Particle surface roughness - its characterisation and impact on dry powder inhaler performance

Dry powder inhalers are commonly formulated as interactive mixtures comprising micronised drug particles and large inert carrier particles. The properties of the carrier surface, particularly surface roughness, are believed to critically affect drug-carrier interactions and ultimately, determine the inhaler performance. Surface roughness has been proposed to influence the: (a) mixing and stability of ordered mixtures, (b) drug-carrying capacity of carriers and (c) adhesion and detachment of drug during aerosolisation. Roughness analysis by mapping of surface topography is seemingly straightforward but practically difficult to achieve with consistency, particularly when handling small particulates with random surface structures. This chapter discusses several considerations, during a roughness measurement and in image-processing, which are essential to obtain roughness parameters that adequately describe particulate surfaces. The surface modification of carrier particles is a method to modulate the drug-carrier interactions and to improve the drug dispersion characteristics. It has been accomplished either during the process of particle formation or by various methods to induce physical changes to existing surfaces. Various methods of carrier surface modification and their effects on the in vitro performance of dry powder inhalers are presented. Generally, surface roughness at greater length scales had a negative impact on fine particle fraction, whereas the opposite was true when surfaces exhibited small-scale surface roughness.