Mechanical properties of human skin equivalents submitted to cyclic tensile forces

As human skin equivalents (SE) represent an important breakthrough for clinical grafts and experimental tools, the various methods for mechanical and functional analyses are also gaining importance in the optimization process. SE characteristics have already been studied through indentation-strength and strain-energy methods. Since repeated tensile loading is a common reality of skin, we present in this paper the evolution of the indentation readings method for SE, including its dermal equivalent (DE) and collagen gel (CG), during cyclic tests.

Briefly, the in vitro production of the disk-shaped (25.4-mm-diameter) SE is based on the culture, under submerged conditions, of keratinocytes seeded on our anchorage-based DE, a human type I+III CG supplemented with elastin and GAGs. All specimens were submitted to quasi-static ramp-deflection cycles induced by means of an actuated hemispherical head (12.7-mm-diameter). We studied the effects of repeated loading through monitoring of the indentation load (P) versus the deflection (Δ) and the relaxation of P over 1000 s.

With the help of the P-Δ indentation curves, we determined characteristics for typical CG, DE and SE. Following the respective loading history for each specimen, we demonstrated that the loading and unloading branches, the residual and the upper P, and the hysteresis shaped by the loading-unloading loop were consistent characteristics in the functional analysis of tissue engineered dermis and epidermis.

The P-Δ indentation curves obtained from a loading history give valuable characteristics that allow accurate description of the mechanical behavior of tissue engineered skins during their optimization process for transplantation or experimental purposes.