Dwell time in contact-free creep tests plays an age-dependent role in the viscoelastic behavior of in vivo human skin

The experimental characterization of human skin in vivo may be highly dependent on the experimental protocol and on the procedure employed for analyzing the experimental data. One way of overcoming these drawbacks is the contact-free indenter at the Laboratoire de Tribologie et Dynamique des Systèmes, Lyon, France. However, it is still necessary to determine the possible influence of the experimental protocol in terms of the dwell time chosen. This paper describes a coupled theoretical/experimental/numerical study on the viscoelastic responses of the human skin at two different ages during contact-free creep tests with dwell times of 0.1 s, 0.5 s, 1 s, 2 s and 100 s, respectively. The 3-D finite element simulations are conducted with the SYSTUS® software. The numerical results point to different behaviors for the young and elderly subjects. The young subject does not appear influenced by dwell times during the numerical creep tests. On the contrary, the response of the elderly subject is highly dependent on dwell time.