Experimental biaxial mechanical characterization of soft biological tissues for cardiac bioprostheses design

Glutaraldehyde (GAF) treatment for biological tissues represents the current standard for cardiac bioprostheses design. An acid-free glyoxal (GAF) fixative devised by the Department of Medical Sciences (University of Turin) allows the obtaining of a more durable tissue, being less prone to calcification after implantation in the human body environment. However, essential requirement in cardiac bioprostheses design is the mechanical performance of the starting material, and each fixation treatment may compromise the tissue structure, consequently deteriorating its mechanical properties. Therefore, the aims of this thesis are (1) to characterize the biaxial mechanical properties of GAF-treated tissue comparing their performances with GLU-treated tissue ones and (2) to determine which treatment produces a more suitable tissue for the bioprostheses. A proper test protocol was developed to allow a straightforward comparison of our results with literature data on physiological valve leaflets mechanical response. Specimens of bovine pericardium and porcine aorta treated with both methods were prepared and stored in physiological saline solution at 4°C. Before testing, the samples thickness was measured detecting a 25-points matrix in the central region of each specimen using a CNC controlled scanning probe. Specimens were then cut in 15x15 mm2 squares with the apex-base and transversal (pericardium) or axial and circular (aorta) directions parallel to the edges. 16 equally spaced sutures were placed on the edges of the specimen and connected to 4 grip fixtures on the planar test bench (TA Instruments). Finally, small markers were drawn in the central region of the specimens, which were filmed during the stress protocol for strain computations. The specimens were immersed in saline solution at 37 °C throughout the test. Load-controlled and displacement-controlled protocols were devised to explore the entire range of physiologic loadings and to obtain adequate data for literature comparisons and for future constitutive modeling. Hysteresis stress-strain curves cycles were acquired and used to compute the energy loss. Moreover, stress-strain curves were used to determine the tissues elastic modulus along the two loading directions, i.e. the curve slope in its toe region. These mechanical indexes are crucial to establish whether GAF-treated and GLU-treated tissues have comparable mechanical properties and which treatment produces a tissue with a mechanical response more similar to the native tissue when subjected to physiological-like stresses.