Polymer dynamics as a mechanism of cartilage flow-independent viscoelasticity

Articular cartilage is the soft tissue consisting mostly of extracellular matrix biopolymers and water that covers the ends of bones in synovial joints. Cartilage functions mechanically: it provides low-friction surfaces for articulation and deforms during joint contact to decrease contact pressure and increase joint stability. As such, understanding the specific molecular origins of cartilage resistance to deformation is necessary to understand cartilage mechanical function.; Most previous cartilage mechanics research has focused on macroscale continuum models, but recent advances have enabled the description and determination of microscale polymeric behavior. The theory of polymer dynamics uses statistical physics to describe the motions and interactions of entangled polymers. The objective of this project was to determine if polymer dynamics is a relevant mechanism in cartilage viscoelasticity, with the underlying hypothesis that modifications to cartilage molecules will result in changes in tissue-level mechanical properties that are predicted by polymer dynamics theory.; Experiments consisted of perturbations to cartilage designed to examine polymeric mechanisms of viscoelasticity, and the results were consistent with polymer theory. Cartilage stress-relaxation data is best described by mathematical models containing both flow-dependent and flow-independent components. Cartilage stress-relaxation proceeds slower at higher volumetric concentrations of matrix molecules. Cartilage exhibits faster stress-relaxation at increased temperature, as well as Gough-Joule-type effects. Enzymatic digestion of specific matrix molecules results in faster stress-relaxation. Increasing the solution ion concentration (decreasing the aggrecan stiffness) results in faster stress-relaxation. The stretched exponential time constant describing cartilage stress-relaxation is negatively correlated with the transverse nuclear magnetic relaxation time of collagen protons.; These data present a complex picture of cartilage mechanics and the polymer dynamics interpretation is consistent with the role of matrix viscoelasticity developed in previous cartilage mechanics studies. Fluid flow appears to be a slow mechanism of cartilage viscoelasticity, and polymer motion may be a mechanism of the faster stress-relaxation which is observed experimentally. Significant further research remains to fully understand how the motions and interactions of specific matrix molecules in conjunction with fluid flow result in the nonlinear viscoelasticity of cartilage.