Ultrasonic Nebulizers
Ultrasonic nebulizers work by applying an alternating electric field to a piezoelectric transducer, which converts the
electrical signal into a periodic mechanical vibration, in contact with the liquid to be nebulized [15]. Ultrasonic
vibrations from the crystal are transmitted to the surface of drug solution where standing waves are formed. Droplets
break free from the crests of these waves and are released as aerosol. The size of droplets produced is inversely
proportional to two thirds of the power of acoustic frequency. Like jet nebulizers, baffles within this nebulizer
remove large droplets and much of the aerosol produced impacts on these, falling back into the drug reservoir [58].
An ultrasonic nebulizer has three components: the power unit, the transducer and a fan. The power unit converts
electrical energy to high-frequency ultrasonic waves at a frequency of 1.3-2.3 megahertz. The power unit also
controls the amplitude of the ultrasonic waves. The transducer vibrates at the frequency of the ultrasonic waves
applied to it (piezoelectric effect). The conversion of ultrasonic energy to mechanical energy by the transducer
produces heat, which is absorbed by the solution over the transducer. A fan is used to deliver the aerosol produced
by ultrasonic nebulizer to the patient, or the aerosol is evacuated from the nebulization chamber by inspiratory flow
of the patient. Small-volume ultrasonic nebulizers are commercially available for delivery of inhaled
bronchodilators and large-volume ultrasonic nebulizers are used to deliver inhaled antibiotics in patients with CF
(e.g. tobramycin) [59-69]. The need for cleaning, inhalation of doses over multiple breaths, and variability in aerosol
generation performance is common to ultrasonic and jet nebulizers [21]. The working mechanism of an ultrasonic
nebulizer is shown in figure 2.