Preparation of cross-linked hyaluronic acid films of low water content

Hyaluronic acid (HA) was chemically cross-linked with poly(ethylene glycol) diglycidyl ether, a diepoxy compound (EX-810), to yield low water content and slowly degradable films when brought into contact with water. The cross-linking reaction was performed under acidic and neutral conditions, since the epoxy group is readily hydrolysed in alkaline media. To allow the reaction to proceed at high HA concentrations, a solution casting method was employed for the cross-linking of HA. The lowest water content of the cross-linked HA films obtained was 60 wt% when swollen with buffered saline at 37 °C. Alginic acid and poly(vinyl alcohol), which possess hydroxyl groups, similar to HA, were also found to undergo cross-linking with the diepoxy compound. Since IR spectra of the cross-linked films had no significantly new absorption, intermolecular formation of ether bonds between the hydroxyl groups belonging to different polysaccharide molecules was assumed to take place. It seemed too difficult to detect the ether bonds in the cross-linked HA films, because the virgin HA film itself contained ether bonds in the molecule. The cross-linked HA film with a water content of 60 wt% exhibited practically no weight loss after 10 days of immersion in phosphate-buffered saline (pH 7.4), while this film underwent in vivo degradation by 30% weight loss after 7 days of subcutaneous implantation in rats. The inflammation reaction elicited around the implanted film was not significant.