What can we learn from atomic force microscopy adhesion measurements with single drug particles?

Frequently solid dosage form formulation manufacture and delivery depend critically on the control and exploitation of interparticulate interactions. Traditional approaches to understand such interactions rely on indirect assessments of adhesion or consider the behaviour of large numbers of particles. In recent years, the possibility of characterizing and perhaps quantifying forces of adhesion between individual micron and sub-micron sized particles has become viable using the atomic force microscope. This has significant potential in formulation development, particularly in the optimization of inhalation and other solid-dosage form based therapies. However, before a widespread acceptance of this approach by pharmaceutical scientists and industry can proceed a number of issues remain to be considered. These include how can single particle events be mapped on to bulk behaviour, the need to understand the sometimes wide variations in adhesion data observed and can formulations be compared quantitatively and perhaps be screened by this approach?