Aerosols in the Health Care Field

:: Section 5

USN

Liquid aerosol can also be produced by methods other than high pressure gas.  For example, an USN (Ultrasonic Nebulizer) uses a piezoelectric crystal to produce an aerosol. The crystal tranducer converts an electrical signal into high-frequency (1.2 to 2.4 MHz) acoustic vibrations. Focused in the liquid above the transducer, the vibrations disrupt the surface and create oscillation waves. With a high frequency and strong amplitude, the oscillation waves will produce a geyser of droplets that break free as fine aerosol particles. Baffles are used to retain the larger droplets.  Fine mist aerosol in the respiratory range is then delivered to the patient.

Compared to conventional jet nebulizers, USNs can produce higher aerosol outputs (0.2-0.5 ml/min) and higher aerosol densities. The output can be adjusted by tuning the amplitude setting: the greater the signal amplitude, the greater is the nebulizer output. Regarding particle size, it is inversely proportional to the frequency of vibrations. As particle size is critical to the delivery, frequency usually is device-specific and is not adjustable by users. For example a DeVilbiss Portasonic nebulizer operating at a frequency of 2.2 MHz produces particles with an MMAD of 2.5 µm, whereas the DeVilbiss Pulmosonic nebulizer operating at 1.25 MHz produces particles in the 4- to 6-µm range. Particle size and aerosol density also depend on the source and flow of gas conducting the aerosol to the patient as discussed previously.