Stable foams are semisolid preparations that entrap air upon application to form a lightweight, flexible matrix with a large surface area of the liquid.
Foams
Stable
foams are semisolid preparations that entrap air upon application to form a
lightweight, flexible matrix with a large surface area of the liquid. Foams are
sometimes used for topical application to areas that are otherwise difficult to
reach, such as hairy scalp, or on sensitive skin, such as in acne.
For
example, Luxiq® aerosol foam is a topical anti-inflammatory cor-ticosteroid
formulation that contains 0.12% w/w betamethasone valerate in a thermolabile
hydroethanolic foam vehicle. This foam vehicle consists of ethanol (~60%),
cetyl alcohol, stearic acid, polysorbate 80, potassium citrate, propylene
glycol, purified water, and cetyl alcohol. It is pressur-ized with a
hydrocarbon (propane/butane) propellant. The foam melts upon contact with warm
skin and is intended for application to the scalp. Similarly, clobetasol 0.05%
foam is an anti-inflammatory corticosteroid formulation intended for
application to the scalp. Clindamycin phosphate foam 1% is a topical antibiotic
preparation for use in acne.
Foams
typically contain a hydrocarbon propellant in the packaging container to
pressurize the drug solution. The drug is dissolved in a low boiling point
vehicle, such as the one containing a high proportion of etha-nol, which also
has a surfactant and a base to dissolve the drug. The vehicle may also contain
preservatives and buffering agents. Evaporation of etha-nol upon aerosolization
leads to expansion of liquid droplets and formation of foam by entrapment of
air.
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