AN ELASTIC-VISCOPLASTIC MODEL FOR EXCISED FACIAL TISSUES

Unified constitutive equations for elastic-viscoplastic materials were modified and used to model the highly nonlinear elastic and rate-depe ndent inelastic response exhibited in recent experiments on excised fa cial tissues. These included the skin and the underlying supportive ti ssue SMAS (the Superficial Musculoaponeurotic System). This study indi cates a number of relevant results: The skin is more strain rate depen dent than the SMAS; the nonlinearity of the elasticity of the skin is greater than that of the SMAS; both tissues exhibit a hardening effect indicated by increased resistance to inelastic deformation due to str ess acting over a time period; the hardening effect leads to a decreas e in time dependence and an increased elastic range, which is more pro nounced for SMAS. Consequently the SMAS can be viewed as the firmer el astic foundation of the more viscous skin. Moreover, the relaxation ti me for the skin is fairly short so the skin would be expected to confo rm to the deformation of the SMAS if it remained attached to the SMAS during stretching. This is relevant when it is undesirable to separate the skin from the SMAS for physiological reasons.