Cosmetics and medical devices are substances or products that are to be applied in direct and prolonged contact with the human skin and mucosae. Consequently they should exhibit no or very low toxicity to the cells that compose the skin or the epithelia.

For this reason, it is necessary to evaluate their cytotoxicity and understand their possible irritant effect. From the analysis of both in vivo data (human volunteers) and in vitro results, it seems to emerge a good correlation both for the effect of the tested substance and for prediction of skin irritation (Osborne et al., 1994). In vitro cell survival can be evaluated by MTT test based on mitochondria activity. The MTT assay, originally developed by Mosman in 1983, is simple, accurate and gives reproducible results (Mosman, 1983).

The key component is 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide or MTT. The principle of the MTT assay is that the decrease in the number of viable cells corresponds to the reduction in mitochondrial activity. The mitochondrial activity of the cells is reflected by the conversion of the MTT tetrazolium salt into formazan crystals that are insoluble in water solution and can be solubilized in isopropanol for homogenous spectrophotometric measurement. Thus, any variation in the cell number can be detected by measuring formazan optical density (OD) at 570 nm.

The MTT assay is suitable for the measurement of the cytotoxicity of ingredients or finished cosmetic products or medical devices: the decrease in cell number reflects cell growth inhibition and the substance or product sensitivity is then usually specified as the concentration required to achieve 50% growth inhibition as compared to the growth of the untreated control (50% inhibitory concentration, IC50) (Vinardell et al., 2008). Sanchez and collegues (Sanchez et al., 2006) proposed also the use of sodium lauryl sulphate (SLS), an anionic detergent, as a positive control, as it is known to be a strong skin irritant in the same experimental model here reported.