Influence of shape-memory stent grafts on local aortic compliance

The effect of repair techniques on the biomechanics of the aorta is poorly understood, resulting in significant levels of postoperative complications for patients worldwide. This study presents a computational analysis of the influence of Nitinol-based devices on the biomechanical performance of a healthy patient-specific human aorta. Simulations reveal that Nitinol stent-grafts stretch the artery wall so that collagen is stretched to a straightened high-stiffness configuration. The high-compliance regime (HCR) associated with low diastolic lumen pressure is eliminated, and the artery operates in a low-compliance regime (LCR) throughout the entire cardiac cycle. The slope of the lumen pressure–area curve for the LCR post-implantation is almost identical to that of the native vessel during systole. This negligible change from the native LCR slope occurs because the stent-graft increases its diameter from the crimped configuration during deployment so that it reaches a low-stiffness unloading plateau. The effective radial stiffness of the implant along this unloading plateau is negligible compared to the stiffness of the artery wall. Provided the Nitinol device unloads sufficiently during deployment to the unloading plateau, the degree of oversizing has a negligible effect on the pressure–area response of the vessel, as each device exerts approximately the same radial force, the slope of which is negligible compared to the LCR slope of the native artery. We show that 10% oversizing based on the observed diastolic diameter in the mid descending thoracic aorta results in a complete loss of contact between the device and the wall during systole, which could lead to an endoleak and stent migration. 20% oversizing reaches the Dacron enforced area limit (DEAL) during the pulse pressure and results in an effective zero-compliance in the later portion of systole.

Leaf Lettuce Yield Is More Sensitive to Low Daily Light Integral than Kale and Spinach

Kale, leaf lettuce, and spinach were grown for 28 days in growth chambers under daily light integrals (DLI) of 8, 10, 12, and 14 mol·m−2·d−1. Fresh weight (FW), dry weight (DW), leaf area, and chlorophyll concentration were measured. Increasing DLI positively influenced lettuce FW; an increase from 1.27 g/plant to 4.33 g/plant was measured. DW for all species increased in a linear fashion under increasing DLI, with lettuce increasing 203%, kale 47%, and spinach 42% as DLI increased from 8 to 14 mol·m−2·d−1. Leaf area response was species-dependent, with lettuce leaf area increasing under increasing DLI while kale leaf area decreased under higher DLI. Chlorophyll levels in kale leaves decreased from DLI of 8 to 12 mol·m−2·d−1, and then increased to 14 mol·m−2·d−1 DLI. Chlorophyll content in kale leaves had a nonlinear response to DLI and the best fit was with a quadratic model. Growers wanting to add supplemental lighting can expect the greatest gains in lettuce yield compared with those of kale and spinach.

ESTIMATION OF SORGHUM SUPPLY ELASTICITY IN SOUTH AFRICA

This study aims to estimate sorghum supply elasticity in South Africa. The study used time series data spanning from 1998 to 2016, obtained from the abstracts of agricultural statistics. The Variance Error Correction Model was employed; the study used two dependent variables, these being area and yield response functions. The results have shown that the area response function was found to be a robust model as most of the variables were significant, responsive and elastic. Maize price, as a competing crop for sorghum, negatively influenced the area allocation; however, the remaining variableshad a positive impact on area allocation in the long-run. The yield response function was found not to be robust and hence not adopted. It was therefore concluded that the area response function is more robust than the yield response function, hence sorghum production has shown more response to areaallocation than yield. The findings further indicated that the error correction term for area and for the yield response function was –1.55 and –1.30, respectively. This indicated that the two models were able to revert to equilibrium. Based on the findings, the study recommends that amongst other methods to enhance sorghum output, producers could use improved varieties or hybrids, as this action would result in allocation of more land to sorghum production, following price change.