| The structure and composition of articular cartilage and other load-bearing biological tissues are highly complex and heterogeneous. As a result, their functional mechanical properties exhibit clear spatial variations. Unlocking the structure-function relationship in these materials is critical for devising strategies to restore tissue impaired by injury or disease and can provide a template for successful implant design. Here, we describe a tissue deformation imaging stage (TDIS) allowing for simultaneous force measurement and visualization of microscale deformation in soft biological tissues under controlled shear strain. In combination with a fast confocal microscope, the TDIS is used to test the microscale response of articular cartilage to shear loading. To obtain the location-specific shear modulus of this tissue, we employ a high-resolution technique that involves tracking the deformation of a line photobleached into a fluorescently stained sample loaded in the TDIS. We find that the quasi-static and dynamic shear moduli are lowest roughly 100 mum below the articular surface. Here, articular cartilage is highly nonlinear, stiffening under increased shear strain and becoming more compliant under increased compressive strain. Using a simple thought model, we relate these results to structural features of the collagen network in articular cartilage. Furthermore, we demonstrate that the region of maximum compliance is also the primary site of shear energy dissipation in articular cartilage. Our findings suggest that damage to or surgical removal of the surface of this tissue will increase the joint's susceptibility to shear-induced damage. Finally, similar experiments are performed on intervertebral disc and growth plate, demonstrating the versatility of our in-situ strain mapping techniques. |
