Evaluate the efficacy of cosmetic products through the microrheological monitoring of ex vivo living skin

The development of innovative formulations, using brand new ingredients that are not extracted from the petrochemical industry or associating specific chemicals to get benefit from unusual sensory properties, is quite tough in cosmetics. Indeed, safety evaluation is mandatory before testing product on human panels leading to a real dilemma for the formula developers as final efficacy, sensory and safety profiles also depend on composition optimization. As a consequence, the resort to characterization techniques that are alternatives to in vivo ones (both animal and human) and in vitro ones (that are too far away from physiological conditions) is at stake. The development of a new ex vivo biosensor using an ultrasonic microrheological technique and that allows a multiscale characterization of living skin viscoelasticity is a solution to answer this problematic. The biosensor is based on a kept-in-life human skin explant that is instrumented with a thickness shear mode quartz (TSM quartz). The ultrasonic microrheology has already been used as a relevant technique to characterize complex fluids like silica gels, forming yoghurts, or cosmetic products. Here, as the skin can be modelled by a multilayer viscoelastic material. The idea is to study the skin-product interactions through the skin viscoelasticity evolution before and after the application of a topic formulation. First experiments have been performed on frozen explants to validate the experimental bench with the dedicated fluidic system required to provide survival medium to the skin. The evolution of the complex viscoelastic modulus and alpha parameter can be linked to the structural changes of the skin layers. Dehydration phenomenon is characterized by the increase of both elastic and viscous modulus. As a consequence the impact of cosmetic creams on the skin can be evaluated by the evolution of the viscoelastic parameters. The tested products are provided by SEPPIC and the results will be compared with ones obtained with organoleptic and viscometric measurements.