In the multidisciplinary field of heart research it is of utmost importance, for the description of phenomena like mechanoelectric feedback or heart wall thickening, to identify accurate myocardium material properties. Therefore this study aims to determine biaxial tensile and triaxial shear properties of the passive human ventricular myocardium. This novel combination of biaxial and shear testing, together with the investigation of the myocardial microstructure, will yield new innovative and essential information about material properties to fulfil the short term goals of constructing realistic myocardial models capable of capturing the mechanics of the heart, as well as aiding the long term goals of improving methods of medical treatment and quality of life for people suffering from heart diseases.
For the biaxial tests, squared specimens (25x25x2mm) were prepared with their sides aligned with the fiber and cross-fiber axis. During the experiments, the specimens were submerged in a cardioplegic solution at physiological conditions and different stretch ratios were applied consecutively. For the triaxial shear testing, three adjoining cubic specimens (4x4x4mm) were prepared with their sides aligned according to the fiber axis, sheet axis and sheet-normal axis. Three cycles of sinusoidal simple shear (0.1-0.5 in 0.1 steps of specimen thickness) were applied to each cubic specimens in two orthogonal directions. A novel combination of optical clearing and multiphoton microscopy was utilized to explore the 3D microstructure of the tissue emphasizing the 3-D orientation and dispersion of the muscle fibers and adjacent collagen fabrics.
The tissue showed pronounced nonlinear and highly orientation dependent behavior. The donor's age was greatly influencing the mechanical behavior of the myocardial tissue. Microstructural investigations affirmed an orthotropic composition of the investigated tissue and showed highly aligned myofibers with small dispersion in the healthy human myocardium. An invariant-based constitutive model showed the ability to give a good representation of both the biaxial tensile and the triaxial shear responses.
The material data from this study is intended to be used in numerical (Finite Element) simulations for better understanding of fundamental underlying ventricular mechanics, a step needed in the improvement of medical treatment of heart diseases.