Nano-mechanical mapping of human and porcine abdominal aortic aneurysm

Abdominal aortic aneurysm (AAA) is characterized by extensive remodeling of the extracellular matrix. The propensity of AAA rupture is dependent on the macro as well as micro and nanoscale heterogeneity in its mechanical properties. Previous studies have shown that AAA tissue comprises of mechanically soft and stiff regions as compared to healthy tissue. However, the correlation of these heterogeneous mechanical properties to its structural components are not well characterized. In this study we used atomic force microscopy (AFM) to analyze human AAA tissue, extracted at the time of vascular surgery. In addition, healthy and diseased aortic tissue from a porcine model of AAA were examined. AFM was used to generate topographical images of tissue sections in a fluid environment, which were identified to be collagenous or non-collagenous regions. Quantitative Nanomechanical Mapping of the corresponding regions was performed to ascertain the local modulus. Transmission electron microscopy (TEM) and histological staining were used to localize calcific deposits. Our results indicate that AAA tissue is mechanically heterogenous with the modulus of collagenous regions being twice that of non-collagenous regions. The moduli of porcine AAA were comparable to that of human AAA and higher than healthy aorta from the corresponding animals. In addition, AAA tissue was interspersed with nanoscale particles of higher moduli, likely representing calcific deposits. The presence of calcific deposits was verified using TEM imaging and a dual histological staining approach consisting of Von Kossa and degraded collagen staining. We thus elucidate the nanoscale mechanical heterogeneity in AAA which can be attributed in part to collagen fibrils and calcific deposits.