Arterial grafts: in depth characterisation of structure and mechanical properties

Motivation. Although polymer-based synthetic bypasses are substituting autografts and homografts in the clinical use, complications that affect their normal performance remain existing. However, the exact causes of this are still unknown. Although many studies aim to characterise the synthetic grafts as well as the native tissue in terms of their structure and mechanical properties, results are very diverse, leading to no uniform conclusions. For these reasons, the aim of this study was to define a workflow to assess, using the same methodologies, the structural and mechanical properties of all these materials. Thanks to a standardized comparative study between synthetic grafts and native tissue, a more robust and intelligent input for the design of an improved biomimetic synthetic graft could be achieved. Materials and methods. Three types of synthetic grafts, Dacron, Gore-Tex and Vectra, have been characterised as well as native tissue. For the 3D structural characterisation, high-resolution microfocus computed tomography (microCT) was used. Strain-controlled planar biaxial tensile testing was used for the mechanical characterization. Forces and displacements of the samples were tracked during all the tests. Also, strain maps were generated to obtain the stresses. Results and discussion. From the visual evaluation of the microCT images, the three native tissue layers within the homograft could be observed in an unprecedented manner. More detailed 3D analysis of these data is, however, still needed. For the synthetic grafts, a full 3D analysis was performed. For Dacron and Gore-Tex, a longitudinally orientation of the matrix was observed, providing a higher failure resistance in this direction. Dacron presented a corrugated structure, similar to the homograft lumen, which provides elasticity longitudinally. Vectra is composed of a foam in which a wire and a sheet are embedded, providing elasticity and a uniform distribution of the stresses. Endothelial cells fixation is anticipated to be more favourable in Dacron due to its higher specific surface area, followed by Gore-Tex and then Vectra. Vectra is the most porous material. Studying the foams, based on the Gibson-Ashby theory of cellular foams, it can be anticipated that Gore-Tex is the stiffest material of the three. However, a compromise between structure and pore thickness must be found to ensure the stiffness. For measuring the thickness of the samples that were tested mechanically, optical images as well as the microCT images were applied. The latter ones were more accurate, since this measurement technique was not influenced by the residual stresses in the materials. The forces registered from the mechanical test demonstrated a large anisotropy in Dacron, Gore-Tex and the homograft. Dacron and Gore-Tex were stiffer in the circumferential direction, while the homograft was stiffer in the longitudinal. However, it should be noted that the exact orientation of the homografts samples still need to be checked. The anisotropy degree in Vectra was lower compared to the others. Viscoelasticity or damage in Dacron, Gore-Tex and the homograft samples was tracked. Dacron and Gore-Tex presented a low resistance to deformation in the longitudinal direction. Furthermore, it was observed that the circumferential direction acquired preconditioning state faster in all the samples, and that the homograft presented, at high strains, a clear elastomeric behaviour. Conclusion. This study shows the need for a combined structural and mechanical characterisation of synthetic graft materials for vascular bypasses, together with native tissue. Only in this way, a direct comparison can be made, and a better understanding of the mismatch in functional behaviour between the synthetic materials and the native tissue can be obtained.