Mechanical and Structural Characterisation of a Calcified Scaffold

Abstract

Atherosclerosis is an inflammatory disease characterised by the formation of plaque in the intimal layer of the artery. The plaque is made of a lipid rich necrotic core which is covered by a collagen rich fibrous cap. If this fibrous cap ruptures, it can lead to sudden thrombotic occlusion of the artery. Rupture of the fibrous cap is linked to many factors. Recently, through high resolution imaging, microcalcifications have been found in the fibrous caps. The role of these microcalcifications in fibrous cap rupture mechanics is a debated theory. Calcifications, by virtue of their high stiffness, are predicted to increase local stresses in the less stiff surrounding collagen tissue by creating stress concentrations. This interaction between collagen and microcalcifications has not been studied extensively. Fibrous cap rupture mechanics can be studied through mechanical tests such as uniaxial tensile tests. Due to limitations in access to human plaque tissues and differences in the mechanisms of cap rupture in animal models, there is a need for an in-vitro platform to study fibrous cap rupture mechanics. In this study, a simplified model of a fibrous cap incorporating two components, collagen and calcifications, for the development of an in-vitromodel of an atherosclerotic fibrous cap was explored. High levels of calcium and phosphate have been found in microcalcifications in fibrous caps. Mesenchymal Stem Cells (MSCs) have been linked to vascular calcifications and have been found to deposit calcium phosphate in collagen scaffolds. Collagen type 1 scaffolds were seeded with MSCs to create calciumphosphate deposits with the aim of emulating an atherosclerotic fibrous cap from the collagen and calcifications aspect. The collagen scaffold constructs were mechanically tested
to study the mechanical properties and effects of the calcium phosphate deposits on the mechanical behaviour. The structure and failure behaviour was studied through histology and scanning electron microscopy. Deposits of calcium phosphate were successfully formed inside the collagen scaffold leading to a calcified scaffold. The calcified collagen scaffoldswere mechanically and structurally characterised. The composition and size of the calcium phosphate deposits were in line with microcalcifications found in atherosclerotic fibrous caps. The failure was characterised by noticeable initial failures, multiple miniature failures and high stretch before the final complete failure. The calcified scaffolds can potentially serve as a baseline for the development of an in-vitro model of an atherosclerotic fibrous cap and for gaining useful insights into fibrous cap rupture mechanics.