Abstract
The oscillatory and steady shear rheological properties of concentrated solutions of proteoglycan subunit (PGS) and aggregate (PGA) from bovine articular cartilage have been studied using a Rheometrics fluids spectrometer. At comparable concentrations in the physiological range tan Ξ΄ increases from 0.5 to 1.0 for PGA as the oscillation frequency (Ο) increases from 10^β1^ to 10^2^ rads / s, compared to a decrease from 40 to 5 for PGS. Thus PGA solutions exhibit predominantly elastic response whereas those of PGS exhibit primarily viscous behavior. PGA solutions show pronounced shearβthinning behavior at all shear rates (Ξ³) in the range 10^β2^ < Ξ³ (s^β1^) < 10 ^2^, whereas PGS solutions exhibit predominantly Newtonian flow. For PGA, the smallβstrain complex viscosity Ξ·* (Ο) is substantially smaller than the steadyβflow viscosity Ξ· (Ξ³) at comparable values of Ο and Ξ³. These observations indicate that the presence of proteoglycan aggregates leads to formation of a transient or weakβgel network. Since aggregation leads to a large increase in molecular hydrodynamic volume and hence in the relaxation times for macromolecular rotation, it appears that role of aggregate formation is to shift the linear viscoelastic response from the terminal viscous flow into the plateau elastomeric regime of relaxational behavior. Normal or pathological changes that produce a decrease in aggregation will result in a loss of elastomeric behavior of the proteoglycan matrix.