Mechanical Characterization of Tissue Engineered Fibrous Cap Constructs

Abstract

Atherosclerosis is a widespread disease characterized by the formation of a plaque within the inner most layer of the arterial wall. This plaque is comprised of a lipid-rich pool containing little to no collagen covered by a collagenous fibrous cap which serves as a barrier containing the plaque from rupture. Plaque rupture is an often fatal phenomenon, not yet fully understood, which can result in arterial blockage causing myocardial infarction or stroke depending on rupture location. Whether a plaque will rupture depends on many interrelated aspects; the biological composition of the plaque, the mechanical properties of these components, and how they are mechanically loaded. The collagenous matrix is the main load bearing structure of the plaque cap, therefore affects plaque stability and should be further investigated. Due to the various limitations when studying ex vivo and in vivo human plaques as well as the significant difference in plaque development in animal models, an in vitro atherosclerotic plaque cap model is necessary to systematically study plaque rupture mechanics. In this study, a simplified collagenous fibrous cap model with a soft inclusion (SI) was developed. The effect of intermittent uniaxial straining (IS) versus no straining (static) during culture was investigated as intermittent straining during culture is linked to an increase in alignment of collagen fibers in the loading direction, which is seen in human fibrous caps. The constructs were mechanically tested with two different clamp designs (commercial and redesigned), to investigate the differences in mechanical behavior and to improve the clamping technique. The local strain patterns were studied using digital image correlation (DIC) preceding rupture. Although there was higher compaction observed in the IS constructs when compared to the static constructs, the mechanical behavior of the IS and static constructs tested in the commercial clamps were comparable. However, DIC analysis demonstrated that the static constructs in the commercial clamps showed homogeneous extension behavior, whereas the IS constructs showed a symmetric “C-shaped” compressive pattern, signifying a possible difference in microstructure. Additionally, the redesigned clamps displayed a consistent two bump rupture pattern correlating to rupture near the SI, as well as higher measured force at comparable stretch, not seen with the commercial clamps. The redesigned clamps demonstrated an improvement in the testing method when compared to the commercial clamps by exhibiting better load transmission from clamp to tissue, creating more homogeneous strain distribution within the region of interest and leading to more consistent rupture near the SI. Interestingly, high extension values were observed at the rupture location in both the redesigned and commercial clamps, possibly demonstrating a linkage between high extension behavior and rupture location. In this study, collagenous constructs with an integrated SI were successfully created exhibiting mechanical properties within the range found in literature and rupturing near the SI-tissue interface, therefore these findings can serve as a guide for future experiments in the development of an atherosclerotic plaque cap in vitro model.