Detailed evaluation of drug powder deposition in swirl-type dry powder inhalers

Abstract

Based on numerical simulations by the Euler/Lagrange approach in combination with the RNG-k-ε turbulence model flow field and particle transport through inhaler devices are studied. Since one major problem in inhaler performance and drug powder release is their deposition on the inhaler walls this issue is analysed in more detail. For computing fine particle motion through the stationary flow field all relevant particle forces and turbulent dispersion are considered. A potential deposition of fine particles is governed by two elementary processes, namely the transport of the fine drug particles towards the walls and their possible wall adhesion. Wall impingement of particles is induced by fluid transport and particle inertia as well as turbulence and for very fine particles also Brownian random motion. The possibility of particle sticking on the inhaler walls is modelled by considering the van der Waals adhesion force. Based on an energy balance for an oblique elastic-plastic wall impingement, a critical impact velocity is derived below which the particles are depositing. This approach accounts for the mechanical properties of wall and particle materials. First, the performance of different deposition models was analysed and the results were compared with literature experiments. The local and global deposition of particles in the considered swirl-type inhalers is determined in dependence of particle size, stationary flow rates (between15 l/min and 90 l/min) and particle release location. The range of particle sizes considered is between 0.05 μm and 10 μm, representing a typical spectrum for fine drug powders in use. Additionally, the spatial distribution of the deposits within the two inhaler types is evaluated for allowing inhaler optimisation and the predicted emitted fraction of fine particles is compared with experimental observations.