Mechanics and fracture properties of soft disordered networks in cartilage and cartilage inspired materials*

Articular cartilage (AC) is a soft tissue that provides a smooth cushion and distributes the mechanical load in joints. AC’s mechanical properties mainly arise from a composite matrix made up of extracellular collagen and aggrecan networks.  As a material, AC is remarkable. It is only a few millimeters thick, can bear up to ten times our body weight over 100-200 million loading cycles despite minimal regenerative capacity, and still avoids fracturing. The simultaneous strength, fracture resistance, and longevity of native AC remain unmatched in synthetic materials. Such properties are desperately needed for tissue engineering, tissue repair, and even soft robotics applications. I will discuss the structural origins of AC’s exceptional mechanical properties using the framework of rigidity percolation theory applied to composite materials and compare our predictions with experiments. Our results provide an understanding of the tissue depth-dependent mechanical properties and how tissue mechanics changes in response to changes in tissue composition during diseases such as osteoarthritis. This framework also offers insights into how structure, composition, and constitutive mechanical properties can be tuned to resist and blunt cracks in AC and cartilage-inspired soft materials. The flexibility in resulting material properties and ease of implementation can be harnessed to fabricate artificial tissue constructs with tunable mechanics. I will discuss results that are an important step towards achieving this future.