The influence of core tablets rheology on the mechanical properties of press-coated tablets

Press-coating (also called compression coating or dry coating) consists of a second compression of an outer layer of material over a preformed tablet core. Despite being old, this technology has returned to popularity due to its widespread use in preparation of chronotherapeutic dosage forms. The literature available on press-coated tablets has mainly investigated drug release kinetics, while there is a lack of information about their mechanical properties. Here we study, for the first time, the effect of material properties and manufacturing parameters on the mechanical characteristics of press-coated tablets. Firstly, we show that the stiffness of the bare core tablets depends on the material type and, in case of viscoelastic materials, also depends on the compression pressure. We then demonstrate that less stiff (i.e. more viscoelastic) core tablets deform to a greater extent upon the second compression and thus allow the formation of less porous, harder coats and with a more homogenous density distribution. Finally, we find that changes in the mechanical properties of press-coated tablets over one month storage are almost negligible. Our data suggest that viscoelastic rather than stiff cores should be used in dry coating, as they promote the formation of more homogenous coats and with better mechanical properties.