GENERATION BY ELECTROSPRAY OF MONODISPERSE WATER DROPLETS FOR TARGETED DRUG-DELIVERY BY INHALATION

An experimental investigation was performed on the feasibility of using an electrospray to produce monodisperse droplets of water in the diameter range 2-10 mum, with the ultimate objective of employing the droplets for targeted delivery of inhaled drugs. Because of the high surface tension of water, the establishment of stable sprays required the use of a sheath flow of CO2 to prevent breakdown in the gas surrounding the spray and its destabilizing consequences on the electrospray performance. With this expedience, stable sprays of water could be operated at flow rates ranging from 5.8 to 42.4 mul min-1, which resulted in the generation of droplets in the diameter range 5-12 mum. Droplet size was controlled primarily by varying the liquid flow rate. An increase in solution conductivity by the addition of a small amount of NaCl showed that this physical variable can also be used to control droplet size and to produce droplets with diameter smaller than 5 mum. Solutions of too large conductivity (> 1.2 x 10(-4) OMEGA-1 cm-1) may, however, produce droplets too small for targeted drug delivery. Since the typical ratio of droplet standard deviation over mean diameter was less than 0.10, the droplets can be considered monodisperse. A prototype delivery system was also developed in which provisions were made for complete or partial neutralization of the droplet charge by corona discharge. A co-flow of air was provided to carry the discharged droplets through a delivery line and to evaluate its effects on the droplet monodispersity. It was found that the droplet size distribution broadens because of size-dependent evaporation effects. However, at typical conditions of inhalation, the effect should not compromise the system efficiency for targeted deposition.