Moritz Fleischhauer1,2, Peter Langguth2 & Herbert Wachtel1,2
1 Boehringer Ingelheim Pharma GmbH & Co. KG, Binger Straße 173, Ingelheim am Rhein 55216, Germany
2 Pharmaceutical Technology & Biopharmaceutics, University of Mainz, Staudingerweg 5, Mainz 55128, Germany
Summary
The Respimat® Soft Mist Inhaler (SMI) utilizes an impinging jet mechanism to generate fine aerosol droplets for pulmonary drug delivery. In this study, this dual jet system (impact angle e.g. 45° with respect to the symmetry plane) was replaced by a single jet impaction mechanism, where a high-velocity liquid jet perpendicularly impacts a solid surface. Maximizing the impact effect aims to reduce the required pressure. This in turn would allow for a higher liquid volume to be pumped within a puff.
A Design of Experiments (DoE) was employed to identify key parameters influencing aerosol performance, specifically particle size and residue—the fraction of liquid that fails to atomize. Droplet size was significantly influenced by nozzle diameter, pressure, and impaction distance. For residue, nozzle diameter, pressure, and baffle diameter were key contributors, while impaction distance showed a marginal effect. Within the tested design space, particle sizes in the range of 3.9–5.7 µm were achieved, confirming the generation of inhalable droplets. However, the DoE did not reveal a clear optimum for minimizing residue. Overall residue remained high, limiting aerosolization efficiency.
To better understand the underlying mechanisms, Computational Fluid Dynamics (CFD) simulations were conducted. These revealed that the baffle diameter is too large, causing deceleration of the liquid film at the edge of the baffle. This results in a large droplet which is not atomized.
Based on these findings, a follow-up DoE is planned, focusing on reducing the baffle diameter to improve atomization efficiency and potentially identify an optimum for minimizing residue.