Test methods for assessing topical formulations

Abstract

Three different techniques have been studied to evaluate the effect of topical formulations on human skin. Measurement of electrical skin impedence as a non-invasive technique for the in-vivo assessment of skin hydration is reassessed, with the aim of evaluating the moisturising effects of various therapeutic treatments. Monitoring of baseline untreated forearm skin suggests wide inter- and intra- subject variations. The equivalent circuit appropriate for modelling the skin has been elucidated. For the electrode configuration commonly used short-circuiting of the electrodes occurs during product assessment. The performance of laser Doppler flowmetry for monitoring cutaneous vasodilation has been evaluated using various nicotinic acid esters as model compounds. Results indicate that the time taken for the nicotinates to elicit a response by the flowmeter, is the most reproducible parameter. This response varies with the concentration of nicotinate applied. The importance of the physicochemical properties of the products is emphasized. The final technique examined here has been developed to assess the irritation potential of surfactants.The concentration of surfactant required to elicit human red blood cell lysis has been used as an index for measuring surfactant irritation and the results compared those obtained by human patch and rabbit eye tests. For a series of surfactants and surfactant mixtures containing different number of moles of ethylene oxide, the results suggest, the higher the concentration required to induce haemolysis, the lower will be the irritation.