Comparative rheological behavior of hyaluronan from bacterial and animal sources with cross-linked hyaluronan (hylan) in aqueous solution.

Using a variety of rheological techniques, the behavior of hyaluronan (M(w) 0.8-2.2 x 10(6)), cross-linked hyaluronan (hylan) (M(w) 1.8-12.5 x 10(6)), and Healon (M(w) approximately 5 x 10(6)) (a proprietary hyaluronan) was studied over a large range of molecular weights. The object was to study the effect of the cross-links in hylan on the various rheological parameters, in comparison with linear hyaluronan. There are significant differences. The Huggins constant and the critical overlap parameter C*[eta] are considerably lower for hylan and an increase in moduli at low frequencies was observed for hylan compared with the hyaluronan samples at all molecular weights studied. The results point to a difference in structure in dilute solution for hylan due to the ability to form networks, which can be removed by pressure filtration. In contrast, we do not find an increase of the steady shear viscosity and elastic modulus at higher concentrations when a homogeneous entangled network is reached. We attribute this behavior to the semirigid character of the hyaluronan chain and to the predominance of entanglements over the cross-link points present in hylan in the semidilute domain. Due to the higher apparent molecular weights that are possible with hylan structures but not with the hyaluronans currently available, a wider range of applications can be achieved with hylans when viscoelasticity is required, particularly for the viscosupplementation of synovial fluid damaged by osteoarthritis.