A Quantitative Study of the Effects of Humidity on the Adhesion of Pharmaceutical Particles to Stainless Steel

Pharmaceutical particle interactions with conveyers, tanks, other particles, and other surfaces are important in the development of processes for drug formulation and production. Relative humidity (RH) has a critical effect on these adhesion phenomena. Model particles, including griseofulvin (model active pharmaceutical ingredients, API) and lactose (model excipient), were studied to investigate their adhesion forces against stainless steel and perspex. Stainless steel, a common conveyor belt material in API production, and Perspex, a common substrate in particle synthesis reactors and mixers, were studied. Relative humidity was controlled in the range of 15 % to 65 %. Particle adhesion forces were generated using colloidal probe microscopy (CPM) with a Veeco atomic force microscope (AFM). Images of the particle were obtained before and after their use in CPM using environmental scanning electron microscopy (ESEM). The images allowed an existing in-house simulation code to be applied to predict adhesion forces as a function of particle size, shape, roughness, composition, and elastic properties, as well as substrate composition and elastic properties. Experimental and modeling results were obtained which show that the primary role of increasing relative humidity was to cause surface annealing of the particles, effectively reducing their roughness, and to reduce their elastic modulus, allowing them to deform more extensively in contact.