How cosmetic tightening products modulate the biomechanics and morphology of human skin.

The active and passive mechanical behavior of a cosmetic tightening product for skin anti-aging is investigated based on a wide range of in vivo and in vitro measurements. The experimental data are used to inform a numerical model of the attained cosmetic effect, which is then implemented in a commercial finite-element framework and used to analyze the mechanisms that regulate the biomechanical interaction between the native tissue and the tightening film. Such a film reduces wrinkles and enhances skin consistency by increasing its stiffness by 48-107% and reducing inelastic, non-recoverable deformations (-47%). The substrate deformability influences both the extent of tightening and the reduction of wrinkle amplitude. The present findings allow, for the first time, to rationalize the mechanisms of action of cosmetic products with a tightening action and provide quantitative evidence for further optimization of this fascinating class of biomaterials.

Analysing the movement of a hair swatch using video and image analysis: a promising technique for exploring the dynamic properties of hair.

OBJECTIVE: The aim of this paper is to present a new instrumental evaluation method of hair movement in order to propose new criteria in the performances of hair products. The in vitro evaluation of hair swatches movement allows us to better understand the phenomena that appear when various hair care and styling technologies are applied upon the mechanisms of hair movements. METHODS: The method consists of a video acquisition system with fast cameras (100 frames/second). The swatch is placed on a motor drive and moved with a back and forth movement. The frequency and amplitude are set by the operator. Facing each camera, a backlight provides high contrast to capture all the fibres of the swatch. A software was specifically developed to analyse the video recordings and provide the physical parameters that characterize the movement. Accordingly, it is possible to follow the surface and shape of the swatch during movement from the 'skeleton' of the swatch and to study the contraction and expansion phases of the swatch(es) during the movement together with 'the envelope' of the swatch. RESULTS: Results show that applying various hair treatments modifies the behaviour of the swatch through the hair shapes contraction or expansion induced by the oscillating movement. Expansion of the swatch suggests that the interactions between hair fibres (adhesions or frictions) play an important role, leading to an 'individualization' of hair swatch (more 'space' between fibres) or to a 'wider' movement related to the increase of the total surface developed by the swatch. As an example, the application of a highly conditioning formula on a hair swatch largely decreases its expansion by a two-fold factor during movements and increases the horizontal amplitude by a two-fold factor, as compared to a commercial bland shampoo. CONCLUSION: In conclusion, this method is complementary to the ones used generally to characterize global visual perceptions (colour, styling, shine). This dynamic component not only opens doors to new qualitative evaluations, linked with sensorial experts and consumers perceptions, but also to new quantitative parameters.