Colloidal characteristics and functionality of rationally designed esculin-loaded hydrogel microcapsules.

This work reports the development of different types of alginate hydrogel microparticles designed specifically for the esculin (ESC) payload. Negatively charged alginate (ALG) microspheres were prepared by the ionotropic gelation technique, and an oppositely charged polyelectrolyte (PE) shell as a compatible polycation (chitosan (CHIT) or gelatin (GEL)) or a synthetic PEs (poly(allylamine hydrochloride) (PAH) and poly(4-styrenesulfonate) (PSS)) were adsorbed using electrostatic complexation. Thorough characterization of microparticles was performed with advanced microscopic techniques (scanning electron, fluorescence and confocal), followed by stability studies, ESC encapsulation efficacy determination and in vitro release kinetics measurements. We provide an in-depth investigation of the relationships between the properties (thickness, viscosity, areal mass, zeta potential) of the outer shell and the retaining and release abilities of the fabricated microcarriers, using quartz crystal microbalance with dissipation monitoring technique (QCM-D), spectroscopic ellipsometry and streaming potential measurements, combined in a new approach that was not attempted before for micrometric particles. The PAH-PSS and GEL coatings provided sufficient protection against ESC release under simulated gastric conditions that followed a two-stage Corrigan-Gallagher model with a marginal release rate in the first (lag) stage. This seems to be an interesting outcome, since it is rather peculiar for a low-molecular weight hydrophilic compound encapsulated in a highly porous microhydrogel to be released in such a manner.