Rheology of hyaluronan solutions under extensional flow.

The extensional viscosity and the steady shear viscosity of sodium type hyaluronan (NaHA) in water with sodium chloride and/or sucrose and in DMSO solvent were measured. The extensional viscosities for HA in aqueous solution (0.05, 0.1, 0.3 w/v%) were constant at lower extensional rates, and then became strain thinning above a critical extensional rate. However, on adding sodium chloride, the extensional viscosity decreased and became strain thickening at higher extensional rates. Sodium ions shield the electrostatic repulsion between carboxyl residues of HA molecules and constrict the coil dimensions. The strain thickening of HA solution in the presence of sodium chloride at higher extension rates is due to the coil stretching. The addition of sucrose increased the extensional viscosity and shifted the critical extensional rate to lower strain rates. With increasing strain (shear) rates, extensional (shear) viscosities for HA aqueous solutions remained constant up to a critical extension (shear) rate; but they showed no plateau and decreased linearly in DMSO. It is clear that molecular interaction of HA in DMSO is stronger than that in aqueous solution. This should be attributed to the different conformations of HA in DMSO and in aqueous solutions.