The biomechanical properties of ascending aortic aneurysms were investigated only in the last decade in a limited number of studies. Indeed, in recent years, there has been a growing interest in this field in order to identify new predictive parameters of risk of dissection, which may have clinical relevance. The researches performed so far have been conducted according to the methods used in the study of abdominal aortic aneurysms. In most cases, uniaxial or biaxial tensile tests were used, while in a smaller number of studies other methods, such as opening angle, bulge inflation, and inflation-extension tests, were used. However, parameters and protocols of these tests are at present very heterogeneous in the studies reported in the literature, and, therefore, the results are not comparable and are sometimes conflicting. The purpose of this review then thence to provide a comprehensive analysis of the experimental methodology for determination of biomechanical properties in the specific field of aneurysms of the ascending aorta to allow for better comparison and understanding of the results.

Planar biaxial testing has been applied to a variety of materials to obtain relevant information for mechanical characterization and constitutive modeling in presence of complex stress states. Despite its diffusion, there is currently no standardized testing procedure or a unique specimen design of common use. Consequently, comparison of results obtained with different configurations is not always straightforward and several types of optimized shapes have been proposed. The purpose of the present work is to develop a procedure for comprehensive comparison of results of biaxial tests carried out on the same soft hyperelastic material, using different types of gripping methods and specimen shapes (i.e., cruciform and square). Five configurations were investigated experimentally using a biaxial test rig designed and built by the authors, using digital imaging techniques to track the displacements of markers apposed in selected positions on the surfaces. Then, material parameters for a suitable hyperelastic law were determined for each configuration examined, employing an inverse method which combines numerical simulations with the finite element method (FEM) and optimization algorithms. Finally, efficiency of examined biaxial configurations was assessed comparing stress reductions factor, degree and uniformity of biaxial deformation, and operative strain ranges.

4D printing is an innovative manufacturing approach that combines 3D printing and stimuli- responsive abilities to produce objects with complex geometry and capable of shapeshifting over time (the fourth dimension). To pursue such an approach this paper proposes to develop re-entrant honeycomb auxetic grids with tunable shape reconfigurable behavior. Particularly, the work combines 3D printing and a photopolymer exhibiting the so-called temperature memory effect (TME), a peculiar shape memory behavior expressing the capability of the material to remember not only the original shape but also the deformation temperature. A thorough experimental activity was carried out on single auxetic unit cells, chosen as representative of the whole auxetic grid, to properly highlight and assess their response upon heating after single-step and multiple-step deformation histories and to describe the recovery process as a function of time and temperature. Results demonstrate the possibility to achieve an easily controlled TME and to successfully exploit it for autonomous, complex hierarchical transformations over a large range of temperatures. As a proof-of-concept, the study of the sequential recovery of an entire auxetic grid subjected to double-step programming allowed highlighting a decoupled in-plane elongation and out-of-plane bending. The behavior of the 4D-printed auxetic structures was simulated by means of finite element (FE) analysis, using a thermoviscoelastic model of the photopolymer and viscoelastic experimental data obtained by time-temperature superposition analysis applied to multifrequency dynamic mechanical tests and to isothermal recovery tests. A good correspondence between experiments and simulations was obtained for all shape memory tests, demonstrating that the proposed FE approach is a suitable tool to support the design of these structures. The combination of 3D printing and TME opens new perspectives to achieve dynamic tunability in mechanical metamaterials, that is a key ingredient in several application fields.