Micromechanical properties of spray-dried core-shell silica aggregates along with drug release tests

In order to enhance the quality of spray-dried products or to adjust material properties for new applications, precise control of the aggregate structure is desirable. For the purpose of preparing hierarchically structured aggregates in the micrometer range, the formulation of the suspension can be specifically designed, utilizing defined nanoparticulate building blocks to achieve a highly uniform structure and porosity. Further adjustments can be made by combining two types of primary particles that differ in size. Thereby, a segregation effect is observed, where nanoparticles with larger particle sizes accumulate rather within the core of an aggregate and those with smaller particle sizes gather mainly near the outer surface, resulting in the formation of a shell. Furthermore, it is possible to produce tailor-made porosities using template particles (e.g. polystyrene) of different sizes as part of the coarse and fine fractions. The removal of these particles by a subsequent tempering process can lead to aggregates with defined porous structures and thus, to different mechanical aggregate properties that can be specifically set by adjusting the process and formulation parameters. As a result, a promising building kit for the hierarchically structure formation via spray drying processes were achieved. For the detailed characterization structural and mechanical material properties were investigated, using e.g. mercury intrusion and SEM. The influence of the formulation parameters of the suspension (primary particle size and template content) on the micromechanical properties of the aggregate structures was systematically investigated by nanoindentation to elucidate structure-property relationships regarding, for example elastic and plastic deformation. As a result, a correlation could be established between the experimentally determined mechanical parameters and the aggregate porosities. Such microstructures with defined properties can be used in a variety of applications, including catalysis or as drug carriers. For instance, these spray-dried aggregates were loaded with ibuprofen as an exemplary active pharmaceutical ingredient and investigated with regard to their drug release behavior.