Jet nebulisers for pulmonary drug delivery

Nebulisers are widely used clinically to produce aerosols for a range of applications. This paper reviews the many factors which determine the particle size of the aerosol and drug output and describes their potential usefulness for novel drug delivery. There are numerous commercially available nebulisers, and their design is an important factor governing aerosol size and fluid output. Recent designs have included developments to reduce the proportion of drug lost during exhalation with traditional continuous output nebulisers. The rate of gas flow driving atomization is a major determinant of aerosol size; there being an inverse relationship between droplet size and flow rate, due to the increased shearing forces at higher flow rates. Although droplet size is largely independent of fill volume, the proportion of available drug increases with increased fill volumes, since some fluid is invariably retained within the nebulisation chamber at the end of atomization. During use the temperature of the fluid within the nebuliser significantly decreases. This may result in precipitation of poorly soluble drugs and produce variability in droplet size due to changes in the physicochemical properties of nebuliser fluids. Mean aerosol size is inversely proportional to viscosity. However, although high viscosity fluids produce small droplets,they require longer to nebulise to dryness and are retained to a greater extent. Reducing the surface tension of fluids tends to produce aerosols of smaller size. Thus, the size and dose of aerosol available for inhalation by a patient is a complex function of all these factors, whilst the dose inhaled and deposited in the airways is highly dependent on patient-related factors.