New regime in the mechanical behavior of skin: strain-softening occurring before strain-hardening.

We report linear and non-linear shear tests on rat skin, evidencing a strain-softening regime, from 1% to 50% strain, followed by a strong strain-hardening regime, leading to a 'deck chair-shaped' stress-strain curve. The strain-softening regime was never reported as such in the literature, possibly mistaken for the linear regime in experiments starting above 1% deformation. The time-dependent response is akin to that of a gel, with a power-law frequency-dependent dynamic shear modulus ranging from ~5.6kPa to ~10kPa between 0.1Hz and 10Hz. We present an analytical non-linear viscoelastic model that accounts for both time-dependent and strain-dependent features of the skin. This eight-parameter model extends the one we proposed for parenchymatous organs by including strain-softening.