Freeze drying and rehydration of alginate fluid gels

Abstract

The aim of this work was to study the effect of freeze drying (FD) and rehydration on alginate fluid gels (AFG). FD was employed as a widely used drying method in order to evaluate whether its application causes any change in the material characteristics crucial for AFG behaviour. Rehydration studies were performed to assess if AFG properties could be restored after drying and rehydration processes. First, it was investigated the influence of the material formulation (alginate (ALG) and calcium chloride (CaCl2) concentrations) on the particle size distribution (PSD) and the rheological properties. The used ALG/CaCl2 ratio demonstrated to be responsible for forming nanoparticles or microparticles. The impact of fluid gel particle dimensions on drying and rehydration processes was also investigated. Overall, particle dimensions do not have a significant effect on drying kinetics. Analyses on rehydrated samples showed that the PSD is not affected by the processing, whereas the same viscosity was completely recovered only for samples formed by ALG and CaCl2 in quantities leading to nanoparticles formation. Preliminary encapsulation experiments were also carried out to highlight the impact of this process on the release behaviour of the loaded active from AFG. Results obtained from in vitro release studies and their model fitting showed that the active release behaviour from AFG was not affected by freeze drying when AFG was formed of nanoparticles. On the contrary, the active release behaviour was affected by freeze drying when AFG was formed by microparticles.