In Vitro Biaxial Stress-Strain Response of Human Skin

Abstract

\s=b\The development of new experimental methods and test equipment has permitted a study of the mechanical characteristics of unembalmed human cadaver skin. Excised tissue specimens were exposed to dynamic, biaxially applied displacements and forces in order to quantify the skin's anisotropic, viscoelastic stress-strain response. In addition to monotonically increasing displacement\x=req-\ time loadings, experiments were performed to measure the tissue's stress relaxation characteristics resulting from a suddenly applied displacement. Data are presented that demonstrate a nonlinear stress-strain relationship for the skin that is notably influenced by displacement boundary conditions imposed on the tissue specimen. Results indicate that the skin is a viscoelastic material whose mechanical loading response is dependent on the anatomic direction in which the strains are applied. (Arch Otolaryngol 1984;110:329-333) Consi derabl e progress has been made in understanding the nature of the dynamic loading response of the skin when exposed to time varying displacements. Experi¬ mental and analytical investigations have properly classified the mechani¬ cal stress-strain characteristics of the skin as a nonlinear, viscoelastic, anisotropie material. As such, it requires unique considerations in experimental protocols designed to quantify its material properties.Early experiments employed uniax¬ ial test methods to measure the loadelongation properties of the skin. Basic studies by Wohlisch1 involved the loading of long, thin strips of skin by hanging static weights and record¬ ing the tissue's displacement. Subse¬ quently, motor-driven loading ma¬ chines were employed in uniaxial test protocols as evidenced by the work of Ridge and Wright.23 Their results identified the nonlinear stress-strain characteristic of the dermis while eliminating the effects of creep by closely controlling the rate of loading. The tissue's stress response to dis¬ placement was visualized as a threephase characteristic.4 Initially linear, the skin's compliance is reduced and eventually demonstrates an exponen¬ tial stress-strain characteristic of rapidly increasing stiffness. The anisotropie nature, or directional dependence on the mechanical re¬ sponse of the skin, was demonstrated in tests along and across Langer's lines. This anisotropie material behavior dictates that a new regimen of experimental protocols are re¬ quired to quantify the physical prop¬ erties of the skin. As pointed out by Fung,5 anisotropy is the rule in biolog¬ ic tissues. The nonlinear, anisotropie features of the skin are shared by many soft tissues, including the mesentery, lung parenchyma, and pericardium. Subsequently, the first biaxial stress-strain studies of in vitro rabbit skin were reported by Lanir and Fung.6 In plane, loads and dis¬ placements were applied to thin square sheets of rabbit abdominal skin in two orthogonal directions. Their experimental results demon¬ strated the directional dependence of the stress-strain characteristic and the nonlinearity and...

Estimating material parameters of human skin in vivo

“…Differences between the subjects were observed. Variations between individuals have previously been reported as being dependent on body locations and age (Schneider et al 1984;Reihsner et al 1995). The current study shows that differences in the stress-strain relationship must also be The material parameters obtained from the three loading and unloading cycles of the experiment on subject 2 are shown in Table 2.…”

Biomechanical Factors in Human Dermal Bitemarks in a Cadaver Model*

“…The use of cadavers to test biomechanical properties of skin is well established. (30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40)(41). Although wound response is not seen in cadavers (edema, inflammation, bruising, and healing), the biomechanical features of the skin are retained for a period of time with properly refrigerated cadavers.…”

Combining Unique Planar Biaxial Testing with Full‐Field Thickness and Displacement Measurement for Spatial Characterization of Soft Tissues

“…Early biaxial tests (Lanir & Fung, 1974a, 1974b) revealed hallmark characteristics of soft tissues that differentiate them from other materials, including nonlinear force‐displacement relationships, pronounced hysteresis, and mechanical anisotropy. Throughout the next two decades, biaxial testing was conducted on many different soft tissues, including skin (Alexander & Cook, 1977; Meijer, Douven, & Oomens, 1999; Schneider, Davidson, & Nahum, 1984), bladders (Gloeckner, Sacks, Chancellor, & deGroat, 1999; Gloeckner et al., 2002; Nagatomi, Chancellor, & Sacks, 2003), arteries (Chuong & Fung, 1986; Chuong & Fung, 1983; Dobrin & Canfield, 1984; Fung et al., 1979), heart valves (Billiar & Sacks, 2000; Lo & Vesely, 1995; Stella & Sacks, 2007), pericardium (Chew, Yin, & Zeger, 1986; Lee, Lewinter, Freeman, Shabetai, & Fung, 1985; Lee et al., 1987), endocardium and epicardium (Humphrey et al., 1990; Kang, Humphrey, & Yin, 1996), and myocardium (Demer & Yin, 1983; Sacks & Chuong, 1993; Yin, Strumpf, Chew, & Zeger, 1987). From these studies, foundational principles integral to the biaxial testing of soft tissues arose, including preconditioning and pseudoelasticity (Fung et al., 1979; Tong & Fung, 1976), residual stresses (Chuong & Fung, 1986), and exponential strain energy functions and strain‐stiffening behavior (Choi & Vito, 1990; Guccione, McCulloch, & Waldman, 1991; Tong & Fung, 1976).…”