Computational modelling of the active mechanical behaviour of soft tissues

This Thesis investigates the numerical modelling of the contractile mechanical behavior of soft tissues, a phenomenon crucial in physiological functions and of major relevance in clinical and biomedical engineering applications. A constitutive model capable of accounting for both active and passive properties of muscle fibers is presented to perform a comprehensive finite element (FE) analysis of soft tissue contraction. Three scenarios are addressed: the age-related impairment of lateral force transmission in skeletal muscle bundles, abdominal wall contraction for optimal mesh selection in ventral hernia repair, and cardiomyocyte contractility for cardiac tissue engineering. In the first study, FE models are used to evaluate how ageing influences lateral force transmission in skeletal muscle bundles by considering changes in fiber architecture and extracellular matrix (ECM) properties. Muscle weakness in the elderly exceeds what is explained by muscle mass reduction alone. To investigate this, FE models of muscle bundles, comprising a limited number of fibers embedded in an ECM layer, are developed for young and elderly groups. Age differences are modeled by varying ECM thickness, consistent with experimental data. Muscle fibers are described through a Hill-type contractile formulation, while the ECM is modeled as an isotropic hyperelastic material. Simulations reveal a 22% reduction in force transmission in elderly models compared to 7.5% in young ones, in agreement with animal studies. The reduction is attributed to decreased shear stiffness caused by ECM thickening, underscoring the central role of ECM structure in muscle mechanics. The second study focuses on abdominal wall biomechanics, aiming to evaluate the mechanical interaction between tissues and surgical meshes in hernia repair. FE models of the abdominal wall are generated from medical imaging, incorporating fasciae, linea alba, aponeuroses, and muscles with fiber-reinforced hyperelastic and Hill-type contractile formulations. Physiological loading is simulated by intra-abdominal pressure (IAP) applied to an internal cavity. An epigastric hernia is introduced and repaired virtually with a surgical mesh. Simulations under varying IAP levels and muscle states demonstrate that meshes increase abdominal wall stiffness, particularly during contraction. These findings stress the necessity of including active muscular behavior in computational models to obtain realistic insights into mesh–tissue interaction, thereby supporting pre-operative planning. The third study addresses the modeling of cardiomyocyte activity within engineered cardiac tissue constructs. The model captures anisotropic fiber orientation, cellular alignment, and contractile force generation, key features for biomimetic cardiac patches. The proposed constitutive model is validated against literature data, and a methodology to construct a numerical model of engineered heart tissue (EHT) is described. Experimental inputs from immunofluorescence imaging and mechanical tests are integrated to provide information beyond experimental accessibility. Although developed with healthy cells, the model holds promise for studying pathological cardiomyocytes associated with altered cardiac ECM, offering a predictive tool for cardiac tissue engineering. Overall, the studies presented demonstrate the necessity of integrating biologically informed constitutive laws, realistic fiber distributions, and active contraction mechanisms into FE simulations of soft tissues. The results contribute to understanding age-related muscle impairment, improving mesh design for hernia repair, and advancing engineered cardiac tissue development.