Determination of Cross-directional and Cross-Wall Variations of Passive Biaxial Mechanical Properties of Rat Myocardium

Heart myocardia are critical to the facilitation of heart pumping and blood circulating around the body. The biaxial mechanical testing of the Left Ventricle (RV) is utilised to build the computa-tional model of the whole heart with little importance given to the unique mechanical properties of Right Ventricle (RV) and Mid-wall (MDW). Most of those studies focussed on the LV of the heart, and then apply the obtained characteristics with a few modifications to the right side of the heart. However, that view has been contested over time with the realisation that the right side of the heart possesses its own unique mechanical properties that are widely distinct from that of the left side of the heart. This paper is aimed at reporting and evaluating the passive mechanical property dif-ferences in the three main walls of the rat heart based on biaxial tensile test data. Fifteen mature Wistar rats weighing 225 ± 25 g were euthanised by inhalation of 5 % halothane. The hearts were excised after which all the top chambers comprising the two atria, pulmonary and vena cava trunks, aorta and valves are all dissected out. Then 5 x 5 mm sections from the middle of each wall were carefully dissected with a surgical knife to avoid over-prestraining the specimens. The specimens were subjected tensile test. The elastic moduli, peak stresses in the toe region and stresses at 40 % strain, anisotropy indices as well as the stored strain energy in the toe and linear region up to 40 % strain are used for statistical significance tests. The following are the main findings of this study: (1) LV and MDW tissues have relatively shorter toe regions of 10 - 15 % strain as compared to RV tissue whose toe region extends up to twice as much as that (2) LV tissues have higher strain energy storage in the linear region despite being lower in stiffness than the RV (3) the MDW has the highest strain energy storage along both directions which might be directly related to its high level of anisotropy. These findings, though for a specific animal species at similar age and around the same body mass, emphasize the importance of application of wall specific material parameters to obtain accurate ventricular hyperelastic models. The findings further enhance our understanding of the desired mechanical behaviour of the different ventricle walls.

The Convolutional Multiple Whole Profile (CMWP) Fitting Method, a Global Optimization Procedure for Microstructure Determination

The analysis of line broadening in X-ray and neutron diffraction patterns using profile functions constructed on the basis of well-established physical principles and TEM observations of lattice defects has proven to be a powerful tool for characterizing microstructures in crystalline materials. These principles are applied in the convolutional multiple-whole-profile (CMWP) procedure to determine dislocation densities, crystallite size, stacking fault and twin boundary densities, and intergranular strains. The different lattice defect contributions to line broadening are separated by considering the hkl dependence of strain anisotropy, planar defect broadening and peak shifts, and the defect dependent profile shapes. The Levenberg–Marquardt (LM) peak fitting procedure can be used successfully to determine crystal defect types and densities as long as the diffraction patterns are relatively simple. However, in more complicated cases like hexagonal materials or multiple-phase patterns, using the LM procedure alone may cause uncertainties. Here, we extended the CMWP procedure by including a Monte Carlo statistical method where the LM and a Monte Carlo algorithm were combined in an alternating manner. The updated CMWP procedure eliminated uncertainties and provided global optimized parameters of the microstructure in good correlation with electron microscopy methods.

Disentangling the groundwater response to Earth and atmospheric tides reveals subsurface processes and properties

<p>In situ quantification of subsurface hydro-geomechanical properties is challenging and requires significant effort. Evolving research illustrates that subtle harmonic components in groundwater head measurements caused by Earth and atmospheric tides can be utilised to explore groundwater systems with little effort compared to traditional investigations. One long standing problem has been that, for dominant tidal components, Earth and atmospheric tides occur at the same frequency which prevents the use of the groundwater response to their individual forcing to infer subsurface properties. While Acworth et al. (2016) offered a way forward, their approach has assumptions that limit the applicability. Here, we illustrate an extended method that disentangles the borehole water level response and attributes magnitude and phase to their individual drivers. As a result, we obtain individual changes in harmonic properties of the drivers and their groundwater response (amplitude ratio and phase shift) using borehole water level records from different locations. In conjunction with groundwater flow and poroelastic theory, these properties can be used to infer the state of confinement, quantify specific storage and hydraulic conductivity as well as barometric efficiency of the formation. Further, because the stresses imposed by Earth and atmospheric tides are volumetric and uniaxial, respectively, their individual responses can be used to reveal strain anisotropy. Our new approach is passive, i.e. it only requires the measurements of atmospheric and groundwater pressure records, and can provide further insight into subsurface processes and properties using information hidden in standard pressure records.</p><p> </p><p>Acworth, R. I., Halloran, L. J. S., Rau, G. C., Cuthbert, M. O., and Bernardi, T. L. ( 2016), An objective frequency domain method for quantifying confined aquifer compressible storage using Earth and atmospheric tides, Geophys. Res. Lett., 43, 11,671–11,678, doi:10.1002/2016GL071328.</p>

Deformation of a cell monolayer due to osmotic treatment: a case study of onion scale epidermis

We performed a combination of experiments and mechanical simulations to assess the importance of cell geometry and wall structure in tissue and cell mechanics. Osmotic treatments combined with live imaging were used to quantify deformations at the tissue, cellular, and subcellular levels. We used the adaxial epidermis of onion scale as a model system. We found that the osmotically induced surface strain in onion is small because outer periclinal walls are thick and stiff, requiring bending stiffness to be considered in our mechanical models. As expected, the mechanical behaviors of the tissue and its component cells are related. Upon changes in internal pressure, cells embedded in the tissue undergo deformation that is different from isolated cells, while the tissue undergoes a somewhat counterintuitive deformation, e.g., shrinking upon pressurization, that depends on cell geometry. At the subcellular level, the amount of deformation and its anisotropy vary within the walls of individual cells, and are affected by the cell shape and vicinity of three-way wall junctions. When the turgor pressure is lost, the protoplast-facing wall surface wrinkles due to buckling, with the pattern of wrinkles depending on the strain anisotropy and the local wall geometry.