Non-destructive characterization of hydrogels.

Hydrogels have been prepared by a freezing-thawing procedure and investigations made of the effect of both number of freezing-thawing cycles and different content of hyaluronic acid (HA) on the mechanical properties of the PVA-HA hydrogels using non-destructive testing. The bulk elastic modulus K of hydrogels has been determined by pulse-echo measurements. It is noted that hydrogel elastic properties improve with the number of the cycles in PVA-HA 100/0; on the other hand samples with a high HA (1,000,000 molecular weight) content, beyond the third cycle, seem to be unaffected by the number of cycles. A bulk elastic modulus fall-off is then observed in samples submitted to an additional overnight freezing between two subsequent cycles. K increases in hydrogels with the highest HA content, when samples undergo pulse-echo measurements soon after their preparation. When hydrogels reach equilibrium, after having been kept in deionized water for 12 h, K values are lower, showing a nearly constant behaviour with different PVA-HA ratios and cycles. Furthermore, by means of scanning laser acoustic microscopy (SLAM) defects have been detected in the hydrogels. In samples which have reached equilibrium, SLAM images show that these defects disappear in PVA-HA hydrogels.