Vertical Diffusion Cell
The Vertical Diffusion Cell (VDC) or Franz Cell is a simple, reproducible test for measuring the drug release from creams, ointments and gels that is rapidly emerging as the apparatus of choice for the in vitro testing of topical semisolid dose forms and skin permeation studies. As well as pharmaceutical applications, the VDC also has value for testing of cosmetics applied to the skin.
Three different designs of VDC are included in USP <1724>, but the basic principle is the same in each case.
The cell comprises two parts (a) the sample holder containing 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. Testing is typically conducted over a period of 6 hours, during which samples of receptor medium are extracted periodically for assay using HPLC or similar technique, with an equal top up of fresh medium keeping the sample bathed in the receptor medium.
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