The effect of hydroxyl radicals on the rheological performance of hylan and hyaluronan

Shear flow, dynamic oscillation and extensional viscosity measurements were used to compare the rheological performance of several hylan samples (M-v 1.6, 3.2, 3.7, 4.7 and 5.6 x 10(6)) and hyaluronan (M-v 1.4 and 1.8 x 10(6)) before and after hydroxyl radicals ((OH)-O-.) induced degradation. It was found that the higher molecular weight cross-linked structure of hylan was more resistant to degradation than hyaluronan and that this superior stability was reflected in various rheological parameters. The (OH)-O-. degradation or the initial hylan and hyaluronan samples produced a range of polysaccharides based on hylan and hyaluronan with molecular weight covering a range from 0.5-5.6 x 10(6). The rheological parameters associated with the polysaccharides could then also be studied. Zero shear values of the complex viscosity (eta*), dynamic viscosity (eta') and shear viscosity (eta) were calculated using the method of Morris(1) and shown to approach the same value at zero shear or frequency. An adaptation of the method of Gibbs et al.(2) gave a 'master curve' for the storage and loss modulus of hyaluronan and hylan, which encompasses a 10-fold molecular weight and a 5-fold concentration variation. In all instances for hylan, the storage modulus predominates over the loss modulus, whereas for hyaluronan, the reverse is true, demonstrating the greater elasticity of hylan throughout the whole experimental range of molecular weights and concentrations. (C) 2000 Elsevier Science B.V. All rights reserved.