Tissue- and microstructural-level deformation of aortic tissue under viscoelastic/viscoplastic loading

Abstract

Mechanical function of tissues in health and disease is regulated through interactions of phenomena spanning across multiple length scales. The multiscale nature of tissue biomechanics should be incorporated into modeling approaches if a full description of soft tissue biomechanics is sought [1-4]. While gross changes in geometry are sufficient for quantifying tissue-level deformations, quantification of microstructural deformations is more challenging [5-6]. There have been some advances over the past decade [7], but there remains many issues regarding how best to characterize changes in microstructure. In this study, we examined the relation between deformation of bovine aortic tissue at the tissue- and microstructural scales, during uniaxial stretch and after loads are removed. Specifically, circumferential aortic samples were subjected to small and large stretches. Some specimens were chemically fixed during stretch, and others released to undergo free retraction before chemical fixation. Specimens were measured macroscopically before/after loading, sectioned and stained histologically, and analyzed to examine microstructure. Image-based analysis of histology images have been effective in quantifying microstructural changes in soft tissues [8-14] and served as guidance to assess tissue microstructure. At the tissue scale, it is observed that sample's elongation is accompanied with width/thickness shrinkage in an isochoric manner. Microstructural investigations reveal straightening of (undulated) fibrillar network when tissue is stretched circumferentially; with variations different at inner and outer layers of wall thickness. Once recovered, samples exhibited larger permanent deformation toward outer layer, which possesses sparse elastic lamina. This study provides a microstructural basis for observations of local permanent stretch in artery tissues, and paves the way for further development of multiscale models of cardiovascular biomechanics. ©2010 Society for Experimental Mechanics Inc.