Mechanical Characterization of Collagen Hydrogels by Quasi-static Uniaxial Tensile Experiments

Collagens constitute the primary structural materials in the human body. As a structural material, it sustains much of external forces and directly imposes the mechanical environment on the cell membranes, which in turn affects cell behavior. Because of the motional complexity in living creatures, it is essential to understand the mechanical properties of biomaterials under various loading conditions. However, previous studies mostly focus on analyzing the mechanical behavior under dynamically-varying compressive or shear loads, but the tensile properties at the quasi-static time scale have been relatively less studied. This work aims to investigate the quasi-static tensile behavior of reconstituted collagen hydrogels under uniaxial tensile stresses. The evolution of the collagen fiber network structures with straining was visually observed using the confocal microscopy equipped with the tensile straining stage. A combination of tensile testing and structural analysis demonstrates the deformation mechanisms predominant at each stage during deformation. While the unfolding of the initially-undulated fibers accommodates the early stage strains, the deformation mechanism continuously changes to the stretching of fibers through the network alignment along the tensile direction. This transition commences with the buckling of a fiber lying perpendicular to the loading direction, which otherwise locks the rotation of adjacent fibers. We present the theoretical model to suggest that the stress required to initiate such transition follows the power-law relation of exponent 4 to the product of fiber diameter and line density of the network.