Age-related changes in mechanical properties of human abdominal fascia.

The purpose of this study is to assess and model age-related changes in the mechanical properties of human fascia. The samples were divided into three age groups: group A-up to 60 years (mean age 52.5 ± 6 years), group B-61-80 years (mean age 70.4 ± 5.2 years), and group C-81-90 years (mean age 83.2 ± 2 years). A uniaxial tensile test was applied to fascia specimens cut perpendicular and parallel to fibers. The secant modulus at 5% strain, the maximum stress, and the stretch at maximum stress were calculated from the stress-stretch ratio curves. The results indicated an increase in the secant modulus with the increased age. The trend is clearer in the longitudinal direction. Considering the strain energy function which accounts the isotropic and non-isotropic response of the fascia where isotropic and anisotropic parts are split, we evaluated which material model is the most suitable to present isotropic mechanical behavior of the tissue. The experimental stress-stretch ratio curves were approximated using Mooney-Rivlin, Yeoh, and neo-Hookean strain energy functions and a good match between theoretical and experimental results was obtained. On the basis of objective function values and normalized mean square root error, we recommend using the Yeoh model to describe the isotropic mechanical behavior of human abdominal fascia. Graphical abstract .

Experimental study of the mechanical properties of human abdominal fascia.

The aim of the study is to characterise mechanical properties of human abdominal fascia according to its direction of loading and localization. The one-dimensional tensile behaviour of human abdominal fascia and its orthotropy has been studied experimentally using human umbilical (UF) and transversalis fascia (FT). The specimens have been cut and stretched parallel and orthogonal to the main fibre bundles. 90 specimens 10 mm wide and up to 70 mm long have been tested. The following mechanical parameters, characterising tensile properties of human abdominal fascia, have been calculated from the obtained stress-stretch ratio curves: maximal stress T(L)(max), stretch ratio at maximal stress λ(T(max)), maximal stretch ratio at failure λ(max), and a secant modulus E(i). The tissue strips obtained from defined areas reveal break stress between 0.63 and 1.99 MPa for FT and 0.93-1.61 MPa for UF. The parameter estimation has shown that in the physiological strain range specimens from both type of fascia can be considered orthotropic material according to their secant module, maximum stress T(L)(max) and stretch at maximum stress. Anisotropy factor AF (ratio of the stress in longitudinal and transverse directions) has been used to establish the level of the orthotropy of material and its variations with the stretch ratio. The maximum AF is 4.3 for FT at 20% deformation and 3.3 for UF at 5% deformation. The differences between the mechanical properties of FT and UF according to localization are not statistically significant thus the mechanical properties of human abdominal fascia are not affected by the localization.

Biomathematical modeling and analysis of blood flow in an intracranial aneurysm.

The basic hypothesis of this study is that the intracranial aneurysm may enlarge and rupture due to dynamic instabilities of the blood flow and pressure inside the aneurysm. The specific question we attempted to answer is: which parameter(s) of aneurysmal geometry can serve as a reliable predictor(s) for aneurysmal rupture? We consider an idealized cylindrical aneurysm of the human common carotid artery and develop a mathematical model of blood flow through a normal artery and aneurysm connected in series. The mathematical model is nonlinear. It comprises nonlinear rheological properties of the normal artery and aneurysmal materials, and the inertial and resistance properties of the blood flow. The model equations were solved numerically and analyzed by methods of nonlinear dynamics. The critical aneurysmal diameter (CAD) is defined as a boundary point between the stable and unstable states of the model equations. The results confirm that a limit point of flow stability can occur only for a certain difference between aneurysmal and artery radii which are pre-disposed from a difference in their material properties. It was shown that CAD is dependent on both aneurysmal length and age of patient. Finally, the results suggest that the ratio between aneurysmal and normal artery diameters is a more reliable predictor of the aneurysmal rupture than the diameter alone. We conclude that an aneurysm diameter twice that of the normal artery could be dangerous.