SKIN FRICTION COEFFICIENT SKIN
, in order to measure in vivo skin friction for different anatomical sites, experiments were carried out using a multiaxial load cell that allows the simultaneous measurement of the normal and tangential forces. In a recent study executed by Ramalho et al. Different experimental techniques have been reported, in order to determine the mechanical properties of skin in vivo, based on measurements of torsion, suction, extensibility or ultrasound. shows that positive feelings are generated when a finger rubs over a rough surface that is smoother than a fingertip negative feelings are generated by coarser surfaces. The main tribological result of aother study performed by Barnes et al. The earliest experimental investigations suggested soft and smooth materials as pleasant, those that were stiff, rough or coarse as unpleasant. , a threshold exists for stress at the nociceptor (sensory receptor) located below the skin surface. Deformation of the skin and/or high coefficients of friction during skin product interactions could cause discomfort, since the subsurface stress and strains within the skin are influenced by skin friction, and according to Xu et al. The analysis of comfort and discomfort in skin product interactions clearly reveals the relative importance of skin friction, since the exploratory procedure that is used to touch a surface, resulting in a somatosensory response/touch perception is a similar to experimentally-determining friction in a reciprocating test. This topic is very interesting for the design of surfaces with a pre-defined tactile feel (smooth, soft) that increase the customer’s satisfaction, but also for developments and applications in robotics. Research topics, like tactile perception and haptics in relation to skin tribology, are largely unstudied and poorly understood. There are, however, still open research questions: the transition from dry to moist skin conditions has so far not been systematically investigated. Other studies investigated gender, age and anatomical sites. Intermediate layers, such as cosmetic applied substances, sweat and sebum excreted from skin into the tribo-interface, also influence the friction behavior of human skin. It was found in recent studies that the friction of skin strongly depends on the operating conditions, like moisture, as well as on the presence of water in the interface between skin (in the stratum corneum (SC) see Figure 1) and a contacting surface, with dry skin showing relatively low coefficients of friction, while moist or wet skin is characterized by significantly higher coefficients of friction. This study will investigate how skin friction (different anatomical regions) varies, rubbing against different types of contacting materials ( i.e., fabrics for medical use) under different contact conditions and their relationship in the formation and prevention of decubitus ulcers. The coefficient of friction of textiles against skin is mainly influenced by: the nature of the textile, skin moisture content and ambient humidity. The risk of developing decubitus ulcers can be predicted by using the “Braden Scale for Predicting Pressure Ulcer Risk” that was developed in 1987 and contains six areas of risk (cognitive-perceptual, immobility, inactivity, moisture, nutrition, friction/shear), although there are limitations to the use of such tools. Decubitus ulcers are one of the most frequently-reported iatrogenic injuries in developed countries. This factor is of particular importance in bedridden patients, since they are not moving about or are confined to wheelchairs. Besides that, friction between the human skin and textiles is a critical factor in the formation of skin injuries, which are caused if the loads and shear forces are high enough and/or over long periods of time. Knowledge of the tribology of human skin is essential to improve and optimize surfaces and materials in contact with the skin.
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