Comparison of surface roughness parameters obtained by scanning probe microscopy for carrier particles of dry powder inhalers

Surface 3-D profiles of carrier particles for dry powder inhalers (DPIs) were obtained by scanning probe microscopy (SPM) and analyzed using various roughness parameters. Different roughness parameters were compared and evaluated for their suitability for distinctively quantifying the surface roughness of the lactose carrier particles. Arithmetic mean roughness (Ra) was found to be the most sensitive for differentiating the surface roughness of various lactose carriers while fractal dimension (FD) and surface ratio (SR) were less sensitive. On the other hand, maximum height (Ry) and 10-point mean roughness (Rz) were less representative of the roughness of the whole surface due to the methods of calculation involved. Scan area was found to be an important factor affecting the roughness results obtained and should be seriously considered in the measurement of surface roughness.