increased with FFV, and the results revealed a direct relationship between impact response and strength. For both impact and dynamic compression, energy absorption per unit volume increased with FFV, further reinforcing the relationship between impact behavior and the dynamic compressive response of ice-templated materials.

Strength enhancement in ice-templated lithium titanate Li4Ti5O12 materials using sucrose

Ice-templating technique enables the synthesis of novel ceramic materials with directional, anisotropic pores, and the templated structure is highly tunable. Another factor that also drives interest in ice-templated ceramics is that for a comparable level of porosity, the uniaxial compressive strength of these materials can be significantly greater in comparison to that of the conventional open-cell foams. However, the strength advantage is significantly reduced in the materials templated from low solid loading suspensions. Toward this limitation, this study sheds insights into the influence of sucrose (a water-soluble additive) and freezing front velocity (FFV) on microstructure and the uniaxial compressive response of ice-templated sintered lithium titanate (LTO, composition Li4Ti5O12). This study used LTO as a model material. Materials were ice-templated from aqueous suspensions with 20 vol.% LTO powder and sucrose content was varied between 1–4 wt.%. The porosity of the sintered materials was in the range of 60–65 vol.%. Materials processed without sucrose exhibited lamellar morphology and low compressive strength. Sucrose had a marked influence on the connectivity between lamella walls, which significantly increased with sucrose content, and morphology became highly dendritic. The microstructural changes remarkably impacted compressive strength, which increased as high as eight-fold. Moreover, materials processed with sucrose exhibited a significant increase in strength in the continuous brittle crushing regime. By adjusting sucrose content in aqueous suspension and FFV, it is possible to synthesize ice-templated sintered LTO materials with tailored connectivity between lamella walls and harness compressive strength within a wide range of 4–80 MPa.

Teichoic acids anchor distinct cell wall lamellae in an apically growing bacterium

The bacterial cell wall is a multicomponent structure that provides structural support and protection. In monoderm species, the cell wall is made up predominantly of peptidoglycan, teichoic acids and capsular glycans. Filamentous monoderm Actinobacteria incorporate new cell-wall material at their tips. Here we use cryo-electron tomography to reveal the architecture of the actinobacterial cell wall of Streptomyces coelicolor. Our data shows a density difference between the apex and subapical regions. Removal of teichoic acids results in a patchy cell wall and distinct lamellae. Knock-down of tagO expression using CRISPR-dCas9 interference leads to growth retardation, presumably because build-in of teichoic acids had become rate-limiting. Absence of extracellular glycans produced by MatAB and CslA proteins results in a thinner wall lacking lamellae and patches. We propose that the Streptomyces cell wall is composed of layers of peptidoglycan and extracellular polymers that are structurally supported by teichoic acids.

Effects of temperature and platelets on lamella wall microstructure, structural stability, and compressive strength in ice-templated ceramics

The current investigation provides insights into microstructure development as a function of sintering temperature within lamella walls of ice-templated ceramics, effects of ceramic platelets on wall microstructure, and the influence of those developments on the compressive mechanical response. The results revealed a profound influence of both on the several aspects of ice-templated ceramics, enabling an improved understanding of the structure-mechanical property relationships in these materials and limitations in the development of the ice-templated ceramic structure. In the materials without platelets, at low-temperature walls were highly porous, at intermediate temperature a pore-free lamella wall with fine-grained microstructure developed, and at high-temperature along with grain growth significant abnormal grain growth occurred as well. More dramatic was the combined effects of temperature and platelets. The effects of platelets were realized at two length scales. Some of the platelets developed lamellar bridges, whereas the platelets that became part of the lamella walls remarkably impacted wall microstructure. The current results strongly suggest that there are significant structural and mechanical strength advantages in the incorporation of large anisometric particles in ice-templated ceramics. The addition of large platelets resulted in a marked increase of compressive strength. A significant structural advantage is that the materials containing platelets exhibited improved stability to structural deformation at higher temperatures compared to the materials without the platelets. Thus, this study shows the importance of deeply probing into the structure-mechanical property relationships of ice-templated ceramics as a function of temperature and composition, providing valuable guidance into the microstructure design of these materials.

Characterization of dynamic and quasistatic compressive mechanical properties of ice-templated alumina–epoxy composites

Abstract

Mechanical Characterization of the Lamellar Structure of Human Abdominal Aorta in the Development of Atherosclerosis: An Atomic Force Microscopy Study.

Atherosclerosis is a major risk factor for cardiovascular disease. However, mechanisms of interaction of atherosclerotic plaque development and local stiffness of the lamellar structure of the arterial wall are not well established. In the current study, the local Young's modulus of the wall and plaque components were determined for three different groups of healthy, mildly diseased and advanced atherosclerotic human abdominal aortas. Histological staining was performed to highlight the atherosclerotic plaque components and lamellar structure of the aortic media, consisting of concentric layers of elastin and interlamellar zones. The force spectroscopy mode of the atomic force microscopy was utilized to determine Young's moduli of aortic wall lamellae and plaque components at the micron level. The high variability of Young's moduli (E) at different locations of the atherosclerotic plaque such as the fibrous cap (E=15.5± 2.6kPa), calcification zone (E=103.7±19.5kPa), and lipid pool (E=3.5±1.2kPa) were observed. Reduction of elastin lamellae stiffness (18.6%), as well as stiffening of interlamellar zones (50%), were detected in the diseased portion of the medial layer of abdominal aortic wall compared to the healthy artery. Additionally, significant differences in the stiffness of both elastin lamellae and interlamellar zones were observed between the diseased wall and disease-free wall in incomplete plaques. Our results elucidate the alternation of the stiffness of different lamellae in the human abdominal aortic wall with atherosclerotic plaque development and may provide new insight on the remodeling of the aortic wall during the progression of atherosclerosis.

Effects of porosity and strain rate on the uniaxial compressive response of ice-templated sintered macroporous alumina

We investigated and thoroughly analyzed compressive response of ice-templated ceramics in quasistatic and dynamic regimes of strain rate. In the high pore volume regime, sintered scaffolds exhibited highly lamellar pore morphology and pore architecture transitioned to dendritic with the decreasing pore volume. Mechanical property measurements in the quasistatic regime of strain rate revealed that with increasing density, compressive response transitioned from a damageable, cellular-like failure to a brittle-like failure. We rationalized the measured results in terms of the propensity of the lamella walls to undergo buckling. Our conjecture is that in the high pore volume regime, lamella walls of the scaffolds are prone to buckling-induced elastic instability, which leads to a compressive response that manifests a gradual decrease of stress beyond peak stress. In contrast, we suggest that in the low pore volume regime thick lamella walls and extensive transverse bridging exhibit marked resistance to buckling-induced instability and scaffolds undergo a global failure, which is manifested in the measured quasistatic compressive response as a sharp drop of stress beyond peak stress. Dynamic compressive response of the scaffolds exhibited measurable differences relative to the quasistatic compressive response. Scaffolds exhibited a relatively gradual decrease of dynamic compressive stress beyond peak stress, and an overall improvement of compressive response and energy absorption capacity was measured under the dynamic loading conditions. We rationalized the measured differences in between the strain rate regimes in terms of the micro-inertia and other effects. This study is of critical significance for applications of ice-templated structures in dynamic environments.

Transcriptome Assembly of the Bast Fiber Crop, Ramie, Boehmeria nivea (L.) Gaud. (Urticaceae)

Ramie (Boehmeria nivea) is a perennial crop valued for its strong bast fibers. Unlike other major bast fiber crops, ramie fiber processing does not include retting, but does require degumming, suggesting distinctive features in pectin and the development and composition of fibers. A comprehensive transcriptome assembly of ramie has not been made available, to date. We obtained the sequence of RNA transcripts (RNA Seq) from the apical region of developing ramie stems and combined these with reads from public databases for a total of 157,621,051 paired-end reads (30.3 billion base pairs Gbp) used as input for de novo assembly, resulting in 70,721 scaffolds (≥200 base pairs (bp); N50 = 1798 bp). As evidence of the quality of the assembly, 36,535 scaffolds aligned to at least one Arabidopsis protein (BLASTP e-value ≤ 10−10). The resource described here for B. nivea will facilitate an improved understanding of bast fibers, cell wall, and middle lamella development in this and other comparative species.

On the brittle fracture characteristics of lamella walls of ice-templated sintered alumina scaffolds and effects of platelets

We attempted to develop a phenomenological understanding of the evolution of the fracture events within fine-grained lamella walls of partially deformed ice-templated sintered Al2O3 scaffolds and revealed the effects of the platelets on the crack propagation characteristics. Detailed microstructural analyses suggest that intergranular cracks within the walls evolved at two orientations, parallel and perpendicular to the loading direction. We proposed probable mechanisms including one based on the so-called wing-crack model to rationalize the observed crack orientations. We presented crack propagation behavior in lamella walls in presence of the platelets and discussed the effects in terms of the crack deflection mechanism.

Debonding of cellular structures with fibre-reinforced cell walls under shear deformation

Many natural structures are cellular solids at millimetre scale and fibre-reinforced composites at micrometre scale. For these structures, mechanical properties are associated with cell strength, and phenomena such as cell separation through debonding of the middle lamella in cell walls are key in explaining some important characteristics or behaviour. To explore such phenomena, we model cellular structures with non-linear hyperelastic cell walls under large shear deformations, and incorporate cell wall material anisotropy and unilateral contact between neighbouring cells in our models. Analytically, we show that, for two cuboid walls in unilateral contact and subject to generalised shear, gaps can appear at the interface between the deforming walls. Numerically, when finite element models of periodic structures with hexagonal cells are sheared, significant cell separation is captured diagonally across the structure. Our analysis further reveals that separation is less likely between cells with high internal cell pressure (e.g. in fresh and growing fruit and vegetables) than between cells where the internal pressure is low (e.g. in cooked or ageing plants).

Influence of anisotropic grains (platelets) on the microstructure and uniaxial compressive response of ice-templated sintered alumina scaffolds

Ice-templated ceramics have unique lamellar pore morphology that offers better compressive mechanical properties in comparison to the typical ceramic foams with isotropic pore morphology. However, for very high-level porosity (>65 vol%) strength difference diminishes. This investigation reveals that by inducing anisotropic grains within the matrix of a fine-grained ceramic, uniaxial compressive response of the ice-templated sintered scaffolds can be markedly enhanced without causing any considerable modification of the total porosity. To address this innovative materials design strategy, we synthesized a series of microstructures by systematically varying the anisotropic grains content in an aqueous suspension and the freezing kinetics to investigate the process-microstructure correlations and understand the structure-property relationships. Microstructural investigations revealed the unique arrangements of the platelets within and out of the lamella walls, where the upward moving ice fronts aligned the platelets' in-plane direction to the ice-growth direction. In the low freezing front velocity regime, platelets were observed to be mainly within the lamella walls, whereas platelets started to develop lamellar bridges with the increasing velocity. As a result, a transition of the pore morphology occurred with the increasing platelets content and the freezing front velocity. A novel method based on the rigorous microstructural analysis is developed to estimate the distribution of the platelets within and out of the walls and the variation of the platelets distribution with the composition and freezing front velocity. A drastic improvement of the compressive mechanical properties (stiffness, strength, energy absorption capacity) was measured due to the platelets' addition, which has been related to the platelets' distribution within and out of the walls and the pore morphology modifications. Results are rationalized based on the role of the platelets during the compressive deformation.

Morphological and structural characterization of the attachment system in aerial roots of Syngonium podophyllum.

The attachment of aerial roots of Syngonium podophyllum involves a multi-step process adjusted by multi-scale structures. Helical-crack root hairs are first found in the attachment system, representing specialized structures for surface anchorage. The morphological variability of attachment organs reflects diverse climbing strategies. One such anchoring mode in clinging-climbers involves the time-dependent interaction between roots and the support: By naturally occurring adhesive roots with root hairs, the plant can ascend on supports of any shape and size. As a typical root-climber, Syngonium podophyllum develops elongate aerial roots at nodes. Here, we studied its attachment behavior from the external morphology to the internal structure in detail. Through SEM and LM observation on several root-substrate interfaces, we suggested that the attachment of aerial roots was mediated by a multi-step process, in which root hairs played significant roles in releasing mucilaginous substance and securing the durable anchorage. We summarized all the types of shape changes of root hairs with particular focus on the abnormal transition from a tube to a helical-crack ribbon. We demonstrated our understanding with respect to the formation of the helical-crack root hairs, based on the structural evidence of cellulose microfibrils orientation on the cell wall lamellae. The helical-crack root hairs serving as energy-dissipating units retard the failure of adhesion under high winds and loads.

Characterization of mechanical properties of lamellar structure of the aortic wall: Effect of aging

Arterial wall tissues are sensitive to their mechanical surroundings and remodel their structure and mechanical properties when subjected to mechanical stimuli such as increased arterial pressure. Such remodeling is evident in hypertension and aging. Aging is characterized by stiffening of the artery wall which is assigned to disturbed elastin function and increased collagen content. To better understand and provide new insight on microstructural changes induced by aging, the lamellar model of the aortic media was utilized to characterize and compare wall structure and mechanical behavior of the young and old human thoracic aortic samples. Such model regards arterial media as two sets of alternating concentric layers, namely sheets of elastin and interlamellar layers. Histological and biaxial tests were performed and microstructural features and stress–strain curves of media were evaluated in young and old age groups. Then using optimization algorithms and hyperelastic constitutive equations the stress–strain curves of layers were evaluated for both age groups. Results indicated slight elevation in the volume fraction of interlamellar layer among old subjects most probably due to age related collagen deposition. Aging indicated substantial stiffening of interlamellar layers accompanied by noticeable softening of elastic lamellae. The general significant stiffening of old samples were attributed to both increase of volume fraction of interlamellar layers and earlier recruitment of collagen fibers during load bearing due to functional loss of elastin within wall lamellae. Mechanical characterization of lamellar structure of wall media is beneficial in study of arterial remodeling in response to alternated mechanical environment in aging and clinical conditions through coupling of wall microstructure and mechanical behavior.

Evaluation of Biaxial Mechanical Properties of Aortic Media Based on the Lamellar Microstructure.

Evaluation of the mechanical properties of arterial wall components is necessary for establishing a precise mechanical model applicable in various physiological and pathological conditions, such as remodeling. In this contribution, a new approach for the evaluation of the mechanical properties of aortic media accounting for the lamellar structure is proposed. We assumed aortic media to be composed of two sets of concentric layers, namely sheets of elastin (Layer I) and interstitial layers composed of mostly collagen bundles, fine elastic fibers and smooth muscle cells (Layer II). Biaxial mechanical tests were carried out on human thoracic aortic samples, and histological staining was performed to distinguish wall lamellae for determining the dimensions of the layers. A neo-Hookean strain energy function (SEF) for Layer I and a four-parameter exponential SEF for Layer II were allocated. Nonlinear regression was used to find the material parameters of the proposed microstructural model based on experimental data. The non-linear behavior of media layers confirmed the higher contribution of elastic tissue in lower strains and the gradual engagement of collagen fibers. The resulting model determines the nonlinear anisotropic behavior of aortic media through the lamellar microstructure and can be assistive in the study of wall remodeling due to alterations in lamellar structure during pathological conditions and aging.

Development of carrot parenchyma softening during heating detected in vivo by dynamic mechanical analysis

Dynamic mechanical analysis was employed to explore the development of carrot parenchyma softening during heating in the range of 30–90 °C. Results indicated that compared with raw carrots the toughness of carrots at 60 °C decreased significantly (P < 0.05) at 0.5 °C/min while there was no significant deference (P > 0.05) at 2 °C/min. Samples at 0.5 °C/min finally obtained the lower toughness. Besides, both loss tangent (LT) and two negative peaks on the temperature–slope plot were heating rate dependent. At 0.5 °C/min, distinct increases of LT started at about 48 °C while at about 67 °C at 2 °C/min. With increasing rate, LT effect decreased obviously. Increasing rates caused a shift of both peaks from low temperatures (49–60 °C) to high ones (67–75 °C). Values of both peaks were significantly higher (P < 0.05) at 0.5 °C/min than at 2 °C/min. Furthermore, the microstructure of samples at 0.5 °C/min was more rigid and irregular shrinkage, accompanied by more small pores as well as bigger cracks in comparison to that at 2 °C/min. And cell walls of samples at 0.5 °C/min seemed to thinner. Therefore, the development of carrot parenchyma softening during heating was both temperature and heating rate dependent. We suggested that the breakdown of pore proteins and middle lamella in cell walls induced the dramatic loss of cell turgidity and the subsequent changes of elastic modulus, collectively causing tissue softening.