ABSTRACT
Introduction 17 Studies in Vitro (ex Vivo) 18
Preparation of Samples 18 Stress-Strain Curves in Vitro 18 Anisotropy of Skin 21 Hysteresis Experiments 21 Relaxation Experiments 22 Isorheological Behavior 23 Repeated Strain 24 Creep Experiments 24 Studies in Skin Wounds 25 Thermocontraction 26
Studies in Vivo 27 Stress-Strain Curves in Vivo 27 Repeated Strain in Vivo 28 In Vivo Recovery after Repeated Strain 29
Conclusions 30 References 31
A significant question is whether animal models are suitable to predict mechanical properties in human skin. Can the results of animal studies be extrapolated to human skin, and are the basic approaches the same? Several attempts have been performed to describe the mechanical properties of skin by mathematical models (Ridge and Wright, 1964, 1965, 1966; Harkness, 1968, 1971; Hirsch and Sonnerup, 1968; Jamison et aI., 1968; Viidik, 1968, 1969, 1973a,b, 1978, 1979; Frisen et aI., 1969a,b; Veronda and Westman, 1970; Danielson, 1973; Soong and Huang, 1973; Wilkes
18 Bioengineering of the Skin: Skin Biomechanics
et a!., 1973; Jenkins and Little, 1974; Lanir and Fung, 1974; Vogel, 1976, 1986; Barbanel and Evans, 1977; Barbanel et a!., 1978; Lanir, 1979; Barbanel and Payne, 1981; BUrlin, 1980, 1981; Fung, 1981; Sanjeevi, 1982; Potts and Breuer, 1983). Most of these authors used models derived from studies in polymers (Ferry, 1970). The simplest mechanical model analogous to a viscoelastic system is a spring combined with a dashpot, either in series (Maxwell element) or in parallel (Voigt or Kelvin element). Combinations of these elements were used to explain the mechanical phenomena in connective tissue, such as stress-strain behavior, relaxation and mechanical recovery, hysteresis, and creep phenomena (Jamison et al., 1968; Frisen et a!., 1969a,b; Hirsch and Sonnerup, 1968; Vogel, 1976a, 1993a,b; Riedl and Nemetscheck, 1977; Vogel and Hilgner, 1979a; Viidik, 1968, 1969, 1973, 1978, 1979). Larrabee (1986), Larrabee and Sutton (1986), and Larrabee and Galt (1986) reviewed the theoretical and experimental mechanics of skin and soft tissue and proposed a mathematical model of skin deformation based on the finite-element method. A finite-element-based method to determine the properties of planar soft tissue was also described by Flynn et a!. (1998). Unfortunately none of these models has been found sufficient to describe all properties of human and animal skin including the mechanical history before measurement and the time dependence during measurement. There is no comprehensive and unequivocally accepted model to describe completely the biorheology of skin.
