Creams, ointments, lotions and gels are topical drugs normally described as semisolid dosage forms. Unlike transdermal drug systems which are applied to the skin for systemic effects (the drug penetrates the skin whereupon it is then absorbed into the drug stream), topical drugs are applied to the skin for local action principally to counter dermatological problems.

Current USP thinking suggests two types of test to characterise semisolids, product quality tests (USP Chapter <3>) and product performance tests (USP Chapter <1724>).

As regards product performance, USP Chapter <1724> SemiSolid Products – Performance Tests suggests 3 apparatuses that may be used to determine drug release from semisolids, the Vertical Diffusion Cell, the Immersion Cell and the Flow Through Cell (Apparatus 4).

Of the apparatuses specified, the Vertical Diffusion Cell (sometimes called the “Franz Cell” after the name of its originator, Dr. T. J. Franz) is rapidly emerging as the preferred choice because of its simplicity and reproducibility, although the Immersion Cell remains in widespread use.

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