Viscoelastic Properties of Aggrecan Aggregate Solutions

Aggrecan is the self-assembling proteoglycan complex whose physiological function is to provide a hydrated gel in cartilage that stabilizes the spatial distribution of collagen fibers and absorbs and resists compressive loads. The linear and non-linear viscoelastic behavior of aggrecan solutions in shear was studied as a function of aggrecan concentration and ionic strength. At physiological ionic strength, a sol-to-gel transition occurs just above the overlap value. Concentrated solutions exhibit a reversible yield point similar to that predicted and observed for close-packed dispersions of soft spheroids. In contrast to the behavior of linear polyelectrolytes, the storage modulus increases with ionic strength until I = 0.75, above which the modulus decreases. In light of the dense polyelectrolyte brush structure of aggrecan, we suggest that this behavior is a manifestation of the theoretically-predicted crossover from "osmotic brush" to " salted brush" conditions.