Biomechanics : mechanical characterization of artery tissue and synthetic arterial grafts

Cardiovascular diseases are one of the leading causes of death in the developed world. The need for accurate mechanical characterization of the native arterial tissue, along with synthetic graft materials, is essential and can provide valuable insights for the correct medical treatment to follow. The general aim of this thesis was to characterize the mechanical properties of native tissue and of one of the clinically available synthetic vascular graft materials, i.e. Dacron. For this purpose, both biaxial extension-inflation testing and 4D micro-CT was used. Preliminary, extension-inflation tests identified parameter of the experimental protocol to be further optimized, such as the proper adjustment of the axial force and transmural pressure, the mounting system, the fluid and the shape of the triaxial cell. Subsequently, two Dacron samples, each tested in different orientations, were imaged using a micro-CT combined with an in situ tensile mechanical loading. This specific experiment is called a 4D micro-CT. First, by visual inspection, a warp knitted Dacron graft was highlighted. This kind of graft is characterized by the longitudinal alignment of the knitted yarns. Then, the mechanical properties of Dacron were evaluated both in longitudinal and circumferential direction. The values of elastic modulus were of 5.75 MPa and 13.65 MPa, respectively. These values highlighted an anisotropy, already identified in some literature papers. Afterwards, the data acquired by the micro-CT scans were processed and a zero-strain test was realized to estimate the amount of error registered by the DVC analysis. Finally, three incremental DVC techniques were tested on Dacron. The "fixed reference image" conventional DVC method was used to visualize the evolution of the strain maps along a maximal displacement of 0.6 millimeter. For the Dacron sample tested in the longitudinal direction, the strain maps represented on the microstructure regions showed a higher strain accumulation in the fold, meaning that the folding induces concentration of stress. For the Dacron sample tested in the circumferential direction, higher tensile strain was identified in the region between the longitudinally warp knitted yarns of the microstrcuture. The thinnest part of the sample was assessed as it was a possible area of failure of the sample. Performing 4D micro-CT on soft tissues, aorta in this work, needs the use of contrast agents that bind with some components of the material to enhance contrast and enable relevant observations. This specific technique is referred as 4D contrast-enhanced micro-CT (4D CE-CT). By visual inspection of the results, an insufficient penetration of the contrast agent through the sample was identified. In addition, the zero-strain test determined a high level of error for the aorta sample, consequence of this lack of staining of the soft tissue. As a conclusion, this work is a first step in the standardization of the experimental protocol of the extension-inflation test. Furthermore, it emphasized the challenges of performing 4D micro-CT on a Dacron graft and 4D CE-CT on native tissue. Further perspectives would be to test more samples for measuring the mechanical properties of different materials.