Failure properties and microstructure of porcine aortic adventitia: fiber level damage vs tissue failure

Abstract

Aortic aneurysm rupture is a sudden local event with high mortality. It is generally accepted that the adventitia acts as the final barrier protecting the aorta from over-expansion. Currently, the knowledge of microscopic structural determinants of the tissue's mechanical response and failure is very limited. The purpose of this study is to provide data on the directional failure properties of the adventitia, combined with micro-structural imaging and structure based constitutive modeling to quantify fiber-scale rupture criteria. Eleven healthy porcine aortas were used in this study. Cylindrical portions of the abdominal section were excised, cut-open longitudinally, the medial and adventitial layers separated methodically. Picrosirius red staining was used to image the collagen fiber morphology via an optical microscope. Subsequently, dog-bone shaped specimens were subjected to uniaxial testing until failure while being recorded by a Nikon digital camera. A fiber-scale damage model was utilized to explain the tissue-scale failure. The ultimate tensile stress in the circumferential and longitudinal directions were recorded to be 0.96 +\- 0.29 MPa and 0.85 +\- 0.36 MPa respectively. Meanwhile, the ultimate stretch to failure in the circumferential and longitudinal directions were recorded to be 1.72 +\- 0.16 and 1.88 +\- 0.13 respectively. Further, correlation between the failure properties of the tissue and mean fiber orientation have been reported. Finally, the critical fiber stretch for damage initiation and eventual tissue failure were identified to be 1.19 +\- 0.07 and 1.24 +\- 0.05 for circumferential and longitudinal specimens respectively. Our approach provides valuable insight into the (patho)physiological mechanical role of collagen fibers at different loading states. This study is useful in enhancing the utilization of structurally motivated material models for predicting arterial tissue failure.