The fluid mechanics of medical sprays: physics-informed design of low-emissions metered dose inhalers*

Pressurised metered dose inhalers (PMDIs) have been used for over 60 years to treat a range of respiratory conditions such as asthma. The microscale inhaled aerosol particles produced by these devices are generated through the flash-evaporation of a volatile propellant. Originally these were chlorofluorocarbons. They were replaced with hydrofluorocarbons (HFCs) by the early 2000s. The Kigali amendment to the Montreal Protocol is now phasing down the use of HFCs worldwide due to their high global warming potential. New propellants are again required. Given the expansive parameter space for pMDI design (droplet chemistry and nozzle design) and broad use cases, trial and error parametric product design is not cost effective or practical. A more nuanced understanding of the fluid mechanics of the pMDI are required to develop new products. We demonstrate how multiphase Large Eddy Simulations can be combined with synchrotron X-ray diagnostics, droplet evaporation models, laser diffraction and high speed imaging measurements to unravel the complex multi-physics problem of volatile propellant evaporation and guide the selection of new propellants, solvents and nozzle designs. We show that sonic flow phenomena and previously hidden standing evaporation wave in the core of the jet plays a dominant role in the primary atomisation process, and that manipulation of nozzle geometry can be used to offset the lower spray momentum of more environmentally friendly propellants with lower vapour pressures and densities. The outcomes of this research are being used to drive the design of new inhaled pharmaceutical products with 10x to 100x lower CO2-equivalent emissions with equivalent or even enhanced performance.