An In-situ Visualization Experimental Study And Analysis Of Friction Behavior Of Human Skin

In recent years,research on skin tribology has attracted widespread attention.The research on the friction properties and friction behavior of human skin and relevant applications has become a hot spot in the fields of biomechanics and tribology,cosmetics,clinical and robot system.In general,such investigations were carried out from either theoretical/ computational aspect or experimental aspect based on various models by many scholars.However,due to the inherent low modulus of elasticity of the skin and high sensitivity to external stimuli,the skin exhibits significant nonlinear characteristics after being loaded.Skin is unlike traditional engineering materials in that it doesn’t follow Amontons’ law and friction varies with contact area and load.In this paper,an in-situ visualization friction experiment is performed in vivo on human skin by using the specially-developed frictional indentation experimental device and curved imaging technology.The friction and deformation behavior of the skin were studied.The device can not only measure the normal force,the lateral force,the normal displacement and the lateral displacement during the friction process,but also evolution images of the skin-indenter contact morphology can be obtained in real time.It was found that the shape of contact area during lateral slippage changed into a special-shape whose geometric characteristics can be regarded as the superposition of two ellipses.Therefore,a double ellipse model is proposed to describe this geometric shape.Moreover,asymmetric deformation of the skin was observed in the friction test because the friction and lateral displacement caused the skin on the posterior and anterior side of the indenter to deform differently to form “skin-uplift”.Since skin-uplift is directly related to lateral resistance,an optimized uplift resistance model which takes the double ellipse model and Neo-Hookean hyperelastic theory into account is proposed based on Greenwood model.The geometric parameters involved in the modeling come from the real measurement of the contact area.In terms of the geometric description of the deformation is better than the Greenwood model;and the model can be degraded into Greenwood model in small deformation.Based on the uplift resistance model,a friction mechanism separation model was proposed after considering the skin uplift effect and the adhesion friction and deformation friction have been separated for different ratios of indentation depth to probe radius.The results show that the friction properties of skin are different under different depth ranges of indentation.Besides,although the deformation friction plays a secondary role in total friction,it is positively correlated with the ratio of indentation depth to probe radius.In order to verify the application range and reliability of this friction research method and friction theory,another type of friction experiments was carried out on the surface of silica gel,and found that the model is also applicable to any other soft material which satisfy hyperelasticity and exhibit asymmetric deformation during friction.Furthermore,in order to study the effect of skin-uplift on the surrounding skin,two types of simulation experiments: static friction and dynamic friction were carried out.By using the 3D digital image correlation technique,the three-dimensional displacement field of the skin area in the front side of the probe was studied.It was found that these displacement fields are characterized by stratification.The stratification of these displacement fields lays the foundation for further study of the continuity and transmission characteristics of skin deformation.