Mechanical structure function properties and fracture toughness of Articular Cartilage modeled as a biopolymer double network

We present results on cracking and fracture toughness of biopolymer double networks, with Articular Cartilage (AC) as our model system. AC is a soft tissue that covers the ends of bones and distributes mechanical loads at the joints in our knees and elbows. Adult AC has very few cells, and its network-like extracellular matrix primarily determines its mechanical response. As a material, AC is remarkable. It is only a few millimeters thick and has minimal regenerative capacity, yet can withstand large forces over our lifetimes during which it undergoes 100-200 million loading cycles without fracturing. The molecular mechanism underlying this exceptional toughness is not well understood. Here we investigate the mechanical structure-function properties underlying the fracture toughness of AC by using a framework that combines a double network model of AC with rigidity percolation theory. We study how the stress relaxation and crack propagation in the double network depend on its composition and on loading conditions. Our results may help to formulate a quantitative criterion for fracture in AC and similar soft and biomaterials akin to the Griffith criterion for fracture of brittle materials.