Indentometric analysis of in vivo skin and comparison with artificial skin models

Background/purpose: The mechanical properties of the skin have been previously analyzed by a number of different techniques including torsional analysis, cutometery, gas-bearing electrodynamometry, etc. The objective of this work is to present quantitative analysis of skin rheology by a technique termed indentometry. Methods: The instrument used was a texture analyzer, which is a mechanical tensiometer simulating the process of touch. The experiments were carried out on human subjects as well as on artificial skin models. They included indentometry tests performed by using spherical probes with various geometrical dimensions as well as stress relaxation and creep experiments. The experimental data were interpreted by using the Hertz theory of contact mechanics and by calculation of fundamental parameters such as the modulus of elasticity. Results: The calculated Young's modulae for skin models ranged from 5.5 x 10(4) to 17.7 x 10(4) N/m(2), while the corresponding values for forearm and facial skin of ten panelists were found to be in the range of 0.7 x 10(4)-3.3 x 10(4) N/m(2). In addition, stress relaxation and creep experiments were conducted, which permitted the assessment of the viscoelastic properties of skin. The results of these measurements were interpreted within the framework of the Kelvin-Voigt model of delayed elasticity leading to the calculation of viscosities and relaxation times. Indentomeric data, obtained by varying the diameter of the indentor and the indentation depth, are also discussed. Conclusion: The indentometric analysis for both in vivo skin and artificial skin models could be interpreted by using the Hertz theory of contact mechanics. The loading and unloading indentometric curves could be used to assess the viscoelasticity of the investigated materials while creep and stress relaxation processes were analyzed quantitatively by the Kelvin-Voigt model with one relaxation time.