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Towards screening of inhalation formulations: measuring interactions with atomic force microscopy

This review charts the progress of atomic force microscopy (AFM) to investigate particle interactions relevant to the performance of inhalers. AFM provides a unique opportunity to examine and quantify single particle behaviour of powdered drugs and excipients in a variety of environmental conditions. An introduction to AFM and particle interactions is given. Comparative experiments that rank adhesion between materials, and quantitative experiments that lead to the measurement of properties such as the work of adhesion and surface energy, are reviewed. The AFM has been widely used to investigate the effects of relative humidity and surface roughness on particle adhesion; these experiments are also reviewed. In the final section, the potential of this approach to screen formulations is discussed.