Water dynamics and thermal properties of tyramine-modified hyaluronic acid - Gelatin hydrogels

Abstract

A new family of injectable hydrogels, enzymatically crosslinked from mixtures of tyramine conjugates of hyaluronic acid (HA) and gelatin (Gel) are characterized in terms of their molecular morphology, miscibility and water sorption ability by employing Field Emission Scanning Electron Microscopy, water sorption/desorption measurements and differential scanning calorimetry technique. The hydrogels were chemically crosslinked by reaction of the phenol groups of the tyramine in the presence of the enzyme horseradish peroxidase and a small amount of hydrogen peroxide. The water fraction in the microporous hydrogels, hw, was up to 0.68 being achieved by equilibration in environment of saturated water vapor. Our measurements reveal good miscibility between HA and Gel components in the whole hydration range studied and that the HA/Gel mixtures exhibit glass transition characteristics, water diffusion dynamics and equilibrium water contents that are independent of the composition. The intermolecular interactions have been found to be dependent on the hydration level and the mixtures behave like the Gel component for hw < 0.20 and like the HA for higher hydration levels. On the other hand, the appearance of gel-sol like transition in all mixtures, like in neat Gel component, points to nanoscale heterogeneities in the hydrogels and the existence of Gel-rich nanodomains. Calorimetric measurements on hydrogels at high hydration levels (hw > 0.40) show that they become partially crystallized at subzero temperatures and reveal that absorbed water molecules form, at least, two discrete forms of ice crystals. In the phase separated hydrogels, next to ice phase, a second phase of polymer/water homogeneous mixture with constant water fraction about 0.25 has been identified.