No abstract

Using morphological and mechanical criteria and applying a method involving paired samples that is widely employed in epidemiology, we obtained an excellent prediction of the mechanical behavior of the calf pericardium used in the construction of cardiac bioprostheses. The method of selection employed in this study may be a highly useful tool for guaranteeing the mechanical resistance of calf pericardium, with a very low level of error.

Horizontal strains were measured with high precision during Crossrail tunnel excavation in central London by means of distributed fibre-optic sensors embedded in shallow transverse and longitudinal surface trenches. Validation of these measurements against manual micrometer stick readings demonstrates the benefits of the fibre-optic sensing: high precision at small strains; significantly smaller spatial resolution; and the possibility for automatised measurements over larger spans. In order to further capitalise on these advantages, two additional potential applications of the fibre-optic measurements were investigated: (a) independent assessment of the vertical ground displacements using transverse strain measurements and analytical settlement trough models; and (b) early prediction of the vertical and horizontal ground displacements, using transverse and longitudinal strain measurements and analytical settlement trough models. While understanding that a single case study can only form a basis for limited conclusions, both applications do appear to be feasible.

We studied the changes in energy consumption of samples of calf pericardium, when joined or not joined by sutures and adhesives, by means of hysteretic cycles. Sixty-four samples were subsequently subjected to tensile stress until rupture. An overlapping suture sewn in the form of a rectangle presented an acceptable mean resistance to rupture of over 10 MPa, although lower than the mean values in an unsutured control series where the mean resistance surpassed 15 MPa. The contribution of an acrylic adhesive to the resistance to rupture was negligible. The sutured samples that were reinforced with adhesives and had not been subjected to hysteretic cycles prior to rupture showed an anisotropic behavior. This behavior appeared to be lost in all the samples that underwent hysteretic cycles. We found an inflection point in the stress/strain curve following the stepwise increase in the load, with a value greater than and proximate to the final load applied. This inflection should be analyzed by means of microscopy. Finally, the mathematical relationship between the energy consumed and the stress applied, the strain or deformation produced and the number of cycles of hysteresis to which the samples were subjected was established as the ultimate objective of this study. The bonding systems provoked a greater consumption of energy, with the greatest consumption corresponding to the first cycle in all the series assayed. An equation relating the energy consumption in a sample to the number of hysteretic cycles to which it was subjected was obtained. Its asymptote on the x-axis indicates the energy consumption for a theoretical number of cycles, making it possible to estimate the durability of the sample.

The mechanical behavior of porcine pericardium was analyzed to compare it with that of calf pericardium employed in valve leaflets for cardiac bioprostheses. Forty samples of pericardium were subjected to uniaxial tensile testing, 20 as controls and 20 exposed to loads increasing stepwise from 0.5 to 1.5 kg and to 3 kg, and thereafter to rupture, with a return to zero load between each new increment. Another 20 samples were used in biaxial tensile tests involving the application of loads increasing stepwise (to 0.5, 1.5, 3 and 5 kg) until rupture with a zero-load interval before each increment. The ultimate stresses were very similar, showing no statistically significant differences when compared in terms of type of assay, controls and study samples or region of pericardial tissue being tested. In the stepwise biaxial assays, the mean stresses at rupture were also very homogeneous. Using morphological and mechanical criteria for sample selection, it was possible to obtain mathematical fits for the stress/strain relationship, with excellent coefficients of determination. The relationship between the area under the stress/strain curve and the load applied or the strain observed was also studied in the biaxial assay as an equivalent to the cycles of hysteresis produced in the test. The increment in the area under the curve (the energy consumed) may be a good parameter for assessing the changes in the collagen fiber architecture of the pericardial tissue, changes that may help to detect early failure.