Novel techniques for characterising inhalers

A.P. McKiernan

Background: While the initial plume formation and expansion during pressurised metered dose inhaler (pMDI) dose release is considered important to drug transport to the lungs, it is not well understood, in part due to the transient nature of the event and the difficulties in accessing the metering chamber. Plume velocity is an important measure of inhaler device performance as large or very fast drug particles tend to deposit in the oropharyngeal region due to the sharp directional flow change.
Methods: Schlieren imaging is an optical technique that is sensitive to refractive index gradients which are often present in pMDI plumes due to gas density variations. We have developed a Schlieren setup and have observed plume expansion up to 400 mm from commercial pMDIs for various actuator/canister/formulation combinations. We have also used phase contrast X-ray imaging at a synchrotron to investigate the stem/sump/orifice of the same pMDIs and a dry powder inhaler (DPI) during dose release.
Results: The plume leading edge dynamics are well described by a high-speed drag model. It was observed that HFA 134a drives faster plumes than HFA 227ea, particularly as the orifice diameter increases. Propellant cavitation behaviour appears dependent on the canister/formulation while the dose duration appears additionally dependent on the actuator design. Individual carrier particles were observed to travel counter to the general airflow inside a DPI.
Conclusions: High speed Schlieren imaging and phase contrast X-ray imaging are suitable techniques to evaluate formulation or actuator modifications, metering chamber behaviour or tooling modifications during inhaler device development.

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