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Numerical modeling of cornea and arteries. The mechanical behavior of these biological tissues reinforced with collagen fibers has been object of an intense study in the last four years. The anisotropic behavior of a two fiber hyperelastic material was implemented in a finite element code which accounts for finite kinematics, expressly developed to describe the behavior of deformable bodies. Applications of the package were the analysis of the mechanical response of the human cornea to the laser refractive surgery and the simulation of the rupture of human arteries damaged by atherosclerotic lesions. Experimental tests (pressurization and tensile tests) on pig corneas are currently underway. Numerical models of skin expansion One of the applications of plastic surgery is an aesthetic treatment of cutaneous tissue anomalies. Skin expansion is a technique widely used in reconstructive surgery (post-mastectomy breast reconstruction, burn care, craniofacial surgery, after tumour removal) to generate new skin. Numerical models were developed in order to simulate plastic surgical procedures and to investigate the mechanism of skin adaptive growth. Numerical and experimental characterisation of soft tissues a) Articular cartilage. The mechanisms underlying the ability of articular cartilage to withstand and distribute the loads applied across diarthrodial joints was comprehensively studied. This tissue is non linear with strain, both in tension and in compression, non linear with direction of stimulus, anisotropic in tension and compression, non-homogeneous with depth, resulting in depth dependent mechanical properties, and presents fluid dependent and fluid independent viscoelasticity. A combined experimentalnumerical approach for the proper description of the cartilage response under confined and unconfined compression was developed. The model successfully simulates the confined and unconfined compression experiments performed on disks of natural and engineered cartilage and is also used to identify parameters of difficult experimental evaluation, such as the collagen stiffness and permeability. c) Thrombus from abdominal aortic aneurysms Computational models, developed to study abdominal aortic aneurysm biomechanics, demonstrated that the presence of an intraluminal thrombus (ILT) can significantly alter the wall stress distribution in the degenerated vessel wall. Although the ILT is subjected to compression in vivo, up to now ILT constitutive models have been based on parameters derived from tensile testing of ILT specimens. ILT samples were tested for unconfined compression and permeation. A poroviscoelastic model was implemented to interpret the experimental data. d) Cerebral aneurysms Cerebral aneurysms are dilations of cerebral arteries, commonly found at apices of arterial bifurcations and outer walls of curved arterial segments. The mechanisms by which cerebral aneurysms develop and rupture are unknown, but it is commonly accepted that the vasculature is subjected to adaptive phenomena as remodelling and/or growth. Finite element models were developed taking into account the anisotropic non-linear constitutive behaviour of arterial tissue. Mechanical tests (uniaxial tensile test) on rabbit internal carotid arteries were performed to characterize the mechanical behaviour of cerebro-vascular tissue and to identify the model parameters. Moreover, a numerical implementation of remodelling mechanisms of arterial wall during aneurysms dilation was implemented. 16 e) Ligaments and planar membranes A constitutive model for the mechanical characterisation of these viscoelastic soft tissues in finite strains was developed. The model is based on the separation of variables, and includes within a single integral framework the contributions of elasticity and of memory by means of two distinct functions. Suitable expressions of the constitutive functions were proposed for different tissues and the relevant parameters were estimated by fitting experimental data. The consistency of the constitutive model with the first and second laws of thermodynamics was also proved. Experimental characterisation of tissues Testing protocols were developed for the determination of the mechanical properties of biological tissues, including both soft collagenous tissues (blood vessels, tendons and ligaments, pericardium, umbilical cord, fetal membranes, dura mater, natural and engineered cartilage) and hard mineralized tissues (cortical and cancellous bone). At present a study on the viscoelasticity of cancellous bone is in progress. The tissue properties were investigated both in tension and in compression, and the effects of the applied strain on the stress relaxation were modelled by a nonlinear function. In vitro models were also designed for studying the biomechanics of surgical procedures such as puncturing of dura mater for spinal anaesthesia and of fetal membranes during amniocentesis.