The physicochemical processes that transform inhalation aerosol between the points of generation and deposition are poorly understood. Knowledge of the time-dependent aerosol properties (size, composition etc.) could improve the efficacy of inhalation therapeutics by guiding the development of improved formulations for active pharmaceutical ingredient (API). In our research on inhalation formulations, we are applying novel aerosol tools to study the dynamical properties of inhalation aerosol, focussing in particular on formulations used in pressurised metered dose inhalers (pMDIs), soft mist inhalers (SMIs) and dry powder inhalers (DPIs). Our objectives are to gain insight into the factors that govern both the capacity and timescale of hygroscopic growth, the dissolution kinetics of crystalline or amorphous aerosol particles when inhaled, and the transport of water to rapidly evaporating volatile droplets generated by pMDI devices.

Novel techniques uniquely developed in Bristol are being used to explore the aerosol microphysics under conditions relevant to aerosol generation and inhalation. More specifically, we are using an electrodynamic balance (EDB), with unique capabilities for isolating single charged aerosol particles in a gas flow and for measuring particle size (<5 ms time-resolution, ±50 nm size accuracy) under conditions of sub- and super-saturated relative humidity. For example, the EDB has been used to measure the hygroscopic response of a wide range of APIs and excipients, investigating both the capacity of aerosol particles to grow at the high humidities and elevated temperatures in the respiratory tract (the thermodynamics, see figure) and how rapidly the size change occurs (the kinetics).