Metered-dose Inhaler - Propellants

Propellants

One of the most crucial components of a MDI is its propellant. The propellant provides the force to generate the aerosol cloud and is also the medium in which the active component must be suspended or dissolved. Propellants in MDIs typically make up more than 99% of the delivered dose, so it is the properties of the propellant that dominate more than any other individual factor. This is often overlooked in literature and in industry because so few propellants are used and their contribution is often taken for granted. Suitable propellants must pass a stringent set of criteria, they must:

• have a boiling point in the range -100 to +30°C
• have a density of approximately 1.2 to 1.5 g cm−3 (approximately that of the drug to be suspended or dissolved)
• have a vapour pressure of 40 to 80 psig
• have no toxicity to the patient
• be non flammable
• be able to dissolve common additives. Active ingredients should be either fully soluble or fully insoluble.