Propellants are the driving force behind aerosol delivery.
They expel the drug and disperse it as a fine mist or foam.
Types of Propellants:
Liquefied Gas Propellants:
Examples: Chlorofluorocarbons (CFCs), Hydrofluorocarbons (HFCs), Hydrocarbons.
Characteristics: These exist in equilibrium between liquid and gas phases inside the container.
Advantages: Provide constant pressure.
Compressed Gas Propellants:
Examples: Nitrogen (N₂), Carbon Dioxide (CO₂), Nitrous Oxide (N₂O).
Characteristics: Stored as compressed gas in the container.
Limitations: Pressure decreases as the product is used.
Selection Criteria:
Safety (non-toxic, non-flammable).
Environmental considerations (avoiding CFCs due to ozone depletion).
Compatibility with the formulation.
Desirable Properties of Propellants
Chemically inert, non-reactive with the drug or container.
Non-toxic and non-irritating.
Acceptable vapor pressure under normal storage conditions.
Stable over the product’s shelf life.