Electrostatic characterisation of inhaled powders: effect of contact surface and relative humidity

Electrostatic charge accumulation on drug and excipient powders arising from interparticulate collisions or contacts between particles and other solid surfaces often leads to agglomeration and adhesion problems during the manufacture and use of dry powder inhaler (DPI) formulations. The aim of this work was to investigate the role of triboelectrification in particle interactions between micronised drug (salbutamol sulphate or ipratropium bromide monohydrate) and excipient (alpha-lactose monohydrate, 63-90 microm) during mixing in cylindrical vessels constructed from stainless steel, polypropylene and acetal under selected relative humidity (rh) conditions (0-86%). The charge was found to depend on both the nature of the powders and the mixing vessel surface. In addition, coating the vessels with drug or excipient removed the influence of the vessel material on charge generation, thus providing a technique to investigate interactions between the drug and excipient substances. A triboelectric series of all materials used, placed ipratropium at the positive end and polypropylene at the negative end. Micronised drug profoundly altered the charging properties of lactose in drug (1.46%, w/w)/lactose DPI formulations. An increase in rh in the range 0-86% produced a corresponding decrease in charge and adhesion values for each drug, lactose and DPI formulation during triboelectrification with each mixing vessel surface. The results provide increased knowledge of the role of electrostatics in DPI technology.