Mechanical Phase Transitions in Articular Cartilage

Articular cartilage is a remarkable material able to sustain millions of loading cycles over decades of use outperforming any synthetic substitute. Crucially, how extracellular matrix constituents determine mechanical performance, particularly in shear, remains poorly understood. Here, I will describe experiments and theory in support of a rigidity percolation framework that quantitatively describes the structural origins of cartilage's shear properties and how they arise from the mechanical interdependence of the collagen and aggrecan networks making up its extracellular matrix. This framework explains that near the cartilage surface, where the collagen network is sparse and close to the rigidity threshold, slight changes in the aggrecan concentrations, and therefore the rigidity of the secondary network, can dramatically alter the tissue modulus. Our results suggest that cartilage is an actively tuned tissue where cells can change the modulus by modifying the production of the aggrecan molecules. More broadly, this framework provides a map for understanding how changes in composition throughout the tissue alter its shear properties and ultimate in vivo function.