Metered atomisation for respiratory drug delivery
An investigation into the factors affecting the metered atomisation of superheated liquids has been carried out. The investigation was aimed primarily at developing an understanding of the factors which affect the performance of. respiratory drug delivery systems (Suspension Pressurised Metered Dose Inhalers). Initial investigations used a semi-empirical sizing technique, representing the human airways, to identify the major variables (formulation and geometric) which affect the performance of the MDI system. Computer models were developed to describe both continuous and metered discharge from a superheated-liquid aerosol generator. These models were based on the concept of thermal and dynamic equilibrium, but they were improved and extended, to describe metered discharge, by including empirical corrections obtained from continuous discharge experiments. Experimental investigations using 'instrumented inhalers' were used to confirm the validity of the computer model. The experimental investigations encompassed the use of conventional CFC's and the new non-chlorinated propellants 134A and 227. The computer models and droplet correlation function developed during these investigations represent powerful tools for use in the design of both current and future HFC/HFA powered metered dose inhaler delivery systems.