MDIs: physics of aerosol formation

Area of Science:

• Pharmaceutical Sciences
• Aerosol Science and Technology
• Respiratory Drug Delivery

Background:

• Metered dose inhalers (MDIs) generate complex aerosol plumes with rapid changes in droplet size and velocity shortly after actuation.
• The interaction between the MDI aerosol cloud and the upper airway geometry significantly influences oral deposition and lung drug delivery.
• Understanding these dynamics is critical for developing effective in-vitro testing methods.

Purpose of the Study:

• To emphasize the importance of accurately characterizing the initial aerosol cloud properties (droplet size and velocity) from metered dose inhalers.
• To highlight the critical role of inlet port design in aerosol sizing instruments for successful in-vitro testing.
• To underscore the limitations of current in-vitro methods that do not adequately account for initial plume dynamics.

Main Methods:

• Analysis of the dynamic behavior of aerosol clouds from metered dose inhalers.
• Investigation of the interaction between aerosol plume characteristics and oropharyngeal geometry.
• Evaluation of the impact of inlet port design on aerosol characterization in in-vitro test methods.

Main Results:

• Significant changes in droplet size and velocity occur within the initial centimeters of the MDI spray plume.
• Inertial mechanisms primarily control oral deposition, directly linking plume dynamics to lung delivery efficiency.
• Inadequate consideration of initial plume velocity and droplet size distribution in in-vitro tests leads to unreliable data.

Conclusions:

• Effective in-vitro test methods for metered dose inhalers must accurately replicate the initial dynamic aerosol cloud.
• Proper design of inlet ports in aerosol sizing instruments is crucial for meaningful in-vitro comparisons and product control.
• Characterizing residual droplets or using large sampling chambers is insufficient for accurate MDI performance evaluation.