Alkylamino hydrazide derivatives of hyaluronic acid: synthesis, characterization in semidilute aqueous solutions, and assembly into thin multilayer films.

A series of biodegradable alkylamino hydrazide hyaluronic acid (HA) derivatives were prepared and used to design new biocompatible films able to release hydrophobic drugs in a controlled manner. The first step of this work thus consisted in optimizing the synthetic conditions of hydrazide HA derivatives bearing pendant hexyl, octyl, decyl, and citronellyl chains with a degree of substitution of 0.05 or 0.10. The behavior in aqueous solution of these water-soluble modified HA samples was then examined in the semidilute regime. The decylamino hydrazide derivatives of HA exhibited remarkable associating properties, giving rise to transparent gels. These gels were found to be more resistant to degradation by hyaluronidase compared to solutions of nonmodified HA at the same concentration. The other derivatives of which the lengths of grafted alkyl chains range from 6 to 8 carbon atoms lead to more or less viscous solutions. Different viscometric features for these derivatives could be observed as a function of the molecular weight of HA. As derivatives prepared from a HA sample of 600,000 g/mol (HA-600) exhibited a much higher tendency to self-aggregate than their counterparts prepared from a HA sample of 200,000 g/mol (HA-200), the latter derivatives were selected for the build up of multilayer films. The topography and z-section of (PLL/HA derivatives) films assembled layer-by-layer were observed by atomic force microscopy (AFM) in liquid and confocal laser scanning microscopy (CLSM) using PLL(FITC) as ending layer. Moreover, the ability of the films made of the different HA derivatives to incorporate the hydrophobic dye nile red (NR) was investigated. Films containing decylamino hydrazide HA derivatives were the most efficient for incorporating and retaining nile red, which confirms the formation of stable hydrophobic nanodomains in the films.