Mechanics and kinematics of soft tissue under indentation are determined by the degree of initial collagen fiber alignment

While several studies have evaluated how the degree of collagen alignment affects the response of soft tissues to tensile loading, the role of fibrillar organization in indentation is less understood. Collagen-based tissue-equivalents (TEs) provide a convenient model system to explore structure-function relationships since their microstructural properties can be easily controlled during fabrication. The purpose of this study was to evaluate the role of initial collagen alignment on the mechanical and structural behavior of soft tissues subjected to indentation using TEs as a model system. Cell-compacted TEs with either isotropic or highly anisotropic fiber alignment were subjected to four-step incremental stress-relaxation indentation tests. The mechanical properties, collagen reorganization and 2D strain patterns were quantified at each indentation step and compared between groups. While no differences were seen in the peak force response, significant differences were seen in relaxation behavior, fiber kinematics and tissue strain. Specifically, highly aligned samples exhibited a slower relaxation rate, smaller changes in collagen fiber orientation, larger changes in strength of alignment, and larger strain magnitudes compared to isotropic samples. Results demonstrate the significant role that microstructural organization plays in mediating the response of soft tissues to a non-tensile (i.e., indentation) mechanical stimulus.