Tribology of skin

Tribological studies of skin provide valuable insight into how the properties of skin are altered with various conditions and chemical treatments. This work reviews the frictional and the electrical properties of the skin surface, with an emphasis on the measurement of the friction coefficient. Many designs have been utilized and past studies have included numerous probe geometries and materials. An advantage of tribological measurements is that they offer a non-invasive way to quantitatively measure skin hydration and health. Skin friction seems to be dependent on several factors such as age, anatomical site, and skin hydration. The design of the measuring instrument and the probe geometry and material also affect the friction measurement. Clinically, it was found that skin with decreased hydration, as with psoriasis or atopic dermatitis, had a reduced friction coefficient and an increased resistance to current flow. Chemical treatments influence skin hydration level and affect the friction coefficient. Application of water and isopropyl alcohol increased and decreased the friction coefficient, respectively. A discussion of emollients and moisturizers is included with an emphasis on describing 'greasiness' and 'stickiness' quantitatively. Tribological measurements are a multi-faceted and non-invasive way to quantify skin hydration and health.