PSI - Issue 18

S.V. Slovikov et al. / Procedia Structural Integrity 18 (2019) 198–204 S.V. Slovikov and O.A.Staroverov / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction The plastic surgery of the soft tissue means not only knowledge of biological aspects of application of ways of restitution, but also understanding of the mechanical processes resulting from intervention. Complexity for a comprehension of mechanics of behavior of living tissues is their non-linear, anelasticity, a wide scatter and difficulty of determination of mechanical parameters. One of the areas of surgery where the use of soft tissue prosthetics is necessary is the surgical repair of hernia defects of the abdominal cavity (hernioplasty). The occurrence of defects is expressed in the destruction of the structure of tissues and is manifested in the form of the formation of hernias. The polymeric meshes made on the basis of monofilament yarns or polyfilament yarns of polypropylene, a polyester, polytetrafluoroethylene are widely used in a hernioplasty of the abdominal area (Zhukovsky, 2011; Coda et al., 2012; Parshikov et al., 2016). Despite the rather long practice of using polymeric meshes in hernioplasty, relapses and complications are still frequent (Ciara, 2016; Latifi et al., 2017). The element of the soft tissues of the abdominal region, which performs the main mechanical function, is aponeurosis, and its mechanical degradation leads to hernia formation (Grigoryuk, 2011). The study of the mechanical properties of soft tissue is difficult and in the literature there is a limited number of publications on this issue. Most often, only the maximum load that soft tissues can withstand is determined. But in this case irreversible processes already occur, and they are unacceptable for a living organism. Or, when conducting research, it greatly simplifies the mechanics of an object's behavior, reducing it to an elastic one and determining only the stiffness ratio of soft tissue (Corey, 2017; Kirilova et al., 2011). Often, tests are carried out on tissues that have become dead or cooled, which changes their mechanical behavior, especially in the initial area of loading diagrams, where significant strain occur without a significant increase in loads (Chow and Zhang, 2011). This is explained by the fact that the collagens entering the aponeurosis structure lose their mechanical properties during cooling and dead. As shown by numerous studies, living tissues have a significant non-linearity of mechanical loading diagrams. Moreover, an increase in the stiffness of tissues with an increase in their strain is a fundamental mechanical property that ensures vital activity (Kobelev et al., 2003; Fung, 1993). This is also evident from the analysis of the complex composite structure of living tissues consisting of cellular elements, collagen and elastic fibers, the reinforcing fibers being in the assembled state and with increasing strain of the tissues unfold, providing an increase in stiffness for large strain (Grigoryuk, 2011; Kirilova et al., 2011). Fung proposed the following expression describing the nonlinearity of the dependence of stress (s) on strain (e) (Fung, 1967; Fung, 1975): (1) where the parameters  and  are determined by approximation of experimental data describing the dependence of stress on strain. There are also many other ways to describe the mechanical behavior of living soft tissue (Shmurak, 2017). At the same time, most technical materials, if they possess nonlinear properties, then with increasing strain, their rigidity decreases. Modern surgical polymeric meshes for the most part have the non-linearity of force-displacement diagrams, which is due to the knitting geometry. This is not done to repeat the non-linearity of the restored soft tissues, but due to the fact that such an interlacing does not dissolve when cut in any direction. In addition, the knitted production method allows varying the porosity in wide ranges, as one of the important factors of engraftment of the polymer mesh that is foreign to living tissue. The studies on the mechanical properties of aponeurotic tissue are few and one of the most detailed is the study of Grasse et al. (2005), where almost complete diagrams of loading of aponeurotic tissues from three regions of the abdominal cavity (supra umbilical, subhepatic, umbilical) are shown in different directions for both sexes and different age groups. ( ), d d       