Page 348 - Advances in Biomechanics and Tissue Regeneration

17.2 BIOMECHANICS IN THE CONTEXT OF THE SKIN 347

FIG. 17.4 Illustration of steps for mechanical tensile testing: (A) the samples are harvested and prepared into specific geometries; (B) typical
stress-strain curve highlighting important measurement points. The typical stress-strain curve is characterized by an initial linear region before
the yield point (called elastic region I) and a postyield nonlinear region (called plastic region II). In the elastic region, the loaded material will effec-
tively return to its original length when the load is removed, that is, there is a negligible permanent extension. The slope of the stress-strain curve
within the elastic region is defined by Hooke’s law and Young’s modulus can be determined. In the plastic region, the material begins to yield, which
mean that the material does not recover to its original shape when the stress is removed. The yield point is called yield stress (σ y ) and corresponds to
the transition from linear (elastic region) to nonlinear behavior (plastic region). The maximum stress (σ max ) is the highest tensile stress that the mate-
rial can support before failure, which means the ultimate tensile strength (UTS) of the material. At the end of the curve the specimen fractures (III).

fibers gradually elongate and tend to align toward the load application direction [4]. The alignment of the collagen
fibers leads to a high resistance the load, which makes the skin tissue to behave as a stiffer material. Thus, the
stress-strain relation turns linear due to the stretching and slippage of the collagen fibers [53, 54]. The third stage begins
after the yield point, for skin considered the strain above 60%–70%. Then the ultimate tensile strength is reached, and
the rupture of the fibers starts to occur due to the loss of fibrillar structure [55]. At this stage, skin tissue loses the capac-
ity to return to its original shape upon removal of the tensile stress.

II. MECHANOBIOLOGY AND TISSUE REGENERATION