The test substance, which may be radiolabelled, is applied to the surface of a skin sample separating the two chambers of a diffusion cell. The chemical remains on the skin for a specified time under specified conditions, before removal by an appropriate cleansing procedure. The receptor fluid is sampled at time points throughout the experiment and analysed for the test chemical and/or metabolites.
The diffusion cell consists of a donor chamber and a receptor chamber between which the skin ispositioned. The use of a physiologically conducive receptor fluid is preferred although others may also be used provided that they are justified. The precise composition of the receptor fluid should be provided. Adequate solubility of the test chemical in the receptor fluid should be demonstrated so that it does not act as a barrier to absorption. In addition, the receptor fluid should not affect skin preparation integrity. Either epidermal membranes (enzymically, heat or chemically separated) or split thickness skin (typically 200-400 μm thick) prepared with a dermatome, are acceptable. The test substance preparation (e.g., neat, diluted or formulated material containing the testsubstance which is applied to the skin) should be the same (or a realistic surrogate) as that to which humans or other potential target species may be exposed. Any variation from the ‘in-use’ preparation mustbe justified. Under normal conditions of human exposure to chemicals, finite doses are usually encountered. Therefore, an application that mimics human exposure, normally 1-5 mg/cm2 of skin for a solid and up to10 l/cm2 for liquids, should be used. The quantity should be justified by the expected use conditions, thes tudy objectives or physical characteristics of the test preparation. For example, applications to the skinsurface may be infinite, where large volumes per unit area are applied. Normal skin temperature is 32±1°C.
The analysis of receptor fluid, the distribution of the test substance chemical in the test systemand the absorption profile with time, should be presented.