On characteristic identification for prestressed human skin

In the paper, we present a model of skin in the form of viscoelastic layer-like structure that is inhomogeneous in thickness and consists in turn of three layers: subcutaneous fat, dermis and epidermis. We also assume the presence of an inhomogeneous uniaxial prestress in the skin layer arisen as a result of relaxation after the skin lifting (tightening) surgery procedure. We formulate the problem on the basis of the general problem statement on steady-state vibrations of an inhomogeneous body, taking into account the initial stress-strain state. Using the correspondence principle, the elastic moduli in the problem statement are replaced by complex analogs corresponding to the model of a standard viscoelastic body. The inverse problem is to determine the mechanical characteristics (complex analogs of the Lame parameters) and prestress using the data on the acoustic response for a periodic probing non-invasive effect on the layer surface. By using the Fourier transform in the longitudinal coordinate, we reduce the original problem to solving a number of simpler problems in transforms. Based on the combination of different loading modes, we present a two-stage scheme for identifying mechanical properties and prestress. Within this scheme, at the first stage, complex analogs of the Lame parameters are successively determined using the iterative approach and the Tikhonov regularization method. At the second stage, by using the functions found at the first stage, prestresses are determined using the proposed projection technique. We illustrate the approach developed to the considered skin integument problem by performing computational experiments showing the efficiency of the techniques proposed in the ranges corresponding to the real values of the sought-for parameters. Additionally, we provide some recommendations on the choice of frequency ranges for the best identification at each stage.