Application of a droplet evaporation model to aerodynamic size measurement of drug aerosols generated by a vibrating mesh nebulizer.
BACKGROUND
Droplet evaporation has been known to bias cascade impactor measurement of aerosols generated by jet nebulizers. Previous work suggests that vibrating mesh nebulizers behave differently from jet nebulizers. Unlike jet nebulizers, vibrating mesh nebulizers do not rely on compressed air to generate droplets. However, entrained air is still required to transport the generated droplets through the cascade impactor during measurement. The mixing of the droplet and entrained air streams, and heat and mass transfer occurring downstream determines the final aerosol size distribution actually measured by the cascade impactor. This study is aimed at quantifying the effect of these factors on droplet size measurements for the case of vibrating mesh nebulizers.


METHODS
A simple droplet evaporation model has been applied to investigate aerodynamic size measurement of drug aerosol droplets produced by a proprietary vibrating mesh nebulizer. The droplet size measurement system used in this study is the Next Generation Impactor (NGI) cascade impactor. Comparison of modeling results with experiment indicates that droplet evaporation remains a significant effect when sizing aerosol generated by a vibrating mesh nebulizer.


CONCLUSIONS
Results from the droplet evaporation model shows that the mass median aerodynamic diameter (MMAD) measured by the NGI is strongly influenced not only by the initial droplet size, but also by factors such as the temperature and humidity of entrained air, the nebulizer output rate, and the entrained air flow rate. The modeling and experimental results indicate that the influence of these variables on size measurements may be reduced significantly by refrigerating the impactor down to 5°C prior to measurement. The same data also support the conclusion that for the case of nebulized drug solutions, laser diffraction spectrometry provides a meaningful droplet sizing approach, that is simpler and less susceptible to such droplet evaporation artifacts.
Rao
Nagaraja
N
Novartis Pharmaceuticals Corporation, San Carlos, California 94577, USA. raj.rao@novartis.com
Kadrichu
Nani
N
Ament
Brian
B
D000336:Aerosols; D016503:Drug Delivery Systems; D006813:Humidity; D008956:Models, Chemical; D009330:Nebulizers and Vaporizers; D010316:Particle Size; D004364:Pharmaceutical Preparations / Q000008:administration & dosage; D012034:Refrigeration; D013057:Spectrum Analysis / Q000379:methods; D013696:Temperature
D000336:Aerosols; D004364:Pharmaceutical Preparations