Indentation Depth-Dependent Hardness of Metal-Organic Framework Crystals: The Effect of Local Amorphization Induced by Indentation.

The hardness of metal-organic frameworks (MOFs) is an important mechanical property metric measuring their resistance to the permanent plastic deformation. The hardness of most MOFs measured from nanoindentation experiments usually exhibits the similar unique indentation depth dependence feature, the mechanism of which still remains unclear. In order to explain the effect of the indentation depth on the hardness of MOFs, we conducted nanoindentation simulations on HKUST-1 by using reactive molecular dynamics simulations. Our simulations reveal that the HKUST-1 material near the indenter can transform from the parent crystalline phase to a new amorphous phase due to the high pressure generated, while its counterpart far from the indenter remains in the crystalline phase. By considering the crystalline-amorphous interface in the energy analysis of MOFs, we derived an analytical expression of the hardness at different indentation depths. It is found that the interface effect can greatly increase the hardness of MOFs, as observed in nanoindentation simulations. Moreover, the proposed analytical expression can well explain the indentation depth-dependent hardness of many MOF crystals measured in nanoindentation experiments. Overall, this work can provide a better understanding of the indentation depth dependence of the hardness of MOFs.

Unexpected no significant soil carbon losses in the Tibetan grasslands due to rodent bioturbation.

The Tibetan grasslands store 2.5% of the Earth's soil organic carbon. Unsound management practices and climate change have resulted in widespread grassland degradation, providing open habitats for rodent activities. Rodent bioturbation loosens topsoil, reduces productivity, changes soil nutrient conditions, and consequently influences the soil organic carbon stocks of the Tibetan grasslands. However, these effects have not been quantified. Here, using meta-analysis and upscaling approaches, we found that rodent bioturbation impacts on the Tibetan grassland soil organic carbon contents were depth-dependent, with significant (P < 0.001) decreasing of 24.4% in the topsoil (0 to 10 cm) but significant (P < 0.05) increasing of 35.9% in the deeper soil layer (40 to 50 cm), and nonsignificant changes in other soil layers. The depth-dependent responses in soil organic carbon content were closely associated with rodent tunnel burrowing, foraging, excrement deposition, and mixing of the upper and deeper soil layers. Rodent bioturbation had shown nonsignificant impacts on soil bulk density, independent of soil layer. Tibetan grasslands totally lose -35.2 Tg C yr-1 (95% CI: -48.5 to -21.1 Tg C yr-1) and -32.9 Tg C yr-1 (-54.2 to -8.6 Tg C yr-1) due to rodent bioturbation in the 0 to 10 or 0 to 30 cm soil layer, while no significant net loss was found over the 0 to 90 cm layer. Our findings highlight the importance of considering depth-dependent factors to robustly quantify the net changes in the terrestrial soil organic carbon stocks resulting from disturbances such as rodent bioturbation.

Influence of Implantation Depth on the Performance of Intracortical Probe Recording Sites.

Microelectrode arrays (MEAs) enable the recording of electrical activity from cortical neurons which has implications for basic neuroscience and neuroprosthetic applications. The design space for MEA technology is extremely wide where devices may vary with respect to the number of monolithic shanks as well as placement of microelectrode sites. In the present study, we examine the differences in recording ability between two different MEA configurations: single shank (SS) and multi-shank (MS), both of which consist of 16 recording sites implanted in the rat motor cortex. We observed a significant difference in the proportion of active microelectrode sites over the 8-week indwelling period, in which SS devices exhibited a consistent ability to record activity, in contrast to the MS arrays which showed a marked decrease in activity within 2 weeks post-implantation. Furthermore, this difference was revealed to be dependent on the depth at which the microelectrode sites were located and may be mediated by anatomical heterogeneity, as well as the distribution of inhibitory neurons within the cortical layers. Our results indicate that the implantation depth of microelectrodes within the cortex needs to be considered relative to the chronic performance characterization.

Impact of Vertical Inhomogeneity on the Charge Extraction in Perovskite Solar Cells: A Study by Depth-Dependent Photoluminescence.

In perovskite solar cells (PSCs), the vertical inhomogeneities which include uneven grains, voids, and grain boundaries are closely linked to the underlying charge transport layer which controls the nucleation and grain growth in the perovskite film. Herein, the vertical inhomogeneity of perovskite films in the device structure is analyzed by depth-dependent photoluminescence (PL) achieved with different excitation wavelengths. An analytical representation between vertical inhomogeneity and depth-dependent PL, parametrized with a factor, b, is introduced to understand the relation between inhomogeneity and charge recombination. Lower values of b correlate to lower vertical inhomogeneity and hence reduced recombination. The analytical representation is validated in two sets of devices that show remarkable differences in perovskite film morphology, device based on mesoporous TiO2 and planar SnO2. By exploring the morphological properties and the PL emission from different depths across the device structures, we show that the lower vertical inhomogeneity leads to more efficient charge carrier extraction in planar SnO2-based devices. Moreover, the SnO2-based devices exhibit lower Urbach energy, which concurs with the slow transient photovoltage decay, suggesting less defects and recombination losses. This work provides a broader understanding of the impact of vertical inhomogeneity on the charge extraction efficiency and presents a methodology to study quantitatively the inhomogeneity of perovskite films in device structures.

Depth-Dependent High Distortion Lens Calibration.

Accurate correction of high distorted images is a very complex problem. Several lens distortion models exist that are adjusted using different techniques. Usually, regardless of the chosen model, a unique distortion model is adjusted to undistort images and the camera-calibration template distance is not considered. Several authors have presented the depth dependency of lens distortion but none of them have treated it with highly distorted images. This paper presents an analysis of the distortion depth dependency in strongly distorted images. The division model that is able to represent high distortion with only one parameter is modified to represent a depth-dependent high distortion lens model. The proposed calibration method obtains more accurate results when compared to existing calibration methods.

Depth-dependent changes in cartilage T2 under compressive strain: a 7T MRI study on human knee cartilage.

OBJECTIVE: To assess the potential of using ΔT2 as an indirect index of cartilage strain by quantifying the relationship between local in situ compressive strain and ΔT2 through the full depth of human tibial and femoral articular cartilage. DESIGN: Osteochondral samples (n = 4) of human tibial and femoral cartilage were harvested from cadavers and imaged in a Bruker 7T research MRI scanner under increasing displacement-controlled compressive strains. T2 was calculated for 3D double echo steady state (DESS) image volumes at each strain level. A decaying exponential model estimated local, depth-dependent strains. Strained image volumes were non-linearly warped back to their unloaded configurations and ΔT2 was calculated by image subtraction. Linear modeling assessed local relationships between strain and ΔT2. RESULTS: Bulk average tibial T2 was 13.2 ms for unstrained cartilage and ranged from 13.0 to 13.1 ms under strain; femoral T2 was 14.0 ms for unstrained cartilage and ranged from 13.5 to 14.8 ms under strain. Local ΔT2 in strained cartilage varied with depth. Linear modeling revealed significant correlations between in situ strain and ΔT2 for both tibial and femoral cartilage; correlation coefficients were higher for tibial cartilage. CONCLUSIONS: Changes in bulk average T2 are unsuitable as a quantitative surrogate measure of cartilage strain because bulk averaging masks important local variations. High-resolution measures of local ΔT2 have potential value as a surrogate for strain; however, their value is limited until we fully understand the influence of factors like age, joint surface and degeneration on the strain vs T2 relationship.

Detection of early osteoarthritis in canine knee joints 3 weeks post ACL transection by microscopic MRI and biomechanical measurement.

PURPOSE: To detect early osteoarthritis (OA) in a canine Pond-Nuki model 3 weeks after anterior cruciate ligament (ACL) transection surgery, both topographically over the medial tibial surface and depth-dependently over the cartilage thickness. METHODS: Four topographical locations on each OA and contralateral medial tibia were imaged individually by magnetic resonance imaging (MRI) at 17.6 µm transverse resolution. The quantitative MRI T2 relaxation data were correlated with the biomechanical stress-relaxation measurements from adjacent locations. RESULTS: OA cartilage was thinner than the contralateral tissue and had a lower modulus compared to the contralateral cartilage for the exterior, interior, and central medial tibia locations. Depth-dependent and topographical variations were detected in OA cartilage by a number of parameters (compressive modulus, glycosaminoglycan concentration, bulk and zonal thicknesses, T2 at 0° and 55° specimen orientations in the magnet). T2 demonstrated significant differences at varying depths between OA and contralateral cartilage. CONCLUSION: ACL transection caused a number of changes in the tibial cartilage at 3 weeks after the surgery. The characteristics of these changes, which are topographic and depth-dependent, likely reflect the complex degradation in this canine model of OA at the early developmental stage.

Characterization of commercial MOSFET detectors and their feasibility for in-vivo HDR brachytherapy.

AIM: The present study was to investigate the use of MOSFET as an vivo dosimeter for the application of Ir-192 HDR brachytherapy treatments. MATERIAL AND METHODS: MOSFET was characterized for dose linearity in the range of 50-1000 cGy, depth dose dependence from 2 to 7 cm, angular dependence. Signal fading was checked for two weeks. RESULT AND DISCUSSION: Dose linearity was found to be within 2% in the dose range (50-1000 cGy). The response varied within 8.07% for detector-source distance of 2-7 cm. The response of MOSFET with the epoxy side facing the source (0 degree) is the highest and the lowest response was observed at 90 and 270 degrees. Signal was stable during the study period. CONCLUSION: The detector showed high dose linearity and insignificant fading. But due to angular and depth dependence, care should be taken and corrections must be applied for clinical dosimetry.

The influence of angle of insonation and target depth on speckle-tracking strain.

BACKGROUND: Speckle-tracking strain is almost universally cited as being independent of angle of insonation, but there are minimal confirmatory studies, and this claim may not be consistent with the known limitations of ultrasound axial and lateral spatial resolution. The aim of this study was to assess the influence of angle and depth on longitudinal peak systolic strain (LPS). METHODS: Thirty-four healthy pediatric subjects (age range, 6-18 years; 47% male) with normal cardiac anatomy and good image quality were prospectively imaged. Angular comparisons of LPS were investigated by examining interangle reproducibility on the basis of one standard and one alternative image acquisition relative to intraobserver reproducibility of two standard views of the same left ventricular segments. A single-window comparison was used to evaluated septal LPS: standard apical four-chamber versus right ventricular centered four-chamber. Two paired standard and alternative window comparisons were as follows: (1) four-chamber: standard apical versus subcostal; and (2) three-chamber: standard apical versus parasternal long-axis. RESULTS: The global LPS intraobserver difference using the paired standard and alternative window comparisons was lower than the interangle difference in global LPS (-1.0 ± 0.1% vs -2.1 ± 2.4%). Intraobserver reproducibility was significantly higher than interangle reproducibility (intraclass correlation coefficient = 0.9 vs 0.29, P < .001). Similar results were found in the segmental strain analysis. Interangle reproducibility was significantly decreased compared with intraobserver reproducibility in the septal single-window comparison. Target depth assessment demonstrated a systematic bias between the near-field and far-field segments. CONCLUSIONS: Echocardiographically derived LPS values were modestly dependent on angle of insonation and target depth in this pediatric population. Normal strain ranges derived from standard apical images should not be applied to strain derived from sub-costal images, off-axis apical imaging, or applications in which a standard window cannot be defined.

Depth-dependent shear behavior of bovine articular cartilage: relationship to structure.

The mechanical behavior of bovine articular cartilage in shear was measured and related to its structure through the depth of the tissue. To make these measurements, we designed an apparatus that could apply controlled shear displacement and measure the resulting shear force on cartilage specimens. Shear displacement and shear strain were obtained from confocal images of photobleached lines on fluorescently stained deformed samples. Depth-dependent collagen structure was obtained using compensated polarized light microscopy. Depth-dependent shear behavior and structure of samples from two animals were measured (group A and B). Both animals were 18-24 months old, which is the range in which they are expected reach skeletal maturity. In mature samples (group A), the stiffest region was located beneath the superficial zone, and the most compliant region was found in the radial zone. In contrast, in samples that were in the process of maturing (group B) the most compliant region was located in the superficial zone. Compensated polarized light microscopy suggested that the animal from which the group A samples were obtained was skeletally mature, whereas the animal yielding the group B samples was in the process of maturing. Compensated polarized light microscopy was an important adjunct to the mechanical shear behavior in that it provided a means to reconcile differences in observed shear behavior in mature and immature cartilage. Although samples were harvested from two animals, there were clear differences in structure and shear mechanical behavior. Differences in the depth-dependent shear strain were consistent with previous studies on mature and immature samples and, based on the structural variation between mature and immature articular cartilage, their mechanical behavior differences can be tenable. These results suggest that age, as well as species and anatomic location, need to be considered when reporting mechanical behavior results.