Influence of Surface Roughness, Material and Climate Conditions on the Friction of Human Skin

The gliding comfort and performance of personal care and wellness products is strongly influenced by the sliding friction behaviour of human skin. In the open literature, most of the results on skin friction are related to the performance of cosmetic products or to the slip and grip properties of surfaces. Experiments were usually carried out on the forearm or the fingertips. The influence of the surface roughness and the material of engineering surfaces have received little attention so far, especially not in sliding contact with the skin of the cheek, or under different climate conditions. A custom-built rotating ring device was used to study the influence of the probe surface roughness (R (a) = 0.1-10 mu m), the probe material (metals, plastics), the climate conditions (21-29 A degrees C, 37-92% RH) and skin hydration on the frictional behaviour of the skin on the cheek and the forearm. The amplitude of the surface roughness has a dominant influence on the friction behaviour: the smoother the surface, the higher the friction. Differences can be as large as a factor 5-10, especially in the range R (a) < 1 mu m. The probe material itself has no significant influence; except for PFTE which reduces the friction by approximately 25% compared to the other materials. In a humid climate, the skin becomes hydrated and the friction is twice as high as in a dry climate. The effect of skin hydration is smaller on the cheek than on the forearm, probably due to the presence of beard stubbles. A simple friction model for human skin is presented, based on adhesion friction, contact mechanics of rough surfaces and the interfacial shear stress of thin organic films. The model explains the effects of the probe surface roughness and skin compliance. Quantitative application of the model indicates that the biomechanical indentation and shearing behaviour of the stratum corneum is influenced by the same physical process, i.e. the intercellular bonding strength of the corneocytes.