Aims: A mounting research effort has recently been devoted to design a biocompatible, sterile and safe material as nucleus polposus substitute.

Several chemical modifications of Hyaluronic acid (HA), a biodegradable linear polysaccharide, have been devised to provide mechanically and chemically robust materials in medical applications.

This study was specially designed to assess whether such a kind of materials are capable to substitute natural NP by suitable viscoelastic properties.

Methods: The NPs were obtained from pigs (L4-L5, L6-L7). Bromide of 2-Hydroxy-4-hydroxyethoxy-2-methyl-propriophenone ester of HA (50% of degree of esterification, cross-linked gel) (HYAFF120p50A8) and dodecylic amide of Hyaluronic acid (11% of carboxyl group substitution of 200 kDa HA) (HYADD3A8) solutions, supplied by FAB, were studied at different ionic strength (0 and 0.15M).

Small amplitude oscillatory shear tests were performed to study linear viscoelastic properties by using a strain controlled rheometer (Bohlin VOR). In particular, the elastic modulus (G’) and the viscous modulus G’’ were evaluated.

Results: Porcine NP showed a rheological behaviour typical of “weak gel”. Indeed, G’ values were always higher than G” ones all over the frequency range and both moduli were almost frequency independent. In particular, at 1 Hz G’ and G” values were respectively 310 and 81 Pa.

The same rheological behaviour was observed for all HA derivatives. In particular, G’ and G’’ were respectively equal to 400 and 91Pa for HYADD3A8 solutions at 1Hz and at 20mg/ml (0,15M), and 210 and 51Pa for HYAFF120p50A8 at 50 mg/ml(0M).

Conclusions: HYADD3A8 and HYAFF120p50A8 solutions seem promising candidates as NP substitute from a viscoelastic point of view. In particular, HYADD3A8 (20mg/ml,0.15M solution) properly matched the rheological behaviour of porcine NPs.