Seeing is believing: using optical diagnostics to investigate MDI sprays and DPI fluidization

Inhaler device modifications directly influence aerosol generation mechanisms, so the development of inhalation products could be advanced effectively if the relevant processes can be studied in situ. However, the ability to visualize, in real time, atomization in a metered dose inhaler (MDI) or fluidization in a dry powder inhaler (DPI) remains challenging. This article reviews high-speed imaging, fluorescence imaging, particle image velocimetry (PIV) and phase-doppler anemometry (PDA) and demonstrates how these techniques can be used to characterize aerosol clouds whilst they are being generated and reveal the nature of aerosol production processes as they unfold inside inhaler devices. The different optical diagnostics generate complementary data including aerosol droplet size, velocity and overall aerosol/spray attributes. Understanding the details of transient events during metering of a drug dose into the inhaled airflow at the appropriate temporal and spatial resolution creates opportunities for targeted interventions to solve problems e.g. excessive device deposition. Moreover, the images generated by optical techniques represent a readily accessible form of information which supports multi-disciplinary collaboration, so, when using optical diagnostics, we can legitimately claim that “Seeing is Believing”.