A Study Of In-situ Measurement Method And Analysis On The Mechanical Properties Of Human Skin

Skin is the largest organ of human body.In-depth research on the mechanical properties of skin will promote the rapid development of mechanics,life science,bionic robot,psychological cognition and other disciplines.In this paper,the relevant research work is carried out on in situ measurement of mechanical properties of in vivo skin,and the experimental methods for measurement of hyperelastic and anisotropic mechanical properties of in vivo skin are systematically studied by bulging method and improved indentation method,respectively.For in situ measurement of in vivo skin mechanical properties,conventional experimental methods such as stretching are no longer applicable.The bulging method shows a better application prospect for this problem.The principle of the bulging method is that the particle at the pole is in an equal biaxial tensile state when a plane membrane is subjected to the lateral pressure.In other word,the bulging method is another experimental method of equiaxial tension.In this paper,an experimental study on the bulging method is carried out using a silicon membrane.The deformation in a neighborhood of the pole is taken to be equibiaxial,and the inflated membrane is viewed to be spherical.In order to verify the reliability of this method,uniaxial and plane tensile tests are performed on the same materials.It is found that the results obtained by bulging method are consistent with the results by the latter two methods.Based on the above study,the governing equations of bulging deformation is established by the energy variational principle,and the numerical prediction results of bulging experiment are presented using the Gent model and GG model,respectively.It is found that the prediction result by the GG model can perfectly matched with the experiment results,proving of the correctness of the numerical method and reliability of the experimental results by bulging method and showing the superiority of GG model.Based on the numerical solution by GG model,the error analysis is conducted due to the basic assumptions used in the bulging experiment.This study provides theoretical support and technical guidance for the measurement of skin mechanical properties by bulging method.On the other side,the skin exhibits the anisotropic property due to fiber reinforcement.The indentation method is just applicable to measure the stiffness and hardness of isotropic materials,and its application region is limited by the only contact force-penetration depth curve.In this paper,the analytical solution of the spherical indenter normally indented on a half space is extended from isotropic material to anisotropic material.In addition,the contact area in the anisotropic indentation theory is no longer circlar but elliptical,and its eccentricity has a dependence on the anisotropic material parameters.In order to verify the rationality of the anisotropic indentation theory,the finite element mothed is used in this paper.By comparison,it is found that the theoretical prediction results are in a good agreement with the numerical simulation results.Moreover,the smaller the penetration depth is,the smaller the relative error is,indicating that the analytical solution is suitable in the small deformation range.And then the indentation test is performed on the human skin of the inner forearm.In order to obtain the contact image during the indentation test,a special optical system is developed.A reasonable material model of skin is presented,in which the skin’s compressibility,anisotropy due to fiber reinforcement and tensile hardening characteristics are considered.Applying the skin material model to the anisotropic indentation theory,a specific analytical expression is abtained.With the use of the contact force-penetration depth curve and the eccentricity of contact area,the relevant material parameters can be determined.In addition,in order to measure the skin’s tensile hardening characteristics,the transparent indentation experiment is also carried out on the same position where the skin is in a pre-stretched state.