Vertical Diffusion Cells

VERTICAL DIFFUSION CELL.  

Creams, ointments, lotions and gels are topical drugs normally described as semisolid dosage forms. Unlike transdermal drug systems (see Note) which are applied to the skin for systemic effects (the drug penetrates the skin whereupon it is then absorbed into the blood 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 (the proposed USP Chapter <1724>).

The Vertical Diffusion Cell or Franz Cell is a simple, reproducible test for measuring the drug release from creams, ointments and gels. It is rapidly emerging as the apparatus of choice for the in vitro testing of drug release of topical semisolid dosage forms. The cell comprises two parts (a) the sample holder containing either 250 mg or 425 mg of the sample to be tested and (b) the reservoir of the diffusion cell itself containing the receptor medium. The two parts are separated by a membrane designed to act as a conduit for diffusion to take place and which serves to contain the test sample whilst ensuring that it remains in contact with the receptor medium. The receptor temperature is usually set to 32 degrees C to approximate normal skin conditions. Normally, no fewer than 6 samples are taken over a 6 hour period, say, 0.5, 1, 2, 4, 5 and 6 hours and analysed using HPLC or similar analytical technique.