Aggrecan, the major cartilage proteoglycan, is the macromolecular species primarily involved in the resiliency of cartilage tissue. Most aggrecan molecules occur in cartilage extracellular matrix as aggregates. Each aggregate has a supramolecular structure, with many highly anionic, brushlike aggrecan subunits noncovalently bound to a hyaluronan chain. To better examine the viscoelastic properties of aggrecan aggregate, contaminating subunits were removed by exclusion chromatography. At physiologic ionic strength, concentrated solutions of purified aggrecan aggregate exhibit predominantly elastic behavior at small shear strains. However, above a critical strain, gamma c, the shear moduli show a pronounced strain-softening transition, where the storage modulus decreases suddenly, and the loss modulus exhibits a maximum. At small stresses, the creep function is also highly elastic, exhibiting an equilibrium compliance and large recoverable compliance. When the stress is amplified, a discrete transition to viscous flow occurs at a yield stress sigma y. These nonlinear responses are similar to previous observations for close-packed colloidal suspensions of soft spheres, such as microgel or emulsion particles, for which a yield transition occurs when the stress and deformation are sufficient to move a particle past its neighbors.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.