Naturally-occurring extracellular matrix (ECM) proteins, e.g. collagen I and fibrin, play an important role in tissues, conferring structural integrity and providing a biochemical environment for eliciting important cellular responses (e.g. migration). Tissue engineers use a variety of matrix polymers as initial scaffolds for seeding cells, sometimes in combination with one another (e.g. collagen-fibrin [1]). For example, our group fabricates arterial tissue equivalents (TEs) by seeding cells in a fibrin gel, which is gradually degraded over time and replaced by cell-produced collagen [2]. While the structure and mechanics of individual ECM proteins have been studied extensively, how multiple fibrillar networks interact to confer overall mechanical behavior remains poorly understood. Narrowing this gap in knowledge of scaffolds comprising multiple fibril networks is crucial in allowing for more rational design in tissue engineering, as cells react differently according to their mechanical environments. For collagen-fibrin networks in particular, early efforts in elucidating interactions between these two fibril networks in co-gels have proven inconclusive due to inconsistent findings from various groups. Recent modeling efforts by our group have shown that simple “series” and “parallel” type interactions provide bounds for the mechanical behavior of collagen-fibrin co-gels [3]. In addition, experiments on pure collagen and fibrin vs. their respective networks from collagen-fibrin co-gels after digestion showed slight differences in mechanical behavior [4]. These previous studies have focused on the composition-function relationship between collagen and fibrin. The objective of the current work is to explore how collagen network architecture changes in the presence of the fibrin network in collagen-fibrin co-gels, thereby providing an added dimension to our understanding of collagen-fibrin systems by elucidating structure-composition-function relationships between collagen and fibrin.

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