Characterisation of adhesional properties of lactose carriers using atomicforce microscopy

The atomic force microscopy (AFM) colloid probe technique was investigated as a method for the characterisation of adhesional properties of pharmaceut ical powder surfaces. Lactose carriers used in dry powder inhaler (DPI) for mulations were chosen for investigation since adhesion between the carrier surface and drug particles has been proposed to affect the dispersion of dr ug particles. Individual adhesion forces were determined by measuring the d etachment forces in air between the colloid probe and the lactose particle surface. The colloid probe consisted of a silica sphere (10 mum diameter) a ttached to a V-shaped silicon nitride cantilever (spring constant, k = 0.42 N/m). Adhesion forces were calculated from individual force-distance curve s using Hooke's Law. Individual forces measured at various adhesion sites w ere observed to be reproducible and stable over 10 min (coefficient of vari ation, CV below 5%). The adhesion force distribution determined from measur ements at multiple sites (n > 50) on each sample followed a log-normal rela tionship (regression coefficient, r(2) ranged between 0.95 and 0.99). This enabled characterisation in terms of the geometric mean adhesion force and a geometric standard deviation (GSD). Significant differences (P < 0.001) i n adhesion force were observed between samples, ranging from 37.47 +/- 1.95 to 117.48 +/- 2.20 nN. This study demonstrates the suitability of AFM as s ensitive technique for the characterisation of adhesional properties of pha rmaceutical particles. (C) 2001 Elsevier Science B.V. All rights reserved.