The influence of extracellular matrix (ECM) micro-structure on the macro- and micro-level biomechanical behavior of tissue constructs and cell-ECM interactions

Cells in vivo reside in a framework known as extracellular matrix (ECM). The ECM guides tissue structure and function by communicating to cells both biochemical and mechanical information about their local micro-environment. It has been well established that mechanical loads applied to tissues influence many fundamental cellular processes; however, pathways that transmit forces from the tissue (macro) to the cellular (micro) level remain to be identified. As such, this dissertation demonstrates new information regarding how ECM microstructure affects the macro- and micro-level biomechanical behavior of tissue constructs and cell-ECM interactions. Since their microstructure can be controlled in vitro, a 3D scaffold of collagen fibrils was used as a model ECM. A tissue construct was then created by seeding fibroblasts, a cell common to connective tissues, inside the collagen ECM. First, the microstructural and macro-level mechanical properties of microstructurally varied collagen ECMs were characterized. To further understand specific mechanisms of the mechanotransduction process, a new experimental system, capable of measuring the three-dimensional (3D) micro-biomechanical behavior of the ECM and cells, was developed. Integrating confocal reflection microscopy with mechanical testing allowed simultaneous 3D visualization of the tissue (macro-level) and of cell and ECM microstructure (micro-level) as measured and controlled mechanical loads were applied to living tissue constructs. Application of a new incremental digital volume correlation algorithm allowed quantification of 3D strains at the micro-level. Combined, these tools allowed study of (1) macro- to micro-level transmission of mechanical strains through tissue constructs and (2) ECM to cell strain transfer inside tissue constructs. Studies revealed that ECM microstructure was a critical determinant in the macro-level stress response, the 3D macro- to micro-level strain transfer properties, and the 3D micro-level strain response of the ECM. Additionally, studies with tissue constructs demonstrated that changes in collagen ECM microstructure altered the transfer of strains from the ECM to resident cells. Applied, this new information will allow the design of microstructurally enhanced ECM constructs for use as functional tissue engineered replacements