New paradigms for relating the microstructure of cartilage to its large scale mechanics: The Roles of Rigidity-Percolation and Double Gel Network Structure in Non-Linear Response

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Cartilage, the smooth tissue that coats bones in joints, is a remarkable material that can last decades and outperform any man-made substance in its unique combination of material properties. There are several aspects of cartilage's microstructure that contribute to its longevity. First, the matrix that gives rise to these mechanical properties is comprised of two interpenetrating polymer networks, a design that has recently been shown to inhibit crack formation. Second, the mechanical properties of cartilage change with depth such that energy damping occurs almost entirely in a thin region near its surface. The PIs have recently developed a theoretical model that explains how variations in the concentration of the polymer networks lead to this localization of energy absorption to the tissue surface. This project uses an array of experimental techniques to test this model and extend its applicability to extreme deformations where cartilage can be damaged. By chemically treating the tissue to remove essential molecular components and testing the tissue response in their absence, the PIs will determine which elements of the matrix are essential for generating cartilage's unique combination of mechanical properties. A better understanding of the macromolecular origins of cartilage mechanical properties will give insight into how diseases such as arthritis develop, guide approaches to tissue repair therapies, and more broadly, provide mechanical design criteria for fabrication of robust materials that can endure extreme loading.Articular cartilage has unique material properties that enable it to endure millions of loading cycles per year while protecting underlying tissues. The structural and compositional origins of its compressive properties have been known for decades. However, there is no equivalent understanding of the underlying mechanisms associated with its shear behavior. Recently the investigators introduced a rigidity percolation theory, previously established to explain the properties of reconstituted polymer networks, to explain the linear response of cartilage shear mechanics. This theory is that the observed orders of magnitude variation in the shear modulus of cartilage arises from small concentration differences of matrix constituents that are poised near a rigidity percolation threshold. Furthermore, the theory makes novel predictions about molecular mechanisms of non-linear shear behavior and gives new insight into mechanical changes that occur from biochemical and mechanical damage. This project will test these predictions and determining if rigidity percolation can be used as a new paradigm for the macromolecular origins of the mechanical shear properties of articular cartilage. Specifically, using chemical and mechanical degradation techniques, predictions made by the model about the critical extracellular matrix elements that give rise to cartilage's macroscale mechanical properties will be tested. Finally, this project will extend the applicability of the model to describe how the microscopic structure of cartilage is altered when it is sheared beyond the linear regime.