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Designing a unit cell to investigate the effect of collagen type IX on mechanical properties of human cartilage

Collagen is one of the most abundant proteins in the human body, which is found in almost all tissues, thus, it has always been considered. Collagen type II fibril is one of the important constituents of cartilage which has the function of providing strength. It has been found that collagen type II is not alone, and collagen type IX periodically attaches to its surface and interacts with other components in cartilage. In this research, the extent and magnitude of their effect are investigated. A unit cell is designed based on the microscopic structure of collagen type IX. These collagen fibers are composed of rod fragments and a flexible non-collagenous kink. Assigning the proper mechanical properties to each part, the mean stress and elasticity tensor were calculated by applying appropriate boundary conditions and loads in different states. The behavior of the resulting unit cell resembles a transversely isotropic material. Computing the equivalent Poisson's ratios and Young's moduli justify the varying mechanical properties of cartilage through the depth.