RHEOLOGICAL BEHAVIOUR OF HYALURONAN, HEALON AND HYLAN IN AQUEOUS SOLUTIONS

Abstract

Rheological properties of hyaluronan solutions are related not only to the molecular weight or concentration, but also to the origin of the samples. Here we present a comparative rheological study of hyaluronan, in aqueous solutions, from bacterial and animal sources with a cross-linked hyaluronan (hylan). Using a variety of rheological techniques, the behaviour of hyaluronan (Mw 0.8 − 2.2 × 106), crosslinked hyaluronan (hylan) (Mw 1.8 − 12.5 × 106) and Healon (Mw ˜ 5 × 106) (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*[η] 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. In the semi-dilute regime, the Newtonian viscosity scales as C4.1 to C3.6 by increasing the polymer concentration. The critical shear rate and the cross-over frequency (ωρ) vary as C-2 M-3. The plateau modulus G ∼ G0 scales as C2.15. We attribute this behaviour, which agrees well with the de Gennes prediction in a good solvent, to the semi-rigid character of the hyaluronan chain and to the predominance of entanglements over the cross-link points present in hylan in the semi-dilute domain. Due to the higher apparent molecular weights which 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.