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Despite observations of massive methane release and geohazards associated with gas hydrate instability in nature, as well as ductile flow accompanying hydrate dissociation in artificial polycrystalline methane hydrates in the laboratory, the destabilising mechanisms of gas hydrates under deformation and their grain-boundary structures have not yet been elucidated at the molecular level. Here we report direct molecular dynamics simulations of the material instability of monocrystalline and polycrystalline methane hydrates under mechanical loading. The results show dislocation-free brittle failure in monocrystalline hydrates and an unexpected crossover from strengthening to weakening in polycrystals. Upon uniaxial depressurisation, strain-induced hydrate dissociation accompanied by grain-boundary decohesion and sliding destabilises the polycrystals. In contrast, upon compression, appreciable solid-state structural transformation dominates the response. These findings provide molecular insight not only into the metastable structures of grain boundaries, but also into unusual ductile flow with hydrate dissociation as observed during macroscopic compression experiments. Sediment-hosted gas hydrates may release vast quantities of methane upon failure, but destabilizing mechanisms at the molecular level are poorly understood. Here, the authors study the deformation using simulations and find that failure differs between single crystals and polycrystalline hydrates.

The upper airway experiences mechanical loads during breathing. Obstructive sleep apnea is a very common sleep disorder, in which the normal function of the airway is compromised, enabling its collapse. Its treatment remains unsatisfactory with variable efficacy in the case of many surgeries. Finite element models of the upper airway to simulate the effects of various anatomic and physiologic manipulations on its mechanics could be helpful in predicting surgical success. Partial 3D finite element models based on patient-specific CT-scans were undertaken in a pilot study of 5 OSA patients. Upper airway soft tissues including the soft palate, hard palate, tongue, and pharyngeal wall were segmented around the midsagittal plane up to a width of 2.5 cm in the lateral direction. Simulations of surgical interventions such as Uvulopalatopharyngoplasty (UPPP), maxillo-mandibular advancement (MMA), palatal implants, and tongue implants have been performed. Our results showed that maxillo-mandibular advancement (MMA) surgery of 1 cm improved the critical closing pressure by at least 212.2%. Following MMA, the best improvement was seen via uvulopalatopharyngoplasty (UPPP), with an improvement of at least 19.12%. Palatal and tongue implants also offered a certain degree of improvement. Further, we observed possible interacting mechanisms that suggested simultaneous implementation of UPPP and tongue stiffening; and palatal and tongue stiffening could be beneficial. Our results suggest that computational modeling is a useful tool for analyzing the influence of anatomic and physiological manipulations on upper airway mechanics. The goal of personalized treatment in the case of OSA could be achieved with the use of computational modeling.

Barlow’s Disease affects the entire mitral valve apparatus causing mitral regurgitation. Standard annuloplasty procedures lead to an average of 55% annular area reduction of the end diastolic pre-operative annular area in Barlow’s diseased valves. Following annular reduction, mitral valvuloplasty may be needed, usually with special focus on the posterior leaflet. An in silico pipeline to perform annuloplasty by utilizing the pre- and -postoperative 3D echocardiographic recordings was developed. Our objective was to test the hypothesis that annuloplasty ring sizes based on a percentage (10%–25%) decrease of the pre-operative annular area at end diastole can result in sufficient coaptation area for the selected Barlow’s diseased patient. The patient specific mitral valve geometry and finite element model were created from echocardiography recordings. The post-operative echocardiography was used to obtain the artificial ring geometry and displacements, and the motion of the papillary muscles after surgery. These were used as boundary conditions in our annuloplasty finite element analyses. Then, the segmented annuloplasty ring was scaled up to represent a 10%, 20% and 25% reduction of the pre-operative end diastolic annular area and implanted to the end diastolic pre-operative finite element model. The pre-operative contact area decrease was shown to be dependent on the annular dilation at late systole. Constraining the mitral valve from dilating excessively can be sufficient to achieve proper coaptation throughout systole. The finite element analyses show that the selected Barlow’s diseased patient may benefit from an annuloplasty ring with moderate annular reduction alone.

Several materials and tissues are characterized by a microstructure composed of fibrous units embedded in a ground matrix. In this paper, a novel three-dimensional (3D) Fourier transform-based method for quantifying the distribution of fiber orientations is presented. The method allows for an accurate identification of individual fiber families, their in-plane and out-of-plane dispersion, and showed fast computation times. We validated the method using artificially generated 3D images, in terms of fiber dispersion by considering the error between the standard deviation of the reconstructed and the prescribed distributions of the artificial fibers. In addition, we considered the measured mean orientation angles of the fibers and validated the robustness using a measure of fiber density. Finally, the method is employed to reconstruct a full 3D view of the distribution of collagen fiber orientations based on in vitro second harmonic generation microscopy of collagen fibers in human and mouse skin. The dispersion parameters of the reconstructed fiber network can be used to inform mechanical models of soft fiber-reinforced materials and biological tissues that account for non-symmetrical fiber dispersion.

Degenerative mitral valve disease is a common valvular disease with two arguably distinct phenotypes: fibroelastic deficiency and Barlow’s disease. These phenotypes significantly alter the microstructures of the leaflets, particularly the collagen fibers, which are the main mechanical load carriers. The predominant method of investigation is histological sections. However, the sections are cut transmurally and provide a lateral view of the microstructure of the leaflet, while the mechanics and function are determined by the planar arrangement of the collagen fibers. This study, for the first time, quantitatively examined planar collagen distribution quantitatively in health and disease using second harmonic generation microscopy throughout the thickness of the mitral valve leaflets. Twenty diseased samples from eighteen patients and six control samples were included in this study. Healthy tissue had highly aligned collagen fibers. In fibroelastic deficiency they are less aligned and in Barlow’s disease they are completely dispersed. In both diseases, collagen fibers have two preferred orientations, which, in contrast to the almost constant one orientation in healthy tissues, also vary across the thickness. The results indicate altered in vivo mechanical stresses and strains on the mitral valve leaflets as a result of disease-related collagen remodeling, which in turn triggers further remodeling.

Nanoindentation test results in the axial direction of mouse femurs were the basis for the current study. Although the majority of the nanoindentation curves showed a reasonable consistency, some curves showed a significantly softer response. Detailed investigation, using focused ion beam-scanning electron microscopy, provided that the softer response is due to subsurface cavities such as lacunae. Finite element models were developed to simulate the nanoindentation of mice femur cortical bone samples with and without the incorporation of a single lacuna underneath the bone surface. Based on the material parameters determined for the cavity-free tissue, numerical simulations were run for different cases of cavity size, shape, and location. Spherical cavities with different size were considered at different distances from the surface. The results showed that subsurface cavities can lead to 50% higher indentation compared to an indentation in cavity-free material. Continuing with ellipsoidal cavities with the center located on the load axis, the results showed a nonlinear dependency of ellipsoid shape. Hence, the shape of the cavity is important for the nanoindentation response. The influence of horizontal and vertical offsets of spherical cavities was studied, thereby the results showed that an increasing horizontal offset caused a decreasing influence of the vertical distance from the surface. In perspective, the present study provides information that may help to get deeper knowledge of nanoindentation load–displacement mechanism taking place in samples with subsurface cavities.

Finite element modeling applied to analyze experimentally determined hydrogel swelling data provides quantitative description of the hydrogel in the aqueous solutions with well-defined ionic content and environmental parameters. In the present study, we expand this strategy to analysis of swelling of hydrogels over an extended concentration of salt where the Donnan contribution and specific ion effects are dominating at different regimes. Dynamics and equilibrium swelling were determined for acrylamide and cationic acrylamide-based hydrogels by high-resolution interferometry technique for step-wise increase in NaCl and NaBr concentration up to 2 M. Although increased hydrogel swelling volume with increasing salt concentration was the dominant trend for the uncharged hydrogel, the weakly charged cationic hydrogel was observed to shrink for increasing salt concentration up to 0.1 M, followed by swelling at higher salt concentrations. The initial shrinking is due to the ionic equilibration accounted for by a Donnan term. Comparison of the swelling responses at high NaCl and NaBr concentrations between the uncharged and the cationic hydrogel showed similar specific ion effects. This indicates that the ion non-specific Donnan contribution and specific ion effects are additive in the case where they are occurring in well separated ranges of salt concentration. We develop a novel finite element model including both these mechanisms to account for the observed swelling in aqueous salt solution. In particular, a salt-specific, concentration-dependent Flory–Huggins parameter was introduced for the specific ion effects. This is the first report on finite element modeling of hydrogels including specific ionic effects and underpins improvement of the mechanistic insight of hydrogel swelling that can be used to predict its response to environmental change.

The influence of static surface properties, such as free energy, toughness, and elasticity, on icephobicity has been extensively studied and documented in existing literature. However, there remains limited understanding of the role played by surface dynamic characteristics in facilitating ice removal. This study investigates the ice adhesion strength of authentic Arctic salmon (Salmon salar) skin, revealing intriguing anisotropic ice adhesion behavior. Results indicate a significant decrease in ice adhesion strength (141 ± 47 kPa) when sheared against the growth orientation of fish scales compared with shearing along this orientation (353 ± 95 kPa). The distinctive structural evolution of fish scales during shearing can lead to a sequential rupture process, thereby diminishing ice adhesion. Additionally, the study highlights the significance of the opening and peeling capacity of fish scales in controlling ice detachment, defined as the ability of unit scales to separate from their underlying structures and adhesives under applied force. Enhancing this capacity could further reduce ice adhesion strength (66 ± 15 kPa), facilitating effortless ice detachment on fish scales. The mechanical robustness of fish scales offers new possibilities for designing hard and durable anti-icing surfaces.

Aqueous microgels are distinct entities of soft matter with mechanical signatures that can be different from their macroscopic counterparts due to confinement effects in the preparation, inherently made to consist of more than one domain (Janus particles) or further processing by coating and change in the extent of crosslinking of the core. Motivated by the importance of the mechanical properties of such microgels from a fundamental point, but also related to numerous applications, we provide a perspective on the experimental strategies currently available and emerging tools being explored. Albeit all techniques in principle exploit enforcing stress and observing strain, the realization differs from directly, as, e.g., by atomic force microscope, to less evident in a fluid field combined with imaging by a high-speed camera in high-throughput strategies. Moreover, the accompanying analysis strategies also reflect such differences, and the level of detail that would be preferred for a comprehensive understanding of the microgel mechanical properties are not always implemented. Overall, the perspective is that current technologies have the capacity to provide detailed, nanoscopic mechanical characterization of microgels over an extended size range, to the high-throughput approaches providing distributions over the mechanical signatures, a feature not readily accessible by atomic force microscopy and micropipette aspiration.

Rotational culture promotes primary human osteoblasts (hOBs) to form three‐dimensional (3D) multicellular spheroids with bone tissue‐like structure without any scaffolding material. Cell‐based bone models enable us to investigate the effect of different agents on the mechanical strength of bone. Given that low dietary intake of both vitamin D and K is negatively associated with fracture risk, we aimed to assess the effect of these vitamins in this system. Osteospheres of hOBs were generated with menaquinone‐4 (MK‐4; 10μM) and 25‐hydroxyvitamin D3 [25(OH)D3; 0.01μM], alone and in combination, or without vitamins. The mechanical properties were tested by nanoindentation using a flat‐punch compression method, and the mineralized extracellular bone matrix was characterized by microscopy. The in vitro response of hOBs to MK‐4 and 25(OH)D3 was further evaluated in two‐dimensional (2D) cultures and in the 3D bone constructs applying gene expression analysis and multiplex immunoassays. Mechanical testing revealed that 25(OH)D3 induced a stiffer and MK‐4 a softer or more flexible osteosphere compared with control. Combined vitamin conditions induced the same flexibility as MK‐4 alone. Enhanced levels of periostin (p < 0.001) and altered distribution of collagen type I (COL‐1) were found in osteospheres supplemented with MK‐4. In contrast, 25(OH)D3 reduced COL‐1, both at the mRNA and protein levels, increased alkaline phosphatase, and stimulated mineral deposition in the osteospheres. With the two vitamins in combination, enhanced gene expression of periostin and COL‐1 was seen, as well as extended osteoid formation into the central region and increased mineral deposition all over the area. Moreover, we observed enhanced levels of osteocalcin in 2D and osteopontin in 3D cultures exposed to 25(OH)D3 alone and combined with MK‐4. In conclusion, the two vitamins seem to affect bone mechanical properties differently: vitamin D enhancing stiffness and K2 conveying flexibility to bone. These effects may translate to increased fracture resistance in vivo. © 2020 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.