A modification of Holzapfel–Ogden hyperelastic model of myocardium better describing its passive mechanical behavior

The passive mechanical behavior of the myocardium is usually mathematically described within the framework of hyperelasticity. One of the most popular models of this kind is that proposed by Holzapfel and Ogden in 2009. It is an orthotropic model formulated in terms of a reasonably selected set of scalar invariants representing different components of the myocardium. Several modifications of the model have emerged over the years. In this paper, we present another one which is characterized by an innovative approach to the modeling of myocardial “sheets”, i.e. lamellar collagenous structures that endow the myocardium with orthotropic mechanical properties. We describe their contribution by means of a less common scalar invariant which expresses the change of area of an oriented planar element (representing the plane of a sheet). To compare our formulation with the original model, we matched both of them to the biaxial tension and simple shear experimental data from the literature using a nonlinear least-squares optimization algorithm. The objective function for each model included both biaxial and simple shear data in order to obtain a single set of parameters for both deformation modes. The results show that our modified model can accurately describe both types of tests. The total residual is lowered by approximately 80% by our modification and $R^2$ increases from 0.877 to 0.978 which demonstrates the significance of our modification on the quality of the fit.