Background/purpose: High molecular weight polymers such as proteins and polysaccharides have been commonly employed in cosmetic practice to induce skin tightness. The effect is perceived by users as an increase in skin tightness (firmness) accompanied by skin smoothing and elimination of wrinkles and lines. The aim of the study was to assess whether high molecular weight synthetic polymers, in the form of simple skin treatment formulations, could modify mechanical properties of natural skin as well as artificial skin models.
Methods: In experiments involving natural skin, the formulations were employed to treat the inner forearm of five panelists, who ranged in age from mid-20s to mid-40s. Indentometric analysis also included rubber skin models, with various viscoelastic characteristics, modified by deposition of thin polymer films on the surface. Polymers such as polyvinylpyrrolidone, polyquaternium-55, polyimide-1, VP/acrylates/lauryl methacrylate copolymer, and sodium alginate were investigated. The effects of the products were quantified by mechanical skin indentation performed at small deformations. Data analysis was performed using the Hertz theory of contact mechanics, which included the calculation of fundamental parameters such as the modulus of elasticity.
Results: The indentometric analysis revealed an increase in the Young's modulae for both artificial and natural skin as a result of treatment with the polymers. The effect was dependent on the amount of deposited product, which was varied in the range from 0.053 to 1.06 mg/cm(2) of polymer. The observed increases in Young's modulae were typically from about 0.7 to 1.4 x 10(4) N/m(2) for untreated skin to 1.4-2.0 x 10(4) N/m(2) for polymer-modified skin, depending on the polymer structure (molecular weight) and the amount of deposition. By the analysis of artificial skin models, it was found that the magnitude of stiffening depends on the mechanical characteristics of the base material. Softer skin models displayed larger increases in Young's modulus after polymer treatment than stiffer skin models. Also, the analysis of skin models suggested that polymeric treatments can lower the viscoelasticity of skin as demonstrated by decreased values of hysteresis loss factors calculated from indentation force vs. penetration depth plots. The performance of the polymers was also shown to be dependent on ambient humidity with the most hygroscopic materials loosing their stiffening effect at high humidity.
Conclusions: The data indicate that high molecular weight polymers can be useful as skin stiffening, firming, or tightening agents. This was demonstrated by indentometric, mechanical analysis of natural skin and artificial skin models, and by determining Young's modulae and viscoelastic parameters.