Initial Collagen Fiber Alignment Determines the Mechanical and Structural Response of Cell-Compacted Tissue-Equivalents Under Indentation

The mechanical behavior of connective soft tissues depends largely on their structural organization, particularly of the collagen network. Previous studies have utilized polarized light imaging techniques to quantify collagen organization and fiber kinematics under tensile load (e.g., [1–2]). Many native tissues function in non-tensile loading environments, however, and the microstructural response to such loads is poorly understood. For example, fiber-reinforced soft tissues can be subjected to indentation in vivo (e.g., supraspinatus tendon in shoulder, flexor tendons that wrap around bones), resulting in a complex combination of compressive (near indentation site) and tensile forces (away from indentation site) being applied to the tissue. In order to understand and predict a tissue’s response to such loading, the respective roles of the collagen and non-fibrillar matrix must be elucidated. In particular, how do the properties of the collagen network (e.g., density/organization) and non-fibrillar matrix (e.g., type/quantity) modulate behavior under load? In order to address these questions, our group has utilized type I collagen gel tissue-equivalents (TEs) as a simplified model system to evaluate properties and relationships of tissue components. TEs are particularly useful because organizational and compositional properties can be controlled during formulation (e.g., mold geometry altered to induce changes in collagen fiber alignment [3]). While our other work has used co-gel tissue analogs to evaluate the contribution of non-fibrillar matrix to indentation [4], the purpose of this study was to evaluate the role of initial collagen organization on tissue behavior in indentation using cell-compacted TEs as a model system.