Mechanical and structural anisotropy of ex vivo and in vivo skin

Abstract

The skin is the first protective layer against the external environment of the body. The body's largest organ, the skin, plays a crucial role in everyday biological and social life. The skin controls the interference of thermal, chemical, biological, and mechanical processes with the external environment. From a mechanical and material science point of view, the skin is multiphasic and multiscale structure enabling the rich set of mechanical properties and constitutive behavior. Understanding skin's anisotropic and nonlinear mechanical behavior has critical importance in cosmetic, reconstructive surgery, skin grafting, healing, and tissue expansions. The three-layered skin structure comprises the epidermis, dermis, and subcutaneous tissue, where the main constituent of the dermis, collagen fibers, govern the mechanical behavior of skin. The lack of knowledge about the skin’s capability of in vivo stretching, its laxity, and anisotropic behavior leads to complications in reconstructive surgeries. The primary objective of the present thesis is to perform the numerical modeling of in vivo human skin to simulate the tissue expansion procedure. For that, the quantitative knowledge of the skin tissue's hyperelastic and anisotropy mechanical behavior under in vivo loading conditions is necessary. In order to model the skin tissue using the structure-based numerical formulations, the quantitative value of collagen dispersion for in vivo human skin is required. Therefore, we have developed the approach to quantify the collagen dispersion, which was validated by numerical modeling. Also, we have developed a full-field measurement surface wave technique to quantify the anisotropy of the epidermis and stratum corneum. This study has aimed to understand the contribution of skin tension lines and primary microrelief lines on the anisotropy of the skin. The uniaxial tensile test is one of the most reported standardized methods to determine the stress-strain relation in soft tissues. In these reported studies, the anisotropy of the excised skin was measured by taking the specimens in different directions concerning the skin's natural tension. However, from the literature, it can be concluded that the types of loading (e.g., Uniaxial, biaxial, and multiaxial) make significant contributions to anisotropic response due to the nature of collagen fibers. Therefore, to apply the more generalized loading and overcome the uniaxial tensile test's limitations, we performed the bulge test with a full-field measurement imaging technique. In the bulge method, the tests were executed using the saline solution at 37°C to mimic in vivo body temperature and fluid conditions. The results of the experiment and histology revealed the significant role of collagen directional distribution intensity on the variation of modulus with the direction. Although this technique explains the complete planar anisotropy of the skin, it fails to explain in vivo mechanical properties of the skin. Therefore, in vivo techniques are essential for transferring the experimental information to the clinical setup. The next part of the work developed a device to perform in vivo tests on human skin noninvasively. This apparatus has been designed to overcome the limitations of existing devices. The available suction-based devices quantify the anisotropy through the displacement field and cannot measure the stress-strain relation in particular directions. Therefore, in the current study, an in vivo full-field measurement suction apparatus was developed to measure the stress and strain of skin in all planar directions through a single experiment. First, this apparatus was tested on silicone substrates of known properties, and then it was used to test the skin of twelve human forearms. Further, to check the effect of hand stability on the measurements, the obtained results of the skin were compared with the results of a standard test performed in the same skin using a steady setup. The consistency between these two results confirms that the stability of the hand does not influence the measurements of skin properties. Also, the direction of maximum moduli was found almost similar to Kraissl's lines' orientation. These results confirm the contribution of skin pre-tension on the anisotropy of the skin. The present test method mimics the tissue expansion procedure for the circular expander. However, the use of different shape test areas can help in the tissue expansion procedure to select the size and shape of the tissue expander. The observations of the above two studies have been combined for the collagen dispersion-based anisotropic numerical modeling of in vivo skin. This approach evaluates the collagen fiber dispersion and concentration parameter, which is helpful for the anisotropic numerical modeling of the skin. This approach overcomes the limitation of conventionally used invasive techniques for the calculation of collagen dispersion. Further, the developed approach was validated in this study by implementing the experimental results in finite element modeling using the Gasser-Ogden-Holzapfel (GOH) model. The stiffness parameters were obtained from the experimental stress-stretch relations, and collagen fibers dispersion was calculated from in vivo study results. The above studies discussed the mechanical anisotropy of the full-thickness skin, where anisotropy is mainly due to the preferential alignment of collagen fibers in the dermis layer. However, we can observe the fine lines on the superficial stratum corneum by a closer look at the skin. The contribution of superficial microrelief lines on the anisotropy of stratum corneum (SC) and epidermis is not well known. Therefore, we have developed a novel full-field surface wave measurement technique to measure the viscoelastic properties of the epidermis and SC in all the direction through a single test. Further, the accuracy and applicability of the developed technique were validated by measuring the viscoelastic properties of the epidermis at an interval of five degrees in planar directions. Then, to estimate the contribution of microrelief lines on epidermis and SC anisotropy, the tests were conducted on the isotropic silicone substrates with surface patterns similar to microrelief lines. This study revealed a significant contribution of microrelief lines on the anisotropy of the stratum corneum. Through this research work, we have achieved a reliable test method and device for in vivo measurement of the structural and mechanical anisotropy of the human skin noninvasively. These devices and methods are potentially useful in medical, research, and cosmetics. In medical applications, mechanical analysis helps in the tissue expansion procedure used for reconstructive surgery and assessment of skin-related diseases. Moreover, the device finds the precise orientation of the natural skin tension line directions. The subject-specific mechanical properties assessment and concentration parameter can be helpful in collagen fibers dispersion-based numerical modeling in FEA.