The mechanical properties of simulated collagen fibrils

Previous theoretical studies of the mechanical properties of tissues such as skin, bone and tendon, have used approaches based on composite materials and have tended to neglect the contribution of individual microscopic components. In this paper, we examine the relationship between the fine structure of a collagen fibril and its relative tensile strength. Collagen is a fibrous protein which provides associated tissues with the majority of their tensile strength. It is present in the form of elongated structures termed fibrils which are created by the self-assembly of rod-like collagen molecules in an entropy-driven process termed fibrillogenesis. Mutations that alter the primary structure of the collagen molecule, interfere with this assembly process and can lead to the potentially fatal brittle bone disease, osteogenesis imperfecta. Here we investigate the mechanical properties of a range of computer-generated aggregates. The aggregates, created by the diffusion limited aggregation of rods, were subjected to a simple tensile test based on local rules of damage accumulation. In the test, core samples are "extracted' from the aggregates, and the network of particles involved in the transmission of stress resolved. Increasing stress applied to the core leads to the removal of individual rods from this network; the tensile strength is determined from the force necessary to form a discontinuous network. Using this approach, we have shown that collagen fibril morphology is critical in determining its tensile strength. We suggest a possible mechanism to account for the increasing severity of osteogenesis imperfecta associated with the distance of mutation from the N-terminal of the collagen molecule.