Dry powder aerosol drug delivery—Opportunities for colloid and surface scientists

Development of dry powder aerosol drug delivery system includes powder production, formulation, dispersion, and delivery of the powder aerosol to the lung. Conventional method of production of fine particles suitable for aerosol delivery involves crystallisation and micronisation by milling. This method has many deficiencies including uncontrolled particle size distribution and amorphous materials. As a result, development of alternative methods such as supercritical fluid technology and high gravity controlled precipitation becomes important. Over the last decade performance of powder formulations has been improved significantly through the use of engineered drug particles and excipient systems which are: (i) of low aerodynamic diameters (using porous particles with low density) and/or (ii) less cohesive and adhesive (by reducing surface energy, using particles with corrugated surfaces, hydrophobic additives and fine carrier particles). Analytical techniques such as the atomic force microscopy (AFM) and inverse gas chromatography (IGC) have been used to probe particle forces and surface energy affecting powder dispersion. Relative humidity (RH) is critical to the performance of dry powder inhalers (DPI) via capillary force and electrostatic interaction. Electrostatic charge of different particle size fractions of an aerosol can now be measured using a modified electrical low-pressure impactor (ELPI). Compared with powders, much less work has been done on the inhaler devices at the fundamental level. Most recently, computational fluid dynamics has been applied to understand how the inhaler design (such as mouthpiece, grid structure, air inlet) affects powder dispersion. Surface and colloid science can be applied to each of these areas to benefit the research and development of dry powder aerosol drug delivery.