Experimental Investigation of the Inelastic Response of Pig and Rat Skin Under Uniaxial Cyclic Mechanical Loading

Abstract

Skin is a highly non-linear, anisotropic, rate dependent inelastic, and nearly incompressible material which exhibits substantial hysteresis even under very slow (quasistatic) loading conditions. In this paper, a series of uniaxial cyclic loading tests of porcine and rat skin at different strain rates and with samples oriented in different directions (with respect to the spine) were conducted to study the effect of strain rate and samples orientations with respect to spine on mullins effect and skin inelastic response. A noteworthy feature of skin is that, similar to certain filled rubbers, its mechanical response shifts after the first extension and exhibits softening and hysteresis when loaded under cyclic tension and mullins effect is observed. The results of these strain-controlled cyclic loading tests also indicated that the extent of softening is different for different strain rates and orientations. Also a substantial hysteresis persists even at very low strain rates indicating inelastic behavior beyond the rate sensitive viscoelastic response. Through this series of experiments, by investigating the effect of strain rate on pig skin and rat skin, we conclude that the skin response is rate dependent but inelastic and shows irreversible changes in fiber orientation which are observed in histology re-

Skin under Strain: From Epithelial Model Tissues to Adult Epithelia

“…Here, we chose to use a strain level of 15% for adult skin instead of 40% for the other models in order to reach similar tissue peak stress levels after the initial ramp stretch between the different models. Moreover, the measured tissue stresses were below stress levels that were previously reported to induce fibrillary orientation in the adult epidermis [ 9 , 11 , 21 ]. Indeed, even though the absolute force levels differ between 15% and 40% for adult rat skin explants, we did not find major differences in curve progression and normalized force relaxation behavior in our experimental setup ( Figure S2B,C ).…”

“…In general, in vivo measurements are performed in this small strain regime but they are often difficult to reproduce whereas measurement devices for in vitro mechanical testing almost invariably perform much better in the high strain and high force regime [ 16 , 17 , 18 ]. Therefore, many publications focused on excised human or animal tissue at high and possibly non-physiological stress levels to characterize the mechanical properties of skin [ 10 , 19 , 20 , 21 ]. Here, monotonic or cyclic uniaxial tensile tests were used to analyze collagen fiber orientation and fracture as well as the influence of strain rate on viscoelasticity and anisotropy.…”

Foundations of viscoelasticity and application to soft tissues mechanics

“…We conclude this brief introductory review by noting that viscoelasticity is not the only time-dependent property of soft tissues. Rate-type effects, such as stiffening and softening as a results of cyclic loading and unloading and ageing are other common effects displayed by biological tissues [29,30].…”