In Vivo Mechanical Characterization Of Human Facial Skin Based On Curved Surface Imaging And Indentation Techniques

Facial skin is one of the most important organs of the human body which is mainly composed of epidermal,dermis and hypodermis,and its internal structure is quite complex.The mechanical properties of human facial skin are of considerable importance for surgery,animation,criminal investigation and cosmetics industry.So far,many researchers have researched on the almost basic structure sand tissues of facial skin,but many results are based on in vitro experimental data.In vitro environment,it is currently not possible to completely recreate in vivo boundary and other environmental conditions.Therefore,it cannot characterize the mechanical properties of skin accurately,objectively and comprehensively.So,in vivo study mechanical properties of facial skin,especially the development of experimental devices and the proposed of new experimental methods is a very meaningful work.In this paper,a new system was developed by measuring mechanical properties of facial skin and soft tissue,which based on indentation and friction device integrated with three industrial cameras CCD.One of the CCD connected transparent indenter is used to observe the contact area and the topography of the test area.The other two CCDs acquire the external image of test area during the experiment,and then obtained the deformation field by using digital image correlation technology(DIC).Compared with the previous indentation and friction experiments,this system can not only obtain the force-displacement curve,but also observe the morphology of the test area and the deformation of the surrounding test area.It can be obtained the mechanical properties of the facial soft tissue more accurately.In this study,an experimental setup was custom-designed for the mechanical characterization combining curved surface optical imaging and indentation techniques.By means of an independently developed transparent indenter,the indenter can normally intented facial skin by imaging analyses,and the contact area and topography of facial skin can be in vivo and in situ captured in real time.A modified formula for the contact area calculation of silica gel,one of the most common materials used to simulate human facial skin,has been proposed.Compared with the formulas in classical Hertz theory,the contact force calculated by the new formula is in better agreement with the experimental results,which also proved its reliability and applicability.Furthermore,we perform the in vivo indentation tests on human facial skin to evaluate the Young's modulus,which shows a good agreement with the previous literature.A better understanding of the adhesive behavior of human facial skin is important for clinic applications.In this study,we based on transparent indenter and curved surface optical imaging technique,and further optimizing light source and indenter and other components,by which the contact area of facial skin can be in vivo and in situ obtained during an indentation test.The real contact radii of human facial skin under different conditions(normal,as-moistened,moistened after 30 minutes)was obtained by indentation experiments on the six volunteers and their adhesive behavior is studied.We analyzed the adhesion forces under different conditions and the differences of theoretical adhesion energy and theoretical debonded radius with the experimental results.Besides,we revise the classical elastic adhesion theory(JKR theory)based on Neo-hookean model,and the contact radii obtained by the modified hyperelastic adhesion theory model shows a better agreement with the experimental values.