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Inhalation OCTOBER2016 13 zles in a metal plate or as holes drilled by a laser through a metal plate. Hole size can be adjusted, as can distance, to form an array that can produce the desired droplet size dis- tribution. However, the principle of generation differs from that of high pressure jet technology. In vibrating plate technology, jet filament production and break up into droplets is performed by a vibrating piezo member bonded to the metal mesh. The preferable configuration is circular and vibration is often in the kilohertz frequency range (Figure 12). A number of versions have been envi- sioned, and to some extent developed, where the piezo ele- ment has been bonded to a silica substrate with etched holes. 28 The technology is affected by viscosity and surface tension, as mentioned previously for conventional and enhanced ultrasonic nebulizers. 29 Piezo horn technology Piezo horn nebulization technology uses the same mesh types as seen in vibrating plate technology: electroformed nozzles or laser-drilled holes in a metal plate. Liquid is placed on the back of the mesh plate and an acoustic horn made of piezo material is placed in contact with the liquid. The horn is vibrated in the kilohertz frequency range, ejecting liquid from the upper side of the mesh (Figure 13). The configuration is also preferably circular. This technology has been implemented in conventional and small handheld nebulizers. 30 Additional technology candidates Other technologies have been explored or developed but have not quite become viable concepts for inhalation therapy, either due to limitations in technology or lack of suitable formulations for use with such technologies. Some of these include electro-hydrodynamic spray tech- nology, condensation liquid aerosol technology and ink jet technology. Looking back, looking ahead This article has discussed the basics of conventional nebu- lization technologies that are available for inhalation ther- apy. 13, 14 It has also presented a number of novel app- roaches that have been implemented or may have the best likelihood for future use in miniaturization of conven- tional nebulizers and development of handheld, single or multiple dose nebulization systems. Looking back to the early 1850s when the first nebulizer system was invented and at the numerous design improvements and new nebulized medications intro- duced since, it can certainly be said that nebulizer devel- opment is still ongoing 9 and holds promise for the future. References 1. Gandevia B. Historical review of the use of parasympa- tholytic agents in the treatment of respiratory disorders. Postgrad Med J; 51(Suppl 7):13-20, 1975. Piezo horn technology Figure 13 Colliding jet technology Figure 10 High pressure spray technology Figure 11 Vibrating plate technology Figure 12 Pressure Vibrating x KHz Vibrating x KHz Plunger Plunger Piezo horn element Piezo element