Aerosols in the Health Care Field

:: Section 4

Aerosolization of Dry Powder

Because fine powder is cohesive and does not flow well, most passive dry powder dispensing systems use a carrier substance (lactose or glucose) mixed into the drug to allow the drug powder to flow out of the device more readily. The powder is available as either micronized (< 5 µm in diameter) drug particles bound into loose aggregates, or micronized drug particles that are loosely bound to large (> 30 µm in diameter) carrier particles. The strong cohesive forces make micronized particles stick to each other and to most surfaces. The addition of the larger carrier particles reduces the adhesive forces in the micronized drug powder. Thus, separation of the aggregates into individual respirable particles (disaggregation) can be achieved more readily. That's why carriers aid the flow of the drug powder from the device. The energy provided by the patient's inhalation is used to strip the bound drug particles from the carrier. Although some people may have reactions to lactose or glucose, cases appear to be fewer than reactions to the surfactants and propellants used in pressurized MDIs. The amount of the carrier is typically much greater than that of the drug. In some cases, it can be 98% or more of the weight per inhaled dose.

Reference

Dhand R. and Fink J. : Dry Powder Inhalers, Respiratory Care 44: 940, 1999.