Friction Dynamics of Human Skin Treated using Polymer Aqueous Solutions

Herein, we evaluated friction dynamics of human skin treated with polyacrylic acid aqueous solutions or gel creams using a sinusoidal motion friction evaluation system to demonstrate the effect of treatment with polymer aqueous solutions on human skin. A polymer aqueous solution or gel cream was applied to the inner forearms of 10 subjects to evaluate temporal changes in friction force under sinusoidal motion. Water content, skin viscoelasticity, and transepidermal water loss were also simultaneously measured to determine the effects on skin conditions. When human skin was treated with the polymer aqueous solution, the friction coefficient immediately after treatment was 0.69-0.99 and the delay time δ, a normalized parameter of the time difference in the delayed response of friction to the movement of the contact probe divided by the friction time T ₀ for one round trip, was 0.171-0.179, which was greater than that of untreated skin. This increase was caused by the swelling and softening of the stratum corneum caused by the penetration of water in the polymer aqueous solution, which increased true contact area between the skin and contact probe. A significant difference was observed in the friction coefficient of the skin immediately after treatment with different polymer aqueous solutions. Among polymers (P1-P4), P4, which has a low-salt resistance and low yield stress, had the lowest friction coefficient because of collapsing of the polymer network structures by shearing and reduced viscosity owing to salts on human skin. The skin treated with a gel cream also exhibited a greater friction coefficient than the untreated skin immediately after treatment and 90 min later. This phenomenon can be caused by the occlusive effect of the oil in the gel cream.