Mechanical behavior of collagen hydrogel network through tensile testing on water surface and microstructure analysis

Abstract

Collagen is at once a main structural material and the most abundant protein in vertebrates. Collagen hydrogel reflects mechanical behavior of human tissues and is the most commonly used scaffold material and extracellular matrix in tissue engineering. As a structural material, it bears and loads stress and affects cell behavior depending on mechano-sensitivity of each cell type. Thus, providing prediction of cell response and mechanical reliability of collagen hydrogel presents significant challenge, so it is essential to understand mechanical properties of aforementioned material. However, soft collagen hydrogels were only tested shear, compressive and rheological properties because they have instability by gravitational forces and dehydrating issues at existing tensile testing methods. In addition, polymer network structure of collagen hydrogel varies under applied stress with showing non-linear stress-strain relationship and mechanism for the non-linearity was not proven yet. To predict hydrogels’ mechanical behavior completely, previously unavailable tensile data is required, and an alternative methodology is necessary to collect tensile properties of collagen hydrogel. In this study, we aim at investigating the mechanical behavior of collagen hydrogels under quasi-static tensile stress by suggesting a solution for tensile testing of soft collagen hydrogel and providing its mechanical response to tensile stress. Tensile testing on water surface imparts hydrating and free-standing environment to collagen hydrogels. Moreover, microstructure analysis of collagen hydrogels using SEM, and nano-scale 3D X-ray tomography demonstrates the mechanism of shown tensile properties. As the fiber diameter increases and the crosslink density decreases with decreasing gelation temperature, the tensile modulus due to the network structure change increases. In addition, the higher the collagen concentration, the more the crosslink density and the elastic modulus increase. After the first regime of mechanical behavior, the entropic deformation of the collagen network is completed and the enthalpic deformation of the network begins. These results illustrate that collagen hydrogels respond in complex manners to tensile stress, either as network or as fiber, which shows non-linear stress-strain relationship. Furthermore, we propose a new approach for tensile testing method for soft and hydrated materials that can be applied for bioengineering field.