Stress-strain curves for skin

The stress-strain curve for skin is shown in Figure 5.126. For strains up to about 30%, the collagen network offers little resistance to deformation, which is similar to... 

Figure 6.6. Stress-strain curve for skin. Stress-strain curves for wet back skin from rat at strain rates of 10 and 50% per minute. The low modulus region involves the alignment of collagen fibers along the stress direction that are directly stretched in the linear region. Disintegration of fibrils and failure occurs at the end of the linear region. (Adapted from Silver, 1987.)...

Determination of Elastic and Viscous Stress-Strain Curves for Skin... 

Figure 7.10. Stress-strain curve for skin. Total stress-strain curves (open boxes) were obtained by collecting all the initial, instantaneous, force measurements at increasing time intervals and then dividing by the initial cross-sectional area and multiplying by 1.0 + the strain. The elastic stress-strain curve (closed diamonds) was obtained by collecting all the force measurements at equilibrium and then dividing by the initial cross-sectional area and multiplying by 1.0 + the strain. The viscous component curve (closed squares) was obtained from the difference between the total and the elastic stresses.

Figure 5.126 Stress-strain curves for wet back skin at various strain rates. Reprinted, by permission, from F. H. Silver and D. L. Christiansen, Biomaterials Science and Biocompatibility, p. 203. Copyright 1999 by Springer-Verlag.

Our last example of the mechanical properties of a protein is that of keratin found in the top layer of skin. The stratum corneum in skin is almost exclusively made up of different keratins that have an a-helical structure. The helices do not run continuously along the molecule so the structure is not ideal. However, the stress-strain characteristics are shown in Figure 6.4 and demonstrate that at low moisture content the stress-strain curve for keratins in skin is approximately linear with a UTS of about 1.8 GPa and a modulus of about 120 MPa. These values are between the values reported for elastin and silk, which is consistent with the axial rise per amino acid being 0.15 nm for the a helix. Thus the a helix with an intermediate value of the axial rise per amino acid residue has an intermediate value of the... 

Table 2.1 lists and defines the terminology of mechanical stress-strain testing. Table 2.2 shows the values for procine skin. The typical stress-strain relationship for human skin is shown in Fig. 2.6 and the E for modulus value is shown as the slope on the linear segment of the curve. 

Elastic and viscous stress-strain curves have been measured for human aorta as well as other arteries. The curves are all similar in that the stress is much lower than that for skin (see Figure 7.11). The lower stress values are consistent with a different network structure of vessel wall compared to skin, which is reflected by the smooth muscle content of aortic tissue. Smooth muscle is absent from skin. The curves for different vessels have similar shapes, however, on careful review the curves have much lower values for the high strain moduli. This relates to the differences in the structure of the media from each of these vessels and potential crosslinking differences. 

The foregoing analysis of the skin-doubler specimen shows that it is essential to know the stiffness characteristics of the adhesive. Since good design practice places bond lines in shear, it was decided that the shear modulus is the primary stiffness parameter. Furthermore, it is recognized that more than the initial portion of the shear stress-strain curve was required. It was clear that the total curve was not linear. It was anticipated that the nonlinearity portion would bear heavily on creep and fatigue performance. Accordingly, the primary requirements for the strain measuring device were set as follows ... 

In this study, the correlation between dF and stiffness was demonstrated by an exponentially fitted curve (Fig. 4). However, our previous study obtained a semilogarithmic curve for a gelatin phantom with brain-like behavior [19]. This discrepancy was due to the differences in the measured objects. Our previous study involved the direct measurement of gelatin with an even structure. On the other hand, in the present study, the measured object had uneven and multiple-layer structures and was investigated via a decompressive cranial window. Although this study was limited because of the obstmctions due to the multiple structural layers including skin, subcutaneous tissues, muscle fascia, and dura, it was shown that the viscoelastic properties of the brain could be evaluated by tactile resonance instead of stress-strain methods. 

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