Tribological Behaviour Of Skin Under Reciprocating Sliding

In everyday life, human skin often rubs on other external surfaces, which has caused many friction problems. Understanding the tribology property of the skin would help to develop living and working implements and biomedical devices, avoid frictional trauma and improve comfortness of the skin.Normal healthy human skin, scar skin and rabbit skin have been tested in vivo under ball-on-flat and flat-on-flat contacts using an UMT- II multi-specimen Biomedical Micro-Tribometer and a homemade ST-1 reciprocal sliding skin friction testing apparatus. The detailed observations and analysis of the skin surface profile, histological structure and physiological response were conducted by using laser confocal scanning microscope (LCSM), optical microscope (Leica, DM RX) and infrared thermal imagers (ThermaCAM^TM30). The friction behaviour, trauma mechanisms and comfortless sensations were investigated by means of tribology, biology, histology and biomechanics. Main conclusions are drawn as follows:1. Three kinds of F_t-D curve: the quasi-closed, elliptic and parallelepipedic cycles have been obtained under different experimental conditions, which were related with the stress-strain law of the human skin. The friction behaviour of skin consisted of three friction regimes: relative sliding regime, intermediate friction regime and sticking regime depending upon three kinds of F_t-D curve. The accommodation of elastic deformation by the skin appeared to be important at the high normal force or low imposed displacement. The friction coefficient increased as the imposed displacement increased or the normal force diminished. The relative sliding zone extended to the intermediate zone and meanwhile, the intermediate zone extended to the sticking zone when the frequency increased and the imposed displacement and normal force remained the same. The friction coefficient increased as the frequency increased in the intermediate friction regime, and had no remarkable change in the relative sliding and sticking regimes. An evaluating indicator of comfortness sensations of the skin concerning the pain, drag and heat has been proposed and investigated. The volunteer felt that it was the most discomfort at the intermediate friction regime from sticking to relative sliding regime. Furthermore, the physiological responses of elastic deformation, friction mark, rubbing heat and traumas occurred on the skin during testing.2. The friction conditions showed remarkable effects on the skin traumas grade and pathological reactions. The inflammatory reactions of the rabbit subcutaneous traumatic tissue caused by reciprocating friction on rabbit skin enhanced with increasing normal load and reciprocal sliding frequency, which aggravated the traumas on the rabbit skin. The friction coefficients of rabbit skin decreased with increasing normal load and reciprocal sliding frequency. In every process of testing, the friction coefficients decreased with frictional time until reaching lubricative stabilization that the extravasation participated.3. The natural differences of the body had different effects on the skin frictional properties. The skin frictional properties were strongly dependent on the age. The differences of friction coefficients at the different anatomical regions became negligible with the increasing of age. The friction coefficients of different ages at the same anatomical regions had significant differences. The skin frictional properties of different gender at the same anatomical region and age have no significant differences. The artifical perspiration had an effect on the friction behaviour of skin by modifying the physicochemical properties of skin surface. The friction behaviours transited from sticking regime to relative sliding regime in the process of the perspiration drying out. The friction coefficient changed from small to peak value, then decreased until reaching stabile value.4. The tribology properties and comfortless sensations differed remarkably between scar skin of residual limb, healthy skin and wearing prosthetic socket skin. The friction coefficient of the scar skin was bigger than the two other skins. The F_t-D curve of the scar skin changed to parallelepipedic cycle and the friction behaviour had intermittent relative slide in the intermediate friction regime and sticking regime of the healthy skin, which resulted in the friction coefficients fluctuate markedly during testing. The scar and healthy skins were sensitive to the comfortless sensations induced by rubbing. The wearing prosthetic socket skin was tolerant to the comfortless sensations. The perspiration caused more comfortless sensations to the volunteers comparing with the dry prosthetic socket condition.