Despite dry powder inhalers (DPIs) being used routinely in the treatment of respiratory illnesses, they remain an ongoing developmental challenge and the subject of significant research activity. The vast majority rely solely on the energy provided by the inhalation action of the patient for successful drug delivery. This can make it difficult to disperse the dose so as to deliver particles of a respirable size, and as a result the efficiency of DPI drug delivery tends to be relatively low. Furthermore, performance can be variable, due to differences in the inhalation profile applied by patients of different physical size, strength and health. 

Conventionally, the respirable fraction of an inhaled drug dose has been defined as having an upper size limit of five microns, with very fine particles, less than one micron often assumed to be exhaled. In recent years these views have been subject to increasing scrutiny, and there is now growing evidence to suggest that particles in the one to two micron, ‘extra fine’, size range, and indeed finer, may in fact be those of most value in inhaled drug delivery.

In this article we consider ...