The viscous limit for Soft Mist Inhalers; an in-silico study.

Abstract

When increasing the dose delivered by means of an SMI, the concentration of the API in the solvent is increased and thus the viscosity of the solution is increased as well. In this work the relation between the viscosity and density of the solvent and the pressure applied by the SMI is investigated. In order to generate jets with sufficient kinetic energy that create sufficiently small droplets, a lower limit for the Reynolds number of 250 is proposed. Below this limit the pressure needs to increase proportionally to the viscosity. Under the assumption that an SMI can apply a pressure of maximally 400 bar, maximal mole fractions of PEG 400 and water mixtures of 3.4 %, 4.1 % and 4.6 % for 25 °C, 30 °C and 35 °C respectively were derived to aid interpretation of these values. Key Message The relation of the viscosity and density of a solvent and the pressure applied by an SMI is investigated. This allows to estimate a viscosity limit and hence the maximal API, excipient or co-solvent concentration.